Characterization Of Compact Polarization Splitters/Rotators In Silicon Slotted-Nanowires

Recently, the importance of photonic integrated circuits (PICs) based on silicon-on-insulator (SOI) system has been widely recognized due to its high index contrast and CMOS-compatible process. However, photonic devices based on SOI are often sensitive to the polarization states of the light. It is well known that different polarizations in a chip would have distinct propagation characteristics, which will degrade the performance of the designed devices and even the systems. Therefore, compact polarization splitters/rotators(PSs/PRs) that can eliminate the effects of polarizations play an important role in the PICs.More recently, silicon slotted-nanowires, which can confine light in nano-scaled low index regions and break through the diffraction limitation, are providing new ideas for forming noval silicon photonic devices. In this dissertation,by using silicon slotted-nanowires,several PSs and PRs are designed and analyzed with features of compact size,high extinction ratio,and low insertion loss. The principles of the designed devices are given, the optimized parameters are presented, and the fabrication tolerances are discussed. The proposed PSs/PRs will have application potential in silicon PIC. The dissertation is organized as follows:In chapter 1,the development of SOI-based PIC is summarized briefly. The structure,principle,and applications of slotted-nanowires are introduced,and then the research progress of the typical PSs/PRs is reviewed.In chapter 2,the common-used mode solver and propagation analysis methods for silicon-based photonics devices are introduced, and their merit and demerit are also evaluated.Then a modified multidomain pseudospectral method for bending waveguides with curved interfaces is proposed. Three structures, i.e., straight ridge waveguide, bending ridge waveguide and bending step-index fiber, are analyzed with this method to validate its effectiveness. The numerical results show that this approach has fast convergence behavior with high accuracy.In chapter 3, two PSs based on asymmetric directional couplers (DC) are designed and analyzed utilizing vertical slot nanowires. By properly optimizing parameters, the two orthogonal polarization states can be separated effectively in both devices. The device based on two-guide DC has a short coupling length of 20.5?m and crosstalk lower than -23.3dB.The device based on three-guide DC has a coupling length of 7.6?m and crosstalk lower than-31dB. For coupling between slot nanowires and silicon nanowires, an adiabatic mode converter is designed to convert the eigenmodes between the two structures efficiently.In chapter 4, two PSs based on multimode interference (MMI) couplers are designed and analyzed utilizing horizontal slot nanowires. To address the issue of long length in MMI PSs,the slot nanowire with high index contrast is used to shorten the length effectively.Simultaneously, the parabolic tapered MMI section and the restricted interference mechanisms are adopted to further reduce the size of the device. The numerical results show that the MMI length is 23.4?m, the polarization extinction ratios (ERs) are higher than 22.3dB,and the insertion losses (ILs) are lower than 0.58dB. To address the issue of compromised ER in MMI PSs, a cascaded PS is proposed, which reaches high ERs for both polarizations. The numerical results show that the MMI length is 42?m, the ERs are higher than 29.8dB, and the ILs are lower than 0.7dB.In chapter 5, cross-slot nanowires are introduced and their mode characteristics are analyzed in detail. Then a PR with compact size and high bandwidth for cross-slot nanowire is designed and analyzed. The device is based on mode hybrid in an L-shaped slot nanowire with slant optical axis, which could realize nearly 100% polarization rotation. The numerical results show that the length of the rotation section is 18.9?m,the ERs are higher than 31.6dB,and the ILs are lower than 0.38dB in C-band. Moreover, the bandwidth of the device is 230nm, which is much higher than the counterparts.In chapter 6, the conclusion of our work is drawn and the outlook for future research is presented.