An alternative variation of four-wave-mixing-based frequency combs is known as Kerr frequency comb. Here, a single laser is coupled into a microresonator (such as a microscopic glass disk that has whispering-gallery modes). This kind of structure naturally has a series of resonant modes with approximately equally spaced frequencies (similar to a Fabry–Pérot interferometer). Unfortunately the resonant modes are not exactly equally spaced due to dispersion. Nevertheless, the four-wave mixing effect above can create and stabilize a perfect frequency comb in such a structure. Basically, the system generates a perfect comb that overlaps the resonant modes as much as possible. In fact, nonlinear effects can shift the resonant modes to improve the overlap with the perfect comb even more. (The resonant mode frequencies depend on refractive index, which is altered by the optical Kerr effect.)