Filter Ring

In its simplest form, an optical ring resonator consists of a straight waveguide and a ring waveguide. The waveguides are placed close to each other, making the light affect each between the two structures. If the propagation length around the ring is an integral number of wavelengths, the field becomes resonant and a strong field builds up in the ring.After propagation around the ring waveguide, some light couples back to the straight waveguide and interferes with the incident light. At resonance, completely destructive interference can be obtained, with no transmitted light. This makes the optical ring resonator an ideal notch filter, blocking the light at the resonant wavelength.
Optical ring resonators are promising building blocks for photonic integrated circuits. Due to the high refractive index contrast, available in, for instance, silicon photonics, very small circuits can be made.