Omitted hut Raman Omitted hut Non-Raman w/Raman Optics Primer Long-Haul Network Trends for Point-to-Point Links Figure 2-37 Trends in Regenerator Spacing regeneration and the 400 km transition to optical networking. Repeater Span ~ 60-100 km Regenerator Spacing ~ 3000 km Repeater Span ~ 40-80 km Regenerator Spacing ~ 3000 km Repeater Span ~ 200 km Metro Network Trends to Optical Connectivity Backbone Network Hub Edge Router Metro-Core Metro-Access Chapter 2 2.9 DWDM Enablers 2.9.1 Thin-Film Filters Thin-film filters can be used to demultiplex wavelengths in a WDM system (see Figure 2-38). Thin-film filters rely on an interference effect created by a highly reflective mirror and a linear variable filter that are coated onto opposite sides of a glass substrate. Typically, several wavelengths are passed through the filter on a single beam of light; as the beam of light encounters the variable filter, the demultiplexing function occurs. The process can also be designed to perform the multiplexing functions. Thin- film filters offer a cost-effective, high-performance solution for low to medium channel counts, and unsurprisingly thin-film technology is the pri- mary method used for networks of 16 to 40 channels with 400-GHz and/or 200-GHz spacing. The perennial manufacturing goal is to continuously improve product yields; add to that the goal of finding economically feasible methods for pro- ducing 100- and 50-GHz channel spacing or channel counts. A solution in support of the goal of narrower channel spacing and higher channel counts is interleaving technology. Figure 2-39 shows how interleaving technology can be used to achieve 50-GHz channel spacing. On the left-hand side of the figure, interleaving combines two separate multiplexing and demultiplex- ing devices. One mux interleaves the odd channels, whereas the second interleaves the even channels. On the right-hand side of the figure, 200- Figure 2-38 Thin-film filters. l 1,2,3,4 Thin-Film Filters Linear Variable Filter Mirror Optics Primer Figure 2-39 Interleavin