Uta-Barbara Goers

Development of a pulsed backscatter-absorption gas-imaging system and its application to the visualization of natural gas leaks.

The design and evaluation of a backscatter-absorption gas-imaging sensor that operates in a pulsed mode is described. It is capable of video visualization of natural gas leaks. Its development was motivated by the need for a methane imaging system to operate at ranges and sensitivities useful to the natural gas industry. The imager employs pulsed laser illumination at a repetition rate of 30 Hz and an average power of ~150 mW to image gas at standoff ranges of as long as 100 m, using a backscatter target with a reflectivity of 0.016 sr(-1). This is a tenfold improvement over an earlier raster-scanned imager. Natural gas leaks as small as 1.6 x 10(-4) standard liters/s [equal to 0.02 standard cubic feet per hour (scfh)] were imaged at short ranges; leaks as low as 7.9 x 10(-4) standard liters/s (0.1 scfh) were observed at long ranges. Data are compared with model predictions, and potential extensions to a fieldable prototype are discussed. The optimization of a direct-injection focal-plane array for detecting short (nanosecond) laser pulses is described.

PPLN-OPO-based backscatter absorption gas imaging (BAGI) system and its application to the visualization of fugitive gas emissions

We report on a laser active imager suitable for the visualization of natural gas leaks and volatile organic compounds emitted by oil refineries. The described backscatter-absorption gas-imaging (BAGI) system employs a raster scanner in conjunction with a tunable continuous wave (cw) laser source. The imager creates real-time video imagery of a scene, while illuminating it with infrared laser light at a wavelength that is absorbed by the gas to be detected. Thus, gas plumes that otherwise cannot be seen by the human eye appear in BAGI images as dark clouds. In order to produce the high intensity infrared light that is needed to image natural gas and refinery by-products, we used a nonlinear frequency-conversion technique that employs the quasi-phase-matched crystal periodically poled LiNbO3. The crystal serves as the active medium in a cw optical parametric oscillator (OPO) that is pumped by a diode-pumped Nd:YAG laser. The output frequencies were selected to coincide with absorption features of general aliphatic species (2935 and 2968 cm-1), aromatics, such as benzene and toluene (3033 cm-1), and methane (3018 cm-1). The crystal was engineered to cover the desired spectral range using a fan-out design. This allows tuning of the OPO between 2832 and 3145 cm-1 in idler wavelength by simply translating the crystal at a fixed temperature. Presented data demonstrate the performance of this system for imaging species of interest at relevant concentrations and ranges up to about 30 m.

PPLN laser-based system for chemical imaging

An infrared-imaging instrument is being developed to provide in situ qualitative and quantitative assessment of hydrocarbon contaminants on metallic surfaces for cleaning verification. A continuous-wave infrared optical parametric oscillator (OPO), based on the quasi-phasematched material periodically poled lithium niobate (PPLN), is interfaced with an InSb focal plane array camera to perform fast, non-invasive analysis by reflectance spectroscopy. The period range of the designed fan-out PPLN crystal determines the range of the output wavelength of the light source. It is able to scan hundreds of wavenumbers positioned in the range of 2820 - 3250 cm-1, which is sufficient to detect functional groups of common organic compounds (-CH, -OH, and -NH). The capability of the instrument has been demonstrated in a preliminary investigation of reflectance measurements for hydrocarbon solvents (methanol and d-limonene) on an aluminum surface. A substantial difference in absorption is obtained for the two solvents at two different laser-illumination wavelengths, thus permitting hydrocarbon detection and molecular species differentiation. Preliminary reflectance spectra of a mixture of aliphatic hydrocarbon lubricants and drawing agents on an aluminum panel are also presented. The relative thickness of the hydrocarbon thin film is determined by the intensity ratio of images acquired at two different laser illumination frequencies.

Development of a compact gas imaging sensor employing a cw fiber-amp-pumped PPLN OPO

Summary form only given. Backscatter absorption gas imaging (BAGI) is a laser technique for real-time video visualization of gas plumes. A scene is illuminated with IR laser radiation as it is imaged by an IR camera. Gases absorbing at the laser wavelength become visible as they attenuate the laser backscatter. We describe the development of a BAGI instrument that is sufficiently portable for use by a walking operator. A compact and electrically efficient format was achieved by developing a miniature PPLN OPO pumped by a fiber amplifier. This effort extends earlier work in which a Nd:YAG-pumped OPO was used in a vehicle-borne imager.

Development of infrared chemical sensors based on quasi-phasematched, periodically poles lithium niobate sources

Periodically poled lithium niobate (PPLN) is a relatively new non-linear optical material which can be used for such processes as second harmonic generation, sum and difference frequency generation and optical parametric oscillation. The use of periodically-poled lithium niobate in spectroscopy and chemical sensing offers many potential advantages over systems employing more traditional laser sources. When pumped by the fundamental of a Nd:YAG laser PPLN offers the promise of high efficiency, high power, broad tunability (1.5–3.5 µm) and compact size. The broad tunability over the C-H stretch region is an important advantage for many chemical sensing applications. At Sandia, we are developing IR sources based on PPLN for both remote and in-situ chemical sensing.

Optical sensors for process control and emissions monitoring in industry

Laser-based sensors, currently under development and testing at Sandia National Laboratories for process control, emissions monitoring, and pollution prevention, are discussed.