Contents 1. Introduction
To dramatically increase the system capacity of the optical fiber communication networks, the optical coherent technology has been successfully utilized in the commercial transmission systems [1]. In such system optical quadrature-phase-shift-keying (QPSK) data-format was implemented as well as the corresponding digital signal processing (DSP) compensation algorithms. To improve the signal quality of QPSK signals in the transmission system, all-optical regeneration technology is a promising approach to directly suppress the noise in the optical domain. Because the information is modulated on the signal's phase, the phase-preserving operation is naturally expected in such all-optical regeneration process. To this end, several novel schemes employing different nonlinear devices have been proposed, e.g. the attenuator-based nonlinear-optical loop mirror (Att-NOLM) [2], the conjugated NOLM structure (Conj-NOLM) [3], the four-wave mixing (FWM) process [4] or the optical parametric amplification [5] in semiconductor optical amplifiers (SOAs). In the view of the integration and the power consumption, the active SOA is more attractive compared to the passive solution, i.e. the highly nonlinear fiber (HNLF) used in the NOLM configuration. Therefore, the SOA-based phase-preserving amplitude regenerator scheme is more suitable to the high-capacity transmission networks which adopts the advanced modulation format. The major issue to use a SOA device to deal with a high-speed signal is the pattern effect coming from its long carrier-recovery time [6]. The conventional regenerators used the continuous-wave (CW) assisted light or the intensity-inverted signal to reduce the pattern effect in SOA. The generation of the inverted signals were accompanied with an extra nonlinear process, e.g. the cross-gain compression (XGC) effect, significantly increasing the complexity of SOA regenerators [7]. Therefore, the CW assisted light scheme is easier implementation, and naturally exist in an FWM regeneration process. In previous works, the very high pump-to-signal power ratio (PSR) is required to sufficiently suppress the pattern effect, e.g. more than 12dB for QPSK cases [5]. This reduces the power level of input signals, consequently the conversion efficiency in the FWM process. In our optical phase conjugation (OPC) work [8], we have thoughtfully investigated the conjugated conversion in a polarization-selected orthogonal-pumped SOA (PSOP-SOA) subsystem. This bidirectional SOA configuration relaxes the requirement of PSR, enabling increasing the optical power of the regenerated signals. To the best of our knowledge, there is still no report on the regeneration results using such scheme. Therefore, in this paper we experimentally investigate the phase-preserving amplitude regeneration for QPSK signals in the PSOP-SOA subsystem. By optimizing the launched optical power and the input optical signal-to-noise ratio (OSNR), we have achieved the maximum signal-quality improvement of 1.84dB in the experiment.
