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.