Grating-Coupling of Thermal Radiation as an Essential Element of a Fully Integrated IR-Absorption Sensor System

摘要

As an alternative to commercially available IR-spectrometers, which are in general expensive and bulky laboratory measurement instruments, miniaturized spectrometers have been investigated by several authors (see, e.g., [1]) in the recent years. However, these devices either require external sources and detectors and hence do not represent fully integrated optical sensor systems or suffer from lacking sensitivities. Since the improvement in micromachining technology enables the efficient thermal insulation of adjacent system components, we propose a fully integrated absorption sensor (see Fig. 1) based on IR-absorption in the evanescent field region of an integrated waveguide utilizing thermally generated and detected IR-radiation, where the absorption is determined at prescribed discrete wavelengths rather than measuring a broad IR-absorption spectrum. Two grating couplers are employed to couple IR-radiation into the waveguide and the attenuated IR-radiation towards a thermal IR-detector. The sensitivity of the entire sensor system is determined by both, an appropriately designed waveguide element and the efficiency of each of the grating couplers. In this contribution, we present detailed considerations concerning the design and the behavior of grating couplers, which are applied to broadband non-polarized thermal radiation.