E 4.49 Example of WDM mux/demux realized with thin film filters. Thin-film filters have also the quality of being low loss and very controlled in shape; thus, they constitute excellent elements for mux/demux. Unfortunately, it is very difficult to obtain thin-film filters suitable for processing DWDM channels, whose distance is 100 or even 50 GHz with a modulation of 10 Gbit/s, while chan- nels modulated at 40 Gbit/s are spaced 100 or 200 GHz. To work at this small spacing, thin-film filters require a too high number of different coatings decreasing the repeatability and generating too great prices to be adopted in real products. Practically all DWDM systems use array waveguide (AWG) multiplexers/demultiplexers Figure 4.50 shows a schematic layout of an AWG demultiplexer; a multiplexer is identical but for the fact that several signals enter from the waveguides that are the demultiplexer output and the WDM comb emerges from the demultiplexer input. The device consists of three main parts which are as follows: 1. Multiple input and output waveguides 2. Two slab waveguide star couplers (also called free propagation region [FPR]) 3. A dispersive waveguide array with the equal length difference between adjacent waveguides Dispersive array Input Constructive Output interference on the correct output port FIGURE 4.50 Scheme of an AWG demultiplexer. Telecommunication Networks TABLE 4.12 Characteristics of a Realistic AWG Used for Multiplexing/Demultiplexing DWDM Channels Number of channel Channel spacing 100 GHz Band profile Gaussian Center wavelengths ITU grid Center wavelength accuracy +5GHz -3dB bandwidth 0.4nm Insertion loss 6.5 dB Cross-talk in adjacent channel -25dB 0.5ps Dispersion -10ps/nm The operation principle of the AWG multiplexer/demultiplexer is described as follows. A DWDM signal launched into one of the input waveguides will be diffracted in the first slab region and coupled into the arrayed waveguide by the first FPR. The length of the AWGs has been designed such that the optical path length difference