  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.
