Measuring of Optical Signal to Noise Ratio in High-Speed Coherent Channels of Optical Transmission Systems

In this paper we discuss the methods for measuring optical signal to noise ratio (OSNR) in high-speed coherent channels of optical transmission systems. The following OSNR measurement methods are presented in this article: out-of-band without channel disabling (but such a method has its own limitations in the field of application) and in-band with or without disabling the optical channel. In-band or in-service methods of measuring of the OSNR in next generation optical dense wavelength division multiplexing (DWDM) systems with coherent optical channels, polarization multiplexing and quadrature amplitude modulation (QAM) format is still under discussion. This article states that it is preferable to calculate OSNR without disabling the optical channels based on an estimation of the correlation relationships between spectral components at predetermined pairs of spaced wavelengths.