The first field demonstration using optical duobinary signalling at 10
Gbit/s over installed fibres is reported. Transmission over 138 km of
standard fibre is demonstrated without repeaters or dispersion compens
ation and with a penalty of only 2.5 dB. This technique significantly
increases the bit rate distance product without changing any specifica
tions of the existing fibre network. The influence of fibre nonlineari
ty on system performance is also investigated.