Hi I have read a paper, where they distinguish between wavelength and frequency modulation. More specifically: "FMS (frequency modulation spectroscopy) can be broken down into two regimes: wavelength modulation (WM) and frequency modulation (FM). In the case of WM, the modulation depth is very large, generating a large number of sidebands, but the modulation frequency is low (<1 MHz); in the case of FM, the modulation depth is small but the frequency is very high (>100 MHz)." I know the math behind phase modulation, so when they say that a large modulation depth leads to many sidebands, I agree with this. But I don't see why the two domains are labelled "WM" and "FM"? Best, Niles.