摘要： We discuss and review in-service Optical-Signal-to-Noise-Ratio measurement techniques with a focus on methods relying on optical spectrum analysis. We briefly review the OSNR definition and the measurement procedure employed in early multi-wavelength systems with in-line amplification, and present in detail the development of the spectrum-based OSNR measurement methods to account for polarized, filtered DWDM signals, and further still, to the current generation of DWDM systems based on coherent detection. We present mathematical implementations for the measurement of polarized signals and their evolution to a reference-based method, suitable for measuring polarization-multiplexed signals independent of coherent transmission formats and receiver metrics. The performance of this reference-based technique is illustrated in a wide range of coherent transmission use cases, thus demonstrating its tolerance to fiber non-linearity induced spectral deformation of the signal. We also explain and demonstrate the ability of this technique to discriminate the amplified spontaneous emission noise due to in-line amplifiers from “Gaussian-like” noise generated in a non-linear operating regime. Finally, we present an extension of this OSNR measurement technique for links where in-line filtering causes significant spectral deformations of the signal and we show how it can be applied to troubleshooting and maintenance-monitoring use cases. The OSNR measurement statistics across all test conditions indicate accuracy levels suitable for use in deployed DWDM networks with ROADMs and coherent transponders.