Fiber-based optical communications can be as simple as a point-to-point link where a single data channel is transmitted between two sites. On the other hand, the demand for capacity and flexibility has led to an evolution of the linear links into more complex networks, including star architectures, rings, and meshed designs. As the complexity increases, so will the demand for additional devices required to support the networking functionality. The most important features for using coarse wavelength division multiplexing (CWDM) in the network include multiplexing and demultiplexing of all optical channels or sub-bands, also the selective add and drop of one or more channels at network nodes. While Chapter 8 will address the role of networking for CWDM in more detail, this chapter clearly

defines itself by focusing on optical filters as key components for multiwavelength optical networking in CWDM systems. After Section 4.1 provides a general overview on the benefits for wavelength multiplexing in the metro and access networks, Section 4.2 reviews the technical options for optical filters, including fiber Bragg gratings (FBGs), arrayed waveguide gratings (AWGs), and thin-film filters (TFFs). The properties of the TFF are discussed in more detail, covering manufacturing steps as well as a performance analysis. Section 4.4 addresses the more technical aspects of TFFs, such as 3-ports, multi-ports, modules, and other solutions aiming at a further reduction of packaging size. Finally, the chapter concludes with the investigation of trends toward higher integration and other challenges in the design of CWDM filters.