Study Of Characteristics And Applications Of Multimode Interference-Based Photonic Crystal Waveguides Coupling System

The realization of photonic integration is all along blocked by the large size and low integration of conventional planar waveguides. However, due to the capacity of controlling the flow of photonic, photonic crystal (PCs), which are periodic dielectric materials, arise as a very promising technology in the path towards photonic integration circuits (PICs). The periods of PCs are the magnitude of the operational wavelength, and there exists low, even zero loss at sharp bends in photonic cryatal waveguides (PCWs). At the same time, the multimode interference (MMI)-based components are important building blocks in PICs, this thesis studied the characteristics and applications of MMI PCWs coupling system through combining the characteristics of the PCWs and conventional MMI couplers.In this dissertation, firstly, multiple single-mode PCWs were put together closely and parallelly to construct waveguide couplers, which were regarded as multimode system. The self-imaging principle was analysized with the case of non-symmetry interference, and the mathematical conditions of the direct and mirror replicas of the input field were obtained in this kind of systems. The multiple eigenmodes in dispersive curves can be nearly degenerated. At the points nearly degenerated, MMI effect is deprived and power is confined along its input direction without observable transferring to other PCWs. This case can be extended to the coupling of more PCWs and the degenerated frequencies are all the while around the decoupled point of two coupled PCWs. Also a coarse wavelength de-multiplexer or multiplexer with compact structures and low cross was designed and simulated.The self-imaging principle with the case of symmetry interference in the structures above was also analysized. The coupling coefficient of two even modes excited by the input field symmetrically entering the multimode system consists of three PCWs may be increased dramatically by reducing the effective index between the neighboring PCWs. This characteristic was used to design a kind of ultrcompact 3dB power splitter. The input power can be split equally with a propagation distance of 1.2μm at the communication wavelength 1.55μm. The devices can be ultracompact, furthermore, high output efficiency and wide bandwidth can be also found in the response spectrum. The distribution of even excited modal fields can be varied by modifying the effective index of the modified region between the coupled PCWs. It leads to the transverse redistribution of the N-fold images in the coupling system composed of over three PCWs. The input power can be freely split into multiple output ports (including uniform splitting) without using cascading structures by exporting the modified N-fold images transversely. The beam splitters can be ultracompact in sizes and the ouput efficiency is very high even if the channel number is very large. At the operating wavelength of 1.55μm, the splitting length of the devices is only 35μm even if the output channel number reaches 20.A new kind of PC multimode waveguide on the basis of total internal reflection (TIR) was studied theoretically. The input field can also be guided along the multimode region with high efficiency due to the combination of TIR effect and distributed Bragg reflection even if there has no photonic band gaps (PBGs) when the effective index of the guided region is larger than ambient ones. A good equivalent model was applied to analysis the designed structures using MMI effect and self-imaging principle, the performance showed that it is a good equivalence. It provides a novel way to realize the polarization-insensitive components by combining PBG and TIR effect in PCWs.This dissertation provides a new and effective way to realize the novel devices having good function. It will find practical and extensive applications in future PICs.