Saturated optical absorption by slow molecules in hollow-core photonic band-gap fibers.

We report on saturated absorption in a hollow-core photonic band-gap fiber filled with 12C2H2 molecules. We find that slow molecules provide a major contribution to the signal in the limit of low optical power and low pressure where the signal deviates significantly from the usual Lorentzian line shape. In particular, we observe a linewidth reduction of about 3 times as compared to the transit-time limited linewidth.