Birefringence control using stress engineering in silicon-on-insulator (SOI) waveguides

We demonstrate that stress engineering is an effective tool to modify or eliminate polarization dispersion in silicon-on-insulator (SOI) waveguide devices, for a wide range of waveguide cross-section shapes and dimensions. The stress-induced effects on the modal birefringence of SOI waveguides are investigated numerically and experimentally. Finite-element simulations show that while the birefringence of ridge waveguides with both slanted and vertical sidewalls can be effectively modified using cladding stress, the birefringence becomes much less sensitive to dimension fluctuations with decreasing sidewall slope. To efficiently simulate the stress-induced effects we propose a normalized plane-strain model which can achieve comparable accuracy as a fully generalized plane-strain model but requires significantly less computational resources. Excellent agreement is achieved between the calculated and measured birefringence tuning using SiO₂ cladding induced stress. Finally, both calculations and experiments confirm that cladding induced stress can be used to eliminate the birefringence in SOI waveguides of arbitrary shapes, for typical SiO₂ film stress values (σ_film≈-100 to -300 MPa) and cladding thicknesses of the order of 1 μm or less.