Date of Award

Fall 2019

Document Type

Open Access Dissertation

Department

Chemistry and Biochemistry

First Advisor

S. Michael Angel

Abstract

Raman and Laser-Induced Breakdown Spectroscopy (LIBS) are optical techniques that provide information about the chemical makeup of a sample without any preparation or physical contact with it. For this reason, Raman and LIBS spectrometers are among the instruments selected for NASA’s Mars 2020 rover mission and are being considered for future missions to the Jovian moons, asteroids and comets. Such future missions will require smaller, more ruggedized Raman and LIBS spectrometers, and the new type of spectrometers discussed here, the spatial heterodyne Raman and LIBS spectrometers are being developed for this purpose. The SHS is a wide-field, Fourier transform, stationary grating interferometer with no moving parts, that is well-suited for miniature Raman and LIBS spectrometers. The spectral resolution of the SHS is not strongly dependent upon entrance aperture size, so the footprint can be very small while still allowing for high resolution spectral measurements. The SHS has a large entrance aperture and a wide acceptance angle that provides much higher light throughput than a dispersive slit-based spectrometer of comparable resolution. Lastly, because the SHS has no moving parts and the spectral resolution is not strongly tied to its size, it can be built monolithically making it both small and robust.

This thesis explores alternative optical configurations of the SHS collection optics that are useful for miniaturizing or extending the capabilities of a remote SHS Raman or LIBS system. A lightweight, low cost Fresnel lens was compared to that of a high-quality telescope for standoff signal collection. This takes advantage of the wide field of view of the SHS, which makes the collection efficiency very forgiving of the quality of the collection optics. We observed that the signal collected by the Fresnel lens, which weighed less than 1% of a comparable glass lens, was only about four times less than that of the telescope. This suggests that Fresnel optics would be useful in applications where size and weight are restricted, such as in instruments designed for spacecraft and planetary landers.

In other studies, a new hyperspectral Raman imaging technique is demonstrated using a spatial heterodyne Raman spectrometer with a microlens array, where the entire hypercube of spatial and spectral information is obtained in a single measurement. Raman images for a variety of sample types are demonstrated where complete Raman spectra, at spectral ranges from 1200 cm-1 to 2800 cm-1, were acquired independently at each spatial point, for 60 to >500 unique spatial points, in a single spectral acquisition. The spectral resolution of the Raman spectra acquired for each spatial point in the images varied from 32 cm-1 to 148 cm-1, dependent on the grating and system magnification and the resolution of the detector. Calculations show that this technique can be extended to include more than 10,000 spatial points with a spectral resolution of 20 cm-1, over a large spectral range.

Rights

© 2019, Ashley Nicole Allen

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