Detailed Introduction of Optical Amplifiers

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.