Jean-Marie Danet

Limitations of Long-Term Stability in a Coherent Population Trapping Cs Clock

Vapor cell atomic clocks exhibit reduced frequency stability for averaging time between about one hundred and a few thousand seconds. Here we report a study on the impact of the main parameters on the mid-to-long term instability of a buffer-gas vapor cell Cs clock, based on coherent population trapping (CPT). The CPT signal is observed on the Cs D1 line transmission, using a double Λ scheme and a Ramsey interrogation technique. The effects on the clock frequency of the magnetic field, the cell temperature, and the laser intensities are reported. We show in particular that the laser intensity shift is temperature dependent. Along with the laser intensity ratio and laser polarization properties, this is one of the most important parameters.

Constructive polarization modulation for coherent population trapping clock

We propose a constructive polarization modulation scheme for atomic clocks based on coherent population trapping (CPT). In this scheme, the polarization of a bichromatic laser beam is modulated between two opposite circular polarizations to avoid trapping the atomic populations in the extreme Zeeman sublevels. We show that if an appropriate phase modulation between the two optical components of the bichromatic laser is applied synchronously, the two CPT dark states which are produced successively by the alternate polarizations add constructively. Measured CPT resonance contrasts up to 20% in one-pulse CPT and 12% in two-pulse Ramsey-CPT experiments are reported, demonstrating the potential of this scheme for applications to high performance atomic clocks.

Temperature dependence of a Cs vapor cell clock: Pressure shift, signal amplitude, light shift

We present the investigation on the temperature dependence of the pressure (collisional) shift of the Cs clock transition in presence of buffer gases (Ne, N2 and Ar). The temperature dependence of the signal amplitude and light shift are also investigated. From the obtained coefficients of temperature dependence it is possible to foresee the buffer gas mixture which cancels the temperature sensitivity at a given temperature. These results as well as the results of the investigation on the signal amplitude and the light shift can be useful for improving the frequency stability of the Cs vapor-cell clocks.

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

Toward the next generation of compact devices, atomic clocks based on coherent population trapping (CPT) offer a very interesting alternative. We present a review of our studies on the short and mid term stability of a compact high performance atomic clock based on CPT in view of portable applications.

A novel scheme for coherent population trapping clock

A new scheme of constructive polarization modulation for coherent population trapping (CPT) clock is proposed. In this scheme, the polarization of a bichromatic laser beam is modulated between two opposite circular polarizations, to avoid trapping the population in the extreme Zeeman sublevels. We show that if an appropriate phase modulation between the two optical components of the bichromatic laser is applied synchronously, the two CPT dark states which are produced successively by the alternate polarizations add constructively. The experimental results show that the observed high contrast CPT signal is favorable for CPT clock application.

Generation of high-purity microwave signals from a dual-frequency OP-VECSEL

Coherent 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 Δν 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.

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

We describe the dual-frequency operation of an optically-pumped vertical external cavity semiconductor laser (VECSEL) stabilized onto an Cs atomic transition. It is based on the simultaneous emission of two cross-polarized adjacent longitudinal modes inside the same laser cavity, which provides a strong correlation between the two laser lines. The frequency difference, in the GHz range, is fixed by the intracavity phase anisotropy, and precisely tuned with an electro-optic modulator (EOM). For this work, we additionally take benefit of the class-A dynamical behaviour of VECSEL which results in a shot-noise limited relative-intensity-noise on a wide spectral range. The GaAs/AlGaAs active structure is pumped with a 1W-fiber coupled laser diode at 670 nm. The laser cavity has been carefully designed for improved thermal and mechanical stability, and compactness. It consists in a 15-mm concave output coupler, a glass Fabry-Perot etalon, a YVO4 birefringent plate and a MgO:SLT EOM. The output power at each frequency reaches 20 mW. The frequency difference is phase-locked to a microwave reference source through the EOM voltage with a MHz bandwidth, resulting in a high-purity optically-carried microwave signal. Simultaneously, one laser line is locked on a Cs atomic hyperfine transition at 852 nm through a low-bandwidth servo-loop on the cavity length. The performance of our laser source is thus fully compatible with the excitation of Cs atoms in coherent population trapping atomic clocks.

Hyperfine coherence and population lifetime in Cs vapor cells

Within the frame of a Raman-Ramsey CPT clock optimisation, we present our investigations on the coherence and population lifetime measurements in Cs vapour cells. To guarantee the reliability of the coherence lifetime measurements, three different methods are compared. Very good agreement is found between two of them and results are discussed for the third. In order to increase the coherence lifetime, two techniques are tested: Firstly a buffer gas can be added in the Cs vapour cell and secondly cell walls can be coated to reduce the destructive collisions against the walls. Buffer gas is tested first and then combined to a coating. First results are presented.