This paper discusses the result of a field trial in which active and passive FTIRs measured plume concentration pathlength (CL). It will be shown that when the coadding duration of passive systems was the same as active systems, nearly identical CL values were obtained. At the field trial, plumes of SF6 were released. The release rate and duration were controlled using a flowmeter. All the FTIR systems were monitoring approximately the same spot in the plume. Weather station data and flowmeter data were used with a gaussian plume model to predict downwind CLs. The active CLs were determined with standard CLS processing. The passive CLs were determined by converting the passive spectra to radiometric spectra. From the radiometric spectra, the air temperature and background temperature were derived. With this information, the absorbance was calculated and the CL was determined from the absorbance peak of the chemical.
The phenomenon of thermal emission form non-volatile liquid surface coatings following pulsed laser heating has been experimentally and theoretically studied with a view to developing a differential thermal imaging scheme for the remote detection of contaminated surfaces. Pulsed UV and IR laser sources have been used to generate radiance profiles from contaminants which are correlated with their characteristic spectra. Data from experiments and numerical simulations are compared and a reasonable level of agreement is demonstrated.
We present a chemical sensor scheme based on selective sensing surfaces and highly sensitive integrated optical transduction methods. Using self-assembly techniques, species selective thin-films are covalently attached to the surface of Si3N4 channel waveguides to produce robust sensor elements. Exposure to targeted analytes results in the selective absorption of these molecules onto the waveguide surface causing a change in the effective refractive index of the guided modes. These relative changes in effective refractive index between TE and TM modes are precisely measured using Zeeman interferometry. Our measurements demonstrate reversible, real time sensing of volatile organic compounds at ppm levels.
We are developing 2-100 kHz repetition rate CO2 lasers with milliJoule pulse energies, rapid acousto-optic tuning and isotopic gas mixes, for differential absorption LIDAR applications. We explain the tuning method, which uses a pair of acousto-optic modulators and is capable of random access to CO2 laser lines at rates of 100 kHz or more. The laser system is also described, and we report on performance with both normal and isotopic gas mixes.
The proliferation of weapons of mass destruction (WMD) continues to be a serious threat to the security of the US. Proliferation of chemical and biological (CB) weapons is particularly disturbing, and the threats posed can be devastating. Critical elements of the US efforts to reduce and counter WMD proliferation include: (1) the location and characterization of WMD facilities and capabilities worldwide; (2) the ability to rapidly detect and identify the use of CB weapons for expeditious warning and reporting on the battlefield; and (3) the capability to mitigate deleterious consequences of a CB incident through effective protective and medical treatment measures. Remote sensing has been touted as a key technology in these efforts. Historically, the role of remote sensing in CB defense has been to provide early warning of an attack from an extended distance. However, additional roles for remote sensing in CB defense, as well as applications in related missions, are possible and should be pursued. This paper examines what has been happening in remote sensing over the past decade to address needs in this area. Accomplishments, emerging technologies, programmatic issues, and opportunities for the future are covered. The Department of Defence chemical- biological, the Department of Energy's Chemical Analysis by Laser Interrogation of Proliferation Effluents, and other agency related programs are examined. Also, the status of remote sensing in the commercial market arena for environmental monitoring, its relevance to the WMD counterproliferation program, and opportunities for technology transfer are discussed. A course of action for the future is recommended.
In this paper, we review some recent advances in optical parametric oscillator (OPO) technology, discuss major coherent source issues and then propose possible solutions that have relevance to remote chemical sensing. The authors discuss their latest result on two OPO schemes. 1) Energies up to 1.2 mJ/pulse and continuously tunable OPO output from 6.7 to 9.8 micrometers was obtained using a 5 X 5 X 25 mm3 type II AgGaS2 crystal, pumped a second OPO using NCPM ZnGeP2 to generate output near 8 micrometers . The tandem OPO produced pulse energies of > 1.5 mJ at 7.8 micrometers with an energy conversion efficiency of 6.8 percent. Finally, we describe schemes for generating multiple photons in the 8-12 micrometers band from one initial 1 micrometers pump photon, and thereby increase the quantum efficiency when OPOs are pumped by Nd:YAG lasers.
The adaptive IR imaging spectroradiometer (AIRIS) is a multispectral imaging system comprising a low-order tunable Fabry-Perot etalon coupled to an IR focal plane array. This low-order interferometer based imaging system provides wide spectral coverage combined with narrow spectral bandwidth, flexible and adaptive sampling and processing of the image to isolate specific spectral features or signatures, high radiance sensitivity, and an extended field-of-view for the survey of wide areas. The adaptive sampling capability of the AIRIS sensor provides the opportunity to rapidly image a scene at only those wavelengths needed for target identification and clutter suppression. We have developed a prototype LWIR AIRIS sensor to perform passive stand-off detection of hazardous chemical vapor plumes. The imaging sensor covers the 10.0 to 11.5 micrometers region and allows identification of numerous compounds, including chemical warfare agents and simulants, on the basis of observed IR spectra. The LWIR AIRIS has a 40 X 40 deg FOV and a NESR equals 2 (mu) W cm-2 sr-1, resulting in a detection limit of 25 ppmv*m for DMMP against a temperature drop of 6 degrees C.
The spectroradiometric performance characteristics of a FT- IR spectrometer are evaluated as a function of temperature, optical mirror velocity, and spectral resolution. The noise equivalent radiance per root Hertz is used in these noise evaluations and is shown to be an important Michelson interferometer figure of merit.
Computer-generated synthetic single-beam spectra and interferograms are used to study signal processing strategies for passive Fourier transform IR (FTIR) sensor. Synthetic data are generated for one-, two-, and four- component mixtures of organic vapors in two passive FTIR remote sensing scenarios. The single-beam spectra are processed using Savitsky-Golay smoothing, first derivative, and second derivative filters of various orders and widths. Interferogram data are processed by Fourier filtering using Gaussian-shaped bandpass digital filters. Pattern recognition of the target analyte spectral signature is performed using soft independent modeling of class analogy. Quantitative models for the target gas integrated concentration-path length product are built using partial least-squares regression and locally weighted regression. Pattern recognition and calibration models of the filtered spectra and interferograms produced similar results. Chemical detection is possible for complex mixtures if the temperature difference between the source and analyte cloud is sufficiently large. Quantitative analysis is possible if the temperature of the analyte cloud is stable or known and is sufficiently different from the background temperature.
Accurate explosives detection would be an effective defense/deterrent against bomb attacks. Chemical vapor sensing has the potential to be both specific and sensitive. The work descried here involves the use of mid-IR laser spectroscopy to detect the presence of nitro-group based explosive vapors. Two different singly-resonant Silver Gallium Selenide OPOs were examined for use as the spectroscopy source: one was pumped with a 1.319 micrometers Nd:YAG laser, and the other was pumped by a 1.57 micrometers KTP/Nd:YAG OPO-laser. Both OPOs can be angle-tuned over wavebands of interest, producing coherent mid-IR idler output power at 5-8 micrometers , as well as near-IR outputs. Current results indicate that output energies of approximately 300 (mu) J in the mid-IR can be generated from 10 mJ of pump energy. The OPO outputs can be used to detect chemical vapors using a sample-reference absorbance experiment. Both OPO outputs are passed through a cell containing the vapor to be measured. The mid-IR output is attenuated by the strong absorption of the vapors, while the near-IR output, being unabsorbed, serves as a reference. The ratio measurement has a shot-noise limited accuracy better than one part per million absorbance.
In the interest of remote sensing research, equipment and techniques have been developed to generate heated plumes with controlled and well-characterized temperature and composition profiles. This report describes the construction of a plume generating device as well as the field operations involved in creating and monitoring heated plumes for research purposes. Performance specifications for the plume generator and its components are provided and typical FTIR validation data are included that illustrate the device's capabilities.
Enactment of the Clean Air Act Amendments of 1990 has resulted in an increased ambient air monitoring needs for industry, some of which may be met efficiently using open- path optical remote sensing (ORS) techniques. Among the most promising of these techniques, we note the Fourier transform spectrometry (FTS). This technique is well suited for the detection of organic and inorganic chemicals since most of them have characteristic absorption bands in one or both of the IR atmospheric window regions. The need for reliable atmospheric pollution monitoring has motivated the development of a number of new chemical analysis approaches. This paper presents an approach for the spectral remote sensing of gaseous emanations from chemical agents, based on the measurements of their weak emission spectra via a passive IR FTS sensor. The method is implemented using such a sensor operating in differential mode between the two inputs, allowing the background to be optically subtracted. A specific algorithm has been developed jointly by Bomem and the Defence Research Establishment of Valcartier to take advantage of this particular setup and allow identification of trace gases in real time. A software research tool named GASEM implements this algorithm and is used with CATSI, a double-beam remote sensing interferometer operating in differential mode in the 3.5-17 micrometers spectral range.
Measurements of gas temperatures and concentrations in combustion and industrial processes, where hot gases are produced, can be carried out using low resolution Fourier transform IR emission spectroscopy. An experimental setup is presented for measuring transmittance as well as emittance spectra of hot gases enclosed in a heated gas cell. It is shown that the measured emissivity of a CO2 and CO mixture at 673 compare well with the absorptivity measured of the gas sample. The influence of the spectral resolution on the detection limit of CO in our experimental setup at 673 K is discussed and illustrated with experimental results and calculations. It is concluded that the most precise CO concentration measurement is obtained at low spectral resolution from a single point in the spectrum. The gas temperature and water vapor content in the hot gas of a power plant boiler can be extracted from low resolution emission spectra which is illustrated with experiments carried out on a power plant.