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(averaged over all spans) are limited by
þD
Pmax þ Pmargin 10logN with N the number of spans and
the channel power (in dBm) averaged over all spans. The mean excursion D
is averaged over the link and is therefore lower than the previously shown excursions of the final span D
(End). For the example shown in Figure 7.14, the nonlinearities would limit the reach to approximately 30 spans (extrapolated). For an assumed overload margin of 9 dB, here the receiver dynamic range could become the more limiting factor. For negative power excursions, OSNR degradation and again the dynamic range of the receiver are responsible for possible outages. The OSNR reduction is roughly half of the power excursion (in dB) in the last span. Figure 7.15 displays the regular link OSNR reduced by the excursion penalties (OSNRpenalty) illustrating the remaining margin for other distortions and system fluctuations for a given number of spans. The noise limitation of the system reach is given by the number of spans where OSNR-OSNRpenalty falls below the required receiver signal-to-noise
7.6 Mitigation of spectral power excursions
Positive Power Excursion (channels region I) 20 Pmax+Pmargin-
(example)
worst case Δ
(dB)
18 remaining nonlin. margin
16 14
Δ
+10logN
12 overload (example)
10 8
Δ
(End)
remaining overload margin
6 4 2
Δ
0 0
5
10
15
20
span count
FIGURE 7.14 Power excursion assessment in region I (blue) – positive excursion Negative Power Excursions (worst channel region II) power (dBm) or OSNR (dB)
40 30
OSNR - OSNRpenalty
20 10
RX OSNR limit (example) Noise Limitation RX Sensitivity Limitation
0 0 -10
5
10
15
20
RX sensitivity(example) p (End)
-20
span count
FIGURE 7.15 Power excursion assessment in region II – negative excursions. p (End) refers to received power for the channel which suffers the largest negative power excursion after the Nth span. The arrows cross the x-axis at the limitations imposed, respectively, by noise and receiver power sensitivity as indicated.
ratio. As with the overload, the excursions may cause outages by exceeding the dynamic range of the receiver. In this case, the reach is given by the point where the negative excursion D
(End) forces the received power level p (End) to drop below the RX sensitivity. As shown before, excursions in region II (see Figure 7.11) can pose a strong limit on the system reach. They directly scale with the Raman tilt in each fiber span. The relevant system parameters as margins, sensitivity, OSNR limit,
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CHAPTER 7 Spectral power fluctuations in DWDM networks
etc., depend on modulation format, Forward error correction (FEC) type, and the receiver type (PIN or APD) and are therefore different for any transmission system. Signals at higher data rate require better receiver OSNR and therefore are more impacted by the power excursions for the same transmission distance.
7.7 SUMMARY AND CONCLUSION Failure events in transparent optical DWDM networks can cause sudden changes of network load conditions. While EDFAs with fast AGC can efficiently suppress excursions in the total channel power, SHB in the amplifiers and SRS in the transmission fibers can lead to fast spectral power fluctuations which can cause BER burst or channel outages. The effects are nonlinear and their impact depends on span count, channel drop, and amplifier saturation level. The influence is low for typical metro/regional networks. Large networks with links consisting of more than eight spans can be severely affected. As long wavelength channels suffer from more degradation, a wavelengthdependent light path assignment can help minimize impairments. Also, a channel replacement circuit that maintains a specific spectral power profile can help.
ACKNOWLEDGEMENTS The authors wish to thank Roman Hartung for his valuable assistance in carrying out the experimental studies. The work reported here has been funded in part by the European Union within the FP6 IST project NOBEL (contract number FP6-506760).
ACRONYMS AGC APD BER DWDM EDFA FEC MD-RODAM NRZ O/E/O OSA OSNR OTN PIN ROADM
Automatic gain control Avalanche photo diode Bit error ratio Dense wavelength-division multiplexing Erbium-doped fiber amplifier Forward error correction Multi-degree reconfigurable optical add/drop multiplexer Non-return to zero Optical/electrical/optical (conversion) Optical spectrum analyzer Optical signal-to-noise ratio Optical transport network p-insulator-n (photodiode) Reconfigurable optical add/drop multiplexer
References
RX SDH SHB SSMF SRS
receiver Synchronous digital hierarchy Spectral hole burning Standard single mode fiber Stimulated Raman scattering
References [1] A.A.M. Saleh, Transparent optical networking in backbone networks, Proc. Optical Fiber Comm. Conf. 2000 (3) (2000) 62e64. [2] K. Motoshima, N. Suzuki, K. Shimizu, et al., A channel-number insensitive Erbiumdoped fiber amplifier with automatic gain and power regulation function, J. Lightw. Techn. 19 (2001) 1757e1759. [3] G. Go¨ger, B. Lankl, Techniques for suppression of Raman and EDFA gain transients in dynamically switched transparent photonic networks, Proc. Europ. Conf. Opt. Comm. 2002, (2002), paper 6.4.7. [4] H. Nakaji, M. Shigematsu, Wavelength dependence of dynamic gain fluctuation in a high-speed automatic gain controlled erbium-doped fiber amplifier, IEEE Phot. Tech. Lett. 15 (2003) 203e205. [5] A.K. Srivastava, Y. Sun, J.L. Zyskind, J.W. Sulhoff, EDFA transient response to channel loss in WDM transmission system, IEEE Phot. Tech. Lett. 9 (1997) 386e388. [6] M. Zirngibl, Analytical model of Raman gain effects in massive wavelength division multiplexed transmission systems, Electron. Lett. 34 (1998) 789e790. [7] E. Desurvire, J.L. Zyskind, J.R. Simpson, Spectral gain hole-burning at 1.53 I¨m in Erbium-doped fiber amplifiers, IEEE Phot. Tech. Lett. 2 (1990) 246e248. [8] C. Fu¨rst, R. Hartung, J.-P. Elbers, C. Glingener, Impact of spectral hole burning and Raman effect in transparent optical networks, Proc. Europ. Conf. Opt. Comm. 2003, (2003), paper Tu4.2.5. [9] A. Pilipetskii, S. Abbott, D. Kovsh, et al., Spectral hole burning simulation and experimental verification in long-haul WDM systems, Proc. Optical Fiber Comm. Conf. 2000 (2) (2003) 577e578. [10] J. Burgmeier, A. Cords, R. Ma¨rz, et al., A black box model of EDFAs operating in WDM systems, J. Lightw. Techn. 16 (1998) 1271e1275. [11] E. Desurvire, J.W. Sulhoff, J.L. Zyskind, J.R. Simpson, Study of spectral dependence of gain saturation and effect of inhomogeneous broadening in erbium-doped aluminosilicate fiber amplifiers, IEEE Phot. Tech. Lett. 2 (1990) 653e655. [12] P.M. Krummrich, R.E. Neuhauser, H.-J. Schmidtke, H. Zech, M. Birk, Compensation of Raman transients in optical networks, Proc. Optical Fiber Comm. Conf. 2000 (1) (2004) 23e27. [13] E. Ciaramella, M. Presi, L. Giorgi, et al., Effective suppression of transient-induced impairments in transparent optical networks, IEEE Phot. Tech. Lett. 17 (2005) 2487e2489. [14] J.-P. Elbers, A. Fa¨rbert, C. Scheerer, et al., Reduced model to describe SPM-limited fiber transmission in dispersion-managed lightwave systems, IEEE J. Sel. Top. Quant. Electron 6 (2000) 276e281.
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CHAPTER
Amplifier Issues for Physical Layer Network Control
8
Daniel C. Kilper, Christopher A. White Bell Laboratories, Alcatel-Lucent
CHAPTER OUTLINE HEAD 8.1. Introduction ................................................................................................ 8.2. Amplifiers in networks: dynamic considerations ............................................ 8.2.1. Constant output power amplifiers ................................................ 8.2.2. Constant gain amplifiers ............................................................. 8.3. Power stability in amplified networks ........................................................... 8.4. Physical layer network control ..................................................................... 8.4.1. Distinct control domains and function ......................................... 8.4.2. Dynamic domains method .......................................................... 8.4.2.1. Fluctuating channel list ....................................................... 8.4.2.2. Triggering control operations (run messages, timeouts, and preemption) ................................................................. 8.4.2.3. DecisiondFractional timeout............................................... 8.4.2.4. Results ............................................................................... 8.5. Conclusions ................................................................................................ Acronyms ........................................................................................................... References .........................................................................................................
221 223 224 231 233 239 239 241 242 243 244 245 249 250 250
8.1 INTRODUCTION Over the past decade transmission systems have evolved from simple point-to-point architectures with a few amplified spans into continental scale, transparently coupled mesh networks. As the client-to-client traffic grows to fill multiple wavelengths, one expects that the traffic engineering capabilities available today in the data layers will become advantageous in the physical or wavelength layer of the network. Today, the limitations of optical amplifiers in their ability to support dynamic loading conditions impede the implementation of such flexibility in largescale transparent networks. Optical amplifiers have enabled wavelength division multiplexing (WDM) in large part due to their ability to provide amplitude regeneration of multiple signals over a wide spectral window using a single device. The advantage of handling Optically Amplified WDM Networks. DOI: 10.1016/B978-0-12-374965-9.10008-1 Copyright Ó 2011 Elsevier Inc. All rights reserved.
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CHAPTER 8 Amplifier issues for physical layer network control
multiple signals in parallel comes with the price of limited control over individual signals. Wavelength dependent gain and loss, either in the amplifier or in other elements in the transmission system, lead to power divergence across the signal band. In most systems the reach of transmitted signals and end terminal signal quality degrade as the power deviates from the design target value. Thus, power divergence translates into a performance penalty, and control over the average gain and the individual channel gain is an important element of transmission system design. Early WDM system deployments used amplifiers in fixed operating modes and often trimmed the power to accommodate different span characteristics using manually applied attenuators (referred to as line break-outs, LBOs). Wavelength dependent gain in erbium-doped fiber amplifiers (EDFAs) was reduced through the use of fixed gain flattening filters. Over time, automated control techniques have been added to increase the transmission reach and compensate for time dependent gain and loss. Common power adjustment technologies include amplifier pump power control, variable optical attenuators, and transmitter output power control. Often these elements can be adjusted at the time of channel provisioning or system commissioning and then left in a fixed state. Long-haul transmission systems (w500þ km distances), particularly at bit rates of 10 Gb/s and above, require tight control of the power divergence and therefore employ power tuning elements that can compensate for the wavelength dependence that varies over time. One approach is to use dynamic gain equalizing filters (DGEF) at selected amplifier sites. These DGEF elements can be used together with continuously or periodically adjusted local amplifier control to achieve stable long-haul transmission. Although upstream power adjustments can impact the power tuning on downstream devices, for pointto-point systems there are no physical mechanisms for positive feedback that can destabilize the system controls. Therefore, in long-haul point-to-point transmission systems, the physical layer system control is often implemented using independent, local controls on the amplifiers and other network elements to manage the optical signal powers along the transmission path. The introduction of transparent networking elements to transmission systems can dramatically change the complexity required of the physical layer control. The most salient impact, which was identified in some of the earliest transparent networking laboratory experiments [1e3], is the so-called transient problem. The existence of transparent paths through optical nodes allows for a disruption caused by a fiber break or similar event to propagate beyond the damaged link. Depending on the system design requirements, controlling these events may place challenging requirements on the control of individual amplifiers (see Chapters 6 and 7). Even with strong compensation in individual amplifiers, other control elements such as dynamic gain equalizing filters and variable optical attenuators will need to be adjusted as well due to the change in the state of the system. Automated provisioning or re-routing (i.e., decommissioning and re-provisioning) of wavelengths requires similar control, although the event can be scheduled and applied gradually to avoid service disruptions. The same transparent coupling that can cause transients to propagate through a network can also cause power fluctuations resulting from
8.2 Amplifiers in networks: Dynamic considerations
routine power adjustments to propagate through a network. For this reason, power control becomes a network-wide optimization problem rather than a local optimization problem. The propagation of power fluctuations in a network is primarily dictated by the active power control elements such as the optical amplifiers. Mechanisms that couple power fluctuations between different WDM channels will also allow fluctuations to extend beyond the originating channels. This introduces the potential for instabilities due to feedback in a mesh network with ring connections. In general, since WDM channels carry traffic to different end users, its critical that events on one channel do not influence the performance of other channels. For example, if a new channel is being placed into service, it should not affect the other channels that are already present and carrying live traffic. Although it is possible to design control elements that limit the interaction between channels in the network elements, the coupling induced between co-propagating channels in an optical fiber may still impact the control algorithm design and overall network stability. This chapter examines the role that optical amplifiers play in the physical layer network control. For systems that can use local amplifier control, such as point-topoint architectures, refer to Chapters 6 and 7 for the operation and control of individual amplifiers. This chapter begins by examining the power dynamics in optical amplifiers that impact network power dynamics, including amplifiers operating with both constant output power and constant gain control. Network topology considerations are described in the second section, and mesh network amplifier control is described in the third section, including a description of the dynamic domains control algorithm.
8.2 AMPLIFIERS IN NETWORKS: DYNAMIC CONSIDERATIONS The dynamic behavior of a given amplifier influences the state of other amplifiers through the transmitted optical power. Neglecting noise contributions, the interactions between amplifiers are mediated by the WDM channels. For the usual case of uni-directional amplifiers, power variations in each amplifier will be carried downstream by each of the respective amplified channels. As these channels reach their destinations, the impact of the original power variations within the network are removed unless there is a mechanism that can allow the power fluctuations to be transferred to other channels. Thus, physical phenomena that couple channel power levels, enabling channels to influence one another, can dramatically change the extent to which a given amplifier can impact the behavior of the network as a whole. Without coupling, each amplifier will have a range of impact that at most extends to the longest channel propagation distance. We will refer to this distance as the channel propagation range. In the absence of power coupling, amplifier dynamics are largely a local problem, although there have been concerns about the growth of large fluctuations in cascade, either from electrical power cycling events [4] or transient events [5]. Even when coupling does exist, the channel propagation range
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CHAPTER 8 Amplifier issues for physical layer network control
is important because the way in which power fluctuations will move through the network is often different depending on whether the fluctuations are inside or outside this range. A wide range of channel power coupling phenomena exist, originating in both healthy and aberrant network operation. One example of a mechanism associated with faulty behavior might include a channel power measurement device (used as feedback to a power controller) that reads incorrect values, dependent on the power in neighboring channelsddue to bleeding effects or saturation. The possibilities for other such events are too varied to consider here. Under normal operation, there are many physical phenomena not related to amplification that can lead to coupling, such as Raman scattering and neighbor channel intensity-dependent crosstalk mediated by the nonlinear refractive index. These effects are usually anticipated and mitigated by design. Raman scattering, however, often cannot be avoided without unduly sacrificing signal-to-noise ratio. The dynamic impact of the Raman effect, which is the change in channel to channel Raman scattering as the channel power and configuration changes, tends to act as a passive lever that modifies other effects. On the other hand, the wavelength dependent loss that results from Raman scattering must be compensated along the transmission path, and this can lead, for example, to a large gain tilt in the amplifiers, which then exacerbates the power coupling in the amplifiers. Finally, coupling mechanisms in the amplifiers tend to be the most important effects, and they depend strongly on the operating mode and state of the amplifiers. Amplifiers that are operated with a constant output power will create coupling between channels through the constant output power constraint: an increase in power for one channel requires a decrease in power for another channel. Constant gain amplifiers, on the other hand, create power coupling through both linear and non-linear wavelength dependent gain ripple. Wavelength dependent gain can also impact constant output power amplifiers, but the effect is usually smaller than the constant power constraint mediated coupling. Figure 8.1 compares the impact of a channel loss event for a 400 node network with mean degree of 2.35 using ideal constant gain amplifiers (i.e., the total power gain is held constant) compared with ideal constant power amplifiers [6]. Here the wavelength dependent gain originates from a tilt placed on the amplifiers to compensate the Raman scattering in the transmission fiber. For the constant power amplifiers, the effect develops quickly and then dies off rapidly outside the channel propagation range (four hops), whereas the impact of the constant gain amplifiers is smaller, grows steadily over the propagation range, and falls off steadily outside the range.
8.2.1 Constant output power amplifiers The strong power coupling associated with constant output power amplifiers was identified in early transparent network experiments and helped motivate the use of constant gain amplifiers. Constant output power can be implemented through either active pump control or passively by operating the amplifiers in deep saturation, which is often the preferred state for other reasons [7,8]. Thus, constant output
8.2 Amplifiers in networks: Dynamic considerations
FIGURE 8.1 Impact of removing a set of channels in a 400 node mesh network quantified by the maximum power change on channels dropped at a given number of hops away from the location at which the channels were cut. Amplifiers are operated in constant gain mode or constant output power mode.
power represents the natural amplifier behavior in the usual saturated configuration and is important to understand from a control perspective. Power coupling between WDM channels occurs because the total power is fixed, and therefore changes in power for one channel must P be compensated by changes in other channels. Thus, for PIj, total output amplifier noise N0, and output power a given input power PIT ¼ j P P POj þ NO ¼ G½ gj PIj þ NI ¼ C when controlled to meet a target POT ¼ j
j
output power C, the power in a given WDM channel at the output of the amplifier can be written in terms of C, average gain G, gain ripple gk, total input-referred noise NI, and the input power of each WDM channel PIj: POk ¼ Ggk PIk ¼ P j
C gk PIk gj PIj þ NI
(Equation 1)
Note that the input referred noise includes the total noise added by the amplifier at the output divided by the average gain plus any noise incident at the input of the amplifier. Another important factor is how the total output power is fixed. In some cases, the channel power spectrum might be monitored and the amplifier pumps adjusted to maintain only the channel power constant, or knowledge of the incident noise and noise added by the amplifier is used to modify the target output power C, such that the total signal power is kept constant (as opposed to the signal power plus
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CHAPTER 8 Amplifier issues for physical layer network control
noise held constant). In each case, the gain G in Equation 1 would need to be modified accordingly. Equation 1 describes the steady-state channel power relationship. Events that occur on time scales much slower than the EDFA response time (>ms) will be dictated by this equation, assuming the amplifier is operated in deep saturation. Examples of typical network events that may occur on these time scales include channel power levelling in reconfigurable optical add/drop multiplexers (ROADMs) or amplifier tilt adjustments. Note that for EDFAs, the gain ripple gk depends on the mean gain G through the average inversion. For the case of identical flat gain (gk ¼ gj ¼ 1) amplifiers in cascade, the new channel power levels generated at the output of the first amplifier will be replicated at the output of all subsequent amplifiers. The constant power constraint in Equation 1 corrects any power fluctuation at the amplifier input and therefore downstream amplifiers will not see a change. Thus, channel power coupling due to constant power amplification does not change in cascade. Note, however, that non-zero gain ripple (gi,k s 1) converts channel power ripple changes into total power changes and can lead to coupling that varies in cascade. This effect occurs for both constant power and constant gain amplification and therefore we treat this gain rippleeinduced channel power coupling separately. The coupling strength or sensitivity can be evaluated between any two channels or groups of channels, by taking the appropriate derivative of Equation 1. Neglecting noise and power dynamics in the amplifier gain, channel power fluctuations at the output of the amplifier can be written in terms of the fluctuations at the input using vPOk Ok a Taylor series expansion: dPOk ¼ vP vPI1 dPI1 þ vPI2 dPI2 þ .. The partial derivatives are evaluated at the desired mean values of the fluctuating quantities and the dP quantities represent deviations from these mean values. A simple example might include two channels (or groups of channels) with input powers PI1 þ dPI1 and PI2 þ dPI2 . The coupling strength for a fluctuation on channel 1 to generate a response in channel 2 is given by: S21 ¼
vPO2 POT g1 g2 PI2 Gg1 g2 ð1 hÞ ¼ ¼ 2 vPI1 ½g1 PI1 þ g2 PI2 þ NI ½g1 h þ g2 ð1 hÞ þ NI =PIT 2 (Equation 2)
where PI1 ¼ hPIT and PI2 ¼ (1h)PIT and PIT, POT are the respective total input and output powers corresponding to the mean gain G. If the noise power is negligible, then the coupling strength depends only on the ratio of the channel powers and the respective wavelength dependent gain. Further neglecting the gain ripple g1 ¼ g2 ¼ 1, S2-1 becomes eG(1h). Of more interest, however, is the coupling strength for a fractional change (or equivalently taking the log one obtains the decibel change in power of channel 2 due to a decibel change in power in channel 1). This quantity is simply S2-1rel ¼ h. Thus, the larger the fraction of power in channel 1, the greater impact a fractional change in channel 1 will have on channel 2. As expected, if PI1<< PI2, then the coupling will be small and changes in channel 1 will have little
8.2 Amplifiers in networks: Dynamic considerations
impact on channel 2 and the converse is also true. Also note that the coupling strength for a power fluctuation on channel 2 at the output induced by a change in channel 2 at the input is modified by the interaction with channel 1 and becomes S22 ¼
vPO2 Cg2 ½g1 PI1 þ NI ¼ /Gh vPI2 ½g1 PI1 þ g2 PI2 þ NI 2
(Equation 3)
Thus, a given fluctuation at the input will be split between outputs by the ratio of the power in the two groups. Consequences of Equation 1 can be propagated through the network and complex relationships are generated depending on the different combinations of add and drop channels. The effect of this steady-state power coupling has been studied through simulations [6,9] and experiment [10]. Consider the case shown in Figure 8.2. Three groups each with eight channels are propagated through four ROADM nodes. Group 1 passing through ROADM1 is cut due to a transient event such as a cable break. The channels in group 2 increase in power in amplifiers 1 and 2 such that the respective total output powers remain constant before and after the transient. Amplifier 3 does not see any change in the number of input channels, but the increase in power of group 2 will cause the newly added channels in group 3 to drop in power at the amplifier output. Neglecting wavelength dependent effects, this disturbance on group 3 will be preserved throughout the cascade. Note that the perturbations on groups 2 and 3 will persist only as long as both groups are present. If group 2 had been dropped at ROADM3, then the disturbance on group 3 would be removed from amplifier 5 onward. Thus, the disturbances persist only through the shared power offset or tilt between two or more channels or groups of channels. Figure 8.3 shows the power excursion experienced by one of the eight group 2 surviving channels for the configuration in Figure 8.2. The excursion generated by the transient event in amplifiers 1 and 2 is dramatically reduced as it is balanced with the newly added channels in group 3 at amplifier 3. The resulting steady-state power deviation for each group can be determined from Equation 1. Using the notation of Equation 2, the change in power for group 2 due to the loss of channels on group 1 is simply PO2 ¼ GhPI2; with h ¼ 0.5, the mean power change is þ3 dB for amplifiers 1 and 2. For amplifiers 3 through 6, the expected mean power excursion in group 2 Group 1 1
Group 3 2
3
5
4
6
Group 2 ROADM1
ROADM2
ROADM3
ROADM4
FIGURE 8.2 Experiment setup to measure transient event power excursions in uncontrolled EDFA cascades with reconfigured channel loading; paths taken by channel groups 1 through 3 indicated the lines with corresponding dash shown in legend.
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CHAPTER 8 Amplifier issues for physical layer network control
(a)
(b)
5
OUT1
4
OUT2
0
Power Excursion (dB)
Power Excursion (dB)
228
3 2 OUT3
1
OUT5
-1
OUT3
-2
OUT6 OUT4
OUT6
0 0.0
0.1
Time (ms)
0.2
0.3
0.0
0.1
0.2
0.3
Time (ms)
FIGURE 8.3 Transient event power excursions in uncontrolled EDFA cascades with reconfigured channel loading shown in Figure 8.2 for a) group 2 and b) group 3. Group 3 is present at ROADMs 2 through 4 before and after group 1 is cut.
becomes dPO2 ¼ 1.5 dB and the power in group 3 becomes dPO3 ¼ 1.5 dB. Note that the channel power excursions in Figure 8.3 do not match these mean calculated values. This is because Figure 8.3 plots the response for one of the eight channels in each group and not the mean power excursion. Furthermore, the gain ripple values are non-zero in the experiment. Notice in particular that the steady-state excursion in amplifiers 1 and 2 for Figure 8.3a) varies from 4.66 dB to 3.67 dB. This reflects a changing tilt that occurred along the cascade. Thus, accurate power evolution requires the complete frequency dependent picture (including the gain dependence of tilt and ripple variations), although the simple model of Equations 2 and 3 describes the rough trends. Using numerical simulation, the impact of channel power coupling has been studied for different network topologies and channel loading. For example, building on the results of Figure 8.3 for a four-node cascade, a 64-node cascade was simulated. The 64-node cascade consisted of 1 amplifier between each add-drop node, and the network was loaded with groups of four channels chosen on random wavelengths out of 100 wavelengths per link. To better understand the impact of power coupling on excursion propagation, all channel groups were chosen to be 10 hops in length. Groups were randomly added at different (bi-directional) nodes until the wavelength assignment became prohibitively difficult due to wavelength blocking. After establishing a steady-state condition with roughly equal channel powers, a 3 dB power change was applied to all channels at a given add location. The resulting power deviation on all other channels at their respective drop locations were then recorded as a function of number of hops from the 3 dB event location, following the new steady-state. Figure 8.4 shows the mean power deviation, where a power deviation is measured relative to the initial steady-state power.
Average Power Deviation (dB)
8.2 Amplifiers in networks: Dynamic considerations
0.6 0.5 0.4 0.3
Mean Path Length LP
0.2 0.1 0.0
LP
-0.1 0
5
10
15
20
25
30
Hops from Event
FIGURE 8.4 Propagation of channel power excursions away from event location in chain of 64 constant power amplifiers and add-drop nodes.
The power excursion increases steadily almost up to the mean path length and then falls off rapidly thereafter with a (hops)1.4 power law [9]. The excursion is observed to oscillate with a period equal to the mean path length, due to the sign change in the power exchange in Equation 2. The mean path length thus influences the location at which the maximum excursion occurs and the evolution of the exchange. Moving to a mesh topology will broaden and tend to shorten the path length distribution of the demands with respect to a given node, due to meandering paths through the network [9]. This assumes that mesh features such as the number of hops around a closed loop are smaller than the mean path length. As a result, the power deviation peak moves closer to the event location and the features observed in Figure 8.4 tend to smooth out. However, the mean path length can still influence the trend [6]. Broadening the path length distribution has a similar effect as the mesh topology. Another interesting observation is that for fixed loading, the maximum power deviation taken over all wavelengths tends to increase as the mean degree of the network increases, whereas the average power deviation tends to decrease [6]. The amplifier gain dynamics of course will play an important role in the channel coupling effects, modifying Equation 1. The steady-state uncontrolled EDFA response is well described by the constant power model, assuming the amplifiers are operated in a saturated condition. Deviations from this simple model, however, are important in the design of telecommunication networks. In particular, over the wide dynamic range required to support large numbers of channels (varying from one to as many as several hundred), it is difficult to design amplifiers for which all stages are heavily saturated under all loading conditions. One important question is how the uncontrolled gain dynamics will shape the time profile of the transition from a given stable initial condition to the new steady-state after a change in power or channel loading has occurred. This response
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CHAPTER 8 Amplifier issues for physical layer network control
along with the associated dynamic system model is essential in the design of amplifier controllers. The harmonic transfer function of an EDFA is dominated by a single pole due to the time lag of the population inversion in the amplifier: HðUÞ ¼
1þ
P j
1 Gj PIj =Psat;j þ ðGp PIp =Psat;p Þ iUs21
(Equation 4)
where Gj is the gain for the channel with input power PIj and saturation power Psat,j, Gp is the gain for the pump with input power PIp, and saturation power Psat,p, s21 is the amplifier spontaneous emission lifetime. This behavior has two important consequences on the dynamic response of uncontrolled amplifiers in cascade: (1) a damped oscillation will form due to the successive lag of each EDFA following the response of the previous amplifiers and (2) the initial derivative on the response will increase in magnitude through the cascade, in proportion to the number of stages/ amplifiers. The second factor is particularly important for EDFA control design in networks. A cascade of 10 identical amplifier stages will be 10 times faster than a single amplifier. Because of this, a controller with an allowable overshoot target of 1 dB must be designed with speed sufficient to meet this target through the longest cascade in the network [11]da far more challenging design problem than the case of a single EDFA. Figure 8.5a) shows the peak derivative of the surviving channel power excursion from the results of experiments shown in Figure 8.3, both with no reconfigurationdFigure 8.2 setup, but with group 1 channels travelling full distance to ROADM 4, no group 3 channels (squares, solid line fit) and with reconfigurationdsetup corresponding exactly to Figures 8.2 and 8.3 (triangles). With reconfiguration, the slope no longer increasesddue to the fact that the surviving channel group is no longer responding to a change, but instead is driving the change. The peak derivative on the excursion of a channel in group 3 is shown in Figure 8.5b).
140
(b)
Transient Duration: OA 1-6 OA 1-2
Peak Derivative (a.u.)
(a) Peak Derivative (a.u.)
230
120 100 80 60 40
-8 -10 -12 -14 -16 -18
20 1
2
3
4
5
Amplifier Number
6
3
4
5
6
Amplifier Number
FIGURE 8.5 Rising edge of power excursion for surviving channels a) group 2 (squares: groups 1 and 2 propagate through full cascade) and b) group 3 from Figure 8.2.
8.2 Amplifiers in networks: Dynamic considerations
The magnitude increases (becomes more negative) in response to the change created by group 2, although it is limited by the speed of the group 2 response and therefore begins to roll over in the last amplifier [10].
8.2.2 Constant gain amplifiers The channel power coupling effects in constant gain amplifiers are typically dominated by wavelength dependent gain phenomena in the amplifier. In some cases this effect is caused by specific aspects of the amplifier physics, such as spectral hole burning. In general, however, channel power coupling can result from the constant gain condition being applied to the total power, which is measured by the input and output power monitors, and not to each individual channel power which is usually not measured or controlled at the amplifier. Because of this total power constraint, any wavelength dependence to the gain will result in channel power interactions, independent of the source of that wavelength dependence. In EDFAs the wavelength dependent gain is often due to imperfect gain flattening, but also can have nonlinear contributions. Often the gain spectrum is intentionally tilted to compensate for spectral tilt generated in the transmission fiber. An important difference between the mechanisms of power coupling in constant gain amplifiers compared with constant power amplifiers is that the coupling and hence the resulting power excursions can grow in cascade. The gain for a channel at the output of a given amplifier can be written POk ¼ fGA0gkPIk, where gk is the gain ripple, GA0 is the total power gain for flat gain (f ¼ 1, neglecting amplifier noise), PIk is the input power, and f is the gain adjustment required to hold the total-power gain G constant, i.e., the change applied to GA0 which results in a constant total power gain of G for the channels that are present. In terms of the total power gain, this becomes P PIj þ NS j Ggk PIk (Equation 5) POk ¼ fGA0 gk PIk ¼ P gj PIj þ NI j
where Ns is the total optical noise incident at the input of the amplifier (as measured by the constant gain controller) and NI is the total input referred optical noise that is emitted from the amplifierdincluding contributions from Ns and the noise added by the amplifier. Neglecting noise and taking the two channel/group case, for a set of n identical amplifiers in cascade with connection losses L, the gain experienced by channel 2 can be written: ðnÞ
PO2 ¼ f ðnÞ f ðn1Þ .f ð1Þ ðGA0 g2 Þn Ln1 PI2 PI1 þ PI2 ¼ GðGLÞn1 n T PI1 þ PI2
(Equation 6)
where T ¼ g1/g2. Note that the gain for channel 2 depends on the input power in channel 1, through the total accumulated tilt in the cascade Tn. The coupling strength
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or sensitivity (taking the derivative of Equation 6 with respect to PI1) can grow in cascade, but will always be less than 1. If on the other hand, the accumulated tilt is periodically compensated, for example by a dynamic gain equalizing filter in a ROADM node, the coupling strength can grow much greater than one. With tilt compensation over a section, the channel power dependent gain is a simple product of response from each compensated section, where n here is the number of such sections: ðnÞ
PO2 ¼ ðfsec GA0 g2 Þn Ln1 PI2 n PI1 þ PI2 ¼ GðGLÞn1 Tsec PI1 þ PI2
(Equation 7)
The channel power dependent gain in Equations 6 and 7 does not rely on any details of the EDFA physics. An important aspect of EDFA gain is that the gain ripple terms gi are dependent on the total power gain (more specifically on the amplifier inversion level). This effect can either reduce the coupling strength or enhance it. Figure 8.6a) shows measurements of the impact on one group of eight channels due to power adjustments on another group of eight channels taken from a ROADM transmission line at the output EDFA of each ROADM [12]. The solid lines show the calculated effect using Equations 6 and 7din terms of the average power for each group of channels. Very good agreement is found after the first amplifier; however, the Raman tilt in the transmission fiber is not included in the calculations resulting in the observed deviations after the third and fifth EDFAs. Also note that for large positive deviation, the measured values roll over because of amplifier maximum output power limits. The peak sensitivity is shown in Figure 8.6b) as a function of the number of amplifiers in cascade.
(b)
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FIGURE 8.6 Channel power coupling due to gain ripple and tilt in a power balanced transmission line a) as a function of location and b) coupling sensitivity as a function of the number of amplifiers with power levelling before amplifiers 3 and 5
8.3 Power stability in amplified networks
Another important aspect of constant gain control is that gain deviations generated by channel power coupling will tend to persist even beyond the region of interaction [13]. Consider the example described in Figure 8.2, but without channel group 3. For constant power operation channel group 2 will be the only group present from amplifier 3 onward. This will cause any power deviation in channel group 2 caused by the loss of group 1 to be corrected as the channels are fixed to the target power in amplifier 3 onward. However, if constant gain control is used, the power deviations in group 2 due to the loss of group 1 will be preserved after amplifier 3 by the constant gain constraint. Thus, with constant gain control, significant feedback is possible even without overlapping groups of channels around the full circumference of a closed loop. Frequent addition and removal of channels along the path, however, will tend to diminish any power errors, assuming the wavelengths are randomly applied relative to any ripple or tilt in the amplifiers [6]. As with any random process, of course, there is always a chance for a worst case configuration to occur, particularly as such transparent network loops will typically have fewer than 10 to 100 nodes in a loop. Because constant gain amplification is realized in EDFAs through gain control methods, the dynamic amplifier response will depend strongly on the controller design. Even in the presence of well designed control, however, channel power coupling will occur due to the wavelength dependent effects described above. Therefore it is critical that the amplifier controller, whether autonomous or cooperative, is not only designed for stable operation of a single amplifier, but also for stable operation in the presence of feedback through transparent paths in a mesh network.
8.3 POWER STABILITY IN AMPLIFIED NETWORKS The potential for power instability in transparent networks was recognized in early network experiments. The obvious case is one in which light at a particular wavelength is allowed to form a closed loop within the mesh network [14e16]. Figure 8.7 shows an example of various paths that can be formed in a network consisting of four connected rings. If the gain around such a loop is greater than 1, then optical lasing can occur. However, gains that even approach 1 will result in unstable behavior. An ^0 path refers to one in which the out of band noise is allowed to propagate. In-band channel paths can also be formed if care is not taken with the optical switch configurations and adjustments. In transparent networks today, these feedback paths are generally designed so that the gain is < 35 dB, particularly within the channel bands. These very low power levels are required to avoid bit errors caused by optical interference. Outside the channel bands, such strong attenuation is not required, but is usually realized due to amplifier gain roll-off and blocking from filtering elements such as ROADM wavelength selective switches. Care must also be taken to avoid closed loop paths as the network is reconfigureddopening and closing switch ports around the network.
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4
Λ0 Cycle 3
Λ0 Cycle 4
1 Λ0 Cycle 1
3
Λ0 Cycle 2 2
FIGURE 8.7 Examples of lambda-0 (out of band ASE) cycles in mesh networks [14]
Even if the mean optical power at a given wavelength is prevented from forming closed loops, there may still be opportunities for power fluctuations to complete a closed loop due to coupling between multiple channels. Lasing will not occur, but this form of closed loop can cause power control devices to become unstable. Perchannel power control is typically used in each ROADM node to correct power offsets between channels from different ports and ripple accumulated in transmission. In constant power amplified networks, strong channel power coupling along a closed loop can cause the resulting power adjustments to feedback to the ROADM input. If the coupling is sufficiently strong, it can de-stabilize the power controller in the ROADM. As shown in Figure 8.4, power adjustments will tend to diminish rapidly beyond the path of the adjusted channels or the channel propagation range. However, this applies when the network is heavily loaded with many different groups of channels being added and dropped along any given path. Simple configurations with a few groups of channels forming closed loops will tend to give stronger feedback. Such feedback will require coupling between at least two groups of channels and two coupling events. In a simple case, a fluctuation from the first group of channels is transferred to a second group at the first coupling event, then the fluctuation is transferred back to the first group at the second coupling event. Using Equation 2, the strength of the coupling will thus be proportional to h(1h), where h is the fraction of power in one of the two groups. This function is a maximum when h ¼ 0.5 and thus is always less than 0.25. In many cases, this damping of the feedback is sufficient to maintain stability. However, resonances in the frequency response of the power controller can compensate for such damping.
8.3 Power stability in amplified networks
Furthermore, the power fractions can vary through the network due to imperfect compensation. Compensation of Wavelength dependent gain and loss along the transmission path can also cause the coupling to be stronger. One common situation that can lead to instability is when multiple independent controllers are present along the feedback path. The channel power coupling can cause adjustments on the different controllers to become coupled, creating competition and/or cooperation that can lead to instability.
(a)
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FIGURE 8.8 a) Experimental setup used to study instabilities in constant power amplified networks; b) resulting power fluctuations on eight channels courtesy of Ben Yoo [17]
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Figure 8.8a shows a configuration in which power instabilities were created due to constant power amplification [17]. Although no channels are allowed to propagate fully around the closed loop, interactions between channels in different sections of the loop cause the power equalization controllers in the wavelength add-drop modules (WADM) to be coupled. Figure 8.8b) shows the resulting power fluctuations on eight channels, which spontaneously occur once the system is configured such that significant feedback is present and two power equalization controllers are running. The coupling is not strong enough to destabilize the control in a single WADM unit. When the amplifiers are operated with constant gain control, the coupling is sufficiently reduced so that the system is again stable. The time delay along the feedback path is also important. Time delay adds an exponential term to the loop gain transfer function. This will create periodic resonances in the response, which can cause instability. The configuration in Figure 8.8a) will be more stable if the system circumference is smaller so that the loop delay is much shorter than the control loop in the WADM devices. The potential for instability in optically transparent WDM networks can be analyzed more generally using linear system theory. A matrix can be constructed in which the gain for each signal along each link in the network is represented by diagonal elements and the channel power coupling is represented by off-diagonal elements. Using this approach, Gorinevsky and Farber [18] determined conditions for stability based on the maximum forward gain and cross-gain for harmonic disturbances on the channel power within the network. This analysis leads to a very conservative requirement on power coupling, which is unlikely to ever be met in practice. For example, analyzing the case in Figure 8.8, the system is predicted to be stable if the cross-coupling is <0.01 dB for all channels given a 1 dB disturbance on any single channel. On the other hand, cross-coupling of 0.02 dB is no longer guaranteed to be stable in this analysis. Given the wide range of channel configurations and wavelength dependence in large transparent mesh systems, such low levels of cross-coupling are impractical. As Gorinevsky and Farber point out, the analysis only guarantees stability if the conditions are met, but does not guarantee instability if the conditions are not met. Careful design of network control can still lead to stable operation even when strong coupling is present. A more detailed analysis was carried out by Pavel [13]. Her approach includes time delay and therefore yields the maximum time delay that can be tolerated before instability occurs. Figure 8.9 shows simulation results for a constant gain amplified ring network, in which constant total-power gain across each amplifier is maintained by instantaneous dynamic gain equalization filters. The network is configured such that power fluctuations can propagate fully around the ring. In Figure 8.9a, the time delay around the ring is short enough that a 1.5 dB tilt perturbation settles after several propagations around the ring. Increasing the time delay by 4x to 16 ms (1 ms longer than the calculated instability condition for this network), however, results in oscillations that do not die out and gradually increase in duration with each round trip.
8.3 Power stability in amplified networks
FIGURE 8.9 a) Growth of power fluctuations due to dynamic gain equalization DGE in a constant gain amplified ring network and b) with 4x longer ring round trip time delay resulting in an unstable condition Courtesy of L. Pavel, [13]
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FIGURE 8.10 Growth of power fluctuations in a constant gain amplified system with gain ripple and tilt operated using a) random DGE triggering, b) synchronous DGE triggering, and c) sequential DGE triggering. Each discrete time step is w10e15 seconds, corresponding to either a spectrum scan or a complete DGE (wavelength selective switch) adjustment on each node.
Sustained oscillations in a network with constant gain controlled amplifiers using pump power feedback have also been observed in experiments [19]. In Figure 8.10, a three ROADM ring network was set up with two overlapping groups of wavelengths and the amplifier tilts were adjusted to create strong coupling. Wavelength selective switch variable power attenuators in the ROADMs were tuned in discrete steps (typically >10 seconds for this experiment) to demonstrate how the coupling creates competition between multiple simultaneously actuated controllers. In Figure 8.10a), all three ROADMs are randomly triggered to operate for a fixed time period. When two or more ROADMs adjust at the same time, the fluctuations in the form of a power offset or tilt between the two channel groups will persist or grow. When all three ROADMs are adjusted continuously and simultaneously, strong oscillations can grow from noise as shown in Figure 8.10b). An opposite and alternating tilt is observed in the two nodes at which the respective groups are added and dropped. Thus, each ROADM applies opposing tilt corrections, which due to the channel coupling results in an over-compensation. Adjustments are further leveraged by the action of the third ROADM. This cooperative action from multiple controllers was also illustrated in [20]. Once the ROADMs are adjusted sequentially as in Figure 8.10c), the network quickly settles to a stable condition. Although this experiment was carried out using slow discrete adjustment steps, in practice ROADM channel power adjustments are made using spectral measurements taken with an optical spectrum analyzer (OSA). An OSA will often require on the order of 1 second to complete a scan. Thus, the ROADM channel power control response time will be
8.4 Physical layer network control
much longer than the amplifier settling time (wms) and the network feedback delay times (w10e100 ms).
8.4 PHYSICAL LAYER NETWORK CONTROL The instabilities that can arise in optical networks due to channel power coupling in optical amplifiers motivate the need for careful network power control design. The need for periodic tuning of network resources due to time or temperature dependent components and physical plant variations will usually dictate the use of a network control algorithm. A variety of parameters need to be adjusted based on the needs of the network. Some networks require frequent control element adjustments to ensure that channel power levels rarely deviate from targeted values. Other networks might provide significant margin allowing channel power levels to vary within a limited range. The timing and frequency of operations are key factors that influence the network control design. For example, a control algorithm may include a maximum time between control adjustments, time for a control adjustment to complete, and a minimum time between control adjustments on the same node. For ROADM based transmission systems, we focus on three key parameters that can be controlled to maintain the steady channel powers often required for long-haul transmission: amplifier gain, amplifier gain tilt (spectral), and channel power. As described above, fast control of the amplifier gain (usually implemented through feedback and/or feedforward control of the pump based on a total power measurement) is used to respond to fast transient events and may run continuously on all amplifiers. The gain tilt might also be adjusted continually toward a target, although the variable optical attenuators commonly used to adjust tilt are usually not as fast as the pump control on the gain. Both the target total power gain and target gain tilt may need to be updated periodically to respond to slow drifts in the system, for example due to temperature variations. The channel power can be adjusted using per-channel attenuators found in the blocker or wavelength selective switch components of the ROADM nodes. Devices such as optical channel monitors can be used in the ROADM node to measure the individual channel powers and provide feedback to the attenuators. The individual channel power control at the ROADMs and the amplifier gain and tilt target updates in the amplifiers can be implemented using a control algorithm that coordinates adjustments throughout the network. This section focuses on the details of such network-level control algorithms.
8.4.1 Distinct control domains and function There are at least two very different control time scales within an optically transparent mesh network. The first of these concerns the control and management of optical power transients. This control must happen very rapidly at the control element experiencing the power transient without coordination with other control elements in the system. This rapid response is required due to the speed at which an
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optical transient propagates through the network. In contrast, the second type concerns the periodic adjustment of control elements to compensate for environmental factors, control element aging, and respond to planned events such as provisioning new wavelengths. This time scale is much longer, allowing node-tonode coordination of control adjustments to maximize network power stability. This control might involve, for example, the adjustment of individual channel powers within a ROADM, or the adjustment of amplifier power or gain targets to optimize network performance. The relatively simple topologies of line and ring systems lead to straightforward algorithms for coordinated network control. Typically, token passing methods can be used to ensure no two control elements will execute simultaneously. Early mesh networks, which consisted of interconnected line and ring networks, could rely on a set of well defined control domains that could execute control operations concurrently. These domains could be provisioned at the network deployment, or derived from the properties of the ROADM nodes. The movement toward full, optically transparent mesh networks gives rise to the need for dynamically defined, adaptive control domains. For small networks, an alternative to token passing is to use small fixed or heavily damped adjustment steps, in some cases incorporating rules to reduce cooperative effects [20]. Such methods limit the size of the swing (e.g., the tilt error shown in Figure 8.10) due to each individual controller and thus for small cascades the cooperative effects will be limited in size as well. However, this damping approach has little effect on the number or strength of channel power interactions in the network. The primary requirements for a coordinated control method are to minimize the interactions between control adjustments while maximizing the number of concurrent control adjustments. These competing goals must be balanced to achieve a fast, stable control algorithm. The relative importance of each requirement depends on the expected size of the network and the relationship between the amount of time to make a single control adjustment and the required frequency of control adjustments. Control methods are particularly important for networks where the number of control elements multiplied by the time to make a single control adjustment approaches the required time interval between adjustments. As outlined in Section 2, interactions between control adjustments can arise from the simultaneous adjustment of multiple control elements in the network. Problems stem from the possibility of two control elements measuring and adjusting the same channel, or indirectly through channel-channel coupling effects in the network. Adjusting only one control element at a time ensures no interactions between concurrent control adjustments; however, such an algorithm would take a very long time to adjust all amplifiers in the network. Even more problematic, as the size of the network grows, the time to adjust all nodes would grow proportionally. Uncoordinated adjustments can maximize concurrent execution. Such a method will execute very rapidly in a total execution time that does not grow with the size of the network. Unfortunately, such a method results in random and potentially
8.4 Physical layer network control
simultaneous adjustments. The power coupling mechanisms described previously may lead to amplifier instabilities or network wide power oscillations. Several other secondary, although related, requirements are desirable: Scalability: Algorithms that require substantial amounts of computation to schedule control adjustments are not suitable for very large mesh networks. No aspect of the algorithm, computation, or storage can grow substantially as the size of the network increases. Guarantee that all elements will execute at least one control action within a period Tmax: The Tmax time period is set by how rapidly a network can drift without control from optimal signal power levels. Local message passing: Algorithms that require significant out of band signaling fail to scale well, and further place significant requirements on the signaling reliability. All messages must be to neighboring nodes; no broadcast messages to all nodes are allowed. No pre-provisioning: Ideally a change in network topology will only require reprovisioning of nodes located in the local environment of the change. The algorithm must adapt to topology changes. These secondary requirements constrain the possible control algorithms. For example, the scalability, message passing, and adaptability requirements drive the need for a distributed control algorithm, rather than an algorithm driven by a central controller. As the size of the network grows, a centralized controller requires computational and message passing resources proportional to the number of nodes in the network. This might require a higher performance processor in the controller (very difficult to change in deployed nodes) and would place a significant burden on the out-of-band signaling system. Adaptability as the network topology and traffic change raises another difficulty for the centralized control model. For example, the interconnection of two pre-existing mesh networks (each with their own centralized controllers) would require significant re-provisioning of existing nodes to create a functioning composite network with a single centralized controller. In the next section we present the dynamic domains method, an attempt to satisfy these requirements with a distributed control method that requires only local communication and storage.
8.4.2 Dynamic domains method The dynamic domains method seeks to provide a coordinated control algorithm that minimizes interactions between nodes while maximizing the number of concurrently executing nodes in the network. This is accomplished in a distributed fashion without a centralized controller. The algorithm does not require any knowledge of the topology of the network beyond the routing from input to output ports within each ROADM. All message passing occurs only downstream (along the light path) and only to nearest neighbor nodes (one hop). The algorithm is fault tolerant and self healing: the loss of out of band communications in the network will degrade the algorithm’s performance, but will not stop the execution of the control adjustments.
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To provide these properties, the dynamic domains method requires four primary components: a fluctuating channel list (FCL), a local timer, a control initiation method, and a decision function. An FCL is a list of channels contained within each control element that indicates which channels are expected to fluctuate due to control element adjustments on other nodes of the network. Much of the dynamic portion of the method exists to maintain a valid FCL for every control element using only local storage and local message passing. The timer maintains the total elapsed time since the last control adjustment. This timer is used to ensure that no control element waits longer than a predefined maximum time (Tmax) between adjustments. A run state propagation method concerns how a control element, when it has completed a control adjustment, might signal neighboring control elements to begin adjusting. This run state propagation method can be viewed as a sequencing method within the dynamically defined control domains. A decision function describes how a control element uses both the FCL information and a run state trigger to initiate a control change. For example, one very conservative decision function might allow a control element to adjust only when its fluctuating channel list is empty. These components represent methods of measuring the current state of the network (timer, FCL), deciding if a control operation will interact with other nodes (the decision function), and a method of signaling other nodes of an ideal time to run. In the subsequent sections, we will describe each of these components in detail.
8.4.2.1 Fluctuating channel list Each control element contains an FCL. Each FCL contains a list of all channels passing through the control element that may not have stable channel power readings. The integrity of this list is maintained through a sequence of messages initiated when a control element initiates a control operation. When a control element on the link connecting node A to node B begins to execute, a pause message1 is sent by node A to node B. This pause message contains a list of all channels propagating from node A to node B that are impacted by the control adjustment. These are the channels that may fluctuate due to the control operation at node A. Within node B, this list is divided into several sub-lists, one for each output port of node B. These sub-lists are created using the internal routing table of node B which shows the output port for each channel arriving from node A. In this manner, each output port of B receives a list of potentially fluctuating channels, exiting node B on that port. Channels that are dropped at node B do not appear in any sub-list. The channels in each of these sub-lists are then added to the FCL within the control elements for each output port of node B. Node B then sends a pause message containing each sub-list to the corresponding nearest neighbor node (i.e., the node connected to the output port linked to the specific sub-list). Messages are sent only to nodes connected to ports with a nonThe name pause message will be described in detail in section 8.4.2.2. The receipt of a pause message by a downstream node may pause control execution in that node. 1
8.4 Physical layer network control
empty sub-list; no action is taken for output ports not impacted by the control operation at node A. The process of subdivision and branching continues throughout the network. At each node, the channels that are dropped at that node are removed from the lists. Eventually all of the channels present in the original list created by node A will be dropped and all of the sub-lists will be empty. At this point, all message passing ceases. Each node on the paths traced by the demands will have received a pause message. This method traces the tree of elements that are impacted by the control change taking place at node A. The number of messages passed is proportional to the size of the demands in the network, not to the number of nodes in the network. In addition, messages are sent only to nearest neighbor nodes directly along the paths defined by the demands passing through node A, so the method essentially determines the network topology based on tracing the path of demands in the network. It does this using only local routing information; no node (or other network entity) need have knowledge of the entire network topology. At the conclusion of a control adjustment in node A, a finish message is sent downstream; again, this message contains all of the channels impacted by the adjustment at node A. For a finish message, these channels will be removed from the control elements FCL. In an analogous manner to the pause message, the list of channels is subdivided into sub-lists and additional finish messages are sent to nearest neighbor nodes in the network. The messages sent to update the FCL can also be used as control initiation and termination messages (as is foreshadowed by the names pause and finish). The impact of these messages on control execution will be described more fully in the next section.
8.4.2.2 Triggering control operations (run messages, timeouts, and preemption) There are several possible methods for initiating a control adjustment. A control adjustment might be initiated based on the receipt of a message from another node, based on a fixed time interval between adjustments, or based solely on the contents of an FCL. Independent of the triggering method, there are several important quantities that provide constraints on how often a control adjustment must be made: Tmin - the minimum time interval separating adjustments on a given control element in the network, and Tmax - the maximum time interval separating adjustments on a given control element in the network. Any triggering mechanism must respect Tmin and Tmax to ensure that no control element will adjust too often (they must wait at least Tmin) nor will an amplifier go too long without running (after Tmax they must run the control). We note in passing that this triggering time is intended to limit the time between longtime scale adjustments (power and gain targets) within amplifiers, not the short-time scale adjustments that maintain the constant power or constant gain behavior of the amplifiers. The dynamic domains method uses a combination of timer based and message based triggers. A control adjustment can be triggered by having the internal timer reach Tmax, or by receiving a finish message from an upstream control element. An
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adjustment triggered by reaching the Tmax timeout value proceeds regardless of the contents of an element’s FCL. The Tmax timeout ensures all elements will execute control within a fixed period of time. The timer-based triggering also provides a fallback event if out of band signaling is lost in the network. In this case, a node that can no longer communicate with neighboring nodes will continue to execute a control operation every Tmax until communication is restored. Triggering by finish message is similar in nature to the token passing methods used in line and ring networks. In this case the token passing follows a tree of nodes based on the demands originating at the base of the tree. Branches in the tree potentially allow the duplication of tokens resulting in concurrent execution of control operations. It is important to note that these dynamically defined domains can overlap and further change shape over time based on changes to the network topology or to the network loading. In addition to initiating control, it is possible for a message to preempt a current control operation. Given that multiple control elements may execute simultaneously, it is possible that an element currently executing a control operation will receive a pause message from an upstream control element. A detailed determination of the optimal pre-emption strategy depends strongly on flexibility and nature of the control. Some control methods may ignore or treat specially potentially fluctuating channels while continuing to adjust, while some may require that all control adjustments be terminated. Other possible methods include ignoring the message and continuing the current control operation or sending an upstream message to keep the upstream node from executing. In all of these cases, an adjustment triggered by a finish message proceeds only if the contents of the element’s FCL satisfies the decision method described in the next section.
8.4.2.3 DecisiondFractional timeout In the simplest implementation, given the existence of an FCL, a control element will make adjustments only after Tmin has passed and it has an empty FCL. If the control operation is executing the control adjustment and its FCL becomes non empty, the control operation will be pre-empted and the control element will revert back to the waiting state. Unfortunately, using this fixed method means that for certain topologies (especially those with high degree nodes), some control elements will never execute a control change. They will always remain in a waiting state. In this case, we can allow a node with a non-empty FCL to make changes after Tmax time has elapsed. This will ensure that all of the amplifiers of the system will adjust. However, since the FCL may have a significant number of channels, the adjustments may have several unintended consequences. Enforcing a strict rule that all control elements must adjust after Tmax is a hard threshold for determining if a control element can make adjustments. Furthermore, in this case, the behavior of the network depends strongly on the initial configuration. The timing of adjustments is defined entirely by the initial execution times for each control element. In a sense, the network is unable to adjust the timing of the algorithm. The introduction
8.4 Physical layer network control
of a fractional timeout is meant to provide some flexibility in the algorithm’s timing. In general, the negative impact of executing a control operation with a nonempty FCL is proportional to the channel fraction contained in the FCL (channels in the FCL / total channels). Given that in many networks reducing the FCL to zero is impractical, the algorithm should attempt to minimize this fraction for all control operations. This allows some nodes to execute with a non-empty FCL, but it will force this behavior only when the relative impact of the adjustment on the network is small. To accomplish this, we compute the fraction of time elapsed since the last control change: Tfrac ¼ ðTlastrun Tmin Þ=ðTmax Tmin Þ
(Equation 7)
This Tfrac will vary from 0 to 1 as the Tlastrun changes from Tmin to Tmax. We then compute the fraction of channels in the FCL: Cfrac ¼ CFCL =Ctotal
(Equation 8)
This will also vary between 0 and 1 as CFCL changes from 0 (an empty FCL) to Ctotal (a full FCL). We now allow control changes to take place if Cfrac < Tfrac. So in this manner, control changes are allowed if only a small fraction of the control element channels is in the FCL. Simulations described in the next section show that the fractional timeout method dramatically reduces the number of channels in the FCL at the time of control executions. This occurs primarily because the fractional method provides some flexibility in execution sequence. This flexibility allows the network to settle into natural patterns of execution based on the topology and traffic. The hard decision point described in the fixed method is trapped in the configuration of executions based on the initial network setup. The fractional method allows the system to settle into an optimal execution strategy.
8.4.2.4 Results To define and optimize this method, simulations were run using the ATOM (A Transparent Optical Mesh) simulation platform [21]. Two distinct types of simulations were performed. The first type looked only at the execution of the dynamic domains method. This was accomplished by running a finite state machine representation of the control. The method tracked transitions in these finite state machines over several days’ worth of simulated network operation. The recorded quantities included the number of nodes executing simultaneously and the fraction of nodes that executed with a non-zero fluctuating channel list. The larger the fraction of channels contained in the FCL, the greater the chance of interactions between the simultaneously interacting nodes. The finite state machine results are a conservative estimate of the efficiency of the method. In many cases the execution of a control change with a non-zero FCL
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FIGURE 8.11 Network configuration used within these simulations.
does not necessarily result in network instability. To examine the actual power levels, we initiated further simulations. This second type of simulation computed dynamic optical power levels in the network using a steady state model of the amplifier physics and fiber propagation for a test network. Two different control methods were applied to adjust the network in Figure 8.11 back to the targeted channel powers from an initial state in which add channel powers were initially set with a random deviation (on the interval 0.5:0.5 dB) from input targets. Time constants in the control were selected to increase the chances of multiple nodes adjusting simultaneously (time to execute control ¼ 10% of the time between adjustments). All of the simulations were performed using the network topology show in Figure 8.11. It contains 31 nodes (average degree ¼ 3), 98 independent control elements on 49 bidirectional links each carrying w20 channels (average demand length ¼ 5 hops). The numbers contained in the small circles show one possible execution order for control adjustments. The control execution order rarely traverses a simple path, showing the difficulty of determining these domains a priori.
8.4.2.4.1 Finite state machine simulations Finite state machine simulations enable rapid, longtime scale simulations of the impact of the control method on the stability of the network. Each control element of the simulated network contains a finite state machine representing the execution of the dynamic control domains method, a fluctuating channel list, and an internal routing table. All messages are transmitted to nearest neighboring elements following the paths of network channels routed to neighboring nodes.
8.4 Physical layer network control
All simulation times are normalized to the duration of a single control adjustment (Tcontrol). In a typical commercial network, this duration is on the order of seconds to minutes depending on the detailed nature of the control hardware. Figure 8.12 shows the results of these simulations for the network in Figure 8.11 [17]. The solid line shows the average fraction of channels in the FCL when a control adjustment occurs. A large fraction of channels indicates a large potential for interactions between control operations. In the fixed method, a control adjustment occurs with a non-empty FCL only when the control element timer reaches the Tmax value. A large number of nodes with a large fraction of channels indicate a failure of the scheduling method. Several nodes in the network execute control adjustments with a high number of FCL. This is typical of high degree nodes in the network that would see impact from a large number of “upstream” amplifier adjustments. The dashed line implements the fractional timeout described in the previous section. Notice that almost all control adjustments occur with less than 5% of the channels in the FCL. This is a dramatic improvement over the fixed decision method. This indicates, somewhat paradoxically, that by allowing a few nodes to execute with a small number of upstream channels under adjustment, the vast majority of nodes will execute with no upstream channels under adjustment. This occurs because the fractional method provides some level of flexibility in the timing and a driving force to execute with a small number of channels in the FCL. Each node adjusts its execution time within the window provided by the fractional timeout to minimize the number of nodes in its FCL. Over time with several control executions at each node of the system, this drives the system to a globally optimized schedule.
FIGURE 8.12 Control element interactions (Tmax ¼ 100)
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FIGURE 8.13 Concurrent control element adjustments (Tmax ¼ 100)
This contrasts with the fixed decision method, which provides no driving force for global optimization, since there is no local driving force for minimizing the number of channels in the FCL. Figure 8.13 shows, for each control element, the number of concurrent control element adjustments enabled by the method. For this network topology and traffic pattern, we achieve between three and four control elements executing simultaneously. This roughly indicates the number of independent control domains in the network. Notice that both the fractional and the fixed decision methods give rise to a high level of concurrency. For the fractional method, this equates to a w3.7 times speedup over a sequential node-by-node adjustment scheme for this case with highly connected domains.
8.4.2.4.2 Time dependent simulations Figure 8.14 shows the results of time dependent simulations contrasting scheduled and un-scheduled control [6]. Figure 8.14a) shows the results from applying control adjustments in a random, unscheduled fashion. During the adjustments, the channel power fluctuates widely, well beyond the initial deviation band of 0.5 dB. Even after 6000 time steps, the channel power has not yet converged. In contrast, Figure 8.14b) shows the results of applying a scheduled control method to the same initial starting conditions. In this case, the channel power approaches the targeted power level, never going outside the initial range.
8.5 Conclusions
FIGURE 8.14 a) Return to target for unscheduled network adjustments; b) return to target for scheduled network adjustments
8.5 CONCLUSIONS Amplified optically transparent transmission systems form a complex network of interacting and fluctuating elements. Introduction of multiple WDM channels, particularly in wide gain bandwidth high-capacity systems, leads to dynamic and static channel power coupling through the amplifier gain. Channel interactions occur in amplifiers with either constant power or constant gain control, although the nature of the power coupling will vary depending on the mechanism. In networks, these interactions can lead to instability if care is not taken in the control design. The dynamic domains network control algorithm provides an example of a method for implementing stable and scalable network control. As transparent networks become more dynamic, for example supporting wavelength re-routing and protection, more advanced network control techniques may be required. Of course one can always argue that heavy-handed techniques such as moving to opaque networks or providing per-channel optical amplification can be used to eliminate the need for network control. Such approaches, however, do not scale and generally remove the cost advantage of parallel wavelength transmission, which has made WDM so advantageous. Indeed, a similar argument can be made for most transmission issues such as dispersion compensation or fiber non-linearities. The challenge is always to find the most cost effective solution; mitigating a problem by design will typically win over adding hardware. Optical power dynamics fall easily into this category of transmission impairments. Transmission over longer distances and supporting wider gain bandwidths will require improved power control techniques. More intelligence and flexibility in the networks will likewise drive more sophisticated network power adaptation.
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ACRONYMS ATOM DGE DGEF EDFA FCL LBO OSA ROADM WADM WDM
A Transparent Optical Mesh simulation tool Dynamic Gain Equalization Dynamic gain equalizing filters Erbium-doped fiber amplifiers Fluctuating channel list Line break-out Optical spectrum analyzer Reconfigurable optical add/drop multiplexer Wavelength add-drop modules Wavelength division multiplexing
References [1] M. Zirngibl, Gain control in erbium-doped fiber amplifiers by an all-optical feedback loop, Electron. Lett. 27 (1991) 560e561. [2] E. Delevaque, et al., Gain control in erbium-doped fiber amplifiers by lasing at 1480 nm with written on fiber ends, Electron. Lett. 29 (1993) 1112e1113. [3] J.L. Zyskind, et al. Fast power transients in optically amplified multiwavelength optical networks. In Proceedings of the OFC’96 Tech. Digest, 1996, PD 31e1. [4] J. Nagel. The dynamic behaviour of amplified systems. In Proceedings of OFC’98 Tech. Digest, ThO3, 1998. [5] Y. Sun, et al., Fast power transients in WDM optical networks with cascaded EDFAs, Electron. Lett. 33 (1997) 313e314. [6] C.A. White, D.C. Kilper. Power stability and control in optically transparent mesh networks. In Proceedings of OFC/NFOEC’08, Tech. Digest, OThI1. Invited paper, 2008. [7] E.L. Goldstein, L. Eskildsen, C. Lin, R.E. Tench, Multiwavelength propagation in lightwave systems with strongly inverted amplifiers, IEEE Photon. Technol. Lett. 6 (1994) 266e269. [8] C.R. Giles, E. Desurvire, Propagation of signal and noise in concatenated erbiumdoped fiber amplifiers, J. Lightwave Technol. 9 (1991) 147e153. [9] D.C. Kilper, C.A. White. Fundamental saturated amplifier dynamics in transparent networks. In Proceedings of ECOC’05, 2005, We4.P.96. [10] D.C. Kilper, S. Chandrasekhar, C.A. White. Transient gain dynamics of cascaded erbium doped fiber amplifiers with re-configured channel loading. In Proceedings of OFC/NFOEC’06 Tech. Digest, 2006, OtuK6. [11] L. Tancevski, L.A. Rusch, A. Bononi, Gain control in EDFA’s by pump compensation, IEEE Photon. Technol. Lett. 10 (1998) 1313e1315. [12] D.C. Kilper, C.A. White, S. Chandrasekhar, Control of channel power instabilities in constant gain amplified transparent networks using scalable mesh scheduling, J. Lightwave Technol. 26 (2008) 108e113.
References
[13] L. Pavel, Dynamics and stability in optical communication networks: a system theoretic framework, Automatica 40 (2004) 1361e1370. [14] K. Bala, C.A. Brackett, Cycles in wavelength routed optical networks, J. Lightwave Technol 14 (1996) 1585e1594. [15] W. Xin, G.K. Chang, B.W. Meagher, S.J.B. Yoo, J.L. Jackel, J.C. Young, et al. Chaotic lasing effect in a closed cycle in transparent wavelength division multiplexed networks. In Proceedings of OFC’99 Tech. Digest, 1999, TuR1e3. [16] P. Kim, S. Bae, S.J. Ahn, N. Park, Analysis on the Channel Power Oscillation in the Closed WDM Ring Network with the Channel Power Equalizer, IEEE Photon. Technol. Lett. 12 (2000) 1409e1411. [17] S.J.B. Yoo, W. Xin, L.D. Garrett, J.C. Young, G. Ellinas, J.C. Chiao, et al., Observation of prolonged power transients in a reconfigurable multiwavelength network and their suppression by gain-clamping of optical amplifiers, IEEE Photon. Technol. Lett. 10 (1998) 1659e1661. [18] D. Gorinevsky, G. Farber, System analysis of power transients in advanced WDM networks, J. Lightwave Technol. 22 (2004) 2245e2255. [19] D.C. Kilper, C.A. White, S. Chandrasekhar. Control of channel power instabilities in transparent networks using scalable mesh scheduling. In Proceedings of OFC/ NFOEC’07 Tech. Digest., 2007, PDP-11. [20] L. Zong, et al. State-based algorithm for power stability control in transparent WDM networks. In Proceedings of OFC/NFOEC’08 Tech. Digest, 2008, JWA117. [21] C.C. Chekuri, et al., Design Tools for Transparent Optical Networks, Bell Labs Tech. J. 11 (2006) 129e143.
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CHAPTER
Advanced Amplifier Schemes in Long-Haul Undersea Systems
9 Alan Lucero
Tyco subcom, USA
CHAPTER OUTLINE HEAD 9.1. Introduction ................................................................................................ 9.2. Raman-assisted systems and experiments..................................................... 9.3. Comparison of Raman- and ROPA-assisted systems........................................ 9.4. Advanced modulation formats and high capacity in hybrid ROPA-EDFA systems 9.5. Conclusion.................................................................................................. Acronyms ........................................................................................................... References .........................................................................................................
253 255 264 271 274 275 275
9.1 INTRODUCTION The majority of traffic in today’s major intercontinental markets is carried over optical undersea transmission systems [1]. Since 1990 alone, over 600,000 km of undersea fiber-optic cable have been installed across the world’s oceans. Lengths of these cable systems range from regional distances (1000 to 5000 km) to trans-Pacific distances (8000 to 13,000 km). In the period since the first undersea fiber-optic systems were installed across the Atlantic and Pacific oceans, the transmission capacity of these cables has grown by several orders of magnitude, from 560 Mg/s to over 3 Tb/s. In terms of a simple digital voice circuit (64 kbit/s), this latter highcapacity cable could carry the equivalent of over 45 million simultaneous phone calls. Intercontinental data traffic, whether phone calls, website viewing, or the vast daily transfer of business data, all ride on this massive sub-sea network. The earliest of the transoceanic cable systems based on fiber optics were actually a combination of optical and electrical technologies. While the data was propagated via pulses of light along optical fibers in the manner similar to the most modern systems, the repeaters were electro-optic devices that first converted the optical pulses to the electrical domain. The pulses were then reshaped by high-speed electronics and finally retransmitted into the following optical fiber section by a semiconductor laser transmitter contained in the same repeater. Despite the capacity restrictions defined by the repeater electronics, regenerated fiber cable transmission capacity eventually increased to 2.5 Gb/s [2]. The first deployment of Optically Amplified WDM Networks. DOI: 10.1016/B978-0-12-374965-9.10009-3 Copyright Ó 2011 Elsevier Inc. All rights reserved.
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erbium-doped fiber amplifiers (EDFAs) removed this capacity bottleneck and ushered in a revolution in capacity growth in international voice and data traffic. In this new system design paradigm, all of the function of the receivers and transmitters in the old repeaters was moved to the terminal points at the shore landings of the cable system. The elimination of complex electronic components provided both a significant cost savings and increased reliability of the system. Not only is the EDFA simpler and less expensive, but it has two other key advantages. First, it is transparent; it amplifies signals irrespective of the bit rate or modulation scheme. Whereas the cost and complexity of regenerative repeaters rises rapidly with the data rate the EDFA cost is independent of the signal bit rate except when a higher pump power might be required. Second, and perhaps more important, the EDFA provides amplification over an appreciable spectral range which is the key enabler for dense wavelength division multiplexing (DWDM) so that a single EDFA can serve as the repeater for many wavelength channels that can be carried on a single undersea fiber. This has been the key to the explosion in the capacity of undersea systems. Since the introduction of the EDFA in the early 1990s, the demonstrated capacity per fiber for long-haul transmission applicable to transoceanic long-haul systems has increased by a factor of up to 6000. A number of technologies have contributed to this extraordinary growth in capacity, but the EDFA, and more recently Raman amplification, have been central to these developments. In conjunction with other enabling technologiesd(see Figure 9.1) [3]: wavelength division multiplexing (WDM), forward error correction (FEC), return to zero (RZ) modulation format, advanced fiber designs, and othersdthe EDFA has been a key feature and in fact the principal enabler in nearly all of the modern sub-sea fiber optic systems. Deploying the EDFA to replace the electro-optic regenerating repeaters has provided both cost savings and performance enhancements. The available gain spectrum of the EDFA 13500
Capacity per fiber (Gb/s)
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p is
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at ul
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x tR en er rs oh fie C pli G m a 0 s at 10 brid y rm H fo n io at ul od s m er K ifi s s r pl er PS ifi fibe D am pl s rid am ned er yb fib nd tte H a la ba f e C on ar s ll i e s at iv er ct rm fe fo ef n e io
2560
Year 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008
FIGURE 9.1 Technology milestones in long-haul optical transmission. The number associated with each point is the resulting aggregate system capacity, i.e., the capacity per channel times the number of supported channels.
9.2 Raman-assisted systems and experiments
provides enough optical bandwidth to support many optical channels using WDM techniques. Since the first optical undersea transmission systems were installed, the cost of transmission capacity in undersea systems has declined by four orders of magnitude [4]. Both total cost of transmission capacity and “first cost” of deployment are principle drivers of system design. Initial cost of long-haul systems can be reduced by further increasing the distance between repeaters. However, simply increasing span length degrades the optical signal-to-noise ratio (OSNR) at the receiver. Motivated by this fundamental restriction, distributed Raman amplification (DRA) and remote optically pumped amplifiers (ROPA), either alone or used in more advanced amplifier schemes together with EDFAs, have been popular subjects of investigation for a number of years [5,6]. Both technologies offer the attractive possibility of increasing the length of the spans between repeaters and making the system more linear as well as enhancing the received OSNR. Both technologies offer a positive synergy when used with many of the other technologies listed in Figure 9.1, particularly large effective area fibers, dispersion flattened fibers, and differential phase shift keying modulation format. This chapter will address the recent history of the development of the DRA and ROPA technologies and their implementation in prominent laboratory demonstrations. We will present a detailed analysis of the performance enhancement of each technology when deployed in hybrid design with EDFAs, and give specific examples for a fair comparison between the designs. The comparison will take into account both the OSNR enhancement and the reduction of nonlinearities characteristic of both technologies. In our own experimental and modeling work [14] we used reasonably simple amplifier architectures for both distributed Raman and ROPA assisted EDFA (DRA/EDFA and ROPA/EDFA) using dispersion slope matched fiber spans with large repeater spacing and increased 980 nm EDFA pump efficiency. Finally, we will review several studies of hybrid systems which were used as a transmission platform for either high spectral efficiency, advanced modulation formats, or both.
9.2 RAMAN-ASSISTED SYSTEMS AND EXPERIMENTS The simplest application of Raman amplification in terrestrial or undersea telecommunications is in the repeaterless design. In currently deployed commercial submarine systems, repeaterless designs occupy an important niche by costeffectively connecting stations separated by a few hundreds of kilometers without the use of periodic in-line amplification and power-feed equipment. Although the terminals for repeaterless systems can be fairly complex, involving high-power amplifiers and Raman pumps, repeaterless systems can offer lower total system cost compared to traditional repeatered systems. Typical applications are coastal festoons, island hopping, and links between oil platforms [7]. For long-haul, repeatered systems, exploiting the gain bandwidth of the DRA alone or in conjunction with the conventional EDFA gain band (C-band) has been
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the object of recent research. A number of ultra-wide bandwidth transmission experiments have been successfully carried out over the last decade using either dual-band EDFAs with L-band (1570 to 1610 nm) in combination with the C-band or applying Raman amplification to extend the signal transmission bandwidth beyond the C-band. This latter scheme could be achieved either with pure distributed Raman amplifiers or with hybrid Raman/EDFAs (DRA-EDFA). The center of the gain band of the DRA is determined by the amplifier pump wavelength. Thus the DRA-EDFA may be used to study simple ways of increasing the bandwidth in long-haul transmission and is the focus of the following example. Foursa et al. [8] have demonstrated that hybrid DRA-EDFAs provide an attractive way to increase the bandwidth of long-haul systems by up to 80 nm of continuous bandwidth. Successful transmission was experimentally demonstrated for a distance of 11,000 km with DRA-EDFAs. Simple single-wavelength unpolarized backward Raman pumping in front of each single-stage EDFA was used in this case. Gain ripple was minimized over a wide continuous bandwidth by selecting pump wavelengths which provided complementary gain spectra of the Raman and EDFA sections. Figure 9.2 shows the block diagram of the hybrid DRA-EDFA. The transmission spans consisted of dispersion matched fibers in a 2:1 ratio. Dispersion matched fibers are any pair of fiber types matched such that their total accumulated chromatic dispersion sums to zero over one span or, alternatively, over a short block of spans. The pump light from two polarization-multiplexed grating-stabilized lasers was coupled into the negative dispersion fiber. The smaller core of the negative dispersion fiber maximized the distributed Raman gain from the 1497 nm pump. The EDFA section of the hybrid amplifier was a highly inverted single-stage 980 nm
Circulator D+
D–
In
980 nm Er-fiber WDM
ISO
GFF
Tap Out
PBS 1497 nm LD
FIGURE 9.2 Block diagram of the hybrid DRA-EDFA. The two 1497 nm Raman pump laser diodes (LD) are combined in a polarization beam splitter (PBS) to produce an unpolarized Raman pump beam. The circulator couples the Raman pump light into the transmission fiber span counter propagating with respect to the signal channels. The EDFA contains a 980 nm pump diode (not shown), a 980 nm WDM to couple the pump light into the erbium-doped fiber, an optical isolator (ISO) to suppress reflections, and a gain flattening filter (GFF) to compensate for spectral gain ripple. The circulator and the Raman pump components are immediately adjacent to the EDFA components (defined by the dotted rectangle).
9.2 Raman-assisted systems and experiments
FIGURE 9.3 Final gain shape of the hybrid DRA-EDFA (circle symbols, left axis); individual gain shapes of the DRA (diamond symbols) and the EDFA sections (triangle symbols)
forward pumped configuration. The gain-flattening filter at the output had a spectral loss profile that was designed to equalize the combined DRA-EDFA gain shape. The graph in Figure 9.3 shows the final gain shape of the hybrid DRA-EDFA as well as the individually measured gain shapes of the DRA section and the EDFA section with 800 mW of total pump power. The signal output power of the fully equalized hybrid amplifier was 18.6 dBm. Typical unequalized and equalized gain shapes are shown in Figure 9.4. To test the performance of this new amplifier concept, a 13-span 525 km chain with 13 DRA-EDFAs was built. OSNR, defined as the ratio of the signal power to the 2.5
Gain, dB
2 1.5 1 0.5 0 -0.5 1520
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Wavelength, nm
FIGURE 9.4 Gain shapes of the hybrid DRA-EDFA before and after gain equalization. The unequalized gain shape (diamond symbols) displays a gain variation of 1.5 to 2.2 dB.
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CHAPTER 9 Advanced schemes in long-Haul undersea systems
power of the amplified spontaneous emission (ASE) measured in a 0.1 nm bandwidth, is a figure of merit for the limit on bit error ratio (BER) performance of an optically amplified link due to ASE-induced optical noise. The OSNR, measured on the chain loaded with a 256 channel signal input, is shown in Figure 9.5. The OSNR value varies across the bandwidth from 26 dB (shortest wavelength) to approximately 28 dB (best part of the L-band). This OSNR as measured through the 525 km amplifier chain is equal or better than what can be achieved with standard C-band EDFAs. The improvement of the OSNR toward the longer wavelengths is attributed to the increased contribution from distributed Raman gain (Figure 9.3). The performance of the new amplifier chain was demonstrated in a transoceanic length transmission experiment using a circulating loop technique. A full 256 channel CþL-band transmitter was built, consisting of 256 signal distributed feedback (DFB) lasers coupled into four return-to-zero modulation paths with neighboring channels combined in orthogonal states of polarization. The received spectrum of the signals after 11,000 km transmission is shown in Figure 9.6. The peak-to-peak power variation across the entire bandwidth is only 7.5 dB, which indicates an average of 28 mdB equalization error per amplifier. The FEC provided error-free decoding of data (BER < 1010, or Q > 16 dB) at the receiver with pre-FEC Q factors of 8.3 dB (BER w4.7103) and above. The average wavelength spacing across the bandwidth from 1527 nm to 1606.6 nm was 0.31 nm. The signals were circulated 21 times to achieve the 11,000 km transmission distance. Subsequent to the period of the studies described above, a more thorough analysis of the hybrid Raman-EDFA system that included details of noise contributions from multipath interference (MPI) in addition to the ASE from both the Raman and the EDFA processes was carried out. The system performance penalty due to MPI is treated in depth in Bromage [9], but the main concept may be summarized as follows. Generally speaking, MPI takes place when light (whether
32 30
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FIGURE 9.5 OSNR of a chain of 13 hybrid DRA-EDFAs (525 km), measured in a 0.1 nm resolution bandwidth
9.2 Raman-assisted systems and experiments
Power, dB
-20 -25 -30 -35 -40 -45 -50 1525 1530 1535 1540 1545 1550 1555 1560 1565 1570 1575 1580 1585 1590 1595 1600 1605 1610
Wavelength, nm
FIGURE 9.6 Received signal spectrum of 256 signal channels after 11,000 km of transmission
from signal or co-propagating ASE) is first back-reflected and then forward-reflected to again co-propagate with the signal. In conventional fiber spans with no Raman gain, this doubly reflected light travels a path with losses orders of magnitude greater than the main path, and the resulting interfering signals at the receiver have only a minor impact on system performance. The backward and forward reflection process in the fiber span, Rayleigh scattering, is caused by microscopic glass composition nonuniformity. This normally weak process becomes problematic when the fiber is a gain medium. In Raman-based systems the principle contributor to MPI is double Rayleigh back scatter (DRBS). The penalty from DRBS is enhanced in the high-gain medium of a distributed Raman amplifier since the weak “single” Rayleigh back-scattered light will experience gain. This amplified single Rayleigh back scattering (SRBS) will then itself be back-scattered and again amplified, thus sending DRBS as incoherent noise co-propagating with the signal. The ratio of the DRBS power to the signal power will determine the system penalty. The magnitude of the DRBS is determined by the gain of the distributed Raman amplifier. As a practical example in the case of a long span (150 km), the MPI due to DRBS begins to dominate the system penalty at less than 20 dB of Raman gain. As part of a methodology offered as an optimizing process for the design of hybrid systems, a recent study [10] also revealed that a significant advantage in the EDFA pump power efficiency of the hybrid systems may be gained by moving the gain flattening filter from the more traditional output end of the EDFA to the input end. The enhanced pump efficiency advantage may be visualized as follows (see Figure 9.7): In the less-efficient configuration with the GFF following the EDFA, the lowest power channels must exit the EDFA at a high enough power so that they still meet the target power after shaping by the filter. The shaded area in Figure 9.7 represents the signal power “wasted” in this configuration. However, if the signal spectrum is first shaped by the GFF before entering the EDFA, then the amplifier need only provide the lower level of power indicated by the lower trace in Figure 9.7, thus consuming much less pump power. Figure 9.8 illustrates the “pre-filter” configuration, in contrast with the span design with a “post-filter” presented in Figure 9.2. EDFAs operate best in saturation,
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FIGURE 9.7 Illustration of the amount of excess signal power that is wasted in the “post-filter” GFF configuration (shaded area). In this example, the lower trace (“Psig out of Filter”) is the target signal spectrum to be launched into the fiber span. In order to achieve that power, over 5.5 dB of excess signal by the upper trace (Psig into Filter) must be output by the EDFA.
D+
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ISO
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980 nm pump
FIGURE 9.8 Diagram of hybrid DRA-EDFA with GFF placed before the EDFA (pre-filter configuration). Similar to the DRA/EDFA in Figure 9.2, the 1450 nm Raman amplifier (RA) pump and WDM are immediately adjacent to the EDFA components, whose boundaries are the two isolators.
in which case their output power is determined mainly by the available 980 nm pump power but not by input power. The output signal powers are limited by realistic pump powers for shorter distance systems and by fiber nonlinearities for longer distance systems. By allowing more efficient use of EDFA pump power, placement of the GFF at the input of the EDFA (pre-filter configuration) provides more signal input power into the Raman section, thus improving the noise performance of the Raman section and consequently the complete amplifier. As a particular example for a 120 km span design, 5.6 dB of additional pump power is needed for the more traditional post-filter configuration due to the loss of the filter when the GFF is placed at the
9.2 Raman-assisted systems and experiments
Table 9.1 Highlighting the benefit of the pre-filter design in hybrid DRA/EDFA systems Post-filtering EDF pump (dBm) with fixed OSNR Received OSNR (dB) with fixed pump Q-Factor (dB) aftr 8.9 Mm with fixed pump
Pre-filtering
Benefit
30.3
24.7
5.6
6.3
12.7
6.5
6.4
11.6
5.2
output of the EDFA (Table 9.1). Equivalently, using equal pump powers, in the prefilter configuration the OSNR is improved by 6.5 dB and the Q-factor performance is increased by 5.2 dB. Using this pre-filtering design architecture for Raman/EDFA hybrid repeaters, the noise performance of 120, 150, and 180 km fiber spans were numerically studied for a signal launch condition designed to provide a constant path average intensity over the full band. ASE noise contributions from erbium-doped fibers (EDF) and Raman amplification and MPI noise from DRBS were taken into account. The EDFA pump wavelength was 980 nm and the Raman pump wavelength was 1450 nm. The effective signal to noise ratio, or OSNReff, can be computed as 1=OSNReff ¼ 1=OSNRedfa þ 1=OSNRRaman þ x=OSNRDRBS where the coefficient x in the term accounting for the noise from DRBS depends on modulation format and receiver bandwidth and had to be computed numerically for the RZedigital binary phase-shift keying (DBPSK) case. For the particular bandwidth and modulation rate (12.5 Gb/s) and format in this experiment, x was very close to 1. Under normal operating conditions (signal power in the linear or nearlinear regime without significant nonlinear pulse broadening), that value appears to be independent of gain. Figure 9.9 shows a breakdown of these three noise contributions, which are plotted for all 96 signal channels for span designs using an EDFA pre-filter and with span lengths from 120 km to 180 km. The signal channels range from 1538 nm to 1563 nm with 33.3 GHz spacing. The noise values are presented in linear units of reciprocal OSNR, or 1/OSNR, on a log scale. Each data point on the graph is representative of the noise of an individual channel. For all but the longest span cases, the dominant source of noise is Raman ASE. As explained below, at optimum performance for the longest spans, the required Raman gain has increased to the point where penalties from MPI become equivalent and even surpass the penalties from ASE. For the longest reach system with the shortest repeater spacing under study (120 km), performance is limited by nonlinear penalties, so less than the maximum available pump power could be used. For 150 km spans the EDFA output power and Raman gain are nearly optimal, and for 180 km and longer spans the systems were
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FIGURE 9.9 Components of noise from MPI or from the ASE generated by the DRA or the EDFA for all 96 channels
pump-power starved. “Optimal” here refers to the signal power operating point at which maximum Q is reached. Although higher powers would provide improved OSNR, penalties from fiber nonlinearities would reduce total Q-factor performance. In Figure 9.10, simulation results of the effective OSNR (OSNReff) for all 96
FIGURE 9.10 For a 150 km span system with 25 dBm of 980 nm pump for the EDFA, the effective OSNR provides a valuable metric for determining the optimum DRA pump power. Here the effective OSNR is displayed for all 96 channels, for all RA pump powers ranging from 24 through 27 dBm. Note the increasing spectral variation across the signal band, for both OSNR and for Raman gain, as the Raman pump power is increased from 24 dBm to 27 dBm.
9.2 Raman-assisted systems and experiments
channels are plotted for the indicated values of Raman pump power ranging from 24 to 27 dBm, when the span length is 150 km and the maximum 980 nm EDFA pump power is 25 dBm. As Raman pump power and thereby Raman gain increases, the relative contribution of ASE noise from the EDFA diminishes, but the relative contribution of MPI noise increases. Plotting the effective OSNR for all channels in this manner allows straightforward selection of the optimum Raman gain for a particular span design, optimized for total noise performance. The power labels in the figure (24 dBm through 27 dBm) refer to the power of the Raman pump. Note that for lower Raman pump power there is little spectral variation in Raman gain and little variation in OSNR across the signal band. However, as Raman pump power is increased, spectral variation in both the Raman gain and the OSNR increase dramatically across the signal band. System performance is dependent not only on the noise performance of the system but also on propagation effects such as penalties arising from chromatic dispersion and optical nonlinearities. To establish the performance of systems based on hybrid Raman/EDFA repeaters, simulations were carried out including the above propagation effects as well as the effects of ASE and MPI induced optical noise. For this series of calculations, maximum pump power limitations were set at 25 dBm each for the 980 nm EDFA pump and the 1450 nm DRA pump. The signal band consisted of 96 channels at 33 GHz spacing with RZ-DBPSK modulation at 10 Gb/s plus 20% FEC overhead. In Figure 9.11, the Q factor vs. system reach of DRA-EDFA systems is presented for two span lengths, with spans comprised of either standard fibers or dispersion-flattened fibers (DFF). System reach in the designs using standard fibers are reduced relative to the DFF designs due to reduced
FIGURE 9.11 System performance versus system length for two different span lengths, either with conventional (single mode fiber [SMF] / non-zero dispersion shifted fiber [NZDSF]) or DFF spans. Note that conventional fiber results are only shown for 150 km span systems.
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FIGURE 9.12 Comparing simulated Q-factors to experimental measurements: 96 channels at 10 Gb/s over 9 Mm system of 150 km spans with hybrid DRA-EDFA
Raman gain efficiency and lowered tolerance to nonlinearities[10]. The system reach is larger for the 120 km spans because, although there are more spans and amplifiers, the span loss is lower and the OSNR is higher. For experimental validation of the modeling, Figure 9.12 shows a comparison of simulated Q-factors to experimental data for a system with 150 km repeater spacing after 9000 km. The fiber spans characteristics (fiber spectral loss, dispersion properties, and nonlinear coefficient) and filter spectral shapes used in the test bed were accurately characterized and used in the simulation, resulting in simulated Q-factors within 0.5 dB of the test bed results for all but a few channels.
9.3 COMPARISON OF RAMAN- AND ROPA-ASSISTED SYSTEMS More recently, as demonstrated in laboratory test beds, increases in repeater spacing can be made possible both by using distributed Raman amplifiers and by ROPAs. In a typical configuration, the ROPA is comprised of a simple short length of erbiumdoped fiber in the transmission line placed a few tens of kilometers before a shore terminal or a conventional in-line EDFA. The remote EDF is backward pumped by a 1480 nm laser, from the terminal or in-line EDFA, thus providing signal gain. There have been laboratory demonstrations of long-haul transmission using 150 km spans with high 980 nm pump efficiency using either DRA/EDFA[10] or ROPA/EDFA [11] hybrid amplification schemes. It was also similarly demonstrated that high spectral efficiency can be achieved at transoceanic distances [12]. The aforementioned amplification schemes can in fact also be used to carry 40 Gb/s per channel traffic
9.3 Comparison of Raman- and ROPA-assisted systems
[12,13]. In this section we will discuss in detail the optimization of a ROPA/EDFA amplification scheme and compare it with DRA/EDFA hybrid amplification [14]. The ROPA/EDFA design is attractive as the ROPA is less sensitive to pump parameters than the DRA is. The optimization of the hybrid amplifiers in this design space must also take into account nonlinear system performance as both schemes in this comparison operate at high span powers. This is made possible because of the EDFA is designed with a pre-filter, which allows for highly efficient use of 980 nm EDFA pump (described in the previous section). The optimized ROPA/EDFA architecture will be shown below both numerically and experimentally to provide superior noise and transmission performance (linear and nonlinear performance). The amplifier and 150 km span architecture for the a) DRA/EDFA and b) ROPA/ EDFA cases are shown in Figure 9.13. Large effective area fiber “Dþ” (110 mm2, þ20 ps/nm/km) and inverse dispersion fiber “D-“ (30 mm2, 40 ps/nm/km) were used with a Dþ / D- span map for the DRA/EDFA spans (Figure 9.13a)) to maximize Raman pump efficiency as described previously. The 1450 nm Raman pump wavelength was chosen to ensure that maximum Raman gain falls in the center of the transmission bandwidth from 1537 to 1563 nm, coinciding with the gain band of the EDFA. In contrast, a Dþ / D- / Dþ span map was used for the ROPA/EDFA spans (Figure 9.13b)). The larger effective area of the Dþ fiber reduced the Raman interaction between the 1480 nm ROPA pump and data channels in the transmission fiber following the ROPA. As an added benefit, this configuration provides a lowerloss path from the 1480 nm pump to the remote EDF, enhancing the efficiency of the ROPA. A study of system performance as a function of the distance between the EDFA and the ROPA determined that the optimum position for the ROPA’s
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FIGURE 9.13 Amplifier and 150 km span schematic for a) DRA/EDFA and b) ROPA/EDFA architectures. In both a) and b), the amplifier boundaries are defined by the WDM on the left and the isolator on the right. As described in the text, the remote amplifier (the ROPA) is part of the 150 km span of mixed fiber types.
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erbium-doped fiber is at the junction between the two fiber types, about two-thirds along the span length from the span input, i.e., one-third of the span length from the downstream repeater site from which the ROPA is pumped. The amplifier in the repeater in both cases is a conventional forward pumped (980 nm) single-stage EDFA in the pre-filtered configuration: the GFF is placed at the amplifier’s input to maximize output signal power and minimize the ASE contributions from Raman (Figure 19.3a)) and ROPA (Figure 9.13b)) amplifiers respectively. In the numerical simulations of this comparative study, 980 nm EDFA pump power was limited to 300 mW; DRA 1450 nm pump power was limited to 316 mW, and ROPA 1480 nm pump power was limited to 316 mW. Both systems used the same 26 nm transmission band, approximately 1537 nm through 1563 nm. As noted previously, with the pre-filtering technique EDFA output power can be large enough so that system performance will be limited not only by ASE accumulation but by nonlinear effects as well. Thus, to achieve optimum performance across the signal band, the path average intensity (PAI) for every wavelength must be equalized by careful selection of EDFA output power. This procedure will result in the best BER performance for every wavelength across the band, but will not necessarily result in equal OSNR. In order to achieve the proper shape of the GFF, all principal gain shaping processes need to be taken into account: EDFA, DRA, and ROPA gains; Raman interaction between data channels (“Raman tilt”); and Raman interaction with the ROPA 1480 nm pump. The EDFA and the ROPA must be designed as an integrated unit to achieve best performance. Thus a carefully coordinated selection of ROPA length (Figure 9.14), ROPA position, and EDFA pump is required for optimization of average effective OSNR (OSNREFF). As noted in section 2, the OSNREFF accounts for all noise sources: ASE from the amplifiers (ROPA, DRA, and EDFA) and multipath interference from different parts of the fiber/amplifier span.
FIGURE 9.14 Optimization of ROPA length for one specific ROPA position and one EDFA pump power. The ROPA gain scales with the ROPA EDF length, as in “standard” EDFAs. This figure was shown and discussed in the oral presentation for [10].
9.3 Comparison of Raman- and ROPA-assisted systems
FIGURE 9.15 Comparison of effective OSNR for a ROPA/EDFA span and for a DRA/EDFA span with identical loss and power budgets
The comparison of single-span OSNREFF for two systems is shown in Figure 9.15. Both systems, one based on ROPA/EDFA and one on DRA/EDFA, have 96 channels with 33 GHz spacing, with the same PAI of the signals in each case. The ROPA/EDFA design has on average 1 dB better OSNREFF than that of the DRA/ EDFA. It should be noted that OSNREFF spectra for both cases are not flat; in fact, in the case of the ROPA/EDFA, it has almost a 2 dB tilt. This latter tilt persists for multispan systems and arises from Raman interaction between the high-power data channels in the Dþ fiber section preceding each ROPA. The fiber loss spectrum exacerbates that tilt, but to a relatively minor extent. The Raman-induced signal power tilt results in higher channel power at longer wavelengths at the ROPA input and thus better noise performance for those longer wavelengths. Figure 9.16 shows the comparison between simulated and measured performance of 33 GHz spaced 10 Gb/s RZ-DBPSK channels versus signal power pre-emphasis (received OSNR) for the channel at 1562 nm after 7300 km of transmission. In this power pre-emphasis study, a channel at the long wavelength end was chosen since this region of the signal band has the weakest channel-to-channel signal power interaction due to spectral hole burning (SHB). It can be seen that there is a good agreement between predicted and measured performance. The OSNR is adjusted by pre-emphasizing this channel, i.e., changing the power of this channel at the input of the link. Below an OSNR of about 12.5 dB the performance is noise dominated, and it improves as OSNR is increased. For higher OSNR, the nonlinear penalties dominate and the performance degrades as the OSNR increases because the pre-emphasis by which the OSNR is increased means that the launch power into each span is increased. Figure 9.17 shows measured performance versus preemphasis for 33 GHz and 17 GHz channel spacing for this same system length.
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FIGURE 9.16 Simulated and measured performance in ROPA/EDFA system versus pre-emphasis (OSNR) for 33 GHz channel spacing
In the case of 17 GHz channel spacing, pre-filtering and orthogonal launch were used. An important conclusion here is that amplifier chains need to be optimized differently for different channel spacings. In this comparison, the peak performance for the 33 GHz system is achieved at a channel power > 1.5 dB higher than the power at optimum performance for the 17 GHz system due to reduced cross-channel penalties in the 33 GHz system.
FIGURE 9.17 Measured performance in ROPA/EDFA chain versus pre-emphasis for 33 and 17 GHz channel spacing
9.3 Comparison of Raman- and ROPA-assisted systems
It bears pointing out that the 17 GHz system operates optimally at a higher total signal power out of the repeater than a 33 GHz system (lower per-channel power, but many more channels). Repeater signal power in the ROPA/EDFA experimental test bed was originally optimized to transmit 200x10 Gb/s channels with 17 GHz channel spacing where the Q-Factor for all channels were measured across the full bandwidth of the system. In contrast, the hybrid DRA/EDFA test bed was optimized for 96x10Gb/s channels with 33 GHz channel spacing. So in order to realize a fair performance comparison between the ROPA/EDFA test bed and the DRA/EDFA test bed at 33 GHz, signal powers for the former would first need to be de-emphasized. This was achieved by using what has now become a standard experimental technique, the tunable comb method with 64 loading tones [15]. In this method, unmodulated loading tones maintain the proper level of inversion and power spectral density across the signal band. Then, to measure the performance of a channel, a few loading tones are replaced by six modulated signals, and the performance of the center channel is measured. In general, this group of channels is sequentially moved across the signal band so that performance at each signal channel wavelength may be evaluated. In this experiment, as the group of six signal channels was tuned to different parts of the band, they were propagated with signal power approximately 2 dB lower than the repeater design target in order to operate at the optimum channel power for the chosen channel spacing. The performance of the channels in the middle of the group was then measured as described above. Figure 9.18 shows this direct experimental performance comparison between the ROPA/EDFA scheme and the DRA/EDFA scheme at 33 GHz channel spacing at transmission distances of 9342 km and 8900 km respectively. When
FIGURE 9.18 System performance for the DRA/EDFA test bed at optimum power level and the ROPA/EDFA test bed with signal power adjusted for optimum performance for the 33 GHz channel spacing
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signal power pre-emphasis is properly set so that the two systems are operating under equivalent conditions, the measured Q-factor performance of the ROPA/EDFA exceeds by 0.2 dB to 0.8 dB the performance of the DRA/EDFA. Further, correcting for the disparity in system lengths, this translates to an advantage of 0.4 dB to 1.0 dB for the ROPA system. Recall (Figure 9.15) that the OSNR advantage of hybrid ROPA over hybrid Raman is approximately 1 dB. Thus we see that the Q performance advantage largely derives from the relative advantage in noise properties. Two earlier papers of note established the first records of transmission over transPacific distances with repeater spans of 100 km [16] and 150 km [17] using Raman assisted EDFAs. The first experiment successfully demonstrated 64 11.4 Gb/s 9000 km transmission with 100 km repeater span by using Raman assisted EDFA for the first time. A unique aspect of this design is that the DRA and the EDFA segments shared a common pump source (Figure 9.19). Two 1460 nm pump diodes were combined by a PBS coupler and directed to the EDF. The residual pump was fed to the transmission line to power the DRA section. Error-free transmission was enabled in that case by using 14% redundancy super FEC in conjunction with the low noise hybrid amplifiers. That result first demonstrated the feasibility of trans-Pacific distance transmission with 100 km repeater spans using Raman assisted EDFAs. In the second paper by the same group [17], by using a narrower portion of the available bandwidth, 32 12.4 Gb/s, 9000 km transmission with 150 km repeater span was demonstrated for the first time. The received optical spectrum (for continuous wave [CW] tones) is presented in Figure 9.20. Both experiments had channel spacing of 37.5 GHz. The small variation in power across the transmission bandwidth allowed for a received OSNR variation of only a few tenths of a dB by the use of simple signal power pre-emphasis at the transmitter, with an average OSNR of 9.5 dB. The received Q-factor was similarly smooth across the bandwidth, with an average Q of 9.5 dB. Those results showed the feasibility of 2.8 dB FEC margin transmission after trans-Pacific distance with 150 km repeater span using Raman assisted EDFA and carrier suppressed return to zero (CSRZ)edigital phase-shift keying (DPSK) modulation format.
FIGURE 9.19 A schematic diagram of the hybrid DRA-EDFA which enabled transpacific transmission with 100 km repeater spacing [16]
9.4 Advanced modulation formats and high capacity
FIGURE 9.20 Optical spectrum for the 150 km span system after 9000 km transmission. Signal power preemphasis at the transmitter provided flat OSNR at the receiver.
9.4 ADVANCED MODULATION FORMATS AND HIGH CAPACITY IN HYBRID ROPA-EDFA SYSTEMS Given the present need for ever-increasing system capacity, it is important to demonstrate the compatibility of hybrid amplifiers with the enabling technologies and techniques that are being developed and deployed, such as the various implementations of digital phase shift key modulation format (DBPSK and digital quadrature phase-shift keying [DQPSK], for example), coherent transmission and detection, higher data rates, and tighter channel spacing. While enabling greater fiber capacity, these technologies in many cases will be more demanding in terms of noise performance because of higher data rates, more complex signal constellations, or greater susceptibility to nonlinear impairments related to closer channel spacing. The improved noise performance of hybrid amplifiers can thus be important for such systems. A number of transmission results with high spectral efficiency 0.6 Bit/Hz/s have already been obtained with ROPA/EDFA scheme using its superior OSNR performance. Foursa et al. [12] demonstrated transmission of 2 Tb/s (200 10 Gb/s) of data over 7300 km with a Q factor that varied from 10.5 to 12.3 dB (Figure 9.21). The 10 Gb/s RZ-DBPSK channels had 16.6 GHz separation and orthogonal launch of neighboring channels, and prefiltering techniques were used to boost performance. In part, the excellent Q performance was due to the noise advantage of the ROPA/EDFA system design over the DRA/EDFA design (see Figure 9.15). A fair comparison of two laboratory test beds, scaling properly for distance and channel power, demonstrates that the ROPA/EDFA noise advantage ranges from 1 to 3 dB across the signal band (Figure 9.22). Similar to the trend in OSNR, the Q-factor increased with wavelength and varied from 10.5 dB in the short wavelength region up to 12.3 dB in the long wavelength region. Over that same loop test bed in a 40 Gb/s experiment, 50 CSRZ-DBPSK channels (2 Tb/s capacity) were transmitted over 3100 km with Q factors all > 4 dB above the FEC threshold of 9.1 dB. As in the 10 Gb/s experiment, measured Q performance varied across the signal band from
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12.9 dB to 14.2 dB (Figure 9.23). The repeater output power was optimized for 40 Gb/s transmission at the chosen distance. An added challenge to long-haul optical paths using 40 Gb/s channels is successful transmission at high spectral efficiency (>0.5 Bit/Hz/s). A recent experiment over a recirculating loop test bed (ROPA/EDFA with 150 km spans)
9.4 Advanced modulation formats and high capacity
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transmitted 2 Tb/s over 5,200 km at 60% spectral efficiency (0.6 Bit/Hz/s). That record was enabled through the use of the polarization multiplexed signals with RZ-DPSK modulation format and an automatic polarization tracking scheme [18].Polarization multiplexed [PM]eRZeDPSK modulation format increases nonlinear tolerance relative to 40 Gb/s RZ-DBPSK signals. Figure 9.24 summarizes the Q performance across the signal band after 5200 km. The automatic polarization
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FIGURE 9.24 Q-factor for PM RZ-DBPSK after 5200 km. Each polarization was measured for 2 minutes, and the average reported here.
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tracking receiver demonstrated long-term stability even in a loop test bed which has a state of polarization which is discontinuous by its nature. Figure 9.24 also shows the FEC threshold for a standard concatenated Reed-Solomon code with a 9.1 dB threshold. In this case, the worst channel in the system has a 3.5 dB average FEC margin.
9.5 CONCLUSION We have presented in this chapter a survey of the recent history of the development of the DRA and ROPA technologies and their implementation in some of the significant laboratory demonstrations in the literature. The hybrid DRA-EDFA provides the advantages of the pump power efficiency and well developed technology of the EDFA combined with the low noise figure and broadband capability of the Raman gain. Furthermore, the Raman section of the hybrid DRA-EDFA can be configured to either provide extra gain in the conventional portion of the signal band or to provide extra signal bandwidth, all determined by selection of the wavelength of the Raman pump laser. Using a simple combination of a singlewavelength pumped DRA and C-band EDFA we have demonstrated 80 nm of continuous bandwidth. This compares favorably to the 18 nm to 35 nm of conventional EDFA alone. Transmission experiments have confirmed high performance of the hybrid DRA-EDFA, demonstrating 11,000-km distances. This good transmission performance has been realized with both RZeon-off keying (OOK) and RZ-DBPSK formats. In the case of DBPSK modulation format transmission experiment, an average of 2.6 dB performance above the FEC threshold was achieved at a transmission distance of 11,000 km with 40% spectral efficiency. A detailed analysis of both the fundamental properties and the signal power and noise performance enhancement of the RA and ROPA technologies when deployed in hybrid design with EDFAs highlights the potential contribution of each to improvement in system performance. Further, the specific examples we have surveyed provided a fair comparison between the designs and lay the groundwork for design decisions between the two amplifier types. In the reviews of our own experimental and modeling work we implemented reasonably simple amplifier architectures for both Distributed Raman and ROPA assisted EDFA (DRA/EDFA and ROPA/EDFA). Consistently, the hybrid amplifier designs allowed for the large repeater spacing, while the simple pre-filter technique allowed increased 980 nm EDFA pump efficiency. Finally, a number of recent published studies which used hybrid amplification techniques have confirmed that the improved OSNR performance of hybrid amplification systems are clearly supportive of the advanced modulation formats, high data rates, and high spectral efficiencies that are moving rapidly into the optical telecommunications industry.
References
ACRONYMS ASE BER C-Band or L-Band CSRZ CW dB dBm DBPSK DPSK DFB DFF DQPSK DRA DRBS EDF EDFA FEC GFF Mm MPI NZDSF OOK OSNR PAI PM Q ROPA RA RZ SMF SRBS WDM
Amplified spontaneous emission Bit error ratio The conventional or the long-wavelength signal bands Carrier suppressed return to zero Continuous wave, i.e., no modulation on the channel Decibel Logarithm of power ratios normalized to a milliwatt Digital binary phase-shift keying Digital phase-shift keying Distributed feedback laser Dispersion-fattened fibers Differential quadrature phase-shift keying Distributed Raman amplifier (or amplification) Double Rayleigh back scatter Erbium-doped fiber Erbium-doped fiber amplifier (or amplification) Forward error correction Gain flattening filter One million meters, or one thousand kilometers (km) Multipath interference Non-zero dispersion shifted fiber On-off keying Optical signal-to-noise ratio Path average intensity Polarization multiplexed pffiffi Q is related to BER as Q ¼ 12ERFC BER 2 Remote optically pumped amplifier Raman amplifier (or amplification) Return to zero Single mode fiber Single Rayleigh back scatter Wavelength division multiplexing
References [1] P. Trischitta, W. Marra, Global Undersea Communications Networks, IEEE Communications Magazine 34 (2) (1996).
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[2] N. Bergano, in: I.P. Kaminow, T. Li (Eds.), Undersea Amplified Lightwave Systems Design, Optical Fiber Telecommunications IIIA, Academic Press, San Diego, CA, 1997, pp. 302e335. [3] A. Pilipetskii, et al., Challenges of High Capacity Undersea Long-Haul Systems, in: Proc. Asia Communications and Photonics Conference, paper THC1, 2009. [4] S. Abbott, Optical Amplifiers in Undersea Systems, in: Proc. Optical Amplifiers and Their Applications, Paper OMC1, 2004. [5] H. Masuda, Hybrid EDFA/Raman Amplifiers, in: M.N. Islam (Ed.), Raman Amplifiers for Telecommunications 1, Springer-Verlag, New York, 2004, pp. 413e444. [6] T. Mizuochi, LEOS 2004 Annual Meeting 2 (2004) 467e468. [7] E.A. Golovchenko, et al., Pushing the Reach of Repeaterless Transmission Systems, in: Proc. Suboptic, paper We.207, 2007. [8] D. Foursa, et al., Ultra wide-band amplifiers for transoceanic length transmission, in: Proc. Optical Amplifiers and Their Applications, Paper OTuD1, 2003. [9] J. Bromage, et al., Multiple Path Interference and Its Impact on System Design, in: M.N. Islam (Ed.), Raman Amplifiers for Telecommunications 2, Springer, New York, 2004, pp. 491e568. [10] A. Lucero, et al., Long-Haul Raman-Assisted EDFA Systems with Ultra-Long Spans, in: Proc. Optical Fiber Communications Conference, Paper OFD2, 2006. [11] D. Foursa, et al., Transmission over 8,900 km with 150-km spans using a novel gain equalization scheme in Raman assisted EDF amplification, in: Proc. European Conference on Optical Communications, post-deadline paper Th4.1.7, 2006. [12] D. Foursa, et al., 2Tb/s (200x10Gb/s) data transmission over 7,300km using 150km spaced repeaters enabled by ROPA technology, in: Proc. Optical Fiber Communications Conference, post-deadline paper PDP-25, 2007. [13] J.-X. Cai, et al., Long-Haul 40 Gb/s RZ-DPSK Transmission over 4,450 km with 150-km Repeater Spacing using Raman Assisted EDFAs, in: Proc. Optical Fiber Communications Conference, paper OWM3, 2007. [14] A. Lucero, et al., Advanced repeater architectures with ultra-long spans for submarine systems, in: Proc. Optical Fiber Communications Conference, paper OTuE3, 2008. [15] J.-X. Cai, et al., A DWDM demonstration of 3.73 Tb/s over 11,000 km using 373 RZ-DPSK channels at 10 Gb/s, in: Proc. Optical Fiber Communications Conference, post-deadline paper PDP-22, 2003. [16] T. Inoue, et al., First Transpacific Distance Transmission Experiment Using Raman Assisted EDF Amplifier with 100 km Repeater Span, ECOC 2004, Th.3.5.2, Stockholm, Sweden, 2004. [17] T. Inoue, et al., 150 km Repeater Span Transmission Experiment over 9,000 km, ECOC 2004, PD4.1.3, Stockholm, Sweden, 2004. [18] J.-X. Cai, et al., 40 Gb/s Transmission Using Polarization Division Multiplexing (PDM) RZ-DBPSK with Automatic Polarization Tracking, in: Proc. Optical Fiber Communications Conference, post-deadline paper PDP-4, 2007.
CHAPTER
Challenges for Long-haul and Ultra-long-haul Dynamic Networks
10 Martin Birk, Kathy Tse AT&T Labs., USA
CHAPTER OUTLINE HEAD 10.1. Photonic network evolution ........................................................................ 10.1.1. Data rate evolution ................................................................. 10.1.2. WDM evolution ...................................................................... 10.1.3. Network architecture evolution ................................................ 10.1.4. Amplifier evolution ................................................................. 10.1.5. Status quo............................................................................. 10.2. Requirements for amplifiers in today’s photonic mesh networks ................... 10.2.1. Optical link control ................................................................ 10.2.2. Static amplifier requirements .................................................. 10.2.3. Dynamic amplifier requirements .............................................. 10.3. The future: requirements for a fully dynamic photonic mesh......................... 10.3.1. Provisioning through pre-cabling ............................................. 10.3.2. Simple photonic restoration techniques ................................... 10.3.3. The holy grail: photonic restoration .......................................... 10.4. Summary................................................................................................... Acronyms ........................................................................................................... References .........................................................................................................
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This chapter is organized into three parts. Part one covers the historic evolution of photonic networksdsome background for understanding how the industry got to where it is today. The second section explains requirements for optical amplifiers used in today’s photonic mesh networks. The third section looks ahead into the future: how the networks may evolve and the requirements for optical amplifiers to support such future networks.
10.1 PHOTONIC NETWORK EVOLUTION Photonic networks will continue to evolve as technology advances and bandwidth demands drive needs for higher transmission capacity at lower costs. With this evolution comes a variety of challenges, both technical and operational. Every Optically Amplified WDM Networks. DOI: Copyright Ó 2011 Elsevier Inc. All rights reserved.
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carrier has its own strategy to meet the demands at the best cost points, and choosing a path forward often depends on the marketplace, the technical alternatives, the willingness to be an early adopter and take risk, and the need for scale. From a technical perspective, amplifiers and their performance are becoming more and more important in meeting the increased demands at higher total system capacities.
10.1.1 Data rate evolution Data rates have increased over time and will continue to increase as the bandwidth demand increases. There is continuous pull between riding the cost curve at a lower bandwidth and jumping to a new, higher bandwidth to accommodate growth and enable new services. Usually, the lower bandwidth offers commodity pricing and high manufacturing volumes, whereas the higher bandwidth may be needed to fulfill the service needs. Today, traffic on carrier networks is still growing by 30% to 60% per year, driven by growth in internet protocol (IP) traffic [3]. This will lead to a network adoption of 100 G per wavelength by 2011.
10.1.2 WDM evolution As the channel data rate is increasing, having more and more wavelengths on a single fiber enables cost improvements and capacity growth (Figures 10.1 and 10.2). Wavelength division multiplexing (WDM) systems have grown from the original two-channel systems of the 1980s to 80-plus channels today. Often it is possible to accommodate even more channels through closer spacing or amplification over a wider spectral range, but with these come trade-offs in reach, cost, and Data rate evolution
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operational issues that must be accounted for. Due to the nonlinear interaction of wavelengths on the optical fiber, closer spacing of wavelengths comes at a penalty to the overall reach, so each carrier has to decide what its typical reach needs to be before determining the ultimate fiber capacity and channel spacing.
10.1.3 Network architecture evolution Network architectures have evolved greatly in the 20-plus years that dense wavelength division multiplexing (DWDM) systems have been deployed. Early systems were point-to-point with terminals at each end. Transponders are used to condition the signal, apply the required overhead and use the appropriate wavelength for the DWDM transmission (Figure 10.3). The optical signal from the end user device (router, Ethernet switch, synchronous optical network [SONET] multiplexer) is received over a short reach client interface, the signal is framed, forward error correction information added and transmitted via a long-reach wavelength signal into the line. For a simple single-channel system, the transponder enables the information to be transmitted over a fiber for a moderate distance (w40 to 80 km) without the need for electrical regeneration (Figure 10.4). When longer distances were needed on these single-channel systems, transponders could be placed back to back to regenerate the signal. In the mid-1980s, these single channel systems were commonly used for enabling fiber optic transmission in carrier networks (Figure 10.5). As bandwidth needs grew above the single channel rates, there was a race between higher speed electronics and the potential for multichannel systems. Early WDM systems arose when the bandwidth need outstripped the electronic
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definition
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FIGURE 10.3 Definition of a transponder
Single wavelength, no amp Outside Plant Transponder
Transponder
Fiber FIGURE 10.4 Simple single channel transport system (1980s)
Single wavelength, no amp, network
Outside Plant n Transponder
Outside Plant Transponder
Transponder
Transponder
Fiber
Fiber Intra-office Fiber
Physical a building A
Physical building B
FIGURE 10.5 Network view of single channel transport system (1980s)
Physical building u C
10.1 Photonic network evolution
development and low channel count systems emerged. These systems were still limited by the distance between expensive regeneration points (Figure 10.6). The main enabler for the DWDM systems we have today was the emergence of the optical amplifier in the early 1990s. For the first time it was not necessary to regenerate the signal at such closely spaced intervals, and the entire payload could go five or more amplified spans. Early systems were limited by the quality of the components available, but systems with eight to 16 wavelengths of OC-48 were normal. Still, at this point it was necessary to fully demultiplex the multichannel signal to regenerate at an optical terminal. This required terminals at every point where add or drop was needed, even if some of the channels did not require signal regeneration at that point (Figure 10.7). To avoid costly and unnecessary regeneration of pass-through wavelengths, some systems allowed for optical pass-through from demultiplexer to multiplexer, via jumper cables. This required a great deal of manual effort, because each terminal site had to be hand-cabled to set up a wavelength at the through locations. In the early 2000s, commercial reconfigurable optical add/drop multiplexer (ROADM) systems started to emerge [1] (Figure 10.8). The early systems had 1x2 switches that allowed a wavelength to be passed through or dropped via a remote command at intermediate sites. For the first time it was possible to limit the “touch” required to set up a wavelength to only the endpoints or regenerator locations, and not every site that any wavelength on the system had to stop for add/drop. At the same time, tunable lasers were becoming more common, allowing a single
WDM Wavelength 1 . . . Wavelength n/2 . . .
Wavelength 1
W D M M U X
W D M
Outside Plant Fiber
D E M U X
Wavelength n
FIGURE 10.6 Wavelength division multiplexing transport system (late 1980s)
. . . Wavelength n/2 . . . Wavelength n
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Amplified WDM Wavelength 1
Wavelength 1 . . . Wavelength n/2 . . .
W D M M U X
W D M
Outside Plant Fiber
Amplifier
Outside Plant D E M U X
Fiber
Wavelength n
. . . Wavelength n/2 . . . Wavelength n
FIGURE 10.7 Amplified WDM transport system (early 1990s)
Simple Optical Add and Drop Wavelength 1 . . .
Wavelength 1 . . .
OSP Fiber
OSP Fiber
. . . Wavelength n
Wavelength n
Wavelength n Wavelength n
282
Local Add/Drop
FIGURE 10.8 WDM transport system with simple MUX/DEMUX optical add/drop (late 1990s)
transponder to serve a spectrum of wavelengths. Technologies such as wavelength blockers and wavelength selective switches further drove down the cost and complexity of ROADM systems, making the ROADM the architecture of choice for modern DWDM networks (Figure 10.9). While 2-degree ROADMs enabled through wavelengths to be expressed through a drop site, many nodes had 3 or more degrees of traffic, i.e. three or more fiber pairs
10.1 Photonic network evolution
ROADM Wavelength 1
Wavelength 1 . . .
OSP
OSP ROADM
Fiber
Fiber
Wavelength x
Wavelength n Wavelength x
Wavelength n
. . .
Any wavelength, any number of Drop or Add
FIGURE 10.9 WDM transport system with ROADM (early 2000s)
meeting at the node. For early ROADM systems, the best that a carrier could do was to position the ROADM in the most advantageous direction, then for the other directions the traffic needed to be dropped and regenerated through a terminal. As ROADM technology evolved, architectures that allowed N > 2 degrees of optical express became commonplace [2]. Early systems were limited to 1x5 or 1x9 switches supporting the architectures, depending on the channel count and spacing. Today’s systems are evolving to support higher and higher degrees, allowing sites to have multiple rails of DWDM in each direction with optical pass-through (Figure 10.10). Given the advances in technologies, today full photonic mesh networks are being built. N-degree ROADMs at each node in the mesh allow any channel to be added, dropped, or expressed anywhere in the network (Figure 10.11). This is the kind of flexible network carriers want: one that does not require special planning or wavelength-specific equipment and that allows the network to grow as technology evolves. For example, as the transponder reach improves, it is now possible to express through sites that may have been drop sites for earlier transponders, continuously improving cost and avoiding network touch points.
10.1.4 Amplifier evolution From the early 1990s to about 1999, optical amplifiers improved their performance rapidly (usable bandwidth, flatness, output power). As the number of WDM channels increased significantly, system designers learned to take many new kinds of effects into account, such as stimulated Raman scattering (SRS). Around 1999, as 10 G direct detection started to move into networks, suddenly the 10 G chromatic dispersion limit required in-line chromatic dispersion compensation modules (DCM). To overcome the loss of these modules, amplifiers with mid-stage access
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OSP Fiber
OSP N-degree ROADM
Fiber
Amplifier
Wavelength x
Amplifier
Fiber
OSP
Amplifier
Multi-degree ROADM
Wavelength x
284
Any wavelength, any number of Drop or Add
FIGURE 10.10 Multidegree ROADM node with add/drop on bottom (after 2005)
Photonic Mesh Network
N-degree ROADM
N-degree ROADM
N-degree ROADM
N-degree ROADM
FIGURE 10.11 Photonic mesh network (2006 and later)
N-degree ROADM
10.2 Requirements for amplifiers in today’s photonic mesh networks
were introduced. With the advent of 40 G coherent transmission in commercial networks in 2008, these DCM modules became unnecessary again for such systems, enabling the industry to go back to an amplifier design without mid-stage access for 40 G and 100 G coherent networks. Raman amplifiers have been deployed primarily to overcome long-reach spans or to give an added channel enhanced reach at an added cost. The additional operational complexities (e.g., fiber cleanliness and eye safety) and cost (Raman pump conversion efficiency is much smaller than EDFA pump conversion efficiency) need to be considered in any network design. Raman amplifiers may be a tool in the future to improve the optical signal-to-noise ratio (OSNR) of transmission at data rates beyond 100 G.
10.1.5 Status quo AT&T has deployed a full photonic mesh network in its backbone. The cartoon in Figure 10.12 shows how this network interconnects the key backbone routers with high-capacity 40 G wavelengths. By using a photonic mesh as described in the previous sections, AT&T has been able to grow the network from 10 G to 40 G and through a second generation of 40 G technology. At each step we have been able to get the best reach and cost without the need to make any changes in the photonic layer (amplifiers and ROADMs). There are no terminals in this network, and each node is capable of full add/drop/express [3]. For AT&T, the main driver to go from 10 G to 40 G was IP data growth. There were many other reasons, such as cost and scalability. Ultimately, these same drivers will push us to go to 100 G in the backbone.
10.2 REQUIREMENTS FOR AMPLIFIERS IN TODAY’S PHOTONIC MESH NETWORKS Design and deployment of a full photonic mesh network (or even islands of photonic mesh) involves careful design and control. Wavelengths can stop and start at any point in the network, and this can lead to unwanted interactions between channels and propagation of effects across the network in unintended ways. The system designer needs to account for these optical effects up front in any design, and the system needs to be modeled and tested to ensure proper operation in the field.
10.2.1 Optical link control Modern transmission systems have a very sophisticated optical link control compared with the systems of the 1980s and 1990s. This is driven, on the one hand, by the availability of lower cost memory and processors, and on the other hand by the need for higher flexibility and performance. The system usually consists of a fast (on the order of tens or hundreds of microseconds) constant gain optical amplifier control loop and possibly a gain tilt
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Seattle
Chicago
Cleveland
Salt Lake City San Francisco
New York Kansas City
Denver
Los Angeles
Herndon
Atlanta Dallas
OC-768 capable DWDMs Backbone Router
Houston Miami
FIGURE 10.12 AT&T photonic mesh network (2006)
control loop for the individual amplifier [4] and a slower (on the order of seconds) control loop to keep the optical link in an optimum state (Figure 10.13). While the goal of the fast control loop is to keep the impact of any failures or fiber cuts on service to a minimum, the slow control loop continuously optimizes the link for best transmission performance, making sure the launch spectra and launch powers after every ROADM and amplifier are as close to the set points as possible. For long-haul transmission, these control loops will be a fundamental part governing the performance of the system. The exact requirements of these control loops will be part (among other things) of the margin allocation for the overall transmission performance. The system designer can trade off certain parameters in the margin allocation to relax the requirements on these control loops. For example, if the amplifier tilt control would require a tilt control device in every amplifier, but the designer wants to save system cost and place that expensive device in only every nth amplifier, a worst case gain tilt penalty can be introduced in the margin allocation. This gain tilt penalty will reduce the reach for certain channels, but lower the overall system cost. For ultra-long-haul (ULH) transmission systems fewer trade-offs will be possible due to the higher optical performance requirement. There are usually local control loops in every amplifier, as well as a separate control loop for every ROADM-to-ROADM link section. The local control loops attempt to keep the amplifier operating parameters such as gain, flatness, etc. in a certain set range, whereas the ROADM-to-ROADM control loop gives commands to the local control loops based on the increased network visibility. For example, if in one ROADM-to-ROADM section, any fiber span has an increased loss (e.g., a large bend loss due to a cable pinch), the local control loop in the following amplifier will
10.2 Requirements for amplifiers in today’s photonic mesh networks
ROADM to ROADM link control loops
N-degree ROADM
Amplifier
Local control loops
Amplifier
N-degree ROADM
Local control loops
FIGURE 10.13 Elements of optical link control (amps, ROADMs)
try to compensate for the added loss, while the ROADM-to-ROADM control loop may increase the launch power from the previous amplifier to help compensate for the larger loss. While these cases are not everyday occurrences in a network, having an intelligent optical infrastructure that keeps the network running as long as possible is a key requirement for carriers.
10.2.2 Static amplifier requirements Based on the description of today’s photonic mesh network, due to the number of concatenated amplifiers, static amplifier performance is key to the overall network performance. Low noise figure, flat gain and low gain ripple, as well as low polarization dependent gain are the most important parameters in a photonic mesh network [5]. In addition, manufacturing repeatability is also very important for being able to model a long chain of amplifiers correctly to design links over the network.
10.2.3 Dynamic amplifier requirements Dynamic amplifier requirements in a photonic mesh network are also important for protecting service from any dynamic events that can happen in a network. For example, if a fiber cut or an amplifier failure occurs in front of a ROADM, the other directions of the ROADM should not be impacted due to the drop in channels and therefore change in loading of the amplifier chains [7,8]. Today’s amplifiers usually contain a control loop to keep the optical gain of the amplifier constant. In a photonic mesh network, due to the impact of fast changes on spectral loading, a way to keep the amplifier flat with a fast gain tilt compensation is also critical [6]. Also, certain impairments can become intensified due to fast changes in the spectral loading of an amplifier (e.g., spectral hole burning) and lead to the need for additional stabilization of the amplifiers during dynamic events [9]. All these control loops need to be designed so that they are unconditionally stable in a network environment, as there are potential feedback paths in a photonic mesh network, where changes in one ROADM-to-ROADM section can propagate through perturbations caused by the changes in other sections.
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10.3 THE FUTURE: REQUIREMENTS FOR A FULLY DYNAMIC PHOTONIC MESH All of the photonic networks that have been discussed above still have one limitation: the transponder has to be cabled to a particular channel port of the multiplexer or demultiplexer, which is then cabled to a particular direction at a multidegree node. Even though the laser on the transponder can be tuned to any of the International Telecommunication Union-grid wavelengths, it is connected to a port that is expecting a particular wavelength number. So for an electrically switched network, it is possible to precable the end user to the network and commit the bandwidth later once it is determined where the service needs to go (also reconfigure and move that bandwidth on the fly if changes are required). For photonic networks, however, the end user is committed to a particular path as soon as the transponder is connected to the system. To avoid this, carriers deploy transponders only when the service is ready to be committed; then provisioning requires a cabling operation at each end and at any regeneration points in the middle. Technologies have existed for a number of years that would allow the photonic network to behave more like an electrically switched one, making the on and off ramps colorless, able to add and drop any wavelength to any port (colorless), and the able to switch the transponder to any of the directions out of a node (non-directional/ steerable). This gives the carrier the ability to pre-cable the transponders to the router or Ethernet switch, then commit the bandwidth, as needed, through remote commands or an optical control plane. Unfortunately these technologies for colorless and non-directional transmission still remain expensive and immature for high-bandwidth and high-capacity applications. But suppliers are starting to offer options for reconfigurability, which are more attractive, and it is only a matter of time until these systems become commonplace in carrier networks. Figure 10.14 shows an early implementation of a two-degree ROADM with a fixed add/drop. In this figure, the wavelengths are split off and dropped, and transponders are connected to ports that are expecting the dropped signals. A wavelength blocker is used to selectively stop the dropped channels from being transmitted on the through path. On the add side, wavelengths are again cabled to ports on the channels that are being added, and those are combined with the through channels to be sent to the next downstream ROADM. Now consider the same architecture with tunable add/drops (Figure 10.15). The transponders can be cabled to any port on the multiplexer or demultiplexer, because it is now possible to drop and add any wavelength to any port. It is no longer required to connect a transponder to a particular port. So if channel 1 is required to be dropped at this site, it would be possible to tune the demultiplexer to receive channel 1 on port M. The blocker would prevent channel 1 from being passed through to the add section. Likewise, on the add side, channel 1 could again be added by tuning the laser to channel 1 into the tunable add multiplexer. This could all be done remotely,
10.3 The future: Requirements for a fully dynamic photonic mesh
Broadcast and select with fixed local add and drop
. . .
Wavelength 1
Wavelength x
Wavelength x
coupler
Wavelength Blocker
. . .
Wavelength 1
coupler
drop
add
FIGURE 10.14 Per-direction fixed optical add/drop
Broadcast and select with tunable local add and drop coupler
coupler
Wavelength Blocker
Tunable drop FIGURE 10.15 Per-direction tunable optical add/drop
Wavelength x
. . .
Wavelength m
tunable
Wavelength x
. . .
Wavelength m
tunable
Tunable add
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N-degree ROADM with fixed nondirectional add/drop
Local add/drop 1
Wavelength x
. . .
Wavelength 1
Wavelength x
. . .
Wavelength 1
N-degree ROADM
Local add/drop 2
FIGURE 10.16 Multidegree ROADM with fixed optical add/drop
N-degree ROADM with tunable non-directional add/drop
N-degree ROADM
Tunable add/drop 1 FIGURE 10.17 Multidegree ROADM with tunable optical add/drop
Wavelength x
. . .
Wavelength m
tunable
Wavelength x
. . .
tunable
Wavelength m
290
Tunable add/drop 2
10.3 The future: Requirements for a fully dynamic photonic mesh
as long as the transponders were cabled to the tunable multiplexers/demultiplexers without the need for a truck roll. The next challenge comes because the channel add and drop is tied to a particular direction. For a two-degree node this is not a problem, but for a multidegree node this requires committing pools of transponders to each direction. Ideally, a carrier would like to pre-connect a transponder and then remotely steer it in any direction out of the node [10]. To do this, a switching mechanism is required to direct the output of the tunable transponder to the desired line direction of the multidegree ROADM (Figures 10.16 and 10.17). Considering the future photonic mesh networks and the requirements of optical amplifiers that can fulfill these architectures, we find three cases of use of photonic mesh technology with varying requirements on the optical amplification chain. The first case will be where the flexibility in the network is mainly used for service provisioning (time scale of hours/minutes), the second case will be re-provisioning after catastrophic failures (time scale of minutes/seconds), and the third case the full photonic mesh restoration (time scale of sub-second).
10.3.1 Provisioning through pre-cabling In an old-style network with manual add/drop, provisioning a wavelength over a large geographic area could take months, due to the many fiber jumpers that had to be
pre-cabling Turn up this path if needed
N-degree ROADM
N-degree ROADM
his
or t
N-degree ROADM
path if ne d
ede
N-degree ROADM
FIGURE 10.18 Pre-cabling at the end points for fast provisioning
N-degree ROADM
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manually placed with a site visit. Modern photonic mesh networks reduce this provisioning time dramatically, since the ROADM sites can be provisioned remotely. Further improvement can be achieved, with additional up-front cost, by using tunable non-directional add/drops instead of fixed wavelengths. Carriers could also have precabled transponders to customers and regenerator-banks inside the network and turn up a wavelength without having to touch the network. This model is already well known from electrical cross-connects, where an extra bandwidth pool is kept reserved to turn up service quickly through point and click, without the need to first install any new hardware (Figure 10.18). The extra bandwidth could be, for example, an extra wavelength in every cross section to support new customer additions. Since the time scale of provisioning in this context is hours or minutes, amplifiers do not need to meet any more stringent requirements than today’s photonic mesh networks. The major improvements needed for this architecture are in higher port count wavelength selective switches, which are in development to improve the economics. We expect to see such architecture in a few years in all major carrier networks.
10.3.2 Simple photonic restoration techniques Another step toward greater flexibility and functionality in the photonic layer is the slow re-provisioning after catastrophic failures, like a fiber cut of amplifier failure. A large number of wavelengths will be affected that need to be re-routed through the network (Figure 10.19). The restoration speed in this case will be on the order of minutes or seconds, making sure that the existing service is not impacted by massive change in the loading of the amplifiers. To achieve this, the reconfigurations and channel turn-ups will be ramped sufficiently slowly that the amplifier control loops
Re-provisioning
N-degree ROADM
N-degree ROADM
N-degree ROADM Slow Re-provisioning
N-degree ROADM
FIGURE 10.19 Simple photonic restoration through re-provisioning
N-degree ROADM
10.3 The future: Requirements for a fully dynamic photonic mesh
will be able to follow. This form of restoration is not suited for sub-second guaranteed restoration, but more for a slower best-effort service (like, e.g., web traffic). Still, being able to restore best-effort traffic can reduce the capacity (and therefore cost) in, for example, the IP router layer, as any failure can be restored within minutes, which may be good enough for certain types of traffic. A tiered system could also make sure that more important real-time traffic gets restored first within seconds and the less important best-effort traffic later. For this case, amplifiers and link control loops need to be able to suppress large changes in channel loading. The re-provisioning algorithm can reduce some of these requirements for example by time staggering at the cost of restoration time.
10.3.3 The holy grail: photonic restoration The third case is full fast photonic mesh restoration with sub-second time scales depending on the application (Figure 10.20). For most modern services, around 100 to 200 millisecond switching time is tolerable. Translating that requirement into the photonic layer means that most elements in today’s transmission system need to be completely re-designed. For example, laser switching speeds of a few milliseconds are required, as well as wavelength selective switches that can re-configure all the channels within tens of milliseconds (today’s devices have a limited control that can re-configure one channel at a time within tens of milliseconds). Transponders would have to be synchronized within milliseconds. Similarly tunable chromatic dispersion elements like 40 G temperature tuned chromatic dispersion devices or coherent signal processing filter adjustments would need to be adjusted within milliseconds. Moreover, the amplifier chain would have to be extremely stable against significant variations in channels in a few milliseconds and still keep the existing channels error free.
Photonic Mesh Restoration
N-degree ROADM
N-degree ROADM
N-degree ROADM
FIGURE 10.20 Full photonic mesh restoration
N-degree ROADM
N-degree ROADM
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The scenario outlined here is still years away, but there is a lot of research going on to ultimately make it happen.
10.4 SUMMARY Optical networks have evolved and continue to evolve in ways that are more and more challenging for the system designers. In large photonic mesh networks, the amplifiers must account for both static and dynamic behavior of the transmission line and accommodate changes without disturbing the traffic quality of service. As more traffic at higher bit rates is placed on these networks, survivability in the face of upstream failures becomes even more important, and the design and control can differentiate one solution from another. Carriers are looking for solutions that have been designed and tested against all possible failure scenarios and can handle network additions and deletions seamlessly. Finally, it is desirable to have a true photonic mesh-restorable network where wavelengths can be treated the same way as the electrical bandwidth and protected in a shared way against catastrophic failure.
ACRONYMS DCM DEMUX DWDM IP ITU MUX OSNR ROADM SONET SRS ULH WDM
Dispersion compensation modules Demultiplexer Dense wavelength division multiplexing Internet protocol International Telecommunication Union Multiplexer Optical signal-to-noise ratio Reconfigurable optical add/drop multiplexer Synchronous optical network Stimulated Raman scattering Ultra-long-haul Wavelength division multiplexing
References [1] J. Berthold, A.A.M. Saleh, L. Blair, J.M. Simmons, Optical Networking: Past, Present, and Future, Journal of Lightwave Technology 26 (9) (2008) 1104e1118. [2] A.A.M. Saleh, Overview of MONET project, Lasers and Electro-Optics. CLEO/ Pacific Rim ’97., Pacific Rim Conference, July 14e18, (1997) pp. 12e12
References
[3] K. Tse, AT&T’s Photonic Network. Optical Fiber Communication/National Fiber Optic Engineers Conference. February 24e28, (2004) pp. 1e6. [4] P.M. Krummrich, M. Birk, 2004. Compensation of Raman transients in optical networks. In: OFC 2004, paper MF 82. [5] P. Wysocki, V. Mazurczyk, Polarization dependent gain in erbium-doped fiber amplifiers: computer model and approximate formulas, Journal of Lightwave Technology 14 (4) (1996) 572e584. [6] P.M. Krummrich, M. Birk, Experimental investigation of compensation of Ramaninduced power transients from WDM channel interactions, IEEE Photonics Technology Letters 17 (5) (2005) 1094e1096. [7] A.K. Srivastava, Y. Sun, J.L. Zyskind, J.W. Sulhoff, EDFA transient response to channel loss in WDM transmission system, IEEE Photonics Technology Letters, 9 (3) (1997) 386e388. [8] Y. Sun, A.K. Srivastava, J.L. Zyskind, J.W. Sulhoff, C. Wolf, R.W. Tkach, Fast power transients in WDM optical networks with cascaded EDFAs, Electronics Letters 33 (4) (1997) 313e314. [9] G. Luo, J.L. Zyskind, J.A. Nagel, M.A. Ali, Experimental and theoretical analysis of relaxation-oscillations and spectral hole burning effects in all-optical gain-clamped EDFA’s for WDM networks, Journal of Lightwave Technology 16 (4) (1998) 527e533. [10] S.L. Woodward, M.D. Feuer, J. Calvitti, K. Falta, J.M. Verdiell, A High-Degree Photonic Cross-Connect for Transparent Networking, Flexible Provisioning and Capacity Growth. 32nd European Conference on Optical Communications. ECOC2006, paper Th1.2.2, (2006) Cannes, France.
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CHAPTER
11
Transport Solutions for Optically Amplified Networks
Werner Weiershausen*, Malte Schneiders *
Luxdyne Ltd., Germany
CHAPTER OUTLINE HEAD 11.1. Introduction .............................................................................................. 11.2. Optical transport networks ......................................................................... 11.2.1. Transport and aggregation networks ......................................... 11.2.1.1. Access networks ........................................................... 11.2.1.2. Metro aggregation networks .......................................... 11.2.1.3. Backbone transport networks ........................................ 11.3. Signal degradation and temporal fluctuations.............................................. 11.3.1. Residual chromatic dispersion ................................................ 11.3.2. Polarization-mode dispersion .................................................. 11.3.2.1. Temporal properties of PMD ......................................... 11.4. Raman amplification in WDM networks ....................................................... 11.4.1. EDFA, Raman and hybrid amplification scenarios ..................... 11.4.2. Laser safety and network implementation issues ....................... 11.5. Summary................................................................................................... Abbreviations...................................................................................................... References .........................................................................................................
297 298 299 301 303 307 313 314 317 320 323 323 332 334 335 338
11.1 INTRODUCTION The chapter gives an overview of state-of-the-art optical transport networks (OTNs) that are introduced by the carriers to adequately address future broadband service aggregation and transport and the growing packet centric traffic. The dramatic traffic increases and special requirements to support new services cost efficiently are driving the evolution of new enabler technologies at all engaged network layers. This chapter mainly focuses on the optical layer encompassing modern OTN/optical transport hierarchy (OTH) architectures and its physical layer, i.e., wavelength division multiplex (WDM) system requirements and approaches from the network point of view. The following section provides an overview of some basic network architectures, which are being developed for national networks in Europe. They will represent Optically Amplified WDM Networks. DOI: 10.1016/B978-0-12-374965-9.10011-1 Copyright Ó 2011 Elsevier Inc. All rights reserved.
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cost-efficient solutions for converged network platforms that carry both circuit and packet switched traffic. Special attention in this section will be on the optically transparent island solutions. The following part will outline some major physical layer problems that limit WDM transmission distances and the size of reconfigurable optically transparent islands, particularly for high-speed channels. In 40 Gbit/s and 100 Gbit/s systems, polarization-mode dispersion (PMD) represents a fundamental limitation for transmission. Furthermore, temporal fluctuations of chromatic dispersion (CD) and PMD require sophisticated and robust transceiver solutions. Some basic approaches, such as adaptive equalization and robust optical modulation formats, are covered in this part. The last part describes the application of different Raman amplification schemes for long-distance links including those for reconfigurable optical add-drop multiplexer (ROADM) based transparent network islands. Distributed and lumped Raman, and hybrid amplification schemes (combination of Raman and erbiumdoped fiber amplification) are introduced, and high power safety related aspects are discussed.
11.2 OPTICAL TRANSPORT NETWORKS In this section we focus on optical network architectures of the modern successors to the first-generation dense wavelength division multiplex (DWDM) networks that were based on simple point-to-point links with electrical termination at each node. Telecommunication services are undergoing fundamental changes due to the dominance of packet switched traffic over the circuit switched traffic. For residential customers, new broadband services such as new video applications are gaining momentum; for enterprises it’s virtual private networks. The enormous growth in bandwidth demand and the increasing need for service transport flexibility, while controlling the costs requires the whole telecommunications industry to optimize transport efficiency of the core and aggregation networks for functionality, flexibility, and scalability. These new requirements of telecommunication networks lead to an overall platform design with strongly interacting parameters from different network layers, local network domains, various service classes, dynamic traffic patterns, and temporal migration scenarios. This makes the optimization of network architectures including the optical layer much more complex when compared with the DWDM networks from the first generation. In legacy networks, the demarcation between their open systems interconnection (OSI) layers allows for a relatively simple individual layer based optimization of traffic engineering, network management and network costs. The optical layer of legacy networks mainly consists of transparent point-to-point links, which are terminated by the terminal multiplexers and transceivers at each switching node, excluding traffic branching within the optical multiplexers or line sections. All traffic switching and routing is done in the electrical domain, e.g., by synchronous optical network (SONET) / synchronous digital
11.2 Optical transport networks
hierarchy (SDH) based cross connects. Some optical add-drop multiplexers (OADM) have been deployed that allow simple all-optical bypassing of the traffic at nodes and permit only a certain percentage of traffic to be dropped. The new network architectures, related to enhanced flexibility and optimized costs, demand common optimization of different entangled protocol layers and a complex physical layer plane. To reduce cost, remotely configurable add-dropmultiplexers (ROADM) are being introduced into the networks so that the traffic switching is no longer restricted to the electrical domain. All-optical switching of WDM channels by ROADMs enables network operators to reduce their capital expenditure (CapEx) and power consumption by avoidance of optical-to-electricalto-optical (OEO) conversions. Unlike the older-generation optical add drop multiplexers (OADMs) with fixed filters, ROADMs can be switched remotely, thus leading to lower operational expenditure (OpEx). In modern transport networks, colorless and directionless ROADM nodes with degrees higher than two can be applied as photonic cross connects (PXC), which allow for low cost traffic routing of wavelength channels. This network approach can, however, benefit only from traffic granularities available in DWDM systems. Data rates at 10 Gbit/s, 40 Gbit/s, and 100 Gbit/s are seen as the wavelength channel bandwidths that will be efficiently transported and switched through the first generation of all-optical meshed networks. In second generation systems, flexible bandwidth support may be possible by techniques like bandwidth adaptable multilevel quadrature amplitude modulation (QAM) formats, sub carrier multiplexing (SCM), or optical orthogonal frequency-division multiplexing (OFDM) for an efficient exploitation of the bandwidth-distance resources near the Shannon limit. The extension of today’s networks from point to pointebased DWDM links toward more seamless meshed all-optical networks, the deployment of new modulation formats such as coherent digital signal processor (DSP)esupported polarization multiplexed quadrature phase shift keying (PM-QPSK), and more flexible bandwidth assignment schemes have consequences on the design of amplification maps and the use of certain types of lumped or distributed optical amplification.
11.2.1 Transport and aggregation networks Network operators have the common basic target to produce cost-efficient telecommunication services. When considering operators from different nations including carriers operating worldwide, a variety of network architecture designs need to be considered. The suitable network design depends on the individual national properties with respect to the telecommunication services to be provided, such as the local population density distributions, the characteristic local residential consumer behavior (e.g., demand for voice telephony, internet protocol [IP], or broadband TV) or the distribution and service level agreement (SLA) requirements of the business customers. The design of the networks is governed by the topology (e.g., ring, star,
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mesh), by the purpose (access, aggregation, transport), by the mean and maximum link distances, and by the density and degree of switching or grooming nodes. All this has a direct impact on the choice of amplification in the optical multiplex section (OMS) of DWDM systems and on the local placement of optical amplifiers. The diameter of networks is one of the most obvious distinctions. Nationwide networks in the United States follow engineering rules different from those applicable to the national backbones in European countries (see Figure 11.1), especially when the design of amplifier maps and the positioning of photonic cross connect (PXC)/ROADM based nodes are considered. The largest diameters within alloptical transport is achieved in submarine cable networks that deploy lumped amplifier span designs with very short distances between adjacent EDFAs and eventually supported by additional distributed Raman amplification. Besides the distance, many other parameters influence decisions for special network layouts, e.g., the local distribution of population and industry to be connected, the traffic patterns and capacity evolution, the telecommunication service kinds and classes, and much more. Also, the deployment choice of lumped inline amplifiers, distributed Raman amplification or hybrid schemes, gain equalizing devices, electrical or optical inline regenerators, and electrical grooming nodes or optically amplified multidegree ROADM nodes is strongly dependent on these multiple factors. In the following part, some network options with consequences for optical amplifier applications will be described against the background of European national networks. Here a variety of requirements force operators to select many different network architectures for different local domains with suitable primary foci to meet optimum transport efficiency and operational performance. The present trend is to consolidate different network domains into a converged platform to simplify the overall network management process.
FIGURE 11.1 A comparison of characteristic link statistics of reference networks in North America and Europe. The distribution function with peak at 600 km belongs to pan-European backbone networks, the other one to backbone networks in North America (the small left satellite peak in the European distribution is not real; it is due to the applied fit algorithm only).
11.2 Optical transport networks
FIGURE 11.2 Schematic of German core network (public reference network [1]) with a table showing summary of the topological network characteristics [1]
European networks cover many scenarios of possible architectures, for ultralong-haul (ULH) pan-European backbone to national European backbone, metro, and access networks. The typical distance characteristics of link lengths between major backbone nodes for North America and pan-European networks are compared by Figure 11.1. In North America the geographical dimensions of backbone networks require ULH systems. The same is applicable in pan-European networks, but the distances are significantly shorter. The backbone links of national networks of the different European states like Germany are even shorter. Figure 11.2 gives characteristic values for a public German reference network. Here the mean fiber link distance between major cities and thus backbone nodes is about 400 km which could be still called “metro.” However, as for the next generation architecture it is intended to intensively apply optically transparent transit nodes (ROADM/PXC), future national networks will also demand systems with a longer reach. In the following sub-sections we will focus on typical modern intranational European network architectures. Future converged telecommunication platforms will comprise access, aggregation, and transport networks. Their design rules depend on their primary purpose: either traffic aggregation or distribution from and to customers, or the transport and routing of large amounts of combined capacity.
11.2.1.1 Access networks Today’s residential access networks deliver services for the mobile and fixed-line telecommunication with the latter dominating in overall traffic capacity. Different
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digital subscriber line technologies (DSL) such as asymetric DSL (ADSL) and very high bit rate DSL (VDSL), demand up to 50 Mbit/s peak traffic per subscriber. DSL backhaul is already realized by fiber-to-the-cabinet (FTTC) solutions, where cabinets are placed in the streets of urban and sub-urban regions. Besides this, fiber-tothe-building (FTTB) and fiber-to-the-home (FTTH) deployments have started in Europe and will secure further bandwidth growth driven by future consumer broadband services. While the carriers migrate from DSL to FTTB/H, they will try to reduce the operational expenditure, which is especially important for the incumbent European carriers. As a consequence, a lot of central offices will eventually be closed. Typical scenarios show a trend toward closure of 90% of the sites, meaning only 10% of the active sites will remain. The extended distance that has to be bridged between the FTTx access nodes and the remaining 10% of active central office sites for backhauling leads to severe problems for network operators when using passive optical network (PON) solutions, which employ gigabit PON (GPON) and Ethernet PON (EPON) standards with tree-type topology. The problem is a combination of the extended distances, greater than 40 km, and a high PON splitting ratio to provide coverage to a large area with many distributed homes and buildings around the little central offices. In addition, when combined with high bandwidth demand, the long distances and high splitting ratios cannot be handled with the classic GPON/EPON technologies. Amplifier extenders can extend the power budget of PON networks with high splitter and fiber transmission losses. These extenders are positioned in semi-active sites using simple outdoor cabinets with a small footprint, without air conditioning or personnel. This way the desired operational cost reduction can be achieved. However, even single-wavelengthebased PON tree networks will not be able to shoulder the whole problem space. An alternative will be colored systems like WDM-PON, or hybrid systems like colored time division multiple access (TDMA)e PON channels on a WDM system. These WDM-PON systems have to be amplified by optical amplifiers (EDFA). Generally, the EDFA is followed by a WDM splitter, e.g., a demultiplexer (DEMUX). It is important that both DEMUX and EDFA can be put in outdoor cabinets without air conditioning. They have to be very tolerant of temperature variations; consequently, athermal arrayed waveguide grating (AWG) filters are used for DEMUX filters. Besides the star and tree topology for WDM-PON, ring topology is also a possibility. WDM rings with fixed optical add-drop multiplexers (OADM, fixed OADM [FOADM], simple filters) can be used to collect the traffic from different access sites and to drop it at a common hub node (hub&spoke to the metro network). The advantage is efficient exploitation of the existing fiber plant, and these rings allow for simple 1þ1 or 1:1 protection by using both ring directions. As the cost of transport of transit traffic through the OADM nodes are relatively small, this is an interesting alternative to stars and trees. To support the ring attenuation budget, for this architecture optical amplifiers (EDFA) have to be used. They can be placed at the multiplexer/demultiplexer/OADM sites.
11.2 Optical transport networks
Low-cost EDFAs are preferred in relatively small access rings with comparably small optical signal-to-noise ratio (OSNR) degradation. Application of the very lowcost passive coarse wavelength division multiplex (CWDM) technology is not suitable because the distances are too long for purely passive rings. This argument, however, applies for the before-mentioned scenario with the strongly reduced number of central offices.
11.2.1.2 Metro aggregation networks The next important network domain of a hierarchical platform is the metro aggregation network. This network connects the regional active central offices to the core sites of a national backbone. The aggregation network has several tasks. First, it is used to aggregate all incoming traffic from the residential customers in the previously described access networks. The aggregation is carried out such that the bandwidth granularity with common destinations is large enough to efficiently fill wavelength channels of DWDM systems. The minimum reasonable channel bandwidth today is 10 Gbit/s (10.7 Gbit/s according to optical channel transport unit (OTU) 2 within the ITU-T G.709 standard for the optical transport hierarchy OTH). Second, the aggregation network can be used to connect network termination (NT) points from business customers to the backbone or other NT points within the same region. Business customers need not be served by the same network like the residential customers, so two parallel network platforms can be dedicated to both applications. While the traffic pattern of business and residential connections are generally very different, requiring a separate network platform definition for them, a trend toward common transport platforms is more suitable due to the uncertainty regarding the future traffic patterns and capacity evolution. Often the future bandwidth evolution, especially of business traffic, is not very distinct, so it is an interesting option to share major traffic platform between the residential and business customers. For the aggregation network, the following architectural alternatives can be seen as major solutions for European national networks.
11.2.1.2.1 Star and tree topologies If a metro network is exclusively dedicated to backhauling of traffic, such as DSL, FTTx or mobile backhaul, a star or tree topology can be applied. The problem is that WDM applications for fiber sharing are not very effective since the traffic is collected by the star from different locations; i.e., the amount of commonly transported traffic is low. Furthermore, a lot of fiber infrastructure is needed, especially if a double star or tree is needed for protection purposes. For intra-regional business traffic with an any-to-any traffic matrix, such networks are not suitable.
11.2.1.2.2 Ring topologies WDM rings with active FOADM or ROADM nodes can solve the previously described problems of star or tree topologies. They are suitable for both service
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classes, residential and business, and the fiber infrastructure can be efficiently exploited by active DWDM (with amplification, e.g., by EDFA). Ring type networks are very suitable for residential traffic aggregation. The traffic pattern within the topological ring can still follow a logical star; i.e., DSL/ FTTx or mobile traffic is backhauled and condensed in a hub node before being transferred into the longer-distance transport backbone. A hub&spoke DWDM ring constellation with one hub OADM and a number of satellite OADM nodes provides good architecture for handling startype traffic pattern including a cost-efficient and easy to manage 1þ1 or 1:1 protection scheme. Protection can be carried out in different layers, for instance in the WDM layer (L1), which is directly concerned about the fast ring direction switching in <50ms in case of a transmission failure (e.g., due to a fiber cable cut). Another very good option is protection in the Ethernet layer (L2). In this case, the WDM system is principally static with respect to protection, and it delivers both ring directions as working and protection paths, which are always active. The Ethernet switches decide and select a suitable protection path. By overbooking the protection path with the working traffic, protection capacity can also be exploited for the active traffic, thus enhancing overall network flexibility. In this architecture, different quality of service (QoS) classes can be defined with or without protection. Besides the traffic from residential DSL and mobile backhaul, business services can also be transported well within the metro ring networks. For these services WDM channels are individually dedicated. Business traffic with a long-haul destination is transported to the hub OADM node and transferred to the backbone the same way as the backhaul traffic. Intra-regional business connections are fed through the hub node via transit WDM channels. The dimensions of the metro DWDM rings are quite different from those of access rings. Reasonable circumferences in central European national networks are in the range of 500 to 1000 km with six to 10 OADM nodes, depending on the traffic source distribution. The attenuation of the fiber links, as well as the OADM insertion loss, is compensated for by optical amplifiers (EDFA) at the OADM nodes. The principal design of the OADMs has to be chosen in a way that prohibits optical instabilities in the ring network. In general, a closed optical loop can generate self oscillations if the loop is optically amplified, e.g., by EDFAs. The ring network could turn into a laser due to amplified spontaneous emission (ASE) from the EDFAs. This is avoided by an OADM broadcast and select (B&S) architecture based on wavelength blockers or wavelength selective switches (WSS). If a blocker is used, the OADM through path is attenuated by a channel filter for all the channels being dropped, so only dedicated transit traffic can pass through the OADM. In the case of wavelength-selective switch based architecture, the WSS defines the drop and through paths. In the OADM designs, either the blocker or the WSS prevents the collision of the transmitted signals (through path) with the newly added signals.
11.2 Optical transport networks
To avoid laser oscillations, the role of a network management system (NMS) is of special importance. It has to ensure correct switching and blocking of optical shortcuts. If a switching error happens in opaque networks, e.g., due to an error by the network management system, only the individual channel is affected. In the case of an optically transparent OADM ring, however, a switching error by the NMS can result in a complete breakdown of the entire transparent network domain. If one channel is optically bypassed, causing laser oscillations, the stimulated emission can deplete the gain of the EDFAs within the ring, resulting in loss of amplification and a possible breakdown of the whole OMS with, e.g., 80 channels. So the impact of NMS errors on the number of affected services is strongly enhanced in this case when compared with an opaque network with OEO transitions. Another crucial issue is that in case of an OMS failure in the working path, the protection path will also be affected because one of the two fibers along the whole ring cannot be used for transmission anymore. For such a breakdown, two switches have to be set wrong, one at the add OADM and the other at the drop OADM. Such a double fault could most likely arise due to an NMS failure, where such an event is not a double fault by definition. The probability of an equipment double fault and of an NMS failure depends on the individual failures in time (FIT) numbers. The following options can help to circumvent the laser oscillations or reduce its probability and impact. Application of an NMS with a low FIT rate. This principally helps to reduce the OMS outage probability. Unfortunately this does not remove the principal existence of a single point of failure (SPoF). In major network domains like backbone and metro networks of national European carriers, typically the avoidance of SPoF can be a fundamental and general request. SPoF avoidance means that a protection is always active, independent of the type and location of a failure. For example, within a 1þ1 or 1:1 protection scheme all essential equipment, including the power supply, has to be backed up or doubled. Furthermore, the equipment for service protection has to be located in a different central office or at least in a separate fire compartment, so that even a fire will not cause failure of the telecommunication connection. Often, the avoidance of SPoF is extended by a dual-vendor strategy; i.e., working and protection system equipments are sourced from different manufacturers. In this strategy, even an NMS software failure of one of the two different systems does not cause any interruption of the connection. Application of a fast transit blocker filter inside the ring that performs fast switch-off of the problematic channel causing the laser oscillation, e.g., triggered by a channel power monitor. Application of an OEO interface inside the ring (applicable only to OADM ring architectures), such that one of the ring nodes is not configured as an optically transparent OADM but as a standard opaque digital node with DEMUX/MUX and transceiver modules. In a backhaul ring network, with hub and spoke architecture, the hub node is predestined for this function because it has to drop
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all channels (or at least all channels with backhaul traffic), and typically the dropped traffic is sent to the next higher hierarchical network domain via an opaque interface for demarcation between NMS and optical domains.
11.2.1.2.3 Meshed topologies The most versatile network topology is the meshed structure. First of all, it can accommodate any kind of traffic pattern; also, effective protection schemes can be applied. Moreover, besides 1þ1 or 1:1 protection, 1:N protection or restoration scenarios are also possible. On the other hand, the universality of the meshed solution can result in high investment costs so that the ring networks may be much more cost effective in specific traffic demands that fit well to that kind of architecture. So, meshed networks are a reasonable approach if either the traffic pattern requires high degree of path flexibility, if network upgrades have to flexibly address an unforeseen topological evolution of source traffic distribution, or if large amounts of traffic are commonly transported. Meshed networks can be equipped with either electrical or optical switching nodes. The optical nodes that have ROADM/PXC can be more cost effective due to sufficient switching of traffic at the wavelength granularity. Electrical switching nodes offer additional network functions, such as a perfect regeneration of the signal, in-line FEC, very comfortable OAM functions, simple fault isolation, control and handling of QoS and service classes, traffic aggregation, switching, grooming, and routing at different traffic granularities. Within the metropolitan aggregation networks, the choice of network architecture is strongly dependent on the existing traffic patterns and service demand and geographical location in the country. For rural regions with little or no industry or other large business, the residential backhaul traffic from DSL, FTTx, and mobile services dominates with its logical star/tree pattern. A traffic increase over the years has to be taken into account for network planning and traffic provisioning processes, but no temporal switching dynamics has to be accommodated. So, the application of OADM-based WDM rings is a very good solution here. They offer an effective use of the fiber infrastructure including an efficient protection scheme at a very attractive cost point. For urban regions with a lot of business traffic requiring a high degree of switching dynamics and characterized by logical any-to-any connections, meshed network architecture with remote-controlled switching nodes is recommended. In regions where average business traffic is considerably lower than the capacity of the used wavelengths, traffic engineering (TE) with traffic aggregation and grooming has to be regarded, requiring opaque node functionalities. At higher business traffic granularities one should take advantage of optically transparent ROADM/PXC switching due to reduced CapEx. To summarize, the optimal metro network architecture strongly depends on the region with its density of population and industry and its specific service and bandwidth requirements. So in a heterogeneous country, typically a combined
11.2 Optical transport networks
application of the described alternative architectures is recommended for typical national European network operators. Some of the described issues for metro networks also apply to the backbone transport domain. The constraints and advantages of meshed architectures with or without optically transparent switching will be described in the next section.
11.2.1.3 Backbone transport networks The mesh network approach described earlier for the metropolitan domain can, in principle, be expanded to the entire flat network platform covering the whole country. In that case, the additional hierarchic layer of a core backbone can be omitted. Such an approach has the advantage of being very flexible to migrate to any kind of network evolution, including the entire capacity growth as well as a local shift of business centers or a general dislocation evolution of source traffic. This approach can also be very cost effective if optical transparency is used to avoid expensive channel-wise OEO conversions. However, such flat meshed network architecture has limitations with respect to transmission technology, network control and costs if it becomes too complex. Then an additional long haul (LH) core network hierarchy (backbone) that transports highly aggregated capacities becomes an attractive option. Also, this backbone network can have different architectures; e.g., with connected DWDM rings or having a DWDM mesh. Interconnected rings are also a kind of mesh network. The backbone itself can become very complex, which may lead to considerations about the placement of optically transparent nodes with low CapEx. In the following part, properties of meshed networks with a focus on optically transparent islands are described, including alternative solutions for network operators. As described earlier, the most versatile network topology is the meshed structure. It can accommodate any kind of traffic pattern, and it can grow very flexibly and thus adapt well to any future evolution; it also provides effective protection schemes that can be applied. So besides 1þ1 protection, shared protection or restoration scenarios are possible. On the other hand, the universalism of the meshed solution can result in high investment costs; i.e., ring networks may be much more cost effective in case of specific traffic demands that fit well with that kind of architecture. So, meshed networks are a reasonable approach if the traffic pattern requires high path flexibility, the network upgrades have to be adaptable to an unforeseen topological evolution, and large amounts of traffic are to be frequently transported. To lower the costs of meshed networks, the replacement of expensive OEOs is an option. Due to cost efficient ROADM and PXC technology this is possible today. As ROADMs are remotely configurable, they offer the same opportunity for low OpEx as systems with opaque switching nodes based on SDH/SONET or OTH (ITU-T G.709) protocols. Both can be easily controlled remotely by the NMS, and by a control plane (CP) or the operator’s central network management sites. Such OpEx advantages cannot be offered to network operators by fixed OADM or fixed multidegree wavelength selective gates.
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Regarding the CapEx for the system equipment, the ROADM or PXC based switching nodes can be much more cost effective than those based on SDH/SONET or OTH due to less OEO transitions. A prerequisite for this is the dominance of traffic with large bandwidth granularity that is transported through multiple nodes as transit traffic to a common destination. In these transit nodes, no grooming with traffic from/to other directions have to take place. The breakthrough with respect to CapEx advantage of PXCs (higher-degree ROADMs) was made by the introduction of WSS-based architectures. The cost of a node roughly scales linearly with its degree, i.e., the number of switched path directions. This was not the case with the former first-generation optical cross connect (OXC) technologies. Multidegree ROADMs can be easily upgraded later when the complexity of the network grows. In some cases, this upgrade can happen “in service”; i.e., the active network connections at the affected node are not interrupted or disturbed during the upgrade process. The CapEx advantages by avoiding OEO within meshed networks are in conflict with the need for higher robustness of WDM systems. For low-complexity meshed networks with a limited number of possible path combinations through different links and PXC nodes, and the option for high number of hops (end-to-end traffic being routed through many node hops), this enhancement of complexity can be handled well. But highly connected meshed domains with a lot of applied node hops will have restrictions, especially when combined with high-speed WDM channel transmission, such as 100 G-Ethernet transport on a dedicated wavelength channel. In this case, there are multiple sources of signal quality degradation. There are two problems. First, a more complex meshed transparent network domain may have longer average link distances and an increased average number of node hops for an end-to-end connection (source-to-destination) which requires higher receiver tolerance. Second, the multiple link/node combinations can lead to high statistical peak values and standard deviations, regarding the accumulation of different physical constraints. Here, the systems have to provide even higher tolerance than when considering only the average values. Since the ROADMs allow for very flexible routing of many wavelength channels (e.g., 80 channels) over different optical fiber paths, it is a special challenge to switch different type of data rates and modulation formats in one optical domain. To hold investment costs as low as needed, telecommunication carriers follow efficient time-to-market strategies. This means that the starting investments shall need to remain low and the major investments scale with the growing traffic demand and thus the revenues. This leads to a reuse of the existing infrastructure for as long as possible and is economical for the installed fiber plant. But even legacy WDM systems have to be filled up with more modern transceivers that offer higher transport capacity per channel. It is a challenge for system manufacturers to enable compatibility for a mixture of different transceivers operating at different bit rates and modulation formats. The amplification map (EDFA map) and the dispersion management map (use of in-line dispersion modules), which will be described later, have to be compatible with the new channels. The link design for
11.2 Optical transport networks
new WDM transceivers is not independent like for the case of a green-field installation. Especially for the layout of an optically transparent island this request for smooth technology migration can complicate the design problem. For some transceiver technologies guard bands are needed that separate them from channels with other technologies. For example, phase-modulated signals, at 40 Gbit/s or differential quadrature phase shift keying (DQPSK) signals, could be transmitted adjacent to a channel at 10 Gbit/s with on-off keying (OOK) modulation. The OOK signals can produce crosstalk to the DQPSK signals in the form of phase noise, by non-linear cross-phase modulation. In an optically transparent domain, manifold switching scenarios may occur, and the NMS has to avoid combinations of forbidden pathchannels. From all the issues previously described, it can be seen that a careful layout of network complexity and an adequate selection of applied transceiver technology is needed for the design of an all-optical island. Following basic architecture directions can be chosen by the network operators: Application of very robust systems that can tolerate the dominant physical impairments limiting fiber transmission. Within European national networks, the overall backbone distances are not very long; i.e., amplifier noise (ASE) degrading the signal OSNR is not the main problem, but the existing fiber infrastructure exhibits high average values for PMD. So PMD is limiting the transmission distance for systems at higher baudrates. As discussed in a later section, the impairment due to PMD begin to limit the reach of signals at 10 Gbaud, and high limitation occurs for systems with 40 Gbaud or more. As a consequence, for new bitrate formats that are needed for certain telecommunication services or IP router interfaces, such as at 40 Gbit/s (or 43 Gbit/s, ITU-T G.709 OTU3) or 100 Gbit/s, multilevel modulation formats will be selected for European national networks like in Germany. By phase modulation via DQPSK, 40 Gbit/s signals require only a 20 Gbaud connection. Another alternative is the application of inverse multiplexing. Here a 40 Gbit/s transmission is represented by four parallel DWDM channels at 10 Gbaud each. However, these four separate DWDM channels have to be resynchronized at the receiver to replicate a single logical bit stream at 40 Gbit/s. Depending on the uncompensated residual dispersion and dispersion slope of the transparent paths taken by the channels, a buffer is needed at the receiver for resynchronization of the bits. This is a disadvantage especially in optically transparent islands with multiple options for channel paths being switched by PXC. Furthermore, the spectral efficiency of inverse multiplex systems is inferior to systems based on other modulation formats such as DQPSK. So, the two-level phase encoded modulation format DQPSK is the best choice for 40 Gbit/s in national European networks. When going one step further to 100 Gbit/s channels (respectively to OTH rates at 112 Gbit/s, 130 Gbit/s), standard two-level encoding is no longer sufficient to keep baud rates low enough. Therefore, the standard DQPSK is not adequate for
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100 Gbit/s transmission. However, by the application of two sub-carriers one can still maintain the two-level modulation format DQPSK obtaining 25 Gbaud. It is possible that the emerging trend for 100 Gbit/s transmission will lead to PM-QPSK modulation with coherent detection. Although quadrature phase shift keying (QPSK) is only a two-level format, the inclusion of the two polarization modes for transmission allows the baud rate to be at 25 Gbaud. In combination with sub-carriers, this value can be reduced even further. Coherent QPSK systems offer additional advantages compared with the above described systems based on DQPSK or inverse multiplex. The use of polarization diversity receivers with coherent detection enables retrieval of amplitude and phase information of the signal in the two orthogonal polarization axes, which allows for a very efficient post processing in the electrical domain by a DSP. The DSP can calculate and compensate for many of the accumulated physical impairment effects, e.g., for the CD or PMD. This is ideal for application in large transparent network domains with many PXCs/ROADMs where the accumulated CD or PMD is dependent on the individual switching state of a DWDM channel along its signal path. So the system robustness and flexibility obtained by PMQPSK is well suited for all-optical islands. Unfortunately, DSP-based PM-QPSK does not compensate for accumulated ASE and related OSNR degradation or cross channel nonlinear effects, such as four wave mixing (FWM) or cross phase modulation (XPM), but this is not a primary issue in the national European networks with their limited dimensions. For bit rates higher than 100 Gbit/s, the robustness of PM-QPSK may no longer be sufficient, and OFDM or QAM formats may become suitable options. For data rates up to 100 Gbit/s, coherent PM-QPSK seems to be an optimal solution for optically transparent islands with PXC/ROADMs to remain in the position to handle these complex network units; however, additional costs for the coherent technology have to be taken into account and compared with alternative solutions. Depending on the size and complexity of the optically transparent network domains, additional cost of extra safety margins of the WDM systems have to be taken into account, which may compensate for the cost advantages due to reduced OEO deployment. So a carrier has to carefully compare CapEx and find the optimal size and complexity of transparent domains. Limit the maximum size and complexity by definition of optically transparent sub-domains that are linked to each other via opaque interfaces representing a technology and protocol-related demarcation. This architectural concept does not automatically imply several network hierarchies; i.e., all optical sub domains can be part of a flat hierarchical plane. But the analogue optical domains (transparent islands) are topologically framed by a digital interface border. This digital frame can be controlled by the NMS and by a CP or a central network management. For instance this frame can be realized by the OTH/G.709 protocol layer with all its carrier-grade supervisory and operation, administration, maintenance (OAM) functions. This kind of architecture provides additional
11.2 Optical transport networks
freedom since it allows for additional network functions such as traffic multiplexing, grooming and routing at the opaque nodes, which cannot be realized by all-optical technology. Furthermore, the opaque nodal borders allow for a clear demarcation between the optical islands from different system manufacturers, and a multivendor strategy can be implemented, which cannot be realized within an all-optical island. Translucent domains inside a network can be defined, and are dominated by optically transparent nodes (ROADM/PXC) and some opaque nodes. These nodes can support traffic that has already reached a certain transmission limit with too much accumulated ASE noise or nonlinear signal distortions and has to be regenerated via an OEO translation. Furthermore, these opaque nodes may reduce network inefficiencies by stranded wavelengths (wavelength blocking) and they can be equipped with higher-layer network functionalities, such as L2 switching and grooming, TE for point-to-multipoint Ethernet services, or IP routing in L3 layer. For the traffic routing of all individual WDM channels, the NMS of the translucent domain has to define the allowed transparent routes through the network, define which channel must pass through an opaque node, circumnavigate wavelength blocking problems, and balance the capacity load of the links. The NMS can limit the maximum number of all-optical hops and coordinate the overall TE. So both in translucent and transparent islands, it is possible to restrict the TE to physically possible and thus allowed channel-linknode combinations, and to enhance WDM channel deployment efficiency by the reduction of wavelength blocking, by the NMS. For this purpose, the NMS must have implemented engineering rules and a routing and wavelength assignment (RWA) algorithm. Thus, by an advanced NMS and network element manager (NEM), it is possible to limit the maximum system robustness and thus the hardware CapEx. However, there will be additional costs in the implementation of more complex NMS and CP if a large number of nodes are connected in an overall meshed transparent or translucent island. The overall CapEx including the NMS and CP has to be considered by the carrier when defining the optimum size and architecture of transparent islands. Application of hierarchical metro-backbone architecture is another possibility. In this case, transitions between the regional metro networks and a long-haul transport backbone are realized by opaque node interfaces, i.e., via transponders, supported by a digital layer L1 protocol (e.g., OTH/G.709). The opaque interfacing between the two network hierarchies allows easy manageability (good OAM, QoS monitoring) and TE. Also, IP routers can be located at these transitions, so that layer L3 routing and MPLS TE can be implemented. In addition, the opaque transition between core and metro networks allows for a technological and economical demarcation between the metro and core network technologies. LH/ULH systems for the backbone are in general more robust against transmission impairments in the physical layer than the metro equipment, but they are also more expensive. So a network operator needs to find
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the best mix for optimizing CapEx and the use of specialized vendors for the different domains, which can be individually selected. If lifetimes of core and metro technologies differ, individual replacement cycles can be applied because of the clear OEO demarcation. Another alternative to the complex optically transparent islands is the opposite, a highly opaque approach. In this case, the switching nodes are exclusively in the electrical domain, such as the OTH cross connects (OTH-XC, ODU-XC) that switch and groom the optical channel data unit (ODU) containers (see ITU-T G.709). To keep the CapEx competitive with an optically transparent island approach, extremely low-cost transceivers are used, particularly in the case of large number of channel deployment. To lower the cost, the following two options are considered: Use of low-cost metro system technology and low-cost pluggable transceiver modules (e.g., XFP) for comparably lower bit rates with shorter reach. In contrast to the required long reach in transparent network islands, where several optical switching nodes (ROADMs, PXCs) have to be bridged, the opaque network needs only a single hop from one switching node to the next. The bit rate per transceiver is kept at a step below the maximum channel bit rate in expensive LH/ULH systems, e.g., at 10 Gbit/s when LH/ULH provides channels at 40Gbit/s. The transceivers at lower bitrate are generally very mature and have lower cost. Use of photonic integrated circuits for transmitters and receivers. For this option, a number of transmitter lasers or receiver diodes are integrated on one semiconductor chip, together with AWG based MUX/DEMUX. Additionally, the electrical signal modulators can be attached by hybrid integration. The cost is reduced because only one fiber-to-chip coupling is needed for a group of wavelength channels. Moreover, packaging cost is reduced and the FIT values are improved over many single channel transceivers. A network based on such low-cost opaque switching nodes still needs in-line amplifiers (typically EDFA) between adjacent switching nodes. These amplifiers will not be replaced by low-cost transceiver based OEO because the optical amplifier that can handle a full OMS with up to, e.g., 80 channels, is one of the most cost efficient devices of the entire WDM technology. The above mentioned opaque digital network approaches are flexible and easy to manage. At every switching node, different traffic types and granularities can be groomed in a flexible manner. There is no wavelength blocking problem at the switch nodes. Furthermore, the OAM properties are as good as those for the old SDH/SONET based systems. At every switching node, the signal quality can be easily tracked and faults can be easily detected and located. Optically transparent island technology with ROADMs is unable to compete with these digital networks when OAM, fault isolation, traffic grooming, and channel blocking efficiency are considered. For the OAM, optical monitors are used at ROADM/PXC nodes to measure basic optical parameters of each channel. The information on signal quality
11.3 Signal degradation and temporal fluctuations
is limited to the measured physical parameters. In most cases, channel power and signal OSNR are measured and the test result is transported by an optical supervisory channel (OSC) to the next digital interface and then directed to the NMS or CP. If, however, DSP-based coherent transceivers are used in the future, it will not be possible to get signal quality information beyond the channel power or OSNR. Monitoring in the time domain to detect linear or nonlinear signal distortions (e.g., if physical effects are subject to temporal fluctuations) will not be possible because in a DSP-based transmission link the channels will not exhibit concrete pulse patterns. The signal information content will be calculated in the digital domain by the receiver DSP. So optical islands will have to be robust against all possible values of physical impairments that lead to signal distortions and are subject to temporal variations, as discussed in a section on chromatic dispersion and polarization-mode dispersion. Furthermore, the network design has to be robust against power transients that can happen if a single or a group of WDM channels are switched off or turned on, or if a failure occurs. To avoid amplified power transients that travel through the entire transparent network domain, the EDFAs need a fast gain or power control. Although there are complex issues to be taken into account, ROADM/PXCbased optically transparent islands promise to become a cost efficient solution for future ultra-broadband transport networks. The all-optical switching will also reduce the power consumption that would otherwise increase dramatically when going toward multiterabit networks.
11.3 SIGNAL DEGRADATION AND TEMPORAL FLUCTUATIONS An important objective of the network operators is to migrate the networks to the next-generation WDM systems with the ability to transport the growing traffic without replacement of the fiber cable infrastructure requiring major investment. Before 2000, a lot of fiber cables were installed worldwide, and this resource continued to be used during the following decade. The spectral efficiency of WDM systems has grown in parallel with the traffic demand. While in 2000, the deployed WDM systems typically transported about 400 Gbit/s capacity over a single fiber, 10 years later up to 8 Tbit/s capacity can be accommodated on the same fiber. It is still sensible from a cost perspective to target the use of the existing fiber infrastructure for higher capacity. Increasing the transmission capacity and the spectral efficiency on the existing fiber plant, however, requires management of signal impairments due to nonlinear fiber effects in addition to the chromatic dispersion and polarization-mode dispersion in the fiber. These tend to limit the maximum transmission distance, especially for high-speed signal transmission. There are schemes to compensate for these effects but the problem is that they are not stable over time and exhibit statistical temporal fluctuations that lead to difficulties in system design and adaptive mitigation schemes. These issues will be discussed in the current section.
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11.3.1 Residual chromatic dispersion The sensitivity of WDM receivers scales as the square of the accumulated CD. Therefore, while CD was of no significance for WDM systems based on 2.5 Gbaud maximum channel baud-rate such as those from before 2000, the modern systems based on 40 Gbaud maximum channel baud-rate and higher are influenced significantly by CD. Figure 11.3 shows typical receiver tolerance for the accumulated CD when return to zero (RZ) modulation format is used. Different fiber types exhibit different CD values. The standard single mode fiber (SMF, ITU-T G.652), which can be found in many legacy networks, has relatively high CD of 17 ps/nm/km. Regarding signal impairments due to CD alone, standard single mode fiber (SMF) is the worst, however; during the last years it has been found that the lower dispersion successors of SMF, such as dispersion shifted fiber (DSF) and non-zero dispersion shifted fiber (NZDSF), suffer from problems with non-linear effects in DWDM systems. DSF, which has been broadly deployed in Asia, can cause severe signal impairments due to strong FWM. In order to overcome this impairment while avoiding expensive new fiber installations, for some systems deployed on DSF fiber plant, the WDM band was shifted away from the C-band where the DSF has very low CD to the L-band by the application of L-band EDFA. However, SMF-based networks still offer a better cost-per-bit efficiency since the Cband amplifier pricing is lower due to economies of scale. Today SMF is regarded as a future-proof technology for efficient transport of high data rate channels to carry strongly growing traffic. Within German fiber plants, SMF is deployed extensively. To accommodate high-speed WDM channels at 10 Gbit/s and 40 Gbit/s in non-coherent systems, dispersion compensation is needed. Two kinds of dispersion compensation schemes are employed in the systems: in-line compensation for dispersion management
FIGURE 11.3 Dispersion limits (at 1 dB bit error ratio [BER] penalty of a typical RZ receiver) for baud rate and transmission distance for different fiber types
11.3 Signal degradation and temporal fluctuations
typically placed between the amplifying stages of mid-stage access in-line amplifiers at repeater sites, and the residual dispersion compensation placed before the receiver. The dispersion management takes into account linear and non-linear effects on the signal. The residual dispersion is purely a linear signal distortion effect, which can be completely mitigated by compensation. In 10 G WDM systems it was sufficient to deploy static dispersion compensation. This has been done by adding a spool of dispersion compensating fiber (DCF) with roughly matching fiber lengths. A fine adjustment is achieved by addition of short pieces of either SMF or DCF depending on the sign of residual dispersion mismatch just before the receiver. To reduce system installation cost, tuneable dispersion compensators such as fiber Bragg gratings (FBGs) have sometimes been deployed. For WDM systems with channel rates of 40 Gbit/s and higher, more adaptive CD compensation is needed because a small amount of uncompensated CD can cause severe BER degradation. As illustrated in Figure 11.3, even a small mismatch away from the zero residual dispersion point, for example from a 4 km length of SMF (or less than 1 km of DCF), will lead to an equivalent OSNR penalty of about 1 dB in a standard RZ receiver. Using commercial DCF modules for the fine adjustment of residual dispersion would therefore be very inefficient. With adaptive CD compensators, fine adjustments of CD compensation can be achieved and system installation is much easier leading to lower costs. Another important reason for the need for adaptive CD compensation is temporal variations of CD. In Germany, fiber cables buried in the ground are typically located 60 to 100 cm below the surface and are therefore subject to seasonal temperature changes. Figure 11.4 [2] shows the temperature variations from the surface to 1 meter below the ground at a location in mid Germany. As the fiber cables are buried deeper than 50 cm, the fast changes due to day-night cycles do not affect the link CD. However seasonal drifts do lead to slow variations of CD that have to be compensated by adaptive CD compensators.
FIGURE 11.4 Temperature variation from surface to 1 meter below ground over several days, measured at a location in the middle of Germany
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The dependence of dispersion D on temperature T is principally given by vD vl0 ¼ ,S; vT vT where S is the dispersion slope and l0 is the zero dispersion wavelength [3,4] of the fiber. The thermal coefficient dl0/dT is nearly same for a wide range of fiber types and is about e0.028 nm/ C [4]. Adaptive CD compensation can be achieved by either optical schemes like fiber Bragg gratings [5] or by electronic mitigation schemes, e.g., finite impulse response (FIR) / infinite impulse response (IIR) filters [6]. An elegant alternative to avoid adaptive schemes is to make the transmitter-receiver unit more robust against residual CD by applying multilevel modulation formats such as DQPSK. By using DQPSK or other multilevel modulation formats, the baud rate can be reduced while maintaining a higher bit rate. From a CD point of view, at 40 Gbit/s data rate, DQPSK is a good choice for national European networks like Germany’s. Figure 11.5 shows the receiver tolerance against accumulated CD for an equivalent OSNR-BER penalty of 2 dB of systems with different modulation formats. At 100 Gbit/s data rate, CD tolerance is low for most of the modulation formats. Even with DQPSK the CD tolerance is only about 20 ps/nm; therefore adaptive CD compensation will be needed. For 100 Gbit/s data rate, coherent PMQPSK is an interesting candidate. This technology uses a local oscillator for coherent detection and DSP which allows for the tolerance of very large dispersion
FIGURE 11.5 Typical system sensitivity on residual chromatic dispersion for different data rates and modulation formats. The numbers are the dispersion for 2 dB of equivalent OSNR penalty at the receiver.
11.3 Signal degradation and temporal fluctuations
values. Even in-line dispersion compensation may not be needed anymore because of powerful DSP. The disadvantage of this technology is the need for more complex electronics entailing increased cost and higher power consumption. Furthermore, the DSP cannot mitigate inter-channel non-linear distortions very well, e.g., XPM, which especially occurs in in-line dispersion compensating modules. For a better time-to-market efficiency, the telecommunication operators favor smooth migration strategies which means that in most cases full system infrastructure is not renewed but only new line cards are added to the existing WDM systems. When 100 Gbit/s line cards will be introduced they will have to run on systems that were laid out for the former 10 Gbit/s and 40 Gbit/s channels, including their in-line amplifier maps and in-line dispersion management. The use of old dispersion maps limits the maximum transmission distance of coherent PM-QPSK systems, to about 1000 km. Due to limited national network dimensions, e.g., in Germany, this is still sufficient. For other countries this may not be the case. This distance limitation has to be considered for the architecture of optically transparent islands using multidegree ROADM/PXC described earlier.
11.3.2 Polarization-mode dispersion In addition to CD, the PMD causes signal impairments in transmission systems, particularly for high data rate channels. The impairments due to PMD are much more difficult to mitigate than CD because PMD exhibits statistical time dependent fluctuations with a time constant that ranges from sub-milliseconds to months. Furthermore, the effect is strongly wavelength dependent such that each WDM channel is affected individually and in a stochastically decoupled way [7]. PMD is due to slight fabrication imperfections in the transmission fiber and other components that break up the symmetry of the two fundamental polarization modes of a single mode fiber. Causes of PMD include geometrical imperfections or stress leading to local birefringence. The two fundamental modes travel at slightly different group velocities, causing pulse broadening and signal distortions. The basic value that characterizes this broadening effect is the differential group delay (DGD) which is generally dependent on wavelength and time. PMD is the average DGD, integrated over a broad spectral range. PMD is a constant fiber parameter, which is usually defined as a fiber property without wavelength dependence, and scales with the square root of the fiber length, so PMD per unit length is quoted in ps/sqrt(km). DGD is also called the first-order PMD. Besides the first order, second- and higherorder PMD terms, being defined as higher-order terms of a Taylor series expansion of DGD versus frequency, can occur and lead to additional receiver penalties. The statistical nature of PMD arises from varying local birefringence properties (birefringence strength and vector orientation on Poincare´ Sphere) along the fiber axis. So mode delay and coupling of light power between the two modes varies constantly, leading to unpredictable results at the end of the fiber link. Only the statistical frame parameters can be predicted, i.e., a Maxwellian statistical distribution of DGD with a certain mean value. In real networks like the backbones of the
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FIGURE 11.6 DGD over wavelength and time, measured within a European fiber infrastructure
European incumbent carriers, even the Maxwellian statistics from ideal PMD theory does not hold, but it can be used as a first approximation to estimate the outage of the telecommunication systems [8]. Figure 11.6 shows the statistical variation of DGD over time and frequency, measured in a European fiber network. Figure 11.7 illustrates the decrease of PMD values of installed fibers [9] over the years due to innovations in fiber manufacturing. As in some European countries
FIGURE 11.7 PMD of installed fibers from 1986 to 2001. Three phases of innovation by the fiber manufacturers can be observed in the late 1980s, early 1990s, and late 1990s [9].
11.3 Signal degradation and temporal fluctuations
a huge amount of fiber cables was already installed during the 1990s, today much of the installed fiber has a relatively high PMD. The PMD of modern fibers is very low so that transmission at 40 Gbit/s data rate is no longer a problem. However, many network operators own large legacy fiber plants, and some of the fiber links in such networks have very high PMD values that lead to problems at these rates, even at 10 Gbit/s for some countries. As the cost of a large scale installation of new fiber cables is very high, network operators look for cheaper alternatives such as the use of modern ULH WDM systems with a high PMD robustness by the application of either special modulation formats or adaptive PMD compensation. Figure 11.8 shows the statistical distribution of PMD in a European network in 2004. The distribution in Figure 11.8 is due to a mixture of fibers from the different production years as illustrated in Figure 11.7. The PMD has been grouped into four classes with respect to the introduction of 40 Gbit/s systems at link distances of 400 km. Class 1 (<2.5 ps) needs no special system adaptation, and simple 40 Gbit/s NRZ can be transmitted over 400km. For Class 2 (<5 ps) simple PMD mitigation schemes are needed, e.g., single-stage optical PMD compensators or electrical filters combined with FEC. For class 3 (<7.5 ps), complex PMD compensators (e.g., with two birefringence stages) are required. Class 4 does not allow for compensation schemes any more. In the last few years, however, this distribution has changed due
FIGURE 11.8 Statistical distribution of PMD and classes for the operation of different 40 Gbit/s systems on 400 km links
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to new fiber installations in the high-speed backbone routes. Besides the mentioned approaches over the years, many new modulation formats have been developed that can relax the baud rate and thus allow for 40 Gbit/s transmission even without compensators.
11.3.2.1 Temporal properties of PMD From Figure 11.6 it can be recognized that DGD is generally stable over a relatively long period of time. On some fiber routes the stability can last even longer, e.g., several days or weeks; however, very fast changes can also happen. The reason for the different time constants is the properties of the infrastructure. In Germany fiber cables are buried 60 to 100 cm deep in the ground. Here temperature variations can lead to slow drifts of DGD. On average, every 80 kms the signal has to be re-amplified by an EDFA inside a telecommunication site. Here the cable leaves the ground and the individual fibers are fed into system racks with optical amplifiers and DCF modules. Slight mechanical vibrations at these sites can cause significant DGD changes at high speed. Also, cable tubes along bridges can cause faster DGD perturbations compared with the buried cable segments. The fast DGD transient time constant are often in the range of 10 ms, but sometimes even at sub millisecond level. Polarization measurements from an installed fiber route collected over many months are shown in Figure 11.9. A special setup allowed for the triggered recording of short transients of polarization changes during a very long observation period. The measurement concept was similar to that published in Krummrich, et. al. [10]. The figure on the left illustrates the temporal fluctuation of the state of polarization (SOP) on the Poincare´ Sphere. The trajectory represents the SOP of the light at the end of the PMD fiber link, before the receiver input. The right figure shows the
FIGURE 11.9 Temporal SOP changes at the output of a fiber link with PMD within an installed fiber plant (buried SMF with amplifier sites). Left: Poincare´ Sphere representation of a varying SOP trajectory. Right: Probability density function of different speed events from changing SOP values.
11.3 Signal degradation and temporal fluctuations
measured SOP dynamics by a statistical probability density function (PDF). The recorded and summarized events are only from significant transients with a SOP shift of more than 90 degrees on Poincare´ Sphere. Typically the variation speeds of DGD and the principal state of polarization (PSP, being the principal effective birefringence axis of the fiber DGD) are on the same order of magnitude as the SOP at the output of the PMD fiber. So the PDF from Figure 11.9 can be taken as a valid measure of the speeds of the changes occurring in DGD and PSP. For network operators who plan to deploy WDM systems with high-speed channels, the absolute value of PMD is important in deciding whether certain bitrate and modulation format can be supported over the link. Moreover, the speed of DGD fluctuations (and principal state of polarization [PSP], SOP variations) is an important parameter, especially in systems with adaptive PMD compensation. There are different PMD compensation or mitigation technologies available including optical and electrical ones. Each is limited to a certain tracking speed of DGD and SOP. In case a DGD or SOP change is faster than the tracking speed, the tracking will be lost and can even degrade the signal quality. This is because during some time intervals the phase lag will cause the correction to be subtracted instead of added to the fiber DGD. There are different adaptive schemes available to compensate for PMD, and their response times differ as well. We distinguish between electrical and all-optical schemes, between PMD mitigators and compensators (PMDC). Most electrical solutions belong to the group of mitigators that do not directly compensate for physical PMD parameters like DGD but that lower the impact of PMD on signal quality. These mitigators are typically part of the receiver electronics after the photodiode and can, for example, be realized as adaptive FIR or IIR filters or combinations of these. The optical compensators manipulate the optical properties of the transmitted signals without any OEO conversion, thus rendering them suitable for use for inline applications to balance the local PMD of large meshed optically transparent domains with many multidegree ROADM nodes. However, the standard application even for optical PMDC is compensation at the receive site, i.e., directly before the photodiodes. The advantage of PMDC at the receiver is the availability of different control parameters for PMDC adaptation, delivered by the receiver itself. For instance, the internal receiver FEC calculation can be used as an additional supporting indicator, making the monitoring part more cost efficient. For inline PMDC typically more complex monitoring devices are needed. For a better control efficiency of any PMDC and to avoid local minimum trapping, often several parameters are detected and analyzed, e.g., degree of polarization (DOP) as well as the FEC calculation values. The speed of adaptive PMDC, which is so important for network operators, depends on the entire control loop and its weakest part. The loop starts by taking parameters from the receiver or optical monitor. The measurement speed is normally not the primary issue, but the kind of physical or technical parameter set being
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chosen. These sets can be very different with respect to control efficiency. We distinguish between two basic concepts for control: one is a direct feed-forward PMDC adjustment, guided by the measured parameters; the other is an iterative feedback loop tracking the PMDC state to optimized monitor values. The feedforward concept ideally needs only one measurement and adjustment step and is, by principle, much faster than a feedback loop that needs 10 or 100 iterations. A disadvantage of the feedforward approach is that special detection parameters are needed for compensator adjustment. They have to be directly related to the physical PMD phenomenon thus requiring complex monitoring approaches. Moreover, aging of the PMDC components can lead to a long-term compensation efficiency decrease. The feedback solution can take advantage of low-cost detection means, e.g., receiver internal FEC overhead or DOP. The speed of the feedback approach strongly depends on the number of loop iterations that are needed when crucial PMD-related signal transients (e.g., fast and strong amplitude changes of SOP before the receiver) occur. Here PMDC speed strongly depends on the choice of monitor parameters and on the optimization algorithm of the control loop. Also important is a smart algorithm for searching a sweet spot of operation in case the PMDC loses tracking. Last but not least, the kind of PMDC design and the components used directly contribute to its transient response speed. Several approaches for optical PMDC exist, depending on the degree of PMD to be compensated. Besides different PMD range capabilities, there are compensators that can compensate for first-order PMD only, while others can compensate for second-order PMD or even higher-order PMD. These higher-order compensators are composed of several individual adaptive sections, typically polarization controllers, and sometimes adjustable delay lines. The control of many separate elements requires very complex and efficient control algorithms, thus having an impact on overall adaptation speed. The selection of PMDC components plays a further important role. The adjustment of variable optical delay lines is comparably slow while the arrangement of a cascade of fixed optical delay lines and variable phase or polarization controllers is much faster, at the same time offering nearly the same functional performance. The choice of material system for the polarization controllers is critical for PMDC speed performance. For instance, liquid crystal is comparatively slow, while Lithium Niobate (LiNbO3) is extremely fast. Unfortunately, LiNbO3 is expensive and therefore avoided for PMDC design. The typical maximum speed of state-of-the-art PMDC is in the range of 1 to 10 milliseconds (where the maximum speed is defined as meaning that the PMD compensation is still robust at Stokes vector rotation speeds of p per given time interval). From the experience of some network operators this may not completely satisfy the requirements as even Figure 11.9 does not illustrate the entire problem. The maximum speed of PMD transients strongly depends on the individual link, but some links exhibit even sub-millisecond DGD and SOP fluctuations. These events do not occur often but they are frequent enough that the PMDC cannot achieve the required link reliability.
11.4 Raman amplification in WDM networks
An alternative to the above described adaptive PMD compensation schemes and their problem of dynamic transient response is the choice of modulation formats that lower the baud rate by multilevel coding and thus enhance the receiver robustness against PMD. In some Western European countries the DQPSK modulation format has become the primary choice for 40Gbit/s transmission, as its dual-level modulation reduces the baud rate by a factor of two. This enables doubling of the link distances. For transmission at 100 Gbit/s, other modulation formats are under consideration since DQPSK is not sufficient. One option is DSPbased coherent PM-QPSK which is very robust against PMD. Modern DSP technology is evolving to reach the calculation speed needed for mitigating the fast DGD transients. In practical networks, even if a system cannot compensate for all impairments due to PMD, a certain residual outage probability may be acceptable. Service providers generally have to guarantee a certain QoS to their customers. This has a direct impact on the required overall survivability design of all OSI layers of the network and the physical layer including layer L1 resilient architectures. So, it is important to know the outage performance of certain systems on a given fiber infrastructure. Deutsche Telekom and NTT have undertaken a common field trial [8] where a procedure has been tested and patented that allows for accelerated measurements of system outages due to PMD, which is very important in specifying the reliability of networks. Besides this approach, standardized test procedures for systems and receivers have been developed that allow the systems to be compared with respect to their sensitivity to PMD to first and higher orders. When networks are expanded to meshed transparent islands, as described earlier, the impact of PMD has to be taken into consideration to avoid instability in operation. Finally, the new robust modulation formats may be the key to solving the PMD problem without the need for new expensive fiber installations.
11.4 RAMAN AMPLIFICATION IN WDM NETWORKS 11.4.1 EDFA, Raman and hybrid amplification scenarios Installed systems in Europe are typically using EDFA-based amplification schemes to achieve required transparent reach for the optical links and are limited to C-band transmission. Staying with 100 GHz and 50 GHz spacing of 10 Gb/s channels results in maximum system capacities of 400 to 800 Gbit/s for metro- and long-haul transport systems, respectively. Some system vendors offer 25 GHz channel spacing DWDM solutions to expand this maximum capacity, giving rise to issues of nonlinear crosstalk from tightly spaced neighboring intensity-modulated 10 Gbit/s channels. Channel data rates of 40 Gbit/s are therefore now available from nearly all system vendors, and 100 Gbit/s already shows up on the roadmaps of most vendors. The growing data rates of the IP-router interfacesdincreasing the transponders’ line rates additionally help reduce operational expenditures by reducing footprint and
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energy consumption per transmitted bitdcontribute to simplified wavelength planning, especially for optically meshed networks with layer 1 protection or restoration. Boosting the spectral efficiency and growing use of ROADM for optically transparent multidegree nodes lead to a strong demand for increase in OSNRbudget. As discussed in the earlier section, Raman amplifiers offer extremely low noise figures compared with conventional EDFAs, which makes them attractive for transport systems with high OSNR-requirements, like high-speed systems and transport links with high optical attenuation. The basic principle behind the Raman effect can be described by the following system of coupled differential equations for two co-polarized waves co-propagating in a lossless fiber [11]: dPP uP g R ¼ , ,PP ðzÞ,Ps ðzÞ dz uS Aeff
(1a)
dPS gR ¼ ,PP ðzÞ,PS ðzÞ dz Aeff
(1b)
Due to inelastic interactions with fiber molecules, a high power wave acts as a pump (PP) for a red shifted wave (stokes wave, PS). This process is available at nearly every transmission band, which makes Raman amplifiers interesting for the extension of available transmission bandwidth. The efficiency of Raman scattering depends on the Raman gain coefficient gR, which is a function of the frequency shift between pump and stokes wave and has a maximum at 13.2 THz. As can be seen from the equations above, the depletion of pump depends on the Stokes power which generally is much smaller than the pump power, assuming the Stokes wave to be an amplified signal channel. Taking into account the influence of fiber doping on gR and the influence of effective area Aeff on the amplification process, it becomes obvious that fiber types with small effective areas show especially high Raman efficiency. So-called lumped Raman amplifiers use small effective area DCF as a high efficiency gain medium to achieve lossless dispersion compensation. The focus here is on distributed Raman amplifiers (DRA), where the amplification process counteracts the influence of losses directly in the transmission fiber, establishing high signal-to-noise performance. This distributed gain is achieved by injecting one or more high power pumps at frequencies around 13 THz above the signal band into the transmission fiber where the distributed gain occurs. Raman pumps can be implemented at the transmitter site (or repeater output for in-line amplifier sites), at the receiver site (or repeater input for in-line amplifier sites), or both, with different advantages and disadvantages: for co-propagating pumping schemes, the main amplification occurs near the transmitter site of the fiber in the region of high signal powers. While this can reduce the generation of ASE noise in the link, it may also give rise to nonlinear signal distortions and to the transfer of relative intensity noise (RIN) from pump to signal. Most commonly,
11.4 Raman amplification in WDM networks
FIGURE 11.10 Generic system model for a hybrid amplified WDM-link with double-stage EDFA and counter propagating Raman pump
backward pumped amplification schemes are used, which introduce the Raman pump light at the in-line amplifier or receiver site at the end of the fiber, where signal powers have already decreased and therefore show only small degrading influence on the pump. In this configuration, pump intensity noise is averaged along the amplifier’s length such that only portions of the pump intensity noise spectrum of less than approximately 10 kHz is transferred to the signal efficiently [12]. Hybrid Ramaneplus EDFA based amplification schemes are used mainly for terrestrial applications. Figure 11.10 shows a typical setup for a hybrid backward pumped WDM link consisting of a number of transmitters followed by a multiplexer/interleaver structure. As discussed earlier, the WDM signal may then be pre-compensated for chromatic dispersion before passing a cascade of transparent transmission sections. Within each section the signal is optically amplified by an EDFA at the input of the transmission fiber and counter propagating Raman pump. An additional EDFA regulates the signal input power to the DCM. Residual dispersion is optimized, before the signal wavelength is extracted by a DEMUX-interleaver structure prior to sending it to a receiver. The impact of Raman scattering is shown in Figure 11.11, which depicts the power profile of a backward pumped DRA with two pumps and 40 channels at 100 GHz spacing: first of all, strong signal amplification occurs in the last kilometers of the fiber, which leads to a deviation from the decrease in power resulting purely from the fiber attenuation, which would be linear on dB scale, as indicated by the dashed line. Usually, the amount of amplification is specified in terms of the Raman on-off gain, which is defined as the ratio of signal output power with and without Raman amplification as marked on the right vertical axis of the figure. The second point to note is the increasing spread in channel powers. This power or gain tilt results from gain variations between the channels, which are due to the spectral shape of the Raman gain on the one hand and Raman interaction between the channels themselves. For large spectral WDM bandwidths, the signals at high frequencies can significantly amplify the lower frequency channels, if the channel powers are high enough. Reduction of the gain tilt for a desired Raman gain is an objective of the optimization of the interactions between pumps, signals, and optical noise, which are strongly nonlinear. A flat gain profile can be achieved by a good choice of the number of injected pumps, their wavelengths and power levels
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FIGURE 11.11 Power profile of a backward pumped transmission fiber (G.652). The two pump scenario is exemplarily shown interacting on a 40 channel WDM signal at 100 GHz spacing. Interaction of the pumps can be observed near the end of the fiber. Depletion of the pumps due to signal powers occurs mainly at high signal power levels close to the transmitter site. The influence of Raman gain is clearly observed here on the second half of the fiber, where the channels’ power profiles rise above the dotted line representing the power profile for an unamplified section. On-off Raman gain is defined as indicated on the right axis; the differences in the received signal power levels for the different channels are mainly due to Raman gain tilt.
[13]. The third aspect is the depletion of the pumps due to the high signal powers at the fiber input and the mutual interaction of the pumps, which can be quite strong, depending on their power levels and frequencies. Analytical solutions for some of the most important parameters like the onoff gain can be derived by neglecting the pump depletion due to the Raman process. The Raman on-off gain in this case depends on the launched pump power PP,in, the effective fiber length Leff,P, the Raman gain coefficient gR, the effective fiber area AP,S for pump, and stokes wavelengths according to the following formula: 10 gR ðfP ; fS Þ 10 , , CR PP;in ,Leff ;P ,PP;in ,Leff ;P ¼ Gon=off ;dB ¼ lnð10Þ Kpol ,AP;S lnð10Þ (2) The polarization factor Kpol takes into account the relative state of polarization between the pump and the signal. For a depolarized counter propagating Raman pump, the value of Kpol ¼ 2. The Raman on-off gain versus pump power from the above analytical expression, neglecting pump depletion, is shown in the Figure 11.12 below. To achieve exact solutions for parameters like gain, OSNR, and
11.4 Raman amplification in WDM networks
Raman Gain - Undepleted Pump Assumption 30
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Pump Power, W
FIGURE 11.12 Raman on-off gain versus pump power for a single-channel, single-pump scenario. Result of estimation is shown for standard single mode fiber of different fiber lengths assuming undepleted pump condition.
noise figure, the differential equations system for the static power distributions along the fiber has to be solved numerically. For making a comparison of the noise impact from a distributed fiber amplifier to that of a lumped device, e.g., an EDFA, an effective Noise Figure is usually introduced. In this definition the amplified fiber is separated into one unamplified transmission fiber followed by a lumped optical amplifier with a gain equivalent to Gon/off. The noise figure, which this theoretical amplifier would need to achieve the same output OSNR as it occurs at the receive end of the distributed amplifier, is the effective noise figure NFeq. This effective noise figure is one essential parameter, that can be derived from the on-off gain Gon/off and the generated noise power PASE measured in the optical bandwidth Bopt: 1 PASE 1þ (3) NFeq ¼ hfBopt Gon=off The artificial separation of fiber loss and amplifier gain within this definition leads to very low values in a logarithmic scale, below 0 dB in most of the relevant cases. Under the assumption of undepleted pump and negligible amount of double Rayleigh backscattering, PASE ¼ hfBopt ,2NASE can be approximated for counter propagating pump, using the following equation, where nsp z 1:13 (complete
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CHAPTER 11 Transport solutions for optically amplified networks
inversion) is the spontaneous emission factor and G(L) is the gain defined as the ratio of signal power at receiver site of the fiber to input power to the transmission fiber: a a a a ; qeap L G ; q ; (4) NASE ðLÞ ¼ nsp GðLÞ 1 þ eq g ap , G ap ap ap RN P with q ¼ CR , P;aP in and Gða; xÞ ¼ sa1 ,expfsgds being the incomplete x gamma function. For high gain and long fiber the equation can be further simplified. Rough estimates can be derived under the assumption of frequency independent attenuation coefficient (a ¼ ap) using the following equation: NASE ðLÞ z nsp
Gon;off ðLÞ lnGðLÞ
(5)
An example of Equivalent Raman Noise Figure versus on-off gain according to the more general approximation from the above equation is depicted in the graph below for a G.652 fiber of 80 km length. Treating the DRA as a discrete device of known effective gain and noise figure offers simplified conditions for the optimization of a link setup, e.g., regarding the amplifier gain in terms of the OSNR. Limitations to the maximum system reach due to ASE noise can be estimated for a link model as shown in Figure 11.13 for the 0.5
equivalent Raman Noise Figure, dB
328
0
-0.5
-1
-1.5
-2
-2.5
-3
0
5
10
15
20
25
Raman on-off Gain, dB
FIGURE 11.13 Estimated Equivalent Raman Noise Figure versus On-Off Gain according to the given equation for 80 km G.652 fiber.
11.4 Raman amplification in WDM networks
general case of a hybrid Raman/ EDFA amplification scheme based on amplifier gains and noise figures using the following generalized analytical formula: PSig ; Nmax; OSNR ¼ 1 1 OSNRmin Feq þ GR F1 1 G1 ,ASMF þ F2 G2 1 hvB0 (6)
where A ¼ ASMF ,AMidstage ¼ G1 G2 GR :
In this formula ASMF and AMidstage are the reciprocal transmission fiber and midstage loss values, which are supposed to be fully compensated by the gains of the first and second amplifier stage G1 and G2 together with the Raman gain GR. Figure 11.14 shows the OSNR-limited system reach versus the required OSNR at the receiver based on the formula shown above for a single- and double stage EDFA amplification scheme as well as for a hybrid double-stage EDFA plus Raman amplification scheme, assuming typical G.652 fiber values, amplifier gains and noise figures. The arrows indicate typical OSNR sensitivities, which can be expected for the given data rates assuming simple NRZ modulation format. For the 40 Gbit/s case, the system reach in terms of ASE noise for the given case would be limited to 300 km using single-stage amplifiers. This graph shows that in order to extend the reach of 40 Gbit/s channels from metro to long haul distances, double-stage amplifiers are 4000
OSNR-limited System Reach, km
single-stage EDFA double-stage EDFA
3500
hybrid Raman/EDFA
3000 2500 2000 1500 1000 500 0 15
40Gb/s
100Gb/s
↓
↓
20
25
30
35
40
OSNRmin, dB
FIGURE 11.14 OSNR-limited system reach versus the required OSNR at the receiver, OSNRmin, for a single-, double-stage EDFA based amplification scheme and a hybrid scheme using additional Raman amplification. Arrows mark typical OSNR sensitivities as they can be expected for the depicted data rates using NRZ modulation.
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CHAPTER 11 Transport solutions for optically amplified networks
mandatory. In the given example the noise limited reach is extended to 2000 km for this case by implementing the second amplifier stage. Longer distances for this example could be achieved by introducing a hybrid amplification scheme, which would allow for more than 4000 km noise limited system reach. Looking at the 100 Gbit/s NRZ case the hybrid amplification scheme would be mandatory to achieve long-haul transmission distances with the given parameters. The graph gives an idea of the capabilities of hybrid Raman supported transmission. In addition to noise, high speed transmission systems are also limited by nonlinear impairments. At 40 Gbit/s intra-channel crosstalk is well known to cause severe impact on the transmission reach. Nonlinear phase shift can be used for quick analytical estimations of the nonlinearity impact in optical transport systems at high data rates [14]. Adding Raman amplification to an EDFA amplified span with fixed fiber input power increases the nonlinear phase shift by adding distributed gain. For backwards pumped DRAs the additional nonlinear impact can be expected to be comparably low because the signal is amplified at the end of the span, where signal powers are fairly low due to fiber attenuation. Neglecting the depletion of the pump, the nonlinear phase shift for a backwards pumped span can be approximated by the following equation: Z LSMF PS ðzÞ dz ¼ g PS; in ,Leff ; (7) fNL; SMF ¼ g z¼0
with a
Leff ¼ ek kaP , and k ¼ q,eaP
aP a
a a Glower ; k Glower ; q aP aP
(8)
LSMF .
The lower incomplete gamma function used here is defined as follows: Z x Glower ða; xÞ ¼ sa1 ,expfsgds; 0 < a < 1
(9)
0
The nonlinear phase shift for a backwards pumped G.652 fiber, as derived in Equation 7, is plotted against the level of pump power in Figure 11.15. Signal transmit power at the input of the 80 km fiber section was set to 0 dBm in this example. It can be seen that for reasonable pump powers (no overcompensation of fiber loss) the increase in nonlinear phase shift can be linearly approximated at a slope of 2$102 rad/W in the given example. Pumping at 500 mW the nonlinear phase shift will be increased by only 40%, while Figure 11.12 shows that a high portion of the fiber loss is compensated for in our given configuration. It is highly desirable to substitute a portion of the EDFA gain by Raman in a hybrid amplification scheme, particularly for system configurations suffering from nonlinear impairments. Substituting EDFA by Raman gain improves OSNR but does not lead to high increase of nonlinear phase shift for moderate pump powers (Figure 11.15). The potential of hybrid Raman plus EDFA transmission will be shown in the following case study, based on fully numerical simulations including all Raman interactions, noise and nonlinear impairments. In this example, the input powers of
11.4 Raman amplification in WDM networks
phi vs. Ppump 0.12
Nonlinear Phase Shift, rad
0.11 0.1 0.09 0.08 0.07 0.06 0.05 0.04 0.03 0.02
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Pump Power, W
FIGURE 11.15 Increase of Nonlinear Phase Shift due to Raman backward pumping on a G.652 80 km fiber span for increasing power of the Raman pump with fixed signal transmit power of 0 dBm.
the two EDFA stages shown in Figure 11.10 have been varied for three scenarios using no Raman pump, pumping at 300 mW and at 500 mW. For all the given combinations the maximum system reach has been calculated. The reach criterion chosen here was based on a maximum system penalty of 3 dB measured at a bit error rate of BER ¼ 103. A 40 Gbit/s RZ system (25% duty cycle) with full inline dispersion post-compensation and optimized dispersion at the receiver was investigated. Figure 11.16 shows the evolution of the power penalty for one point of operation of the EDFAs with Raman pump powers at 0, 300 and 500mW. The results of the optimization of EDFA output power and DCF input power for each of the Raman pump power condition is also shown in the figure as contour plots. Without Raman pumping the maximum reach is limited to 9 spans in the optimum case, which was found between 0 and 2 dBm input power into the transmission fiber and 3 to 6 dBm into the DCM. For lower power levels the transmission reach is mainly limited by ASE noise, whereas fiber nonlinearities have highest impact at higher power levels. Introducing 300 mW of Raman pump light enables the system designer to increase the system reach by 33 %, in this example. The optimum point of operation shifts to EDFA powers which are about 2 to 3 dB lower than in the pure EDFA scheme. A portion of the gain of the EDFA stage which transmits the signal to the transmission fiber, is substituted by Raman gain. This leads to lower impact from nonlinearities and increased OSNR due to the lower
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CHAPTER 11 Transport solutions for optically amplified networks 4
3.5
SMF28, 40 Gb/s, RZ, Gauß, d = 0.25, PRaman = 0.0W
0
7
8
0.0
8
9
-4
6
-6 -8 -4
100 200 300 400 500 600 700 800
8 10
6
8
10
10
12
10
4
SMF Input Power, dBm
6
8
6
-6
10
2
8
-4
-8
8
0
6 -2
-10 -10
-8
6
-2
8
-8 -4
12
-6
6 8
4
-4
0
4
6
-2
6
4
6
10
6
8
4
SMF28, 40 Gb/s, RZ, Gauβ, d = 0.25, PRaman = 0.5W
4
2
6
DCF Input Power, dBm
4
6
2
4
4
SMF28, 40 Gb/s, RZ, Gauβ, d = 0.25, PRaman = 0.3W
0
0
SMF Input Power, dBm
Transmission Distance, km 4
9
-2
10 -6
-4
6
8
0
2
7
8
6
7
5 -2
6
0.5
5
6
6 7
7
1.0
5
6
1.5
5
8
DCF Input Power, dBm
2.0
2
9
System Penalty, dB
2.5
4
4
PRaman = 0.0W PRaman = 0.3W PRaman = 0.5W
3.0
DCF Input Power, dBm
332
-2
0
2
4
6
SMF Input Power, dBm
FIGURE 11.16 Results of numerical Optimization of EDFA output powers in terms of maximum transparent reach for 40 Gbit/s RZ25 single channel transmission with hybrid EDFA/ Raman amplification scheme. The upper left curve shows the evolution of system penalty along the link for three Raman pump power conditions (0, 300 and 500mW). The contour plots depict optimization of SMF input power and DCF input power (ref. Fig. 11.10) for maximum number of spans under the condition of max. system penalty @BER=10-3, under the three Raman pump power conditions.
noise figure, increasing the total system reach. Increasing the pump to 500 mW shifts the optimum operation point further to lower power levels, but does not improve the total system reach any further. This can be explained in part by the large increase in signal power at the fiber output, which will be close to the fiber input power. In addition ASE noise and double Rayleigh backscattering may occur at the high Raman gain. This design example shows the typical mechanisms and design issues for hybrid EDFA/ Raman links; the absolute parameters strongly depend on fiber parameters, data rates and modulation formats.
11.4.2 Laser safety and network implementation issues Two major issues hinder Raman amplifiers from significant deployment in the German national backbone networks. First of all, there was no real need for
11.4 Raman amplification in WDM networks
comparable high investment as point-to-point topologies are feasible for transmission distances and infrastructure in the national environment. However, future OTN network structure will have higher optical transparency by the implementation of ROADM based topologies offering more benefits by deployment of Raman amplifiers. In addition, there are still reservations concerning operational aspects like reliability and laser safety. Especially for all-Raman amplified systems and also for hybrid amplification schemes, field safety issues related to the relatively high output power of the pump module have to be addressed. Regarding the laser safety, it has to be ensured, that no danger may occur for individuals or infrastructure. Most likely, high power levels may injure persons, damage connectors or cause fire in the central office. If automatic laser shut-down mechanisms fail, fiber fuse effect can destroy the transmission fiber over several kilometers [12]. Raman pumps have typical output powers in the range of 300 mW to above 1 W, whereas EDFA output powers typically do not exceed 200 mW. Automatic power reduction or shut-down (ALS) is therefore required to fulfil hazard level 1M specifications for optical fiber communication systems according to IEC60825-2 or ITU-T G.644. The mechanism has to reliably reduce or shutdown the output power fast enough (within 1 s for many cases) to avoid radiation hazards. The mechanism has to detect any type of fiber break or open connector. Pure detection of Optical Supervisory Channel (OSC) is not sufficient since the power may be detected from amplified optical noise even in the case of a fiber cut. Raman ALS mechanisms therefore use pilot tone based approaches, detection of back-reflection pump energy or a combination of several detection approaches. Especially for high-loss spans Raman amplified systems require a fast but secure restart procedure, because the usual procedures may fail due to the low power levels at the receive site before the Raman pump power has been restored. The afore mentioned laser safety aspects increase the system complexity compared to purely EDFA based transmission links, therefore raising concerns about the reliability of the system. Another issue for Raman amplifiers arises from the local losses from connectors or bad splices in the gain region of the fiber which can severely reduce the pump efficiency. Raman amplified spans should therefore be spliced all through. Increased fiber attenuation from aging, in most cases due to repairs, may degrade the signal quality over the years. Addressing the question of need for Raman amplification, the following figure (Figure 11.17) shows a histogram of the length distribution of amplifier sections for a typical German backbone transport network infrastructure based on EDFA amplified DWDM transmission systems. Section lengths in the range of 30 to 109 km occur with a mean value of 72.9 km. The graph shows that only a few spans longer than 80 km appear, therefore the common link design scheme based on EDFAs is quite adequate. The small number of exceptions will not lead to special requirements on the transmission equipment due to the fact that an individual span with higher loss will be no problem, as long as total link length and OSNR margin do not exceed the margins. Due to the fact that there is no lack of sites to install
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CHAPTER 11 Transport solutions for optically amplified networks
25 Mean = 72.9 km Max. = 109.0 km Min. = 30.0 km
20
Count
15 10 5
>115
110-115
95-100
105-110
90-95
85-90
80-85
75-80
70-75
65-70
60-65
55-60
50-55
45-50
40-45
35-40
0 30-35
334
Amplifier section length [km]
FIGURE 11.17 Histogram of the length distribution amplifier sections for a typical national German transport backbone network based on EDFA supported optically transparent DWDM links [15]
amplifiers with sufficient spacing, there is no real demand for the introduction of low noise Raman amplifiers in such a network structure. Nevertheless, some of the emerging trends in different carrier network strategies might lead to new issues which could be addressed by hybrid EDFA/ Raman amplification. One of those trends is the implementation of ROADM based optically transparent meshed or ring structures. To deliver real benefits, any restrictions on optically transparent system reach should be minimal to enable flexible lightwave paths through the network, not only supporting the shortest paths connecting pairs of nodes but also allowing for detours which may occur in protection scenarios or due to limited capacity along the shortest route. At the same time the increase of line rates continues and with it the introduction of more complex modulation formats with more demanding OSNR requirements. Depending on the mix of modulation formats and symbol rates along the link, there might be increased OSNR requirements or the need for solutions to mitigate impact from fiber nonlinearities. Another point could be increased span lengths in the case of hut-skipping, e.g., driven by the need to save OpEx from power or space. The potential for high data rate transmission over a transmission link with high-loss fiber sections was shown for a link typical of German fiber infrastructure within the scope of a field trial [16], [17]. This work reports the transmission of 170 Gbit/s over 185 km of field fiber plus 25 km of lumped SSMF, with a total loss of 61 dB, using Raman assisted transmission.
11.5 SUMMARY Telecommunication services are subject to fundamental changes, due to the growing displacement of circuit switched traffic by packet switched traffic for residential customers, and new broadband services. It is a challenge for the
Abbreviations
service providers to meet the enormous growth in bandwidth demand and the increasing need for service transport flexibility, while the budget of customers for telecommunication services remains limited. This enforces industrywide optimization of transport efficiency of the core and aggregation networks. It also calls for robustness and simplified network operation, and reduced capital and operational expenditures for carriers. In order to address these cost requirements, different architectures for European national networks have been discussed with special focus on the optically amplified transparent optical layer with WDM transmission via point-to-point fiber links or ROADM supported all-optical network islands. Modern WDM systems are equipped with high channel capacities up to 100Gb/s in order to address high-speed services, to connect 100G IP router ports, and to enhance the spectral efficiency of the fiber. Physical effects in optical fibers (e.g., chromatic dispersion, polarization-mode dispersion, and nonlinear intra- and inter-channel distortions) and amplifier noise limit the transmission speed. Some of the effects are subject to large statistical temporal fluctuations, so that adaptive compensation schemes have to be applied for high-speed transmission over backbone network distances. Alternatives include robust modulation schemes, such as multilevel modulation with coherent detection and digital signal processing. For the suppression of nonlinear fiber impairments and to extend the system reach special amplification schemes have been proposed, including EDFAs and distributed Raman based amplification.
ABBREVIATIONS ADSL ALS ASE AWG BER B&S CapEx CD CP CWDM dB DCF DCM DEMUX
Asymmetric digital subscriber line Automatic laser shut-down Amplified spontaneous emission Arrayed waveguide grating Bit error ratio Broadcast and select Capital expenditures Chromatic dispersion Control plane Coarse wavelength division multiplex Decibel Dispersion compensating fiber Dispersion compensation module Demultiplexer
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CHAPTER 11 Transport solutions for optically amplified networks
DGD DOP DQPSK DRA DSF DSL DSP DWDM EDFA EPON FBG FEC FIR FIT FOADM FTTB FTTC FTTH FTTx FWM Gbaud Gbit/s 2.5 Gbit/s 10 Gbit/s, 40 Gbit/s, 100 Gbit/s GPON IIR IP ITU L1, L2, L3 LH MPLS MUX NEM NMS NRZ NT NZDSF OA OADM OAM ODU ODU-XC
Differential group delay Degree of polarization Differential quadrature phase shift keying Distributed Raman amplifier Dispersion shifted fiber Digital subscriber line Digital signal processor Dense wavelength division multiplex Erbium-doped fiber amplifier Ethernet passive optical network Fiber Bragg grating Forward error correction Finite impulse response Failure in time (number of failures in 10exp9 hours) Fixed optical add-drop multiplexer Fiber to the building Fiber to the cabinet Fiber to the home Fiber to the X (anything) Four wave mixing Giga-baud Giga-bit per second In most cases the numbers 2.5, 10, 40, 100 are placeholders for their whole bitrate family, e.g., 40Gbit/s stands for the whole family encompassing native 40.0 Gbit/s, 39.813 Gbit/s (OC768/STM-256), 43.018 Gbit/s (OTU3), 44.571 Gbit/s (OTU3e) and other formats. Gigabit passive optical network Infinite impulse response Internet protocol International telecommunication union Layer 1, 2, 3 (OSI-layers) Long haul Multiprotocol label switching Multiplexer Network element management Network management system Non return to zero Network termination Non-zero dispersion shifted fiber Optical amplifier Optical add-drop multiplexer Operation, administration, and maintenance Optical channel data unit Optical channel data unit cross connect
Abbreviations
OEO OFDM OMS OOK OpEx OSC OSI OSNR OTH OTH-XC OTN OTU OXC PDF PMD PM-QPSK PON PSP PXC QAM QoS QPSK RDS RIN ROADM RWA RZ SCM SDH SLA SMF/SSMF SONET SOP SPoF TE TDMA ULH VDSL WDM WSS XFP XPM
Optical-electrical-optical (conversion) Orthogonal frequency division multiplex Optical multiplex section On-off keying Operational expenditures Optical supervisory channel Open systems interconnection reference model Optical signal-to-noise ratio Optical tansport hierarchy Optical transport hierarchy cross connect Optical tansport network Optical channel transport unit (OTU1/OTU2/OTU3 for 2.7G, 10.7G, 43G) Optical cross connect Probability density function Polarization mode dispersion Polarization multiplexed quadrature phase shift keying Passive optical network Principal state of polarization Photonic cross connect Quadrature amplitude modulation Quality of service Quadrature phase shift keying Relative dispersion slope Relative Intensity noise Reconfigurable optical add-drop multiplexer Routing and wavelength assignment Return to zero Subcarrier modulation Synchronous digital hierarchy Service level agreement Standard single mode fiber Synchronous optical network State of polarization Single point of failure Traffic engineering Time division multiple access Ultra long haul Very high bitrate digital subscriber line Wavelength division multiplex Wavelength selective switch 10 gigabit small form factor pluggable Cross phase modulation
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CHAPTER 11 Transport solutions for optically amplified networks
aDCF aTrans Dx DPre DRes DCF Dfopt EDFA Fx g Gx LDCF LTrans N NZDSF OADM OSNR PSig PASE Rx RDS SASE SMF Tx
Attenuation coefficient of DCF Attenuation coefficient of the transmission fiber Local fiber dispersion of the named device Precompensation at the transmitter Residual dispersion per section Dispersion compensation fiber Optical bandwidth Erbium-doped fiber amplifier Noise figure of the named (x) device Nonlineare fiber coefficient Gain of the named (x) device Length of the DCF Length of the transmission fiber Number of sections Non zero dispersion shifted fiber Optical add drop multiplexer Optical signal-to-noise ration Signal power Amplified spontaneous emission power Receiver Ratio dispersion slope Noise power density Single mode fiber Transmitter
References [1] T.P. Christina, H. Herbert, A.S. Dominic, Svetoslav Duhovnikov, Gottfried Lehmann, Alexandros Stavdas, et al., Integrated Design and Operation of a Transparent Optical Network: A Systematic Approach to Include Physical Layer Awareness and Cost Function, IEEE Commun. Mag. 45 (2) (Feb 2007) 40e47. [2] Sascha Vorbeck, Malte Schneiders, Werner Weiershausen, Cornell Gonschior, Franko Kueppers, 100GEthernet for Aggregation and Transport Networks, SPIE Photonics West 2009, Conf. on Optical Metro Networks and Short-Haul Systems, San Jose, California, USA, January 29, 2009. Proceedings Vol. 7235, ISBN 978-0-81947481-0. [3] M.J. Hamp, J. Wright, M. Hubbard, B. Brimacombe, Investigation into the Temperature Dependence of Chromatic Dispersion in Optical Fiber, IEEE Photon. Technol. Lett. 14 (11) (Nov. 2002) 1524e1526. [4] T. Kato, Y. Koyano, M. Nishimura, Temperature dependence of chromatic dispersion in various types of optical fiber, Opt. Lett. 25 (16) (August 2000) 1156e1158.
References
[5] D. van den Borne, V. Veljanovski, E. Gottwald, G. D. Khoe1 and H. de Waardt Fiber Bragg Gratings for In-line Dispersion Compensation in Cost-effective 10.7-Gbit/s Long-Haul Transmission, Proceedings Symposium IEEE/LEOS Benelux Chapter, 2006, Eindhoven [6] Henning Bu¨low, Fred Buchali, and Axel Klekamp Electronic Dispersion Compensation Journal of Lightwave Technology, Vol. 26, Issue 1, pp. 158e167 [7] Werner Weiershausen, Ralph Leppla, Frank Kuppers and H. Scholl Polarization Mode Dispersion in fiber transmission: theoretical approach, impact on systems and suppression of signal-degradation effects, in European Conference on Optical Communication Conference (ECOC 1999), Sep 26e30, Nice, France, Invited Paper We C3.1. [8] Werner Weiershausen, Ralph Leppla, Ottokar Leminger, Frank Rumpf, Ralf Herber, Arnold Mattheus, et al PMD outage measurements in a joint field trial of a 43-Gbit/s NTT WDM transmission system within DT’s installed fiber environment in Optical Fiber Communication Conference (OFC 2004), Feb 22e27, Anaheim, CA, USA), paper WP3. [9] Measurements of PMD in the installed fiber plant of Deutsche Telekom D. Breuer, H.-J. Tessmann, A. Gladisch, H.M. Foisel, G. Neumann, H. Reiner, H. Cremer, Holey Fibers and Photonic Crystals/Polarization Mode Dispersion/Photonics Time/ Frequency Measurement and Control, Digest of the LEOS 2003 Summer Topical Meetings Volume, Issue, 14e16 July 2003 Page(s): MB2.1/5-MB2.1/6 [10] P. Krummrich, E.-D. Schmidt, W. Weiershausen, A. Mattheus, Field Trial Results on Statistics of Fast Polarization Changes in Long Haul WDM Transmission Systems, Conference on Optical Fiber Communication (OFC 2005), March 6e11, Anaheim, CA, USA, paper OThT6 [11] G.P. Agrawal, Fiber-Optic Communication Systems, third ed., John Wiley & Sons, Inc., N.Y., 2002. [12] M.N. Islam, Raman Amplifiers for Telecommunications, IEEE Journal of Selected Topics in Quantum Electronics Vol. 8 (No. 3) (May/June 2002). [13] D. Breuer, M. Schneiders, S. Vorbeck, R. Freund, A. Richter, Design analysis and upgrade strategies from single to double and triple wavelength-band WDM-transmission, APOC, Bejing, 2004. invited paper. [14] S. Vorbeck, M. Schneiders, R. Leppla, Tolerances and Engineering Rules for Performance Estimation and System Design in 160 Gbit/s Transmission Systems, Proc. of Optics East ITCOM, Philadelphia, USA, 2004. [15] A. Gladisch, A. Betker, R.-P. Braun, D. Breuer, A. Ehrhardt, H.eM. Foisel, et al., Evolution of Terrestrial Optical Networks in the Context of Network Architecture, Proceedings of the IEEE, Volume 94, Issue 5, May 2006 Page(s): 869e891 [16] M. Schneiders, S. Vorbeck, R. Leppla, E. Lach, M. Schmidt, S.B. Papernyi, et al., Field Transmission of 8 x 170 Gbit/s over high loss SSMF link using third order distributed Raman amplification, Proc. of OFC, PDP 31, Anaheim, USA, 2005. [17] M. Schneiders, S. Vorbeck, R. Leppla, E. Lach, M. Schmidt, S. Papernyi, et al., Field transmission of 8x170 Gbit/s over high loss SSMF link using third order distributed Raman amplification, Journal of Lightwave Technology Vol. 24 (Issue 1) (January 2006) 175e182.
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CHAPTER
Optical Amplifier for Maintenance Friendly Fiber Networks
12
Glenn A. Wellbrock, Tiejun J. Xia
CHAPTER OUTLINE HEAD 12.1. Fiber maintenance today ............................................................................ 12.2. Next generation optical transport network................................................... 12.3. The maintenance friendly fiber network ...................................................... 12.4. Fiber switch technologies .......................................................................... 12.5. Optical amplifier designs for MFFN ............................................................. 12.6. MFFN trial with hybrid switch design .......................................................... 12.7. Summary................................................................................................... Acronyms ........................................................................................................... References .........................................................................................................
342 343 349 351 352 354 358 358 359
Physical changes in the fiber plant are inevitable. Manmade events such as road construction, railroad maintenance, bridge or tunnel work, pipeline repair, and industrial spills, as well as natural events such as floods, fires, mud slides, and damaging winds can impact network fiber and require maintenance. This fiber maintenance also must be coordinated with other fiber work such as overbuild projects, system repair, and upgrades. Regardless of the event, the fiber shares the same network protection resources. Couple this with the fact that each fiber now carries an ever increasing amount of traffic and protection domains are expanding geographically as conventional synchronous optical network (SONET) rings are replaced with mesh network designs and you can see the challenge of maintaining a reliable, resilient fiber network. When responding to fiber events in a modern network, traffic is moved off an active fiber via a SONET switch. This approach not only limits the coordination and exposure of a simultaneous event occurring to one ring, it also avoids switching a fiber with active trafficdgenerally referred to as a “hot cut.” However, this controlled domain approach will not work in future optical transport network (OTN)ebased mesh networks that typically employ multidegree reconfigurable optical add drop multiplexer (ROADM) nodes supporting optical bypasses at intermediate locations. The line rates are also increasing from 10 Gb/s to 40 Gb/s and, eventually, to 100 Gb/s, making the task of moving traffic without disruption Optically Amplified WDM Networks. DOI: 10.1016/B978-0-12-374965-9.10012-3 Copyright Ó 2011 Elsevier Inc. All rights reserved.
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even more critical. In addition, OTN-based mesh networks increasingly use internet protocol (IP) routers, which take longer than conventional time division multiplexing (TDM) switches to converge when a disruption occurs. One way to address this foreseeable problem is to introduce a new mechanism for fiber maintenance. Instead of switching the traffic to the protection route at the end points, one can consider switching the traffic, temporarily or permanently, from the active fiber to a spare fiber between the two nearest optical amplifiers without imposing serious interruption to the data flow. Switching traffic between two amplifiers at a composite fiber level eliminates the need to identify the circuit end points or to coordinate with other network events. To introduce this change, however, the optical amplifier would have to be redesigned to quickly perform the function described above. With this change, a fiber network equipped with new amplifiers could be considered a maintenance friendly fiber network (MFFN). Consequently, moving fibers would no longer be such a heavy burden to the network operation team [3]. Obviously, modifying the optical amplifier design is the key to making MFFN a reality. In this chapter the following topics will be covered:
Fiber maintenance today Next generation optical transport network The maintenance friendly fiber network Fiber switch technologies Optical amplifier design for MFFN MFFN trial with hybrid fiber switches
12.1 FIBER MAINTENANCE TODAY Figure 12.1 shows a typical SONET ringebased network design illustrating fiber maintenance today. The customer circuit traverses multiple rings that are divided into separate protection domains. If a fiber move is required between amplifier j and jþ1, as shown in this example, the network operator simply performs a manual switch to a protection path from the two nearest add/drop multiplexer (ADM) nodes (location Q and S in this example). This action removes traffic from the target fiber so work can be performed with minimal interruption to the customer (SONET standards mandate less than 50 ms for a protection switch). Granted, the customer is susceptible to an outage while in simplex or unprotected mode because of the inherent probability of a fiber or equipment failure, but the exposure is limited. After the fiber work is completed, the protection switch is removed and the traffic returns to the normal working path. A fiber may carry many individual SONET systems because of dense wavelength division multiplex (DWDM) transport technologies, but each system has an associated ADM switching point that allows the network operator to schedule work in each domain simultaneously without impacting customer traffic. Care must be taken to avoid impacting the protection path while in simplex mode to avoid an outage. This is yet another reason an MFFN adds value for SONET-based systems.
12.2 Next generation optical transport network
R
Optical Amplifier
ADM Q ADM
Long Haul
S ADM
Metro ADM ADM CPE
j
ADM j+1
Fiber maintenance planned
Location A
Metro ADM CPE Location B
FIGURE 12.1 Fiber maintenance work on a SONET network
12.2 NEXT GENERATION OPTICAL TRANSPORT NETWORK Although SONET-based networks provide valuable maintenance attributes as previously defined, they are not an economical solution for carrying data-centric traffic that is growing at double digit rates [4,5]. As discussed previously, SONETbased networks will be replaced with OTN-based mesh networks. Meshed networks, however, are generally designed to avoid boundaries by employing direct end-to-end paths whenever possible. The protection capacity can be dedicated or shared, based on platform capabilities and user preference. Either way, the problem of clearing all active traffic from more than one physical path at a time becomes complex because there is no direct relationship among the circuit paths. Consequently, only one maintenance event can occur in the network at a time unless a significant amount of preparation is done to guarantee two or more events are not related to the same circuit path or its associated protection path. One could use a probability matrix or complex modeling tool, but both require time and carry some level of risk. Figure 12.2 shows an example of a ROADM-based mesh network. Here, path 1 and path 2 both start from node A but end at node H and node E, respectively. Path 3 bridges communications between node C and node G. When fiber maintenance is required between node D and node G, as indicated in the figure, all three paths are affected. Now imagine 80 or more DWDM channels on each fiber at data rates as high as 100 Gb/s each and the difficulty of tracing all the paths to the end points and switching them to alternative routes for every fiber maintenance event in the network becomes evident. Even if it were possible, the time and effort to identify and switch each individually is prohibitive. Consequently, a new method must be employed to localize the event and avoid network-wide implications.
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ROADM Based Optical Transport Network
A
C B
D
E G Path 2
F H ROADM
Path 3 Fiber maintenance planned Path 1
FIGURE 12.2 Fiber maintenance for ROADM-based optical transport network
To increase capacity of the transport network, several approaches have been studied. Of these methods, three stand out: using higher data rates, wider amplifier bandwidth, and/or narrower channel spacing. Because higher data rates can be supported on existing systems, this is the most likely first step. To that end, Verizon conducted three separate field trials using 100 Gb/s per wavelength. The intent is to establish performance expectations and push the industry toward similar solutions so the entire ecosystem of component, sub-system and system suppliers work together to bring products to market quicker and at better cost points. While the amplifier has always been an integral part of any line system, it becomes even more important when advanced modulation formats and very high-end signal processing are used to mitigate fiber impairments like polarization mode dispersion (PMD) and chromatic dispersion (CD). In this regime, system reach is dictated primarily by optical signalto-noise ratio (OSNR) tolerance. Granted, very sophisticated forward error correction (FEC) algorithms are being developed, but the amplifier is a key building block to achieve ultra-long-haul (ULH) distances. 100 Gb/s technology development is a major breakthrough for the next generation transport network. In the foreseeable future, internet traffic is expected to grow at a fast pace because of bandwidth-hungry services, such as video services, largescale data storage and mirroring, increased social networking, real-time gaming, and other services taking advantage of broadband communications. In the past several years U.S. broadband services have grown about 40% annually [4]. In the next several years, global internet traffic will likely maintain a similar, if not higher, growth rate [5]. Figure 12.3 shows global IP traffic predictions up to 2012. In 2012,
12.2 Next generation optical transport network
50
Global IP Traffic (EB/month)
Source: Ref. [5]
Mobolity IP Business IP Consumer IP
40
30
20
10
0 2006
2007
2008
2009
2010
2011
2012
FIGURE 12.3 Global IP traffic growth
global IP traffic is expected to exceed 40 ExaBytes (1018 bytes) per month, of which consumer IP traffic is the largest portion. Increased internet traffic growth is driving large carriers to provide enough bandwidth to meet market demand. While carriers and service providers feel the urgency to develop more powerful networks, equipment suppliers also feel that urgency. In the optical transport equipment community, technology development is chasing the pace of bandwidth demand growth. In terms of channel data rates, 100 Gb/s is the next step. Most recently, 100 Gb/s development has gained a huge momentum [6e24]. Figure 12.4 shows the trajectory of 100 Gb/s DWDM evolution based on published papers from major optical communication conferences. If transport capability is defined as capacity times distance in a unit of Pb/s-km, 100 Gb/s capability quickly grows from below 1 Pb/s-km to near 100 Pb/s-km in less than three years. This is a result of tremendous industrial investment. This result proves the 100 Gb/s optical channel is able to match, if not exceed, the performance of the traditional 10 G channel but with 10 times the capacity for each fiber. In 2007, the first real-time traffic carried by a single wavelength 100 Gb/s channel over a deployed long haul system was accomplished [25e28]. This trial demonstrated that 100 Gb/s channels can be overlaid onto an existing in-service DWDM infrastructure, which would provide notable economic advantages for carriers. In a joint field trial with Verizon and Alcatel-Lucent, a 107 Gb/s channel carrying live video traffic traveled over a 504 km in-service DWDM route between Tampa and Miami, Florida. The 100 Gb/s channel propagated together with nine commercial 10 Gb/s channels. This long-haul-system is a 50 Ghzespaced Ramanpumped DWDM system. The 100 Gb/s channel was added at the Tampa ROADM as an alien wavelength and dropped at a ROADM in Miami. Figure 12.5 shows the configuration of the trial.
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100
100G Capacity-Distance (Pb/s-km)
346
10
1 100G DWDM ≥ 50-GHz ch. space Source: Ref. [6-24] 0.1 2006
2007
2008
2009
FIGURE 12.4 100 G DWDM transport capacity development
Verizon national video service network
Tampa
Florida, USA
100G field trial route Miami
FIGURE 12.5 Route of the first single wavelength 100 G real-time field trial
The modulation format used in this 100 Gb/s trial was return-to-zero differential quadrature phase shift keying (RZ-DQPSK) at 53.5 Gbaud with all the necessary real-time signal processing functions. At the transmitter, an OC192 signal, which contained live HDTV traffic in a GbE channel, was tapped optically from Verizon’s
12.2 Next generation optical transport network
DP-QPSK Transmitter
DP-QPSK Receiver
π/2 Laser
Data generator
π/2
ADC & DSP
Laser
Phase modulator
90° hybrid
FIGURE 12.6 DP-QPSK transmitter and receiver proposed for 100 Gb/s transmission
national video service network and fed to the client port of the 100 Gb/s equipment. The 107 Gb/s RZ-DQPSK signal was then fed into a ROADM and transmitted over 504 km. Then, the signal was dropped using a different ROADM and fed into the 100 Gb/s receiver. The original OC-192 signal containing the live HDTV video traffic was then reconstructed in the receiver. The OC192 was fed into a ADM to re-create the GbE channel, which was then fed into a video test set to extract different HDTV channels for display. During the trial, neither SONET errors nor video signal defects were observed on the 100 Gb/s wavelength and all 10 Gb/s channels remained error free. Many modulation formats have been proposed for long distance 100 Gb/s transmission. Taking advantage of mature DWDM technology and balancing capacity and reach distance, dual polarization quadrature phase shift keying (DPQPSK) with coherent detection is gaining more attention over other modulation formats for 100 Gb/s transport equipment [29]. The baud of DP-QPSK 100 Gb/s channel is a quarter of the data rate, so the channel easily fits into a 50 GHz spaced channel plan. Coherent detection with ultra-high speed analog-to-digital conversion (ADC) and digital signal processing (DSP) improve the requirement for OSNR and help the channel reach a long-haul, or even an ultra-long-haul, distance. Figure 12.6 shows a diagram of a DP-QPSK transmitter and receiver. In the transmitter, the 100 Gb/s signal is generated by two phase modulators with the same wavelength while the orthogonal polarizations are combined by a polarization beam combiner. At the receiver, the 100 Gb/s signal is arbitrarily split into two polarizations. The 90 hybrid interferometers help obtain amplitude and phase information of each polarization. The detected signals are then converted into digital formats. With the help of the DSP, the received signal is reconstructed in time with phase, amplitude, and polarization information. With coherent detection and powerful digital processing, all linear fiber impairments (such as CD and PMD) can, in principle, be corrected at the receiver. In another 100 Gb/s trial, jointly carried out by Verizon and Nokia-Siemens Networks, DP-QPSK was shown to travel over a long-haul distance with significant
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tolerance for fiber impairment [30,31]. In this trial, the optical transmission performance of a 111 Gb/s coherently demodulated polarization multiplexed RZQPSK channel with electronic post-processing (100 Gb/s) is characterized. The 100 Gb/s channel traveled, neighbored by both 10.7 Gb/s OOK channels (10 Gb/s) and 43 Gb/s DPSK channels (40 Gb/s) over 1040 km of field fiber (13 spans). The 1040km link had one ROADM at each end and one center-span ROADM. The 10 Gb/s, 40 Gb/s, and 100 Gb/s channels were fed into the ROADMs at the ends of the link. The composed optical signals traveled for 80 km on each span, then were fed into an amplifier with mid-stage dispersion compensation, using an optimized 10 Gb/s dispersion map. No Raman amplification was used, and the optical amplifiers consisted of EDFAs. The 100 Gb/s equipment consists of a full C-band tunable RZ pulse shaped DP-QPSK transmitter and a coherent receiver. The transmitter was fed by two 27.75 Gb/s pseudo random bit sequence (PRBS) signals with lengths of 216 -1 bits. The received data was then captured by a 50 GS/s digital storage oscilloscope and processed on a computer. In this trial, the 100 Gb/s channel was surrounded evenly by two 40 Gb/s channels and eight 10 Gb/s channels with 50 GHz channel spacing. To examine the impact of the neighboring channels on the 100 Gb/s channel, the bit error rate of the 100 Gb/s channel was analyzed along the input power of the 10 Gb/s and 40 Gb/s channels. The results showed the performance of the 100 Gb/s channel could be optimized by carefully choosing the power levels of the neighboring 10 Gb/s and 40 Gb/s channels. This trial confirms the suitability of 100 Gb/s DP-QPSK for multirate operation in existing systems on deployed fiber infrastructures. Using coherent detection has another advantage because of its tremendous tolerance to fiber impairment, as mentioned above. Verizon and Nortel conducted a field study showing a significant PMD tolerance for a 100G b/s-like channel [32,33]. The trial involved 92 Gb/s, 46 Gb/s and 10.7 Gb/s channels for comparison. The 92 Gb/s channel employed dual-subcarrier DP-QPSK modulation while the 46 Gb/s channel used single-carrier DP-QPSK modulation, and the 10.6 Gb/s channel used standard on-off keying (OOK) modulation format. The 92 Gb/s channel used two subcarriers, which together occupied only one wavelength on the 50 GHz grid. The field fibers used for this trial were four aged spare fibers between two field sites for a span of 36 km. In the trial, the spare fibers were patched at one of the sites and looped back to another site, where the 92 Gb/s transmitter and receiver sat. The four spare fibers exhibited different mean differential group delay (DGD) values. The spare fibers were patched in different combinations to find those with high PMD values. In the trial, a pair of fibers with mean DGD of 65 ps were used. The wavelength of the channels were tuned to the Telecommunication Standardization Sector of International Telecommunication Union (ITU-T) grid with a high instantaneous DGD value, then the bit error ratio (BER) of the channel was measured. The performance of the channels was measured by the error seconds of the OC-192 tributary signal, which was fed to the client ports of the transport channels. Figure 12.7 shows the measured error seconds (ES) for the channels versus instantaneous DGD values. The 10.7 Gb/s channel begins to fail when the value is
12.3 The maintenance friendly fiber network
Error Seconds (ES)
100% 92 Gb/s 46 Gb/s 10.7 Gb/s
80%
60%
40%
20%
0% 0
30
60
90
120
Instantaneous DGD (ps)
FIGURE 12.7 Measured error seconds of the 92 Gb/s, 46 Gb/s, and 10.7 Gb/s channels
more than 50 ps, while 92 Gb/s and 46 Gb/s channels maintain error-free performance for DGD values, up to and beyond 100 ps. Compared with current 10 Gb/s technology, 100 Gb/s technology provides almost 10 times the capacity to the transport network, but imposes a challenge when it comes to fiber maintenance. While we can argue how quickly bandwidth demand will grow in the future, there is no doubt it will. Regardless of which technology we choose to meet that demand (higher line rates, wider amplifier bandwidths, or narrower channel spacing), the result is more capacity on each trunk fiber. Consequently, moving all traffic off a fiber before doing maintenance will cause a significant amount of activity in the network and lead to less than optimal signal flows during the event. Not to mention, the overall network costs will increase to compensate for the disruption. Effectively, each local maintenance event will employ network protection, thereby impacting the entire national, or even global, network as ever increasing amounts of data flow over each fiber. The objective of an MFFN is to keep the local event isolated so network protection can be reserved for equipment and/or facility failures, thus significantly reducing the likelihood of a catastrophic effect.
12.3 THE MAINTENANCE FRIENDLY FIBER NETWORK The concept of a maintenance friendly fiber network is incorporating a really fast photonic switch into the amplifiers so all traffic is moved from the active fiber to a standby fiber with minimal interruption. Ideally, the switch would be fast enough to avoid triggering a layer 1, or higher, protection switch anywhere in the network. Assuming this could be accomplished, it would be time consuming to identify all circuits on a given fiber path or to understand how these circuits might impact other end-to-end paths on other fibers that also require fiber maintenance during the same
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Concept of Maintenance Friendly Fiber Network (MFFN) Spare fiber
Optical amplifier
Working fiber
ROADM
FIGURE 12.8 Schematic drawing of the MFFN concept
time frame. This is equally important for maintenance occurring during equipment or facility failures given the lack of domains or controlled areas where it is easy to determine if maintenance would interrupt traffic already on protection or restoration paths. In most cases, fiber maintenance work involves a piece of fiber cable between two adjacent optical amplifiers. Figure 12.8 shows a schematic drawing of the MFFN concept. Here, “no traffic interruption” means data networking will not be interrupted. For example, if customer traffic is carried by a router and transported through an Ethernet circuit to the OTN transport client interface and the OTN signal rides on a wavelength on a DWDM long-haul system, then no traffic interruption occurs during the time when the traffic shifts between fibers. As a result, there is no DWDM system shutdown, no OTN searching for protection routes, no reporting link down to the IP layer from the Ethernet switch, and no re-routing of the traffic at the IP layer. The key component to success is a low loss and fast photonic switch with a relatively wide wavelength range. Low loss is required to avoid negatively impacting the reach between amplifiers. The transmit end can be compensated by increasing the output power of the amplifier since it looks like flat loss of any other component. But the receive end is tougher as it looks like span loss. The faster the switch, the better. This is important in order to avoid race conditions with the existing layer 1, 2, or 3 protection protocols or triggering the laser safety shutdown mechanism. In addition, coordinating the switch between the two amplifiers is key to achieving nearly hitless switching. But this can be accomplished using a loss of light trigger at the receive end. This technique avoids precise time coordination between the two amplifiers by using a simple photo detector to trigger the switch when an event is coming. The wavelength range also is important to ensure a flat response across the entire operating window, including Raman pumps and the optical service channel. The good news is photonic switches that meet or exceed these basic high level specifications are becoming available at reasonable prices that allow commercialization.
12.4 Fiber switch technologies
12.4 FIBER SWITCH TECHNOLOGIES Fiber switches are referred to as photonic switches because they do only optical to optical switching [34]. Steady and consistent advances in fiber switching technology over the last several years have dramatically reduced optical losses, improved switching speeds, and significantly lowered costs. Microelectromechanical systems (MEMS), optical collimator steering (OCS) and robotic fiber connection (RFC) are three technologies commonly seen in fiber switches that have a large number of input and output ports [35e37]. Figure 12.9 shows schematics for these three technologies used for large fiber switches. MEMS technology relies on a set of micro-mirrors to steer the optical beam from an input collimator to an output collimator. The MEMS switch offers fast switching (<20 ms), relatively low loss (1 to 2 dB), and several hundreds input and output ports. OCS switches make optical connections between the input and output ports by using peizo-electric elements to move input and output collimators so they point to each other. This type of fiber switch has less than 1 dB insertion loss and less than 10 ms switch time. OCS switches with port counts close to 100 already have been seen in the market. The fiber switch based on RFC technology is quite different from MEMS and OCS switches. RFC uses robots to physically mate fiber connectors. The performance is very close to a manual fiber connection, except that the connection is made by robots. The advantage of RFC is insertion loss similar to that of a manual connection, plus the connection does not disappear if there is a loss of electrical power. Because it may take several seconds to make connections and disconnections, RFC’s drawback is the amount of time to complete the task. The different attributes of each fiber switch has to be considered with respect to MFFN. The fiber switches mentioned above can be called large fiber switches because they have a significant number of input and output ports. By contrast, there are fiber MEMS
OSC
FIGURE 12.9 Examples of technologies used for large fiber switches
RCF
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switches with only a few input and output ports but much faster switch speeds. For example, 1x2 or 2x2 fiber switches based on magneto-optic technology can have 20 ms or less switching time [36]dmuch faster than large fiber switches. Another type of 1x2 and 2x2 fiber switch based on a ceramic electro-optic material has an even faster switch speed, below 100 ns [38]. Other small fiber switches based on waveguide technologies can switch the port in a few nanoseconds, but the insertion loss kills the application [34]. These fast small switches with low insertion loss can be used for MFFN by creating a spare port for optical amplifiers [3].
12.5 OPTICAL AMPLIFIER DESIGNS FOR MFFN Improvements in optical amplifier design are a major step in next generation optical transport equipment design moving toward MFFN. The main design change is the integration of a fiber switch in the amplifier, allowing the amplifier to choose the outside plant (OSP) fiber automatically or by remote control. The amplifier also should be able to coordinate fiber switches to avoid traffic interruptions when the switches shift traffic from one fiber to another. Below we discuss two possible amplifier modifications as examples for implementing the MFFN concept. One amplifier improvement is to integrate a large fiber switch with the amplifier, as shown in Figure 12.10. During fiber maintenance, the operations team can temporarily connect a spare fiber to the switch or use an existing fiber between the two amplifiers as a spare fiber. Then the traffic in the working fiber can be switched to the spare fiber only between the two amplifiers. When the working fiber is traffic free, it can be cut, moved, and re-spliced. The traffic on the spare fiber is then switched back to the maintained fiber and the spare fiber is released. In this scenario, switch speed is critical. Large fiber switches have 10 to 20 ms switch time, and this event produces an optical signal interruption of at least
Fibers
Large fiber switch Test sets
Gain module
Amplifier j
FIGURE 12.10 Amplifier integrated with large port count fiber switch
Test sets
Amplifier j+1
12.5 Optical amplifier designs for MFFN
10 to 20 ms. Under current amplifier design, this length of interruption is considered a serious loss of signal (LOS). To avoid harm to anyone working on the fiber, the amplifier initiates an automatic power reduction (APR) to lower the optical power to a certain level in a certain time period, in accordance with safety standards for optical communications [39]. While the APR causes the DWDM system to shut down temporarily, the DWDM system recovers in a few seconds, similar to a false alarm. However, this interruption of a few seconds is a severe event for the transport layer because all upper layers are affected by the interruption. The OTN may seek a protection route, layer 2 may report link-down to layer 3, and layer 3 may start rerouting traffic as if the link is gone. To avoid the upper layer disturbance, the amplifier is modified to distinguish a planned fiber switch from a real traffic interruption. Of course, the maintenance fiber should be tested before moving traffic to assure it is within the maintenance limits of the system. This can be done with external or built-in test equipment, before the maintenance switch is performed, as shown in Figure 12.10. When a fiber maintenance job is planned to access the fiber between amplifier j and amplifier jþ1, network management prepares the affected amplifiers for a fiber switch. The test sets in the amplifier remotely control a pre-switching test to assure the spare fiber is in good condition to carry the channels. Then network management issues the command to the amplifier to switch the traffic to the spare fiber. Because the amplifiers understand this is a fiber switching, not a fiber cut, the amplifiers along the line will wait until the optical signal returns. The safety standard requires that power reduction must be realized in less than one second if the total power is less than 650 mWdor even shorter if the power is higher [39]. A DWDM system can withstand a 20 ms power reduction, which is about 2% of the total time required to reduce the power. For upper layers, the time to report layer 2 to layer 3 should be set to be more than 20 ms to avoid IP re-routing. Coordination of two amplifiers to simultaneously perform fiber switching is a challenge. The ordinary data communication channel (DCC) is not reliable because data transmission may have delayed fluctuations. Other approaches for synchronizing the amplifiers also are uncertain. One approach to consider is the traveling wave control. In this approach, the upstream amplifier is considered the master amplifier and the downstream amplifier is the slave amplifier. When coordinating a fiber switch, the downstream amplifier is aware of the fiber ports from which the switching starts and ends. After the upstream amplifier initiates switching the fiber, the downstream amplifier detects a signal loss at the first port and a signal emerging at the second port. The downstream amplifier detects the power changes in the two ports and immediately switches them. In this way the interruption duration to the signal flow is only the switch time itself. The signal transmission delays in both fibers have canceled each other as both travel in the two fibers at almost the same time. Using small fast fiber switching can further reduce the interruption time, considering that large switches create 10 ms or more interruption while small switches generate only tens of ms or even sub-ms interruption. The small fast switch,
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Fibers
Large fiber switch Small fast switch Test sets
Test sets
Amplifier j
Amplifier j+1
FIGURE 12.11 Amplifier integrated with small fast switch and large fiber switch
however, must work with the large fiber switch because the large fiber switch provides the flexible spare fiber selection. Hence, the second amplifier design for MFFN is an optical amplifier integrated with a hybrid switch, which consists of a small fast switch and large fiber switch. The advantage of this design is the lack of speed requirement for the large switch. Consequently, a switch with long switching time, such as the robotic fiber connection, could be used for MFFN if desired. In addition, this design significantly reduces the impact to the network. Figure 12.11 shows this second amplifier design. In this section we have discussed two designs for modifying amplifiers that serve the MFFN function.
12.6 MFFN TRIAL WITH HYBRID SWITCH DESIGN The concept of MFFN has been tested in several trials [3,40]. In one trial the concept is tested with SONET traffic fed into a ULH optical transport system. Two hybrid switches, which work with two optical amplifiers, and SONET test equipment are used for the trial. The hybrid switches help switch all channels from one fiber to another, and the SONET test set monitors the traffic interruption of the communication circuits. Figure 12.12 shows the setup for the trial. Three different OC-48 protection schemes are fed into a DWDM ULH system. The SONET network elements form a two-node 1þ1 linear circuit, a three-node unidirectional path switched ring (UPSR), and a three-node bidirectional line switched ring (BLSR) with an automatic protection switch (APS). One span of each protection scheme is connected through the ULH system. As shown in Figure 12.12, the 1þ1 linear traffic and the UPSR
12.6 MFFN trial with hybrid switch design
Large fiber switch 1x2 fast switch
Transponder /muxponder
1+1 linear
BLSR
UPSR
1+1 linear
Spare fiber
Optical amplifer
BLSR
working fiber
UPSR
ROADM
SONET circuits
FIGURE 12.12 Ultra-long-haul system set-up for MFFN trial (eastbound only)
traffic are fed into one tributary side of an OC-192 multiplexing card at the transponder, which is carried by a long reach DWDM channel of the ULH system. The BLSR traffic is fed to another multiplexing card. The two 10 Gb/s channels carrying the SONET traffic are combined with 17 DWDM channels in the terminal shelf. The channels range from 192.325 to 195.45 THz with 50 GHz or 25 GHz channel spacing and are sent through the ULH system. The DWDM ULH system is constructed with two terminals with transponders and muxponders, 10 line amplifiers and one ROADM, eastbound and westbound, respectively. The trial is performed on the eastbound route. The third span of the eastbound route is a 57 km field fiber used as the working fiber. Six other field fibers with the same length are used as the spare fibers. The optical amplifiers involved in this trial are connected by a hybrid configuration of fiber switches. The field fibers are selected from a metro ring cable in the Dallas area. The insertion loss of the field fiber ranges from 11 dB to 14 dB. The total distance of the ULH system is 656 km in the eastbound direction. The remaining spans of the 1þ1 linear circuit, the UPSR, and the BLSR APS configurations not going through the ULH system are not shown in the figure. The hybrid switch configuration comprises a large fiber switch and a small fast switch. The large fiber switch, AFM-400, developed by FiberZone Networks, is a non-blocking 200x200 switch based on robotic fiber connections. It has low insertion loss (about 0.5 dB) and is able to maintain the connections even during a power outage. At 30 seconds to change a connection, it is relatively slow, but that does not limit this application as its major function is to provide a flexible spare fiber selection. The small fast switch used in this trial, developed by Primanex Corporation, is based on magneto-optic technology. The switch speed is about 20 ms per connection
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with an insertion loss of about 1 dB. To function within the MFFN, the hold-off time of the network element along the channel path should be longer than the duration of the interruption. It should be noted that the length of the interruption impacting a channel is usually longer than the pure optical interruption caused by the switches because extra processing time is required after the interruption. The attainable interruption for various network elements has a wide range, from less than 20 ms to more than 800 ms for SONET equipment [40]. Because of the gap between the available switching time of large fiber switches and the switching time needed for SONET network elements, the small fast switch is needed. In the hybrid switch, the large fiber switch selects spare fiber only and the small fast switch executes traffic switching. To coordinate fast switches at both ends of the amplifier span, the downstream fast switch has a fast photo detector at the port connected to the spare fiber. Prior to fiber switching there is no light at the port. When the optical signals are switched to the spare fiber at the upstream node, the detector in the downstream node sees light emerging at the spare fiber port once the switched signals arrive. The downstream node then executes fiber switching immediately. In this coordination configuration the signal delays in both fibers (working and spare) cancel each other and the total optical interruption is insensitive to the span distance. Figure 12.13 shows the switch configuration and the measured optical interruption. The optical signal interruption is only 44 ms, which is much shorter than, and independent of, the span signal delay of 280 ms. In this trial, tributary signals from a SONET test set are fed to each SONET protection scheme. The tributary signal used for 1þ1 linear circuit is an OC-12 containing an synchronous transport signal with level 3 concatenation (STS3c), a UPSR OC-3 containing an STS3c, and a BLSR OC-3 containing an STS3c. The test signals are either looped back or spanned over to measure the interruption of the tributary signal caused by the fiber switching. Even though the optical interruption time is about 44 ms, the resulting tributary signal interruption varies. During the trial,
(a) Upstream
Downstream amplifier
amplifier
(b)
working fiber
Spare fiber Small fast switch with PD
FIGURE 12.13 Switch configuration in the trial: a) downstream fast switch with photo detector; b) measured optical interruption due to fiber switching
12.6 MFFN trial with hybrid switch design
the traffic was switched from the working fiber to each of the six spare fibers. The performance of the DWDM transponders, the interruption to the SONET tributary signals, and the status of the protection routes in each protection scheme were monitored. The results of the trial are shown in Figure 12.14. Figure 12.14 (a) shows the interruption to 1þ1 linear circuit when the traffic was switched from the working fiber to each of the spare fibers. Figure 12.14(b) shows the impact to the UPSR and Figure 12.14(c) shows that the impact to the BLSR. The protection tributaries of the 1þ1 linear circuit and the UPSR only have a few milliseconds of disturbance, while the BLSR tributaries have a much longer interruption, which may be caused by longer software processing time. In all cases, however, there is no SONET automatic protection switching of the OC-48 traffic path or line and no DWDM ULH system traffic interruption. In other words, all traffic on the fiber is switched between two line amplifiers without any protection switching on the transport network. This trial shows the configuration is “friendly” for maintenance work required on the fiber.
(a)
(b)
1+1 linear
Interruption (ms)
Interruption (ms)
UPSR
2.5
2.5 2 1.5 1 0.5
2 1.5 1 0.5 0
0 0
2
4
0
6
2
Spare Fiber #
4
6
Spare Fiber #
(c)
BLSR
Interruption (ms)
50 40 30 20 10 0 0
2
4
6
Spare Fiber #
FIGURE 12.14 Interruption time of SONET circuits measured in the trial: a) 1þ1 linear circuit; b) UPSR; c) BLSR
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In this MFFN proof-of-concept trial, 19 lit DWDM 10 Gb/s channels, three SONET OC-48 tributaries fed into two DWDM channels, two hybrid switches, and seven field fibers were involved. The total traffic can be freely switched among the fiber spans between two line amplifiers without causing an interruption severe enough to trigger automatic traffic protection. The results show MFFN is a feasible concept.
12.7 SUMMARY Future generation meshed-based networks will have very high capacity links that support multiple service types, each with its own protection/restoration schemes. Consequently, moving traffic off a particular link in an orderly fashion to do physical layer maintenance will be very time consuming and expensive because it increases the amount of spare capacity needed in the network. The concept of an MFFN using fast optical switches provides a means to localize the inevitable fiber moves such that no spare capacity is used, no coordination is required, and no extra latency is added to the circuit path. This is particularly important given the sophisticated nature of multiple nested protection and restoration layers. In addition, the added latency can impact customer circuits when traffic is rerouted to accommodate maintenance events that affect fiber cables every day in every global network. In this chapter we have discussed the concept of MFFN, in which optical amplifiers are modified to integrate fiber switches. Two designs are presented: one with the large fiber switch only, which requires the DWDM system to tolerate a 10 ms or longer optical interruption; and one that uses a small fast switch with a large fiber switch, significantly reducing the optical interruption. A preliminary proof-of-concept trial is presented as well, demonstrating that the concept of MFFN is feasible and can be included in the next generation of optical amplifier designs.
ACRONYMS ADC ADM APR APS BER BLSR CD DCC DGD DSP DP-QPSK
Analog-to-digital conversion Automatic power reduction Automatic protection switch Bit error ratio Bidirectional line switched ring Chromatic dispersion Data communication channel Differential group delay Digital signal processing Dual polarization quadrature phase shift keying
References
DWDM FEC LOS MEMS MFFN OCS OOK OSNR OTN PMD RFC ROADM RZ-DQPSK SONET TDM ULH UPSR
Dense wavelength division multiplex Forward error correction Loss of signal Microelectromechanical systems Maintenance friendly fiber network Optical collimator steering On-off keying Optical signal-to-noise ratio Optical transport network Polarization mode dispersion Robotic fiber connection Reconfigurable optical add drop multiplexer Return-to-zero differential quadrature phase shift keying Synchronous optical network Ultra-long-haul Unidirectional path switched ring
References [1] D.Z. Chen, et al., New field trial distance record of 3040 km on wide reach WDM with 10 and 40 Gbps transmission including OC-768 traffic without regeneration, OFC/ NFOEC 2006 PDN1. [2] E.B. Basch, et al., Architectural tradeoffs for reconfigurable dense wavelength-division multiplexing systems, IEEE J. Selected Topics in Quant. Electron 12 (2006) 1e12. [3] G.A. Wellbrock, et al., First Trial of Maintenance Friendly Network (MFN) by Switching Spare Field Fibers without Traffic Interruption, OFC/NFOEC 2008 (2008) NMB2. [4] S. Elby, Bandwidth Flexibility and High Availability, OFC/NFOEC 2009 Service Provider Summit (2009). [5] Cisco, Cisco Visual Networking IndexdForecast and Methodology, 2007e2012. White paper (2008). [6] G. Raybon, et al., 10 107-Gbit/s Electronically Multiplexed and Optically Equalized NRZ Transmission over 400 km, OFC/NFOEC 2006 (2006) PDP32. [7] P.J. Winzer, et al., 10 x 107 Gb/s electronically multiplexed NRZ transmission at 0.7 bits/s/Hz over 1000 km non-zero dispersion fiber, ECOC 2006 (2006). Tu1.5.1. [8] P.J. Winzer, et al., 2000-km WDM transmission of 10 x 107-Gb/s RZ-DQPSK, ECOC 2006 (2006) Th4.1.3. [9] A. Sano, et al., 14-Tb/s (140 x 111-Gb/s PDM/WDM) CSRZ-DQPSK Transmission over 160 km Using 7-THz Bandwidth Extended L-band EDFAs, ECOC 2006 (2006) Th4.1.1. [10] H. Masuda, et al., 20.4-Tb/s (204 111 Gb/s) Transmission over 240 km Using Bandwidth-Maximized Hybrid Raman/EDFAs, OFC/NFOEC 2007 (2007) PDP20. [11] C.R. Fludger, et al., 10 x 111 Gbit/s, 50 GHz Spaced, POLMUX-RZ-DQPSK Transmission over 2375 km Employing Coherent Equalisation, OFC/NFOEC 2007 (2007) PDP22.
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[12] K. Schuh, et al., 1 Tbit/s (10x107 Gbit/s ETDM) NRZ Transmission over 480km SSMF, OFC/NFOEC 2007 (2007) PDP23. [13] P.J. Winzer, et al., 10 x 107-Gb/s NRZ-DQPSK Transmission at 1.0 b/s/Hz over 12 100 km Including 6 Optical Routing Nodes, OFC/NFOEC 2007 (2007) PDP24. [14] K. Schuh, et al., 8 x 107 Gbit/s Serial Binary NRZ/VSB Transmission over 480 km SSMF with 1 bit/s/Hz Spectral Efficiency and without Optical Equalizer, ECOC 2007 (2007) Mo2.3.1. [15] A. Sano, et al., 30 x 100-Gb/s all-optical OFDM transmission over 1300 km SMF with 10 ROADM nodes, ECOC 2007 (2007) PD 1.7. [16] K. Schuh, et al., 8 Tbit/s (80x107 Gbit/s) DWDM ASK-NRZ VSB transmission over 510 km NZDSF with 1bit/s/Hz spectral efficiency, ECOC 2007 (2007) PD 1.8. [17] C. Sethumadhavan, et al., Hybrid 107-Gb/s Polarization-Multiplexed DQPSK and 42.7-Gb/s DQPSK Transmission at 1.4-bits/s/Hz Spectral Efficiency over 1280 km of SSMF and 4 Bandwidth-Managed ROADMs, ECOC 2007 (2007) PD 1.9. [18] X. Zhou, et al., 2Tb/s (20107 Gb/s) RZ-DQPSK Straight-Line Transmission over 1005 km of SSMF without Raman Amplification, OFC/NFOEC 2008 (2008) OMQ3. [19] G. Charlet, et al., Transmission of 16.4Tbit/s Capacity over 2,550km Using PDM QPSK Modulation Format and Coherent Receiver, OFC/NFOEC 2008 (2008) PDP3. [20] J. Yu, et al., 20112Gbit/s, 50GHz spaced, PolMux-RZ-QPSK straight-line transmission over 1540km of SSMF employing digital coherent detection and pure EDFA amplification, ECOC 2008 (2008) Th.2.A.2. [21] J. Renaudier, et al., Experimental Analysis of 100Gb/s Coherent PDM-QPSK LongHaul Transmission under Constraints of Typical Terrestrial Networks, ECOC 2008 (2008) Th.2.A.3. [22] A. Sano, et al., 13.4-Tb/s (134 x 111-Gb/s/ch) No-Guard-Interval Coherent OFDM Transmission over 3,600 km of SMF with 19-ps average PMD, ECOC 2008 (2008) Th.3.E.1. [23] H. Masuda, et al., 13.5-Tb/s (135 x 111-Gb/s/ch) No-Guard-Interval Coherent OFDM Transmission over 6,248 km Using SNR Maximized Second-Order DRA in the Extended L-Band, OFC/NFOEC 2009 (2009) PDPB5. [24] G. Charlet, et al., 72x100Gb/s Transmission over Transoceanic Distance, Using Large Effective Area Fiber, Hybrid Raman-Erbium Amplification and Coherent Detection, OFC/NFOEC 2009 (2009) PDPB6. [25] Verizon, Verizon Successfully Completes Industry’s First Field Trial of 100 Gbps Optical Network Transmission, Press release, 2007 November 19. [26] T.J. Xia, et al., Transmission of 107-Gb/s DQPSK over Verizon 504-km Commercial LambdaXtreme Transport System, OFC/NFOEC 2008 (2008) NMC2. [27] G. Wellbrock, et al., Field Trial of 107-Gb/s Channel Carrying Live Video Traffic over 504 km In-Service DWDM Route, 21th IEEE/LEOS Annual Meeting, WH1, Newport Beach, Calif., USA, 2008. November 9e13, 2008. [28] P.J. Winzer, et al., 100-Gb/s DQPSK transmission: from laboratory experiments to field trials, Journal of Lightwave Technology 26 (20) (2008) 3388. [29] Optical Internetworking Forum, 2009. 100G Ultra Long Haul DWDM Framework Document. June 30. [30] Verizon, Verizon and Nokia Siemens Networks Set New Record for 100 Gbps Optical Transmission, Press release, 2008 September 25. [31] T.J. Xia, et al., Multi-Rate (111-Gb/s, 2x43-Gb/s, and 8x10.7-Gb/s) Transmission at 50GHz Channel Spacing over 1040-km Field-Deployed Fiber, ECOC 2008 (2008) Th.2.E.2.
References
[32] Verizon, Verizon Confirms Quality of 100G Transmission, Press release, 2008 October 6. [33] T.J. Xia, et al., 92-Gb/s Field Trial with Ultra-High PMD Tolerance of 107-ps DGD, OFC/NFOEC 2009 (2009) NThB3. [34] T.S. El-Bawab, Optical Switching, Springer, New York, 2006. [35] R. Helkey, et al., Design of Large MEMS-Based Photonic Switches, Optics and Photonics News, 2002 May, pp. 43e45. [36] T.J. Xia, et al., Field Trial of Photonic Switches for Efficient Fiber Network Operation and Maintenance, OFC/NFOEC 2007 (2007) NTuC6. [37] S. Roskes, et al., Costs per Home Connected: The Impacts of Automated Fiber Management on Fiber-to-the-Home Deployments, OFC/NFOEC 2008 (2008) NThB2. [38] BATi, NanonaÔ High Speed & Low Loss Optical Switch 2009. Product sheet. [39] IEC, 2004. Safety of laser productsdPart 2: Safety of optical fiber communication systems, third ed. IEC 60825-2. [40] T.J. Xia, G. Wellbrock, Study of Impact of Photonic Switch Speed on Transport Networks, OFC/NFOEC 2007 (2007) NTuC4.
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CHAPTER
13
Low Cost Optical Amplifiers
Bruce Nyman*, Gregory Cowlex
*
Tyco Electronics Subsea Communications, Eatontown, NJ, x JDS Uniphphase, Milpitas, CA
CHAPTER OUTLINE HEAD 13.1. Introduction .............................................................................................. 363 13.1.1. Erbium Doped Fiber Amplifier (EDFA)...................................... 364 13.1.2. Erbium-doped waveguide amplifier (EDWA) .............................. 365 13.1.3. Semiconductor optical amplifier (SOA) .................................... 366 13.2. Erbium-doped fiber amplifiers..................................................................... 367 13.2.1. Cost structure ........................................................................ 367 13.2.2. Cost reduction ....................................................................... 370 13.3. Waveguide based amplifiers....................................................................... 373 13.3.1. Erbium-doped waveguide amplifier .......................................... 373 13.3.1.1. Buried waveguides...........................................................373 13.3.1.2. Channel waveguides ........................................................374 13.3.2. Erbium-doped bulk amplifiers ................................................. 375 13.3.3. PLC and erbium fiber amplifiers .............................................. 375 13.4. Cost summary............................................................................................ 376 13.5. Access applications of low cost amplifiers.................................................. 378 13.5.1. High Power Amplifiers ............................................................ 378 13.5.2. WDM PON............................................................................. 379 13.6. Future directions ....................................................................................... 381 13.6.1. Single stage optical amplifiers................................................. 381 13.6.2. Integration ............................................................................ 382 Acronyms ........................................................................................................... 383 References ......................................................................................................... 383
13.1 INTRODUCTION Optical amplification caused a revolution in the cost of fiber optic systems in the early 1990s. The advent of erbium-doped fiber amplifiers (EDFAs) allowed the deployment of long-haul wavelength division multiplexed (WDM) systems. A single EDFA replaced the electrical regenerator required for each wavelength. For the first eight channel systems two pump lasers replaced eight high speed receivers Optically Amplified WDM Networks. DOI: 10.1016/B978-0-12-374965-9.10013-5 Copyright Ó 2011 Elsevier Inc. All rights reserved.
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and transmitter lasers. As the number of wavelengths increased the cost advantage of EDFAs increased. However, the overall cost of the amplifier is a constant concern in system designs. There are also specific applications such as the access market where amplifier cost is an issue in system design. In this chapter we examine the issues concerning obtaining low cost amplifiers. These include issues such as component and manufacturing costs. We consider both fiber and waveguide based amplifiers including semiconductor based amplifiers [1]. In the past the focus was on erbium-doped waveguide amplifiers; today the focus is on using waveguide based devices to implement the passive and possibly some active components. We will consider amplifier designs that contain a single gain stage in Sections 2 through 4. These amplifiers are primarily used in long-haul and metro applications. We will also look at applications where the amplifier is used in cable access television (CATV) distribution systems, Section 5. The first application is for high power amplifiers used in CATV distribution systems. The second application is amplified spontaneous emission (ASE) sources for the next generation of WDMbased passive optical networks (PONs).
13.1.1 Erbium Doped Fiber Amplifier (EDFA) The EDFA is the basic building block of advanced WDM systems. It provides gain across a wide wavelength range of over 30 nm in either the C or L bands. It is bit rate independent for any current transmission rate. A block diagram of a very basic amplifier is shown in Figure 13.1. This is a co-propagating design where the pump and signal propagate in the same direction. The basic design is that the 980 or 1480 nm pump power is coupled through the WDM and into the erbium-doped fiber. The optical isolators prevent reflections from reaching the erbium fiber and being amplified. In a transmission system the reflections can come from connectors or even the Rayleigh scattering from the transmission spans [2]. This design illustrates the basic functionality of an EDFA. There are a number of design additions that may be included. For example input and output taps are used to control the amplifier gain. An additional pump may be used to improve reliability. And finally a gain flattening filter can be added to increase the usable bandwidth. Er Isolator WDM Fiber
Pump
FIGURE 13.1 EDFA block diagram
Isolator
13.1 Introduction
Now let’s consider the technologies required for each of the components. The optical isolators use bulk optics. The WDM will be either a bulk optic or a fused fiber device. The pump laser is either a GaAs or InP based device in a standard package such as a butterfly. The erbium-doped fiber is made with outside vapor deposition (OVD) or modified chemical vapor deposition (MCVD) processes. Given the various technologies there is little opportunity for integration. Thus we cannot use the traditional integration approach to reduce costs.
13.1.2 Erbium-doped waveguide amplifier (EDWA) As erbium-doped fiber amplifiers became widely accepted there was immediate interest in finding ways to reduce the cost and size of the amplifier. The erbium fiber has a limit on the coil diameter of about 2 inches to obtain high reliability. This limits the overall size of the device. One approach is to replace the erbium fiber with an erbium-doped waveguide. Passive waveguides were an active area of research throughout the 1980s and 1990s. The first major products were 1N splitters where N is greater than eight. These were used in CATV distribution systems. The device technology was based on either ion diffused waveguides or silica based etched waveguides. Using these technologies researchers demonstrated taps and WDMs. The only remaining element for an amplifier is the gain medium. Extensive research resulted in obtaining erbium-doped glasses in both technologies [3,4]. Optical isolators were mounted externally to the waveguide devices. Waveguide based isolators are still an active area of research [5]. The EDWA technology also allows the possibility of creating amplifier arrays. Figure 13.2 shows a schematic layout of an array of four amplifiers developed by Inplane Technologies. The device includes the input signal taps, WDMs, a pump
WDM
Pump 2 Pump 1
Gain section
1
Optical Signals
2 3 4
980 filter Isolators
Input PIN-Tap VOA
Output PIN-Tap 25mm
• One-sided chip – input and output on the same side. • Individual amplifier outputs are controlled through VOAs. • The control electronics provides constant power or constant gain modes.
FIGURE 13.2 Inplane Technologies GEM 2000 EDWA
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variable optical attenuator, a 980 nm filter, and output taps. All four amplifiers fit in a chip less than 25 mm long. Now what’s missing from this picture? There are two important things: the pump lasers and optical isolators. Typically these are pigtailed devices that would limit the overall size. One issue with these amplifiers is that the erbium-doped waveguide is not as efficient as erbium-doped fiber. This leads to higher required pump powers that lead to increased costs. We will discuss this in more detail in Section 13.3.
13.1.3 Semiconductor optical amplifier (SOA) The first optical amplifier technology for optical communications was based on using a semiconductor device. Here the gain medium is a waveguide structure similar to that used in a laser. Both ends of the waveguide are anti-reflection coated. The performance of this coating will determine any gain ripple. This is similar to a very weak Fabry-Perot cavity. To reduce the back reflection even further the waveguide is angled or curved with respect to the end of the chip. The waveguide is also designed for low loss coupling to optical fibers. And finally the waveguide is designed to reduce the differences in gain with respect to polarization. This is usually done by providing some type of material strain in the multiple quantum wells of the active region. A typical package for these devices is a variant on the 14 pin butterfly laser package. The package has both input and output pigtails on opposite ends of the package. The SOA is operated by supplying a current to the device. The combination of simple direct current (DC) current drive and a small package makes the SOA the smallest of the optical amplifier technologies. The SOA can also be integrated with other waveguide devices such as lasers and modulators on a single chip. This type of arrangement is used to build non-linear processing elements such as wavelength converters and regenerators. While SOAs have been commercially available for many years they have never been deployed for inline amplification. This is primarily due to the higher noise figure and the gain transient effects as well as higher costs. Like the other amplifiers the key characteristics of an SOA are the gain, output power, noise figure, and gain ripple. Additional characteristics are optical bandwidth, polarization dependent gain, and gain transients. The typical performance parameters of a commercially available SOA from CIP Technologies (Table 13.1) Table 13.1 CIP SOA-L-OEC-1550 for 500 mA current, 1535–1560 nm Parameter Gain Polarization dependent gain Noise figure Saturated output power ASE ripple
Min.
Typ.
10
14 1 6.5 16
Max.
7.5 0.2
Unit dB dB dB dBm dB
13.2 Erbium-doped fiber amplifiers
show reasonable gain and output power [6] compared with an EDFA. However, the polarization dependent gain is higher at 1 dB and the noise figure is also at least 1 dB higher than an EDFA. A key parameter in multichannel operation of an SOA is the gain transient. An SOA has a gain response on the order of a few hundred ps. For bit rates up to 10 Gb/s the SOA can respond to the leading edge of the transition from a zero to a one. As the gain is compressed on one wavelength the power at another wavelength can be affected [7]. This effect can be overcome by operating the SOA in the linear regime or using a reservoir channel or a gain clamped setup. One interesting characteristic of the SOA is that its bandwidth can be much larger than an erbium-based device. For example a bandwidth of 60 nm is possible. The SOA can also be designed for operation in other wavelength bands besides the C and L band. One can easily design a device for 1310 or 1480 nm operation. This allows amplification for coarse WDM (CWDM) channels which are spaced at 20 nm increments from 1270 to 1610 nm [8] by using multiple SOAs. Another interesting application may be in WDM passive optical networks for fiber to the home applications that operate at 1310, 1490, and 1550 nm.
13.2 ERBIUM-DOPED FIBER AMPLIFIERS Cost reduction in erbium-doped amplifiers has been a topic since the amplifiers were put into production. In this section we review the costs drivers in a typical single stage amplifier. Then we look at some of the strategies to reduce costs. These strategies follow the standard technology S curve for electronics: first improve yield, second reduce component costs by leaning on the suppliers and opening up the specifications, and finally move the production to a location with low labor costs. One thing that has not happened is automation. A good way of handling the various fiber pigtails was never developed. A person is still needed to coil and route the fiber pigtails.
13.2.1 Cost structure In this section we will examine the cost structure of the single stage amplifier optical block diagram of Figure 13.3. This optical configuration is typical of an industry
Tap
Er Isolator WDM Fiber
PD
PD Pump
FIGURE 13.3 EDFA block diagram
Isolator Tap
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standard amplifier that is defined by a multisource agreement (MSA) [9]. The MSA specifies common performance and footprint. In this case the amplifier measures 70x90x12 mm. It has output power of greater than 15 dBm and a noise figure of 6 dB. It has input and output taps with monitor photodiodes. The amplifier has no control electronics; the user must control the pump and monitor all the photodiodes. The production cost of an EDFA can be divided into five categories not including any recovery of the initial R&D costs. The categories are passive components, erbium fiber, electro-optic parts, mechanical parts, and labor. Each category presents a number of options; for example, in-house versus externally sourced components, component reliability, component qualification, and the amount of testing. In this amplifier we have five passive components: the input tap, output tap, WDM, an input isolator, and an output isolator. From the block diagram it may seem there is no difference between the input and output components. This may not be strictly true or false for that matter. It depends on the design of the amplifier and the statistics of the part parameters. Consider the isolators. The loss at the input of the EDFA is directly added to the noise figure. This means that you want lower loss isolators at the input. Now consider the insertion loss parameter of the isolator. The distribution of the insertion loss will most likely be a Gaussian that is truncated at the maximum allowed insertion loss. If the center of the distribution is near the maximum allowed insertion loss, then the supplier has lots of devices that cannot be shipped. If the center of the distribution is much lower than the maximum insertion loss, then more of the devices can be shipped. This higher yield should correspond to a lower price since more units can be shipped. In the first case we may have to choose the better isolators for the input and the higher loss ones for the output. In the second case we may be able to use any isolator for either input or output. Now the question for the supplier in the second case is whether it should take the lowest loss parts and call that a premium part and charge more. There is an interesting interplay between the design specifications and the part statistics that drive the part’s cost. The technologies used in the passive components are bulk optics for the isolators and fused fiber devices for the taps and WDM. In this example a 980 nm pump is used. This drives the choice of the WDM to be fused fiber technology. The average cost of these components is about $105 in large volumes. One immediate question then is whether there is a way to combine these components into a single device. The answer is technically yes but will it result in a cost savings? We will discuss this in Section 13.2.2. The erbium fiber cost is really dependent on the choice of supplier and the total annual quantity used. The erbium fiber pricing is done in either of two ways. One is a straight cost per meter. This approach assumes the gain per meter of the fiber is constant from lot to lot. The alternative, which is more common for volume production, is a cost per gain module. This approach allows for variation in the gain per meter of the fiber. In this model the fiber supplier specifies the correct erbium length for a specific gain. For the example amplifier the cost of the erbium gain module is $50.
13.2 Erbium-doped fiber amplifiers
There are three electro-optic parts in this amplifier: two monitor photodiodes and one pump laser. The monitor photodiodes are simple positive intrinsic negative (PIN) devices packaged in TO can with a pigtail. For the monitoring application the photodiode specifications are quite basic. There is no requirement for speed beyond a few 100 kHz. These diodes cost a few dollars each in volumes. The other component is a 980 nm pump laser. This laser is a GaAs based semiconductor laser with an external Bragg stabilization grating. The laser is packaged in either a 14 pin butterfly or a mini-DIL package. The butterfly package usually has a thermoelectric cooler (TEC). The pricing for these lasers is on the order of $300. The price depends on the output power. The mechanical parts of the EDFA include some type of metal housing; fiber routing guides, printed circuit board (PCB) for mounting the electro-optic components, and strain relief boots for the input and output fibers. The metal housing is usually a machined piece of aluminum. One surface is needed as a heat sink for the pump laser. A molded part is typically not used due to insufficient volume to justify the cost of the mold. The rest of the mechanical parts are some type of plastic. These are obtained from high volume suppliers and are not custom parts. The cost of the mechanical parts including the PCB is on the order of $50. The final cost category is labor. This includes all the tasks required to build and test an EDFA. For a fiber based amplifier the biggest part of the labor is fusion splicing and routing the fibers between devices. For this amplifier there are eight component splices and two more if input and output pigtails are included. Each splice and fiber routing takes about 30 minutes. For a U.S.-based operation, assemblers cost at least $40 per hour when benefits are included. This leads to a cost of $160 to assemble the amplifier. Another 30 minutes is needed to test the amplifier. For this device a simple single channel test is done at one power level to confirm the device performance. The test is not designed to be exhaustive, just to check that the device has been assembled correctly. This adds another $40 in costs. Now all of these numbers have assumed a 100% first pass yield. The actual costs will be higher to account for failed units and rework. In summary, the EDFA costs are shown in Table 13.2 based on 2008 prices. From this data we see that the pump laser dominates the overall cost with the assembly
Table 13.2 Cost by category for EDFA Category
Cost
Assembly and test Electro-Optic Erbium fiber Mechanical Passives Total
$200 $300 $50 $50 $105 $705
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labor second. However, all aspects of the EDFA design are candidates for cost reduction efforts. We should note for completeness that this cost does not include overhead. Overhead covers a wide range of expenses including costs for sales, administration, research, development, and depreciation on capital equipment. This can add up to 40% to the cost.
13.2.2 Cost reduction In this section we will look at a number of strategies for reducing the overall cost. These are divided into two distinct areas, component costs and labor costs. We will not address issues related to reduction in overheads. Those can be found in many business books. The EDFA consists of five passive components. At the input there are three components in series and at the output there are two components in series. The question then is whether these components can be combined into a single component. The answer is yes, but will it lower the costs? These integrated components are referred to as hybrid components and are based on adding functionality to an optical isolator. The idea is that once you have a collimated beam it is not difficult to add an extra filter and fiber pigtail to the device. An additional filter can be added at the input, output, or both ends of an isolator. Functionally the filter is either a tap or a WDM, Figure 13.4. There are three advantages to using a hybrid component. The first is space: the hybrid component is just slightly bigger than an isolator and it has only eliminated the intermediate pigtails between the devices. This leads to the second advantagedreduction in labor costs. Using a hybrid tap/isolator/WDM component eliminates two splices or one hour of labor. The third advantage may be better performance for parameters such as insertion loss, polarization dependent loss
Tap
Isolator
WDM
Input
Output
5% Tap
Pump Laser In
Isolator Signal In
WDM
Tap
Pump
FIGURE 13.4 Hybrid component for EDFA input
Out
13.2 Erbium-doped fiber amplifiers
(PDL), temperature dependent loss (TDL), etc. For example, the loss of a fused fiber tap plus an isolator including the splice is 1.1 dB. The loss for the equivalent hybrid part is 0.8 dB. Now the one possible problem is that the hybrid component may cost more than the individual parts. This can be true if the yield on the hybrid part is not high or if the part is not produced in large volumes. The other optical components are the erbium fiber and the electro-optics parts. The erbium fiber is a commodity item for the most part. The price will depend primarily on volume and specifications. One issue is to make sure that the supplier has all the intellectual property licenses necessary. For the electro-optic parts the photodiodes are a commodity item. These will be standard TO can devices. For the pump laser there may be some advantages in pricing for different types of packages. The most common package is a 14 pin butterfly. This package includes a TEC. An alternative package is a miniedual in line (DIL) that does not have a TEC. This package may be cheaper in piece parts, but the final price will depend on the volumes. Some people prefer this package since it requires less electrical power to operate the EDFA. The other choice affecting pump pricing is the power specification. Pump power is usually specified relative to a kink in the light-current (LI) curve. For high reliability you operate the laser well below this kink point. If you are less concerned with reliability you operate closer to the kink and thus can buy nominally lower power devices. Since the pump pricing is done as $/Watt, this can save money. This is yet another interesting design trade-off. The final components are the mechanical parts. Here the costs are related to labor costs. The use of large automated machine shops can reduce the costs for high labor regions. Moving to a lower cost labor region can be helpful but only if the machining is located near the assembly operation. The shipping costs of parts must be considered. The labor costs for assembly and testing are crucial for controlling the amplifier cost. There are a number of strategies that companies use to lower this cost. These strategies have lots of names in business books but they all boil down to reducing the amount of time required to build a certain number of parts and then reducing the per hour labor cost. There are three ways to reduce the amount of time required to build anything. The first is to reduce the number of parts that have to be assembled. We have already discussed the use of hybrid components to reduce the number of splices required. In our EDFA design the number of splices goes from eight to five. This reduces the assembly time by 1.5 hours. The second is to increase the number of units that are built per unit time. Here we assume that we cannot make people work faster but better. That is, we are focused on first pass manufacturing yield. We want to increase the number of parts that pass the first time they are tested and do not require rework. This is done by improving manufacturing processes such as fiber splicing. It can also be done by opening up the specifications to allow for a lower minimum level of performance. The third strategy is to reduce the amount of testing required. For example, the typical final test of an amplifier is a measurement of the gain and noise
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figure at a few conditions. The number of input levels and perhaps the number of wavelengths can be optimized. Another test that can be quite expensive is temperature testing. Remember that testing impacts cost through both the actual testing labor and the capital cost of the test hardware. In summary, these labor cost reduction approaches are part of ongoing struggle to reduce labor costs. Another option to reduce labor costs may be to move the assembly and test operations to a lower cost location. This assumes that the product development is not already being done in a low cost environment. This option is usually available only if there is sufficient volume. The cost savings of the move must be larger than the transfer costs. The transfer costs can be significant. There are two cases: setting up a new location or working with a contract manufacturing. The comments here are a simple overview; a detailed analysis must be done for each circumstance. Note that the location of lower cost labor may vary over time. For example, in the late 1990s labor in Canada was about 50% of the cost in the United States. Around 2003 the labor cost in China was less than 20% of the cost in the United States. These cost comparisons will change over time and depend on fluctuations in currencies. Setting up a new location in a lower wage location can entail many expenses. First is finding an appropriate location that has an available labor pool. Then a local management team needs to be hired. They may require training. After this a local staff is hired and trained. Now all of this does not happen without support from the original manufacturing location. They may have to spend significant time training and transferring the technology. Other issues that can come up include the need for export licenses, employee retention, and management of intellectual property. The other option is to use a contract manufacturer (CM). In this case the production process is transferred to the CM which provides space, staff, training, and material management. The CM may also be able to obtain lower component costs due to the larger total volume of parts they buy. Some CMs also offer design services for things such as printed circuit boards and manufacturing equipment. The profit margins in this business are typically 5% to 10% according to the annual reports of large companies. For EDFAs the dominant CM is Fabrinet in Thailand. According to a 2006 presentation their fully loaded labor costs and overhead were a factor of eight less than in the United States and Europe. This type of arrangement works well with a mature product that is in high volume. This model may not work with a low volume high mix product line. In summary, the use of low cost labor can save money. However, do not forget to consider the total costs such as taxes, shipping, and longer lead times. Also, there is another cost that is rarely considered. When the manufacturing and development are in different locations innovation may suffer. Sometimes there is a lot that the development group can learn from manufacturing problems. All of these strategies to reduce component and labor costs have been used in the last 10 years to reduce amplifier costs. This has lead to the commoditization of component pricing with low margins for suppliers. And it has led to the movement of manufacturing to China and other Asian countries such as Thailand.
13.3 Waveguide based amplifiers
13.3 WAVEGUIDE BASED AMPLIFIERS In the introduction to this chapter we mentioned the idea of replacing the erbiumdoped fiber with an erbium-doped waveguide. We also will also examine two other waveguide based devices. One amplifier concept is to use a bulk piece of doped material as the gain medium. The other approach uses waveguides to integrate many of the components onto a single chip. This chip might have taps, WDMs, and variable attenuators. The isolators and pump lasers are mounted external to the chip. The gain medium is still erbium fiber. This approach offers some interesting architecture flexibility and may reduce component costs. This component chip is referred to as a planar lightwave circuit (PLC). We will examine the EDWA and PLC in more detail and mention the bulk amplifier version. The bulk amplifier version has not been very successful. This approach has been successful in lasers where the signal passes many times through the cavity. The work on waveguide amplifiers has been driven by a number of goals. The most important was the reduction in cost that waveguide devices offer. If you can make multiple devices on a large silicon wafer the production cost can be reduced. The waveguide approach also allows for reductions in size as well as the possibility of arrays of devices. Trying to put more than one fiber based amplifier in a single package is quite difficult and expensive. Creating an array of waveguide amplifiers is just an issue of mask design. And finally the waveguide devices are based on silicon device processing. This will, one hopes, simplify the required manufacturing effort.
13.3.1 Erbium-doped waveguide amplifier Erbium-doped waveguides have been developed in two different geometries. One approach creates buried waveguides in an erbium-doped glass substrate while the other fabricates channel waveguides on a silicon or silica substrate. Both approaches can result in similar optical performance. One difference compared with erbiumdoped fiber amplifiers is the shape of the gain spectrum. The waveguide and bulk amplifiers may use a different type of glass instead of erbium-doped fibers. This will change the size and wavelength of the gain peak. For example, amplifiers made with erbium-ytterbium co-doping will have a gain peak at 1535 nm compared with 1532 nm for erbium-doped fibers.
13.3.1.1 Buried waveguides The buried waveguide technology was commercialized by Teem Photonics. It is based on using an erbium-doped phosphate glass as the substrate. Buried waveguides are formed in the glass using ion exchange techniques. The passive components are formed in undoped piece of glass. The two pieces are then attached to form a complete device [10]. An example of this is shown in Figure 13.5 where the erbium-doped waveguide guide section is combined with an eight-channel passive splitter [11]. In single channel applications the undoped section could contain a pump combiner.
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FIGURE 13.5 EDWA with an integrated eight-channel splitter
This approach takes advantage of the relatively low complexity required to fabricate the ion exchange waveguides. The waveguides are buried below the glass surface. This allows good mode matching to the optical fiber at the input and output. However, one limitation in this approach is the waveguide bend radius. The waveguide bend radius is limited so these amplifiers tend to be straight devices, thus increasing their length. Following on our discussion of the MSA EDFA in Section 13.2.1 we compare a similar performance EDWA. The device as output powers of either 10 or 15 dBm. The noise figure is less than 7 dB which is a little worse than the EDFA. The device size is significantly smaller at 81x35x12 mm. Two EDWA devices would fit in the space of one EDFA. Note that this device does include a pump laser packaged in a mini-DIL and input and output isolators. Another interesting version of the EDWA is a four channel array version. This device has performance similar to the single channel with output power of 10 dBm at each amplifier output. However, the overall size is only 95x55x12 mm, which is still smaller than a single channel EDFA.
13.3.1.2 Channel waveguides The concept of using channel waveguides for making passive components has been around for a very long time. Extensive effort was invested in developing low loss waveguides and designing passive components such as WDMs and taps. The bulk of this work was done on either silica or silicon substrates using a variety of deposition techniques [12]. As erbium-doped glasses were developed for optical fibers they were adapted for use in channel waveguides. Much of this effort occurred at universities such as the Technical University of Denmark and industrial companies such as Bell Laboratories [13]. All of this effort gave rise to two commercial ventures, Cisilias in Denmark and Inplane in the United States. Cisilias merged with a passive waveguide company Ionas to form NKT integration. This was sold in 2005 to Ignis which makes only passive components. Inplane was sold in 2007 to CyOptics and is the only remaining supplier of EDWAs using channel waveguides. An Inplane device is shown in Figure 13.2 [14]. The pump lasers and optical isolators are fiber coupled to the PLC. On the PLC the signal path has an input tap
13.3 Waveguide based amplifiers
with a photodiode mounted on the chip. It is then combined with the pump signal and transmitted to the gain section. Due to the high index contrast of the channel waveguides and the low gain per unit length the gain section is a spiral structure [15]. An output tap and integrated photodiode completes the device. The device has over 20 dB single channel gain with a noise figure of less than 6.9 dB. The four amplifiers required 400 mW of pump power.
13.3.2 Erbium-doped bulk amplifiers Another approach for a low cost amplifier was to replace the erbium fiber erbiumdoped waveguide with a piece of erbium-doped glass. One requirement is that the erbium-doped glass has very high gain in a short length. Erbium-doped phosphate glasses have demonstrated gains of up to 3 dB/cm [16,17]. This is similar to that used in the Teem Photonics waveguide amplifiers. In the bulk design the design of the waveguide is larger than that used in a single mode design. This allows for a free space coupling of the pump into the gain medium. The idea is that this can reduce the cost of the pump packaging. It can also reduce the cost of the pump by moving from single spatial mode to multiple spatial mode devices. For example, you could move to a multiwatt high power pump with a 100 mm stripe at a lower cost than a single spatial mode device. An example of this type of design was the amplifier developed by Molecular Optoelectronics [18,19]. In this design an erbium-doped glass core of 15 mm was surrounded by a lower refractive index cladding material. The pump light was coupled from the side of the device. The end of the device is polished at 45 degrees and is coated with a pump reflector. The signal was coupled to the gain medium using a lens and was effectively a collimated beam through the gain medium. Gains of over 8 dB were measured at 1535 nm with a noise figure of less than 6 dB. The coupled pump power was 350 mW from a 1.5W multi-mode pump. At the time this product was developed it looked like an attractive way to get lower costs due to the lower cost of the pump laser and elimination of some passive components. However, the fabrication cost of the gain medium is a concern for the overall cost of this structure. Also, the gain medium of this device used an erbium ytterbium-doped glass so the spectrum had a sharp gain peak at 1535 nm. This device has not been manufactured since approximately 2003.
13.3.3 PLC and erbium fiber amplifiers As we have already discussed a PLC can contain many passive components such as taps and WDMs. This leads to another amplifier implementation where the PLC contains the passives and the photodiodes, and erbium fiber is used as the gain medium. This approach combines the low cost and compact size of the PLC with the flexibility of changing the erbium fiber length. As in the EDWA the isolators and pump lasers are external to the PLC.
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In addition, active devices such as variable attenuators, variable splitters, and even switches can be integrated on the PLC. For silica-based waveguides these devices are based on the thermo-optic effect. That is they are varied by changing the local temperature to change the index of refraction. For example, a PLC developed by JDSU included a variable pump splitter, tunable gain flattening filter, and a tilt filter [20]. One component to consider further is the optical isolator. If this component is fiber coupled to the PLC the space savings will be limited. To overcome this issue isolators have been developed that can mount to the side of the PLC and be operated in reflection. With proper optical coupling design the waveguide on the PLC replaces the optical fibers. These devices are still based on a free space isolator core. They can also be made into arrays to further decrease the required space. This approach to amplifier design is most cost efficient as the number of passive components increases. The cost of additional components such as taps, variable splitters, and variable attenuators is a small increment of the total PLC cost. This is not the case for a discrete component design. However, for a simple single stage low, cost amplifier the discrete components may be more cost effective.
13.4 COST SUMMARY In the previous sections we have examined three technologies for low cost amplifiers: EDFAs, EDWAs, and SOAs. We will look at the relative costs trade-offs for a simple single stage amplifier for a single channel application. The amplifier design is a single stage that does not include any control electronics. The cost is the sales price for a medium volumedsay 100 units in a single purchase. The sales price for each technology is shown in Table 13.3. For each technology there are a number of issues that will affect the overall cost structure. A common issue for any manufactured product is first pass yield. That is how many of the products meet specification the first time. A major goal of all manufacturing operations is to reduce product variation. A low first pass yield increases labor costs and testing costs. It may also lead to increased scrap costs if components have to be replaced. For example, in an EDFA, what happens if the
Table 13.3 Amplifier Price Comparison Parameter
EDFA
EDWA
SOA
Unit
Gain Output Power Noise Figure Size Price Range
24 15 6 70 90 12 $750 to $1,000
13 10 6.5 81 35 12 $900
14 10 6.5 12.6 73 8.5 $1,600
dB dBm dB mm USD
13.4 Cost summary
erbium fiber length is incorrect? If it is too long, it can be cut back, but this adds labor and the device needs to be re-tested. If it is too short, the erbium fiber may have to be thrown away. For all the technologies, loss will be a major contributor to first pass yield. In the EDFA it is component and splice loss. In the SOA and EDWA, it will be coupling loss. For all three, variation in the gain medium will also affect first pass yield. For the EDFA it is instructive to compare the sales price in Table 13.3 with the component costs in Table 13.2. Here we see that the lowest sales price is very close to the component costs. This shows that there is little margin in the assembly of simple EDFAs. The margins are similar to the less than 10% obtained by CMs. So to make money in this business it helps to be vertically integrated. In this way the supplier gets to keep all the margins associated with each component. This is especially true of the pump lasers which are one of the largest costs. For the SOA the costs can be divided into two categories, chip cost and packaging cost. The chip cost is not a simple calculation. The first thing to consider is whether you have a wafer fab or it is done externally. If you have a wafer fab there are large sunk costs to keep it operating. These include both capital equipment as well as ongoing operating costs. The actual wafer cost will also depend on the number of wafers that are run through the fab. For an external fab-based operation there is a significant cost associated with each wafer run. There is a fixed cost for the epitaxial growth as well as for the processing. And typically more than one wafer is processed at a time. Each wafer will generate over 1,000 devices. So how do you allocate the cost of these devices to a customer who may want only 10 units? You either end up with a very expensive chip or lots of chip inventory. Another issue: what are the costs of wafers that do not meet specification allocated? The issues with chip fabrication tend to drive toward a few standard designs. The other cost is packaging. The bulk of the cost is the package itself. A butterfly-style package with inputs on both ends is a relatively expensive custom part. The first pass yield is an important determinant of the device costs. Depending on the coupling process it may not be possible to repair a defective unit. Scrapping expensive packages does not help reduce costs. For EDWAs the cost structure will depend on the waveguide fabrication technology. The ion exchange waveguides require basic photolithography tools and some relatively low cost processing equipment. The channel waveguide approach requires the capabilities found in a silicon fab plus the capability to deposit the erbium-doped glass. This becomes a question of a dedicated fab facility or an external fab as we discussed for the SOA. The erbium-doped glass adds a number of steps to processing compared with the fabrication of the SOA. One thing to note for fab-based devices is the inherent conflict between volume and cost. Initially the volume and yields are low, leading to high device cost. However, the customers are unwilling to pay those initial costs. They want the cost when the fab is in high volume production and yields are up. And since there is usually no guarantee a program will reach high volume production much less actually hit the yield targets, there may be a reluctance to agree to the lower pricing
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needed to make the product development go forward. This has been an ongoing conundrum for product managers for many years. We have summarized the technologies and cost drivers. What are the advantages of each technology? The EDFA provides the maximum flexibility for customization. New designs are readily manufactured by changing the erbium length and the pump power. The EDWA can offer a smaller device size and the option of compact arrays of amplifiers. The SOA can provide the most compact device. It is also the only technology that can be used for additional wavelength bands besides the C- and L-bands.
13.5 ACCESS APPLICATIONS OF LOW COST AMPLIFIERS In the previous sections the applications for the amplifiers are mainly for long-haul and metro telecommunications. There are also applications for low cost amplifiers in the access part of the network. These include very high power amplifiers (>1W) for hybrid fiber coax (HFC) and fiber to the X (FTTX) networks as well as ASE sources for WDM PONs. In the access market there is significant cost pressures. This is due to the large number of units required and the low number of customers served per unit.
13.5.1 High Power Amplifiers HFC networks were developed by the CATVoperators to take advantage of the lower losses and higher bandwidths that can be obtained by using optical fiber rather than coax cable. In an HFC network the video signal is generated at a head end and then transmitted using 1550 nm optics to feed multiple nodes (Figure 13.6). At each node the optical signal is converted to a radio frequency (RF) signal that is transmitted to 50 to 200 homes. The number of nodes served from the head end is limited by the available optical power. One way to increase the number of nodes served is to use a high power optical amplifier at the head end. For example amplifiers with 1W (30 dBm) of optical output power can be split into 16 or more signals. In FTTX
FIGURE 13.6 HFC network diagram Source: http://www.iec.org/online/tutorials/hfc_dwdm/topic01.asp
13.5 Access applications of low cost amplifiers
FIGURE 13.7 Keopsys V groove side pumping component and functionality
architectures the amplifier serves a similar purpose to provide the 1550 nm video signal to multiple PONs. The technology for the high power amplifiers is based on erbium ytterbium codoped fiber [21]. The addition of Yb allows the use of higher pump powers than erbium alone. A double clad geometry can be used to increase the amount of pump power that can be coupled into the fiber. A tapered fiber bundle can be used to couple multiple multimode pumps into the double clad fiber from the ends. Keopsys uses an alternative approach that allows pumping along the length of the fiber. This approach initially developed by Lew Goldberg at the Naval Research Lab couples light into the fiber through the cladding by reflecting it off a V groove in the cladding (Figure 13.7). Amplifier output powers of up to 2W are possible. These amplifiers are available from a number of vendors including IPG, Keopsys, and Emcore. For example IPG offers amplifiers with more than 1W of output power packaged with a splitter with up to 36 outputs. Keopsys offers a 32 channel unit with 17 dBm output from each channel. The cost for a 30 dBm module without an integrated splitter is less than $10,000 USD. With 32 outputs the cost per channel comes to approximately $315. This is much less than the cost of any single amplifier.
13.5.2 WDM PON WDM PONs offer a way to provide individual users increased bandwidth instead of current GPONs. In the WDM PON each user gets a dedicated wavelength. This approach can be very expensive if a fixed wavelength source is used at both the transmitter and receiver. To reduce the cost a number of architectures have been developed. These use spectrally sliced sources to generate the transmit wavelengths and injection locked Fabry-Perot lasers or reflective SOAs at the customer’s location [22]. Figure 13.8 shows the return path for a WDM PON. The transmitter is an ASE source, either a super luminescent diode or an erbium-based device. The light is combined through a circulator and transmitted to the arrayed waveguide grating (AWG) near the customers. The AWG slices the light into different wavelengths.
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RCV A W G
A W G
RCV
RSOA
RSOA
ASE
FIGURE 13.8 WDM PON return path
The light is then injected into the reflective SOA. This causes the SOA to have gain at this wavelength [23]. The reflective semiconductor optical amplifier (RSOA) is modulated, and the signal is transmitted back through the system and routed by the AWG to the correct receiver. If the receive path is at one wavelength band the transmit path must be at another wavelength band. For example, one can be C-band and the other perhaps S- or L-band. This may be an advantage for SOAs as they can be fabricated to operate in the S-band while erbium fiber cannot. More details on the use of SOAs in this application can be found in the chapter by Spiekman and Piehler. However, we will consider the ASE source. If each WDM PON serves 16 to 32 customers there will be a large number of ASE sources required for even a modest sized network. Thus the cost will be important. An ASE source based on erbiumdoped fiber is a modified version of a single stage EDFA. The main change is that the input isolator is replaced with a reflector (Figure 13.9). The reflector can be something as simple as a gold-coated fiber end. This simple design will provide an output with the same spectral shape as the erbium-doped fiber [24]. Gain flattening filters can be used if required to smooth out the spectrum. The erbium-based ASE source has many desirable properties. The ASE is almost completely unpolarized. The limit is the polarization-dependent loss of the output components. This is an important parameter in the application. In order to get light coupled into the RSOA it must be in the same polarization state as device Er Fiber
WDM
Reflector Pump
FIGURE 13.9 Erbium fiberebased ASE source
Isolator
13.6 Future directions
waveguide. Because the ASE is unpolarized some amount of light will always be in the correct polarization state. The next important parameter is that the erbium-based ASE source has very low relative intensity noise (RIN). This is an important parameter in the overall system performance. The one major limitation is that an erbium-based ASE source works best in the C-band. L-band designs require significantly more pump power and erbium fiber length, thus increasing the costs. Also, there is no S-band version. The costs for erbium-based ASE source are similar to that of the single stage amplifier. The other ASE source is a superluminescent diode (SLD). The SLD is fundamentally an SOA with a high reflector on the rear facet or a laser diode with a low reflection coating on the output facet. There are two drawbacks to the using an SLD as the ASE source: the output light is well polarized and it will have higher RIN. To obtain unpolarized light the output from two devices must be polarization multiplexed. Thus the cost of the SLD-based source will include two SLDs, a polarization multiplexer, and an optical isolator. The total cost of the assembled module is over $1,000. One advantage is that the SLD is available for almost all the wavelength bands, not just C-band.
13.6 FUTURE DIRECTIONS The future direction for optical amplifiers is a continuation of the need for smaller devices that use less power and cost less. This will encompass most of the product requirements. The disruptive technology will be the use of EDWA and PLC to increase the level of integration at almost no additional cost.
13.6.1 Single stage optical amplifiers Over the last few years there has been a shift in how transmission equipment is designed. Initially a system supplier would buy all the components and integrate them onto circuit boards. As optical amplifiers became a commodity they were sold as modules. The next step in this evolution is the supply of completed circuit packs or blades. In this model the component supplier will integrate an optical amplifier and a wavelength selective switch (WSS) on a single card. This card will include complete electronics to control the amplifier and WSS. The electronics will communicate with the system network element manager. This model puts a premium on the size and power dissipation of the optical amplifier. Another application that is driving the size and power requirement is advanced modulation schemes for 40 Gb/s and higher speeds. In these systems each channel needs an amplifier to keep the optical power at the receiver constant. For EDFAs the reduction in size will require the use of hybrid components as we have previously discussed. A current goal is an amplifier half the size of the current MSA: 45x35x12 mm. Suppliers are beginning to address this requirement. One supplier has introduced a 30x50x8 mm unit using a fiber-based design while other suppliers are introducing similar units. EDWAs may also offer a path to size
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reduction. For example, you could build an EDWA inside an XFP module. An XFP is an MSA pluggable module that is normally used to implement a transceiver. Finally, SOAs can do this today but with worse optical performance and at a higher cost. As the amplifiers are integrated into a single card power dissipation becomes an issue. This requirement will drive the use of pumps without thermoelectric coolers (TECs). These have been mini-DIL packaged devices rather than a traditional butterfly. Removing the TEC can save an entire Watt of power consumption and heat dissipation. Another option to address size and power may be to develop EDWA or SOA based arrays. These could be very compact devices. However, this approach might require a change to the system architecture in how the functions are distributed across the blades.
13.6.2 Integration From the discussion in Section 3 we see that it is possible to integrate multiple passive components along with the erbium-doped waveguides on a single PLC. An interesting example of this is a lossless bus interface module that contains over 200 passive components (Figure 13.10) [25]. This module includes an AWG, optical switches, taps, and an EDWA. The application is for routing signals in an optical bus. The concept is very similar to what is needed in a basic reconfigurable optical add drop node. We also saw that the PLC-plus fiber approach allows for integration of multiple passive and thermo-optic tunable components. A ROADM could be added to the amplifier components on the PLC. Since the erbium fiber and most likely the gain flattening filters are located off the PLC one device could be used for many different designs. This would allow the PLC to reach high volume production. This would drive fab utilization and could result in PLC chips for, say, $100. If the additional ROADM functionality does not add much area to the chip then it is effectively almost free. This approach could result in a disruption in the cost structure for what is now in a blade.
FIGURE 13.10 Highly integrated: lossless bus interface module
References
ACRONYMS ASE AWG CATV CM CW DC DIL EDFA EDWA FTTx GPON HFC MCVD MSA OVD PCB PDL PIN PLC PON R&D RF RIN ROADM RSOA SOA TDL TEC WDM
Amplified spontaneous emission Arrayed waveguide grating Cable access television Contract manufacturer Continuous wave Direct current Dual in line Erbium-doped fiber amplifier Erbium-doped waveguide amplifier Fiber to the X Gigabit passive optical network Hybrid fiber coax Modified chemical vapor deposition Multisource agreement Outside vapor deposition Printed circuit board Polarization dependent loss Positive intrinsic negative Planar lightwave circuit Passive optical network Research and development Radio frequency Relative intensity noise Reconfigurable optical add drop multiplexer Reflective semiconductor optical amplifier Semiconductor optical amplifier Temperature dependent loss Thermoelectric cooler Wavelength division multiplexed or wavelength division multiplexer
References [1] D.R. Zimmerman, L.H. Spiekman, Amplifiers for the Masses: EDFA, EDWA, and SOA Amplets for Metro and Access Applications, J. of Light. Tech. 22 (2004) 63e70. [2] U.S. patent 5,204,923. [3] J.-M.P. Delavaux, S. Granlund, O. Mizuhara, L.D. Tzeng, D. Barbier, M. Rattay, et al., Integrated optics erbium-ytterbium amplifier system in 10-Gb/s fiber transmission experiment, Photonics Technology Letters, IEEE 9 (2) (1997) 247e249.
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[4] J. Shmulovich, A. Wong, Y.H. Wong, P.C. Becker, A.J. Bruce, R. Adar, Er3þ glass waveguide amplifier at 1.5 mu m on silicon, Electronics Letters 28 (13) (1992) 1181e1182. [5] T.R. Zaman, X. Guo, R.J. Ram, Semiconductor Waveguide Isolators, J. Light. Tech. 26 (2008) 291e301. [6] CIP Technologies, SOA_L_OEC_1550 Rev. H datasheet. [7] L.H. Spiekman, A.H. Gnauck, J.M. Wiesenfeld, L.D. Garrett, WDM transmission using semiconductor optical amplifiers, Lasers and Electro-Optics Society 2000 Annual Meeting, IEEE 1, Rio Grande, Puerto Rico, 2000. November 13e16, pp. 271e272. [8] ITU-T G.694.2. [9] Nortel, Agere, Alcatel Optronics, Press release, 2002, February 13. [10] D. Barbier, R.L. Hyde, Erbium-Doped Glass Waveguide Devices, in: E.J. Murphy (Ed.), Integrated Optical Circuits and Components, Marcel Dekker Inc., 1999. [11] Y. Jaouen, L. du Mouza, D. Barbier, J.-M. Delavaux, P. Bruno, Eight-wavelength Er-Yb doped amplifier: combiner/splitter planar integrated module, Photonics Technology Letters, IEEE 11 (9) (1999) 1105e1107. [12] T. Tamir (Ed.), Integrated Optics (Topics in Applied Physics), Springer-Verlag, Berlin, 1983. [13] J. Shmulovich, A.J. Bruce, G. Lenz, P.B. Hansen, T.N. Nielsen, D.J. Muehlner, et al., Integrated planar waveguide amplifier with 15 dB net gain at 1550 nm, Optical Fiber Communication Conference and the International Conference on Integrated Optics and Optical Fiber Communication, OFC/IOOC ’99, Technical Digest, San Diego, California (1999), pp. PD42/1ePD42/3. [14] MIT Center for Integrated Photonics, Photonics and Roadmapping Spring Conference. cips.mit.edu/conference04/TWG3_Shmulovich.pdf. 2004. [15] D. Portch, R.R.A. Syms, W. Huang, Folded-Spiral EDWAs With Continuously Varying Curvature, IEEE Photon. Tech. Lett. 16 (2004) 1634e1636. [16] M.R. Lange, E. Bryant, M. Myers, J. Myers, High Gain Coefficient Phosphate Glass Fiber Amplifier, NFOEC 2003 (2003), paper 126 [17] B.-C. Hwang, S. Jiang, T. Luo, K. Seneschal, G. Sorbello, M. Morrell, F. Smektala, et al., Performance of High-Concentration Er3þ-Doped Phosphate Fiber Amplifiers, IEEE Photon. Tech. Lett. 13 (2001) 197e199. [18] B.L. Lawrence, L.P. Clow, S.E. Flint, M.C. Mendrick, T.P. Maney, J.L. Schulze, et al., Versatile waveguide design for optical amplification. Optical Fiber Communication Conference and Exhibit, Anaheim, California. OFC 2001 3 2001, pp. WDD271eWDD27-3. [19] U.S. Patents 6,236,793 and 6,384,961. [20] M. Bolshtyansky, H. Cheng, P. Colbourne, Z.-W. Dong, D. Dougherty, K.-Y. Huang, et al., Planar Waveguide Integrated EDFA, San Diego, California, OFC 2008 (2008), postdeadline paper PDP17 [21] Y.J.-G. Deiss, C.M. McIntosh, G.M. Williams, J.-M.P. Delavaux, Gain flatness of a 30 dBm tandem Er3þ-Er3þ/Yb3þ double-clad fiber amplifier for WDM transmission. Optical Fiber Communication Conference and Exhibit, Anaheim, California, OFC 2002 (2002) 249e251. [22] F. Payoux, P. Chanclou, N. Genay, WDM-PON with colorless ONUs. Optical Fiber Communication and the National Fiber Optic Engineers Conference, OFC/NFOEC 2007 (2007) Paper OTuG5.
References
[23] K.Y. Cho, Y. Takushima, Y.C. Chung, Enhanced Operating Range of WDM PON Implemented by Using Uncooled RSOAs, Photonics Technology Letters, IEEE 20 (18) (2008) 1536e1538. [24] P.F. Wysocki, M.J.F. Digonnet, B.Y. Kim, Spectral characteristics of high-power 1.5 mm broad-band superluminescent fiber sources, Photonics Technology Letters, IEEE 2 (3) (1990) 178e180. [25] A. Bruce, S. Frolov, J. Shmulovich, Highly Integrated, Lossless Bus Interface Modules for Avionic Fiber Optic Communications Networks, Avionics Fiber-Optics and Photonics, IEEE Conference September 12e14, Annapolis, Maryland (2006), pp. 60e61.
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CHAPTER
Semiconductor Optical Amplifiers for Metro and Access Networks
14
Leo Spiekman*, David Piehler** *
Alphion Corp.
**
Fields and Waves
CHAPTER OUTLINE 14.1. Introduction .............................................................................................. 14.2. The passive optical network....................................................................... 14.3. SOAs in PONs ............................................................................................ 14.3.1. Extending the PON link budget with optical amplifiers .............. 14.3.2. Why semiconductor optical amplifiers? .................................... 14.3.3. The utility of extended link-budget PONs ................................. 14.3.4. Network design with extended reach PONs ............................... 14.4. Basic properties of semiconductor optical amplifiers................................... 14.5. Features of the SOA in optical communication applications.......................... 14.6. SOAs APPLIED IN PON AMPLIFICATION ........................................................ 14.6.1. Standardization and management............................................ 14.7. Other applications of SOAs IN OPTICAL COMMUNICATION NETWORKS ........... Acronyms ........................................................................................................... References .........................................................................................................
387 388 390 390 393 394 396 397 402 404 408 409 414 415
14.1 INTRODUCTION Passive optical networks (PONs) have become a dominant optical access technology for broadband service. Businesses and residential customers are connected to the central office of their telecommunications service provider via a fiber plant that is fully passive in the field. For various economic and business reasons, service providers wish to extend the reach of these passive networks beyond what can be provided given the optical power budgets of the used components, and therefore are looking at optical amplification. This chapter discusses the semiconductor optical amplifier (SOA) as a potentially suitable technology for this purpose. The SOA, basically a semiconductor laser with frustrated feedback, is a compact and low cost amplifier, and more importantly one that provides gain in the wavelength bands where such access networks typically operate. We will discuss the properties that make these devices suitable for amplifying and extending PON networks, and we will address some topics regarding other metro/access architectures. Optically Amplified WDM Networks. DOI: 10.1016/B978-0-12-374965-9.10014-7 Copyright Ó 2011 Elsevier Inc. All rights reserved.
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14.2 THE PASSIVE OPTICAL NETWORK A telephone central office (or a cable television head end) communicates to the outside world through several connections to core or metro networks. These connections have high security, high equipment, and link redundancy, and use 2.5, 10, and 40 Gb/s data rates on multiple dense wavelength division multiplexd (DWDM) wavelengths. The same central office (CO) may offer business and residential communication services to over 10,000 customers. While the technologies used on the core side of the CO can be extended to the customer side, the economics are vastly different. The equipment required to connect each customer to the CO must be several orders of magnitude less expensive (per connection) than the equipment connecting the CO to the core or metro network. The sheer number of connections dictates that the access equipment and architectures must have simple network engineering rules and simple deployment procedures. The operating costs per connection must also be low, which requires the carrier to note the real estate and powering costs, as well as the reliability, expected lifetime and maintenance schedules of the outside plant. The combination of the above issues have led many access network providers to focus on the PON as the optimal choice for the access network [1]. Figure 14.1 illustrates the PON. The PON network uses a shared medium, in this case optical fiber. A common downstream signal travels through the fiber at a defined wavelength, lD, from the CO to all customers. At an optical splitter, this common broadcast stream is routed to each customer’s ONU. The media access control (MAC) function in each ONU sorts the data destined for the individual ONU from the whole broadcast stream. Data in the upstream travels from each ONU at a common wavelength lU. In the upstream the OLT MAC assigns each ONU a unique time slot for data transmission. This time division multiple access (TDMA) scheme eliminates the possibility of upstream data
FIGURE 14.1 Schematic diagram of a point-to-multipoint passive optical network. The passive optical splitter can be either centralized as shown or distributed (e.g., into separate splitters in the CO and the outside plant). OLT: optical line terminal (CO); ONU: optical network unit (customer premises).
14.2 The passive optical network
collisions. The broadcast downstream, TDMA upstream architecture is common to other access systems using a shared medium, including hybrid fiber-coax networks (cable modems [2]), and wireless networks (e.g. WiMAX [3]). The critical advantages of the PON1 are as follows: It is a high reliability, fully passive outside plant. There are no active electronics between the CO and the customer. Active electronics require powering, monitoring, and a serviceable location. A passive optical splitter requires no power, can be placed in a wide variety of environments (including under water), with a mean time before failure (MTBF) measured in thousands of years. Compared with copper loops, optical fiber is impervious to damage from moisture and corrosion, and requires minimal maintenance and replacement schedules. Single-mode optical fiber transmits more data over greater distances than either copper loops or coaxial cable. Fiber can support the transmission of data at rates exceeding 10 Tb/s, while complex electronics are needed to raise the data rates over copper pairs to speeds greater than several 10s of Mb/s. Fiber attenuation is measured in tenths of dBs per km, enabling transmission distances in excess of 20 km. Both copper pairs and coaxial cable have frequency-dependent attenuations that require distances to be held to less than a few kilometers, as long as active repeaters are not used. A single optical line terminal port feeds many customers. A single transmitter and receiver within the CO can be connected to over 100 customers. This significantly reduces the floor-space requirements and fiber management issues associated with a point-to-point optical network. In addition, space limitations in fiber ducts can limit the number of fibers that exit the central office. Optical fiber can support next-generation technology. Finally, the very high bandwidth capacity of optical fiber enables a carrier to overlay next-generation equipment to deploy higher bandwidth services, by only modifying the terminal equipment. PON standards from both the IEEE and ITU-T allow Gb/s-rate PONs to be seamlessly upgraded to 10 Gb/s-rate PONs using the identical outside plant with both data rates sharing the same fiber. The IEEE and ITU-T standard bodies have specified equipment that enables Gb/s data rates over passive optical networks. The first standard to be widely deployed was the broadband PON (BPON), defined by ITU-T Recommendation G.983, which supported data rates of 0.655 Gb/s in the downstream and 0.155 Gb/s in the upstream. Next the IEEE standardized an extension to the Ethernet definition (IEEE 802.3ah), which included a gigabit Ethernet PON (GE-PON) supporting (shared) data rates of 1.0 Gb/s in both directions2. The most recent PON to be widely deployed is the gigabit-enabled PON (GPON) defined by ITU-T recommendation 1
Strictly speaking, a PON can be any network without active electronics in the outside plant. An optical network of point-to-point Ethernet connections with one fiber per connection qualifies as a PON under this broad definition. We follow the common usage where such a network is called a point-to-point network, and PON refers to a point-to-multipoint network. 2 The GE-PON follows the Gb-Ethernet’s use of 8B/10B encoding, and as a result, a physical line rate of 1.25 Gb/s is required to carry a 1.0 Gb/s data stream.
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G.984 in which the 2.5 Gb/s data rate is shared in the downstream, with 1.25 Gb/s in the upstream. The GPON and the GE-PON are widely deployed and serve millions of homes as of this writing. Most notable are the NTT GE-PON deployment in Japan, and the Verizon GPON deployment in North America. Both NTT and Verizon plan to overbuild the vast majority of their existing copper access networks with PONs. Both IEEE and ITU-T organizations have worked hard to extend their standards to support higher data rates. As of the writing of this chapter, the IEEE 802.3av (10G-EPON) specification is nearly complete. The 10G-EPON specification supports the coexistence of a 10G-EPON and legacy GE-PON equipment on the same network. The ITU-T next-generation PON (which will be standardized under G.987), supports similar data rates, and coexistence on the same network as the legacy GPON equipment. Table 14.1 compares key features of the PON flavors mentioned above. As of this writing the ITU-T G.987 specifications have not reached sufficient maturity for inclusion in the table. All of the PONs mentioned above use time division multiplexing (TDM) in the downstream and TDMA in the upstream to distinguish separate CO to customer data streams. As per customer bandwidth requirements increase, the optimal technology needed for optical access will evolve. In general there is a desire to use (or re-use) the point-to-multipoint architecture over a shared passive optical network. In addition to the TDM/TDMA approach, visions of next-generation access networks include wavelength division multiplexing (WDM) PON [4], hybrid TDMCWDM (coarse wavelength division multiplexing) PON [5], optical code division multiple access (CDMA) PON [6], coherent PON [7], and RF subcarrier multiplexed PON, which can vary from the reproduction of standard cable television (CATV) signals on fiber [8] to orthogonal frequency division multiplexing (OFDM) PON [9]. The next section deals with the use of optical amplification in the access network. While the material in this section applies to TDM/TDMA PONs, optical amplification can be applied to each of the above listed PONs.
14.3 SOAs in PONs 14.3.1 Extending the PON link budget with optical amplifiers Network designers often need to create PONs with link budgets in excess to those listed in Table 14.1. There are two basic approaches to increasing the link budget. The first and simplest is to improve the performance of the PON OLTs and ONUs. Transmitter power can be increased by using higher power lasers, forward error correction (FEC) codes, or booster optical amplifiers. Receiver sensitivity can be increased by the use of improved receiver electronics, FEC codes, or optical preamplification. In an effort to share the costs of such an improvement over all subscribers, it is desirable to concentrate the improvements in the shared OLT. In addition, one can envision an improved OLT that is compatible with legacy deployed
Table 14.1 Comparison of BPON (ITU-T G.983), GE-PON (IEEE 802.3ah), GPON (ITU-T G.984), and 10G-EPON (IEEE 802.3av) characteristics. Each specification includes definitions for various link budgets. In the chart we use the PX 20 specification for the GE-PON, the class Bþ specification for the GPON, and the PX30 specification for the 10G-EPON. BPON
GE-PON
GPON
10G-EPON
Standardization Downstream line rate (Gb/s) Upstream line rate (Gb/s) Downstream wavelength (nm) Upstream wavelength (nm) Downstream laser at OLT Upstream laser at ONU Downstream receiver at ONT Upstream burstmode receiver at OLT Link budget class Link budget (dB) Fiber penalty (downstream / upstream) (dB)
ITU-T G.983 0.622
IEEE 802.3ah 1.25
ITU-T G.984 2.5
IEEE 802.3av 10
0.155
1.25
1.25
10
1480–1500
1480–1500
1480–1500
1575–1581
1260–1360
1260–1360
1290–1330
1260–1280
direct mod. DFB
direct mod. DFB
direct mod. DFB
external mod. DFB
direct mod. FP or DFB
direct mod. FP or DFB
direct mod. DFB
direct mod. DFB
PIN
PIN
APD
APD
APD
APD
APD
APD
B 25
PX20 30
Bþ 28 0.5
PX30 29
(Continued )
14.3 SOAs in PONs
Common Name
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392
Common Name
BPON
GE-PON
GPON
10G-EPON
Minimum distance range (km) OLT transmitter power range (dBm) ONU transmitter power range (dBm) OLT receiver power range (dBm) ONU receiver power range (dBm) line coding BER requirement forward error correction
20
20
20,60
0–10
0.5 to þ 4.0
þ2 to þ7
þ1.5 to þ5
5.5 to þ2
1 to þ4
þ0.5 to þ5
8 to 31.5
3 to 24
28 to 8
26.5 to 6.0
6 to 27
27 to 8
scrambled NRZ 1010 no
8B/10B 1012 no
scrambled NRZ 1010 optional
64B/66B 1012 mandatory
Notes: Line rates: ITU-T BPON and GPON recommendations support multiple line rates; only those listed have been implemented and deployed. 10G-EPON is intended to simultaneously support 1.25 Gb/s upstream (with identical properties to the legacy GE-PON upstream) in addition to the 10 Gb/s upstream. Only the 10Gb/s upstream is dealt with here. Upstream wavelength: Upstream wavelength range for GPON is the Dl ¼ 40 nm variant listed in ITU-T G.984.5. Lasers: Most common implementations are show. DFB ¼ distributed feedback laser. FP ¼ Fabry-Pe´rot laser. Receivers: The most common implementations shown. PIN ¼ positive-intrinsic-negative semiconductor photodiode. APD ¼ avalanche photodiode. Link budget class: Each specification supports multiple classes; listed classes are either the most deployed, or exemplary. The link budget and the fiber power penalties define the operating optical power ranges of the OLT and ONU transmitters and receivers.
CHAPTER 14 Semiconductor optical amplifiers
Table 14.1 Comparison of BPON (ITU-T G.983), GE-PON (IEEE 802.3ah), GPON (ITU-T G.984), and 10G-EPON (IEEE 802.3av) characteristics. Each specification includes definitions for various link budgets. In the chart we use the PX 20 specification for the GE-PON, the class Bþ specification for the GPON, and the PX30 specification for the 10G-EPON. Continued
14.3 SOAs in PONs
FIGURE 14.2 Potential locations of optical amplifiers in the passive optical network: a) mid-span amplifier, b) optical pre-amplifier, c) optical booster amplifier
ONUs. It can be shown, however, that overall PON performance is most often limited by the upstream link, and that for Gb/s-rate links, the cumulative effects of the listed techniques offer an improvement of several dBs at most in OLT receiver sensitivity. As shown in Table 14.1, most PON transmitters and receivers support links up to about 30 dB. Many applications call out for a major extension of PON link budget by 10 or 20 dB. The use of mid-span optical amplifiers can accomplish this task. In fact, the optimal positioning for a mid-span amplifier is at the splitter location. In this chapter we will consider the optical amplifier at three different locations in the PON. This is illustrated in Figure 14.2. For today’s GE-PON and GPON systems, the mid-span amplifier offers clear advantages for link extension. For next generation PON networks (including the 10 Gb/s-rate PONs) there are good reasons to consider booster amplifiers and pre-amplifiers at the OLT in addition to in-line amplifiers.
14.3.2 Why semiconductor optical amplifiers? There are several compelling reasons to use semiconductor optical amplifiers for PON extension. The most obvious one is that commercial SOAs exist at each of the upstream and downstream wavelengths defined in the various PON standards. In addition, since SOAs are fabricated out of semiconductor material structures identical to Fabry-Pe´rot (FP) or Distributed feedback (DFB) lasers, the availability of laser sources at a particular wavelength certainly indicates that an SOA for the corresponding wavelength can be fabricated. Even if erbium-doped fiber amplifiers (EDFAs) are available at the desired wavelength, SOAs often represent a better choice from both economic and technical perspectives. From an economic standpoint, we believe that SOAs offer advantages over EDFAs in both capital and operating expenditures. At the time this chapter is written, one can purchase “gain-equivalent” SOAs and EDFAs in low quantities for similar prices. The EDFA price is based on the high-volume market and the volume cost-reductions associated with the EDFAs’ ubiquitous presence in modern optical networks. The SOA, on the other hand, has not yet experienced such wide deployments. One would expect the SOA price to be very elastic with respect to the demand for use in access networks, compared with EDFAs.
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When DWDM systems were first introduced, the SOA was at a disadvantage relative to high end EDFAs, due to its modest specifications for noise figure, output power, and polarization-dependent gain. Another drawback for long-haul DWDM applications was the fast gain dynamics in SOAs, which resulted in gain modulation by high bit rate signals giving rise to intersymbol interference and cross channel intermodulation. The requirements for these parameters are less extreme in the access network, and nobody is willing to pay a premium to obtain high-end EDFA performance or even specification-similar EDFA performance, which, as argued, will come at a premium in high volumes. In addition, with respect to operational economics, the electrically pumped SOA has better wall-plug efficiency compared with the semiconductor laser-pumped erbium-doped fiber amplifier. And given that line card space is often tight, the smaller footprint of the SOA is also a major advantage. There are other technical advantages of SOAs over EDFAs that are are based on operating wavelength and the population inversion dynamics internal to the amplifier. Not only are SOAs available over a wider range of wavelengths, but they also have a broader operating wavelength range. In fact, SOAs have been shown to simultaneously amplify signals over a range of 140 nm [10]. As we will detail in a later part of the chapter, the excited state lifetime in an SOA is on the order of 200 ps, while that of the active Er3þ ion the fiber amplifier is on the order of 10 ms. This makes the EDFA unsuitable for burst-mode operation, unless complex transient gain control electronics are included. The SOA gain, on the other hand, will equilibrate in less than one bit-period after it passes a multibit data packet, making it immediately ready to process another data packet with any arbitrary delay. (The same will be true for long-haul packetebased networks. When and if these come about, the use of SOAs might be reconsidered, although complex transient control is more affordable in the long haul.)
14.3.3 The utility of extended link-budget PONs There are a number of reasons that a service provider may want to entend the link budget of a PON, with an active device such as the bi-directional semiconductor optical amplifier indicated by a) in Figure 14.2. Later sections in this chaper will demonstrate the technical feasibility. The economic feasibility of remote amplification in general needs to be compared with the overall costs of alternative solutions for providing service to a given customer set. Alternative solutions include deploying shorter-reach PONs from closer central offices, deploying cabinethardned remote OLTs, or in some cases optical-to-electrical-to-optical (OEO) repeaters. In addition to the product’s initial cost, the energy consumption, and other operating expenses (which include installation), maintenance and real estate must also be considered. These factors are dealt with in quantitative detail by Rand-Nash, et al. [11] and by Vaughn, et al. [12]. In the remainder of this section we give details of the attractive features of PON link extension from a network design perspecitive. Figure 14.3 shows several scenarios in which extending the link budget of a passive network offers benefit. Many of the most interesting network designs use
14.3 SOAs in PONs
FIGURE 14.3 Various applications for PON reach extension using a mid-span PON extender
link extenders to enable either the closing of existing central offices, or not building new central offices. In some rural deployments there may be small customer clusters too far from any existing central office to be reached with a standard PON. Without a PON reach extension technology, the carrier may need to build a new central office or rent space to deploy a new PON OLT. A reach-extension device, placed in a cabinet
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FIGURE 14.4 Calculated central office reduction in Brittany enabled by improving a GPON link budget from 28 dB to 54 dB [13]. Each circle in a) represents a France Telecom central office. By deploying class Bþ GPON OLTs in each of the 820 central offices, 100% of customers can be reached. In b) class Bþ OLTs are deployed only in the 45 indicated central offices. PON reach extenders deployed as shown enable 45 central offices to serve 100% of the customer premises.
or mounted on a pole, saves real estate or construction costs. In many telephone networks, the location of central offices is based on the copper loop length limitations of a decades-old analog telephone network. The use of PON extension systems allow the carrier to centralize PON OLT locations, and slate other central offices for closure in the future as fiber replaces copper. For example, France Telecom has analyzed the situation in Britanny on the west coast of France (Figure 14.4). By using standard PON technology, it can reach 100% of its existing customers with class Bþ GPONs located in each of the 840 central offices located in the region. Using reach extension technology would allow the location of GPON OLTs into only 45 central offices throughout the region and replace existing central offices with remote extenders located either in remote cabinets or in leased office space.
14.3.4 Network design with extended reach PONs The optical link budget between the OLT and its subscriber ONUs is perhaps the most critical property of a PON. The optical link budget sets a hard limit on the distance the PON can cover, and on the optical splitting ratio, which sets a limit on the number of subscribers per OLT. Roughly speaking, the insertion loss (in dB) of a 1 N passive optical splitter is 10 log10 N þ 1/2 log2 N. In the PONs defined in Table 14.1, the upstream wavelengths are all within the O-band (1260 to 1360 nm) and experience a higher loss than the downstream wavelength. The intrinsic fiber loss of most commercial single mode (ITU-T G.652) fiber is about 0.32 dB/km in the O-band. Most network providers will bundle in various other losses (splices,
14.4 Basic properties of semiconductor optical amplifiers
Table 14.2 The total link budgets are estimated for various combinations of fiber distance and optical split ratios using assumptions given in the text. Shaded cells in the upper-left quadrant represent PON configurations with total loss budgets 28 dB. The unshaded cells in the center represent additional PON configurations with total loss budgets 32 dB. With the assumption (which we will justify in a later section) that technology exists to increase the total link budget to 49 dB, all PON configurations except those represented by unshaded cells in the lower-right corner are permissible. distance (km) 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70
loss (dB) 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28
split ratio loss (dB)
1×4 7
1×8 10.5
1 × 16 14
1 × 32 17.5
1 × 64 21
1 × 128 24.5
1 × 256 28
7 9 11 13 15 17 19 21 23 25 27 29 31 33 35
10.5 12.5 14.5 16.5 18.5 20.5 22.5 24.5 26.5 28.5 30.5 32.5 34.5 36.5 38.5
14 16 18 20 22 24 26 28 30 32 34 36 38 40 42
17.5 19.5 21.5 23.5 25.5 27.5 29.5 31.5 33.5 35.5 37.5 39.5 41.5 43.5 45.5
21 23 25 27 29 31 33 35 37 39 41 43 45 47 49
24.5 26.5 28.5 30.5 32.5 34.5 36.5 38.5 40.5 42.5 44.5 46.5 48.5 50.5 52.5
28 30 32 34 36 38 40 42 44 46 48 50 52 54 56
connectors, etc.) as well as a design margin and use a value of 0.40 dB/km. These assumptions allow the construction of the matrix in Table 14.2 below, which assigns a total link budget to various PON configurations. From Table 14.2 we see that a class Bþ PON (28 dB link budget) will support the optical splitter/distance combinations shaded in the upper left quadrant. While improvement in OLT technology to support a 32 dB link budget is possible, it enables only the extra design choices represented by the unshaded boxes forming a diagonal across the center. The use of a mid-span link, extender, however moves the vast majority of design scenarios to become possible.
14.4 BASIC PROPERTIES OF SEMICONDUCTOR OPTICAL AMPLIFIERS We have argued that the SOA is the technology that enables these design scenarios consisting of long transmission distance and/or high split ratio, and has several properties that make it eminently suitable for use in PON amplification. One of the
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FIGURE 14.5 a) An indium phosphide wafer containing about 2000 SOA chips, b) a single SOA chip, c) a pigtailed device in an industry standard butterfly package
fundamental features of the semiconductor optical amplifier is that its manufacturing process is a wafer-scale technology. The SOA is manufactured on a semiconductor substrate, with many chips on a wafer. Figure 14.5a) shows a typical SOA wafer, fabricated on a 2-inch indium phosphide (InP) substrate. The wafer contains about 2,000 devices. A single chip (Figure 14.5b)) is smaller than a grain of sand, comparable in size to a semiconductor laser chip, with which it shares most of its fabrication technology. The main feature in the figure is the metal pad used to inject the operating current. The devices are pigtailed and assembled into packages like the one shown in Figure 14.5c), ready for use in a module or subsystem. A feature that is not very well visible in Figure 14.5b) but is essential for the operation of the SOA is the optical waveguide that runs along the entire length of the chip. It is in this waveguide that the physical processes creating optical gain take place: recombination of carriers and stimulated emission. To facilitate these processes, both the optical mode and the electrical current are tightly confined to the same space.
14.4 Basic properties of semiconductor optical amplifiers
FIGURE 14.6 Confinement of carriers and photons in a double heterostructure waveguide consisting of a lower band gap, higher refractive index material (the active layer), sandwiched between cladding layers of higher band gap material with a lower index
This is illustrated in Figure 14.6. An optical waveguide of higher refractive index than its surroundings confines the light to the active layer. The carriers are confined by the fact that this layer has a lower band gap than the surroundings, and by appropriately applying p- and n-doping to the cladding layers, a diode structure is formed in which excited states are created by injecting a forward current. A population inversion is created where the conduction and valence bands of the semiconductor material are filled with free electrons and holes, respectively. The band gap determines the energy by which these bands are separated, and therefore the minimum energy associated with an interband transition. Such transitions can occur spontaneously or can be stimulated. In the first case, a free electron in the conduction band recombines with a hole in the valence band, emitting a photon of a wavelength corresponding to the transition energy. This event is called spontaneous emission, and it happens continuously in any type of optical amplifier based on an excited medium, giving rise to optical noise. Stimulated transitions occur under the influence of a photon traveling through the inverted medium. In this case, recombination of an electron-hole pair causes the
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FIGURE 14.7 a) Typical amplified spontaneous emission spectrum of a traveling wave SOA; b) ASE ripple caused by residual reflections from the SOA chip facets
stimulated emission of a photon with identical wavelength, phase, and state of polarization. This is the process that is responsible for the optical gain in the medium. Spontaneous emission occurs in a wavelength range determined by the band gap of the medium and the energy level to which the bands are filled with free carriers. Photons created by spontaneous emission get further amplified while traveling down the optical waveguide. The combination of spontaneous emission and gain gives rise to the amplified spontaneous emission (ASE) spectrum of the device, an example of which is shown in Figure 14.7a). In a perfectly reflection-free amplifier, the ASE spectrum is fully determined by the band structure of the semiconductor. However, in practice, reflections from the chip facets give rise to a residual round-trip component in the amplified light, which leads to minima and maxima in the spectrum [14]. This can be seen in Figure 14.7b), which is an enlarged part of Figure 14.7a). Reflections are usually minimized by having the gain stripe running at an angle, a few degrees off normal with respect to the chip facets, and by applying anti-reflection coatings to the facets. Combining these methods, overall reflectivity values approaching 106 are possible. Stimulated emission amplifies not only the noise, but also optical signals coupled into the amplifier; this is after all the effect that we are after. Figure 14.8 shows the gain spectrum of a typical SOA. The shape of the gain spectrum closely follows the shape of the ASE spectrum, because both are brought about by the same mechanism. The 3-dB gain bandwidth typically is 60 to 100 nm. The absolute value of the gain depends on the thickness of the active layer (more precisely, on the confinement factor that describes the fraction of the guided modal
14.4 Basic properties of semiconductor optical amplifiers
FIGURE 14.8 Gain vs. wavelength in a SOA with low polarization dependence
power that resides in the active layer), and can be tuned for a given waveguide design by choosing the chip length and proportionally scaling the injection current. The gain is limited only by the facet reflectivity, and can reach values as high as 36 dB. It must be noted that since the shape of the gain medium is not invariant for a rotation of 90 about the propagation axis, as it is in an erbium-doped fiber, gain is not automatically polarization independent. Special measures have to be taken in order to insure that the polarization-dependent gain (PDG) is as small as possible. One method, albeit not very practical from the fabrication perspective, is to design a square active layer [15]. A much more commonly used method is to introduce crystal strain in the quantum wells [16] or in the bulk active layer [17]. This modifies the band structure in such a way that gain in one of the polarization directions is enhanced over the other one. The application of tensile strain enhances the material gain for the transverse-magnetic (TM) mode, the modal gain of which would be reduced in an unstrained structure. Figure 14.8 shows that judicious application of an appropriate amount of strain can make the SOA gain close to polarization independent. A PDG less than 0.5 dB can be obtained for most commercial SOAs. The SOA amplifies incoming signals, but at the same time adds a background of ASE noise, and therefore deteriorates the optical signal-to-noise ratio (OSNR) at the output. The amount of deterioration is described by the noise figure (NF) of the amplifier, which in an optically amplified system dominated by signal-spontaneous emission beat noise, can be written as a constant times the noise power spectral density at the wavelength of interest, divided by the gain [18]. The theoretical minimum noise figure for an optical amplifier is 3 dB, and the NF for practical SOAs is in the 6 to 8 dB range.
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14.5 FEATURES OF THE SOA IN OPTICAL COMMUNICATION APPLICATIONS Most SOAs designed for application in optical communication systems are fabricated using the InGaAsP / InP material system. InxGa1-xAsyP1-y is a quaternary III/V semiconductor, in which choosing the element ratios x and y provides two degrees of freedom. By appropriately parametrizing x and y, the first degree of freedom keeps the band gap of the semiconductor constant, and is used for setting the crystal lattice parameter for the introduction of tensile strain as discussed previously in the context of PDG. The other dimension, while keeping the lattice parameter constant, allows tuning the band gap over a large range. A SOA fabricated in InGaAsP can exhibit a gain peak in the range 1200 to 1650 nm. This is shown in Figure 14.9, where ASE spectra of devices of different design have been plotted together. This is an extremely useful feature for the access networks, where as we have seen the downstream and upstream signal wavelengths are around 1490 and 1310 nm, respectively. SOAs can be designed to provide gain at each of these wavelengths. The gain of an SOA can be very high, but the amount of output power is limited by gain saturation: When free carriers are depleted, the gain will be compressed. This process is shown in Figure 14.10, where the gain of an SOA is plotted against its output powerda common depiction that enables visualization of the small-signal gain value of the devices as well as its saturation output power. Figure 14.10 is a steady-state picture, measured on a time scale that is extremely long compared with the characteristic times associated with the dynamics of the
FIGURE 14.9 ASE spectra of different SOA chips with widely different gains and peak wavelengths in O-, S-, and C-band. Changing the composition of the InGaAsP active layer can set the gain peak anywhere from 1200 to 1650 nm.
14.5 Features of the SOA in optical communication applications
FIGURE 14.10 Saturation curve of an SOA. At small powers, the gain approaches the value of the smallsignal gain Gss. The output power corresponding to a gain compression of a factor of two is the 3 dB saturation output power P3dB.
SOA. These characteristics can be revealed by compressing the gain of a device using a short pulse, and observing the gain on a streak camera using a probe wavlength. This has been done in Figure 14.11: When a strong pump pulse arrives, large numbers of free carriers are swept out of the active region, and the gain gets compressed immediately. The gain recovery, on the other hand, takes much longer; 25 to 250 ps is typical, dependent on the design of the active layer and the magnitude
FIGURE 14.11 a) Gain recovery experiment in which an intense pulse (the pump) compresses the gain of a SOA, which is measured by a weak probe beam; b) gain compression and recovery curve. A typical gain recovery time of 140 ps is observed.
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of the injection current. Note that these dynamics are eight orders of magnitude faster than those associated with the gain recovery of the erbium ions in an erbiumdoped fiber amplifier, which occurs over w10 ms. The fast carrier dynamics of the SOA allow the device to be directly modulated by means of the injection current, which allows the application of the device as a fast switch or as an intensity modulator. The modulation speed is usually limited by electrical parasitics in the chip and the package as opposed to the carrier dynamics. The effect of gain saturation and its dynamics can be used for high-speed alloptical processing of signals [19], as will be seen in the next section. However, for linear amplification of optical wavelengths carrying data, saturation usually has to be avoided. This can be accomplished by limiting operation to the range of output powers where gain compression is small (less than approximately 1 dB, the so-called linear regime). Alternatively, gain clamping schemes can be devised in which lasing action is introduced into the device in a controlled way, fixing the gain to a value corresponding to the round-trip loss of the clamping laser. A final important property of the SOA is its ability to be monolithically integrated with other optical circuit elements to yield devices with significantly higher levels of functionality. Integration with passive waveguides and passive waveguide devices, phase shifters, and distributed Bragg reflectors has yielded devices such as tunable lasers, multi-wavelength lasers, and wavelength converters.
14.6 SOAs APPLIED IN PON AMPLIFICATION For the purpose of PON extension, we are going to concentrate on the in-line architecture of Figure 14.12. A PON extension module containing two SOAs is
Optical Trunk Line (OTL)
GPON OLT
Optical Distribution Network (ODN)
1490 1310
PON Extension Module
GPON ONT GPON ONT GPON ONT GPON ONT GPON ONT GPON ONT GPON ONT GPON ONT GPON ONT
FIGURE 14.12 Amplified PON using an extension module located at the site of the passive splitter. The module consists of a downstream amplifier in the 1490 nm band, and an upstream amplifier at 1310 nm, plus management and control electronics.
14.6 SOAs applied in PON amplification
placed right in front of the passive splitter in the field. This introduces active elements in the field-installed part of the access network, requiring the supply of power and a (small) containing space, but this is a price network operators are willing to pay in view of the considerable capital and operational cost savings brought about by having to maintain a smaller number of central offices. The task of the SOAs is to supply the access network with additional power margin to either or both allow for extended transmission reach (a longer optical trunk line (OTL in Figure 14.12), and allow for a larger split ratio in the optical distribution network (ODN). The transmission line can be considered in a simplified way as a single amplifier between a transmitter and a receiver, with optical loss on either side. (See Figure 14.13a).) In the downstream direction, the loss in front of the SOA (L1) is the fiber transmission loss of the OTL, and the loss after the SOA (L2) is the sum of the splitting loss and the fiber transmission loss in the ODN. For the upstream SOA, the situation is reversed.
(a) L1
Tx
(b)
L2 (dB) limited by SOA power power amp
in -li ne
with SOA
Psat - PRx
limited by SOA gain
PTx - PSOA+Rx
pre amp
L1+L2 = L
Rx
PTx = Tx optical power PSOA+Rx = sensitivity of SOA+Rx PRx = sensitivity of Rx Psat = max output power of SOA G = gain of SOA L = PTx – PRx = link budget without SOA L1, L2 = optical losses
L1+L2 = G + L
am p
no SOA
L2
SOA
limited by SOA noise figure and optical filter width L1 (dB)
FIGURE 14.13 a) Simplified view of an amplified access network in each of the signal propagation directions. L1 and L2 are losses in the OTL and the ODN and vice versa, in the downstream and upstream directions, respectively. b) Diagram showing the ways the properties of the SOA limit the power budget of the overal system. L2 is limited by the output power of the SOA, while L1 is limited by its noise figure. Their sum is limited by the SOA gain.
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The resulting power budget limitations are illustrated qualitatively in the diagram in Figure 14.13b). If the loss L1 in front of the SOA dominates, the device acts essentially as a pre-amplifier. In this case, system performance is limited by its noise figure. The transmitted power PTx must, after experiencing the loss L1, stay above the sensitivity presented by the combination of the SOA and the receiver, indicated as PSOAþRx in the figure. If, on the other hand, the loss after the SOA dominates, the amplifier in essence plays the role of a booster. In this case, the limiting factor is the maximum output power of the SOA, indicated as Psat because the limitation is presented by the non-linearities introduced by the saturation of the amplifier. The loss L2 is bounded by this power minus the sensitivity of the receiver PRx. If L1 and L2 are reasonably balanced, the PON link budget without SOA (equal to PTx e PRx) is enhanced by an amount corresponding to the gain G of the SOA. As the diagram shows, this is the case in which the increase of the link budget is largest. Since the loss of the passive splitter in a PON is significant for non-trivial split ratios, this situation can be approached by placing the PON extender right in front of this splitter. The overall link budget can be increased by simply using higher gain amplifiers, limited only by their Psat and NF. These bounds of operation are demonstrated in practice in Figure 14.14, which shows the 10-9 bit error rate contours measured for appropriate values of the losses before and after the SOA. The three regimes of limitation by the SOA noise figure, its gain, and its maximum output power, are easily distinguished. Considering both downstream and upstream transmission, and taking into account realistic amplifier properties as well as the operating power ranges at the transmitter and the receiver, a range of OTL and ODN losses can be given as indicated in Figure 14.15a) trunk distance (km) = L2 / (0.4 dB/km)
406
distribution loss = L1 (dB)
FIGURE 14.14 Bit error rate (BER) contour diagrams, for three different SOAs, measured for varying losses L1 and L2 as indicated in Figure 14.13a). The contours correspond to a BER of 109 of a PRBS of length 2311, transmitted at a bit rate of 2.5 Gb/s [20].
14.6 SOAs applied in PON amplification
(a)
40 35 30
OTL (dB)
25 20 U
15
D 10 5 0 0
5
10
15
20
25
30
35
40
ODN (dB)
(b)
120
100 D
OTL (km)
80
60
40 U 20
0 0
5
10
15
20
25
30
35
40
ODN (dB)
FIGURE 14.15 Operating regions for downstream (D) and upstream (U) amplified PON transmission using typical amplifiers, with loss in OTL and ODN expressed in dB a), or with OTL reach specified in km b). The overlap of the upstream and downstream regions represents the area in which an amplified PON can realistically be operated.
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(a)
SOA Conditions Pin = 0 dBm Gss = 24 dB Psat-dc = +10 dBm
1.25 Gb/s PRBS
1310 nm DFB
SOA (α)
(b)
out of transmitter (α)
without filter
with filter
60 km fiber
Rx (β)
(γ)
after SOA (β)
without filter
oscilloscope
with filter
after SOA + 60 km fiber (γ)
without filter
with filter
FIGURE 14.16 a) Experimental setup simulating upstream transmission in an amplified GPON using a directly modulated DFB laser; b) Eye diagrams after the transmitter, after the SOA, and before the receiver. The directly modulated laser exhibits significant overshoots at the 01 transitions, which are only visible in the optical eye diagram, not after optimal filtering in the electrical domain.
over which a PON can be operated using a PON extension module. Since the fiber transmission loss is different at 1490 nm (downstream) and at 1310 nm (upstream), the areas for upstream and downstream operation are offset as shown in Figure 14.15b). The intersection of both areas represents the design space available for such an amplified PON. Among other things, it is possible to extend the trunk reach to 60 km and the split ratio to 1:128. This is significant, because these numbers represent the logical limits of GPON, determined by the GPON MAC and the protocol field sizes. The bounds in Figure 14.15 on the lower end of the loss ranges are determined by the saturation behavior of the SOA, which sets an upper limit to the input power that may be fed into the amplifier. The non-ideal eye shape of the signal after the directly modulated lasers, typically used in a PON, with large overshoots at the 0-1 transitions, may augment the problem, because the SOA with its 100-ps carrier lifetime is fast enough to resolve these overshoots. (See Figure 14.16.) However, proper filtering in the electrical domain shows that very little distortion occurs even with the SOA in heavy saturation. The amplifier being much faster than the bit rate, the SOA can be used as a limiting amplifier in the upstream direction, boosting packets from weak (far away) ONTs by a larger amount than packets from a stronger, closer by ONT.
14.6.1 Standardization and management The ITU has issued a recommendation describing the architecture and parameters of GPON systems with extended reach [21]. Both regenerators and optical amplifiers
14.7 Other applications of SOAs in optical communication networks
are allowed by the recommendation. A maximum reach of 60 km, with loss budgets of over 27 dB in both the OTL and the ODN are foreseen. The recommendation requires that the PON extension module be fully manageable in terms of its configuration, performance monitoring, and fault reporting. This can be accomplished by including an embedded ONT in the module, which makes the PON extender addressable as one of the ONTs in the PON.
14.7 OTHER APPLICATIONS OF SOAs IN OPTICAL COMMUNICATION NETWORKS Gain compression and recovery at a time scale comparable to the bit rate in on-off keying (OOK) digital transmission leads to inter-symbol interference (ISI) in single channel transmission and inter-channel crosstalk in multichannel wavelength division multiplexing (WDM) transmission. However, operating the SOAs in the linear regime enables transmission with capacity-distance products that are significant for metro or access network environments. As an example, Figure 14.17 shows eight dense WDM channels (spaced 200 GHz) modulated at 10 Gb/s, transmitted 240 km over multiple fiber spans in a transmission system in which all the optical amplifiers are SOAs [22]. All channels are received essentially error-free, with bit error rate penalties of around 1 dB with respect to a back-to-back measurement. Due to wide band gain of the SOA, the device lends itself to amplification of coarse WDM systems, in which channel spacings are kept wide (20 nm) to allow for the use of uncooled lasers and cheap filters. The common 80-nm bandwidth of a typical SOA permits amplification of a band of four CWDM channels, while the design freedom of the gain peak wavelength allows the gain band to be suitably adjusted to cover the entire CWDM band. For systems with limited margin requirement, it is feasible to amplify a band of eight contiguous CWDM channels, as depicted in Figure 14.18 [10].
FIGURE 14.17 Transmission of eight WDM channels modulated at 10 Gb/s across 240 km of standard fiber, amplified using SOAs. Transmission penalty is around 1 dB for all channels, without error floors.
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FIGURE 14.18 Input a) and output b) spectra, showing an eight-channel CWDM grid amplified by a single SOA. This particular experiment demonstrates an upgrade in one of the CWDM channels with a combination of eight DWDM channels.
A transmission link with amplifiers working in the linear regime is very suitable for handling dynamic loadingevarying number of channels or packet or burst data, as in the upstream direction of a PON; it does not suffer from transients often experienced in links using EDFAs operating in saturation, and can be used without any transient control. This has been demonstrated using a 16-channel link, in which
14.7 Other applications of SOAs in optical communication networks
FIGURE 14.19 a) Extinction ratio of a SOA used as a gating switch. Shown are three different injection currents. In the “off” state, the signal can not be discerned in the noise, demonstrating an extinction ratio in this case better than 80 dB. b) Switching speed. Horizontal scale 1 ns / div.
eight channels were dynamically added and dropped [23]. The CWDM link shown above in Figure 14.18, in which one of the CWDM channels was upgraded to a comb of eight DWDM channels, is an example of in-service upgrade, without disruption to the other seven CWDM channels. Modulating the SOA injection current allows tuning of the gain of the device, and setting the current to zero makes the SOA strongly absorbing [24]. This allows the device to be used as a gating switch with extremely high (60 dB or more) extinction ratio. (See Figure 14.19a).) Switching speed is limited by the capacitance of the chip (the metallization pads on the chip, or its p- and n-doped contact layers, form a capacitor that can be approximately the size of the chipdsee Figure 14.5b)dor by the inductance of bond wires inside the package. A standard SOA not optimized for high speed operation accomplishes one to two ns switching speed
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FIGURE 14.20 Non-blocking 3x3 switching matrix built up from SOA gate switches
(see Figure 14.19b)), which is suitable for packet switching; an optimized structure can provide an order of magnitude faster switching. By integrating SOAs with passive waveguides, non-blocking switching matrices can be built. (See Figure 14.20.)
FIGURE 14.21 a) Mach-Zehnder interferometer with a SOA in each arm, which can be used as a wavelength converter or as a 2-R regenerator; b) interferometric switching curve of a MZI. Because of its non-linear shape, extinction ratio from input to converted output can be improved.
14.7 Other applications of SOAs in optical communication networks
A sufficiently fast SOA switch can be used as an OOK optical modulator. In a WDM-PON, when supplied with a cw wavelength from the head end, such a device can be applied as a wavelength-agnostic transmitter for the upstream data. For this purpose, a reflective SOA is used: a device that has only one facet pigtailed with a fiber that serves as both the input and the output, while the other facet reflects the signal for a second pass through the device [25]. Monolithic integration of SOAs with passive waveguides and other elements gives rise to devices with functionalities such as wavelength conversion [26] and 2-R regeneration (see Figure 14.21), optical time division multiplexing [27], tunable and wavelength selectable lasers [28], and even entire WDM transmitters [29]. (See Figure 14.22.) Tunable lasers and integrated WDM transmitters have been and are being commercialized, but the rise of SOA applications in all-optical signal processing is lagging, despite a strong technical push around 10 years ago. The most important factor seems to be that continuous and strong development in high-speed electronics has thus far rendered all-optical techniques unnecessary. However, as electronic processing speed goes up, so does power consumption. Therefore, in the end, it may not be the unavailability of electronics at ultra-high speeds, but rather their growing hunger for power that will prompt the introduction of all-optical techniques.
FIGURE 14.22 Integrated DWDM transmitter
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ACRONYMS ASE BER BPON CATV CO CDMA CWDM DFB DWDM EDFA FEC FP GE-PON GPON ISI MAC MTBF NF ODN OEO OFDM OLT ONT ONU OOK OSNR OTL PDG PON SOA TDM TDMA TM WDM
Amplified spontaneous emission Bit error rate Broadband passive optical network Cable television Central office Code division multiple access Coarse wavelength division multiplex Distributed feedback Dense wavelength division multiplex Erbium-doped fiber amplifiers Forward error correction Fabry-Pe´rot Gigabit Ethernet passive optical network Gigabit-enabled passive optical network Inter-symbol interference Media access control Mean time before failure Noise figure Optical distribution network Optical-to-electrical-to-optical Orthogonal frequency division multiplexing Optical line terminal Optical network terminal Optical network unit On-off keying Optical signal-to-noise ratio Optical trunk line Polarization dependent gain Passive optical network Semiconductor optical amplifier Time division multiplexing Time division multiple access Transverse magnetic Wavelength division multiplexing
References
References [1] M. Abrams, P.C. Becker, Y. Fujimoto, V. O’Byrne, D. Piehler, FTTP Deployments in the United States and Japan-Equipment Choices and Service Provider Imperatives, J. Lightwave Technol. 23 (1) (2005) 236e246. [2] CableLabs, Data Over Cable Interface Specification DOCSIS 3.0 http://www. cablemodem.com/specifications/specifications30.html. [3] IEEE, IEEE 802.16: IEEE Standard for Local and Metropolitan Area NetworksdPart 16: Air Interface for Broadband Wireless Access SystemsdAdvanced Air Interface. [4] W.R. Lee, M.Y. Park, S.H. Cho, J. Lee, C. Kim, G. Jeong, et al., Bidirectional WDMPON based on gain-saturated reflective semiconductor optical amplifiers, IEEE Photon, Technol. Lett. 17 (11) (2005) 2460e2462. [5] P.P. Iannone, H.H. Lee, K.C. Reichmann, X. Zhou, M. Du, B. Pa´lsdo´ttir, et al., Four Extended-Reach TDM PONs Sharing a Bidirectional Hybrid CWDM Amplifier, J. Lightwave Technol. 26 (1) (2008) 138e143. [6] G.C. Gupta, M. Kashima, H. Iwamura, H. Tamai, T. Ushikubo, T. Kamijoh, A Simple One-System Solution COF-PON for Metro/Access Networks, J. Lightwave Technol. 25 (1) (2007) 193e200. [7] S. Narikawa, H. Sanjoh, N. Sakurai, K. Kumozaki, T. Imai, Coherent WDM-PON using directly modulated local laser for simple heterodyne transceiver. 31st European Conference on Optical Communication: ECOC 2005 3, 449e450, September 25e29, 2005. [8] G.C. Wilson, T.H. Wood, J.A. Stiles, R.D. Feldman, J.-M.P. Delavaux, T.H. Daugherty, et al., FiberVista: An FTTH or FTTC system delivering broadband data and CATV services, Bell Labs Tech. J. 4 (1) (1999) 300e322. [9] T.N. Duong, N. Genay, M. Ouzzif, J. Le Masson, B. Charbonnier, P. Chanclou, et al., Adaptive Loading Algorithm Implemented in AMOOFDM for NG-PON System Integrating Cost-Effective and Low-Bandwidth Optical Devices, IEEE Photon. Technol. Lett. 21 (12) (2009) 790e792. [10] Iannone, P.P., Reichmann, K.C., Spiekman, L., 2003. In-service upgrade of an amplified 130-km metro CWDM transmission system using a single LOA with 140-nm bandwidth. Optical Fiber Communication Conference Atlanta, Georgia, USA, March 23e28, 2003. Optical Society of America, Paper ThQ3, 548e549. [11] Rand-Nash, T., Roth, R., Ram, R., Kirchain, R., 2008. Characterizing the CapEx and OpEx Tradeoffs in Next Generation Fiber-to-the-Home Networks. Optical Fiber communication/National Fiber Optic Engineers Conference 2008, San Diego, California, USA, February 24, 2008. Optical Society of America, Paper NthD2. [12] M.D. Vaughn, D. Kozischek, D. Meis, A. Boskovic, R.E. Wagner, Value of reach-andsplit ratio increase in FTTH access networks, J. Lightwave Technol. 22 (11) (2004) 2617e2622. [13] Chanclou, P., Belfqih, Z., Charbonnier, B., Duong, T., Frank, F., Genay, N., et al., 2008. Optical access evolutions and their impact on the metropolitan and home networks. 34th European Conference on Optical CommunicationdECOC 2008. September 21e25, 2008. Paper We.3.F.1. [14] M.J. O’Mahony, Semiconductor laser optical amplifiers for use in future fiber systems, J. Lightwave Technol 6 (4) (1988) 531e544.
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[15] P. Doussie`re, P. Garabedian, C. Graver, D. Bonnevie, T. Fillion, E. Derouin, et al., 1.55 mm polarisation in-dependent semiconductor optical amplifier with 25 dB fiber to fiber gain, IEEE Photon. Technol. Lett. 6 (2) (1994) 170e172. [16] K. Magari, M. Okamoto, Y. Noguchi, 1.55 mm polarization-insensitive high- gain tensile-strained-barrier MQW optical amplifier, IEEE Photon. Technol. Lett. 3 (11) (1991) 998e1000. [17] J.Y. Emery, T. Ducellier, M. Bachmann, P. Doussie`re, F. Pommereau, R. Ngo, et al., High performance 1.55 mm polarisation-insensitive semiconductor optical amplifier based on low-tensile- strained bulk GaInAsP, Electron. Lett. 33 (12) (1997) 1083e1084. [18] N.A. Olsson, Lightwave systems with optical amplifiers, J. Lightwave Technol 7 (7) (1989) 1071e1082. [19] J.M. Wiesenfeld, Gain dynamics and associated nonlinearities in semiconductor optical amplifiers, Int. J. High-Speed Electronics and Systems 7 (1) (1996) 179e222. [20] Spiekman, L., Piehler, D., Iannone, P., Reichmann, K., Lee, H.-H., 2007. Semiconductor Optical Amplifiers for FTTx. In: Proceedings of the International Conference on Transparent Optical Networks (ICTON 2007), July 1e5, 2, pp. 48e50. [21] International Telecommunication Union, 2008. ITU-T Recommendation G.984.6: Gigabit-capable Passive Optical Networks (GPON): Reach extension. [22] L.H. Spiekman, J.M. Wiesenfeld, A.H. Gnauck, L.D. Garrett, G.N. van den Hoven, T. van Dongen, et al., 8 10 Gb/s DWDM transmission over 240 km of standard fiber using a cascade of semiconductor optical amplifiers, IEEE Photon. Technol. Lett. 12 (8) (2000) 1082e1084. [23] Gnauck, A.H., Spiekman, L.H., Wiesenfeld, J.M., Garrett, L.D., 2000. Dynamic add/drop of 8-of-16 10 Gb/s channels in 440 km semiconductor-optical-amplifier-based WDM system. Optical Fiber Communication Conference 2000, March 7e10, 4, pp. 284e286. [24] E. Almstrom, C.P. Larsen, L. Gillner, W.H. van Berlo, M. Gustavsson, E. Berglind, Experimental and analytical evaluation of packaged 4 4 InGaAsP/InP semiconductor optical amplifier gate switch matrices for optical networks, J. Lightwave Technol 14 (6) (1996) 996e1004. [25] Mottet, S., Buldawoo, N., Gadonna, M., 2000. A semiconductor laser amplifier reflector for DWDM access networks. In: Optical Amplifiers and their Applications 2000, Quebec, Canada, July 9e12. Paper OMD13, pp. 75e78. [26] T. Durhuus, C. Joergensen, B. Mikkelsen, R.J.S. Pedersen, K.E. Stubkjaer, All optical wavelength conversion by SOAs in a Mach-Zehnder configuration, IEEE Photon. Technol. Lett. 6 (1) (1994) 53e55. [27] S. Nakamura, Y. Ueno, K. Tajima, J. Sasaki, T. Sugimoto, T. Kato, et al., Demultiplexing of 168- Gb/s data pulses with a hybrid-integrated symmetric Mach-Zehnder all-opitcal switch, IEEE Photon. Technol. Lett. 12 (4) (2000) 425e427. [28] Lal, V., Masanovic, M., Wolfson, D., Fish, G., Coldren, C., Blumenthal, D.J., 2005. Monolithic widely tunable optical packet forwarding chip in InP for all-optical label switching with 40 Gbps payloads and 10 Gbps labels. In: European Conference on Optical Communications 2005, Glasgow, Scotland, UK, September 25e29. Paper Th4.3.1, 6, pp. 25e26. [29] R. Nagarajan, M. Kato, V.G. Dominic, C.H. Joyner Jr., R.P. Schneider, A.G. Dentai, et al., 400 Gbit/s (10 channel 40 Gbit/s) DWDM photonic integrated circuits, Electron. Lett. 41 (6) (2005) 347e349.
CHAPTER
Market Trends for Optical Amplifiers
15 Daryl Inniss Ovum Inc.
CHAPTER OUTLINE HEAD 15.1. Ovum view: more of the same..................................................................... 15.1.1. Demand for core amplification continues as does the demand for lower cost......................................................................... 15.1.2. Market led by strong suppliers................................................. 15.1.3. Demand complicated by array of options, favors incumbent suppliers ............................................................................... 15.1.4. Waiting, watching, preparing for the amplifier market game changer................................................................................. 15.2. Strong demand for optical amplifiers .......................................................... 15.2.1. Traffic grows in network core................................................... 15.2.2. Metro moving to 50 GHz......................................................... 15.2.3. Agility as a major amplifier requirement ................................... 15.2.4. 40 and 100 Gbps raises demand for single channel amplifiers... 15.2.4.1. 100 Gbps ethernet ready to come out of the gates......... 15.2.4.2. 100 Gbps DWDM.......................................................... 15.2.4.3. 40 and 100 Gbps DWDM on a new transmission fiber ... 15.2.5. Amplification in access networks ............................................. 15.2.5.1. Amplification in cable TV............................................... 15.2.5.2. Amplification for radio frequency over fiber (RFoG)........ 15.2.5.3. WDM PON.................................................................... 15.2.5.4. 10 Gbps PON ............................................................... 15.2.6. Optical amplifier opportunity summary..................................... 15.3. Optical amplifier component supplier review ............................................... 15.3.1. Strength goes to the captive pump laser suppliers..................... 15.3.2. Controlling functional modules also offer competitive advantages 15.3.2.1. Dispersion compensators for mid-stage access.............. 15.3.2.2. ROADMs for mid-stage access ...................................... 15.3.3. Raman amplifiers have a role in this market ............................. 15.3.4. CATV and FTTX amplifier ........................................................ 15.4. System vendor challenges optical amplifier suppliers.................................. 15.4.1. Strong pricing pressures ......................................................... 15.4.2. Strong demand for all different kinds of amplifiers .................... Optically Amplified WDM Networks. DOI: 10.1016/B978-0-12-374965-9.10015-9 Copyright Ó 2011 by Daryl Innis.
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15.5. Optical amplifier supplier analysis.............................................................. 15.5.1. Incumbents dominate optical amplifier market ......................... 15.5.1.1. Optical amplifiers or better defined as wavelength management market..................................................... 15.5.1.2. Ovum finds too many suppliers ..................................... 15.5.1.3. Market led by Oclaro (formerly Bookham and Avanex), JDSU, and Furukawa .................................................... 15.5.2. Cost reduction strategies ........................................................ 15.5.3. Stagnant optical amplifier suppliers......................................... 15.5.3.1. Little new competition ................................................... 15.5.3.2. Raising barrier to entry.................................................. 15.5.3.3. While pump lasers present high barrier to entry, its role has changed................................................................. 15.5.3.4. Diversity of amplifiers also challenges suppliers ............. 15.5.3.5. Consolidation of the behemoths incumbents raises the bardOclaro formed by Bookham acquiring Avanex ....... 15.5.4. Incumbents support semiconductor optical amplifiers ............... 15.5.5. Little outside investment ........................................................ 15.5.5.1. Non-existent venture capital spending ........................... 15.6. Outlook: more of the same ......................................................................... 15.6.1. Incumbents are vertically integrated and have built high barriers to entry ..................................................................... Acronyms ........................................................................................................... References .........................................................................................................
434 434 434 435 436 436 437 437 438 438 438 439 440 440 440 440 440 441 442
Strong demand for dense wavelength division multiplexing (DWDM) and broadband optical amplification exists to support modern communication networks. Functional integration of amplifiers with reconfigurable optical add/drop multiplexers (ROADMs), optical performance monitors (OPMs), and possibly dispersion compensating modules (DCMs) are current strategies that lower cost to system vendors and raise the barrier to entry for competitors. This market is very competitive and dominated by incumbent suppliers. Opportunities exist for new entrants and technologies, particularly for per channel and narrowband amplification. But there is little outside spending in this market, and the emerging opportunities do not appear big enough to change course. Hence, expect an active competitive environment to continue with more evolutionary rather than revolutionary trajectory.
15.1 OVUM VIEW: MORE OF THE SAME 15.1.1 Demand for core amplification continues as does the demand for lower cost Bandwidth demand continues to grow on core networks worldwide [1]. Traffic growth is driven by an increase in both the number of users and the per user
15.1 Ovum view: more of the same
bandwidth requirement. This is evidenced by the proliferation of high speed internet users [2], the increasing amount of bandwidth on even the simplest web sites, and transport of bandwidth-hungry video on these networks. Both wired and wireless networks are contributing to the demand. In wireless networks, for example, traffic is often backhauled from the cellular tower to a wired network and the per user capacity is continually increasing as operators transition to 3G and 4G networks [3]. And bandwidth growth is being felt worldwide such that developing countries are deploying high capacity network to support the demand [4]. The relatively low cost and high bandwidth capacity of DWDM on optical fiber is being exploited to support the bandwidth growth. Network operators worldwide are using DWDM and optical communications to mitigate the bandwidth demand problem. Network operators are also continuously pushing to reduce the network cost, a requirement whose sensitivity increases for the deployment of optics deep into the network edge (in enterprises and to consumer premises). DWDM component and equipment suppliers are then challenged with delivering lower cost solutions. While these price pressures are normal, the telecom market implosion in 2001 [5] has left a number of suppliers still struggling to build a sustainable and profitable business. Optical amplifier suppliers’ current response to lower cost is to incorporate the optical amplifier with other wavelength management functions. Fundamentally the suppliers are taking functions like switching (ROADM), DCM, and optical channel monitoring (OCM) and co-packaging them with amplifiers [6]. These products sell for less than the sum of the individual ones. The product is furthermore attractive as it can decrease footprint, power consumption, and time to market. Attacking wavelength management products holistically is further attractive to amplifier suppliers because while the revenue growth rate for core amplifiers is flat at best through 2014, Ovum estimates that wavelength management products like ROADMs will nearly double. Wavelength management products satisfy demand in the network core. But new strategies and approaches will likely be needed to support the demand at the network edge.
15.1.2 Market led by strong suppliers Optical amplifiers are dominated by incumbent suppliers who have been manufacturing amplifiers for over 10 years. These suppliers have exploited their technical and business leadership and built reasonable businesses with fairly high barriers to entry. They created a high entry barrier first by acquiring the pump lasers and now by developing highly complex subsystems and integrating numerous functional features with the amplifier. We have seen strong growth for optical amplifiers since the telecom bubble collapse of 2001. Amplifiers have evolved, but there has been no fundamental or significant performance or architectural shift. The market is principally driven by the
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CHAPTER 15 Market trends for optical amplifiers
demand for lower cost. One trend has been to introduce to the market higher power pump lasers which lowered the power consumption and amplifier cost. In addition, suppliers have cut cost by transitioning their manufacturing operations to contract manufacturers and/or to low-cost manufacturing regions [7]. While the optical component market is challenging, the leading amplifier suppliers are among or near the top 10 suppliers in the market [8]. Optical amplification is one of numerous product segments supported by them. These are among the strongest companies in the market, and they are continually investing to maintain their position against challengers.
15.1.3 Demand complicated by array of options, favors incumbent suppliers Optical amplifier designs are varied, and there does not appear to be any convergence. Much of the manufacturing process is manual, and testing is a time consuming and costly process. The market can be characterized as one of low volume, high variability, and medium (greater than 20%) gross margin. The customer demand ranges from networking companies that prefer to purchase discrete components and design and manufacture their own amplifiers to those that purchase network ready subsystems that are near plug and play units. The per customer range of amplifier design includes pre, booster, and line amplifiers as well as variations on these for different applications and customers. The supply space is stacked advantageously for the incumbent amplifier suppliers and for business to continue as usual.
15.1.4 Waiting, watching, preparing for the amplifier market game changer The amplifier market “game changer” is the same today as it was almost 10 years ago. Investors and technologists alike believed that the market would need small amounts of amplification for a large number of applications. Semiconductor optical amplifiers (SOAs) [9] and planar waveguide based optical amplifiers [10] were pursued aggressively. But the telecom market crash of 2000e2001 was a sufficiently massive blow to the market that the demand did not materialize, and most of the companies developing these technologies dissolved. The fact remains that as optical technology pushes out from the core network to the edge, to enterprises and to consumers, as the end-user demand for bandwidth grows so will the demand for amplification. Moreover many of these applications would require a small amount of amplification and the price target would need to be factors of 10 smaller than the amplifiers that currently support the core network. But there are a number of open questions: When will this demand materialize? Will the market price/performance demand require new approaches to amplification?
15.2 Strong demand for optical amplifiers
Will the opportunity be sufficiently large to justify new investment in this market? Will the incumbent suppliers be able to meet the market demand? Islands of demand are showing up in numerous areas at the network edge, albeit in early stages. For example, there are some signs in access as carriers are pushing for 10 Gbps passive optical network (PON) which may require amplifiers. Some enterprises are already using coarse wavelength division multiplexing and dense wavelength division multiplexing, but if demand for cloud computing, for example, grows substantially then higher bandwidth connections and amplification to enterprises may be required. And the move to 40 and 100 Gbps Ethernet will also drive the need for more amplifiers in enterprises. Optical communications will push further into end-user networks, but the timing for this growth is unclear. The market has to monitor the progress and prepare for the low cost, low gain, high volume amplifier demand to materialize.
15.2 STRONG DEMAND FOR OPTICAL AMPLIFIERS 15.2.1 Traffic grows in network core The demand for optical amplification fueled by the growth of IP traffic in the core of the network is as strong as ever. Figure 15.1 is an example to illustrate the challenges and the implication on the demand for amplification. It shows the average IP traffic in North America from 1993 to 2008 [11]. Taken alone, and recognizing that wavelength density multiplexed (WDM) systems are used to support this demand, it suggests strong demand for amplifiers.
Capacity
Cost
IP traffic
10000
10000
100 100
10
10
1 0.1 0.01
19
93 19 94 19 95 19 96 19 97 19 98 19 99 20 00 20 01 20 02 20 03 20 04 20 05 20 06 20 07 20 08
1
IP traffic (PB/month)
Cost ($/Gbps/km) Capacity (Gbps)
1000 1000
0.1
0.001
FIGURE 15.1 IP traffic in NA core network, optical equipment capacity, and cost. Source: Ovum, IP traffic data from Minnesota internet traffic studies http://www.dtc.umn.edu/mints/igrowth.html
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Also shown in Figure 15.1 is the maximum capacity of DWDM systems that support core networks. The capacity has been capped at 1.6 Tbps (160 channels at 10 Gbpsdthis is based on 80 channels in the C-band and 80 in the L-band, and the L-band is rarely deployed. For practical purposes 800 Gbps is the realistic system capacity since 2001. Only recently have 40 Gbps systems been introduced. So in fact as the traffic demand increased, carriers deployed the same WDM line side capacity systems. System vendors did increase the client side significantly so they can more efficiently pack traffic onto the network as networks migrate to internet protocol (IP). Finally, Figure 15.1 shows the cost of these DWDM systems over time [12]. To the credit of component and system vendors, there have been strong cost reductions to support the demand. While the demand for amplification has increased and the cost for these devices has gone down, performance requirements of the amplifier have also changed. Carriers have transitioned from point to point networks to rings and now prefer mesh networks (see Figure 15.2) [13]. ROADMs were introduced to support this network. These were available in the 1999 and 2000 [14] period but the market did not really develop for these components until 2005e2006. They are now considered a mainstream product for DWDM systems. The second biggest change is the demand for higher data rates. The market is entering widespread adoption of 40 Gbpsdit is still challenged with high prices,
Point to point
Mesh
FIGURE 15.2 End to end network showing point-to-point (old), ring (current), and mesh (future). Identify network changes and where amplifiers sit. Source: Ovum
15.2 Strong demand for optical amplifiers
and many are clamoring for 100 Gbps. 40 Gbps will become a mainstream transmission rate over the next few years, supporting both metro and long-haul networks. 100 Gbps will be introduced during this time period. The demand for amplifiers continues to support these data rates, the biggest change being the growing demand for single channel amplifiers at most receive sites and for some transmission sites.
15.2.2 Metro moving to 50 GHz Another major change in the core network is the increased demand for 50 GHz channel spacing in metro networks. One characteristic demarcation between metro and long haul was the number on DWDM channels per fiber and hence the channel spacing. But the increased bandwidth demand, the growth in mesh networking, and the uncertainty of where the next bandwidth hungry application will come from is driving carriers to increase the capacity in metro networks. The keen interest in 50 GHz is that operators can double the network capacity without doubling the cost. This change increases the channel loading per amplifier and drives modification of metro amplifier designs.
15.2.3 Agility as a major amplifier requirement ROADMs have insertion losses greater than 4.5 dB and require amplification. Architecturally they are located between two stages of an amplifier. The first stage is a preamplifier to account for the loss in the transmission line. The signal goes through the ROADM and then it is followed by a booster amplifier. The ROADM can add and drop wavelengths on demand. This requirement changes the response time requirements of amplifiers. Adding and dropping wavelengths can create transients. Measures must be taken to suppress the transients as they can destabilize the amplifier and the transmission line. These amplifiers are often designed with digital signal processors to rapidly respond to changing conditions. Herein we will refer to these as “agile” amplifiers. They are designed to maintain the gain setting independent of the changing number of incoming wavelengths. Rapid transient suppression, on the order of tens of microseconds, and variable settable gain are the main features. There had been an open question as to who should develop the circuitry for these amplifiers. These functions can be designed by the amplifier manufacturer or the system vendor can purchase a “fixed” amplifier or gain block and implement the circuitry to support these features. Since the gain blocks are cheaper, some system vendors prefer to design the circuit. In addition, some system vendors feel more in control of the amplifier when they offer this feature themselves. Component vendors have varying degrees of capabilities. The function desired here (fast gain control) was once considered a major differentiator, but is simply table stakes today.
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15.2.4 40 and 100 Gbps raises demand for single channel amplifiers The optical communications market has just started to deploy 40 Gbps transmission gear in appreciable volumes in 2008 (over a few thousand DWDM interfaces were deployed). These lines are designed to run on the same optical fiber and common equipment as the 10 Gbps lines. This means that the new 40 Gbps has to run on the existing amplifiers and dispersion compensators that were deployed for 10 Gbps. Since the line was designed for 10 Gbps and because there are no commercially available avalanche photodiode based receivers for 40 Gbps, both additional dispersion correction and amplification are needed at the receive site for 40 Gbps today. Per channel tunable dispersion compensators and single channel amplifiers are used. In addition, there are circumstances in which per channel amplification may be required at transmission sites. For example, some 40 Gbps modulation formats use multiple modulators that results in attenuation of the transmission signal. Per channel amplification may be required in these situations to match the transmission power of the other signals on this line. This situation occurs less frequently than amplification required at the receiver. To get an estimate of how big the demand for single channel amplifiers can get for 40 Gbps, we can look at the 10 Gbps DWDM market. The volumes for 10 Gbps DWDM are over 100,000 per year today. As 40 Gbps can be considered a replacement for 10 Gbps then the 10 Gbps volumes can be used as a guide to how large the single channel amplifiers can get for this application. Vendors like Oclaro (formerly Avanex) are advantageously designing subsystems with a tunable dispersion compensator and amplifier for these applications [15]. The market will probably prefer the electronic dispersion compensator (EDC) in the long run (five years or longer) but as these are a few years away from commercialization, the amplifier/dispersion compensator subsystem makes a good product. Even once the EDC is available, they are power hungry components so their use would be weighed against performance. Nonetheless they have gained significant favor for 100 Gbps as the market, lead by the recommendation of the Optical Internetworking Forum (OIF), is supporting coherent detection. Meanwhile the amplifier demand at the receiver will probably continue.
15.2.4.1 100 Gbps ethernet ready to come out of the gates Although the standards are not finalized, there is demand for 100GBase-LR4 and 100GBase-ER4. These modules support 100 Gbps transmission for 10 and 40 km respectively. In each case four wavelengths (1295, 1300, 1305, and 1310 nm) are multiplexed. The 10 km may be just short enough not to require amplification, but all the 40 km links would need amplifiers. SOAs are the leading candidates to support this demand. The volumes in this market will likely reach tens of thousands and this will take five to seven years after the product is introduced.
15.2 Strong demand for optical amplifiers
15.2.4.2 100 Gbps DWDM For 100 Gbps DWDM the market will be similar to that of 40 Gbps. The 100 Gbps transmission must exist on the same lines designed for 10 and 40 Gbps. Amplification and tunable optical dispersion compensators are likely required at the receiver of non-coherent based systems. This market will take a few years to achieve the few thousand units that the 40 Gbps market is enjoying today. But end users (carriers and multiple system operators [MSOs]) are anxiously awaiting the arrival of this higher data rate.
15.2.4.3 40 and 100 Gbps DWDM on a new transmission fiber New systems designed for 40 and/or 100 Gbps will also be introduced to the market. These will be designed for optimal transmission at these data rates. Raman amplification is a likely technology of choice as the carriers can realize better performance from distributed gain that Raman amplifiers offer. Raman amplification is in the toolbox of all system vendors. It is used occasionally when link lengths need to be extended. But it is used as an exception, not as a rule. Deployment of new lines optimized for 40 and/or 100 Gbps will present this amplifier product category an opportunity to be significantly larger, but it will be some time before Raman becomes a serious contender for the rule in amplification over the erbium-doped fiber amplifier. More likely, Raman will be used in conjunction with erbium in the hybrid Raman/EDFA (erbium-doped fiber amplifier).
15.2.5 Amplification in access networks Much of the amplification discussed so far is in the core of the network. Each line represents large volumes of traffic so the cost of amplification is shared. But cost requirements intensify sharply for equipment in the metro and in access segments of the market. As a consequence of the reduced number of users per line, the price demands are more stringent than those in the core. Optics has proliferated to the end user in access networks and the cost pressures are even more severe as it is a per user cost that has to be minimized. To date, the transmission rates and distances are sufficiently reasonable that vendors don’t need amplifiers at the end user. But the bandwidth requirements are increasing and the market is reaching the point where amplification may be needed in the next few years at the end user. When this happens, the current technology may not be sufficient to support the low cost required and the market will start to look for innovations to put it on a new cost curve.
15.2.5.1 Amplification in cable TV Cable TV providers use high power amplifiers to support the large number of node splits required for the operating region. Amplifiers based on co-doping the fiber core with ytterbium and erbium are typically used. These amplifiers are also pumped with multimode pump lasers and the fiber is based on a double cladding structure. The first core is to support the signal to be amplified and the second core to support the pump wavelength.
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15.2.5.2 Amplification for radio frequency over fiber (RFoG) Fiber is the preferred infrastructure for data networks in new Greenfield building development, particularly in North America. This infrastructure favors the telecom operators who are deploying fiber to the X (FTTX) access technologies, and is disadvantageous to the cable operators whose infrastructure is the hybrid fiber coax. Cable operators, however, are offering radio frequency (RF) over fiber so they can use their existing terminal equipment and provide services on fiber. Optical amplification is used in these networks. See IPG, for example [16].
15.2.5.3 WDM PON WDM PON is one of the next generation passive optical network (PON) technologies promising high bandwidth, secure service, and symmetrical bandwidth transmission. It still remains to be standardized so there are different variants being discussed in the market. Herein we are referring to the LG Ericsson (originally Novera, then LG Nortel before being acquired by Ericsson) architecture where an array of Fabry Perots is injection locked to provide the transmission signal at the optical line terminal (OLT). The OLT terminates the optical local loop at the edge of the network. It also receives upstream signals from the customer premises or the optical network unit (ONU). In this case a broadband light source based on erbium fiber is used with wavelength tuned filters to lock the lasers. This market is in its infancy. The critical bit is a low cost transmitter. The LG Nortel approach is attractive, but the upfront cost is high due to the required sharing of the broadband light source, even if there are only a few subscribes on the give line. There are many other low cost approaches also being developed. There is general market excitement about this technologydit competes attractively with coarse wavelength division multiplexing (CWDM) for second mile applications and is a high bandwidth solution for enterprise broadband access needs. But until standardization is completed the market will continue to be plagued with ambiguity. Meanwhile the market needs to be monitored to understand the role that amplifiers and erbium plays.
15.2.5.4 10 Gbps PON The market is also working on 10 Gbps as higher bandwidth PON, which is believed to be needed for the next round. Much of the testing is happening in Japan [17]. Demand for multiple HDTV channels is one of the reasons this is being supported. Amplification would likely be needed to support the 20 km and 1x32 split. We need to keep our eye on the development here too.
15.2.6 Optical amplifier opportunity summary Table 15.1 summarizes the above optical amplifier opportunity discussion. The main market opportunity is to support the long haul and metro networks. While there are numerous other opportunities, they are distributed. Some will use erbium-doped fiber, but not necessarily all. In addition, many of the single channel opportunities can be satisfied by SOAs. Finally, the total volume of these amplifier
15.3 Optical amplifier component supplier review
Table 15.1 Optical amplifier opportunity summary Demand
Type of amplifier
Comment
Long haul and metro core 40 Gbps DWDM
Broadband erbium-doped fiber
Dominates current market revenues. Supported by subsystem, agile, gain blocks, and discrete components Greenfield is an emerging area. Receiver is supported by module or subsystem at receiver, some opportunity at transmitter.
WDM PON
Hybrid Raman/EDFA for Greenfield systems. Amplification at the receiverdcan be supported by erbium-doped fiber or SOA. Hybrid Raman/EDFA for Greenfield systems. Amplification at the receiverdcan be supported by erbium doped fiber or SOA. Likely semiconductor optical amplifier Erbium and erbium/ytterbium double clad Erbium and erbium/ytterbium double clad Unclear
10 Gbps PON
Candidate for SOA
100 Gbps DWDM
100 Gbps Ethernet CATV (cable television) RFoG
A new area that has not started. Can be supported by module or subsystem at receiver, some opportunity at transmitter
A new area that is just starting Stable, mature market. Subsystem or gain blocks Newly emerging area, supported by subsystem or gain blocks Newly emerging area. Currently uses broadband amplified stimulated emission (ASE) source Emerging area
Source: Ovum
opportunities is unclear. So the market has to be vigilant. But as price, size, and low power consumption are big factors for equipment and components in access networks, SOAs may provide the best solution for these opportunities.
15.3 OPTICAL AMPLIFIER COMPONENT SUPPLIER REVIEW 15.3.1 Strength goes to the captive pump laser suppliers The intent of this section is to identify the suppliers and technologies that are currently used in optical amplifiers. All amplifiers must be pumped. The simplest amplifier is illustrated in Figure 15.3. This type of design is needed for amplifying a single channel. This type of amplifier might be used as a booster when signals are transmitted or as a preamplifier when the signals are received.
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Erbium fiber
Signal
Pump
FIGURE 15.3 Schematic of representative narrowband amplifier showing discrete components Source: Ovum
Pump lasers are important because their cost can be as high as 65% of the total amplifier. They also have a big impact on the performance and reliability of the module. So having a captive supply of pump lasers is important in this market. There are both 1480 and 980 nm pump lasers. Figure 15.4 shows the suppliers and the type of lasers that they make. Furukawa and JDSU are the only suppliers that manufacture both 1480 and 980 nm pump lasers. The 980 nm laser space appears overcrowded with eight suppliers. The strongest 980 nm suppliers are Oclaro (formerly Bookham) and JDSU. 3S Photonics was divested from Avanex (now Oclaro).
980nm
EM4 3S Photonics Archom Axcel Photonics Oclaro (Bookham) Coset
1480nm
Furukawa JDSU
Anritsu
FIGURE 15.4 Pump laser suppliers Source: Ovum
15.3 Optical amplifier component supplier review
Erbium fiber
Signal
Erbium fiber
Signal
Mid-stage access Pump
Pump
FIGURE 15.5 Schematic of representative amplifier with mid-stage access Source: Ovum
15.3.2 Controlling functional modules also offer competitive advantages Figure 15.5 is a schematic of a broadband amplifiers with mid stage access. ROADMs, dynamic gain equalizer filters (DGEFs), and dispersion compensators would nominally sit at this site.
15.3.2.1 Dispersion compensators for mid-stage access It is advantageous to minimize the loss at the mid-stage access and keep the loss constant independent of the compensation required. Table 15.2 shows the dispersion compensator suppliers, the technologies, and the insertion loss listed on their data sheets for these products. The fiber solutions show a range as the amount of dispersion is dependent on the fiber length as is the loss. This is the technology most often used because it provides the largest dispersion range of the applied technologies to date. But there is considerable market interest in using an approach that has Table 15.2 Dispersion compensator suppliers by technology, typical insertion loss, and other issues Insertion loss
Dispersion Range (ps/nm)
Fiber Bragg Grating Fiber
<8
314 to 2157
3.5-9.8
2000 to 1937
Fixed dispersion, IL (insertion loss) is fiber and dispersion range dependent
Etalon
<6.5
þ/ 1000
Tunable dispersion
Etalon
<4
þ/ 1700
Tunable, incorporated with transponder
Company
Technology
Accelink Oclaro (formerly Avanex)
Padtec (Civcom)
Comment
(continued on next page)
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Table 15.2 Dispersion compensator suppliers by technology, typical insertion loss, and other issues (continued) Company
Technology
Enablence
Planar light circuit Fiber Bragg Grating Fiber
E/O Land Fujikura
Furukawa
Fiber
Proximion
Fiber Bragg Grating Fiber Bragg Grating Fiber
Sumitomo Electric
Teraxion
Fiber Bragg Grating
Insertion loss
Dispersion Range (ps/nm)
Comment
<6.5
þ/ 2100
Tunable
<7.8
to 1520
Fiber and dispersion range dependent Fixed
850 to 450
Tunable
þ/ 700 <10.7
to 1700
Fixed dispersion, insertion loss is fiber and dispersion range dependent
<6
þ/ 1800
Fixed and tunable
Source: Ovum and company reports.
fixed insertion loss to minimize amplifier requirements and to decrease design complexity. The fiber Bragg grating and the etalon stand out as options because the loss can be constant independent of the amount of dispersion or slope. These have not taken over the DCM solutions yet but as transmission rates increase the demand for these types of solution will also increase. Also listed are the tunable dispersion compensators that are nominally used today for 40 and 100 Gbps transmission at the receiver. Coherent optical communications implemented with advanced signal processing could provide a wide range of dispersion compensation and obviate the need for some or all of the dispersion compensation at amplifier sites.
15.3.2.2 ROADMs for mid-stage access Table 15.3 lists ROADM suppliers, the technology they use, and the insertion loss listed on their product sheets. As the market is interested in new configurations that may require cascading ROADMs, those with the lower insertion loss have a decided advantage in this regard. Note, however, that features like broad pass band are also critical so designers have to optimize a number of factors.
15.3 Optical amplifier component supplier review
Table 15.3 ROADM suppliers, technology, typical insertion loss, and other issues Company
Technology
Agiltron
Free space and micro integrated MEMS Liquid crystal MEMS PLC Liquid crystal on silicon PLC PLC MEMS PLC DLP Liquid crystal
Capella CoAdna Dicon Fiberoptics Enablence Finisar Furukawa JDSU NeoPhotonics Nistica Oclaro (formerly Avanex) Optoplex Polatis Xtellus (now Oclaro)
Micro optic and micro actuator Beam steering Liquid crystal and 1-axis MEMS
Insertion Loss (dB)
Comment Single channel
7 10.5
11 7.5 6.5
6.5
Wavelength selective switch Wavelength selective switch Single channel Integrated planar light circuit Wavelength selective switch
Wavelength selective switch Integrated planar light circuit Wavelength selective switch Wavelength selective switch Single channel
1.4 6
Wavelength selective switch
Source: Ovum and company reports
Wavelength selective switch (WSS)ebased ROADMs can also be used to equalize the signal power across the transmission band. This function is usually performed by a DGEF which sits mid-stage of an optical amplifier after every five or six amplifier huts. As this functionality can be supported by ROADMs, the market size (volumes) for standalone DGEF is not as great as previously anticipated.
15.3.3 Raman amplifiers have a role in this market The schematic of a Raman amplifier is shown in Figure 15.6. Suppliers include JDSU, Furukawa, IPG Photonics, Accelink, WXZTE, and RedC Networking. Products are sold as discrete pump lasers, subsystems, and modules. The product is nominally pump lasers at specified wavelengths with their output optical radiation coupled together into a single fiber. Although suppliers of the lasers have a commanding advantage, RedC Networking has been able to build a subsystem product with revenues without owning the pump lasers. Note, though, that they do have a supply agreement with JDSU although we do not know if these pump lasers are part of that agreement.
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Transmission fiber
Signal
Pumps FIGURE 15.6 Schematic of a Raman amplifier Source: Ovum
15.3.4 CATV and FTTX amplifier Figure 15.7 is a schematic of a high power optical amplifier that is based on the Er/ Yb double clad fiber. Emcore and IPG Photonics are two suppliers of this type of amplifier. Optical systems that require a large number of passive splits have a need for amplification. A pump laser for this double clad fiber is unique because it needs to be both high power and multimode. Suppliers include JDSU, Oclaro (Bookham), and IPG Photonics. Erbium fiber
Signal
Er/Yb Pump Multimode pumps
FIGURE 15.7 Schematic of a high power amplifier Source: Ovum
15.4 SYSTEM VENDOR CHALLENGES OPTICAL AMPLIFIER SUPPLIERS 15.4.1 Strong pricing pressures System vendors in the core and metro segments of the optical communication market are much larger companies than the optical component vendors ($16.3 billion for optical networking compared with $4.8 billion for optical component
15.5 Optical amplifier supplier analysis
vendors in 2008). More importantly, the concentration of these vendors is higher than that of the optical component suppliers (top 10 system vendors have 81% market share while to 10 OC vendors have 66% market share) and hence the system vendors have better leverage to control pricing. The large number of optical component vendors, particularly in the optical amplifier segment, means that the system vendors can set up competition among the vendors thereby driving down prices. The system vendors’ strength has increased over the past few years as a consequence of industry consolidation. For example, Alcatel acquiring Lucent created an even stronger long-haul DWDM market leader. And the past few years have seen the emergence of Huawei as a formidable competitor in both the metro and long-haul space, further intensifying the competition for design wins and business.
15.4.2 Strong demand for all different kinds of amplifiers While the overall demand for amplifiers is strong, the type of amplifier is a customer specific decision. The options are gain block, electronically controlled or agile module, and subsystem. Cisco, for example, would love the subsystem. It is focused on plug and play, cost reduction, and time to market; it finds this type of product suits its needs well. Subsystems are modules that are rack mounted either directly or through attachment to a card. Subsystems contain substantial software for operational functions, include network management (simple network management protocol [SNMP]/transaction language (TL1) to support functions such as monitoring and alarm, and use designs owned by the component vendor. The footprint, electrical connections, optical functions, and software are common. Software interfaces are included that enable the network equipment manufacturer to customize unit operation. But Fujitsu, Ericsson, and Huawei would prefer to purchase discrete components or to purchase gain blocks and make the rest of the network element themselves. As Huawei has grown its market share over the past few years, the demand for discrete components has increased. We believe this trend will continue because Huawei continues to win business in emerging areas and there appears to be a preference for these components in those regions. A gain block is a module that has all the optics and pumps integrated into the module. There are photodiodes that are connected to the printed circuit board (PCB) with the pumps. But the amplifier is controlled entirely by an external board designed by the system vendor. Meanwhile a large segment of the market prefers to purchase the agile optical amplifiers. These are gain blocks with integrated electronics in the module that can operate without intervention once powered. It is typically equipped with digital signal processor (DSPs) and firmware and can have automatic gain control and transient control. These trends will stay with the market for some time to come and it are unlikely to change.
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15.5 OPTICAL AMPLIFIER SUPPLIER ANALYSIS 15.5.1 Incumbents dominate optical amplifier market 15.5.1.1 Optical amplifiers or better defined as wavelength management market Total demand for optical amplification is $300 million in 2008 declining at a 1% compound annual growth rate through 2014. The decline is largely due to the worldwide macroeconomic crisis that started to impact telecommunications in the fourth quarter of 2008. The two types of optical amplifiers considered in this analysis are the erbium-doped fiber based and the Raman amplifiers. More than 90% of the market in 2008 was erbium-doped fiber based. The total demand is larger than the merchant market because there are amplifier end customers who do not take amplifiers but purchase the piece parts and design and build their own. Companies in the category include Huawei, Ericsson, and Fujitsu. Ovum estimates that the merchant market for optical amplification was $219 million in 2008, representing only 69% of the total demand. There is definitely an opportunity for amplifier vendors to win business that system vendors currently control, thereby increasing the merchant market opportunity. Optical amplifier suppliers can further increase the revenue opportunity to include constituent parts of the amplifier and modules that reside at the mid-stage of amplifiers and optical performance monitors. Figure 15.8 shows the total demand for these components. The $1,070 million market is led by amplifiers and pump lasers, with ROADMs following in 2008. Discrete components include filters, isolators, taps, and couplers that are part of the optical amplifier assembly.
2008: $1.1 billion wavelength management market Filters 7%
OPMs 8% Amps 30%
ROADMs 19%
DCMs 9%
2014: $1.6 billion wavelength management market Filters 4%
OPMs 10%
Amps 19%
ROADMs 26%
Pumps 16%
Pumps 20% Discrete components 7%
DCMs 20%
Discrete components 5%
FIGURE 15.8 Total 2008 demand for wavelength management products Source: Ovum
15.5 Optical amplifier supplier analysis
The total wavelength management market outlined in Figure 15.8 is slated to grow at a 6% compounded annual growth rate through 2014 to $1,560 million. Dispersion compensators, particularly the tunable dispersion compensators, are to grow at the fastest rate followed by ROADMs.
15.5.1.2 Ovum finds too many suppliers Ovum finds there are 28 optical amplifier suppliers worldwide for a $300 million market. Clearly there are too many suppliers to share this market. In fact, the suppliers are not equal by a long shot. One of the most important parts of the amplifier is the pump laser. Most of the large amplifier suppliers also have captive supply of pump lasers. These companies have been supplying pump lasers and amplifiers for over 19 years. They tend to be vertically integrated and support the market with both low cost and the highest functional amplifiers sold. These companies are also among the strongest companies of the suppliers from a financial perspective. Figure 15.9 illustrates the relative positioning of the optical amplifier suppliers. Optical amplifier supplier strength includes the length of time it is selling amplifiers (i.e., strong is characterized as shipping amplifiers since before 2000), captive pump lasers, vertically integrated, and substantive subsystem supplier, and support of low cost amplifiers. A company earns one point for each attribute and can earn as many as five total points. Oclaro (formerly Bookham) and JDSU are the only suppliers with the highest optical amplifier strength. Avanex (now part of Oclaro) would have been the next 4
JDSU
Financial strength
3
Emcore
Fujikura
2
SDO Northlight Manlight Lightwave 2020 Lightel Bayspec Ascentta
1
0 0
Oclaro
Furukawa
IPG
RedC Networks WXZTE Dowslake Luxpert Titan Keopsys AOC
1
Licomm
Accelink
Amonics Browave NuPhoton Opto-Link
O/E Land ONet
2
3
4
5
6
Optical amplifier strength
FIGURE 15.9 Optical amplifier supplier relative strength Source: Ovum
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CHAPTER 15 Market trends for optical amplifiers
strongest only missing having a captive supply of pump lasers (Avanex divested its fab in 2006 to 3S Photonics). Now that Bookham and Avanex have merged, Oclaro occupies the highest optical amplifier strength. The y axis, the financial strength, includes ability to raise capital (public companies or private ones that have a positive track record on this matter), revenues over $100 million per year, merger and acquisition aggressiveness, and profitability. The highest possible score is four points. No company attained the highest level. Four companies attained a score of three points including JDSU, Oclaro (formerly Bookham and Avanex), Furukawa, and Emcore. Furukawa was short on its mergers and acquisition (M&A) aggressiveness and all others were short on profitability. The intent of the financial strength axis is to show companies that have a history or the financial strength to improve their market positions. The sizes of the bubbles represent the approximate revenues for these companies. The revenue of the optical component business unit is used where known. The revenue of Furukawa and Fujikura is significantly larger than shown here. The revenue of IPG represents the total company revenue.
15.5.1.3 Market led by Oclaro (formerly Bookham and Avanex), JDSU, and Furukawa A read of Figure 15.9 shows that the optical amplifier market is led by Oclaro (formerly Bookham and Avanex), JDSU, and Furukawa. These companies also have the highest optical amplifier revenues based on Ovum estimates. They are among the first suppliers in this market. They are vertically integrated and have or had their own pump lasers. Much of Oclaro’s (formerly Bookham) business comes from acquisition of Nortel’s optoelectronics business unit in 2002. It also benefited from the acquisition of Onetta, which was a start-up amplifier supplier specializing in agile amplifiers. It is further strengthened in amplifiers and wavelength management products by the acquisition of Avanex, which closed during the writing of this paper. IPG’s emergence comes from attacking the fiber laser business. It became a public company in 2006. Its fiber lasers business use technologies that can be used for high gain amplifiers like those used in CATV (see Figure 15.7). Emcore’s amplifier product is the consequence of an acquisition from JDSU for the CATV market. We believe Emcore is solely focused on the CATV market for amplification purposes. Accelink and WXZTE are interesting because they represent the low cost manufacturing region. While most amplifier suppliers manufacture in these regions, these two have local ties and can win business within their own countries. They do not have captive pump lasers. However, they do offer all types of amplifiersdgain blocks, agile modules, and subsystems.
15.5.2 Cost reduction strategies Reducing cost is one of the main goals of optical component suppliers. Figure 15.10 illustrates some of the necessary components to be a low cost supplier. The leading
15.5 Optical amplifier supplier analysis
Incumbent
Oclaro (Avanex + Bookham) Furukawa JDSU
Vertically integrated
Accelink O/E Land
Low-cost region Amonics Browave Licomm Luxpert
O-Net Communications Opto-Link WXZTE
FIGURE 15.10 Low cost strategies of optical amplifier manufacturers Source: Ovum and company reports
suppliers have to both be vertically integrated and to transition manufacturing to low cost regions. Companies are considered vertically integrated if they sell/manufacture three of the following components: DCMs, ROADMs, pump lasers, gain flattening filters, OPMs. Suppliers in low cost regions and vertically integrated manufacturers are also summarized in the schematic. In this analysis, O/E Land is found to be a vertically integrated supplier as it makes DCM, pump lasers, and gain flattening filters. Accelink and O-Net Communications distinguish themselves as being both vertically integrated and residing in a low cost region. They both sell DCMs, gain flattening filters, and optical channel monitors. O-Net Communications also sells ROADMs.
15.5.3 Stagnant optical amplifier suppliers 15.5.3.1 Little new competition Optical amplification is dominated by erbium-doped fiber amplifiers and the leading suppliers have been shipping amplifiers for 10 years or longer. These companies include Oclaro (formerly Bookham and Avanex), JDS Uniphase, and Furukawa. Ovum estimates these companies enjoy more than 60% market share of the nearly $200 million dollar merchant erbium-doped fiber amplifier market in 2008. There are another 25 companies fighting for the remaining revenues. Twenty-one of the remaining optical amplifier companies that still exist today started between 1997 and 2003. All the amplifier suppliers in low cost regions started between 1998
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CHAPTER 15 Market trends for optical amplifiers
and 2002. And only two new amplifier suppliers have entered the market since 2003, Manlight and Titan Photonics.
15.5.3.2 Raising barrier to entry The optical amplifier barrier to entry is an ever increasing target, a situation that’s good for the incumbent suppliers. First pump lasers protected the incumbents, but all pump laser suppliers must sell to the merchant market to overcome a large fixed cost. Second, and unlikely that this was a planned strategy, there is a large array of amplifiers to support even one customer. Subsystem is one class of amplifiers that initially appears to set itself in a class above all the other types. As most suppliers at least offer this type of amplifier, the market has now introduced functional integration of numerous modules in the amplifier subsystem, thereby raising the bar again. And the incumbent suppliers also make a number of the wavelength management products providing an opportunity of stronger control if these subsystems become a substantial part of the market.
15.5.3.3 While pump lasers present high barrier to entry, its role has changed The leading suppliers also hold the distinction of being the leading pump laser suppliers. The merchant pump laser market was over $100 million in 2008. The pump laser is the most expensive component in the amplifier and it is the largest contributor to the reliability of the amplifier. Manufacturing ownership of this component can provide cost, performance, supply, and reliability advantages. But as it is expensive to support a semiconductor fabrication facility, supporting the merchant market with pump lasers is also an important business. Pump laser suppliers need to sell as much product as possible so their operations have enough revenues and margins to remain viable. Over the years a number of these facilities have run into financial trouble and were jettisoned or mothballed (Lasertron is one example) as a result [18]. The changing role of the pump laser was seen in 2006 when Avanex (now Oclaro) decided to divest its pump laser facility. Avanex (now Oclaro) moved a valuable asset to the merchant market thereby supporting some of its competitors. Avanex was comfortable enough that it could get what it needs from 3S Photonics or elsewhere and did not feel that it was enabling its competition or putting its own amplifier business at risk [19]. Avanex’s goal was to reduce its fixed cost and continue to run a viable business. 3S Photonics and amplifier suppliers without pump lasers were the beneficiaries.
15.5.3.4 Diversity of amplifiers also challenges suppliers It is not only pump lasers, but numerous components required in an amplifier that add to its cost. Components like filters, isolators, and gain flattening filters, were all critical in the early years and now have become fairly easy to acquire on the merchant market. But there are numerous designs that the market looks for that fall into three general categories: gain block, agile amplifiers, and subsystems. It once appeared
15.5 Optical amplifier supplier analysis
that these subsystems would raise a barrier sufficiently large that few suppliers would be able to enter, but today nearly all suppliers have this product as part of their portfolio. Ovum believes, however, that most subsystem sales are from the established or incumbent suppliers. The amplifier barrier was raised again when JDSU introduced the agile optical networking superblade. On a single 1U (the height of equipment intended for mounting) rack mountable blade, JDSU integrated the amplifier, the WSS-based ROADM and the OPM. A large amount of component and networking expertise is necessary to design, build, and support such a subsystem.
15.5.3.5 Consolidation of the behemoths incumbents raises the bardOclaro formed by Bookham acquiring Avanex The creation of Oclaro by Bookham merging with Avanex is an atypical yet important example in the amplifier space. It is atypical because there have been very few mergers and acquisitions involving amplifiers since 2004. It is important because the combined entity will certainly form the largest amplifier supplier in the world. When JDS Uniphase merged with SDL Inc. in 2001 it combined a strong pump laser supplier with a strong amplifier supplier. The U.S. Department of Justice (DOJ) forced JDSU to divest its Zurich pump laser facility because the combined entity would have about 80% of the pump laser market [20]. The Zurich facility was diverted to Nortel Optronics which Oclaro (formerly Bookham) has since acquired.
O
Accelink
D
Fujikura
JDSU
Oclaro D
Furukawa
IPG Photonics
Legend:
P
O
R
D
P*
P
O
R
D
P
O
R
D
RedC Networking
WXZTE
P
O
P = Pump lasers; O = Optical channel monitor;
R
D
R = ROADMs; D = Dispersion compensators
Note: *IPG manufactures pump lasers but they are used in internally designed amplifiers. These pumps are not sold on the merchant market as a stand-alone product today.
FIGURE 15.11 Erbium-doped fiber amplifier suppliers and some of their other products Source: Ovum and company reports
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CHAPTER 15 Market trends for optical amplifiers
The creation of Oclaro did not present a threat to the market in the DOJ’s view. Indeed, Oclaro (formerly Avanex) divested its fab well before the merger so the level of concern is perhaps not as great. The strength of the newly formed company for wavelength managed products is shown in Figure 15.11. The leading amplifier suppliers and other significant players wavelength management products are summarized. Oclaro, joining Bookham and Avanex, joined Furukawa and JDSU which have the most complete wavelength management products as a group. This analysis does not speak to the strength of the supplier in a given area. JDSU, for example, is one of the strongest ROADM suppliers while Oclaro is just starting to penetrate this market with its WSS products after the recent acquisition of Xtellus.
15.5.4 Incumbents support semiconductor optical amplifiers Semiconductor optical amplifiers may be the solution of choice for channelized devices at 10 G, 40 G, or 100 G and for channel amplification in access and enterprise networks. The incumbent amplifiers manufacture InP for other optoelectronic productsdthis technology can be applied to a semiconductor optical amplifier if required. Even if the amplification market was to demand high volumes for these applications, incumbent suppliers are well positioned to support the demand.
15.5.5 Little outside investment 15.5.5.1 Non-existent venture capital spending Optical amplification has little venture capital spending. Indeed the last was some five years ago in 2003 when InPlane Photonics was able to raise $10 million for integrated optical components including an erbium-doped waveguide amplifier [21] and Kamelian raised $6.7 million for a semiconductor optical amplifier [22]. At the time, these technologies were the leading candidates to compete with the erbiumdoped fiber amplifier. Neither technology has been able to displace the incumbent nor have these companies. It is true that SOAs play an important role in integrated optics, but they do not generally compete directly with traditional erbium-doped fiber amplifiers.
15.6 OUTLOOK: MORE OF THE SAME 15.6.1 Incumbents are vertically integrated and have built high barriers to entry The optical amplifier market is distributed; that is, there are many different designs and product types and this distribution favors the market being controlled by incumbent suppliers. The incumbents are getting stronger as they invest through mergers and acquisitions, develop new wavelength management products, and introduce high end products like wavelength managed functionally integrated subsystems.
Acronyms
But the optical amplifier market appears trapped in a vicious cycle. There is not enough profit or rewards to support long-term innovative approaches. The need for amplification at the network edge and in access is becoming an increasing reality although the market has not hit the tipping point yet. New solutions are desperately needed to put the market on a new cost curve to support these demands and enable widespread proliferation. JDSU has recently introduced a photonic integrated amplifier that combines planar light circuit (PLC) expertise with erbium-doped fiber and amplifier designs with the result being a highly integrated amplifier. The footprint is smaller and the amplifier can be used as a booster, preamplifier, or an in-line amplifier, thereby reducing the number of amplifiers required. It is integrated and replaces 50 discrete components with a single chip. And JDSU touts higher performance. While this is not a revolutionary advance, it reduces the number of designs and simplifies the manufacturing while improving the performance. This product introduction is certainly in the right directiondit can support low cost and high performance. In short, we do not see substantive changes in the market over the next few years. We see the incumbents continuing to control the market. We see little new innovation being introduced. We see gradual growth in demand and evolutionary cost reduction to support the demand and the competitive nature of the market. We believe that the network edge, access, and enterprise will present new opportunities that require innovation and hence opportunities for new entrants.
ACRONYMS 1U ASE CATV CWDM dB DCMs DGEF DOJ DSP DWDM EDC EDFA Er/Yb FTTX Gbps IL
Height of equipment intended for mounting Amplified spontaneous emission Cable television Coarse wavelength division multiplexing Decible Dispersion compensating modules Dynamic gain equalization filters Department of Justice Digital signal processor Dense wavelength division multiplexing Electronic dispersion compensation Erbium-doped fiber amplifier Erbium and ytterbium Fiber to the x Gigabits per second Insertion loss
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CHAPTER 15 Market trends for optical amplifiers
InP M&A MSO OCM MEMS OIF ONU OLT OPM nm PCB PLC PON Ps RF RFoG ROADM SNMP SOA TL1 WSS
Indium phosphide Mergers and acquisition Multiple system operator Optical channel monitor Micro electro-mechanical systems Optical Internetworking Forum Optical network unit Optical line terminal Optical performance monitor Nanometer Printed circuit board Planar light circuit Passive optical network Picosecond Radio frequency Radio frequency over glass Reconfigurable optical add/drop multiplexer Simple network management protocol Semiconductor optical amplifier Transaction language 1 Wavelength selective switch
References [1] B. Swanson, G. Gilder, Estimating the Exaflood: The Impact of Video and Rich Media on the Internet (2008). Available from
References
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[15]
[16] [17]
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[21] [22]
release, February 5, 2009. Available from:
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Index A Access networks, 301e303 amplification demand in, 425e426 10Gbps PON, 426 for cable TV, 425 radio frequency over fiber, 426 WDN PON, 426 low cost amplifiers for, 378e381 high power amplifiers, 378e379, 378f, 379f WDM PON, 379e381, 380f metro aggregation networks dimensions v., 304 PONs for, 388e390, 388f SOAs for, 19 Active pump control, for constant output power amplifiers, 224e225 Actuators, in WSS, 32e33, 33f ADC. see Analog-to-digital converter Add coupler, in WB, 27 Add ports, with WSS, 32 Add-drop multiplexers (ADMs), synchronous digital hierarchy, in early DWDM, 24 Additive white Gaussian noise (AWGN), in “Shannon limit,” 61 ADMs. see Add-drop multiplexers ADSL. see Asymmetric digital subscriber line AGC. see Automatic gain control Agile amplifiers, 3e7, 4f, 7f, 11, 423 Amplified spontaneous emission (ASE) in backbone transport networks, 309 in DRA-EDFAs, 257e258, 258f ROPA/EDFAs v., 265f, 266 in EDFAs, 87 in fiber capacity estimation, 64 improved fiber link for reduced, 48 MPI and, 258e259 in power excursion, 207f, 208 Raman pumps and, 324e325, 325f for replacement signals, 170 self-phase modulation and, 72 in “Shannon limit,” 61e62 with SOAs, 401e402, 402f in WDM PONs, 380e381, 380f Amplifier modules, in ROADM node, 37e38, 39f Amplifying stages, in DWDM system with EDFA, 94, 94f Analog-to-digital converter (ADC), high-speed, for high-capacity transmission system, 61
Angular deflection, with LCoS actuators, 33 APC. see Automatic power control APS. see Automatic protection switch Arrayed waveguide gratings (AWG) PLC for, 29 for WDM-POM, 302 ASE. see Amplified spontaneous emission Asymmetric digital subscriber line (ADSL), 302 AT&T photonic mesh network, 285, 286f Automatic gain control (AGC), 120 for EDFAs, power excursion and, 204e206, 204f electrical feedback and feedforward for, 121, 122f Automatic power control (APC), 120 Automatic protection switch (APS), for MFFN trial, 354e355, 355f AWG. see Arrayed waveguide gratings AWGN. see Additive white Gaussian noise
B Backbone transport networks, 307e313 DQPSK for, 309e310 hierarchical metro-backbone architecture for, 311e312 OEO replacement in, 307e308 opaque approach to, 312 optically transparent sub-domains in, 310e311 PM-QPSK for, 310 QPSK for, 310 robust systems for, 309e310 time-to-market strategies for, 308e309 translucent domains in, 311 Bandwidth capacity, increased demand for, 23, 25 BER. see Bit error ratio Bidirectional line switched ring (BLSR), for MFFN trial, 354e357, 355f, 357f Binary phase-shift keying (BPSK), OSNR penalty, 57e58, 57t, 58f Birefringent wedge, LC polarization based switches and, 34 Bit error ratio (BER) of DRA-EDFAs, 257e258, 258f ROPA/EDFAs v., 266 after FEC, 159 OSNR and, 57 power transients and, 202 for SOAs, 406, 406f
445
446
Index
Black box model, of power excursions with channel loss, 209e212, 211f Block turbo code (BTC), with forward-error correction, 71 BLSR, for MFFN trial. see Bidirectional line switched ring Booster modules, in ROADM node, 37e38, 39f BPON. see Broadband PON BPSK. see Binary phase-shift keying Broadband optical power coupler, in WB, 27 Broadband PON (BPON), 389, 391e392t BTC. see Block turbo code Buried waveguides, for EDWA, 373e374, 374f
C Cable TV amplification demand for, 425 amplifier for, 432 C-band DRA-EDFA and, 255e256 DWDM channels in, 95 EDF gain in, 93, 93f CD. see Chromatic dispersion Central office (CO), communication services through, 388 Channel coupling, amplifier gain dynamics and, 229e231, 230f Channel data rate, in optical transmission systems, 48 Channel deactivation component failures and, 162 fiber cuts and, 161e162 Channel demultiplexing structure, for PLC, 29 Channel gain, SHB and, 103e104 Channel interactions, 249 Channel loading, power excursions with reconfigured, 227e228, 227f, 228f Channel loss, power excursions with, 209e212, 211f substituting missing channels for, 214f, 215e216 Channel number, changes in, 160 Channel population, in EDFAs, 105 Channel power control of, 171, 188e189 excursions, propagation of, 228e229, 229f fast optical control of, for power excursion mitigation, 215 OSNR and, 159 pump power and, 156e157, 157f reduction of, 167
SHB and, 164e165, 165f SRS and, 166e167, 167f substitution of missing, 214f, 215e216 Channel power coupling, 224 with constant power amplification, 226 due to gain ripple and tilt, 232, 232f impact of, 228e229, 229f strength between, 226e227 Channel propagation range, 223e224 Channel spacing in DRA-EDFA v. ROPA/EDFAs study, 267e268, 268f hybrid ROPA/EDFAs and, 271 PLC-ROADM network and, 30 Channel waveguides, for EDWA, 362f, 374e375 Channel-to-channel Raman interaction, noise loss with, 92 Chromatic dispersion (CD) data rate increase and, 344 with linear fiber impairments, 60, 60t modulation format and detection scheme and, 49 in OTNs, 314e317, 314f, 315f, 316f of PSCF, 69 in “Shannon limit,” 61e62 solutions for, 298 WDM and, 2 Closed loop systems feedback in, 234e235 optical lasing in, 233e234, 234f power fluctuations in, 234 CM. see Contract manufacturer CMA. see Constant modulus algorithm CO. see Central office Coarse wavelength division multiplexing (CWDM), 367 SOAs amplification of, 409, 410f Coding techniques, novel, 71e72 Coherent detection digital, 51e52, 51f, 61 fiber impairment and, 348 for hybrid ROPA/EDFAs, 271 Coherent optical orthogonal frequency-division multiplexing (CO-OFDM), 52e54 high-speed DAC for, 61 key features of, 53e54, 54f SPM effect in, 72 Coherent transmission, for hybrid ROPA/EDFAs, 271 Coherent-detection, with BPSK, for OSNR penalty, 57e58, 57t, 58f
Index
Colorless add/drop ports creation of, 41 requirements for, 288 for ROADM, 41 with WSS, 32, 41 Component supplier, for optical amplifiers, review of, 427e432 Constant gain amplifiers, 231e233, 232f channel power coupling phenomena and, 224, 225f sustained oscillations with, 236e237, 238e239f control, feedback with, 233 mode, for EDFAs, 106f, 107e108 Constant modulus algorithm (CMA), for PDM, 52 Constant output power amplifiers, 224e231, 227f, 228f, 229f, 230f channel power coupling phenomena and, 224, 225f Constant power amplification channel power coupling with, 226 power instabilities with, 235e236, 235f Constellation diagrams for digital coherent detection, 51, 51f for DPSK, 50, 50f Contract manufacturer (CM), for EDFA production, 372 Control algorithms, 241 dynamic domains method, 241e248 Control domains, for optical power transients, 239e241 Control initiation method, for dynamic domains method, 242 Control operations, triggering of, in dynamic domains method, 243e244 CO-OFDM. see Coherent optical orthogonal frequency-division multiplexing Correlation coefficient, of SHB, 101 Correlation function, of SHB, 101, 101f Cost reduction of EDFA components, 370e372, 370f of optical amplifier components, 436e437, 437f of optical amplifiers, 8e9, 8f Coupling strength, between channels, 226e227 CPM. see Cross-phase modulation Cross saturation, in EDFAs, 105, 111e112 Cross-phase modulation (CPM), 6 Cross-phase modulation (XPM), inter-channel, 73 CWDM. see Coarse wavelength division multiplexing
D
DAC. see Digital-to-analog converter Data rates. see also Channel data rate demand for, 422e423 evolution of, 278 increase in, 7, 344e345 DBPSK. see Differential binary phase-shift keying; Digital phase shift key modulation format DCD. see Digital coherence detection DCF. see Dispersion compensating fiber DCM. see Dispersion compensating modules DD. see Direct detection Decision function, for dynamic domains method, 242 Decisionefractional timeout, in dynamic domains method, 244e245 Deflection angle actuator array technologies, for WSS, 33 Demultiplexer modules, in ROADM node, 38, 39f Demultiplexing filter structure, in WB, 27 Dense wavelength division multiplexing (DWDM), 23 central office use of, 388 challenges with, 83 cost over time of, 421f, 422 current systems, 49 EDFAs for, 3, 83, 93e96, 93f, 94f, 254 amplifying stages for, 94, 94f evolution of, 279e283, 280f, 281f, 282f, 283f, 284f maximum capacity of, 421f, 422 OMS of, 300 performance of, 95 point-to-point, 24, 24f power transients in, 5 requirements of, 111 SDP in, 104 Detection schemes, high spectral-efficiency, 49e52, 50f, 51f DFB lasers. see Distributed feedback lasers DGEF. see Dynamic gain equalizing filters DGT. see Dynamic gain tilt Differential binary phase-shift keying (DBPSK), 49 Differential phase-shift keying (DPSK) constellation diagrams for, 50, 50f PAM with, 50 transmission based on, 49 Differential quadrature phase-shift keying (DQPSK), 50 for backbone transport networks, 309e310
447
448
Index
Differential quadrature phase-shift keying with polarization multiplexing (PM-DQPSK), transient suppression and, 191 Differentially coherent optical transmission systems, 49 Diffraction grating, in WB, 28, 28f Digital coherence detection (DCD), 50e51 constellation diagrams for, 51, 51f high-speed ADC for, 61 polarization diversity for, 51e52, 51f spectral efficiency with, 55t, 56 Digital light processing (DLP) mirror arrays, for WSS, 33e34 Digital phase shift key modulation format (DBPSK), for hybrid ROPA/EDFAs, 271 Digital quadrature phase-shift keying (DQPSK), for hybrid ROPA/EDFAs, 271 Digital subscriber line (DSL), 302 Digital-to-analog converter (DAC), high-speed, for high-capacity transmission system, 61 Direct detection (DD) spectral efficiency with, 55t, 56 transmission based on, 49 Directionless add/drop ports, for ROADM, 41e43, 42f, 43f Dispersion compensating fiber (DCF) for DCM, 94e95 losses of, 95 Raman gain coefficient and, 167 Dispersion compensating modules (DCM) amplifier evolution and, 283, 285 in DWDM, 94e95, 94f EDFA for, 92 Dispersion compensators, for optical amplifiers, 429, 430t Dispersion matched fibers, for DRA-EDFA, 256e257 Dispersive element, in WB, 28, 28f Distributed feedback (DFB) lasers with DRA-EDFAs, 258 for replacement signals, 170 Distributed Raman amplifiers (DRAs) EDFAs with, 92 gain coefficient calculation of, 90e91 peak, 91 spectra for, 89e90, 90f gain shape of, 257, 257f gain spectra of, 96 introduction to, 89 investigation into, 255 noise performance of, 91e92
on-off gain of, 92 OSNR and, 328e329, 328f, 329f properties of, 84 spectral flexibility of, 90 DLP mirror arrays. see Digital light processing mirror arrays Double Rayleigh back scatter (DRBS), MPI and, 259 DP-QPSK. see Dual polarization quadrature phase shift keying DPSK. see Differential phase-shift keying DQPSK. see Differential quadrature phase-shift keying; Digital quadrature phase-shift keying DRA-EDFA. see Hybrid Raman/EDFAs DRAs. see Distributed Raman amplifiers Drop coupler, in WB, 27 Drop ports, with WSS, 32 DSL. see Digital subscriber line Dual polarization quadrature phase shift keying (DP-QPSK) fiber impairment and, 348 for long distance transmission, 347 DWDM. see Dense wavelength division multiplexing Dynamic domains method, 241e248 components of, 242 decisionefractional timeout, 244e245 FCL, 242e243 results with, 245e248, 246f finite state machine simulations, 246e248, 247f, 248f time dependent simulations, 248, 249f triggering control operations, 243e244 Dynamic gain control, of optical amplifiers, 84 Dynamic gain equalizing filters (DGEF) for constant gain amplifiers, 232 for optical amplifiers, 431 for power divergence control, 222 Dynamic gain tilt (DGT), retilting and, 168f, 170 Dynamic loading, with SOAs, 410e411 Dynamic optical filters, for power excursion mitigation, 215 Dynamic optical networking, 2
E
EDF. see Erbium-doped fiber EDFAs. see Erbium-doped fiber amplifiers EDWA. see Erbium-doped waveguide amplifier Electrical cross-connects, in early DWDM, 24
Index
Electrical feedback circuit, for EDFAs, 120e124, 122f, 123f, 124f Electrical feedforward circuit, for EDFAs, 121e124, 122f, 123f, 124f Electrically switched network, photonic mesh network v., 288 Electronic gain control block diagram of, 158, 158f component failures, 161e162 error due to gain spectrum ripple, 163e164, 163f fiber cuts, 161e162 gain variations due to SHB, 164e165, 165f optical power transients and, 157e162, 158f, 160f, 162f from SRS, 171 point-to-point configuration transition to meshed transparent photonic networks, 160 remaining effects, 162e168 stimulated Raman scattering in transmission fiber, 165e168, 166f, 167f in WDM, 158e159 WDM channel number change, 160 Electronic mitigation, for EDFAs, 120 Electronic switching fabrics for optical networks, 26 wavelength-specific filtering v., 24 Electro-optic devices, for undersea transmission systems, 253 Electro-optic parts, of EDFA cost reduction of, 371 production costs of, 369, 369t Embedded monitoring, with ROADM networks, 26e27 EPON standards. see Ethernet passive optical network standards Erbium (Er3+) concentration, in EDF GSHB, 129, 130f, 131, 131f, 132f, 142e144, 143f second hole depth dependence of, 136, 138, 139f Stark energy structure of, 142, 143f Erbium-doped fiber (EDF) cost reduction in production of, 371 gain coefficient, spectra of, 93, 93f GFF for, 93 GSHB in, 125e139 discussion for, 131e139 Er3+ concentration and saturation signal input power dependence of second hole character, 129, 130f, 131, 131f, 132f Er3+ concentration dependence of second hole depth, 136
experiment for, 126e128, 126f at low temperature v. room temperature, 132e134, 133f, 134f measurement scheme at 77K for, 127e128, 127f multigain hole structure in, 141e142, 141f numerical model of, 146e150 positive gain change at room temperature, 143f, 144e145, 145f results for, 128e131 sample of, 126e127, 127f saturating wavelength dependences of main and second hole depth, 134, 135f saturating wavelength dependences of main and second hole widths, 135e136 signal wavelength dependence of second hole depth, 136e139, 137f, 139f wavelength dependence of main and second hole, 129, 130f wavelength dependence of second hole character, 128e129, 128f, 129f production costs of, 368, 369t SHB in, 124e125 Erbium-doped fiber amplifiers (EDFAs), 84e89, 85f, 364e367. see also Hybrid Raman/ EDFAs AGC for, 122e124, 122f, 123f, 124f block diagram of, 364, 364f challenges with, 83 channel population in, 105 components of, 364e365 cost reduction of, 370e372, 370f cost structure for, 367e370, 367f, 369t cost summary for, 376e378, 376t cross saturation in, 105 for DRA-EDFA v. ROPA/EDFAs setup, 265f, 266 DRAs with, 92 for DWDM, 3, 83, 93e96, 93f, 94f, 254 amplifying stages for, 94, 94f dynamic effects in, 104e108, 106f modeling of, 104e108, 106f SHB and, 108e109, 108f energy level scheme of, 84e85, 85f full fiber transmission band with, 70e71 gain in average, 107 calculation of, 85e86 column depth of erbium and, 88e89 constant gain mode for, 106f, 107e108 dynamics, 204
449
450
Index
Erbium-doped fiber amplifiers (EDFAs) (Continued ) optical noise with, 87e88 saturation in, 86 shape of, 257, 257f spectrum of, 36 spontaneous emission with, 87 GEQ in, 119 GSHB in, 125 Er3+ concentration and, 142e144, 143f Er3+ Stark energy structure and, 142, 143f experimental, 140 multigain hole structure in EDF, 141e142, 141f principles of, 139e144 results and discussion for, 141e144 history of, 2 introduction to, 83e85, 85f ISI in, 104 optical power surge in, 119e124 cause of, 119e120, 120f feedback and feedforward control, 121e124, 122f, 123f, 124f feedback control, 120, 121f optical power transients with, 156 output power with, 88e89 in chain, 105e107, 106f cross saturation and, 105 PLC and, 375e376 power excursions and, 204e205, 204f production cost of, 368e370, 369t properties of, 84 SDP in, 104 Size reduction of, 381e382 SOAs v., 104, 393e394 spectral hole burning in, 84, 103e104, 119 dropped channel and, 108e109 simulated evolution of, 110, 110f transient response in inhomogeneous effects in, 108e111, 108f, 110f suppression of, 203 for undersea transmission systems, 253e254, 254f wavelength dependent gain in, 222 in WDM networks, 323e332, 325e329f, 331f, 332f WDM-POM for, 302 Erbium-doped phosphate glasses, for EDWA replacement, 375 Erbium-doped waveguide amplifier (EDWA), 365e366, 365f, 373e375
buried waveguides, 373e374, 374f channel waveguides, 362f, 374e375 cost summary for, 376e378, 376t size of, 374 Ethernet, failure protection mechanism of, 7 Ethernet passive optical network (EPON) standards, 302 Europe, link lengths in, 300f, 301
F Fabry-Perot cavity, SOA and, 366 Fast carrier dynamics, of SOAs, 404 Fast gain control, for power level transients, 6, 7f, 11 FCL. see Fluctuating channel list FEC. see Forward-error correction Feedback in closed loop systems, 234e235 with constant gain control, 233 Fiber Bragg grating, 429 Fiber capacity, estimation of, 62e65, 63f results and implications, 65e67, 66t, 67f Fiber cuts channel deactivation and, 161 with OXC node, 161, 162f power transients and, 161e162, 162f Fiber impairment coherent detection and, 348 DP-QPSK and, 348 Fiber loss coefficient, OSNR gain and, 68, 69f Fiber maintenance current, 342, 343f MFFN for, 341e342 for ROADM, 343, 344f Fiber switches, for MFFM, 351e352, 351f Fiber-to-the-building (FTTB), 302 amplification for, 378e379 amplifier for, 432 Fiber-to-the-cabinet (FTTC), 302 Fiber-to-the-home (FTTH), 302 amplifier for, 432 Figure of merit (FOM), transmission distance and, 69e70 Fill signals, for optical power transient compensation, 189e191 Finish message, in dynamic domains method, 244 Finite state machine simulations, for dynamic domains method, 246e248, 247f, 248f First pass yield, of optical amplifiers, 376e377 Fixed filters, for OADMs, 24e25, 25f Fixed gain flattening devices, SHB and, 119
Index
Fluctuating channel list (FCL), 242e243 decisionefractional timeout with, 244e245 for dynamic domains method, 242 triggering control operations with, 243e244 FOM. see Figure of merit Forward-error correction (FEC) to approach Shannon limit, 71 BER after, 159 data rate increase and, 344 for DRA-EDFAs, 258, 259f of hybrid ROPA/EDFAs, 271e274, 273f for OSNR in high-capacity transmission, 58e59 Four photon mixing (FPM), 6 Four-wave mixing (FWM) effect, 72e73 fill signals and, 191 FPM. see Four photon mixing FTTB. see Fiber-to-the-building FTTC. see Fiber-to-the-cabinet FTTH. see Fiber-to-the-home FWM effect. see Four-wave mixing effect
G Gain in EDFAs average, 107 calculation of, 85e86 column depth of erbium and, 88e89 constant gain mode for, 106f, 107e108 optical noise with, 87e88 saturation in, 86 spontaneous emission with, 87 of SOAs, 402e404, 402f, 403f Gain clamping, for EDFAs, 120 Gain coefficient with DRAs calculation of, 90e91 peak, 91 polarization Raman, 91 spectra for, 89e90, 90f of EDF, spectra of, 93, 93f Gain compression, SHB and, 102e103, 103f Gain deviation, with SHB, 119 Gain dynamics, of optical amplifiers, channel coupling, 229e231, 230f Gain equalizer (GEQ), gain deviation by, 119 Gain flattening filter (GFF) for DRA-EDFA, 257, 257f for DRA-EDFA v. ROPA/EDFAs setup, 265f, 266 for DRA-EDFAs, 259e261, 260f, 261t in DWDM, 94e95, 94f for EDF, 93
Gain ripple, with DRA-EDFA, 256 Gain shape, of DRA-EDFA, 257, 257f Gain spectrum in DWDM, 93f, 94e95 of erbium-doped fiber optical amplifiers, 36e37 gain variations due to SHB, 164e165, 165f with optical amplifiers, 36 Gain spectral hole burning (GSHB), 125 in EDF, 125e139 discussion for, 131e139 Er3+ concentration and saturation signal input power dependence of second hole character, 129, 130f, 131, 131f, 132f Er3+ concentration dependence of second hole depth, 136 experiment for, 126e128, 126f at low temperature v. room temperature, 132e134, 133f, 134f measurement scheme at 77K for, 127e128, 127f numerical model of, 146e150, 147f, 148f, 149f results for, 128e131 sample of, 126e127, 127f saturating wavelength dependences of main and second hole depth, 134, 135f saturating wavelength dependences of main and second hole widths, 135e136 signal wavelength dependence of second hole depth, 136e139, 137f, 139f wavelength dependence of main and second hole, 129, 130f wavelength dependence of second hole character, 128e129, 128f, 129f in EDFAs, 125 Er3+ concentration and, 142e144, 143f Er3+ Stark energy structure and, 142, 143f experimental, 140 multigain hole structure in EDF, 141e142, 141f principles of, 139e144 results and discussion for, 141e144 positive gain change at room temperature, 143f, 144e145, 145f Gain spectrum ripple, electronic gain control error due to, 163e164, 163f Gain transient, of SOAs, 367 Gain/loss ripple, with linear fiber impairments, 60, 60t GDR. see Group-delay ripple Generalized multi-protocol label switching (GMPLS), with ROADM, 40, 118
451
452
Index
Generalized nonlinear Schrodinger equation (GNSE) for fiber capacity estimation, 64 for “Shannon limit,” 62 GE-PON. see Gigabit Ethernet PON GEQ. see Gain equalizer Germany, link lengths in, 301, 301f GFF. see Gain flattening filter Gigabit Ethernet PON (GE-PON), 389, 391e392t Gigabit passive optical network (GPON) standards, 302 10G-EPON, 389, 391e392t market demand for, 421 Gigabit-enabled PON (GPON), 389, 391e392t GMPLS. see Generalized multi-protocol label switching GNSE. see Generalized nonlinear Schrodinger equation Gordon-Mollenauer phase noise, 72e73 GPON. see Gigabit-enabled PON GPON standards. see Gigabit passive optical network standards Group-delay ripple (GDR), with linear fiber impairments, 60, 60t GSHB. see Gain spectrum hole burning Guard-band spacing, OFDM and, 54
H
HFC network. see Hybrid fiber coax network Hierarchical metro-backbone technology, for backbone transport networks, 311e312 High power amplifiers, for access networks, 378e379, 378f, 379f High spectral frequency, hybrid ROPA/EDFAs and, 272e273 High spectral-efficiency modulation formats and detection schemes, 49e52, 50f, 51f High-capacity transmission systems, 12e13 emerging technologies for, 68e73 fiber nonlinearity compensation, 72e73 novel coding techniques, 71e72 novel optical amplification schemes, 70e71 novel optical transmission fibers, 68e70 photonic integrated circuits, 71 spatial multiplexing, 73 introduction to, 48e49 recent developments in, 49e56 high spectral-efficiency modulation formats and detection schemes, 49e52, 50f, 51f orthogonal frequency-division multiplexing, 52e54, 53f, 54f recent demonstrations for, 54e56, 55t
“Shannon limit” for, 61e67 background on, 61e62 capacity estimation framework, 62e65, 63f capacity estimation results and implications, 65e67, 66t, 67f summary for, 73e74 technical challenges in, 57e61 implementation challenges, 60e61 linear fiber impairments, 48, 60, 60t nonlinear fiber impairments, 48, 59 OSNR requirements, 57e59, 57t, 58f, 59t High-order distributed Raman amplification, 71 High-spectral-efficiency optical modulation formats, generation of, 48 Hybrid components, for EDFA production, 370e371, 370f Hybrid fiber coax (HFC) network, 378, 378f Hybrid Raman/EDFAs (DRA-EDFA), 3, 92 first trans-Pacific transmission with, 270, 270f, 271f gain shape of, 257, 257f hybrid ROPA/EDFAs v., 264e270 channel spacing, 267e268, 268f OSNR of, 267, 267f PAI of, 266, 266f power pre-emphasis, 267, 268f setup for, 265e266, 265f signal power in, 269e270, 269f potential of, 330e332, 331f, 332f spectral hole burning in, 104 for undersea transmission systems, 255e264 bandwidth increase with, 256e257, 256f, 257f GFF for, 259e261, 260f, 261t MPI in, 258e259 noise performance for, 261, 262f pump power for, 261e263, 262f simulated Q-values v. experimental data, 264, 264f system performance for, 263e264, 263f testing of, 257e258, 258f, 259f in WDM networks, 323e332, 325e329f, 331f, 332f Hybrid ROPA/EDFAs (ROPA/EDFA) advanced modulation formats and high capacity in, 271e274 hybrid Raman/EDFAs v., 264e270 channel spacing, 267e268, 268f OSNR of, 267, 267f PAI of, 266, 266f power pre-emphasis, 267, 268f setup for, 265e266, 265f signal power in, 269e270, 269f
Index
I
IEC. see International Electrotechnical Commission IFFT. see Inverse fast Fourier transform Inband crosstalk, with linear fiber impairments, 60, 60t Inbound optical amplifiers, for ROADM networks, 35e36 Input isolator, of EDFA cost reduction of, 370e371, 370f production costs of, 368, 369t Input power, in OAs, 156e157, 157f Input tap, of EDFA cost reduction of, 370e371, 370f production costs of, 368, 369t International Electrotechnical Commission (IEC), 9e11 Intersymbol interference (ISI) in EDFAs, 104 OFDM and, 52 Inverse fast Fourier transform (IFFT), for OFDM, 52 IP traffic, in North America, 421, 421f ISI. see Intersymbol interference
J Joint self-phase modulation compensation (J-SPMC), 72 J-SPMC. see Joint self-phase modulation compensation
K Kerr fiber nonlinearity, in “Shannon limit,” 61e62
L Labor, of EDFA cost reduction of, 371e372 production costs of, 369, 369t Large core area fiber, 68e69, 70f Large fiber switch, for MFFN, 352e353, 352f trial for, 354e358, 355f, 356f, 357f Laser oscillations, in metro aggregation networks, 305e306 Laser safety, in WDM networks, 332e334, 334f Lasers, narrow-linewidth, for digital coherent detection, 61 L-band, DRA-EDFA and, 255e256 LC polarization based switches. see Liquid crystal polarization based switches LCoS phase modulators. see Liquid crystal on silicon phase modulators
Linear fiber impairments, mitigation of, 48, 60, 60t Linearized system, for optical power transients compensation of, 191e192 suppression of, 168e169, 168f Link budget, extending, with PONs, 390, 393, 393f network design with, 391e392, 396e397, 397t utility of, 393f, 394e396, 395f, 396f Liquid crystal on silicon (LCoS) phase modulators colorless add/drop ports and, 41 strengths and weaknesses of, 34e35 for WSS, 33 Liquid crystal (LC) polarization based switches colorless add/drop ports and, 41 strengths and weaknesses of, 34e35 for WSS, 33e34 Local message passing in dynamic domains method, 241e242 for network control, 241 Local timer, for dynamic domains method, 242 Long-haul dynamic networks, challenges for, 16e17, 277e294 Long-haul transmission systems higher data rates on, 345e349, 346f, 347f, 349f power divergence in, 222 Long-haul undersea systems advanced amplifier schemes in, 16, 253e274 DRA-EDFAs for bandwidth increase with, 256e257, 256f, 257f GFF for, 259e261, 260f, 261t MPI in, 258e259 noise performance for, 261, 262f pump power for, 261e263, 262f simulated Q-values v. experimental data, 264, 264f system performance for, 263e264, 263f testing of, 257e258, 258f, 259f first trans-Pacific transmission, 270, 270f, 271f introduction to, 253e255, 254f Raman amplification for, 255e264
M Mach-Zehnder interferometers (MZI) PLC and, 29 switching speed with, 29 Maintenance friendly fiber networks (MFFN), 17e18, 349e350, 350f current fiber maintenance, 342, 343f fiber switch technologies for, 351e352, 351f
453
454
Index
Maintenance friendly fiber networks (MFFN) (Continued ) hybrid switch design trial for, 354e358, 355f, 356f, 357f introduction for, 341e342 optical amplifiers for, 352e354, 352f, 354f large fiber switch, 352e353, 352f small fast fiber switch, 353e354, 354f summary for, 358 MCVD. see Modified chemical vapor deposition Mechanical parts, of EDFA cost reduction of, 371 production costs of, 369, 369t MEMS mirror arrays. see Micro electromechanical systems mirror arrays Meshed transparent photonic network for backbone transport networks, 307e313 failure scenarios of, 161e162 for metro aggregation networks, 306e307 point-to-point links transition to, 160, 160f Message based triggers, for dynamic domains method, 244 Metro aggregation networks, 303e307 50 GHz demand in, 423 access network dimensions v., 304 laser oscillations in, 305e306 meshed topologies for, 306e307 ring topologies, 303e306 SOAs for, 19 star and tree topologies, 303 MFFN. see Maintenance friendly fiber networks Micro electro-mechanical systems (MEMS) mirror arrays colorless add/drop ports and, 41 for MFFN, 351, 351f strengths and weaknesses of, 34 MIMO. see Multi-input and multiple-output Mirror arrays DLP. see Digital light processing mirror arrays MEMS. see Micro electro-mechanical systems mirror arrays Modified chemical vapor deposition (MCVD), for EDF production, 365 Modulation formats high spectral-efficiency, 49e52, 50f, 51f for long distance transmission, 347 MPI. see Multipath interference Multi-input and multiple-output (MIMO), 73 Multipath interference (MPI) in DRA-EDFA v. ROPA/EDFAs study, 266 in DRA-EDFAs, 258e259 DRBS and, 259
Multiplexer modules, in ROADM node, 38, 39f Multisource agreement (MSA), for EDFAs, 368 MZI. see Mach-Zehnder interferometers
N Narrow-linewidth lasers, for digital coherent detection, 61 NCM. see Network control and management system NEM. see Network element manager Network architecture, evolution of, 279e283, 280f, 281f, 282f, 283f, 284f Network control and management (NCM) system, 6 Network element manager (NEM), for backbone transport networks, 311 Network management system (NMS) for backbone transport networks, 311 for metro aggregation networks, 305e306 NMS. see Network management system Noise performance with channel-to-channel Raman interaction, 92 for DRA-EDFAs, 261, 262f of DRAs, 91e92 of hybrid ROPA/EDFAs, 271 with optical amplifiers, 87 of Raman amplification in WDM networks, 327e330, 328f, 329f Non zero dispersion shifted fiber (NZDSF), Raman gain coefficient and, 167 Non-dispersion shifted fiber (NZDSF), peak Raman gain coefficient with, 91 Nonlinear fiber impairments compensation for, 72e73 mitigation of, 48, 59 of Raman amplification in WDM networks, 330 signal distortions with, 66e67, 67f North America IP traffic in, 421, 421f link lengths in, 300f, 301 Numerical model, of EDF GSHB, 146e150, 147f, 148f, 149f calculated results of, 149e150 description of, 146e149, 147f, 148f, 149f NZDSF. see Non zero dispersion shifted fiber; Non-dispersion shifted fiber
O
OA. see Optical amplifiers OADMs. see Optical add-drop multiplexers OCM modules. see Optical channel monitor modules OCS. see Variable optical attenuator (VOA)
Index
ODN. see Optical distribution network OFDM. see Orthogonal frequency-division multiplexing OMS. see Optical multiplex section On-off gain of DRAs, 92 Raman, 326e327, 327f Raman Noise Figure v., 327e328, 328f On-off-keying (OOK) channels for backbone transport networks, 309 in DWDM systems, 49 SOAs for, 413 OOK channels. see On-off-keying Operational life, of deployed network, ROADMs and, 26 Optical add-drop multiplexers (OADMs), 24e25, 25f fiber cuts with, 161 key problems with, 28 limitations of, 25, 298e299 Optical amplification EDFAs for, 84e85, 85f novel schemes for, 70e71 amplification to cover full fiber transmission band, 70e71 high-order distributed Raman amplification, 71 Optical amplifier gain control algorithms, with optical amplifiers, 37 Optical amplifiers (OA). see also Waveguide based amplifiers array of options, 420 challenges and opportunities, 13 components of, 428f constant gain, 231e233, 232f channel power coupling phenomena and, 224, 225f constant output power, 224e231, 227f, 228f, 229f, 230f channel power coupling phenomena and, 224, 225f cost reduction and commoditization of, 8e9, 8f cost structure of, 376e378, 376t demand for, 418e419, 433e434, 434f in access networks, 425e426 agility, 423 metro moving to 50 GHz, 423 single channel amplifiers, 424e425 strength of, 421e427 summary for, 426e427, 427t traffic growth in network core, 421e423, 421f, 422f
dispersion compensators for, 429, 430t dynamic changes in, 119 dynamic considerations of, 223e233, 225f constant gain amplifiers, 231e233, 232f constant output power amplifiers, 224e231, 227f, 228f, 229f, 230f dynamic gain control of, 84 EDWA, 365e366, 365f evolution of, 281, 283, 285 first pass yield of, 376e377 in fixed operating modes, 222 future directions for, 381e382 integration, 382, 382f single stage, 381e382 gain dynamics of, channel coupling, 229e231, 230f introduction to, 221e223 key limitation of, 36 with laser oscillations at multiple different wavelengths, 170 limitations of, 221 for long-haul undersea systems, 16, 253e274 introduction to, 253e255, 254f low cost, 18e19, 363e382 access applications, 378e381 cost summary, 376e378, 376t demand for, 418e419 EDFA, 367e372, 367f, 369t, 370f EDWA, 365e366, 365f SOA, 366e367, 366t waveguide based amplifiers, 373e376 for maintenance friendly fiber networks, 341e358 market trends for, 19e20, 417e441 game changer of, 420e421 for MFFN, 17e18, 352e354, 352f, 354f mid-stage access for, 429, 429f optical power transients with, 156, 157f photonic mesh networks requirements for, 285e287 dynamic, 287 optical link control, 285e287, 287f static, 287 in physical layer control, 15e16, 221e240 power fluctuations and, 223 present status of, 11 recent developments in, 3e11 agile amplifiers, 3e7, 4f, 7f wideband amplifiers, 3 ROADM for, 429e430, 431t in ROADM networks, 14, 35e37
455
456
Index
Optical amplifiers (OA) (Continued ) SOA. see Semiconductor optical amplifiers standardization of, 9e11, 10f static changes in, 119 summary for, 249 suppliers of analysis of, 433e440 barriers to entry, 440e441 components, review for, 427e432 consolidation of, 439e440, 439f cost reduction, 436e437, 437f diversity of, 438e439 outside investment, 440 quantity of, 434e436, 435f SOAs and, 440 stagnant, 437e440 strong, 419e420 system vendor challenges, 432e433 for WDM networks, 221e222 next generation, 1e11 Optical attenuator, for PLC, 29 Optical bandpass filtering, with linear fiber impairments, 60, 60t Optical channel monitor (OCM) modules, for ROADM networks, 38, 39f, 239 Optical collimator steering (OCS), for MFFN, 351, 351f Optical crossconnects (OXC), fiber cuts with, 161, 162f Optical distribution network (ODN), PON loss in, 406, 407f, 408 Optical isolator, with PLC, 376 Optical lasing, in closed loop systems, 233e234, 234f Optical link control, for photonic mesh networks, 285e287, 287f Optical mitigation, for EDFAs, 120 Optical multiplex section (OMS), of DWDM systems, 300 Optical networks. see also Photonic networks efficiency of, 155 power transients in, 14e15, 156e168, 157f electronic gain control, 157e162, 158f, 160f, 162f Optical noise, with gain in EDFAs, 87e88 Optical nonlinear effects, 6 Optical power control, with ROADM networks, 26e27, 36 Optical power monitors in PLC-ROADM networks, 29e30 in ROADM networks, 27
Optical power surge in EDFAs, 119e124 cause of, 119e120, 120f feedback and feedforward control, 121e124, 122f, 123f, 124f feedback control, 120, 121f in optical amplifiers, 119 Optical power transients. see also Power level transients acceptable limits to, 159e160 compensation of, fill signals for, 189e191 compensation of SHB, 188e189 compensation of SRS, 171e188 experimental investigation, 183, 185e188, 185f, 186f, 187f, 188f numerical investigation of, 172e173, 173f, 174f simulation results and discussion, 173e179, 175f, 176f, 177f in transparent photonic networks, 179e183, 180f, 182f, 184f control and management of, 239e241 electronic gain control and, 157e162, 158f, 160f, 162f block diagram of, 158, 158f component failures, 161e162 error due to gain spectrum ripple, 163e164, 163f fiber cuts, 161e162 gain variations due to SHB, 164e165, 165f optical power transients and, 157e162, 158f, 160f, 162f point-to-point configuration transition to meshed transparent photonic networks, 160 remaining effects, 162e168 stimulated Raman scattering in transmission fiber, 165e168, 166f, 167f in WDM, 158e159 WDM channel number change, 160 introduction to, 14e15, 156e168, 157f summary for, 192e193 suppression of, 161, 168e171, 168f individual channel power control, 168f, 171 linearized systems, 168e169, 168f replacement signal, 168f, 169e170 retilting element, 168f, 170 Optical reach limitations, in DWDM deployments, 24 Optical signal-to-noise ratio (OSNR) in backbone transport networks, 309 channel power and, 159
Index
data rate increase and, 344 DRA and, 328e329, 328f, 329f of DRA-EDFAs, 257e258, 258f ROPA/EDFAs v., 267, 267f in DWDM systems, 95 gain and fiber loss coefficient, 68, 69f high-capacity requirements for, 57e59, 57t, 58f, 59t modulation format and detection scheme and, 49 with multiple amplifiers, 89 power transients and, 202, 207f, 208, 209f, 213, 213f Raman amplifiers for, 285 for ROADM networks, 36 span length and, 255 WDM and, 2 2x1 Optical space switch, for PLC, 29 Optical spectrum analyzer (OSA), for transient compensation monitoring, 186 Optical supervisor channel (OSC), in DWDM, 94, 94f Optical transmission fibers, novel, 68e70 large core area fiber, 68e69, 70f shorter span length, 69e70 ultralow-loss fiber, 67f, 68, 69f Optical transmission systems channel data rate in, 48 differentially coherent or self-coherent, 49 high-capacity. see High-capacity transmission systems Optical transport networks (OTNs), 17, 298e313 fiber event response in, 341e342 increase capacity of, 344 introduction to, 297e298 meshed topologies, 306e307 next generation, 343e349, 344f, 345f, 346f, 347f, 349f Raman amplification in, 323e334 DRA-EDFA, 330e332, 331f, 332f EDFA, Raman and hybrid amplification scenarios, 323e332, 325f, 326f, 327f laser safety and network implementation issues, 332e334, 334f noise impact in, 327e330, 328f, 329f nonlinear impairments of, 330 signal degradation and temporal fluctuations in, 17, 313e323 PMD, 317e323, 318f, 319f residual chromatic dispersion, 314e317, 314f, 315f, 316f
summary for, 334e335 transport and aggregation networks, 299e313, 300f, 301f access networks, 301e303 backbone transport networks, 307e313 metro aggregation networks, 303e307 Optical trunk line (OTL), PON loss in, 406, 407f, 408 Optical waveguide, of SOAs, 398e399, 399f Optically attenuating elements, in WB, 28, 28f Orthogonal frequency-division multiplexing (OFDM), 52e54, 53f, 54f benefit of, 54 transient suppression and, 191 OSA. see Optical spectrum analyzer OSC. see Optical supervisor channel OSNR. see Optical signal-to-noise ratio OTL. see Optical trunk line OTNs. see Optical transport networks Outbound optical amplifiers, for ROADM networks, 35e36 Output isolator, of EDFA cost reduction of, 370e371, 370f production costs of, 368, 369t Output ports, multiple with LCoS and LC based actuators, 34e35 with MEMS, 34 Output power with EDFAs, 88e89 in chain, 105e107, 106f cross saturation and, 105 in OAs, 156e157, 157f Output tap, of EDFA cost reduction of, 370e371, 370f production costs of, 368, 369t Outside vapor deposition (OVD), for EDF production, 365 OVD. see Outside vapor deposition OXC. see Optical crossconnects
P
PAI. see Path average intensity PAM. see Pulse-amplitude modulation Passive components, of EDFA cost reduction in, 370e371, 370f production costs of, 368, 369t Passive optical networks (PONs), 302, 388e390, 388f 10 Gbps, 426 advantages of, 389
457
458
Index
Passive optical networks (PONs) (Continued ) for broadband service, 387 extended link budget, 390, 393, 393f network design with, 391e392, 396e397, 397t utility of, 393f, 394e396, 395f, 396f operation of, 388e389, 388f SOAs in, 390e397 amplification, 404e409, 404f, 405f, 406f, 407f, 408f reason for, 393e394 standardization and management, 408e409 WDM, 379e381, 380f amplification demand for, 425 Path average intensity (PAI), of DRA-EDFA v. ROPA/EDFAs, 266, 266f PDG. see Polarization-dependent gain PDL. see Polarization-dependent loss PDM. see Polarization-division multiplexing PDM-OFDM. see Polarization-division multiplexing orthogonal frequencydivision multiplexing Photonic cross connects (PXC), 299 Photonic integrated circuits (PIC), 71 Photonic mesh networks. see also Meshed transparent photonic network amplifier requirements for, 285e287 dynamic, 287 optical link control, 285e287, 287f static, 287 of AT&T, 285, 286f electrically switched network v., 288 evolution of, 283, 284f fiber event response in, 341e342 requirements for dynamic, 288e294, 289e290f photonic restoration, 293e294, 293f provisioning through pre-cabling, 291e292, 291f simple photonic restoration techniques, 292e293, 292f transition to, 422, 422f Photonic networks evolution of, 277e285 amplifiers, 281, 283, 285 data rates, 278 network architecture, 279e283, 280f, 281f, 282f, 283f, 284f status quo, 285, 286f WDM, 278e279, 278f, 279f summary for, 294
Photonic restoration for dynamic photonic mesh networks, 293e294, 293f ROADM for, 40 Photonic switches. see Fiber switches Physical layer control, 239e248 distinct control domains and function, 239e241 dynamic domains method, 241e248 components of, 242 decisionefractional timeout, 244e245 FCL, 242e243 results with, 245e248, 246f triggering control operations, 243e244 optical amplifiers in, 15e16, 221e240 transparent networking elements and, 222e223 PIC. see Photonic integrated circuits Planar light wave circuitry (PLC), 373 channel waveguides and, 365f, 374e375 erbium fiber amplifiers and, 375e376 functional diagram of, 29f limitations of, 30 node architecture with, 29 ROADM, 29e30, 29f PLC. see Planar light wave circuitry PMD. see Polarization-mode dispersion PM-DQPSK. see Differential quadrature phase-shift keying with polarization multiplexing Point-to-point links meshed transparent photonic network transition from, 160, 160f transition from, 422, 422f Polarization diversity, for digital coherent detection, 51e52, 51f Polarization Raman gain coefficient, with DRAs, 91 Polarization rotation of light, LC polarization based switches and, 34 Polarization-dependent gain (PDG), with linear fiber impairments, 60, 60t Polarization-dependent loss (PDL), with linear fiber impairments, 60, 60t Polarization-division multiplexing (PDM), 52 Polarization-division multiplexing orthogonal frequency-division multiplexing (PDMOFDM), 52e53, 53f Polarization-mode dispersion (PMD) data rate increase and, 344 limitations from, 298 with linear fiber impairments, 60, 60t modulation format and detection scheme and, 49
Index
in OTNs, 317e323, 318f, 319f solutions for, 298 temporal properties of, 318f, 320e323, 320f WDM and, 2 PONs. see Passive optical networks Population inversion, in SOAs, 399 Port isolation with LCoS and LC based actuators, 34e35 with MEMS, 34 Power combiner, for colorless add/drop ports, 41 Power control, network-wide, 222e223 Power coupling in constant gain amplifiers, 231e233, 232f in constant output power amplifiers, 225 Power divergence in long-haul transmission systems, 222 with optical amplifiers, 221e222 Power excursions. see also Spectral power fluctuations channel propagation of, 228e229, 229f with reconfigured channel loading, 227e228, 227f, 228f Power fluctuations in closed loop systems, 234 optical amplifiers and, 223 Power instabilities with constant power amplification, 235e236, 235f time delay and, 236, 237f Power level management, with optical amplifiers, 37 Power level transients in DWDM systems, 5 fast gain control for, 6, 7f mitigation of, with optical amplifiers, 37 slew rate of, 5 Power pre-emphasis, for DRA-EDFA v. ROPA/EDFAs study, 267, 268f Power pumps, without TECs, 382 Power splitter, for colorless add/drop ports, 41 Power stability, in transparent networks, 233e238, 234f, 235f, 237f Pre-cabling, provisioning through, 291e292, 291f Pre-provisioning, network control and, 241 PSCF. see Pure-silica-core fiber Pseudo-linear regime of transmissions, 65 Pulse-amplitude modulation (PAM), DPSK with, 50 Pump laser barrier to market entry, 437e438 changing role of, 438 suppliers of, 427e428, 428f
Pump power channel power and, 156e157, 157f control features for, 119 for DRA-EDFAs, 261e263, 262f feedforward and feedback signal and, 122e123 output power and, 88e89 Raman gain v., 91 Raman on-off gain v., 326e327, 327f Pump wiggle. see Site dependent pumping Pure-silica-core fiber (PSCF), 68e69 PXC. see Photonic cross connects
Q Q performance, of hybrid ROPA/EDFAs, 271e273, 272f, 273f QAM. see Quadrature amplitude modulation QPSK. see Quadrature phase-shift keying Quadrature amplitude modulation (QAM) constellation diagrams for, 51, 51f transient suppression and, 191 Quadrature phase-shift keying (QPSK) in backbone transport networks, 310 constellation diagrams for, 51, 51f
R Radio frequency over fiber, amplification demand for, 425 Raman amplification. see also Hybrid Raman/ EDFAs evolution of, 285 full fiber transmission band with, 70e71 high-order distributed, 71 for repeaterless systems, 255 for ROADM, 298 schematic for, 430e432, 431f suppliers of, 430e432 for undersea transmission systems, 254, 254f, 255e264 bandwidth increase with, 256e257, 256f, 257f GFF for, 259e261, 260f, 261t MPI in, 258e259 noise performance for, 261, 262f pump power for, 261e263, 262f simulated Q-values v. experimental data, 264, 264f system performance for, 263e264, 263f testing of, 257e258, 258f, 259f in WDM networks, 323e334 DRA-EDFA, 330e332, 331f, 332f EDFA, Raman and hybrid amplification scenarios, 323e332, 325f, 326f, 327f
459
460
Index
Raman amplification (Continued ) laser safety and network implementation issues, 332e334, 334f noise impact in, 327e330, 328f, 329f nonlinear impairments of, 330 Raman effect, 324 Raman gain coefficient DCF and, 167 NZDSF and, 167 SRS and, 165e166, 166f Raman Noise Figure, on-off gain v., 327e328, 328f Raman pumps, 324e325, 325f Raman scattering channel power coupling phenomena and, 224 impact of, 325e326, 326f Rayleigh scattering cause of, 259 ultralow-loss fiber and, 68 Reconfigurable optical add/drop multiplexer (ROADM), 12, 23e44, 24f, 25f, 201e202 agility of, 4, 11, 423 benefits of, 25e27, 299 control loops for, 286e287, 287f development of, 2 dynamic amplifier requirements for, 287 EDFA for, 92 emerging applications and uses of, 39e43 colorless add/drop architectures, 41 directionless add/drop architectures, 41e43, 42f, 43f evolution of, 27e35, 281e283, 283f, 284f planar light wave circuitry, 29e30, 29f wavelength blocker, 27e28, 27f, 28f wavelength selective switch, 30e35, 30f, 31f, 32f, 33f for fiber capacity estimation, 63e64 fiber event response in, 341e342 fiber maintenance for, 343, 344f GSHB in, 146 increased density and functional integration of, 37e38, 39f introduction of, 422 miniaturization of, 38, 39f for optical amplifiers, 14, 35e37, 83e84, 429e430, 431t physical layer control for, 239e248 pump power control features for, 119 Raman amplification for, 298 reconfiguration and restoration times of, 6e7
requirements of, 111 spectral hole burning in, 104 addressing, 119 summary for, 43e44 system model of, 118, 119f transient effects in, 111e112 Relative intensity noise (RIN) with DRAs, 91 with linear fiber impairments, 60, 60t Raman pumps and, 324e325, 325f Remote optically pumped amplifiers (ROPA). see also Hybrid ROPA/EDFAs composition of, 264 investigation into, 255 Repeaterless systems, Raman amplification for, 255 Replacement signal, for optical power transient suppression, 168f, 169e170 Re-provisioning, for dynamic photonic mesh networks, 292e293, 292f Retilting element, for optical power transient suppression, 168f, 170 Return-to-zero differential quadrature phase shift keying (RZ-DQPSK), for long distance transmission, 346e347 RFC. see Robotic fiber connection RIN. see Relative intensity noise Ring topology for access networks, 302 for metro aggregation networks, 303e306 for optical networks, 26 for ROADM system, 118, 119f transition to/from, 422, 422f ROADM. see Reconfigurable optical add/drop multiplexer Robotic fiber connection (RFC), for MFFN, 351, 351f ROPA. see Remote optically pumped amplifiers RZ-DQPSK. see Return-to-zero differential quadrature phase shift keying
S Saturation signal power, in EDF GSHB, 129, 130f, 131, 131f, 132f main and second hole depths and, 134, 135f main and second hole widths and, 135e136 Saturation signal wavelength, in EDF GSHB, second hole depths and, 136e139, 137f, 139f SBS. see Stimulated Brillouin scattering Scalability, for network control, 241
Index
SDH. see Synchronous digital hierarchy SDP. see Site dependent pumping SE. see Spectral efficiency Self-coherent optical transmission systems, 49 Self-phase modulation (SPM), amplified spontaneous emission and, 72 Self-phase modulation compensation (SPMC), 72 Semiconductor optical amplifiers (SOAs), 366e367, 366t ASE noise in, 401e402, 402f basic properties of, 397e401, 398f, 399f, 400f, 401f BER for, 406, 406f cost summary for, 376e378, 376t CWDM amplification with, 409, 410f dynamic loading with, 410e411 EDFAs v., 104, 393e394 fast carrier dynamics of, 404 features of, 402e404, 402f, 403f gain of, 402e404, 402f, 403f compression and recovery in, 409, 409f transient, 367 tuning of, 411e412, 411f incumbent suppliers and, 440 manufacture of, 397e398 for metro and access networks, 19 monolithic integration of, 412f, 413, 413f for OOK channels, 413 operation of, 399e401, 400f, 401f optical waveguide of, 398e399, 399f performance parameters of, 366e367, 366t in PONs, 387, 390e397 amplification, 404e409, 404f, 405f, 406f, 407f, 408f extending link budget, 390, 393, 393f reason for, 393e394 standardization and management, 408e409 population inversion in, 399 spontaneous emission in, 400, 400f stimulated emission in, 400e401, 401f stimulated transitions in, 399e400 switching matrices with, 411e412, 412f in WDM PONs, 379e381, 380f “Shannon limit,” 61e67 background on, 61e62 capacity estimation framework for, 62e65, 63f results and implications, 65e67, 66t, 67f SHB. see Spectral hole burning Signal degradation, in OTNs, 17, 313e323 PMD, 317e323, 318f, 319f residual chromatic dispersion, 314e317, 314f, 315f, 316f
Signal distortions, with fiber nonlinearities, 66e67, 67f Signal power, in DRA-EDFA v. ROPA/EDFAs study, 269e270, 269f Signal quality, OSNR and, 57 Signal-to-noise ratio (SNR), in “Shannon limit,” 61 Simple photonic restoration techniques, for dynamic photonic mesh networks, 292e293, 292f Single channel amplifiers, demand for, 424e425 Site dependent pumping (SDP), in EDFAs, 104 SLD. see Superluminescent diode Small fast fiber switch, for MFFN, 353e354, 354f trial for, 354e358, 355f, 356f, 357f SNR. see Signal-to-noise ratio SOAs. see Semiconductor optical amplifiers SONET. see Synchronous optical networks Spatial multiplexing, 73 Spectral bandwidth, wavelength selective switch and, 34 Spectral efficiency (SE) with digital coherence detection, 56, 56t with direct detection, 56, 56t modulation format and detection scheme and, 49 Spectral flexibility, of DRAs, 90 Spectral hole burning (SHB), 96e104, 101f, 102f, 103f channel gain and, 103e104 correlation coefficient of, 101 correlation function of, 101, 101f dropped channel and, 108e109 in EDF, 124e139 discussion for, 131e139 Er3+ concentration and saturation signal input power dependence of second hole character, 129, 130f, 131, 131f, 132f Er3+ concentration dependence of second hole depth, 136 experiment for, 126e128, 126f at low temperature v. room temperature, 132e134, 133f, 134f measurement scheme at 77K for, 127e128, 127f numerical model of, 146e150, 147f, 148f, 149f results for, 128e131 sample of, 126e127, 127f saturating wavelength dependences of main and second hole depth, 134, 135f
461
462
Index
Spectral hole burning (SHB) (Continued ) saturating wavelength dependences of main and second hole widths, 135e136 signal wavelength dependence of second hole depth, 136e139, 137f, 139f wavelength dependence of main and second hole, 129, 130f wavelength dependence of second hole character, 128e129, 128f, 129f in EDFAs, 84, 96e97, 97f, 119 Er3+ concentration and, 142e144, 143f Er3+ Stark energy structure and, 142, 143f experimental, 140 multigain hole structure in EDF, 141e142, 141f principles of, 139e144 results and discussion for, 141e144 gain compression and, 102e103, 103f gain variations due to, 164e165, 165f mechanism of, 97e99, 98f modeling of, 99e101 positive gain change at room temperature, 143f, 144e145, 145f power excursions and, 15, 204e205, 204f black box modeling of, 209e212, 211f experimental investigation for, 206e209, 206e209f power transient compensation for, 188e189 prediction of, 101e103, 102f in ROADM, 104 addressing, 119 simulated evolution of, 110, 110f Spectral power fluctuations, 15, 201e218 black-box modeling of channel loss with, 209e212, 211f experimental investigation for, 206e209 results, 207e209, 207f, 208f, 209f setup, 206e207, 206f introduction to, 201e203, 202f mitigation of, 215e217 fast optical control of channel power levels, 215 substitution of missing channels, 215e216 wavelength-dependent reach and light path assignment, 216e217, 217f origins of, 203, 203t parameterization of wavelength dependent, 212e213, 213f, 214f physical effects of, 203e206, 203t, 204f, 205f summary for, 218
SPM. see Self-phase modulation SPMC. see Self-phase modulation compensation Spontaneous emission with gain in EDFAs, 87 in SOAs, 400, 400f SRS. see Stimulated Raman scattering Star topology for access networks, 302 for metro aggregation networks, 303 Stark broadening, in EDFAs, 85, 85f Stark energy structure, of Er3+ and EDF GSHB, 142, 143f Stark splitting in EDFAs, 85, 85f spectral hole burning and, 99 Steady-state channel power relationships, 226 Stimulated Brillouin scattering (SBS), fill signals and, 191 Stimulated emission, in SOAs, 400e401, 401f Stimulated Raman scattering (SRS) amplifier evolution and, 283 channel power and, 166e167, 167f power excursions and, 15, 204e205, 204f black box modeling of, 209e212, 211f experimental investigation for, 206e209, 206e209 power transient compensation for, 171e188 experimental investigation, 183, 185e188, 185f, 186f, 187f, 188f numerical investigation of, 172e173, 173f, 174f simulation results and discussion, 173e179, 175f, 176f, 177f in transparent photonic networks, 179e183, 180f, 182f, 184f retilting and, 168f, 170 tilt accumulation with, 84, 165e166 in transmission fiber, 165e168, 166f, 167f Stimulated transitions, in SOAs, 399e400 Superluminescent diode (SLD), as ASE source, 381 Suppliers, of optical amplifiers analysis of, 433e440 barriers to entry, 440e441 consolidation of, 439e440, 439f cost reduction, 436e437, 437f diversity of, 438e439 incumbent, 420 outside investment, 440 quantity of, 434e436, 435f
Index
SOAs and, 440 stagnant, 437e440 strong, 419e420 system vendor challenges, 432e433 Sustained oscillations, in constant gain amplifier networks, 236e237, 238e239f Switching matrices, with SOAs, 411e412, 412f Switching speed, with MZI, 29 Synchronous digital hierarchy (SDH), add-drop multiplexers, in early DWDM, 24 Synchronous optical networks (SONET) in early DWDM, 24 evolution from, 5e6 failure protection mechanism of, 7 maintenance on, 342, 343f replacement of, 341
T
TCM. see Trellis-coded modulation TECs. see Thermoelectric coolers Temperature dependence, of GSHB, 132e134, 133f, 134f positive gain change and, 143f, 144e145, 145f Temporal fluctuations, in OTNs, 17, 313e323 PMD, 317e323, 318f, 319f residual chromatic dispersion, 314e317, 314f, 315f, 316f Thermoelectric coolers (TECs), pumps without, 382 Thin film optical filters, SHB and, 119 Tilt accumulation, SRS and, 84, 165e166 Tilt compensation, in constant gain amplifiers, 232 Time delay, power instability and, 236, 237f Time dependent simulations, for dynamic domains method, 248, 249f Timer based triggers, for dynamic domains method, 244 Traffic topologies, ROADM and, 25e26, 40 Training symbols (TSs), for OFDM, 52e53 Transient compensation fill signals for, 189e191 linearized system for, 191e192 for SHB, 188e189 for SRS tilt, 171e188 experimental investigation, 183, 185e188, 185f, 186f, 187f, 188f numerical investigation, 172e173, 173f, 174f simulation results and discussion, 173e179, 175f, 176f, 177f in transparent photonic networks, 179e183, 180f, 182f, 184f
Translucent domains, in backbone transport networks, 311 Transmission distance, figure of merit and, 69e70 Transmission systems, high-capacity. see High-capacity transmission systems Transparent networking elements, physical layer control and, 222e223 Transparent photonic network, 201e202. see also Meshed transparent photonic network challenges with, 202 economic benefits of, 202 power stability in, 233e238, 234f, 235f, 237f transient propagation in, 179e183, 180f, 182f, 184f Tree topology for access networks, 302 for metro aggregation networks, 303 Trellis-coded modulation (TCM), 72 TSs. see Training symbols Tunable add/drop ports, in photonic mesh network, 288, 289f, 291 Tunable filter array, for colorless add/drop ports, 41
U
ULH. see Ultra-long-haul Ultra-long-haul (ULH) dynamic networks, challenges for, 16e17, 277e294 Ultralow-loss fiber, 67f, 68, 69f Undersea transmission systems. see Long-haul undersea systems Unidirectional path switched ring (UPSR), for MFFN trial, 354e357, 355f, 357f UPSR. see Unidirectional path switched ring
V Variable optical attenuator (VOA) in DWDM, 94e95, 94f for retilting, 168f, 170 Variable wavelength channel passbands with LCoS and LC based actuators, 34e35 with MEMS, 34 VDSL. see Very high bit rate digital subscriber line Very high bit rate digital subscriber line (VDSL), 302 VOA. see Variable optical attenuator
W Waveguide based amplifiers, 373e376 EDWA, 373e375 erbium-doped bulk amplifiers, 375 PLC and erbium fiber amplifiers, 375e376
463
464
Index
Wavelength banding, for OADMs, 25 Wavelength blocker (WB) diagram of ROADM with, 27f functional diagram of, 28f functional stages of, 28 in ROADM networks, 27e28, 288, 289f Wavelength channel power excursions of, 212e213, 213f, 214f shape of, wavelength selective switch and, 34 suppression of, wavelength selective switch and, 34 Wavelength dependent gain channel power coupling phenomena and, 224, 225f in EDFAs, 222 Wavelength dispersive device, in WSS, 32e33, 33f Wavelength division multiplexing (WDM), 23 deployment of, 2 in DWDM, 94, 94f of EDFA cost reduction of, 370e371, 370f output power, 88e89 production costs of, 368, 369t electronic gain control in, 158e159 evolution of, 278e279, 278f, 279f for fiber capacity estimation, 62e65, 63f limitations of, 298 optical amplifiers for, 1e11, 221e222 PONs, 379e381, 380f amplification demand for, 425 Raman amplification in, 323e334 DRA-EDFA, 330e332, 331f, 332f EDFA, Raman and hybrid amplification scenarios, 323e332, 325f, 326f, 327f
laser safety and network implementation issues, 332e334, 334f noise impact in, 327e330, 328f, 329f nonlinear impairments of, 330 Wavelength division multiplexing-passive optical network (WDM-POM), 302 Wavelength routing, with ROADM networks, 26e27 Wavelength selective switch (WSS) application of, 31 colorless add/drop port with, 41 directionless add/drop port with, 42, 42f, 43f functional construction of, 32e33, 33f functional diagram of, 30f node architecture with alternative, 31e32, 32f common, 30e31, 31f with optical amplifier on single switch, 381 for optical amplifiers, 430, 431t performance parameters for, 34 for ROADM network, 30e35, 30f, 31f, 32f, 33f Wavelength-specific filtering electrical switching fabrics v., 24 for traffic managing, 12, 23e24 WB. see Wavelength blocker WDM. see Wavelength division multiplexing WDM-POM. see Wavelength division multiplexing-passive optical network Wideband amplifiers, 3 WSS. see Wavelength selective switch
X
XPM. see Cross-phase modulation
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