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Metrological characterization of custom-designed 894.6 nm VCSELs for miniature atomic clocks
First-order cancellation of the Cs clock frequency temperature-dependence in Ne-Ar buffer gas mixture
Through the detection of Coherent Population Trapping (CPT) resonances, we demonstrate the temperature-dependence cancellation of the Cs clock frequency in microfabricated vapor cells filled with a mixture of Ne and Ar. The inversion temperature at which the Cs clock frequency temperature sensitivity is greatly reduced only depends on the partial pressure of buffer gases and is measured to be lower than 80±C as expected with simple theoretical calculations. These results are important for the development of state-of-the-art Cs vapor cell clocks with improved long-term frequency stability.
ac Stark shift in double resonance and coherent population trapping in a wall-coated cell for compact Rb atomic clocks
We present a comparative study of the light-shifts (ac Stark shift) in a Rb vapour cell using two possible schemes for Rb atomic clocks: double resonance (DR) and coherent population trapping (CPT). For both schemes, the same wall-coated cell in a compact atomic resonator was used. The light-shift resulting from a monochromatic (DR) or a non-monochromatic (CPT) optical excitation was measured as a function of the laser intensity and the laser frequency and compared with existing theoretical results.
Light-shift and temperature-shift studies in atomic clocks based on coherent population trapping
Les horloges atomiques à cellules exploitent une transition de l’état fondamental d’un atome alcalin afin d’obtenir une référence de fréquence stable. La cellule contenant ces atomes est au le cœur de ce type d’horloges. Une horloge atomique à cellule peut être réalisée selon deux principes différents: La Double Résonnance (DR) et le Piégeage Cohérent des Populations (CPT). Dans le cas de la DR, les atomes sont pompés optiquement dans l’état désiré, puis interrogés par un champ micro-onde résonnant avec la transition dite “horloge”. Dans le cas de la CPT, les deux états fondamentaux de l’atome alcalin sont couplés à un état excité commun en utilisant deux champs électromagnétiques cohérents (ici, la fréquence de transition micro-onde est appliquée à travers la différence de fréquence entre les deux champs électromagnétiques cohérents). Les atomes sont ainsi «piégés» dans une superposition cohérente des deux états fondamentaux, appelée «état noir». La fréquence d’une horloge atomique à cellule est sensibile aux variations de divers paramètres, tels que: les propriétés de la lumière utilisée pour l’excitation (à travers l’effet du «déplacement radiatif»), de la pression du gaz tampon dans la cellule, de la température de la cellule, etc. Il est donc très important de stabiliser tous ces paramètres et de trouver une configuration qui minimise la sensibilité de l’horloge aux variations de ces paramètres.
Cette thèse décrit l’étude de deux horloges atomiques de laboratoire, basées sur la DR et la CPT, utilisant des cellules de tailles et contenus différents. L’impact de la source lumineuse y est également étudié. La partie principale de cette thèse se concentre sur l’étude du déplacement radiatif et du décalage en température (temperature-shift) des deux types d’horloges atomiques. Ces études incluent un développement théorique ainsi que des mesures expérimentales de ces phénomènes, montrant une bonne correspondance entre eux. Deux nouvelles approches de suppression de ces taux de variation de fréquence sont proposées pour les horloges CPT. Le premiére est la suppression du déplacement radiatif en fonction de la température de la cellule. La seconde est la suppression du taux de variation de fréquence en température. La suppression du taux variation de fréquence en température est obtenue à une température coïncidant avec la température de la cellule à laquelle le déplacement radiatif d’intensité est supprimé. De plus, cette température se trouve est dans une gamme favorable à l’opération de l’horloge. La stabilité en fréquence des horloges DR et CPT est mesurée avec deux types de cellules: des cellules avec gaz tampon et des cellules avec revêtement. La première mesure à long-terme de stabilité en fréquence avec une cellule à revêtement est présentée dans cette thèse. Les résultats obtenus permettront la réalisation de nouvelles horloges atomiques miniatures basées sur le principe CPT, ayant des stabilités en fréquence de l’ordre de 10-11 à 10 000s, ce qui permettra de produire une nouvelle génération d’instruments portables pour le positionnement, la navigation, la synchronization des réseaux de distribution d’energie (smart-grids) ou encore les télécommunications. De plus, les résultats discutés dans cette thèse ont ètè utilisés dans l’élaboration de la première horloge atomique miniature Européenne., Vapor-cell atomic clocks exploit the ground-state microwave transition in an alkali atom to provide a stable frequency reference. At the heart of a conventional vapor-cell atomic clock is a cell containing the alkali atoms. A vapor-cell atomic clock can be realized using two different basic principles: Double Resonance (DR) and Coherent Population Trapping (CPT). In DR, the alkali atoms are optically pumped in order to be placed in the desired states, and the microwave resonances are excited by a microwave field resonant with the clock transition. In the CPT principle, the two ground-state atomic levels of the alkali atom are coupled to a common excited state, using two coherent electromagnetic fields (here, the microwave transition frequency is present in the frequency difference of the two coherent electromagnetic fields). The atoms are trapped in a coherent superposition of the ground states, called a dark state. The frequency of the vapor-cell atomic clocks can shift due to various residual variations such as: the properties of the light field (through the light-shift effect), the buffer gas pressure in the resonance cell, the temperature of the clock cell, and others. It is therefore critical to carefully stabilize all these parameters and to find a clock operational scheme that minimizes the sensitivity of the clock frequency towards them.
This thesis presents the studies on two laboratory atomic clocks: based on DR and CPT, using different vapor-cell content and size and different laser wavelengths. The main part of this thesis is focused on the study of the light-shift and temperature-shift phenomena in DR and CPT atomic clocks. These studies include theoretical developments and experimental measurements of these phenomena, showing a good agreement between them. Two novel approaches for the CPT vapor-cell atomic clock frequency shift suppression are proposed. The first is on the light-shift suppression as function of the miniature vapor-cell temperature. The second is on the temperature-shift suppression in the miniature single buffer gas vapor-cell. This temperature-shift suppression is obtained for the temperature that coincides with the cell temperature for suppressed intensity light-shift and it is also in the range of suitable temperatures for clock operation. DR and CPT clocks frequency stabilities are measured using both: buffer gas and wall-coated cell. A first ever wall-coated clock long-term frequency stability measurement is shown here. These results potentially provide possibilities for a number of applications like realization of novel CPT miniature atomic clocks with frequency stability of ~10-11 at 10 000s, which will enable a new generation of portable instruments for positioning, navigation, smart-grid synchronization and telecommunication applications. Also, the results discussed in this thesis are used in devising the first European integrated miniature atomic clock.
Quadratic dependence on temperature of Cs 0-0 hyperfine resonance frequency in single Ne buffer gas microfabricated vapour cell
Presented is the observation of a quadratic temperature dependence of the Cs 0-0 ground state hyperfine resonance frequency in a single Neon (Ne) buffer gas vapour microcell. The inversion temperature, expected to be theoretically independent of the buffer gas pressure, is measured to be about 80-C for two different samples. A proposal to develop chip scale atomic clocks with improved long-term frequency stability, simpler configuration (a single buffer gas instead of a buffer gas mixture) and then relaxed constraints on pressure accuracy during the cell filling procedure is presented.
AC Stark-shift in CPT-based Cs miniature atomic clocks
We report on studies on the light-shift in caesium miniature atomic clocks based on coherent population trapping (CPT) using a micro-fabricated buffer-gas cell (MEMS cell). The CPT signal is observed on the Cs D1-line by coupling the two hyperfine ground-state Zeeman sublevels involved in the clock transition to a common excited state, using two coherent electromagnetic fields. These light fields are created with a distributed feedback laser and an electro-optical modulator. We study the light-shift phenomena at different cell temperatures and laser wavelengths around 894.6 nm. By adjusting the cell temperature, conditions are identified where a miniature CPT atomic clock can be operated with simultaneously low temperature coefficient and suppressed light-shift. The impact of the light-shift on the clock frequency stability is evaluated. These results are relevant for improving the long-term frequency stability of CPT-based Cs vapour-cell clocks.
Ac Stark shift in double resonance and coherent population trapping in a wall-coated cell for compact Rb atomic clocks
We present a comparative study of the light-shifts (ac Stark shift) in a Rb vapour cell using two possible schemes for Rb atomic clocks: double resonance (DR) and coherent population trapping (CPT). For both schemes, the same wall-coated cell in a compact atomic resonator was used. The light-shift resulting from a monochromatic (DR) or a non-monochromatic (CPT) optical excitation was measured as a function of the laser intensity and the laser frequency and compared with existing theoretical results
Metrological characterization of custom-designed 894.6 nm VCSELs for miniature atomic clocks
We report on the characterization and validation of custom-designed 894.6 nm vertical-cavity surface-emitting lasers (VCSELs), for use in miniature Cs atomic clocks based on coherent population trapping (CPT). The laser relative intensity noise (RIN) is measured to be 1×10−11 Hz−1 at 10 Hz Fourier frequency, for a laser power of 700 μW. The VCSEL frequency noise is 1013 ƒ −1 Hz2/Hz in the 10 Hz < ƒ < 105 Hz range, which is in good agreement with the VCSEL’s measured fractional frequency instability (Allan deviation) of ≈ 1 × 10−8 at 1 s, and also is consistent with the VCSEL’s typical optical linewidth of 20–25 MHz. The VCSEL bias current can be directly modulated at 4.596 GHz with a microwave power of −6 to +6 dBm to generate optical sidebands for CPT excitation. With such a VCSEL, a 1.04 kHz linewidth CPT clock resonance signal is detected in a microfabricated Cs cell filled with Ne buffer gas. These results are compatible with state-of-the-art CPT-based miniature atomic clocks exhibiting a short-term frequency instability of 2–3×10−11 at τ = 1 s and few 10−12 at τ = 104 s integration time.