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Optica

Optical frequency transfer over submarine fiber links

Cecilia Clivati, Anna Tampellini, Alberto Mura, Filippo Levi, Giuseppe Marra, Pauline Galea, André Xuereb, Davide Calonico

Published Jul 23, 2018• 55 citations
Publisher

Abstract

Bidirectional phase-stabilized optical fiber links allow state-of-the-art optical clocks to be compared on a continental scale.However, intercontinental comparisons are still based on satellite techniques, preventing optical clocks from being compared without degradation.We performed the first optical frequency transfer experiment over submarine links, measuring levels of environmentally induced noise substantially lower than for on-land links.From these measurements, we predict the transfer stability that can be achieved over transoceanic links.We also performed the first optical frequency transfer over fibers installed in power cables, observing optical perturbations caused by the high-voltage field.Finally, we show that the low background noise of fibers on the seafloor allows applications in geophysical sensing.

DOI
10.1364/optica.5.000893

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Bidirectional phase-stabilized optical fiber links allow state-of-the-art optical clocks to be compared on a continental scale.However, intercontinental comparisons are still based on satellite techniques, preventing optical clocks from being compared without degradation.We performed the first optical frequency transfer experiment over submarine links, measuring levels of environmentally induced noise substantially lower than for on-land links.From these measurements, we predict the transfer stability that can be achieved over transoceanic links.We also performed the first optical frequency transfer over fibers installed in power cables, observing optical perturbations caused by the high-voltage field.Finally, we show that the low background noise of fibers on the seafloor allows applications in geophysical sensing.
Optical clock comparison via optical fibre links has been achieved over continental scales, but has not yet been demonstrated intercontinentally. The transfer of ultra-stable optical frequencies over transoceanic distances is a challenging task, as the seafloor cable architecture prevents the use of environmental noise suppression techniques currently employed on land-based metrological links. As a fundamental first step towards devising suitable frequency transfer techniques to enable future clock comparison on a global scale, here we show the free-running frequency stability and phase noise measurements of a transatlantic seafloor optical link between the UK and Canada. To the best of our knowledge, these are the first ever measurements of an intercontinental optical fibre link.
The stability of the transmitted optical frequency in a single bidirectionally operated fiber was compared with stability in a link consisting of two separate unidirectional fibers. The measurements were performed in a standard closed-loop system canceling the fiber induced phase noise. It was found that phase fluctuations in the higher frequency band (acoustic noise) are well suppressed in both configurations (single bidirectional fiber and separate unidirectional fibers). The long-term drifts however are not fully compensated in a link consisting of two separate unidirectional fibers, but still about two orders of magnitude noise reduction was observed in comparison with not compensated transfer.
Optical clocks are considered as promising candidates for redefining the second in the International System of Units. Compared with microwave clocks, optical clocks are powerful tools for the fundamental research such as the constancy of the fundamental constants, the validity of Einstein’s theory of general relativity, and the predictions of quantum electrodynamics. Recently two research groups have demonstrated the optical clocks with an unprecedented precision level of 10-18, which is two orders better than the present primary frequency standard. Using two Sr optical clocks and three Cs fountain clocks, SYRTE group has demonstrated the definition of second with optical clocks.#br#For redefining the second with optical clocks in the future, the optical clocks from the remote laboratories should have a high precision and the frequency of the optical clocks need to be transferred over a long distance, with extremely high precision. Unfortunately the conventional means of frequency transfer such as two-way satellite time and frequency transfer can reach a 10-16 level in one day which is far below the requirement for an optical clocks. Various methods have been developed to transfer optical frequency signal via optical fibers. Especially a research group from Germany has achieved a frequency transfer stability of 10-19 level in hundreds of seconds with a fiber length of 1840 km.#br#We demonstrate the recent development of optical frequency transfer over a 70-km fiber spool at National Time Service Center. The measurement shows that the compensation for the fiber noise is close to the limitation induced by the fiber delay for the Fourier frequency from 1 Hz to 250 Hz. The transfer stability (Allan deviation) of the fiber link is 1.2×10-15 in 1 s averaging time, and 1.4×10-18 in 10000 s. A preliminary test of the optical frequency transfer over a 100-km spooled fiber is achieved with a stability of roughly one order worse than the 71 km result, 5×10-15 in 1 s.#br#We demonstrate a new scheme of remote compensation for optical frequency transfer via fibers against conventional local compensation method. This new scheme has the advantage of great simplification of the local site, which can find applications in massive extension of star network. The key feature is that we transfer the mixture of the round-trip signal and local reference to the remote user’s end via an auxiliary fiber. At remote site, the fiber noise is measured and compensated by AOM2 accordingly.#br#Transfer stabilities of 13×10-15 in 1 s averaging time and 4.8×10-18 in 10000 s are achieved with the remote fiber noise compensation via a 25 km fiber spool. The demonstrated transfer stability is comparable to that obtained by the local fiber noise compensation method.#br#The future star fiber network of optical frequency transfer can benefit from this method, because the simpler local setup is required and even can be shared in the central site for multitudinous remote users.
High-quality frequency transfer based on existing telecommunication fiber links allows state-of-the-art microwave and optical clocks to be compared on a continental and potentially even intercontinental scale. We present a half-a-year-long data set of unidirectional optical frequency transfer over a 2 × 43 k m urban fiber link. We observe a relative frequency instability of 8.0 × 10 − 16 at 1 s and at best 6.0 × 10 − 18 at 10 6 s integration time. Our results show that the unidirectional two-way method gives the possibility to perform comparisons of international primary standards over fiber links faster than over satellite links. Moreover, we investigate the major limiting factors of such a unidirectional setup.
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.