One of the basic building blocks in silicon integrated optics is the ring resonator.  Such resonators can be used to enhance nonlinear effects, as a modulator, as part of a detector, as a filter, or for a variety of other applications.  Building high-Q resonators in silicon-on-insulator used to be quite challenging, and the Q-factor of the resonators was a good measure of the performance of a waveguide fabrication process.