Stress engineering for polarization control in silicon-on-insulator waveguides and its applications in novel passive polarization splitters/filters

Over the last decade, devices in silicon-on-insulator (SOI) have gained tremendous attention due to their potential for making highly compact monolithic integrated opto-electronic circuits. As the waveguide cross-section reduces to less than the wavelength of light, polarization control becomes increasingly challenging in the device design and operation and is one of the main obstacles to the development of viable microphotonic commercial products. This thesis explores and develops a method of polarization control in SOI: stress engineering. A systematic and comprehensive study of the geometrical and stress-induced effects on waveguide modal birefringence is presented. A new calculation scheme, the Normalized Plane-Strain Model, is proposed and implemented for stress analysis with excellent accuracy and efficiency. Both calculations and experiments confirm that the cladding induced stress can effectively modify or eliminate polarization dispersion in SOI waveguides of arbitrary shapes, for typical SiO2 cladding film stress (sigmafilm = -100 to -300 MPa) and cladding thicknesses on the order of 1 mum or less.; An important application of stress engineering is in passive polarization splitters/filters. The modeling, fabrication and characterization of several novel polarization splitters and filters in the SOI platform by employing stress engineering are presented. The designs are mainly focused on two configurations: the Mach-Zehnder interferometer (MZI) and the zero-order arrayed waveguide grating (AWG). The main advantage of these devices is the simplicity in polarization control - only one single postfabrication step is required. In addition, stress engineering provides the freedom to decouple the birefringence constraints from the waveguide cross-section design such that the waveguides can be independently optimized for its insertion loss, coupler performance and bend loss. Furthermore, the wavelength independent characteristics of stress engineering ensure the broadband performance of the proposed devices. Functioning stress-induced polarization splitters are demonstrated experimentally in both the MZI- and zero-order AWG-based devices, exhibiting broadband polarization splitting as high as -14 dB for both output polarizations from 1460 to 1570 nm. The device sizes are 2.5 mm x 16 mum and 12 mm x 4 mm for the MZI- and the zero-order AWG-based devices, respectively. This work represents the first reported use of stress engineering for making SOI polarization splitters/filters.