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Hollow glass waveguides are an increasingly popular fiber for the delivery of high-power IR laser radiation. At CO(2) laser wavelengths the measured and theoretical losses agree, but at the 3-microm Er: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 Ag/AgI 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 microm.

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Hollow glass waveguides provide an attractive means for the delivery of IR laser power and for broadband temperature and chemical fiber sensor applications. These waveguides are made using liquidphase chemistry techniques to deposit a thin dielectric film of AgI over metallic Ag on the inside of small-bore silica tubing. By controlling the thickness of the AgI film, we can tailor the optical response of the guides to give low loss at selected IR laser wavelengths or to produce a broadband response. In either case, the losses depend on the thickness uniformity of the AgI film. Initially, the AgI film thickness varies as much as 14% from end to end of a 6-m long guide. A faster AgI deposition rate reduces this variation to only 1.3% over a 5-m long waveguide. This leads to a reduction in the total loss for the hollow guides in the 2 to 12 m region.

Hollow glass waveguides with losses as low as 0.2 dB/m at the 10.6 m are an attractive fiber delivery system for a broad range of IR wavelengths. These guides are made by depositing, using liquidphase chemistry methods, silver and silver iodide films on the inside of silica tubing. The wet-chemistry techniques used in the process reduce somewhat the mechanical properties of the hollow-silica guides compared to the uncoated tubing used to make the guides. In particular, the mean failure stress is reduced from 8.4 GPa for uncoated, as-drawn, 530-m bore silica tubing to 6.12 GPa for the Ag/AgI coated, 530-m bore waveguide. In addition, the Weibull modulus decreases from 96.9 to 17. This means that the bending radius of the hollow guides, which is inversely proportional to the mean failure stress, increases from about 1 to about 1.5 cm. The reason for the decrease in strength for the processed tubing is the slow reaction of the aqueous coating solutions with the silica surface. Nevertheless, the bending radii for the guides is still quite small and the fatigue behavior of the guides, as measured by the crack growth parameter n, is essentially the same for processed or uncoated tubing.