Optical communication

(communications) The use of electromagnetic waves in the region of the spectrum near visible light for the transmission of signals representing speech, pictures, data pulses, or other information, usually in the form of a laser beam modulated by the information signal.

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. See also Electromagnetic radiation; Light.

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 also Laser; Light-emitting diode; Optical fibers.

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. See also Waveguide.

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 also 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. See also Photodiode.

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 also 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. See also Heterodyne principle; Radio receiver.

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 also Optical information systems.

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Optical communication is any form of telecommunication that uses light as the transmission medium.

An optical communication system consists of a transmitter, which encodes a message into an optical signal, a channel, which carries the signal to its destination, and a receiver, which reproduces the message from the received optical signal.

Contents 1 Forms of optical communication 2 Optical fiber communication 3 Free-space optical communication 4 See also 5 References

Forms of optical communication

Early optical communication

There are many forms of non-technological optical communication, including body language and sign language.

Techniques such as semaphore lines, ship flags, smoke signals, and beacon fires were the earliest form of technological optical communication.

The heliograph uses a mirror to reflect sunlight to a distant observer. By moving the mirror the distant observer sees flashes of light that can be used to send a prearranged signaling code. Navy ships often use a signal lamp to signal in Morse code in a similar way.

Distress flares are used by mariners in emergencies, while lighthouses and navigation lights are used to communicate navigation hazards.

Aircraft use the landing lights at airports to land safely, especially at night. Aircraft landing on an aircraft carrier use a similar system to land correctly on the carrier deck. The light systems communicate the correct position of the aircraft relative to the best landing glideslope.

Optical fiber is the most common medium for modern digital optical communication.

Free-space optical communication is also used today in a variety of applications.

Optical fiber communication

Main article: Fiber-optic communication.

Optical fiber is the most common type of channel for optical communications, however, other types of optical waveguides are used within communications gear, and have even formed the channel of very short distance (e.g. chip-to-chip, intra-chip) links in laboratory trials. The transmitters in optical fiber links are generally light-emitting diodes (LEDs) or laser diodes. Infrared light, rather than visible light is used more commonly, because optical fibers transmit infrared wavelengths with less attenuation and dispersion. The signal encoding is typically simple intensity modulation, although historically optical phase and frequency modulation have been demonstrated in the lab. The need for periodic signal regeneration was largely superseded by the introduction of the erbium-doped fiber amplifier, which extended link distances at significantly lower cost.

Free-space optical communication

Main article: Free space optical communication.

Free Space Optics (FSO) systems are generally employed for 'last mile' communications and can function over distances of several kilometers as long as there is a clear line of sight between the source and the destination, and the optical receiver can reliably decode the transmitted information. IrDA is an example of low-data-rate, short distance free-space optical communications using LEDs.

See also

• Laser
• Optical telegraph
• Telegraph
• Jun-Ichi Nishizawa an inventor of optical communication.
• Opto-isolator
• Fiber tapping
• Modulating retro-reflector

References

• Encyclopedia of Laser Physics and Technology
• Fiber-Optic Technologies by Vivek Alwayn

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