Modulation instability of narrow-band nanosecond pulses propagating in anomalous-dispersion fibre

We demonstrate that, in the presence of noise, propagation of narrow-band 100-ns pulses through a 6-km long optical fibre sharply reduces their amplitude when a threshold power (∼0.2 W) is exceeded. The effect is due to the development of modulation instability and is sensitive to the noise level in the spectral region ∼100 GHz in width around the central frequency.