| Fiber optic sensor with anti-electromagnetic interference, anti-corrosion, anti-radiation and other advantages, has been paid more and more attention. As one of fiber optic sensor, Sagnac fiber optic vibration sensor has broad prospects with its advantages of large dynamic range, high sensitivity, long-distance real-time monitoring.This paper is engaged in the research on the optical fiber vibration sensing system which is based on the Sagnac interferometer and the phase generated carried modulation. How to accurately demodulate the external vibration signal through the system is mainly studied. What’s more, in order to extract useful signal and improve the demodulation accuracy, the signal is processed by wavelet de-noising and moving average filter.Based on the principle of Sagnac interferometer, optical fiber vibration sensor system which includes data acquisition, demodulation and signal processing is designed. Firstly, according to analysis the effect of external vibration signal on light phase, fiber optic phase modulation formula is given. The principle of the system demodulation and positioning is analyzed. To show the null frequency and verify the effect of the delay fiber length on null frequency, it is simulated by matlab. Secondly, it is theoretically illustrated that the phase carrier demodulation is feasibility. The signals including triangular wave sine and square wave, are used to simulate the disturbance and are demodulated by PGC demodulation method successfully.The amplifier circuits of PIN photo detector, including the preamp circuit, the second amplifier and a high pass third order Butterworth filter circuit, are analyzed and designed by Protues, as well as printed circuit board is designed by Altium designer13. The spectral width of the circuit is 500 k Hz and the magnification is 125,000.The data acquisition and storage software based on Lab VIEW platform had been developed. The spectrum of demodulated signal is processed by Wavelet threshold denoising and moving average filtering. The selection of threshold function and how to determine the threshold are theoretically analyzed. Then, the denoising effects of different ways of threshold quantization on the invasion vibration signal from Sagnac sensors are compared and analyzed, to obtain the best threshold denoising method which is suitable for the system. To obtain the appropriate method by comparing the moving average filter effects of different data smoothing methods. The output signal is processed by the combined method to retain the approximate information of external vibration signal, and to get a clear null frequency in the frequency spectrum. It is helpful to improve positioning accuracy effectively.Fiber optic vibration sensor system based on Sagnac which is consist of ASE light source, optical circulator, 3 × 3 coupler, PZT phase modulator, Faraday rotator mirror and PIN photo detectors has been built, and the demodulation detection experiments have been accomplished. In the experiment, the signal generator is used to produce high frequency carrier signal and the signal is launched in the system by PZT. The output interference signal with external disturbance information is acquired by NI USB-6361, demodulated and denoised. As a result, the signal to noise ratio has been improved. In addition, the polarization instability is eliminated by using the Faraday rotator mirror. How to choose the amplitude and frequency of the carrier signal is theoretically analyzed and the proper frequency and amplitude of the carrier signal is selected. Finally the stability of demodulated signal is greatly enhanced. |
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