A Study On Silicon-based Multimode Interference Couplers With Subwavelength Gratings

As an important element,widely used since the 1990 s,the multimode interference(MMI)coupler has the advantages of broad bandwidth and large fabrication tolerances.However,the rapid development of the information technology has brought great demands for high-speed and large-capacity optical communication systems.Optical communication systems require devices with higher level of integration and performance.Apparently,MMI couplers based on conventional waveguides need to be further optimized.Subwavelength periodic structures,which have significant advantages in improving device performance and affording flexibility in designing,are landmark discoveries in the field of photonics.Recently,subwavelength structures are widely applied to construct various kinds of silicon-based photonic devices due to the development of high-precision lithography techniques.The introduction of subwavelength structures can efficiently control polarization and modal behaviors,which means they are suitable for the design of polarization-insensitive power splitters and mode-order converters.The former can realize the elimination of typical strong-polarization-dependence on the SOI platform and the split of optical power,and are common devices in optical communication systems.The latter,which are the core components in mode division multiplexing technology,are used to convert fundamental mode to high-order mode.In this thesis,a novel and high-performance polarization-insensitive power splitter and a novel mode order converter based on subwavelength-grating-based MMI couplers on the SOI platform are proposed.Firstly,the development and the significance of photonics integrated circuits(PICs)are reviewed.The current status of power splitters and mode order converters are introduced in detail.The application of subwavelength grating in silicon-based passive devices is briefly described.Finally,the contents and innovation of this paper are presented.Secondly,the working principles of the MMI couplers and subwavelength structures are introduced in brief.The basic principles,boundary conditions and development process of several commonly used numerical techniques for analyzing photonic devices are also summarized.Afterwards,the modal characteristics and light propagation features of the subwavelength-grating-based MMI couplers and the related structures of MMI couplers are presented.Next,a polarization-insensitive power splitter with an SWG embedded in an MMI coupler is proposed.By adjusting the duty cycle,polarization-insensitive transmission is achieved.With the help of tapers,cut-right-angled multimode waveguides,and inverse-tapered SWG structures,the polarization-dependent loss can be quite low.From the results,this device has superiorities of broad bandwidth,low loss and large fabrication tolerances.Then,a mode order converter based on the subwavelength-grating-based MMI coupler is proposed and analyzed to realize the conversion from the fundamental mode to the first-order mode of TE polarization.Two sets of subwavelength gratings are used to modulate the phases of images in the multimode waveguide.The position,size and duty cycle of subwavelength gratings are determined by the 3D-FDTD method.The final conversion efficiency can reach98%.Based on this scheme,the conversion from the fundamental mode to the second-order mode of the TE polarization can also be realized.Moreover,the conversion efficiency is further improved and the insertion loss is reduced by cutting the right angles at both corners of the used MMI coupler,and the final conversion efficiency is 94.8%.These two devices are evaluated by conversion efficiency,insertion loss and crosstalk,and the analysis results show that these two devices have the advantages of compact size,broad bandwidth and large fabrication tolerance.Finally,the conclusion of this paper is drawn and the outlook of the future research is presented.