Fabrication of diffractive optics for dense optical interconnects in optical computing

Abstract

We designed and fabricated an optical system containing high efficiency diffractive optical elements (DOEs) with large numerical apertures (NA) for an all-optical gate, based on a Symmetric Self-Electro-Optic Effect Device (S-SEED) technology. The S-SEEDs are the active elements that perform the optical switching in the optical interconnect. Multiple, off-axis DOEs are used to collect and focus light onto the S-SEEDs and the Input/Output optical fibers. Each S-SEED has at least seven input signals, two alignment signals, and two output signals. Each signal uses a DOE. DOE fabrication is relatively mature and utilizes the precise lateral alignment inherent in photolithography to produce arrays compatible with dense optical interconnects. Losses across the system have a negative impact on the S-SEED switching speeds. The primary challenge of DOEs is the diffractive optic efficiency that corresponds to high NAs. Lower efficiencies, due to requirements for large deflection angles, lead to extremely small feature sizes in the outer zones of the DOEs. We optimize DOE efficiency with modifications to the blaze geometry and by selecting the appropriate number of levels for specific deflection angles. The system layout is modified to reduce complexity by working in collimated space between the S-SEEDs instead of imaging onto relay mirrors. This reduces the spatial frequency of the DOEs and increases system tolerance by not imaging mirror defects. Finally, we quantify the effects of lithographic masks misalignment and look at the step geometry deviations and their effects on DOE efficiency.