Publications

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We discuss a maritime surveillance and detection concept based on Raman scattering of water molecules. Using a range-gated scanning lidar that detects Raman scattered photons from water, the absence or change of signal indicates the presence of a non-water object. With sufficient spatial resolution, a two-dimensional outline of the object can be generated by the scanning lidar. Because Raman scattering is an inelastic process with a relatively large wavelength shift for water, this concept avoids the often problematic elastic scattering for objects at or very close to the water surface or from the bottom surface for shallow waters. The maximum detection depth for this concept is limited by the attenuation of the excitation and return Raman light in water. If excitation in the UV is used, fluorescence can be used for discrimination between organic and non-organic objects. In this paper, we present a lidar model for this concept and discuss results of proof-of-concept measurements. Using published cross section values, the model and measurements are in reasonable agreement and show that a sufficient number of Raman photons can be generated for modest lidar parameters to make this concept useful for near-surface detection.

In this report we discuss a new maritime surveillance and detection concept based on Raman scattering of water molecules. Using a scanning lidar that detects Raman scattered photons from water, the absence or change of signal indicates the p resence of a non - water object. With sufficient spatial resolution a negative two dimensional imag e of the object can be generate d by the scanning lidar . Because Raman scatt er ing is an inelastic process with a relatively large wavelength shift for water , this concept completely avoids the problematic elastic sc attering for objects at or very close to the water surface . Elastic scattering makes it difficult to disc riminate between water and dark objects at or near the water surface especially when automated detection is required . It is also difficult to deal wit h elastic scattering from the bottom surface for shallow waters. The maximum detection depth for this concept is limited by the attenuation of the excitation and return Raman light in water. If excitation in the UV is used, fluorescence can be used for dis crimination between organic and non - organic objects. Range gating can be used for this concept for detection of objects below a specified depth. In this report we develop a lidar model for this concept to estimate the number of detected Raman photons fo r variable lidar parameters and depths in the presence of the solar background . We also report on the results of proof - of - concept measurements using the Sandia Ares lidar with excitation at 355 nm. The measurements show good agreement with the lidar mode l predictions. The detected number of photons for typical lidar parameter shows the concept is viable and applicable to a variety of day and nighttime detection scenarios. This concept has many potential applications including ne ar - surface mine detection, swimmer detection for security purposes, wide area search, as well as other civilian applications.

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A short-standoff bistatic lidar system coupled with an aerosol chamber has been built to measure aerosol optical backscatter and laser induced fluorescence cross-sections. Preliminary results show good sensitivity across all channels with high signal-to-noise ratio. © OSA 2012.

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Laser-induced fluorescence measurements of cuvette-contained laser dye mixtures are made for evaluation of multivariate analysis techniques to optically thick environments. Nine mixtures of Coumarin 500 and Rhodamine 610 are analyzed, as well as the pure dyes. For each sample, the cuvette is positioned on a two-axis translation stage to allow the interrogation at different spatial locations, allowing the examination of both primary (absorption of the laser light) and secondary (absorption of the fluorescence) inner filter effects. In addition to these expected inner filter effects, we find evidence that a portion of the absorbed fluorescence is re-emitted. A total of 688 spectra are acquired for the evaluation of multivariate analysis approaches to account for nonlinear effects.

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The Mg-Li-N-H system is a very promising hydrogen storage material due to its high capacity, reversibility and moderate operating conditions. Some of thermodynamic and structural properties for this system are characterized here. Pressure-composition isotherms are measured and presented in this paper for absorption-desorption at 220, 200 and 180 C. Powder X-ray diffraction (XRD) and Fourier Transform Infrared (FTIR) analysis were carried out for samples at various degrees of hydrogenation. These results provide information about the structural changes during absorption/desorption. The mixture of (2LiNH{sub 2} + MgH{sub 2}) partially converts to (Mg(NH{sub 2}){sub 2} + 2LiH) when heated at 220 C and 100 bar of hydrogen without undergoing desorption. Based on two distinct parts which appear in all of the pressure-composition isotherms (180-220 C), two reactions taking place isothermally in hydrogen absorption/desorption are proposed for the material starting with (2LiNH{sub 2} + MgH{sub 2}) or (Mg(NH{sub 2}){sub 2} + 2LiH). These reactions include a single solid-phase reaction, corresponding to the sloping region for hydrogen weight percent (Hwt%) smaller than 1.5%, and a multiple-phase reaction, corresponding to a plateau region for Hwt.% > 1.5 in the isotherms. During hydrogen absorption/desorption, the single-solid-phase reaction corresponds to the forming/consuming of NH{sub 2} which is bonded to Li and the multiple-solid-phase reaction corresponds to forming/consuming Mg(NH{sub 2}){sub 2} and LiH. A mechanism for the sorption reactions has been proposed.

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A number of industrial combustion systems are adopting oxygen-enhanced firing to improve heat transfer characteristics and reduce emissions. The exhaust gas from these systems is dominated by H2O and CO2 and therefore has substantially different gas properties from traditional combustion exhaust. In the past, laser-induced breakdown spectroscopy (LIBS) has been successfully used for the evaluation of alkali aerosol concentrations in air-based combustion systems. This paper presents results of LIBS measurements of alkali concentrations in a laboratory calibration setup and in an oxygen/natural gas container glass furnace. It shows how both gas conditions (composition and temperature) and the molecular form of the alkali species affect the LIBS signals. The paper proposes strategies for mitigating these effects in future applications of LIBS in oxygen-enhanced combustion systems.

Laser-induced breakdown spectroscopy (LIBS) was used in the evaluation of aerosol concentration in the exhaust of an oxygen/natural-gas glass furnace. Experiments showed that for a delay time of 10 {micro}s and a gate width of 50 {micro}s, the presence of CO{sub 2} and changes in gas temperature affect the intensity of both continuum emission and the Na D lines. The intensity increased for the neutral Ca and Mg lines in the presence of 21% CO{sub 2} when compared to 100% N{sub 2}, whereas the intensity of the Mg and Ca ionic lines decreased. An increase in temperature from 300 to 730 K produced an increase in both continuum emission and Na signal. These laboratory measurements were consistent with measurements in the glass furnace exhaust. Time-resolved analysis of the spark radiation suggested that differences in continuum radiation resulting from changes in bath composition are only apparent at long delay times. The changes in the intensity of ionic and neutral lines in the presence of CO{sub 2} are believed to result from higher free electron number density caused by lower ionization energies of species formed during the spark decay process in the presence of CO{sub 2}. For the high Na concentration observed in the glass furnace exhaust, self-absorption of the spark radiation occurred. Power law regression was used to fit laboratory Na LIBS calibration data for sodium loadings, gas temperatures, and a CO{sub 2} content representative of the furnace exhaust. Improvement of the LIBS measurement in this environment may be possible by evaluation of Na lines with weaker emission and through the use of shorter gate delay times.