  More than 4 orders of magnitude of cavity-linewidth narrowing in a
rare-earth-ion-doped crystal cavity, emanating from strong intracavity
dispersion caused by off-resonant interaction with dopant ions, is
demonstrated. The dispersion profiles are engineered using optical pumping
techniques creating significant semipermanent but reprogrammable changes of the
rare-earth absorption profiles. Several cavity modes are shown within the
spectral transmission window. Several possible applications of this phenomenon
are discussed.
