Planar waveguide circulating Gaussian beam resonators on a silicon photonic chip

A silicon (Si) slab waveguide resonator design with a circulating Gaussian-like cavity mode is described and characterized, for both all-pass and add-drop configurations and several different input/output coupling strengths. The circulating beam propagates in a slab waveguide with no lateral confinement. Three straight mirrors and one curved mirror define a folded two-dimensional Gaussian cavity. Light is coupled to and from the resonator by beam splitters formed by a narrow gap between a cavity mirror and the input slab waveguides. The coupling is determined by the gap width and is wavelength independent and lossless. For a L = 100 μm path length cavity, resonance line widths of λ_3 dB = 5 pm with Q-values of Q = 310000 were measured. The resonator drop spectrum exhibited a comb of almost identical resonance lines across a 100 nm tuning range. This resonator design is capable of broadband operation and is less susceptible to sidewall roughness and defect scattering-induced loss and mode splitting, when compared with Si rings formed from narrow single-mode waveguides.