Cavity-resonator-integrated grating input/output couplers for WDM optical-interconnect system in package

A key component for high-density and ultra-broad-band intra-board chip-to-chip signal transmission is an integrated-optic coupler of several-micron aperture coincident with the emission diameter of typical single-transverse-mode VCSEL. A new type of coupler named cavity-resonator integrated grating input/output coupler (CRIGIC), recently proposed by some of the authors, has remarkable features of small aperture and vertical coupling with high coupling efficiency. CRIGIC consists of one grating coupler (GC) between two distributed Bragg reflectors (DBRs) with phase-shifting spaces. In an output coupling of a guided wave to a radiation wave, an incident forward guided wave is partially reflected by the front DBR, partially coupled out by GC to radiation waves, and reflected by the rear DBR to backward guided wave. A phase-shifting space between GC and the rear DBR is determined so that the backward guided wave from the rear DBR is radiated by GC in phase with the radiation of the forward guided wave. The coupling efficiency of the front DBR and another phase-shifting space between GC and the front DBR are chosen so that the transmission of the backward guided wave from GC is canceled by the reflection of the forward guided wave by the front DBR. Thus CRIGIC generates neither transmission nor reflection, providing 100% radiation even if GC has too small aperture to give high efficiency in a usual one-way propagation scheme. In order to demonstrate the operation principle of CRIGIC, a simple structure of 20 μm aperture length was designed, fabricated and characterized. The out-coupled power from CRIGIC was estimated to be 80% of that from conventional GC of 500 μm coupling length, indicating almost 20 times miniaturization in aperture was achieved. Integration of CRIGIC and a different-guided-mode-coupling DBR was discussed to construct a free-space-wave add/drop multi-/demultiplexer (ADM) in a thin film waveguide for an application to WDM optical-interconnect system in package. Eight-wavelength multiplexing within 20 nm wavelength range was considered. ADMs were designed and their performance was simulated theoretically.