Engineered Raman Gain for Mode-Locking Transition in Raman-Kerr Frequency Combs

We propose and experimentally demonstrate that by engineering Raman gain one can control the effective dissipation of cavity modes in a Raman-active silica microcavity, and selectively switch from a Raman dominant state to a mode-locked soliton state. Distinct regimes involving mode-locked Kerr comb and multimode Raman lasing can be triggered by changing the driving power or frequency detuning of control light, and we realize a broadband Kerr comb covering S-band, C-band, and L-band in a 1.3-mm-diameter fused silica microrod cavity. The mode locking and coherence of Raman-Kerr frequency combs is confirmed by a narrow RF beat signal of 10 Hz. Moreover, by selectively exciting different orders of axial modes, we attain a Raman assisted four wave mixing (RFWM) comb in the normal dispersion regime. Our work demonstrates an effective approach to engineer strong Raman scattering for microcavity soliton generation.