Abstract:
An spectrometer including Raman and LIBS spectroscopy capabilities is disclosed. The spectrometer includes a laser source configurable to produce a lased light directable towards a target substance, the laser source having a single wavelength and having sufficient power to cause a portion of the target to emit Raman scattering and sufficient to ablate a portion of the target substance to produce a plasma plume. A separate remote light collector is optically configurable to collect light emitted from the portion of the target emitting Raman scattering and from the portion of the target producing the plasma plume. A filter is optically coupled to the remote light collector to remove reflected light and Rayleigh-scattered light, and a spectroscope is optically coupled to the filter and configured to separate the collected and filtered light into a frequency spectrum comprising a Raman spectrum and a laser-induced breakdown spectrum. Finally, an electronic light sensor is used to record the frequency spectrum.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a non-provisional application claiming priority from U.S. Provisional Application Ser. No. 61/204,373, filed Jan. 5, 2009, and incorporated herein by reference in its entirety. 
     
    
     STATEMENT OF GOVERNMENTAL SUPPORT 
       [0002]    At least a portion of the invention disclosed and claimed herein was made in part utilizing funds supplied by Office of Naval Research under Grant No. N00014-08-1-0351. The United States Government has certain rights in this invention. 
     
    
     FIELD OF THE DISCLOSURE 
       [0003]    The present disclosure relates generally to spectrometry and more particularly to methods and apparatus for remote Raman and Laser-Induced Breakdown Spectrometry. 
       BACKGROUND OF RELATED ART 
       [0004]    Spectrometry is a technique used to assess the concentration or amount of a given material. The apparatus that performs such measurements is a spectrometer or spectrograph. Spectrometry is often used in physical and analytical chemistry for the identification of substances through the spectrum emitted from or absorbed by them. Spectrometry is also often used in remote sensing applications, such as explosive detection. 
         [0005]    One example method of spectrometry is Raman spectroscopy. Raman spectroscopy uses the inelastic scattering of light to analyze vibrational and rotational modes of molecules. Because vibrational information is specific to the chemical bonds, atomic mass of the atoms in the bond and symmetry of molecules, Raman spectroscopy provides a “fingerprint” by which the molecule can be identified. 
         [0006]    Another example method of spectrometry is Laser-induced breakdown spectrometry (LIBS). LIBS uses a high-power laser focused onto the surface of a sample to produce plasma. Light from the plasma is captured by spectrometers and the characteristic spectra of each element can be identified, allowing concentrations of elements in the sample to be measured. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is an illustration of an example apparatus suitable for measuring remote LIBS and Raman spectra. 
           [0008]      FIG. 2  is a graph illustrating one example of both Raman, and Raman/LIBS spectra as measured by the example apparatus of  FIG. 1 . 
           [0009]      FIG. 3  is a graph illustrating one example of LIBS spectra as measured by the example apparatus of  FIG. 1 . 
           [0010]      FIG. 4  is graph of another example data set measured by the example apparatus of  FIG. 1 . 
           [0011]      FIG. 5  is graph of another example data set measured by the example apparatus of  FIG. 1 . 
           [0012]      FIG. 6  is graph of another example data set measured by the example apparatus of  FIG. 1 . 
           [0013]      FIG. 7  is graph of another example data set measured by the example apparatus of  FIG. 1 . 
           [0014]      FIG. 8  is graph of another example data set measured by the example apparatus of  FIG. 1 . 
           [0015]      FIG. 9  is graph of another example data set measured by the example apparatus of  FIG. 1 . 
           [0016]      FIG. 10  is graph of another example data set measured by the example apparatus of  FIG. 1 . 
           [0017]      FIG. 11  is graph of another example data set measured by the example apparatus of  FIG. 1 . 
           [0018]      FIG. 12  is graph of another example data set measured by the example apparatus of  FIG. 1 . 
           [0019]      FIG. 13  is graph of another example data set measured by the example apparatus of  FIG. 1 . 
           [0020]      FIG. 14  is graph of another example data set measured by the example apparatus of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    The following description of the disclosed examples is not intended to limit the scope of the invention to the precise form or forms detailed herein. Instead the following description is intended to be illustrative of the principles of the invention so that others may follow its teachings. 
         [0022]    Raman spectroscopy typically uses either a continuous wave (CW) or a pulse visible laser of modest average power (e.g., approximately 100-700 mW/cm 2 ) to identify the molecular fingerprint of the sample from its Raman spectrum. For a continuous wave laser based remote Raman system, there are generally two significant issues: interference of the high-ambient light background during the day; and long-lived fluorescence with the Raman spectra of the sample. However, as described herein, by utilizing a pulsed laser system and gated receiver, these limitations may be overcome. Pulsed Raman spectroscopy offers two important benefits: the ability to discriminate against unwanted light; and the ability to discriminate against long-lived fluorescence emissions from the sample. 
         [0023]    In general, LIBS uses a pulsed laser having a typical wavelength of 1064 nm, and a high peak power (e.g., &gt;1 GW/cm 2 ) for ablating material from the surface of a sample and to probe elemental composition. During LIBS, a small amount of the target is ablated and atomized, and the resulting atoms are excited to emit light. The emitting elements are identified by their unique spectral peaks, and the process yields semi-quantitative abundances of major, minor, and trace elements simultaneously. Advantageously, laser ablation profiles through dust and weathering layers, meaning that the LIES technique is effective in cleaning sample surfaces. By repetitive sampling with a high intensity laser at the same spot, the LIES technique permits ablation through weathered surfaces to reach the underlying bulk rock. 
         [0024]    Both remote Raman and remote LIES spectroscopic techniques interrogate samples using pulsed laser beams and both employ dispersive spectrographs over over-lapping spectral ranges with approximately the same spectral resolution. Therefore, as described herein, it is possible to combine the two techniques into one system. In particular, at low laser power density at the sample, one obtains the Raman spectra and at high laser power, with high optical power density, one can also generate the LIES plasma spectra and record the LIBS spectra from the same material. Both the Raman and LIBS spectra can be recorded simultaneously with a focused laser where the area falling in the high energy density region produces the LIBS spectra and molecules exited by the edges of the focused beam corresponding to a low energy density region generates the Raman spectra. 
         [0025]    By combining remote Raman spectroscopy and remote LIBS in a single apparatus a more complete analysis of a sample in terms of mineralogy and elemental composition is possible. For example, Raman spectroscopy can determine whether the sample is calcite or aragonite, both of which have the same chemical composition but different mineral structures that often indicate different geological formation processes. LIBS analysis is sensitive to typical cationic elements and is also capable of remotely detecting hydrogen, carbon, oxygen, and sulfur. Raman spectroscopy mostly distinguishes mineral structure and anionic species from Raman active lattice modes of the mineral, and can provide only indirect information about cations from internal vibrational mode of polyatomic cations that are sensitive to cationic composition. Usually Raman active lattice modes are much more sensitive to composition and polymorphism than the internal modes of vibrations. 
         [0026]    Additionally, for the detection of organic compounds, Raman spectroscopy is a more desirable technique than LIES as it can distinguish between various hydrocarbons such as benzene, naphthalene, methane, and their various chemical isomers (e.g., o-, m-, p-xylene, etc.); and different kind of organic molecules, such as protein, lipids, amino acids, complex molecules such as pigments, mycosporines, etc. 
         [0027]    Moreover, LIBS can detect small amounts of impurities in a bulk sample, but the trace analysis is difficult with Raman spectroscopy because the Raman signal is proportional to the number of molecules excited by the laser in the sample. A few molecules of impurity produce a very small Raman signal compared to the signal produced by the host bulk sample. 
         [0028]    Accordingly, there is a desire to combine the two techniques and to develop various methods and apparatus that include a single instrument capable of analyzing a sample from a remote distance and capable of obtaining both Raman and LIBS spectra. The combined Raman and LIBS methods and apparatus described herein may be used for a variety of applications, including, for example, in the remote identification of explosives, explosive residue, chemical, minerals, gases, fume clouds, and/or other chemicals of interest. Additionally, by providing for a remote sensing capability, the described apparatus and/or operator may be located a distance away from the sample, providing for a greater range of detection and/or safety. 
         [0029]    Referring now to  FIG. 1 , a diagram illustrating an example combined Raman spectroscopy and LIBS apparatus  10  is shown. The example apparatus  10  generally includes a laser source  12 , an emitted light collector  14 , and a spectrograph  16 . Specifically, in this example, the laser source  12  emits a lased light such as, for example, a laser beam  18 . The example laser beams  18  may travel through a beam expander  20  to focus and shape the laser beam as desired. The resultant laser beam  18  is then directed toward the intended remote target  22 . In this example, a plurality of prisms  24  are used to direct the laser beam  18 , however, any suitable object, device, and/or means of directing the beam  18  may be utilized including, for example, a splitter, mirror(s), etc. In each example, the laser beam  18  is of sufficient power to cause the generation of an emission signal  26  at the target  22  having at least one of Raman excitation and LIBS plasma formation as described herein. 
         [0030]    As noted, Raman excitation generally relies upon inelastic scattering, or Raman scattering, of the laser beam  18 , such as, for instance, the vibrational, rotational, and/or other low-frequency excitation modes in the target  22 . In particular, the laser light  18  interacts with molecular vibrations or other excitations in the target  22 , resulting in the energy of the laser photons being shifted up or down. The shift in energy gives detectable information (e.g., the signal  26 ) about the Raman active vibrational and rotational modes in the target  22 . 
         [0031]    Similarly, as described, LIBS operates by focusing the laser beam  18  onto a small area at the surface of the target  22 . During LIES, the laser beam  18  is of sufficient power to ablate at least a portion of the target  22  to generate a detectable plasma plume. In particular, as the plasma plume expands and cools, the plume emits atomic emission lines (e.g., the signals  26 ) characteristic of the elements comprising the target  22 . 
         [0032]    After the laser beam  18  strikes the target  22 , the emitted signals  26  (e.g., at least one of Raman and/or LIES) excited by the laser beam  18  are collected by the collector  14 . The example collector  14  may be any suitable collector, including, for instance, a telescope. In this example, the telescope includes a beam finder  27  which may be used to align the telescope with the beam  18 . The collected signals  26  are then optically directed into the spectrograph  16  for processing. In at least one example, however, the collected signals  26  are first passed through a filter  28 , such as, for example, a notch filter to remove the reflected and Rayleigh-scattered light from the signal  26 . Additionally, in this example, the signal  26  is focused into the spectrograph  16  by passing through at least one lens  30 . The example signal  26  may then be processed by the spectrograph  16 . In particular, the spectrograph  16  is configured to separate the signal  26  into a detectable frequency spectrum. For example, to detect the generated frequency spectrum, the example apparatus  10  includes an intensified charge-coupled device (ICCD)  32  optically coupled to the spectrograph  16 . Once detected by the ICCD  32 , the frequency spectrum may be processed by any suitable process, including, for example, by spectrum analysis software. 
         [0033]    In one example of the integrated remote Raman and LIES apparatus  10 , the laser source  12  is a frequency doubled mini Nd:YAG laser source, such as a Model ULTRA CFR, Big Sky Laser, 532 nm, 20 Hz. The use of a double pulse separated, by, for example approximately 0.15 μs to approximately 1.0 μs may enhance the LIBS signal by at least a factor of 10. The example  532  nm pulsed laser beam is focused to a spot (e.g., 300 μm diameter) on the target  22  placed a distance (e.g., nine meters) away with a 10× beam expander  20 . For simple Raman measurements the laser power at 532 nm may be electronically adjusted to approximately 25 mJ/pulse, and for combined LIBS and Raman measurements the laser power may be increased to approximately 35 mJ/pulse. 
         [0034]    In still other examples, the laser source  12  may produce a lased light with any suitable wavelength, such as, for example, 248 nm, 266 nm, 532 nm, 785 nm, and 830 nm for Raman spectra, and 532 nm and 1064 nm for LIES spectra. Still further, it will be appreciated that a Raman spectra may first be produced at a lower laser power and/or by defocusing the laser beam, thereby giving a lower optical power density which is not enough to decompose the sample. The LIBS spectra may then be obtained after the Raman measurement on the same target with a higher laser power and/or a focused laser beam, thereby giving a high enough optical power density to ablate the sample. 
         [0035]    In one example for the combined Raman and LIBS measurements, the signals  26  excited by the laser source  12  are collected with a 203-mm diameter reflecting telescope in 180-degree geometry, such as, for example, a Meade LX200R Advanced Ritchey-Chrétien, 203 mm clear aperture, f/10. In other example, the laser source  12  may be collected with any suitable light collector, including, for example, a suitable camera lens. Additionally, in one example, the collected signal  26  passes through a 532-nm holographic super-notch filter (NF) to remove the reflected and Rayleigh-scattered laser light, and enters the spectrograph  16 , which in this example is a Kaiser F/1.8 HoloSpec transmission-grating spectroscope with a 100-μm slit coupled to an intensified CCD detector, such as a Princeton Instruments, PI-MAX. The HoloSpec spectrometer measures the Raman spectra from 70 cm −1  to 4500 cm −1  when excited with a 532 nm laser. The grating covers the spectral region between 534 nm to 699 nm, which is sufficient for identifying some of the major atomic emission lines of measured targets  22  from their LIBS spectra. It will be understood, however, that the components and/or operating parameters of the example apparatus  10  may be suitably changed, combined, and/or configured as desired. 
         [0036]      FIG. 2  is a graph illustrating one example of both Raman, and Raman/LIBS spectra as measured by the example apparatus  10 . In particular,  FIG. 2  shows a Raman spectra  200  and a combined Raman and LIBS spectra  250  of calcite (CaCO 3 )  200 A,  250 A and of Gypsum (CaSO 4 .2H 2 O)  200 B,  250 B located at approximately nine meters in air and in the 534-615 nm wavelength range. In this example, the Raman spectra  200  of these samples were excited with 25 mJ/pulse of the 532 nm laser and accumulated for one second in the gated mode with a 2 μs gate. As illustrated, the lattice modes  202  of calcite at 155 cm −1  and 282 cm −1  are clearly visible, along with internal modes  204  of carbonate ions at 711 (ν 4 ) cm −1 , 1085 (ν 1 ) cm −1 , and 1434 (ν 3 ) cm −1 . In addition, a combination mode  206  of carbonate ions at 1748 cm −1  and the symmetric stretching modes  208  of atmospheric O 2  and N 2 , respectively at 1556 cm −1  and 2331 cm −1  are also detected. 
         [0037]    In the illustrated Raman spectrum of gypsum  200 B, the strongest Raman peaks  210  observed at 1006 cm −1  originate from the symmetric stretching vibrations of the SO 4   2−  ion, ν 1  (SO 4 ), and its position depends on degree of hydration. The other internal mode of vibration of sulfate ions  212  appear at 1136 (ν3) cm −1 , 415 cm −1  and 495 (ν2) cm −1 , and 621 cm −1  and 671 (ν4) cm −1  are clearly visible. 
         [0038]    In the combined LIES and Raman spectra  250  of calcite  250 A and of gypsum  250 B the Raman lines are marked with the letter “R”. In the example spectrum of calcite  250 A, the 1085 cm −1  Raman line of carbonate  252  appears only as a weak line. In the example combined LIES and Raman spectra of gypsum  250 B, most of the prominent Raman lines of sulfate  254  are clearly visible indicating that the LIES spectrum is produced only by a few hot spots in the focused laser beam. As both calcite and gypsum contain Ca cations, the LIBS lines  250  in  FIG. 2  originate from Ca I excited states. In the combined LIBS Raman spectrum of calcite  250 A the LIBS lines  256  at 588.99 and 589.59 nm indicate presence of trace amount of Na ion in the calcite sample. 
         [0039]    Similarly,  FIG. 3  is a graph illustrating one example of LIBS spectra as measured by the example apparatus  10 . Specifically,  FIG. 3  shows an example of a remote LIBS spectrum  300  of minerals magnetite (Fe 3 O 4 )  302 , hematite (Fe 2 O 3 )  304 , α-quartz (α-SiO 2 , Qz)  306 , and forsterite (Mg 2 SiO 4 , Fo)  308 , respectively. The example graph is limited to the 540-690 nm spectral range, the respective target was located approximately nine meters away, and was excited with a 532 nm laser pulse of 35 mJ/pulse. The LIBS illustrated spectra  300  of magnetite  302  and hematite  304  and are dominated by the Fe I emission lines  310 . Both of these iron oxides contain a few ppm of Na as indicated by the presence of Na I emission lines  312  at 589.0 and 589.6 nm. 
         [0040]    Additionally, the remote LIBS spectra of Fo  308  and Qz  310  show strong emission lines of Si  314  at 634.7 and 637.1 nm, and weak emission lines of Na  316  and Li  318  indicate the presence of these elements in trace amounts. The Fo LIBS spectrum  308  contains strong emission lines of Mg  320  at 552.8 nm and a weak line  322  at 571.1 nm. A number of weak Fe emission lines  324  are also observed in the LIES spectrum of Fo indicating that this sample is indeed Mg-rich olivine (Fo  92 ) containing ˜8% of iron. 
         [0041]    In another example data set reproduced as a graph  400  in  FIG. 4 , another operation of the example apparatus  10  is illustrated. In this example, the apparatus  10  was used to detect the Raman shift of calcite at a distance of approximately ten meters, using only a Raman spectrum. In a first operating example  410 , the apparatus  10  was operated in a continuous wave mode with room lights on and having an average room light of 13 W/m 2 . In a second operating example  420 , the apparatus  10  was again operated in a continuous wave mode with the room lights turned off. Finally, in a third operating example  430 , the apparatus  10  was operated in a gated mode, with the room lights turned on and having an average room light of 13 W/m 2 . 
         [0042]    In another example use of the apparatus  10  illustrated as a graph  500  in  FIG. 5 , the apparatus  10  was used to detect the Raman shift of a homemade explosive chemical at a distance of approximately fifty meters. The laser  12  of the example apparatus  10  was operated at 532 nm, 20 Hz, 30 mJ/pulse, and included a 50 ns gate width. In a first example  510 , the apparatus  10  was utilized to detect potassium perchlorate (KClO 4 ). In a second example  520 , the apparatus  10  was utilized to detect potassium nitrate (KNO 3 ). Finally, in a third example  530 , the apparatus  10  was used to detect ammonium nitrate (NH 4 NO 3 ). In each example, the apparatus  10  provided clear, sharp peaks for chemical identification, with a high signal-to-noise ratio. 
         [0043]    In yet another example use of the apparatus  10  illustrated as a graph  600  in  FIG. 6 , the apparatus  10  was used to detect the Raman shift of various chemicals through a container at a distance of approximately fifty meters. In a first example  610 , the apparatus  10  was utilized to detect water (H 2 O). In a second example  620 , the apparatus  10  was utilized to detect isopropyl alcohol (2-propanol). Finally, in a third example  630 , the apparatus  10  was used to detect acetone (OC(CH 3 ) 2 ). In each example, the apparatus  10  was able to detect the various liquids through plastic and/or glass containers. 
         [0044]    In still another example use of the apparatus  10  illustrated as a graph  700  in  FIG. 7 , the apparatus  10  was used to detect and distinguish between the Raman shift associated with similar chemical, during daytime and at a distance of approximately fifty meters. In a first example  710 , the apparatus  10  was used to detect nitrobenzene (C 6 H 5 NO 2 ). In a second example  720 , the apparatus  10  was utilized to detect ethylbenzene (C 6 H 5 CH 2 CH 3 ). Finally, in a third example  630 , the apparatus  10  was used to detect benzene (C 6 H 6 ). 
         [0045]    As illustrated in a graph  800  in  FIG. 8 , in another example, the apparatus  10  was used to detect gypsum at approximately fifty meters. In this example, the apparatus  10  was used to detect the Raman shift of both the target gypsum and/or the ambient atmosphere. In particular, in a first example  810 , the apparatus  10  was used to detect both the ambient atmosphere and the gypsum target. In a second example  820 , the apparatus  10  was used to detect only the ambient atmosphere. Lastly, in a third example  830 , the apparatus  10  was used to detect only the gypsum target. 
         [0046]    In another example use of the apparatus  10  illustrated as a graph  900  in  FIG. 9 , the example apparatus  10  was used both in a single pulse mode  910 , and in a one-second, twenty pulse mode  920 . In this example, the apparatus was used to detect the Raman shift of cyclohexane (C 6 H 12 ) at a distance of approximately fifty meters. As is illustrated, the detected signals  910  and  920  are proportional to the number of laser pulses. 
         [0047]    In still another example use of the apparatus  10  illustrated as a graph  1000  in  FIG. 10 , the example apparatus  10  was used to detect the Raman Shift of naphthalene (C 10 H 8 ). In a first example  1010  the apparatus  10  was placed approximately fifty meters from the target naphthalene. In a second example  1020  the apparatus  10  was placed approximately one hundred meters from the target naphthalene. As is shown, the detected signals  1010  and  1020  are substantially similar in shift detection and proportional in intensity. 
         [0048]    In another example data set reproduced as a graph  1100  in  FIG. 11 , the example apparatus  10  was used to detect the Raman shift of a target at a distance of approximately one hundred meters. In a first operating example  1110 , the apparatus  10  was used to detect ammonium nitrate (NH 4 NO 3 ). In a second operating example  1120 , the example apparatus  10  was used to detect Potassium perchlorate (KClO 4 ). 
         [0049]    In another example data set reproduced as a graph  1200  in  FIG. 12 , the example apparatus  10  was used to detect the Raman shift of an organic target at a distance of approximately one hundred meters. In a first operating example  1210 , the apparatus  10  was used to detect nitromethane (CH 3 NO 2 ). In a second operating example  1220 , the example apparatus  10  was used to detect Nitrobenzene (C 6 H 5 NO 2 ) and is graphed at ×0.2. 
         [0050]    In an example data set reproduced as a graph  1300  in  FIG. 13 , the results of the operation of the example apparatus  10  is illustrated. In this example, the apparatus  10  was used to detect the LIBS spectrum of Trinitrotoluene (TNT) (C 6 H 2 (NO 2 ) 3 CH 3 ). In this example, the LIES spectrum was produced with a single pulse excitation of a 532 nm laser powered to 35 mJ. The target was placed at approximately nine meters from the apparatus  10 . Additionally, in this example, the gate size of the detector  32  was 8 μs, and the produced laser spot on the target TNT was approximately 629 μm. As can be seen from the graph  1300 , the notch filter  28  creates an area  1310  of filter (e.g., zero) data. 
         [0051]    In an example data set reproduced as a graph  1400  in  FIG. 14 , the result of the operation of the example apparatus  10  is illustrated. In this example, the apparatus  10  was used to detect the LIBS spectrum of eight percent Cyclotrimethylenetrinitramine (8% RDX) on silica. In this example, the LIES spectrum was produced with a single pulse excitation of a 532 nm laser powered to 35 mJ. The target was placed at approximately nine meters from the apparatus  10 . As can be seen from the graph  1400 , the notch filter  28  creates an area  1410  of filter (e.g., zero) data. 
         [0052]    Although certain example methods and apparatus have been described herein, the scope of coverage of this patent is not limited thereto. On the contrary, this patent covers all methods, apparatus, and articles of manufacture fairly falling within the scope of the appended claims either literally or under the doctrine of equivalents.