Integrated polarimeter in an optical line system

An optical line device for use in an optical line system, wherein the optical line device is configured to connect to a second optical line device via a transmit fiber and a receive fiber, the optical line device comprising:

• a transmitter connected to the transmit fiber via an output port of the optical line device, wherein the transmitter is configured to transmit a polarization probe signal at a wavelength outside of a band of wavelengths used for traffic-bearing channels in the optical line signal, to a second polarimeter receiver at the second optical line device;
• and
• a polarimeter receiver connected to the receive fiber via an input port of the optical line device, wherein the polarimeter receiver is configured to receive a second polarization probe signal from a second transmitter transmitted from the second optical line device and to derive a measurement of State of Polarization (SOP) on the receive fiber based on the second polarization probe signal.