Ultra-low-frequency fiber optic hydrophone array based on push-pull unbalanced Michelson interferometers.

High-sensitivity detection of ultra-low-frequency (ULF, <10 Hz) signals is a critical challenge for advanced underwater acoustic monitoring, particularly when scaling to large arrays where performance degradation is a primary bottleneck. This paper demonstrates a time-division multiplexed (TDM) ULF fiber optic hydrophone array whose architecture is specifically designed to overcome this integration challenge. The scheme integrates a push-pull sensing structure to enhance the low-frequency acoustic pressure response, an external triggering synchronization mechanism that ensures precise channel demultiplexing while suppressing timing-jitter-induced phase noise, and the phase-generated carrier differential-cross-multiplication (PGC-DCM) algorithm for stable synchronous demodulation. Experimental results show that the developed array exhibits high element-to-element consistency, with the standard deviation of array response amplitude being better than ±0.35 dB. Critically, the array shows exceptional long-term stability, with phase amplitude fluctuations below 0.043 rad over 12 hours of continuous operation. It demonstrates a flat frequency response from 1 Hz to 2 kHz, with an average acoustic pressure sensitivity of -134.61 dB re rad/μPa. Notably, the time-domain response quality at 1 Hz is significantly superior to that of a standard reference hydrophone. The array's equivalent noise pressure (ENP) at 10 Hz is approximately 1.17 mPa/√Hz, which is nearly 8 dB lower than typical ambient sea noise. At 1 kHz, the ENP is 64.51 µPa/√Hz, below the deep-sea state zero noise level. Furthermore, inter-channel crosstalk is less than -60 dB, and the root-mean-square error (RMSE) of two-dimensional direction-of-arrival (DOA) estimation is 2.37°. These key performance indicators confirm that this scheme provides an effective technical pathway for developing high-performance, practical, large-scale ultra-low-frequency fiber optic hydrophone arrays.