ill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Transport Technologies Transport Technologies available for the purpose of photodetection, they are perfectly adequate for the requirements of most optical systems. In cases where they are not considered sensitive enough for high-performance systems, they can be coupled with a preamplifier to increase the overall sensitivity. Avalanche Photodiodes (APD) APDs work as optical signal amplifiers. They use a strong electric field to perform what is known as avalanche multiplication. In an APD, the elec- tric field causes current accelerations such that the atoms in the semi- conductor matrix get excited and create, in effect, an avalanche of current to occur. The good news is that the amplification effect can be as much as 30 to 100 times the original signal; the bad news is that the effect is not altogether linear and can create noise. APDs are sensitive to temperature and require a significant voltage to operate them-30 to 300 volts depending on the device. However, they are popular for broad- band systems and work well in the gigabit range. We have now discussed transmitters, fiber media, and receivers. In the next section, we examine the fibers themselves, and how they have been carefully designed to serve as solutions for a wide variety of networking challenges and to forestall the impact of the nonlinearities described in this section. Optical Fiber As was mentioned briefly in a prior section, fiber has evolved over the years in a variety of ways to accommodate both the changing requirements of the customer community and the technological chal- lenges that emerged as the demand for bandwidth climbed precipitously. These changes came in various forms of fiber that presented different behavior characteristics to the market. Modes: An Analogy The concept of "modes" is sometimes difficult to understand, SO let me pass along an analogy that will help.