Research On Wavelength Demodulation Method Of Fiber Grating Based On F-P Etalon

Fiber Bragg Grating（FBG）sensors use wavelength changes to reflect the size of the measurand,so it is of great significance to improve the wavelength demodulation accuracy of FBGs.However,affected by factors such as device temperature drift,system noise interference,and spectral distortion in the demodulation system,the demodulation accuracy of the existing FBG demodulation methods still needs to be further improved.Therefore,in view of the above problems,this paper proposes a high-precision,broadband multi-peak composite reference FBG wavelength demodulation method.The method is based on the tunable Fabry-Perot（F-P）filter detection method,which combines an Acetylene（C₂H₂）gas cell and an F-P etalon to achieve FBG wavelength demodulation.In this paper,the sensing characteristics of FBG are studied,and the advantages and disadvantages of the composite reference wavelength demodulation system composed of F-P etalon,reference grating,interferometer and gas cell are analyzed and compared.On this basis,a FBG wavelength demodulation system with a C₂H₂ gas cell with a differentially balanced photodetector and an F-P etalon composite reference is designed,which reduces the vertical distortion of the gas cell transmission spectrum affected by the spectral flatness of the light source.And the characteristics of the main functional devices of the system are analyzed and selected,which further improves the demodulation performance of the system.In order to reduce the influence of noise on the spectral signal and further improve the vertical distortion of the spectrum,a spectral signal preprocessing method is proposed in this paper.The noise reduction effect of the wavelet threshold method and the moving average filtering method is studied and analyzed,and the collected original spectrum is denoised by combining the characteristics of the two noise reduction algorithms;A spectral signal shaping method including baseline leveling,logarithm,decimation and interception is proposed,and more ideal spectral data are obtained,which is beneficial to the subsequent multi-peak detection work.Aiming at the problem of spectral lateral distortion caused by the narrow spectral reference range of the gas cell and the nonlinear scanning of the tunable F-P filter,a FBG wavelength demodulation algorithm based on the full spectral range of the F-P etalon is proposed.A threshold-hysteresis segmentation method is designed to make up for theambiguity and low efficiency of Hilbert transform and Mexican hat wavelet transform in the identification of dense multi-peak regions;The spectral correction algorithm in the gas cell band is studied,and the spectral wavelength in the gas cell band is obtained by the X-d B bandwidth method,the gas cell reference peak selection method and the gas cell multi-peak reference correction method;A distortion demodulation algorithm for the spectrum outside the band of the gas cell is proposed,by using the fitting method,extrapolation method and data fusion method,the center wavelength of the FBG outside the band of the gas cell is obtained,and the high-precision and wide-band FBG wavelength demodulation.Experiments test the repeatability and temperature stability of the system,and the results show that in the range of 1510～1590nm,the maximum wavelength repeatability error of FBG is 2.51pm,and the linearity can reach more than 99.9%;under the working environment of-20℃to 60℃,the maximum full-scale error does not exceed±1.71pm,which is similar to only Compared with the demodulation method using the F-P etalon reference,the gas cell composite reference method improves the accuracy by 64.8%and reduces the standard deviation by 75.6%.