The present invention has an object to provide an optical fiber testing method and an optical fiber testing device capable of measuring the delay ratio between the modes at each position of a fiber over a long distance in which a plurality of modes propagate.
An optical fiber testing method and its device according to the present invention, measure the change amount for the wave number k of a Brillouin Frequency Shift ν in stimulated Brillouin scattering generated in the same acoustic mode, with respect to each target propagation mode. Thereby, the ratio of the change amount measured at each propagation mode is acquired as the group delay ratio between the modes.