1,658 research outputs found

    Fibre-optic delivery of time and frequency to VLBI station

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    The quality of Very Long Baseline Interferometry (VLBI) radio observations predominantly relies on precise and ultra-stable time and frequency (T&F) standards, usually hydrogen masers (HM), maintained locally at each VLBI station. Here, we present an operational solution in which the VLBI observations are routinely carried out without use of a local HM, but using remote synchronization via a stabilized, long-distance fibre-optic link. The T&F reference signals, traceable to international atomic timescale (TAI), are delivered to the VLBI station from a dedicated timekeeping laboratory. Moreover, we describe a proof-of-concept experiment where the VLBI station is synchronized to a remote strontium optical lattice clock during the observation.Comment: 8 pages, 8 figures, matches the version published in A&A, section Astronomical instrumentatio

    High spectral purity microwave sources based on optical resonators

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    L'optique constitue aujourd'hui une solution performante pour la réalisation de sources très pures en hyperfréquences, en particulier grâce à l'approche de l'oscillateur électro-optique (OEO). La pureté spectrale de ces sources est essentielle pour les applications spatiales, militaires et pour la métrologie du temps et des fréquences. Durant cette thèse, nous avons réalisé et étudié différents types d'OEO à résonateur optique en vue d'optimiser le bruit de phase de ce système. Nous avons en particulier orienté nos travaux vers une approche originale utilisant un anneau résonant fibré (ARF) passif. Ce type de résonateur présente en effet des coefficients de qualité optiques supérieurs à 109 pour des longueurs de fibre restant relativement faibles (L ~ 10 m) et facilement intégrables dans un support planaire. En parallèle, nous avons mené un travail important sur les oscillateurs à base de résonateurs optiques 3D. Concernant l'OEO à ARF, des progrès spectaculaires ont pu être obtenus grâce à une meilleure compréhension des phénomènes de bruit intrinsèques à ce système. Les deux types de bruit prépondérants étaient la conversion du bruit du laser (FM et AM) en bruit de phase RF par différentes non-linéarités (dont la photodiode) et le déclenchement d'effets non-linéaires optiques à l'intérieur du résonateur. Le contrôle de ces effets a permis en particulier d'éliminer des remontées importantes de bruit sur le spectre de l'oscillateur, et d'atteindre un niveau de bruit de phase de -128 dBc/Hz à 10 kHz de la porteuse à 10.2 GHz en utilisant un OEO à base d'un ARF passif de 100 mètres de longueur, optimisé et immunisé contre les effets non-linéaires optiques.Optics represents an elegant and reliable solution to generate high spectral purity microwave signals, especially the approach using the optoelectronic oscillator (OEO). The spectral purity of these sources is very important for space and military applications and also for time and frequency domain metrology. During this thesis, we have fabricated and studied many types of resonator based OEO in order to optimize the system phase noise. We have especially investigated an original approach using a passive fiber ring resonator (FRR). This resonator type can feature optical quality factors higher than 109 when only few meters of optical fibers are used (L ~ 10 m) and it can be easily integrated in a planar setup. Moreover, we have performed an important work on 3D WGM resonators based oscillators. In the FRR based OEO, spectacular progresses have been achieved thanks to a good understanding of the system intrinsic noise phenomena. Actually, we have found that the most important noise parameters were the laser FM and AM noise conversion into RF phase noise by means of different nonlinearities in the system (like the photodiode nonlinearity), but also by the generation of nonlinear optical effects inside the resonator. By controlling these effects, we have been able to reduce the OEO phase noise level and to reach a -128 dBc/Hz noise level at 10 kHz offset frequency from a 10.2 GHz carrier. This has been achieved using an OEO based on a 100m-long passive FRR, which has been optimized and immunized against different nonlinear optical effects

    Atmospheric Clock Transfer Based on Femtosecond Frequency Combs

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    Avoiding Aliasing in Allan Variance: an Application to Fiber Link Data Analysis

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    Optical fiber links are known as the most performing tools to transfer ultrastable frequency reference signals. However, these signals are affected by phase noise up to bandwidths of several kilohertz and a careful data processing strategy is required to properly estimate the uncertainty. This aspect is often overlooked and a number of approaches have been proposed to implicitly deal with it. Here, we face this issue in terms of aliasing and show how typical tools of signal analysis can be adapted to the evaluation of optical fiber links performance. In this way, it is possible to use the Allan variance as estimator of stability and there is no need to introduce other estimators. The general rules we derive can be extended to all optical links. As an example, we apply this method to the experimental data we obtained on a 1284 km coherent optical link for frequency dissemination, which we realized in Italy

    Characterization of optical frequency comb bades me asurements and spectral purity transfer for optical atomic clocks

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    Tesis doctoral inédita cotutelada por l´Université de La Sorbonne y por la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Física de Materiales. Fecha de lectura: 15-03-202

    Femtosecond combs for optical frequency metrology

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    This thesis is dedicated to femtosecond combs as a tool for optical frequency metrology and as an integral part of an optical clock. After an overview of optical frequency measurement techniques, the design of two frequency combs based on mode-locked femtosecond lasers as they were at the beginning of my project is described. The first comb is based on an Er:fibre laser operating at a central wavelength of 1550 nm with a repetition rate of 100 MHz. The second is a Ti:sapphire-laser-based comb operating at a central wavelength of 810 nm with a repetition rate of 87 MHz. Improvements to the original design of the Ti:sapphire comb are detailed in the next chapter. A novel f-to-2f self-referencing scheme based on a pair of Wollaston prisms and employing a PPKTP crystal for SHG results in up to 20 dB enhancement of the signal to noise ratio in the carrier-envelope offset frequency beat signal f0 and in up to 15 dB lower phase noise in the f0 beat signal compared to a Michelson interferometer based system. Next, the factors influencing the stability and accuracy of the microwave reference signal and the performance of two synthesisers used for the stabilisation of the frequency combs were investigated. It is shown that stability of the maser reference signal is reduced by the distribution system by factor of 1.5. A fractional frequency change of 4.1(0.7) × 10−16 (K/h)−1 was measured for the better of the two synthesisers (an IFR 2023A) indicating that for accurate frequency measurements the synthesiser signal should be monitored to enable systematic frequency corrections to be made. Finally, an absolute frequency measurement of the electric quadrupole clock transition in a frequency standard based on a single 171Yb+ trapped ion is described. The result f = 688 358 979 309 310 ± 9 Hz agrees with an independent measurement made by the PTB group within the uncertainty of the measurements

    Absolute frequency measurement of an 171Yb lattice clock and optical clock comparisons

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    The measurement of time and frequency is at the heart of many technological applications and scientific measurements alike. In fact, the SI-unit the second is by quite a margin the SI-unit with the best relative uncertainty (ca. 10^{-16}), given by the accuracies of Cs fountain clocks probing the F = 3 - F = 4 ground-state transition in 133Cs. Still, demands for even higher accuracy and especially stability (a Cs fountain needs up to two weeks for the statistics to reach its declared uncertainty) are uttered in support of technological advancements (e.g. geodesy and GNSS systems) as well as fundamental science (physics beyond the standard model, tests of relativity). Nowadays optical lattice clocks confining a large number of neutral atoms in Stark shift free optical traps (the Stark shift free condition is characterised by a so-called magic wavelength of the trap) propose good candidates for a future redefinition of the SI-second in terms of an optical transition. Their accuracy and stability already surpass the Cs-fountains by two and three orders of magnitude, respectively. With further improvements to be expected in the near future, the application of optical lattice clocks to relativistic gravimetry, quantum computing, quantum simulation and fundamental physics keeps evolving. This thesis describes the development and characterisation of an 171Yb lattice clock at INRIM as well as its first frequency measurement campaigns and technolo- gies towards improved optical frequency measurements. The lattice clock confines cold atoms in a 1D optical dipole trap at the magic wavelength, which also cancels any Doppler- and recoil-related effects on the ultra-narrow clock transition. The first chapter offers a general overview of the physics behind lattice clocks and opti- cal frequency measurements. In the second chapter the 171Yb lattice clock developed during this work is expounded, including the trapping, state-preparation and state-probing of ultracold atoms inside the optical lattice. An exhaustive uncertainty budget for the clock transition is given and discussed showing already a performance beyond state-of- the-art Cs fountain clocks. An absolute frequency measurement obtained during this work is laid out. The result represents the lowest uncertainty achieved in a measurement of this transition against a primary frequency standard so far and is in agreement with previous values obtained by other groups around the world. A proof-of-principle experiment demonstrating for the first time the feasibility of transportable optical lattice clocks for geodesy and metrology applications outside of laboratory environments is described in chapter three. This experiment was conducted in collaboration with PTB and NPL and included a geodetic measurement with a transportable optical lattice clock that agreed with conventional methods as well as an optical 171Yb-87Sr frequency ratio measurement, enlarging the database on this particular ratio and thereby contributing to a possible redefinition of the SI-unit the second in terms of an optical transition or frequency-ratio matrix in the future. The fourth chapter discusses improvements added to the Yb lattice clock after the aforementioned measurements, in particular the stabilisation of the cooling and trapping lasers on a single stable low-drift cavity using mirrors coated for three disparate wavelengths across the optical spectrum. The simultaneous offset sideband locking and a throughout characterisation of the cavity are discussed. The last chapter is about the characterisation and optimisation of the NPL universal oscillator, which was conducted during my secondment at the NPL research facilities in the UK. The universal oscillator consists out of a femtosecond frequency comb, an ultra stable master laser and six slave oscillators. The femtosecond comb is transferring the stability of the superior master oscillator cavity to all six slave oscillators, which includes five lasers ranging from the infrared to the visible region. The principle of operation is explained and the obtained high performance of the spectral purity transfer set forth and discussed. This experiment demonstrated an unprecedented spectral purity transfer performance in a multi-branch configuration, opening the way for the interrogation of whole clock ensembles by just one master oscillator

    Coherent fibre-optic link: applications in Time and Frequency metrology, Geodesy, Radio Astronomy and Seismology

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    L'abstract è presente nell'allegato / the abstract is in the attachmen

    Integrated optical frequency comb for 5G NR Xhauls

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    : We experimentally demonstrate the use of optical frequency combs (OFCs), generated by a photonic integrated circuit (PIC), in a flexible optical distribution network based on fiber-optics and free-space optics (FSOs) links, aimed at the fifth generation of mobile network (5G) Xhauls. The Indium Phosphide (InP) monolithically integrated OFC is based on cascaded optical modulators and is broadly tunable in terms of operating wavelength and frequency spacing. Particularly, our approach relies on applying the PIC in a centralized radio access network (C-RAN) architecture, with the purpose of optically generating two low-phase noise mm-waves signals for simultaneously enabling a 12.5-km of single-mode fiber (SMF) fronthaul and a 12.5-km SMF midhaul, followed by a 10-m long FSO fronthaul link. Moreover, the demonstrator contemplates two 10-m reach 5G wireless access networks operating in the 26 GHz band, i.e. over the frequency range 2 (FR2) from the 5G NR standard. The proposed integrated OFC-based 5G system performance is in accordance to the 3rd Generation Partnership Project (3GPP) Release 15 requirements, achieving a total wireless throughput of 900 Mbit/s
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