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Joint Conference of the European Frequency and Time Forum and IEEE International Frequency Control Symposium (EFTF/IFCS)

Ultra-Stable Atmospheric Short Link for the Optical Frequency Signal Transfer

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

Published Jul 7, 2021• 2 citations
Publisher

Abstract

The 17 m open-air link for coherent transfer of ultrastable optical frequency signal have been demonstrated. The fractional frequency instability caused by air turbulence is suppressed by more than 3 orders of magnitude with the help of active noise compensation system and reaches 1.7∙10-19after averaging time of 1000 s. The link contribution to uncertainty is decreased by more than 400 times and is equal to 5∙10-20.

DOI
10.1109/eftf/ifcs52194.2021.9604310

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The 17 m open-air link for coherent transfer of ultrastable optical frequency signal have been demonstrated. The fractional frequency instability caused by air turbulence is suppressed by more than 3 orders of magnitude with the help of active noise compensation system and reaches 1.7∙10-19after averaging time of 1000 s. The link contribution to uncertainty is decreased by more than 400 times and is equal to 5∙10-20.
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.
We demonstrate the long-distance transmission of an ultrastable optical frequency derived directly from a state-of-the-art optical frequency standard. Using an active stabilization system we deliver the frequency via a 146-km-long underground fiber link with a fractional instability of 3 x 10(-15) at 1 s, which is close to the theoretical limit for our transfer experiment. After 30,000 s, the relative uncertainty for the transfer is at the level of 1 x 10(-19). Tests with a very short fiber show that noise in our stabilization system contributes fluctuations that are 2 orders of magnitude lower, namely, 3 x 10(-17) at 1 s, reaching 10(-20) after 4,000 s.
We report an optical link of 540 km for ultrastable frequency distribution over the Internet fiber network. The phase-noise compensated link shows a fractional frequency instability in full bandwidth of 3 × 10 −14 at one second measurement time and 2 × 10 −18 at 30 000 s. This work is a significant step towards a sustainable wide area ultrastable optical frequency distribution and comparison network. Time transfer was demonstrated simultaneously on the same link and led to an absolute time accuracy (250 ps) and long-term timing stability (20 ps) which outperform the conventional satellite transfer methods by one order of magnitude. Current development addresses the question of multiple users distribution in the same metropolitan area. We demonstrate on-line extraction and first results show frequency stability at the same level as with conventional link. We also report an application to coherent frequency transfer to the mid-infrared. We demonstrate the frequency stabilisation of a mid-infrared laser to the near-infrared frequency reference transferred through the optical link. Fractional stability better than 4 × 10 −14 at 1 s averaging time was obtained, opening the way to ultrahigh resolution spectroscopy of molecular rovibrational transitions.
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.
We demonstrate the coherent transfer of an ultrastable optical frequency reference over a 490 km noisy field fiber link. The fiber-induced phase noise power spectrum density per-unit-length at 1 Hz offset frequency can reach up to 510 rad2⋅Hz−1⋅km−1, which is much higher than the fiber noise observed in previous reports. This extreme level of phase noise is mainly due to the fiber link laying underground along the highway. Appropriate phase-locked loop parameters are chosen to complete the active compensation of fiber noise by measuring the intensity fluctuation of additional phase noise and designing a homemade digital frequency division phase discriminator with a large phase detection range of 212 π rad. Finally, a noise suppression intensity of approximately 40 dB at 1 Hz is obtained, with fractional frequency instability of 1.1 × 10−14 at 1 s averaging time, and 3.7 × 10−19 at 10000 s. The transfer system will be used for remote atomic clock comparisons and optical frequency distribution over a long-distance communication network established in China.