MONITORING SYSTEM FOR PHOTO REPEATER

PURPOSE:To make it possible to monitor the input power to a photo repeater device and the operating state of LDs in a terminal station simultaneously, by adopting a mark-rate modulating device. CONSTITUTION:When a monitoring signal is outputted, a monitoring-signal identifying circuit 25 controls a photo switch 21. A signal, which is sent out to an upward bound transmission path 12, is coupled to a downward bound transmission path 13 by way of a loop back circuit 26, and a monitoring loop for returning the signal to a terminal station is formed. At the same time, a ''1'' is outputted to an AND circuit 23. Input power and an oscillating frequency in a DC-DC converter 22 have a constant relationship, which can be obtained in advance. Therefore, the transmitting frequency in the DC-DC converter 22 is adopted as the data for the magnitude of the input power to a repeater device. Meanwhile, which of an active LD19 or a back-up LD is in operation can be determined by monitoring the logic level of a control lines connecting a control circuit 24 and the active LD and the back-up LD20.