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In this study, an Erbium Doped Fiber Amplifier (EDFAs) simulation program has been written in Matlab to characterize Noise Figure variations of a forward pumped EDFAs operating in C band (1525-1565 nm) as functions of Er 3+ fiber length, injected pump power, signal input power and Er 3+ doping density. The effect of multiple amplified stage on the noise figure was student at different Fiber length Noise Figure Characteristics of Erbium Doped Fiber Amplifiers at single and ... 45 and pumping power. The numerical results shows that EDFAs could be operated in saturation regimes leading minimum noise figure when the EDFAs is supplied with sufficient pump power, Moreover a continuous increasing of the noise figure value was demonstrated by increasing all of amplified stage number and fiber length. IIntroduction One of the most important factors limiting the transmission distance in fiber optical communication systems is the optical power loss caused by scattering and absorption mechanisms in optical fiber. Electrical repeaters, which require optical-electrical signal conversion, have previously been used to compensate the power losses increasing with distance. The use of such repeaters in optical communication systems have made the systems more complex and increased their installation costs [1,2] . The invention of the Erbium Doped Fiber Amplifier (EDFAs) in the late eighties was one of the major events in the history of optical communications. It provided new life to the optical fiber transmission window centered at 1550nm and the consequent research into technologies that allow high bit-rate transmission over long distances. High bit-rates were also possible with the aid of different dispersion compensation schemes [3] . EDFAs made by doping the silica fiber with erbium ions can operate in a broad range within the 1550 nm window at which the attenuation of silica fiber is minimum and therefore it is ideal for the optical fiber communication systems operating at this wavelength range. According to the research performed in recent years, it is known that the pumping of erbium doped fiber at 980 nm or 1480 nm is the most efficient way. High gain (3050 dB), large bandwidth (≥ 90 nm), high output power (1020dBm) and low noise figure (NF=35 dB) can be obtained using an erbium doped fiber amplifier optimised for 1.55 μm range [4] . The principal source of noise in EDFAs is Amplified Spontaneous Emission (ASE), which has a spectrum approximately the same as the gain spectrum of the amplifier. As well as decaying via stimulated emission, electrons in the upper energy level can also decay by spontaneous emission, which occurs at random, depending upon the silica structure and inversion level. Those photons from spontaneous emission may also interact with the dopant ions, and the initial spontaneous emission is therefore amplified in the same manner as the signals, hence becomes (ASE). The noise will propagate with the signal (Amplified Stimulated Emission) to the receiver and cause a degradation of the system performance [2,5,6] . EDFAs are currently attracting increased interest, especially for use in optical communication wavelength-division Dr. Ragheed M. Ibrahim & Nawzat S. Saadi & Saad F. Ramadhan 46 multiplexed (WDM) systems. The main reason is the possibility of compensating for optical fiber losses in broad wavelength ranges and developing a large-capacity, long-distance transmission system. The performance of the transmission system is strongly influenced by the gain and noise of used EDFAs. Therefore, detailed information about these EDFAs characteristics is the key for advanced design of WDM systems [3,7] . IIEDFAs Configuration The basic configuration of EDFAs is shown in figure (1). EDFAs consist of optical isolators, Wavelength Division Multiplexer (WDM) coupler to combine pump and signal lights, active fiber that consist of a short section of fiber where the core of a silica fiber is doped with trivalent Erbium ions (Er +3 ), forward pumping lasers to pump the erbium ions into higher energy levels and achieve population inversion. The gain characteristics of EDFAs depend mainly on their pumping schemes. EDFAs can be pumped at 980 nm or 1480 nm, and with different configurations: backward pumping, forward pumping or dual (bi-directional) pumping. In forward pumping, both of the signal and pump lights propagate in the same direction through the fiber whereas in the backward pumping they propagate in the opposite direction. The forward pumping direction provides the lowest noise figure. In fact, the noise is sensitive to the gain and the gain is the highest when the input power is the lowest. Backward pumping provides the highest saturated output power. dual (bi-directional) pumping ,combines the two schemes, has a higher performance than the other two by combining the lowest noise figure and the highest output power advantageous although it requires two pump lasers [5,6,8] . IIITheoretical part A common way to characterize the performance of a preamplifier is through its noise figure. The noise figure (NF) is defined as the ratio of Input Signal Amplified Output Isolator Isolator Erbium-Doped Fiber Laser Diode Pump

Noise Figure Characteristics of Erbium Doped Fiber Amplifiers at single and multiple amplified stage