Optical ring resonators
type of optical resonator
An optical ring resonator, or micro-ring resonator (MRR) is a set of waveguides in which at least one is a closed loop coupled to light input and output. The input and output can be, but are not limited to being, waveguides. Geometries behind optical ring resonators are the same as those behind whispering galleries except that they use light and obey the properties behind constructive interference and total internal reflection. When light of the resonant wavelength is passed through the closed loop from the input waveguide, the light builds up in intensity over multiple round-trips owing to constructive interference and is coupled to the output waveguide. Because only a narrow wavelength range will be at resonance within the loop geometry, the optical ring resonator functions as a filter. Additionally, two or more ring waveguides can be coupled to each other to form an add/drop optical filter or to send light in a preferred direction.
Background
Optical ring resonators work on the principles behind total internal reflection, constructive interference, and optical coupling.
Total internal reflection
The light travelling through the waveguides in an optical ring resonator remains within the waveguides due to the phenomenon known as total internal reflection (TIR). TIR is an optical phenomenon that occurs when a ray of light strikes the boundary of a medium and fails to refract through the boundary. Given that the angle of incidence is larger than the critical angle (with respect to the normal of the surface) and the refractive index is lower on the other side of the boundary relative to the incident ray, TIR will occur and no light will be able to pass through the boundary.
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