Hollow waveguides present an attractive alternative to other solid-core IR fibers. The key features of hollow guides are their ability to transmit wavelengths well beyond 20 mm, their inherent advantage of having an air core for high-power laser delivery, and their relatively simple structure and potential low cost. Initially these waveguides were developed for medical and industrial applications involving the delivery of CO2 laser radiation, but more recently they have been used to transmit incoherent light for broadband spectroscopic and radiometric applications. In general, hollow waveguides enjoy the advantages of high laser power thresholds, low insertion loss, no end reflection, ruggedness, and small beam divergence. Their potential disadvantages, however, include an additional loss on bending and a small NA. Nevertheless, they are today one of the best alternatives for both chemical and temperature sensing as well as for power delivery in IR laser surgery or in industrial laser delivery systems, with losses as low as 0.1 dB/m and transmitted cw laser powers as high as 2.7 kW.
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