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This paper investigates the accuracy of deriving the composite optical constants of binary mixtures from only the complex indices of refraction of the neat materials. These optical constants enable the reflectance spectra of the binary mixtures to be modeled for multiple scenarios (

The Pacific Northwest National Laboratory (PNNL) gas-phase database is a compilation of quantitative experimental (5, 25, and 50 °C) infrared spectra of ca. 500 molecules, designed for in situ, standoff or remote sensing of gases and vapors at or near atmospheric pressure. The data are characterized by calibration on both the wavenumber and intensity axes. Recent papers have called into question the PNNL intensity values for isobutane, [2-methylpropane, HC(CH3)3], suggesting discrepancies of 30-40%. In this study, we remeasure and re-examine the intensity values of isobutane using both similar and alternate methods to those used to generate the original PNNL database spectra. Indirect confirmation from literature data of homologous molecules and direct confirmation from new results confirm that for many band integrals across the isobutane spectrum, the original PNNL data are indeed accurate to within the reported 3% experimental uncertainty.

In this study we capture and identify pyrolysis gases from prescribed burns conducted in pine forests with a shrub understory using a manual extraction device. The device selectively sampled emissions ahead of the flame front, minimizing collection of oxidized gases, with the captured gases analyzed in the laboratory using infrared absorption spectroscopy. Results show that emission ratios (ER) relative to CO for ethene, and acetylene were significantly greater than previous fire studies, suggesting that the sample device was able to collect gases prior to ignition. Further evidence that ignition had not begun was corroborated by novel infrared detections of several species, in particular naphthalene. With regards to oxygenated species, several aldehydes (acrolein, furaldehyde, acetaldehyde, formaldehyde) and the carboxylic acids (formic, acetic) were all observed; results show that ERs for acetaldehyde were noticeably greater while ERs for formaldehyde and acetic acid were lower compared to other studies. The acetylene-to-furan ratio also suggests that high temperature pyrolysis was the dominant process generating the collected gases. This hypothesis is further supported by the presence of HCN and the absence of NH3.

Volatile organic compounds (VOCs) are emitted from many sources, including wildland fire. VOCs have received heightened emphasis due to such gases' influential role in the atmosphere, as well as possible health effects. We have used extractive infrared (IR) spectroscopy on recent prescribed burns in longleaf pine stands and herein report the first detection of five compounds using this technique. The newly reported IR detections include naphthalene, methyl nitrite, allene, acrolein and acetaldehyde. We discuss the approaches used for detection, particularly the software methods needed to fit the analyte and multiple (interfering) spectral components within the selected spectral micro-window(s). We also discuss the method's detection limits and related parameters such as spectral resolution.

Pyrolysis is the first step in a series of chemical and physical processes that produce flammable organic gases from wildland fuels that can result in a wildland fire. We report results using a new time-resolved Fourier transform infrared (FTIR) method that correlates the measured FTIR spectrum with an infrared thermal image sequence, enabling the identification and quantification of gases within different phases of the fire process. The flame from burning fuel beds composed of pine needles (Pinus palustris) and mixtures of sparkleberry, fetterbush, and inkberry plants was the natural heat source for pyrolysis. Extractive gas samples were analyzed and identified in both static and dynamic modes synchronized to thermal infrared imaging: a total of 29 gases were identified including small alkanes, alkenes, aldehydes, nitrogen compounds, and aromatics, most previously measured by FTIR in wildland fires. This study presents one of the first identifications of phenol associated with both pre-combustion and combustion phases using ca. 1 Hz temporal resolution. Preliminary results indicate ∼2.5× greater phenol emissions from sparkleberry and inkberry compared to fetterbush, with differing temporal profiles.

Pyrolysis is the first step in a series of chemical and physical processes that produce flammable organic gases from wildland fuels that can result in a wildland fire. We report results using a new time-resolved Fourier transform infrared method that correlates the measured FTIR spectrum to an infrared thermal image sequence enabling identification and quantification of gases within different phases of the fire process. The flame from burning fuel beds composed of pine needles (Pinus palustris) and mixtures of sparkleberry, fetterbush and inkberry plants was the natural heat source for pyrolysis. Extractive gas samples were analyzed and identified in both static and dynamic modes synchronized to thermal infrared imaging: A total of 29 gases were identified including small alkanes, alkenes, aldehydes, nitrogen compounds and aromatics, most previously measured by FTIR in wildland fires. This study presents one of the first identifications of phenol associated with both pre-combustion and combustion phases, using ca. 1 Hz resolution. Preliminary results indicate ~ 2.5× greater phenol emission from sparkleberry and inkberry compared to fetterbush, with differing temporal profiles.

Uranium dioxide (UO2) is a material with historical and emerging applications in numerous areas such as photonics, nuclear energy, and aerospace electronics. While often grown synthetically as single-crystal UO2, the mineralogical form of UO2 called uraninite is of interest as a precursor to various chemical processes involving uranium-bearing chemicals. Here, we investigate the optical and chemical properties of a series of three UO2 specimens: synthetic single-crystal UO2, uraninite ore of relatively high purity, and massive uraninite mineral containing numerous impurities. An optical technique called single-angle reflectance spectroscopy was used to derive the optical constants n and k of these uranium specimens by measuring the specular reflectance spectra of a polished surface across the mid- and far-infrared spectral domains (ca. 7000-50 cm-1). X-ray diffractometry, scanning electron microscopy, and energy-dispersive X-ray spectroscopy were further used to analyze the surface composition of the mineralogical forms of UO2. Most notably, the massive uraninite mineral was observed to contain significant deposits of calcite and quartz in addition to UO2 (as well as other metal oxides and radioactive decay products). Knowledge of the infrared optical constants for this series of uranium chemicals facilitates nondestructive, noncontact detection of UO2 under a variety of conditions.

A method for deriving the optical constants ( n / k ) of organic powdered materials using pressed pellets in the mid-infrared spectral range is introduced that combines variable angle spectroscopic ellipsometry and transmission spectroscopy. The approach is applied to anhydrous lactose, in which three different forms of pellets were pressed and measured: a pure lactose pellet and a mixed lactose/potassium bromide (KBr) pellet with a large analyte percentage were used for ellipsometric measurements, and a KBr transmission pellet with only a small analyte percentage was used for transmission measurements. The transmittance data provide an initial set of oscillators and improve the spectral fitting of weak absorption features ( k < 0.01 ). Ellipsometric data for the pure and mixed pellets are then fit simultaneously to derive the final n / k values for lactose from 6000 − 400 c m − 1 . An alternative method just using the ellipsometric data from the mixed pellet and the transmittance data is also presented and shows good agreement with the multi-sample analysis, providing a simpler method for powders that do not press easily into pure pellets. Finally, the derived optical constants were used to model the reflectance data, demonstrating a good match with the measured reflectance spectra if non-idealities are included.

Since solids are only sometimes seen en masse in a pure bulk form, and for liquids other than water almost never, a capability to model reflectance spectra from analytes deposited on various substrates would be highly advantageous. If available, the real, n ( ν ) , and imaginary, k ( ν ) , components of the complex refractive index, n∼ = n + ik, can be used to simulate infrared spectra, accounting for reflection, refraction, and absorption phenomena as a function of wavelength. We focus on using the Pacific Northwest National Laboratory (PNNL) derived n / k vectors for solid and liquid analytes deposited as thin layers on different types of substrates including conductors, such as aluminum, and inorganic dielectrics, such as glass. The model is an adaptation of the Monte Carlo ray trace modeling program, TracePro, extended through the use of its macrolanguage. The model is tested using thin films of organic liquids including silicone oil and no. 2 diesel fuel, as well as organic solids such as caffeine and acetaminophen on aluminum and glass. The predicted spectra for the solid films were compared to experimental hemispherical reflectance data measured using a Fourier transform spectrometer with an integrating sphere. The thickness of the calculated layer is a parameter for predicting the (transflectance) spectra and is obtained using the areal density measured from gravimetric methods to generate the thin-layer samples. Comparison of the calculated spectra with experimental hemispherical reflectance data shows excellent agreement, indicating promise for the use of measured n / k data to synthesize reference spectral data. In particular, accounting for the inhomogeneity of the deposits greatly improved the match with experimental data. Finally, the theoretical modeling shows that for thicker layers (ca. 20 to 100 μm) of typical organics possessing moderately strong k values, the longwave infrared features are often saturated and better spectral contrast is obtained from the overtone/combination bands in the shortwave infrared.

Broadband high-speed absorption spectroscopy using swept-wavelength external cavity quantum cascade lasers (ECQCLs) is applied to measure multiple pyrolysis and combustion gases in biomass burning experiments. Two broadly-tunable swept-ECQCL systems were used, with the first tuned over a range of 2089-2262 cm−1 (4.42–4.79 µm) to measure spectra of CO2, H2O, and CO. The second was tuned over a range of 920-1150 cm−1 (8.70–10.9 µm) to measure spectra of ammonia (NH3), ethene (C2H4), and methanol (MeOH). Absorption spectra were measured continuously at a 100 Hz rate throughout the burn process, including inhomogeneous flame regions, and analyzed to determine time-resolved gas concentrations and temperature. The results provide in-situ, dynamic information regarding gas-phase species as they are generated, close to the biomass fuel source.

Volatile organic compounds (VOCs) are emitted from many sources, including wildland fire; VOCs have received heightened emphasis due to such gases' influential role in the atmosphere, as well as possible health effects. We have used extractive infrared (IR) spectroscopy on recent prescribed burns in longleaf pine stands and herein report seminal detection of five compounds using this technique. The newly reported IR detections include naphthalene, methyl nitrite, allene, acrolein and acetaldehyde. We discuss the approaches used for detection, particularly the software methods needed to fit the analyte and multiple (interfering) spectral components within the selected spectral micro-window(s). We also discuss the method's detection limits and individual species' context in terms of atmospheric chemistry.

Pyrolysis of intact wildland fuels in the southern United States is being measured at bench, wind tunnel, and field scales as part of a larger research project to measure and model pyrolysis of wildland fuels to improve models used to predict prescribed fire behavior. Traditional pyrolysis experiments typically use dried, ground samples in either an inert or oxidizing environment subject to uniform heating and heat transfer. Fletcher and others are presenting results of pyrolysis experiments using a flat-flame burner to heat intact foliage from 14 species of plants native to the southern U.S. at this conference. The fuel beds in prescribed burns in southern pine forests are composed of a mixture of dead pine needles and a large variety of herbaceous and woody plants. We have burned 73 fuel beds composed of Pinus palustris needles and mixtures of Lyonia lucida, Ilex glabra, and Vaccinium darrowii plants in a wind tunnel facility with and without a moderate wind speed of ~ 1 m s-1. The flame from the spreading fire is the heat source for the pyrolysis experiment. Canister samples of pyrolysis and flaming combustion gases within the fuel bed have been collected for offline analysis. Open path spectrometers nonintrusively measured composition of pyrolysis gases in real-time. Thermocouples, a Schmidt-Boelter heat flux sensor, a nadir thermal IR camera, and background-oriented Schlieren photography estimated heat transfer and air flow around the plants. This presentation will present results of this ongoing work.

Uranium dioxide (UO

We have developed methods to measure the directional-hemispherical (ρ) and diffuse (ρd) reflectances of powders, liquids, and disks of powders and solid materials using a commercially available, matte gold-coated integrating sphere and Fourier transform infrared spectrometer. To determine how well the sphere and protocols produce quantitative reflectance data, measurements were made of three diffuse and two specular standards prepared by the National Institute of Standards and Technology (NIST), LabSphere Infragold and Spectralon standards, hand-loaded sulfur and talc powder samples, and water. Relative to the NIST measurements of the NIST standards, our directional hemispherical reflectance values are within ±4% for four of the standards and within ±7% for a low reflectance diffuse standard. For the three diffuse reflectance NIST standards, our diffuse reflectance values are within ±5% of the NIST values. For the two specular NIST standards, our diffuse reflectance values are an order of magnitude larger than those of NIST, pointing to a systematic error in the manner in which diffuse reflectance measurements are made for specular samples using our methods and sphere. Sources of uncertainty are discussed in the paper.

Hyperspectral imaging (HSI) continues to grow as a method for remote detection of vegetation, materials, minerals, and pure chemicals. We have used a longwave infrared (7.7 to 11.8 μm) imaging spectrometer in a static outdoor experiment to collect HSI data from 24 minerals and background materials to determine the efficacy with which HSI can remotely detect and distinguish both pure minerals and mineral mixtures at a 45-deg tilt angle relative to ground using two different backgrounds. Measurements were obtained separately for the minerals and materials mounted directly on both a bare plywood board and a board coated with aluminum foil: 19 powders (3 mixtures and 16 pure mineral powders) held in polyethylene bottle lids as well as five samples in rock form were taped directly to the boards. The primary goal of the experiment was to demonstrate that a longwave infrared library of solids and minerals collected as directional-hemispherical reflectance spectra in the laboratory could be used directly for HSI field identification along with simple algorithms for a rapid survey of the target materials. Prior to the experiment, all 24 mineral/inorganic samples were measured in the laboratory using a Fourier transform infrared spectrometer equipped with a gold-coated integrating sphere; the spectra were assimilated as part of a larger reference library of 21 pure minerals, 3 mixtures, and the polyethylene lid. Principal component analysis with mean-centering was used in an exploratory analysis of the HSI images and showed that, for the aluminum-coated board, the first principal component captured the difference between the signal that resembled a blackbody and the highly reflective aluminum background. In contrast, the second, third, and fourth principal components were able to discriminate the materials including phosphates, silicates, carbonates, and the mixtures. Results from generalized least squares target detection clearly showed that laboratory reference spectra of minerals could be utilized as targets with high fidelity for field detection.

Single-angle infrared (IR) reflectance spectroscopy is a proven and effective method for determining the complex optical constants n and k of condensed matter. The modern method uses a Fourier transform IR spectrometer to record the quantitative reflectance R(ν) spectra followed by application of the Kramers–Kronig transform (KKT) to obtain the complex optical constants. In order to carry out the KKT, it is essential to measure the reflectance spectra to as high and low a frequency (wavenumber) as possible. Traditionally, the reflectance spectra of solid specimens consist of large (typically>10 mm diameter) polished single-crystal faces free of defects or voids. The requirement of a large polished face, however, is not a realistic expectation for many synthetic, geologic, or rare specimens where the size is usually small and the morphology can vary. In this paper we discuss several improvements and considerations to both the hardware and far-IR measurement protocols that lead to more accurate R(ν) values and thus to more accurate n/k values, especially for small (millimeter-sized) specimens where the R(ν) spectrum is concatenated from multiple independent R(ν) spectra from overlapping hardware/spectral domains. Specifically, the improved hardware and analyses introduced here include the following: (1) providing a set of far-IR calibration standards; (2) custom-designing and manufacturing low reflectivity, stray-light reducing sample masks for small specimens; (3) minimizing stray light interaction between the sample mask, the interferometer Jacquinot stop, and the detector; (4) optimizing the methods to “splice” together the spectra from independent domains; (5) discussing what methods one can use to obtain or calculate the important R(0 cm−1) value; (6) using a quartic relationship to extrapolate from the measured R data to R(0); and (7) accounting for the limiting effects of diffraction for the spot size at the sample mask and detector for millimeter-sized specimens, especially at the very long wavelengths. These seven considerations are all highly interconnected and are discussed in turn, as well as their strong interdependencies. This paper presents a holistic approach for determining reliable n/k values of millimeter-sized samples using single-angle reflectance in the mid- and far-IR.

Activation of the renin-angiotensin system (RAS) is thought to promote myocardial fibrosis. However, it is unclear whether this physiological fibrotic response results from chronic hemodynamic stress or from direct cellular signaling. Male C57B/6 mice were randomly assigned to receive angiotensin II (AngII) (2.0 μg kg(-1) min(-1)), AngII+hydralazine (6.9 μg kg(-1) min(-1)) or saline (control) via osmotic pumps for 7 days. Blood pressure was measured via noninvasive plethysmography. Hearts were harvested and processed for analysis. Cellular infiltration and collagen deposition were analyzed using histological staining. Molecular mediators were assessed using quantitative RT-PCR. As previously described, animals that received AngII developed hypertension and multifocal cellular infiltration by SMA(+)/CD133(+) fibroblast progenitors followed by collagen deposition. The coadministration of hydralazine with AngII completely inhibited the hypertensive effects of AngII (P0.01) and resulted in minimal cellular infiltration and minimal collagen deposition. These findings were in the context of persistent RAS activation, which was evidenced by elevation in serum aldosterone levels in animals that received AngII or AngII+hydralazine compared with animals that received saline. At the molecular level, infusion of AngII resulted in the significant upregulation of profibrotic factors (connective tissue growth factor-7.8±0.7 fold), proinflammatory mediators (TNFα-4.6±0.8 fold; IL-1β-6.4±2.6 fold) and chemokines (CCL2-3.8±1.0 fold; CXCL12-3.2±0.4 fold), which were inhibited when hydralazine was also infused. We provide evidence that myocardial infiltration by fibroblast progenitor cells secondary to AngII and the resultant fibrosis can be prevented by the addition of hydralazine. Furthermore, the beneficial effects of hydralazine were observed while maintaining RAS activation, suggesting that the mechanism of fibrosis is blood pressure dependent.

Background: Activated Protein C (aPC) is a naturally produced circulating anticoagulant with anti-inflammatory and cytoprotective properties proposed to have beneficial effects on the myocardium. Myocardial fibrosis is a pathophysiologic process evident in many cardiovascular diseases and is believed to directly contribute to eventual organ failure. Using a well-described model of myocardial fibrosis after Angiotensin II (AngII) infusion our aim was to investigate the novel therapeutic function of aPC in the development of fibrosis. Methods: C57Bl/6 mice were continuously infused with AngII (2.0 μ g/kg/min), AngII and aPC (0.4 μ g/kg/min), or saline for a 3d period. Hearts were harvested and processed for analysis. Cellular infiltration and collagen deposition were analyzed using histologic staining. Cellular apoptosis was assessed using a TUNEL assay. Quantitative RT-PCR was used to assess transcript levels of molecular mediators. Results: Infusion of AngII for 3d resulted in multifocal areas of myocardial cellular infiltration associated with significant collagen deposition compared to saline control animals (p≤0.01). The addition of aPC with AngII infusion inhibited this fibrotic response. Co-administration with aPC also inhibited the upregulation of the pro-fibrotic cytokine CTGF expression seen in AngII infused animals (p≤0.01). Apoptosis was also significantly inhibited when aPC was co-administered based on a decrease of in TUNEL positive cells and an increase expression ration of BCL/BAX suggesting an anti-apoptotic effect (p≤0.01), Furthermore, aPC also inhibited the up-regulation of the cell adhesion molecule p-selectin seen in AngII infused animals (p≤0.01). Conclusion: The co-administration of aPC in a model of myocardial fibrosis was able to abrogate completely the fibrotic response. The mechanism of action of aPC appears to be a decreased cellular adhesion molecule expression in the myocardium leading to decreased cellular infiltration and enhanced cellular survival. This data suggests that aPC has the potential as a therapeutic agent in cardiovascular diseases.

Remote infrared (IR) sensing provides a valuable method for detection and identification of materials associated with nuclear proliferation. Current challenges for remote sensors include minimizing the size, mass, and power requirements for cheaper, smaller, and more deployable instruments without affecting the measurement performance. One area that is often overlooked is sensor calibration design that is optimized to minimize the cost, size, weight, and power of the payload. Yet, an on-board calibration system is essential to account for changes in the detector response once the instrument has been removed from the laboratory. The Calibration Systems project at Pacific Northwest National Laboratory (PNNL) is aimed towards developing and demonstrating compact quantum cascade (QC) laser-based calibration systems for infrared sensor systems in order to provide both a spectral and radiometric calibration while minimizing the impact on the instrument payload. In FY05, PNNL demonstrated a multi-level radiance scheme that provides six radiance levels for an enhanced linearity check compared to the currently accepted two-point scheme. PNNL began testing the repeatability of this scheme using a cryogenically cooled, single-mode quantum cascade laser (QCL). A cyclic variation in the power was observed that was attributed to the thermal cycling of the laser's dewar. In FY06, PNNLmore » continued testing this scheme and installed an auxiliary liquid nitrogen reservoir to limit the thermal cycling effects. Although better repeatability was achieved over a longer time period, power fluctuations were still observed due to the thermal cycling. Due to the limitations with the cryogenic system, PNNL began testing Fabry-Perot QCLs that operate continuous-wave (cw) or quasi-cw at room temperature (RT) in FY06. PNNL demonstrated a multi-level scheme that provides five radiance levels in 105 seconds with excellent repeatability. We have continued testing this repeatability in FY07. A burn-in effect appears in which the power increases over a certain time period. Repeatability better than 1%, however, is demonstrated for most of the radiance levels after this initial burn-in. In FY06, PNNL also began investigating a fiber-coupled RT QCL for a compact IR calibration source. PNNL demonstrated a uniform beam profile by measuring a time-averaged response and modulating the fiber optic with a motor to minimize the effects of speckle. In FY07, PNNL examined the power stability of fiber-coupled QCLs. Feedback appears to degrade the stability so that anti-reflective coatings for fibers may be essential. In FY07, PNNL continued to investigate the stability of room temperature QCLs as well as the measurement technique to provide a quantitative estimate for the measurement uncertainty. We designed and built a custom environmental enclosure to reduce the measurement uncertainty. After an initial burn-in, we have achieved uncertainties better than 0.1% for data collected over almost 100 hours of operation. We also built a bench-top system to demonstrate how the QC laser can be used to calibrate a microbolometer array and illustrated the importance of a multi-point calibration.« less

The purpose of Task 3.b under PL02-OP211I-PD07 (CBW simulant detection) was to demonstrate the applicability of the sensor work developed under this project for chemical and biological weapons detection. To this end, the specific goal was to demonstrate the feasibility of detection of chemical agents via that of simulants (Freons) with similar spectroscopic features. This has been achieved using Freon-125 as a simulant, a tunable external cavity quantum cascade laser (ECQCL), and a Herriott cell-based sensor developed at Pacific Northwest National Laboratory (PNNL) specifically for this task. The experimentally obtained spectrum of this simulant matches that found in the Northwest Infrared (NWIR) spectral library extremely well, demonstrating the ability of this technique to detect the exact shape of this feature, which in turn indicates the ability to recognize the simulant even in the presence of significant interference. It has also been demonstrated that the detected features of a typical interferent, namely water, are so different in shape and width to the simulant, that they are easily recognized and separated from such a measurement. Judging from the signal-to-noise ratio (SNR) of the experimental data obtained, the noise equivalent absorption sensitivity is estimated to be 0.5 x 10-7 to 1 x 10-6more » cm-1. For the particular feature of the simulant examined in this work, this corresponds to a relative concentration of 50 to 25 parts-per-billion by volume (ppbv). The corresponding relative concentrations of other chemical targets would differ depending on the particular transition strengths, and would thus have to be scaled accordingly.« less

We have recently developed vetted methods for obtaining quantitative infrared directional-hemispherical reflectance spectra using a commercial integrating sphere. In this paper, the effects of particle size on the spectral properties are analyzed for several samples such as ammonium sulfate, calcium carbonate, and sodium sulfate as well as one organic compound, lactose. We prepared multiple size fractions for each sample and confirmed the mean sizes using optical microscopy. Most species displayed a wide range of spectral behavior depending on the mean particle size. General trends of reflectance versus particle size are observed such as increased albedo for smaller particles: for most wavelengths, the reflectivity drops with increased size, sometimes displaying a factor of 4 or more drop in reflectivity along with a loss of spectral contrast. In the longwave infrared, several species with symmetric anions or cations exhibited reststrahlen features whose amplitude was nearly invariant with particle size, at least for intermediate and large size sample fractions: that is, ≳150 μm. Trends of other types of bands (Christiansen minima, transparency features) are also investigated as well as quantitative analysis of the observed relationship between reflectance versus particle diameter.

The Calibration Systems project at Pacific Northwest National Laboratory (PNNL) is aimed towards developing and demonstrating compact Quantum Cascade (QC) laser-based calibration systems for infrared imaging systems. These on-board systems will improve the calibration technology for passive sensors, which enable stand-off detection for the proliferation or use of weapons of mass destruction, by replacing on-board blackbodies with QC laser-based systems. This alternative technology can minimize the impact on instrument size and weight while improving the quality of instruments for a variety of missions. The potential of replacing flight blackbodies is made feasible by the high output, stability, and repeatability of the QC laser spectral radiance.

The principal goal of Pacific Northwest National Laboratory's (PNNL's) long wave infrared (LWIR) cavity enhanced sensor (CES) task is to explore ultra-sensitive spectroscopic chemical sensing techniques and apply them to detecting proliferation of weapons of mass destruction (WMD). Our primary application is detecting signatures of WMD production, but LWIR CES techniques are also capable of detecting chemical weapons. The LWIR CES task is concerned exclusively with developing novel point sensors; stand-off detection is addressed by other PNNL tasks and projects. PNNL's LWIR CES research is distinguished from that done by others by the use quantum cascade lasers (QCLs) as the light source. QCLs are novel devices, and a significant fraction of our research has been devoted to developing the procedures and hardware required to implement them most effectively for chemical sensing. This report details the progress we have made on LWIR CES sensor development.

The Laser Development Task of Pacific Northwest National Laboratory's (PNNL) Remote Spectroscopy project (PL211I) is focused on the development of novel laser technology for a new generation of standoff and in-situ chemical sensors for detecting the proliferation of nuclear weapons. These lasers will improve the sensitivity, flexibility, or range of active standoff sensors, enable ultra-trace in situ sensors with enhanced selectivity, as well as greatly improve calibration of passive standoff sensors. In particular, laser transmitters with minimal size, weight, and power consumption (SWAP) are needed to meet the requirements for a variety of in situ or short-range stand-off sensors and sensors for small UAVs or other platforms. These laser transmitters need to be rugged and free of requirements for consumables such as liquid nitrogen. Many sensing techniques also require lasers that produce a single narrow wavelength (single longitudinal mode). Lasers that provide high continuous-wave (CW) output power on a single line at operating temperatures accessible with thermoelectric (TE) cooling are therefore essential for sensor applications.

This report summarizes the preliminary tests that PNNL has performed to date for the Quantum Cascade (QC) Laser Alignment System that is being developed for the Using Client for a classified application. PNNL is designing, fabricating, assembling, and testing the QC Laser Alignment System and has a subcontract with Maxion Technologies, Inc. for development and production of the QC laser devices to be used in this system. The QC lasers furnished by Maxion will be incorporated into the QC Laser Alignment System by PNNL. The QC Laser Alignment System consists of five Alignment Source Assemblies (ASAs) and a computer control system with graphical user interface (GUI). Each ASA has two QC lasers along with a temperature sensor. The system design also includes an optical detector for each QC laser to measure the output power of the rear facet for additional stabilization. The system will monitor the voltage across the QC laser, the temperature, the current, and the signal from the optical detectors to ensure the system is within the tolerances specified in the System Specifications. The System Specifications require that the relative power between lasers on the same ASA be maintained at {+-}1% and among the lasers on different ASAsmore » at {+-}2.5%. For the tests reported here, we attempted to examine the power stability of the QC laser as well as the variability of the optical detectors to ensure the system will adhere to these specifications. These preliminary tests did not incorporate the actual ASA mounting scheme or the QC lasers that are being fabricated by Maxion to operate at the specified temperature of -50 C. Thus, we expect the performance from these results to differ from the actual results that can be achieved in the QC Laser Alignment System. Current limitations with the mounting scheme created problems with the power stability due to thermal cycling. Short-term power stability where thermal cycling was not a problem showed power fluctuations within the 1 % tolerance limits. Once thermal cycling was introduced, however, larger variations resulted although most measurements were still within the 2.5% tolerance limits. Unfortunately, problems with the mount worsened, which contributed to a drop in power for the long-term measurements, until finally we lost electrical contact with the device. The variability tests for the optical detectors were encouraging and show that monitoring the light from the rear facet of the QC laser should provide sufficient feedback to maintain the QC lasers within the system specifications.« less

Fabry–Perot quantum cascade lasers (QCLs) were characterized following irradiation by high-energy (64 MeV) protons and Cobalt-60 gamma rays. Seven QCLs were exposed to total accumulated radiation doses that varied from 20 to 46.3 krad(Si), which are typical exposure levels for electronic components in a space environment. The QCLs did not show any measurable changes in threshold current or slope efficiency suggesting the suitability of QCLs for use in space-based missions.

Modeling done at Pacific Northwest National Laboratory (PNNL) in FY2000 predicted improved sensitivity for remote chemical detection by differential absorption lidar (DIAL) if frequency-modulated (FM) lasers were used. This improved sensitivity results from faster averaging away of speckle noise and the recently developed quantum cascade (QC) lasers offer the first practical method for implementing this approach in the molecular fingerprint region of the infrared. To validate this model prediction, a simple laboratory bench FM-DIAL system was designed, assembled, tested, and laboratory-scale experiments were carried out during FY2001. Preliminary results of the FM DIAL experiments confirm the speckle averaging advantages predicted by the models. In addition, experiments were performed to explore the use of hybrid QC - CO2 lasers for achieving sufficient frequency-modulated laser power to enable field experiments at longer ranges (up to one kilometer or so). This approach will allow model validation at realistic ranges much sooner than would be possible if one had to first develop master oscillator - power amplifier systems utilizing only QC devices. Amplification of a QC laser with a CO2 laser was observed in the first hybrid laser experiments, but the low gain and narrow linewidth of the CO2 laser available for these experimentsmore » prevented production of a high-power FM laser beam.« less

An external-cavity (EC) quantum cascade (QC) laser using optical feedback from a partial-reflector is reported. With this configuration, the otherwise multi-mode emission of a Fabry-Perot QC laser was made single-mode with optical output powers exceeding 40 mW. A mode-hop free tuning range of 2.46 cm(-1) was achieved by synchronously tuning the EC length and QC laser current. The linewidth of the partial-reflector EC-QC laser was measured for integration times from 100 μs to 4 seconds, and compared to a distributed feedback QC laser. Linewidths as small as 480 kHz were recorded for the EC-QC laser.