The Acousto-Optic Tunable Filter (AOTF) devices are used in a variety of applications including hyperspectral and spectrapolarimetric based imaging. The AOTF devices provide several advantages including speed and random access to desired spectral bands and enable all-solid-state operation that are attractive for space applications. In this paper, the performance of Tellurium Dioxide (TeO2) and Mercurous Bromide (Hg2Br2) AOTF devices in space environment carried out under the NASA’s Materials International Space Station Experiments 11 (MISSE-11) mission is discussed. The purpose of the MISSE-11 mission was to study materials and devices subjected to space conditions for long term while attached to the International Space Station (ISS). The TeO2 AOTF was developed for Short Wavelength IR (SWIR) operation while the Hg2Br2 AOTF device was developed for Long Wavelength IR (LWIR) operation. These devices were attached to the ISS platform for more than a year. Pre-flight and post-flight performance characterization were performed on these devices. In the case of TeO2 AOTF, the preflight efficiency was around 87% with a center frequency at 68MHz while the post-flight efficiency was around 83%. However, the optical transmission of Hg2Br2 AOTF had deteriorated considerably. The details of performance testing and analysis of both AOTF devices are discussed.
A non-contact, laser-based technology is deployed to monitor the detailed mechanical operations of the various chambers and valves of the heart. The high sensitivity, optical speckle-tolerant laser technology enables cardiac signature detection from the skin vascular network at all locations throughout the patient’s body surface, even in the presence of light-blocking surface coverage such as clothing and shoes. In experiments, observed signal features are identified with specific cardiac activity and corroborated with other modalities including electrocardiography. To demonstrate applicability, cardiac monitoring signals were obtained from patients with widely varying ethnical backgrounds. Abnormal signal from one patient exhibiting sinus arrhythmic symptoms was collected and analyzed, indicating the technology’s potential for medical diagnostics.
Recently, a novel optical technology, LIBS in mid-IR (MWIR/LWIR) region was developed to capture the infrared molecular emission signatures from those vibrationally excited intact sample molecules excited by laser-induced plasma. Mid-IR LIBS is the first mid-IR emission spectroscopy that can complement LIBS and Raman as rapid, in situ, and standoff chemical characterization probes without the need of any sample preparation. With all the advantages of the conventional UVN LIBS, the UVN + LWIR LIBS spectrometer can rapidly and unambiguously reveal both the elemental composition and molecular makeups of the sample that is meters away without any sample preparation required and without the need to unscramble the spectral fingerprints of targets from the irregular and cluttered background. UVN + LWIR LIBS is able to provide in-situ, real-time/near-real-time chemical detection and identification regardless of the shapes and conditions of the sample surface while requiring no need for any sample preparations. One does not need to do anything to the target sample, just point the laser at the intended target and get the spectral signatures back within a millisecond.
Mercurous halides; mercurous iodide (Hg2I2), mercurous bromide (Hg2Br2) and mercurous chloride (Hg2Cl2) are high figure of merit materials for fabricating Acousto-optic (AO) devices that operate in the visible and infrared regions. Single crystals of mercurous halides were grown by physical vapor transport (PVT) method. Thermal expansion as well as the effect of annealing on the material are discussed. Mercurous halides show positive thermal expansion along "a" axis whereas no significant thermal expansion along "c" axis. The coefficients of thermal expansion of Hg2Cl2, Hg2Br2 and Hg2I2 are 6.72 × 10-5 °C-1, 6.44× 10-5 °C-1 and 6.08 × 10-5 °C-1, respectively. The optical band gap of Hg2Cl2 was calculated using the transmission spectra as 2.9 eV.
Tellurium dioxide is the most widely used uniaxial crystal for acousto-optic devices. Acousto-optic tunable filters based on this material can cover spectral range from UV to MWIR in a non-collinear configuration. The diffracted narrow band output beams have orthogonal linear polarizations, propagating in different directions, allowing the filter to act as polarizing beam splitter/analyzer as well. To achieve full electronic tuning, two liquid crystal variable retarders are used to measure all six polarization states used in the calculation of Stokes vector. We will present the design of the instrument, test results, and performance considerations.
We report on the development of an acousto-optic tunable filter (AOTF) based novel, high speed spectropolarimeter system operating over the visible and near-IR spectral bands to extract Stokes and Mueller matrices. Developed primarily for planetary composition and analysis applications, the wavelength tunable polarimetric system is configured with tellurium dioxide based AOTF and liquid crystal based variable retarders (LCVR) with no movable mechanical parts. Fitted with a standard silicon camera for operation up to 900 nm and a Mercury Cadmium Telluride (MCT) camera for operation up to 2500 nm, the spectropolarimetric system is currently configured for passive operation. The operation of this spectropolarimetric system is fully automated with an interactive and user friendly graphical user interface, and accordingly provides a snapshot polarimetric measurement capability in minutes.
An acousto-optic devices were designed and fabricated using high quality single crystals of mercurous halide (Hg2X2) that were grown by physical vapor transport method (PVT). The orientation and the crystalline quality of the grown crystals were determined using high resolution x-ray diffraction (HRXRD) technique. The full width at half maximum (FWHM) of the grown mercurous bromide (Hg2Br2) crystals was measured to be 0.13 degrees for (004) reflection, which is the best that has been achieved so far for PVT grown mercurous halide single crystals. The extended defects of the crystals were also analyzed using high resolution x-ray diffraction topography. Preliminary studies were carried out to evaluate the performance of the crystals on acousto-optic modulator (AOM) and acousto-optic tunable filter (AOTF) applications. The results indicate the grown mercurous halide crystals are excellent materials for acousto-optic modulator as well as acousto-optic tunable filter device fabrications. The diffraction efficiencies of the fabricated AOM device with 1152 and 1523nm wavelength lasers polarizing parallel to the acoustic wave were found to be 35% and 28%, respectively. The diffraction efficiencies of the fabricated AOTF device with 10600 nm wavelength laser found to be 26%.
Time multiplexed spectral images of burning aluminum particles from two experiments using a hyperspectral imaging system (HIS) coupled to a high speed video (HSV) camera were investigated. The first experiment looks at ignited aluminum particles generated by a welding torch that were continuously funneled into the imaging plane of the HISHSV system. The HIS was set to hop between two wavelengths at a rate of 300 frames per second (fps): 485.7 nm, the peak emission of aluminum monoxide, and 502.3nm, the bottom of the same emission peak. The second experiment images ignited AlO from the burn of an aluminized ammonium perchlorate solid propellant hoping between the wavelength of 486.3nm and 480.0nm at 2100 fps.
Recently, a mercury-cadmium-telluride (MCT) linear array detection system that is capable of rapidly capturing (~1-5 second) a broad spectrum of atomic and molecular laser-induced breakdown spectroscopy (LIBS) emissions in the longwave infrarμed region (LWIR, ~5.6 to 10 μm) has been developed. Similar to the conventional Ultraviolet (UV)-Visible (Vis) LIBS, a broad band emission spectrum of condensed phase samples covering the entire 5.6 to 10 μm region can be acquired from just a single laser-induced micro-plasma or averaging a few single laser-induced micro-plasmas. This setup has enabled probing samples “as is” without the need for extensive sample preparation and also offers the possibility of a simultaneous UV-Vis and LWIR LIBS measurement. A Martian regolith simulant (JSC Mars-1A) was studied with this novel Vis + LWIR LIBS array system. A broad SiO2 vibrational emission feature around 9.5 μm and multiple strong emission features between 6.5 to 8 μm can be clearly identified. The 6.5 to 8 μm features are possibly from biological impurities of the simulant. JSC Mars-1A samples with organic methyl salicylate (MeS, wintergreen oil) and Dimethyl methyl-phosphonate (DMMP) residues were also probed using the LWIR LIBS array system. Both molecular spectral signature around 6.5 μm and 9.5 μm of Martian regolith simulant and MeS and DMMP molecular signature emissions, such as Aromatic CC stretching band at 7.5 μm, C-CH3O asymmetric deformation at 7.6 μm, and P=O stretching band at 7.9 μm, are clearly observed from the LIBS emission spectra in the LWIR region.
Laser-induced breakdown spectroscopy is a powerful diagnostic tool for detection of trace elements by monitoring the atomic and ionic emission from laser-induced plasmas. Besides elemental emissions from conventional UV-Vis LIBS, molecular LIBS emission signatures of the target compounds were observed in the long-wave infrared (LWIR) region in recent studies. Most current LIBS studies employ the fundamental Nd:YAG laser output at 1.064 μm, which has extremely low eye-damage threshold. In this work, comparative LWIR-LIBS emissions studies using traditional 1.064 μm pumping and eye-safe laser wavelength at 1.574 μm were performed on several energetic materials for applications in chemical, biological, and explosive (CBE) sensing. A Q-switched Nd: YAG laser operating at 1.064 μm and the 1.574 μm output of a pulsed Nd:YAG pumped Optical Parametric Oscillator were employed as the excitation sources. The investigated energetic materials were studied for the appearance of LWIR-LIBS emissions (4-12 μm) that are directly indicative of oxygenated breakdown products as well as partially dissociated and recombination molecular species. The observed molecular IR LIBS emission bands showed strong correlation with FTIR absorption spectra of the studied materials for 1.064 μm and 1.574 μm pump wavelengths.
Conventional laser induced breakdown spectroscopy (LIBS) mostly uses silicon-based detectors and measures the atomic emission in the UV-Vis-NIR (UVN) region of the spectrum. It can be used to detect the elements in the sample under test, such as the presence of lead in the solder for electronics during RoHS compliance verification. This wavelength region, however, does not provide sufficient information on the bonding between the elements, because the molecular vibration modes emit at longer wavelength region. Measuring long-wave infrared spectrum (LWIR) in a LIBS setup can instead reveal molecular composition of the sample, which is the information sought in applications including chemical and explosive detection and identification. This paper will present the work and results from the collaboration of several institutions to develop the methods of LWIR LIBS for chemical/explosive/pharmaceutical material detection/identification, such as DMMP and RDX, as fast as using a single excitation laser pulse. In our latest LIBS setup, both UVN and LWIR spectra can be collected at the same time, allowing more accurate detection and identification of materials.
The three most important desirable features in the search for room temperature semiconductor detector (RTSD) candidate as an alternative material to current commercially off-the-shelf (COTS) material for gamma and/or thermal neutron detection are: low cost, high performance and long term stability. This is especially important for pager form application in homeland security. Despite years of research, no RTSD candidate so far can satisfy the above 3 features simultaneously. In this work, we show that mercurous halide materials Hg2X2 (X= I, Cl, Br) is a new class of innovative compound semiconductors that is capable of delivering breakthrough advances to COTS radiation detector materials. These materials are much easier to grow thicker and larger volume crystals. They can detect gamma and potentially neutron radiation making it possible to detect two types of radiation with just one crystal material. The materials have wider bandgaps (compared to COTS) meaning higher resistivity and lower leakage current, making this new technology more compatible with available microelectronics. The materials also have higher atomic number and density leading to higher stopping power and better detector sensitivity/efficiency. They are not hazardous so there are no environmental and health concerns during manufacturing and are more stable making them more practical for commercial deployment. Focus will be on Hg2I2. Material characterization and detector performance will be presented and discussed. Initial results show that an energy resolution better than 2% @ 59.6 keV gamma from Am-241 and near 1% @ 662 keV from Cs-137 source can be achieved at room temperature.
Laser-induced breakdown spectroscopy (LIBS) is a powerful analytical technique to detect the elemental composition of solids, liquids, and gases in real time. For example, recent advances in UV-VIS LIBS have shown great promise for applications in chemical, biological, and explosive sensing. The extension of conventional UVVIS LIBS to the near-IR (NIR), mid-IR (MIR) and long wave infrared (LWIR) regions (~1-12 μm) offers the potential to provide additional information due to IR atomic and molecular signatures. In this work, a Q-switched Nd: YAG laser operating at 1064 nm was employed as the excitation source and focused onto several chlorate and nitrate compounds including KClO3, NaClO3, KNO3, and NaNO3 to produce intense plasma at the target surface. IR LIBS studies on background air, KCl , and NaCl were also included for comparison. All potassium and sodium containing samples revealed narrow-band, atomic-like emissions assigned to transitions of neutral alkali-metal atoms in accordance with the NIST atomic spectra database. In addition, first evidence of broad-band molecular LIBS signatures from chlorate and nitrate compounds were observed at ~10 μm and ~7.3 μm, respectively. The observed molecular emissions showed strong correlation with FTIR absorption spectra of the investigated materials.
Low cost, power and bandwidth UGS can be used to fill the growing need for surveillance in remote environments. In
particular, linear and 2D thermal sensor systems can run for up to months at a time and their deployment can be scaled to
suit the size of the mission. Thermal silhouette profilers like Brimrose's SPOT system reduce power and bandwidth
requirements by performing elementary classification and only transmitting binary data using optimized compression
methods. These systems satisfy the demands for an increasing number of surveillance operations where reduced
bandwidth and power consumption are mission critical.
The long chain of a polymer molecule often has intrinsic linear birefringence and/or dichroism, which lead to macroscopic properties when the molecules have a preferred orientation under nonequilibrium conditions, such as flow of a polymer melt. Measurements of properties, including linear birefringence, in a polymer melt can in turn lead to valuable insights into its structure, morphology, and rheology. We demonstrate that a spectropolarimeter based on an acousto-optic tunable filter can measure linear birefringence in real time during the manufacture of polymers, at hundreds of wavelengths, within 5 min, and thus provides more information than use of a spectrometer or laser-based polarimetry alone. In this work, optical properties of different molten ethylene vinyl acetate samples were measured with a spectropolarimeter based on an acousto-optic tunable filter and a photoelastic modulator in the near infrared. Regression models of the results indicate good correlation between the melt indices of the samples and the measured spectra.
AOTF spectrometers have found useful role in space exploration due to their ruggedness, tunability and absence of moving parts. We discuss their
space qualification issues.
We demonstrate experimentally an optical scanning technique for measuring the step heights of surface features without using conventional optical interferometers. This technique involves the deployment of the so-called photo-EMF sensors that are capable of sensing the presence of step-like features on an otherwise optically flat surface. Scanning of the target surface is achieved by rotating the object being investigated while keeping the laser beam stationary. Theoretical modeling and experimental data will be presented indicating the resolution of step-like features with merely 15 nm in height.
Controlling/monitoring the thickness of applied paint in real time is important to many situations including painting ship and submarine hulls in dry docks for maintaining health of ships and submarines against the harshness of the sea, in automobile and aerospace industries, and in a variety of other industries as a control sensor that plays significant role in product quality, process control, and cost control. Insufficient thickness results to inadequate protection while overspray leads to waste and pollution of the environment. A rugged instrumentation for the real time non-contact accurate measurement of wet and dry paint film thickness measurement will be immensely valuable. As paint is applied with several layers of the same or different type, thickness of each newly sprayed wet layer is of most interest, but measurement on dry paint is also useful. In this study, we use acousto-optic tunable filter-based near infrared spectrometer to obtain the absorption spectrum of layers of paint sprayed on sand blasted steel surface and thus measure the thickness of coating under both wet and dry situations. NIR spectra are obtained from 1100 to 2300 nm on four sample of different thickness of paint up to 127 micron. Partial least squares model built with the spectra shows good correlation with standard error of prediction within ~ 0.7 micron. Results indicate that the spectra also respond to the amount of organic solvent in the wet paint and can be used to monitor the degree of dryness of the paint in real time.
This paper describes a simple, inexpensive multispectral imaging system for image cytometry applications. The system is based on an acousto-optical tunable filter (AOTF), a monochrome CCD camera, and a research-grade fluorescence microscope. The selected TeO2 AOTF has a 10x10 mm2 entrance aperture and operates within the spectral range of 447-750 nm. The bandpass of the filter varies between 1.4 nm at 450 nm and 5.1 nm at 690 nm. The control software works within the environment of a popular image-acquisition and -processing package Image Pro-Plus, making this system easy to integrate with many existing fluorescence microscopes and cameras. Since image-cytometry applications do not require very high spatial resolution, AOTF-based systems may become an interesting alternative to more complex and expensive LCTF or pushbroom methods.
Candidate weapon systems have conservative environmental and service life limits to ensure both performance reliability and ordnance safety. One important element that must be monitored is chemical
indicators of propellant degradation. Chemical degradation of energetic compounds in propellants can result in reduced performance and potential instability and auto-ignition in extreme circumstances. The current method for testing for chemical indicators of propellant degradation consists of removing a missile from its sub, disassembling it, and performing HPLC testing. An improvement to the current system is to use near-infrared (NIR) spectral analysis to measure chemical indicators of propellant degradation. An AOTF multi-channel spectrometer with reflectance probes can simultaneously scan different areas of a propellant. A study has shown clear spectral differences in samples of M1MP propellant with two different
concentrations of the chemical diphenyl amine (DPA). DPA is very similar to many important chemical indicators of propellant degradation. The spectral differences provide the basis for correlating spectral data to DPA concentration using a multivariate regression technique.
KEYWORDS: Solar cells, Multijunction solar cells, Resistance, Indium gallium arsenide, Space operations, Silicon, Silicon carbide, Doping, Solar energy, Metalorganic chemical vapor deposition
In this work, we study a high efficient four-junction solar cells using AlAsSb(1.91eV), AlInAs(1.46eV), ALGaInAs(1.0eV) and InGaAs(0.75eV) materials. The >35% efficiency is calculated under AM0, and the thickness of each layer is adjusted to equalize the short circuit current. We also experimentally fabricated a 1.0eV cell and studied its characteristics.
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