Mid-Infrared Soliton and Raman Frequency Comb Generation in Silicon   Microrings
  We numerically study the mechanisms of frequency comb generation in the mid-infrared spectral region from cw pumped silicon microring resonators. Coherent soliton comb generation may be obtained even for a pump with zero linear cavity detuning, through suitable control of the effective lifetime of free-carriers from multiphoton absorption, which introduces a nonlinear cavity detuning via free-carrier dispersion. Conditions for optimal octave spanning Raman comb generation are also described. 
Tobias Hansson, Daniele Modotto, and Stefan Wabnitz
physics.optics nlin.PS