Design of silicon-wire waveguide ultracompact racetrack resonators: geometrical parameters for optimal coupling.

In this paper, it is shown that a suitable choice of the geometrical parameters of a silicon-wire waveguide microracetrack resonator structure can lead to a substantial improvement in the control of coupling coefficients and, hence, the design of ultracompact devices for high-performance channel add-drop filters and all-optical switching applications. On the one hand, some simple theoretical arguments and simulation results indicate that the reduction of the silicon-wire rectangular waveguide cross-section area (width×height) is possible, from standard (450  nm×220  nm) to (380  nm×200  nm) on both the bus and the resonator waveguides; this action, apart from still guaranteeing a quasi-TE single-mode operation, would provide an effective improvement into scale-of-integration by a 1.30 factor per device volume. On the other hand, it will be shown by a semianalytical method (analytical calculation + numerical simulation) that achieving the waveguide-racetrack optimal coupling condition for a particular application can be reduced to a prime calculation of the main resonator geometrical parameters (bend radius, straight length, air gap and overall coupling length). In particular, the design of high-performance ultracompact waveguide-racetrack resonator structures, with pre-established Q factor (Q≥2000), free spectral range (FSR≥15  nm), full width at half-maximum (FWHM≤5  nm), finesse (F≥40) or extinction ratio signals (ER≥20  dB) can be systematically obtained with this procedure.