Research And Implementation Of High Speed Fiber Bragg Grating Demodulation System

Fiber Bragg Grating(FBG)is widely used in structural health monitoring,physical measurement of harsh environments and condition monitoring of large mechanical equipment due to its small size,low cost,corrosion resistance,high sensitivity and anti-electromagnetic interference.However,in the field of high-frequency signal measurement such as dynamic detection of rotating machinery,not only the dynamic demodulation frequency higher than the high-frequency component of the device is required,but also the wavelength demodulation should not be affected by the change of the light intensity.Among the commonly used demodulation techniques,wavelength demodulation technology is suitable for dynamic and static measurement,but the cost is high,the demodulation rate is limited to only several thousand Hz;the light intensity demodulation method can reach more than 10 kHz,but the fluctuation of FBG reflected light intensity will seriously affect the measurement result.And the demodulation range is limited.Therefore,this paper studies and implements a fast,wavelength-demodulated fiber grating demodulation system based on the Fourier domain mode-locked frequency-swept laser.The main contents are as follows:(1)Study the optical path correlation theory of high speed fiber grating demodulation system and design the optical path.The sensing mechanism of fiber grating is analyzed,and the key points and difficulties of high speed fiber grating system are proposed.The principle of the Fourier domain mode-locked frequency-swept laser is studied,and the Fourier domain mode-locked laser is constructed.By analyzing the nonlinear effect of the Fourier domain mode-locked laser,the influence of laser nonlinear effect on the accuracy of demodulation results is explored.The theory of optical beat frequency based on Mach-Zehnder interferometer is studied,a nonlinear calibration method based on optical beat frequency technology and single peak filter is proposed and implemented.(2)Hardware design and implementation of high speed fiber grating demodulation system.Aiming at the problems of slow peak search and large data volume of software peak finding,in the design of the lower computer board,a high-speed hardware real-time peak finding scheme is proposed,and the photoelectric signal conditioning circuit is designed to convert the fiber grating optical pulse signal into an edge signal that can be captured by a Field Programmable Gate Array(FPGA).Inside the FPGA,a multi-channel clock splicing and counting module is designed to speed up the sampling frequency and improve the resolution of the demodulation wavelength.In the aspect of data transmission,the hierarchical structure of the hardware protocol stack is studied,and the TCP/IP hardware protocol stack is built with the FPGA design function module to meet the data transmission requirements generated by the high speed fiber grating demodulation.(3)System performance is tested through static and dynamic experiments.The stability of the system was tested by setting up a constant temperature environment.The static and dynamic demodulation performance of the high speed fiber grating demodulation system was verified by static demodulation experiments and dynamic demodulation experiments.The experimental results show that the high-speed demodulation system has a resolution of 2 pm,a linearity of 0.9994,an accuracy of 5 pm,a stability of 6 pm,a frequency of 40 kHz,and the nonlinear calibration method based on optical beat frequency technology can significantly improve the system demodulation accuracy in the 20 nm demodulation range.