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arXiv:2402.08899 · Physical Review Applied

Free-space optical-frequency comparison over rapidly moving links

Shawn McSorley, Benjamin P. Dix-Matthews, Alex M. Frost, Ayden McCann, Skevos Karpathakis, David R. Gozzard, Shane M. Walsh, Sascha Schediwy

Published Feb 18, 2025• 10 citations
arXivHTMLPDF

Abstract

The comparison of optical clocks from ground to space will provide unprecedented tests of fundamental physics. Such comparison is limited, though, by the relative motion between the clocks. The experiment in this Letter demonstrates a method to compensate for Doppler shifts over a rapidly moving free-space optical drone link, using an electro-optic modulator. The authors demonstrate Doppler-shift compensation up to 18 MHz, providing a path toward ground-to-space optical comparison.

DOI
10.1103/physrevapplied.23.l021003

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The comparison of optical clocks from ground to space will provide unprecedented tests of fundamental physics. Such comparison is limited, though, by the relative motion between the clocks. The experiment in this Letter demonstrates a method to compensate for Doppler shifts over a rapidly moving free-space optical drone link, using an electro-optic modulator. The authors demonstrate Doppler-shift compensation up to 18 MHz, providing a path toward ground-to-space optical comparison.
Geopotential and orthometric height differences between distant points can be measured via timescale comparisons between atomic clocks. Modern optical atomic clocks have residual instabilities on the order of 10^-18}, allowing height differences of around 1 cm to be measured. Frequency transfer via free-space optical links will be needed for measurements where linking the clocks via optical fiber is not possible, but requires line of sight between the clock locations, which is not always practical due to local terrain or over long distances. We present an active optical terminal, phase stabilization system, and phase compensation processing method robust enough to enable optical frequency transfer via a flying drone, greatly increasing the flexibility of free-space optical clock comparisons. We demonstrate a residual instability of 2.5x10^-18 after 3 s of integration, corresponding to a height difference of 2.3 cm, suitable for applications in geodesy, geology, and fundamental physics experiments.
The continued remarkable advances in optical clocks and oscillators has led to a parallel strong development of optical clock networks [1]. Such networks have the potential to support a wide range of applications from basic time/frequency dissemination, to clock-based geodesy, to tests of general relativity [1]. To support optical clocks/oscillators at their highest accuracy and precision, these networks must rely on optical signals between the nodes to carry the timing signals. Networks based on the reciprocal transmission across fiber optics have allowed frequency comparisons between remote clocks across Europe and Japan [2, 3]. However, there are many potential applications, both terrestrial and space-based, for which the clocks will not be connected via an optical fiber and the signals must be sent across free space optical links. To this end, we have been developing techniques for the free-space transfer of time and frequency [4-6]. Moreover, rather than pursuing only frequency comparison, we have developed an approach that allows time comparison and even real-time synchronization between nodes. Such a capability can support the wide variety of proposed applications of mobile optical clock networks.
High-precision time–frequency is widely used in time measurement, satellite navigation, scientific research, and other fields. With the rapid development of optical clock technology, the fractional frequency instability and uncertainty of optical clock have reached 10−18 orders of magnitude, which is expected to contribute to generating the International Atomic Time and may even be used to redefine the “second” in the future. Therefore, the long-distance transfer of time–frequency signals between optical atomic clocks is of great significance. Free-space optical frequency transfer technology is one of the important technologies for solving the space-based optical clock comparison because of its high transfer precision and easy networking characteristics. In order to solve the long-distance space-based optical clock comparison, this paper investigates a free-space active phase noise compensation method using an Acousto-Optic Modulator (AOM), based on the traditional optical fiber phase noise compensation scheme. This new method is more flexible and scalable than the optical fiber time–frequency transfer technology. The optical frequency transfer over a 64 m free-space link is demonstrated. The fractional frequency transfer instability during free running is 9.50 × 10−16 at 1 s, and 4.44 × 10−16 at 2000 s, and the fractional frequency instability after compensation is 7.10 × 10−17 at 1 s, 3.07 × 10−19 at 2000 s, which is about 1–3 orders of magnitude better than that in free running, and provides a feasible scheme for space-based optical clock comparison.
High-precision optical frequency transfer through free-space links will enable advances in fields ranging from coherent optical communication and satellite Doppler ranging, to tests of general relativity and fundamental physics. Previous work has focused on two-way time and frequency transfer techniques using optical frequency combs. Here the authors show that a continuous-wave coherent link can exceed the fractional frequency stability of frequency-comb methods by an order of magnitude. They also highlight and discuss the technical hurdles to implementing similar links over the distances required to reach low Earth orbit.
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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.