Prospects and challenges for microphotonic waveguide components based on Si and SiGe

Optical properties of silicon-on-insulator (SOI) and SiGe waveguides are reviewed. Design and fabrication challenges are highlighted in view of the demand for downscaling of waveguide circuits. Scaling laws governing the mode number, optical power isolation, bend loss, effective index and the birefringence are discussed, for waveguide dimensions ranging from several microns down to sub-micron scale. In particular, variations in the effective index and birefringence with waveguide width and depth for ridge waveguides with rectangular and trapezoidal cross-sections are investigated. It is shown that trapezoidal waveguides with sidewall angles of ∼54° are 10 times less sensitive to dimension fluctuations, but suffer from a large residual birefringence. Local stress engineering is demonstrated as an effective method for circumventing this problem and eliminating birefringence in SOI waveguides, as well as opening other design possibilities for Si and SiGe components. Using cladding stress, the birefringence in SOI arrayed waveguide grating (AWG) devices was reduced from 1 × 10^-3 to 5 × 10^-5. Design and experiments on SiGe waveguide photodetectors for high-speed and high responsivity operations are also discussed.