Research On High-precision FBG Demodulation Method Based On HCN Absorption Spectrum For Temperature Changing Environment

The spacecraft usually works in the outer space with vacuum and severe temperature changing conditions.Besides,the fiber sensing system for monitoring of international oil pipelines may also face the challenge of severe temperature and humidity changes.Fiber Bragg grating(FBG)sensors are widely used in aerospace,bridges,tunnels and other monitoring fields due to their light size,high sensitivity,electromagnetic interference resistance,ability of multiplex and other advantages.In this paper,a high-precision FBG demodulation system based on tunable Fabry-Perot(F-P)filter and hydrogen cyanide(HCN)gas cell in the variable-temperature environment is studied,aiming at the distortion mechanism of HCN absorption peaks caused by F-P filter transmittance function.A spectral iterative deconvolution algorithm is proposed to eliminate the impact of F-P filter transmittance function,and the recovered HCN absorption spectrum is obtained.The FBG demodulation experiments are carried out applying the recovered HCN absorption spectrum as the wavelength reference.The effect of this method on improving the demodulation precision and expanding the demodulation range with high-precision is verified.The main works done are as follows:1.A FBG demodulation system based on tunable F-P filter and HCN gas cell is built.The mechanism of broadening,lightening and asymmetric distortion of HCN absorption spectrum caused by the F-P transmittance function is studied.The impact of the distortion on the threshold setting and peak localization in the peak-finding process of HCN absorption spectrum is discussed.The peak-finding error of the centroid method is clarified.2.A spectral iterative deconvolution algorithm is proposed to realize the recovery of HCN absorption spectrum,thus eliminating the broadening,lightening and asymmetric distortion caused by F-P filter function.Compared with that of the detected HCN absorption spectrum without recovery,the peak-finding standard deviation of the recovered HCN absorption peaks is significantly reduced,especially on the edges of Cband of 1525-1530 nm and 1560-1565 nm and near the two "marking points" of 1541.4-1543.5nm,where the absorption peaks are naturally shallower.The peak-finding standard deviation of these spectral regions is reduced by an average of 54.9 %,59.0%,56.1%,respectively.3.Five FBGs are selected to conduct experiments in stable-temperature and variable-temperature environment respectively.The Bragg wavelengths of FBGs are in different spectral regions.The effectiveness of the method employing the recovered HCN absorption spectrum as the wavelength reference of the demodulation system is verified.Compared with the method employing the F-P etalon transmission spectrum,the results show that the demodulation stability in variable-temperature environment is greatly improved.Compared with employing the detected HCN absorption spectrum,the demodulation precision is obviously improved,especially near the "marking points" and on the edges of the C-band,where the absorption peaks are naturally shallower.The wavelength measurement error of these spectral regions is reduced by an average of52.8%,and the standard deviation is reduced by an average of 47.6% in temperature changing environment.The wavelengths standard deviation on the whole C-band can be limited within 0.996 pm.Therefore,the spectral range that can be accurately demodulated is expanded from the spectral regions with deep absorption peaks to the whole C-band while ensuring the temperature stability.