258 CHAPTER 6. OPTICAL AMPLIFIERS
Figure 6.18: Schematic of an EDFA designed to provide uniform gain over the 1530–1570-
nm bandwidth using an optical filter containing several long-period fiber gratings. The two-
stage design helps to reduce the noise level. (After Ref. [85];©c1997 IEEE; reprinted with
permission.)
Although fabrication of such a filter is not simple, several gain-flattening techniques
have been developed [55]. For example, thin-film interference filters, Mach–Zehnder
filters, acousto-optic filters, and long-period fiber gratings have been used for flattening
the gain profile and equalizing channel gains [82]–[84].
The gain-flattening techniques can be divided into active and passive categories.
Most filter-based methods are passive in the sense that channel gains cannot be adjusted
in a dynamic fashion. The location of the optical filter itself requires some thought
because of high losses associated with it. Placing it before the amplifier increases the
noise while placing it after the amplifier reduces the output power. Often a two-stage
configuration shown in Fig. 6.18 is used. The second stage acts as a power amplifier
while the noise figure is mostly determined by the first stage whose noise is relatively
low because of its low gain. A combination of several long-period fiber gratings acting
as the optical filter in the middle of two stages resulted by 1977 in an EDFA whose
gain was flat to within 1 dB over the 40-nm bandwidth in the wavelength range of
1530–1570 nm [85].
Ideally, an optical amplifier should provide the same gain for all channels under
all possible operating conditions. This is not the case in general. For instance, if the
number of channels being transmitted changes, the gain of each channel will change
since it depends on the total signal power because of gain saturation. The active control
of channel gains is thus desirable for WDM applications. Many techniques have been
developed for this purpose. The most commonly used technique stabilizes the gain
dynamically by incorporating within the amplifier a laser that operates outside the used
bandwidth. Such devices are called gain-clamped EDFAs (as their gain is clamped by
a built-in laser) and have been studied extensively [86]–[91].
WDM lightwave systems capable of transmitting more than 80 channels appeared
by 1998. Such systems use the C and L bands simultaneously and need uniform ampli-
fier gain over a bandwidth exceeding 60 nm. Moreover, the use of the L band requires
optical amplifiers capable of providing gain in the wavelength range 1570–1610 nm.
It turns out that EDFAs can provide gain over this wavelength range, with a suitable
design. An L-band EDFA requires long fiber lengths (>100 m) to keep the inver-
sion level relatively low. Figure 6.19 shows an L-band amplifier with a two-stage