Portable hyperspectral imagers enable real time decision-making in application areas such as threat detection, forensics, environmental and agricultural monitoring and biomedical screening. High spectral and spatial resolution provide far more actionable information than obtainable by multispectral imagers. High-resolution VNIR reflectance imagers characterize photosynthetic productivity via solar-induced fluorescence (SIF) monitoring, representing a substantial improvement over earlier multispectral analytics such as normalized difference vegetation index (NDVI). Handheld Raman instruments enable high speed screening for explosives, narcotics and hazardous materials. A line scan/area imaging approach detects trace quantities of target materials, such as small particles of explosives or narcotics, within a bulk sample.
Raman spectral imaging is increasingly becoming the tool of choice for field-based applications such as threat, narcotics and hazmat detection; air, soil and water quality monitoring; and material ID. Conventional fiber-coupled point source Raman spectrometers effectively interrogate a small sample area and identify bulk samples via spectral library matching. However, these devices are very slow at mapping over macroscopic areas. In addition, the spatial averaging performed by instruments that collect binned spectra, particularly when used in combination with orbital raster scanning, tends to dilute the spectra of trace particles in a mixture. Our design, employing free space line illumination combined with area imaging, reveals both the spectral and spatial content of heterogeneous mixtures. This approach is well suited to applications such as detecting explosives and narcotics trace particle detection in fingerprints. The patented High Throughput Virtual Slit1 is an innovative optical design that enables compact, inexpensive handheld Raman spectral imagers. HTVS-based instruments achieve significantly higher spectral resolution than can be obtained with conventional designs of the same size. Alternatively, they can be used to build instruments with comparable resolution to large spectrometers, but substantially smaller size, weight and unit cost, all while maintaining high sensitivity. When used in combination with laser line imaging, this design eliminates sample photobleaching and unwanted photochemistry while greatly enhancing mapping speed, all with high selectivity and sensitivity. We will present spectral image data and discuss applications that are made possible by low cost HTVS-enabled instruments.
Remote sensing has moved out of the laboratory and into the real world. Instruments using reflection or Raman imaging modalities become faster, cheaper and more powerful annually. Enabling technologies include virtual slit spectrometer design, high power multimode diode lasers, fast open-loop scanning systems, low-noise IR-sensitive array detectors and low-cost computers with touchscreen interfaces. High-volume manufacturing assembles these components into inexpensive portable or handheld devices that make possible sophisticated decision-making based on robust data analytics. Examples include threat, hazmat and narcotics detection; remote gas sensing; biophotonic screening; environmental remediation and a host of other applications.
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-1 and tuning can be accomplished at rates of <25 cm-1 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
The requirements for standoff detection of Explosives and CWA/TICs on surfaces in the battlefield are challenging because of the low detection limits. The variety of targets, backgrounds and interferences increase the challenges. Infrared absorption spectroscopy with traditional infrared detection technologies, incandescent sources that offer broad wavelength range but poor spectral intensity, are particularly challenged in standoff applications because most photons are lost to the target, background and the environment. Using a brighter source for active infrared detection e.g. a widely-tunable quantum cascade laser (QCL) source, provides sufficient spectral intensity to achieve the needed
sensitivity and selectivity for explosives, CWAs, and TICs on surfaces. Specific detection of 1-10 μg/cm2 is achieved
within seconds. CWAs, and TICs in vapor and aerosol form present a different challenge. Vapors and aerosols are present at low
concentrations, so long pathlengths are required to achieve the desired sensitivity. The collimated output beam from the
QCL simplifies multi-reflection cells for vapor detection while also enabling large standoff distances. Results obtained by the QCL system indicate that <1 ppm for vapors can be achieved with specificity in a measurement time of seconds, and the QCL system was successfully able to detect agents in the presence of interferents. QCLs provide additional capabilities for the dismounted warfighter. Given the relatively low power consumption, small package, and instant-on capability of the QCL, a handheld device can provide field teams with early detection of toxic agents and energetic materials in standoff, vapor, or aerosol form using a single technology and device which makes it attractive compared other technologies.
Block Engineering has developed a widely tunable quantum cascade laser (QCL) spectrometer, a probe, and algorithms
specific to detecting low levels of surface contamination. This paper discusses the basic technology of the QCL
spectrometer both in a standoff and probe based configuration. It provides information on the algorithms and probes
developed for this application. The paper compares the QCL based technique to other approaches for detecting surface
Block Engineering has developed an absorption spectroscopy system based on widely tunable Quantum Cascade Lasers
(QCL). The QCL spectrometer rapidly cycles through a user-selected range in the mid-infrared spectrum, between 6 to
12 μm (1667 to 833 cm-1), to detect and identify substances on surfaces based on their absorption characteristics from a standoff distance of up to 2 feet with an eye-safe laser. It can also analyze vapors and liquids in a single device. For
military applications, the QCL spectrometer has demonstrated trace explosive, chemical warfare agent (CWA), and toxic
industrial chemical (TIC) detection and analysis.
The QCL's higher power density enables measurements from diffuse and highly absorbing materials and substrates.
Other advantages over Fourier Transform Infrared (FTIR) spectroscopy include portability, ruggedness, rapid analysis,
and the ability to function from a distance through free space or a fiber optic probe. This paper will discuss the basic
technology behind the system and the empirical data on various safety and security applications.
This paper reports the design, fabrication, and characterization of a millimeter diameter, surface micromachined
Micro-Electro-Mechanical-Systems (MEMS) mirror, which is assembled perpendicular to the substrate and can be
linearly and repeatedly traversed through 600 μm. The moving mirror, when combined with a fixed mirror and
beamsplitter, make up a monolithic MEMS Michelson interferometer; all are made on the same substrate and in the
same surface micromachined fabrication process. The beamsplitter has been specifically designed such that the
motion of the mirror enables modulation of light over the 2-14 μm spectral region. The rapid scan MEMS
Michelson interferometer is the engine behind a miniaturized, Fourier transform infrared (FTIR) absorption
spectrometer. The FTIR measures the absorption of infrared (IR) radiation by a target material, which can be used
for the detection and identification of gases, liquids, or solids. The fabrication of the mirror with the ability to
displace 600 μm along the optical axis enables the miniaturized system to have species identification resolution,
while leveraging wafer scale batch fabrication to enable extremely low system cost. The successful fabrication of
the millimeter diameter mirrors and beamsplitter with interferometric alignment over the range of travel of the
moving mirror promises unprecedented sensitivity relative to the size of the FTIR spectrometer system.
We describe the development of a MEMS-based correlation radiometer for remote detection of chemical species. The radiometer utilizes a new type of MEMS programmable diffraction grating called the Polychromator. The Polychromator contains an array of 1024 electrostatically actuated reflective beams that are 10 microns wide by 1 cm long, and have a vertical travel of approximately 2 - 4 microns. The Polychromator grating is used to replace the reference cell of conventional correlation radiometry. Appropriate programming of the deflection profile of the grating array enables the production of any spectral transfer function desired for the correlation measurement. Advantages of this approach to correlation radiometry include the ability to detect multiple chemical species with a compact
instrument, the ability to optimize the reference spectra to eliminate chemical interferences, and the ability to produce
reference spectra for hazardous and transient species.