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We report bit error rate (BER) measurements, as a function of the extinction ratio, for an optical fibre transmission experiment in which a Ti: LiNbO3 directional coupler was used to externally modulate a 1.3 Â¿m multilongitudinal-mode laser. We derive the experimental dependence of the power penalty at the receiver against extinction ratio at 180 and 417 Mbit/s NRZ data rates. We also show that a simple BER probabilistic calculation provides good agreement with our experimental data. Finally, we consider the use of polarisation-maintaining fibre as a means of ensuring a stable input polarisation.