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Synchronization of clocks through 12km of strongly turbulent air over a city



Laura C. Sinclair, William C. Swann, Hugo Bergeron, Esther Baumann, Michael A. Cermak, Ian R. Coddington, Jean-Daniel Deschenes, Fabrizio R. Giorgetta, Juan Juarez, Isaac H. Khader, Keith G. Petrillo, Katherine T. Souza, Michael L. Dennis, Nathan R. Newbury


We demonstrate real-time, femtosecond-level clock synchronization across a low-lying, strongly turbulent, 12-km horizontal air path by optical two-way time transfer. For this long horizontal free-space path, the integrated turbulence extends well into the strong turbulence regime corresponding to multiple scattering with a Rytov variance up to 7 and with the number of signal interruptions exceeding 100 per second. Nevertheless, optical two-way time transfer is used to synchronize a remote clock to a master clock with femtosecond-level agreement and with a relative time deviation dropping as low as a few hundred attoseconds. Synchronization is shown for a remote clock based on either an optical or microwave oscillator and using either tip-tilt or adaptive-optics free-space optical terminals. The performance is unaltered from optical two-way time transfer in weak turbulence across short links. These results confirm that the two-way reciprocity of the free-space time-of-flight is maintained both under strong turbulence and with the use of adaptive optics. The demonstrated robustness of optical two-way time transfer against strong turbulence and its compatibility with adaptive optics is encouraging for future femtosecond clock synchronization over very long distance ground-to-air free-space paths.
Applied Physics Letters


frequency comb, two-way optical time frequency transfer, synchronization


Sinclair, L. , Swann, W. , Bergeron, H. , Baumann, E. , Cermak, M. , Coddington, I. , Deschenes, J. , Giorgetta, F. , Juarez, J. , Khader, I. , Petrillo, K. , Souza, K. , Dennis, M. and Newbury, N. (2016), Synchronization of clocks through 12km of strongly turbulent air over a city, Applied Physics Letters, [online], (Accessed May 26, 2024)


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Created October 11, 2016, Updated November 10, 2018