Improvement of phase modulation laser Doppler shift measurement method

Abstract

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. In this paper, this kind of phase-modulated laser-Doppler-shift measurement method is improved to simultaneously utilize the useful information in the DC and beat signal for the Doppler-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 Doppler-shift measurement. The signal light intensity terms in DC and beat signal can be eliminated, so the improved phase-modulated laser-Doppler-shift measurement method does not need to measure the signal light intensity, which makes its structure further simplified and a noise channel eliminated. 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. This theoretical result shows that the improved phase-modulated laser-Doppler-shift measurement method will keep the same working mode as un-improved one, and can inherit its advantages. In theory, by comparing the measurement sensitivity curves, it is proved that the improved phase-modulated laser-Doppler-shift measurement method has higher measurement sensitivity and dynamic range than the un-improved one. The useful information included in the DC signal is the modulated signal light intensity transmittance of Fabry-Perot interferometer. So the improvement is essential to introduce the advantages of edge-technique laser-Doppler-shift measurement method based on the Fabry-Perot interferometer into the phase-modulated method for achieving higher performance. Two phase-modulated laser-Doppler-shift measurement methods before and after improvement are separately used to measure the frequency-shifted controllable signal light reflected by a hard object. The experimental results are in accordance with the theoretical analysis results very well. The comparison of experimental result between the two methods shows that the improved phase-modulated laser-Doppler-shift measurement method can approximately double the measurement dynamic range and reduce about 35% measurement standard deviation compared with the un-improved one.