Analysis of Photoelectric Detection Phase Polarity of Fiber-Optic Hydrophones Based on 3 × 3 Coupler Demodulation Technique

Phase consistency among hydrophones in fiber-optic hydrophone (FOH) arrays is crucial for effective beamforming. In this study, we investigate the photoelectric detection phase characteristics of FOHs based on the 3 × 3 coupler demodulation technique. We develop a theoretical model combining the 3 × 3 coupler demodulation algorithm with coupled-mode theory to analyze acoustic signal responses. Our model reveals that phase shifts from coupler-to-photodetector and coupler-to-sensing-arm connections arise from different mechanisms, and both are capable of causing π rad phase inversions in demodulated signals. We demonstrate that distinct connection configurations can be classified into groups yielding identical polarity outcomes, and that the input port selection for incident light does not affect output signal phase polarity. Experimental results validate these theoretical predictions. This work establishes critical hardware-level prerequisites for phase polarity consistency in FOH arrays, complementing existing calibration techniques and enhancing array performance in underwater target detection and localization.