Recent Advances and Applications of External Cavity-QCLs towards Hyperspectral Imaging for Standoff Detection and Real-Time Spectroscopic Sensing of Chemicals

Abstract

External-cavity quantum cascade lasers (EC-QCL) are now established as versatile wavelength-tunable light sources for analytical spectroscopy in the mid-infrared (MIR) spectral range. We report on the realization of rapid broadband spectral tuning with kHz scan rates by combining a QCL chip with a broad gain spectrum and a resonantly driven micro-opto-electro-mechanical (MOEMS) scanner with an integrated diffraction grating in Littrow configuration. The capability for real-time spectroscopic sensing based on MOEMS EC-QCLs is demonstrated by transmission measurements performed on polystyrene reference absorber sheets, as well as on hazardous substances, such as explosives. Furthermore, different applications for the EC-QCL technology in spectroscopic sensing are presented. These include the fields of process analysis with on-or even inline capability and imaging backscattering spectroscopy for contactless identification of solid and liquid contaminations on surfaces. Recent progress in trace detection of explosives and related precursors in relevant environments as well as advances in food quality monitoring by discriminating fresh and mold contaminated peanuts based on their MIR backscattering spectrum is shown.Keywords: quantum cascade lasers; external cavity quantum cascade lasers; MOEMS grating; quantum cascade laser based spectroscopy; imaging laser backscattering spectroscopy; inline spectroscopic analysis

Microoptoelectromechanical systems-based external cavity quantum cascade lasers for real-time spectroscopy

“…In pulsed operation, the typical spectral width 20 of its emission lies between 1 and 2 cm −1 . Changing to a cw-capable laser [chip #3, see Fig.…”

“…With a tripod-mounted system with a large-aperture collection optics, small amounts of explosives and related precursors could be detected over distances of about 20 m with spectral scanning times on the order of 10 to 30 s. 20 Here, we present our first results obtained with a QCL-based handheld scanner designed to detect a wide range of substances on arbitrary surfaces over shorter distances of about 0.5 to 2 m. These developments are driven by the need of first responders for portable and simple-to-use detection hardware for fast identification of suspicious, potentially hazardous substances. Importantly, the technique works at eye-safe power levels.…”

“…In this paper, we report on recent results obtained with our fast scanning EC-QCL system, [19][20][21] employing a MOEMS grating optimized specifically for broad spectral tuning in the mid-IR. The MOEMS grating is resonantly driven at a rate of 1 kHz.…”

Hyperspectral data acquisition and analysis in imaging and real-time active MIR backscattering spectroscopy

“…However, as the hyperspectral image bands are sequentially generated, a relative motion of the sensor and scene could potentially induce non-linear spectral mixing of adjacent materials in the resulting data set. In [10] a fast scanning single-element detector variant of the same principle is proposed that measures single spectra at a timescale of 1 ms and the required spatial information is obtained sequentially. It is shown that the measurement speed is sufficient for acquisition of hyperspectral observations in dynamic scenes (i.e.…”

High performance direct absorption spectroscopy of pure and binary mixture hydrocarbon gases in the 6–11 $$\upmu$$ μ m range

“…The development of quantum cascade lasers (QCL) and interband cascade lasers (ICLs) provides laser emission in the MIR and opened the important chemical fingerprint region to this technique [4][5][6][7][8]. In addition, external cavity quantum cascade laser (EC-QCL)-based spectroscopy proved to be suitable for highly resolved and wide-range spectroscopic sensing of a multitude of gases [9][10][11][12][13][14].…”