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Conference on Lasers and Electro-Optics

Optical Frequency Transfer over Submarine Fibers

Cecilia Clivati, Giuseppe Marra, Filippo Levi, Alberto Mura, André Xuereb, Davide Calonico

Published Jan 1, 2020• 2 citations
Publisher

Abstract

We characterized the phase noise and instability of submarine optical fibers and estimate achievable performances of trans-oceanic fiber-based optical frequency transfer, in view of intercontinental atomic clock comparisons. We also show that the low noise of the submarine environment allows exploitation of long-haul telecom fibers as distributed sensors for underwater earthquakes.

DOI
10.1364/cleo_si.2020.sm2n.1

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We characterized the phase noise and instability of submarine optical fibers and estimate achievable performances of trans-oceanic fiber-based optical frequency transfer, in view of intercontinental atomic clock comparisons. We also show that the low noise of the submarine environment allows exploitation of long-haul telecom fibers as distributed sensors for underwater earthquakes.
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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 telecommunications industry’s deployment of billions of kilometers of optical fiber has created a vast global network that can be exploited for additional applications such as environmental sensing, quantum networking, and international clock comparisons. However, for reasons such as the unidirectionality of long-haul fiber links, telecom fiber networks cannot always be adapted for important applications beyond data transmission. Fortunately, new multicore optical fibers create the opportunity for application coexistence with data traffic, creating multifunctional networks. Toward that end, we propose and demonstrate the faithful transfer of ultrastable optical signals through deployed multicore fiber in a way that is compatible with the unidirectionality of long-haul fiber optic systems, demonstrating fractional frequency instability of 3×10 −19 at 10,000 s. By supporting state-of-the-art optical atomic clocks, subsea multicore fibers can break the distance barrier for ultrastable optical frequency transfer, opening the door to intercontinental optical clock comparisons at the highest level, with applications in fundamental physics, relativistic geodesy, and the redefinition of the second.
We have investigated the performance of an optical carrier transfer system for a remote comparison of optical clocks. In the frequency transfer system, technical problems and fundamental principles limit the coherent transfer on a long-haul optical fiber. They contribute to noise processes that come from the propagation delay in the fiber, the design of an interferometer, burst length fluctuations of the fiber, asymmetrical propagation characteristics, and so on. The feasibility of a full optical link with high stability is discussed. The phase detection with a high degree of precision and cancellation of the fiber-induced noise on the round-trip light are achieved with a well-known standard technique using an all-fiber-based interferometer. We report the characteristics of these noise processes with results on a spooled fiber. An Allan deviation of the compensated link which is extended to 90 km reaches 5 × 10-18 close to the theoretical limit imposed by the delay-unsuppressed fiber noise at averaging time of 103s.
SummaryIn this paper, to the best of our knowledge, the first long-term stable dissemination of the optical frequency via an aerial fiber link with a length of 148 km with passive phase noise cancellation and the first characterization of phase noise PSD characteristics about optical frequency transmission over aerial fiber links is demonstrated in this paper. Different from buried fiber, the phase noise PSD of the aerial link in the free-running case is empirically with a slope of 1⁄f3, at Fourier frequency between 1Hz and 1kHz. The level of the phase noise PSD tested overhead fiber optic links is hundreds or even thousands of times that of similar buried links, but our final frequency transmission results are not much different from those of buried fiber optic cables of the same magnitude, and a transfer frequency instability of 7×10−19at 4000 s are achieved, This result means that aerial fiber links are also a viable option when building long-haul fiber optic networks, when buried fiber optic cables are not available or are too costly.