FIBER OPTICS People have used light to transmit information for hundreds of years. However, it was not until the 1960s with the invention of the laser that widespread interest in optical (light) systems for data communications began. The invention of the laser prompted researchers to study the potential of fiber optics for data communications, sensing, and other applications. Laser systems could send a much larger amount of data than the telephone, microwave, and other electrical systems. The first experiment with the laser involved letting the laser beam transmit freely through the air. Also, researchers conducted experiments that transmitted the laser beam through different types of waveguides. Glass fibers, gas-filled pipes, and tubes with focusing lenses are examples of optical

waveguides.

Glass fibers soon became the preferred medium for fiber-optic research. Initially, the large losses in the optical fibers prevented coaxial cables from being replaced. 

Loss is the decrease in the amount of light reaching the end of the fiber. Early fibers had losses around 1,000 dB/ km, making them impractical for communications use. In 1969, several scientists concluded that impurities in the fiber material caused the signal loss in optical fibers. The basic fiber material did not prevent the light signal from reaching the end of the fiber. These researchers believed it was possible to reduce the losses in optical fibers by removing the impurities. By removing the impurities, researchers made possible the construction of low-loss optical fibers.

Developments in semiconductor technology that provided the necessary light sources and detectors furthered the development of fiber optics. Conventional light sources, such as lamps or lasers, were not easily used in fiber-optic systems. These light sources tended to be too large and required lens systems to launch light into the fiber. In 1971, Bell Laboratories developed a small area light-emitting diode (LED). This light source was suitable for a low-loss coupling to optical fibers. Researchers could then perform source-to- fiber jointing easily and repeatedly. Early semiconductor sources had operating lifetimes of only a few hours; however, by 1973, projected lifetimes of lasers advanced from a few hours to greater than 1,000 hours. By 1977, projected lifetimes of lasers advanced to greater than 7,000 hours. By 1979, these devices were available with projected lifetimes of more than 100,000 hours.

In addition, researchers also continued to develop new fiber-optic parts. The types of new parts developed included low-loss fibers and fiber cables, splices, and connectors. These parts permitted demonstration and research on complete fiber-optic systems.

Advances in fiber optics have permitted the introduction of fiber optics into present applications. These applications are mostly in the telephone long-haul systems but are growing to include cable television, computer networks, video systems, and data links. Research should increase system performance and provide solutions to existing problems in conventional applications. The impressive results from early research show there are many advantages offered by fiber-optic systems.

FIBER-OPTIC SYSTEMS System design has centered on long-haul communications and the subscriber-loop plants. The subscriber-loop plant is the part of a system that connects a subscriber to the nearest switching center. Cable television is an example. Also, limited work has been done on short-distance applications and some military systems. Initially, central office trunking required multimode optical fibers with moderate to good performance. Fiber performance depends on the amount of loss and signal distortion introduced by the fiber when it is operating at a specific wavelength. Two basic types of optical fibers are used in industry: multimode fibers and single mode fibers.

Future system design improvements depend on continued research. Researchers expect fiber-optic product improvements to upgrade performance and lower costs for short-distance applications. Future systems center on broadband services that will allow transmission of voice, video, and data. Services will include television, data retrieval, video word processing, electronic mail, banking, and shopping.

ADVANTAGES AND DISADVANTAGES OF FIBER OPTICS Fiber-optic systems have many attractive features that are superior to electrical systems. These include improved system performance, immunity to electrical noise, signal security, and improved safety and electrical isolation. Other advantages include reduced size and weight, environmental protection, and overall system economy. Table 6-1 details the main advantages of fiber-optic systems. Despite the many advantages of fiber-optic systems, there are some disadvantages. Because of the relative newness of the technology, fiber-optic components are expensive. Fiber-optic transmitters and receivers are still relatively expensive compared to electrical interfaces. The lack of standardization in the industry has also limited the acceptance of fiber optics. Many industries are more comfortable with the use of electrical systems and are reluctant to switch to fiber optics; however, industry researchers are eliminating these disadvantages.

Standards committees are addressing fiber-optic part and test standardization. The cost to install fiber-optic systems is falling because of an increase in the use of fiber-optic technology. Published articles, conferences, and lectures on fiber optics have begun to System Performance Greatly increased band. width and capacity Immunity to Electrical Immune to noise Noise (electromagnetic interference [EMI] and radio-frequency interference [RFI]) Signal Security Electrical Isolation educate managers and technicians. As the technology matures, the use of fiber optics will increase because of its many advantages over electrical systems.

Table 6-1.- Advantages of Fiber Optics

Lower signal attenuatior (loss)

No cross talk Low bit error rates

Difficult to tap Nonconductive (does no radiate signals)

No common ground required

Freedom from short circuit and sparks

Size and Weight Reduced size and weigh cables

Environmental Protection Resistant to radiation and corrosion

Resistant to temperature variations

Improved ruggedness and flexibility

Less restrictive in harsh environments

Overall System Economy Low per-channel cost Lower installation cost Silica is the principal abundant, and inexpensive material (source is sand)