Properties of Periodic Gas Lenses

The beam-waveguide described by Goubau1 appears as a promising device to transmit light over long distances. However, to reduce the power loss due to absorption and reflection, which is inevitable with lenses made of solid dielectrics, gas lenses have been proposed2 3 instead of the solid lenses used by Goubau. Two earlier papers3-4 discussed the properties of a particular type of gas lens. This tubular gas lens consists of a warm tube into which a cooler gas is blown. The thermal gradients in the gas lead to density gradients which give the structure the properties of a positive lens. Ref. 4 discusses the focal length and principal surface of this gas lens for the case that the gas enters the lens at a constant temperature. It was shown that this device, when operated under optimum conditions, acted as an optically rather thin lens with moderate lens distortions. The present paper extends the earlier analysis in several ways. We consider periodic lens structures. Such a structure results if hot and cold tubes are alternated to form a long, periodic structure. The gas is heated and cooled periodically giving rise to periodically arranged positive and negative lenses. A periodic structure of this type represents an alter2083