Miniaturized integrated evanescent field IR-absorption sensor: Design and experimental verification with deteriorated lubrication oil

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 of maintenance or increase productivity as well as production quality. Since the absorption measurement in the mid-IR-region is a powerful method to determine the chemical composition of fluids, we propose a fully integrated absorption sensor based on IR-absorption in the evanescent field region of an integrated mono-mode 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 towards 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 frequency. We report on the design of the fully integrated infrared absorption sensor, which is capable to determine chemical properties of, e.g., lubrication oil by investigating infrared absorption. Beside the theory related to an optimum design of the mono-mode waveguide, we discuss the applicability of the proposed concept to the monitoring of deterioration of lubrication oil.