Towards a compact cesium coherent population trapping clock: dual-frequency dual-polarization laser source and miniature optoelectronic bench

The Coherent Population Trapping (CPT) phenomenon with high contrast is particularly promising for the development of a compact and high frequency stability clock. An original compact clock architecture based on a miniature optoelectronic bench and a dual-polarization dual-frequency laser is proposed.

A miniature bench (volume below 10 L) with the optical functions and the servo-controls necessary to the stabilization of the laser field is developed and studied. The miniature optical components are meticulously aligned to guarantee noise reductions comparable to laboratory setups.

A complete study of the optical power stabilization is made, revealing the electronic limitations at short-term and the thermal sensitivities of the setup at long-term. The use of a dual-frequency generator allows to validate the bench by obtaining the CPT spectroscopy on the D2 line of cesium in continuous and pulsed regime.

A Vertical-External-Cavity Surface-Emitting Laser (VECSEL) is implemented to generate the two optical frequencies necessary for the CPT interrogation. The design choices for the VECSEL, in particular to obtain the emission of two modes with strongly correlated and in-phase optical intensities, are detailed. The addition of elements in the cavity leads to additional losses and limits the emitted optical power. The emission of two polarization modes around 852 nm (and 895 nm) with a tunable frequency difference of a few gigahertz is obtained. The understanding of the correlations between the cavity eigenstates allows to model and optimize the reduction strategies of the laser noises. With in-phase intensity fluctuations, the stabilization of the optical power leads to similar noise reductions for both polarization modes, but limited by the amplitude of the correlations.

Simultaneous control of the optical frequency as well as of the frequency difference between the modes is demonstrated for the first time using two electro-optical crystals integrated in the laser cavity. The contributions of the laser noises to the frequency instabilities of the future compact clock are estimated. The intensity and phase noises participate limits the stability to only a few 10^-13 at 1 s, in line with the targeted objectives.

CPT atomic clock, optoelectronic servo-loop, frequency stability, semiconductor laser, noise correlations

Full document (FR) : HAL-04221413