A 1 x 2 multimode interferometer beam splitter based on mid-infrared emitting chalcogenides waveguides is designed. This device multiplexes mid-infrared light in two channels whose respective passbands overlap either CO 2 or CO absorption bands, respectively between 4.20-4.32 μm and 4.50-4.86 μm. The proposed device offers a low-cost solution for monolithic combination of broadband on-chip mid-infrared light emission with dispersive spectroscopic element devoted to mid-IR multigas sensing applications. Based on restrictive interference mechanism in a 1 x 2 multimode interferometer, the multimode section dimensions (width and length) are engineered to increase the imbalance between the two ports for the two passbands and consequently to increase the output contrast ratio. Tolerances to variations from the optimum device design resulting from processing conditions (materials fabrication and sputtering, photolithography and dry etching steps) are assessed. In particular, the 1 x 2 multimode interferometer diplexer spectral transmission is investigated as a function of deposited film refractive index and multimode section dimensions (width and length) deviation from designed values. Input and output ports tapering is introduced to reduce the device insertion loss.