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On-chip parallel Fourier transform spectrometer for broadband selective infrared spectral sensing

Abstract : Optical spectrometers enable contactless chemical analysis. However, decreasing both their size and cost appears to be a prerequisite to their widespread deployment. Chip-scale implementation of optical spectrometers still requires tackling two main challenges. First, operation over a broad spectral range extending to the infrared is required to enable covering the molecular absorption spectrum of a broad variety of materials. This is addressed in our work with an Micro-Electro Mechanical Systems (MEMS)-based Fourier transform infrared spectrometer with an embedded movable micro-mirror on a silicon chip. Second, fine spectral resolution Δλ is also required to facilitate screening over several chemicals. A fundamental limit states that Δλ is inversely proportional to the mirror motion range, which cannot exceed the chip size. To boost the spectral resolution beyond this limit, we propose the concept of parallel (or multi-core) FTIR, where multiple interferometers provide complementary optical paths using the same actuator and within the same chip. The concept scalability is validated with 4 interferometers, leading to approximately 3 times better spectral resolution. After the atmospheric contents of a greenhouse gas are monitored, the methane absorption bands are successfully measured and discriminated using the presented device. Miniaturization of sensors based on MEMS technologies is now a proven option for performing measurements in low-cost and wide-scale deployments. Their integration in very large volume markets such as smartphones, cars and many other consumer products opens up new prospects for the internet-of-things (IoT). However, this success is still limited to the measurement of physical parameters only. Chemical parameters are so numerous that one can hardly consider having as many chemical sensors as chemical substances, which are quite diverse. To address this difficulty, a paradigm shift regarding implementing concepts of analytical chemistry on a silicon chip has been introduced. Although micro-gas chromatography 1 is among the most interesting options that have been thoroughly investigated, optical spectro-scopy seems to be the only way to carry out contactless, remote chemical analysis. Therefore, microscale optical spectroscopy is attracting increasing interest 2-11. Many of the reported solutions are limited to the visible spectral range 2-4 , while those addressing the near-infrared range have a very limited spectral range 5,7,8. In addition, very few attempts extend to the mid-infrared 9,10 , and those that do still have a very limited bandwidth. In fact, MEMS spectrometers can be based on different configurations 12 such as diffraction gratings 13 , digital micro-mirror devices (DMDs) 11 , Fourier transform interferometers 14 , linear variable filters and tuneable Fabry-Pérot filters 15. Fourier transform infrared (FTIR) spectrometers appear to be a promising option since their broad spectral range enables covering the absorption molecular spectrum of a broad variety of materials, including gases, liquids and solids. In addition, a high signal-to-noise ratio (SNR) can be achieved due to the detection of all wavelengths simultaneously with a single detector 16. In its benchtop form, it is currently used in medical analysis 17-21 , food quality control 22-25 and soil analysis 26 , to name a few application areas. A vast majority of these applications
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Alaa Fathy, Yasser Sabry, Sebastien Nazeer, Tarik Bourouina, Diaa Khalil. On-chip parallel Fourier transform spectrometer for broadband selective infrared spectral sensing. Microsystems & Nanoengineering, Springer Nature, 2020, 6 (1), ⟨10.1038/s41378-019-0111-0⟩. ⟨hal-02478146⟩

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