In spite of increasing research efforts in the field of stand-off detection, the chemical identification of hazardous substances such as explosives over safe distances of several meters is still a challenging task. Electromagnetic radiation, particularly in the mid-infrared (MIR) wavelength region, is well suited to tackle this challenge, as characteristic, finger-print like molecular absorption lines lie in that spectral range. It is well known that laboratory-based MIR finger-print spectroscopy, using nowadays mostly Fourier-Transform IR (FTIR) spectrometers, allows the identification of almost any organic or inorganic chemical compound. FTIR spectrometers, coupled to a MIR focal plane array (FPA) camera rather than a single element detector, are now in use also for passive hyperspectral MIR imaging, e.g. allowing for spatially resolved detection of plumes of gases. Switching from passive to active hyperspectral imaging further increases the wealth of information to be gained in such measurements. This holds in particular if chemical compounds are present as residual surface contaminations of solid substances on objects made from other materials. For these purposes, such as the detection and identification of residues of explosives e.g. on auto body parts, imaging MIR backscattering spectroscopy using a spectrally tuneable laser source for active illumination has been proven to be a powerful technique.

While in previous works tuneable external cavity quantum cascade lasers (EC-QCL) were used for wavelength selective illumination of the object under investigation, in the present article by Goyal et al. a multi-wavelength QCL array, comprising 15 individually addressable QCLs, was employed for that purpose. The QCL array allows inherently faster wavelength scans by just electronically addressing different lasers, compared to classical EC-QCLs utilizing moveable mechanical parts for wavelength tuning. A diffraction-grating-based wavelength beam combiner was used in the present work to direct the output beams of the individual tapered QCLs, featuring integrated DBR sections for wavelength stabilization, into a single output beam. The QCLs were operated in low repetition rate (5 kHz) short pulse (200 ns) mode with a peak power in the 1-2 W range. On the detection side a digital-pixel FPA camera was employed, providing on-chip time-gating and background-subtraction capabilities. Using this combination of multi-wavelength QCL array and digital-pixel FPA camera, active hyperspectral imaging over a distance of 5 m was demonstrated, exploiting the diffusively backscattered laser radiation from a thin layer of diethyl phthalate deposited on a gold coated substrate. Furthermore, the high-speed capabilities of the above active imaging system was demonstrated using a mixture of sand and KClO3 particles, moving with respect to the sensing laser beam at speeds of up to 10 m/s. A clear differential image was generated using just two strategically selected laser wavelengths for sequential illumination.

The drawbacks of the multi-wavelength QCL array based sensing system in its present implementation are the limited wavelength range (here 9.21-9.78 µm) and the limited number of different wavelength channels (here 15). The latter drawback was actually compensated by applying a bias current to the DBR sections of the QCL, allowing a fine tuning of each lasing wavelength by Δλ/ λ of 0.3 %. Many real-world applications require a wider spectral coverage to achieve sufficient sensitivity and selectivity, in particular if the target substance is mixed with other materials exhibiting similar finger-print spectra. State-of-the-art EC-QCLs can provide in practical sensing systems already a spectral coverage of Δλ/ λ >25%, with a maximum coverage of Δλ/ λ ~40% demonstrated in the lab. It can be expected that multi-wavelength QCL array technology will rapidly catch up with this development. On the other hand, EC-QCL employing optical MEMS components as tuneable wavelength selective elements will become available soon, thus removing the two major drawbacks of EC-QCL technology, which are a limited tuning speed and the use of moveable mechanical parts.

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