The invention concerns a method for characterizing mode group properties of multimodal light traveling through an optical component, comprising: providing a Mode Group Separating optical fiber in an optical path between a light source and said optical component; launching reference pulses of light with a wavelength λt from said light source through said Mode Group Separating optical fiber into said optical component at discrete intervals between a core center and a core radius of said fiber. The Mode Group Separating optical fiber is a multimode fiber with an α-profile graded index core with an α-value chosen such that said fiber satisfies the following criterion at the wavelength λt:  Δτ  · L Δ ⁢ ⁢ T REF > 4 where: Δτ is a time delay difference between consecutive mode groups; L is a length of said fiber; ΔTREF is a Full Width at Quarter Maximum of said reference pulses.