  We experimentally study the factors that influence the span in frequency
combs derived from the crystalline whispering gallery mode resonators. We
observe that cavity dispersion plays an important role in generation of combs
by cascaded four wave mixing process. We observed combs from the resonators
with anomalous dispersion and nearly zero dispersion at the pump wavelength. In
addition, the comb generation efficiency is found to be affected by the
crossing of modes of different families. The influence of Raman gain is
discussed as well as the role of cavity diameter and pump power. Copyright 2014
California Institute of Technology. Government sponsorship acknowledged.
