Scrambling-Type Three-Mode PLC Multiplexer Based on Cascaded Y-Branch Waveguide With Integrated Mode Rotator

A novel scrambling-type mode multiplexer (MUX) is proposed for future mode-division multiplexing with large number of modes. The proposed MUX does not aim at high modal extinction ratio as in a conventional-mode MUX, rather, all the modes are moderately mixed assuming that an MIMO-based receiver is used. The proposed configuration is superior to the conventional-mode MUX based on asymmetric directional couplers in terms of the fabrication tolerance of the waveguide width and scalability to a large number of modes. The wavelength dependence is also small since no coherent coupling devices are used, except for mode rotators. A three-mode multiplexer design based on a cascaded Y-branch waveguide and an integrated mode rotator on a silica planar lightwave circuit shows that low-loss and small wavelength dependence multiplexing is possible and a proof-of-concept device is fabricated.