Ther are a lot of good responses here, I would like to jump in first of all Joe50 is right please specify dBM dBmv, ect.. if your telemetry is dropping about 17 dB as read on your sweep meter when this happens verify light power input at the receiver so you can eliminate the fiber link as a potential problem if your light is dropping verify the optical Tx, connectors, fiber. if light is ok shoot a CW from the node and use it to troubleshoot the Rx and the reverse combining.