Hyperspectral imaging with sufficient resolution and sensitivity for scientifically useful space-based mapping of trace gases has long required large and expensive satellite instruments. Miniaturizing this capability to a CubeSat configuration is a major challenge, but opens up more agile and far less expensive observing strategies. A major step in this direction is our development of NACHOS, an ultra-compact (1.5U instrument, 3U complete CubeSat) hyperspectral imager covering the 300-500nm spectral range in 400 channels. Here we describe laboratory and field performance characterization of this new instrument. Laboratory tests demonstrate spatial and spectral resolutions of <0.8 mrad and 1.3 nm, respectively, with good resolution of the spectral lines of our SO2 and NO2 target gases. Outdoor field tests under realistic illumination conditions provide real-world signal-to-noise benchmarks, and yield hyperspectral images displaying high quality solar and atmospheric spectra. To estimate on-orbit gas retrieval sensitivities, we computationally implanted plumes of varying concentrations into acquired hyperspectral datacubes. Applying our adaptive matched filter gas-retrieval algorithms to the generated scene, we predict NACHOS will be able to distinguish 35 and 7 ppm⋅m plumes of SO2 and NO2 (respectively) with high sensitivity; a capability well-suited to address scientific goals related to monitoring both passive SO2 degassing from volcanoes and NO2 emissions from anthropogenic sources. Lastly, we will show findings from thermal and vibrational environmental tests, performed in preparation for a scheduled early-2022 launch, demonstrating the extremely robust spectrometer design is well-suited for satellite-based deployment.
The Nano-satellite Atmospheric Chemistry Hyperspectral Observation System (NACHOS) is a high-throughput (f/2.9), high spectral resolution (~1.3 nm optical resolution, 0.6 nm sampling) Offner-design hyperspectral imager operating in the 300-500 nm spectral region. The 1.5U instrument payload (1U optical system, 0.5U electronics module) is hosted by a 1.5U LANL-designed CubeSat bus to comprise a 3U complete satellite. Spectroscopically similar to NASA’s Ozone Monitoring Instrument (OMI), which provides wide-field global mapping of ozone and other gases at coarse spatial resolution, NACHOS fills the complementary niche of targeted measurements at much higher spatial resolution. With 350 across-track spatial pixels and a 15-degree across-track field of view, NACHOS will provide spectral imaging at roughly 0.4 km per pixel from 500 km altitude. NACHOS incorporates highly streamlined gas-retrieval algorithms for rapid onboard processing, alleviating the need to routinely downlink massive hyperspectral data cubes. We will discuss the instrument design, challenges in achieving mechanical robustness to launch vibration in such a compact instrument, the onboard calibration system, and gas-retrieval data downlink strategy. We will also discuss potential science missions, including monitoring of NO2 as an easily detected proxy for anthropogenic fossil-fuel greenhouse gases, monitoring lowlevel SO2 degassing at pre-eruptive volcanoes, H2CO from wildfires, and characterization of aerosols. The long-term vision is for a many-satellite constellation that could provide both high spatial resolution and frequent revisits for selected targets of interest. As an initial technology demonstration of this vision, the NACHOS project is currently slated to launch two CubeSats in early 2022.
Los Alamos is currently working toward demonstrating Cubesat-based hyperspectral detection of gas-phase chemical plumes, a goal initially pursued in the internally funded Targeted Atmospheric Chemistry Observations from Space (TACOS) project, and now advancing toward space deployment with the NASA-funded Nanosat Atmospheric Chemistry Hyperspectral Observation System (NACHOS). This paper will present a general overview of these projects. Bandwidth considerations prevent full datacube downloads, and so processing algorithms include an on-board processing component to provide matched-filter and RX images for the gases of interest. The downlinked data will additionally include the full spectrum for a small sample of pixels, and one of the challenges for ground-based analysis will be to incorporate these different but incomplete "views" of the datacube into a more physical interpretation/analysis of the downlinked data.
We describe the development and implementation of plume detection algorithms under severe bandwidth and processing constraints imposed by a CubeSat architecture. In particular, two ideas will be presented: one employs onboard processing to reduce the data that is downlinked, and one employs the Sparse Matrix Transform (SMT) to speed up the onboard computation of an approximate Mahalanobis distance.
Programmable spectral filters based on digital micromirror devices (DMDs) are typically restricted to imaging a 1D line across a scene, analogous to conventional "push-broom scanning" hyperspectral imagers. In previous work, however, we demonstrated that, by placing the diffraction grating at a telecentric image plane rather than at the more conventional location in collimated space, a spectral plane can be created at which light from the entire 2D scene focuses to a unique location for each wavelength. A DMD placed at this spectral plane can then spectrally manipulate an entire 2D image at once, enabling programmable matched filters to be applied to real-time video imaging. We have adapted this concept to imaging rapidly evolving gas plumes. We have constructed a high spectral resolution programmable spectral imager operating in the shortwave infrared region, capable of resolving the rotational-vibrational line structure of several gases at sub-nm spectral resolution. This ability to resolve the detailed gas-phase line structure enables implementation of highly selective filters that unambiguously separate the gas spectrum from background spectral clutter. On-line and between-line multi-band spectral filters, with bands individually weighted using the DMD's duty-cycle-based grayscale capability, are alternately uploaded to the DMD, the resulting images differenced, and the result displayed in real time at rates of several frames per second to produce real-time video of the turbulent motion of the gas plume.
The performance of a solid-state optical refrigerator is the result of a complex interplay of numerous optical and thermal parameters. We present a first preliminary study of an optical cryocooler using ray-tracing techniques. A numerical optimization identified a non-resonant cavity with astigmatism. This geometry offered more efficient pump absorption by the YLF:10%Yb laser-cooling crystal compared to non-resonant cavities without astigmatism that have been pursued experimentally so far. Ray tracing simulations indicate that ~80% of the incident pump light can absorbed for temperatures down to ~100 K. Calculations of heat loads, cooling power, and net payload heat lift are presented. They show that it is possible to cool a payload to a range of 90–100 K while producing a net payload heat lift of 80 mW and 300 mW when pumping a YLF:10%Yb crystal with 20 W and 50 W at 1020 nm, respectively. This performance is suited to cool HgCdTe infrared detectors that are used for sensing in the 8–12 μm atmospheric window. While the detector noise would be ~6× greater at 100 K than at 77 K, the laser refrigerator would introduce no vibrations and thus eliminate sources of microphonic noise that are limiting the performance of current systems.
Rapidly programmable spatial light modulation devices based on MEMS technology have opened an exciting new arena
in spectral imaging: rapidly reprogrammable, high spectral resolution, multi-band spectral filters that enable
hyperspectral processing directly in the optical hardware of an imaging sensor. Implemented as a multiplexing spectral
selector, a digital micro-mirror device (DMD) can independently choose or reject dozens or hundreds of spectral bands
and present them simultaneously to an imaging sensor, forming a complete 2D image. The result is a high-speed, highresolution,
programmable spectral filter that gives the user complete control over the spectral content of the image
formed at the sensor. This technology enables a wide variety of rapidly reprogrammable operational capabilities within
the same sensor including broadband, color, false color, multispectral, hyperspectral and target specific, matched filter
imaging. Of particular interest is the ability to implement target-specific hyperspectral matched filters directly into the
optical train of the sensor, producing an image highlighting a target within a spectrally cluttered scene in real time
without further processing. By performing the hyperspectral image processing at the sensor, such a system can operate
with high performance, greatly reduced data volume, and at a fraction of the cost of traditional push broom hyperspectral
instruments. Examples of color, false color and target-specific matched-filter images recorded with our visible-spectrum
prototype will be displayed, and extensions to other spectral regions will be discussed.
The role of transition-metal impurities in Yb3+-doped YLiF4 (YLF) laser-cooling crystals is studied. Divalent 3d transition-metal ions, in particular Fe2+, are found to have strong absorptions at the laser cooling pump wavelength and degrade the cooling efficiency by introducing background absorption. A set of eight substitutional and chargecompensated defects that form upon introduction of 1+, 2+, and 3+ transition-metal ions into the YLF crystal lattice is proposed. A calculation of solution energies for each defect type and for a range of 3d ions is carried out. It indicates that divalent 3d ions preferentially substitute for Y3+ accompanied by a fluoride vacancy for charge compensation. An electron paramagnetic resonance (EPR) study of a YLF crystal identifies Fe2+ in the crystal lattice, in agreement with the elemental analysis and the computational results. A strategy for purifying the YF3, LiF, and YbF3 starting materials for the YLF:Yb crystal growth is discussed. Chelate-assisted solvent extraction purification with pyrrolidine dithiocarbamate (APDC) for Y, Li, and Yb as well as ethylenediaminetetraacetic acid (EDTA) for Li was carried out.
Rapidly programmable micromirror arrays, such as the Texas Instruments Digital Light Processor (DLP®) digital micromirror device (DMD), have opened an exciting new arena in spectral imaging: rapidly reprogrammable, high spectral resolution, multiband spectral filters that perform spectral processing directly in the optical hardware. Such a device is created by placing a DMD at the spectral plane of an imaging spectrometer and by using it as a spectral selector that passes some wavelengths down the optical train to the final image and rejects others. Although simple in concept, realizing a truly practical DMD-based spectral filter has proved challenging. Versions described to date have been limited by the intertwining of image position and spectral propagation direction common to most imaging spectrometers, reducing these instruments to line-by-line scanning imagers rather than true spectral cameras that collect entire two-dimensional (2-D) images at once. Here, we report several optical innovations that overcome this limitation and allow us to construct full-frame programmable filters that spectrally manipulate every pixel, simultaneously and without spectral shifts, across a full 2-D image. So far, our prototype, which can be programmed either as a matched-filter imager for specific target materials or as a fully hyperspectral multiplexing Hadamard transform imager, has demonstrated over 100 programmable spectral bands while maintaining good spatial image quality. We discuss how diffraction-mediated trades between spatial and spectral resolution determine achievable performance. Finally, we describe methods for dealing with the DLP’s 2-D diffractive effects and suggest a simple modification to the DLPs that would eliminate their impact for this application.
Hyperspectral imaging sensors have proven to be powerful tools for highly selective and sensitive chemical detection applications, but they have significant operational drawbacks including slow line-scanning acquisition, large data volume of the resulting images, and a detection time lag due to the computational overhead of the matched-filter analysis. We have recently developed and demonstrated a high-speed, high-resolution, programmable spectral filter based on the Texas Instruments DLP® digital micromirror device (DMD) that is capable of performing matched-filter image processing across a two-dimensional (2-D) field-of-view directly in the optical hardware and will enable real-time chemical detection without slow scanning, large data volumes or expensive postprocessing requirements. Based on traditional optical techniques, our spectral filter encodes the spectral information orthogonal to the spatial image information, enabling the DMD to encode matched-filter information into the spectral content of the scene without disturbing the underlying 2-D image. With this new technology, everything from simple multiband filters to very complicated hyperspectral matched filters can be implemented directly in the optical train of the sensor, producing an image highlighting a target signature within a spectrally cluttered scene in real time without further processing. We will first describe the implementation of a DMD as a multiplexing spectral selector for a 2-D field-of-view, discuss its utility as a multiband spectral filter, and show how the DLP’s® duty cycle-based grayscale capability enables the direct measurement of the adaptive matched filter. We will also show examples of multispectral and hyperspectral matched-filter images recorded with our visible spectrum prototype.
Rapidly programmable micro-mirror arrays, such as the DLP® digital micro-mirror device (DMD), have opened an exciting new arena in spectral imaging: rapidly reprogrammable, high spectral resolution, multi-band spectral filters that perform spectral processing directly in the optical hardware. Such a device is created by placing a DMD at the spectral plane of an imaging spectrometer, and using it as a spectral selector that passes some wavelengths down the optical train to the final image and rejects others. While simple in concept, realizing a truly practical DMD-based spectral filter has proved challenging. Versions described to date have been limited by the intertwining of image position and spectral propagation direction common to most imaging spectrometers, reducing these instruments to line-by-line scanning imagers rather than true spectral cameras that collect entire two-dimensional images at once. Here we report several optical innovations that overcome this limitation and allow us to construct full-frame programmable filters that spectrally manipulate every pixel, simultaneously and without spectral shifts, across a full 2D image. So far, our prototype, which can be programmed either as a matched-filter imager for specific target materials or as a fully hyperspectral multiplexing Hadamard transform imager, has demonstrated over 100 programmable spectral bands while maintaining good spatial image quality. We discuss how diffraction-mediated trades between spatial and spectral resolution determine achievable performance. Finally, we describe methods for dealing with the DLP’s 2D diffractive effects, and suggest a simple modification to the DLP that would eliminate their impact for this application.
KEYWORDS: Optical filters, Image filtering, Digital micromirror devices, Sensors, Mirrors, Hyperspectral imaging, Data acquisition, Multiplexing, Signal detection, Chemical detection
Hyperspectral imaging sensors have proven to be powerful tools for highly selective and sensitive chemical detection applications, but have some significant operational drawbacks including a detection time-lag due to the large computational overhead of the matched filter analysis of the resulting data cubes. For applications where only a single chemical is of interest or real-time detection is desired, an intelligently designed multispectral sensor can trade high resolution and continuous spectral coverage for an in-line optical matched filter, enabling snapshot chemical detection with nearly no image processing requirements. Such a system can operate with little loss of performance, greatly reduced data volume, and at a fraction of the cost. We have recently developed a high-speed, high-resolution, programmable spectral filter based on a DLP® digital micro-mirror device (DMD) that mimics a conventional band-pass filter by operating on the spectrum without disturbing the underlying image. Our DMD-based filter can independently choose or reject dozens or hundreds of spectral bands and present them simultaneously to an imaging sensor, forming a complete 2D image. With this new technology, even very complicated matched filters can be implemented directly into the optical train of the sensor, producing an image highlighting the target chemical within a spectrally cluttered scene in real-time without further processing. Examples of matched-filter images recorded with our visible-spectrum prototype will be displayed, and extensions to other spectral regions will be discussed. Finally, we will discuss strategies for implementing more sophisticated clutter-suppressing matched filters on the DMD-based system, including schemes that approximate the subtlety of post-processing algorithms by utilizing the DMD’s duty-cycle-based gray-scale capability.
Hyperspectral imaging (HSI), in which each pixel contains a high-resolution spectrum, is a powerful technique that can
remotely detect, identify, and quantify a multitude of materials and chemicals. The advent of addressable micro-mirror
arrays (MMAs) makes possible a new class of programmable hyperspectral imagers that can perform key spectral
processing functions directly in the optical hardware, thus alleviating some of HSI's high computational overhead, as
well as offering improved signal-to-noise in certain important regimes (e.g. when using uncooled infrared detectors). We
have built and demonstrated a prototype UV-Visible micro-mirror hyperspectral imager that is capable not only of
matched-filter imaging, but also of full hyperspectral imagery via the Hadamard transform technique. With this
instrument, one can upload a chemical-specific spectral matched filter directly to the MMA, producing an image
showing the location of that chemical without further processing. Target chemicals are changeable nearly
instantaneously simply by uploading new matched-filter patterns to the MMA. Alternatively, the MMA can implement
Hadamard mask functions, yielding a full-spectrum hyperspectral image upon inverting the transform. In either case, the
instrument can produce the 2D spatial image either by an internal scan, using the MMA itself, or with a traditional
external push-broom scan. The various modes of operation are selectable simply by varying the software driving the
MMA. Here the design and performance of the prototype is discussed, along with experimental results confirming the
signal-to-noise improvement produced by the Hadamard technique in the noisy-detector regime.
KEYWORDS: Clouds, LIDAR, Signal detection, Diffusion, Mass attenuation coefficient, Signal attenuation, Geometrical optics, Multiple scattering, Optical filters, Data analysis
The Wide-Angle Imaging Lidar (WAIL), a new instrument that measures cloud optical and geometrical properties by means of off-beam lidar returns, was deployed as part of a multi-instrument campaign to probe a cloud field at ARM (Atmospheric Radiation Measurement) Southern Great Plain (SGP) site on March 25, 2002. WAIL is designed to determine physical and geometrical characteristics using the off-beam component of the lidar return that can be adequately modeled within the diffusion approximation. Using WAIL data, we estimate the extinction coefficient and geometrical thickness of a dense cloud layer; from there, we infer optical thickness. Results from the new methodology agree well with counterparts obtained from other instruments located permanently at the SGP ARM site and from the WAIL-like airborne instrument that flew over the site during our observation period.
At most optical wavelengths, laser light in a cloud lidar experiment is not absorbed but merely scattered out of the beam, eventually escaping the cloud via multiple scattering. There is much information available in this light scattered far from the input beam, information ignored by traditional 'on-beam' lidar. Monitoring these off-beam returns in a fully space- and time-resolved manner is the essence of our unique instrument, Wide Angle Imaging Lidar (WAIL). In effect, WAIL produces wide-field (60-degree full-angle) 'movies' of the scattering process and records the cloud's radiative Green functions. A direct data product of WAIL is the distribution of photon path lengths resulting from multiple scattering in the cloud. Following insights from diffusion theory, we can use the measured Green functions to infer the physical thickness and optical depth of the cloud layer, and, from there, estimate the volume-averaged liquid water content. WAIL is notable in that it is applicable to optically thick clouds, a regime in which traditional lidar is reduced to ceilometry. Here we present recent WAIL data on various clouds and discuss the extension of WAIL to full diurnal monitoring by means of an ultra-narrow magneto-optic atomic line filter for daytime measurements.
Full utilization of hyper-spectral imagery in the thermal infrared region requires a high-quality calibration, and the calibration quality requirement increases with the power and sophistication of the retrieval algorithms to be employed. Here we examine calibration issues associated with an airborne imaging Michelson Fourier Transform Infrared (FTIR)Spectrometer operating in the long-wave infrared (LWIR) region. In addition to the fundamental challenge of extracting a weak signal of interest from a complex background, problems which arise in such an instrument include pointing jitter, detector non-linearity, sampling position errors and etalon effects within the focal plane array. In each case, a frequent, high-quality calibration can ameliorate these problems. We discuss several hyper-spectral data analysis techniques, and how our calibration strategy, incorporating both ground and on-board calibration systems, improves the sensitivity of the retrievals.
Traditional lidar provides little information on dense clouds beyond the range to their base (ceilometry), due to their extreme opacity. At most optical wavelengths, however, laser photons are not absorbed but merely scattered out of the beam, and thus eventually escape the cloud via multiple scattering, producing distinctive extended space- and time-dependent patterns which are, in essence, the cloud's radiative Green functions. These Green functions, essentially 'movies' of the time evolution of the spatial distribution of escaping light, are the primary data products of a new type of lidar: Wide Angle Imaging Lidar (WAIL). WAIL data can be used to infer both optical depth and physical thickness of clouds, and hence the cloud liquid water content. The instrumental challenge is to accommodate a radiance field varying over many orders of magnitude and changing over widely varying time-scales. Our implementation uses a high-speed microchannel plate/crossed delay line imaging detector system with a 60-degree full-angle field of view, and a 532 nm doubled Nd:YAG laser. Nighttime field experiments testing various solutions to this problem show excellent agreement with diffusion theory, and retrievals yield plausible values for the optical and geometrical parameters of the observed cloud decks.
We report the successful preparation of a solid solution of C60 in a silicon dioxide (SiO2) glass matrix by means of sol-gel chemistry. Raman spectroscopy and x-ray diffraction were used to verify that our synthetic route produced glasses with a homogeneous dispersion of intact fullerenes. The vibrational spectrum of C60 is preserved in the C60/SiO2 gel glass. Raman and X-ray diffraction data confirm that the C60 is microscopically dispersed, and does not form detectable phase-separated, microcrystalline regions. We report preliminary observations of optical limiting in these gels, with intensity and concentration dependence consistent with that observed for C60 in solution.
Luminescence spectra, both emission and excitation, and the excitation dependence of the resonance Raman (RR) spectra have been measured for a 1-dimensional charge-density-wave solid, [Pt(L)2Cl2][Pt(L)2](ClO4)4; L equals 1,2- diaminoethane. The luminescence experiments support the existence of tail states in the band gap region, which indicate the presence of disorder. In contrast, the RR measurements conclusively demonstrate that the effects of static structural disorder on the vibrational spectroscopy can be neglected. This apparently paradoxical result can be explained by considering the zero-point motion of the lattice. Our experimental results are compared to recent theoretical models.
Resonance Raman techniques, together with lattice-dynamics and Peierls-Hubband modelling, are used to explore the electronic and vibrational dynamics of the quasi-one-dimensional metal-halogen chain solids [Pt(en)2][Pt(en)2X2](ClO4)4, (en equals C2H8N2 and X equals Cl, Br), abbreviated 'PtX.' The mixed-halide materials PtCl1-xBrx and PtCl1-xIx consist of long mixed chains with heterojunctions between segments of the two constituent materials. Thus, in addition to providing mesoscale modulation of the chain electronic states, they serve as prototypes for elucidating the properties to be expected for macroscopic heterojunctions of these highly non-linear materials. Once a detailed understanding of the various local vibrational modes occurring in these disordered solids is developed, the electronic structure of the chain segments and junctions can be probed by tuning the Raman excitation through their various electronic resonances.
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