Non-Hermitian mode cleaning in waveguides and amplifiers

We propose an all-optical spatial non-Hermitian mode-cleaning scheme for parabolic waveguides and nonlinear fiber amplifiers supported by analytical and numerical studies. The effect is triggered by a unidirectional coupling among the modes, resulting from the simultaneous modulation of the refractive index and gain/loss along graded-index multimode waveguides. Moreover, the proposal also enables mode conversion, and in the case of amplifiers, the antisymmetric non-Hermitian potential allows for a flexible modal management.