TABLE OF CONTENTS Optical amplifiers are widely used by the long-haul transmission systems to compensate for the scattering and absorption losses in the optical fibers. Erbium-doped Fiber Amplifiers (EDFAs) are best suited for long-haul communications system applications. The basic principle of the EDFA states that the sum of signal photons pump photons and erbium atoms is equal to the applied signal photons [9]. When pumped by a 980- to 1600-nm laser, the energy level of erbium atoms jumps. Some atoms return to their ground state via spontaneous emission, producing a noise in the signal. However, most of them are knocked back to their ground state by signal photons with wavelengths somewhere between 1510 and 1600 nm. When this happens, the erbium ions emit a photon that has a wavelength identical to the signal, thereby providing a gain. EDFAs currently are used in communications systems to boost signals from 1530 to 1565 nm (C-band). Specialized optical fibers have extended the EDFAs to operate in the L-band region from 1570 to 1620 nm spectrum (Figure 6.10).
In most communications systems, EDFAs with flat-gain response over wideband are preferred (Figure 6.10). In certain applications, both the flat gain and low noise figure are required over 40 to 60 nm spectral region. Note a WDM multiplexing scheme using an EDFA is more efficient and cost-effective compared to a DWDM system, because a DWDM...
