For the online characterization of fluids regarding their chemical composition, the miniaturization of an IR-absorption
sensor at application-specific distinguished wavelengths for the mid-IR-region promises outstanding features. Utilizing
micromachining technology facilitates the integration of all required components (including thermal emitter and
detector) into a complete sensor system. The absorption is sensed in the evanescent field of an appropriately designed
slab mono-mode waveguide (ZnSe, n=2.42) residing on a BaF2-substrate (n=1.44), which represents the central element
of the system. A typical application for such a system is, e.g., the characterization of engine oil oxidation in terms of the
absorption at 5.85 μm as an indicator for deterioration. The thermal generation and detection of mid-IR-radiation is
preferred over expensive and sophisticated quantum well devices. However, the spatial and non-coherent character of
thermally generated IR-radiation requires an extension of the numerical methods established for coherent light sources
for a proper design of the system's grating couplers, which act as key elements determining the system performance.
These couplers yield efficient coupling into and out of the sensing waveguide and provide the required spectral filtering
at the same time. In the actually projected implementation, a multilayer waveguide Si/BaF2/ZnSe is used, where the
silicon substrate practically represents a rear-reflector in the grating region featuring several advantages compared to
simpler grating couplers. In this contribution we discuss the modelling of the coupling of non-coherent, thermally
generated and detected IR-radiation by means of these multilayer grating couplers in the context of a fully integrated
IR-absorption sensor system.