Optical Communication

communication by means of electromagnetic waves in the optical region—generally frequencies of 10¹³-10¹⁵ Hz. The use of light in very simple, or low information, communication systems has a long history, which includes such interesting inventions as the optical telegraph. The appearance of lasers opened up the possibility of applying to light waves the principles and means of obtaining, processing, and transmitting information that had been developed for radio waves.

Today’s tremendous growth in the volume of transmitted information and almost total exhaustion of the capacity of the radio-frequency region have imparted great urgency to the problem of using light waves for communication. The principal advantages of optical communication over radio-frequency communication result from light waves’ high frequencies, or short wavelengths, and include the great width of the frequency band used to transmit information, a width 10⁴ times greater than the frequency band of the entire radio-frequency region, and the high directivity of radiation with input and output apertures much smaller than the apertures of antennas for radio waves. This high directivity makes it possible to use relatively low-power generators in the transmitters of optical communication systems and provides greater noise immunity and secrecy of communication.

An optical communication line is structurally similar to a radio communication line. There are two ways to modulate the oscillation of an optical generator. Either the oscillation process is controlled by manipulating the power source or the generator’s optical resonator, or supplementary external devices that alter the output radiation as necessary are used. The radiation is shaped by the output unit into a low-divergence beam that travels to the input unit, where it is focused on the active surface of a photoconverter. Electrical signals pass from the photocon-verter to a processor. The selection of the carrier frequency in an optical communication system is a difficult and complicated task—attention must be paid to the conditions of propagation of the optical radiation in the transmission medium and to the technical characteristics of the lasers, modulators, light detectors, and optical assemblies.

Two methods of signal detection—direct detection and heterodyne reception—find application in optical communication systems. The heterodyne method of reception has a number of advantages, chief among which are higher sensitivity and discrimination of background noise, but it is technically much more complicated than direct detection. The dependence of signal magnitude at the output of the photodetector on the characteristics of the signal’s path is a serious shortcoming of the heterodyne method.

According to their operating range, optical communication systems can be divided into the following classes: open overland short-range systems using the lower layers of the atmosphere for light propagation; overland systems that use closed light guides, such as fiber light guides and light-guiding mirror-lens structures, for high-information communication between automatic telephone exchanges, electronic computers, or cities; high-information communication lines—primarily relay lines—that operate in space in the vicinity of the earth; and long-range communication lines in space.

Considerable experience has been acquired in the USSR and abroad in open optical communication lines using lasers in the lower layers of the atmosphere. The great dependence of communications reliability on atmospheric conditions, which determine optical visibility, in the line of propagation has been shown to limit the use of open optical communication lines to such relatively short distances as a few kilometers. Moreover, open lines are restricted to such uses as backups for existing cable lines or use in low-information mobile systems and signaling systems. Open optical communication lines, however, are a promising means of communication between the earth and space. For example, information can be transmitted by a laser beam over a distance of about 10⁸ km at a rate of up to 10⁵ bits/sec, while microwave equipment permits at these distances a transmission rate of just 10 bits/sec. In principle, optical communication in space is possible for up to 10¹⁰ km, a distance inconceivable for other communication systems. It is technically extremely difficult, however, to establish optical communication lines in space.

Under earth conditions closed light-guide structures are the most promising approach for optical communication systems.

The possibility of producing glass light guides with a signal attenuation of no more than a few decibels per kilometer was demonstrated in 1964. At present, light-emitting semiconductor diodes operating in both laser, or coherent, and incoherent modes and cables with fiber cores and semiconductor detectors can be used to construct trunk lines carrying thousands of telephone channels with retransmitters separated by distances of about 10 km. Intensive work toward designing laser sources with lifetimes of the order of 10⁴–10^s hours and the development of highly sensitive broadband receivers, of more efficient light guide structures, and of the techniques for manufacturing extremely long light guides apparently will make optical communication competitive with communication over existing cable and relay lines within the next decade. Optical communication can be expected to assume an important place in the national communications network alongside other types of communication. Optical communication systems with light-guide lines may, in the long run, become the chief means of communication between and within cities because of their information capacity and their cost per unit of information.