Opt. Lett. 38, 1790–1792 (2013)

Despite the recent interest in generating frequency combs from high-Q Kerr microresonators pumped by a continuous-wave laser, little is known about the dependence of the comb's characteristics on the pump resonator. Now, by applying the Lugiato–Lefever equation to Kerr combs, Stéphane Coen and Miro Erkintalo from the University of Auckland in New Zealand have derived universal scaling laws that allow the comb bandwidth to be analytically estimated. They obtained a simple equation for estimating the achievable 3-dB comb bandwidth for given pump-resonator characteristics. The findings indicate that the bandwidth is determined solely by a resonator's finesse (or losses), nonlinearity and group-velocity dispersion coefficient. Interestingly and counter-intuitively, the comb bandwidth does not depend directly on the free spectral range (or the mode separation) of the resonator: a larger mode spacing does not result in a broader comb, but simply leads to the same overall bandwidth with fewer modes. They also showed that the route to a stable Kerr frequency comb passes through chaotic states, and that it is critical to control the cavity phase detuning parameter to reach a stable comb. The findings are likely to be very useful for future applications of chip-integrated frequency combs.