# In-Band OSNR Measurement Method for All-Optical Regenerators in Optical Domain
## 1 Introduction  
With 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 aremany reports about the in-band OSNR measurements by means of optical delay interferometer [11], polarization-nulling method [12], spectrum analysis techniques [13], the nonlinear approach [14], and data stream intensity autocorrelation [15]. But, these electrical methods are dependent on complex measurement devices, and difficult to be applied in the intermediate transmission links. In addition, many digital signal processing (DSP) based in-band OSNR measurement methods, such as neural network algorithms [16; 17; 18], training sequence assisted technique [19], were also studied. Certainly, an additional photoelectric noise introduced by the coherent receiver has an influence on the accuracy of OSNR measurement. Moreover, the expensive hardware cost and high-speed sampling devices needed in digital coherent receivers hinder the practical application of the electrical method to some extent. In this paper, we present an all-optical measurement method for the output OSNR of all-optical regenerators, which can be calculated from the signal gain and the total gain of the degraded signal, as well as the input OSNR. The proposed method is validated by simulating the all-optical QPSK regenerator consisting of a phase-sensitive amplifier (PSA) and amplitude reshaping unit. Our results show that the OSNRs obtained by this method are in agreement with the calculated data by the error vector magnitude (EVM) of received QPSK signals, and the OSNR can be improved by more than 3 dB in the input OSNR range of 10-26 dB. The remainders of this paper are organized as follows. Section 2 explains the gain characteristics of signal and noise and puts forward the optical-domain measurement method of OSNR for all-optical regenerators. In Section 3, an optical QPSK regenerator is taken into account to verify the feasibility of the proposed measurement method by simulation.