The Research On The Key Technologies Of OFDR For Fiber Grating Sensing

Due to the advantages of wavelength coding, wide measurement range, anti-electromagnetic interference, long-term stability, corrosion resistance, array multiplexing especially, etc, fiber Bragg grating(FBG) sensing has been widely applied in many fields such as the monitoring of engineering structures, petrochemical industry, power systems and so on, and it is the most developed fiber sensor so far. But because of the limitation of the spectral width of lasers and the bandwidth of sensor, high-density multiplexing of sensor array hasn’t been realized. So it will be the most effective method to achieve high-density multiplexing of sensor array that OFDR is applied to the demodulation of FBG sensing. For the high-density FBG sensing, this thesis studied laser characteristic, optical structure and the key technologies in demodulation algorithm of OFDR.The main works of this thesis were as follows:1. This thesis expounded the basic principle of OFDR. The factors, which affect the dynamic range and the spatial resolution, were analyzed and discussed by deriving OFDR theory model. The theory model of laser phase noise was set up and its impact on the beat signal was analyzed by simulation in the different laser coherence length and connector reflectivity. The methods to inhibition spectral polarization decline in OFDR system were analyzed.2. The signal demodulation algorithms of OFDR were analyzed and designed. The nonlinear tuning laser effects and several solutions were introduced. The matching condition between laser sweep rate and light path delay was obtained in the trigger sampling method based on auxiliary interferometer.3. The non-uniform Fourier transformation(NUFFT) and cubic spline interpolation algorithm to eliminate the nonlinear tuning effects was verified by simulation results and the system spatial resolution was improved. Using cubic spline interpolation method based on adding hamming windows FFT, which could effectively reduce the energy spread of signal, was proposed to further improve the system spatial resolution.4. The demodulation experiments of high-density FBG sensing based on OFDR were illustrated. On the one hand, the signal processing software was written to obtain the correct signal of OFDR. 1cm spatial resolution of OFDR system was achieved by using the compensation algorithms of NUFFT and Cubic spline interpolation method based on adding windows FFT. On the other hand, by writing the demodulation algorithms of FBG’s wavelength shift, temperature test experiments was carried out in four FBG sensors which have consistent wavelength, 15 cm spacing and weak reflectivity. OFDR system successfully demodulate the wavelength shift of FBG at different temperatures, and then the correctness of the demodulation algorithm was verified by compared with actual measurement results of FBG’s wavelength on the optical spectrum analyzer. Temperature sensitivity of FBG sensing system was 9.83 /Cpm ? in the case of non-sensitizing. The experimental results show that the system can be applied to demodulation of high-density FBG sensing.