Fluctuation-resisted Demodulation Technique For The Water Surface Acousto-optic Coupled Fiber-optic Hydrophone

As the optical fiber probe of the traditional fiber-optic hydrophone must be immersed in the water for detection, it limits the use in the wide range of the sea surface remote sensing and complex seabed conditions. The water surface acousto-optic coupled fiber-optic hydrophone overcomes these shortcomings, making the hydrophone more flexible and applicable. However in the actual detection environment, the demodulation signal fades or even dissapears when the water fluctuates, which can seriously affect the sensitivity and the dynamic range of the system, causing great trouble for the later data analysis.A novel method was put forward to weaken the signal fading phenomenon of the water surface acousto-optic coupled fiber-optic hydrophone when the water fluctuates. The method compensates the signal amplitude based on the light intensity of the signal arm. It divides into two algorithm according to the amendment sequence in the demodulation process:amendment before and after the demodulation. The amendment process for the demodulation signal was analysed in theory and by numerical simulation. A fiber optic hydrophone system based on π/2phase demodulation technique was constructed to test the method for water constant vibration signal, Compared with the original demodulation signal, the amplitude of the amended signal is stabilized, the degree of distortion is reduced, the phase noise is suppressed, the signal to noise ratio is improved and the time range of the identifiable signal is extended to some extent. The results show that, this method can effectively overcome the signal fading phenomenon caused by surface fluctuation and optimize the signal quality. Finaly the cause of the low frequency interference component in the demodulation signal was discussed and analysed, and effective measures to reduce the interferences of signal demodulation was proposed, further enhancing the demodulation ability of resisting water surface fluctuation for the system.