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Hollow sapphire tubing has been used to deliver nearly single-mode CO(2) laser energy. The hollow single-crystal A1(2)O(3) fiber has an n < 1 cladding at 10.6 microm. Measured losses at 10.6 microm are as low as 0.49 dB/m, and the minimum bend radius of the 580, microm x 380 microm fiber is 14 cm. Bending losses are shown to vary as 1/R, and essentially only the HE(11) mode propagates in the bent guide.
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Harrington et al. (1990) studied this question.
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