Device for selective increasing higher-order modes' losses

A device for selective increasing higher-order modes' losses (3), characterized in that it comprises an optical fiber taper executed on a multi-mode fiber multi-mode at a certain wavelength, and that the fiber taper(7) has separated regions, i.e. non-tapered fiber regions of (a1) and (a2) in total lengths, in which fiber diameter equals (d1), transition regions of (b1) and (b2) in total lengths, in which fiber diameter is reduced/increased, respectively; and a taper waist region of (c) in total length, in which fiber diameter equals (d2), and the taper level is defined as R=((d1-d2)/d1)·100% and equals at least 20%, and the length of the transition regions is (b1), at least 0.5 mm, (b2) is equal or larger than 0 mm, the length of the taper waist is at least 0.5 mm, and the taper area is coated with a filtering substance with attenuating properties.