Contrast and linewidth of the coherent population trapping transmission hyperfine resonance line in ${}^{87}\mathrm{Rb}:$ Effect of optical pumping

Coherent population trapping is studied in ${}^{87}\mathrm{Rb}$ in a buffer gas. Experimental results are reported on the contrast of the field independent hyperfine resonance line and on its linewidth. A physical model, which includes an optical pumping process that traps atoms in a Zeeman sublevel of the ground state, is developed. The model explains the general behavior of the experimental observations.