Hollow Metallic and Dielectric Waveguides for Long Distance Optical Transmission and Lasers

During recent years the potentially large frequency range made available to communications by the development of the optical maser has stimulated much interest in efficient methods for long distance transmission of light. The most promising contenders for long distance optical transmission media consist of sequences of lenses or mirrors, highly reflective hollow metallic pipes, and dielectric waveguides. 1-10 In this paper we present an analysis of the field configurations and propagation constants of the normal modes in a hollow circular waveguide which, because of its simplicity and low loss, may become an important competitor. The guiding structure considered here may consist of an ordinary metallic pipe of precision bore whose inner surface is highly reflective, or of a hollow dielectric pipe -- i.e., one in which the metal is replaced with dielectric. Although the transmission characteristics of metallic waveguides are well known for microwave frequencies, this theory is invalidated for operation at optical wavelengths, because the metal no longer acts as a good conductor but rather as a dielectric having a large dielectric constant. In the subsequent analysis, therefore, both the dielectric and metallic guide are considered as special cases of a general hollow circular waveguide having an external medium made of arbitrary isotropic material whose optical properties are characterized by a finite complex refractive index. If the free-space wavelength is much smaller than the internal radius of the tube, the energy propagates not in the external medium but essentially within the tube, bouncing at grazing angles against the wall.