There is provided a method to discriminate NLE-induced signal deformation from ASE-noise on polarization multiplexed signals, in order to measure the OSNR under NLE conditions and/or characterize the NLE-induced signal deformation. In accordance with one aspect, the method is based on the acquisition of optical spectrum traces when the (data-carrying) optical communication signal is partially or completely extinguished (ASE-noise only), as well as with a live optical communication signal. Comparing traces acquired with different conditions and/or at different dates allows discrimination of the signal contribution, the ASE-noise contribution and the NLE-induced deformations on the SUT.