Determining a physical property of a device under test--DUT--includes receiving an optical scatter signal returning from the DUT in response to a probe signal launched into the DUT, wavelength dependent separating a first response signal and a second response signal from the scatter signal, determining a first power information of the first response signal and a second power information of the second response signal, time-adjusting the first power response and the second power response to each other in order to compensate a group velocity difference between the first response signal and the second response signal within the DUT, and determining the physical property on the base of the time-adjusted power responses.