Impact of modulation instability on high bit rate optically-amplified transmission systems

Modulation instability (MI) can have a significant impact on optically-amplified transmission systems operated in the anomalous dispersion regime. MI is a nonlinear four wave mixing (FWM) interaction whereby a probe signal (in this case spontaneous emission noise) is parametrically amplified and a pump signal (optical carrier) depleted. This effect therefore results in a degraded SNR and will limit maximum optical amplifier output power. As the amplified noise has a non-white spectral density, its impact on system performance can be catastrophic as the TDM bit rate increases. This paper addresses the potential system limitations over dispersion shifted fibre (DSF) and non-dispersion shifted fibre (NDSF). Modelling results are obtained using a nonlinear numerical simulator and the predicted results show good agreement with experiments. The onset of MI at moderately low power levels (4 to 8 mW) show that this effect will have a significant impact on high speed TDM transmission over both NDSF and DSF, when operated in the anomalous regime.