One can now visualize the source of modal dispersion: a light ray that enters parallel to the central axis of a fiber will normally travel a shorter distance than a ray that enters at an angle and caroms from side to side as it travels down the fiber channel. As a result a light pulse made up of a combination of rays becomes spread out over time.
To overcome pulse-spreading many of today's fibers have a core whose refractive index is graded, or shaped, to compensate for the different distances the rays travel. In such fibers the refractive index decreases with radial distance away from the center. In the regions of lower refractive index the light travels faster. It is possible to arrange the radial decrease in refractive index so that all rays arrive at their destination at more nearly the same time. In a fiber of uniform refractive index the pulse-spreading amounts to about 25 X 10-9 second per kilometer, which is equivalent to about 500 centimeters. Graded-index fibers now being tested in the field reduce this dispersion by a factor of 25, and in laboratory samples an improvement of 100 to one has been demonstrated. The first high-transparency fibers were made by the Corning Glass Works out of a material whose principal component was silicon dioxide. The first successful fiber with a graded refractive index was made by the Nippon Sheet Glass Co., Ltd. In a process developed by Bell Laboratories a graded-index fiber is made by heating and collapsing a three-foot tube of quartz glass that has previously been coated on the inside with dozens of precisely controlled layers of silicon dioxide doped with germanium. Each layer is only about a hundredth of a millimeter thick. The tube is collapsed into solid rod called a preform, which is.then drawn into a fiber a few kilometers long.
In the best fiber specimens the transmission losses can be as low as one decibel per kilometer, which is equivalent to 80 percent transmission of the input energy. Such low losses, however, cannot be achieved at the operating frequency of available light sources; a morerealistic average loss figure is four or five decibels per kilometer, or about 30 percent transmission of the input energy. Even at this value laser-light pulses can be transmitted a distance of 14 kilometers before amplification is needed. (At that distance only 10-7 of the input energy survives.) Undoubtedly as the sources and detectors are "tuned" to the region of the spectrum where the fibers show a minimum loss at wavelengths somewhat longer than one micrometer, and as the fibers themselves are improved, the distance between amplifiers can be extended substantially beyond 14 kilometers.
The hair-thin light guides are readily assembled into cables. After they are coated for protection against humidity, abrasion and losses due to bending, the fibers are assembled into fiat, color-coded ribbons, each ribbon containing 12 fibers. Up to a dozen ribbons are enclosed in a cable that cushions and protects the individual fibers against damage in field service. Considerable ingenuity was required to devise efficient splicing methods. A technique was finally developed that can align all the fiber ends in a cable to an accuracy of within two micrometers.
Light guides offer a number of advantages over transmission by metallic conductors. Since the light in a light-guide transmission system is tightly confined to the inner core of each fiber. signals cannot leak between adjacent fibers and give rise to "cross talk." Moreover. since light guides are not affected by electrical interference from other sources. lightwave systems should show advantages in carrying information in electrically noisy environments. such as between switching apparatus in telephone central offices.
Cables for light-wave communications offer distinct savings in materials compared with metallic cables of equivalent capacity. At present the optical fibers are much more expensive than copper wire. but this is to be expected when a technologically complex new product is first put into production. Just as there are two kinds of light source for light-wave communications. so there are two types of detector in service. Both are solid-state devices. One is a simple device of the junction type known as a PIN detector. rather similar to a solar cell. in which photons of light generate an electric current. (The letters P, I and N stand for the electronic properties of the semiconductors used in the junction of the detector.) The other device is the avalanche photodetector mentioned above. All signal detectors have a background noise that increases in proportion to their operating speed. For example. the background noise in a PIN detector increases from 10-11 watt when the device is operated at one megabit per second to 10-9 watt at 100 megabits per second. At the same operating speeds the background noise in an avalanche detector is lower by a factor of 10. It follows that the transmission distances for low-speed systems are greater than those for high-speed systems. In light-wave communications the signal detector is the first stage in a receiver mod ule that contains the circuitry needed to adapt the signals for transmission through the existing telecommunication network.