Counting the cycles of light using a self-referenced optical microresonator

Phase coherently linking optical-to-radio frequencies with femtosecond mode-locked laser frequency combs has 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. By using a temporal dissipative single soliton state in an ultrahigh-Q crystalline microresonator that is broadened in highly nonlinear fiber, an optical frequency comb is generated that is self-referenced, allowing us to phase coherently link a 190 THz optical carrier directly to a 14 GHz microwave frequency. Our work demonstrates that precision optical frequency measurements can be realized with compact high-Q microresonators. (C) 2015 Optical Society of America