Study Of Multimode Interference-based Artificial Microstructural Waveguides Coupling System

Over the past two decades, materials of artificial microstructure, such as photonic crystals(PtCs), photonic quasicrystals(PtQCs), phononic crystals(PnCs), phononic quasicrystals(PnQCs) and so on, have been proven to present enormous potential in controlling the flow of electromagnetic(EM) wave or acoustic energies on a scale comparable to the corresponding wavelength of wave because of their unique properties. Self-imaging phenomena is a property of multimode waveguide system by which an input field profile is reproduced in single or multiple images at regular intervals along the propagation direction. The multimode interference(MMI) effect results in the occurrence of replicas(include single and multiple images). MMI-based devices are important components of optoelectronic integrated circuit(OEIC) because of their simple structure, low polarization dependence, low loss, and large bandwidth. This property of a multimode waveguide is extensively investigated in conventional optical waveguides as well as in photonic crystal waveguides(PtCWs). However, studies on self-imaging in acoustic waveguides are rarely reported. In this paper, we investigate self-imaging effect in photonic quasicrystal waveguides(PtQCWs) based on the coupled-mode theory and the finite element method(FEM). For the first time, MMI effect in optical waveguide is extended to the acoustic waveguide.Research indicate that the self-imaging phenomena still can be achieved in photonic crystal waveguides(PtQCWs). As a potential application, a new kind of compact 3dB PtQCWs-based power splitter based on MMI effect for terahertz(THz) waves with symmetric interference is presented and analyzed. The proposed device provides a new compact model for exporting efficiently THz wave with a broad bandwidth to two channels averagely and can be extended to new designs of PtQCWs devices.An acoustic multimode waveguide structure that consists of M parallel phononic crystal waveguides(M-PnCWs) is constructed. To achieve the dispersions and replicas of the input acoustic waves produced along the propagation direction, the finite element method is used to M-PnCWs, which support M guided modes within the target frequency range. The simulation results show that single images(including direct and mirrored images) and N-fold images(N is an integer) are identified along the propagation direction with asymmetric and symmetric incidences discussed separately. The simulated positions of the replicas agree well with the calculated values that are theoretically decided by self-imaging conditions based on the guided mode propagation analysis. The self-imaging principle remains applicable for M-PnCWs just as it does for optical multimode waveguides. Moreover, the potential applications based on this self-imaging phenomenon for ultrasonic power splitters in the acoustic field are presented accordingly. This device can export freely ultrasonic waves to N channels and present practical applications in the acoustic wave circuits.