Towards an Integrated IR Absorption Microsensor for the Online Monitoring of Fluids
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In industry, a vast variety of processes can be optimized by means of online measurement of chemical properties of fluids in order to reduce costs for maintenance or increase productivity as well as production quality. In this contribution, we report on the design of a fully integrated infrared absorption sensor, which is capable to determine chemical properties by investigating infrared absorption at selected distinct wavelengths in the mid infrared region. For example, in order to reduce the frequency of scheduled maintenance of lubrication oil, e.g., in combustion engines, the quality of the deteriorated oil has to be determined, which has been selected as pilot application for our development. Based on results of transmission spectroscopy, we show that by means of a properly designed optical waveguide structure, a sensitivity similar to that obtained by transmission spectroscopy, as used in the laboratory, can be achieved. To achieve a highly sensitive absorption element, a planar mono mode waveguide is utilized which requires an appropriate coupler to couple the infrared light into the waveguide. The employed grating couplers have been modeled in order to derive a design yielding the desired sensor performance. As the targeted wavelength is located in the mid-infrared region, a suitable infrared source or detector, respectively, can be fabricated by thermal components. Finally we report on the ongoing technological development to establish the required thin-film processes.