Sinusoidal phase-modulated signal light through the Fabry-Perot interferometer can produce a beat signal. Moreover, its amplitude monotonically changes with the signal light frequency. So the beat signal amplitude can be used to measure laser-Doppler-shift. In addition to the beat signal, the phase-modulated signal also contains a direct current(DC) signal, and it still contains a large amount of Doppler-shift information, but the information is not utilized, resulting in the waste of Doppler information. I...