er (MMF) base. CWDM modules based on 840-nm wavelength Vertical Cavity Sur- face-Emitting Lasers (VCSELs) or 1,310-nm DFB lasers have already been developed, enabling 4X2.5 Gbps transmission over 100-m MMF or 300-m Chapter 6 MMF, respectively, both with a 62.5-um core diameter. However, recent demonstrations of 10-Gbps VCSEL transmitters and MMF receivers together with the progress of silicon germanium (SiGe) electronics show the feasibility of a straightforward, serial, high-speed solution without the added complexity of optical multiplexing and demultiplexing. Indeed, 10- Gbps data transmissions over record distances of 1.6 km or 2.8 km of a new high-bandwidth 50-um core diameter MMF have been reported recently.6,7 Subcarrier multiplexing and multilevel coding as alternative upgrading methods have not yet been demonstrated in sufficient quality and are not considered currently competitive. Likewise, today's parallel optical mod- ules, already with an aggregate data rate up to 30 Gbps through 12 chan- nels, seem unsuitable due to the high cost of a few 100-m-long fiber ribbon cable.9 With a further increase of bandwidth demand beyond the direct cur- rent modulation capabilities of laser transmitters, CWDM approaches will gain importance for building backbone links. Researchers have already demonstrated a 4-channel, 10-Gbps CWDM link over 310 m of high-perfor- mance MMF cable.10,1 The second application is in support of Ethernet in the First Mile (EFM) work of the IEEE. Figure 6-9 shows a CWDM application in the point-to- point Ethernet mux application,5 and Figure 6-10 shows a CWDM applica- tion in the Ethernet passive optical network (EPON).5 Others advocate what we call a poor man's metropolitan network, which is a network achieved by extending the optics of GbE to cover longer distances on the assumption that dark fiber can be secured by an end user who will assume the economic burden. Typically, these networks are not carrier class, do not have the requisite security, and are difficult to scale and manage. T