High sensitivity stand-off detection and quantification of chemical mixtures using an active coherent laser spectrometer (ACLaS)

Abstract

High sensitivity detection, identification and quantification of chemicals in a stand-off configuration is a highly sought after capability across the security and defense sector. Specific applications include assessing the presence of explosive related materials, poisonous or toxic chemical agents, and narcotics.Real world field deployment of an operational stand-off system is challenging due to stringent requirements: high detection sensitivity, stand-off ranges from centimeters to hundreds of meters, eye-safe invisible light, near real-time response and a wide chemical versatility encompassing both vapor and condensed phase chemicals. Additionally, field deployment requires a compact, rugged, power efficient, and cost-effective design.To address these demanding requirements, we have developed the concept of Active Coherent Laser Spectrometer (ACLaS), which can be also described as a middle infrared hyperspectral coherent lidar. Combined with robust spectral unmixing algorithms, inherited from retrievals of information from high-resolution spectral data generated by satellitebased spectrometers, ACLaS has been demonstrated to fulfil the above-mentioned needs.ACLaS prototypes have been so far developed using quantum cascade lasers (QCL) and interband cascade lasers (ICL) to exploit the fast frequency tuning capability of these solid state sources. Using distributed feedback (DFB) QCL, demonstration and performance analysis were carried out on narrow-band absorbing chemicals (N 2 O, H 2 O, H 2 O 2 , CH 4 , C 2 H 2 and C 2 H 6 ) at stand-off distances up to 50 m using realistic non cooperative targets such as wood, painted metal, and bricks. Using more widely tunable external cavity QCL, ACLaS has also been demonstrated on broadband absorbing chemicals (dichloroethane, HFC134a, ethylene glycol dinitrate and 4-nitroacetanilide solid) and on complex samples mixing narrow-band and broadband absorbers together in a realistic atmospheric background.

Standoff Chemical Detection Using Laser Absorption Spectroscopy: A Review

“…In standoff detection, the optical pathlength depends on the standoff distance that is usually variable in real-world applications, and hence, the performance is often quantified using the pathlength-integrated unit of ppm•m [9]. Further normalization with bandwidth renders a detection limit in the unit of ppm•m•Hz −1/2 to account for the SNR improvement with averaging in the domain that white noise dominates [7].…”

“…The up to date results of LHR detection are summarized, as shown in Table 6. However, passive LHS derives its sensitivity from thermal contrast when used in transmission mode, which limits its ability to provide ppb level detection in any realistic standoff detection configuration [7]. Because of its passive nature, LHR is not sensitive enough for terrestrial remote detection of trace chemical plumes over distances of tens to hundreds of meters.…”

“…According to the working principle of the ACLaS, a forward model has been established to describe the relationship between the directly measured spectral signal and the unknown state vector of molecular mixing ratios, with consideration of instrument parameters [213]. The OEM algorithm originally used in LHR was adapted to retrieve mixing ratios using the synthetized spectral trace calculated by the forward model as input [7,213]. A "cost" function is defined in OEM consisting of weighted differences between the measured data and the simulated data by the forward model, and between the current state vector and the a-priori vector.…”

“…The solution minimizing the cost function is output as the optimal solution. In addition, the OEM also enables full error propagation analysis to estimate the level of confidence and further detection sensitivities [7]. According to the working principle of the ACLaS, a forward model has been established to describe the relationship between the directly measured spectral signal and the unknown state vector of molecular mixing ratios, with consideration of instrument parameters [213].…”

“…Although the precise requirements vary with specific applications, the key common requirements are summarized by N. Macleod and D. Weidmann for standoff chemical detection [7], including multi-species detection, high detection sensitivity, large dynamic range, flexible standoff detection ranges, rapid temporal response, eye-safe laser operation, compact and rugged design, as well as cost-effectiveness. Obviously, these requirements put forward a formidable challenge to detection technology.…”