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Before the introduction of optical amplifiers, reach extension was achieved via repeaters or regenerators. A regenerator is also called optical-electrical-optical (OEO) device since it converts the optical signal to an electrical signal, processes this signal (re-amplify, reshape and retime) and then converts back to an optical signal so that the signal can then cover longer distances. This process is not only expensive but also restricts the useable optical bandwidth due to the limitations of the electronics.
The introduction of optical amplifiers in the 1990s, conquered the regenerator technology and opened doors to the WDM technology. There are various types of optical amplifiers depending on the technique of amplifying, namely SOA (semiconductor optical amplifier), EDFA and Raman amplifier.
The SOA is used in arrayed of applications such as wavelength conversion, signal regeneration, pulse reshaping and power limiting . It is a resourceful component that is capable of high integration and volume manufacturing due to which it is space and power efficient. SOA has been very useful in longhaul, metro, and access high-speed optical networks since SOAs are commertially available in different form factors and according to the applications.
SOAs amplify incident light through stimulated emission. An electrical pump current is used to excite the electrons in the active region of the SOA. When the optical signal travels through the active region, it causes these electrons to lose energy in the form of photons and get back to the ground state. The stimulated photons have the same wavelength as the optical signal, thus amplifying the optical signal. The basic working principle of a SOA is the same as a semiconductor laser but without feedback.
The key parameters used to characterize a SOA are gain, gain bandwidth, saturation output power and noise. Gain is the factor by which the input signal is amplified and is measured as the ratio of output power to input power (in dB). The optical gain of the unit is controlled by the SOA current. A higher gain results in higher output optical signal. Gain bandwidth defines the range of bandwidth where the amplification functions.
A wide gain bandwidth is desirable to amplify a wide range of signal wavelengths. Saturation output power is the maximum output power attainable after amplification beyond which no amplification is reached. It is important that the SOA has a high power saturation level to remain in the linear working region and to have higher dynamic range. Noise defines the undesired signal within the signal bandwidth which arises due to physical processing in the amplifier. A parameter called noise figure is used to measure the impact of noise which is typically around 5dB.