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适用范围：This part of IEC 61280 provides a parameter definition and a test method for obtaining opticalsignal-to-noise ratio (OSNR) using apparatus that measures the optical spectrum at amultichannel interface. Because noise measurement is made on an optical spectrum analyzer,the measured noise does not include source relative intensity noise (RIN) or receiver noise.Three implementations for an optical spectrum analyser (OSA) are discussed: a diffractiongrating-based OSA, a Michelson interferometer-based OSA, and a Fabry-Perot-based OSA.Performance characteristics of the OSA that affect OSNR measurement accuracy areprovided.A typical optical spectrum at a multichannel interface is shown in Figure 1. Importantcharacteristics are as follows.• The channels are placed nominally on the grid defined by ITU RecommendationG.694.1.[4]• Individual channels may be non-existent because it is a network designed with opticaladd/drop demultiplexers or because particular channels are out of service.• Both channel power and noise power are a function of wavelength.For calculating the OSNR, the most appropriate noise power value is that at the channelwavelength. However, with a direct spectral measurement, the noise power at the channelwavelength is included in the signal power and is difficult to extract. An estimate of thechannel noise power can be made by interpolating the noise power value between channels.The accuracy of estimating the noise power at the signal wavelength by interpolating thenoise power at an offset wavelength can be significantly reduced when the signal spectrumextends into the gap between the signals and when components such as add-dropmultiplexers along the transmission span modify the spectral shape of the noise. Theseeffects are discussed in further detail in Annex B, and can make the method of this documentunusable for some situations. In such cases, where signal and noise cannot be sufficientlyseparated spectrally, it is necessary to use more complex separation methods, likepolarization or time-domain extinction, or to determine signal quality with a differentparameter, such as RIN. This is beyond the scope of the current document.