Objective A weak fiber Bragg grating(WFBG) array is fabricated online via wire drawing by a drawing tower, grating etching using a lithography platform, and primary coating by a ultra-violet(UV) curing device, wherein thousands of WFBGs are multiplexed. A WFBG array has the tensile strength of an ordinary fiber because of the array with no fusion point. It can be coated directly outside the UV-curable coating layer of the array to increase the underwater acoustic sensitivity and form hydrophones, which is expected to result in a towed line array with fine size, large aperture, and strong gain. There are two types of traditional theoretical analyses in case of a WFBG secondary coating. The first method is the two-layer model, which considers the fiber and the primary coating as the first layer and the second coating layer as the second layer. The two-layer model is considerably rough because of the small difference in fiber diameter, primary coating thickness, and secondary coating thickness. The second method is the three-layer model, which comprises an optical fiber layer, a primary coating layer, and a secondary coating layer. The stress values associated with the primary and secondary coating layers are directly equal to the external sound pressure, which exhibits a large error of fiber strain. In this study, a three-layer model according to the actual structure of the WFBG hydrophone with secondary coating is established. Further, the functional relation between the strain of the fiber layer and change in sound pressure can be established according to the boundary conditions of stress and displacement. The phase change law of optical pulse transmission in a fiber affected by sound pressure is studied, which provides the theoretical support required for the preparation of a WFBG hydrophone with secondary coating sensitization.

Methods In this study, a three-layer model for a WFBG hydrophone is established according to the actual structure comprising an optical fiber layer, a primary coating layer, and a secondary coating layer. The undetermined coefficients are used to obtain the stress, strain, and radial displacement in the three-layer regions, which are obtained according to the boundary conditions of radial displacement, radial stress, and axial stress. Further, the law of fiber strain affected by acoustic pressure is obtained, and the phase change rule with respect to the optical pulse in an optical fiber can be understood. High-density polyethylene (HDPE) is considered to be the secondary coating material in the theoretical model simulation, and a 0.4-mm-diameter HDPE-coated WFBG hydrophone is prepared. A 50-m-long WFBG hydrophone is rolled into a 6-cm-diameter ring and placed in a vibrating liquid column. The phase-sound pressure sensitivities of the hydrophone are measured at frequencies of 5, 7.5, 10 Hz, which are compared with those of the bare WFBG array to verify the sensitization effect of the hydrophone.

Results and Discussions The simulation results indicate that the phase-sound pressure sensitivity of the hydrophone increases with the increasing secondary coating thickness. The sensitivity remains unchanged when the radius of the hydrophone reaches 1 mm[Fig.2(a)]. The sensitivity decreases with the increasing elastic modulus of the secondary coating material [Fig.2(b)], indicating that the larger the elastic modulus, the smaller will be the axial strain caused by sound pressure change and the smaller will be the phase change. The sensitivity increases with the increasing Poisson’s ratio[Fig.2(c)], indicating that the larger the Poisson’s ratio, the greater will be the transverse strain caused by sound pressure change and the greater will be the phase change. Theoretical analysis shows that sensitivity can be increased by 19.8 dB with an HDPE coating (Fig.3). The sensitivities of a 50-m-length bare WFBG array are -176.26 dB (1 rad·μPa ^-1)@5 Hz(Fig.6), -170.53 dB@7.5 Hz(Fig.7), and -160.96@10 Hz(Fig.8), whereas those of a 50-m-long and 0.4-mm-diameter HDPE-coated WFBG hydrophone are -132.74 dB@5 Hz(Fig.9), -126.93 dB@7.5 Hz(Fig.10), and -126.04 dB@10 Hz (Fig.11). When compared with the sensitivities of the bare WFBG array, the comprehensive sensitivity of WFBG is greater by approximately 40 dB (Table 2).

Conclusions Thus, a WFBG hydrophone with secondary coating sensitization is proposed in this study. The selection of secondary coating material and thickness of the WFBG array is guided by a three-layer composite stress model. Simulation results show that HDPE (elastic modulus is 0.84 and Poisson’s ratio is 0.38) as the coating material can increase sensitivity by 19.8 dB. The sensitivities of a 0.4-mm-diameter WFBG hydrophone under frequencies of 5, 7.5, 10 Hz are measured using a vibrating liquid column. The overall sensitization effect is approximately 40 dB. Simulation and experimental results show that a high-sensitivity hydrophone can be obtained by coating a WFBG array with 50-m grating spacing via HDPE. The sensitivities are -132.74 dB@5 Hz, -126.93 dB@7.5 Hz, and -126.04 dB@10 Hz, and the fluctuation in frequency response is 6.7 dB.