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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Cascaded optical link on a telecommunication fiber network for ultra-stable frequency dissemination

Olivier Lopez, N. Chiodo, Fabio Stefani, Fabrice Wiotte, Nicolas Quintin, Anthony Bercy, Christian Chardonnet, Giorgio Santarelli, Paul-Éric Pottie, Anne Amy‐Klein

Published Mar 10, 2015• 4 citations
Publisher

Abstract

The transfer of ultra-stable frequencies between distant laboratories is required by many applications in time and frequency metrology, fundamental physics, particle accelerators and astrophysics. Optical fiber links have been intensively studied for a decade and brought the potential to transfer frequency with a very high accuracy and stability thanks to an active compensation of the propagation noise. We are currently developing an optical metrological network using the fibers of the French National Research and Education Network. Using the so-called dark-channel approach, the ultrastable signal is copropagating with data traffic using wavelength division multiplexing. Due to significant reflections and losses along the fibers, which cannot be compensated with amplifiers, we have developed some repeater stations for the metrological signal. These remotely-operated stations amplify the ultrastable signal and compensate the propagation noise. The link is thus composed of a few cascaded spans. It gives the possibility to increase the noise correction bandwidth, which is proportional to the inverse of the fiber length for each span. These stations are a key element for the deployment of a reliable and large scale metrological network. We report here on the implementation of a two-spans cascaded link of 740 km reaching a relative stability of a few 10-20 after 103 s averaging time. Extension to longer links and alternative transfer methods will be discussed.

DOI
10.1117/12.2083830

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The transfer of ultra-stable frequencies between distant laboratories is required by many applications in time and frequency metrology, fundamental physics, particle accelerators and astrophysics. Optical fiber links have been intensively studied for a decade and brought the potential to transfer frequency with a very high accuracy and stability thanks to an active compensation of the propagation noise. We are currently developing an optical metrological network using the fibers of the French National Research and Education Network. Using the so-called dark-channel approach, the ultrastable signal is copropagating with data traffic using wavelength division multiplexing. Due to significant reflections and losses along the fibers, which cannot be compensated with amplifiers, we have developed some repeater stations for the metrological signal. These remotely-operated stations amplify the ultrastable signal and compensate the propagation noise. The link is thus composed of a few cascaded spans. It gives the possibility to increase the noise correction bandwidth, which is proportional to the inverse of the fiber length for each span. These stations are a key element for the deployment of a reliable and large scale metrological network. We report here on the implementation of a two-spans cascaded link of 740 km reaching a relative stability of a few 10-20 after 103 s averaging time. Extension to longer links and alternative transfer methods will be discussed.
We are currently developing a cascaded optical frequency link between Paris and Strasbourg using the French National Research and Education Network RENATER. The ultrastable signal is copropagating with data traffic using wavelength division multiplexing. We have developed some repeater stations to amplify and filter the ultrastable signal between each sections of the link and compensate the propagation noise. The remote operation of these stations is a key element for the deployment of a reliable and large scale metrological network. We report on the first implementation of such a remote station on a 540-km cascaded link between Paris and Reims. The stability (modified Allan deviation with Λ-type counter) reaches a few 10−20 at 103 s averaging time.
We transfer the frequency of an ultra-stable laser over a cascaded optical link comprising two compensated links of 150 km and a repeater station. Each link passes through two important nodes of the telecommunication network and includes 114 km of Internet fiber simultaneously carrying data traffic, through a dense wavelength division multiplexing scheme. The metrological signal is inserted in and extracted from the communication network using bidirectional optical add-drop multiplexers. The repeater station is working independently without any remote control. The phase noise on the two links is compensated with the usual round-trip technique. The 300-km multiplexed cascaded link shows an Allan deviation of 3×10-15at one second and 7×10-20at 20 hours. This work paves the way to a wide dissemination of ultra stable optical clock signals between distant laboratories via the Internet network.
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].
Summary form only given. The transfer of ultra-stable frequencies between distant laboratories is a fundamental issue for a wide range of high-sensitivity experiments in advanced metrology and fundamental physics. Since a few years, several experiments of optical frequency transfer were reported over dedicated fiber, with a record distance of 920 km [1] recently demonstrated in Germany. The challenge is now to build a wide frequency dissemination network connecting laboratories and research agencies in Europe.For that purpose we exploit the existing fine meshing of the Internet fiber network, connecting laboratories via the National Research Networks (NRENs). In that scheme, the ultra-stable frequency signal is copropagating simultaneously with the Internet traffic in the same fiber using a dedicated channel in a dense wavelength division multiplexing (DWDM) approach. Optical add-drop multiplexers enable to insert and extract the ultra-stable frequency signal in and from the network.We have demonstrated an ultra-stable optical link over 540 km, including 470 km of public fiber network of the French NREN (RENATER) simultaneously carrying Internet data traffic [2]. The signal bypasses four amplification stages and two Data Center Facilities using multiplexers and custom-made bidirectional erbium-doped fiber amplifiers. One specific point of this experiment is that with Internet fibers, we have a very limited control on the attenuation and noise. For instance the Erbium fiber doped bidirectional amplifiers gain is drastically limited to avoid lasing oscillations due to parasitic reflections due to connectors, splices and more fundamental Rayleigh backscattering. Nevertheless the results are comparable with those obtained with dedicated fibers. Figure 1 shows the measured fractional frequency stability, we obtain 6×10-15 at 1 s and below 10-18 after 10000 second integration time. Such optical link will enable a broad range of high-sensitivity measurements, including the search for fundamental constants variation and gravitational mapping. This concept of fiber link has been very recently used to implement a giant fiber gyroscope [3]
The transfer of ultra-stable frequencies between distant laboratories is required by many applications in time and frequency metrology, fundamental physics, particle accelerators and astrophysics. This thesis describes the development of ultra-stable optical links over distances from 86 km to 300 km. The long-term objective is to connect the main European National Metrology Institutes with optical links extending up to 1000 km. The optical link is based on the transfer of the optical phase of an ultra-stable laser emitting light of wavelength 1542 nm through an optical fiber of the telecommunication network. For that purpose, ultra-stable cavity-stabilized lasers and fiber-stabilized lasers have been developed. The fractional instability of cavity-stabilized laser is approximately 10-15 at 1s integration time. The fiber-stabilized lasers are robust, compact, simple, tunable and their frequency noise is comparable to that of the cavity-stabilized laser for Fourier frequencies higher than a few tens of Hertz. Optical links were first demonstrated on 86 km and 172 km using dedicated fibers connecting the laboratories LNE-SYRTE and LPL. Fractional frequency instability in the range of or below 10-19 at 104 s integration time has been obtained. For larger distances, the ultra-stable laser signal is transferred on the Internet network simultaneously with the data traffic but on a dedicated frequency channel, using wavelength division multiplexing. Multiplexed links were demonstrated over distances up to 300 km, with fractional frequency instabilities at the same level or better than with dedicated fibers.
High-resolution time and frequency transfer between remote laboratories are of major interest for many fundamentals applications, such as tests of general relativity or temporal variation of fundamental constants performed by comparing clocks signals. We report on the progress of a metrological fiber network on the French territory, with extensions to Europe, which aims to provide an ultra-stable frequency signal to around 20 laboratories. This network is based on the dissemination of an ultra-stable signal using optical links through the academic fiber internet network. As a first step, we have demonstrated robust links of up to 1480 km with stability at 104s and accuracy below 10−19.