Search

The purpose of this document is to describe some “open-hardware” aspects of the demonstrations related to a robust optical frequency dissemination with a dual-polarization coherent receiver. &nbsp

Digital signal processing is supporting novel in-field applications of optical interferometry, such as in laser ranging and distributed acoustic sensing. While the highest performances are achieved with field-programmable gated arrays, their complexity and cost are often too high for many tasks. Here, we describe an alternative solution for processing optical signals in real-time, based on a dual-core 32-bit microcontroller. We implemented in-phase and quadrature demodulation of optical beat-notes resulting from the interference of independent laser sources, phase noise analysis of deployed optical fibers covering intercity distances, and synchronization of remote acquisitions via optical trigger signals. The embedded architecture can efficiently accomplish a large variety of tasks in the context of optical signal processing, being also easily configurable, compact and upgradable. These features make it attractive for applications that require long-term, remotely-operated, and field-deployed instrumentation.

published paper "Improving the Resolution of Comb-Based Frequency Measurements Using a Track-And-Hold Amplifier", https://doi.org/10.1103/PhysRevApplied.18.064010, by M. Risaro et al., Phys. Rev. Applied18, 064010 (2022). Support datasets available athttps://doi.org/10.5281/zenodo.7484196

Optical clock comparison via optical fibre links has been achieved over continental scales, but has not yet been demonstrated intercontinentally. The transfer of ultra-stable optical frequencies over transoceanic distances is a challenging task, as the seafloor cable architecture prevents the use of environmental noise suppression techniques currently employed on land-based metrological links. As a fundamental first step towards devising suitable frequency transfer techniques to enable future clock comparison on a global scale, here we show the free-running frequency stability and phase noise measurements of a transatlantic seafloor optical link between the UK and Canada. To the best of our knowledge, these are the first ever measurements of an intercontinental optical fibre link.

Among the most powerful techniques for the exploration of the Universe is very long baseline interferometry (VLBI), which is based on the simultaneous observation of radio sources in the sky with arrays of distant ground-based antennas. One of the effects currently limiting its ultimate sensitivity is the phase-instability of the reference clocks adopted at each antenna.This term can be made negligible delivering the same clock signal to multiple telescope sites using optical fibers. We realized such an infrastructure by disseminating a coherent optical frequency signal to two distant radio telescopes using a 1739-km-long fiber. We performed a 24 h geodetic VLBI campaign in which the same clock reference was used at both telescopes and analyzed it using standard VLBI procedures. The results were consistent with the expectations, confirming that the proposed approach is feasible and configures as a novel tool for studying the role of clocks, troposphere, and systematic effects in the ultimate VLBI resolution.

Time synchronization and time stamping traceability are becoming rapidly a key enabling technology. Accurate time distribution is no more used only in navigation, but is becoming a key element in a large variety of industrial and scientific fields. E.g. telecommunications, mobile 5G technology, power grids, finance and trading, aerospace and research centers rely on accurate timing. Global Navigation Satellite Systems (GNSS) is today the most widespread timing source, although widely known problems such as spoofing, jamming, space weather vulnerability and lack of traceability to Universal Time Coordinated (UTC). Unlike GNSS, commercial telecommunication fiber-optic networks are intrinsically immune to the above mentioned problems and therefore represent a reliable, safer alternative distribution mean for Time and Frequency (TF) reference signal, for all those field that are do not require navigation and mobile devices. Since several years, INRIM has been developing a national fiber network to distribute a coherent frequency reference signal, spanning over 2000 km connecting several industrial and scientific research centers all over the country. This network is also reaching the Italian border with France, connecting INRIM to other European NMIs (SYRTE in Paris, and from there NPL in the UK and PTB in Germany), for accurate comparison of atomic clocks. On the same network we have recently started to distribute timing signal, on the same fiber cable, using bidirectional paths in single fiber, based on dense wavelength division multiplexing channels Different techniques were proposed in the last decade to address accurate timing on optical fibers. To compensate the delay and the variation of the fiber length due to thermal and mechanical stress two parallel fiber (TX and RX) are used. showing good level of stability, but some criticalities in the accuracy statement. In fact, on long hauls, one of the main limitation is always the non-reciprocity of the optical path at meter level. To overcome this problem, we implemented a modified White Rabbit set-up. White Rabbit is an open-source project for TF dissemination using fiber-optical links on Ethernet networks, combining Synchronous Ethernet and Precise Time Protocol to transport the traffic over medium, that was originally developed at CERN. We propose in-field deployment and performance demonstration of solution for TF dissemination, based on White-Rabbit Precision Time Protocol (WR-PTP), as extension of IEEE1588-2018 standard. The system was implemented on single shared fiber-optic link, utilized together with other data traffic and signals. This method allows to have a completely bidirectional optical path, making the delay compensation much more accurate and stable, being the chromatic differential group velocity the leading term in timing accuracy budget. The test bed for timing is a fiber haul spanning 230 km, connecting the Istituto Nazionale di Ricerca Metrologica in Turin and the final user, Leonardo S.p.A. in Nerviano (near Milan) – a global high technology company and key player in Aerospace, Defense and Security. At the conference, we will describe key system elements, network architecture, multiplexing technique and introduce coherent signal for frequency distribution applied on the same fiber. We will present solution for in-field calibration of link delays and results achieved in term of accuracy and showing our approach to evaluate performance in terms of stability.

Supplemental document

arxive containing supplementary material and related figures

arxive containing supplementary material and related figures

Differential comparisons of distant optical clocks with uncertainties in the 10−18 level linked by phase-stabilized optical fibers can be used to probe the Geoid potential at the centimeter sensitivity. Here, we present the realization and characterization of a coherent optical link that will enable gravitational potential measurements on a baseline of 90km and at 1000m of difference in altitude. This is a proof-of-principle experiment that aims at exploring extensive interconnections between geodesy and frequency metrology.

We stabilized the frequency of a compact planar-waveguide external cavity laser (ECL) on a Fabry-P\'erot cavity (FPC) through a Pound-Drever-Hall scheme. The residual frequency stability of the ECL is 1E-14, comparable to the stability achievable with a fiber laser (FL) locked to a FPC through the same scheme. We set up an optical link of 100 km, based on fiber spools, that reaches 1E-19 relative stability, and we show that its performances using the ECL or FL are comparable. Thus ECLs could serve as an excellent replacement for FLs in optical links where cost-effectiveness and robustness are important considerations.

We present the first field test of the implementation of a coherent optical fiber link for remote antenna synchronization realized in Italy between the Italian Metrological Institute (INRIM) and the Medicina radio observatory of the National Institute for Astrophysics (INAF). The Medicina VLBI antenna participated in the EUR137 experiment carried out in September 2015 using, as reference systems, both the local H-maser and a remote H-maser hosted at the INRIM labs in Turin, separated by about 550 km. In order to assess the quality of the remote clock, the observed radio sources were split into two sets, using either the local or the remote H-maser. A system to switch automatically between the two references was integrated into the antenna field system. The observations were correlated in Bonn and preliminary results are encouraging since fringes were detected with both time references along the full 24 hours of the session. The experimental set-up, the results, and the perspectives for future radio astronomical and geodetic experiments are presented.

We realized a UV laser spectrometer at 253.7 nm for Doppler broadening thermometry on the 1 S 0 − 3 P 1 intercombination line in mercury vapors. Our setup is based on the two-stage duplication of a 1014.8 nm diode laser in a fiber-coupled periodically poled lithium niobate waveguide crystal and a beta-barium borate crystal in enhancement cavity, and we exploit injection locking of a 507.4 nm diode laser to boost the available optical power after the first duplication. Our setup addresses spectroscopic features that allow the thermodynamic temperature determination of the atomic sample from the absorption profile with 10 − 6 accuracy. The realized UV laser source has 1 × 10 − 4 relative intensity stability, Gaussian shape, and over 10 GHz mode-hop-free tunable range. These features are crucial for the practical realization of the kelvin in the new International System of Units through a spectroscopic technique.