Ch. Andreeva

Saturation effects in the sub-Doppler spectroscopy of cesium vapor confined in an extremely thin cell

Saturation effects affecting absorption and fluorescence spectra of an atomic vapor confined in an extremely thin cell (cell thickness L<1 {mu}m) are investigated experimentally and theoretically. The study is performed on the D{sub 2} line ({lambda}=852 nm) of Cs and concentrates on the two situations L={lambda}/2 and L={lambda}, the most contrasted ones with respect to the length dependence of the coherent Dicke narrowing. For L={lambda}/2, the Dicke-narrowed absorption profile simply broadens and saturates in amplitude when increasing the light intensity, while for L={lambda}, sub-Doppler dips of reduced absorption at the line-center appear on the broad absorption profile. For a fluorescence detection at L={lambda}, saturation induces narrow dips, but only for hyperfine components undergoing a population loss through optical pumping. These experimental results are interpreted with the help of the various existing models and are compared with numerical calculations based upon a two-level modeling that considers both a closed and an open system.

Ground-state magneto-optical resonances in cesium vapor confined in an extremely thin cell

Experimental and theoretical studies are presented related to the ground-state magneto-optical resonance observed in cesium vapor confined in an extremely thin cell (ETC), with thickness equal to the wavelength of the irradiating light. It is shown that utilization of the ETC allows one to examine the formation of a magneto-optical resonance on the individual hyperfine transitions, thus distinguishing processes resulting in dark (reduced absorption) or bright (enhanced absorption) resonance formation. We report experimental evidence of bright magneto-optical resonance sign reversal in Cs atoms confined in an ETC. A theoretical model is proposed based on the optical Bloch equations that involves the elastic interaction processes of atoms in the ETC with its walls, resulting in depolarization of the Cs excited state, which is polarized by the exciting radiation. This depolarization leads to the sign reversal of the bright resonance. Using the proposed model, the magneto-optical resonance amplitude and width as a function of laser power are calculated and compared with the experimental ones. The numerical results are in good agreement with those of experiment.

Frequency stability comparison of diode lasers locked to Doppler and sub-Doppler resonances

We present a systematic quantitative comparison of the performance parameters of two Extended Cavity Diode Lasers (ECDL), stabilized to Doppler and sub-Doppler profiles. The experimental study is carried out on Rb atomic vapour cells. The frequency shift of the resonance under investigation with respect to a reference resonance is measured and analyzed, in dependence on the modulation amplitude, cell temperature, laser power, applied magnetic field and misalignment of the pump-probe beams angle. The Allan variance is measured for both locking methods. It shows that the stabilization to the Doppler profile results in about one order of magnitude lower stability than the one to SA resonances for the short-term measurement. However, the stability achieved using both methods is sufficient for many applications and for some of them the simplicity of the Doppler locking method can be advantageous.

Reducing light-shift effects in optically-pumped gas-cell atomic frequency standards

We investigate a novel method to suppress the light-shift effect in laser-pumped passive rubidium frequency standards. The reduction of the light-shift due to additional frequency components in the pumping light is studied theoretically and compared to experimental results obtained with a frequency-modulated DBR laser diode as a pumping light source. In this way, a reduction of the light shift by a factor of 55 was successfully demonstrated. Complete cancellation of the light shift can be reached with a suitably adjusted modulation index and frequency.

Towards a simple and performing CPT based magnetometer: optimization of experimental paramaters

A simple set-up for observation of CPT effect at single hyperfine ground state excitation has been examined for application in measurements of non-vanishing magnetic fields. The CPT is prepared using low-frequency modulated (in the kHz range) diode laser. Magnetic field measurements down to 500 pT sensitivity and reproducibility were obtained for integration time of 30 ms. One of the important characteristics of the proposed methodology is a proper Cs excitation in order to overcome effectively the huperfine optical pumping effefct. This is possible working in narrow ranges of laser beam intensity, buffer gas pressures and Cs vapor densities. The best conditions have been experimentally determined and discussed.

Sub-Doppler spectroscopy and coherence resonances in submicron Cs vapour layer

The absorption and fluorescence spectra of a submicron Cs vapour layer under resonant excitation on the D2 line are measured. Sub-Doppler features centered at the resonance frequency of the hyperfine (hf) transitions have been observed. Substantial changes in the amplitude and width of the sub-Doppler resonances for individual hf transitions were recorded in dependence on the intensity of the incident laser radiation. In the fluorescence spectra narrow dips due to the effects of saturation have been observed. We also report about the observation of coherence resonances in Hanle configuration obtained at cell thickness of the order of the wavelength of the laser light.

Light shift reduction in atomic clocks

In this paper we present a detailed description of a method for reduction of the light shift in laser-pumped gas-cell atomic clocks. The method consists in using a multi-frequency optical pumping obtained through frequency modulation of the laser spectrum with precisely controllable parameters. Experimental evidence of a strong reduction of the frequency dependence of the LS in an optical pumping Rb gas-cell clock is demonstrated in a large frequency interval, which is in a very good agreement with the numerical estimations reported.

Light shift and laser sidebands in gas-cell atomic clocks using optical pumping and coherent population trapping

Theoretical and experimental investigation of the light shift in optical pumping and coherent population trapping based gas-cell atomic frequency standards is reported. A possibilty for realization of a minimial (possibly zero) light shift configuration is discussed.

Coherent spectroscopy in Cs for precise magnetic field measurements

Investigation on coherent population trapping and electromagnetic induced absorption effects has been made in Cs vapors. A detailed analysis of degenerate and non-degenerate cases is reported. Possible application to precise magnetic field measurements is discussed and preliminary results described.

Temperature dependence of coherent resonances in Na and Cs cells

Temperature dependences of coherent resonances in Na and Cs atoms prepared by two coherent laser frequencies or two polarizations of single frequency laser field are presented. It is shown that the temperature dependence of the amplitude or contrast of the resonances in most cases exhibits a maximum. For both types of coherent resonances, a reason for the contrast reduction at higher temperature is the increased absorption of the laser light along the gas cell due to the optical thickness of the medium. Spin-exchange collisions lead to resonance contrast reduction more effectively in a cell with pure alkali atoms.