Propagation and density effects in the coherent-population-trapping maser

A fully self-consistent theory of the coherent population trapping (CPT) maser in the case of an optically thick active medium and cell length comparable with the microwave wavelength was made. It was found that the laser fields propagation inside a cell of length L≈λ leads in general to a shift of the emission profile with respect to the hyperfine transition frequency. In the low-density case, this shift may be controlled through a proper choice of the cell length while in the higher density case a coated cell or a suitably modulated laser spectrum were probably the only solution to reduce this shift that otherwise represents a serious drawback for the realization of a frequency standard with high-medium-term stability.