  Microresonator-based frequency combs (microcombs or Kerr-combs) can
potentially miniaturize the numerous applications of conventional frequency
combs. A priority is the realization of broad-band (ideally octave spanning)
spectra at detectable repetition rates for comb self referencing. However,
access to these rates involves pumping larger mode volumes and hence higher
threshold powers. Moreover, threshold power sets both the scale for power per
comb tooth and also the optical pump. Along these lines, it is shown that a
class of resonators having surface-loss-limited Q factors can operate over a
wide range of repetition rates with minimal variation in threshold power. A
new, surface-loss-limited resonator illustrates the idea. Comb generation on
mode spacings ranging from 2.6 GHz to 220 GHz with overall low threshold power
(as low as 1 mW) is demonstrated. A record number of comb lines for a microcomb
(around 1900) is also observed with pump power of 200 mW. The ability to
engineer a wide range of repetition rates with these devices is also used to
investigate a recently observed mechanism in microcombs associated with
dispersion of subcomb offset frequencies. We observe high-coherence,
phase-locking in cases where these offset frequencies are small enough so as to
be tuned into coincidence. In these cases, a record-low microcomb phase noise
is reported at a level comparable to an open-loop, high-performance microwave
oscillator.
