Key Technology Of Eliminating Polarization-induced Fading In The Fiber Gratting Hydrophone

At present,the part of fiber gratting hydrophone under water has been know basically.Elimination of polarization-induced fading has become one of the key technologies of fiber gratting hydrophone.The traditional fiber optic hydrophone always use Faraday rotating mirrows to realize the elimination of polarization-induced fading,but the fiber grating hydrophone is unable to use the Faraday rotating mirrors because of its special structure.Therefore,finding a new method to eliminate the polarization-induced fading becomes an urgent need.As an effective technology of eliminating polarization fading for fiber gratting hydrophone,polarization switching can eliminate the visibility fading and the phase noise caused by the double refraction effects in the system.Polarization switching has become the first choice to eliminate polarization fading for fiber grating hydrophone.It has been applied in the seismic wave detection system in Norway in 2008.The research on technology of polarization switching has not been reported yet in China.In this paper,we do the following research on technology of polarization switching :1.Theoretical and experimental studies on the technology of polarization switching are carried out.A theoretical model based on the four channel polarization switching is established.The experimental system of continuous optical polarization switching based on Michelson interferometer and the experimental system based on the FP cavity are set up.The experimental results show that visibility of the synthesized channel is stable at 0.9after four channels are sythesized.Visibility of the synthesized channel will not produce the fading phenomenon.2.The problem of four channels synthesised is solved by using the complex cross multiplication method and the synthesis channel is demodulated.Demodulated from the synthesised signal and the original signal are the same.The amplitude of the signal demodulated from the synthesis channel fluctuate with 0.6dB.So the complex cross multiplication method is feasible.3.The background noise of the system is measured under the condition of no disturbance and additive.Under the condition of no disturbance,the background noise of the single channel is about-100 dB,and the background noise of the synthesis channel is about-100 dB too.Under the condition of disturbance,the noise of the single channel is about-95 dB,and the background noise of the synthesis is about-100 dB.The experimental results show that the polarization switching can eliminate the phase noise caused by the double refraction effects.