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Research On FBG Demodulation Technology Based On A Tunable Fabry-Perot Interferometer

Posted on:2009-04-02Degree:DoctorType:Dissertation
Country:ChinaCandidate:X Y YuFull Text:PDF
GTID:1118360278966949Subject:Measuring and Testing Technology and Instruments
Abstract/Summary:PDF Full Text Request
Fiber Bragg gratings are one type of developing fast passive fiber devices in recent years. As sensing element, they encode the sensing information in a wavelength form, which are their distinguished advantages over other transducers. FBG sensors have the characteristics of compact structure, easy to integrate and bury. Also they are modulated by wavelength and immune to electromagnetic interference and light intensity fluctuation. Multi-sensing like stress and temperature sensing are achieved by wavelength, time and space division multiplexing technologies. FBG sensors are mainly applied in the fields of civilian, aviation, ship, electric power and petroleum. Many demodulation techniques and methods for FBG sensors have been reported such as the fiber unbalanced Mach-Zehnder interferometer, edge filters, and matched fibre gratings filters and tunable fiber Fabry-Perot (F-P) interferometer. Due to the advantages of high sensitivity, wide tunable range, the fiber F-P interferometer demodulation uses micro-driving device to change the microcavity for scanning spectrum of the FBGs, which is more sutable for multi-point demodulating. Therefore, a FBG demodulation method based on the tunable fiber F-P interferometer is deeply studied.Absorption loss in the fiber interfaces, effects of the finite multi-beam interference and the unparalleled reflection plates on the F-P interferometer performance are given here. Then the parameters and design requirements of the tunable F-P interferometer are presented. A driver based on gradient magnetic field force and a driver based on electric field force are designed by simulation experiments. The micro-displacement driver is designed on the base of analyzing the features of giant magnetostrictive material (GMM) and piezoelectric (PZT) micro-displacement drivers. In this thesis, a tunable F-P interferometer structure is designed to demodulate FBG sensors. Aim at keeping the temperature stability of the F-P microcavity length and decreasing the system complexity, an F-P interferometer structure with low temperature drift is studied by using the ANSYS software. Firstly, according to the experiments, the temperature-length curve of the F-P microcavity is built. The feasibility and correctness of the finite element modeling simulation are verified by contrasting the experiments and simulation curves. Then a new structure of low temperature drift F-P interferometer is designed by using the composite materials (Invar alloy and carbon steel), which compensate the thermal expansion differences of the PZT module. Simulating results show that the composite structures eliminate the temperature drifts.In order to get the accurate time of the output signal peak of the demodulation system, signal reconstracting arithmetics are presented to filter the noises. By analyzing the frequency spectrum of the output signal, we get the frequency range of noises and design a low-pass digital filter using Kaiser window. Because the low-pass digital filter can't filter the noise completely, local Gaussian fitting filter is designed according to the spectral reflective characteristics of FBG.Series and parallel correction methods are analyzed, and we choose and imprvove the parallel one. A multi-point FBG sensor demodulation system based on the tunable F-P interferometer is built. The PZT driving voltage-wavelength function is built according to the 48 reference points using fast method. This system can calibrate the nonlinear problems of the micro-displacement driver and the structural errors of tunable F-P interferometer in real-time, that is to say, the measurement precision is improved. Furthermore, circuits by using field programmable gate array(FPGA) as core device are designed, which integrates control circuits, data compression circuits and filter circuits in one FPGA chip and ensures the system work in real-time.
Keywords/Search Tags:Fabry-Perot interferometer, fiber Bragg grating sensor, demodulation, digital signal processing
PDF Full Text Request
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