Figure 1: Artist’s rendering of an ultrahigh-$Q$ fused silica microtoroid resonator. An optical wave is coupled from a tapered fiber waveguide into the periphery of the microtoroid in a whispering-gallery mode (named after its acoustic counterpart). Whispering galleries can be utilized in several geometries, most notably disks, rings, and spheres. Taking advantage of the tight mode confinement and high $Q$ of the WGM, efficient nonlinear interactions can generate a comb of frequencies from a single input laser.