A bi-directional optical signal transmission network comprises an outbound (12) and an inbound (14) fibre each including an optical amplifier/repeater (18/24). An optical coupling (30, 38, 40) communicates between one of the fibres (12), to the output side of the amplifier/repeater (18) and the other fibre (14). The coupling includes a filter (40) which is arranged to permit transfer of a test signal wavelength between the outbound (12) and the inbound (14) fibre but which prevents transfer of unwanted traffic signal.