Optical resonator with steep internal dispersion

We present an optical resonator with modified properties due to a nonabsorbing highly dispersive medium. The steep nonabsorbing dispersion is created with an additional pump field in an atomic beam using the effect of coherent population trapping. The linewidth of such a resonator depends on the slope of the dispersion line, which in turn depends on the atomic density and the intensity of pump and probe field. In the experiments presented here, the cavity linewidth is reduced by a factor of more than 50 relative to the linewidth of the empty resonator. We have studied the influence of the relative intensities of pump and probe field on the line profile. Due to the dispersion of the medium, the resonance frequency is nearly independent of the geometrical length of the resonator.