Intermodal Interference In Microstructure Fibers And Mode Coupling Between Cavities

Due to the tailoring flexibility of the cladding microstructure, the optical characteristics of microstructure fiber (hereinafter referred to as MF) are obviously different from those of the traditional fiber. The MF has become a heatedly discussed topic in the optical filed due to these characteristics. The thesis attempts to conduct an elementary research concerning some characteristics of MF.Firstly, a general review about the research situation on the area of MF has been given. The review includes the introduction of the concept of MF, the similarities and differences between the MF and the traditional fiber, variety of the dispersion characteristics of MF, polarization and birefringence in MF, MF application in nonlinear optics and the multiplicity of the MF fabrication materials and the fabrication methods, intermodal interference in MF, etc.Then, a number of methods for the study of mode and intermodal interference in MF have been introduced briefly. The methods include: the effective-index method, the localized function method, the full vector plane wave decomposition method, the biorthonormal-basis method, the multi-pole method, the Fourier decomposition method, the finite-difference method, and the finite element method, etc. Each method has its respective advantages and disadvantages. One can choose appropriate method with flexibility depending on the research object and the requirement of calculation accuracy in the actual research work.Further more, the different mode dispersion characteristics of various structure parameter MF have been studied using the effective-index method. The results indicate that the mode and dispersion characteristics of MF are determined by its structure parameter sensitively. The zero dispersion points of different modes in multi-mode MF distribute on one wavelength region. Some nonlinear effects, which can only take place in the vicinity of the zero dispersion point, take place more easily in the multi-mode MF. The supercontinuum spectrum generated by the combination of various nonlinear effects in the multi-mode MF is a typical example.Intermodal interference in MF has been studied intensively in the following section. The intermodal interference phenomenon in a MF which only supports two guided modes has been studied firstly by using the effective index method. The theoretic result coincides well with the experimental measurement reported in the reference. This coincidence confirms that the theoretical method chosen is a feasible one for the present research. More importantly, the fine structure in two-mode intermodal interference in frequency domain is predicted by the theoretical calculation. The fine structure is induced by the flexibility tailoring of the cladding microstructure. The fine structure has improved the interference measurement preciseness by one degree. Then, a novel phenomenon in the MF which is made by our group has been explained. A continuum spectral is generated in a certain wavelength region when a femtosecond pulse generated by the Ti:sapphire laser is coupled into a MF. Then, along the direction of the fiber core, a pseudo-periodic distribution of some colors intensively spectral line in some wavelength emerged. A similar phenomenon has been observed in a random distribution air hole multicore MF at the beginning stage of MF fabrication. At that time, the phenomenon was considered as the result of polarization-related interference induced by the asymmetry of the random air holes. But now, the MF is well symmetric, the phenomenon should be explained by other mechanism. Numerical calculation shows that the MF can only support four guided modes. The intermodal interference in the four mode MF can very well explain the phenomenon. Finally, a contrast and comparison of multi-mode intermodal interference between the MF and the traditional waveguide has been conducted. The potential application areas of multi-mode intermodal interference in MF have been shown.The characteristic of the mode coupling between cavities has been studied in the last section of the doctoral thesis. The fractions of power coupling between the injecting cavity's fundamental mode and the following cavity's fundamental mode, the first transverse mode, second transverse mode, and third transverse mode have been studied. The dependence of these fractions of power coupling on the relative offset and tilting between the cavity axes are given in an analytical expression. The conditions that must be satisfied for these fractions of power coupling to get the maximum are given finally. This piece of theoretical work may be used as a guideline in the process of optical cavity adjustment.