Thin Fiber-Optic Hydrophone Towed Array for Autonomous Underwater Vehicle

The fiber-optic hydrophone (FOH) provides new possibilities for underwater acoustic detection and surveillance, particularly when integrated with autonomous underwater vehicles (AUVs). Its lightweight, high sensitivity, and ease of multiplexing are significant advantages. Miniaturizing the FOH and incorporating multiple units into a thin towed array suitable for AUVs, however, presents a challenge. In this study, we present the design and implementation of a 48-element thin FOH towed array, boasting an outer diameter of only 20 mm. The average pressure sensitivity of the FOH is approximately −140.2 dB re rad/<inline-formula> <tex-math notation="LaTeX">$\mu $ </tex-math></inline-formula>Pa, with a fluctuation of less than ±1 dB across a frequency range of 20 Hz–4 kHz. This wide operating frequency range and high sensitivity align well with the demands of AUVs for underwater acoustic detection and surveillance. The average minimum detectable pressure of the array meets practical application requirements, even when compared to ambient sea noise. Noise characterization was assessed through towing tests in real-world environments. Results indicate that flow noise, dominant in the low-frequency band (<200 Hz), is unlikely to be a limiting factor in practical applications, as the design frequencies of the array are located outside this range. The directional of arrival estimation of the fixed and moving sources was successfully demonstrated. Bearings obtained using a conventional beamformer (CBF) closely matched the true values, confirming the feasibility of the thin FOH towed array for practical direction of arrival (DOA) estimation. These findings are instrumental for the future implementation of thin FOH towed arrays on AUVs.