Physics / mechanism
A ring resonator is a closed-loop optical waveguide evanescently coupled to a straight bus waveguide. Light at resonant wavelengths (where the round-trip path equals an integer multiple of the guided wavelength) builds up via constructive interference and is dropped from the bus. Key parameters: free spectral range (FSR = λ²/nₘL), Q-factor (10⁴–10⁶ in silicon photonics, >10⁷ in SiN), extinction ratio (>20 dB achievable), and coupling gap (100–300 nm, lithography-limited). State of the art: silicon ring modulators at 100+ Gb/s per channel; thermal tuning via integrated microheaters adds ~10 mW per ring. Primary integration platforms are Si (220 nm SOI), SiN, and LiNbO₃ thin-film.
Competitive landscape
Direct competitors for filtering/modulation: Mach-Zehnder interferometers (MZIs), arrayed waveguide gratings (AWGs), and Fabry-Pérot cavities. MZIs offer broader bandwidth and lower thermal sensitivity but larger footprint; AWGs suit dense WDM demux but are passive-only; FP cavities deliver high finesse but are harder to integrate. For sensing, whispering-gallery-mode resonators (microspheres, microdisks) compete on Q but lack planar fab compatibility.
Companies using
Connected ideas
Sources
Frontier (open questions)
- To be added.