During population inversion phenomenon and as spontaneous emission occurs in all modes supported by the fiber (guided and unguided). Clearly, some of these photons would appear from time to time in the same fiber mode occupied by the signal field. Such spontaneously emitted photon perturbs both the amplitude and the phase of the optical field in a random fashion. These random perturbations of the signal are the source of amplifier noise in EDFAs and results in ASE.
For long haul links, generally EDFA’s are cascaded to overcome fiber losses in the link. Due to these cascading structures, amplifier induced noise buildup and impacts the performance of Amplifier. The ASE accumulates over many amplifiers and degrades the optical SNR. Also, as the level of ASE grows, it begins to saturate optical amplifiers and reduce the gain of amplifiers located further down the fiber link. The net result is that the signal level drops further while the ASE level increases. So, it’s obvious that if the number of amplifiers is large, the SNR will degrade so much at the receiver that the BER will become unacceptable.