Delay Distortion in Generalized Lens-Like Media

Optical fibers have assumed considerable importance for potential use as transmission media wherever wire pairs or coaxials are now used. 177 We give here some theory related to increasing the information capacity of such fibers. Conventional fibers have a core of uniform index of refraction n, surrounded by a cladding of slightly lower index of refraction. The cladding serves to isolate the outer fiber surface from the optical field, which is confined to the core, thus permitting the fiber to be handled and bundled into cables without affecting the information transmission. More recently, fibers having continuously graded index of refraction, with maximum on the fiber's axis and lower values at increasing radial distances, were proposed and realized in practice. 1-3 Graded-index fibers have image-focusing properties3 and provide modulated-carrier transmission with less delay distortion than conventional fibers having a step-index change at the core-cladding boundary. 4-6 Recent experimental work verified the potential of low delay distortion.7 Current studies of graded-index fibers from Corning Glass Works show that total transmission losses under lOdB/kilometer and low delay distortion (approximately one nanosecond per kilometer) can be realized simultaneously in graded-index fibers.8,9 This brings into focus the need for detailed knowledge about delay distortion related to the shape of the index gradient in the transverse plane. The following sections relate to that need. This study was done in parallel with that of D.