. If the three frequencies are part of a perfectly spaced frequency comb, then the fourth frequency is mathematically required to be part of the same comb as well.

Starting with intense light at two or more equally spaced frequencies, this process can generate light at more and more different equally spaced frequencies. For example, if there are a lot of photons at two frequencies , four-wave mixing could generate light at the new frequency . This new frequency would get gradually more intense, and light can subsequently cascade to more