The Control System Of Distributed Optical Fiber Temperature Sensor Based On FPGA

Distributed optical fiber sensor is a real-time sensor system which is used to monitor the spatial distribution of temperature or strain. The environment parameters are measured according to both the scattering in optical fiber and the conventional optical time domain reflectometry (OTDR) technology. However, the improvement of the system performance will be limited by the problem of weak spontaneous scattered light detection. In addition to the interferometric detection method, the use of the pump encoding technology in the direct detection is also an effective approach for solving the problem. Meanwhile, the system control of the distributed optical fiber temperature sensor plays an important role in ensuring the sensor stability and providing a good support for the accurate measurement.Distributed optical fiber sensor is researched from the two aspects of theoretical analysis and experimental verification. Firstly, the basic theories of optical fiber sensors including OTDR and inelastic scattering in optical fiber are introduced. And a distributed optical fiber sensor is numerical modeled and analyzed by using Brillouin-scattering coupled amplitudes equations. Then, the application of matrix-based encoding pump in fiber sensors is introduced, and a novel structure of encoding matrix is proposed on the basis of the analysis of the S matrix characteristics. Both S matrix and the new matrix encoding pump in a distributed fiber sensor are numerical analyzed by taking advantages of Brillouin-scattering model. Finally, in order to design the distributed optical fiber temperature sensing experiment successfully, a data acquisition card is cooperatively developed in view of the architecture of the ADC+FPGA+DSP. The distributed optical fiber sensor controlled system based on FPGA is completed. And, the online real-time test functions of SignalTapⅡembedded logic analyzer self-carried in QuartusⅡsoftware are employed to verify the correctness of the FPGA design in the distributed optical fiber temperature sensing experiment.The available results are as follows:the distribution of spontaneous Brillouin-scattering power in the fiber and the curve of scattering power received in the start point of fiber are obtained. And on the basis of the numerical results of the scattering light power changing linearly with the temperature located on different positions of the fiber, the Brillouin temperature coefficient is fitted to meet the experiment result. The value of the coefficient is related with the location of the fiber. Actually, farther from the fiber end, smaller the corresponding coefficient is. The coding gain of an encoding matrix is related to the quadratic sum of all the elements of the decoding matrix. And, the coding gain is improved with the decreasing quadratic sum. The encoding performance is directly impacted by the structure of the encoding matrix and the value of the elements in the matrix. Many expected advantages are available by using the new matrix as greater coding gain, fewer transmission times for pumping light and simplified coding process. Then the power of scattering signal and the coding gain are improved with the increasing code length. At the same time, the temperature uncertainty can also be decreased. We can adjust the code length to obtain the required system performance. The design for the control system of distributed optical fiber temperature sensor based on FPGA can be performed in system controlling, timing controlling and data buffering in a data acquisition card according to the ADC+FPGA+DSP architecture. The expected results have been obtained in the distributed optical fiber temperature sensing experiment.