Optical Networks - _7_2 Optical Line Amplifiers_88

Optical Networks - _7_2 Optical Line Amplifiers_88 - 438...

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438 WDM Network Elements l OSC l OSC l l l 1 2 , , . . ., W Receiver Laser OADM Gain stage Gain stage Dispersion compensator Raman pump laser Figure 7.3 Block diagram of a typical optical line amplifier. Only one direction is shown. The amplifier uses multiple erbium gain stages and optionally includes dispersion com- pensators and OADMs between the gain stages. A Raman pump may be used to provide additional Raman gain over the fiber span. The OSC is filtered at the input and termi- nated, and added back at the output. 7.2 Optical Line Amplifiers Optical line amplifiers are deployed in the middle of the optical fiber link at periodic intervals, typically 80–120 km. Figure 7.3 shows a block diagram of a fairly standard optical line amplifier. The basic element is an erbium-doped fiber gain block, which we studied in Chapter 3. Typical amplifiers use two or more gain blocks in cas- cade, with so-called midstage access. This feature allows some lossy elements to be placed between the two amplifier stages without significantly impacting the overall noise figure of the amplifier (see Problem 4.5 in Chapter 4). These elements include dispersion compensators to compensate for the chromatic dispersion accumulated along the link, and also the OADMs, which we will discuss next. The amplifiers also include automatic gain control (see Chapter 5) and built-in performance monitoring of the signal, a topic we will discuss in Chapter 8. There are also Raman amplifiers, where a high-power pump laser is used at each amplifier site to pump the fiber in the direction opposite to the signal. The optical supervisory channel is filtered at the input and terminated, and added back at the output. In a system using C- and L-bands, the bands are separated at the input to the amplifier and separate EDFAs are used for each band. 7.3 Optical Add/Drop Multiplexers Optical add/drop multiplexers (OADMs) provide a cost-effective means for handling passthrough traffic in both metro and long-haul networks. OADMs may be used at
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7.3 Optical Add/Drop Multiplexers 439 Add/Drop Node A Node B Node B Node C (a) Add/Drop Node A Node C (b) Transponder OLT OADM Optical passthrough Figure 7.4 A three-node linear network example to illustrate the role of optical add/drop multi- plexers. Three wavelengths are needed between nodes A and C, and one wavelength each between nodes A and B and between nodes B and C. (a) A solution using point-to-point WDM systems. (b) A solution using an optical add/drop multiplexer at node B. amplifier sites in long-haul networks but can also be used as stand-alone network elements, particularly in metro networks. To understand the benefits of OADMs, consider a network between three nodes, say, A, B, and C, shown in Figure 7.4, with IP routers located at nodes A, B, and C. This network supports traffic between A and B, B and C, and A and C. Based on the network topology, traffic between A and C passes through node B. For simplicity, we will assume full-duplex links and full-duplex connections. This is the case for most networks today. Thus the network in Figure 7.4 actually consists of a pair of fibers carrying traffic in opposite directions.
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