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 EDFAOptical Amplifiers, 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.

In its most basic form the EDFA consist of a length of EDF (typically 10-30m), a pump laser, and a component (often referred to as a WDM) for combining the signal and pump wavelength so that they can propagate simultaneously through the EDF. In principle EDFA’s can be designed such that pump energy propagates in the same direction as the signal (forward pumping), the opposite direction to the signal(backward pumping), or both direction together. The pump energy may either by 980nm pump energy, 1480nm pump energy, or a combination of both. Practi cally, the most common EDFA configuration is the forward pumping configuration using 980nm pump energy , as shown in Figure 2. This configuration makes the most efficient use of cost effective, reliable and low power consumption 980nm semiconductor pump laser diodes, thus providing the best overall design with respect to performance and cost trade-offs.

Besides the three basic components described above, Figure 2 also shows additional optical and electronic components used in a basic single stage EDFA. The signal enters the amplifier through the input port, and then passes through a tap which is used to divert a small percentage of the signal power (typically 1-2%) to an input detector. The signal then passes through an isolator, before being combined with pump energy emitted by the 980nm pump laser diode. The combined signal and pump energy propagate along the EDF, where signal amplification occurs, and then the amplifi ed signal exits the EDF and passes through a second isolator. The purpose of the two isolators, which allow light to pass only in a single direction, is to ensure that lasing cannot take place within theCATV amplifier. Furthermore, the output isolator also acts as a filter for 980nm light propagating in the forward direction, thus stopping the 980nm light from exiting the amplifier output port.

In a multi-channel WDM amplifier, a Gain Flattening Filter (GFF) is usually placed following the output isolator in order to flatten the gain spectrum, as shown in Figure 3. The attenuation spectrum of the GFF is designed to match the Gain spectrum of the EDF (operating at a given fixed gain), such that the combination of the two produces a flat gain. Following the GFF the signal passes through an output tap used to divert a small percentage of the output power (typically 1-2%) to the output detec tor. The output and input detectors are used to monitor the input and output power respecti vely, and thus provide feed-back to the control unit, which controls the amplifier by setting the pump laser current, and thus the amount of pump power injected into the EDF. The control unit also provides exter nal communication, for example via an RS232 interface.

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