The influence of optical feedback on the longitudinal mode stability of microchip Nd:YAG lasers is demonstrated. Under optical feedback, the longitudinal mode output of microchip lasers relies strongly on the external cavity length, and mode suppression can be achieved. The intensity modulation waveform varies at different external cavity lengths. At higher feedback levels, the laser flips between orthogonal polarizations. Three important factors determining the influence of optical feedback are summarized, which are the external cavity length, longitudinal mode competition, and the external reflectivity.
A self-mixing laser Doppler velocimeter is designed based on dual frequency laser. We can receive two beat signals and analyze them in frequency domain. The value of Doppler frequency shift is half of the distance between the two components in the power spectrum while the sign of Doppler frequency shift can be easily determined by the relative position of the two components in the power spectrum.