THE OPTICAL FIBER COMMUNICATION

The optical components of fiber communication are, in simple term, a light emitter, which initiates the optical signal, a fiber transmits it, and a detector which receives it and converts it into an electrical equivalent. If several fibers need to be joined, end to end, the couplers must ensure that the fibers are correctly aligned and butted, to reduce any joining losses to a minimum. Each of these components has essential ancillary parts; the detector and emitter are driven by stabilized voltages, and mounted in such a way that maximum transfer of light between them and the fiber is achieved. The fiber itself must be clad in short protective coating and made up into a cable that withstand the rigours of installation over long distance.

Optical fiber communications present the most exciting, and probably the most challenging, aspect of modern systems. Fibers are exciting because they seem to offer so many benefits ---low cost, enormous bandwidth, very small attenuation, low weight and size, and very good security again external interference. Physically, fibers occupy very little space, and they are so flexible that they can be used in places that would not be accessible to conventional cable. An optical fiber is, in essence, a dielectric wave guide. It has been known for a long time that high-frequency electromagnetic energy can be transmitted along a glass or plastic rod and, indeed, observation shows that short rods are translucent to light. However, two factors prevented that knowledge from being used to product useful light guides: (1) energy leaked from outside of the dielectric to the surrounding air (2) the attenuation was so large that worthwhile lengths could not be achieved. The first difficulty though virtually insurmountable at microwave frequencies, can be overcame in the optical and infrared parts of spectrum by enclosing the guide in cladding of similar material, but which has slightly smaller refractive index. The boundary between the cladding and the core acts as a reflecting surface to transmitted light. The second problem that of high attenuation, could be reduced only by refining the methods of producing and drawing the glass so that the impurities and irregularities were reduced to a minimum. The attenuation now achievable in the laboratory is almost as low as possible, at about 0.2 db/Km. Fiber of varying quality are used for communications, but when distance are significant, care is taken to ensure that lowest attenuation possible is achieved. This involves choosing the best operating frequency for particular fiber material, and ensuring that any contaminating elements are remove from the glass during manufacture. Before considering the loss mechanism inherent in any fiber, we will look at the different fibers used, and examine, with the help of ray theory, the way in which light propagates along an optical waveguide.

****Source : Telecommunication Engineering ; J Dunlop and D G Smith; Chapman &Hall; London