Rapid and Sensitive Quantification of Isotopic Mixtures Using a Rapidly-Swept External Cavity Quantum Cascade Laser

Long-wave infrared pulsed external-cavity QCL spectrometer using a hollow waveguide gas cell

A spectrometer built using an external cavity pulsed quantum cascade laser is described. The spectrometer has a tuning range from 10 - 13 µm (1,000 - 769 cm-1) and is designed to target volatile organic compounds (VOCs) which often exhibit water-free molecular absorption within the region. The spectrometer utilizes a hollow silica waveguide gas cell which has an internal volume of a few millilitres, a fast response time (∼1 s), and is advantageous when only low sample volumes, similar to the cell volume, are available. Propane is used as a test gas because it is easy to handle, and its spectral profile is comparable to VOCs of interest. Its absorption in the region is primarily within the ν21 band which spans from 10.55 - 11.16 µm (948 - 896 cm-1). Spectral measurements at a range of concentrations show good linearity and an Allan deviation of absorbance values recorded over a 100-minute period indicates a minimum detectable absorbance of 3.5×10-5 at an integration time of 75 s.

In-situ measurement of pyrolysis and combustion gases from biomass burning using swept wavelength external cavity quantum cascade lasers

Broadband high-speed absorption spectroscopy using swept-wavelength external cavity quantum cascade lasers (ECQCLs) is applied to measure multiple pyrolysis and combustion gases in biomass burning experiments. Two broadly-tunable swept-ECQCL systems were used, with the first tuned over a range of 2089-2262 cm^-1 (4.42-4.79 µm) to measure spectra of CO₂, H₂O, and CO. The second was tuned over a range of 920-1150 cm^-1 (8.70-10.9 µm) to measure spectra of ammonia (NH₃), ethene (C₂H₄), and methanol (MeOH). Absorption spectra were measured continuously at a 100 Hz rate throughout the burn process, including inhomogeneous flame regions, and analyzed to determine time-resolved gas concentrations and temperature. The results provide in-situ, dynamic information regarding gas-phase species as they are generated, close to the biomass fuel source.

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.

Fast Swept-Wavelength, Low Threshold-Current, Continuous-Wave External Cavity Quantum Cascade Laser

We present a low threshold-current and fast wavelength-tuning external cavity quantum cascade laser (EC-QCL) using a scanning galvanometer in the Littman-Metcalf cavity geometry. The EC-QCL could repeatedly swept at 100 Hz over its full tuning range of about 290 nm (2105 cm^-1 to 2240 cm^-1), providing a scan rate of 59.3 μm s^-1. The continuous-wave (CW) threshold current of the EC-QCL was as low as 250 mA and the maximum output power was 20.8 mW at 400 mA for a 3-mm-long QCL gain chip. With a sawtooth wave modulation, a scan resolution of < 0.2 cm^-1 can be achieved within the tuning range. The low power-consumption and fast swept-wavelength EC-QCL will be beneficial to many applications.

Quantitative isotopic measurements of gas-phase alcohol mixtures using a broadly tunable swept external cavity quantum cascade laser

Isotopic quantification of gas-phase mixtures is performed using a swept external cavity quantum cascade laser and broadband infrared spectral analysis.