Optical EngineeringMechanical properties of hollow glass waveguides
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Hollow glass waveguides with losses as low as 0.2 dB/m at the 10.6 µm are an attractive fiber delivery system for a broad range of IR wavelengths. These guides are made by depositing, using liquid- phase 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-µm bore silica tubing to 6.12 GPa for the Ag/AgI coated, 530-µm 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.