Propagation losses are determined for 100 μm × 100 μm square, hollow waveguides constructed from glass capillaries. The small size makes it possible to observe optical effects not easily seen with larger waveguides. The depletion of higher-order even modes creates a large, nonlinear loss. Over a distance of a meter the loss approaches the smaller, linear value expected for the fundamental mode. Additionally, the lowest two even modes beat to produce an oscillatory loss with a period of ～2 cm. Making the focal radius 0.35 the waveguide width minimizes these two effects. In a related study, 50-μm waveguides embossed in polydimethylsiloxane are shown to have losses similar to glass capillaries.
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