The use of a tunable midinfrared external cavity quantum cascade laser for the standoff detection of explosives at medium distances between 2 and 5 m is presented. For the collection of the diffusely backscattered light, a high-performance infrared imager was used. Illumination and wavelength tuning of the laser source was synchronized with the image acquisition, establishing a hyperspectral data cube. Sampling of the backscattered radiation from the test samples was performed in a noncooperative geometry at angles of incidence far away from specular reflection. We show sensitive detection of traces of trinitrotoluene and pentaerythritol tetranitrate on real-world materials, such as standard car paint, polyacrylics from backpacks, and jeans fabric. Concentrations corresponding to fingerprints were detected, while concepts for false alarm suppression due to cross-contaminations were presented

In this work we demonstrate imaging standoff detection of solid traces of explosives using infrared laser backscattering spectroscopy. Our system relies on active laser illumination in the 7 µm-10 µm spectral range at fully eye-safe power levels. This spectral region comprises many characteristic absorption features of common explosives, and the atmospheric transmission is sufficiently high for stand-off detection. The key component of our system is an external cavity quantum cascade laser with a tuning range of 300 cm(-1) that enables us to scan the illumination wavelength over several of the characteristic spectral features of a large number of different explosives using a single source. We employ advanced hyperspectral image analysis to obtain fully automated detection and identification of the target substances even on substrates that interfere with the fingerprint spectrum of the explosive to be detected due to their own wavelength-dependent scattering contributions to the measured backscattering spectrum. Only the pure target spectra of the explosives have to be provided to the detection routine that nevertheless accomplishes reliable background suppression without any a-priory-information about the substrate

Broadband tunable external cavity quantum cascade lasers (EC-QCL) have emerged as attractive light sources for midinfrared (MIR) "finger print" molecular spectroscopy for detection and identification of chemical compounds. Here we report on the use of EC-QCL for the spectroscopic detection of hazardous substances, using stand-off detection of explosives and sensing of hazardous substances in water as two prototypical examples. Our standoff-system allows the contactless identification of solid residues of various common explosives over distances of several meters. Furthermore, results on an EC-QCL-based setup for MIR absorption spectroscopy on liquids are presented, featuring a by a factor of ten larger single-pass optical path length of 100 μm as compared to conventional Fourier transform infrared spectroscopy instrumentations.

We investigated the beam‐steering phenomenon of quantum‐cascade lasers emitting at ∼7.8 µm wavelength, operated in pulsed mode at heat sink temperatures around room‐temperature. The lasers have a dimension of 12 µm × 1500 µm. The observed beam‐steering phenomenon has been explained by considering spatial hole burning in the population inversion of conduction electrons along the lateral direction. Rate equations have been set up to describe the lateral hole burning effect. The simultaneous oscillation of at least two lateral modes, which are a consequence of the lateral hole burning and at the same time possible cause of beam steering, has been verified by high‐resolution spectroscopy. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

We point out the difference between the two mostly used methods for calculating the effective refractive index of mid-infrared quantum cascade lasers, namely by solving the Maxwell's wave equation and by analyzing the frequency spacing of the longitudinal modes of the Fabry-Perot cavity. The effective refractive indices obtained by these methods are shown to be different, as one refers to the phase effective refractive index while the other refers to the group effective refractive index, respectively. Dispersion relationships for these two effective refractive indices are deduced for mid-infrared quantum cascade lasers, which show an increase in group refractive index and a decrease in phase refractive index with wavelength. Experiments are conducted to obtain the group effective refractive indices of quantum cascade lasers emitting at various wavelengths, and good agreement has been achieved between the experimental data and theory