Optical frequency link between Torino and Firenze for remote comparison between Yb and Sr optical clocks
Davide Calonico, Cecilia Clivati, Giovanni Antonio Costanzo, Aldo Godone, Filippo Levi, Marco Marchetti, Alberto Mura, M. Prevedelli, M. Schioppo, Guglielmo Maria Tino, Massimo E. Zucco, N. Poli
The Italian Institute of Metrology (INRIM), the European Laboratory for NonLinear Spectroscopy (LENS), the Physics and Astronomy Department of the University of Firenze (UNIFI) and the Politecnico of Torino started a project to establish a 650 km optical fiber link that will enable remote frequency comparisons between high accuracy microwave and optical clocks of INRIM and UNIFI-LENS and an absolute measurement of the UNIFI-LENS Sr clock versus INRIM Cesium fountains. We report the overall architecture of the project, the present status of the experiment and the preliminary tests performed on the equipments.
The Italian Institute of Metrology (INRIM), the European Laboratory for NonLinear Spectroscopy (LENS), the Physics and Astronomy Department of the University of Firenze (UNIFI) and the Politecnico of Torino started a project to establish a 650 km optical fiber link that will enable remote frequency comparisons between high accuracy microwave and optical clocks of INRIM and UNIFI-LENS and an absolute measurement of the UNIFI-LENS Sr clock versus INRIM Cesium fountains. We report the overall architecture of the project, the present status of the experiment and the preliminary tests performed on the equipments.
We describe LinkO, an experimental set up at the Italian National Metrology (INRiM), Torino, in cooperation with the Politecnico di Torino with the aim to perform the frequency dissemination on optical fibers of ultrastable frequency signals at a European scale. this system allows the frequency comparisons of optical clocks at the relative level of some parts in 10^15 with a 1s integration time, overcoming the capabilities of the satellite based techniques. Up to now the relative stability of the optical link is at the level of 2x10^-15 on a protorype system based on 50km fiber spools inside the lab
This thesis describes the realization of a facility for the high-accuracy frequency dissemination on a continental scale. This project was developed at the Optics division of the Italian National Metrology Institute (INRIM), which is in charge of the realization and operation of the atomic primary frequency standards, of their dissemination, and cooperates to the International Atomic Time generation. Frequency dissemination is now based on satellite techniques; however, the resolution provided by these methods does not cope with the accuracy and stability of newgeneration frequency standards. For instance, more than 10 days of measurement are needed to effectively compare primary standards, and years would be necessary to compare optical clocks via satellite links. On the other hand, it has been demonstrated that systems based on optical fibers can improve the resolution by about 5 orders of magnitude with respect to satellite techniques. This is the reason why many national metrology institutes in Europe are cooperating to develop a network for frequency dissemination and clocks comparisons all across the continent. This technique is based on the transmission of a narrow-linewidth laser along a standard telecom fiber. At the metrological laboratory, the frequency is measured with respect to a frequency standard, while at the remote link end it is extracted and used as a frequency reference. However, temperature changes, acoustic noise, air flows can degrade the stability of the delivered frequency, and must therefore be compensated. This is done through a Doppler cancellation technique, in which the light travels twice in the optical fiber in opposite directions. This is different from what happens in telecom links, which are inherently bidirectional. Dedicated instruments have therefore been developed. This thesis describes the realization of a phase-stabilized optical link between INRIM and the European Laboratory for Non Linear Spectroscopy in Florence, where an optical clock based on Sr atoms was under development at the time of measurements. This clock has been compared to the INRIM primary standard via the optical link. I worked on the realization of the ultrastable laser sources, on the development and characterization of the optical and electronics apparatus, and at the development of the metrological chain for the remote clocks comparison. This thesis also describes some novel techniques which we investigated in the framework of this project, and new applications of optical fiber links. During my studies I spent two months at the Laboratoire National d'Essais-Systemes de Référence Temps-Espace (LNE-SYRTE) in Paris, where I cooperated to the realization of an ultrastable laser source based on an optical delay line. This is reported in the thesis as well
Optical fiber links offer the best performance to transfer time and frequency. They are beneficial to primary metrology, and there is a strong effort to develop a European network of T&F fiber links. In Italy, we realized a coherent optical fiber link from the National Metrological Institute INRIM to the INAF radio-antennas in Medicina, Bologna and the physics laboratories in Sesto Fiorentino, for a total haul of 642 km. We demonstrated the transfer of primary frequency references to the radio-antenna in Medicina, with potential impact for future development in radioastronomy and VLBI. The extension of this link targets to arrive to Matera, at the Space Geodesy Center of ASI. We describe here the space metrology interests of the optical fiber link, the present set-up of the experiment the preliminary results and the future developments.
Nowadays, optical fiber links are key elements in optical metrology, spectroscopy, quantum communication and geodesy. In geodetic Very Long Baseline Interferometry, a local maser is responsible for providing time and frequency reference at radiotelescope. Here, we present our recent results on frequency dissemination using a coherent fiber link 1800 km long from Turin to Medicina and Matera, Italy. Metrological reference disseminated via fiber link improve the stability of about two order with respect to the local H-maser clocks. This kind of dissemination paves the way to VLBI observation using a remote clock reference on the Italian and European radio observatories.
Abstract Two 87Sr optical lattice clocks (OLCs) are being developed and operated at the National Institute of Metrology (NIM), located on different campuses that are 40 km apart. In order to compare the optical frequencies of these two Sr OLCs, a 58-km noise canceled fiber link is built to transfer both a 1542-nm transfer laser and a microwave reference from Changping campus to Hepingli campus. Two commercial optical frequency combs (OFCs) with adapted single-branch 698/1542 nm outputs coherently unite the two 698-nm clock lasers and the 1542-nm transfer laser. The fractional instability of the comparison yields 3.1 × 10−17 at 10000 s averaging time. The measured fractional frequency difference between these two Sr OLCs was evaluated to be 1.9(3.2) × 10−17, which is within their claimed uncertainties. This result demonstrates the consistency of their frequencies when they serve as optical frequency standards. Our remote comparison demonstrates the feasibility of optical clock comparison through a long-distance fiber link and contributes to the progress of redefinition of the SI second.
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].