Study On Sagnac Fiber-Optic Hydrophone

The Sagnac interferometer is used to measure both the angular velocity of a moving body with a fiber optic gyroscope and the reciprocity signal of asymmetric disturbance.This paper presents theoretical and experimental investigations on the key techniques of a fiber-optic hydrophone based on the Sagnac interferometer.The response characteristics of a Sagnac fiber-optic hydrophone to acoustic signals are firstly discussed,and main influence factors are also analyzed.The results show that the phase sensitivity of the Sagnac fiber-optic hydrophone changes periodically.The changing period is inversely proportional to the length of the delay fiber in the system,and the sensitivity is proportional to the sensing fiber in the probing unit.Simulations also show that the sensing fiber should be as longer as possible when designing a practical Sagnac fiber-optic hydrophone,and the length of delay fiber should be suitable.To reduce system noise,a low-coherence light source should be utilized if possible.And a Lyot depolarizer can be used in the system to reduce the influence of polarization fading.This paper offers a detailed theoretical and experimental study of the signal demodulation techniques of Sagnac fiber-optic hydrophones based on different modulation methods of phase bias.Take into account the characteristics of the Sagnac optic fiber hydrophone array,we presented a pulse phase modulation signal demodulation solution based on active phase bias. This solution can not only eliminate the impacts of light intensity fluctuation,lower the requirements for light source stability,but also effectively inhibit low frequency circuit noise and optical path noise,and provide direct current phase compensation for the system.The principle on Sagnac fiber-optic hydrophones pickup and null rung method suppression have been studied in detail,and experimental investigations on the effects of the null rung method on pickup suppression have been carried out.The results show that the optimal working range of a pickup suppression system is that the sum of response amplitudes of the hydrophone to all signals should be within the range of[0,π].If the bias phase is set to beπ/2,the pickup suppression system will enjoy a relatively large dynamic range and better suppression.The system's pickup suppression capability will be improved with the increase of signal frequencies as well as the increase in signal's phase modulation amplitude.The pickup suppression of the Sagnac fiber-optic hydrophone can be as high as 30dB in this experiment.The research results of the sensing characteristics of the Sagnac fiber-optic hydrophone,and signal demodulation techniques as well as pickup suppression presented in this paper will lay a solid foundation for the practical application of Sagnac fiber-optic hydrophones.