  Optical frequency combs have recently been demonstrated in micro--resonators
through nonlinear Kerr processes. Investigations in the past few years provided
better understanding of micro--combs and showed that spectral span and mode
locking are governed by cavity spectrum and dispersion. While various cavities
provide unique advantages, dispersion engineering has been reported only for
planar waveguides. In this Letter, we report a resonator design that combines
dispersion control, mode crossing free spectrum, and ultra--high quality
factor. We experimentally show that as the dispersion of a MgF2 resonator is
flattened, the comb span increases reaching 700 nm with as low as 60 mW pump
power at 1560 nm wavelength, corresponding to nearly 2000 lines separated by 46
GHz. The new resonator design may enable efficient low repetition rate coherent
octave spanning frequency combs without the need for external broadening, ideal
for applications in optical frequency synthesis, metrology, spectroscopy, and
communications.
