Design And Implementation Of High-speed Fiber Bragg Grating Demodulation System Based On Dispersion-compensating Fiber

Fiber Bragg grating(FBG)sensing technology has been gaining greater popularity in such areas as ultrasonic flaw detection and high-speed rotation or vibration detection,which imposes higher requirement on sensor demodulation rate.In this paper,the dispersion effect of dispersion-compensating fiber(DCF)was employed to turn information on wavelength shift of FBG into time-domain information,which realized the high-speed and high-precision demodulation of quasi-distributed fiber grating sensing systems,combined with optical time-domain reflectometry(OTDR).By introducing reference grating and adding reference channels,this system has reduced the demodulation errors induced by affecting temperature fluctuations.Advanced calibration is no longer required when demodulating,an optical pulse is enable to realize the high-speed synchronous decoupling of the information on both the wavelength and position of the grating.Main research works are as follows:(1)Analysis of the temperature and strain sensing theories on FBG sensors,and investigation of both the theories and properties of present typical high-speed demodulation methods of FBG like Fourier-domain mode locked(FDML)swept method,edge filter demodulation and nonsymmetrical Mach-Zehnder interferometry.In addition,the theory on common dispersion devices has been analyzed and the principle of transformation between grating wavelength and time domain has also been discussed.(2)A high-speed FBG demodulation system based on DCF has been realized from the perspectives of optical path construction,circuit design and peak-detection algorithms optimization.The selection and location of the optical devices,as well as the construction of the optical path,has been decided through experimental investigation.Pulse pattern generating circuit,semiconductor optical amplifier-driven circuit,constant temperature-controlled circuit and photoelectric signals conversion circuit have been included in the circuit design process.An optimized peak-detection algorithm has been introduced after the analysis of the properties of the present peakdetection algorithms,and completed the algorithm design of the demodulation system.(3)Theoretical analysis of high-speed FBG demodulation based on DCF has been made,the feasibility of which has been demonstrated through subsequent stress test.Two optimization methods of the demodulating systems has been introduced and validated.On one hand,reference-grating method was employed by calculating the time difference between the sensing and reference FBG,which reduced the length fluctuations of DCF induced by environment temperature.On the other hand,reference channel has been added to the demodulation system.Two equations of wavelength and time difference have been established through double channels,the solution of which could be obtained by simultaneous equation.This method reduced the environmentinduced fluctuations of quasi-distributed FBG sensing network.(4)The feasibility,precision,and stability of optimized high-speed demodulation system has been validated,analyzed,and tested,respectively.The experimental investigation indicated that degree of linearity of the demodulated FBG wavelength was good,and that the demodulation rate could be as high as 1MHz.The degree of linearity could reach 0.9998 after being averaged 10 times with the demodulation precision and stability estimated to be 8.8pm and 7.2pm,respectively.