Optical Proximity Communication With Passively Aligned Silicon Photonic Chips

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Abstract
We report high-fidelity 10-Gb/s optical-proximity-communication using reflecting mirrors micro-machined into silicon and co-integrated with low-loss silicon-on-insulator waveguides for packaged chip-to-chip communication. Device integration was carried out by dry etching a rib waveguide 8 mum wide that was tapered to a width of 13 mu m and subsequently truncated with a wet-etched, micro-mirror facet forming a 54deg angle with the (100) surface. Light in waveguides on a bottom chip can couple to waveguides on a top chip upon face-to-face positioning so that the reflecting mirrors form a coupled pair and complete an optical proximity hop. High-speed link measurements were accomplished with chips aligned with a six-axis nano-positioner stage and compared with results for a new precision alignment approach that packages silicon chips for proximity signaling. Our new packaging approach is based on the co-integration of pyramidal etch pits micro-machined into silicon that match a precision micro-sphere for accurate chip alignment. Assemblies of chips can self-align in the package using chip placement that is initially coarse. The final chip alignment accuracy of our new packaging approach is limited by photolithographic resolution. Additionally, multichip arrays can be aligned together with global positioning having similar precision. Nonreturn-to-zero data was fiber launched into the waveguides and transported across a package consisting of a three-chip assembly with two optical proximity hops for inter-chip communication. Continuous wave optical losses, eye diagrams, bit error rates, and power penalties were measured. A passively aligned chip-to-chip optical proximity hop in the package was measured to have optical coupling loss of 4.0 dB, which was 1 dB more than when measured with precision-aligned chips with a nano-positioning stage. RMS jitter and amplitude metrics for the eye quality are shown to be nearly identical (to within 1%) when OPxC hops associated with the package is inserted into 10-Gb/s links. This self-aligned mechanism enables chip packages for several classes of proximity communication.

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