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 f 1 , f 2 , four-wave mixing could generate light at the new frequency 2 f 1 − − f 2 . This new frequency would get gradually more intense, and light can subsequently cascade to more and more new frequencies on the same comb.