Karla M. Armstrong

Broadly tunable, mode-hop-tuned cw optical parametric oscillator based on periodically poled lithium niobate

We describe a broadly tunable, cw optical parametric oscillator (OPO) based on periodically poled lithium niobate. The OPO can be tuned over a broad region in the mid IR (2900-3100 cm(-1)) covering the important C-H stretch region while a high spectral resolution (<0.1 cm(-1)) is maintained. The OPO is the light source for a field-portable photoacoustic spectrometer for gas-phase monitoring of volatile organic compounds.

Refinery Evaluation of Optical Imaging to Locate Fugitive Emissions

Abstract Fugitive emissions account for approximately 50% of total hydrocarbon emissions from process plants. Federal and state regulations aiming at controlling these emissions require refineries and petrochemical plants in the United States to implement a Leak Detection and Repair Program (LDAR). The current regulatory work practice, U.S. Environment Protection Agency Method 21, requires designated components to be monitored individually at regular intervals. The annual costs of these LDAR programs in a typical refinery can exceed US

Long-wave IR chemical sensing based on difference frequency generation in orientation patterned GaAs

1,000,000. Previous studies have shown that a majority of controllable fugitive emissions come from a very small fraction of components. The Smart LDAR program aims to find cost-effective methods to monitor and reduce emissions from these large leakers. Optical gas imaging has been identified as one such technology that can help achieve this objective. This paper discusses a refinery evaluation of an instrument based on backscatter absorption gas imaging technology. This portable camera allows an operator to scan components more quickly and image gas leaks in real time. During the evaluation, the instrument was able to identify leaking components that were the source of 97% of the total mass emissions from leaks detected. More than 27,000 components were monitored. This was achieved in far less time than it would have taken using Method 21. In addition, the instrument was able to find leaks from components that are not required to be monitored by the current LDAR regulations. The technology principles and the parameters that affect instrument performance are also discussed in the paper.

PPLN laser-based system for chemical imaging

Lightsources employing quasiphasematched (QPM) nonlinear materials have demonstrated unique attributes for chemical sensing in the near- to mid-infrare spectral range (1 - 5 micrometers ). The advent of patterned-growth GaAs allows the first practical extension of QPM materials to operation in the long-wave IR (5 - 12 micrometers ). That wavelength range is particularly attractive for chemical sensing because it contains an atmospheric window, many molecular groups absorb there at distinct frequencies, and their absorptions tend to be strong relative to those in the near- and mid-IR. Here, the application of orientation-patterned GaAs (OPGaAs) for use in a continuous wave (cw) difference frequency spectrometer is described. The outputs of two external- cavity diode lasers operating in the 1.3 and 1.5 micrometers telecom bands are mixed in a OPGaAs crystal, producing tunable radiation at wavelengths near 8 micrometers . The application of the source to the measurement of a water vapor rovibrational absorption line is presented.

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

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.

Detection of Surface Contaminant Residue by Tunable Infrared Laser Imaging

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

We report the development of a new, real-time non-contacting monitor for cleaning verification based on tunable infrared-laser methods. New analytical capabilities are required to maximize the efficiency of cleaning operations at a variety of federal (Department of Defense [DoD] and Department of Energy [DOE]) and industrial facilities. These methods will lead to a reduction in the generation of waste streams while improving the quality of subsequent processes and the long-term reliability of manufactured, repaired or refurbished parts. We have demonstrated the feasibility of tunable infrared-laser imaging for the detection of contaminant residues common to DoD and DOE components. The approach relies on the technique of infrared reflection spectroscopy for the detection of residues. An optical interface for the laser-imaging method was constructed, and a series of test surfaces was prepared with known amounts of contaminants. Independent calibration of the laser reflectance images was performed with Fourier transform infrared (FTIR) spectroscopy. The performance of both optical techniques was evaluated as a function of several variables, including the amount of contaminant, surface roughness of the panel, and the presence of possible interfering species (such as water). Finally, detection limits for generic hydrocarbon contaminants were evaluated as a function of system noise level.

Tunable light source for use in photoacoustic spectrometers

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.