Optical frequency combs have recently been demonstrated in micro--resonatorsthrough nonlinear Kerr processes. Investigations in the past few years providedbetter understanding of micro--combs and showed that spectral span and modelocking are governed by cavity spectrum and dispersion. While various cavitiesprovide unique advantages, dispersion engineering has been reported only forplanar waveguides. In this Letter, we report a resonator design that combinesdispersion control, mode crossing free spectrum, and ultra--high qualityfactor. We experimentally show that as the dispersion of a MgF2 resonator isflattened, the comb span increases reaching 700 nm with as low as 60 mW pumppower at 1560 nm wavelength, corresponding to nearly 2000 lines separated by 46GHz. The new resonator design may enable efficient low repetition rate coherentoctave spanning frequency combs without the need for external broadening, idealfor applications in optical frequency synthesis, metrology, spectroscopy, andcommunications.
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