A method of distinguishing whether a detected change in reflected power in an optical time domain reflectometer (OTDR) measurement carried out in a fiber optic transmission system using an OTDR is caused by an event causing actual attenuation or a change in a mode field diameter, comprising: a step of emitting a succession of first sampling light pulses of a first wavelength into the fiber optic transmission system while a pumping signal with a second wavelength is emitted into the fiber optic transmission system , and measuring a first OTDR trace resulting from the reflection of the first sampling light pulses in the fiber optic transmission system , such that the first sampling light pulses and their reflections interact with the pumping signal via stimulated Raman scattering; a step determining, based at least on information extracted from the first OTDR trace , whether the detected change is mainly due to an event causing actual attenuation or to a change in the mode field diameter in the fiber optic transmission system ; wherein the step of determining whether the detected change is mainly due to an event causing actual attenuation or to a change in the mode field diameter is carried out by checking for consistency of information about the first , and optionally, second and/or third OTDR traces with empirical or calculated model data, wherein said model data comprises - first model data representing a first scenario in which the change in reflected power is due to an event causing actual attenuation only, and - second model data representing a second scenario in which the change in reflected power is due to a change in the mode field diameter of the fiber optic transmission system only. a step of emitting a succession of first sampling light pulses of a first wavelength into the fiber optic transmission system while a pumping signal with a second wavelength is emitted into the fiber optic transmission system , and measuring a first OTDR trace resulting from the reflection of the first sampling light pulses in the fiber optic transmission system , such that the first sampling light pulses and their reflections interact with the pumping signal via stimulated Raman scattering; a step determining, based at least on information extracted from the first OTDR trace , whether the detected change is mainly due to an event causing actual attenuation or to a change in the mode field diameter in the fiber optic transmission system ; wherein the step of determining whether the detected change is mainly due to an event causing actual attenuation or to a change in the mode field diameter is carried out by checking for consistency of information about the first , and optionally, second and/or third OTDR traces with empirical or calculated model data, wherein said model data comprises - first model data representing a first scenario in which the change in reflected power is due to an event causing actual attenuation only, and - second model data representing a second scenario in which the change in reflected power is due to a change in the mode field diameter of the fiber optic transmission system only. - first model data representing a first scenario in which the change in reflected power is due to an event causing actual attenuation only, and - second model data representing a second scenario in which the change in reflected power is due to a change in the mode field diameter of the fiber optic transmission system only. <EOS>