City Research Online

Abstract

Many integrated optical subsystems incorporate guided wave photonic devices and connecting optical waveguides with two-dimensional confinement and a high index contrast between the core and the cladding. The modes present in such waveguides are not purely of the TE or TM type but are hybrid in nature, with all six components of the electric and magnetic fields being present, which makes the overall system to be polarization dependent. In present high- performance photonic components and optoelectronic subsystems, this polarization issue is a major issue to be tackled with for further enhancement of their performances.

This thesis describes the design, analysis and optimisation of such polarization-based waveguide devices such as polarization rotators, polarization splitters and polarization controllers. To characterize the polarization properties of such devices a fully vectorial approach is necessary. In this work, the most versatile and accurate full vectorial H-field based finite element method (FEM) is used to simulate complex waveguide structures in order to optimize and evaluate novel devices, prior to fabrication. This method can accurately calculate the propagation constants of both polarized modes and consequently these are used to calculate the half-beat length, which is an important parameter when designing waveguide devices involving polarization issues. Many important photonic devices, such as polarization splitters, polarization rotators and polarization controllers may be fabricated by combining several butt-coupled uniform waveguide sections. The least squares boundary residual (LSBR) method is used to obtain both the transmission and the reflection coefficients of all the polarized modes by considering both the guided and radiated modes.

In this work, a combination of the FEM and LSBR methods has been extensively used to obtain the TE to TM or TM to TE polarized power transfer efficiency in semiconductor waveguides and the polarization crosstalk in high index contrast silica waveguides. When designing polarization rotators or identifying the possible polarization crosstalk, it is necessary to calculate the modal ‘hybridity’ of a mode. In this study, it is identified that when the waveguide lacks structural symmetry, the modal hybridity is enhanced, and thereby a considerably high polarization conversion is expected. A compact 400pm long passive polarization rotator (PR) with cascaded asymmetrical waveguide sections is designed with a very low insertion loss of 0.2dB. A more compact 320pm long, much improved PR is designed and analysed by using only a single slanted walled rib waveguide section. Furthermore, an extensive study of fabrication tolerances of a compact, single-stage PR is undertaken. A 99.8% polarization conversion is achieved with a very low crosstalk value of- 29dB. The design of a compact 1.6mm long single-section polarization splitter in a deeply etched semiconductor MMI waveguide is also presented. An extensive analysis of polarization crosstalk is carried out using high index contrast planar silica waveguides. The origin of polarization crosstalk in silica waveguides is explained and it is shown that a significant polarization conversion can take place in a long, high index contrast silica waveguide. It is also learnt that non-verticality of the sidewalls causes the significant polarization crosstalk in many silica-based components. Finally a novel design concept of an active polarization controller is presented using twin electrodes with both biasing and controlling signals. The asymmetry is introduced by incorporating a non-symmetric modulating electric field in order to control the polarization conversion. Both the phase matching and polarization conversion are achieved simultaneously.

Publication Type:	Thesis (Doctoral)
Subjects:	T Technology > TK Electrical engineering. Electronics Nuclear engineering
Departments:	School of Science & Technology > School of Science & Technology Doctoral Theses Doctoral Theses School of Science & Technology > Engineering

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