# Optical Amplifiers for Access and Passive Optical Networks: A Tutorial
## 3 Optical Amplifiers in Telecommunications Networks
### Semiconductor Optical Amplifiers--SOAs  
SOAs are another possible solution for data transfer in optical communications. An excellent review is provided in [68]. Note that SOAs were explored in the 1960s, when semiconductor lasers were invented. While the principle of the laser dates to 1958, a solid-state ruby laser was demonstrated in 1960, and the semiconductor laser was subsequently considered. Early SOAs used GaAs/AlGaAs, but more complex InGaAsP/InP materials, which operate in the 1300 nm to 1600 nm wavelength window, were subsequently introduced for use in optical transmission systems. SOAs are important devices in many optoelectronic systems, such as optical recording or high-speed printing. In the reality of telecommunication networks, SOAs were deployed in the 1980s, but they exhibited some drawbacks, such as a rather high noise figure and polarization  
Figure 7: Schematic diagram of an erbium-doped fiber amplifier (EDFA) with backward and forward pumping. sensitivity, as well as serious problems when amplifying more than one signal due to effects such as cross-phase modulation. On the other hand, SOAs can be manufactured in specific ways and are able to function in nearly every optical band, covering an almost empty spectral window 1460-1530 nm, the so-called the S-band (no silica host glass rare element-based amplifiers can operate in this band); additionally, these amplifiers can be integrated on chips. For this reason, SOAs are reused for high-speed 100 GE transceivers, where four SOAs are integrated within transceivers and each SOA amplifies only one 25 Gb/s optical signal. SOAs can be used as all-optical wavelength converters and even all-optical switches [59]. SOAs have a structure similar to that of Fabry-Perot lasers (see Figure 8). However, this Fabry-Perot configuration is practically improper for data transmission applications because the available bandwidth is very small (less than 10 GHz). To make SOAs suitable for the data world, conversion to traveling wave (TW) devices must be accomplished, which can be performed by the suppression of reflections from the end facets of an SOA with antireflection coatings. The reflectivity must be very small (less than 0.1%) to achieve the desirable behavior. For this reason, other techniques for suppressing reflections were invented, for example, angled-facet or tilted-stripe structures [59]. SOAs are small and electrically pumped (in contrast to EDFAs/PDFFAs or Raman amplifiers) and can be easily integrated with other semiconductor elements and devices, such as lasers and modulators. Undesirable properties, such as a high noise figure, low output power and polarization sensitivity, restrain SOAs from massive deployment as amplifiers, even when many techniques, such as series, parallel or double-pass configurations, have been introduced and studied. Other novel areas exist where SOAs can find potential use; examples include wavelength conversion, optical demultiplexing of very-high-speed (100 Gb/s) signals to low-speed (10 Gb/s) tributary signals or optical clock recovery units. However, commercial equipment based on these principles is not available. The gain of semiconductor amplifiers is not generated in the fiber optic material but is generated in the structure of a semiconductor amplifier. Pumping is not optically performed but must support electrical energy (electrical field). Typical materials used for SOA amplifiers are GaAs, AlGaAs, InGaAs, InGaAs, InGaAsP, InAlGa As and InP. These materials have excellent quantum efficiency, which provides a maximum number of generated photons. The principle of SOA operation is similar to that of photon emission in lasers [60]:  
Figure 8: Schematic diagram of a semiconductor optical amplifier (SOA). * Media excitation. Excitation of a semiconductor medium in the P-N transition is the result of energy pumping and depends on stimulated absorption. Absorbed energy is transferred to an electron in the valence band, which is excited to a higher energetic layer in the valence band.