We theoretically and experimentally investigated the transition betweenmodulation instability and Raman gain in a small silica microcavity with alarge free-spectral range (FSR), which reveals that we can selectively switchfrom a four-wave mixing dominant state to a stimulated Raman scatteringdominant state. Both the theoretical analysis and the experiment show that aRaman-dominant region is present between transitions of Kerr combs withdifferent free-spectral range spacings. We can obtain a stable Kerr comb and astable Raman state selectively by changing the driving power, coupling betweenthe cavity and the waveguide, and laser detuning. Such a controllabletransition is achieved thanks to the presence of gain competition betweenmodulation instability and Raman gain in silica whispering gallery modemicrocavities.
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