Optimization of silicon photonic devices for polarization diversity applications

This thesis discusses two important designs, analysis and optimization of polarization-based devices such as polarization rotator and splitter. Many optical sub-systems integrate with guided wave photonic devices with two-dimensional confinement and high contrast between the core and cladding. The modes present in such waveguides are not purely of the TE or TM type. They are hybrid in nature, where all six components of the magnetic and electric fields are present. This causes the system fully to be polarization dependent. Currently, the polarization issue is a major topic to be dealt with during the design of high efficiency optoelectronic subsystems for further enhancement of their performances. To characterize the device polarization properties a vectrorial approach is needed. In this work, the numerical analysis has been carried out by using the powerful and versatile full vectorial H-field based finite element method (FEM). This method has been proved to be one of the most accurate numerical methods to date for calculating the modal hybridness, birefringence and consequently to calculate the device length, which is an important parameter when designing devices concerning the polarization issues. Polarization devices may be fabricated by combining several butt-coupled uniform waveguide sections. The Least Squares Boundary Residual (LSBR) method is used to obtain transmission and reflection coefficients of all the polarized modes by considering both the guided and the radiated modes. On the other hand, finite element method cannot calculate the power transfer efficiency directly, hence the LSBR method is used along with the FEM for this purpose. The LSBR method is rigorously convergent, satisfying the boundary conditions in the least square sense over the discontinuity interface. Using this method, the power transfer from the input to the coupler section and at the output ports can be evaluated. When designing polarization rotators, it is necessary to calculate the modal hybridness of a mode. In this research, it is identified that when the symmetric waveguides are broken, the modal hybridity is enhanced, and thereby a high polarization conversion is expected. This work is devoted to the study of design optimization of a compact silicon nanowire polarization device. An interesting and useful comparison is made on their operating properties such as the crosstalk, device length, polarization dependence, and fabrication tolerances of the polarization in directional coupler based devices. In this study initially the H-field modal field profile for a high index contrast silicon nanowire waveguide is shown. The effects of waveguide’s width on the effective indices, hybridness, power confinement in the core, and the cladding have been investigated. The modal birefringence of such silicon nanowire waveguides also is shown. It is presented here that for a silicon nanowire waveguide with height of 220 nm, fundamental and second modes exist in the region of the width being 150 – 300 nm, and 500 – 600 nm, respectively. A compact 52.8 μm long passive polarization rotator (PR) using simple silicon nanowire waveguides is designed with a power transfer of 99 % from input TE to output TM power mode, with cross-talk better than – 20 dB and loss value lower than 0.1 dB. Furthermore, an extensive study of fabrication tolerances of a compact (PR) is undertaken. The design of an ultra-compact polarization splitter (PS) based on silicon-on-insulator (SOI) platform is presented. It is shown here that a low loss, 17.90 μm long compact PS, and wide bandwidth over the entire C-band can be achieved.