On the unique possibility to increase significantly the contrast of dark   resonances on D1 line of $^{87}$Rb
  We propose and study, theoretically and experimentally, a new scheme of excitation of a coherent population trapping resonance for D1 line of alakli atoms with nuclear spin $I=3/2$ by bichromatic linearly polarized light ({\em lin}$||${\em lin} field) at the conditions of spectral resolution of the excited state. The unique properties of this scheme result in a high contrast of dark resonance for D1 line of $^{87}$Rb. 
A.V. Taichenachev, V.I. Yudin, V.L. Velichansky, and S.A. Zibrov