  We implement double electromagnetically-induced transparency (double EIT) in
rubidium vapor, using a tripod-shaped energy level scheme consisting of
hyperfine and magnetic sublevels of the 5S1/2 to 5P1/2 transition. We show
experimentally that through the use of double EIT one can control the contrast
of transparency windows as well as group velocities of the two signal fields.
In particular, the group velocities can be equalized, which holds promise to
greatly enhance nonlinear optical interaction between these fields.
