6.4. ERBIUM-DOPED FIBER AMPLIFIERS 259
Figure 6.19: Schematic of an L-band EDFA providing uniform gain over the 1570–1610-nm
bandwidth with a two-stage design. (After Ref. [92];©c1999 IEEE; reprinted with permission.)
design [92]. The first stage is pumped at 980 nm and acts as a traditional EDFA (fiber
length 20–30 m) capable of providing gain in the range 1530–1570 nm. In contrast, the
second stage has 200-m-long doped fiber and is pumped bidirectionally using 1480-nm
lasers. An optical isolator between the two stages passes the ASE from the first stage
to the second stage (necessary for pumping the second stage) but blocks the backward-
propagating ASE from entering the first stage. Such cascaded, two-stage amplifiers can
provide flat gain over a wide bandwidth while maintaining a relatively low noise level.
As early as 1996, flat gain to within 0.5 dB was realized over the wavelength range of
1544–1561 nm [93]. The second EDFA was codoped with ytterbium and phosphorus
and was optimized such that it acted as a power amplifier. Since then, EDFAs providing
flat gain over the entire C and L bands have been made [55]. Raman amplification can
also be used for the L band. Combining Raman amplification with one or two EDFAs,
uniform gain can be realized over a 75-nm bandwidth covering the C and L bands [94].
A parallel configuration has also been developed for EDFAs capable of amplifying
over the C and L bands simultaneously [95]. In this approach, the incoming WDM
signal is split into two branches, which amplify the C-band and L-band signals sep-
arately using an optimized EDFA in each branch. The two-arm design has produced
a relatively uniform gain of 24 dB over a bandwidth as large as 80 nm when pumped
with 980-nm semiconductor lasers while maintaining a noise figure of about 6 dB [55].
The two-arm or two-stage amplifiers are complex devices and contain multiple compo-
nents, such as optical filters and isolators, within them for optimizing the amplifier per-
formance. An alternative approach to broadband EDFAs uses afluoridefiber in place
of silica fibers as the host medium in which erbium ions are doped. Gain flatness over
a 76-nm bandwidth has been realized by doping atelluritefiber with erbium ions [96].
Although such EDFAs are simpler in design compared with multistage amplifiers, they
suffer from the splicing difficulties because of the use of nonsilica glasses.
Starting in 2001, high-capacity lightwave systems began to use the short-wavelength
region—the so-called S band—extending from 1470 to 1520 nm [97]. Erbium ions
cannot provide gain in this spectral band. Thulium-doped fiber amplifiers have been
developed for this purpose, and they are capable of providing flat gain in the wave-
length range 1480–1510 nm when pumped using 1420-nm and 1560-nm semiconduc-
tor lasers [98]. Both lasers are needed to reach the^3 F 4 state of thulium ions. The gain
is realized on the^3 F 4 →^3 H 4 transition. Raman amplification can also be used for the
S band, and such amplifiers were under development in 2001.