An optical amplifier is a device that amplifies an optical signal directly,
without the need to first convert it to an electrical signal. An optical
amplifier may be thought of as a laser without an optical cavity, or one in
which feedback from the cavity is suppressed. Optical amplifiers are
important in optical communication and laser physics.
There are several different physical mechanisms that can be used to amplify a
light signal, which correspond to the major types of optical amplifiers. In
doped fibre amplifiers and bulk lasers, stimulated emission in the amplifier's
gain medium causes amplification of incoming light. In semiconductor optical
amplifiers, electron-hole recombination occurs. In Raman amplifiers, Raman
scattering of incoming light with phonons in the lattice of the gain
medium produces photons coherent with the incoming photons. Parametric
amplifiers use parametric amplification. Laser amplifiers
Almost any laser active gain medium can be pumped to produce gain for light at
the wavelength of a laser made with the same material as its gain medium. Such
amplifiers are commonly used to produce high power laser systems. Special types
such as regenerative amplifiers and chirped-pulse amplifiers are used to
amplify ultrashort pulses. = Doped fiber amplifiers=
Doped fibre amplifiers are optical amplifiers that use a doped optical
fibre as a gain medium to amplify an optical signal. They are related to
fibre lasers. The signal to be amplified and a pump laser are multiplexed into
the doped fibre, and the signal is amplified through interaction with the
doping ions. The most common example is the Erbium Doped Fibre Amplifier, where
the core of a silica fibre is doped with trivalent erbium ions and can be
efficiently pumped with a laser at a wavelength of 980 nm or 1,480 nm, and
exhibits gain in the 1,550 nm region. An erbium-doped waveguide amplifier is
an optical amplifier that uses a waveguide to boost an optical signal.
Amplification is achieved by stimulated emission of photons from dopant ions in
the doped fibre. The pump laser excites ions into a higher energy from where
they can decay via stimulated emission of a photon at the signal wavelength
back to a lower energy level. The excited ions can also decay
spontaneously or even through nonradiative processes involving
interactions with phonons of the glass matrix. These last two decay mechanisms
compete with stimulated emission reducing the efficiency of light
amplification. The amplification window of an optical
amplifier is the range of optical wavelengths for which the amplifier
yields a usable gain. The amplification window is determined by the
spectroscopic properties of the dopant ions, the glass structure of the optical
fibre, and the wavelength and power of the pump laser.
Although the electronic transitions of an isolated ion are very well defined,
broadening of the energy levels occurs when the ions are incorporated into the
glass of the optical fibre and thus the amplification window is also broadened.
This broadening is both homogeneous and inhomogeneous. Homogeneous broadening
arises from the interactions with phonons of the glass, while
inhomogeneous broadening is caused by differences in the glass sites where
different ions are hosted. Different sites expose ions to different local
electric fields, which shifts the energy levels via the Stark effect. In
addition, the Stark effect also removes the degeneracy of energy states having
the same total angular momentum. Thus, for example, the trivalent erbium ion
has a ground state with J = 15/2, and in the presence of an electric field splits
into J + 1/2 = 8 sublevels with slightly different energies. The first excited
state has J = 13/2 and therefore a Stark manifold with 7 sublevels. Transitions
from the J = 13/2 excited state to the J= 15/2 ground state are responsible for
the gain at 1.5 µm wavelength. The gain spectrum of the EDFA has several peaks
that are smeared by the above broadening mechanisms. The net result is a very
broad spectrum. The broad gain-bandwidth of fibre amplifiers make them
particularly useful in wavelength-division multiplexed
communications systems as a single amplifier can be utilized to amplify all
signals being carried on a fibre and whose wavelengths fall within the gain
window. = Basic principle of EDFA=
A relatively high-powered beam of light is mixed with the input signal using a
wavelength selective coupler. The input signal and the excitation light must be
at significantly different wavelengths. The mixed light is guided into a section
of fibre with erbium ions included in the core. This high-powered light beam
excites the erbium ions to their higher-energy state. When the photons
belonging to the signal at a different wavelength from the pump light meet the
excited erbium atoms, the erbium atoms give up some of their energy to the
signal and return to their lower-energy state. A significant point is that the
erbium gives up its energy in the form of additional photons which are exactly
in the same phase and direction as the signal being amplified. So the signal is
amplified along its direction of travel only. This is not unusual - when an atom
“lases” it always gives up its energy in the same direction and phase as the
incoming light. Thus all of the additional signal power is guided in the
same fibre mode as the incoming signal.There is usually an isolator
placed at the output to prevent reflections returning from the attached
fibre. Such reflections disrupt amplifier operation and in the extreme
case can cause the amplifier to become a laser. The erbium doped amplifier is a
high gain amplifier. = Noise=
The principal source of noise in DFAs is Amplified Spontaneous Emission, which
has a spectrum approximately the same as the gain spectrum of the amplifier.
Noise figure in an ideal DFA is 3 dB, while practical amplifiers can have
noise figure as large as 6–8 dB. As well as decaying via stimulated
emission, electrons in the upper energy level can also decay by spontaneous
emission, which occurs at random, depending upon the glass structure and
inversion level. Photons are emitted spontaneously in all directions, but a
proportion of those will be emitted in a direction that falls within the
numerical aperture of the fibre and are thus captured and guided by the fibre.
Those photons captured may then interact with other dopant ions, and are thus
amplified by stimulated emission. The initial spontaneous emission is
therefore amplified in the same manner as the signals, hence the term Amplified
Spontaneous Emission. ASE is emitted by the amplifier in both the forward and
reverse directions, but only the forward ASE is a direct concern to system
performance since that noise will co-propagate with the signal to the
receiver where it degrades system performance. Counter-propagating ASE
can, however, lead to degradation of the amplifier's performance since the ASE
can deplete the inversion level and thereby reduce the gain of the
amplifier. = Gain saturation=
Gain is achieved in a DFA due to population inversion of the dopant ions.
The inversion level of a DFA is set, primarily, by the power of the pump
wavelength and the power at the amplified wavelengths. As the signal
power increases, or the pump power decreases, the inversion level will
reduce and thereby the gain of the amplifier will be reduced. This effect
is known as gain saturation – as the signal level increases, the amplifier
saturates and cannot produce any more output power, and therefore the gain
reduces. Saturation is also commonly known as gain compression.
To achieve optimum noise performance DFAs are operated under a significant
amount of gain compression, since that reduces the rate of spontaneous
emission, thereby reducing ASE. Another advantage of operating the DFA in the
gain saturation region is that small fluctuations in the input signal power
are reduced in the output amplified signal: smaller input signal powers
experience larger gain, while larger input powers see less gain.
The leading edge of the pulse is amplified, until the saturation energy
of the gain medium is reached. In some condition, the width of the pulse is
reduced. = Inhomogeneous broadening effects=
Due to the inhomogeneous portion of the linewidth broadening of the dopant ions,
the gain spectrum has an inhomogeneous component and gain saturation occurs, to
a small extent, in an inhomogeneous manner. This effect is known as spectral
hole burning because a high power signal at one wavelength can 'burn' a hole in
the gain for wavelengths close to that signal by saturation of the
inhomogeneously broadened ions. Spectral holes vary in width depending on the
characteristics of the optical fibre in question and the power of the burning
signal, but are typically less than 1 nm at the short wavelength end of the
C-band, and a few nm at the long wavelength end of the C-band. The depth
of the holes are very small, though, making it difficult to observe in
practice. = Polarization effects=
Although the DFA is essentially a polarization independent amplifier, a
small proportion of the dopant ions interact preferentially with certain
polarizations and a small dependence on the polarization of the input signal may
occur (typically 5 THz, and the gain is reasonably flat over a wide wavelength
range. However, a number of challenges for
Raman amplifiers prevented their earlier adoption. First, compared to the EDFAs,
Raman amplifiers have relatively poor pumping efficiency at lower signal
powers. Although a disadvantage, this lack of pump efficiency also makes gain
clamping easier in Raman amplifiers. Second, Raman amplifiers require a
longer gain fiber. However, this disadvantage can be mitigated by
combining gain and the dispersion compensation in a single fiber. A third
disadvantage of Raman amplifiers is a fast response time, which gives rise to
new sources of noise, as further discussed below. Finally, there are
concerns of nonlinear penalty in the amplifier for the WDM signal channels.
Note: The text of an earlier version of this article was taken from the public
domain Federal Standard 1037C. Optical parametric amplifier
An optical parametric amplifier allows the amplification of a weak
signal-impulse in a noncentrosymmetric nonlinear medium). In contrast to the
previously mentioned amplifiers, which are mostly used in telecommunication
environments, this type finds its main application in expanding the frequency
tunability of ultrafast solid-state lasers. By using a noncollinear
interaction geometry optical parametric amplifiers are capable of extremely
broad amplification bandwidths. Recent achievements
The adoption of high power fiber lasers as an industrial material processing
tool has been ongoing for several years and is now expanding into other markets
including the medical and scientific markets. One key enhancement enabling
penetration into the scientific market has been the improvements in high
finesse fiber amplifiers, which are now capable of delivering single frequency
linewidths (