One‐mirror, one‐grating spatial heterodyne spectrometer for remote‐sensing Raman spectroscopy

• Published in: Journal of Raman spectroscopy 2020-09, Vol.51 (9), p.1794-1801
• Main Authors: Egan, Miles J., Acosta‐Maeda, Tayro E., Angel, S. Michael, Sharma, Shiv K.
• Format: Article
• Language: English
• Subjects: Biomarkers; Diffraction; Fourier transforms; Gratings (spectra); interferometry; Michelson interferometers; Minerals; Photons; planetary surface exploration; Raman spectra; Raman spectroscopy; Remote sensing; spatial heterodyne spectrometer; Spectral resolution; Spectrum analysis
• ISSN: 0377-0486; 1097-4555
• DOI: 10.1002/jrs.5788

Recently, we evaluated a new type of Fourier transform Raman spectrometer, the spatial heterodyne Raman spectrometer (SHRS), which provides high‐resolution Raman spectra without the need for an entrance slit. An SHRS is a variant of a Michelson interferometer in which the mirrors in the arms of a Michelson interferometer are replaced by two stationary diffraction gratings. Instead of sampling path length differences temporally, as in the case of a Michelson interferometer, SHRS samples path length differences spatially at a two‐dimensional array detector. Applying a Fourier transform to the resulting interferogram recovers the desired spectrum. In the modified SHRS (mSHRS) instrument used in the present work, one of the diffraction gratings has been replaced by a stationary λ/10 mirror. This modification has a few effects. First, the detector records a greater number of photons, as spectral light is not lost into unused diffraction orders in the mirror arm of mSHRS. Second, the free spectral range (wavelength coverage) is doubled, whereas the spectral resolution is cut in half. In this work, the authors present remote‐sensing Raman spectra of minerals, organics, and biomarkers using this mSHRS for the first time. In this work, the authors demonstrate the ability to measure the Raman spectra of minerals at up to 18.25‐m distance using a modified spatial heterodyne Raman spectrometer (SHRS) that contains one mirror and one grating. These measurements represent the longest known Raman spectra of minerals with any spatial heterodyne spectrometer known to the authors.