Optical two-way time and frequency transfer over free space

Published

April 28, 2013

Author(s)

Fabrizio R. Giorgetta, William C. Swann, Laura C. Sinclair, Esther Baumann, Ian R. Coddington, Nathan R. Newbury

Abstract

The transfer of high-quality time-frequency signals between remote locations underpins many applications, including precision navigation and timing, clock-based geodesy, long-baseline interferometry, coherent radar arrays, tests of general relativity and fundamental constants, and future redefinition of the second. However, present microwave-based time-frequency transfer is inadequate for state-of-the-art optical clocks and oscillators that have femtosecond-level timing jitter and accuracies below 1 × 10⁻¹⁷. Commensurate optically based transfer methods are therefore needed. Here we demonstrate optical time-frequency transfer over free space via two-way exchange between coherent frequency combs, each phase-locked to the local optical oscillator. We achieve 1 fs timing deviation, residual instability below 1 × 10⁻¹⁸ at 1,000 s and systematic offsets below 4 × 10⁻¹⁹, despite frequent signal fading due to atmospheric turbulence or obstructions across the 2 km link. This free-space transfer can enable terrestrial links to support clock-based geodesy. Combined with satellite-based optical communications, it provides a path towards global-scale geodesy, high-accuracy time-frequency distribution and satellite-based relativity experiments.

Citation

Nature Photonics

Volume

7

Issue

6

Pub Type

Journals

Download Paper

https://doi.org/10.1038/nphoton.2013.69

Keywords

frequency combs, frequency transfer, time transfer

Citation

Giorgetta, F. , Swann, W. , Sinclair, L. , Baumann, E. , Coddington, I. and Newbury, N. (2013), Optical two-way time and frequency transfer over free space, Nature Photonics, [online], https://doi.org/10.1038/nphoton.2013.69 (Accessed October 27, 2025)