Investigation of the Suppression Effect on Laser Phase Noise in Very-Low-Frequency Elliptic-Fitting Phase Difference Measurement System

For hydrophones with very-low frequency (VLF) and high detection resolution, elliptic-fitting phase-difference demodulation technology (EF-PDDT) will be investigated for suppressive effect on laser phase noise (LPN) and advantage of measuring VLF signal. The suppressive effect was analyzed by examining two types of phase noise resulting from center-frequency drift and linewidth of laser. The experimental results demonstrate that EF-PDDT achieves a suppressive effect of 44 dB on the phase signal, and the effect of 48 dB can be achieved on a wider linewidth laser. In the detection of VLF signals, EF-PDDT has higher information-utilization efficiency compared to quadrature phase demodulation technology, resulting in a reduction of data volume by 66 dB and circumventing the limitations imposed by long-term sampling due to equipment performance, thereby enhancing spectral resolution. Hence, EF-PDDT is an effective solution for improving the detection resolution of VLF signals and finding applications in fields, such as underwater communication and seismic-wave monitoring.