High-speed optical telecommunication is enabled by wavelength-division multiplexing, whereby hundreds of individually stabilized lasers encode information within a single-mode optical fibre. Higher bandwidths require higher total optical power, but the power sent into the fibre is limited by optical nonlinearities within the fibre, and energy consumption by the light sources starts to become a substantial cost factor1. Optical frequency combs have been suggested to remedy this problem by generating numerous discrete, equidistant laser lines within a monolithic device; however, at present their stability and coherence allow them to operate only within small parameter ranges2,3,4. Here we show that a broadband frequency comb realized through the electro-optic effect within a high-quality whispering-gallery-mode resonator can operate at low microwave and optical powers. Unlike the usual third-order Kerr nonlinear optical frequency combs, our combs rely on the second-order nonlinear effect, which is much more efficient. Our result uses a fixed microwave signal that is mixed with an optical-pump signal to generate a coherent frequency comb with a precisely determined carrier separation. The resonant enhancement enables us to work with microwave powers that are three orders of magnitude lower than those in commercially available devices. We emphasize the practical relevance of our results to high rates of data communication. To circumvent the limitations imposed by nonlinear effects in optical communication fibres, one has to solve two problems: to provide a compact and fully integrated, yet high-quality and coherent, frequency comb generator; and to calculate nonlinear signal propagation in real time5. We report a solution to the first problem.
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This work was supported by the Marsden Fund Council and Julius von Haast Fellowship from government funding, managed by Royal Society Te Apārangi of New Zealand, and the Max Planck Institute for the Science of Light, Erlangen, Germany.
Nature thanks Pauline Kuo and the other anonymous reviewer(s) for their contribution to the peer review of this work.
Extended data figures and tables
a, COMSOL simulation of the 3D copper microwave cavity. The colour bar indicates the microwave electrical field distribution inside the microcavity. In the side and top view, the localization close to the rim of the WGM is apparent. Dashed lines indicate the diameter of the WGM. The microwave field is coupled through a pin coupler to the WGM. b, Top and bottom halves of the copper cavity. The lithium niobate WGM resonator is mounted in the top half and the silicon prism in the bottom half.
This file contains Supplementary Sections A-E, including Supplementary Figures 1-2 and Supplementary References.