Optical-frequency synthesizers, which generate frequency-stable light from a single microwave-frequency reference, are revolutionizing ultrafast science and metrology, but their size, power requirement and cost need to be reduced if they are to be more widely used. Integrated-photonics microchips can be used in high-coherence applications, such as data transmission1, highly optimized physical sensors2 and harnessing quantum states3, to lower cost and increase efficiency and portability. Here we describe a method for synthesizing the absolute frequency of a lightwave signal, using integrated photonics to create a phase-coherent microwave-to-optical link. We use a heterogeneously integrated III–V/silicon tunable laser, which is guided by nonlinear frequency combs fabricated on separate silicon chips and pumped by off-chip lasers. The laser frequency output of our optical-frequency synthesizer can be programmed by a microwave clock across 4 terahertz near 1,550 nanometres (the telecommunications C-band) with 1 hertz resolution. Our measurements verify that the output of the synthesizer is exceptionally stable across this region (synthesis error of 7.7 × 10−15 or below). Any application of an optical-frequency source could benefit from the high-precision optical synthesis presented here. Leveraging high-volume semiconductor processing built around advanced materials could allow such low-cost, low-power and compact integrated-photonics devices to be widely used.
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We thank Srico, Inc. for use of the waveguide PPLN device, Aurrion Inc. for use of the III–V/Si tunable laser, and D. Hickstein, T. Dunker, A. Wallin, D. Carlson and Z. Newman for comments on the experiment. N.V. acknowledges support from the Swiss National Science Foundation (SNSF). This research is supported by the Defense Advanced Research Projects Agency DODOS program and NIST. We thank R. Lutwak and the DODOS program management team for discussions throughout the experiment.Reviewer Information
Nature thanks M. Lipson, D. Moss and the other anonymous reviewer(s) for their contribution to the peer review of this work.
Extended data figures and tables
a, Typical tuning map of the III–V/Si tunable laser’s peak wavelength in nanometres versus current applied to each heater above the ring resonators. b, Normalized optical spectra showing >40 dB of side-mode suppression ratio across the tuning range. c, Typical unlocked RF beat notes between the tunable laser and the auxiliary comb for two different biases of the phase section. Careful control of the heater is required to reach all wavelengths in the tuning range, and reduction of the laser linewidth (blue to red) through longitudinal mode alignment and the optical feedback effect42 is required to achieve the best phase-locking performance to the microcombs. RBW, resolution bandwidth, VBW, video bandwidth.
a, Output optical spectrum of the THz microcomb showing dual-dispersive waves, as measured on two optical spectrum analysers. b, Comparison of electro-optic repetition rate detection43 when using the same III–V/Si laser (black) and external cavity diode laser (ECDL, red) from the main experiment to pump the THz microcomb.