T. Zanon

Observation of Raman-Ramsey fringes with optical CPT pulses

The Ramsey method has been applied by means of optical coherent population trapping (CPT) pulses through a cesium vapor cell with N/sub 2/ buffer gas at room temperature, using two phase-locked lasers. With this method, CPT resonance spectral widths are no longer limited by optical saturation and collision effects, but only depend on free evolution time between the two pulses. A fringe width below 100 Hz is reported. Experimental Raman-Ramsey fringes are analyzed using the classical wavefunction formalism.

Raman–Ramsey Interaction for Coherent Population Trapping Cs Clock

A gas cell atomic frequency standard based on coherent population trapping (CPT) with Ramsey interrogation pulses is developed. The clock resonance is detected in a Cs cell with buffer gas using two phase-locked diode lasers tuned to the Dl line. A preliminary short-term stability of 3.5 10-12t is reported. The CPT specificity allows a new interrogation-detection sequence, i.e., the continuous pulse train, which, with phase modulation, could lead to better short-term stability compared to usual continuous CPT clocks. The parameters optimizing the stability include a high Rabi frequency, a very short measurement time, and an interrogation time equal to the lifetime of the hyperfine coherence

Frequency Stability Measurement of a Raman-Ramsey Cs Clock

CPT clocks are studied worldwide as miniature frequency standards for portable applications. To improve their frequency stability, we have investigated the optimum operating conditions (cell temperature, laser intensity, interrogation method) and measured the corresponding frequency shifts and stability. Compared to the continuous CPT interrogation, the pulsed interrogation reduces the light shift by a factor 300 and improves the frequency stability by a factor 10. The short term frequency stability is 9 x 10 -3 at 1 s, limited to 2 x 10 -3 after 300 s.

Coherent population trapping with cold atoms

Atomic clocks based on CPT are interesting because of the narrow linewidths achievable at very low intensities. The linewidth of a CPT resonance in a sample of cold atoms mainly depends on the optical saturation by two lasers and the different relaxation terms responsible for destroying the hyperfine coherence. We propose a complete analytical solution for the excited population based on the classical Bloch formalism to determine the line shape of the resonance. Finally we present the future development of a cold atoms clock based on CPT linewidth.

Recent results on a pulsed CPT clock

CPT clocks are very attractive for their miniaturization potentiality. Nevertheless, two main effects limit today their performance: the optical pumping associated with circularly polarized lasers and the line broadening due to saturation effects. We propose to overcome these limitations by the use of orthogonal linear laser polarizations and a pulsed interrogation sequence. The optimization of the time sequence leading to the best characteristics (contrast, linewidth) of the dark resonance is discussed. Very preliminary measurements of the frequency stability demonstrate an interesting performance of 3.5 10-12 tau-frac12

Development of compact laser cooled atomic frequency standards

Two projects of compact cold atom clocks are currently developed: a clock with atoms directly cooled in the microwave cavity and a clock based on coherent population trapping. For each device, different interrogation and detection schemes are evaluated and reported