Abstract
We present experimental results for the effect of an increased supervisory signal power in a high‐loss loopback supervisory system in an optically amplified wavelength division multiplexing (WDM) transmission line. The study focuses on the investigation of increasing the input power for the supervisory signal and the effect on the co‐propagating WDM data signals using different channel spacing. This investigation is useful for determining the power limitation of the supervisory signal if extra power is needed to improve the monitoring. The study also shows the effect of spacing on the quality of the supervisory signal itself because of interaction with adjacent data signals.