High-resolution Dual-frequency Fiber Grating Laser Hydrophone Based On Corrugated Diaphragm

Hydrophones have found a wide range of applications in defense,underwater communication and navigation as well as resource exploration.Fiber optic hydrophones have presented advantages including high sensitivity,high compactness and light weight.However,the existing techniques usually utilize an unbalanced interferometer for phase demodulation,which suffers polarization fluctuation and inconvenience in multiplexing.To solve this problem,we propose and demonstrate hydrophones based on dual-frequency fiber grating lasers.In this scheme,the laser is employed as sensing element and a corrugated elastic diaphragm is used to translate acoustic pressure into lateral stress subjected on the fiber.As a result,the acoustic wave can be detected by reading out the frequency variation of the beating signal between the two orthogonal laser modes.Compared with the exiting methods,the fiber laser hydrophone can be wavelength/frequency-division multiplexed in a single fiber.Radio-frequency demodulation is relatively mature and low-cost.The noise of the beat signal is weaker than that of a single polarization mode,as a result of the self-compensation effect.In order to achieve high-resolution acoustic detection,our work begins with of the following aspects: As to the sensing element,the general sensitivity characteristics have been investigated,particularly the effect of laser cavity length as well as the loading length.The mechanism of the frequency noise is analyzed,and its dependence on cavity length has been clarified.The theoretical study indicates that the signal-to-noise ratio is inversely proportional to the square root of the grating separation(which approximately equals the effective cavity length)when point-loading the optical fiber.On the other hand,we have designed the geometry of the elastic diaphragm and studied how to translate the acoustic pressure more effectively into the lateral force on the fiber.The result suggests that,for a corrugated diaphragm,the shape factor q can largely determines conversion efficiency and the resonant frequency.The corrugated diaphragm with higher H/h enables higher conversion efficiency and wider working bandwidth.Based on the above works,a high-resolution fiber laser hydrophone is demonstrated.Experimental result shows an average acoustic sensitivity 185.7 kHz/Pa with a grating separation of 3 mm.By shortening the cavity length,the detection limit reaches 74 ?Pa/Hz1/2 at 1 kHz,which is comparable to 0-th order background noise of deep sea.The working bandwidth of the hydrophone is 1.2 kHz and the dynamic range is up to 112 dB.The comparison study shows that shorter cavity length can enhance detection capability of the hydrophone.The comparison between the flat and the corrugated diaphragm as a transducer confirms the superiority of the corrugated diaphragm in terms of conversion efficiency and working bandwidth.