Dick effect in a pulsed atomic clock using Coherent Population Trapping

The Dick effect can be a limitation of the achievable frequency stability of a passive atomic frequency standard when the ancillary frequency source is only periodically sampled. Here we analyze the Dick effect for a pulsed vapor cell clock using coherent population trapping (CPT). Due to its specific interrogation process without atomic preparation nor detection outside of the Ramsey pulses, it exhibits an original shape of the sensitivity function to phase noise of the oscillator. Numerical calculations using a three-level atom model are successfully compared with measurements; an approximate formula of the sensitivity function is given as an easy-to-use tool. A comparison of our CPT clock sensitivity to phase noise with a clock of the same duty cycle using a two-level system reveals a higher sensitivity in the CPT case. The influence of a free-evolution time variation and of a detection duration lengthening on this sensitivity is studied. Finally this study permitted to choose an adapted quartz oscillator and allowed an improvement of the clock fractional frequency stability at the level of 3.2x10-13 at 1

Tunable high-purity microwave signal generation from a dual-frequency VECSEL at 852 nm (orale)

International audienceWe demonstrate the dual-frequency emission of a diode-pumped vertical external-cavity semiconductor laser operating at 852 nm, dedicated to the coherent population trapping of cesium atoms for compact atomic frequency references. It is based on a single laser cavity sustaining the oscillation of two adjacent, cross-polarized, modes. The output power reaches 10 mW on each frequency. The frequency difference and the absolute laser frequencies are simultaneously precisely tuned and stabilized on external references, resulting in the generation of a high-purity optically-carried microwave signal. The laser design has focused on stability and compactness

Emission bifréquence d'un laser à semiconducteur en cavité externe à 852 nm pour les horloges atomiques a césium (orale)

National audienceNous décrivons l'émission simultanée en phase, sur deux fréquences optiques polarisées perpendiculairement, d'un laser à semiconducteur en cavité externe pompé optiquement. L'émission est accordable autour de la raie D2 du césium à 852,14 nm avec une puissance optique d'environ 13 mW sur chaque polarisation. La différence de fréquence est ajustée grâce à un modulateur électro-optique autour de 9,2 GHz. Nous évaluons la source réalisée en vue de son application au piégeage cohérent de population d'atomes de césium dans une horloge atomique

Evaluation of the noise properties of a dual-frequency VECSEL for compact Cs atomic clocks (Poster)

International audienceWe evaluate a dual-frequency and dual-polarization optically-pumped semiconductor laser emitting at 852 nm as a new laser source for compact atomic clocks based on the coherent population trapping (CPT) technique. The frequency difference between the laser modes is tunable to 9.2 GHz corresponding to the ground state hyperfine-split of Cs. Impact of the laser noise has been investigated. Laser relative intensity noise is limited by the pump-RIN transfer to a level of-110 dB/Hz. Laser frequency noise shows excess mechanical and technical noise resulting in a laser linewidth of 1 MHz at 1 s in lock operation. The noise performance and spectral properties of the laser are already adequate to realize CPT experiments and should result in Allan standard-deviation of the clock below 1 × 10-12 at 1 second

Two-cross-polarized-frequency VECSEL at 852nm for CPT-based Cs clocks (poster)

International audienceWe demonstrate a tunable high-purity microwave signal generation from a cross- polarized dual-frequency diode-pumped vertical external-cavity semiconductor laser operating at 852 nm for the coherent population trapping of cesium atoms in compact atomic frequency references

Low-noise dual-frequency laser for compact Cs atomic clocks

International audienceWe report the dual-frequency and dual-polarization emission of an optically-pumped vertical external-cavity semiconductor laser (OP-VECSEL). Our laser source provides a high-purity optically-carried RF signal tunable in the GHz range, and is specifically designed for the coherent population trapping (CPT) of Cs atoms in compact atomic clocks. The laser spectrum is stabilized onto a Cs atomic transition at 852.1 nm, and the frequency difference is locked to a local oscillator at 9.2 GHz. Special attention has been paid to the evaluation of the frequency, intensity and phase noise properties. A maximum phase noise of - 90 dBrad2/Hz has been measured. Finally, we estimate the contribution of the laser noise on the short-term frequency stability of a CPT atomic clock, and predict that a value below 3 × 10-13 over one second is a realistic target

Generation of high purity microwave signal from a dual-frequency OP-VECSEL (orale)

International audienceCoherent population trapping (CPT) is an interesting technique for the development of compact atomic frequency references. We describe an innovating laser source for the production of the two cross-polarized coherent laser fields which are necessary in CPT-based atomic clocks. It relies on the dual-frequency and dual-polarization operation of an optically-pumped vertical external-cavity semiconductor laser. This particular laser emission is induced by intracavity birefringent components which produce a controllable phase anisotropy within the laser cavity and force emission on two cross-polarized longitudinal modes. The laser emission is tuned at the Cs D2 line (λ = 852.14 nm), and the frequency difference ∆ν between the two laser modes is tunable in the microwave range. The laser line wavelength is stabilized onto an atomic hyperfine transition, and concurrently the frequency difference is locked to an ultra-low noise RF oscillator at 9.2 GHz. The high spectral purity of the optically-carried microwave signal resulting from the beatnote of the two cross-polarized laser lines is assessed through its narrow spectral linewidth (<30 Hz) as well as its low phase noise (≤ -100 dBrad2/Hz). The performance of this laser source is already adequate for the interrogation of atoms in a CPT atomic clock, and should result in an estimated relative stability of 3.10-13τ-1/2 - one order of magnitude better than commercial atomic clocks

High-purity microwave signal from a dual-frequency semiconductor laser for CPT atomic clocks (poster)

Coherent population trapping (CPT) of metal-alkali atoms is an interesting technique for the development of compact atomic frequency references; it relies on the excitation of the atoms by two phase-coherent laser fields. We describe the design and operation of an innovating dual-frequency laser source dedicated to Cs CPT atomic clocks, based on the direct dual-frequency and dual-polarization operation of an optically-pumped semiconductor laser at 852 nm. The phase noise of beatnote generated by the laser source is at maximum of -90 dBrad²/Hz with active stabilization, and the relative intensity noise (RIN) has been measured at -115 dB/Hz. It would potentially results in a clock frequency stability of 1.6 .10^-12 at 1 second, limited by the laser RIN. With proper adjustments in the laser and clock set-up, we target a stability of 3.10^-13 at 1 second

Horloge atomique à piégeage cohérent de population du césium en cellule : limitations à la stabilité de fréquence

This report refers to the frequency stability study of a compact clock using coherent population trapping. The frame of such a study is firstly to deal in depth with the understanding of the systematic effects affecting the frequency of a coherent population trapping resonance. A second goal is to build a state-of-the-art compact atomic clock. Because of a pulsed interrogation and laser beams linearly and orthogonally polarized, our prototype would present a fractional frequency stability distinctly below 10^-13 at 1 s integration if it was shot-noise limited. Further to a setup description, the first part of this report is devoted to study the noise sources which limit the short-term stability of the clock. A special attention has been paid to model, experiment and reduce the transfer of local oscillator frequency noise and of laser intensity noise to microwave frequency noise. It led to measure an interesting stability measurement at the level of 3.2x10^-13 at 1 s. An experimental and theoretical study of the frequency shift due to laser intensity fluctuation is then presented. Beyond the influence on this shift of dark resonance overlapping that has been enlighted, this study gives the basics to understand the insensibilization method of the frequency to power fluctuations presented in the last chapter.Ce mémoire porte sur l’étude de la stabilité de fréquence d’une horloge atomique à piégeage cohérent de population. Le cadre de cette étude est d’une part d’approfondir la connaissance du piégeage cohérent de population en cellule de vapeur et d’autre part de construire un prototype d’horloge démontrant une stabilité de fréquence à l’état de l’art des meilleures horloges compactes de laboratoire. Grâce à une interrogation impulsionnelle et un schéma d’excitation en polarisations linéaires et orthogonales, cette horloge présenterait une stabilité de fréquence relative nettement inférieure à 10^-13 à 1 s si elle était limitée par un bruit fondamental tel que le bruit de photon. Après une présentation du montage expérimental, la première partie de ce mémoire est consacrée à l’étude des différentes sources de bruit limitant la stabilité de fréquence court-terme. Le soin particulier donné à la modélisation, à la caractérisation expérimentale et à la réduction des transferts de bruit de fréquence de l’oscillateur local (effet Dick) et du bruit d’intensité du laser en bruit de fréquence de l’horloge, a permis de mesurer une stabilité de fréquence au niveau de 3.2x10^-13 à 1 s. Dans un deuxième temps une étude théorique et expérimentale du déplacement de fréquence micro-onde en fonction de la puissance laser est présentée. Au-delà de la mise en évidence du caractère clé de la déformation de la raie dans l’explication de ce déplacement, elle a posé les bases de la dernière partie de ce mémoire qui propose une méthode d’insensibilisation du déplacement de fréquence aux fluctuations de puissance

Atomic clock using coherent population trapping in a cesium cell : limitations to the frequency stability

Ce mémoire porte sur l’étude de la stabilité de fréquence d’une horloge atomique à piégeage cohérent de population. Le cadre de cette étude est d’une part d’approfondir la connaissance du piégeage cohérent de population en cellule de vapeur et d’autre part de construire un prototype d’horloge démontrant une stabilité de fréquence à l’état de l’art des meilleures horloges compactes de laboratoire. Grâce à une interrogation impulsionnelle et un schéma d’excitation en polarisations linéaires et orthogonales, cette horloge présenterait une stabilité de fréquence relative nettement inférieure à 10^-13 à 1 s si elle était limitée par un bruit fondamental tel que le bruit de photon. Après une présentation du montage expérimental, la première partie de ce mémoire est consacrée à l’étude des différentes sources de bruit limitant la stabilité de fréquence court-terme. Le soin particulier donné à la modélisation, à la caractérisation expérimentale et à la réduction des transferts de bruit de fréquence de l’oscillateur local (effet Dick) et du bruit d’intensité du laser en bruit de fréquence de l’horloge, a permis de mesurer une stabilité de fréquence au niveau de 3.2x10^-13 à 1 s. Dans un deuxième temps une étude théorique et expérimentale du déplacement de fréquence micro-onde en fonction de la puissance laser est présentée. Au-delà de la mise en évidence du caractère clé de la déformation de la raie dans l’explication de ce déplacement, elle a posé les bases de la dernière partie de ce mémoire qui propose une méthode d’insensibilisation du déplacement de fréquence aux fluctuations de puissance.This report refers to the frequency stability study of a compact clock using coherent population trapping. The frame of such a study is firstly to deal in depth with the understanding of the systematic effects affecting the frequency of a coherent population trapping resonance. A second goal is to build a state-of-the-art compact atomic clock. Because of a pulsed interrogation and laser beams linearly and orthogonally polarized, our prototype would present a fractional frequency stability distinctly below 10^-13 at 1 s integration if it was shot-noise limited. Further to a setup description, the first part of this report is devoted to study the noise sources which limit the short-term stability of the clock. A special attention has been paid to model, experiment and reduce the transfer of local oscillator frequency noise and of laser intensity noise to microwave frequency noise. It led to measure an interesting stability measurement at the level of 3.2x10^-13 at 1 s. An experimental and theoretical study of the frequency shift due to laser intensity fluctuation is then presented. Beyond the influence on this shift of dark resonance overlapping that has been enlighted, this study gives the basics to understand the insensibilization method of the frequency to power fluctuations presented in the last chapter