Observation of BER degradation due to fading in long-distance optical amplifier system

4500 km optical fibre transmission at 5 Gbit/s was successfully carried out using 138 erbium-doped fibre amplifiers. However, the long term performance measurement shows BER fluctuation. It is found that such BER fluctuation can be correlated to the fading of the optical signal-to-noise ratio presumably due to the polarisation effect.