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

Phase-coherent frequency comparison of optical clocks using a telecommunication fiber link

H. Schnatz, Osama Terra, Katharina Predehl, T. Feldmann, Thomas Legero, B. Lipphardt, Uwe Sterr, Gesine Grosche, Theodor W. Hänsch, Ronald Holzwarth, Thomas Udem, Zhilan Lü, L. Wang, Wolfgang A. Ertmer, J. Friebe, A. Pape, Ernst Maria Rasel, Matthias Riedmann, Temmo Wübbena

Published Apr 1, 2009• 9 citations
Publisher

Abstract

We have explored the performance of two ldquodark fibersrdquo of a commercial telecommunication fiber link for a remote comparison of optical clocks. These fibers establish a network in Germany that will eventually link optical frequency standards at PTB with those at the Institute of Quantum Optics (IQ) at the Leibniz University of Hanover, and the Max Planck Institutes in Erlangen (MPL) and Garching (MPQ). We demonstrate for the first time that within several minutes a phase coherent comparison of clock lasers at the few 10-15level can also be accomplished when the lasers are more than 100 km apart. Based on the performance of the fiber link to the IQ we estimate the expected stability for the link from PTB to MPQ via MPL that bridges a distance of approximately 900 km.

DOI
10.1109/freq.2009.5168297

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European Frequency and Time Forum•Apr 1, 2010•0 citations

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Stefan Droste, Christian Grebing, Julia Leute +6 more

A global positioning system (GPS) carrier-phase frequency transfer link along a baseline of 450 km has been established and is characterized by comparing it to a phase-stabilized optical fiber link of 920 km length, established between the two endpoints, the Max-Planck-Institut für Quantenoptik in Garching and the Physikalisch-Technische Bundesanstalt in Braunschweig. The characterization is accomplished by comparing two active hydrogen masers operated at both institutes. The masers serve as local oscillators and cancel out when the double differences are calculated, such that they do not constitute a limitation for the GPS link characterization. We achieve a frequency instability of in 30 s and for long averaging times. Frequency comparison results obtained via both links show no deviation larger than the statistical uncertainty of . These results can also be interpreted as a successful cross-check of the measurement uncertainty of a truly remote end fiber link.

arXiv:1505.02144•New Journal of Physics•Aug 26, 2015•29 citations
We have explored the performance of two ldquodark fibersrdquo of a commercial telecommunication fiber link for a remote comparison of optical clocks. These fibers establish a network in Germany that will eventually link optical frequency standards at PTB with those at the Institute of Quantum Optics (IQ) at the Leibniz University of Hanover, and the Max Planck Institutes in Erlangen (MPL) and Garching (MPQ). We demonstrate for the first time that within several minutes a phase coherent comparison of clock lasers at the few 10-15level can also be accomplished when the lasers are more than 100 km apart. Based on the performance of the fiber link to the IQ we estimate the expected stability for the link from PTB to MPQ via MPL that bridges a distance of approximately 900 km.
We have explored the performance of 2 "dark fibers" of a commercial telecommunication fiber link for a remote comparison of optical clocks. These fibers establish a network in Germany that will eventually link optical frequency standards at PTB with those at the Institute of Quantum Optics (IQ) at the Leibniz University of Hanover, and the Max Planck Institutes in Erlangen (MPL) and Garching (MPQ). We demonstrate for the first time that within several minutes a phase coherent comparison of clock lasers at the few 10(-15) level can also be accomplished when the lasers are more than 100 km apart. Based on the performance of the fiber link to the IQ, we estimate the expected stability for the link from PTB to MPQ via MPL that bridges a distance of approximately 900 km.
We have explored the performance of two “dark fibers” of a commercial telecommunication fiber link for a remote comparison of optical clocks. The two fibers, linking the Leibniz University of Hanover (LUH) with the Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig, are connected in Hanover to form a total fiber length of 146 km. At PTB the performance of an optical frequency standard operating at 456 THz was imprinted to a cw transfer laser at 194 THz, and its frequency was transmitted over the fiber. In order to detect and compensate phase noise related to the optical fiber link we have built a low-noise optical fiber interferometer and investigated noise sources that affect the overall performance of the optical link. The frequency stability at the remote end has been measured using the clock laser of PTB’s Yb+ frequency standard operating at 344 THz. We show that the frequency of a frequency-stabilized fiber laser can be transmitted over a total fiber length of 146 km with a relative frequency uncertainty below 1×10−19, and short term frequency instability given by the fractional Allan deviation of σ y (τ)=3.3×10−15/(τ/s).
We have characterized the 24Mg optical frequency standard at the Institute of Quantum Optics (IQ), Hanover, using a clock laser at the Physikalisch-Technische Bundesanstalt (PTB), Braunschweig, via a noise compensated 73 km fiber link and present preliminary results for the stability of the Mg standard. The stability of the clock laser (λ = 657 nm) is transferred with a femtosecond frequency comb to a telecommunication laser at λ = 1542 nm. The signal is then transmitted from PTB through the fiber link to IQ. A second comb at IQ (the remote end) is used to compare the transmitted laser frequency with that of the Mg clock laser λ = 914 nm. The frequency ratio of the clock lasers νMg/νCa shows a relative instability < 10-14 at 1 s. The upper limit for the contribution of the fiber link to the frequency instability is measured by connecting another optical fiber following the same 73 km route at Hanover computer center. The comparison performed at PTB between the local and the transmitted signal after a round trip length of 146 km showed a relative uncertainty below 1 x 10-19 and a short term instability σy(τ)= 3.3 x 10-15 / (τ/s).
Chronometrie levelling [1-3] is a technique of deriving gravitational potential differences from clock frequency comparisons allowing the determination of height differences. Interferometric optical fibre links (IFL) [4] enable chronometric levelling between distant labs housing optical clocks [5]. An IFL is a phase stabilized telecom fibre link connection establishing a fixed optical phase relationship between two locations up to ~1000 km apart [6]. To perform a chronometric levelling campaign between the stationary optical clocks of the Physikalisch-Technische Bundesanstalt (PTB) and PTB's transportable Strontium clock [7] placed at the Max Planck Institute of Quantum Optics (MPQ) we have established a new 940 km long IFL between the two locations. At both link ends the frequencies of the clock lasers are compared to the 1,5 pm link transfer laser via fs-combs. To enable chronometric levelling experiments at the level of 10 cm height resolution, we aim a fractional frequency uncertainty of the frequency transfer of-18for averaging times of ~40 ks. To monitor the frequency transfer over the complete 940 km long fibre link the signal is sent back from the receiving end to the sender on a second fibre, which allows an out-of-loop characterization using data of the 1880 km loop. We study two different link setups, cf. Fig. 1a: i) a single-span loop configuration with the option to tap off the signal at MPQ with a passive frequency extraction unit [8]. This setup requires monitoring at the sending end only and allows us to investigate an ultralong IFL, even longer than the previous record [9]. And ii) an antiparallel configuration of two concatenated, individually stabilized interferometric fibre links. This setup offers lower instabilities due to a decreased delay-limit [10] but requires setting up and maintaining a stabilization system at both link ends. To compensate the attenuation of the fibres we used fibre Brillouin amplifier modules (FBAM) developed inhouse providing a typical gain of > 40 dB.
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.
A global positioning system (GPS) carrier-phase frequency transfer link along a baseline of 450 km has been established and is characterized by comparing it to a phase-stabilized optical fiber link of 920 km length, established between the two endpoints, the Max-Planck-Institut für Quantenoptik in Garching and the Physikalisch-Technische Bundesanstalt in Braunschweig. The characterization is accomplished by comparing two active hydrogen masers operated at both institutes. The masers serve as local oscillators and cancel out when the double differences are calculated, such that they do not constitute a limitation for the GPS link characterization. We achieve a frequency instability of in 30 s and for long averaging times. Frequency comparison results obtained via both links show no deviation larger than the statistical uncertainty of . These results can also be interpreted as a successful cross-check of the measurement uncertainty of a truly remote end fiber link.