Theoretical And Technical Researches On The Optical Fiber Grating Sensors

This thesis focuses on the basic theoretical and technical researches on the optical fiber grating sensors, including coupled mode analysis of optical fiber gratings; the fabrication methods of optical fiber gratings; the sensing mechanism of fiber gratings; the increasing and decreasing of temperature and pressure sensitivities of optical fiber gratings, the fabrication of wide band light source for fiber grating sensors and the demodulation technologies for the wavelength signals of fiber grating sensors. Firstly, the recent developments in the area of optical fiber grating sensors are reviewed. The coupled mode equations are built by means of expanding perfect waveguide modes. Using the coupled mode theory, the relationships between the reflective spectrum characteristics of fiber Bragg gratings and chirped gratings (the maximum reflectivity and the bandwidth) and their structural parameters (grating length, coupled coefficient, chirped coefficient, etc.) are studied. A mathematical model about refractive index distribution of fiber Bragg gratings under saturation is ? proposed Using this model, the first- and second-order diffraction characteristics of fiber Bragg gratings under saturation are studied and the formation processes of the first- and second-order diffraction are simulated. The simulated results are in excellent agreement with the experimental results Supported by Chinese National Nature Science Foundation (No. 69878001) ?? ~~1 The key problem of optical fiber gratings to be wide application and batch production is the techniques of fiber grating fabrication. In this thesis, various fabrication methods of fiber gratings are described systematically. The advantages -~ and disadvantages of each method are analyzed and compared. The research work mainly focuses on the phase mask method. The characteristics of near field diffraction of the phase mask are analyzed with scalar diffraction theory. The properties of grating formation are studied specifically when the writing light beam incidents normally and slantingly, respectively. Some important conclusions are obtained. The experimental study is accomplished to fabricate the fiber Bragg gratings with phase mask. The temperature, strain and pressure sensing mechanisms of fiber Bragg gratings and long-period gratings are studied systematically. The influences on the measurement results caused by the second-order temperature and strain sensitivities as swell as temperature-strain cross sensitivity are analyzed. The methods and mechanisms of increasing and decreasing of temperature sensitivity as well as increasing of pressure sensitivity are also studied In addition, various schemes of simultaneously measuring strain and temperature with fiber grating sensors are described. The advantages and disadvantages of these schemes are compared. The systematic errors of these schemes are also discussed. The temperature and stress sensing characteristics of fiber Bragg gratings are studied experimentally. The response curves of wavelength-temperature and wavelength-stress are measured in the range of 250C-1000C and O?80g, respectively. Broad band light source and wavelength demodulation technologies are two...