Fiber-Optic Technologies

Chapter Description

Vivek Alwayn discusses in this chapter the increasing demand of optical-fiber and its wide spread applications ranging from global networks to desktop computers.


Fiber optics have become the industry standard for the terrestrial transmission of telecommunication information. Fiber optics will continue to be a major player in the delivery of broadband services. Carriers use optical fiber to carry POTS service across their nationwide networks. Today more than 80 percent of the world's long-distance traffic is carried over optical-fiber cables. Telecommunications applications of fiber-optic cable are widespread, ranging from global networks to desktop computers. These involve the transmission of voice, data, and video over distances of less than a meter to hundreds of kilometers, using one of a few standard fiber designs in one of several cable designs. Carriers use optical fiber to carry analog phone service. Cable television companies also use fiber for delivery of digital video services. Intelligent transportation systems and biomedical systems also use fiber-optic transmission systems. Optical cable is also the industry standard for subterranean and submarine transmission systems.

The principle of total internal reflection is used to propagate light signSummaryals. Light is guided through the core, and the fiber acts as an optical waveguide. SMF and MMF cables are constructed differently. MMF has a larger core diameter as compared to SMF. There are two types of propagation for fiber-optic cable: multimode or single mode. These modes perform differently with respect to both attenuation and time dispersion. SMF cable provides better performance than MMF cable. The three primary propagation modes include multimode step index, single-mode step index, and multimode graded index propagation.

In an optical communications system, information from the source is encoded into electrical signals that can drive the transmitter. The transmitter consists of an LED or laser and is pulsed at the incoming frequency. The transmitter performs an EO conversion. The fiber acts as an optical waveguide. At the detector, the signals undergo an OE conversion, are decoded, and are sent to their destination. Fiber-optic system characteristics include attenuation, interference, and bandwidth characteristics. Fiber-optic systems are also secure from data tapping, and tampering can be detected far more easily than with metallic-based transmission medium or free-space propagation. Furthermore, the relatively smaller cross section of fiber-optic cables allows room for substantial growth in the capacity of existing conduits. Attenuation characteristics can be classified as intrinsic and extrinsic. Intrinsic attenuation occurs because of substances inherently present in the fiber, whereas extrinsic attenuation occurs because of external influences such as bending.

Decibel loss at the connector interface is directly proportional to the alignment accuracy and rigidity of the connector. Many types of optical connectors are in use. The one you use depends on the equipment you use it with and the application you use it on. Seamless permanent or semipermanent optical connections require fibers to be spliced. Fiber-optic cables might have to be spliced together for any of a number of reasons. One reason is to realize a link of a particular length. Connecting two fiber-optic cables requires precise alignment of the mated fiber cores or spots in a single-mode fiber-optic cable. This is required so that nearly all the light is coupled from one fiber-optic cable across a junction to the other fiber-optic cable. The two main splicing techniques in use are mechanical and fusion splicing.

Optical loss, or total attenuation, is the sum of the losses of each individual component between a transmitter and receiver. Loss-budget analysis is the calculation and verification of a fiber-optic system's operating characteristics. This encompasses items such as fiber routing, electronics, wavelengths, fiber type, and circuit length. Attenuation and nonlinear fiber characteristics are the key parameters for fiber span analysis. Transmitter launch power, receiver sensitivity, and the dynamic range of the receiver are crucial numbers used in span analysis.

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