Research On Interference Demodulation For Fiber Bragg Grating Sensor

As a passive device, Fiber grating has many characteristics such as small size, high sensitivity, immunity to electromagnetic interference and so on, which plays an important role in the modern sensing field. As fiber grating sensor responses to reflected or transmitted wavelength change, how to accurately and conveniently demodulate the wavelength shift is one of the key techniques of fiber grating sensing. In this thesis, the interferometer demodulation methods for fiber grating reflected wavelength were studied, then a simple and accurate demodulation structure was proposed.In this thesis, the fiber grating sensing mechanism was analyzed by using coupled-mode theory and the effects of various parameters on the reflection spectrum were studied by numerical simulation. Temperature and strain sensing models were established respectively according to the resonance wavelength formula.An interferometer demodulation scheme based on unbalanced M-Z waveguide modulator was proposed. Theoretical analysis and experimental verification were carried out. Then the demodulation scheme was compared with another one which was based on unbalanced M-Z polarization maintaining fiber interferometer. The differences between them were studied through structural analysis and experimental results comparing.Finally, a fiber Bragg grating (FBG) temperature sensing and demodulation system based on unbalanced M-Z lithium niobate waveguide modulator was designed. The temperature sensing and interferometer demodulation experiments were carried out respectively. The experiments showed that, in the range of25～85℃, there was an approximately linear relationship between FBG resonance wavelength and the ambient temperature with a rate of10.74pm/℃, which was consistent with the theoretical analysis. The demodulation accuracy was about1.66pm and the demodulation range was about2.2nm, which proved that this system had high precision and good stability. The study in this thesis has great significance for the widely use of fiber Bragg grating sensors.