Phase-coherent microwave-to-optical link with a self-referenced microcomb
Precise measurements of the frequencies of light waves have become common with mode-locked laser frequency combs1. Despite their huge success, optical frequency combs currently remain bulky and expensive laboratory devices. Integrated photonic microresonators are promising candidates for comb generators in out-of-the-lab applications, with the potential for reductions in cost, power consumption and size. Such advances will significantly impact fields ranging from spectroscopy and trace gas sensing to astronomy, communications and atomic time-keeping. Yet, in spite of the remarkable progress shown over recent years, microresonator frequency combs ('microcombs') have been without the key function of direct f–2f self-referencing, which enables precise determination of the absolute frequency of each comb line. Here, we realize this missing element using a 16.4 GHz microcomb that is coherently broadened to an octave-spanning spectrum and subsequently fully phase-stabilized to an atomic clock. We show phase-coherent control of the comb and demonstrate its low-noise operation.
Additional Information© 2016 Macmillan Publishers Limited. Received 12 January 2016; accepted 27 April 2016; published online 6 June 2016. This work is supported by the National Institute of Standards and Technology, the National Physical Laboratory, the California Institute of Technology, the Defense Advanced Research Projects Agency Quantum—Assisted Sensing and Readout programme, the Air Force Office of Scientific Research and the National Aeronautics and Space Administration. P.D. acknowledges support from the Humboldt Foundation. D.C.C. acknowledges support from the National Science Foundation Graduate Research Fellowship Program under grant no. DGE 1144083. Author Contributions: P.D., S.B.P. and S.A.D. conceived the experiments. P.D. and A.C. designed and performed the experiments. T.F. contributed to the fceo stabilization. K.B. and D.C.C. contributed to the nonlinear spectral broadening. K.Y.Y., H.L. and K.J.V. provided the microresonator. P.D. and S.A.D. prepared the manuscript, with input from all co-authors. The authors declare no competing financial interests.
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