  Phase coherently linking optical to radio frequencies with femtosecond
mode-locked laser frequency combs enabled counting the cycles of light and is
the basis of optical clocks, absolute frequency synthesis, tests of fundamental
physics, and improved spectroscopy. Using an optical microresonator frequency
comb to establish a coherent link between optical and microwave frequencies
will extend optical frequency synthesis and measurements to areas requiring
compact form factor, on chip integration and comb line spacing in the microwave
regime, including coherent telecommunications, astrophysical spectrometer
calibration or microwave photonics. Here we demonstrate a microwave to optical
link with a microresonator. Using a temporal dissipative single soliton state
in an ultra-high Q crystalline microresonator that is broadened in highly
nonlinear fiber an optical frequency comb is generated that is self-referenced,
allowing to phase coherently link a 190 THz optical carrier directly to a 14
GHz microwave frequency. Our work demonstrates precision optical frequency
measurements can be realized with compact high Q microresonators.
