Detailed Introduction of Optical Amplifiers

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Optical Amplifier

 

WHAT IS OPTICAL AMPLIFIER?

Optical
Amplifier is a device referred to as a repeater
accomplished the re-amplification where optical signal is attenuated when
traveling through an optical fiber in long-distance applications. While the
technology available today eliminates the need for repeaters, the optical
amplifiers are now used instead of repeaters. An optical amplifier can amplify
optical signal directly without electric and electric optical transformation.
The originally dominating application of fiber amplifiers was in optical fiber
communications over large distance, where signals need to be periodically
amplified, with the developments of optical amplifier, there are more and more
types to choose for customers’ different needs, even some high-power fiber
amplifiers are now used in laser material processin.

 

Tips: A repeater is basically a receiver
and transmitter combined in one package. The receiver converts the incoming
optical energy into electrical energy. The electrical output of the receiver
drives the electrical input of the transmitter. The optical output of the
transmitter represents an amplified version of the optical input signal plus
noise.

 

(1) Doped fiber amplifier (eg. EDFA)

 

The first is Doped Fiber Amplifier.
Stimulated emission in the amplifier’s gain medium causes amplification of
incoming light. The most common version is Erbium-Doped Fiber Amplifier (EDFA).
EDFA Amplifier is generally used for very long fiber links such as undersea
cabling. It uses a fiber that has been treated or “doped” with erbium,
and this is used as the amplification medium.

 

The pump lasers operate at wavelength below
the wavelengths that are to be amplified. The doped fiber is energized with the
laser pump. As the optical signals is passed through this doped fiber, the
erbium atoms transfer their energy to the signal, thereby increasing the energy
or the strength of the signal as it passes. With this technique, it is common
for the signal to be up to 50 times or 17dB stronger leaving the EDFA than it
was when it entered. EDFA may also be used in series to further increase the
gain of the signal. Two EDFA amplifiers used in series may increase the input
signal as much as 34dB.

 

Principle: 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 fiber 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 fiber mode
as the incoming signal.There is usually an isolator placed at the output to
prevent reflections returning from the attached fiber. 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.

 

(2) Semiconductor optical amplifier (SOA)

 

SOA Amplifier uses a semiconductor to
provide the gain medium. These amplifiers have a similar structure to
Fabry–Pérot laser diodes but with anti-reflection design elements at the end
faces. Recent designs include anti-reflective coatings and tilted wave guide
and window regions which can reduce end face reflection to less than 0.001%.
Since this creates a loss of power from the cavity which is greater than the
gain, it prevents the amplifier from acting as a laser.

 

SOA amplifiers are typically made from
group III-V compound semiconductors such as GaAs/AlGaAs, InP/InGaAs,
InP/InGaAsP and InP/InAlGaAs, though any direct band gap semiconductors such as
II-VI could conceivably be used. Such amplifiers are often used in
telecommunication systems in the form of fiber-pigtailed components, operating
at signal wavelengths between 0.85 µm and 1.6 µm and generating gains of up to
30 dB.

 

High optical nonlinearity makes SOA
amplifiers attractive for all optical signal processing like all-optical
switching and wavelength conversion. There has been much research on SOA
amplifiers as elements for optical signal processing, wavelength conversion,
clock recovery, signal demultiplexing, and pattern recognition.

 

Compared with EDFA: The SOA amplifier is of
small size and electrically pumped. It can be potentially less expensive than
the EDFA and can be integrated with semiconductor lasers, modulators, etc.
However, the performance is still not comparable with the EDFA. The SOA has
higher noise, lower gain, moderate polarization dependence and high
nonlinearity with fast transient time. The main advantage of SOA is that all
four types of nonlinear operations (cross gain modulation, cross phase
modulation, wavelength conversion and four wave mixing) can be conducted.
Furthermore, SOA can be run with a low power laser. This originates from the
short nanosecond or less upper state lifetime, so that the gain reacts rapidly
to changes of pump or signal power and the changes of gain also cause phase
changes which can distort the signals. This nonlinearity presents the most
severe problem for optical communication applications. However it provides the
possibility for gain in different wavelength regions from the EDFA.

 

(3) Fiber Raman amplifier (eg. DRMA)

 

Fiber Raman amplifier scattering of
incoming light with phonons in the lattice of the gain medium produces photons
coherent with the incoming photons. The most common version is Distributed
Multi-pump Raman Amplifier (DMRA). However, unlike EDFA amplifiers, this
technique does not use doped fiber, just a high power pumping laser. The laser
is operated at wavelengths 60nm to 100nm below the desired wavelength of the
signal. The laser signal energy and the photons of the transmitted signal are
coupled, thereby increasing the signal strength. The principal advantage of
Raman amplification is its ability to provide distributed amplification within
the transmission fiber, thereby increasing the length of spans between
amplifier and regeneration sites.

 

The Raman gain spectrum of optical fiber
exhibits a broad continuum shapes due to amorphous nature of the material. The
peak value of Raman gain coefficient is inversely proportional to pump
wavelength. In other word, Raman gain shape is wavelength/frequency dependent.
In the Fiber Raman amplifier, when the signal and a high-power pump are
injected into a fiber together, and the signal is within the Raman gain region
of the pump, the signal will be amplified.

 

(4) Fiber optical parametric amplifier
(FOPA)

 

Fiber optical parametric amplifier in
according with the four-wave mixing. In quantum-mechanical terms, FWM occurs
when photons from one or more waves are annihilated and new photons are created
at different frequencies such that the net energy and momentum are conserved
during the parametric interaction. We can see it is s bandwidth of several
hundred nanometers by use of silica fibers and just one or two pumps with power
of the order of a few watts. Arbitrary center wavelength by changing the
zero-dispersion wavelength of the fiber. It is easy to obtain large gain (pump
power & fiber length). The noise of a phase-sensitive FOPA can actually
approach 0 dB. Wavelength conversion is accompanied by spectral inversion. this
is a quite important advantage. The Fiber optical parametric amplifier gain two
pump photons annihilate themselves to produce a signal photon and an idler
photon. Comparison of other optical amplifiers shown as following figure.

 

 

SUMMARY:

 

Each amplification technique has advantages
and disadvantages. Remember to keep in mind the amplification that the
amplifier is being used in. For example, if a signal needed amplification but
noise was an issue, a DMRA would most likely be the best choice. If the signal
needed to be amplified by just a small amount, the SOA might be best.

 

All of these amplification methods have one
big advantage: optical amplifiers will amplify all signals on a fiber at the
same time. Therefore, it is possible to simultaneously amplify multiple
wavelengths. But it is important to keep in mind that the power levels must be
monitored carefully because the amplifiers can become saturated, thereby
causing incorrect operation.