We demonstrate a novel, fully-integrated approach to spectral sensing in the near-infrared range suitable for analyzing the chemical composition of organic materials. The sensor consists of 16 detector pixels, each forming a resonant-cavity enhanced photodetector consisting of an InGaAs/InP photodiode and a tuning layer enclosed in a planar cavity formed by two metal mirrors. For wavelengths meeting the resonance condition of the optical cavity, the absorption in the photodiode is enhanced, which leads to a wavelength-specific response of the photodetector. As the thickness of the tuning layer is varied throughout the pixels, each of the 16 photodetectors features an individual complex spectral response with several peaks of about 50 nm linewidth and responsivity above 0.1 A/W. All pixels together cover the whole wavelength range from 900 nm to 1700 nm, allowing for the analysis of broad spectral features typical for diffuse reflectance spectra of organic materials in the near-infrared range. The photocurrents read-out from the spectral sensors can be combined with chemometric analysis methods to determine the material composition. We demonstrate the performance of the spectral sensor for the determinate of moisture in rice grains, leading to a coefficient of determination of R² = 0.97. Other demonstrated applications include the quantification of the sugar content in tomatoes, fat and protein content in raw cow milk and the classification of different types of plastic. With a size of 1.5 mm by 1.5mm and a fabrication scheme based on optical lithography, this on-chip spectral sensor yields potential for large-scale production. Together with the mechanical stability of the sensor, this approach is an important step towards portable, low-cost spectral sensing solutions.
We have developed a static Fabry Perot based filter array operating in the mid-infrared range from ~2 to 4.5 μm, matching the absorption region of several volatile organic compounds. Each filter consists of a cavity system formed by two distributed bragg reflectors with a tuning element with varying length in between. The filters with high transmission (>80%) and controllable transmission peak width are integrated with an array of pyroelectric detectors. The robustness, easy-fabrication and the possibility to tune the optical response to a specific application make the integrated filter arrays suitable for compact sensing systems.
We demonstrate a near-infrared (900-1650nm) spectral sensor based on an array of 16 pixels for classifying and quantifying materials and their composition. These pixels consist of resonant-cavity enhanced photodetectors containing a thin absorbing layer, tuning element and cavity. Using a wafer-scale optical lithography process, we achieve a tunable, wavelength-specific response with narrow linewidths of 50nm and high responsivity (R>0.1A/W). The customizability of the response, small-size and robustness make it suitable for portable spectroscopic solutions in a wide variety of applications. The sensing performance is demonstrated on the prediction of moisture in rice with a coefficient of determination of R^2=0.95.
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