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Aiming at a handheld sensor to characterize chemical compositions, we propose a fully integrated absorption sensor based on infrared (IR) absorption in the evanescent field region of an integrated monomode waveguide utilizing thermally generated and detected IR radiation. For the coupling, two grating couplers are employed, which couple broadband thermal IR radiation into the waveguide and the attenuated IR radiation at the waveguide output toward a thermal IR detector, respectively. To achieve a spectroscopic measurement, we utilize the dispersive feature of the grating coupler, which couples the IR beams out of the waveguide at angles depending on the wavelength. Depending on the application, the spectral resolution might be more or less critical. In this paper, we report on the design of the grating couplers associated with the proposed system configuration influencing the spectral resolution. We also show measurements confirming the results obtained by numerical simulations aiming at the investigation of the dispersive feature of the grating couplers.