High Precision And Wide Range Of Grating Sensing Demodulation System Based On Tunable Filter

Fiber Prague grating (FBG) sensor is not affected by power fluctuation of the lightsource and not sensitive to electromagnetic. And it is the main device of grating sensing field.The accurate measurement of grating center wavelength shift decides the demodulationaccuracy of sensing signal. Intensity modulated grating demodulation system has very highdemodulation speed, but the nonlinear characteristics of optical filter in the system wouldreduce the demodulation resolution and sensitivity in the full scale range. Increasing the slopeof the optical filter can improve the accuracy of demodulation, but it will reduce the effectiverange of dynamic strain. The different demodulation technology affects the sensor workingmode and performance.This paper describes a design of a high precision and wide range sensing demodulationsystem based on tunable filter, Through controlling the adaptive scanning voltage of F-P filterwhich narrow linewidth output wavelength will real-time lock the spectrum of-3dB inGrating Reflective by means of dynamic tracking and locking, in order to realize the highsensitivity and wide range of fiber Bragg grating sensor and demodulation. The demodulationsystem is a good solution to the low precision problem in the full range caused by the tunablefilter scanning nonlinearity, and effectively improves the sensor range.This paper first introduces the development and application of fiber grating sensor andanalysis of the strain and temperature sensing principle of fiber Prague grating, thenintroduced several methods of grating sensor demodulation which commonly used and theiradvantages and disadvantages are analyzed; Choose the F-P filter method as the basicdemodulation method in this paper, and design the overall scheme of the grating sensordemodulation system; Due to F-P filter inherent hysteresis and creep problems, we proposedan adaptive scanning voltage which control the F-P filter, and a method to improve theaccuracy and range of the fiber grating sensing demodulation through the real-time trackingthe variation of reflection spectrum of FBG sensor. Build and debug optical part of the wholesystem and accomplish the calibration of grating reflection wavelength by using etalon.Completed the hardware circuit design using Xilinx company’s FPGA chip XC4VLX25as themain controller, and write controlling procedures for A/D and D/A with VHDL language, achieving the output data acquisition of sensor and F-P filter scanning voltage. We designedthe Ethernet data communication using DM9000A, then send the sensor data to the PC whichusing the data calibration of demodulation part and demodulation algorithm for the vibrationsignal demodulation. We can see that the range of grating demodulation system can be morethan2nm for the dynamic strain signals at low speed from the results based on the test of thesystem; The wavelength shift in the static system was±1.5pm, and the fitting coefficient was0.9997; The external vibration signal can be demodulated by the system up to2KHz whichproves the feasibility and stability of the system.