Observation of Unique Coupling-Independent Resonances in Coupled Spiral Resonators

A coupled spiral interferometer (CSPIN) is a coupled resonator optical waveguide (CROW) that consists of a waveguide coiled upon itself in a spiral with adjacent arms close enough that light is continuously coupled between arms. This distributed coupling gives CSPINs a distinct set of properties, some similar to a ring resonator and to CROWs, others unique, depending on the number of arms and the intra-arm coupling distribution. In this paper, we show via numerical simulations that as a result of the waveguide's inherent modal index dispersion, for any large enough value of the intra-arm coupling the wavelength can always be adjusted such that light is trapped optimally in the spiral. Similarly, when used as a sensor for any intra-arm coupling there are wavelengths where the sensitivity (or slope of a resonance) is maximum. This set of properties gives the CSPIN a significant edge over other resonators, which require a precise coupling ratio to accomplish these same functions, which is difficult to achieve reproducibly in practice. To verify these and other CSPIN properties predicted in earlier publications, we report the fabrication and characterization of the first CSPINs. The devices, fabricated on a silicon-on-insulator substrate, exhibit the resonance spectra predicted by theory, as well as the aforementioned independence of sensitivity on coupling.