Tunable OTDR based on self-locked RSOA for in-line monitoring of WDM-PON is proposed and demonstrated. The
characteristics of self-locked RSOA were experimentally studied. The tunable OTDR was successfully demonstrated for
a range of 25 nm, having a dynamic range of about 15 dB. Under in-service state of 2.5-Gb/s data, the power penalty
induced by the line monitoring channel was about 0.1 dB at BER of 10<sup>-9</sup>. The proposed tunable OTDR is cost-effective
and useful to monitor the status of WDM-PON.