The first optical-frequency hollow waveguides were simple parallel-plate guides. The early pioneering work in this field was done by Garmire et al. Her group made a rectangular waveguide using aluminum strips separated by about 0.5 mm with bronze shim stock. This structure is shown in Fig. 3.2. Even when the aluminum was not well polished, these guides worked surprisingly well, with losses at 10.6 mm well below 1 dB/m. As early as the late 1970s, Garmire demonstrated the high power-handling capability of an air-core guide by delivering more than 1 kW of CO2 laser power through this simple structure. These rectangular waveguides, however, never gained much popularity, primarily because their overall dimensions (about 0.5x10 mm) were quite large in comparison to circular cross-section guides and also because the rectangular guides cannot be bent uniformly in any direction. As a result, hollow circular waveguides with diameters of 1 mm or less fabricated using metal, glass, or plastic tubing are the most common guide today.
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