Method of assessing a power penalty at a given bit error rate of a multimode fiber, comprising at least: - a step of differential mode delay measurement, measuring a set of elementary fiber responses corresponding respectively to different offset launches (r) of light over the core radius into multimode fiber, - a step of generating a global fiber response by applying, to the set of elementary fiber responses, a set of weighting coefficients and time delays respectively depending on the different offset launches (r) of the elementary fiber responses, and - a step of computing a parameter representative of a fiber power penalty from the global fiber response, wherein, in the step of generating a global fiber response, the set of weighting coefficients includes several subsets (W(r, 1) to W(r, 6)) of weighting coefficients that are time delayed relatively to one another, and wherein at least one relative time delay that is not set to zero, and wherein the weighting coefficients of each subset respectively depend on the different offset launches (r) of the elementary fiber responses and wherein: each subset (W(r, 1) to W(r, 6)) of weighting coefficients is applied, to the same set of elementary fiber responses, to generate a partial fiber response, wherein each fiber response corresponds to only one transverse mode of a transversely multimode laser diode, wherein different subsets of weighting coefficients correspond to different transversal modes of the laser diode, and wherein a time delay is applied to each partial fiber response corresponding to only one transverse mode of a laser source, and said global fiber response is built as a sum of the different delayed partial fiber responses and wherein the respective time delays are computed as the products of the respective differences between wavelengths of the respective transverse modes of the laser source and central wavelength of the laser source multiplied by the chromatic dispersion of the fiber. - a step of differential mode delay measurement, measuring a set of elementary fiber responses corresponding respectively to different offset launches (r) of light over the core radius into multimode fiber, - a step of generating a global fiber response by applying, to the set of elementary fiber responses, a set of weighting coefficients and time delays respectively depending on the different offset launches (r) of the elementary fiber responses, and - a step of computing a parameter representative of a fiber power penalty from the global fiber response, each subset (W(r, 1) to W(r, 6)) of weighting coefficients is applied, to the same set of elementary fiber responses, to generate a partial fiber response, wherein each fiber response corresponds to only one transverse mode of a transversely multimode laser diode, wherein different subsets of weighting coefficients correspond to different transversal modes of the laser diode, and wherein a time delay is applied to each partial fiber response corresponding to only one transverse mode of a laser source, and said global fiber response is built as a sum of the different delayed partial fiber responses and wherein the respective time delays are computed as the products of the respective differences between wavelengths of the respective transverse modes of the laser source and central wavelength of the laser source multiplied by the chromatic dispersion of the fiber.