An apparatus (100, 200) comprising: a digital source of test signals (110, 210); a coherent optical transmitter (COT, 120, 220) to transmit a light signal carrying different portions of the test signals on orthogonal polarizations thereof; a coherent optical receiver (COR, 130, 230) capable of separately measuring different polarization components of light received therein and to output signals indicative thereof; characterized in that the apparatus (100, 200) also comprises: an optical switch (160, 260) operable to selectively optically connect the COT (120, 220) and the COR (130, 230) to an end of an optical fiber link (150, 250) in a first configuration of the optical switch and to optically back-to-back connect the COT (120, 220) and the COR (130, 230) in a second configuration of the optical switch (160, 260); and an electrical processor (140, 240) connected to receive the signals from the COR (130, 230) and configured to estimate a gain imbalance between a first and a second of the orthogonal polarizations in at least one of the COT (120, 220) and the COR (130, 230) based on the signals received from the COR (130, 230) in the second configuration, and to measure a portion of the light signal backscattered by the optical fiber link (150, 250) in the first configuration. a digital source of test signals (110, 210); a coherent optical transmitter (COT, 120, 220) to transmit a light signal carrying different portions of the test signals on orthogonal polarizations thereof; a coherent optical receiver (COR, 130, 230) capable of separately measuring different polarization components of light received therein and to output signals indicative thereof; characterized in that the apparatus (100, 200) also comprises: an optical switch (160, 260) operable to selectively optically connect the COT (120, 220) and the COR (130, 230) to an end of an optical fiber link (150, 250) in a first configuration of the optical switch and to optically back-to-back connect the COT (120, 220) and the COR (130, 230) in a second configuration of the optical switch (160, 260); and an electrical processor (140, 240) connected to receive the signals from the COR (130, 230) and configured to estimate a gain imbalance between a first and a second of the orthogonal polarizations in at least one of the COT (120, 220) and the COR (130, 230) based on the signals received from the COR (130, 230) in the second configuration, and to measure a portion of the light signal backscattered by the optical fiber link (150, 250) in the first configuration. an optical switch (160, 260) operable to selectively optically connect the COT (120, 220) and the COR (130, 230) to an end of an optical fiber link (150, 250) in a first configuration of the optical switch and to optically back-to-back connect the COT (120, 220) and the COR (130, 230) in a second configuration of the optical switch (160, 260); and an electrical processor (140, 240) connected to receive the signals from the COR (130, 230) and configured to estimate a gain imbalance between a first and a second of the orthogonal polarizations in at least one of the COT (120, 220) and the COR (130, 230) based on the signals received from the COR (130, 230) in the second configuration, and to measure a portion of the light signal backscattered by the optical fiber link (150, 250) in the first configuration.