8.1 Introduction

The invention of the laser in 1960 revolutionized the area of telecommunications, as it made it possible to modulate light waves at high frequencies. Initially, modulated laser beams propagating through open space were used to transmit information from one place to another. However, due to environmental disturbances, such as rain, fog, dust, temperature variations, and so on, such a transmission system was neither convenient nor reliable to use. Present-day optical communication systems use optical fibers through which information is transmitted in the form of optical pulses from one place to another. In the following, we discuss the basic propagation characteristics of optical fibers.

An optical fiber is a cylindrical structure, and the simplest (and the most extensively used) optical fiber is the step-index fiber, in which the refractive index distribution (in the transverse direction) is given by

(8.1)

where n₁ and n₂ (< n₁) represent the refractive indices of the core and the cladding, respectively, and a represents the radius of the core (see Fig. 8.1).

The cladding is usually pure silica, while the core is usually silica doped with germanium; doping by germanium results in an increase in refractive index. The guidance of the light beam (through the optical fiber) takes place because of the total internal reflection at the core-cladding interface. The rays for which the angle φ is more than the critical angle φ_c, where

(8.2)