Chapter 3: Dense WDM and Demultiplexers

3.1.1 Dense WDMs Are Making Optical Network Design Practical

High density wavelength division multiplexing (HDWDM), typically 32 channels at 100-GHz spacing, is widely used in telecommunication optical networks. The commercial availability of very high density wavelength division multiplexing (VHDWDM), typically 96 channels, 50-GHz spacing was announced in 2000 (Figure 3.1). The feasibility of a switchless network at a national scale with 800 wavelengths with a spacing down to a few gigahertz is now being studied within a new European research program called ACTS 3rd, SONATA.

Figure 3.1: 96 channels, 50-GHz-spacing, athermal dense multiplexer WDM. ( Source: HighWave Optical Technologies.)

Why is there such an explosive growth rate? WDMs multiply network capacity at affordable costs and provide unmatched possibilities for optical routing and optical switching. WDMs also provide signal transparency and large network design flexibility. Some years ago, bit rates of a few Gbps, available on SM fibers without optical multiplexing, seemed to be more adequate. However, they failed to meet the needs of the new multimedia communications appearing now. Today, networks need to deal with the Terabits/s rates required by growing needs for computer networking through private links, TV and HDTV broadcasting, Internet data communications, and video conference on the public networks. Increasing volumes of signals have to be transmitted, switched, processed, and exchanged through the different nodes of multi-Gbps optical systems.

It would be interesting to try to guess how much bandwidth would be necessary to get as much information as...