Rapidly-swept external-cavity quantum cascade lasers (swept-ECQCLs) provide the broad IR spectral coverage and high spectral brightness required for sensitive multi-chemical standoff detection. The laser source characteristics of swept- ECQCLs can be applied to a variety of standoff applications where gas-phase chemical species concentrations are changing rapidly in time. Some of these applications include standoff measurements in turbulent or chemically reactive plumes, monitoring of industrial emissions from stacks, and real-time monitoring of combustion processes. We have demonstrated and characterized a swept-ECQCL design that is capable of broadband spectral acquisition (~100 cm-1) at a 5 ms acquisition rate with a high spectral resolution (<0.2 cm-1). Laser systems based on this swept-ECQCL design have been applied to open-path and standoff measurements in turbulent and reactive chemical plumes of time-varying chemical and isotopic composition. Chemical plume studies have demonstrated the capability of this technology for open-path and standoff chemical sensing applications. In this talk we will present our current progress on the development and application of swept-ECQCL technology for sensitive and rapid multi-chemical open-path measurements in MeOD/MeOH plumes with time-varying isotopic composition at a 40 Hz rate. Preliminary results will also be provided for a 5 m standoff detection of hot CO and CO2 in a turbulent flame using a rapidly-swept mode-hop free (MHF) ECQCL design that is capable of collecting 1-2 cm-1 MHF spectral windows at an acquisition rate of 1 kHz with a spectral resolution of ~0.001 cm-1.
We demonstrate standoff detection of turbulent mixed-chemical plumes using a broadly tunable external cavity quantum cascade laser (ECQCL). The ECQCL was directed through plumes of mixed methanol/ethanol vapor to a partially reflective surface located 10 m away. The reflected power was measured as the ECQCL was swept over its tuning range of 930 to 1065 cm−1 (9.4 to 10.8 μm) at rates up to 200 Hz. Analysis of the transmission spectra though the plume was performed to determine chemical concentrations with a time resolution of 0.005 s. Comparison of multiple spectral sweep rates of 2, 20, and 200 Hz shows that higher sweep rates reduce effects of atmospheric and source turbulence, resulting in lower detection noise and more accurate measurement of the rapidly changing chemical concentrations. Detection sensitivities of 0.13 ppm*m for MeOH and 1.2 ppm*m for EtOH are demonstrated for a 200-Hz spectral sweep rate and normalized to a 1-s detection time.
We present recent results on development of swept external cavity quantum cascade lasers (ECQCLs) for rapid and precise measurements of trace gases. Rapid tuning of a swept-ECQCL over its entire tuning range at rates up to 100 Hz and a spectral resolution of <0.2 cm-1 provides an important capability for detection of gas mixtures in transient sources such as chemical plumes. We present examples of using the swept-ECQCL for quantitative measurements of multicomponent gas-phase mixtures in turbulent plumes for standoff detection applications.
We describe a versatile mid-infrared (Mid-IR) spectroscopy system developed to measure the concentration of a wide range of gases with an ultra-low sample size. The system combines a rapidly-swept external cavity quantum cascade laser (ECQCL) with a hollow fiber gas cell. The ECQCL has sufficient spectral resolution and reproducibility to measure gases with narrow features (e.g., water, methane, ammonia, etc.), and also the spectral tuning range needed to measure volatile organic compounds (VOCs), (e.g., aldehydes, ketones, hydrocarbons), sulfur compounds, chlorine compounds, etc. The hollow fiber is a capillary tube having an internal reflective coating optimized for transmitting the Mid-IR laser beam to a detector. Sample gas introduced into the fiber (e.g., internal volume = 0.6 ml) interacts strongly with the laser beam, and despite relatively modest path lengths (e.g., L ~ 3 m), the requisite quantity of sample needed for sensitive measurements can be significantly less than what is required using conventional IR laser spectroscopy systems. Example measurements are presented including quantification of VOCs relevant for human breath analysis with a sensitivity of ~2 picomoles at a 1 Hz data rate.
In this paper, we present the results of long-term operational testing of several quantum cascade laser (QCL) variants to illustrate their robustness and long lifetimes. Performance factors are investigated including power and spectral stability over different timescales ranging from days to years. The effects of burn-in, packaging, mounting, and facet coatings are considered with respect to their influence on long-term laser performance. In addition, the results from the several years’ operation of a custom external cavity quantum cascade laser-based trace gas sensor are presented to highlight the reliable performance of QCL-based sensor systems. This sensor monitored the laboratory air for multiple chemicals and operated continuously for two years without any evidence of degradation in performance. The data from all of these experiments will be discussed to demonstrate the reliability and robust performance of QCLs.
KEYWORDS: Chemical analysis, Absorbance, Time metrology, Liquids, NOx, Chemical species, Data acquisition, Modulation, Quantum cascade lasers, Wavelength tuning
A rapidly-swept external-cavity quantum cascade laser with an open-path Herriott cell is used to quantify gas-phase chemical mixtures of D2O and HDO at an update rate of 40 Hz (25 ms measurement time). The chemical mixtures were generated by evaporating D2O liquid near the open-path Herriott cell, allowing the H/D exchange reaction with ambient H2O to produce HDO. Fluctuations in the ratio of D2O and HDO on timescales of < 1 s due to the combined effects of plume transport and the H/D exchange chemical reaction are observed. Based on a noise equivalent concentration analysis of the current system, detection limits of 147.0 ppbv and 151.6 ppbv in a 25 ms measurement time are estimated for D2O and HDO respectively with a 127 m optical path. These detection limits are reduced to 23.0 and 24.0 ppbv with a 1 s averaging time for D2O and HDO respectively. Detection limits < 200 ppbv are also estimated for N2O, F134A, CH4, Acetone, and SO2 for a 25 ms measurement time.
We describe a prototype trace gas sensor designed for real-time detection of multiple chemicals. The sensor uses an
external cavity quantum cascade laser (ECQCL) swept over its tuning range of 940-1075 cm-1 (9.30-10.7 μm) at a 10 Hz
repetition rate. The sensor was designed for operation in multiple modes, including gas sensing within a multi-pass
Heriott cell and intracavity absorption sensing using the ECQCL compliance voltage. In addition, the ECQCL
compliance voltage was used to reduce effects of long-term drifts in the ECQCL output power. The sensor was
characterized for noise, drift, and detection of chemicals including ammonia, methanol, ethanol, isopropanol, Freon-
134a, Freon-152a, and diisopropyl methylphosphonate (DIMP). We also present use of the sensor for mobile detection
of ammonia downwind of cattle facilities, in which concentrations were recorded at 1-s intervals.
Using infrared hyperspectral imaging, microscopy of small particles of the explosives compounds RDX, tetryl, and PETN with near diffraction-limited performance is demonstrated. The custom microscope apparatus includes an external cavity quantum cascade laser illuminator scanned over its tuning range of 9.13 to 10.53 μm in 4 s, coupled with a microbolometer focal plane array to record infrared transmission images. The hyperspectral microscopy technique is used to study the infrared absorption spectra of individual explosives particles, and demonstrate subnanogram detection limits.
We present experimental demonstration of a new chemical sensing technique based on intracavity absorption in an external cavity quantum cascade laser (ECQCL). This new technique eliminates the need for an infrared photodetector and gas cell by detecting the intracavity absorption spectrum in the compliance voltage of the laser device itself. To demonstrate and characterize the technique, we measure infrared absorption spectra of chemicals including acetone and Freon-134a. Sub-ppm detection limits in one second are achieved, with the potential for increased sensitivity after further optimization. The technique enables development of handheld, high-sensitivity, and high-accuracy trace gas sensors for in-field use.
Infrared scattering scanning near-field optical microscopy (s-SNOM) is an apertureless superfocusing technique that uses the antenna properties of a conducting atomic force microscope (AFM) tip to achieve infrared spatial resolution below the diffraction limit. The instrument can be used either in imaging mode, where a fixed wavelength light source is tuned to a molecular resonance and the AFM raster scans an image, or in spectroscopy mode where the AFM is held stationary over a feature of interest and the light frequency is varied to obtain a spectrum. In either case, a strong, stable, coherent infrared source is required. Here we demonstrate the integration of a broadly tunable external cavity quantum cascade laser (ECQCL) into an s-SNOM and use it to obtain infrared spectra of microcrystals of chemicals adsorbed onto gold substrates.
Residues of the explosive compound tetryl was deposited onto gold substrates. s-SNOM experiments were performed in the 1260-1400cm-1 tuning range of the ECQCL, corresponding to the N02 symmetric stretch vibrational fingerprint region. Vibrational infrared spectra were collected on individual chemical domains with a collection area of ~500 nm2 and compared to ensemble averaged far-field reflection-absorption infrared spectroscopy (RAIRS) results.
We present experimental demonstrations using a broadly tunable external cavity quantum cascade laser (ECQCL) to
perform Reflection-Absorption InfraRed Spectroscopy (RAIRS) of thin layers and residues on surfaces. The ECQCL
compliance voltage was used to measure fluctuations in the ECQCL output power and improve the performance of the
RAIRS measurements. Absorption spectra from self-assembled monolayers of a fluorinated alkane thiol and a thiol
carboxylic acid were measured and compared with FTIR measurements. RAIRS spectra of the explosive compounds
PETN, RDX, and tetryl deposited on gold substrates were also measured. Rapid measurement times and low noise were
demonstrated, with <1E-3 absorbance noise for a 10 second measurement time.
Reliable active and passive hyperspectral imaging and detection of explosives and solid-phase chemical residue on
surfaces remains a challenge and an active area of research. Both methods rely on reference libraries for material
identification, but in many cases the reference spectra are either not available or do not sufficiently resemble the
instrumental signals of light reflected, scattered, or emitted from real-world objects. We describe a physics-based model
using the complex dielectric constant to explain what is often thought of as anomalous behavior of scattered or nonspecular
signatures encountered in active and passive sensing of explosives or chemicals on surfaces and show modeling
and experimental results for RDX.
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.
We present a study of the spectral and angular dependence of the diffuse scatter of mid-infrared (MIR) laser light from
explosives residues on surfaces. Experiments were performed using an external cavity quantum cascade laser (ECQCL)
tunable between 7 and 8 μm (1270 to 1400 cm-1) for surface illumination. A mercury cadmium telluride (MCT) detector
was used to detect backscattered spectra as a function of surface angle at a 2 meter standoff. A ferroelectric focal plane
array was used to build hyperspectral images at a 0.5 meter standoff. Residues of RDX, tetryl, and TNT were
investigated on surfaces including a painted car door for angles between zero (specular) and 50 degrees. We observe
spectral signatures of the explosives in the diffuse scattering geometry which differ significantly from those observed in
transmission geometries. Characterization of the scattered light spectra of explosives on surfaces will be essential for
understanding the performance of standoff explosives detection instruments and developing robust spectral analysis
techniques.
Using infrared hyperspectral imaging, we demonstrate microscopy of small particles of the explosives compounds RDX,
tetryl, and PETN with near diffraction-limited performance. The custom microscope apparatus includes an external
cavity quantum cascade laser illuminator scanned over its tuning range of 9.13-10.53 μm in four seconds, coupled with a
microbolometer focal plane array to record infrared transmission images. We use the hyperspectral microscopy
technique to study the infrared absorption spectra of individual explosives particles, and demonstrate sub-nanogram
detection limits.
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 experimental results on the detection of explosives residues with active hyperspectral imaging by
illumination of the target surface using an external cavity quantum cascade laser (ECQCL) and imaging using an
uncooled microbolometer camera. Explosives have rich absorption features in the molecular fingerprint region that
spans 1500 to 500 wavenumbers and is easily probed by the wavelength range of quantum cascade lasers (QCL), which
can be fabricated to emit from 3300 to 400 wavenumbers. Our laboratory-built ECQCL consists of a Fabry-Pérot laser
with anti-reflection coated front facet that is arranged in a Littman-Metcalf configuration. The ECQCL was operated
quasi-CW with a 100 kHz repetition rate, 50% duty cycle drive signal and tuning range from 1102.95 to 983.8
wavenumbers. The active hyperspectral imaging technique forms an image hypercube by recording one image for each
tuning step of the ECQCL. For the experiments reported here, each wavelength band was 2 wavenumbers wide and 60
bands of image data were acquired in 2 seconds. The resulting hyperspectral image contains the full absorption
spectrum produced by the illumination laser at each pixel in the image which can then be used to identify the explosive
type and relative quantity using the rich library of spectral identification approaches developed initially in the remote
sensing community. These techniques include spectral feature fitting, matched filtering, and mixture tuned matched
filtering. Mixtures of materials can be evaluated using linear spectral unmixing approaches and matched filtering or
mixture tuned matched filtering. We provide examples of these methods using ENVI, a commercial spectral image
processing software package.
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 demonstrate the use of a tunable external cavity quantum cascade laser system for measurement of broad absorption
features in the mid-infrared spectral region. The thermoelectrically cooled external cavity laser was tuned over a 65 cm-1range centered at 8.7 microns using stepper motor control. Pulsing the laser at a high duty cycle provided 2-4.5 mW
average output power over the tuning range, and enabled phase-sensitive detection of amplitude-modulated signals. We
used the laser system to measure the absorption spectra of Freon-125 using a Herriott cell. In addition, the absorption
spectrum of water in the laboratory air was measured. The measurements showed excellent agreement with reference
spectra, in both wavelength and amplitude. The measured scan resolution of 0.14 cm-1 is suitable for measurement of the
absorption features of complex molecules as well as simple molecules with atmospherically broadened lines. We
discuss the limits to the scan resolution due to effects of spectral chirp and mode-hops during pulsed operation.
We demonstrate the performance of a novel long-wave infrared photoacoustic laser absorbance
spectrometer for gas-phase species using an amplitude modulated (AM) quantum cascade (QC) laser and a
quartz tuning fork microphone. Photoacoustic signal was generated by focusing the output of a Fabry-Perot
QC laser operating at 8.41 μm between the legs of a quartz tuning fork which served as a transducer for the
transient acoustic pressure wave. The QC laser was modulated at the resonant frequency of the tuning fork
(32.8 kHz). This sensor was calibrated using the infrared absorber Freon-134a by performing a
simultaneous absorption measurement using a 35 cm absorption cell. The NEAS of this instrument was
determined to be 2 x 10-8 W • cm / √Hz, and the fundamental sensitivity of this technique is limited by the
noise floor of the tuning fork itself.
We demonstrate the performance of a novel long-wave infrared photoacoustic laser absorbance spectrometer for gas-phase species using an amplitude modulated (AM) quantum cascade (QC) laser and a quartz tuning fork microphone. Photoacoustic signal was generated by focusing the output of a Fabry-Perot QC laser operating at 8.41 μm between the legs of a quartz tuning fork which served as a transducer for the transient acoustic pressure wave. The QC laser was modulated at the resonant frequency of the tuning fork (32.8 kHz). This sensor was calibrated using the infrared absorber Freon-134a by performing a simultaneous absorption measurement using a 35 cm absorption cell. The NEAS of this instrument was determined to be (see equation in manuscript), and the fundamental sensitivity of this technique is limited by the noise floor of the tuning fork itself.
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