Photonic synthesis of radiofrequency (RF) waveforms revived the quest for unrivalled microwave purity because of its ability to convey the benefits of optics to the microwave world. In this work, we perform a high-fidelity transfer of frequency stability between an optical reference and a microwave signal via a low-noise fibre-based frequency comb and cutting-edge photodetection techniques. We demonstrate the generation of the purest microwave signal with a fractional frequency stability below 6.5 × 10⁻¹⁶ at 1 s and a timing noise floor below 41 zs Hz^−1/2 (phase noise below −173 dBc Hz⁻¹ for a 12 GHz carrier). This outperforms existing sources and promises a new era for state-of-the-art microwave generation. The characterization is achieved through a heterodyne cross-correlation scheme with the lowermost detection noise. This unprecedented level of purity can impact domains such as radar systems, telecommunications and time–frequency metrology. The measurement methods developed here can benefit the characterization of a broad range of signals.

Full article:

Nature Photonics Vol. 11, p. 44 (2017)

doi:10.1038/nphoton.2016.215