4.3 Gain spectra and DWDM applications 93 4.3 GAIN SPECTRA AND DWDM APPLICATIONS 4.3.1 DWDM amplifiers and average inversion The most widespread applications of EDFAs are in DWDM systems in which channels at closely spaced wavelengths (typically about 40 channels at 100 GHz channel spacing for metro/regional systems or about 80 channels at 50 GHz channel spacing for long-haul systems) are amplified at the end of each span. The spectra of the gain coefficient, g(l), the gain per unit length, in units of dB/m, is given in Figure 4.3 for a typical erbium-doped fiber with aluminum co-doping in the core. The gain spectrum of the EDF in an EDFA can be determined by multiplying the gain coefficient corresponding to length averaged inversion of the erbium-doped fiber by the length of the erbium doped fiber. It can be seen that if the inversion is reasonably high, there will be significant gain in the so-called C-band between about 1525 nm and 1565 nm. From Figure 4.3 it can also be seen that over the C-band the gain as a function of wavelength will be quite non-flat. At high inversion there will be a sharp peak near 1530 nm and at lower inversion there is a broader peak at a wavelength greater than 1550 nm. To obtain the required uniformity of signal power and OSNR over the