Phase Off-Axis Modulation Metasurface for Orbital Angular Momentum Mode Multiplexing/Demultiplexing

Orbital angular momentum (OAM) mode multiplexing provides a promising solution for enlarging communication capacity density. Although various OAM mode multiplexing technologies have been investigated, (de)multiplexers possessing a wide spectral response and polarization insensitivity to combine wavelength division multiplexing (WDM) and polarization division multiplexing (PDM) remain elusive. Herein, we propose a phase off-axis modulation strategy for OAM mode (de)multiplexing by encoding the binary Dammann optical vortex grating (DOVG) phase into a geometric phase metasurface that consists of metallic nanoantennas with spatially varying orientations on a dielectric-metal substrate. We show that four OAM modes (−2, −1, 1, 2) were successfully multiplexed and demultiplexed by virtue of the multilevel diffraction of DOVG and its reverse process. The working wavelength of this metasurface ranges from 1260 nm to 1675 nm due to the dispersion-free geometric phase, moreover, the mode symmetry of DOVG and the conjugate modulation of the geometric phase yield polarization insensitivity. As a proof of concept, an OAM mode multiplexing communication system combining WDM and PDM was constructed, and 1.56 Tbit/s QPSK signals were transmitted with the bit-error-rate approaching 10−6. These results demonstrate that this phase off-axis modulation metasurface can effectively (de)multiplex OAM modes and shows good compatibility with WDM and PDM, which may promote the practical application of OAM communication and all-optical interconnection.