Multimode 3 dB Coupler Based on Symmetrically Coupled Waveguides for On-Chipbrk Mode Division Multiplexing

Mode division multiplexing on chips can enhance optical interconnect capacity tremendously, avoiding expensive wavelength division multiplexing technology. It is a critical functionality to switch or process different mode channels together for future optical network-on-chip based on mode division multiplexing. However, controlling different modes simultaneously in the traditional wide multimode waveguide is very difficult due to very different optical properties of the modes. Here, we introduce a novel concept to realize compact multimode 3 dB couplers, which can process multiple modes of a densely-packed waveguide array bus waveguide simultaneously. A dual-mode 3 dB coupler with two input and two output bus waveguides, each of which supports two transverse electrical modes, is designed with simulated insertion losses of less than 0.23 dB and crosstalks of ~-18.1 dB for both modes at the central wavelength. The relative phases at the two outputs are 90° for both modes theoretically. The designed device has a short coupling length of 21 μm with SiO
₂
cladding. The fabricated one demonstrates insertion losses of ~0.3 dB and ~0.5 dB and crosstalks of -19.6 dB and -14.1 dB for the two modes, respectively. A triple-mode 2 × 2 3 dB coupler, consisting of six coupled waveguides in the coupling region of 87 μm length, shows average insertion losses and crosstalks for all three modes of '0.25 dB and ~-14 dB, respectively. The present design method is also extended to 3 dB couplers for more modes theoretically.