Study On Polarization Induced Signal Fading Elimination And Crosstalk Reduction Of Optical Fiber Hydrophone Array Based On Fiber Bragg Gratings

With the development of underwater acoustic counterwork and the increasing of the need for submarine detection,developing the low-cost,small size,lighter and high performance acoustic sensor system has been more and more important.The inline interferometric fiber sensor array based on fiber Bragg gratings(FBGs)provides the potential to develop the large-scale,high-performance acoustic sensor array with low-cost and small size simultaneously.However,two problems limit the application of the such system,which are the polarization induced signal fading and the multi-reflection(MR)induced crosstalk.The research focuses on the two problems and tries to find out the method to solve them.The thesis starts with the design and analysis of inline interferometric fiber sensor array.To solve the problem of the polarization induced signal fading,the polarization switching(PS)method is proposed and researched.The theory of the MR induced crosstalk is analyzed in detail.And Fabricating the FBG array with the ultra-weak FBGs is proposed to achieve low crosstalk.The layer peeling(LP)algorithm for eliminating the crosstalk is researched.Finally,a 2-WDM×4-TDM inline interferometric fiber sensor array based on fiber Bragg gratings is made and the performance is experimentally tested.The main results of this thesis are as follows:1.A phase-mask and pulling force matching method is proposed to realize the online fabrication of the FBG array and a high reflectivity FBG is used as the reference to measure the reflectivity of the low-reflectivity FBG,which provides the experimental foundation for the key technology research.2.The polarization switching(PS)method is proposed and researched to eliminate the polarization induced signal fading and polarization induced phase noise.The basic principle of the method is analyzed.The detailed process of the method combined with the phase generated carrier(PGC)scheme is derived,where the thought of converting the interference intensities of four polarization channels into complex-exponent form is creatively proposed to solve the synthesis of the interference intensities of four polarization channels.The experimental result shows that the synthesized complex signal can achieve the maximum equivalent visibility and the polarization induced phase noise can be eliminated successfully.3.An interference synthesis method was presented to reduce the MR-induced crosstalk.The experimental results showed that a crosstalk reduction of 18 dB could be achieved with the FBG reflectivity of 5%.4.The layer peeling algorithm based on the time-domain interferometric intensities is researched to eliminate the crosstalk between the sensors.The thought of converting the interferometric intensities to complex-exponent form is applied to the algorithm,which realized the theory of the algorithm for both the polarization maintaining(PM)fiber sensor system and the single-mode(SM)fiber system.The experimental result shows that 8 dB-15 dB crosstalk in the PM 4-TDM sensor system can be achieved with the FBG reflectivity of 5%.5.A low-crosstalk and polarization independent inline interferometric optical fiber sensor array with 2-WDM × 4-TDM channels was demonstrated.The low level of the MR induced crosstalk was realized by the ultra-weak(<1%)fiber Bragg gratings(FBGs).The PS method based on the PGC scheme was applied to eliminate the polarization induced signal fading and the input-polarization-induced phase noise simultaneously.The application of the PS method to multiple channels was validated.The background noise could reach-97 dB/?Hz at 1 kHz.The MR-induced crosstalk level of lower than-45 dB was achieved for the 4-TDM sensor array.And the crosstalk between different wavelength channels was lower than-60 dB.