Page 204 - Optical Communications Essentials

Optical Amplifiers

194 Chapter Eleven

Difference-frequency generation in waveguides is based on the mixing of two
input waves. Here the nonlinear interaction of the material is with a pump and
a signal wave. For example, one demonstration of this technique has shown the
simultaneous conversion of eight input wavelengths in the 1546- to 1560-nm
region to a set of eight output wavelengths in the 1524- to 1538-nm region.

11.7. Summary

Optical amplifiers have become an essential component in modern lightwave
communication systems. The three basic technologies are semiconductor opti-
cal amplifiers (SOAs), doped-fiber amplifiers (DFAs), and Raman amplifiers.
SOAs are based on the same operating principles as laser diodes, whereas the
other two types employ a fiber as the gain mechanism. Among the DFAs,
erbium-doped fiber amplifiers (EDFAs) are used widely in the C- and L-bands
for optical communication networks. In contrast to an EDFA which uses a spe-
cially constructed fiber for the amplification medium, a Raman amplifier makes
use of the transmission fiber itself.
The attractiveness of an SOA is that since it is essentially an InGaAsP laser
that is operating below its threshold point, it can operate in every fiber wave-
length band extending from 1280nm in the O-band to 1650nm in the U-band.
SOAs are particularly useful in the O-band where other optical amplifier tech-
nologies are more difficult to implement. However, the limitation is that the
SOA is practical only for single-wavelength amplification. Table 11.1 lists typi-
cal values of SOA performance parameters for devices that can be housed in a
standard 14-pin butterfly package.
The most common optical fiber amplifier for long-haul telecommunication
applications is a silica fiber doped with erbium, which is known as an erbium-
doped fiber amplifier, or EDFA. Originally the operation of an EDFA by itself
was limited to the 1530- to 1560-nm region, which is the origin of the designa-
tion conventional band or C-band. However, improvements in erbium-doped
fiber designs and the use of pump lasers operating at wavelengths different
from those used in the C-band have allowed the extension of EDFAs into the
L-band. An EDFA needs to be pumped optically, which is done with 980- and
1480-nm lasers for C-band operation and by 960-nm lasers for L-band use.
Table 11.2 compares the performances of these two laser types, and Table 11.3
lists the characteristics of couplers used to combine the pump power wave-
lengths with the C-band and/or L-band transmission signals.
Whereas an EDFA requires a specially constructed optical fiber for its opera-
tion, a Raman amplifier makes use of the transmission fiber itself as the ampli-
fication medium. A Raman amplifier is based on an effect called stimulated
Raman scattering (SRS). This process transfers optical energy from a strong
laser pump beam to a weaker transmission signal that has a wavelength which
is 80 to 100nm higher than the pumping wavelength. For example, pumping at
1450nm will lead to a signal gain at approximately 1530 to 1550nm. By using
counterdirectional pumping, the amplification is distributed within the last

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