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Optical Networks: A Practical Perspective - Part 20

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Optical Networks: A Practical Perspective - Part 20. 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. | 160 Components Amplifier site Amplifier site Figure 3.38 Distributed Raman amplifier using a backward propagating pump shown operating along with discrete erbium-doped fiber amplifiers. amplifier as well as a distributed amplifier. In the lumped case the Raman amplifier consists of a sufficiently long spool of fiber along with the appropriate pump lasers in a package. In the distributed case the fiber can simply be the fiber span of interest with the pump attached to one end of the span as shown in Figure 3.38. Today the most popular use of Raman amplifiers is to complement EDFAs by providing additional gain in a distributed manner in ultra-long-haul systems. The biggest challenge in realizing Raman amplifiers lies in the pump source itself. These amplifiers require high-power pump sources of the order of 1 W or more at the right wavelength. We will study some techniques for realizing these pump sources in Section 3.5.5. The noise sources in Raman amplifiers are somewhat different from EDFAs. The Raman gain responds instantaneously to the pump power. Therefore fluctuations in pump power will cause the gain to vary and will appear as crosstalk to the desired signals. This is not the case with EDFAs. We will see in Section 3.4.6 that the response time of the gain is much slower on the order of milliseconds in those devices. Therefore for Raman amplifiers it is important to keep the pump at a constant power. Having the pump propagate in the opposite direction to the signal helps dramatically because fluctuations in pump power are then averaged over the propagation time over the fiber. To understand this first consider the case where the pump propagates along with the signal in the same direction. The two waves travel at approximately the same velocity. In this case when the pump power is high at the input the signal sees high gain and when the power is low the signal sees a lower gain. Now consider the case when the signal and pump travel in opposite directions. To .