Research On Spectral Sampling Distortion And Its Peak-finding Algorithm In High Speed FBG Demodulation System

With the rapid development of aerospace industry,structure health monitoring of spacecraft has become a necessary method to ensure the safe operation of spacecraft.FBG sensors are widely used in spacecraft structure monitoring because of their unique advantages,such as small size,light weight,passivity,high sensitivity,easy to reuse,insensitive to electromagnetic interference,and easy to be embedded inside materials.This paper focuses on the high speed FBG demodulation method based on scanning light source,and studies the FBG spectral sampling distortion in the system and its peak-finding technique.Based on the physical mechanism of FBG spectral sampling distortion,the distortion caused only by the scanning light source and together by the scanning source and the detection circuit are studied respectively.The influence of the scanning frequency of the light source and the gain of the detection circuit on the distortion is explained.A fitting peak-finding method which is suitable for Gaussian and distorted FBG spectra is proposed based on the temporal response of photodetectors,which is also verified by experiments.The concrete work is as follows:1.The performance like flatness and nonlinearity of sampled grating distributed Bragg reflector(SGDBR)scanning light source and Fabry-Perot(F-P)scanning light source are described respectively.The effects of two non-ideal scanning light sources under different frequency on FBG spectral sampling distortion are analyzed.2.A high speed FBG demodulation system based on F-P scanning light source and gain-adjustable photodetector is built.The temporal response model of the photodetector is established and simulated.An asymmetry coefficient is also applied to describe the FBG spectral sampling distortion quantitatively,which is caused by the scanning light source and the detection circuit.3.A fitting peak-finding algorithm based on photodetector temperal response model is proposed.Experiments with varying degrees of FBG sampling distortion are carried out and results show that the algorithm is generally applicable to the demodulation of Gaussian and distorted FBG sampling spectra.When the scanning frequency of light source is 3200 Hz and the gain of the detection circuit is 70 dB,the measurement error of the temporal-response based method is limited within ± 1.777 pm and the standard deviation is less than 0.940 pm.Compared with the traditional Gaussian fitting method,the measurement error and the standard deviation of the temporal-response method are respectively improved by 6.87 times and 4.38 times.This method improves the stability of FBG high speed demodulation and effectively improves the disadvantage of traditional methods for FBG asymmetric distortion demodulation.