Saturated absorption in acetylene and hydrogen cyanide in hollow-core photonic bandgap fibers.

Saturated absorption is studied in overtone transitions of C2H2 and H13CN molecules confined in the hollow core of a photonic bandgap fiber. The dynamics of filling and venting the fiber is markedly different for the two molecules owing to the presence of a permanent dipole moment in one of them. Saturation is observed for input power down to 10 mW, and well resolved Lamb dips limited by transit time broadening across the 10 ?m core diameter are observed with a counter-propagating probe beam.