Silicon microring resonators with 1.5-microm radius.

We demonstrate a junction between a silicon strip waveguide and an ultra-compact silicon microring resonator that minimizes spurious light scattering and increases the critical dimensions of the geometry. We show cascaded silicon microring resonators with radii around 1.5 microm and effective mode volumes around 1.0 microm(3) that are critically coupled to a waveguide with coupled Q's up to 9,000. The radius of 1.5 microm is smaller than the operational wavelength, and is close to the theoretical size limit of the silicon microring ring resonator for the same Q. The device is fabricated with a widely-available SEM-based lithography system using a stitch-free design based on a U-shaped waveguide.