An apparatus comprising means for detecting polarization dependent loss, PDL, induced quality of transmission, QoT, variations, wherein detecting PDL induced QoT variations includes at least one of detecting a difference between QoT measures obtained for two polarization tributaries of electrical field and detecting a temporal correlation between QoT variations and state of polarization, SOP, variations; and means for performing a SOP adjustment using an iterative process, wherein an iteration includes determining transmitter SOP settings (SOP Tx ) based on QoT measures, sending the transmitter SOP settings to an optical transmitter and updating the QoT measures.