  We theoretically and experimentally investigated the transition between
modulation instability and Raman gain in a small silica microcavity with a
large free-spectral range (FSR), which reveals that we can selectively switch
from a four-wave mixing dominant state to a stimulated Raman scattering
dominant state. Both the theoretical analysis and the experiment show that a
Raman-dominant region is present between transitions of Kerr combs with
different free-spectral range spacings. We can obtain a stable Kerr comb and a
stable Raman state selectively by changing the driving power, coupling between
the cavity and the waveguide, and laser detuning. Such a controllable
transition is achieved thanks to the presence of gain competition between
modulation instability and Raman gain in silica whispering gallery mode
microcavities.
