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The transmission of a 10.6-microm TEM(00) beam through a hollow circular-cross-section waveguide is modeled in terms of the excitation and propagation of the two lowest-order circularly symmetric EH(1n) modes. At points along the guide axis where the are in phase the TEM(00) input field is shown to be regenerated, but midway between these points, transverse-mode profiles that have a doughnut shape are produced. It is proposed that these dramatic field variations should cause variations in the effective attenuation coefficient along the length of the waveguide. The first direct experimental measurements to our knowledge of the guide-length-dependent attenuation characteristics of a 1.0-mm-bore hollow silica waveguide support this hypothesis by revealing a strong periodic component in addition to the anticipated exponential decay.

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