SiGe-based dual-wavelength demultiplexers and polarization splitters

The simulation and performance of dual-wavelength demultiplexers fabricated in SiGe are presented. The device design is a symmetric directional coupler optimized for 1.3/1.55 mu m demultiplexing. Modeling results using the Beam Propagation Method are presented as a means of examining the fabrication tolerances and design considerations of the devices, initially the duplexers were fabricated with a Si.97Ge.03 core using chemical etching, and displayed crosstalk of -19 dB and -15 dB in the 1.3 and 1.55 mu m wavelength channels, respectively. This performance was enhanced by thermal tuning, resulting in isolation of -21 dB and -18 dB. The high degree of strain in pseudomorphic SiGe layers results in highly birefringent waveguides. This characteristic restricts effective duplexer operation to a single polarization, but suggests that the same devices can be modified to act as polarization splitters for a chosen wavelength. Modeling of the devices in this configuration is also presented. The simple device design used for demultiplexers and splitters was chosen to evaluate the potential for fabrication of SiGe-based optoelectronic devices using standard VLSI processing relying on the local oxidation of silicon (LOCOS). A second set of devices was successfully fabricated using LOGOS and effectively separated 1.15 and 1.3 mu m wavelength signals. This performance is compared to similar devices that were wet-etched.