The Design Of High-frequency Demodulation For Fiber Bragg Gratings Sensor

Fiber Bragg Grating sensor is widely used in the fields of civil engineering, aviation, shipping, electric power and petroleum because of its small size, high sensitivity, high reliability, low cost, and that it is immune to electromagnetic interference and easy to integrate and ambuscade. The demodulation of grating is the key technology of the application of grating sensor. The highest frequency of the demodulation currently applied in china is 200HZ, but in some practical applications, this frequency can not satisfy the actual need if detecting the vibratile or revolving entity with high-speed in real time is necessary. Therefore, in this paper the author designs a system in which frequency of demodulation is 2000hz.Firstly, the research on the fiber grating sensing principle and demodulating methods is done. After analysis on several kinds of grating demodulation method is done, the demodulation method of Fiber F-P (Fabry - Perot) Tunable Filter is selected. The key problems of this method would be solved in this paper, including the driving of F-P ,the optical-electrical conversion circuit and the light source circuit. The data acquisition part ,sawtooth wave output control part and the data processing part of high-speed signal are also important parts of this paper. The acquisition frequency of the signal is directly related to the measurement accuracy of the entire system. The higher the acquisition frequency to the signal, the better the detection accuracy of the system is. Sawtooth wave output control part is mainly used to generate sawtooth wave to drive the F-P. The processing part of the signal is the core part of the whole system, which consists of the FIR(finite impulse response) filter, peak-detection module ,and the offset computing module of center wavelength during the processing part of the signal. The major role of the FIR filter is to filter out the noises of high frequency, in order to improve the input signal noise ratio of the peak-detection algorithm. Peak-detection algorithm module is the key part of the signal processing part because of that the peak-detection error has a direct influence on the accuracy of the whole system, and a kind of peak-detection algorithm in which the average peak-detection error is smaller than 0.5pm when the input signal noise ratio is not bad. In the offset computing module of center wavelength the offset of center wavelength would be figured out by the peak information had be get in the modules before, which is used to figure out the change of the temperature and pressure.Finally, detecting results of the entire system are analyzed, and the simulating results of the FIR filter and peak-detection algorithm are given. The results of the experiment show that the demodulator works well and the detecting accuracy of the system could satisfy the engineering needs under the conditions that the bandwidth is 50nm and the scanning frequency is 2000Hz.