Ultrafast mode-locked optical frequency Kerr comb oscillators

Recently, continuous-wave pumped monolithic microresonators have been examined as an alternative platform for frequency comb and ultrashort pulse generation. With a delicate balance of anomalous group velocity dispersion (GVD) and self-phase modulation, optical solitons can be formed in these microresonators and remarkable broad optical bandwidths and RF-optical stability have been demonstrated. Here we report the first observations of coherent broadband frequency combs and ultrashort pulse generation from on-chip normal group-velocity dispersion microresonators. The observation is supported by phase noise characterization, direct frequency-resolved optical gating pulse measurement, and first-principles nonlinear coupled-mode modeling. All comb spectral lines are collected in our frequency-resolved optical gating pulse measurement and the phase-retrieved pulses demonstrate mode-locking down to 74 fs, one of the shortest frequency comb pulses on-chip to date. Nonlinear coupled-mode modeling of the comb growth and dynamics, with the measured normal group velocity dispersion and quality factor, confirms the feasibility of frequency comb and ultrashort pulse generation. We show, through measurements and theory, the physical mechanisms and basis for ultrafast mode-locking in Kerr comb oscillators.