Study of the laser linewidth influence on "lin ∥ lin" Coherent Population Trapping

We investigated, theoretically and experimentally, the influence of the laser spectrum (in particular the linewidth) on the Coherent Population Trapping (CPT) resonance in ⁸⁷Rb buffer gas cell. We investigated the case of linearly polarized laser fields. The experiments have been performed by using either a current-modulated Vertical Cavity Surface Emitting Laser (VCSEL) or a Phase-Locked-Laser (PLL) system. A model based on the density matrix approach has been used for the data interpretation. We found that, under experimental conditions suitable for compact clock applications, the fact that VCSELs have a broader spectrum than PLLs does not influence significantly the CPT signal amplitude. We demonstrate that a not negligible role is played by the destructive interference between different CPT excitation channels and propose further investigations to evaluate this contribution with respect to the other causes. Finally, we discuss the application of this scheme in compact atomic clocks.