Hard and Soft Excitation Regimes of Kerr Frequency Combs
  We theoretically study the stability conditions and excitation regimes of hyper-parametric oscillation and Kerr frequency comb generation in continuously pumped nonlinear optical resonators possessing anomalous group velocity dispersion. We show that both hard and soft excitation regimes are possible in the resonators. Selection between the regimes is achieved via change in the parameters of the pumping light. 
Andrey B. Matsko, Anatoliy A. Savchenkov, Vladimir S. Ilchenko, David   Seidel, and Lute Maleki
physics.optics