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- PublicationAccès libreGNSS-grade space atomic frequency standards: Current status and ongoing developments(2021)
; ; ; We present an overview on the current state of Global Navigation Satellite Systems (GNSS)-grade or better space atomic frequency standards’ (SAFS) technologies and discuss their applications. We estimate that a total of more than 1000 such standards were sent to space so far, the vast majority consisting of rubidium-cell frequency standards, Cs atomic beam frequency standards, and passive hydrogen masers. Finally, we review a variety of ongoing developments in view of future new generations of GNSS-grade SAFSs. - PublicationAccès libreMetrological and stability studies in high-performance rubidium vapor-cell atomic clocksCette thèse présente les études métrologiques et de stabilité des horloges atomiques compactes de haute performance basées sur une cellule à vapeur de rubidium (Rb) et la spectroscopie à double résonance (DR)1. Nous abordons les limites de la stabilité de fréquence d'horloge à court (à 1 s) et moyen-long (jusqu'à 1 jour) termes en utilisant deux différentes horloges à cellule à vapeur de Rb. Les études expérimentales des effets physiques qui perturbent la fréquence d'horloge et dégradent la stabilité à long terme sont réalisées pour une première horloge Rb à double résonance à pompage optique pulsée (POP-DR) au moyen d'une diode laser (LD) émettant à 780 nm. Une seconde horloge Rb exploitant un schéma DR à onde continue (CW-DR) est utilisée pour évaluer le potentiel d'application des lasers de télécommunications émettant à 1560 nm doublés en fréquence. Diverses applications, tel que les systèmes de navigation par satellite, la métrologie industrielle ou fondamentale, les télécommunications, qui nécessitent une référence de fréquence compacte exploitent les horloges à cellule à vapeur. En particuliers pour les systèmes de navigation par satellite, une taille compacte d'un volume de quelques litres et une haute performance de stabilité de fréquence autour de 1 10-14 à 1 jour (équivalent à 1 ns/jour) sont essentiels.
À long terme, les fluctuations des paramètres expérimentaux et environnementaux affectent la stabilité de fréquence des horloges par le biais de différents processus physiques. Les impacts individuels de ces processus sont caractérisés par un coefficient quantifiant la sensibilité de la fréquence d'horloge à un paramètre donné et par les fluctuations de ce dernier. Pour notre horloge POP-DR, nous avons mesuré une instabilité de fréquence plus bas que 2 10-14 jusqu'à 1 jour de temps d'intégration. La stabilité de la fréquence d'horloge est optimisée pour les échelles de temps à long terme. L'impact de l'effet Zeeman du second ordre limitant la stabilité d'horloge est réduit et celui des fluctuations de température est consolidé, aux niveaux de quelques 10-15 à 1 jour. Le bilan établi des sources d'instabilité à long terme présenté dans cette thèse indique que la contribution dominante provient des effets induits par la lumière.
La stabilité de fréquence à court terme des horloges à cellule à vapeur à pompage optique par un laser, est limitée par diverses contributions de bruit, parmi lesquelles le bruit du laser (intensité et/ou fréquence) est le plus souvent le facteur dominant. Pour réduire l'impact du bruit du laser, nous évaluons une source optique basée sur une LD de télécommunication à faible bruit émettant à 1560 nm. La fréquence d'émission de la LD de télécommunication est doublée à 780 nm et stabilisée en utilisant une cellule à vapeur de Rb. Le faible bruit d'intensité et surtout de fréquence de ce système laser présente des avantages significatifs pour les horloges à cellule de vapeur de Rb et de nombreuses autres applications de haute résolution qui manipulent des atomes Rb à 780 nm.
Afin d'améliorer la stabilité de fréquence d'horloge à court terme, nous avons implémenté le système laser optimisé basé sur la diode laser de télécommunication à faible bruit dans une horloge à cellule à vapeur de Rb exploitant un schéma d'interrogation CW-DR. Nous présentons une instabilité de la fréquence d'horloge à court terme de 2.5 10-13 à 1 s comparable aux meilleures horloges actuelles et établissons le bilan de bruit exhaustif en tenant compte des contributions du bruit du laser et de la source micro-onde. Le faible bruit de fréquence du laser de télécommunication permet d'assouplir les exigences relatives aux conditions de fonctionnement de l'horloge, notamment en ce qui concerne la fréquence du pompage optique et la température de la cellule à vapeur qui influencent la conversion du bruit de fréquence du laser en bruit d'intensité. Néanmoins, les fluctuations de la puissance optique à la sortie de 780 nm du système laser deviennent une source d'instabilité importante à 10 s, dégradant la stabilité d'horloge par l'effet de décalage de fréquence dû à l'intensité de la lumière.
Les résultats de cette thèse permettent d'exposer les limites de la stabilité de fréquence des prototypes d'horloges étudiés et de proposer de nouvelles approches pour les surmonter. Nous démontrons également que les références de fréquence optique basées sur un laser de télécommunication et doublage en fréquence bénéficiant de la stabilité de la fréquence des transitions atomiques Rb présentent un fort potentiel d'implémentation dans les applications d'horloges à cellules à vapeur de Rb et d'autres applications aux deux longueurs d'onde (1560 nm et 780 nm). Ces résultats sont bénéfiques pour le développement de la prochaine génération d'horloges compactes à cellule de vapeur de Rb utilisant une source laser et garantissant un niveau de stabilité de fréquence de 1 10-14 à 1 jour.
1 Ce travail de thèse a été réalisé au sein du Laboratoire Temps-Fréquence of the University of Neuchâtel. Ce travail a été soutenu par le Fonds National Suisse de la recherche scientifique (SNF) : "Precision double resonance spectroscopy and metrology with stabilised lasers and atomic vapours: applications for atomic clocks and magnetometers", n° 156621. Ce travail a également été financé par le programme de recherche et d'innovation Horizon 2020 de l'Union Européenne, dans le cadre de la convention de subvention n° 820393 (Quantum Technology Flagship, projet macQsimal).
Summary
This thesis presents the metrological and stability studies in compact and high-performance atomic clocks based on a rubidium (Rb) vapor cell and double-resonance (DR) spectroscopy1. We address the limitations of the clock frequency stability on short (at 1 s) and medium-to-long (up to 1 day) timescales using two separate Rb vapor-cell clocks. First, the experimental studies on the physical effects that perturb the clock frequency and degrade the long-term stability are performed for a pulsed optically pumped DR (POP-DR) Rb clock based on a laser diode (LD) emitting at 780 nm. Secondly, a Rb clock operated in a continuous-wave DR (CW-DR) scheme is used to assess the application potential of frequency-doubled telecom lasers initially emitting at 1560 nm. Various applications, such as satellite-based navigation systems, industrial or fundamental metrology, telecommunications, that require a compact frequency reference widely exploit the vapor-cell clocks. In particular for satellite navigation systems, a compact size with a volume of few liters and a high-frequency stability performance at the level of 110-14 at 1 day (equivalent to 1 ns/day) are essential.
On long-term timescales, the fluctuations of the experimental and environmental parameters impact the clock frequency stability via various physical processes. The individual impacts of these processes are characterized by a coefficient quantifying the clock frequency sensitivity to the fluctuating parameter and the fluctuations of the sensitive parameter. Using our POP-DR clock, we measured a frequency instability below 2 10-14 up to 1 day of averaging time. The clock frequency stability is optimized for long-term timescales. The impact of the second-order Zeeman effect limiting the clock stability is reduced to and that of the temperature fluctuations is consolidated at the level of few 10-15 at 1 day. The established long-term instability budget presented in this thesis indicates that the dominant contribution arises from light-induced effects.
The short-term frequency stability of laser-pumped vapor-cell clocks is limited by several noise contributions, among which the laser (intensity and/or frequency) noise is most often the dominant factor. To reduce the impact of the laser noise, we evaluate an optical source based on a low-noise telecom LD emitting at 1560 nm. The emission frequency of the telecom LD is doubled to 780 nm and stabilized using a Rb vapor cell. The lower intensity noise and particularly the lower frequency noise of this laser system present significant advantages for Rb vapor-cell clocks and many other high-resolution applications that manipulate Rb atoms at 780 nm.
In order to improve the short-term clock frequency stability, we implemented the optimized laser system based on the low-noise telecom laser diode in a Rb vapor-cell clock operating in a CW-DR scheme. We present a short-term clock frequency instability of 2.510-13 at 1 s, at the level of the state-of-the-art and established the exhaustive noise budget including the contributions from the laser and the microwave source noise. The low-frequency noise of the telecom laser allows for relaxed requirements regarding the clock operation conditions, particularly regarding the optical-pump frequency and the vapor-cell temperature that affect the laser frequency noise conversion to intensity noise. Nevertheless, the optical power fluctuations at the 780-nm output of the laser system become an important instability source at 10 s, degrading the clock stability via the intensity light-shift effect.
The results of this thesis serve to expose the limits of the frequency stability of the studied clock prototypes and to instigate new approaches to overcome these limitations. We also demonstrate that the optical frequency references based on a frequency-doubled telecom laser that benefits from the stability of Rb atomic transition frequencies present a high implementation potential in Rb vapor-cell clock applications, as well as for other applications at both wavelengths (1560 nm and 780 nm). These outcomes are beneficial for the development of the next generation of compact Rb vapor-cell clocks using a laser source and ensuring a frequency stability performance at the level of 1 10-14 at 1 day. 1 This thesis work was done in the Laboratoire Temps-Fréquence of the University of Neuchâtel. This work was supported by the Swiss National Science Foundation (SNF): “Precision double resonance spectroscopy and metrology with stabilised lasers and atomic vapours: applications for atomic clocks and magnetometers,” n° 156621. This work also received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement no. 820393 (Quantum Technology Flagship, project macQsimal). - PublicationMétadonnées seulementLong-Term Stability Analysis Towards < 10-14 Level for a Highly Compact POP Rb Cell Atomic Clock(2019-4-14)
; ; ; ; Long-term frequency instabilities in vapor-cell clocks mainly arise from fluctuations of the experimental and environmental parameters that are converted to clock frequency fluctuations via various physical processes. Here, we discuss the frequency sensitivities and the resulting stability limitations at one day timescale for a rubidium vapor-cell clock based on a compact magnetron-type cavity operated in air (no vacuum environment). Under ambient laboratory conditions, the external atmospheric pressure fluctuations may dominantly limit the clock stability via the barometric effect. We establish a complete long-term instability budget for our clock operated under stable pressure conditions. Where possible, the fluctuations of experimental parameters are measured via the atomic response. The measured clock instability of < 2·E10.14 at one day is limited by the intensity light-shift effect, which could further be reduced by active stabilization of the laser intensity or stronger optical pumping. The analyses reported here show the way towards simple, compact, and low-power vapor-cell atomic clocks with excellent long-term stabilities. ≤ 10.14 at one day when operated in ambient laboratory conditions. - PublicationAccès libreLong-Term Stability Analysis Towards <10-14 Level for a Highly Compact POP Rb Cell Atomic ClockLong-term frequency instabilities in vapor-cell clocks mainly arise from fluctuations of the experimental and environmental parameters that are converted to clock frequency fluctuations via various physical processes. Here, we discuss the frequency sensitivities and the resulting stability limitations at one-day timescale for a rubidium vapor-cell clock based on a compact magnetron-type cavity operated in air (no vacuum environment). Under ambient laboratory conditions, the external atmospheric pressure fluctuations may dominantly limit the clock stability via the barometric effect. We establish a complete longterm instability budget for our clock operated under stable pressure conditions. Where possible, the fluctuations of experimental parameters are measured via the atomic response. The measured clock instability of <2 × 10-14 at one day is limited by the intensity light-shift effect, which could further be reduced by active stabilization of the laser intensity or stronger optical pumping. The analyses reported here show the way toward simple, compact, and low-power vapor-cell atomic clocks with excellent long-term stabilities ≤10-14 at one day when operated in ambient laboratory conditions.
- PublicationAccès libreRb vapor-cell clock demonstration with a frequency-doubled telecom laserWe employ a recently developed laser system, based on a low-noise telecom laser emitting around 1.56 μm, to evaluate its impact on the performance of an Rb vapor-cell clock in a continuous-wave double-resonance scheme. The achieved short-term clock instability below 2.5·10−13·τ−1∕2 demonstrates, for the first time, the suitability of a frequency-doubled telecom laser for this specific application. We measure and study quantitatively the impact of laser amplitude and frequency noises and of the ac Stark shift, which limit the clock frequency stability on short timescales. We also report on the detailed noise budgets and demonstrate experimentally that, under certain conditions, the short-term stability of the clock operated with the low-noise telecom laser is improved by a factor of three compared to clock operation using the direct 780-nm laser.
- PublicationAccès libreCharacterization of Frequency-Doubled 1.5-μm Lasers for High-Performance Rb ClocksWe report on the characterization of two fiber-coupled 1.5- μm diode lasers, frequency-doubled and stabilized to Rubidium (Rb) atomic resonances at 780 nm. Such laser systems are of interest in view of their implementation in Rb vaporcell atomic clocks, as an alternative to lasers emitting directly at 780 nm. The spectral properties and the instabilities of the frequency-doubled lasers are evaluated against a state-of-the-art compact Rb-stabilized laser system based on a distributed-feedback laser diode emitting at 780 nm. All three lasers are frequency stabilized using essentially identical Doppler-free spectroscopy schemes. The long-term optical power fluctuations at 780 nm are measured, simultaneously with the frequency instability measurements done by three beat notes established between the three lasers. One of the frequency-doubled laser systems shows at 780 nm excellent spectral properties. Its relative intensity noise <10−12 Hz−1 is one order of magnitude lower than the reference 780-nm laser, and the frequency noise <106 Hz2/Hz is limited by the laser current source. Its optical frequency instability is <4 × 10−12 at τ = 1 s, limited by the reference laser, and better than 1 × 10−11 at all timescales up to one day. We also evaluate the impact of the laser spectral properties and instabilities on the Rb atomic clock performance, in particular taking into account the light-shift effect. Optical power instabilities on long-term timescales, largely originating from the frequency-doubling stage, are identified as a limitation in view of high-performance Rb atomic clocks.
- PublicationAccès libreImpact of microwave-field inhomogeneity in an alkali vapour cell using Ramsey double-resonance spectroscopyWe numerically and experimentally evaluate the impact of the inhomogeneity of the microwave field in the cavity used to perform double-resonance (DR) Ramsey spectroscopy in a buffer gas alkali vapour cell. The Ramsey spectrum is numerically simulated using a simple theoretical model and taking into account the field distribution in a magnetron-type microwave resonator. An experimental evaluation is performed using a DR pulsed optically pumped (POP) atomic clock. It is shown that the sensitivity to the micro-wave power of the DR POP clock can be reproduced from the combination of two inhomogeneities across the vapour cell: microwave field inhomogeneity and atomic ground-state resonance frequency inhomogeneity. Finally, we present the existence of an optimum operation point for which the microwave power sensitivity of our DR POP clock is reduced by two orders of magnitude. It leads into a long-term frequency stability of 1 × 10-14.