External Cavity Quantum Cascade Laser Research Articles

Spectroscopic study of plasma nitrocarburizing processes with an industrial-scale carbon active screen

The active screen plasma nitrocarburizing technology is an improvement of conventional plasma nitrocarburizing by providing a homogeneous temperature distribution within the workload and reducing soot formation. In this study, an industrial-scale active screen (AS) made of carbon-fibre-reinforced carbon serves as the cathode as well as the carbon source for the plasma-chemical processes taking place. The pulsed dc discharge was maintained at a few mbar of pressure while simultaneously being fed with a mixed gas flow of hydrogen and nitrogen ranging from 10 to 100 slh. Using in situ infrared laser absorption spectroscopy with lead salt tuneable diode lasers and external-cavity quantum cascade lasers, the temperatures and concentrations of HCN, NH3, CH4, C2H2, and CO have been monitored as a function of pressure and total gas flow. To simulate industrial treatment conditions the temperature of the sample workload in the centre of the reactor volume was kept at 773 K by varying the plasma power at the AS between 6 and 8.5 kW. The resulting spectroscopically measured temperatures in the plasma agreed well with this value. Concentrations of the various species ranged from 6 × 1013 to 1 × 1016 cm−3 with HCN being the most abundant species.

Standoff chemical plume detection in turbulent atmospheric conditions with a swept-wavelength external cavity quantum cascade laser.

Rapid and sensitive standoff measurement techniques are needed for detection of trace chemicals in outdoor plume releases, for example from industrial emissions, unintended chemical leaks or spills, burning of biomass materials, or chemical warfare attacks. Here, we present results from 235 m standoff detection of transient plumes for 5 gas-phase chemicals: Freon 152a (1,1-difluoroethane), Freon 134a (1,1,1,2-tetrafluoroethane), methanol (CH3OH), nitrous oxide (N2O), and ammonia (NH3). A swept-wavelength external cavity quantum cascade laser (ECQCL) measures infrared absorption spectra over the range 955-1195 cm-1 (8.37- 10.47 µm), from which chemical concentrations are determined via spectral fits. The fast 400 Hz scan rate of the swept-ECQCL enables measurement above the turbulence time-scales, reducing noise and allowing plume fluctuations to be measured. For high-speed plume detection, noise-equivalent column densities of 1-2 ppm*m are demonstrated with 2.5 ms time resolution, improving to 100-400 ppb*m with 100 ms averaging.

External Cavity Quantum Cascade Laser-Based Mid-Infrared Dispersion Spectroscopy for Qualitative and Quantitative Analysis of Liquid-Phase Samples.

Acquisition of classical absorption spectra of liquids in the mid-IR range with quantum cascade lasers (QCLs) is often limited in sensitivity by noise from the laser source. Alternatively, measurement of molecular dispersion (i.e., refractive index) spectra poses an experimental approach that is immune to intensity fluctuations and further offers a direct relationship between the recorded signal and the sample concentration. In this work, we present an external cavity quantum cascade laser (EC-QCL) based Mach-Zehnder interferometer setup to determine dispersion spectra of liquid samples. We present two approaches for acquisition of refractive index spectra and compare the qualitative experimental results. Furthermore, the performance for quantitative analysis is evaluated. Finally, multivariate analysis of a spectrally complex mixture comprising three different sugars is performed. The obtained figures of merit by partial least squares (PLS) regression modelling compare well with standard absorption spectroscopy, demonstrating the potential of the introduced dispersion spectroscopic method for quantitative chemical analysis.

Dual-spectroscopy technique based on quartz crystal tuning fork detector

Quartz crystal tuning fork (QCTF) as a sensitive element and valuable component has been widely used for developing mass and stress sensor, pressure and temperature sensor, optoacoustic transducer and photoelectric detector, due to its high quality factor, high stability and precision, and low power consumption. In this paper, QCTF was reported for simultaneous detection of both acoustic signal induced by photoacoustic effect (i.e. photoacoustic spectroscopy) and light signal from molecular absorption process (i.e. direct absorption spectroscopy). A broadband tunable external cavity quantum cascade laser (ECQCL) and two QCTFs with same resonant frequency was demonstrated for dual-spectroscopy technique detection of gas concentration. The results indicated that the proposed dual-spectroscopy technique can be efficiently utilized to enhance sensitivity of QCTF based gas sensors. Comparing to the common QEPAS technique, a sensitivity enhancement of 4–7 times was obtained for the proposed dual-spectroscopy technique.

Optical Extraction Efficiency for External Cavity Quantum Cascade Lasers

Optical Extraction Efficiency for External Cavity Quantum Cascade Lasers

Sensitive and interference-immune formaldehyde diagnostic for high-temperature reacting gases using two-color laser absorption near 5.6 µm

In this work we present a novel two-color differential absorption diagnostic near 5.6 μm using an external-cavity quantum-cascade-laser (EC-QCL) for formaldehyde (CH2O) detection in high-temperature reacting gases. This diagnostic utilizes the narrow mid-infrared absorption features of CH2O in the R branch of the C=O stretching band. Specifically, the online and offline colors were chosen as 1787.05 cm-1 and 1787.85 cm-1, respectively. Compared to the previous Stanford work that targeted the 3.5 μm band of CH2O, this new diagnostic achieved higher sensitivity, better species selectivity and immunity to interference absorption from other combustion gases, especially small hydrocarbons (e.g. methane). Absorption cross sections of CH2O at the two colors were measured in shock-heated 1,3,5-trioxane/argon mixtures over a wide range of temperatures and pressures: 870 K ≤ T ≤ 1800 K, 0.7 atm ≤ P ≤ 4.5 atm. These cross section measurements were further expressed as analytical functions of temperature and pressure with 2σ statistical uncertainties of ± 4.9%. Initial application of the diagnostic was demonstrated in a shock tube study of the thermal decomposition of methanol/argon mixture at 1427 K and 2.77 atm, where the measured CH2O time-history was compared with simulations from multiple combustion mechanisms. We expect this new diagnostic to be useful in future combustion studies.

Infrared Spectroscopy with a Fiber-Coupled Quantum Cascade Laser for Attenuated Total Reflection Measurements Towards Biomedical Applications.

The development of rapid and accurate biomedical laser spectroscopy systems in the mid-infrared has been enabled by the commercial availability of external-cavity quantum cascade lasers (EC-QCLs). EC-QCLs are a preferable alternative to benchtop instruments such as Fourier transform infrared spectrometers for sensor development as they are small and have high spectral power density. They also allow for the investigation of multiple analytes due to their broad tuneability and through the use of multivariate analysis. This article presents an in vitro investigation with two fiber-coupled measurement setups based on attenuated total reflection spectroscopy and direct transmission spectroscopy for sensing. A pulsed EC-QCL (1200–900 cm) was used for measurements of glucose and albumin in aqueous solutions, with lactate and urea as interferents. This analyte composition was chosen as an example of a complex aqueous solution with relevance for biomedical sensors. Glucose concentrations were determined in both setup types with root-mean-square error of cross-validation (RMSECV) of less than 20 mg/dL using partial least-squares (PLS) regression. These results demonstrate accurate analyte measurements, and are promising for further development of fiber-coupled, miniaturised in vivo sensors based on mid-infrared spectroscopy.

Jet-cooled rovibrational spectroscopy of methoxyphenols using two complementary FTIR and QCL based spectrometers.

Methoxyphenols (MPs) are a significant component of biomass burning emissions which mainly exists in our atmosphere in the gas phase where they contribute to the formation of secondary organic aerosols (SOAs). Rovibrational spectroscopy is a promising tool to monitor atmospheric MPs and infer their role in SOA formation. In this study, we bring a new perspective on the rovibrational analysis of MP isomers by taking advantage of two complementary devices combining jet-cooled environments and absorption spectroscopy: the Jet-AILES and the SPIRALES setups. Based on Q-branch frequency positions measured in the Jet-AILES Fourier-transform infrared (FTIR) spectra and guided by quantum chemistry calculations, we propose an extended vibrational and conformational analysis of the different MP isomers in their fingerprint region. Some modes such as far-IR out-of-plane -OH bending or mid-IR in-plane -CH bending allow us to assign individually all the stable conformers. Finally, using the SPIRALES setup with three different external cavity quantum cascade laser sources centered on the 930-990 cm-1 and the 1580-1690 cm-1 ranges, it was possible to proceed to the rovibrational analysis of the ν18 ring in-plane bending mode of the MP meta isomer providing a set of reliable excited state parameters, which confirms the correct assignment of two conformers. Interestingly, the observation of broad Q-branches without visible P- and R-branches in the region of the C-C ring stretching bands was interpreted as being probably due to a vibrational perturbation. These results highlight the complementarity of broadband FTIR and narrowband laser spectroscopic techniques to reveal the vibrational conformational signatures of atmospheric compounds over a large infrared spectral range.

Determining a Line Strength in the ν3 Band of the Silyl Radical Using Quantum Cascade Laser Absorption Spectroscopy.

Silane (SiH4) plasmas are widely used for the deposition of hydrogenated amorphous silicon (a-Si:H) films. Nevertheless, the chemical processes governing film deposition are still incompletely understood. Moreover, there is still no general method available to determine the absolute concentration of the silyl radical (SiH3), which is the accepted chemical precursor of a-Si:H films. In this study, a 10% silane in helium RF plasma was spectroscopically investigated between 2085 and 2175 cm-1 using an external cavity quantum cascade laser (EC-QCL) based spectrometer. This led to the identification of 4 distinct species from their absorption features: SiH4, disilane (Si2H6), SiH3, and an unassigned short-lived species. Furthermore, 17 absorption features of SiH3 were identified and unambiguously assigned. Fast spectral scanning of selected absorption features belonging to the four species in a 10 Hz pulsed RF plasma enabled the measurement and interpretation of their temporal behavior in terms of plausible chemical reactions involving silicon containing species. By quantitatively measuring the decay of the SiH3 a ← a pP4 (5) transition at 2151.3207 cm-1 after the discharge was stopped, its line strength (S) was determined to be (7.5 ± 5.5) × 10-20 cm2 cm-1 mol-1.

PH titration of β-lactoglobulin monitored by laser-based Mid-IR transmission spectroscopy coupled to chemometric analysis.

A novel external cavity-quantum cascade laser (EC-QCL)-based setup for mid-IR transmission spectroscopy in the amide I and amide II region was employed for monitoring pH-induced changes of protein secondary structure. pH titration of β-lactoglobulin revealed unfolding of the native β-sheet secondary structure occurring at basic pH. Chemometric analysis of the dynamic IR spectra was performed by multivariate curve resolution-alternating least squares (MCR-ALS). Using this approach, spectral and abundance distribution profiles of the conformational transition were obtained. A proper post-processing procedure was implemented allowing to extract information about pure protein spectra and spurious signals that may interfere in the interpretation of the system. This work demonstrates the potential and versatility of the EC-QCL-based IR transmission setup for flow-through applications, benefitting from the high available optical path length.

A fast approximation for adaptive wavelength selection for tunable infrared chemical sensors

Active mid-infrared (MIR) spectroscopy with tunable lasers is a leading technology for standoff detection of trace chemicals. Traditionally, the laser wavelength is swept to sample its tuning range uniformly without regard to how informative particular wavelengths may be. In contrast, this paper presents an adaptive technique to control the sequence of wavelengths, which the system samples in order to maximize chemical identification accuracy while minimizing measurement time and power consumption. Realtime adaptive wavelength selection is enabled by an approximation, which operates ∼40 000× faster than the full calculation. Application of this technique to synthetic data suggests that it can reduce the number of wavelengths required (and hence the measurement time) by a factor of two relative to an evenly spaced grid, with even higher gains for chemicals with weak signatures. While this paper focuses on MIR sensors using external cavity quantum cascade lasers, the technique is applicable to any system that has a tunable element and appropriate data characteristics.

Experimental-based comparison between off-axis integrated cavity output spectroscopy and multipass-assisted wavelength modulation spectroscopy at 77 µm

A mid-infrared trace gas detection system based on off-axis integrated cavity output spectroscopy (OA-ICOS) is demonstrated for accurate and sensitive detection of N2O in combination with a continuous wave external-cavity quantum cascade laser (EC-QCL) working around 7.7 µm. A 13-times improvement in signal-to-noise ratio is achieved using a re-injection mirror and a minimum detection limit of 70 ppbv in less than 10 s averaging time is achieved, which yields a noise-equivalent absorption sensitivity (NEAS) of 6×10−9 cm−1 Hz−1/2. For comparison, a compact multipass cell is deployed to measure the same absorption line of N2O using wavelength modulation spectroscopy with second harmonic detection (WMS-2f). An enhancement factor of 20 in comparison to direct absorption spectroscopy (DAS) is achieved, yielding a minimum detection limit of 15 ppbv in less than 10 s averaging and a NEAS of 1×10−9 cm−1 Hz−1/2. A comprehensive comparison between the two systems is carried out in terms of residual amplitude noise (RAM), linearity, long-term stability, detection limit, spectral fitting, reproducibility, and background variations. The proposed sensor based on OA-ICOS is potentially advantageous for trace gas sensing in outdoor applications and harsh environments due to its robustness and flexibility of alignment.

Characterization of high-explosive detonations using broadband infrared external cavity quantum cascade laser absorption spectroscopy

Infrared laser absorption spectroscopy provides a powerful tool for probing physical and chemical properties of high-explosive detonations. A broadly tunable swept-wavelength external cavity quantum cascade laser operating in the mid-wave infrared (MWIR) spectral region is used to measure transmission through explosive fireballs generated from 14 g charges of 4 different explosive types detonated in an enclosed chamber. Analysis of time-resolved transmission and emission at a 2 μs sampling rate shows the evolution of fireball infrared opacity in the first 10 ms after detonation. Broadband high-resolution absorption spectra acquired over the spectral range of 2050–2300 cm−1 (4.35–4.88 μm) at a 100 Hz rate are used to measure properties of fireball evolution over longer time scales out to 100 s. Path-integrated concentrations of combustion products CO, CO2, H2O, and N2O are measured and show evolutions over multiple time scales and significant differences between explosive types. Spectral analysis is used to characterize gas temperature and to measure broadband attenuation from absorption and scattering of particulates. Analysis of the results provides information on the MWIR optical properties, gaseous detonation/combustion products, and particulates throughout the explosive process including initial detonation, fireball expansion and cooling, and diffusive mixing in the chamber.

Comb-calibrated sub-Doppler spectroscopy with an external-cavity quantum cascade laser at 7.7 μm.

We study the frequency noise and the referencing to a near-infrared frequency comb of a widely tunable external-cavity quantum-cascade-laser that shows a relatively narrow free-running emission linewidth of 1.7 MHz. The frequency locking of the laser to the comb further narrows its linewidth to 690 kHz and enables sub-Doppler spectroscopy on an N2O transition of the ν1 band near 7.7 μm with sub-MHz resolution and absolute frequency calibration. The combined uncertainty on the measured transition center is estimated to be less than 50 kHz.

A New Quantum Cascade IR-Laser Online Detector: Chemical-Sensitive Size-Exclusion Chromatography Measurement at Unprecedented Low Levels.

Online chemically sensitive detectors for size exclusion chromatography (SEC) through coupled setups based on infrared (IR) (or NMR) spectrometers present new possibilities through unprecedented levels of polymer detail with respect to molecular weight and chemical composition. Herein, a new external cavity quantum cascade laser (EC-QCL) mid-IR spectrometer as a chemically sensitive online detector for SEC is custom-designed, built, and tested. This unique spectrometer features multiple broadly tunable EC-QCL sources, which can be operated in continuous wave and pulsed mode, accompanied with balanced liquid nitrogen cooled mercury cadmium telluride (MCT) detectors and a new custom-built transmission flow cell. Automated data analysis is done with a self-written MATLAB code. The limit of detection (LOD) is measured online, coupled with SEC chromatography, where on average, one carbonyl functionality in 530 000 g mol-1 at chromatographic conditions for SEC could be detected. It is possible to detect 0.46 µg (LOD) PMMA, which is approximately a factor of 30 lower than that reported for SEC-Fourier transform infrared.

External cavity quantum cascade laser-based QEPAS for chlorodifluoromethane spectroscopy and sensing

Detection of chlorodifluoromethane (R-22) with unresolved rotational–vibrational absorption lines was demonstrated using quartz-enhanced photoacoustic spectroscopy (QEPAS) and an external cavity quantum cascade laser (EC-QCL). A spectral region spanning 1230–1420 cm−1 was measured. A mini quartz tuning fork with high resonance frequency of ~ 75 kHz is first used to improve the QEPAS sensitivity. At a laser optical power of 2 mW, a 1σ-normalized noise-equivalent absorption of 1.9 × 10−8 cm−1 W Hz−1/2 was achieved. The analysis of a mixture of two broadband absorbers (R-22 and acetone) with overlapping absorption spectra was demonstrated. The good linearity response of the measured QEPAS signal amplitude as a function of R-22 concentration indicates the QEPAS-based sensor has a great potential for continuous and noninvasive real-time monitoring of broadband absorbers.

Implementation and characterization of a thermal infrared laser heterodyne radiometer based on a wavelength modulated local oscillator laser

This article presents the first implementation and the experimental characterization of a thermal infrared wavelength modulation laser heterodyne radiometer (WM-LHR) based on an external cavity quantum cascade laser. This novel WM-LHR system has demonstrated calibration-free operation, a superior signal to noise ratio and, more importantly, has opened the door for cost-efficient wide spectral range laser heterodyne radiometry in the near future.

High-pressure, high-temperature optical cell for mid-infrared spectroscopy

The design, characterization, and operation of a new high-temperature, high-pressure optical cell for infrared spectroscopy is presented. The optical cell uses 16 cm CaF2 window rods to penetrate the temperature gradient between the heated optical path length and the ambient conditions surrounding the system. Along the 21.3 cm optical path length, good temperature uniformity was measured with deviations being less than 2.4% of the mean absolute temperature. Further characterization of the optical cell revealed usable transmission up to ∼ 8 µm and pressure stability from vacuum to over 30 atm. Moreover, stable operation of the optical cell during simultaneous heating and pressurization to 800 K and over 30 atm was demonstrated. To illustrate the utility of the new facility, high-resolution measurements of the R-branch in the fundamental absorption band of nitric oxide near 5.2 µm were collected at several pressures (1-34 atm) and temperatures (294–802 K) using an external cavity quantum cascade laser. The measured spectra were compared with Voigt line shape simulations using the HITRAN/HITEMP databases. At high gas densities, deviations from the Voigt line shape model are observed, emphasizing the need to further study infrared molecular spectra at high temperatures and pressures.

High resolution quantitative multi-species hydrocarbon gas sensing with a cw external cavity quantum cascade laser based spectrometer in the 6–11 μm range

We report multi-species spectroscopy of hydrocarbons with a continuous wave external-cavity quantum cascade laser based spectrometer providing tunability from 6 to 11μm to measure direct absorption spectra of the first 7 alkanes and their mixtures. The gas spectra were acquired in the range from 1440 to 1480cm−1 at a reduced pressure of 50 mbar and at a temperature of 323 K. By linearization of the measured wavelengths with a custom-made highly temperature stable air spaced etalon, a high spectral accuracy of ±0.001cm−1 is achieved for the whole spectral range. The simultaneous high resolution of 0.001cm−1 yields spectra of unprecedented richness of detail for the heavier alkanes (C3–C5) and allows the discrimination of narrow spectral features for the lighter ones (C1–C2). Thereby, the measured spectra reveal the influences of collisional broadening effects among the measured species. Quantitative spectroscopic multi-species gas sensing relies on the comprehension of the extent of spectral broadening. Studying the spectral broadening in combination with highly accurate reference spectra is mandatory for highly sensitive and specific multi-species gas analyzers. The quantitative results that can be obtained with our approach are presented for an exemplary calibrated mixture of all 7 components and reveal an absolute accuracy below 0.5 vol. % for the determination of the mole fraction of each gas.

Determination of Olefin Contents in Liquid Hydrocarbons Using a Quantum Cascade Laser and a Photoacoustic Detector

The determination of olefin contents in liquid hydrocarbons by means of laser-based infrared spectroscopy was demonstrated for the first time in this work. This new technique utilizes a commercial mid-infrared external-cavity quantum cascade laser and a photoacoustic detector to monitor characteristic olefin C═C absorption bands in the ∼1640–1680 cm–1 (5.95–6.1 μm) region. It is suitable for measurements of olefin concentrations in a variety of samples, including those derived from partially upgraded bitumen. To confirm this capability, olefin concentrations in the solutions of model compounds, thermally cracked and hydrotreated naphthas, and actual olefin-rich distillate samples were measured. This investigation showed that infrared lasers create a viable alternative to the conventional NMR and chemical methods widely used to determine the olefin contents in hydrocarbon feedstocks and products.