How Cherenkov radiative losses can improve optical frequency combs: Broader optical frequency combs on a photonic chip can help refine time standards

The idea of a frequency comb seems relatively simple, yet substantial technical efforts are required for one to be generated with the high accuracy and stability needed for metrology applications. The ideal frequency comb would be a set of discrete equidistant frequency (f) components separated by intervals Δf. Typically Δf is in the microwave range so that the separation of the comb teeth can be measured and controlled electronically. However, stabilization of Δf is not sufficient for applications; having a nearly octave-wide comb is necessary for self-referencing the comb (1, 2). On page 357 of this issue, Brasch et al. (3) show how to create a wideband comb spectrum to help realize this goal.