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arXiv:1710.04063 · Physical Review Letters

Comparing Optical Oscillators across the Air to Milliradians in Phase and 10−17 in Frequency

Laura C. Sinclair, Hugo Bergeron, William C. Swann, Esther Baumann, Jean-Daniel Deschênes, Nathan R. Newbury

Published Jan 29, 2018• 69 citations
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Abstract

We demonstrate carrier-phase optical two-way time-frequency transfer (carrier-phase OTWTFT) through the two-way exchange of frequency comb pulses. Carrier-phase OTWTFT achieves frequency comparisons with a residual instability of 1.2×10^{-17} at 1 s across a turbulent 4-km free space link, surpassing previous OTWTFT by 10-20 times and enabling future high-precision optical clock networks. Furthermore, by exploiting the carrier phase, this approach is able to continuously track changes in the relative optical phase of distant optical oscillators to 9 mrad (7 as) at 1 s averaging, effectively extending optical phase coherence over a broad spatial network for applications such as correlated spectroscopy between distant atomic clocks.

DOI
10.1103/physrevlett.120.050801

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We demonstrate carrier-phase optical two-way time-frequency transfer (carrier-phase OTWTFT) through the two-way exchange of frequency comb pulses. Carrier-phase OTWTFT achieves frequency comparisons with a residual instability of 1.2×10^{-17} at 1 s across a turbulent 4-km free space link, surpassing previous OTWTFT by 10-20 times and enabling future high-precision optical clock networks. Furthermore, by exploiting the carrier phase, this approach is able to continuously track changes in the relative optical phase of distant optical oscillators to 9 mrad (7 as) at 1 s averaging, effectively extending optical phase coherence over a broad spatial network for applications such as correlated spectroscopy between distant atomic clocks.
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Abstract Timescale comparison between optical atomic clocks over ground-to-space and terrestrial free-space laser links will have enormous benefits for fundamental and applied sciences. However, atmospheric turbulence creates phase noise and beam wander that degrade the measurement precision. Here we report on phase-stabilized optical frequency transfer over a 265 m horizontal point-to-point free-space link between optical terminals with active tip-tilt mirrors to suppress beam wander, in a compact, human-portable set-up. A phase-stabilized 715 m underground optical fiber link between the two terminals is used to measure the performance of the free-space link. The active optical terminals enable continuous, cycle-slip free, coherent transmission over periods longer than an hour. In this work, we achieve residual instabilities of 2.7 × 10 −6 rad 2 Hz −1 at 1 Hz in phase, and 1.6 × 10 −19 at 40 s of integration in fractional frequency; this performance surpasses the best optical atomic clocks, ensuring clock-limited frequency comparison over turbulent free-space links.
We demonstrate optical two-way time-frequency transfer (O-TWTFT) across a three-node free-space network. The network spans 28 km of outdoor air and relays time and frequency from a master node, through an intermediate fielded node, to an end node. Despite strong atmospheric turbulence along the close-to-ground free-space links and large ambient temperature swings at the fielded node, the network transfers time with relative timing offsets below 5 fs. Additionally, the network transfers frequency with instability below 10-18at averaging times greater than 200 s-i.e. below the level needed to compare state-of-the-art optical atomic clocks.
A global network of optical atomic clocks will enable unprecedented measurement precision in fields including tests of fundamental physics, dark matter searches, geodesy, and navigation. Free-space laser links through the turbulent atmosphere are needed to fully exploit this global network, by enabling comparisons to airborne and spaceborne clocks. We demonstrate frequency transfer over a 2.4 km atmospheric link with turbulence comparable to that of a ground-to-space link, achieving a fractional frequency stability of 6.1×10^{-21} in 300 s of integration time. We also show that clock comparison between ground and low Earth orbit will be limited by the stability of the clocks themselves after only a few seconds of integration. This significantly advances the technologies needed to realize a global timescale network of optical atomic clocks.
SummaryHigh-precision time comparison and synchronization is required with the increasing application of the optical clocks in precise navigation, gravitational sensing, and relativity experiments. However, previous works at femtosecond-level time synchronization did not extend beyond dozens of kilometers. Here we demonstrate real-time synchronization of two optical clocks over a turbulent 113-km free space link via optical two-way time and frequency transfer. Despite optical power attenuation of 80dB due to atmospheric turbulence, the time deviation between two synchronized time scales is below 50 fs over a 15-hour measurement period, and a stability of less than 1.6 × 10−19at 10,000 s is achieved through an out-of-loop time-frequency transfer link. The ability to synchronize two distant optical clocks can support future precision navigation, and it also provides technical foundation for the future femtosecond clock synchronization over ground-to-satellite free-space paths.