Selective guided mode coupling via bridging mode by integrated gratings for intraboard optical interconnects

Abstract

Growing attention is paid to intraboard optical interconnects among multichip modules because of its potential to solve so-called pin bottleneck problem in constructing ultrahigh speed information processing unit. Vertical integration of waveguides and their coupling as well as input/output coupling of guided waves from/to free- space waves are key issues. Three types of high performance coupling via bridging modes by integrated gratings are reviewed. Use of radiation mode as bridging mode is discussed with advantage of selective coupling between two waveguides among multistory structure. Three waveguides were integrated with grating couplers, and wavelength-division demultiplexing with 5 nm separation and 0.5 nm selectivity from one waveguide to two other waveguides was demonstrated. Utilization of supermode as bridging mode is shown for optical add/drop multiplexing (OADM) function between two waveguides. Device was designed to have high power transfer efficiency of almost 100% with high wavelength selectivity of 2 nm, while preliminary experimental results were 40% efficiency with 1 nm selectivity. OADM between guided wave and free-space wave is also discussed with utilization of supermode. Power transfer efficiency of 25% with wavelength selectivity of 4 nm was obtained in coupling of guided wave to free-space wave, while theoretically predicted efficiency was 70% with 2 nm selectivity.