Proc. SPIE. 10183, Chemical, Biological, Radiological, Nuclear, and Explosives (CBRNE) Sensing XVIII
KEYWORDS: Signal to noise ratio, Standoff detection, Detection and tracking algorithms, Deep ultraviolet, Sensors, Spectroscopy, Raman spectroscopy, Directed energy weapons, Spectral resolution, Explosives
Deep-ultraviolet Raman spectroscopy is a very useful approach for standoff detection of explosive traces. Using two simultaneous excitation wavelengths improves the specificity and sensitivity to standoff explosive detection. The High Technology Foundation developed a highly compact prototype of resonance Raman explosives detector. In this work, we discuss the relative performance of a dual-excitation sensor compared to a single-excitation sensor. We present trade space analysis comparing three representative Raman systems with similar size, weight, and power. The analysis takes into account, cost, spectral resolution, detection/identification time and the overall system benefit.
A promising approach to stand-off detection of explosive traces is using resonance Raman spectroscopy with Deepultraviolet (DUV) light. The DUV region offers two main advantages: strong explosive signatures due to resonant and λ- 4 enhancement of Raman cross-section, and lack of fluorescence and solar background. For DUV Raman spectroscopy, continuous-wave (CW) or quasi-CW lasers are preferable to high peak powered pulsed lasers because Raman saturation phenomena and sample damage can be avoided. In this work we present a very compact DUV source that produces greater than 1 mw of CW optical power. The source has high optical-to-optical conversion efficiency, greater than 5 %, as it is based on second harmonic generation (SHG) of a blue/green laser source using a nonlinear crystal placed in an external resonant enhancement cavity. The laser system is extremely compact, lightweight, and can be battery powered. Using two such sources, one each at 236.5 nm and 257.5 nm, we are building a second generation explosive detection system called Dual-Excitation-Wavelength Resonance-Raman Detector (DEWRRED-II). The DEWRRED-II system also includes a compact dual-band high throughput DUV spectrometer, and a highly-sensitive detection algorithm. The DEWRRED technique exploits the DUV excitation wavelength dependence of Raman signal strength, arising from complex interplay of resonant enhancement, self-absorption and laser penetration depth. We show sensor measurements from explosives/precursor materials at different standoff distances.
A key challenge for standoff explosive sensors is to distinguish explosives, with high confidence, from a myriad of unknown background materials that may have interfering spectral peaks. To meet this challenge a sensor needs to exhibit high specificity and high sensitivity in detection at low signal-to-noise ratio levels. We had proposed a Dual-Excitation- Wavelength Resonance-Raman Detector (DEWRRED) to address this need. In our previous work, we discussed various components designed at WVHTCF for a DEWRRED sensor. In this work, we show a completely assembled laboratory prototype of a DEWRRED sensor and utilize it to detect explosives from two standoff distances. The sensor system includes two novel, compact CW deep-Ultraviolet (DUV) lasers, a compact dual-band high throughput DUV spectrometer, and a highly-sensitive detection algorithm. We choose DUV excitation because Raman intensities from explosive traces are enhanced and fluorescence and solar background are not present. The DEWRRED technique exploits the excitation wavelength dependence of Raman signal strength, arising from complex interplay of resonant enhancement, self-absorption and laser penetration depth. We show measurements from >10 explosives/pre-cursor materials at different standoff distances. The sensor showed high sensitivity in explosive detection even when the signalto- noise ratio was close to one (~1.6). We measured receiver-operating-characteristics, which show a clear benefit in using the dual-excitation-wavelength technique as compared to a single-excitation-wavelength technique. Our measurements also show improved specificity using the amplitude variation information in the dual-excitation spectra.
Raman spectroscopy is a widely used spectroscopic technique with a number of applications. During the past few years,
we explored the use of simultaneous multiple-excitation-wavelengths (MEW) in resonance Raman spectroscopy. This
approach takes advantage of Raman band intensity variations across the Resonance Raman spectra obtained from two or
more excitation wavelengths. Amplitude variations occur between corresponding Raman bands in Resonance Raman
spectra due to complex interplay of resonant enhancement, self-absorption and laser penetration depth. We have
developed a very sensitive algorithm to estimate concentration of an analyte from spectra obtained using the MEW
technique. The algorithm uses correlations and least-square minimization approach to calculate an estimate for the
concentration. For two or more excitation wavelengths, measured spectra were stacked in a two dimensional matrix. In a
simple realization of the algorithm, we approximated peaks in the ideal library spectra as triangles. In this work, we
present the performance of the algorithm with measurements obtained from a dual-excitation-wavelength Resonance
Raman sensor. The novel sensor, developed at WVHTCF, detects explosives from a standoff distance. The algorithm
was able to detect explosives with very high sensitivity even at signal-to-noise ratios as low as ~1.6. Receiver operating
characteristics calculated using the algorithm showed a clear benefit in using the dual-excitation-wavelength technique
over single-excitation-wavelength techniques. Variants of the algorithm that add more weight to amplitude variation
information showed improved specificity to closely resembling spectra.
Proc. SPIE. 8710, Chemical, Biological, Radiological, Nuclear, and Explosives (CBRNE) Sensing XIV
KEYWORDS: Signal to noise ratio, Explosives detection, Detection and tracking algorithms, Deep ultraviolet, Sensors, Spectroscopy, Raman spectroscopy, Explosives, Algorithm development, Signal detection
Deep-ultraviolet resonance Raman spectroscopy (DUVRRS) is a promising approach to stand-off detection of explosive traces due to: 1) resonant enhancement of Raman cross-section, 2) λ-4-cross-section enhancement, and 3) fluorescence and solar background free signatures. For trace detection, these signal enhancements more than offset the small penetration depth due to DUV absorption. A key challenge for stand-off sensors is to distinguish explosives, with high confidence, from a myriad of unknown background materials that may have interfering spectral peaks. To address this, we are developing a stand-off explosive sensor using DUVRRS with two simultaneous DUV excitation wavelengths. Due to complex interplay of resonant enhancement, self-absorption and laser penetration depth, significant amplitude variation is observed between corresponding Raman bands with different excitation wavelengths. These variations with excitation wavelength provide an orthogonal signature that complements the traditional Raman signature to improve specificity relative to single-excitation-wavelength techniques. As part of this effort, we are developing two novel CW DUV lasers, which have potential to be compact, and a compact dual-band high throughput DUV spectrometer, capable of simultaneous detection of Raman spectra in two spectral windows. We have also developed a highly sensitive algorithm for the detection of explosives under low signal-to-noise situations.
Deep-ultraviolet resonance Raman spectroscopy (DUVRRS) is a promising approach to stand-off detection of explosive
traces due to large Raman cross-section and background free signatures. In order to design an effective sensor, one must
be able to estimate the signal level of the DUVRRS signature for solid-phase explosive residues. The conventional
approach to signal estimation uses scattering cross-sections and molar absorptivity, measured on solutions of explosives
dissolved in an optically-transparent solvent. Only recently have researchers started to measure solid-state cross-sections.
For most solid-phase explosives and explosive mixtures, neither the DUV Raman scattering cross sections nor the
optical absorption coefficient are known, and they are very difficult to separately measure. Therefore, for a typical solid
explosive mixture, it is difficult to accurately estimate Raman signal strength using conventional approaches. To address
this issue, we have developed a technique to measure the Raman scattering strength of optically-thick (opaque)
materials, or “Raman Albedo”, defined as the total power of Raman-scattered light per unit frequency per unit solid
angle divided by the incident power of the excitation source. We have measured Raman Albedo signatures for a wide
range of solid explosives at four different DUV excitation wavelengths. These results will be presented, and we will
describe the use of Raman Albedo measurements in the design and current construction of a novel stand-off explosive
sensor, based on dual-excitation-wavelength DUVRRS.
Deep-ultraviolet resonance Raman spectroscopy (DUVRRS) is a potential candidate for stand-off detection of
explosives. A key challenge for stand-off sensors is to distinguish explosives, with high confidence, from a myriad of
unknown background materials that may have interfering spectral peaks. To address this, we have investigated a new
technique that simultaneously detects Raman spectra from multiple DUV excitation wavelengths. Due to complex
interplay of resonant enhancement, self-absorption and laser penetration depth, significant intensity variation is observed
between corresponding Raman bands with different excitation wavelengths. These variations with excitation wavelength
provide a unique signature that complements the traditional Raman signature to improve specificity relative to singleexcitation-
wavelength techniques. We have measured these signatures for a wide range of explosives using amplitudecalibrated
Raman spectra, obtained sequentially by tuning a frequency-doubled Argon laser to 229, 238, 244 and 248
nm. For nearly all explosives, these signatures are found to be highly specific. An algorithm is developed to quantify
the specificity of this technique. To establish the feasibility of this approach, a multi-wavelength DUV source, based on
Nd:YAG harmonics and hydrogen Raman shifting, and a compact, high throughput DUV spectrometer, capable of
simultaneous detection of Raman spectra in multiple spectral windows, are being investigated experimentally.
Structural Health Monitoring (SHM) is required for early detection of damage in structural components to improve the
safety, reduce the cost, and increase the performance and efficiency of aircrafts. Currently available techniques have a
number of deficiencies prohibiting wide spread of SHM in aerospace applications. In this contribution we will present
the initial results of development at Luna Innovations of an all-fiber optic ultrasonic airframe SHM system that will be
able to address the deficiencies of solutions suggested/developed to date. In this contribution we will present the details
on design, development and testing of the prototype fiber optic SHM system.
We investigate the phase transitions of intense ultrashort laser-heated solids, from the cold solid to the hot dense plasma state, by measuring the complex electrical conductivity (or refractive index) transients at terahertz (1 THz = 1012 Hz) frequencies. Using optical-pump, terahertz-probe spectroscopy, we measured the phase shifts and absorption of terahertz probe pulses that were reflected from the warm dense plasma. To characterize the THz field, we developed and used a single-shot, high-temporal-resolution THz diagnostic capable of measuring free-space electromagnetic pulse fields in time and space. Due to relatively large focal spot sizes of the THz probe (~mm), mainly limited by the diffraction properties of THz radiation, the optical pump pulse was weakly focused onto the target in order to overfill the THz probe spot size with a peak intensity of ~1013 W/cm2. In contrast to the previous measurements of conductivities at optical frequencies, our THz non-contact probe method can directly measure quasi-DC electrical conductivities, providing insight into the transport nature of warm dense matter and any present discrepancies with the Drude model. In case of warm dense aluminum, we observe a noticeable deviation from the Drude model even in the ~1013 W/cm2 laser intensity regime. In addition, we observe strong coherent THz emission produced by a current surge in the laser-produced plasma.