Mode interaction aided excitation of dark solitons in microresonators   constructed of normal dispersion waveguides
  Kerr frequency combs from microresonators are now extensively investigated as a potentially portable technology for a variety of applications. Most studies employ anomalous dispersion microresonators that support modulational instability for comb initiation, and mode-locking transitions resulting in coherent bright soliton-like pulse generation have been reported. However, some experiments show comb generation in normal dispersion microresonators; simulations suggest the formation of dark pulse temporal profiles. Excitation of dark pulse solutions is difficult due to the lack of modulational instability in the effective blue-detuned pumping region; an excitation pathway has been demonstrated neither in experiment nor in simulation. Here we report experiments in which dark pulse combs are formed by mode-interaction-aided excitation; for the first time, a mode-locking transition is observed in the normal dispersion regime. The excitation pathway proposed is also supported by simulations. 
Xiaoxiao Xue, Yi Xuan, Yang Liu, Pei-Hsun Wang, Steven Chen, Jian   Wang, Dan E. Leaird, Minghao Qi, and Andrew M. Weiner
physics.optics