In-Band OSNR Measurement Method for All-Optical Regenerators in Optical Domain

Abstract

We propose an in-band measurement method of optical signal-to-noise ratio (OSNR) output from an all-optical regeneration system with a nonlinear power transfer function (PTF) according to the fact that there are different average gains of signal and noise. For the all-optical quadrature phase-shift keying (QPSK) regenerator as an example, the output OSNR is derived from the input OSNR and the total gain of the degraded QPSK signal. Our simulation shows that the OSNR results obtained by this method are in agreement with those calculated from the error vector magnitude (EVM) formula. The method presented here has good applicability for different data rates but is also useful for analyzing the OSNR degradation of other nonlinear devices in optical communication links. IntroductionWith the exponential growth of data information, optical communication networks are becoming more and more intelligent and elastic [1]. Optical performance monitoring (OPM), especially in optical signal-to-noise ratio (OSNR), will play an important role in optical link diagnosis, flexible bandwidth allocation, and fast network service deployment [2]. In high-speed long-haul optical transmission systems, amplified spontaneous emission (ASE) noise from erbium-doped fiber amplifier (EDFA) is one of the main factors resulting in the degradation of the transmitted signals. Different from fiber dispersion and nonlinearity, it cannot be eliminated by some compensation methods. In this case, all-optical regeneration technology has attracted much attention [3], especially to high-order modulation signal regeneration, such as multi-level amplitude regeneration based on nonlinear interference structure [4,5], multi-stage phase regeneration based on phase-sensitive four-wave mixing (FWM) [6,7], and several regeneration schemes of quadrature amplitude modulation (QAM) signals [8,9].Just like in-line EDFAs, all-optical regenerators are usually integrated into the intermediate nodes of optical links. However, most of the previous studies paid more attention to the regeneration performance in the electrical domain, instead of the optical domain. In other words, the OSNR improvement for all-optical regenerators is seldom considered in experiments due to a lack of more accurate OSNR measurement methods in the optical domain. As is well-known, the traditional OSNR measurement method used in WDM systems is based on the spectral interpolation technique by optical spectrum analysis [10]. Unfortunately, the use of optical filters or all-optical regenerators can give rise to the out-band distortion of the optical spectrum distribution, and then the validation of the interpolation measurement. Secondly, the coherent receivers are extensively used in high-order modulation systems such as those of quadrature phase-shift keying (QPSK) or QAM signals, so the performance of all-optical regenerators is often evaluated in the electrical domain as well. In fact, there are