  We consider the Autler-Townes effect when a strong coupling field is applied
in the hyperfine manifold of an alkali atom. Explicit solution is obtained in
the case of the D1-line. We show how the hyperfine interaction modifies the
dressing effects associated with the strong field as well as the sample
susceptibility with respect to the probe mode. Particularly, if the strong
field is far detuned from the atomic resonance line the Autler-Townes structure
differs significantly from the prediction of the Lambda-type approximation. We
also find that tuning the strong field in between the upper state hyperfine
components enhances the Autler-Townes effect. The results are discussed in the
context of quantum memory protocols based on the stimulated Raman process or
EIT effect.
