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We demonstrate a sensor based on tunable diode laser absorption spectroscopy for the detection of hydrogen fluoride (HF) gas at ambient pressure. Absorption from the HF R(1) ro-vibrational peak at ν = 4038.962 cm−1 (2.476 µm) in the fundamental (Δν = 1) band is measured. A quantitative spectral fit based on HITRAN data is used to account for overlapping spectral peaks of HF and water vapor, with an rms residual noise of 5 × 10−4 absorbance units. The sensor is optimized for the detection of transient variations in HF concentration. We measure noise-equivalent concentrations for HF of 38 parts-per-trillion by volume (ppt) for 1-s integration times and 2.3 ppt for 10-min integration times.

We describe a high accuracy frequency stepping method for a tunable diode laser to improve a three dimensional (3D) imaging approach based upon interferometric speckle imaging. The approach, modeled after Takeda, exploits tuning an illumination laser in frequency as speckle interferograms of the object (specklegrams) are acquired at each frequency in a Michelson interferometer. The resulting 3D hypercube of specklegrams encode spatial information in the x-y plane of each image with laser tuning arrayed along its z-axis. The specklegrams are processed by Fast Fourier Transformation (FFT) along the z-axis of the hypercube and the center of the peak in the resulting power spectrum for each pixel encodes its surface height. Alternatively, Takeda’s method can be followed which uses the phase of the FFT, unwraps it, and determines the surface height encoded in the slope of a line fitted to the phase. Wraparound of modulations above the Nyquist limit results in ambiguity in the optical path difference (OPD) between test and reference surfaces. Wraparound also amplifies measurement noise caused by errors and jitter in frequency stepping the illumination laser. By locking the laser frequency to successive cavity modes of a reference confocal interferometer, tuning is precisely controlled resulting in dramatically improved imaging quality/. We present laboratory data of before and after results showing enhanced 3D imaging resulting from precise laser frequency control.

We present the design, modeling, construction, and results of a three-dimensional imager based upon multiplewavelength speckle interferometry. Speckle imaging used in non-destructive evaluation is well-known but requires a precisely acquired reference image and can measure excursions only within the 2π ambiguity range determined by the illumination wavelength. Our approach is based upon earlier efforts pioneered by Takeda, but with updated illumination, imaging, and processing tools, in which a surface under test is illuminated with tunable laser light in a Michelson interferometer configuration. A speckled image is acquired at each laser frequency step creating a data hypercube. Interference between the reference wavefront and light from the object causes the amplitude of the speckles to cycle with laser tuning. Fourier transforming the hypercube in the laser frequency dimension reveals periods that map heights of surface features. Height resolution is determined by the maximum tuning range of the laser, which for our 16-nm tuning range provides approximately 18 micron resolution without any efforts at interpolation. The largest height without wraparound depends on the smallest tuning steps, which for our laser is 15 cm for 0.002 nm (1 GHz) tuning steps. In this way, objects with large discontinuous steps or holes can be imaged without confusion. Also, due to the illumination beam being normal to the surface under test, shadowing is eliminated. To inform our design and better understand our system’s limitations, we have developed extensive numerical models based upon Monte Carlo ray tracing in which speckle patterns are produced after scattering from model surfaces by coherent summing of rays at the detector plane. Data acquired by the system as well as modeling results will be shown.

A multi-channel laser-based chemical sensor platform is presented, in which a modular architecture allows the exchange of complete sensor channels without disruption to overall operation. Each sensor channel contains custom optical and electronics packages, which can be selected to access laser wavelengths, interaction path lengths and modulation techniques optimal for a given application or mission. Although intended primarily to accommodate mid-infrared external cavity quantum cascade lasers and astigmatic Herriott cells, channels using visible or near infrared lasers or other gas cell architectures can also be used, making this a truly versatile platform. Analog and digital resources have been carefully chosen to facilitate small footprint, rapid spectral scanning, low-noise signal recovery, fail-safe autonomous operation, and in-situ chemometric data analysis, storage and transmission. Results from the demonstration of a two-channel version of this platform are also presented.

Stand-off detection of hazardous materials ensures that the responder is located at a safe distance from the suspected source. Remote detection and identification of hazardous materials can be accomplished using a highly sensitive and portable device, at significant distances downwind from the source or the threat. Optical sensing methods, in particular infrared absorption spectroscopy combined with quantum cascade lasers (QCLs), are highly suited for the detection of chemical substances since they enable rapid detection and are amenable for autonomous operation in a compact and rugged package. This talk will discuss the sensor systems developed at Pacific Northwest National Laboratory and will discuss the progress to reduce the size and power while maintaining sensitivity to enable stand-off detection of multiple chemicals.

We describe the performance of a sensor system designed for simultaneous detection of multiple chemicals with both broad and narrow absorption features. The sensor system consists of a broadly tunable external cavity quantum cascade laser (ECQCL), multi-pass Herriott cell, and custom low-noise electronics. The ECQCL features a fast wavelength tuning rate of 2265 cm-1/s (15660 nm/s) over the range of 1150-1270 cm-1 (7.87-8.70 μm), which permits detection of molecules with broad absorption features and dynamic concentrations, while the 0.2 cm-1 spectral resolution of the ECQCL system allows measurement of small molecules with atmospherically broadened absorption lines. High-speed amplitude modulation and low-noise electronics are used to improve the ECQCL performance for direct absorption measurements. We demonstrate simultaneous detection of Freon-134a (1,1,1,2-tetrafluoroethane), ammonia (NH3), and nitrous oxide (N2O) at low-ppb concentrations in field measurements of atmospheric chemical releases from a point source.

We describe and characterize an external cavity quantum cascade laser designed for detection of multiple airborne chemicals, and used with a compact astigmatic Herriott cell for sensing of acetone and hydrogen peroxide.

We present results demonstrating real-time sensing of four different fluorocarbons at low part-per billion (ppb) concentrations using an external cavity quantum cascade laser (ECQCL) designed for infrared vibrational spectroscopy of molecules with broad absorption features. The ECQCL was repeatedly swept at 20 Hz over its full tuning range of 1145-1265 cm(-1) providing a scan rate of 3535 cm(-1) s(-1), and a detailed characterization of the ECQCL scan stability and repeatability is presented. The ECQCL was combined with a 100 meter path length multi-pass cell for direct absorption spectroscopy. A portable sensor system is described, which was deployed on a mobile automotive platform to provide spatially-resolved detection of fluorocarbons in outdoor experiments. Noise-equivalent detection limits of 800-1000 parts-per-trillion (ppt) are demonstrated for 1 s integration times.