250 CHAPTER 6. OPTICAL AMPLIFIERS
Figure 6.14: Measured gain profile of a Raman amplifier with nearly flat gain over an 80-nm
bandwidth. Pump frequencies and powers used are shown on the right. (After Ref. [30];©c 2001
IEEE; reprinted with permission.)
signal that can be useful for dispersion compensation (see Section 7.7). Fiber amplifiers
can also be made using stimulated Brillouin scattering (SBS) in place of SRS [29]. The
operating mechanism behind Brillouin amplifiers is essentially the same as that for fiber
Raman amplifiers in the sense that both amplifiers are pumped backward and provide
gain through a scattering process. Despite this formal similarity, Brillouin amplifiers
are rarely used in practice because their gain bandwidth is typically below 100 MHz.
Moreover, as the Stokes shift for SBS is∼10 GHz, pump and signal wavelengths nearly
coincide. These features render Brillouin amplifiers unsuitable for WDM lightwave
systems although they can be exploited for other applications.
6.4 Erbium-Doped Fiber Amplifiers
An important class of fiber amplifiers makes use ofrare-earth elementsas a gain
medium by doping the fiber core during the manufacturing process (see Section 2.7).
Although doped-fiber amplifiers were studied as early as 1964 [50], their use became
practical only 25 years later, after the fabrication and characterization techniques were
perfected [51]. Amplifier properties such as the operating wavelength and the gain
bandwidth are determined by the dopants rather than by the silica fiber, which plays the
role of a host medium. Many different rare-earth elements, such as erbium, holmium,
neodymium, samarium, thulium, and ytterbium, can be used to realize fiber ampli-
fiers operating at different wavelengths in the range 0.5–3.5μm. Erbium-doped fiber
amplifiers (EDFAs) have attracted the most attention because they operate in the wave-
length region near 1.55μm [52]–[56]. Their deployment in WDM systems after 1995
revolutionized the field of fiber-optic communications and led to lightwave systems
with capacities exceeding 1 Tb/s. This section focuses on the main characteristics of
EDFAs.