I. Historical Introduction
Communication implies transfer of information from one point to another. When it is necessary to transmit information, such as speech, images, or data, over a distance, one generally uses the concept of carrier wave communication. In such a system, the information to be sent modulates an electromagnetic wave such as a radio wave, microwave, or light wave, which acts as a carrier. This modulated wave is then transmitted to the receiver through a channel and the receiver demodulates it to retrieve the imprinted signal. The carrier frequencies associated with TV broadcast (~ 50–900 MHz) are much higher than those associated with AM radio broadcast (~ 600 kHz–20 MHz). This is due to the fact that, in any communication system employing electromagnetic waves as the carrier, the amount of information that can be sent increases as the frequency of the carrier is increased. Obviously, TV broadcast has to carry much more information than AM broadcasts. Since optical beams have frequencies in the range of 1014 to 1015 Hz, the use of such beams as the carrier would imply a tremendously large increase in the information-transmission capacity of the system as compared to systems employing radio waves or microwaves.
In a conventional telephone hookup, voice signals are converted into equivalent electrical signals by the microphone and are transmitted as electrical currents through metallic (copper or aluminum) wires to the local telephone exchange. Thereafter, these signals continue to travel as electric currents through metallic wire cable (or for long-distance transmission as radio/microwaves to another telephone exchange) usually with several repeaters in between. From the local area telephone exchange, at the receiving end, these signals travel via metallic wire pairs to the receiver telephone, where they are converted back into corresponding sound waves. Through such cabled wire-pair telecommunication systems, one can at most send 48 simultaneous telephone conversations intelligibly. On the other hand, in an optical communication system that uses glass fibers as the transmission medium and light waves as carrier waves, it is distinctly possible today to have 35,000 or more simultaneous telephone conversations (equivalent to a transmission speed of about 2.5 Gbit/s) through one glass fiber no thicker than a human hair. This large information-carrying capacity of a light beam is what generated interest among communication engineers and caused them to explore the possibility of developing a communication system using light waves as carrier waves.
The idea of using light waves for communication can be traced as far back as 1880 when Alexander Graham Bell invented the photophone (see Figure 7-2) shortly after he invented the telephone in 1876. In this remarkable experiment, speech was transmitted by modulating a light beam, which traveled through air to the receiver. The flexible reflecting diaphragm (which could be activated by sound) was illuminated by sunlight. The reflected light was received by a parabolic reflector placed at a distance of about 200 m. The parabolic reflector concentrated the light on a photoconducting selenium cell, which formed a part of a circuit with a battery and a receiving earphone. Sound waves present in the vicinity of the diaphragm vibrated the diaphragm, which led to a consequent variation of the light reflected by the diaphragm. The variation of the light falling on the selenium cell changed the electrical conductivity of the cell, which in turn changed the current in the electrical circuit. This changing current reproduced the sound on the earphone.
Figure 7-2 Schematic of the photophone invented by Bell. In this system, sunlight was modulated by a vibrating diaphragm and transmitted through a distance of about 200 meters in air to a receiver containing a selenium cell connected to the earphone.
After succeeding in transmitting a voice signal over 200 meters using a light signal, Bell wrote to his father: "I have heard a ray of light laugh and sing. We may talk by light to any visible distance without any conducting wire." To quote from Maclean: "In 1880 he (Graham Bell) produced his 'photophone' which, to the end of his life, he insisted was '
The modern impetus for telecommunication with carrier waves at optical frequencies owes its origin to the discovery of the laser in 1960. Earlier, no suitable light source was available that could reliably be used as the information carrier. At around the same time, telecommunication traffic was growing very rapidly. It was conceivable then that conventional telecommunication systems based on, say, coaxial cables, radio and microwave links, and wire-pair cable, could soon reach a saturation point. The advent of lasers immediately triggered a great deal of investigation aimed at examining the possibility of building optical analogues of conventional communication systems. The very first such modern optical communication experiments involved laser beam transmission through the atmosphere. However, it was soon realized that shorter-wavelength laser beams could not be sent in open atmosphere through reasonably long distances to carry signals, unlike, for example, the longer-wavelength microwave or radio systems. This is due to the fact that a laser light beam (of wavelength about 1 mm) is severely attenuated and distorted owing to scattering and absorption by the atmosphere. Thus, for reliable light-wave communication under terrestrial environments it would be necessary to provide a "guiding" medium that could protect the signal-carrying light beam from the vagaries of the terrestrial atmosphere. This guiding medium is the optical fiber, a hair-thin structure that guides the light beam from one place to another as was shown in Figure 7-1. The guidance of the light beam through the optical fiber takes place because of total internal reflection, which we discuss in the following section.
Figure 7-3 A typical fiber optic communication system: T, transmitter; C, connector; S, splice; R, repeater; D, detector
In addition to the capability of carrying a huge amount of information, optical fibers fabricated with recently developed technology are characterized by extremely low losses (< 0.2 dB/km), as a consequence of which the distance between two consecutive repeaters (used for amplifying and reshaping the attenuated signals) could be as large as 250 km. We should perhaps mention here that it was the epoch-making paper of Kao and Hockham in 1966 that suggested that optical fibers based on silica glass could provide the necessary transmission medium if metallic and other impurities could be removed. Indeed, this 1966 paper triggered the beginning of serious research in developing low-loss optical fibers. In 1970, Kapron, Keck, and Maurer (at Corning Glass in USA) were successful in producing silica fibers with a loss of about 17 dB/km at a wavelength of 633 nm. (Kapron, Keck, and Maurer) Since then, the technology has advanced with tremendous rapidity. By 1985 glass fibers were routinely produced with extremely low losses (< 0.2 dB/km). Figure 7-3 shows a typical optical fiber communication system. It consists of a transmitter, which could be either a laser diode or an LED, the light from which is coupled into an optical fiber. Along the path of the optical fiber are splices, which are permanent joints between sections of fibers, and repeaters that boost the signal and correct any distortion that may have occurred along the path of the fiber. At the end of the link, the light is detected by a photodetector and electronically processed to retrieve the signal.
