..n X1 2 13 .An 12 13 .44 13...14 5...An Fiber Multiplexer Demultiplexer Arrayed Waveguides Arrayed Waveguides Input Output Input Output Coupler Coupler Coupler Coupler Phased Array Phased Array AL = Constant Fiber AL = Constant Input Planar "free space" Coupler Planar "free space" Coupler Input Waveguides Waveguides 1,2...n 1, 2 n WDM can be viewed as a sublayer to SONET. WDM can replace a num- ber of distinct SONET transmission systems with a single WDM transmis- sion system and a single-fiber pair that support the (numerous) constituent SONET end systems, as shown in Figure 2-32. DWDM systems saw major deployment in the United States in the late 1990s. 16 Figure 2-33 shows the optical window of operation for WDM. Table 2-8 provides a view of the sys- tem-level evolutions over time. Table 2-9 reviews the evolution of all-optical networks, whereas Figure 2-34 shows technology generations directly applicable to WDM. Finally, Table 2-10 summarizes the entire space of opti- cal technologies on a time line. Using FDM techniques on a fiber transmis- sion system, pair gain was achieved where multiple signals operating at different frequencies could each carry OC-48 or OC-192 channels between endpoints. Signal spacing improved to the 100-, 50-, and even 25-GHz range. In long-haul applications, optical amplification conjoined with WDM enables the network planner either to space out hub, regeneration, or amplification sites (as shown in Figure 2-35) or to skip amplification at some sites using techniques such as Raman amplification (as shown in Fig- ure 2-36). Figure 2-37 shows another view in the trend towards all-optical networks. Optics Primer Before WDM Figure 2-32 DWDM designs. 1310nm 1310nm 1310nm 1310nm repeater repeater repeater repeater SONET SONET < - 40km > < 40 km - > < 40km- > < 40km < 40 km - > SONET SONET SONET SONET SONET SONET SONET SONET After WDM SONET SONET SONET SONET SONET SONET SONET SONET SONET SONET Table 2-8 Phase 1 (1977-present) Asynchronous TDM (optical DS3s and SO on) System-Level Evolu- Pha