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Development of a Miniature Fiber Optic Raman Spectrograph for Planetary Landers, 15-9981

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Principal Investigators
Martin P. Wüest
Ralph H. Hill
Robert J. Beeson
David C. Slater

Inclusive Dates: 11/01/96 - 06/30/99

Background - The ability of Raman spectroscopy to probe chemical bonds and the crystal lattice of some minerals and chemicals makes it useful for studying the chemical and mineralogical composition of a planetary surface. Raman spectroscopy has been used in the laboratory to study the chemical composition and petrographical structure of meteorites, interplanetary dust particles, synthesized comet particles, and lunar materials. Recent electro-optic advances make it possible to assemble a fiber-optic multichannel spectrograph suitable for use in a planetary lander, which has restrictive mass and volume requirements and limited power resources.

Approach - The goal of this program is to design and assemble a laboratory prototype model of the fiber-optic spectrograph. In a trade-off study, the advantages and disadvantages of different designs were evaluated. Special emphasis was placed on spectrograph design requirements including: 1) a minimum spectral resolution of 5 cm-1, 2) a high signal-to-noise ratio, 3) reduced size and weight, and 4) reduced optical complexity. Following the trade-off study, the selected design was thoroughly characterized.

Accomplishments - The trade-off study resulted in a flat field, imaging spectrograph design with a single aberration corrected, concave holographic grating. To reduce costs, commercial off-the-shelf components were used, which meant that the focal length was longer than could be obtained in a custom-designed instrument. Also the spectral resolution was degraded to 16 cm-1. The spectrograph has been optically characterized, and Raman spectra were obtained at 785 and 831-nm incident laser wavelength. In addition, Raman spectra of a meteorite and a Mars soil analog were obtained with a commercial research-grade Raman spectrograph at 780-nm incident laser wavelength. Also, a literature database was established with Raman spectra of minerals that might be encountered in future space missions.

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