Low-Frequency Optical Hydrophone Based on Active Optical Path Compensation and Low-Coherence Interference

Traditional interferometric optical hydrophones typically employ narrow-linewidth lasers. Although their long coherence length facilitates interference, it also makes the system highly susceptible to spurious interference and phase noise. Low-coherence sources can effectively suppress such parasitic noise; however, their extremely short coherence length imposes stringent stability requirements on the optical path difference (OPD). In underwater environments, ambient disturbances easily cause OPD drift beyond the coherence length, resulting in loss of interference. To address this issue, we propose and experimentally demonstrate a closed-loop hydrophone system that combines low-coherence interferometry with dynamic OPD compensation using a programmable fiber delay line (FDL). Numerical simulations and experiments confirm that the closed-loop compensation algorithm maintains the OPD within the coherence length under environmental perturbations, thereby ensuring long-term stable interference. The prototype achieves a sensitivity of −117.82 dB re rad/µPa at 171 Hz, a noise-equivalent sound pressure spectral density of 8.69 dB re 1 µPa/√Hz, a measured peak phase amplitude of 130 rad, a minimum detectable phase of 14 µrad, and a corresponding logarithmic dynamic range of 139 dB. These results verify the feasibility of integrating low-coherence interferometry with active OPD compensation for low-frequency underwater acoustic detection, under laboratory quasi-static environmental conditions, the proposed closed-loop compensation maintained stable interference throughout a continuous 1.5-h experiment, presenting a differentiated and worthy-of-further-engineering-exploration technical path for high-sensitivity and low-noise optical hydrophones.