EDFA is an optical amplifier that uses a doped optical fiber as a gain medium to amplify an optical signal. The signal which is to be amplified and a pump laser are multiplexed into the doped fiber, and the signal is amplified through interaction with the doping ions. EDFA is the best known and most frequently used optical amplifier suited to low loss optical window of silica based fiber. 

A particular attraction of EDFAs is their large gain bandwidth, which is typically tens of nanometers and thus actually it is more than enough to amplify data channels with the highest data rates without introducing any effects of gain narrowing. A single EDFA may be used for simultaneously amplifying many data channels at different wavelengths within the gain region. Before such fiber amplifiers were available, there was no practical method for amplifying all channels between long fiber spans of a fiber-optic link. One had to split all data channels, detect and amplify them electronically, optically resubmit and again combine them. The introduction of fiber amplifiers thus brought an enormous reduction in complexity, along with a corresponding increase in reliability. 

In WDM systems by multiplexing, a stream of wavelength channels particularly in C and L-band regimes can simultaneously amplify to the desired power level where the amplification of any particular channel is dependent on the signal wavelength, the number of signals present in the system, the input signal powers and its absorption and emission cross-sections .

The gain-flattened erbium-doped fiber amplifier (EDFA) is a key component in long-haul multichannel lightwave transmission systems such as the Wavelength Division Multiplexing (WDM). One difficulty in implementing a WDM system including EDFA’s is that the EDFA gain spectrum is wavelength-dependent. In a WDM system, the EDFA does not necessarily amplify the wavelength of the channels equally. EDFA in a WDM system is often required to have equalized gain spectra in order to achieve uniform output powers and similar signal-noise ratios (SNR). There are several methods in designing a flat spectral gain EDFA such as by controlling the doped fiber length and the pump power ,  proper choosing of optical notch filter’s characteristic, by using an acousto-optic tunable filter, and by employing an inhomogeneously broadened gain medium. This paper achieves gain flatness of EDFA by controlling the doped fiber length and the pump power for a given input power of -26dBm and the desired output power of more than 8dBm.