Standoff detection of chemical plumes from high explosive open detonations using a swept-wavelength external cavity quantum cascade laser

Abstract

A swept-wavelength external cavity quantum cascade laser (ECQCL) is used to perform standoff detection of combustion gases in a plume generated from an outdoor high-explosive (HE) open detonation. The swept-ECQCL system was located at a standoff distance of 830 m from a 41 kg charge of LX-14 (polymer-bonded high explosive) and was used to measure the infrared transmission/absorption through the post-detonation plume as it propagated through the beam path. The swept-ECQCL was operated continuously to record broadband absorption spectra at a 200 Hz rate over a spectral range from 2050 to 2230 cm−1 (4.48–4.88 μm). Fitting of measured spectra was used to determine time-resolved column densities of CO, CO2, H2O, and N2O. Analysis of visible video imagery was used to provide timing correlations and to estimate plume dimensions, from which gas mixing ratios were estimated. Measured emission factors and modified combustion efficiency show good agreement with previously reported values.

Mid-Infrared Reflectance Spectroscopy Based on External Cavity Quantum Cascade Lasers for Mineral Characterization

“…Recent work showed improved performance of ECQCL over tuning speed covering hundreds of cm −1 within a millisecond. 30–32 Combined with fast thermoelectrically cooled mercury cadmium telluride detectors, acquisition of hundreds of spectra per second could be achieved, allowing high resolution mapping of mineral surfaces and point-counting applications where a large number of measurement points is needed on short time scales. Such application is unique to ECQCL-based MIR spectroscopy and shows where it advantageously discerns itself from conventional FT-IR instruments and capabilities.…”

Standoff Infrared Measurements of Chemical Plume Dynamics in Complex Terrain Using a Combination of Active Swept-ECQCL Laser Spectroscopy with Passive Hyperspectral Imaging

“…The swept-ECQCL standoff detection hardware is a custom-built system that has been described previously in experiments at standoff ranges up to 900 m [18,38]. In the measurements here, the swept-ECQCL beam was directed to a retroreflector located at a standoff distance of 1.5 km by expanding the output beam to a 1/e 2 diameter of ~50 mm using a negative lens with −15 mm focal length and off-axis parabola (OAP) with 230 mm focal length and 75 mm diameter.…”

“…The swept-ECQCL system architecture and operation have been described in detail previously [9,18,38,[40][41][42][43][44][45], and the reader is directed to these references for more information. Briefly, the swept-ECQCL used here included a QCL device (Thorlabs QE, Jessup, MD, USA) designed for broadband operation in the LWIR and which provided an overall tuning range of 910-1215 cm −1 (8.23-10.99 µ m).…”

“…The NECD for SF 6 and F152a is 1.0 and 2.6 ppm × m, respectively, for a 2.5 ms averaging time. Although the plumes did not show fluctuations over these short time scales, the high measurement rate provides benefits for the detection of species in flames or chemicals associated with explosive events [38,40]. For the plume measurements here, the 1 s averaging time was used to match the timescales of the plume variations, which improves the NECD to 0.2-0.5 ppm × m for SF 6 and F152a.…”

“…The second plume detection method used a custom-built swept-wavelength external cavity quantum cascade laser (swept-ECQCL) operated in a standoff configuration [18,38] and collocated with the LWIR-HSI system. The swept-ECQCL source operated in the LWIR spectral region and was directed to a retroreflector located 1.5 km away from the laser transmitter and ~100 m downwind of the plume release points, defining a line path for sensing.…”