  Cavity linewidth narrowing via double-dark resonances has been experimentally
observed using the 87Rb Zeeman splitting sublevels. With the steep dispersion
led by the interacting dark resonances in the tripodtype electromagnetically
induced transparency system, we narrow the cavity linewidth to 250 KHz at room
temperature. Furthermore, the position of this ultranarrow cavity linewidth
could be tuned in a 60 MHz coupling field detuning range.
