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

Ralf Ostendorf,Frank Fuchs,André Merten,Quankui Yang,Klaus Rieblinger,Jan Jarvis,Thorsten Tybussek,Marcel Rattunde,Lorenz Butschek,Joachim Wagner,Christian Schilling,Jan Grahmann,Dusan Boskovic,Stefan Hugger

doi:10.3390/photonics3020028

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.

Highlights

In this paper we report on recent advances in external-cavity quantum cascade laser (EC-quantum cascade lasers (QCLs))technology at our institute [1,2], focusing on real-time applications for broadband mid-infrared (MIR) spectroscopy
We report on recent advances in fast scanning External-cavity quantum cascade lasers (EC-QCL) employing a custom designed and fabricated MOEMS scanning grating [1,14,24]
The following section reports on three test cases for the spectroscopic application of broadband tunable EC-QCLs in real-world scenarios

Summary

Introduction

In this paper we report on recent advances in external-cavity quantum cascade laser (EC-QCL). This mirror was inserted into the optical beam path between the collimating lens, collecting the emission from the anti-reflection coated QCL chip facet and the diffraction grating, the latter providing wavelength selective optical feedback This way scan rates of up to 5 kHz were achieved with a tuning range of up to 7 cm1 [20]. The highest tuning speed for an external cavity quantum cascade laser so far was reported by Lyakh et al [23] They used an acousto-optic modulator (AOM) to control the emission wavelength of a long-wave EC-QCL tunable from ~8.5 μm to ~9.8 μm with a switching time between any two wavelengths within that range of less than 1 μs. We report recent advances in the application of EC-QCL for spectroscopic analysis and control of chemical processes [14], as well as for imaging stand-off detection of explosives [1,15,16,17,18]

Setup of the MOEMS

Determination of the Spectral Linewidth and Wavelength Reproducibility

Case Studies for the Use of EC-QCL in Spectroscopic Sensing

Inline-Monitoring of Chemical Reactions

Initial Results