Abstract:
A method of performing Raman spectroscopy, and an apparatus for performing the method, including irradiating, with laser light, a trace amount of a target substance disposed on a painted surface, receiving reflected laser light and a Raman return signal from the trace amount of the target substance, processing the Raman return signal using, at least, a spectrograph and camera to obtain a Raman signature for the target substance, and identifying the target substance based on the Raman signature for the target substance and a database of Raman signatures. The laser light wavelength may be selected based on a ratio of Raman return signal power to fluorescence power.

Description:
[0001]    This application claims the benefit of U.S. Provisional Application No. 61/242,933, filed Aug. 17, 2010, which is incorporated herein by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention is related to Raman spectroscopy, and more particularly to systems and methods for performing stand-off Raman spectroscopy using laser light in the deep ultraviolet spectral region. 
       BACKGROUND OF THE INVENTION 
       [0003]    The danger of exposure to chemical or biological agents can be severe. Whether a result of unintended release, by way of, e.g., an accident, or a result of intentional delivery, it is desirable to quickly and accurately identify (1) the type of agent that has been released and (2) the precise area of contamination. Early and accurate detection of such dangerous substances can be a significant factor in reducing potential casualties and limiting further spreading of the agent, e.g., by wind, human and animal contact, etc. 
         [0004]    It is likewise important to be able to quickly and accurately identify the presence of explosive materials. Such detection is particularly important in security and military settings. The sooner detection can take place, the sooner personnel can be evacuated or other appropriate measures taken. 
         [0005]    Prior remote sensor technologies designed to measure surface-deposited chemical contamination used vehicles and associated test equipment that retrieved a physical sample of the contaminant and then used an extremely cumbersome and time-consuming process to analyze the sample, e.g., a mechanical sampling wheel system coupled with a mass spectrometer to test for chemical agents. 
         [0006]    In view of the complexity of such approaches, newer, simpler technologies were desired. One emerging technology in response to this desire is referred to, generally, as “standoff surface detection,” and refers to a category of technologies that permit detection of a substance without actually having to physically come in contact with the substance. The goal of these detection systems is to provide the capability to detect, identify, locate, quantify, warn, and report chemical and biological threats and thereby give military forces or civilian personnel sufficient early warning to avoid (further) contamination and deploy appropriate counter measures. 
         [0007]    One of the technologies that enables standoff surface detection is Raman spectroscopy. Raman spectroscopy is a technique used to characterize materials and thereafter to identify such materials. Typically, a laser transmitter serves as a spectrally narrow light source with high irradiance. The laser illuminates a known or an unknown substance, such as a chemical compound, biological agent, or explosive material, among others. A portion of the incident light is Raman scattered by the substance. This light is scattered in all spatial directions as well as shifted spectrally to discrete wavelengths. These wavelength shifts correspond to unique vibrational energies associated with the molecular bonds in the substance. 
         [0008]    In conventional standoff Raman spectroscopy systems, the Raman scattered light is collected by a telescope and is coupled into a dispersive optical system. The telescope focuses the collected light onto, e.g., an optical fiber bundle. At the opposite end of the fiber bundle, individual fibers are oriented linearly to form an entrance slit for a grating-based spectrograph. An electro-optical detector array records the optical spectrum of the Raman scattered light. This spectrum serves as a “fingerprint” for the known or unknown substance. An analysis computer stores the reference spectral information along with information identifying the known substance, and more often, employs pattern-matching algorithms to identify the unknown substance from a spectral library of previously analyzed and “fingerprinted” reference substances. 
         [0009]    Laboratory-based Raman spectroscopy systems have been in use for many years. Recently, portable Raman systems have become possible as a result of components that have decreased in size and unique integration techniques. A description of one such portable system can be found in U.S. Pat. No. 6,608,677, which is incorporated herein by reference. A backpack implemented man-portable Raman sensor has also recently been fielded by ITT (Wilmington, Del.). Aspects of that system are described in U.S. Pat. No. 7,796,251, entitled Method, Apparatus and System for Rapid and Sensitive Standoff Detection of Surface Contaminants, which is also incorporated herein by reference. Truck mounted Raman sensors have also been fielded to date. Reference may also be made to U.S. Pat. No. 6,788,407, also incorporated herein by reference, for still further discussion of Raman spectrometry. 
         [0010]    Notwithstanding these known systems, there remains a need to provide different forms of Raman sensors to meet the needs of both civilian and military personnel responsible for identifying, especially, trace amounts or residues of explosive substances. 
       SUMMARY OF INVENTION EMBODIMENTS 
       [0011]    The present invention provides a unique implementation of a Raman spectroscopy system that can be employed to avoid having to be in the immediate vicinity of dangerous substances or items. More specifically, with embodiments in accordance with the present invention, personnel are provided the capability to perform remote yet sensitive site exploitation including inspection of buildings, equipment, vehicles, aircraft, and other manmade or natural surfaces for the presence of unknown substances or items including, but not limited to, explosive material (e.g., TNT and RDX), homemade explosives and their precursors, and chemicals including chemical warfare agents (CWAs), and toxic industrial chemicals (TICs). One significant advantage of embodiments of the present invention is the ability to search for and detect improvised explosive devices (IEDs) or traces of explosives on vehicles without having to place personnel in the same location in which explosives might be hidden. 
         [0012]    Other advantages of embodiments of the present invention include the fact that the technology (1) is a non-contact standoff system, (2) allows mapping of the surface contamination, and (3) provides quick response (typically only seconds to perform detection). 
         [0013]    These and other features of embodiments of the present invention and their attendant advantages will be more fully appreciated upon a reading for the following detailed description in conjunction with the associated drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a schematic functional diagram of a Raman spectroscopy system in accordance with an embodiment of the present invention. 
           [0015]      FIG. 2  depicts a plot of Raman photon efficiency factor including three identified resonance regions. 
           [0016]      FIG. 3A-C  depict plots of fluorescence spectra recorded from vehicle panels used in connection with experiments conducted in accordance with an embodiment of the present invention. 
           [0017]      FIGS. 4A  and B show Raman to fluorescence power ratio data for explosive material PETN and Semtex. 
           [0018]      FIGS. 5A and 5B  depict plots of spectra of selected vehicle panels used in connection with experiments conducted in accordance with an embodiment of the present invention. 
           [0019]      FIGS. 6A and 6B  depict plots of the Raman shift for PETN and TNT using 248-nm excitation at a 10 meter standoff. 
           [0020]      FIGS. 7A and 7B  depict plots of the Raman shift for AN residues on aluminum-coated glass and a surface of a vehicle panel in connection with methodologies according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    Raman signatures tend to be distinctive and unique to a material; thus this “optical fingerprinting” technique for identifying explosives at standoff distances has received much attention. Raman signals can be generated with laser excitation, and numerous studies have been conducted to evaluate Raman spectroscopy for explosives detection in the near-infrared, visible, and UV spectral regions. This flexibility has spurred the development of Raman systems, some capable of detecting explosives from tens of meters. 
         [0022]    In this regard, many academic studies have focused on only one aspect of Raman spectroscopy for explosives detection, such as demonstrating the feasibility of long standoff distances or the potential of a particular dual-sensor concept. By and large, these studies have been performed under experimental conditions that are not particularly analogous to real-world field operations. For example, the study presented in Pettersson, A.; Wallin, S.; Östmark, H.; Ehlerding, A.; Johansson, I.; Nordberg, M.; Ellis, H.; Al-Khalili, A.  Proc. of SPIE,  2010, 7664, 76641K-1 demonstrated that ammonium nitrate (AN) can be detected outdoors from a distance of 470 meters with a 532-nm Raman system, but the data were recorded using essentially laboratory conditions. Specifically, a 500-mL container of bulk AN was interrogated for a period of 1-10 seconds with pulse energies of 320 mJ at a repetition rate of 10 Hz (i.e., ˜3.2 Watts of laser power). 
         [0023]    Increasing the distance and detecting many types of explosives at very low concentrations continues to be an area of interest. Described herein is a UV Raman sensor that has been shown to detect TNT, PETN, and AN at concentrations as low as 3-50 μg/cm 2  from a standoff distance of 10 meters using moderate laser power (−100 mW). Importantly, this level of sensitivity has been demonstrated for explosive residues on field relevant backgrounds in a timeframe on the order of seconds. These are key points because real-world backgrounds such as painted surfaces of vehicle panels often limit the sensitivity of Raman spectroscopy by overwhelming the Raman signal with fluorescence. Furthermore, the choice of excitation wavelength is related to the amount of material present. 
         [0024]    Detecting some explosives can be particularly challenging because Raman scattering from these materials is insufficient to overcome the limitations imposed by absorption. In general, strong absorption limits penetration into the target by the incident laser beam, minimizing the volume of material that can contribute to the Raman signal. The emerging Raman signal is also absorbed by the target along its path to the receiver. Consequently, the amount of material that can contribute to the observed Raman signal is limited to a thin layer for strongly absorbing explosives. RDX and HMX absorb strongly between 225 nm and 275 nm, and the thickness of material that contributes to the signal is predicted to be less than 0.3 μm (for 257-nm excitation). Ultimately, signal strength depends on the efficiency of Raman scattering as well as absorption. For example, because Raman scattering from TNT is ˜100× greater than from RDX with 257-nm excitation, Raman signals obtained from TNT are considerably larger than those from RDX, despite the fact that absorption limits the theoretical interaction depth for TNT to ˜100 nm. 
         [0025]    Embodiments of the present invention use a deep UV laser to generate Raman scattering from the surface chemical. Raman scattering is generally much stronger with UV laser light than with visible and IR light due to the well-known ν 4  effect and the onset of a phenomenon known as resonance enhancement. Importantly, unlike the Raman signals obtained with visible and IR lasers, resonance enhancement tends to emphasize the spectral features of the targeted explosives as compared to the spectral features obtained from the background, making it possible to detect small quantities of explosives. Moreover, the system is solar blind and interference from background fluorescence is minimal. A single-frequency laser can be employed, and processing does not require picture-to-picture registration is required for differential imaging techniques. 
         [0026]      FIG. 1  depicts a Raman spectroscopy system in accordance with one possible implementation of the present invention. A scanner  170  includes a UV laser  10  that is tuned to a single frequency in the wavelength region below 400 nm, preferably below 250 nm, and more preferably around 248 nm in order to avoid fluorescence from relevant surfaces, as will be discussed in more detail below. The laser&#39;s outgoing UV beam is permitted to radiate to a target substance via a safety shutter  20  and beam conditioning optics  30 . The beam conditioning optics  30  adjust the beam&#39;s characteristics to match the interrogation spot size of a receiver telescope  70 . This beam radiates from the system to a surface on which trace amounts of, e.g., an unknown chemical, such as an explosive material or precursor, have been deposited. The phrase “trace amounts” as used herein is meant to refer to substance residues, smears and the like, and is meant to be distinguished from bulk material. A trace amount may be on the order of less than, e.g., 100 μg/cm 2 . 
         [0027]    The UV laser light interacts with the target surface and surface deposited chemical. The return light consists of Rayleigh scattered and Raman scattered light from the surface and the target chemical. In parallel with the UV laser, a rangefinder  40  detects the range from the system to the interrogated surface. Data from the rangefinder  40  can be used by control electronics  50  to control the beam conditioning optics  30  and, via an autofocus control module  60 , the focus of the receiving telescope  70 . 
         [0028]    The telescope  70  receives the Rayleigh and Raman scattered light and focuses it first onto optical filtering and beam shaping or conditioning module  80 . The optical filtering component filters the Rayleigh scattered (basic UV laser light) and focuses the resulting filtered light onto the end of a fiber optic bundle  90 . The near end of the fiber optic bundle is arranged in a nearly round configuration to maximize coupling. The far end, the end closest to a spectrograph  100  is preferably arranged in a linear array of individual fibers, fashioning a slit. Depending on the specific design, the linear array may be arranged in a shallow v-arrangement to offset optical effects that occur in the spectrograph  100 . The spectrograph  100  uses a grating and optical components to disperse the received light into its constituent wavelengths and focus the light onto a Charge Coupled Device or other array detector, referred to generally as camera  110 . Camera  110  may be an intensified CCD (ICCD) in order to amplify the Raman return signal. The camera  110  converts the optical signal into an electrical waveform that represents the amplitude of the light in each wavelength bin. This signal is then processed and compared against a library of signatures in an analysis computer  120 . Processing may include removal of any residual baseline from the received signal and pre-conditioning the library spectra with the system&#39;s spectral response so that the measured signal and library spectra appear to have been measured by the same system. When the analysis computer  120  determines a match, it provides the operator with visual, audible, and/or textual notification (indicated by reference numeral  130 ) indicating the presence, name, etc. of the detected material. 
         [0029]    The scene to be analyzed may be viewed using a video camera  140 . The system illuminates the telescope&#39;s field of view using a designator  150 , which indicates to the system operator the interrogation area. In some applications the beam of the range finder  40  may be used as the designator. The operator directs the system to scan the area of interest using a scan control computer  160  that controls the overall scanner  170 . Scanning may be accomplished either manually while watching the video scene or the operator can designate an area on the video display which he wishes to scan. The scan control computer  160  then calculates the scanning path of the UV laser  10  and telescope  70 . Scanning can also be used as part of mapping functionality to map an area of interest in regard to the presence of one or more target substances. 
         [0030]    As will be appreciated by those skilled in the art, this system can be used to remotely interrogate surfaces for specific chemicals, including explosives and their precursors. Other chemicals could also be detected if their spectra are included in the active library used by the algorithm. In one possible implementation, the system may be deployed on a movable or mobile vehicle. 
         [0031]    An important aspect of embodiments described herein is the selection of the laser light wavelength. Minimizing interference from fluorescence on real-world backgrounds, while simultaneously avoiding sample degradation, was determined to be a primary driver for selecting a suitable laser wavelength in a system that does not use more complicated fast gating detectors. Phenomenology studies were undertaken to spectroscopically characterize explosives on real-world backgrounds, including 32 vehicle panels (several of which are listed in the table below). As will be discussed in more detail below, laser excitation at 248 nm adequately minimizes fluorescence-interference and sample-degradation. 
         [0032]    Raman responses from several explosives were surveyed using laser excitation between 225 and 275 nm. It was concluded that excitation near 257 nm might be well-suited for detecting explosives residues for the following reasons: 
         [0033]    1) Preliminary data recorded from a few randomly selected vehicle panels indicated that fluorescence generated by laser excitation at wavelengths longer than 257 nm would greatly reduce sensitivity towards trace explosives. 
         [0034]    2) Simple predictions indicated that the strength of the Raman signal favored longer excitation wavelengths, with 257 nm being near the maximum for some of the explosives tested. 
         [0035]    3) Strongly absorbing samples tended to yield Raman signatures that showed evidence for photodegradation when recorded with laser wavelengths shorter than 257 nm. 
         [0036]    These factors generally describe the “Raman photon efficiency factor,” which is the Raman signal strength divided by excitation wavelength. While Albrecht theory can predict the wavelength for highest Raman cross section, this approximation alone fails to take into account the effect of re-absorption and light penetration into the sample on Raman signal strength.  FIG. 2 , is a plot of the Raman photon efficiency factor, which can be described in 3 regions:
       Off Resonance wavelength: Dominance of ν 4  dependence; minor variances due to molar absorptivity.   Approaching or Pre Resonance: Dramatic increase in Raman scattering &amp; relatively lower increase in molar absorptivity.   High Resonance, High absorptivity: Modest increasing Raman scattering strength relative to molar absorptivity resulting in higher probability of photo/thermal induced change or degradation.       
 
         [0040]    What was found is that as the excitation wavelength approaches resonance, Raman signal strength is balanced by increased scattering strength, but lower photon-molecule interactions. As the excitation wavelength moves closer to optimum resonance, Raman signal strength increasing further is inhibited by much lower photon-molecule interactions to the point at which signal strength is competing with system noise/interference or efficiency. While energetic/explosive materials can be classified in a small set of groups, each material will have a unique Raman photon efficiency profile. Experiments to collect data on signal strength vs. laser wavelength showed that excitation near 257 nm was near the maximum signal possible (theoretically) for explosive materials PETN, and explosive material components UN and AN for samples that were 100 microns thick. Similar data for explosive materials TNT, RDX, and HMX showed a smaller degree of variation for 100-micron samples, but the signal strength was always better at longer wavelengths. 
         [0041]    To obtain additional data, further studies compared 257-nm Raman signals of explosives with spectra obtained from randomly selected vehicle panels to determine if fluorescence would severely limit sensitivity. Some of the vehicle panels exhibited overwhelming fluorescence under 257-nm illumination. As such, three additional laser wavelengths were also evaluated: 253 nm, 248 nm, and 244 nm. Because the excitation wavelength was being incremented in small steps (Δλ=4 nm or 5 nm), it was possible to identify a “Raman to Fluorescence Power Ratio” at which the Raman signal was optimal in regard to interfering signals from the background and the Raman photon efficiency factor. 
         [0042]    Shown in  FIGS. 3A-C  are fluorescence spectra obtained from 17 randomly selected painted vehicle surfaces, recorded with a Ti:sapphire laser operating at 257 nm, 253 nm, and 248 nm, respectively. As can be seen, the strength of the observed fluorescence decreases significantly as the excitation wavelength is shifted from 257 nm to 248 nm. Importantly, studies have shown that the intensity of the Raman signal from the explosives of interest do not change as dramatically with excitation wavelength. Thus, the Raman to Fluorescence Power Ratio that is larger with 248-nm excitation is of importance. This point is underscored by the data presented in  FIGS. 4A and 4B , which show Raman to Fluorescence power ratios for PETN and Semtex (a formulation including PETN and RDX), respectively, disposed on the indicated vehicle as a background for the explosive material. 
         [0043]    Details of vehicles used in experiments described herein include: 
         [0000]                                                    Panel #   Year   Make &amp; Model   Color                           2   1995   Lincoln Continental   dark blue           4   1994   Ford Econoline   silver            5*   1994   Ford Thunderbird   teal           15*   1993   Pontiac Firebird   orange           31*   1995   VW Jetta   red           32*   1991   Jaguar X86   burgundy                        
Panels from vehicles denoted by an asterisk are employed in separate experiments as discussed further below.
 
         [0044]    The Raman to Fluorescence Power ratios in the waveband of interest was derived by computing absolute spectrum variance over the Raman signal waveband of 700-2000 wavenumbers (cm −1 ). It is of note that including the Carbon-Hydrogen bond (CH) region of the signature (waveband 2000-3000) would decrease values even further, but the predominant information used in detecting and identifying the explosive residue is contained in the waveband used and therefore a better system design metric. 
         [0045]    Experiments have shown the ability to detect TNT, PETN, and AN at concentrations as low as 3-50 μg/cm 2  from a standoff distance of 10 meters using moderate laser power (˜100 mW). Notably, positive detections were reported for samples containing masses as low as 45 μg of PETN, 5 μg of RDX, and 500 ng AN. 
         [0046]    More specifically, vehicle panels #5, #15, #31, and #32 (see table above) were randomly selected for testing with deposited explosive residues. The spectra obtained from these panels are shown in  FIGS. 5A and 5B . This set of Raman spectra qualitatively represents the four types of spectra that were observed. Note that the intensity of the recorded signal from vehicle panel #15 was found to be considerably stronger than other vehicle panels. The spectrum of #31 has been vertically offset in  FIG. 5B  for clarity. Raman bands attributable to ambient nitrogen and oxygen are marked. The oxygen Raman band is obscured by overlap with Raman bands from each surface. 
         [0047]    Shown in  FIGS. 6A and 6B  and  FIGS. 7A and 7B  are spectra obtained from residues of PETN ( FIG. 6A ), TNT ( FIG. 6B ) and AN ( FIGS. 7A and 7B ) deposited onto various surfaces at 10-meter standoff using 248-nm excitation. Each spectrum shown in  FIGS. 6A and 6B  corresponds to the average of 25 contiguous acquisitions (for a total integration time of 850 msec), and those in  FIGS. 7A and 7B  correspond to varying amounts of signal averaging. More specifically,  FIG. 7A  depicts UV spectra of AN residues on aluminum-coated glass, and  FIG. 7B  shows a plot of vehicle surface #31. The corresponding number of acquisitions, each 34 msec in duration, is shown on the right-hand side of  FIG. 7B . Traces have been vertically offset for clarity. Estimated surface concentrations of AN for  FIG. 7A  and  FIG. 7B  were ˜10 μg/cm 2  and ˜20 μg/cm 2 , respectively, which are barely detectable by the eye. 
         [0048]    In all cases it was possible to find a portion of the dataset that yielded signal-to-noise ratios greater than 10 for the prominent band of AN in less than one second of total integration (25 acquisitions). 
         [0049]    As noted, embodiments of the present invention employ “deep uv” laser wavelengths, namely those at 250 nm or below. The use of this particular wavelength range is non-trivial, since it has been established that fluorescence at 262 nm, and even perhaps for all wavelengths from 250 nm to 1000 nm, will render detection of trace explosives at significant stand off range (&gt;5 meters) difficult if not impossible. The Raman sensor described herein also has the capability to interrogate bulk samples of larger than trace amounts. 
         [0050]    As mentioned, detectable substances include explosive materials, toxic chemicals, biologicals, and chemical warfare agents, and other materials that have Raman signatures. 
         [0051]    The systems and methods described herein may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The foregoing embodiments are therefore to be considered in all respects illustrative and not meant to be limiting.