Semiconductor optical amplifier space switch BER improvement and guard-time reduction through feed-forward filtering
Introduction
The exponential data traffic increase of the last decade [[1], [2]] is driving the demand for faster communication including data centers interconnects and their routing switches. Semiconductor Optical Amplifier (SOA) based interconnects are promising for high capacity networks [[3], [4]]. In this sense, SOA-based systems are being considered in 5G [5], all-optical IP [[6], [7]], radio-over-fiber [8], and wavelength multiplexed networks [[9], [10]]. Moreover, SOA-based space switches have low latency [[11], [12]] and can be integrated [[13], [14], [15]], which makes them interesting for the surge of integrated photonic devices [[16], [17], [18]]. However, such features come with some drawbacks, such as relatively lower energy efficiency [19] and nonlinearities [[20], [21], [22]].
Among these undesired nonlinearities, the off-on switching overshoot saturates the photoreceiver during high-state symbols, while also raising the amplitude of low-state symbols. Its occurrence is followed by ringing oscillations, which delays the SOA-based space switch stabilization on the on-state [23]. As a consequence, both these effects increase the guard-time interval (GTI), motivating the GTI reduction with experimental post-processing analysis here presented, including the investigation of the mitigation potential of Feed-Forward Filtering (FFF). Although the usual SOA GTI is lower than a few nanoseconds, data centers interconnects can benefit from any latency reduction.
The bit error rate (BER) was estimated via a software algorithm described here including its capacity to control the equalization parameters. The GTI is assumed as the number of symbols spared between the precise moment when the SOA switch turns on and the moment when the signal starts to be considered in the BER estimation. In this context, the direct impact of an SOA-based space switch GTI on BER was experimentally analyzed here.
A Wiener filter was employed aiming the GTI reduction, and the influence of the filter length was investigated (and compared to the non-filtered signal). Next, the optical fiber link operating parameters influence are characterized as function of the SOA GTI for several fiber spans, bias currents, switching techniques, and pre-impulse durations.
Section snippets
Experimental setup
The experimental setup to acquire the space switched signals and to characterize the SOA switching performance is illustrated in Fig. 1. It is a SOA-based switching setup, in which different fiber spans and operational parameters were employed to evaluate their impact on the system performance through offline signal processing and comparative analysis.
Offline Digital Signal Processing (DSP)
Instead of running a real-time analysis through a bit error rate tester (BERT), we estimated the BER by dividing the electrical signal at the PRBS generator output and acquiring it alongside the PD output, taking into account the delay between channels for each different span. This approach to obtain a BER estimation has advantages and drawbacks. On one hand, we can control our algorithm so it can stop counting errors when the signal is switched-off, enabling qualitative analysis of individual
Results
The analysis assumes a large number of variables, e.g., the number of taps on the Feed-Forward Filter, the switching technique and configuration, and the fiber span characteristics. This section presents the results starting from the simplest setup and most independent variables, and gradually increasing the complexity by including other parameters. For this purpose, the SSMF (which had comparable results to back-to-back measurements) is used first to analyze the filtering impact, switching the
Conclusions
The Wiener filter has shown a consistently positive impact on decreasing the guard-time required for the switched optical signal to achieve the BER FEC limit. Comparing with the non-filtered reference, the Wiener filter with only 2 taps was able to provide BER reduction in all tested scenarios, including variations in the SOA operating parameters (such as bias current and switching technique) and optical link configuration (especially optical fiber length and type). We believe that it helps to
Acknowledgments
This work was partially supported by National Council for Scientific and Technological Development (CNPq — projects 140058/2016-8, 402184/2014-9, 301409/2017-0, 402923/2016-2, and 574017/2008-9) and São Paulo Research Foundation (FAPESP — projects 2017/20121-8, 2014/18791-7, 2007/56024-4, 2015/50063-4, and 2015/24517-8).
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