  Pulse propagation is studied in an EIT medium with the control field having a
periodically varying phase (chirp). Based both on numerical calculations and on
an approximate approach neglecting absorption and nonadiabatic effects, it is
shown that transparency occurs for probe pulses having a sufficiently narrow
envelope and an appropriately suited chirp. For other chirped pulses the
evolution of their spectra along a sample is studied. Using a projection method
one can divide a pulse into a part for which the medium is transparent and a
rest for which it is opaque.
