g Technologies Interleaving technologies. 200 Ghz 1 X 8 200 Ghz 1 X 4 1,2,3 1 X 8 1,2,3, .8 1 X 4 200 Ghz @ 50 Ghz @ 50 Ghz 1 X 4 1 X 8 200 Ghz 1 X 4 GHz spacing is interleaved to support 50-GHz applications. This approach adds channels in groups of four instead of two. One benefit of interleaving is that it enables a carrier to add capacity on an as-needed basis. Remem- ber that this approach tends to become costly in the higher channel count systems. In addition, as the channel count increases beyond 16, insertion loss increases in the overall system. 2.9.2 Fiber Bragg Grating (FBG) FBG is a technique used to separate and combine wavelengths. It does SO by changing the structural core of standard single-mode fiber. The fiber strand is exposed to a high-power ultraviolet light, which creates a periodic change in the refractive index along the core by creating ridges on a short section of fiber. The periodic change in the fiber core enables narrowband wave- length selection. Each fiber grating isolates a single wavelength (see Figure 2-40). Advantages of FBG technology include wavelength uniformity and low insertion loss. The primary limitation is that it performs better at mod- erate channel counts (under 16 channels), and it is difficult to scale for the higher channel count systems. 2.9.3 Arrayed Waveguide Grating (AWG) AWGs are painstakingly built by depositing carefully engineered and com- posed glass layers on a silica or silicon substrate. To guide light pulses through the gratings, glass layers are patterned and etched with standard semiconductor processes. Figure 2-41 illustrates how the input signal, Chapter 2 Figure 2-40 Fiber Bragg Grating 1,2,3,4 2,3,4 Figure 2-41 Arrayed waveguides. Arrayed Waveguides Waveguide Array; Guides Differ in Length Coupler Combines Light Input Coupler Splits Light l 1,2,3,4 Among Many Diffraction Waveguides Separates Wavelengths arriving from a distant destination over a single fiber, is fractioned by the origination coupler and passed on to multiple curved waveguides. Eac