A highly sensitive, low-power, low-noise multifunctional active-excitation spectral analyzer (MAESA) that would span the wavelength range of 0.5 to 2.5 µm and would operate near room temperature is undergoing development. The fully developed MAESA is expected to be a portable and highly miniaturized version of a prototype of the instrument that has been demonstrated in a laboratory. The MAESA is intended primarily for use in remote sensing of chemical compositions of mineral surfaces on planets or on Earth.
The MAESA (see figure) would include a laser and associated optics for generating a beam of monochromatic light to illuminate a point or a line on a target. Other optics in the MAESA would image the target onto a rectangular focal-plane array (FPA) of InGaAs photodetectors, whereon a pixel or a column of pixels, would correspond to the illuminated target point or the illuminated target line, respectively. At the target, the illumination would excite Raman scattering, the spectrum of which would depend on (and which could be analyzed to estimate) the chemical composition of the target. The light returning from the target would be long-wavelength-pass filtered to remove the laser wavelength component, then focused onto a convex diffraction grating, which would spectrally disperse the remaining Raman-scattered light along a row of the FPA. The InGaAs FPA (which could be a commercially available unit) would be hybridized with a custom-made silicon complementary metal oxide/semiconductor (CMOS) readout multiplexer integrated circuit. The output of the readout multiplexer would be sent to pixel-data-processing circuits.
The wavelength of the spectral component impinging on each pixel would be a known function of the position of the pixel along the row. Hence, by processing the readouts from all the pixels in this row at the same time, one could obtain a characteristic spectrum for estimating the chemical composition of the illuminated point on the target. Optionally, the readout multiplexer and the downstream data-processing circuitry could be made to repeat the readout and processing in a temporal sequence corresponding to successive rows of pixels, thereby building up spectrally resolved information about the chemical compositions of the target at all the target pixel locations along the column axis. A further option would be to deactivate the laser and replace the convex diffraction grating with a conventional curved mirror, in which case the MAESA would function as a conventional camera recording images in ambient visible and/or infrared light.
The advantage of using an FPA made of InGaAs (in contradistinction to photodetector arrays made of other semiconductors) are that InGaAs offers the potential for superior detectivity (D*), without need for cooling, over the wavelength range of 0.5 to 2.5 µm, which range is not spanned by any other single active detector material. The high D* enables high spectral (or alternatively, temporal) resolution and makes possible the wide dynamic range needed for detection of weak visible and infrared Raman spectra characteristic of materials of interest.
The convex diffraction grating would consist, more precisely, of three concentric subgratings. Each subgrating would provide spectral dispersion free of distortion in one of three wavelength subbands: 0.5 to 1.0, 0.8 to 1.6, and 1.25 to 2.5 µm, respectively. The grating would be fabricated in a micromachining process that is based on electron-beam lithography.
This work was done by Quiesup Kim of Caltech for NASA’s Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free online at www.nasatech.com/tsp under the Physical Sciences category.
This Brief includes a Technical Support Package (TSP).
A Multifunctional Active-Excitation Spectral Analyzer
(reference NPO-21143) is currently available for download from the TSP library.
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