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Optical two-way time and frequency transfer over free space



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


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.
Nature Photonics


frequency combs, frequency transfer, time transfer


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], (Accessed May 22, 2024)


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Created April 28, 2013, Updated November 10, 2018