Fiber optic hydrophones are a kind of underwater acoustic information sensor built on optical fiber and optoelectronic technology platform. They mainly use optical coherence detection technology to convert an underwater acoustic signal into an optical signal and transmit it to the signal processing system through optical fiber so that useful sound data can be obtained. They have the advantages of high sensitivity, wide dynamic range, resistance to electromagnetic dry radiation, flexible structure, being ready to form large-scale arrays, and being suitable for extremely harsh conditions. Thus, since their advent, they have developed rapidly under the impetus of huge military strategy and civilian value and become an important development direction of modern optical sensing technology and underwater acoustic sensing technology. In recent years, with the increase in the frequency of maritime shipping and seabed mining activities, there are higher requirements for the detection ability of underwater targets, and the research focus of fiber optic hydrophones has gradually moved closer to the array multiplexing technology. However, as one of the key technologies of the underwater acoustic detection system, the hydrophone unit technology cannot be ignored, and the probe technology is particularly important. Fiber optic hydrophone probes will be deformed by the hydrostatic pressure in a deep-sea environment, which affects the detection performance of underwater acoustic signals and even makes the probe unstable and cannot continue to operate in the deep-sea environment. For the sake of ensuring that the hydrophones can work stably and effectively, a high-precision shape measurement method is urgently needed to provide an accurate and reliable approach to detecting the structural deformation of the fiber optic hydrophone probe. Traditional electrical sensors are difficult to be used for the deformation monitoring of hydrophone probes due to factors such as size and electromagnetic interference. Fiber Bragg grating cannot perform the fully distributed monitoring of small fiber optic hydrophone probes due to low spatial resolution. In response, this paper proposes a method with high spatial resolution and high precision based on optical frequency domain reflectometry to monitor the deformation of the hydrophone probe.

In the experiment, two protection methods are adopted to improve the pressure resistance of the probe, namely covering probe 1 with soft rubber and encapsulating probe 2 by a metal skeleton. The optical fiber is tightly wound on the surface of the hydrophone probes. To make the probes pressurized as evenly as possible, we use a special metal pressure tank, which can be filled with water to simulate an actual water pressure environment. After setting the system parameters, we gently place the two hydrophone probes protected by different methods in the pressure tank and fix them at the same position. Then, we gradually increase the pressure distribution in the tank to 0.5, 1.0, 2.0, 3.0, 4.0, 5.0, and 6.0 MPa. The self-developed distributed optical fiber shape monitoring analyzer is used to measure the wavelength shift of the optical fiber Rayleigh scattering spectrum due to the deformation of the probes in two different protection modes.

Affected by external pressure, the deformation of the hydrophone probes by extrusion will cause the optical fiber wound on the thin-walled cylinders of the probes to shrink inward, which induces the optical fiber to strain and results in a "blue shift" of the Rayleigh backscattering spectrum in the optical fiber (Fig. 6). Then, the two-dimensional shape reconstruction of the probes is realized using the relationship of the spectral wavelength shift, strain, and the radius of the probe cylinder by the method of inverse integration (Fig. 9). The results show that the diameters of the hydrophone probes gradually decrease with hydrostatic pressure increasing, and there is an approximately linear relationship (Fig. 7), with periodic depressions appearing in the circumferential direction. When the hydrostatic pressure reaches 6 MPa, the depth of three periodic depressions is up to 124 μm (Fig. 10). It is further verified that the performance of the probe encapsulated by a metal skeleton is slightly better than that of the probe covered with soft rubber. The experimental results are consistent with the simulation results of pressure resistance based on ANSYS (Fig. 5).

In the present study, we propose a method for measuring the deformation of fiber optic hydrophone probes based on optical frequency domain reflectometry. The pressure analysis of the probe with ANSYS indicates that there are three periodic depressions in the circumferential direction of the probes under static pressure. In terms of theoretical research, we establish a hydrophone probe deformation reconstruction method based on the wavelength shift in the optical fiber Rayleigh scattering spectrum. In terms of experiments, the optical fiber is tightly wound on the surface of the fiber optic hydrophone probes to sense the strain generated by the probe deformation. The high-precision distributed optical fiber shape measuring instrument independently developed by the laboratory is used to measure the wavelength shift of the optical fiber Rayleigh scattering spectrum with static pressure, and the deformation of the hydrophone probes under static pressure is further reconstructed. The experimental results show that when the static pressure reaches 6.0 MPa, the circumferential depression depth of the hydrophone probes is up to 124 μm. This research provides powerful support for the development and optimization of the material and structure of fiber optic hydrophone probes, which can thereby reduce the cost and cycle of developing hydrophone probes.