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.

In ocean engineering, the demand for fiber-optic vector hydrophones (FOVHs) is increasing. The performance of a FOVH depends on phase consistency between its pressure and acceleration channels, which should match the acoustic field’s properties. Phase polarity, which refers to the alignment of the output signal with the acoustic field direction, is critical. Incorrect phase polarity during sensor assembly can disrupt phase consistency and invalidate directional measurements. This study investigates phase polarity in mandrel FOVHs that use the Phase Generated Carrier (PGC) technique. We develop a theoretical model combining the PGC algorithm with elastic mechanics to analyze the response of acoustic signals. Our model shows that correct demodulated signal polarity requires a specific physical setup: the pressure sensor’s long arm should be on the inner mandrel and the short arm on the outer, while the accelerometer’s positive axis should follow the vector from the long to its short arm. These results are validated through standing wave tube experiments and lake tests. This research provides practical guidelines for the installation and calibration of FOVHs, ensuring phase consistency in underwater acoustic sensing.

We propose a photoelectric common-mode noise (CMN) suppression method based on an initial phase locking scheme for phase-generated-carrier (PGC)-based time-division-multiplexing (TDM) fiber-optic hydrophone (FOH) arrays. Based on the phase noise transfer model, we analyze the influence of the initial phases of the sensing and reference interferometric signals on the phase noise. The results show that the effectiveness of suppression is closely related to the initial phase difference, and locking the initial phases enables stable suppression under balanced or properly calibrated channel conditions. On this basis, we use a 3 × 2 coupler as the reference interferometer to lock the initial phase values. The experimental results show that the proposed method achieves an average phase noise reduction of 8.8 dB in the frequency range from 50 Hz to 1 kHz, which is lower than that obtained by the conventional cross-multiplication and subtraction suppression method under the same experimental conditions. We also experimentally verify the stability of the suppression method, its ability to suppress the transmission noise, and its effect on acoustic signal detection. The experimental results indicate that the method enables long-term and stable CMN suppression within the residual imbalance of the experimental system without affecting acoustic signal detection. The proposed method can effectively reduce noise in PGC-based systems and has broad application potential.

The phase consistency of mandrel fiber-optic hydrophones (MFOHs) is essential for underwater acoustic detection. However, the phase characteristics of such hydrophones based on heterodyne detection are poorly understood. In this study, we propose a method for determining the phase characteristics of MFOHs using heterodyne detection technology, which effectively eliminates the π rad phase difference between hydrophones caused by incorrect configurations. The key novelty of this work lies in revealing that the phase response is jointly governed by two coupled factors: the optical frequency configuration of the Mach–Zehnder interferometer (MZI) and the mandrel winding configuration of the Michelson interferometer (MI). Specifically, when the central optical frequency of the MZI short-arm pulse exceeds that of the long-arm pulse, the phase response is determined by the optical phase difference between the MI long- and short-arm pulses. Under this condition, configuring the inner mandrel as the long arm and the outer mandrel as the short arm ensures that the phase response remains consistent with the underwater acoustic pressure signal. When the MZI frequency relationship is reversed, the roles of the inner and outer mandrels are correspondingly interchanged. Experimental validation confirmed these findings, with the phase deviation between the MFOH probe and the reference hydrophone remaining within 0.03 rad under properly configured conditions. These results establish a robust framework for characterizing MFOH phase responses and provide a critical foundation for advancing heterodyne-based acoustic sensing technologies.

Process parameters affect the performance of fiber-optic hydrophones (FOHs), like sensitivity and hydrostatic pressure resistance. High sensitivity and high hydrostatic pressure resistance are important for deep-sea engineering applications. However, optimizing one typically comes at the expense of the other. We used orthogonal experiments to investigate the effects of several key parameters—including the oxide layer thickness of the elastic tube, the tension pulled on the optical fiber, the adhesive curing temperature, and the cured layer thickness—on both hydrostatic pressure resistance and acoustic pressure sensitivity. The range analysis showed that the oxide layer thickness of the elastic tube has the most significant effect on the hydrostatic pressure resistance, while the fiber tension is the dominant process parameter influencing the sensitivity. The optimal process parameter set for overall performance was determined through a comprehensive scoring method. After optimization, the hydrostatic pressure resistance remained above the experimental average, while the acoustic pressure sensitivity improved from -141.57 dB (re 1 rad/μPa) to -139.70 dB (re 1 rad/μPa). This study provides practical guidance for optimizing the fabrication process of high-sensitivity deep-sea FOHs. It is helpful for fabricating robust, reliable sensors for deep-sea environment.