Non-Hermitian coupling of orbital angular momentum modes in optical waveguides

We investigate the optical coupling of orbital angular momentum (OAM) modes in a multimode waveguide assisted by a lossy single-mode waveguide. By utilizing coupled mode theory, we show that the coupling coefficients between two OAM modes with opposite rotation directions become imaginary-valued in cylinder waveguides, which further gives rise to polarization-locked wave dynamics. Any injections with different polarizations finally evolve into linear polarization (LP) modes, as the output modes are synthetized by antisymmetric supermodes of two OAM modes. In addition, non-Hermitian asymmetric couplings are induced in elliptical waveguides, which stem from the interaction between synthetical gauge fields and dissipation. By increasing the ellipticity of waveguides, the degree of asymmetric coupling increases, and the system experiences a phase transition, which can be reflected from propagation characteristics of OAM modes. The study provides a promising approach for controlling modal coupling of OAM modes and may find potential applications in OAM generation and polarization conversion.