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The transceiver power budget can only deliver up to a certain length of transmission length. Previously, to solve this issue, a repeater or a regenerator would be used. A regenerator 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.
The introduction of optical amplifiers in the 1990s, especially the EDFA (Erbium doped fiber amplifier) conquered the regenerator technology and opened doors to the WDM technology. The EDFA, unlike the repeater, is transparent to the number of channels, bit-rate, protocol, and modulation formats thus revolutionizing the backhaul network (for transoceanic distance) and uplifting the optical communication field and community at large. There are various types of amplifiers depending on the technique of amplifying, namely SOA (semiconductor optical amplifier), EDFA and Raman amplifier.
An optical amplifier can be used in different sections of a link. A booster power amplifier or post-amplifier is used directly after the optical transmitter or in case of WDM, before the signal is launched into a transmission fiber. It operates in a high saturation regime with high gain to boost the signal power level. The output ASE noise does not play a crucial role since the signal input to the amplifier is directly after the transmitter, i.e. it has not experienced any loss. Thus the OSNR at the output of the amplifier is extremely high.
An inline amplifier is a high output power low noise device used as a repeater between fiber spans to compensate for the losses in the network and to increase the regeneration lengths. Since the input signals to these amplifiers are weak, one has to take care of ASE level and thus a low noise figure is desired. When designing a link with multiple channels, the dynamic range of the inline amplifier is also very important. Thus to simultaneously achieve high gain, high output power, and a low noise figure with wide dynamic range, in practice a multistage amplification is designed in which the first stage is provides an efficient low noise pre-amplification and the subsequent stage delivers boosted power.
A pre-amplifier is a very low noise amplifier that is used just before an optical receiver to improve its sensitivity. The input signal is very weak thus it is important that the amplifier is not only low noise but also has low insertion loss, low coupling loss and high isolation. In WDM communication, a multistage amplification is designed that provides low noise figure and high-gain. A band pass filter is inserted between the two amplifiers to remove the out of band noise.