A Study On Compact Silicon-based Polarization Multiplexer/Beam Splitters Using Subwavelength Gratings

Recently,photonics integrated circuits(PICs)have attracted much interest in on-chip large-capacity and high-speed interconnect systems.PICs based on silicon-on-insulator(SOI)platform is the most popular and promising candidate for such system due to its advantages of compact size,high integration,CMOS compatibility.Polarization multiplexer and polarization beam splitter are fundamental and essential components for silicon-based PICs.The former can achieve the multiplexing of two polarization modes and thus double the transmission capacity.Meanwhile,it can be used with other multiplexing techniques for further improvement of communication capacity.The latter is used to solve the polarization dependence problem in silicon-based optical device by efficiently splitting two polarizations.In this thesis,novel polarization multiplexer and polarization beam splitter utilizing subwavelength gratings(SWG)structure in SOI platform are proposed and analyzed in detail,providing reference for building high performance PICs.Firstly,the background and significance of the photonic integration technology are summarized,together with brief comments on advantages and disadvantages of silicon photonics.Afterwards,current status of hybrid multiplexing techniques and polarization beam splitters are reviewed.Key performance parameters of silicon optical devices are also given.Secondly,the principle of SWG structure and several commonly used numerical methods are briefly introduced.With finite-difference frequency-domain(FDFD)method and finite-difference time-domain(FDTD)method,the modal characteristics and transmission properties of SWG with different parameters are analyzed.Then,the loss characteristics of SWG mode converter and the coupling characteristics of SWG directional coupler are analyzed in detail.The advantages of multi-dimensional and flexible index manipulation for SWG structure are presented,providing a firm foundation for further designing novel SWG-based devices.Next,a compact silicon-based polarization multiplexer is proposed and analyzed,in which the SWG structure is embedded into the microring resonator.By carefully optimizing such parameters as duty cycle and pitch,the proposed device has advantages of compact size,low-loss,and high extinction ratio.In addition,the fabrication tolerances to key structural parameters are also analyzed.Furthermore,a novel polarization beam splitter utilizing a special directional coupler composed of two narrow strip silicon waveguides and SWG structure is proposed and analyzed.The special directional coupler can overcome the common drawback of the long coupling length for TE mode in conventional directional coupler,and thus the length of the device is greatly reduced.Simulation results shows that the total coupling length is as short as 9μm,while excellent extinction ratio and low insertion loss of both polarizations are achieved,covering whole C-band in optical communication system,indicating that the present device has great potential applications in building ultracompact diversity schemes in PICs.Finally,the conclusion is drawn and the outlook of the future work is also presented.