Defense R&D
Optical systems to detect and identify chemical, biological, nuclear, explosive, and other threats have seen continuous advancement in defense and security research in recent years. The sensitivity and selectivity of the sensors are improving, while system cost and complexity show downward movement.
Developers of reliable sensor systems for defense and security applications face many challenges, but some of the more promising technologies being developed are for point and standoff detection of chemical, biological, and explosives (CBE) agents.
Among a variety of classical and emerging technologies for point detection, some have potential for affordable and reliable combined CBE detection. For standoff detection, there are a small number of passive or active methods at infrared frequencies for chemical detection. However, standoff detection and identification of explosives and biological agents at operationally significant ranges continues to be a difficult problem to solve.
Traditional CBE sensors are often segmented according to a complex space of agents (C, B, E), sensor placement (point, standoff, remote), and level of analysis (screen, classify, identify). Point detection refers to sensing modalities that physically sample the threat agent in a medium, such as through a sample pump or aerosol collector. Standoff detectors analyze threats from a distance, typically measured in tens to thousands of meters. They are often optically-based systems, which may employ a laser for interrogation (active sensing) or not (passive sensing).
Remote detection, a tactical means to remove the operator from the threat environment, may employ point sensors, standoff sensors, or a combination. Level of analysis simply refers to the agent detail provided by the sensor and is often dictated by the objectives of the mission. But it may also be indicative of technological limitations. For example, laser or UV-induced fluorescence sensors merely indicate the presence of biological material with little to no differentiation between normal flora and pathogens. Polymerase chain reaction sensors, on the other hand, provide biological, species-level agent identification through DNA matching.
Perhaps the ultimate detector is one that can precisely identify a combination of C, B and E threat agents at operationally significant standoff ranges. However, we are not there today, at least not with any deployable or affordable solution. Consequently, developers seek to optimize detection parameters within the (C, B, E, point, standoff, remote, screen, classify, identify) space, according to a customer’s unique requirements. In addition to these variables, a developer is challenged to simultaneously manage other design parameters such as size, weight, and power (SWAP); as well as cost (purchase price as well as cost of ownership), detection time, sensitivity, false alarm performance, reliability, supportability, and maintainability, to name a few.
Historically, chemical and explosives point detection came in two general forms: Mass spectrometry (MS) if the operator needed high fidelity measurements and had the space, money, and time to accommodate a complex system; and ion mobility spectrometry (IMS) if the operator required a mobile, lightweight detector and was willing to accept a high level of false alarms or missed detections.
Over the last few years, MS systems have seen an evolutionary reduction in size and complexity, while maintaining relatively high-fidelity performance. This trend will continue, and perhaps one day we will see “handheld” MS detectors.
For IMS, where performance is roughly proportional to the length of the ion drift tube, higher fidelity is achieved with larger or multiple drift tubes with added size and cost penalties. Alternatively, IMS performance can improve incrementally through the addition of temporal sample concentrators and statistical signal processing algorithms to improve analyte detection and false alarm rejection.
Recently, an emerging technology called differential mobility spectrometry (DMS), somewhat of a cross between MS and IMS in terms of the ion analyzer, has shown promise for accurate detection of trace-level compounds for chemical and explosives detection in a handheld configuration. Additionally, surface plasmon resonance (SPR) detectors and biogel-based optical waveguides, traditionally designed for biological detection, have been applied to the chemical point detection problem.
A portable, affordable, and reliable combined chemical and biological point detector would have considerable utility for military and homeland security operations.
There are a small number of optically based modalities for detection of chemical vapors at standoff distances. Passive infrared, hyperspectral approaches such as Fourier Transform Infrared spectroscopy provide reasonable, broadband spectral analysis but are range- and wavelength-limited based on atmospheric conditions. Thanks to advances from the reconnaissance and surveillance community, hyperspectral sensors with imaging capability (with similar weather-range limitations) provide additional confidence in standoff detection of vapor clouds. Range limitations on standoff detection may be overcome by using one or more lasers for active interrogation of a sample volume.
Generally, increased standoff performance requires higher power from the laser(s) and a subsequent higher price tag. Traditional laser-based, standoff chemical detection has been accomplished using differential absorption LIDAR (DIAL), a technique that measures molecular absorption ratios for laser wavelengths on and off chemical absorption peaks. DIAL requires a large number of laser wavelengths for multiple analyte detection, and advancements in this technology have been progressing slowly over the years.
The recent developments of spectrally tuned quantum cascade lasers (QCL) show promise for smaller, lower-cost DIAL systems, which may compete with the passive infrared technologies. Other active interrogation methods, such as laser-induced breakdown spectroscopy (LIBS), Raman spectroscopy, and terahertz spectroscopy require continued development to overcome standoff performance and system maturity limitations, but they show promise for future combined standoff detection of chemical and biological agents.
Several detector options are available for biological point detection and may be generally segmented into ones which interrogate chemical content (such as tryptophan or NADH), specific antigen-antibody binding (immunoassay), or DNA content. Today the benchmark for nonspecific biological screening is UV-induced fluorescence, while the benchmark for specific biological identification is immunoassay or PCR.
Due to the complexity and cost of ownership of immunoassay and PCR sensors, system developers often combine a continuous, real-time, screening detector with an identification sensor that operates only after the screening detector alarms, an effective but complex and expensive approach. Recent advancements in miniature, label-free, optically based sensors such as SPR and biogel-based optical waveguides show promise for single-step detection systems.
For all approaches to biological point detection, significant systems-level work remains in integrating efficient, low-power collector/concentrators and small, efficient fluidics-management subsystems.
Standoff biological detection continues to be a challenging problem, and current approaches are limited to active interrogation at optical or terahertz frequencies.
Conventional LIDAR may be used to detect and track aerosol (potentially biological) clouds at significant standoff range but provides no discrimination between biological and non-biological aerosol particles. UV laser-induced fluorescence (LIF) may be employed in a standoff mode, similar to point detection, to provide discrimination between biological and non-biological particles, but it requires substantial laser and optical collection power and may be severely range-limited. A combined LIDAR/LIF system overcomes the deficiencies of each approach. This is potentially a complex and costly solution that still provides limited information.
Recently, biological species-level discrimination was demonstrated at standoff ranges using differential scattering with long-wave infrared LIDAR. The major advance came from the incorporation of a Support Vector Machine algorithm for detection and identification.
This is a breakthrough advance, but additional development will be required before fielding. Other advancements in active standoff interrogation methods such as LIBS, Raman and terahertz may provide alternative solutions to standoff biological and perhaps combined chemical and biological standoff detection. However, significant work is still required at the system level.
Explosives detection is a form of chemical detection with unique challenges.
While many technologies for standoff detection of chemical vapors may be applied to the standoff detection of explosives, vapor and particulate signatures from explosives are so small that reasonable standoff detection ranges have been challenging. Additional fundamental research and system development is required to make any advances in this area.
Over the last nine years, there have been continuous advances in capabilities for the detection and identification of threat agents. This year’s SPIE Defense+Security Symposium in Orlando, FL, expanded its conference on chemical and biological sensing to include threat detections of radiological, nuclear, and explosive materials as well.
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