Raman - Ramsey pulsed excitation of coherent population trapping resonances in a 87Rb cell with a buffer gas

We report the results of experimental investigation of the Raman - Ramsey method of pulsed excitation of coherent population trapping (CPT) resonances in a rubidium vapour cell filled with a mixture of Ar and Ne buffer gases. For the development of compact quantum microwave frequency standards, this method is an alternative to the pulsed optical pumping method recently implemented using the same rubidium cell. Both methods have advantages over the traditional double radio-optical resonance technique, the main of which is the substantial suppression of the light shift of the clock transition frequency, since the evolution of the coherence of atomic states forming the clock transition occurs in the absence of laser radiation in the cell. The narrow (110 - 240 Hz) Raman - Ramsey resonances are obtained using the scheme of pulsed excitation of CPT resonances on the D-1 lines of the Rb-87 atom with the same linear (lin parallel to lin) polarisations of the bichromatic laser radiation components. The process of optimising the central fringe linewidth, its contrast, and the linewidth-to-contrast ratio is described. The magnetic dependence of the Raman - Ramsey line central fringe linewidth is experimentally investigated.