  We demonstrate high optical depths (50+/-5), lasting for hours in
Rubidium-filled hollow-core photonic band-gap fibers, which represents a 1000X
improvement over operation times previously reported. We investigate the vapor
generation mechanism using both a continuous-wave and a pulsed light source and
find that the mechanism for generating the Rubidium atoms is primarily due to
thermal vaporization. Continuous generation of large vapor densities should
enable measurements at the single-photon level by averaging over longer time
scales.
