Optical Networks: A Practical Perspective - Part 33. This book describes a revolution within a revolution, the opening up of the capacity of the now-familiar optical fiber to carry more messages, handle a wider variety of transmission types, and provide improved reliabilities and ease of use. In many places where fiber has been installed simply as a better form of copper, even the gigabit capacities that result have not proved adequate to keep up with the demand. The inborn human voracity for more and more bandwidth, plus the growing realization that there are other flexibilities to be had by imaginative use of the fiber, have led people. | 290 Transmission System Engineering today to increase the available bandwidth and hence the number of wavelengths in a single fiber. Amplifiers are used in three different configurations as shown in Figure . An optical preamplifier is used just in front of a receiver to improve its sensitivity. A power amplifier is used after a transmitter to increase the output power. A line amplifier is used typically in the middle of the link to compensate for link losses. The design of the amplifier depends on the configuration. A power amplifier is designed to provide the maximum possible output power. A preamplifier is designed to provide high gain and the highest possible sensitivity that is the least amount of additional noise. A line amplifier is designed to provide a combination of all of these. Unfortunately the amplifier is not a perfect device. There are several major imperfections that system designers need to worry about when using amplifiers in a system. First an amplifier introduces noise in addition to providing gain. Second the gain of the amplifier depends on the total input power. For high input powers the EDFA tends to saturate and the gain drops. This can cause undesirable power transients in networks. Finally although EDFAs are a particularly attractive choice for WDM systems their gain is not flat over the entire passband. Thus some channels see more gain than others. This problem gets worse when a number of amplifiers are cascaded. We have studied optically preamplified receivers in Section . In this section we will study the effect of gain saturation gain nonflatness noise and power transients in systems with cascades of optical amplifiers. Gain Saturation in EDFAs An important consideration in designing amplified systems is the saturation of the EDFA. Depending on the pump power and the amplifier design itself the output power of the amplifier is limited. As a result when the input signal power is increased Transmitter D. Receiver Power .
