Research And Implementation Of PGC Demodulation Algorithm For Optical Fiber Seismometer

Optical fiber sensor(OFS)has the advantages of high sensitivity,high resolution,antielectromagnetic interference,anti-corrosion and so on.And fiber-optic interferometric sensors(FOISs)are widely used in seismometers because of their extremely sensitive to the external environment.In FOISs,the demodulation methods directly affect the detection performance of OFS system.Phase generated carrier(PGC)demodulation algorithm is one of the key demodulation techniques in FOISs.Differential-and-cross-multiplying approach of phase generated carrier(PGC-DCM)algorithm and arctangent approach of phase generated carrier(PGC-Arctan)algorithm are two conventional PGC demodulation algorithms which have the important applications in the FOISs demodulation system.However,due to the existence of light intensity disturbance(LID),nonideal characteristics of low-pass filter,dynamic change of phase modulation depth,carrier phase delay and other factors,it will bring nonlinear distortion to PGC demodulation results and even lead to demodulation failure.Therefore,this paper mainly studies the PGC demodulation algorithm and proposes two algorithms to suppress the nonlinear distortion of the PGC demodulation system.The So C demodulation system of the optical fiber seismometer is implemented based on the optimization algorithms.It includes the following points:1)In this paper,the sensing principle of FOISs is studied.The demodulation algorithms of PGC-DCM and PGC-Arctan are introduced.The nonlinear distortion in the modulation algorithms is analyzed.2)Carrier phase delay synchronization(CPDS)algorithm based on orthogonal phaselocked algorithm is proposed to solve the problem of carrier phase delay in PGC-Arctan demodulation algorithm.The CPDS module is introduced to the PGC-Arctan demodulation algorithm to calculate the carrier phase delay.Thus,the carrier phase delay compensation is achieved by adjusting the initial phase of the fundamental and the second harmonic of the carrier.The algorithm can suppress the nonlinear distortion of PGC-Arctan demodulation algorithm.And the simulation results verify the effectiveness of CPDS algorithm.3)A self-calibration PGC-Arctan(PGC-Arctan-SC)demodulation algorithm is proposed to solve the problem of phase modulation depth C which random drift and fluctuation in PGC-Arctan demodulation algorithm.The conventional PGC-Arctan demodulation algorithm relies on the stability of C,which requires a specific value to work normally.However,in the actual working environment,the C value will change with the change of laser wavelength,ambient temperature and ambient humidity.The PGC-Arctan-SC demodulation algorithm can suppress the nonlinear distortion by the ellipse fitting algorithm(EFA),and estimate the C value by the ellipse correction parameters.Then the C value will be calibrated to the optimal working value by the proportional integration differentiation(PID)closed-loop feedback control algorithm to suppress the nonlinear distortion further.The simulation results are consistent with the theoretical analysis,which verifies the effectiveness of PGC-Arctan-SC in suppressing nonlinear distortion.4)On the basis of the proposed optimization algorithm,the hardware demodulation system of optical fiber seismometer based on Mach-Zehnder interferometer and Xilinx zynq-7100 So C hardware development platform is designed and implemented.The effectiveness of CPDS algorithm and PGC-Arctan-SC algorithm in suppressing nonlinear distortion is verified on the hardware system.And the detailed function and performance test for the hardware system is completed.The index requirements are all achieved.