Research On Wavelength Demodulation Technique Of Fiber Bragg Grating Based On Photorefractive Volume Grating

A novel wavelength demodulation technique of fiber Bragg grating is presented in this thesis on basis of adequate research on the diffraction properties of photorefractive volume grating. Where, the photorefractive volume grating works as a filtering component by making use of the wavelength selectivity to realize the linear filtering of Bragg wavelength signal and achieve the purpose of wavelength demodulation. Compared with the available demodulation techniques, the method possesses many merits: different photorefractive volume gratings can be easily and flexibly designed according to practical requirements, such as the measurement resolution and range; photorefractive volume grating can be thermally or electrically erased and can be repeatedly used; multiple photorefractive volume gratings can also be recorded in the same photorefractive crystal to realize the wavelength demodulations of multiple sensing fiber Bragg gratings; the simple configuration and cheap cost are also the merits of this kind of fiber Bragg grating sensor, and so on.Encircling the demodulation technique, the principal work in the thesis is as follows:(1) The reflective spectra of fiber gratings in single mode silica fiber, including uniform fiber Bragg grating, long periodic fiber grating, linearly chirped fiber grating and apodization fiber grating are numerically simulated and the relationships between the spectra and the length of grating as well as the refractive index modulation are investigated. The sensing properties for temperature and strain of fiber Bragg grating are theoretically analyzed and numerically simulated.(2) On basis of the Kogelnik's coupled wave theory, the angle selectivity and wavelength selectivity of photorefractive volume grating are theoretically analyzed. Furthermore, the photorefractive volume gratings are experimentally recorded in the LiNbO₃:Fe crystals with the holographic method and the experimental results of angle selectivity are presented under the conditions of different crystal thicknesses and recording angles.(3) A novel method for measuring the refractive index modulation depth and effective thickness of photorefractive volume grating is presented. By measuring the relationship between diffraction efficiency and Bragg angle deviation of volume grating, the refractive index modulation depth and effective thickness of photorefractive volume grating can be calculated according to the 3dB bandwidth, relative peak value of side lobe and its corresponding Bragg angle deviation in the relationship curve. The experimental results with LiNbO₃:Fe crystal are well in agreement with the theoretic analysis.(4) According to the refractive index ellipsoid of uniaxial crystal, the relationship between diffraction efficiency of volume grating and polarization state of readout beam is theoretically analyzed. The direction of refractive light beam and corresponding refractiveindex modulation will be both changed due to the variation of polarization state of readout beam. This may lead to the strong variation of the diffraction efficiency of volume grating. This kind of polarization dependent diffraction efficiency of volume grating in anisotropy crystal is extremely disadvantageous for some applications. A method to suppress the polarization dependent diffraction efficiency with double volume gratings is presented, and the experiments with LiNbO₃ Fe crystal are also demonstrated. The experimental results indicate that this method can well suppress the polarization dependent diffraction efficiency of volume grating. Furthermore, the diffraction properties of double volume gratings are almost independent of the polarization state of readout beam. The relative values of diffraction peaks are calculated on the basis of the relationship between index modulation and polarization state. The experimental values are well in agreement with the theoretic analyses.(5) The angle and wavelength selectivity of photorefractive volume grating for near-infrared wavelength is experimentally researched by using two-lambda method. The...