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Optical Networks - _3_4 Optical Amplifiers_38

Optical Networks - _3_4 Optical Amplifiers_38 - 3.4 Optical...

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3.4 Optical Amplifiers 157 any periodic filter can be used as an interleaver by matching its period to the de- sired channel spacing. For example, a fiber-based Mach-Zehnder interferometer is a common choice. These devices are now commercially available, and interleaving is becoming a popular approach toward realizing high channel count multiplexers and demultiplexers. 3.4 Optical Amplifiers In an optical communication system, the optical signals from the transmitter are at- tenuated by the optical fiber as they propagate through it. Other optical components, such as multiplexers and couplers, also add loss. After some distance, the cumulative loss of signal strength causes the signal to become too weak to be detected. Before this happens, the signal strength has to be restored. Prior to the advent of optical amplifiers over the last decade, the only option was to regenerate the signal, that is, receive the signal and retransmit it. This process is accomplished by regenerators. A regenerator converts the optical signal to an electrical signal, cleans it up, and converts it back into an optical signal for onward transmission. Optical amplifiers offer several advantages over regenerators. On one hand, re- generators are specific to the bit rate and modulation format used by the communi- cation system. On the other hand, optical amplifiers are insensitive to the bit rate or signal formats. Thus a system using optical amplifiers can be more easily upgraded, for example, to a higher bit rate, without replacing the amplifiers. In contrast, in a system using regenerators, such an upgrade would require all the regenerators to be replaced. Furthermore, optical amplifiers have fairly large gain bandwidths, and as a consequence, a single amplifier can simultaneously amplify several WDM signals. In contrast, we would need a regenerator for each wavelength. Thus optical ampli- fiers have become essential components in high-performance optical communication systems. Amplifiers, however, are not perfect devices. They introduce additional noise, and this noise accumulates as the signal passes through multiple amplifiers along its path due to the analog nature of the amplifier. The spectral shape of the gain, the output power, and the transient behavior of the amplifier are also important considerations for system applications. Ideally, we would like to have a sufficiently high output power to meet the needs of the network application. We would also like the gain to be flat over the operating wavelength range and to be insensitive to variations in input power of the signal. We will study the impact of optical amplifiers on the physical layer design of the system in Chapters 4 and 5. Here we explore their principle of operation.
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158 Components E 1 E 2 Absorption Stimulated emission Stimulated emission Optical signal Figure 3.33 Stimulated emission and absorption in an atomic system with two energy levels.
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