  We describe a subluminal laser which is extremely stable against
perturbations. It makes use of a composite gain spectrum consisting of a broad
background along with a narrow peak. The stability of the laser, defined as the
change in frequency as a function of a change in the cavity length, is enhanced
by a factor given by the group index, which can be as high as 105 for
experimentally realizable parameters. We also show that the fundamental
linewidth of such a laser is expected to be smaller by the same factor. We
first present an analysis where the gain profile is modeled as a superposition
of two Lorentzian functions. We then present a numerical study based on a
physical scheme for realizing the composite gain profile. In this scheme, the
broad gain is produced by a high pressure buffer-gas loaded cell of rubidium
vapor. The narrow gain is produced by using a Raman pump in a second rubidium
vapor cell, where optical pumping is used to produce a Raman population
inversion. We show close agreement between the idealized model and the explicit
model. A subluminal laser of this type may prove to be useful for many
applications.
