Semiconductor optical amplifier space switch BER improvement and guard-time reduction through feed-forward filtering

Abstract

Analysis of the guard-time interval impact on bit error rate (BER) of a linear SOA-based space switch aiming the guard-time reduction is presented, including effects of fiber propagation. The characterization employed feed-forward filtering and optical switching techniques, through several types and lengths of fibers (with and without dispersion compensation). The Wiener filter (with 2 taps) consistently showed a switching performance improvement, with a guard-time (GT) reduction enough to achieve the BER pre-FEC limit. In most configurations the average GT was of $429\text{ps}$ (3 bits GT at $7\text{Gbps}$). The overall best GT result achieved was using $100\text{mA}$ of SOA bias current and applying the Wiener Filter, resulting in a GT of $286\text{ps}$ (2 bits GT at $7\text{Gbps}$). The space-switch operating parameters with focus on the bias current and switching technique configuration were also analyzed, aiming at reducing the SOA-based electro-optical space switch guard-time, allowing the achievement of high commutation rates.

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.