Model for bidirectional transmission in an open cascade of optical amplifiers

We derive a model for signal propagation and the generation and propagation of amplified spontaneous emission (ASE) in a completely open bidirectional cascade of optical amplifiers. The effect of single Rayleigh scattering of both signal and ASE is included. Our model predicts and our experiments substantially verify that the dominant gain saturating mechanism is the Rayleigh backscattered ASE centered around the 1535 nm peak in the ASE spectrum and that the spectral shape of the output ASE becomes much more sharply peaked around the 1535 nm gain peak. In an experimental open cascade of three EDFAs the ASE spectral density at the 1535 nm gain peak is increased by as much as 8-9 dB, resulting in gain compression ranging from 1-2 dB near 1550 nm to as much as 35 dB nearer 1538 nm. This combined with multiple Rayleigh scattering of the ASE around our 1538 and 1540 nm signal wavelengths results in Rayleigh scattering induced reductions of the optical signal to ASE ratio of 1-2 dB at the output of the first amplifier increasing to 36 dB at the output of the third amplifier each signal traverses.