Research On High Performance Quasi-Distributed Fiber Acoustic Sensor System For Under Water Acoustic Sensing

Fiber-optic hydrophone was proposed in the 1970 s,it is firstly developed for the military applications.In the recent years,fiber-optic hydrophone array is widely used in the other applications such as scientific research and seabed resources detection.At present,traditional fiber-optic hydrophones can not adapt the changing of the application requirements and shows problems of insufficient of multiplexing scale,high noise level and the oversize of the sensor array.However,the possibility of enhancing the system performance by using the traditional technique is little.On the other hand,in the recent ten years,many new techniques which can be used to enhance the system performance has been developed in distributed acoustic sensors(DAS)area.Because DAS technique has the same sensing function as the hydrophone array system,these techniques may be used in the research of enhancing the performance of fiber hydrophone array.Such as the coherent detection technique,which is developed after the commercialization of narrow linewidth laser;the pulse compression technique and frequency division multiplexing technique,which is proposed after the development of digital system;and the reflection enhancement technique,such as the weak fiber Bragg grating array,weak reflector array and the Rayleigh backscatter scattering enhancement technique,which is proposed after the development of the laser processing technique.These techniques can be used to enlarge the multiplexing scale and enhance the sensor resolution.However,directly using the DAS technique or relevant technique in to the hydrophone system can not meet the requirement of the application.This work aims at developing a large-scale multiplexed fiber hydrophone array system and make it meet the requirement of the underwater acoustic sensing application.The research is mainly about enhancing the key performance indicator such as the multiplexing scale,sensing resolution and frequency response.In the same time to provide a solution of miniaturization of the sensor element.This research can be divided into two parts,the first part is to propose a coherent detection based fiber hydrophone system with weak reflector array as the sensing fiber,the second part is to use advanced modulation and demodulation methods to furtherly enhance the system performance.The innovations of these works are:(1)In the work of fiber hydrophone array system with phase noise compensated coherent detection,we proposed a coherent detection based fiber hydrophone system with weak reflector array as the sensing fiber and a phase noise compensation configuration.The system has a sensing distance of 20 km,a sensor spacing of 10 m,a noise level of-71.2dB re rad/?Hz@10-2500 Hz.The phase noise compensation configuration reduces the low frequency phase noise of 28 dB in average.(2)In the work of pulse compression based fiber hydrophone system with ultra high resolution,we proposed a pulse compression based phase noise compensation configuration and give an analysis of the system noise.This system has a sensing distance of 20 km,a sensor spacing of 20 m and a noise level of-93.16 dB re rad/?Hz@500-2500 Hz.The phase noise compensation configuration gives 12 dB reduction to the phase noise floor.(3)In the work of OFDM and Vernier method based system with high frequency response,we propose the OFDM method and the Vernier method.The system has a sensing distance of 50 km and can detect the ultrasound signal with a frequency of 25 k Hz at maximum.(4)In the work of femtosecond laser inscription method of large scale multiplexed weak reflector array,we proposed a automatic processing system.Based on the system,we develop a weak reflector inscribing method with high reflectivity and low transmission loss.The produced weak reflector array has a length of 9.8 km,a spacing of 10 m,a reflectivity of-42 dB and a transmission loss of-0.34 dB/km.