 associated with its backup, whether in a CO or a wiring closet, reduces operating costs. 6.2.3 Comparable Reliability The reliability of DFB lasers used in DWDM and CWDM transport has been proved in both cooled and uncooled designs. The difference between the two laser designs is the number of additional components, including the Peltier cooler, thermister, and associated electronics in DWDM lasers. How- ever, manufacturers generally claim that no reliability difference exists between the two types of systems in real-world applications. Coarse Wavelength Division Multiplexing (CWDM) Technology 6.2.4 Smaller Physical Size CWDM lasers are significantly smaller than DWDM lasers. Uncooled lasers are typically constructed with a laser die and monitor photodiode mounted in a hermetically sealed container with a glass window. These con- tainers are aligned with a fiber pigtail or an alignment sleeve that accepts a connector. The container and sleeve form a cylindrical package called a Transmitter Optical Subassembly (TOSA). A typical TOSA is approxi- mately 2 cm in length and 0.5 cm in diameter. Cooled lasers are offered in either a butterfly or dual-inline laser package and contain the laser die, monitor photodiode, thermister, and Peltier cooler. These lasers are about 4 cm long, 2 cm high, and 2 cm wide. These devices are almost always pig- tailed, requiring fiber management, a heatsink, and a corresponding moni- tor and control circuitry. The size of a DWDM laser transmitter typically occupies about five times the volume of a CWDM transmitter. 6.2.5 Up to 16 Wavelengths CWDM systems supporting two to eight wavelengths have been commer- cially available for some time. Newer systems (see Figure 6-7) now scale to dB/cm Bands Figure 6-7 Optical bands. Conventional Fiber Loss Conventional Fiber Spectrum EWB Fiber Spectrum EWB Fiber Loss Wavelength Chapter 6 16 wavelengths in the 1,290- to 1,610-nm spectrum. Today most CWDM systems are based on 20-nm channel spacing from 1,470 to 1,610 nm with some development occurring 