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High performance frequency comparisons over optical fibre

Gesine Grosche, Katharina Predehl, S. M. F. Raupach, H. Schnatz, Osama Terra, Stefan Droste, Ronald Holzwarth

Published Aug 1, 2011• 0 citations
Publisher

Abstract

We aim at distributing the most precise frequency references provided by optical clocks, both from specialized labs to end users, and between specialized labs for comparing optical clocks. Transmitting an optical carrier over standard telecom fibre, we have transported frequency information over up to 900 km distance, with fidelity close to theoretical limits and reaching a precision of a few parts in 1018.

DOI
10.1109/iqec-cleo.2011.6193614

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We aim at distributing the most precise frequency references provided by optical clocks, both from specialized labs to end users, and between specialized labs for comparing optical clocks. Transmitting an optical carrier over standard telecom fibre, we have transported frequency information over up to 900 km distance, with fidelity close to theoretical limits and reaching a precision of a few parts in 1018.
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.
The transfer of ultra-stable frequencies between distant laboratories is required by many applications from fundamental metrology to high-precision measurements. For that purpose, optical fibre links have been intensively studied over the last decade. They have demonstrated impressive results far beyond the GPS capabilities on distances up to 2000 km, thanks to an active compensation of the fiber propagation noise.Up to now, optical links has been mainly implemented between two labs, when future applications will require the development of metrological fiber networks. Towards this goal, we have developed a few techniques which allow us to distribute the ultrastable optical signal to many users simultaneously. We have designed repeater laser stations, which can be used to build a cascaded link or to distribute the ultrastable signal among two optical links [1]. These stations allow us also to compare the output end signals of two optical links. These versatile stations will be a key-component of the national metrological network currently being developed in France within the Refimeve+ project. To complete the network, we have demonstrated the extraction of the ultrastable signal from a main link and its seeding to a secondary link [2].Optical links are already been successfully used for clocks comparison. Recently we were able to compare the atomic clocks of the French and German National Metrology Institutes through two connected optical links of total length 1415-km [3]. We found that the two fully independent Sr lattice optical clocks at the two sites agreed to better than 5x10-17 limited only by the clocks uncertainties. A fractional statistical uncertainty of 3x10-17 was reached after only 1000 s averaging time, which is 10 times better, and more than four orders of magnitude faster than with any other existing frequency transfer method. The atomic fountain primary frequency standards from the two institutes were also compared and they were found to agree well within their combined uncertainties of a few 10-16 [4].
Comparison of microwave frequency standards by satellite techniques is currently limited in instability at the 10-15level at one day [1]. However, the best optical frequency standards have already demonstrated an instability of ~ 5 × 10-17at 2000 s and an estimated uncertainty of 9 × 10-18[2]. The predicted improvement in optical frequency standards motivates the need for a new transfer method capable of performing below these predicted levels of instability. The transmission of cw optical frequencies through optical fibre networks is the most promising alternative to satellite methods when it comes to comparing remote optical frequency standards. Intra-lab comparison of optical frequency standards can already be carried out with the use of femtosecond optical frequency combs, which can easily bridge the frequency gap between the different standards under test. However, a particularly more challenging task is to compare different standards at remote European laboratories. The comparison of remote standards, particularly like-for-like systems (such as the Yb+clocks at NPL and PTB), will be important in determining the relative uncertainty and reproducibility of these clocks. For this remote comparison method to operate below the instability level of the optical standards, a highly stable phase-coherent local oscillator is needed so as not to degrade the stability of the atomic reference being compared.
The unprecedented accuracy of modern optical clocks has spurred the development of matching frequency comparison techniques, with optical frequency transfer over fibre emerging as the method of choice. However, the generation and dissemination of the international reference time scale, as well as many applications outside metrology, require reliable and economically sustainable time transfer in addition to frequency transfer capabilities. To address these needs, a new EMPIR-funded project TiFOON – Time and Frequency Over Optical Networks - has been set up with the aim of advancing fibre-based frequency transfer capabilities in Europe towards a universal tool for time and frequency metrology and other demanding applications.