11.1 Fundamental Optical Amplifier Types
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order of 1–100 ps. This rapid response results in both advantages and limitations.
The advantage is that SOAs can be implemented when both switching and signalprocessing functions are called for in optical networks. The limitation is that the
rapid carrier response causes the gain at a particular wavelength to fluctuate with
the signal rate for speeds up to several Gb/s. Because this fluctuation affects the
overall gain, the signal gain at other wavelengths also fluctuates. Thus the rapid gain
response gives rise to crosstalk effects when a broad spectrum of wavelengths must
be amplified simultaneously.
In DFAs, the active medium for operation in the S-, C-, and L-bands is created
by lightly doping a silica (silicon dioxide) or tellurite (tellurium oxide) fiber core
with rare-earth elements such as thulium (Tm), erbium (Er), or ytterbium (Yb). The
DFAs for the O-band are achieved through doping fluoride-based fibers (rather than
silica fibers) with elements such as neodymium (Nd) and praseodymium (Pr). The
important features of DFAs include the ability to pump the devices simultaneously
at several different wavelengths, low coupling loss to the compatible-sized fiber
transmission medium, and very low dependence of gain on light polarization. In
addition, DFAs are highly transparent to signal format and bit rate, because they
exhibit slow gain dynamics, with carrier lifetimes on the order of 0.1–10 ms. The
result is that, in contrast to SOAs, the gain responses of DFAs are basically constant
for signal modulations greater than a few kilohertz. Consequently, they are immune
from interference effects (such as crosstalk and intermodulation distortion) between
different optical channels when wavelength channels in a broad spectrum (e.g., in
a 30-nm spectral band ranging from 1530 to 1560 nm) are injected simultaneously
into the amplifier.
A Raman optical amplifier is based on a nonlinear effect called stimulated Raman
scattering (SRS), which occurs in fibers at high optical powers (see Chap. 12).
Whereas a DFA requires a specially constructed optical fiber for its operation, Raman
amplification takes place within a standard transmission fiber. The Raman gain mechanism can be achieved through either a lumped (or discrete) amplifier or a distributed
amplifier. In the lumped Raman amplifier configuration, a spool of about 80 m of
small-core fiber along with appropriate pump lasers is inserted into the transmission
path as a distinct packaged unit. For the distributed Raman amplifier application, one
or more Raman pump lasers convert the final 20–40 km of the transmission fiber into
a preamplifier. Because the Raman gain in a particular spectral range is derived from
the SRS-induced transfer of optical power from shorter pump wavelengths to longer
signal wavelengths, these amplifiers can be designed for use in any wavelength band.
Table 11.1 lists some possible optical amplifier structures and their operating
ranges. The following sections give details on their characteristics.
11.2 Semiconductor Optical Amplifiers
A semiconductor optical amplifier is essentially an InGaAsP laser that is operating
below its lasing threshold point [6, 7]. Analogous to the construction of a laser diode,
441
order of 1–100 ps. This rapid response results in both advantages and limitations.
The advantage is that SOAs can be implemented when both switching and signalprocessing functions are called for in optical networks. The limitation is that the
rapid carrier response causes the gain at a particular wavelength to fluctuate with
the signal rate for speeds up to several Gb/s. Because this fluctuation affects the
overall gain, the signal gain at other wavelengths also fluctuates. Thus the rapid gain
response gives rise to crosstalk effects when a broad spectrum of wavelengths must
be amplified simultaneously.
In DFAs, the active medium for operation in the S-, C-, and L-bands is created
by lightly doping a silica (silicon dioxide) or tellurite (tellurium oxide) fiber core
with rare-earth elements such as thulium (Tm), erbium (Er), or ytterbium (Yb). The
DFAs for the O-band are achieved through doping fluoride-based fibers (rather than
silica fibers) with elements such as neodymium (Nd) and praseodymium (Pr). The
important features of DFAs include the ability to pump the devices simultaneously
at several different wavelengths, low coupling loss to the compatible-sized fiber
transmission medium, and very low dependence of gain on light polarization. In
addition, DFAs are highly transparent to signal format and bit rate, because they
exhibit slow gain dynamics, with carrier lifetimes on the order of 0.1–10 ms. The
result is that, in contrast to SOAs, the gain responses of DFAs are basically constant
for signal modulations greater than a few kilohertz. Consequently, they are immune
from interference effects (such as crosstalk and intermodulation distortion) between
different optical channels when wavelength channels in a broad spectrum (e.g., in
a 30-nm spectral band ranging from 1530 to 1560 nm) are injected simultaneously
into the amplifier.
A Raman optical amplifier is based on a nonlinear effect called stimulated Raman
scattering (SRS), which occurs in fibers at high optical powers (see Chap. 12).
Whereas a DFA requires a specially constructed optical fiber for its operation, Raman
amplification takes place within a standard transmission fiber. The Raman gain mechanism can be achieved through either a lumped (or discrete) amplifier or a distributed
amplifier. In the lumped Raman amplifier configuration, a spool of about 80 m of
small-core fiber along with appropriate pump lasers is inserted into the transmission
path as a distinct packaged unit. For the distributed Raman amplifier application, one
or more Raman pump lasers convert the final 20–40 km of the transmission fiber into
a preamplifier. Because the Raman gain in a particular spectral range is derived from
the SRS-induced transfer of optical power from shorter pump wavelengths to longer
signal wavelengths, these amplifiers can be designed for use in any wavelength band.
Table 11.1 lists some possible optical amplifier structures and their operating
ranges. The following sections give details on their characteristics.
11.2 Semiconductor Optical Amplifiers
A semiconductor optical amplifier is essentially an InGaAsP laser that is operating
below its lasing threshold point [6, 7]. Analogous to the construction of a laser diode,