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Quantum Electronics

Frequency transfer via an ultra-stable free-space link

K. S. Kudeyarov, D. S. Kryuchkov, Gulnara A. Vishnyakova, Nikita O. Zhadnov, Ksenia Yu. Khabarova, N. Kolachevsky

Published Mar 1, 2020• 5 citations
Publisher

Abstract

Abstract An optical frequency transfer at a wavelength of 1542 nm via an ultra-stable 5-m free-space link with active compensation of the phase noise caused by atmospheric fluctuations is demonstrated. The link-induced phase noise and its contribution to the frequency transfer instability are investigated. It is shown that, with the phase compensation system switched on, the link contribution to the relative transfer instability in terms of Allan deviation reaches 1.7 × 10−19 for a 5000-s averaging time.

DOI
10.1070/qel17258

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Abstract An optical frequency transfer at a wavelength of 1542 nm via an ultra-stable 5-m free-space link with active compensation of the phase noise caused by atmospheric fluctuations is demonstrated. The link-induced phase noise and its contribution to the frequency transfer instability are investigated. It is shown that, with the phase compensation system switched on, the link contribution to the relative transfer instability in terms of Allan deviation reaches 1.7 × 10−19 for a 5000-s averaging time.
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.
In this paper, we present a remote time-base-free technique for a coherent optical frequency transfer system via fiber.At the remote site, the thermal noise of the optical components is corrected along with the link phase noise caused by environmental effects.In this system, a 1 × 2 acousto-optic modulator (AOM) is applied at the remote site, with the first light being used to eliminate the noise of the remote time base and interface with remote users while the zeroth light is used to establish an active noise canceling loop.With this technique, a 10 MHz commercial oscillator, used as a time base at the remote site, does not contribute to the noise of the transferred signal.An experimental system is constructed using a 150 km fiber spool to validate the proposed technique.After compensation, the overlapping Allan deviation of the transfer link is 7.42 × 10 -15 at 1 s integration time and scales down to 1.07 × 10 -18 at 10,000 s integration time.The uncertainty of the transmitted optical frequency is on the order of a few 10 -19 .This significantly reduces the time-base requirements and costs for multi-user applications without compromising transfer accuracy.Meanwhile, these results show great potential for transferring ultra-stable optical frequency signals to remote sites, especially for point-to-multi-users.
SummaryPrecise and stable coherent optical frequency transfer over long-haul fiber link is required by diverse scientific applications, such as large-scale optical clock networks, global positioning and timing, very-long baseline interferometry, and high-sensitivity tests of fundamental physics. To meet the needs of the above applications, active compensation schemes for cancelling fiber-induced phase noise has developed considerably over the past ten years. However, optical signal severely reduces across distant fiber-optic networks, which degrades optical beat signal and feedback control.Here, we adopt active phase compensation and bidirectional optical frequency locking to achieve ultra-stable optical carrier transfer. The phase noise of the fiber link is effectively suppressed, meanwhile, the ultra-stable optical frequency signal is regenerated accurately at the remote site.Preliminary experiment over 100 km fiber link indicates that the additional frequency stability exhibits 1.9×10-16and 2.5×10-17at an averaging time of 1 s and 100 s, respectively. This approach has a capacity of extending to the parallel transfer of microwave and optical frequency, and free-space optical frequency transfer.