Two kinds of light source are currently in service. The first is a refined version of the LED's that are found in the visual-display units of pocket calculators. For light-wave communications what is needed is a source that is not only much more intense than that of visual displays but also comparable in size to the optical fiber, which is only a few hundredths of a millimeter in diameter. In LED's designed for light-wave communications a small hole, or well, is etched into the face of the LED in order to bring the fiber as close as possible to the active region of the semiconductor junction, where the light originates. The fibers have their lowest loss in the infrared region of the spectrum, so that one selects a semiconductor material that emits infrared radiation. LED's made from gallium arsenide, which emit at a wavelength of about .8 micrometer, are satisfactory, but they would be even better if their emission wavelength were somewhat longer. Semiconductor materials that promise a better wavelength match for present optical fibers are under active investigation.
The other type of light source is the semiconductor diode laser, which has a more complex structure than the lightemitting diode. No bigger than a grain of salt, a diode laser consists of several layers of semiconductor material, each one of a different composition. The sandwich structure helps to establish the conditions necessary for laser action; it provides a region that confines the charge carriers giving off light when they recombine and also helps to guide the light in a preferred direction.
It was once difficult to lay down successive layers of material in such a laser without spoiling the crystal structure of the layers. The early devices were therefore notorious for their rapid decay in light-emitting efficiency; some failed completely within a few hours. New techniques were gradually developed so that the composite structures can now be fabricated without introducing imperfections in the crystalline layers. Accelerated aging tests demonstrate that the most recently developed devices should last for several years at room temperature. Ultimately laser diodes should be fully as reliable as other solidstate devices.
The laser source has two main advantages. The first is its directionality. Because the stimulated emission from the laser emerges in a narrow beam it is possible to couple a large fraction of its radiation directly into the end of an optical fiber. The second advantage is its small spread in color, or wavelength, which is typical of a laser source. In traveling through a light guide rays of different wavelengths travel at slightly different velocities; hence the broadening of the pulses in a light guide varies directly with the width of the band of wavelengths transmitted. Laser sources are therefore able to transmit pulses at a higher rate over a given distance than LED's, which emit a broader band of wavelengths. The spectrum of a typical diode laser has a width of only 20 angstroms, compared with 35 0 angstroms for a light-emitting diode of the type suitable for light-guide communications. After traveling a kilometer through an optical fiber, the laser pulse will show a dispersion in time of 200 X 1012 second, equivalent to a dispersion in distance of four centimeters at the reduced velocity with which light travels through glass. For an LED source the dispersion is nearly 20 times greater. This dispersion due to a lack of spectral purity places an important limitation on the pulse rate and thus on the information capacity of light-wave communication systems. Another major limitation is a type of pulse-spreading due to modal dispersion, which arises because some of the rays entering an optical fiber travel slightly longer paths than other rays. As we shall see, modal dispersion can be greatly minimized but not entirely eliminated.
In order to obtain the extraordinary transparency needed for a light guide the optical fibers are designed so that the light never comes near the outside surface of the fiber, where dust, scratches or contact with other surfaces would cause serious losses. Each fiber actually consists of three layers. The outer layer is a coating, usually of plastic, that provides protection from scratches and abrasion, which could weaken the fiber and lead to breakage under stress. Within the protective coating the glass fiber itself has a core region with an index of refraction slightly higher than that of its surrounding cladding. Because of this higher refractive index, rays that enter the end of the fiber at a shallow angle to the central axis are reflected back into the core when they strike the interface between the core and the cladding_ Rays that enter the fiber at large angles to the axis simply escape without being reflected. One can see from geometric considerations that if a ray is reflected back into the core at its first encounter with the interface, it will continue to be confined indefinitely, provided there are no sharp bends in the fiber. Such bends can be avoided by carefully encasing several fibers in a fairly stiff cable sheath.