We present vertically-integrated multimode interferometers that optically couple between waveguide layers of
a three-dimensional photonic circuit. Coupling between these layers can be restricted to certain regions by
selectively fabricating a silicon channel between them, resulting in an isolated multimode waveguide section.
Simulations reveal that complete coupling between two waveguides that are 2 μm square is achieved over a
length of 236 μm, when the separating silicon channel is 1 μm thick. Standard photolithography and etching
techniques are used to fabricate a proof-of-concept device consisting of one waveguide coupling into a silicon
waveguide that is vertically multimode.
We present preliminary experimental results for silicon-on-insulator polarization rotators with asymmetric external waveguiding layers. These devices consist of a waveguide with vertical and sloped sidewalls and are fabricated using a combination of plasma and chemical etching techniques. For a device length of 3256 µm, a TE-to-TM polarization conversion efficiency of 75% was measured.
We report simulation results for a directional coupler between silicon waveguides in different layers of a three-dimensional (3D) optical circuit. The coupling length is 1.4 mm. The device is manufacturable using standard CMOS technology provided individual waveguide layers can be vertically stacked. In simulations of coupling efficiency the design exhibits negligible loss with respect to translational and rotational misalignments of up to 0.5 μm. Efficiency degradation is less than 5% for etch depth and waveguide width variations of 0.4 μm, and less than 1 dB over the range of standard lithographic tolerances for variations from layer to layer in feature width, depth, and alignment.