Portable high speed mid-IR spectrometer

Abstract

The absorption spectra of mols. in the mid-IR (mid-IR) region are used for mol. anal., and the 5-15m wavelength range is sometimes called the fingerprint region, as it enables specific identification of mols. Mol. anal. using a mid-IR light source is typically performed using a Fourier-Transform IR Spectroscopy (FTIR) system. The delicate and complex optical alignment of such instruments limits the miniaturization of mid-IR spectrometers and increases the cost and size of the spectrometer. To solve this problem, we have developed a more robust spectrometer that combines a thermal imaging camera with a hyperspectral filter. This Fabry-Perot filter consists of two high-reflectivity Si/air mirrors that are assembled with a slight angle, so that the gradual change in the cavity gap between the two mirrors changes the resonant frequency of the filter. The variable filter disperses the broadband mid-IR light source to different frequencies, each assigned to a different pixel in the microbolometer array. The high refractive index contrast between silicon and air provides a large free spectral range, wide enough to cover a detection range of from 7 mm to 14 mm. By using a com. thermal imaging camera as detector and a Diesel engine glow plug as a source, we have demonstrated an inexpensive and robust spectrometer with no moving parts and rapid spectral acquisition. We present absorption spectra of various types of polymers, biomols. and drugs in the mid-IR region, obtained within 1 s, and compare these to spectra acquired with a conventional FTIR system.

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