Bi-Directional Coupler as a Mode-Division Multiplexer/Demultiplexer

A bi-directional coupler supported with Bragg grating is proposed as a mode-division multiplexer/demultiplexer. The structure can demultiplex 3 modes with only two waveguides and a Bragg grating. The input waveguide is multimode, while the output waveguide is single mode. Both first- and second-order modes of the input waveguide are coupled to the output waveguide, propagating at opposite directions, while the fundamental mode is kept in the input waveguide. A theoretical analysis of the proposed demultiplexer is developed based on the perturbative mode-coupled theory. The insertion losses of both first- and second-order modes are derived under suitable phase-matched conditions. Expressions of the insertion losses are obtained for simple slab-waveguides coupler. Numerical results of the proposed demultiplexer are presented for the slab-waveguides coupler under different design parameters. Furthermore, the wavelength dependence of the device is addressed under mismatching conditions and a Lorentzian model of the Silicon material. In addition, FDTD simulation of proposed demultiplexer is performed to prove the validity of the proposed concept. Return loss due to reflection to the source is taken into consideration in the simulation in addition to the device wavelength dependence.