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IEEE Photonics Conference

Optical Two-Way Time-Frequency Transfer across a Three-Node Free-Space Network

Martha I. Bodine, Jennifer L. Ellis, William C. Swann, Sarah A. Stevenson, Jean-Daniel Deschênes, Emily D. Hannah, Paritosh Manurkar, Nathan R. Newbury, Laura C. Sinclair

Published Sep 1, 2020• 0 citations
Publisher

Abstract

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.

DOI
10.1109/ipc47351.2020.9252482

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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.
We demonstrate frequency-comb-based optical two-way time-frequency transfer across a three-node clock network. A fielded, bidirectional relay node connects laboratory-based master and end nodes, allowing the network to span 28 km of turbulent outdoor air while keeping optical transmit powers below 5 mW. Despite the comparatively high instability of the free-running local oscillator at the relay node, the network transfers frequency with fractional precision below 10−18 at averaging times above 200 s and transfers time with a time deviation below 1 fs at averaging times between 1 s and 1 h. The successful operation of this network represents a promising step toward the operation of future free-space networks of 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.
Abstract As the superiority of quantum two-way time transfer (Q-TWTT) has been proved convincingly over fiber links, its implementation on free-space links becomes an urgent need for remote time transfer expanding to the transcontinental distance. In this paper, the first Q-TWTT experimental demonstration over a hybrid link of 2 km-long turbulent free-space and 7 km-long field fiber is reported. Despite the significant loss of ∼30 dB and atmospheric turbulence, reliable time transfer performance lasting for overnights has been realized with time stability in terms of time deviation far below 1 picosecond. This achievement shows the good feasibility of quantum-enhanced time transfer in the space-ground integrated optical links and nicely certifies the capability of Q-TWTT in comparing and synchronizing the state-of-the-art space microwave atomic clocks.
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.
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.
Abstract Optical clock networks play important roles in various fields, such as precise navigation, redefinition of “second” unit, and gravitational tests. To establish a global-scale optical clock network, it is essential to disseminate time and frequency with a stability of 10^−19 over a long-distance free-space link. However, such attempts were limited to dozens of kilometers in mirror-folded configuration. Here, we take a crucial step toward future satellite-based time-frequency disseminations. By developing the key technologies, including high-power frequency combs, high-stability and high-efficiency optical transceiver systems, and efficient linear optical sampling, we demonstrate free-space time-frequency dissemination over two independent links with femtosecond time deviation, 3 × 10−19 at 10,000 s residual instability and 1.6 × 10^−20 ± 4.3 × 10^−19 offset. This level of the stability retains for an increased channel loss up to 89 dB. Our work can not only be directly used in ground-based application, but also firmly laid the groundwork for future satellite time-frequency dissemination.