Underwater Acoustical Problems Of Large-aperture Line Fiber Optic Hydrophone Array

A concern which arises, when considering the use of a very large aperture line fiber optic hydrophone array in order to achieve high array gain, is that the performance of array may have been degraded by the coherence loss of signal received at widely separated sensors. The key underwater acoustical problems have been investigated in this dissertation for application of large-aperture fiber optic hydrophone array. First, summarizes and analyseses the important schemes of multiplexing techniques. Next, the principle of horizontal longitudinal coherence and the performance of a large-aperture,horizontal line array in an ocean multipath environment have been investigated at first. A subspace projection method based on normal mode wave-number is proposed for enhancing array gain. A numerical study of the effects of a solitary internal wave on the signal coherence and conventional beamformer(CBF) is performed in a shallow water waveguide with a thermocline.The mostly points of this dissertation are as follows:·Theoretical and experimental results show that the horizontal spatial correlation in the shallow water is degraded over range for higher frequency signal and oscillation for lower frequency signal.·When there exist more than 1 effective normal modes, the output of the conventional beamformer can be equivalent to the whole summation of the output of plane waves from several different directions.·Theoretical analysis shows that there is a limitation for the array gain loss in an ocean multipath environment that exists more than 1 effective normal modes, which is an advantage for the application of a large-aperture line array.·A new subspace projection method based on normal mode wave-number is presented for improving array gain in an ocean multipath environment. Compared with the conventional matched-field beamformer, which requires 3 dimension search, this proposed method saves computational time for requiring only one dimension search. Furthermore, this method is not sensitive to bottom parameters mismatch.·A numerical study of the effects of a solitary internal wave on the signal coherence and output of CBF shows that the signal correlation coefficient and array gain of CBF are oscillated with time.