An Inestigaton On Silicon-based Compact Mode-order Converters

To meet the bandwidth requirements of data center and super computer growing,various kinds of multiplexing technologies have been proposed,e.g.,wavelength division multiplexing(WDM),polarization division multiplexing(PDM),and mode division multiplexing(MDM)technology,greatly improving the transmission capacity of the optical communication system.Meanwhile,the development of multiplexing technology also requires the development of optical integrated circuits(PICs),in which silicon-based PIC has the advantages of high refractive index contrast and CMOS-compatible processing.In addition,mode division multiplexing has become one of the hot topics in current research,where the MOC is the key component.In this thesis,several silicon-based compact mode order converters are proposed and designed in detail.The performance is assessed and the tolerances to key structural parameters are presented.Firstly,from the demand of optical communication transmission capacity and the development of photonic integration technology,the development of optical communication technology and the state-of-art for PICs are briefly reviewed and challenges they will face up are discussed.The development and current status of MOCs are reviewed indetail.Next,starting from the Maxwell equations,two commonly used numerical techniques for analyzing integrated optical devices are introduced,including frequency domain finite difference method(FDFD)and finite-difference time-domain(FDTD)method.The principle of FDFD method to analyze modal charactertics for optical waveguides and that of FDTD method to analyze light propagation features in photonic devices are presented.By using FDFD method,the modal characteristics of slot waveguide are analyzed in detail.It is found that the slot waveguide can strongly confine the light field to the low refractive index slot region.By using such properties,various kinds of novel photonic devices can be developed.Then,using diverse slot waveguide structures,an interferometric MOC for TE polarization and a phase modulation MOC for TM polarization are proposed and analyzed in detail.Numerical results show that the conversion of fundametal mode to first-order mode is realized at the wavelength of 1.55?m,where the conversion efficiency of the former MOC is 83.5%and that of the latter MOC is 67.4%.In addition,the insertion loss with respect to the angle of the Y-branch or that of the individual waveguide is discussed in detail.Moreover,based on the principle of self-imaging,a multimode interference(MMI)MOC for TM polarization is proposed,in which the fundamental mode can be converted into first-order mode.By carefully optimizing the structural parameters,the conversion efficiency is up to 92.3%.Self-imaging effect in multi-modes waveguide is analyzed in detail and the optimized structural parameters for the proposed MOC are given.By 3D-FDTD simulation,the insertion loss and crosstalk of the output port as the functions of the key structural parameters are also presented.In addition,a MMI MOC based on multi-slots waveguide is also proposed,and the conversion from fundamental mode to first-order mode for TM polarization is realized with a conversion efficiency of 91%.A detailed analysis of the modal characteristics for multiple slotted-waveguide is given,the variation of the optical confinement factor and power density with the thickness of the slot is presented,and the insertion loss and crosstalk with respect to thickness of the slot are also given.Finally,the summary of the thesis and the outlook of the future work are given.