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11 Basics of Optical Amplifiers
11.10 Summary
The creation and development of optical amplifiers has provided significant increases
in information capacity over longer transmission distances, which offer attractive
economical advantages compared to electronic-based signal amplification. Optical
amplifiers have found widespread use in applications ranging from ultra-long
undersea links to short links in access networks.
The three main optical amplifier types are semiconductor optical amplifiers
(SOAs), active fiber or doped-fiber amplifiers (DFAs), and Raman amplifiers. All
optical amplifiers increase the power level of incident light through a stimulated
emission or an optical power transfer process. In SOAs and DFAs the mechanism for
creating a population inversion is the absorption of energy supplied from an external
source called the pump. The pump supplies energy to electrons in an active medium,
which raises them to higher energy levels to produce the population inversion. An
incoming signal photon will trigger these excited electrons to drop to lower levels
through a stimulated-emission process. Because one incoming trigger photon stimulates a cascade effect in which many excited electrons emit photons of equal energy
as they drop to the ground state, the result is an amplified optical signal.
In contrast to the amplification mechanisms used in an SOA or DFA, in Raman
amplification there is a transfer of optical power from a high-power pump wavelength
(e.g., 500 mW at 1480 nm) to lightwave signals at longer wavelengths (e.g., a -25dBm signal around 1550 nm). This Raman amplification mechanism is done without
the need for a population-inversion process.
The dominant noise generated in an optical amplifier is called amplified spontaneous emission (ASE) noise. The origin of this noise is the spontaneous recombination of electrons and holes in the amplifier medium. This recombination occurs
over a wide range of electron–hole energy differences and thus gives rise to a broad
spectral background of noise photons that get amplified along with the optical signal
as they travel through the EDFA. This is shown in Fig. 11.8 for an EDFA amplifying
a signal at 1540 nm.
When analyzing a transmission link that has a series of optical amplifiers in it,
an important point is that the light signal entering the optical receiver may contain
a significant level of ASE noise that has been added by the preceding cascade of
optical amplifiers. In this case one has to evaluate the optical signal-to-noise ratio
(OSNR). This parameter is defined as the ratio of the average EDFA optical signal
output power P ave to the unpolarized ASE optical noise power P ASE .
A Raman optical amplifier is based on a nonlinear effect called stimulated Raman
scattering (SRS), which occurs in fibers at high optical powers. Whereas an EDFA
requires a specially constructed optical fiber for its operation, a Raman amplifier
makes use of the standard transmission fiber itself as the amplification medium.
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
11 Basics of Optical Amplifiers
11.10 Summary
The creation and development of optical amplifiers has provided significant increases
in information capacity over longer transmission distances, which offer attractive
economical advantages compared to electronic-based signal amplification. Optical
amplifiers have found widespread use in applications ranging from ultra-long
undersea links to short links in access networks.
The three main optical amplifier types are semiconductor optical amplifiers
(SOAs), active fiber or doped-fiber amplifiers (DFAs), and Raman amplifiers. All
optical amplifiers increase the power level of incident light through a stimulated
emission or an optical power transfer process. In SOAs and DFAs the mechanism for
creating a population inversion is the absorption of energy supplied from an external
source called the pump. The pump supplies energy to electrons in an active medium,
which raises them to higher energy levels to produce the population inversion. An
incoming signal photon will trigger these excited electrons to drop to lower levels
through a stimulated-emission process. Because one incoming trigger photon stimulates a cascade effect in which many excited electrons emit photons of equal energy
as they drop to the ground state, the result is an amplified optical signal.
In contrast to the amplification mechanisms used in an SOA or DFA, in Raman
amplification there is a transfer of optical power from a high-power pump wavelength
(e.g., 500 mW at 1480 nm) to lightwave signals at longer wavelengths (e.g., a -25dBm signal around 1550 nm). This Raman amplification mechanism is done without
the need for a population-inversion process.
The dominant noise generated in an optical amplifier is called amplified spontaneous emission (ASE) noise. The origin of this noise is the spontaneous recombination of electrons and holes in the amplifier medium. This recombination occurs
over a wide range of electron–hole energy differences and thus gives rise to a broad
spectral background of noise photons that get amplified along with the optical signal
as they travel through the EDFA. This is shown in Fig. 11.8 for an EDFA amplifying
a signal at 1540 nm.
When analyzing a transmission link that has a series of optical amplifiers in it,
an important point is that the light signal entering the optical receiver may contain
a significant level of ASE noise that has been added by the preceding cascade of
optical amplifiers. In this case one has to evaluate the optical signal-to-noise ratio
(OSNR). This parameter is defined as the ratio of the average EDFA optical signal
output power P ave to the unpolarized ASE optical noise power P ASE .
A Raman optical amplifier is based on a nonlinear effect called stimulated Raman
scattering (SRS), which occurs in fibers at high optical powers. Whereas an EDFA
requires a specially constructed optical fiber for its operation, a Raman amplifier
makes use of the standard transmission fiber itself as the amplification medium.
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