Acoustics detection is significant for national marine military defense, resource exploration, and disaster monitoring. Hydrophone technology is a key means of underwater acoustic detection and has been extensively researched. Compared to traditional electronic hydrophones, fiber optic hydrophone (FOH) has advantages such as resistance to electromagnetic interference, small size, light weight, and high detection sensitivity, making them a research hotspot for the new generation of hydrophones. Compared to traditional point-type FOH arrays, FOH based on fiber-optic distributed acoustic sensing technology (DAS), has advantages such as large capacity, high consistency, miniaturization, and low cost. It has gradually become a research hotspot in recent years. In 2017, due to its unique advantages, the DAS technology was first introduced into the field of underwater acoustic detection. This new type of FOH was continuously wound with one fiber, without any fusion splice and other optical components, which greatly reduces the complexity of the system and the manufacturing process and shows the potential application as the distributed FOH array. In 2022, a short tested distributed fiber-optic hydrophone towing array was proposed and demonstrated in a sea trial. We elaborate the comprehensive study on the development of a large-scale distributed fiber-optic hydrophone towing array with 192 independent sensing units, including the optimization of array-structure design, the integration of attitude perception system, the large-scale manufacture technology, the demodulation methods, and the calibration testing. The lake trial test with the performance evaluation on the array towing attitude, the noise test, the spatial gain, and the localization of the artificial target is presented as well.

The distributed FOH array is composed of an acoustic sensing cable sandwiched between front and rear vibration-isolation cables. Each vibration isolation cable is 25 m long, with different designed density ratios. The acoustic sensing cable consists of two 50 m long sections, including the high acoustic pressure sensitivity sensing units, the vibration damping modules, the attitude perception modules, the signal transmission fibers, and the Kevlar tension ropes, with a neutral buoyancy in water. The acoustic sensing unit is specially designed with a fiber evenly wrapped on the composite material structure to enhance acoustic pressure sensitivity. The 50 m long sensing cable includes 96 sensing units, which are wound by a single fiber. Based on the automation of the fiber length control, the constant tension maintenance, and the winding curing process, we achieve the highly efficient and consistent manufacture of the large-scale sensing unit. The attitude perception module has an oil pressure hole to sense the hydraulic pressure and evaluate the depth of the array with an error of less than 2 cm. Besides, the local incline angle can be acquired by the attitude perception chip embedded in the module. With the home-developed signal acquisition terminal and display software, the attitude angle and the depth of the cable can be obtained in real time.

The lake trial test of the developed large-scale distributed fiber-optic hydrophone towing array is carried out in Dongjiang Lake in Hunan Province. When the towing speed is 6 kn, the depth at the front of the array is 14.7 m, while the depth at the tail is 31.3 m. The inclination angle of the acoustic section of the array is about 7.8°, maintaining a good level (Fig. 7). The measured results of the array depth match well with the simulation results. The variation of towing noise with the towing speed shows that the hydrophone towing array has good noise suppression capabilities. The phase time domain spectrum of the 600 Hz signals from 12 channels of the array, as well as the PSD results of the 400 Hz, 500 Hz, and 600 Hz signals are presented in Fig. 9. Although different channels suffer different levels of noise, the response to the line spectrum signal exhibits excellent consistency. Traditional beamforming (CBF) signal processing is performed and the spatial gain of the whole array can be achieved as 16.87 dB (Fig. 10). The arrival angle estimation (DOA) is also conducted at 400 Hz. The relatively bearing time trajectory of the hydroacoustic target is obtained by continuously recording the DOA data (Fig. 11), which indicates that the distributed FOH towing array can achieve target trajectory tracking.

We elaborate on the design, manufacture, and testing process of a large-scale distributed fiber-optic hydrophone towing array, introduce the uDAS signal demodulation system, and analyze the lake trial. The conclusions are listed below:

1) The large-scale distributed fiber-optic hydrophone towing array has an average acoustic pressure sensitivity of -127.44 dB (re rad/uPa) with a standard deviation of 1.2 dB, within the frequency range of 20-1000 Hz.

2) The large-scale distributed fiber-optic hydrophone towing array has real-time attitude perception capability and can maintain good array posture underwater.

3) At a towing speed of 6 kn, the large-scale distributed fiber-optic hydrophone towing array shows good towing noise suppression capability. By array signal processing, the spatial gain of the hydrophone array reaches nearly 17 dB and the DOA estimation is achieved. It can achieve underwater acoustic target location and trajectory tracking.