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Gharavipour, Mohammadreza
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Gharavipour, Mohammadreza
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- 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 libreRamsey spectroscopy in a Rubidium vapor cell and realization of an ultra-stable atomic clock(2018)
; Divers domaines d’application dans l’industrie, les télécommunications, la navigation et l’espace exigent des étalons de fréquence fiables, compacts et performants avec un niveau de stabilité de <1×10−14 à 105s (équivalent à <1 ns/jour). Avec la technologie des lasers à semi-conducteurs, la technique du pompage optique par laser a ouvert de nouveaux schémas d’interrogation basés sur la double résonance (DR) laser et micro-ondes comme le pompage optique continu (CW) et le pompage optique pulsé (POP) qui sont utilisé pour faire de nouvelle horloges atomiques à cellules.
Dans cette thèse, nous présentons les performances d’un prototype de laboratoire d’horloge atomique à cellule à vapeur de Rubidium (Rb) fonctionnant avec un schéma Ramsey-DR (basé sur POP). L’horloge utilise une cavité micro-ondes compacte de type magnétron avec un volume de seulement 45 cm3 et un faible facteur de qualité (≈ 150). Le schéma Ramsey-DR utilise deux champs électromagnétiques résonants pour interroger les atomes - le champ optique pour polariser une population d’atomes par pompage optique, et le champ micro-ondes pour interroger la transition hyperfine de l’état fondamental qui est la fréquence atomique de référence. Les impulsions optiques et micro-ondes sont séparées dans le temps dans le schéma Ramsey-DR ; par conséquent, l’effet de biais de fréquence du à la lumière (light shift LS) peut être fortement réduit, ce qui améliore la stabilité de l’horloge. La cavité micro-ondes de type magnétron est conçue, développée et construite en collaboration avec le Laboratoire d’Electro- Magnétique et d’Acoustique (LEMA) de l’Ecole Polytechnique Fédérale de Lausanne (EPFL)1. Une cellule de verre nouvellement fabriquée au LTF avec un volume queusot 10 fois plus petit que la version précédente est remplie de 87Rb et de gaz tampon d’Argon et Azote. Un queusot plus petit réduit le coefficient de température queusot d’environ un ordre de grandeur, ce qui a été un facteur limitant pour la stabilité de l’horloge à moyen et long terme.
Les caractérisations et performances détaillées du signal d’horloge (frange centrale du signal Ramsey) sont présentées dans cette étude2. Nous obtenons un signal d’horloge avec un contraste allant jusqu’à environ 35% et une largeur à mi-hauteur d’environ 160 Hz obtenu en optimisant les différents paramètres impliqués dans le schéma Ramsey-DR. Avec notre cavité plus petite, ces réalisations ne sont pas triviales, car les exigences élevées en matière d’homogénéité sur l’ensemble du volume de la cellule sont plus difficiles à satisfaire. Dans cette étude, on obtient une stabilité à court terme (1 s à 100 s) de 2.4×10-13τ−1/2 ce qui est comparable à l’état-de-l’art en utilisant le schéma CW-DR et/ou en utilisant le schéma POP avec une cavité micro-ondes TE011 plus grande avec un facteur de qualité plus élevé. Le biais de fréquence dû à la lumière (light shift) est quantifié dans notre horloge atomique Ramsey-DR Rb. De plus, nous présentons un modèle préliminaire basé sur le modèle du light-shift en mode continue (CW-DR-LS) et estimons l’impact de l’intensité de la lumière (intensity light shift) dans le schéma Ramsey-DR. De plus, une nouvelle expression analytique est développée pour prédire la stabilité à court terme de l’horloge en considérant la durée de détection optique dans le schéma Ramsey-DR. A partir de cette formule, nous estimons également le meilleur temps Ramsey pour optimiser la stabilité à court terme de l’horloge.
Cette thèse contient en outre une étude plus fondamentale sur les temps de relaxation de la population et de la cohérence (T1 et T2, respectivement) de la "transition d’horloge" 87Rb. Cette étude a été réalisée en collaboration avec l’Institut de physique de Belgrade (Université de Belgrade)3. Les temps de relaxation sont une donnée importante de notre horloge atomique Rb, car ils limitent le "temps de Ramsey" utilisable dans le schéma Ramsey-DR. Une méthode expérimentale de Echo de Spin Optiquement Détecté (Optically-Detected Spin-Echo ODSE), inspirée de l’écho de spin classique de la résonance magnétique nucléaire, est développée pour mesurer les temps de relaxation du rubidium 87 dans notre cellule. La méthode ODSE permet d’accéder au T2 intrinsèque (spécifique pour la transition d’horloge) en supprimant la décohérence résultant de l’inhomogénéité du champ C à travers la cellule. Le T2 mesuré avec la méthode ODSE est en accord avec la prédiction théorique.
Ce travail a été réalisé au Laboratoire Temps-Fréquence de l’Université de Neuchâtel, en collaboration avec l’EPFL-LEMA pour la cavité micro-ondes de type magnétron, l’Istituto Nazionale di Ricerca Metrologica (INRIM) pour l’oscillateur local (LO) et l’Institut de physique de l’Université de Belgrade pour les mesures des temps de relaxation.
1 Dans le cadre des projects du Fonds National Suisse (FNS):"Microwave Cavities for High Performance Double Resonance Atomic Clocks and Sensors" no. 140712 (2012-2015) et "Study of microwave cavities for high performance pulse pumped double resonance atomic clocks" no. 162346 (2015-2018).
2 Dans le cadre du projet MClocks: "Compact and High-Performing Microwave Clocks for Industrial Applications", EMRP (European Metrology Research Programme, Programme of Euramet) project IND55-Mclocks (2013-2016). Le EMRP est conjointement financé les pays participant au EMRP qui dépend de EURAMET de l’union européen.
3 Dans le cadre du project FNS (SCOPES): "Ramsey spectroscopy in Rb vapor cells and application to atomic clocks" no. 152511 (2014-2018)., Various application fields in industry, telecommunication, navigation and space demand reliable, compact, high-performance frequency standards with a stability level of <1×10−14 at 105s (equivalent to <1ns/day). Thanks to semiconductor technology, optical pumping technique with a laser has opened up new schemes based on laser-microwave double-resonance (DR), such as continuous-wave (CW) and pulsed optical pumping (POP) to operate vapor cell atomic clocks.
In this thesis, we demonstrate the performances of a vapor cell Rubidium (Rb) atomic clock operating in a Ramsey-DR (based on POP) scheme in an ambient laboratory using a compact magnetron-type microwave cavity with a volume of only 45 cm3 and a low quality factor of ≈ 150. The Ramsey-DR scheme involves two resonant electromagnetic fields to interrogate the atoms - the optical field to polarize a population of atoms by optical pumping, and the microwave field to drive the ground-state hyperfine clock transition that serves as an atomic frequency reference. The applied optical and microwave pulses are separated in time in the Ramsey-DR scheme; therefore, the light shift (LS) effects can be strongly reduced which results in improving the clock stability. The magnetron-type microwave cavity is designed, developed and built in collaboration with Laboratory of Electro Magnetics and Acoustics (LEMA) at Ecole Polytechnique Fédérale de Lausanne (EPFL)1. A newly homemade vapor cell with a 10 times smaller stem volume compared to the previous contains 87Rb and buffer gases of Argon and Nitrogen. The smaller stem results in reducing the stem temperature coefficient by about one order of magnitude, which has been a limiting factor for the medium- to long-term scales clock stability.
Detailed characterizations and performances of the clock signal (Ramsey central fringe) are presented in this study2. We obtain a clock signal with a contrast up to approximately 35% and a linewidth of approximately 160 Hz by optimizing the various parameters involved in the Ramsey-DR scheme. In our smaller cavity, these achievements are not trivial, because of the high requirements on field homogeneity over the entire cell volume are more challenging to meet. In this work, a short-term stability (1 s to 100 s) of 2.4×10-13τ−1/2 is achieved which is comparable to the state-of-the-art results using the CW-DR scheme and/or using the POP scheme with a larger TE011 microwave cavity with a higher quality factor. The LS effect is quantified in our Ramsey-DR Rb atomic clock. In addition, we present a preliminary model based on the CW-DR LS theory and estimate the intensity LS coefficient in the Ramsey-DR scheme. Moreover, a new analytical expression is developed to predict the clock’s short-term stability by considering the optical detection duration in the Ramsey-DR scheme. From this formula, we also estimate the best Ramsey time to improve the short-term stability of the clock.
This thesis, in addition, contains a more fundamental investigation on the measurements of the population and coherence relaxation times (T1 and T2, respectively) of the 87Rb "clock transition". This study has been performed in collaboration with the Institute of Physics Belgrade (University of Belgrade)3. These relaxation times are relevant for our Rb atomic clock, since they limit the usable "Ramsey time" in the Ramsey-DR scheme. An experimental method of Optically-Detected Spin-Echo (ODSE), inspired by classical nuclear magnetic resonance spin-echo, is developed to measure the ground-state relaxation times of 87Rb atoms held in our buffer gas vapor cell. The ODSE method enables accessing the intrinsic (T2 (specific for the clock transition) by suppressing the decoherence arising from the inhomogeneity of the C-field across the vapor cell. The measured T2 with the ODSE method is in good agreement with the theoretical prediction.
This work has been done at the Laboratoire Temps-Fréquence of University of Neuchâtel, in col- laboration with the EPFL-LEMA for the magnetron-type microwave cavity, Istituto Nazionale di Ricerca Metrologica (INRIM) that provided the Local Oscillator (LO) and Physics Institute of Belgrade University for relaxation times measurements.
1 Within the projects Fonds National Suisse (FNS):"Microwave Cavities for High Performance Double Resonance Atomic Clocks and Sensors" no. 140712 (2012-2015) and "Study of microwave cavities for high performance pulse pumped double resonance atomic clocks" no. 162346 (2015-2018).
2 Within the MClocks project: "Compact and High-Performing Microwave Clocks for Industrial Applications", EMRP (European Metrology Research Programme, Programme of Euramet) project IND55-Mclocks (2013-2016). The EMRP is jointly funded by the EMRP participating countries within EURAMET and the European Union.
3 Within the project FNS (SCOPES): "Ramsey spectroscopy in Rb vapor cells and application to atomic clocks" no. 152511 (2014-2018). - PublicationMétadonnées seulementOptically-detected spin-echo method for relaxation times measurements in a Rb atomic vapor(2017-6-26)
; ; ; ; - PublicationMétadonnées seulementDouble-resonance spectroscopy in Rubidium vapour-cells for high performance and miniature atomic clocks(2017-2-16)
; ; ; Kang, SWe report our studies on using microwave-optical double-resonance (DR) spectroscopy for a high-performance Rb vapour-cell atomic clock in view of future industrial applications. The clock physics package is very compact with a total volume of only 0.8 dm3. It contains a recently in-house developed magnetron-type cavity and a Rb vapour cell. A homed-made frequency-stabilized laser system with an integrated acousto-optical-modulator (AOM) – for switching and controlling the light output power– is used as an optical source in a laser head (LH). The LH has the overall volume of 2.5 dm3 including the laser diode, optical elements, AOM and electronics. In our Rb atomic clock two schemes of continuous-wave DR and Ramsey-DR schemes are used, where the latter one strongly reduces the light-shift effect by separation of the interaction of light and microwave. Applications of the DR clock approach to more radically miniaturized atomic clocks are discussed. - PublicationAccès libreHigh performance vapour-cell frequency standardsWe report our investigations on a compact high-performance rubidium (Rb) vapour-cell clock based on microwave-optical double-resonance (DR). These studies are done in both DR continuous-wave (CW) and Ramsey schemes using the same Physics Package (PP), with the same Rb vapour cell and a magnetron-type cavity with only 45 cm3 external volume. In the CW-DR scheme, we demonstrate a DR signal with a contrast of 26% and a linewidth of 334 Hz; in Ramsey-DR mode Ramsey signals with higher contrast up to 35% and a linewidth of 160 Hz have been demonstrated. Short-term stabilities of 1.4×10-13 τ-1/2 and 2.4×10-13 τ-1/2 are measured for CW-DR and Ramsey-DR schemes, respectively. In the Ramsey-DR operation, thanks to the separation of light and microwave interactions in time, the light-shift effect has been suppressed which allows improving the long-term clock stability as compared to CW-DR operation. Implementations in miniature atomic clocks are considered.
- PublicationMétadonnées seulementStability limitations from optical detection in Ramsey-type vapour-cell atomic clocks(2015-10-29)
; ; ; In today's state of the art compact vapour-cell atomic clocks relying on the pulsed Ramsey-type interrogation, optical detection noise is a major limitation to the achievable short-term stability. In this communication, the influence of the optical detection time on the clock's short-term stability is investigated and a new analytical expression is developed to precisely predict the stability performance, taking into account the details of the optical detection phase of a Ramsey-type atomic clock. The theory is in good agreement with the experimental results. It is applied for evaluating the clock's shot-noise limit. - PublicationMétadonnées seulementHigh performance vapour-cell frequency standards(: Journal of Physics: Conference Series 723, 2015-10-27)
; ; ; ; ; ; We report our investigations on a compact high-performance rubidium (Rb) vapour-cell clock based on microwave-optical double-resonance (DR). These studies are done in both DR continuous-wave (CW) and Ramsey schemes using the same Physics Package (PP), with the same Rb vapour cell and a magnetron-type cavity with only 45 cm3 external volume. In the CW-DR scheme, we demonstrate a DR signal with a contrast of 26% and a linewidth of 334 Hz; in Ramsey-DR mode Ramsey signals with higher contrast up to 35% and a linewidth of 160 Hz have been demonstrated. Short-term stabilities of 1.4×10^-13 τ^-1/2 and 2.4×10^-13 τ^-1/2 are measured for CW-DR and Ramsey-DR schemes, respectively. In the Ramsey-DR operation, thanks to the separation of light and microwave interactions in time, the light-shift effect has been suppressed which allows improving the long-term clock stability as compared to CW-DR operation. Implementations in miniature atomic clocks are considered. - PublicationAccès libreDemonstration of a high-performance pulsed optically pumped Rb clock based on a compact magnetron-type microwave cavity(2015-3-12)
; ; ; ; We demonstrate a high-performance pulsed optically pumped (POP) Rb vapor-cell clock based on a magnetron-type microwave cavity of only 44 cm3 external volume. Using optical detection, an unprecedented 35% contrast of the Ramsey signal has been obtained. Both the signal-to-noise ratio (of 30 000) and the estimated shot-noise limit of 1.7 × 10−14 τ−1/2 are at the same level as those found with a bigger cylindrical TE011 cavity (100 cm3 inner volume) and are sufficient for achieving excellent clock stability. Rabi oscillations are measured and indicate a sufficiently uniform microwave magnetic field distribution inside the cavity. The instability sources for the POP clock's performance are analyzed. A short-term stability of 2.1 × 10−13 τ−1/2 is demonstrated which is consistent with the noise budget. - PublicationAccès libreStability limitations from optical detection in Ramsey-type vapour-cell atomic clocksIn today’s state of the art compact vapour-cell atomic clocks relying on the pulsed Ramsey-type interrogation, optical detection noise is a major limitation to the achievable short-term stability. In this communication, the influence of the optical detection time on the clock’s short-term stability is investigated and a new analytical expression is developed to precisely predict the stability performance, taking into account the details of the optical detection phase of a Ramsey-type atomic clock. The theory is in good agreement with the experimental results. It is applied for evaluating the clock’s shot-noise limit.
- PublicationAccès libreDemonstration of a high-performance pulsed optically pumped Rb clock based on a compact magnetron-type microwave cavityWe demonstrate a high-performance pulsed optically pumped (POP) Rb vapor-cell clock based on a magnetron-type microwave cavity of only 44 cm3 external volume. Using optical detection, an unprecedented 35% contrast of the Ramsey signal has been obtained. Both the signal-to-noise ratio (of 30 000) and the estimated shot-noise limit of 1.7×10-14τ-1/2 are at the same level as those found with a bigger cylindrical TE011 cavity (100 cm3 inner volume) and are sufficient for achieving excellent clock stability. Rabi oscillations are measured and indicate a sufficiently uniform microwave magnetic field distribution inside the cavity. The instability sources for the POP clock’s performance are analyzed. A short-term stability of 2.1×10-13τ-1/2 is demonstrated which is consistent with the noise budget.