J. Jarvis

Broadband-tunable external-cavity quantum cascade lasers for the spectroscopic detection of hazardous substances

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.

Infrared hyperspectral standoff detection of explosives

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.

Quantum cascade laser based active hyperspectral imaging for standoff detection of chemicals on surfaces

We employ active hyperspectral imaging using tunable mid-infrared (MIR) quantum cascade lasers for contactless identification of solid and liquid contaminations on surfaces. By collecting the backscattered laser radiation with a camera, a hyperspectral data cube, containing the spatially resolved spectral information of the scene is obtained. Data is analyzed using appropriate algorithms to find the target substances even on substrates with a priori unknown spectra. Eye-save standoff detection of residues of explosives and precursors over extended distances is demonstrated and the main purpose of our system. However, it can be applied to any substance with characteristic reflectance / absorbance spectrum. As an example, we present first results of monitoring food quality by distinguishing fresh and mold contaminated peanuts by their MIR backscattering spectrum.

Real-time spectroscopy enabled by external cavity QCLs with MOEMS diffraction gratings

In this contribution, we report on real-time mid-IR spectroscopy enabled by rapidly tunable External Cavity Quantum Cascade Lasers (EC-QCLs). High speed spectral scanning in a Littrow-type resonator is realized by employing a resonantly driven micro-opto-electro-mechanical-systems (MOEMS) grating as wavelength selective element. Oscillating at a frequency of 1 kHz with mechanical amplitudes of up to 10°, the MOEMS grating is able to cover the whole spectral range provided even by broad-gain QCL chips in just 500 μs. In addition to the high spectral scanning frequency, the MOEMS approach also allows for a miniaturized and rugged design of the EC-QCL. An evaluation of this laser source with regard to spectral reproducibility of consecutive scans, pulse intensity noise, and spectral resolution will be given. Furthermore, we present spectroscopic measurements in backscattering as well as in transmission geometry, demonstrating the real-time capability in different scenarios.

Broadband tunable external cavity quantum cascade lasers for standoff detection of explosives

We demonstrate contactless detection of solid residues of explosives using mid-infrared laser spectroscopy. Our detection scheme relies on active laser illumination, synchronized with the collection of the backscattered radiation by an infrared camera. The key component of the system is an external cavity quantum cascade laser with a tuning range of 300 cm-1 centered at 1220 cm-1. Residues of TNT (trinitrotoluene), PETN (pentaerythritol tetranitrate) and RDX (cyclotrimethylenetrinitramine) could be identified and discriminated against non-hazardous materials by scanning the illumination wavelength over several of the characteristic absorption features of the explosives.

Recent advances of external cavity QCLs with MOEMS diffraction gratings (Conference Presentation)

The combination of spectral broadly tunable quantum cascade laser chips in an external cavity (EC-QCL) with a rapid scanning MOEMS grating as wavelength selective element has attracted a lot of attention in recent years. Spectral tuning ranges of more than 350 cm-1 in the mid-infrared fingerprint region combined with scan frequencies of up to 1 kHz for a complete wavelength scan have enabled several new sensing applications such as contactless real time identification of chemical substances via backscattering spectroscopy. Moreover, the technological approach of a MOEMS EC-QCL allows for a dense integration of the electro optical components resulting in a footprint size for the laser source comparable to that of a matchbox. This makes the MOEMS EC-QCL especially attractive for handheld sensing systems. In this talk we present the recent advances on the MOEMS EC-QCL technology made at Fraunhofer IAF and IPMS within the European projects MIRPHAB, Aquarius and Chequers. A detailed analysis of spectral reproducibility of consecutive scans, amplitude noise, and spectral resolution in pulsed and cw operation of the laser source is shown and several showcase applications from online process control in chemical and pharmaceutical industry such as transmission measurements on liquids and gases are discussed.

Hyperspectral imaging for standoff trace detection of explosives using quantum cascade lasers (Conference Presentation)

Reliable standoff detection of traces of explosives is still a challenging task. Imaging MIR backscattering spectroscopy has been shown to be a promising technique for non-contact detection of traces of explosives on various surfaces. This technique, which is eye-safe, relies on active imaging with MIR laser illumination at various wavelengths. Recording the backscattered light with a MIR camera at each illumination wavelength, the MIR backscattering spectrum can be extracted from the three-dimensional data set recorded for each point within the laser illuminated area. Applying appropriate image analysis algorithms to this hyper-spectral data set, chemically sensitive and selective images of the surface of almost any object can be generated. This way, residues of explosives can be clearly identified on the basis of characteristic finger print backscattering spectra and separated from the corresponding spectra of the underlying material. To achieve a high selectivity, a large spectral coverage is a key requirement. Using a MIR EC-QCL with a tuning range from 7.5 μm to 9.5 μm, different explosives such as TNT, PETN and RDX residing on different background materials, such as painted metal sheets, cloth and polyamide, could be clearly detected and identified. For short stand-off detection distances (<3 m), residues of explosives at an amount of just a few 10 μg, i .e. traces corresponding to a single fingerprint, could be detected. For larger concentration of explosives, stand-off detection over distances of up to 20 m has already been demonstrated. During the European FP7 projects EMPHASIS and HYPERION several field tests were performed at the test site of FOI in Sweden. During these tests realistic scenarios were established comprising test detonations of IEDs. We could demonstrate the potential of QCL-based imaging backscattering spectroscopy for the detection of trace amounts of hazardous substances in such scenarios.

Imaging standoff trace detection of explosives using IR-laser based backscattering

We perform active hyperspectral imaging using tunable mid-infrared (MIR) quantum cascade lasers for contactless identification of solid and liquid contaminations on surfaces. By collecting the backscattered laser radiation with a camera, a hyperspectral data cube, containing the spatially resolved spectral information of the scene is obtained. Data is analyzed using appropriate algorithms to find the target substances even on substrates with a priori unknown spectra. Eye-save standoff detection of residues of explosives and precursors over extended distances is demonstrated and the main purpose of our system. Using a MIR EC-QCL with a tuning range from 7.5 μm to 10 μm, detection of a large variety of explosives, e.g. TNT, PETN and RDX and precursor materials such as Ammonium Nitrate could be demonstrated. In a real world scenario stand-off detection over distances of up to 20 m could be successfully performed. This includes measurements in a post blast scenario demonstrating the potential of the technique for forensic investigations.

Hyperspectral image analysis for standoff trace detection using IR laser spectroscopy

In the recent past infrared laser backscattering spectroscopy using Quantum Cascade Lasers (QCL) emitting in the molecular fingerprint region between 7.5 μm and 10 μm proved a highly promising approach for stand-off detection of dangerous substances. In this work we present an active illumination hyperspectral image sensor, utilizing QCLs as spectral selective illumination sources. A high performance Mercury Cadmium Telluride (MCT) imager is used for collection of the diffusely backscattered light. Well known target detection algorithms like the Adaptive Matched Subspace Detector and the Adaptive Coherent Estimator are used to detect pixel vectors in the recorded hyperspectral image that contain traces of explosive substances like PETN, RDX or TNT. In addition we present an extension of the backscattering spectroscopy technique towards real-time detection using a MOEMS EC-QCL.

Standoff trace detection of explosives with Infrared Hyperspectral Imagery

In this work we present a hyperspectral image sensor based on MIR-laser backscattering spectroscopy for contactless detection of explosive substance traces. The spectroscopy system comprises a tunable Quantum Cascade Laser (QCL) with a tuning range of 7.5 μm to 9.5 μm as an illumination source and a high performance MCT camera for collecting the backscattered light. The resulting measurement data forms a hyperspectral image, where each pixel vector contains the backscattering spectrum of a specific location in the scene. The hyperspectral image data is analyzed for traces of target substances using a state of the art target detection algorithm (the Adaptive Matched Subspace Detector) together with an appropriate background extraction method. The technique is eye-safe and allows imaging detection of a large variety of explosive substances including PETN, RDX, TNT and Ammonium Nitrate. For short stand-off detection distances (<3 m), residues of explosives at an amount of just a few 10 μg, i.e. traces corresponding to a single fingerprint, could be detected. For larger concentration of explosives, stand-off detection over distances of up to 20 m has already been demonstrated.