microring

A microring is a small, ring-shaped optical resonator that guides light in whispering-gallery modes. It consists of a thin, closed-loop waveguide, typically a few micrometers in radius, that is placed close to a straight bus waveguide to enable evanescent coupling. Light couples into and out of the ring at specific wavelengths where resonance occurs, while other wavelengths are largely suppressed, enabling on-chip filtering and routing.

The resonance condition for a microring is approximately mλ = n_eff(λ)·2πR, where m is an integer, λ is the wavelength, R is the ring radius, and n_eff is the effective refractive index of the guided mode. The circulating light forms high-Q resonances, and the device can function as an add-drop filter when a second bus waveguide is coupled to the ring, allowing selective dropping or adding of wavelength channels.

Materials and geometry vary, but common platforms include silicon on insulator (SOI), silicon nitride, and III–V semiconductors. The performance is described by the quality factor (Q) and the free spectral range (FSR), with Δλ ≈ λ^2/(n_g·2πR) where n_g is the group index. Loss mechanisms include scattering from sidewall roughness, absorption, and bending losses; smaller radius rings experience higher bend losses.

Applications span on-chip photonic signal processing and sensing. Microrings are used as compact wavelength filters, multiplexers/demultiplexers, and modulators or switches in integrated photonic circuits. They are also employed in sensing, where resonance shifts indicate changes in temperature, refractive index, or ambient media. In nonlinear optics, high circulating intensities enable Kerr-based phenomena such as frequency comb generation.