The hydrophone which use DFB fiber laser as sensing element has many advantages such as thin diameter, high sensitivity, easy to multiplex array, wide dynamic range, immunity to electromagnetic interference and no need of impedance matching. A 4-element fiber laser hydrophone towed linear array is studied. A sleeve with grooves type fiber laser hydrophone is designed and realized. The sensitivity experiment of the hydrophone element shows that the sensitivity is about -135dB(re 1rad/μPa)in the range of acoustic frequency from 20Hz to 1000Hz, and the amplitude fluctuate of the frequency response curve is less than±1.5dB. Based on the hydrophone of this structure and the WDM technology, the 4-element fiber laser hydrophone towed linear array that is encapsulated with polyurethane sleeve is established, and its diameter is 15mm. The experiments of the hydrophone towed linear array in the lake test show that the localization and tracking of the target can be realized by the towed linear array.

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In order to obtain the robust high-resolution beamforming, a high order cross sensor processing (CSP) approach is developed. According to the relation ship between the target bearing and the phase difference of each element receiving signal, this method exploits the property that the same diagonal of covariance matrix with the same phase difference and obtains (2 M −1)(N −1) virtual elements (N is the original array number) by executing M order CSP. The extended virtual elements can effectively increase the physical aperture of linear array, reduce the main lobe width of beam-forming, and improve the bearing resolution. The CSP method accumulates the data on the same sub-diagonal of the covariance matrix, which can decrease the impact of background noise on beam-forming. The theoretical analysis and experimental results both show that this method has high resolution in bearing estimation, compared with the MUSIC method, which has better robustness under the lower signal-to-noise ratio (SNR).

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