6.4. ERBIUM-DOPED FIBER AMPLIFIERS 251
Figure 6.15: (a) Energy-level diagram of erbium ions in silica fibers; (b) absorption and gain
spectra of an EDFA whose core was codoped with germania. (After Ref. [64];©c1991 IEEE;
reprinted with permission.)
6.4.1 Pumping Requirements.....................
The design of an EDFA looks similar to that shown in Fig. 6.10 with the main differ-
ence that the fiber core contains erbium ions (Er^3 +). Pumping at a suitable wavelength
provides gain through population inversion. The gain spectrum depends on the pump-
ing scheme as well as on the presence of other dopants, such as germania and alumina,
within the fiber core. The amorphous nature of silica broadens the energy levels of
Er^3 +into bands. Figure 6.15(a) shows a few energy levels of Er^3 +in silica glasses.
Many transitions can be used to pump an EDFA. Early experiments used the visible
radiation emitted from argon-ion, Nd:YAG, or dye lasers even though such pumping
schemes are relatively inefficient. From a practical standpoint the use of semiconductor
lasers is preferred.
Efficient EDFA pumping is possible using semiconductor lasers operating near
0.98- and 1.48-μm wavelengths. Indeed, the development of such pump lasers was
fueled with the advent of EDFAs. It is possible to realize 30-dB gain with only 10–
15 mW of absorbed pump power. Efficiencies as high as 11 dB/mW were achieved by
1990 with 0.98-μm pumping [57]. The pumping transition^4 I 15 / 2 →^4 I 9 / 2 can use high-
power GaAs lasers, and thepumping efficiencyof about 1 dB/mW has been obtained
at 820 nm [58]. The required pump power can be reduced by using silica fibers doped
with aluminum and phosphorus or by usingfluorophosphate fibers[59]. With the avail-
ability of visible semiconductor lasers, EDFAs can also be pumped in the wavelength
range 0.6–0.7μm. In one experiment [60], 33-dB gain was realized at 27 mW of pump
power obtained from an AlGaInP laser operating at 670 nm. The pumping efficiency
was as high as 3 dB/mW at low pump powers. Most EDFAs use 980-nm pump lasers
as such lasers are commercially available and can provide more than 100 mW of pump