Research On Fiber Bragg Grating & Its Application In Sensing Field

This dissertation describes the progression in theory, experiment and related works about the fiber Bragg grating (FBG) and its application in sensing field, which I have carried out during the past three years. Here are the main points:1.The coupled-mode equations of fiber Bragg gratings are derived using Maxwell's equations, the principle of superposition, and considering the expansion of the perturbative vector fields in the eigen modes which perform orthogonality relation mutually. The boundary conditions of the equations are introduced. To chirped fiber gratings, the phenomenon of broad band reflection is qualitatively explained. Based on the equations, the Bragg law is derived and has been demonstrated by the writing-grating experiment.2.FBGs have been successfully written in both single-mode and multimode fibers. The angle between the multimode fiber and the axis of phase-mask is found to be the key factor to produce the difference transmittivities at Bragg wavelengths in transmission spectrum.3.The linearly tunable FBG that utilizes a cantilever beam is introduced. The strain on the beam has been analyzed theoretically. A formula described the tuning is suggested. The beam's structure has been optimized through calculating the wavelength-shifts of different cantilever beams that are with same sizes and different cross-section shapes. T-shaped section beam is pointed to be the best one to act as a linear tuner of FBG. After optimized, to a 10cm length beam, if 0.01N force is applied, it can provide over 0.28nm wavelength shift. Introducing a correction indexη=0.65, experimental result coincides with the theoretical value perfectly.4.The ability of linearly tunable of"T"shaped section cantilever beams that are with different thickness has been analyzed theoretically. Employing an appropriate beam size, the linearly tuning of Bragg wavelength of fiber grating has been demonstrated experimentally. The chirped output caused by the strain gradient along the beam-axis has been observed. Selecting short grating can reduce broadening effect.5.An uniform density and thickness isosceles triangle cantilever beam is designed to realize linearly tuning of Bragg wavelength of fiber grating with no-chirped by adjusting the deflection or the applied force of the free end that is the vertex part of the triangle. The tuning is demonstrated experimentally within the range of 4.52 nm. After introducing a correction indexη=0.71, experiment results match the theoretical value well. It was found that the reflection spectrum highly depends on the gluing quality...