Mode Multiplexer With Cascaded Vertical Asymmetric Waveguide Directional Couplers

We report a mode (de) multiplexer based on the structure of cascaded vertical waveguide directional couplers, where the spatial modes of a few-mode core are coupled to various single-mode cores placed above the few-mode core. Using five cascaded directional couplers, we design and fabricate a six-mode (de) multiplexer with polymer material that can spatially combine or separate the E-11, E-21, E-12, E-22, E-31, and E-13 modes. A typical fabricated device, which has a length of 29 mm, provides coupling ratios varying from 62% to 90% and modal crosstalks from-28.2 to-11.6 dB in the wavelength range from 1530 to 1565 nm with weak polarization dependence. The proposed structure is simple and scalable and could be developed into a range of mode-controlling devices for mode-division-multiplexing applications based on few-mode fibers.