6.4. ERBIUM-DOPED FIBER AMPLIFIERS 257
Relatively low noise levels of EDFAs make them an ideal choice for WDM light-
wave systems. In spite of low noise, the performance of long-haul fiber-optic commu-
nication systems employing multiple EDFAs is often limited by the amplifier noise.
The noise problem is particularly severe when the system operates in the anomalous-
dispersion region of the fiber because a nonlinear phenomenon known as the modu-
lation instability [29] enhances the amplifier noise [76] and degrades the signal spec-
trum [77]. Amplifier noise also introduces timing jitter. These issue are discussed later
in this chapter.
6.4.5 Multichannel Amplification
The bandwidth of EDFAs is large enough that they have proven to be the optical ampli-
fier of choice for WDM applications. The gain provided by them is nearly polarization
insensitive. Moreover, the interchannel crosstalk that cripples SOAs because of the
carrier-density modulation occurring at the channel spacing does not occur in EDFAs.
The reason is related to the relatively large value ofT 1 (about 10 ms) compared with
the carrier lifetime in SOAs (<1 ns). The sluggish response of EDFAs ensures that the
gain cannot be modulated at frequencies much larger than 10 kHz.
A second source of interchannel crosstalk is cross-gain saturation occurring be-
cause the gain of a specific channel is saturated not only by its own power (self-
saturation) but also by the power of neighboring channels. This mechanism of crosstalk
is common to all optical amplifiers including EDFAs [78]–[80]. It can be avoided by
operating the amplifier in the unsaturated regime. Experimental results support this
conclusion. In a 1989 experiment [78], negligible power penalty was observed when
an EDFA was used to amplify two channels operating at 2 Gb/s and separated by 2 nm
as long as the channel powers were low enough to avoid the gain saturation.
The main practical limitation of an EDFA stems from the spectral nonuniformity of
the amplifier gain. Even though the gain spectrum of an EDFA is relatively broad, as
seen in Fig. 6.15, the gain is far from uniform (or flat) over a wide wavelength range. As
a result, different channels of a WDM signal are amplified by different amounts. This
problem becomes quite severe in long-haul systems employing a cascaded chain of
EDFAs. The reason is that small variations in the amplifier gain for individual channels
grow exponentially over a chain of in-line amplifiers if the gain spectrum is the same
for all amplifiers. Even a 0.2-dB gain difference grows to 20 dB over a chain of 100
in-line amplifiers, making channel powers vary by a factor of 100, an unacceptable
variation range in practice. To amplify all channels by nearly the same amount, the
double-peak nature of the EDFA gain spectrum forces one to pack all channels near
one of the gain peaks. In a simple approach, input powers of different channels were
adjusted to reduce power variations at the receiver to an acceptable level [81]. This
technique may work for a small number of channels but becomes unsuitable for dense
WDM systems.
The entire bandwidth of 35–40 nm can be used if the gain spectrum is flattened
by introducing wavelength-selective losses through an optical filter. The basic idea
behind gain flattening is quite simple. If an optical filter whose transmission losses
mimic the gain profile (high in the high-gain region and low in the low-gain region) is
inserted after the doped fiber, the output power will become constant for all channels.