A standoff chemical detection system is being developed to detect and identify a wide range of trace chemicals on a variety of natural and artificial surfaces. The system is based on active mid-infrared (MIR) hyperspectral imaging in which the target surface is illuminated using miniature, rapidly tunable, external-cavity quantum cascade lasers (ECQCLs). These lasers are tuned across the wavelength range of 7.7 – 11.8 μm while a HgCdTe camera captures images of the reflected light. Hypercubes with 128x128 pixels and more than 130 wavelengths are captured within 0.1 s. By operating the camera in sub-window mode, hypercubes with 16x96 pixels and 138 frames are captured in only 14 ms. To the best of our knowledge, these represent the world’s fastest acquisition of active MIR hypercubes. Raster-scanning of the laser beam is used to scan large regions. In this talk, we will present results for detecting traces of solid chemicals (with loadings on the order of 100 μg) on natural outdoor surfaces such as roofing shingles, concrete, sand, and asphalt at a standoff distance of 5 m. The measured spectra are found to correlate very well with those of reference measurements made of pure chemicals after accounting for the substrate reflectance.
Results are presented for the detection of trace explosive residues on real-world surfaces using active mid-infrared (MIR) hyperspectral imaging. The target surface is illuminated using miniature, rapidly tunable, external-cavity quantum cascade lasers (EC-QCLs) and the reflected light is imaged using a HgCdTe camera with a spatial resolution of 70 μm. Hypercubes with 128x128 pixels are captured with more than 256 wavelengths that span 7.7 – 11.8 μm. The samples consisted of PETN residues which were applied to keyboard keys at various levels of chemical loading. We estimate a limit of detection of less than 6 ng per pixel for the as-deposited chemical. The explosive residue remains detectable by HSI even after wiping the surface several times using isopropyl alcohol. Simple signature models for solid particles (i.e., Mie scattering) and thin-films account for the many of the spectral features observed in the chemical signatures.
Algorithms for standoff detection and estimation of trace chemicals in hyperspectral images in the IR band are a key component for a variety of applications relevant to law-enforcement and the intelligence communities. Performance of these methods is impacted by the spectral signature variability due to presence of contaminants, surface roughness, nonlinear dependence on abundances as well as operational limitations on the compute platforms. In this work we provide a comparative performance and complexity analysis of several classes of algorithms as a function of noise levels, error distribution, scene complexity, and spatial degrees of freedom. The algorithm classes we analyze and test include adaptive cosine estimator (ACE and modifications to it), compressive/sparse methods, Bayesian estimation, and machine learning. We explicitly call out the conditions under which each algorithm class is optimal or near optimal as well as their built-in limitations and failure modes.
Laser-based, mid-infrared (MIR) hyperspectral imaging (HSI) has the potential to detect a wide range of trace chemicals on a variety of surfaces under standoff conditions. The major challenge of MIR reflection spectroscopy is that the reflection signatures for surface chemicals can be complex and exhibit significant spectral variability. This paper describes a MIR Hyperspectral Simulator that is being developed to model the reflectance signatures from surfaces including the effects of speckle and other sources of spectral variability. Simulated hypercubes will be compared with experiments.
We report on a standoff chemical detection system using widely tunable external-cavity quantum cascade lasers (ECQCLs) to illuminate target surfaces in the mid infrared (λ = 7.4 – 10.5 μm). Hyperspectral images (hypercubes) are acquired by synchronously operating the EC-QCLs with a LN<sub>2</sub>-cooled HgCdTe camera. The use of rapidly tunable lasers and a high-frame-rate camera enables the capture of hypercubes with 128 x 128 pixels and >100 wavelengths in <0.1 s. Furthermore, raster scanning of the laser illumination allowed imaging of a 100-cm<sup>2</sup> area at 5-m standoff. Raw hypercubes are post-processed to generate a hypercube that represents the surface reflectance relative to that of a diffuse reflectance standard. Results will be shown for liquids (e.g., silicone oil) and solid particles (e.g., caffeine, acetaminophen) on a variety of surfaces (e.g., aluminum, plastic, glass). Signature spectra are obtained for particulate loadings of RDX on glass of <1 μg/cm<sup>2</sup>.
Standoff detection and identification of chemical threats has been the "holy grail" of detection instruments. The advantages of such capability are well understood, since it allows detection of the chemical threats without contact, eliminating possible operator and equipment contamination and the need for subsequent decontamination of both. In the case of explosives detection, standoff detection might enable detection of the threat at safe distances outside the blast zone. A natural extension of this capability would be to also detect and identify biological threats in a standoff mode and there are ongoing efforts to demonstrate such capability.
Widely tunable quantum cascade lasers (QCLs) spanning the long-wave infrared (LWIR) atmospheric transmission window and an HgCdTe detector were incorporated into a transceiver having a 50-mm-diameter transmit/receive aperture. The transceiver was used in combination with a 50-mm-diameter hollow retro-reflector for the open-path detection of chemical clouds. Two rapidly tunable external-cavity QCLs spanned the wavelength range of 7.5 to 12.8 m. Open-path transmission measurements were made over round-trip path-lengths of up to 562 meters. Freon-132a and other gases were sprayed into the beam path and the concentration-length (CL) product was measured as a function of time. The system exhibited a noise-equivalent concentration (NEC) of 3 ppb for Freon-132a given a round-trip path of 310 meters. Algorithms based on correlation methods were used to both identify the gases and determine their CLproducts as a function of time.
Block MEMS/Engineering develops mid-infrared spectroscopy systems based on both Fourier transform infrared (FTIR)
spectrometers and quantum cascade lasers (QCLs). Our recently developed miniaturized external-cavity QCLs are
widely tunable over a spectral range of >250 cm<sup>-1</sup> and tuning can be accomplished at rates of <25 cm<sup>-1</sup> per millisecond. This enables high-speed mid-infrared spectroscopy of gases and surface contaminants for a variety of military and
commercial applications. This paper provides an overview of our FTIR and QCL systems and their defense-related
We investigated the signature phenomenology of long-wave infrared (LWIR) reflectance of contaminated surfaces using
a quantum-cascade laser (QCL) that tunes from λ = 9.1 to 9.8 μm and a HgCdTe focal-plane-array (FPA) with custom
read-out integrated circuit (ROIC). A liquid chemical, diethyl phthalate (DEP), was applied to a variety of substrates
such as diffusely reflecting gold, concrete, asphalt, and sand. Multispectral image-cubes of the scattered radiation were
generated over 81 wavelengths in steps of 1 cm<sup>-1</sup> at standoff distances ranging from 0.1 to 5 meters. For idealized
substrates such as diffusely reflecting gold, the experimentally measured signatures are in good agreement with
theoretical calculations. Clear signatures were also obtained for contaminated concrete, asphalt, and sand. These
measurements demonstrate the potential of this technique for detecting and classifying chemicals on native outdoor
Wavelength beam combining was used to co-propagate beams from 28 elements in a linear array of distributedfeedback
quantum cascade lasers (DFB-QCLs). The overlap of the beams in the far-field is improved using
wavelength beam combining; the beam-quality product of the array, defined as the product of near-field spot
size and far-field divergence for the entire array, was improved by a factor of 21. We measured the absorption
spectrum of isopropanol at a distance of 6 m from the laser arrays, demonstrating the efficacy of wavelength
beam combined DFB-QCL arrays for remote sensing.
We report on a novel geometry for electrically driven semiconductor lasers called the GRISSL. The laser cavity is
formed between two mirrors that are external to the semiconductor chip and the laser beam intercepts the quantum well
(QW) gain region at a grazing angle-of-incidence. In this first demonstration of the GRISSL, the laser structure was
grown on an n-type GaAs substrate and the gain region comprises three InGaAs QWs. The external cavity consists of a
pair of lenses, a flat high-reflectivity mirror, and a flat R = 70% output coupler. Lasers emit a power of 30 mW CW in a
single-mode, 35-μm-diameter beam at λ ~ 0.98 μm. Under pulsed conditions (1 μsec, 1 kHz), a peak output power of
0.37 W was measured. The beam is single-mode near threshold but becomes multi-mode at the maximum drive
currents. The slope efficiency of these devices is about 10 times lower than the design value of 1 W/A. This
discrepancy can be accounted for by higher than anticipated losses in the substrate and a poor overlap of the laser beam
with the pumped region. Methods for overcoming both of these factors to regain a high wall-plug efficiency are
We have been developing a high power, high brightness semiconductor diode laser concept, the Slab-Coupled
Optical Waveguide Laser (SCOWL). This laser concept is based upon slab coupling, in which a large, multimode
waveguide is converted to a large, single mode waveguide by means of slab coupling of the higher order waveguide
modes. SCOWL devices feature large, nearly circular mode sizes (≈4 x 4 &mgr;m and larger) and low modal loss, leading
to low gain per unit length, allowing for the construction of long (≈1 cm cavity length) devices. These characteristics
allow for high single mode output power. For 980-nm AlGaAs/InGaAs/GaAs-based SCOWL devices, we have
demonstrated > 1 W CW output power in a single spatial mode, with brightness levels of > 100 MW/cm<sup>2</sup>-str. We have
constructed high power arrays of SCOWL devices with bar widths of 1 cm and cavity lengths of 3 mm, and have
demonstrated > 90 W under CW operation. By using the technique of wavelength beam combining (WBC), which is
analogous to wavelength division multiplexing in optical communications, we have been able to combine the outputs
from the elements of a SCOWL array to obtain 50 W peak power (30 W CW) with nearly diffraction-limited beam
quality. These SCOWL arrays combined by WBC have demonstrated record single bar brightness levels, 3.6 GW/cm<sup>2</sup>-
str. The WBC SCOWL approach is inherently scalable, and offers a route to obtaining kW-class, nearly diffraction
limited output from an all-diode laser source. We have also recently extended single SCOWL devices to the multi-Watt
regime, demonstrating 2.8 W CW output power from a 980-nm SCOWL with a novel design.
This paper discusses a high-brightness multi-laser source developed at Polaroid for such applications as coupling light to fibers, pumping fiber lasers, pumping solid state lasers, material processing, and medical procedures. The power and brightness are obtained by imaging the nearfields of up to eight separate multi-mode lasers side by side on a multi-faceted mirror that makes the beams parallel. The lasers are microlensed to equalize the divergences in the two principal meridians. Each laser is aligned in a field- replaceable illuminator module whose output beam, focused at infinity, is bore-sighted in a mechanical cylinder. The illuminators are arranged roughly radially and the nearfields are reimaged on the mirror, which is produced by diamond machining. The array of nearfields is linearly polarized. A customizable afocal relay forms a telecentric image of the juxtaposed nearfields, as required by the application. The lasers can be of differing powers and wavelengths, and they can be independently switched. Light from other sources can be combined. The output can be utilized in free space or it can be coupled into a fiber for transport or a fiber laser for pumping. A linearly polarized free space output can be obtained, which allows two units to be polarization combined to double the power and brightness.
Heterojunction bipolar transistors (HBTs) are capable of producing very high speed digital integrated circuits operating as high as 40 GHz. In this paper we introduce a potentially low cost technique of monolithically integrating in-plane lasers with HBT circuits. A multifunctional epitaxial structure is used which is essentially the same as that for a standard high-speed HBT with modifications made to allow for efficient light amplification. Unlike previous multifunctional epitaxial structures, compromise in the transistor's performance is minimal. The schematic energy band diagrams of the HBT/laser structure biased as an HBT and laser are depicted. Light amplification is achieved by forward biasing the HBT's base- collector junction. The optical gain media is placed in the GaAs collector and consists of strained InGaAs quantum wells (QWs). Under normal HBT operation, the base-collector junction is reverse biased and serves as a sink for electrons which have diffused across the base. To confine electronic carries to the gain region when this junction is forward biased, the subcollector and base consist of a wider bandgap AlGaAs relative to the GaAs collector.