As optical signals travel through an Optical Fiber, they are attenuated. In long-distance applications, the signal is attenuated to the point where re-amplification is required. Traditionally, a device is offten referred to as a repeater accomplished this re-amplification.
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.
The technology available today eliminates the need for repeaters. Passive Optical Amplifier are now used instead of repeaters. A passive optical amplifier amplification optical signal directly without need to electric and electric optical transformation.
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 (SOAs), 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.
Erbium Doped Fiber Amplifier (EDFA)
Eribium doped fiber amplifier (EDFA) is gererally used for very long fiber links such as undersea cabling. The EDFA Optical Amplifier 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.
Figure 1 shows CATV EDFA Fiber Optic Amplifier
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.
EDFA may also be used in series to further increase the gain of the signal. Two EDFAs used in series may increase the input signal as much as 34dB.
Figure 2 shows Configuration for Erbium Doped Fiber Amplifier (EDFA)
Semiconductor optical amplifiers (SOAs)
Semiconductor optical amplifiers (SOAs) use a technique similar to that of EDFAs but without doping the optical fiber. Unlike the EDFA, which is energized with a laser pump, the SOA is energized with electrical current. The SOAs use an optical waveguide and a direct bandgap semiconductor that is basically a Fabry–Pérot laser to inject light energy into the signal, as shown in Figure 2.
Figure 3 shows Semiconductor optical amplifiers (SOAs)
One problem with SOAs are that the gain is very hard to control. By using the semiconductor technique and a waveguide, the signal may deplete the gain of a signal at another wavelength. This can introduce crosstalk among channels by allowing the signal at one wavelength to modulate another.
Raman Amplification
Raman amplification is a method that uses pump lasers to donate energy to the signal for amplification. However, unlike EDFAs, 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.
Figure 4: Raman amplification principle
The principal advantage of Raman amplification is its ability to provide distributed amplification within the transmission fibre, thereby increasing the length of spans between amplifier and regeneration sites.
In 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 Raman amplifier 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.
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