  Optical frequency combs are crucial for both fundamental science and
applications demanding wide frequency spanning and ultra-precision resolutions.
Recent advancements of nonlinear Kerr effect based optical frequency combs in
microcavities open up new opportunities in a compact platform, however,
internal cavity-enhanced nonlinearities are still unclear. Here we demonstrate
transient nonlinear dynamics during optical frequency comb formation inside a
Kerr microcavity. We show that gain spiking forms due to nonlinear phase
modulation, causing comb lines' self-detuning nearby a cavity resonance, this
introduces one key mechanism to stabilize optical frequency combs. Moreover,
nonlinear beating has be observed by injecting an external probe to exam
nonlinear cross-phase modulation between comb lines. Nonlinear transient
dynamics here reveal the hidden nonlinear features of Kerr based optical
frequency combs, leading to a new direction for ultrawide, ultrastable and
ultrafast frequency comb generation in microcavities.
