Coupling Intensity Effects in Ladder-type Electromagnetically Induced Transparency of Rb atoms

We have studied the polarization effects and the coupling intensity effects of electromagnetically induced transparency(EIT) in the 5S(1/2)5P(3/2)-5D(5/2) ladder system of Rb. We obtained the EIT spectrum with the hyperfine structure of 5D(5/2) transitions and the minimal width of the measured EIT spectrum was 6.5 MHz. We observed the change of the relative magnitudes of the hyperfine structure of EIT according to not only the polarizations of lasers but also the intensity of the coupling laser. The cause of the coupling intensity effects is that the EIT signal nonlinearly increases to the coupling intensity.