The length dependence of group delay differences between orthogonally polarised modes has been investigated experimentally for real single-mode fibres ranging from 01 km to 100 km. The mean group delay difference is a constant value of 0-67 ps, over a length of more than 1 km. The mode coupling coefficient of random mode mixing which causes pulse broadening has been estimated to be 6-3 × 10-3 m-1. A light pulse will broaden by 17ps after traversing a 100 km-long fibre. This may restrict single-mode fibre systems to a bit rate of 10 GHz for lengths over 150 km.
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