An optical fiber filter includes an optical fiber, a pair of first coupling regions, and a phase-shift region. The optical fiber includes a core and a cladding. An optical signal can pass through the optical fiber. The first coupling regions are formed in the optical fiber, at a predefined mutual distance, for producing power transfer between first and second propagation modes of the optical signal. The phase-shift region is defined by a section of the optical fiber, disposed between the first coupling regions, for producing a phase shift between the first and second propagation modes of the optical signal. In the first coupling regions, the optical fiber includes, in cross-section, an asymmetrical refractive index profile. A related optical fiber filtering device, optical fiber, process for producing an optical filter, optical amplifier, optical telecommunications system, optical fiber modal coupler, and method for filtering an optical signal are also disclosed.