  An optical comb is shown to arise from a whispering gallery mode resonator
pumped by two optical frequencies. Two externally excited modes couple due to
Kerr nonlinearity to initially empty modes and give rise to new frequency
components. This thresholdles process is much more efficient than the
previously reported single-pump four-wave mixing. As a result, a few milliwatt
pump is sufficient to generate strong secondary fields, that efficiently
generate higher-order frequency components and so on, in a cascade process
leading to an optical comb.
