Nature 557, 81–85 (2018)

Optical-frequency synthesizers using table-top mode-locked-laser frequency combs are useful for ultrafast photonics and metrology. However, their wide use is constrained by their bulky size, and high power requirement and cost. Now, exploiting dissipative Kerr-soliton combs and silicon photonics, Daryl Spencer and colleagues have realized a compact, low-power, on-chip synthesizer. Their synthesizer consists of a heterogeneously integrated III–V/silicon ring-resonator tunable laser and a fully stabilized dual comb composed of an octave-bandwidth comb with 1 THz mode spacing and a C-band-spanning comb with 22 GHz mode spacing. The former comb is generated in a silicon nitride planar waveguide-coupled resonator while the latter in a silicon oxide wedge-based whispering-gallery-mode resonator with a quality factor of 180 million, which facilitates low-power operation. By phase-stabilizing the comb spacing and offset frequency, the team managed to achieve highly precise phase-coherent multiplication from an electronic clock at 10 MHz to the optical domain with a multiplication factor of 19,403,904. Frequency synthesis across a 4 THz band near 1,550 nm with 1 Hz resolution and an uncertainty of better than 7.7 × 10–15 was obtained. The researchers are optimistic that their high-precision, high-accuracy, compact synthesizer will be of great use to any applications that need an optical-frequency source.