Optical communications

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Optical communications

The transmission of speech, data, video, and other information by means of the visible and the infrared portion of the electromagnetic spectrum.

Optical communication is one of the newest and most advanced forms of communication by electromagnetic waves. In one sense, it differs from radio and microwave communication only in that the wavelengths employed are shorter (or equivalently, the frequencies employed are higher). However, in another very real sense it differs markedly from these older technologies because, for the first time, the wavelengths involved are much shorter than the dimensions of the devices which are used to transmit, receive, and otherwise handle the signals.

The advantages of optical communication are threefold. First, the high frequency of the optical carrier (typically of the order of 300,000 GHz) permits much more information to be transmitted over a single channel than is possible with a conventional radio or microwave system. Second, the very short wavelength of the optical carrier (typically of the order of 1 micrometer) permits the realization of very small, compact components. Third, the highest transparency for electromagnetic radiation yet achieved in any solid material is that of silica glass in the wavelength region 1–1.5 μm. This transparency is orders of magnitude higher than that of any other solid material in any other part of the spectrum.

Optical communication in the modern sense of the term dates from about 1960, when the advent of lasers and light-emitting diodes (LEDs) made practical the exploitation of the wide-bandwidth capabilities of the light wave. See Laser, Light-emitting diode

Optical fiber communications

With the development of extremely low-loss optical fibers during the 1970s, optical fiber communication became a very important form of telecommunication almost instantaneously. For fibers to become useful as light waveguides (or light guides) for communications applications, transparency and control of signal distortion had to be improved dramatically and a method had to be found to connect separate lengths of fiber together.

The transparency objective was achieved by making glass rods almost entirely of silica. These rods could be pulled into fibers at temperatures approaching 3600°F (2000°C).

Reducing distortion over long distances required modification of the method of guidance employed in early fibers. These early fibers (called step-index fibers) consisted of two coaxial cylinders (called core and cladding) which were made of two slightly different glasses so that the core glass had a slightly higher index of refraction than the cladding glass. By reducing the core size and the index difference in a step-index fiber, it is possible to reach a point at which only axial propagation is possible. In this condition, only one mode of propagation exists. These single-mode fibers can transmit in excess of 1011 pulses per second over distances of several hundred miles.

The problem of joining fibers together was solved in two ways. For permanent connections, fibers can be spliced together by carefully aligning the individual fibers and then epoxying or fusing them together. For temporary connections, or for applications in which it is not desirable to make splices, fiber connectors have been developed.

Almost every major metropolitan area in the United States has a light-wave transmission system in service connecting telephone central offices. These systems typically operate at a wavelength of either 1.3 or 1.55 μm (where silicon fibers have a minimum loss). It is anticipated that light-wave systems will gradually be installed in the telephone loop plant—that is, the portion of the telephone plant which connects the individual subscriber to the telephone central office. See Data communications, Facsimile

Optical transmitters

In principle, any light source could be used as an optical transmitter. In modern optical communication systems, however, only lasers and light-emitting diodes are generally considered for use. The most simple device is the light-emitting diode which emits in all directions from a fluorescent area located in the diode junction. Since optical communication systems usually require well-collimated beams of light, light-emitting diodes are relatively inefficient. On the other hand, they are less expensive than lasers and, at least until recently, have exhibited longer lifetimes.

Another device, the semiconductor laser, provides comparatively well-collimated light. In this device, two ends of the junction plane are furnished with partially reflecting mirror surfaces which form an optical resonator. As a result of cavity resonances, the light emitted through the partially reflecting mirrors is well collimated within a narrow solid angle, and a large fraction of it can be captured and transmitted by an optical fiber.

Both light-emitting diodes and laser diodes can be modulated by varying the forward diode current.

Optical receivers

Semiconductor photodiodes are used for the receivers in virtually all optical communication systems. There are two basic types of photodiodes in use. The most simple comprises a reverse-biased junction in which the received light creates electron-hole pairs. These carriers are swept out by the electric field and induce a photocurrent in the external circuit. The minimum amount of light needed for correct reconstruction of the received signal is limited by noise superimposed on the signal by the following circuits.

Avalanche photodiodes provide some increase in the level of the received signal before it reaches the external circuits. They achieve greater sensitivity by multiplying the photogenerated carriers in the diode junction. This is done by creating an internal electric field sufficiently strong to cause avalanche multiplication of the free carriers. See Microwave solid-state devices, Optical detectors

Coherent communication

The transmission systems described above are all incoherent systems. That is, the signal is transmitted and detected without making use of the phase of the emitted light. Many lasers are capable of transmitting light with the phase sufficiently stable that coherent techniques such as homodyne and heterodyne detection can be used exactly as they are used for radio detection. Coherent systems offer the potential for a tremendous increase in bandwidth along with a modest increase in sensitivity.

Photonic interconnects

Advances in technology have opened a new application for optical communication; transmission of very large amounts of data over relatively short distances. Devices for this purpose are known as photonic interconnects. These devices are only a few centimeters in length but they are massively parallel; that is, they carry a very large number (millions or even billions) of individual channels from one chip on an integrated circuit board to another chip on the same or near-by board. See Optical information systems

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