The flank array of fiber laser hydrophone takes the fiber laser hydrophone as the sensitive element. A new type of fiber laser hydrophone with double diaphragms is designed, the results of anechoic tank test show that the average sound pressure sensitivity of fiber laser hydrophone is -136.9dB in the middle and high frequency range of 2.5 ~ 10kHz, and the fluctuation amplitude of sensitivity is not more than ±0.5dB. Taking the hydrophone as the sensing unit, a 64-element fiber laser hydrophone flank array is constructed based on wavelength division / time division multiplexing technology and passive homodyne interference detection technology. The lake test shows that the beam forming of the flank array is stable, the target bearing estimation is correct, and the root mean square error of horizontal target bearing estimation is 1.68°.
A fiber laser hydrophone enhancing sensitivity through polyurethane end surface pulling is presented. After the relation between acoustic pressure sensitivity, dynamic performance and polyurethane material of the hydrophone are respectively described by theoretical model and ANSYS simulation, the polyurethane used for enhancing whose parameters are optimized is prepared. Prototypes of the hydrophone are fabricated and tested in vibration liquid column and Qiandao Lake experimental field. The average acoustic pressure sensitivity is -142.70dB(0dB=1rad/μPa)with the fluctuation less than ±2dB in the frequency range of 10~2000Hz. The result shows that DFB fiber laser encapsulated by polyurethane end surface ensures the high sensitivity, its fluctuation used in underwater sound detection is restrained at the same time, which is of great significance to promote the engineering application of fiber laser hydrophone.
We have developed a 64-element fiber laser sensing system based on 8 wavelength and 8 space division multiplexing. The array consists of four 980nm pump source which illuminate eight group distributed feedback fiber lasers(DFB FL) and eight optical switches which addressing in turns. The wavelength shifts of the DFB FL are converted to phase changes by routing the reflected signal through a nearly path balanced fiber Michelson Interferometer(MI). Through introducing an improved symmetric phase recovering method based on a 3×3 coupler, the influence of light intensity and splitting ratio is decreased. A 64 channels real-time synchronous demodulation system has been realized based on FPGA.
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