P.E. Powers

Continuous tuning of a continuous-wave periodically poled lithium niobate optical parametric oscillator by use of a fan-out grating design.

We report on a new periodically poled lithium niobate grating design with a continuous grating-period change (fan-out). We observed 350cm(-1) (80 nm at 1.5microm) of complete spectral coverage at a constant temperature in a cw optical parametric oscillator. Complete spectral coverage is demonstrated by measurement of an absorption band of CO(2) .

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.

Demonstration of differential backscatter absorption gas imaging

Backscatter absorption gas imaging (BAGI) is a technique that uses infrared active imaging to generate real-time video imagery of gas plumes. We describe a method that employs imaging at two wavelengths (absorbed and not absorbed by the gas to be detected) to allow wavelength-differential BAGI. From the frames collected at each wavelength, an absorbance image is created that displays the differential absorbance of the atmosphere between the imager and the backscatter surface. This is analogous to a two-dimensional topographic differential absorption lidar or differential optical absorption spectroscopy measurement. Gas plumes are displayed, but the topographic scene image is removed. This allows a more effective display of the plume image, thus ensuring detection under a wide variety of conditions. The instrument used to generate differential BAGI is described. Data generated by the instrument are presented and analyzed to estimate sensitivity.

Remote imaging of controlled gas releases using active and passive infrared imaging systems

The results of field tests of an active backscatter absorption gas imaging (BAGI) system and a passive imager based on a Ga:Si infrared focal-plane array are presented. Both imagers allow real-time video imaging of gas emissions. The former system images gases through their attenuation of backscattered laser illumination; the latter images gases through temperature or emissivity differences. The results represent the first side-by-side comparison of an active and passive imager and the first BAGI field trial involving the imaging of plumes of controlled concentration and dimension.

Two-tone frequency modulation spectroscopy from laser light scattered off a hard target

Summary form only given. The results of two-tone frequency-modulated (FM) laser light scattered off a target to look at atmospheric absorptions are presented. The results are a first-time demonstration of the two-tone FM measuring technique used for monostatic (single-ended) applications involving diffusely scattering targets.

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.

Development of a compact cavity-ringdown spectrometer using a PPLN-based OPG/OPA lightsource

Summary form only given. Cavity-ringdown laser absorption spectroscopy (CRLAS) is a powerful technique for quantifying weak absorptions in gases. It is performed by injecting laser light into a high-finesse optical cavity containing the gas and monitoring the rate of loss of the radiation from the cavity. Measurement of loss as a function of the laser wavelength provides an absorption spectrum of the gas sample. This paper describes the performance of an analytical instrument that uses CRLAS for quantitative gas measurements in the mid-IR. This device differs from past CRLAS systems in its use of a novel mid-IR light source based on periodically-poled lithium niobate (PPLN).

Differential backscatter absorption gas imaging

Backscatter gas absorption imaging (BAGI) has been demonstrated as a useful technique for visualizing gas leaks. BAGI uses active imaging in the infrared to generate a laser- illuminated video image of a scene. A dark cloud in the image is formed when a gas absorbs the illuminating laser radiation in the vicinity of the plume. The sensitivity of the technique is limited by the ability of the operator to distinguish scene contrasts from gas contrasts. To improve its performance, we have developed a differential absorption system to subtract off scene contrasts that can obscure a gas plume. This system is essentially an imaging differential absorption LIDAR (DIAL) that allows one to focus on contrast in a scene due to absorption from a gas plume instead of contrast due to variations in the reflectivity of the target. Practical aspects of this system are presented along with results taken in real-world settings. The noise floor for a differential image is shown to be dominated by uncorrelated speckle fluctuations -- not contrasts in the scene.