Mode Division Multiplexing and De-multiplexing (MDMux / MDDeMux) has been considered a promising technique for increasing link capacity in optical interconnection. One of the most promising structures currently proposed for this application is the Asymmetric Directional Coupler ADC designed to match the propagation constant of the fundamental mode in one guide to that of the higher order modes of the adjacent guides in the coupler. Such an ADC has been used extensively in the literature to demonstrate the mode Mux for up to 16 modes in planar optical circuits. One of the key parameters in the design of such Mux / DeMux is the crosstalk (CT) due to the leakage of an undesired mode in a specific waveguide / channel. The evaluation of this parameter is usually done using numerical techniques such as the BPM or the FDTD. In this work, we apply the Coupled Mode Theory (CMT) on the ADC for the evaluation of the cross talk between guides. The results of the CMT are compared with those obtained by the 2D Beam Propagation Method, and a very good agreement is obtained. The CT is also calculated as a function of the wavelength and the location of the minimum is determined with accuracy better than 2.5 nm at the wavelength of 1550 nm. The developed CMT formulation allows very rapid structure optimization due to the great reduction in the design cycle.
Mode Division Multiplexing / De-Multiplexing (MD-MUX/De-MUX) is currently investigated as an effective and attractive technique for increasing channel capacity in optical communication networks. The core of such mode multiplexing is usually based on an optical mode converter. Different integrated optical structures have been proposed for mode conversion in planer technology. One of the difficulties in this direction is the design of a mode converter for higher order fiber modes where the energy is distributed in the two transverse dimensions of the guide cross section. In this work, we make use of the two dimensional multi-mode interferencein 2D multimode waveguide (2D MMI) for building a mode converter to convert a fundamental mode LP10 of single mode fiber to the third order LP21 mode in multimode fiber. The operating principle of the structure is based on using the 2D MMI with a proper length to create the 4 images on the SM input fiber into the output of the 2D symmetric MMI. The 4 images are distributed in the space in both the x and y directions as a 2x2 matrix form. For these images to form the field distribution of the fiber LP21 mode, a proper phase shift should be associated with each image. This is done using a section of phase shifters based on the control of the waveguide width that allows controlling its relative propagation constant. The proposed design is tested using the 2D Beam Propagation Method BPM. The obtained performance is quite encouraging.