G. Théobald

Investigation of microwave π transitions in cesium beam clocks operated with U‐shaped H plane waveguide cavities

In this article, we present the characteristics of the ΔF=±1, ΔmF=±1, π microwave transitions which appear in the response of cesium beam resonators operating with U‐shaped waveguide Ramsey cavities bent in the plane of the H field. Such resonances are due to the presence in the cavity of microwave magnetic induction perpendicular to the static field direction. It is shown that the π resonance feature is derived from a pure two‐level atomic system interacting with four spatially separated oscillating fields. Good agreement is found between experimental data and theoretical predictions when we take into account the actual transverse microwave field profile in the cavity obtained with tridimensional electromagnetic field computations.

Ground state Zeeman coherence effects in an optically pumped cesium beam

Abstract The population trapping in a cesium beam induced at low magnetic field by σ polarized light from a broadband laser diode has been investigated. The dip observed in the fluorescence is interpreted in terms of Zeeman coherences in the ground state which trap the atomic populations. A quantitative description is given through the analysis of the atom light interaction with a rate equation model including Zeeman coherences.

Frequency shifts in cesium beam clocks induced by microwave leakages

The anomalous sensitivity of an optically pumped cesium beam clock to microwave power and the unexpected frequency shifts observed are demonstrated to result from microwave leakages outside the arms of the Ramsey cavity. A new design of the cavity associated with a careful realisation provides very weak microwave leakages and negligible related frequency offsets. We have established a theoretical model that allows us to calculate the frequency shifts due to microwave field components propagating along the beam axis in regions which are free-field in an ideal Ramsey cavity. This results in first order Doppler effect shifts. The order of magnitude of the frequency shift can be predicted and agrees with the measured one when the amplitude of the leakage magnetic field is about a 1000 times smaller than the amplitude of the microwave interrogation field in the cavity.

Measurement of the Signal-to-noise Ratio in an Optically Pumped Caesium-beam Frequency Standard

We have measured a figure of merit for a caesiumbeam frequency standard in which optical pumping and optical detection are accomplished by a single semiconductor laser source. The value of this figure of merit is 29, leading to an estimated fractional frequency stability of 2.6 × 10-12 τ-1/2. This result compares very favorably with the frequency stability of the best manufactured caesium-beam frequency standards.

Comparison of pumping a cesium beam tube with D1 and D2 lines

The operation of a cesium beam tube optically pumped with cesium D1 and D2 lines is theoretically and experimentally investigated. In a single laser diode configuration using the D2 line, the most efficient excitation scheme is found to be the 3→3σ(D2) scheme leading to a clock signal‐to‐noise ratio equal to 15 000 in a one‐hertz bandwidth. In the case of the D1 line, the predicted best excitation schemes are the 4→3π(D1) and the 4→3σ(D1) schemes. However, these two configurations lead to an extra fluorescence noise presumably due to unpumped atoms. The 3→4π(D1) scheme is experimentally found to be the best with S/N performances comparable to the 3→3σ(D2) configuration. Nevertheless, in the case of the 3→3σ(D2) scheme, optical pumping efficiency decreases when the magnetic field is too low whereas 3→4π(D1) features are independent of the magnetic‐field amplitude. Moreover, the well‐separated fluorescence lines in the case of the D1 line allow a more accurate locking of the laser frequency on the top of th...

Frequency stability of an optically pumped cesium beam frequency standard

Results obtained in an experimental optically pumped cesium beam frequency standard in which a single semiconductor laser is used for the state selection and the atom detection are reported. The separation between the two interaction regions is equal to 21 cm. This gives a 500 Hz linewidth which is observed with a signal to noise ratio equal to 10,000 in a 1 Hz noise bandwidth. A quartz crystal oscillator is frequency controlled by the atomic transition. The measured short term frequency stability is given by σy(τ)=2×10−12τ−1/2 for 1 s<τ≲500 s. Prospects for improvement of this frequency stability are discussed.

Noise in the optical detection of atoms in a beam

Abstract The noise processes which affect the optical detection of atoms in a beam are identified. A noise factor of the atom to fluorescence photon conversion is defined in the case where the counting statistics of the fluorescence photons can be assumed poissonian. The signal to noise ratio of the atom detection is given in the case where a photomultiplier or a silicon photocell is used. It is shown that the noise added to the actual atomic beam shot noise can be represented in terms of the shot noise of an equivalent fictitious flux of incident atoms. Results given are applied to the optical detection of cesium atoms.

Influence of Low Magnetic Field on Pumping Efficiency in an Optically Pumped Cesium Beam Resonator

In this paper, we report our investigation on the influence of the magnetic field on the optical pumping of a cesium beam using coherent excitation with a broadband laser. The generation of Zeeman coherences in the ground state level when optical transitions of the cesium D2 line are excited by a linearly a polarized laser may result in a decrease of the efficiency of the optical pumping and lead to an intensity minimum in the fluorescence LJp as the applied magnetic field decreases to zero value. Accordingly the population difference An and LJp are found to depend on the magnitude B of the magnetic field. Here we describe physical basis which indicate that the fluorescence intensity variation results from the coupling of the lower level ∆m = ± 2 coherences to the excited level population. Comparison of theoretical predictions with experimental records of the fluorescence are then made. Finally guidelines are given which may influence the magnetic field design in the light-atom interaction zones of an efficient optically pumped cesium beam frequency standard.

Effect of anisotropy on atomic fluorescence: Influence on the detection of a hyperfine transition in a cesium beam

The problem of the fluorescence anisotropy resulting from alignment in the excited state is investigated in the case of a cesium beam illuminated by a broadband laser diode tuned to the cesium D2 line. Its effect on the collection efficiency is derived when photon fluorescence collection is performed inside a small solid angle about the quantization axis. Excitation schemes leading to a noticeable increase of the fluorescence yield are pointed out and experimental results agree quite well with the predictions.