Temperature Dependence of Dark Lines in Coherent Population Trapping

We measure the signal amplitude and linewidth of a dark line in coherent population trapping in the Rb vapour cell filled with mixed buffer gas N₂ and Ar as a function of cell temperature. We find that the dark line signal amplitude increases with temperature up to a maximum at 49°C and then drops at higher temperatures due to quenching effects of N₂. The linewidth of the dark line remains basically constant, at 1080Hz. We also measure the linewidth of the dark line as a function of laser intensity. The linewidth increases linearly with laser intensity. An intrinsic linewidth (FWHM=896Hz at 3.4GHz) of the Rb cell is obtained.