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Detecting ocean-floor seismic activity is crucial for our understanding of the interior structure and dynamic behavior of the Earth. However, 70% of the planet’s surface is covered by water and seismometers coverage is limited to a handful of permanent ocean bottom stations. We show that existing telecommunication optical fiber cables can detect seismic events when combined with state-of-the-art frequency metrology techniques by using the fiber itself as the sensing element. We detected earthquakes over terrestrial and submarine links with length ranging from 75 to 535 km and a geographical distance from the earthquake's epicenter ranging from 25 to 18,500 km. Implementing a global seismic network for real-time detection of underwater earthquakes requires applying the proposed technique to the existing extensive submarine optical fiber network., peer-reviewed

Extreme Energy Events (EEE) is an extended Cosmic Rays (CRs) Observatory, composed of about 60 tracking telescopes spread over more than 10 degrees in Latitude and Longitude. We present the metrological characterization of a representative set of actually installed EEE GPS receivers, their calibration and their comparison with respect to dual-frequency receivers for timing applications, as well as plans for a transportable measurement system to calibrate the currently deployed GPS receivers. Finally, the realization of an INRIM Laboratory dedicated to EEE, aimed at hosting reference telescopes and allowing timing studies for Particle Physics/Astrophysics experiments, is presented, as well as the possibility of synchronizing already deployed telescopes utilizing White Rabbit Technique, over optical fiber links, directly with the Universal Time Coordinated time scale, as realized by INRIM (UTC(IT)).

Frequency dissemination in phase-stabilized optical fiber networks for metrological frequency comparisons and precision measurements are promising candidates to overcome the limitations imposed by satellite techniques. However, in an architecture shared with telecommunication data traffic, network constraints restrict the availability of dedicated channels in the commonly-used C-band. Here, we demonstrate the dissemination of an SI-traceable ultrastable optical frequency in the L-band over a 456 km fiber network with ring topology, in which data traffic occupies the full C-band. We characterize the optical phase noise and evaluate a link instability of 4.7 × 10-16 at 1 s and 3.8 × 10-19 at 2000 s integration time, and a link accuracy of 2 × 10-18. We demonstrate the application of the disseminated frequency by establishing the SI-traceability of a laser in a remote laboratory. Finally, we show that our metrological frequency does not interfere with data traffic in the telecommunication channels. Our approach combines an unconventional spectral choice in the telecommunication L-band with established frequency-stabilization techniques, providing a novel, cost-effective solution for ultrastable frequency-comparison and dissemination, and may contribute to a foundation of a world-wide metrological network., Optics Express, 29 (16), ISSN:1094-4087

Quantum key distribution (QKD) based on entangled photon pairs holds the potential for repeater-based quantum networks connecting clients over long distance. We demonstrate long-distance entanglement distribution by means of polarisation-entangled photon pairs through two successive deployed 96 km-long telecommunications fibres in the same submarine cable. One photon of each pair was detected directly after the source, while the other travelled the fibre cable in both directions for a total distance of 192 km and attenuation of 48 dB. The observed two-photon Bell state exhibited a fidelity 85% $\pm$ 2% and was stable over several hours. We employed neither active stabilisation of the quantum state nor chromatic dispersion compensation for the fibre., 7 pages, 3 figures

International audience; Following a pilot study initiated by the CCTF WG on TWSTFT and coordinated by the BIPM, since the summer of 2016 several laboratories have been testing a new type of TWSTFT receiving equipment based on a Software Defined Radio. This type of receiver has been developed with the aim of mitigate some of the time transfer instabilities observed in the TWSTFT links based on SATRE modem. One of the major feature of the SDR receiver is the low AM-PM conversion, thank to the particular design of the software correlator. At present the SDR system is only used as a receiver in parallel with the regular TWSTFT SATRE system and the investigation is mainly focused on the instability improvement. Nevertheless, considering that in the frame of the TAI network the TWSTFT links are mostly used for their accuracy, it is useful to develop a calibration method for such devices to allow a direct comparison with the traditional links, and to investigate the long term instability.

We describe a VLBI experiment in which, for the first time, the clock reference is delivered from a National Metrology Institute to a radio telescope using a coherent fibre link 550 km long. The experiment consisted of a 24-hours long geodetic campaign, performed by a network of European telescopes; in one of those (Medicina, Italy) the local clock was alternated with a signal generated from an optical comb slaved to a fibre-disseminated optical signal. The quality of the results obtained with this facility and with the local clock is similar: interferometric fringes were detected throughout the whole 24-hours period and it was possible to obtain a solution whose residuals are comparable to those obtained with the local clock. These results encourage further investigation of the ultimate VLBI performances achievable using fibre dissemination at the highest precision of state-of-the-art atomic clocks.

Two-way frequency transfer on optical fibers is a powerful technique for the comparison of distant clocks over long and ultra-long hauls. In contrast to traditional Doppler noise cancellation, it is capable of sustaining higher link attenuation, mitigating the need of optical amplification and regeneration and thus reducing the setup complexity. We investigate the ultimate limitations of the two-way approach on a 300 km multiplexed fiber haul, considering fully independent setups and acquisition systems at the two link ends. We derive a theoretical model to predict the performance deterioration due to a bad synchronisation of the measurements, which is confirmed by experimental results. This study demonstrates that two-way optical frequency transfer is a reliable and performing technique, capable of sustaining remote clocks comparisons at the resolution, and is relevant for the development of a fiber network of continental scale for frequency metrology in Europe.

Global Positioning System (GPS) dissemination of frequency standards is ubiquitous at present, providing the most widespread time and frequency reference for the majority of industrial and research applications worldwide. On the other hand, the ultimate limits of the GPS presently curb further advances in high-precision, scientific and industrial applications relying on this dissemination scheme. Here, we demonstrate that these limits can be reliably overcome even in laboratories without a local atomic clock by replacing the GPS with a 642-km-long optical fiber link to a remote primary caesium frequency standard. Through this configuration we stably address thesup1/supSsub0/sub-sup3/supPsub0/subclock transition in an ultracold gas ofsup173/supYb, with a precision that exceeds the possibilities of a GPS-based measurement, dismissing the need for a local clock infrastructure to perform beyond-GPS high-precision tasks. We also report an improvement of two orders of magnitude in the accuracy on the transition frequency reported in literature.

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.

Optical fiber links are beneficial not only for frequency metrology, but also for other fields of physical research, such as radioastronomy and geodesy. We realized the first optical fiber link from a National Institute of Metrology to a Radiotelescope used for Very Long Baseline Interferometry and for geodetic measurements, over a distance of 544 km. We performed a remote calibration of the hydrogen-maser used as a frequency reference at Medicina Radiotelescopes, in central Italy, against the Italian Cs fountain primary frequency standard. The comparison was limited by the statistical uncertainty of the hydrogen-masers. This experiment demonstrates that optical links can provide radioastronomical facilities with very accurate and stable frequency references, in perspective better than the currently used hydrogen-masers. This opens new perspectives in the ultimate limits of VLBI and a more precise space geodesy.

We realize a coherent fiber link for application in very long baseline interferometry (VLBI) for radio astronomy and geodesy. A 550-km optical fiber connects the Italian National Metrological Institute (INRIM) to a radio telescope in Italy and is used for the primary Cs fountain clock stability and accuracy dissemination. We use an ultrastable laser frequency- referenced to the primary standard as a transfer oscillator; at the radio telescope, an RF signal is generated from the laser by using an optical frequency comb. This scheme now provides the traceability of the local maser to the SI second, realized by the Cs fountain at the 1.7 × 10(-16) accuracy. The fiber link never limits the experiment and is robust enough to sustain radio astronomical campaigns. This experiment opens the possibility of replacing the local hydrogen masers at the VLBI sites with optically-synthesized RF signals. This could improve VLBI resolution by providing more accurate and stable frequency references and, in perspective, by enabling common- clock VLBI based on a network of telescopes connected by fiber links.

In this paper we analyze Distributed Raman Amplification (DRA) for phase-coherent optical links, showing the promising features as well as the existing limits for such a new technique. After the demonstration of an optical frequency transfer on 200 km in single fiber span in our experiment, with a frequency instability of 3×10 -19 at 100-1000 s, we present an analysis aimed in particular at extending the technique on significantly longer hauls and at overcoming detrimental nonlinear effects.

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We performed a two-way remote optical phase comparison on optical fiber. Two optical frequency signals were launched in opposite directions in an optical fiber and their phases were simultaneously measured at the other end. In this technique, the fiber noise is passively cancelled, and we compared two optical frequencies at the ultimate 10−21 stability level. The experiment was performed on a 47km fiber that is part of the metropolitan network for Internet traffic. The technique relies on the synchronous measurement of the optical phases at the two ends of the link, that is here performed by digital electronics. This scheme offers some advantages with respect to active noise cancellation schemes, as the light travels only once in the fiber.

In this paper we present a giant-area fiber-optic gyroscope (GAFOG) combining a Raman distributed temperature sensor (RDTS) for metrological monitoring of Earth rotation rate fluctuations. The GAFOG exhibits sensitivity limits in Earth rotation sensing associated to temperature variations along the fiber. Distributed temperature measurements providing an accurate fiber temperature mapping, enable tracking of the signal phase drifts. This allows reduction of the phase noise at low Fourier frequencies, and a corresponding enhancement of the Earth rotation rate variation sensitivity beyond the currently achieved 2·nrad/s for integration times of 1000 s. © 2014 SPIE.

To significantly improve the frequency references used in radio-astronomy and precision measurements in atomic physics, we provide frequency dissemination through a 642 km coherent optical fiber link, that will be also part of a forthcoming European network of optical links. We obtained a resolution of 3e-19 at 1000 s on the frequency transfer, and an accuracy of 5e-19. The ultimate link performance has been evaluated by doubling the link to 1284 km, demonstrating a new characterization technique based on the double round-trip on a single fiber. The arming of a second fiber is avoided: this is beneficial to long hauls realizations in view of a continental fiber network for frequency and time metrology. The data analysis is based on the Allan deviation; its expression is theoretically derived for the observed noise power spectrum, which is seldom found in the literature.

This work describes the realization of a fiber optic gyroscope with a sensitivity of 10-8 (rad/s)/√Hz. Such a high sensitivity is obtained using a 47 km long single mode fiber of the urban optical telecommunication network, that is shared with other users and encloses an area of 20 km2. This setup has been used to investigate ground motion in occurrence of earthquakes.

We report on the realization of the coherent optical link for time and frequency dissemination developed in Italy. The fiber backbone connects scientific laboratories that need accurate time and frequency measurements located in Torino, Milano, Bologna and Firenze. We briefly describe the technique, and report the status of installation and preliminary characterization. © 2013 IEEE.

We report on the design and the realization of a digital architecture based on a tracking direct digital synthesizer (DDS) driven by a Field Programmable Gate Array (FPGA) for the implementation of a coherent optical link for time and frequency dissemination. The realized digital system has been implemented and characterized on a real fiber link on a 47 km metropolitan optical telecommunication network using a Dense Wavelength Division Multiplexing, that is shared with the internet data traffic. We describe the technique, the hardware and the characterization of the link, and we compare the new digital system with the analogic one, that uses common fiber links electronics.

We describe the application of Raman Optical-fiber Amplification (ROA) for the phase coherent transfer of optical frequencies in an optical fiber link. ROA uses the transmission fiber itself as a gain medium for bi-directional coherent amplification. In a test setup we evaluated the ROA in terms of on-off gain, signal-to-noise ratio, and phase noise added to the carrier. We transferred a laser frequency in a 200 km optical fiber link with an additional 16 dB fixed attenuator (equivalent to 275 km of fiber on a single span), and evaluated both co-propagating and counter-propagating amplification pump schemes, demonstrating nonlinear effects limiting the co-propagating pump configuration. The frequency at the remote end has a fractional frequency instability of 3e-19 over 1000 s with the optical fiber link noise compensation.

The examples of controls in a high-precision scientific laboratory are countless and most scientific applications rely on controls of the powerful proportional-integral-derivative (PID), first introduced by Minorsky in 1922. In recent years a novel Active Disturbance Rejection Control (ADRC) has been proposed. The ADRC is a linear, simple, intuitive, and inherently robust form of control. It is based on a linear extended state observer (LESO) that estimates and compensates the disturbance of the system in real time with a certain bandwidth. The bandwidths of the control and of the observer are the only parameters needed for tuning. The design of the ADRC is model independent and requires only some approximate knowledge of the fast frequency response of the system. We applied the ADRC to the stabilization of the temperature of optical ultra-stable cavities. Such cavities are used for laser frequency stabilization for high-resolution spectroscopy and in optical clocks. In our case a 578 nm yellow laser is locked to the cavities and it will be used for the spectroscopy of ytterbium atoms. The temperature of the cavities has to be stabilized to prevent drift in their length and thus in the frequency of the laser. We will present the details of the ADRC and show the performance of the temperature stabilization. The ADRC shows attractive features that motivates our choice: it is easy to design and to implement digitally, insensitive to environmental changes, and avoids typical problems encountered with PID like overshoots and integral windup.

The Unit of Time in the International Systems of Units is realized at the relative frequency accuracy level of few parts in 10-16 by laser cooled Cesium fountains [1]; since 2003 INRIM has operated a laser cooled Cesium fountain with relative frequency accuracy at the level of 5×10-16 and has contributed with this frequency standard to the generation of the International Atomic Timescale of Bureau International des Poids et des Mesures (BIPM). Since 2009 a cryogenic Cesium fountain is under operation and its metrological characterization is in progress, with a target accuracy of 1×10{-16}.

An experiment aimed at the determination of the constant G was started in 1995 at the Politecnico di Torino, Torino, Italy, based on measurement of the period of a simple pendulum. Two source masses are periodically moved, orthogonally to the swinging plane, between two fixed positions. An optical system detects the pendulum perigee, and an electronic circuit measures the period. This paper describes recent progress and results, in which an unexpected effect, which is currently under investigation, limits the desired 10-4 uncertainty level to 3.2%