1. Introduction

Semiconductor optical amplifiers (SOAs) have long been attractive as small and potentially low-cost amplification devices for metro and regional network systems. Duobinary modulation formats are generally characterized by high dispersion tolerance which can be exploited to allow simple uncompensated transmission over longer distances that are practical for these networks. The most commonly used form of duobinary is low-pass filtering (LPF) duobinary [1]. Another variant is optical duobinary which is generated when a transmitted DPSK signal is demodulated with a narrowband optical filter in the receiver [2]. Moreover, DPSK signals have been shown under some conditions to suppress SOA-induced nonlinear impairments [3,4]. Consequently, the optical duobinary signal format has previously been investigated for WDM transmission over metro and regional systems with in-line SOAs [5,6].

In [7], we extended earlier system studies by demonstrating uncompensated single channel and WDM transmission for both LPF and optical duobinary systems over a 470 km hybrid fiber link with 6 in-line SOAs. Here we expand upon those basic results by describing in detail the limiting nonlinear impairments from the SOAs and the capability of the MLSE receiver to compensate for these effects in both duobinary formats. MLSE digital signal processing technology has been widely studied recently for extending the uncompensated reach of 10 Gb/s signals and providing equalization for linear and nonlinear impairments [8–11]. We find that the MLSE receiver is able to not only partially compensate for some of the accumulated dispersion of about 3300 ps/nm, but also some SOA-induced nonlinearities, especially self-gain modulation (SGM) in both duobinary variants. The optical filter used in these experiments to demodulate the DPSK signals and convert them to optical duobinary is sub-optimal in its shape and bandwidth. Previous results in [10] have shown that MLSE may be effective in partially equalizing impairments caused by non-optimal optical demodulation filters, so the MLSE receiver provides some benefit for the optical duobinary system in this way too. Here we demonstrate transmission over the 470 km hybrid fiber link with 3–4 dB margins over the forward error correction (FEC) threshold with both duobinary formats, although the LPF duobinary exhibits slightly better performance. This appears due in part to a greater sensitivity of the transmitted DPSK signals to SOA nonlinear effects with significant accumulated dispersion as well as non-optimality of the demodulation filter used for optical duobinary. We also observed that for LPF duobinary, the WDM system was primarily limited by cross-gain modulation (XGM) for which MLSE offered only a small nonlinear advantage in comparison to a standard receiver. On the other hand, the optical duobinary WDM system was still largely limited by SGM effects which admitted to significant compensation by MLSE. Therefore the MLSE receiver had a bigger impact on the optical duobinary WDM performance, although it still slightly underperformed the LPF duobinary WDM system.

2. Experimental configuration

The transmission system experimental set-up is shown schematically in Fig. 1. Up to eight wavelengths in the 1530–1536 nm range and spaced by 100 GHz were modulated together with a Mach-Zehnder modulator (MZM) in either the DPSK or LPF duobinary format. The bit rate was 10.7 Gb/s and we used a pseudo-random bit sequence (PRBS) of length 2³¹ -1. The channels were first fully de-correlated with 10 km of standard single-mode fiber and then launched into the first span with total launch power controlled by an erbium doped fiber amplifier (EDFA) and variable optical attenuator (VOA). The launch power was varied to determine the optimal condition balancing OSNR and nonlinear impairments.

 

Fig. 1. Experimental set-up for uncompensated transmission over 470 km hybrid fiber link with 6 SOAs.

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The hybrid fiber link was comprised of two 75 km spans of standard single-mode fiber and four 80 km spans of NZ-DSF (Corning^® LEAF^® fiber). This link might reasonably represent part of a metro/regional network architecture where different fiber types may be present. For example, the 150 km of standard single-mode fiber might be located in the metro network, while the NZ-DSF may be deployed in a regional network with longer link distances. Each span was made to have 17 dB total loss with a VOA at the end of each span. After transmission through the hybrid fiber link, the measurement channel was selected with a 0.6 nm grating filter (GF), amplified, and then filtered again with either a 0.25 nm GF for LPF duobinary signals or a tunable Fabry-Perot (F-P) filter with 5.5 GHz bandwidth to demodulate the DPSK signals. A polarization controller (PC) was adjusted to launch the worst-case polarization into the first fiber span in terms of measured BER at the receiver for all measurements. As mentioned earlier, this F-P filter is not optimal in terms of its shape and bandwidth characteristics but it produces OSNR sensitivity and dispersion tolerance performance close to LPF duobinary and its tunability allows measurement of all 8 channels. Figure 2 demonstrates the close performance of the two transmitters in terms of required OSNR to achieve a BER value of 1×10^-3 as a function of uncompensated dispersion for single channel linear transmission with a standard receiver.

 

Fig. 2. Dispersion tolerance data for LPF duobinary and optical duobinary transmitters measured as the required OSNR to achieve a BER of 1×10^-3 with a standard receiver.

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The SOAs used in these experiments were based on a standard buried heterostructure design utilizing a dilute mode method [12] with low noise figure (typically<7 dB) and high 1 dB saturation power (measured >12 dBm at 1530 nm). The small signal gain at 1530 nm was >16 dB, with polarization dependent gain (PDG) <0.5 dB. The SOA gain values were not quite sufficient to fully compensate for the span losses, with the net loss depending on the input power to the SOAs. The net link loss measured for 3 of the 8 channels from the input of the hybrid fiber link to the output of SOA6 at the end of the link is shown in Fig. 3 as a function of the total launch power into the first span. The average net link loss varies from <1 dB/span for small launch powers to about 1.3 dB/span for a total launch power of 14 dBm.

 

Fig. 3. Net link loss as a function of total launch power into first span of the hybrid fiber link

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Two commercially available receivers were compared in the system experiments to determine and illustrate the compensation capability of MLSE for SOA-induced nonlinear effects. One was a standard receiver with PIN photodetector, trans-impedance amplifier, and associated clock and data recovery circuitry. The second receiver was from the same manufacturer, but had MLSE-EDC circuitry in the back-end electronics. The MLSE receiver digital equalizer comprises a 3 bit A/D converter operating at up to 25 Gsamples/s and a four-state (memory m=2) Viterbi decoder.

3. Experimental results

We first evaluate the effect of MLSE in the link transmission performance for both a single channel system and an 8-channel WDM system by comparing the performance of the MLSE receiver against that of the standard receiver. The BER results measured for channel #5 at 1533.47 nm as a function of the total launch power into the first span for the optical duobinary single channel and WDM systems are shown in Figs. 4(a) and 4(b), respectively.

 

Fig. 4. BER values vs. total launch power into first span for optical duobinary signals over 470 km hybrid fiber link. (a) Single channel, (b) WDM system.

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For low launch powers in the SOA linear regime, the performance advantage of the MLSE receiver with the optical duobinary signals in both single channel and WDM systems is due to partial compensation of chromatic dispersion and the non-optimal demodulation filter impairments. As the launch power into the first span is increased, the single channel system is limited by SGM from the SOAs with the standard receiver to an optimal launch power of about -2 dBm into the first span. The MLSE receiver can significantly increase the SGM tolerance for the single channel optical duobinary as was also observed earlier [11], in this case allowing a higher optimal launch power by about 7 dB. For the 8-channel WDM system, the MLSE receiver again significantly alleviates SGM effects which still appear to limit the launch power with the standard receiver. In this case, the MLSE allows a higher optimal launch power by about 5 dB, and the signals are ultimately limited by XGM rather than SGM with the MLSE receiver. For both single channel and WDM systems, the MLSE receiver produces BER values at optimal launch powers more than an order of magnitude lower than the standard receiver.

The same results measured for BER as a function of total launch power into the first span with the LPF duobinary signals are shown in Fig. 5. In this case, we observe that the MLSE receiver allows a higher optimal launch power for the single channel system by about 4–5 dB by mitigating the SGM impairment incurred in the SOAs. For the WDM system, the launch power is mainly limited by XGM for both the standard and MLSE receivers and the MLSE appears to offer only a minor nonlinear advantage of 1-2 dB in optimal total launch power. In this case, the lower BER obtained with the MLSE receiver is largely by virtue of partial dispersion compensation.

 

Fig. 5. BER values vs. total launch power into first span for LPF duobinary signals over 470 km hybrid fiber link. (a) Single channel, (b) WDM system.

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A direct comparison of the WDM systems using the MLSE receiver with the two duobinary formats is shown in Fig. 6. The results in Fig. 6(a) for channel #5 show the optimal total launch power was about 11–12 dBm for both modulation formats, corresponding to OSNR values of about 24 dB. The launch power was mainly limited by cross-channel nonlinear effects in the SOAs, and perhaps partially by nonlinear phase noise for the optical duobinary format [3,4]. These results suggest that any XGM tolerance advantage of DPSK in the back-to-back state may be lost or reversed in the presence of the large uncompensated dispersion of the link. All 8 channels were measured at the optimal launch power levels and the WDM results expressed as Q values are given in Fig. 6(b). Both formats had Q values well above the assumed enhanced FEC threshold with margins of about 3 dB for optical duobinary and 4 dB for LPF duobinary.

 

Fig. 6. (a) BER vs. total launch power into first span for channel #5 with both WDM systems, (b) Q values for all 8 channels with both formats.

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4. Summary and conclusions

We have demonstrated uncompensated WDM transmission over a 470 km hybrid fiber link and through 6 in-line SOAs using two forms of duobinary and an MLSE receiver with significant margin over the FEC threshold. The hybrid fiber link represents a potential configuration that might be found in a metro/regional network architecture. The MLSE receiver was essential to successful transmission over the link, providing partial dispersion compensation as well as significantly increased tolerance to SOA-induced nonlinear effects, especially for the optical duobinary format using a non-optimal narrowband optical demodulation filter. The results demonstrated that SGM impairments in both formats were amenable to compensation to a significant degree with MLSE. We also observed that with a standard receiver, the optical duobinary WDM transmission results appeared to be limited to a large extent by SGM rather than XGM, consistent with the larger WDM nonlinear tolerance advantage afforded by MLSE for this format. On the other hand, the LPF duobinary WDM system was limited by XGM with the standard receiver and MLSE offered only a small nonlinear advantage in that case. Overall, the slightly better performance of the LPF duobinary WDM system may reflect a higher sensitivity of optical duobinary to nonlinear SOA impairments in the presence of significant accumulated dispersion.