Highly integrated optical microsystem for particle concentration measurement

Abstract

We present our research on integrated optical Talbot interferometers for particle mass concentration measurements. For optimum integration of highly sensitive optical measurement systems we apply a planar emitter-receiver-unit with a vertical cavity surface emitting laser (VCSEL) at 850 nm as light source. The optical system is integrated into a planar transparent PMMA (polymethylmethacrylate) substrate. We suggest a planar integrated free-space optical system for monitoring the particle mass concentration of polydisperse suspensions. Thorough simulations of the optical sensor show that for the required regime of particle concentration and particle size distribution (PSD) turbidity measurements where the attenuation of a light beam is evaluated for example at different wavelengths do not provide the required measurement precision. We therefore propose a system where the probe beam even though sent through the system is blocked before reaching the detector by an interferometric setup. The stray light originating from the particles is exploited for the measurement. For our application we focus on particles with sizes in the range 1 -120 μm and particle mass concentrations in the range of 1-10 mg/L. In this case significant strength of the scattering signal only appears in small angles relative to the incident probe beam (forward scattering). The probe beam and the stray light thus overlap to a large extent.Our sensor concept is based on a monolithically integrated Talbot interferometer. Two properly aligned diffraction gratings are used to remove the primary beam. We use a stripe detector as second grating. The stray light causes perturbations within the formation of the self image of the grating. These perturbations are visualized as speckles on a detector and exploited for particle concentration measurements. The potential of the sensor concept is presented on the example of a modular Talbot interferometer using a HeNe laser at 633 nm to measure particle mass concentrations between 1 mg/L and 250 mg/L of Arizona test dust. We present the results of our investigations concerning the generation of Talbot self images in the planar configuration using a diverging multimode VCSEL light source. Furthermore we discuss the design and demonstrate the fabrication of a planar optical test system containing the integrated passive optical elements necessary for forming an integrated Talbot interferometer. Light source and sensor are positioned on a separate chip.