Crosstalk-limited transmission distance in bidirectional fibre optic systems

Optical bidirectional transmission systems, such as subscriber systems, where two signals are carried over a single fibre are subject to optical crosstalk. This crosstalk causes an unwanted increase of the bit error rate at the receivers in both channels. To evaluate the crosstalk effects, measurements of the bit error rate were performed. It is revealed that, to ensure a bit error rate of 10-9 or better with a negligible power penalty, a 15 dB separation is required between the signal and the interference powers. It is found that the receiver power penalties due to interference signals are expressed by a simple empirical formula which expresses a quadratic curve. An evaluation method which gives the transmission distance under a given crosstalk interference level is proposed and some examples are shown. It is found that there is an optimum launch power which gives a maximum transmission distance, depending on the level of crosstalk interference.