We propose a novel optical supervisory system for the wavelength division multiplexing (WDM) transmission link including in-line erbium-doped fiber amplifiers (EDFA). By employing a newly proposed EDFA structure supporting bi-directional transmission at pre-determined wavelength and modified optical time domain reflectometry technique, it was possible to accurately monitor the status of transmission fiber link under in-service condition. A proof of the concept experiment with 8 channel, 320 kin WDM transmission in-service state has been successfully carried out giving a clear loss/gain/reflection profile of the link with negligible BER power penalty less than 0.3 dB. (C) 2002 Elsevier Science (USA). All rights reserved.