Bit synchronisation for optical networks.

A submarine optical telecommunications system has two shore based transmitting stations whose signals are combined at a passive, submerged junction. Phase errors are caused by variations in transmission times to the junction. An error signal is generated at the first repeater after the junction. The error signal, which represents the phase errors, is transmitted to a shore station where it is used to adjust the sending phase. A surface acoustic wave filter tuned to half the frequency of the combined signal is the preferred way to generate the error signal.