## 1 Introduction  
With the application of high-speed and large-capacity optical communication technologies such as reconfigurable optical add-drop multiplexer (ROADM), dense wavelength division multiplexing (DWDM), polarization multiplexing, and coherent optical communication to optical communication systems, the capacity and rate of optical networks are changing at a constantly changing speed. In the future, the optical network will continue to develop towards the goal of being more flexible, high-speed and dynamic, however, with the increasing complexity of network structure and the aggravation of signal impairment, the reliability of optical fiber communication system and optical network will be reduced. It is predicted that the future optical communication system will no longer be a relatively static network system that operates completely following established norms. The dynamic link structure will change with the temperature, component replacement, aging and optical fiber maintenance, and the optical nodes that can achieve \"Plug and Play\" will be required to better allocate optical network resources. This phenomenon improves the transparency of an optical system but increases the complexity of network management [1]. The capacity of optical networks has been growing steadily, and the network architectures are becoming more dynamic, complex, and transparent. Optical signals in high-speed optical fiber networks are vulnerable to various transmission impairments, which can change dynamically with time. Therefore, an appropriate monitoring mechanism must be established throughout the optical network to provide accurate and real-time information on the quality of transmission links and the health of optical signals. optical performance monitoring (OPM) technology detects the performance parameters of transmission links or optical network nodes in optical networks, and understands the network status and signal transmission status in real time, and therefore processes and ensures the normal network operation and signal transmission. It is of great significance to guarantee the performance of signal transmission and optimize the allocation and management of network resources. Therefore, in order to realize the next generation dynamic reconfigurable optical network, it is necessary to monitor the important parameters, isolate faults in time, and optimize the processing to ensure the transmission quality of the network [2; 3]. The optical signal-to-noise ratio (OSNR) is one of the key parameters of OPM. It can realize fault management of an optical transmission system and network, including fault diagnosis and location [4]. It can be used to estimate the bit error rate (BER) and signal quality of the system, and it can directly indicate the channel quality and may realize the management, configuration, and optimization of dynamic reconfigurable optical networks. OSNR can intuitively reflect the change of signal power and noise power of the transmitted optical signal, and it is related to the BER at the end of the optical signal transmitted through the optical amplification link. OSNR is transparent to the bit rate and modulation format of optical signals and suitable for dynamic reconfigurable optical networks. Therefore, OSNR has become an important indicator of optical path performance in OPM and can be used to directly reflect signal quality and system performance. Moreover, understanding OSNR can help effectively utilize available network resources. OSNR is a direct indication of link transmission performance in all kinds of impairment to be monitored, regardless of modulation format. Therefore, the monitoring of OSNR is essential for the overall effective operation of network management system (NMS) and network [5]. At present, most of the practical application of OSNR monitoring methods are out-of-band OSNR testing methods, including the monitoring method based on tunable optical filter [6], arrayed waveguide grating [7], and so on.