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Infrared

Infrared light transmissions have existed for many years in something of a backwater in the world of transmission systems, their use having been limited to TV remote controls and wireless slide projector remote controls. However, they now are assuming a position of some, if still limited, importance. Infrared systems use the infrared light spectrum (TeraHertz, or THz, range) to send a focused light beam to a receiver, much as would a microwave system, although no reflective dish is used. Rather, a pair of lenses are used, with a focused lens employed in the transmitting device and a collective lens in the receiving device (Figure 3.6). Infrared is an airwave, rather that a conducted transmission system. Although generally used in short-haul transmission, they do offer substantial bandwidth, but with risks of interference.


Figure 3.6  Infrared transmission system.

Advantages include rapid deployment, especially as there are no licensing requirements as typically is the case with microwave. Additionally, infrared offers fairly substantial bandwidth of [le]6.312 Mbps ([le]T2, or T1 × 4) at relatively low cost. However, infrared systems require line-of-sight and suffer from environmental interference, as do microwave systems; error performance is in the range of 10-8 [3-14]. Additionally, infrared is distance limited, typically [le]2 miles. However, infrared often is an attractive alternative to leased lines or private cabled systems for building-to-building connectivity in a campus environment. Infrared transmission also is used in certain wireless LAN systems and is incorporated into some PDAs (Personal Digital Assistants).

Fiber Optics

The concept of using light waves for communications originally was trialed by Alexander Graham Bell, who in the late 1800s invented and experimented with the photophone, a system utilizing sunlight and mirrors for audio transmission. While his experiment was successful in transmitting very poor quality voice over very short distances, the technique was clearly impractical. During World War II, the Nazi military experimented with similar but more advanced systems, which also proved impractical [3-6].

During the 1940s, experiments with waveguides first were conducted, with both microwave radio and light. Waveguides are rigid, insulated pipes, which serve to contain the electromagnetic energy and channel it from end-to-end, while offering protection from outside interference. While significant transmission speeds can be realized through this process, it is generally impractical and, therefore, is seldom used.

Flexible glass wires offered much more potential as a transmission medium for light. The efforts of the American Optical Corporation resulted as early as the 1950s in optical fiber cable which could carry light signals a few feet. It was at Standard Telecommunications Laboratories in 1966 that Charles Kao and George Hockman developed the first practical conceptual breakthrough—the purity of the glass was the issue. During the early 1970s, the first practical fiber optic systems were developed. These systems were made possible by the manufacturing of pure glass fibers with a silica content high enough (few impurities) to permit the transmission of light over long distances with little loss. At roughly the same time, AT&T Bell Laboratories invented laser diodes, which serve as high speed transmit devices. Since that time, fiber optic development has progressed to the point that virtually all high speed networks are based on fiber optic technology. According to Corning Telecommunications Products [3.15], a major producer of fiber optic cables, demand was estimated at 14.5 million miles in 1995, an increase of 20% to 25% over the previous year.

Fiber optic transmission systems are opto-electric in nature. In other words, a combination of optical and electrical electromagnetic energy is involved. The signal originates as an electrical signal, which is translated into an optical signal, which subsequently is reconverted into an electrical signal at the receiving end.

Configuration

Fiber optic systems consist of light sources, cables and light detectors, as depicted in Figure 3.7. In a simple configuration, one of each is used. In a more complex configuration over longer distances, many such sets of elements are employed. Much as is the case in other transmission systems, long haul optical communications involves a number of regenerative repeaters. In a fiber optic system, repeaters are optoelectric devices. On the incoming side of the repeater, a light detector receives the optical signal, converts it into an electrical signal, boosts it, converts it into an optical signal, and places it onto a fiber, and so on. There may be many such optical repeaters in a long haul transmission system, although typically far fewer than would be required using other transmission media.


Figure 3.7  Fiber optic system, consisting of a light source, a glass fiber, and a light detector.

Light Sources

These are of two types, Light Emitting Diodes (LEDs) and Diode Lasers or (Semiconductor Lasers).

Light Emitting Diodes (LEDs) are common household semiconductor components, which are found in clocks, calculators, and a plethora of other devices. The LEDs used in fiber optic transmission are, of course, much more sophisticated. LEDs predominated in early fiber optic systems largely because of their much lower costs of acquisition and operation. However, they provide less bandwidth and generate infrared light waves that are capable of traveling only relatively short distances with acceptable error performance. LEDs have found continuing application in short haul transmission systems.

Diode Lasers are similar in structure to LEDs, although they are much more difficult and expensive to manufacture. They also are associated with more expensive and complex supporting electronics. They require cooling and consume more power than do LEDs. However, they offer much more bandwidth, as they can pulse on and off in a fraction of a nanosecond, one billionth of a second. Further, contemporary diode lasers create light frequencies which attenuate much less; hence, the signal can travel much farther without being repeated.


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