We experimentally demonstrate fiber-longitudinal monitoring that visualizes polarization-wise signal power profiles. The method is based on linear least squares and successfully estimates the position and amount of PDL in a multi-span optical link. The demonstration experiment for a 150 km transmission successfully estimated low-level PDL values ranging from 3 dB to 0.5 dB at a spatial granularity of 1 km. Results show the effectiveness of this method for PDL monitoring.