Study on mechanism of amplitude fluctuation of dual-frequency beat in microchip Nd:YAG laser

Abstract

In the laser heterodyne interferometry based on the microchip Nd:YAG dual-frequency laser, the amplitude of the beat note periodically fluctuates in time domain, which leads to the instability of the measurement. On the frequency spectrums of the two mono-frequency components of the laser and their beat note, several weak sideband signals are observed on both sides of the beat note. It is proved that the sideband frequencies are associated with the relaxation oscillation frequencies of the laser. The mechanism for the relaxation oscillations inducing the occurrence of the sideband signals is theoretically analyzed, and the quantitative relationship between the intensity ratio of the beat note to the sideband signal and the level of the amplitude fluctuation is simulated with the derived mathematical model. The results demonstrate that the periodical amplitude fluctuation of the beat note is actually induced by the relaxation oscillation. And the level of the amplitude fluctuation is lower than 10% when the intensity ratio is greater than 32 dB. These conclusions are beneficial to reduce the amplitude fluctuation of the microchip Nd:YAG dual-frequency laser and improve the stability of the heterodyne interferometry.

Demodulation of Polarimetric Fiber Laser Ultrasonic Sensor With Intensity Noise Cancellation

“…The relaxation oscillation gives rise to an in-phase intensity modulation on each orthogonal polarization mode. This intensity modulation gives rise to a sum and difference frequency shift of the polarization beat frequency through heterodyning the two modes and has been observed in a Nd:YAG microchip laser [25]. A second mechanism is through direct frequency modulation of the laser modes due to small perturbations of the mode's effective refractive indices.…”

“…Based on the analysis of the Nd:YAG microchip laser [25], we treated the intensity noise of the DFB fibre laser as a single frequency signal. The total intensity noise can be treated as the superposition of multiple single noise signals.…”

“…Unlike previous reports using DBR fibre lasers in Refs [7], [21]- [23], a pair of sidebands can be observed near the beat frequency. To confirm the modulation sidebands are due to the relaxation oscillation intensity noise [25], the laser pump power was increased and the frequency difference of the sidebands to the beat frequency showed a direct correlation with the relaxation oscillation RIN peak. The RIN of the DFB fibre laser with different driving currents was measured and shown in Fig.…”