Optical ring resonators consist of a waveguide in a closed loop coupled to one or more input/output (or bus) waveguides. When light of the appropriate wavelength is coupled to the loop by the input waveguide, it builds up in intensity over multiple round-trips due to constructive interference. It can then be picked up by a detector waveguide. Since only some wavelengths resonate in the loop, it functions as a filter.

Ring resonators are increasingly becoming a popular area of research, and at least one company, Little Optics, Inc., sells filters based on optical ring resonators. They are especially useful for exotic nonlinear optics experiments, and may eventually lead to photonic logic .

Numerous research groups have come up with exciting research in optical computing, filters, and photonic logic based on ring resonators, most prominently at University of Maryland, College Park, University of Southern California, and the Heinrich Hertz Institut .