High Coupling Efficiency Silicon Waveguide to Metal–Insulator–Metal Waveguide Mode Converter

Conventional silicon optical waveguides can have efficient coupling to optical fibers using grating couplers, but there is no structure of comparable coupling efficiency, optical bandwidth, and small size for direct coupling from optical fibers to metal slot optical waveguides. In this paper, we propose two structures for efficient coupling from conventional silicon strip waveguide to metal-insulator-metal (MIM) waveguide. The previously reported approach of mode conversion using taper-funnel structures to gradually squeeze the strip mode into metal slot has a big difference in effective indices and a large mismatch between the modes, thus limiting their coupling efficiency. Here, we demonstrate that adding another silicon strip-slot mode converter before the silicon taper structure can improve the coupling efficiency. Furthermore, the reversed taper-funnel structure has lower metallic absorption loss, thus further increasing the coupling efficiency. We experimentally measured 3 dB/coupler loss for coupling to a 200-nm-wide MIM slot, and 4 dB/coupler in the case of coupling to an 80-nm-wide MIM slot at 1640-nm wavelength.