Research On Transmission Characteristics And Applications Of Coupled Devices Based On Photonic Crystal Fibers And Optical Waveguides

This dissertation demonstrates several novel-function dual-core photonic crystal fibers and their applications in designs of mode converters, directional couplers, polarization beam splitters, and refractive index sensors are analyzed by various numerical methods. The main contents are as follows:It’s generally difficult to achieve equal power in three outputs for a conventional1x3directional coupler due to its limitation of structure. As a result, a design for a novel1x3directional coupler which is based on an asymmetric three-core photonic crystal fiber (PCF) is proposed, where the energy is equally divided in three cores. It’s finally found that a part of energy coupling between the outside cores and the center core can be realized by introducing the enlargement of air holes of the side cores in three-core PCF. The impacts of the inner cladding air holes around the core and the periods on energy are analyzed in detail. In addition, the proposed coupler shows large tolerance to the fiber parameters.The reported mode converter can’t work well due to their narrow bandwidth or low conversion efficiency. To solve it, a design for a novel broadband mode converter is proposed by applying semi-vectorial beam propagation method. By adjusting the air-hole diameters of the inner rings in the cores and the index of the down-doped silica rods, broadband index-matched coupling between LPoi and LP02can be achieved. Numerical investigations demonstrate that the novel mode converter can work with broad operating wavelengths. In addition, a novel optical polarizer based on flexible design of photonic crystal fibers is proposed. It can be achieved by coupling the unwanted polarization from one core to the other core, thus allowing the required polariztion existing in the original core.Generally, there are two methods to achieve polarization splitting. One is to make full coupling between the two polarizations, and then, meet certain ration between the corresponding coupling lengths. On the other hand, another kind of polarization splitters was designed with asymmetric dual-cores, in which one polarization is entrapped in the incident core while the other polarization can be freely coupled between the two cores. We also proposed a novel polarization splitter. Based on mode coupled theory, the fiber is designed such that index-matched coupling between the two cores can be achieved for one polarization state while only a part of energy could be coupled for the other polarization state. As a result, the coupling length in one polarization is twice longer than that in the other polarization. This allows for adjusting the optical fiber structure parameters to achieve polarization splitting easily. The modes of the proposed fibers are solved by a semivectorial beam propagation method. In addition, the numerical results are in good agreement with those of the multipole method.The sensitivity is relatively low for detecting analyte with low refractive index (Generally, the refractive index of the analyte is lower than that of the fiber background). However, it has the potential in chemical and biological applications by taking advantage of the exponential dependence of intercore coupling on analyte index. We demonstrate design strategies for high-sensitivity refractive index sensors, which are based on the principle of wavelength-selective resonant coupling in dual-core photonic crystal fibers. Phase matching at a single wavelength can be achieved between an analyte-filled microstructured core and a small core with a down-doped rod or one small air hole in the center, thus enabling selectively directional resonant-coupling between the two cores. As a result, the transmission spectra of the output light presents a notch at the index-matched wavelength, yielding a resonant wavelength depending on the refractive index of the analyte. The impact on the shift of the resonant wavelength and the full width at half maximum of the resonance are analyzed in detail. In addition, the PCF coupler is influenced by the environmental temperature and the tolerance of the structure parameters.Optical properties of silica-air PCFs can be extended by filling the cladding air holes with materials such as analyte, liquid crystal, semiconductor, or metal etc, and the corresponding tunable devices can be achieved. The polarization characteristics of a dual-core photonic crystal fiber (DC-PCF) with a metal wire filled into the cladding air hole between the two cores have been investigated. The coupling length of the proposed dual-core PCF is always shorter than that of the dual-core PCF without filling metal wire in the center. Furthermore, the coupling length reaches a maximum. In addition, we theoretically investigate dual-core photonic crystal fibers with metal wires between the two cores. Numerical simulations demonstrate that a polarization splitter with broad band, high extinction ratio, and good output mode field can be achieved by selectively filling metal wires.The light can be confined in the subwavelength SOI waveguide due to its high refractive index difference, while the optical waveguides based on surface plasmon resonance can break the optical diffraction limit and the light can also be confined in subwavelength waveguide. To take advantage of them, we propose a polarization beam splitter which is composed of a horizontally slotted waveguide and a hybrid plasmonic waveguide. The splitter is designed such that index-matched coupling between two waveguides can be achieved for one polarization state while huge effective index difference can be allowed for the other polarization state. As a result, an ultrashort device with low extinction ratio can be used to photonic integration.