Daryl T. Spencer, Tara Drake, Travis C. Briles, Jordan Stone, Laura C. Sinclair, Connor Fredrick, Qing Li, Daron Westly, B. Robert Ilic, Aaron Bluestone, Nicolas Volet, Tin Komljenovic, Lin Chang, Seung Hoon Lee, Dong Yoon Oh, Myoung-Gyun Suh, Ki Youl Yang, Martin H. P. Pfeiffer, Tobias J. Kippenberg, Erik Norberg, Luke Theogarajan, Kerry Vahala, Nathan R. Newbury, Kartik Srinivasan, John E. Bowers, Scott A. Diddams & Scott B. Papp1,2
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 microwaveto-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, lowpower and compact integrated-photonics devices to be widely used.