Photodetectors for the non-visible region of the electromagnetic spectrum are vital for security, defense and space as well as industrial and scientific applications. The research activities at Fraunhofer IAF contribute to Europe’s non-dependence on critical components and support the European strategy for critical space technologies. A broad range of III-V material systems is developed to address the spectral region adjacent to the visible regime. For the ultraviolet (UV) spectral region, AlGaN is the material of choice with an adjustable bandgap between 3.4 and 6.0 eV, depending on the Al content, addressing the wavelength regime between 365 to 210 nm. The short-wavelength infrared (SWIR) region from 0.9 up to 3.0 µm is covered by two approaches: Lattice matched InGaAs absorber material on InP substrates for a cut-off wavelength at 1.7 µm and InGaAsSb lattice matched on GaSb substrates for 1.7 up to 3.0 µm. Through the choice of appropriate layer thickness, InAs/(In,Ga)(As,Sb) type-II superlattices (T2SLs) can be tailored to cover the wavelength range from mid- to long- up to very-long-wavelength infrared (MWIR, LWIR, VLWIR) in the spectrum of 3-15 µm.
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.
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.
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.
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.
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.
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 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.
We present standoff detection of various explosives by backscattering spectroscopy, using a sensing system based on
mid-IR external-cavity quantum cascade lasers (EC-QCL) with a broad tunable range of about 300 cm-1. Traces of TNT
(trinitrotoluene), PETN (pentaerythritol tetranitrate) and RDX (cyclotrimethylenetrinitramine) as well as different nonhazardous
substances were investigated by illuminating them with the EC-QC laser and collecting the diffusely
backscattered light. Tuning the EC-QCL across the characteristic absorption spectra enables us to detect and identify the
explosives against a background of non-hazardous materials.
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.
Different approaches to power scaling of 4.5- to 5-µm emitting quantum cascade (QC) lasers by multiemitter beam combining are investigated. Spectral beam combining of linear arrays of QC lasers consisting of several individual emitters located side by side is demonstrated as a first variant, using an external cavity equipped with a diffraction grating and a partially transmitting output mirror providing wavelength-selective feedback to each emitter. In this way, spectral beam combining of up to eight individual QC lasers is achieved with an optical coupling efficiency of 60% for an array of six emitters. The resulting beam quality (M2 < 2 for both fast and slow axes) is close to that observed for single emitters. As a second approach, a linear array of QC lasers is coupled to a custom-made array of silicon microlenses positioned in front of the output facets of the QC lasers. This technique produces a set of closely spaced parallel output beams, strongly overlapping in the far field, without introducing any coupling losses. The resulting beam divergence is given by the aperture size of the microlenses, which is limited by the center-to-center spacing of the QC lasers (500 µm in our case).
We report large beam steering effects, observed in the far-field pattern of InP-based mid-infrared quantum-cascade lasers
along the slow axis. Changing the temperature by a few degrees around room temperature or varying the drive current
strongly affects the lateral direction of the output beam. The position of maximum intensity in the far-field-distribution
changes by more than 20°. This beam steering effect is correlated to changes in the lateral mode distribution, as revealed by
time-resolved spectroscopy of the lasing spectrum.
In this contribution we present the results of an imaging stand-off detection system based on a mid-IR external-cavity
quantum cascade laser (EC-QCL) with a broad tunable range of 200 cm-1. Traces of TNT (trinitrotoluene) and PETN
(pentaerythritol tetranitrate) as well as various non-hazardous substances such as flour or skin cream on different
substrate-materials were investigated by illuminating them with the EC-QC laser and collecting the diffusely
backscattered light. By tuning the EC-QCL across the significant absorption spectra we were able to detect the
explosives
Results on the detection of traces of trinitrotoluene (TNT) on different substrate-materials like Aluminum and
standard car paint are presented. We investigated different samples with a movable imaging standoff detection
system at angles of incidence far away from specular reflection. The samples were illuminated with a tunable
mid-infrared external-cavity quantum cascade laser. For collection of the diffusely backscattered light a highperformance
infrared imager was used. Trace concentrations of TNT corresponding to fingerprints on realworld-
substrates were detected, while false alarms of cross-contaminations were successfully suppressed.
An overview of quantum cascade detector technology for the near- and mid-infrared wavelength range will be given.
Thanks to photovoltaic instead of photoconductive operation, quantum cascade detectors offer great opportunities in
terms of detection speed, room temperature operation, and detectivity. Besides some crucial issues dealing with
fabrication and general characteristics, some possibilities for performance improvement will also be briefly presented. In
a theory section, some basic considerations adopted from photoconductive detectors confirm the necessity of various
trade-offs for the optimization of such devices. Nevertheless, we will show several possible measures to push the key
performance figures of these detectors closer to their physical and technological limits.
We report on the concept, realization and performance data of infrared semiconductor laser modules serving as compact
and robust laser sources for a Directed Infrared Countermeasures (DIRCM) system. While the 2-2.5 μm atmospheric
transmission window is covered by a GaSb-based optically pumped semiconductor disk laser (OPSDL), delivering a
continuous-wave (cw) or temporally modulated output of ≥ 1 W with a high beam quality (M2 < 3), an external cavity
(EC) quantum cascade (QC) laser module is used to cover the 4.5-5 μm spectral range. The EC-QC laser concept allows
efficient spectral beam combining of the output of several QC laser located side-by-side on the same semiconductor chip,
while preserving the high-quality output beam of a single emitter. Both the OPSDL and the EC-QC laser have been integrated
into rugged laser modules, comprising also all necessary power supply and control electronics, ready for use in
field trials.
We present experimental results on a Quantum cascade laser (QC laser) embedded in an external cavity. These results were obtained with a broadly tunable laser exceeding 80 cm-1 covering a characteristic absorption band of trinitrotoluene
(TNT). By combining the laser source with a high performance IR imager a stand-off detection setup based on multi-
spectral MIR backscattering spectroscopy has been realized. With this technique TNT surface-contaminations of as low
as 10 μg/cm2 could be detected on surfaces such as an aluminum-sheet and standard car paint. The contrast of the
detection technique depends on the reflectance of the surface. A surface leading to mirror-like reflectance of the IR laser radiation leads to absorbance-like signatures of the TNT contamination, while surfaces showing high absorbance of the laser light may induce a contrast-reversal in the resulting image of the TNT coverage. This effect can be explained by a theoretical model for thin film coated substrates taking into account differences in the reflectance. Limitations and
further work needed to explore the full potential of the IR backscattering technique are also discussed.
In this contribution, we demonstrate that spectral beam combining in an external cavity (EC), a technique which has been
applied previously to shorter wavelength diode laser bars [1], is also applicable to mid-infrared QC lasers. Within this
concept, the output of multiple emitters from a 4.6 μm emitting QC laser chip is combined in a single, collinear beam.
The average power of an EC-QC laser module realized that way surpasses the output of a corresponding single emitter
by more than a factor of 4. Furthermore, the EC-concept allows a certain degree of wavelength tuning during operation.
The EC, consisting of a collimating lens, a grating and a partially reflecting outcoupling mirror, forces each laser to emit
at a unique wavelength defined by its offset relative to the main optical axis. The EC approach further ensures the
collinear directional and spatial overlap of the individual QC laser output beams forming a single combined output beam.
We report on the development and characteristics of infrared semiconductor lasers as compact and robust light sources
for Directed Infrared Countermeasures (DIRCM). The short-wavelength side of the 2-5 μm wavelength band of interest
can be covered by GaSb-based optically pumped semiconductor disk lasers (OPSDLs), delivering a continuous-wave
(cw) or temporally modulated multiple-Watt output with a high beam quality (M2<3). For the 3.7-5 μm wavelength
range InP-based quantum cascade (QC) lasers are the best suited semiconductor laser source, delivering several hundreds
of mW of average output power in a nearly diffraction limited output beam (M2<2). Further up-scaling of the output
power can be achieved for OPSDLs by intra-cavity coupling of several semiconductor chips as gain elements in a
multiple-disk cavity arrangement. For a 2.3 µm emitting dual-disk OPSDL, a doubling of the maximum roomtemperature
output power compared to that of a comparable single-chip OPSDL has been demonstrated. For QC lasers
power scaling by beam-quality-preserving beam combining has been demonstrated via polarization coupling of the
output beams of two individual QC lasers, yielding a coupling efficiency of 82%.
We report on the experimental study of the structural, electronic and thermal properties of state-of-art Sb-based quantum-cascade lasers (QCLs) operating in the range 4.3-4.9 µm. This information has been obtained by investigating the active region band-to-band photoluminescence signals, detected by means of an GaInAs-array detector. This technique allowed to probe the spatial distribution of conduction electrons as a function of the applied voltage and to correlate the quantum design of devices with their thermal performance. We demonstrate that electron transport in QCLs based on Sb-ternary barriers may be insufficient, thus affecting the tunneling of electrons and the electronic recycling and cascading scheme. Finally, we present the first measurement of the electronic and lattice temperatures and the electron-lattice coupling in Sb-based QCLs based on a quaternary-alloy. We extracted the thermal resistance (RL = 8.9 K/W) and the electrical power dependence of the electronic temperature (Re = 11.7 K/W) of Ga0.47In0.53As/Al0.62Ga0.38As1-xSbx structures operating at 4.9 µm, in the lattice temperature range 50 K - 80 K. The corresponding electron-lattice coupling constant ( = 10.8 Kcm2/kA) reflects the reduction of the electron-leakage channels associated with the use of a high conduction band-offset.
We report on the experimental study of the electronic and thermal properties in state of art Sb-based quantum-cascade lasers (QCLs) operating in the range 4.3-4.9 &mgr;m. This information has been obtained by investigating the band-to-band photoluminescence signals, detected by means of an InGaAs-array detector. This technique allowed to probe the spatial distribution of conduction electrons as a function of the applied voltage and to correlate the quantum design of devices with their thermal performance. We demonstrate that electron transport in these structures may be insufficient, thus affecting the tunneling of electrons and the electronic recycling and cascading scheme. Finally, we present the first measurement of the electronic and lattice temperatures and of the electron-lattice coupling in Sb-based QCLs based on a
quaternary-alloy. We extracted the thermal resistance (RL = 9.6 K/W) and the electrical power dependence of the
electronic temperature (Re = 12.5 K/W) of Ga0.47In0.53As/Al0.62Ga0.38As1-xSbx structures operating at 4.9 &mgr;m, in the lattice temperature range 60 K - 90 K. The corresponding electron-lattice coupling &agr;= 9.5 Kcm2/kA) reflects the efficient electronic cooling via optical phonon emission. The experimental normalized thermal resistance RL* = 3.9 Kxcm/W
demonstrates the beneficial use of quaternary thicker barriers in terms of device thermal management.
We present a study of the spectral characteristics of Fabry-Perot quantum cascade lasers in pulsed mode operation applying a time resolution of 3 ns in combination with a high spectral resolution of 0.02 cm-1. With this technique the laser spectra were investigated applying pulse lengths ranging between 100 ns and 20 μs and duty cycles between 0.01% and 10%. Depending on the current density and operation temperature, the spectra exhibit complex line patterns, which indicate mode competition caused by gain saturation effects.
We present pulsed operation of index-coupled distributed feedback quantum cascade lasers based on the GaInAs/AlInAs/InP materials system emitting at a wavelength around 5.4 μm. The emission is single mode in the entire investigated temperature range between 240K and 350K with a side mode suppression ratio larger than 27 dB. These devices are employed in a fast gas detection experiment for the quantitative detection of nitric oxide. With the present measurement system minimum noise equivalent concentrations between 16.7 ppbv and 23.3 ppbv are obtained, corresponding to minimum detectable optical densities between 4.7•10-5 and 6.5•10-5.
Trap centers and minority carrier lifetimes are investigated in InAs/(GaIn)Sb superlattices used for photodetectors in the far-infrared wavelength range. In our InAs/(GaIn)Sb superlattice photodiodes, trap centers located at an energy level of ~1/3 band gap below the effective conduction band edge could be identified by simulating the current-voltage characteristics of the diodes. The simulation includes diffusion currents, generation-recombination contributions, band-to-band coherent tunneling, and trap assisted tunneling. By including the contributions due to trap-assisted tunneling, excellent reproduction of the current voltage curves is possible for diodes with cut-off wavelength in the whole 8-32 μm spectral range at temperatures between 140 K and 25 K. The model is supported by the observation of defect-related optical transitions at ~2/3 of the band-to-band energy in the spectra of the low temperature electroluminescence of the devices. With the combination of Hall- and photoconductivity measurements, minority carrier lifetimes are extracted as a dependence of temperature and carrier density.
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