Abstract: The present invention relates to optical repeaters in a two-way optical line link, e.g. one included in a teletransmission system. The optical repeaters are implemented such as to provide a system which, inter alia in fault localizing, utilizes the line link transmission capacity optimally by completely optical through-connection of signals on the optical fibre cables. In fault localizing, one repeater at a time is checked, by an end terminal in the link transmitting a test signal including an address for selecting a repeater for testing and a test pattern. An optical amplifier amplifies the test signal in every repeater through which the signal passes, and when it reaches the repeater indicated by the address, a control circuit controls an optical switch such that the test signal is switched (loop connected) and is returned to the transmitting terminal where fault analysis takes place.