Study On The Technology To Eliminate The Polarization-induced Fading Based On Polarization Switching In Fiber Bragg Grating Hydrophone System

The fiber Bragg grating array(FBG) hydrophone array based on path-match interferometry has many advantages, such as small size, simple structure, high reliability and high sensitivity. It has been a development orientation of the large-scale optical fiber hydrophone array. The polarization-induced fading(PIF) is a main factor to affect the interference stability of the hydrophone in the interference system. Pulse polarization switching technology is a main method to solve the PIF in the FBG hydrophone array, which can be realized by the PS-LN modulator at different time slots with different polarizations. The output pulses interference with each other with different polarizations to guarantee the visibility and then the detection sensitivity of the system was improved.For the problem of the instability of the working point of the PS-LN modulator, factors to affect the stability were analyzed based on the modulation theory of the PS-LN modulator. The modulation character and stability character of the PS-LN modulator made by Photline Company are tested. The feedback control system of the working point was built, the control algorithm was designed using Labview software and then the stable control of the working point was realized. Finally, the polarization switching technology and the working point stable control technology were used in a four-element FBG hydrophone array and the interference waveforms of different polarization channels were obtained. The visibility of bigger than 0.7 was guaranteed and the PIF in the FBG hydrophone array could be eliminated.This paper will provide important theoretical and experimental references for the practical applications in engineering and will provide strong technical support to solve the PIF in the large-scale optical fiber hydrophone array.