Cascaded inline semiconductor laser amplifiers in a coherent optical fibre transmission system

A coherent optical transmission system with two broadband inline optical amplifiers was investigated. The main cause of system degradation was the generation of an interfering signal owing to backward scattered waves in the amplifier chain. The perturbation was eliminated by an isolator or sufficient attenuation between both amplifiers. It is possible to convert the effective 30 dB gain of both amplifiers fully into an increase of the transmission span.