  We investigate resonant nonlinear optical interactions and demonstrate
induced transparency in acetylene molecules in a hollow-core photonic band-gap
fiber at 1.5$\mu$m. The induced spectral transmission window is used to
demonstrate slow-light effects, and we show that the observed broadening of the
spectral features is due to collisions of the molecules with the inner walls of
the fiber core. Our results illustrate that such fibers can be used to
facilitate strong coherent light-matter interactions even when the optical
response of the individual molecules is weak.
