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How is spectral purity of a laser assessed?

Spectral purity expresses the degree of monochromaticity of a radiation, and has been early identified as one of the main remarkable characteristics of lasers even before the experimental demonstration of the first laser source. However, since no generic standard definition is available, complementary aspects have to be considered in order to assess spectral purity of a laser, mostly depending on the approach and on the application aimed with a laser set-up. Linewidth is probably the most usual way of quantifying…