We investigate the performance of a maximum likelihood sequence estimation (MLSE) receiver at 10.7 Gb/s in the presence of two optical nonlinear impairments, cross-phase modulation (XPM) and stimulated Brillouin scattering (SBS). We find that the tolerance to both nonlinearities decreases with larger levels of uncompensated dispersion. Our results also suggest that the MSLE receiver loses its linear regime advantage in comparison to a standard receiver at some dispersion levels in the presence of the nonlinear effects. We demonstrate that long uncompensated links up to 160 km may show better tolerance to the nonlinear effects with a lower dispersion fiber when using an MLSE receiver.
© 2009 OSA
The past several years have witnessed a significant level of research and commercial activity in the application of electronic signal processing techniques for the compensation of optical signal distortion in optical fiber communications systems. For direct detection systems, MLSE digital signal processing has emerged as one of the most effective receive-side electronic compensation technologies. It has been extensively studied and shown to be effective in compensating for linear optical signal distortions such as chromatic dispersion [1–3], polarization-mode dispersion (PMD) , and narrowband optical filtering [5,6], as well as nonlinear distortions such as self-phase modulation (SPM) [7,8] and self-gain modulation (SGM) in semiconductor optical amplifiers . In particular, the ability of MLSE to compensate for chromatic dispersion has drawn considerable interest, approximately doubling the optically-uncompensated reach for nonreturn-to-zero (NRZ) signals as compared to standard receivers.
In comparison to the signal distortion impairments listed above, fewer experimental studies have addressed the performance of an MLSE receiver in the presence of noise-like nonlinear impairments such as cross-gain modulation (XGM), XPM or SBS. In , the behavior of MLSE was measured for three different modulation formats in the presence of strong XGM. The tolerance to XGM of the MSLE receiver varied according to the format and level of uncompensated dispersion, but in general the MLSE showed somewhat less tolerance with larger dispersion. Another study examined the dispersion compensation capability of an MLSE receiver under conditions of SPM alone and SPM plus XPM . In that work, it was found that the presence of XPM significantly diminished the ability of MLSE to compensate for dispersion compared to both the linear regime and to the case in which the signal was impaired with only SPM.
In the work reported here, we further investigate the performance of an MLSE receiver under conditions of noise-like nonlinear impairments. Specifically, we address XPM and SBS effects on NRZ signals transmitted at 10.7 Gb/s, with an approach of looking at the relative nonlinear tolerance for varying levels of uncompensated dispersion. We find that the MLSE receiver generally shows greater degradation, or lower tolerance, to the nonlinearities with increasing dispersion. We also compare the MSLE receiver to a standard adjustable threshold receiver. For a transmission distance of about 75 km over standard single-mode fiber, the MLSE receiver appears to suffer more than the standard receiver from the nonlinearities, losing the OSNR sensitivity advantage it holds in the linear regime. We also find that the relative tolerance to nonlinearity between the MLSE and standard receivers can differ somewhat for XPM and SBS effects depending on the dispersion.
2. Experimental configuration
The general experimental setup used is illustrated in Fig. 1 . The measurement channel was a distributed feedback (DFB) laser at 1552.12 nm externally modulated at 10.7 Gb/s in an NRZ format signal using an Mach- Zehnder modulator (MZM). The pseudorandom bit sequence (PRBS) used was of length 231-1. For dense wavelength-division multiplexing (DWDM) experiments, the measurement channel was combined in the middle of 7 other channels modulated by a separate MZM in a 50 GHz grid. The 7 interfering channels were de-correlated from each other and the measurement channel by passing through a 15 km span of standard single-mode fiber before combining with the test channel. The polarization of the test channel was adjusted to be best aligned with the other channels. For single-channel tests the 7 extra channels were disconnected. The channel(s) were then amplified and the launch power into the fiber span under test was controlled with a variable optical attenuator (VOA). After transmission through the fiber span, the channel(s) were noise-loaded with amplified spontaneous emission (ASE) to control the optical signal-to-noise ratio (OSNR). The measurement channel was then filtered, amplified and filtered again before detection. For most tests, the required OSNR to achieve a BER value of 1x10−3 was determined as a measure of system performance. In some experiments, optical dispersion compensation fiber (DCF) was employed directly after the span to reduce the residual dispersion before detection.
Two receivers from the same manufacturer were compared in the experiments. One was a standard optimal threshold receiver with a PIN photodetector, transimpedance amplifier, and clock and data recovery. The second receiver had MLSE circuitry in the back-end electronics. The MLSE digital equalizer comprises a 3-bit analog-to-digital converter operating at up to 25 Gsamples/s and a four-state (memory m = 2) Viterbi decoder. The bit error rate (BER) of the measurement channel was measured with a BER tester.
3. Experimental results
We initially examine the relative tolerance of the MLSE and standard receivers for three different levels of uncompensated dispersion. The first test was transmission over 125 km of G.652-compliant standard single-mode fiber followed by optical dispersion compensation for 100 km, leaving a residual level of chromatic dispersion corresponding to 25 km. The other tests were uncompensated transmission over 75 km and 100 km fiber spans. Both single-channel experiments limited by SBS and multi-channel experiments limited by XPM were run and received by both the MLSE and standard receivers. Figure 2 shows the results for the single channel and DWDM transmission tests for the residual dispersion level of 25 km.
The data of Fig. 2 shows that the sensitivity of the MLSE and standard receivers to XPM and SBS nonlinearities are essentially the same for the small dispersion case corresponding to a residual dispersion equal to 25 km of fiber, although the MLSE receiver has a small penalty relative to the standard receiver that we have previously reported for near-fully compensated systems . While this penalty may be particular to the specific receivers that we used, there have some suggestions of similar behavior under some conditions elsewhere [1,10]. In any case, we do not find any observable difference in sensitivity to these nonlinear impairments between the two receivers when the residual dispersion is well within the normal tolerance for an NRZ signal with a standard receiver.
Similar transmission experiments were conducted over uncompensated systems with span lengths of 75 km and 100 km to evaluate nonlinear sensitivity differences between the MLSE and standard receivers for these larger dispersion values. The results for the 8-channel DWDM experiments and single channel experiments for both distances are shown in Fig. 3a and Fig. 3b, respectively.
The results in Fig. 3 suggest that for 75 km uncompensated dispersion, the MLSE receiver displays a somewhat lower tolerance to both XPM and SBS than the standard receiver. This is reflected by the fact that while the MLSE receiver starts out with an advantage of about 1 dB in required OSNR in the linear regime because of its dispersion compensation capability, this OSNR sensitivity advantage is lost in the highly nonlinear regime for both impairments. In the XPM-limited tests, the required OSNR for the MLSE and standard receivers becomes nearly equal for channel powers ≥ 10 dBm, while for the SBS-limited signals the required OSNR with MLSE actually exceeds that for the standard receiver for launch power values > 13 dBm. Another noteworthy feature observed for both nonlinearities is that while the MLSE receiver at 100 km and the standard receiver at 75 km have essentially the same required OSNR in the linear regime, the MSLE receiver also degrades more quickly with increasing channel launch power in comparing these two cases. On the other hand, there is some difference in relative performance between the two receivers at 100 km for the XPM- and SBS-limited systems. For both systems, the standard receiver suffers a large dispersion penalty in the linear regime compared to the MLSE receiver. However, for XPM impairments the standard receiver performance degrades more quickly with increasing channel power than the MLSE receiver for this high level of dispersion, while for single channel transmission the standard receiver benefits from SPM-induced eye opening not experienced by the MLSE receiver and thus shows a somewhat smaller nonlinear penalty relative to the linear regime out to a channel power of 13.5 dBm. The results of Fig. 3 are summarized in Table 1 in terms of the channel launch power values measured for a 1 dB penalty relative to linear transmission.
A comparison of the MLSE receiver’s tolerance to nonlinearity in the 1 channel and 8 channel systems for various fiber spans and uncompensated dispersion lengths up to 160 km is provided in Fig. 4 . The data for launch power at 1 dB penalty shows decreasing XPM tolerance for larger dispersion values, a finding generally in agreement with the results in ref . which showed diminished dispersion compensation capability with significant XPM impairment. The SBS-limited single channel data shows a similar trend although the decrease in launch power threshold with increasing dispersion is not as steep as for the XPM-limited DWDM system.
Finally, uncompensated DWDM transmission over 160 km of standard G.652 fiber and an ultra-low-loss G.652 fiber, (Corning SMF-28® ULL fiber) using the MLSE receiver is compared in Fig. 5 . BER was measured as a function of channel launch power without additional noise-loading. In addition to having a 160 km span loss value smaller by more than 4 dB, the ultra-low-loss fiber span also had a smaller residual dispersion at this distance by about 300 ps/nm. These two attributes facilitate better performance by SMF-28® ULL fiber in both the linear and nonlinear regimes using the MSLE receiver for dispersion compensation. In fact, the minimum BER obtained from the standard fiber is matched by the ultra-low-loss fiber at a launch power almost 8 dB lower. The difference in BER at high launch powers largely reflects the MLSE performance with the different dispersion levels.
4. Summary and conclusions
We have studied the relative tolerance of MLSE and standard receivers to XPM and SBS nonlinearities for various levels of dispersion. For small residual dispersion well within the dispersion tolerance of a standard receiver such as 25 km, we observed no differences in the nonlinear sensitivity between the standard and MLSE receivers. However, the MLSE receiver shows a generally decreasing nonlinear tolerance to both XPM and SBS with increasing dispersion, and also a lower tolerance than a standard receiver at around 75 km of residual dispersion. For a long 160 km uncompensated span, an ultra-low-loss fiber with lower attenuation and dispersion provided better system performance in both the linear and nonlinear regimes for an NRZ signal in a DWDM system received with the MLSE receiver.
References and links
1. A. Farbert, S. Langenbach, N. Stojanovic, C. Dorschky, T. Kupfer, C. Schulien, J. P. Elbers, H. Wernz, H. Griesser, and C. Glingener, “Performance of a 10.7 Gb/s Receiver with Digital Equaliser using Maximum Likelihood Sequence Estimation,” European Conference on Optical Communications (ECOC 2004), Stockholm, Sweden, Paper Th4.1.5, (2004).
2. H. Haunstein, and R. Urbansky, “Application of Electronic Equalization and Error Correction in Lightwave Systems,” European Conference on Optical Communications (ECOC 2004), Stockholm, Sweden, Paper Th1.5.1, (2004).
3. J. D. Downie, M. Sauer, and J. Hurley, “Experimental measurements of uncompensated reach increase from MLSE-EDC with regard to measurement BER and modulation format,” Opt. Express 14(24), 11520–11527 ( 2006). [CrossRef] [PubMed]
4. I. L. Lobato Polo, and D. van den Borne, E. gottwald, H. de Waardt, and E. Brinkmeyer, “Comparison of Maximum Likelihood Sequence Estimation equalizer performance with OOK and DPSK at 10.7 Gb/s,” European Conference on Optical Communications (ECOC 2006), Cannes, France, Paper We2.5.3, (2006).
5. M. Rubsamen, P. J. Winzer, and R.-J. Essiambre, “MLSE Receivers for Narrow-band Optical Filtering,” Optical Fiber Communication Conference and Exhibition and The National Fiber Optic Engineers Conference on CD-ROM) (Optical Society of America, Washington, D.C., 2006), paper OWB6 (2006).
6. J. D. Downie, J. Hurley, M. Sauer, S. Lobanov, and S. Raghavan, “Experimental Measurements of the Effectiveness of MLSE against Narrowband Optical Filtering Distortion,” Optical Fiber Communication Conference and Exhibition and The National Fiber Optic Engineers Conference on CD-ROM) (Optical Society of America, Washington, D.C., 2007), paper OMG4 (2007).
7. S. Chandrasekhar and A. H. Gnauck, “Performance of MLSE receiver in a dispersion-managed multispan experiment at 10.7 Gb/s under nonlinear transmission,” IEEE Photon. Technol. Lett. 18(23), 2448–2450 ( 2006). [CrossRef]
8. J. D. Downie, J. Hurley, and M. Sauer, “Behavior of MLSE-EDC with Self-Phase Modulation Limitations and Various Dispersion Levels in 10.7-Gb/s NRZ and Dduobinary Signals,” IEEE Photon. Technol. Lett. 19(13), 1017–1019 ( 2007). [CrossRef]
9. J. D. Downie and J. Hurley, “Performance of an MLSE-EDC Receiver with SOA-Induced Nonlinear Impairments,” IEEE Photon. Technol. Lett. 20(15), 1326–1328 ( 2008). [CrossRef]
10. T. Kupfer, S. Langenbach, N. Stojanovic, S. Gehrke, and J. Whiteaway, “Performance of MLSE in Optical Communication Systems,” European Conference on Optical Communications (ECOC 2007), Berlin, Germany, Paper Th9.1.1, (2007).