Evolution of field spectrum due to fiber-nonlinearity-induced phase noise in in-line optical amplifier systems

The spectral evolution of light propagating in in-line optical amplifier systems is investigated. Phase noise, which is caused by amplified spontaneous emission through the nonlinear Kerr effect, does not result in simple linewidth broadening but changes the lineshape of the Lorentzian carrier wave. Tails of the field spectrum are raised over a wide frequency range. It is experimentally confirmed that the phase noise is proportional to the third power of the number of amplifiers. Intensity-modulation direct-detection systems, in addition to coherent systems, are affected by the phase noise when a large number of amplifiers are cascaded.>