Theoretical Study On Mode Propagation Characteristics Of A-Axis Single Crystal Fibers

With the development of optical fiber communication technology, single crystal fibers have attached more and more attentions recently. a-axis step-index single crystal fibers have large electro-optic coefficient and nonlinear effect, which make them wide application in electrical field modulation, holographic storage and grating fabrication. A lot of experimental research has been done on a-axis step-index single crystal fibers. High quality a-axis single crystal fibers have been grown, and the properties of them have been studied. Because the theoretical calculation of a-axis single crystal fibers is very complex, there are few theoretical studies on them. In this thesis, the theoretical studies of the mode characteristics for the a-axis single crystal fiber were presented in detail.On the basis of Maxwell's equations, under the weakly anisotropic condition, the electromagnetic field equations of the a-axis step-index single crystal fiber were solved by means of the perturbation method. According to the boundary conditions, the expressions of the transverse field components and the dispersion equations were obtained. The dispersion relations and mode characteristics of the fiber were analyzed numerically. It is found with the increase of the normalized frequency, the number of the propagation modes of the weakly anisotropic a-axis step-index single crystal fiber increases. As the normalized frequency is invariant, with the decrease of the normalized propagation contant, the mode is farther from linear polarization.The dispersion equations of the mth order elementary modes for the strongly anisotropic a-axis step-index single crystal fiber were obtained under more accurate boundary condition. The dispersion equations were discussed for the case of faring from cut-off region, nearing cut-off region and in the guided mode region, respectively. Numerical results of the mth order elementary modes were presented for the strongly anisotropic a-axis step-index single crystal fiber. It was found that some mth order elementary modes for the strongly anisotropic a-axis step-index single crystal fiber may be approximately regarded as the mth order linearly polarized mode (LPmn). Some modes are corresponding to a same linearly polarized mode, which are the sub-modes of the linearly polarized mode. The degree of linear polarization of the sub-mode changes with the normalized propagation constant b.The changes of the dispersion relation and the propagation modes of the a-axis lithium niobate single crystal fiber with applied electrical field were studied on the basis of the electro-optical effect of the lithium niobate crystal. The dispersion relation and the mode characteristics of the second order elementary modes were discussed numerically for the a-axis lithium niobate single crystal fiber with applied electrical field. It is found that the number of the modes and the cut-off frequency and the ratio of the peak of |Ex| to |Ey| of a same mode change with applied electrical field. This provides a way to realize electrical control of the propagation in a-axis single crystal fiber.The mode propagation characteristics of electro-optic modulator and optical filter based on a-axis single crystal fibers were studied. The a-axis single crystal fiber modulator can be used to construct the all optical fiber transmitting module with the fiber laser. LP01 mode is admitted without electrical field, and LP01 mode is cut-off while the electrical field is applied. The single mode propagation is realized by applying electrical field. The a-axis single crystal fiber filter can be used to control the mode propagation by filtering out some mode with applied electrical field.