Processing and characterization of silver films used to fabricate hollow glass waveguides

Hollow glass waveguides are an increasingly popular fiber for the delivery of high-power IR laser radiation. At CO₂ laser wavelengths the measured and theoretical losses agree, but at the 3-μmEr:YAG laser wavelength the losses remain higher than expected. The reason for this is the surface roughness of the silver film used to form the first layer of the AgyAgI thin-film structure. We found that the roughness of the silver film increases fivefold as silvering times increase from 5 to 80 min. This increased surface roughness produces a concomitant linear increase in the attenuation coefficient for the silver-only guides for wavelengths shorter than approximately 5 μm.