 The operating wave- lengths of DWDM systems are defined according to a standardized frequency/wavelength grid developed by the International Telecommunica- tions Union (ITU), which was discussed in Chapter 5. DFB lasers are used as sources in DWDM systems. The laser wavelength drifts about 0.08 nm/o with the temperature. DFB lasers are cooled to sta- bilize the wavelength from drifting outside the passband of the multiplexer and demultiplexer filters as the temperature fluctuates in DWDM systems. CWDM systems use DFB lasers that are not cooled. They are specified to operate from 0 to 70°C with the laser wavelength drifting only about 5 to 6 nm over this range (for example, 0.1 nm/oc). This wavelength drift coupled with the variation in laser wavelength of up to +3 nm due to laser die man- ufacturing processes yields a total wavelength variation of about 12 nm. The optical filter passband and laser channel spacing must be wide enough to accommodate the wavelength variation of the uncooled lasers in CWDM systems (see Figure 6-5). Channel spacing in these systems is typically 20 nm with a channel bandwidth of 13 nm. CWDM systems offer some key advantages over DWDM systems for applications that require channel counts on the order of 16 or less. These benefits include better costs, power requirements, and size. Coarse Wavelength Division Multiplexing (CWDM) Technology Figure 6-4 Lambda spacing. Typical Transmittance for a 4-channel Coarse Wavelength Demultiplexer 1570 1580 Wavelength (nm) Typical Transmittance for an 8-channel Dense Wavelength Demultiplexer 7XXXXXXXX Wavelength (nm) The spacing between the individual wavelengths transmitted through the same fiber serves as the basis for differentiating dense wavelength division multiplexing from coarse WDM (CWDM). Spacing in CWDM systems is typically 20 nm, while most DWDM systems today offer 0.8 nm (100 GHz) wavelength separation. 6.2.1 Less Expensive Hardware The cost difference between CWDM and DWDM systems can be attributed to hardware and operating costs. Althou