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

In recent years,on-chip optical interconnect technique,with photonic integrated circuits(PICs)as the core,has been widely emphasized and is anticipated to break through the bottleneck problems of traditional electronic interconnect systems.Among them,PICs based on silicon-on-insulator(SOI)platform have become a kind of popular technology for building high-integration,high-speed and high-capacity optical interconnect systems,due to their compact structures and compatibility with CMOS manufacturing processes.Polarization beam splitter and polarization rotator are the two basic passive devices for building PICs.The former can realize the multiplexing and demultiplexing of the two polarization modes to increase the communication capacity,while the latter is mainly used to convert one polarization mode to the other one,reducing the number of polarization-sensitive devices in system and thus improving integration.In this thesis,novel polarization beam splitters/rotators based on subwavelength gratings(SWG)structure are proposed and optimized in detail,providing references for building high-performance and highly integrated PICs.Firstly,the development background and research significance of PICs are briefly reviewed.The features and advantages of silicon photonics,together with the problems and challenges that will be faced,are also generalized.Afterwards,current status of polarization beam splitters/rotators are introduced in detail,and the crucial performance parameters of these two silicon passive devices are presented.Secondly,two numerical analysis methods,finite-difference time-domain(FDTD)method and finite-difference frequency-domain(FDFD)method,are introduced for device design and optimization.The structure features of SWG are described,and the transmission and mode characteristics of SWG structures are discussed,using FDTD and FDFD,respectively,verifying the effective suppression of diffraction and reflection,as well as the property of tunable refractive index.A foundation for the subsequent device design and optimization is established.Next,a silicon-based polarization beam splitter,making use of SWG and coupled mode theory,is proposed and analyzed.In the asymmetric directional coupler embedded with SWG-assisted waveguide and slots,the TM mode is set to meet the phase matching condition while the TE mode is not,thus the splitting of the two polarizations is realized.Simulations and calculations are performed to optimize the structural parameters of the SWG-assisted strip waveguide.The results show that the device has low insertion loss(IL),high extinction ratio(ER),large operating bandwidth,and compact footprint.Then,with the combination of tilted and typical SWG and the effect of adiabatic evolution,the other silicon-based polarization beam splitter is proposed.Based on the birefringence property of tilted SWG,the TE mode is guided into the waveguide with higher equivalent refractive index,whereas the TM mode is confined in the original waveguide.As a result,field distributions of the two modes are effectively separated.The results show that the insertion loss of the device is only 0.10 d B,with the extinction ratio larger than 30 d B at the wavelength of 1.55μm,and the bandwidth over 240 nm for ER > 15 d B,which is of high application value in polarization diversity systems with multiple wavelength channels.Furthermore,a silicon-based polarization rotator,employing structures of etched waveguide and SWG is proposed and analyzed.The combination of taper-etched strip waveguide and SWG breaks the symmetry,which enables the mixing and coupling of the two polarization modes,realizes the conversion from TM mode to TE mode,and reduces the wavelength dependence of conversion effect.The results show that the polarization rotation is achieved with low insertion loss,high extinction ratio,and high conversion efficiency in a wide wavelength range.The device is also cascaded with the polarization beam splitter in Chapter 4 to form a polarization beam splitter-rotator,with insertion loss of 0.17 d B at the wavelength of 1.55μm and the bandwidth for ER > 15 d B exceeding 240 nm.Finally,the work of the full text is summarized and the outlook on the subsequent research is also presented.