Research On High Precision Fiber Bragg Grating Demodulation System Based On Spectrum Segmentation

The optical passive and active components based on the optical fiber have been widely studied since optical fiber was invented. Fiber Bragg grating (FBG), which can be used for sensing by converting the change of external environment factors into the Bragg wavelength shift, is one of the typical optical passive components. It has many advantages, such as the immunization to electromagnetic interference and the external instability environment. Moreover, the FBG based system is low cost. Therefore, it attracts more and more attentions. To measure the Bragg grating wavelength shift accurately has become the most important point in the FBG based sensing demodulation system.Firstly, the mode coupling theory was studied in the thesis. After that, a fiber Bragg grating temperature and strain sensing model were established. By comparing the advantages and disadvantages of existing demodulation methods, the Fabry-Perot filtering method was used to demodulate the wavelength shift in our work. However, in the demodulation system based on Fabry-Perot filtering, the filter tuning would greatly affect the stability and precision of system. In order to eliminate this impact, a high precision fiber Bragg grating demodulating system based on spectrum segmentation is proposed. A Fabry-Perot (F-P) etalon is added to dynamically calibrate the read out value of filter wavelength, which can eliminate measurement errors caused by the temperature drift effect and the peristaltic effect of F-P filter. In addition, the ASE spectral characteristics are used to segment the wavelength for demodulation, which can greatly reduce the effects of the nonlinear effect of F-P filter to improve the demodulation accuracy and enhance the stability of the demodulation system. To verify the performance of the proposed system, a high-speed control and acquisition circuit was designed using the DSP2812 chip. Besides, the Lab VIEW and Matlab software are combined to design the date processing program and the interface of the system, which can enrich the system functions and reduce the difficulty of operation.Finally, the performance of the system was tested and the measurement error was also analyzed. The results show that the stability of the system is 0.97 pm while the resolution is 0.33pm. In addition, the error of temperature is 0.03～0.13℃ and the linearity is 0.9999.Compared to the traditional method, the precision and stability of the system have been significantly enhanced, which validates that a real-time temperature sensing can be achieved by the system.