Adiabatic and nonadiabatic preparation of a ground-state coherence in an optically thick lambda medium

Fractional stimulated Raman adiabatic passage (f-STIRAP) is used to develop a method for preparation of a two-photon coherence between long-lived ground states of an optically thick ensemble of lambda atoms. Despite strong depletion and reshaping of preparation pulses in the course of propagation, this method provides a stable predetermined value of the coherence uniformly established along the sample, from input to the output. Both adiabatic and nonadiabatic preparation regimes are considered.