cted with standardized interfaces, which were discussed in Section 4.6. As we discussed in Section 4.2.5, optical systems transmit light energy pulses to the fiber; they do not use light as a carrier the same way as in radio communications. In bidirectional systems two fibers, one for each transmis- sion direction, are needed as shown in Figure 4.33. However, development of semiconductor laser technology has made narrow bandwidth lasers avail- able and several parallel optical signals at different wavelengths can use the same fiber. This wavelength-division multiplexing (WDM) uses an optical coupler to combine optical signals (WDM multiplexer) and optical filters (WDM demultiplexer) to separate optical signals at the receiving end as shown in Figure 4.33. 4.8.7 WDM Many single-mode fiber cables have been installed and technical solutions that increase fiber capacity without installation of new cable have become very attractive as the demand for transmission capacity increases. Particularly in long-distance systems, WDM has become popular and it can increase fiber capacity by a factor from 10 to 100. Optical Fiber 1 Electrical signal Electrical signal Fiber 2 Optical signal signal Optical signals at different A1 A2 A3 Demux wavelengths Optical signals Demux A1 A2 A3 at different wavelengths Figure 4.33 Optical fiber system and WDM. Introduction to Telecommunications Network Engineering Cooled distributed feedback (DFB) lasers are available in precisely selected wavelengths. The ITU has defined a laser grid for point-to-point DWDM systems based on 100-GHz wavelength spacing. There are 45 defined wavelengths in a range from 196.1 THz (1,528.77 um) to 191.7 THz (1,563.86 um) in the third window ("L" band in Figure 4.30), which is a compatible range for the EDFAs discussed later. Manufacturers can deviate from the grid by extending the upper or lower bounds or by spacing wave- lengths more closely, typically at 50 GHz or even down to 25 GHz to double or triple the number of channels. Each optical channel can be used f