Directional Control in Light-Wave Guidance

In research on techniques for transmitting light waves over appreciable distances for communication it has become evident that control of direction of propagation is an important and difficult problem. Electromagnetic waves in free space travel in a straight line. In a medium that is sheltered from atmospheric effects, frequent changes in direction are necessary to follow vertical terrain contours and to conform to a horizontal path avoiding physical obstacles and regions of high-cost installation. The wave guiding medium must provide these direction changes. In this paper some simple relations are derived to give the order of magnitude of the direction-determining factors, bending radius and abrupt tilt angle, for any wave guiding structure as a function of wavelength and the transverse dimension of the guided electromagnetic wave beam. These simple relations are then compared to the corresponding more precisely defined quantities for specific waveguides: (1) a sequence of lenses,1 (2) the hollow-dielectric waveguide, 5 and (3) round waveguides for circular electric waves. I I . DERIVATION O F G E N E R A L W A V E G U I D E DIRECTIONAL S E N S I T I V I T Y