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.