The method involves monitoring an optical coupling arrangement by combining the optical signals directed to the coupling inputs with a low frequency and low amplitude recognition signal. The outputs of the coupler are connected to optical rectifiers with a branch output connected to an opto-electrical transducer. The diverted optical signal at the output of the coupling is monitored for the amplitude and frequency of the recognition signal. The recognition signal in the non-diverted part of the optical signal is suppressed after being optically rectified and sent to an optical fibre amplifier.
