Coupled Spiral Interferometers

Coupled resonant optical waveguides (CROWs) exhibit unique abilities to support broadband slow light and steeply sloped resonances, which have important applications in many branches of photonics. These features, however, generally require coupling a large number N of resonators, which compromises compactness and thermal stability. Here, we introduce a class of optical interferometers that exhibit similar properties as CROWs, while occupying a much smaller area (~1/N) and being consequently more stable. Called a coupled spiral interferometer (CSPIN), it consists of a spiral waveguide with continuous coupling between the spiral's arms, which induces radial energy exchange between arms. We show through numerical simulations that with suitable design, this coupling mechanism efficiently traps light inside the spiral. With proper choice of N and the sequence of coupling coefficients between arms, a CSPIN can be engineered to behave like a ring resonator or various types of CROWs and to support strong slow light, fast light, broadband slow light, and/or light of negative group index.