Vibration displacement immunization model for measuring the free spectral range by means of a laser self-mixing velocimeter

In this paper, a vibration-displacement immunization model is proposed to measure the free spectral range of a resonant cavity by using a laser self-mixing velocimeter. The validity of this method is demonstrated by the experimental results, which can effectively get rid of low measurement accuracy related to the self-mixing vibration system due to the vibratory displacement. According to the periodic waveform separation characteristic of the self-mixing velocity signal, the free spectral range of a multilongitudinal mode diode laser is calculated to be 88.24 GHz. Moreover, the influences of different target velocities and signal sampling frequencies on the free spectral range have been analyzed in detail from the theoretical analysis. In the case of high signal sampling rate and low velocity, from which the undistorted velocity signal waveform at the integral order external cavity mode keeps stable, it is possible to obtain relatively accurate measured results.