252 CHAPTER 6. OPTICAL AMPLIFIERS
power. Pumping at 1480 nm requires longer fibers and higher powers because it uses
the tail of the absorption band shown in Fig. 6.15(b).
EDFAs can be designed to operate in such a way that the pump and signal beams
propagate in opposite directions, a configuration referred to as backward pumping to
distinguish it from the forward-pumping configuration shown in Fig. 6.10. The per-
formance is nearly the same in the two pumping configurations when the signal power
is small enough for the amplifier to remain unsaturated. In the saturation regime, the
power-conversion efficiency is generally better in the backward-pumping configura-
tion [61], mainly because of the important role played by the amplified spontaneous
emission (ASE). In the bidirectional pumping configuration, the amplifier is pumped
in both directions simultaneously by using two semiconductor lasers located at the two
fiber ends. This configuration requires two pump lasers but has the advantage that the
population inversion, and hence the small-signal gain, is relatively uniform along the
entire amplifier length.
6.4.2 Gain Spectrum
The gain spectrum shown in Fig. 6.15 is the most important feature of an EDFA as it de-
termines the amplification of individual channels when a WDM signal is amplified. The
shape of the gain spectrum is affected considerably by the amorphous nature of silica
and by the presence of other codopants within the fiber core such as germania and alu-
mina [62]–[64]. The gain spectrum of erbium ions alone is homogeneously broadened;
its bandwidth is determined by the dipole relaxation timeT 2 in accordance with Eq.
(6.1.2). However, the spectrum is considerably broadened in the presence of randomly
located silica molecules. Structural disorders lead to inhomogeneous broadening of
the gain spectrum, whereasStark splittingof various energy levels is responsible for
homogeneous broadening. Mathematically, the gaing(ω)in Eq. (6.1.2) should be av-
eraged over the distribution of atomic transition frequenciesω 0 such that the effective
gain is given by
geff(ω)=∫∞−∞g(ω,ω 0 )f(ω 0 )dω 0 , (6.4.1)wheref(ω 0 )is the distribution function whose form also depends on the presence of
other dopants within the fiber core.
Figure 6.15(b) shows the gain and absorption spectra of an EDFA whose core was
doped with germania [64]. The gain spectrum is quite broad and has a double-peak
structure. The addition of alumina to the fiber core broadens the gain spectrum even
more. Attempts have been made to isolate the contributions of homogeneous and inho-
mogeneous broadening through measurements ofspectral hole burning. For germania-
doped EDFAs the contributions of homogeneous and inhomogeneous broadening are
relatively small [63]. In contrast, the gain spectrum of aluminosilicate glasses has
roughly equal contributions from homogeneous and inhomogeneous broadening mech-
anisms. The gain bandwidth of such EDFAs typically exceeds 35 nm.
The gain spectrum of EDFAs can vary from amplifier to amplifier even when core
composition is the same because it also depends on the amplifier length. The reason
is that the gain depends on both the absorption and emission cross sections having dif-
ferent spectral characteristics. The local inversion or local gain varies along the fiber