Science 361, 1358–1363 (2018)

Mode-locked cavities have long been exploited where laser pulses with high stability or precision are required, such as in optical atomic clocks. However, electro-optic modulation arguably offers higher repetition rates than mode-locking and could thus lead to increased precision. Now, David Carlson and co-workers at the National Institute of Standards and Technology and the University of Colorado in Boulder, Colorado, report electro-optic generation of 100-pJ pulses at rate of 30 GHz and also show sub-optical-cycle timing precision. The total output power is about 1.1 W contained in spectra that span from 750 nm to about 2,700 nm in wavelength. A 1,550-nm continuous-wave laser was phase- and intensity-modulated by a commercial 10-GHz dielectric resonant oscillator locked to a microwave cavity. The set-up involves an intensity modulator in series with multiple phase modulators in order to create a deterministic cascade of sidebands (comb lines). Broadband thermal noise, a characteristic of electro-optic modulation, is optically filtered by a Fabry–Pérot cavity, and spectral broadening then takes place in a nonlinear fibre. Analysis confirms that the pulses are up to 15 ps in duration (2.8 optical cycles).