Influence of longitudinal modes on self-mixing signals in Nd:YVO4 feedback interferometers

The influence of longitudinal modes on self-mixing signals in Nd:YVO₄ feedback interferometers has been investigated theoretically and experimentally. When laser feedback interferometers (LFI) are used for large scale measurement, periodic envelope usually appears on the measuring signals, which may reduce the measuring accuracy and even disable the system. Experiments are implemented to investigate the mechanism of this phenomenon through changing the conditions of external cavity and the pumping power. The model of threshold gain modulation is adopted for theoretical analyses. It is found that the periodic envelope is attributed to the synthetic influences of the external cavity length, the number of longitudinal modes, and the relative intensity coefficient of the adjacent longitudinal modes. The theoretical analyses are in good agreement with the experiment results. This paper offers guidance to advance the development of laser feedback interferometers with a large measurement scale.