Thermal loading in launching and butt connection with hollow waveguides for CO2 laser beam delivery

We investigate the thermal loading of a bare silver-silver halide hollow waveguide with 1 mm core diameter and 1 m length when used to guide carbon-dioxide laser radiation with a power of about 100 W. It is shown that the heating of the waveguide results from two factors: losses at the input face of the fiber caused by beam 'clipping;' and distributed beam attenuation inside the waveguide. Both effects produce a temperature highest at the input face and decreasing along the fiber. The effects were studied theoretically and by experiment. It is found that the 'clipping' effect contributes more thermal loading than the beam attenuation, and also the greater part of the temperature gradient is caused by the 'clipping.' The thermal response to transverse misalignment at the launching is also investigated; it is found that the temperature increase at the front end of the waveguide is much more sensitive to misalignment than is the power transmission. The performance of butt-jointed fiber also is investigated. It is shown that while the transmission is sensitive to transverse alignment of the joint, it is relatively insensitive to the longitudinal misalignment (gap between the fibers); the second fiber normally has higher transmission than the first one due to the absence of the launch coupling loss. There is also no obvious temperature penalty at the joint. Therefore it is possible to use the standard connectors developed for silica fiber to extend the hollow waveguide length with little penalty.