Research On High Stability And Low Harmonic Distortion PGC Demodulation Algorithm

The rapid development of optical fiber technology has promoted the application of optical fiber sensors in different fields.Among many types of optical fiber sensors,interferometric optical fiber sensors are widely used in mine safety,aerospace,oil exploration and other fields because of their high sensitivity,strong anti-interference ability and corrosion resistance.In the interferometric fiber optic sensor system,the phase demodulation technology directly affects the dynamic range and measurement accuracy of the sensing system.Among them,the phase generated carrier(PGC)demodulation algorithm is one of the commonly used demodulation techniques for interferometric fiber optic sensors,including differential cross multiplication(PGC-DCM)algorithm and arctangent(PGC-Arctan)algorithm.However,in practical applications,the PGC demodulation algorithm will be affected by factors such as phase modulation depth fluctuation,light intensity disturbance,carrier phase delay and environmental disturbance,resulting in nonlinear distortion of the demodulation results.In this thesis,the nonlinear distortion factors in the PGC demodulation algorithm are studied in depth,and two improved PGC demodulation algorithms are proposed.Through experimental verification,the improved algorithm effectively suppresses nonlinear distortion.The main research contents of this paper are as follows:Firstly,the principles of PGC-DCM and PGC-Arctan demodulation algorithms are introduced in detail by combining Michelson interferometric optical fiber sensor and PGC internal modulation technology.Aiming at the nonlinear distortion problem existing in the practical application of the PGC demodulation algorithm,the research on Ellipse Fitting Algorithm(EFA)based on Least Square Method(LSM)and Extended Kalman Filter(EKF)is carried out.Secondly,a PGC-EFA-DCM demodulation algorithm based on PID control is proposed.For small-amplitude signal measurement,the EFA based on LSM cannot be used and the modulation depth fluctuates.In this scheme,a low-frequency triangular wave signal is added to the laser modulation,which ensures the effectiveness of the LSM-based EFA in the measurement of small-amplitude signals.At the same time,the modulation depth is calculated by using ellipse fitting parameters,and calibrated in real time through the PID module,which effectively reduces the fluctuation of the modulation depth.The demodulation algorithm in this paper is programmed on LabVIEW,and a Michelson interferometric optical fiber sensor experimental platform is built for testing and verification.The experimental results show that the signal-to-narrative ratio(SINAD)of the demodulation result of this scheme is 13.61 dB higher than that of the PGC-DCM algorithm and the total harmonic distortion(THD)is always lower than 0.1%,and the standard deviation of the recorded 1000 phase modulation depth is low to 0.0058 rad.Finally,an EFA double cosine modulation PGC demodulation algorithm based on EKF is proposed.Aiming at the problems that LSM is susceptible to noise and poor robustness when measuring small-amplitude signals.In this scheme,a phase shift greater than π is generated by modulating the laser with a double cosine signal to improve the fitting accuracy of EFA.Compared with LSM fitting,EKF fitting has higher fitting accuracy and robustness.The experimental results show that the eccentricity of the ellipse after EKF fitting correction is as low as 1/2 of that of LSM,and the SINAD of the demodulated signal of this scheme is26.84 dB higher than that of the PGC-Arctan demodulation algorithm under the same excitation signal.