19 November 2020
For the comparison of frequency standards and for applications related to time-frequency metrology, we are developing in collaboration with the Laboratoire de Physique des Lasers a new technique of time-frequency transfer by optical fiber. The starting point is an ultra-stable laser at the Telecom wavelength referenced to the optical and microwave clocks of the SYRTE using a frequency comb. The metrological signal transported by the fiber is the phase of the laser. Part of the ultra-stable laser light is injected into an optical fiber. The perturbations brought to the metrological signal by the fiber (mainly due to thermal fluctuations and acoustic noise) are measured after a round trip in the fiber and compensated by an active electronic system.
In order to take advantage of the existing fiber infrastructure, we have developed a collaboration with RENATER and developed a technique for parallel transfer of the data flow, without disturbing either the metrological signal or the data. We are thus working on the construction of a metrological network that we are seeking to have labeled as a national research infrastructure. This is the investment project of Avenir REFIMEVE+.
We seek in the laboratory to develop the technology, to extend the range of optical links, to further improve the performance level, to find other performance/cost trade-offs and finally to increase the link capacities (RF transfer, time transfer, etc...) and their field of application, especially in Earth and Universe sciences.
As early as 2012, we demonstrated the simultaneous transfer of time and frequency over 540 km of fiber deployed between Paris and Reims. We are currently working to develop the technology and include it in the REFIMEVE network.
Stakes
An essential activity of Time-Frequency metrology is the comparison of the clock frequencies of a set of clocks made independently: the comparison allows us to look for biases and ensure their accuracy, and to learn about the drift of fundamental constants in physics. Thus the comparison between primary frequency standards at the best level requires a means of comparison whose resolution is better than the noise floor of the clocks we wish to compare.
Currently remote clock comparisons are performed either by GPS or by telecommunications satellites. Comparisons are limited in relative stability to some 10-15 for one day of measurement, which is insufficient for the ultimate performance of current atomic frequency standards.
For applications of ultra-high resolution spectroscopy and synchronization of large instruments - measurements of fundamental physics, measurement of atmospheric pollutants, geodesy, astronomy,- the quality of the frequency references is an important element of the experimental system and can be a limiting factor for the resolution of the measurements. However, the implementation and cost of operating a high-performance frequency standard is often prohibitive for the user. The commonly employed strategy is to compare the frequency of a local oscillator with a higher quality frequency standard and use the result of the comparison to discipline the local oscillator on the frequency reference. The most convenient solution is to exploit GPS signals. Here again, progress in laboratories is such that the resolution of the comparison becomes a limiting factor in the resolution of the measurements.
Objectives
The objective of our activity is to enable the comparison of clocks on a European scale at the level of 10-18.
and to disseminate nationally an optical frequency reference for about 20 laboratories interested in very high resolution measurements (project REFIMEVE+).
- Project of the network in the Paris suburb area (REMIF).
Translated with www.DeepL.com/Translator (free version)
Fiber Optic Link
The technology developed, in close collaboration with the Laboratoire de Physique des Lasers (LPL, CNRS / Université Paris 13), consists in coupling in an optical fiber an ultra-stable laser referenced to the optical and microwave clocks of SYRTE. The metrological signal transported by the fiber is the phase of the laser. The perturbations brought to the metrological signal by the fiber (mainly due to thermal fluctuations and acoustic noise) are measured after a round trip in the fiber and compensated by an active electronic system. For a better rejection, it can be shown that a link subdivided into smaller segments allows a better noise rejection. Demonstrated as early as 2009->https://www.osapublishing.org/abstract.cfm?URI=ol-34-10-1573], extended and improved, the technique has been demonstrated with laboratory prototypes up to Strasbourg. This is the key technology of REFIMEVE+.
A radio-frequency signal can also be transferred by adding amplitude modulation to the laser [4,6]. We have recently shown that a synchronization signal and a time reference can also be transferred by adding phase modulation. The result of the study already shows much better performance than GPS [1].
Optical demodulation by a Mach-Zehnder interferometer has been shown to further improve performance while reducing cost and complexity.
- Simultaneous time and frequency transfer over 540 km with a public network parallel to data traffic.
- Frequency transfer over 540 km with a public network parallel to data traffic.
The limiting factors of this technique are the attenuation of the signal and the bandwidth of the correction when the length of the fiber becomes very large (typically beyond a few hundred km).
We have worked on the development of repeater stations in the optical domain, without degradation of the metrological signal [3]. These stations are now fully automatic and remotely controllable. The know-how has been transferred to an industrial consortium, whose leader is MuQuans. The bi-directional amplifiers are also products marketed by Lumibird-Keopsys.
We have developed a complete transfer of know-how to the MuQuans company, which made the first coherent optical link with their commercial products on the Paris-Lille link.
Multiplexed optical link
The second difficulty is the availability of a fiber for metrological applications.
The use of a dedicated fiber requires the expensive rental of a pair of fibers from an operator. However, since our signal is not - or very weakly - modulated, it is not necessary to use a dedicated fiber, a single channel of the ITU grid is more than sufficient. We have shown that by wavelength division multiplexing, we can transmit the metrological signal in parallel with an Internet data stream, without degradation of the quality of the metrological signal, and without disturbing the data stream [5].
- Optical link
We have established a collaboration with RENATER, the National Telecommunications Network for Technology, Education and Research, to use the infrastructure of the national academic network. A specific equipment is nevertheless necessary, which we have installed (end 2020) on the lines Paris-Reims-Nancy-Strasbourg-Besançon, Paris-Lille, Paris-Lyon-Grenoble-Modane. We also have a SYRTE-LPL-NPL link. Thus the SYRTE is optically linked to its German, English and Italian counterparts and the French network allows intercomparisons between any pair of clocks hosted at these 4 metrology institutes.
Clock comparisons
The first comparison of optical and microwave clocks took place in 2015 between SYRTE and PTB, and the results published in Nature Communication and Metrologia. The network was extended in 2016 to the NPL, then to INRIM in 2020. The clock network allows restricted relativity tests, which we published in PRL.
The graph above shows the stability of the comparison of the SYRTE and PTB Sr clocks for the March and June 2015 campaigns (blue dots), and the stability of the fiber optic link during these campaigns. The noise of the link is negligible in the comparison from the first seconds of acquisition.
Extraction - Distribution of the signal to multiple users
For a large number of network users, a point-to-point connection between each user and the SYRTE is not an optimum solution. We have developed a technique for online extraction of the metrological signal, following the original proposal of Gesine Grosche. The results were published in 2014 and 2016, and were transferred to MuQuans, REFIMEVE’s industrial partner.
Fundamental limits
We study on a deployed fiber and on coils the fundamental limits of optical links, comparing active and passive compensation techniques (Two-Way) and their combination in hybrid schemes. These studies have allowed us to highlight noise and bias due to the interferometer, to implement strategies to minimize and compensate for it, and to better understand the limiting factors related to polarization. The noise correlations between uni-directional and bi-directional architectures are studied.
Dark matter
We provide resources to the theory team for dark matter research in the laboratory. This is the project DAMNED.
The tapuscrit of Etienne Savalle’s thesis will be available soon.
White Rabbit
In collaboration with CERN and industrial manufacturers such as SevenSolution and OPNT, we are developing the White Rabbit solution for time and frequency reference optical fiber transfer (see also the European project WRiTE).
Namneet Kaur’s thesis showed possible ways to improve the technology, with a direct tuning system and to increase the servo bandwidth, as well as the feasibility of implementation on unidirectional architectures of long range telecommunication networks.
Link to Namneet Kaur’s thesis
>https://hal.archives-ouvertes.fr/tel-01909292]
Consortium REFIMEVE+ (equipment of excellence)
Laboratory of Laser Physics :
Anne Amy-Klein, Christian Chardonnet (PI), Olivier Lopez, Etienne Cantin.
LNE-SYRTE :
Michel Abgrall, Baptiste Chupin, Michel Lours, José Pinto, Paul-Eric Pottie (co-PI), Rodolphe Le Targat, Laurent Volodimer
Digital Photonics and Nanoscience Laboratory (LP2N) :
Giorgio Santarelli (co-PI)
RENATER :
Thierry Bono, Karim Boudjemaa, Emilie Camisard, Laurent Gydé, Nicolas Quintin
Current members:
Mads Tonnes, doctoral student
Maxime Mazouth-Laurol
Jérôme Fils
Alumni :
Anthony Bercy, PhD student co-supervising LPL/SYRTE (2012-2015)
Sacha Schediwy, visiting scientist UWA, 2012
Fabio Stefani, post-doc (2012-2015)
Won-Kyu Lee, visiting scientist KRISS (2014-2015)
Namneet Kaur, doctoral student (2014-2017)
Florian Frank, engineer (2016-2020)
Etienne Cantin, engineer (2018-2020)
Eva Bookjans, engineer (2017-2020)
Dan Xu, post-doctoral fellow (2016-2020)
European projects:
JRP NEAT FT
Co-Coordinator: Harald Schnatz, PTB
JRP OFTEN
Co-Coordinator: Harald Schnatz, PTB
GN3+ ICOF
Co-Coordinator: Harald Schnatz, PTB
JRP WRiTE
Co-ordinator: Davide Calonico, INRIM
JRP TiFOON
Co-ordinator: Jochen Kronjaeger, NPL
EC INFRA-INNOV CLONETS
Co-ordinator: Philip Tuckey, Paris Observatory
NB: Project ’cousins’ on clocks:
JRP OC18
JRP ROCIT
Internal collaboration
Within the framework of fiber optic time transfer, we work with the "References Microwave and time scales" team of the laboratory :
Philip Tuckey, Michel Abgrall, Baptiste Chupin, Joseph Achkar, Daniele Rovera (PhD).
We work with the theory team in the laboratory for fundamental physics tests, and dark matter research:
<Pacôme Delva, Peter Wolf, Etienne Savalle.
The project is supported by the LNE, CNES, ANR, EURAMET, BPI France, the Ile de France region (Nano-k, SIRTEQ), the Observatory’s scientific council, GRAM.
The REFIMEVE+ project is part of the Future Investments program.
Contact
Paul-Eric Pottie
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<e-mail : paul-eric.pottie (at) obspm.fr
<Tel: +33 (0) 1 40 51 22 22
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