Research On PGC Demodulation Method Of DFB Fiber Laser Hydrophone

With the continuous improvement of silent submarine technology,fiber optic hydrophone technology has been developed rapidly.Among them,the distributed feedback fiber laser hydrophone has been used widely due to its excellent characteristics,such as low noise,narrow line width,small volume,and good output spectrum characteristics.The probe of a distributed feedback fiber laser hydrophone is a phase-shifted fiber grating which has been encapsulated,and it directly writes a Bragg grating with ? phase shift amount on a doped fiber.The length of the packaged phase-shifted fiber grating is only a few centimeters,but the detection sensitivity of the distributed feedback fiber laser hydrophone has reached the detection level of hundreds of meters of interference fiber-optic hydrophones.At present,the development of packaging technology based on phase-shifted fiber gratings is relatively mature,but less research on demodulation technology.The demodulation system is the core of the entire hydrophone system and its performance will affect the detection accuracy of the hydrophone.At the same time,it can also solve problems that cannot be solved only by packaging technology,such as environmental noise of underwater.In this paper,the purpose of research is to design a distributed feedback fiber laser hydrophone demodulation system,and the following issues are mainly done:Firstly,we first analyzed the working principle and sensing principle of distributed feedback fiber laser.And discussed the parameters that affect the transmission characteristics of phase-shifted fiber gratings based on the coupling mode theory and optical transmission matrix.As an important part of fiber laser,phase-shifted fiber gratings,the performance of which will directly determine the performance of the hydrophone system.Secondly,we analyzed the demodulation principle of traditional PGC algorithm and analyzed the causes of distortion of its demodulation results,and proposed a new demodulation algorithm based on these shortcomings,which named single-path differential division.This algorithm can effectively reduce demodulation distortion caused by optical disturbance and variation of modulation depth.And the reliability of the algorithm is verified by Matlab software simulation.Thirdly,An experimental demodulation system was set up based on the Michelson non-equilibrium interferometer,and transfer the collected data to several algorithms based on Labview platform on PC with NI data acquisition card.The demodulation results show that the SINAD of the improved algorithm is 22.3d B,which is 14.4d B and 5.4d B higher than the demodulation results of the PGC-DCM algorithm and the PGC-Arctan algorithm.