Both the phase modulation and frequency shift evaluation on laser vibrometer

Abstract

Sinusoidal phase-modulated signal light through the 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 frequency shift. In addition to the beat signal, the phase-modulated signal also contains a direct current signal, and it still contains a large amount of frequency shift information, but the information is not utilized, resulting in the waste of information. In this paper, this kind of phase-modulated laser-Doppler frequency shift measurement method is improved to simultaneously utilize the useful information in the DC and beat signal for the frequency shift measurement. The specific method is to use the ratio of beat signal amplitude to DC signal amplitude to define a new parameter used in frequency shift measurement. The signal light intensity terms in DC and beat signal can be eliminated, so the improved measurement method does not need to measurement the signal light intensity, which makes its structure further simplified. By comparing the frequency change curves between the newly defined parameter and the beat signal amplitude theoretically, we find that they have the same distribution rule. In theory, by comparing the measurement sensitivity curves, it is proved that the improved phase-modulated frequency shift measurement method has higher measurement sensitivity and dynamic range than the unimproved one.