Laser system for detection and identification of chemical and biological agents and method therefor

FIELD OF THE INVENTION

[0001] The present invention relates to standoff detection and identification of chemical and biological agents in a gaseous form, and more particularly to detection and identification of chemical and biological agents in a gaseous form from a standoff distance using a micro-doppler ladar system.

BACKGROUND OF THE INVENTION

[0002] It is often desirable to know the constituents or substances in a gaseous cloud from a distance before any personnel enter or venture near to the gaseous cloud. In particular, in modern times many chemical and biological agents, such as those for chemical and biological warfare, can be emitted into the atmosphere as a gaseous or amorphous cloud. When these substances are emitted into the atmosphere it is generally unknown what substances are present and what precautions must be taken. It is also desirable to know, also from a standoff distance, what substances might be present in a chemical plume such as from a chemical fire or spill. Though these are merely exemplary, it is understood that it is generally desirable to know the constituents and substances present in a gaseous cloud or region before any individual comes in contact with, or close proximity to, the cloud or region.

[0003] Though lab techniques for identifying different compounds and biological agents are generally well known in the art, doing such identification at a distance without confining the substance in any manner is often very difficult. Many times chemical spills, accidents, or even intentional acts, which release harmful chemical and biological agents into the atmosphere, are done well outside of controlled circumstances that allow for easy testing and identification of the chemical agents in the plume. It is desirable to identify unknown substances so that the personnel who must approach the plumes can take appropriate precautions.

[0004] One method uses raman back scattering spectroscopy. In this method, a monochromatic laser is pulsed through a cloud or region of suspicious gas. The internal vibrational characteristics of the molecules back scatter some of the photons without changing them if the photons match the vibrational characteristics of the molecules. The signals from a raman back scattering spectroscopy are generally very weak and therefore require that a very powerful laser be used for this purpose.

[0005] Other techniques such as Differential Absorption Lidar (DIAL) use infrared wavelengths to identify chemical species in a manner similar to conventional infrared absorption spectroscopy. An infrared laser beam is shined at a cloud or region of gas and light is reflected from the cloud. As the light is scattered back, different conditions act on the reflected light, such as range absorption of the suspicious region and atmosphere scattering, all allowing for the characterization of absorption lines to characterize the various chemical species present in the cloud or plume.

[0006] These systems, however, have inherent drawbacks such as requiring high powered lasers with very specific wavelength capabilities and highly absorbable infrared radiation. Many techniques that are generally used in the lab provide for highly specific characterizations of unknown molecules which these previously known methods and systems do not provide. One known technique can identify unknown gaseous molecules and even quantify the same photo acoustic spectroscopy (PAS). With reference to FIG. 1, a well-known PAS apparatus <b>10</b> is shown. The PAS apparatus <b>10</b> can be used to identify and quantify a gaseous substance which has been placed in a sample cell <b>12</b>.

[0007] Briefly, PAS apparatus <b>10</b> works by providing a black body <b>14</b> which emits a ray <b>16</b> of infrared (IR) light. This IR light is focused by a mirror <b>18</b> towards the sample cell <b>12</b>. A tuning source <b>19</b>, such as a diffraction grading, limits the frequency of infrared light reaching the sample cell <b>12</b> at any given moment. Therefore, the frequency of infrared light reaching the sample cell <b>12</b> is known. The tuning apparatus <b>19</b> can tune the infrared ray of light <b>16</b> over a plurality of frequencies. A chopper <b>20</b> chops the ray of light <b>16</b> before it enters the sample cell <b>12</b>. This produces an intermittent ray <b>16</b><i>a </i>of light. The sample cell <b>12</b> includes a gaseous material which, when heated by a specific frequency of the ray <b>16</b><i>a </i>of IR light, expands. Therefore, when the ray of light <b>16</b> is tuned to a specific frequency it will cause an expansion of the sample in the sample cell <b>12</b>, if the sample absorbs that specific frequency of light. If the sample in the sample cell <b>12</b> expands, it will also then contract as that frequency of light is changed. This expansion and contraction of the sample cell produces a photo acoustic effect.

[0008] Microphones <b>22</b> and <b>24</b> detect the photo acoustic effect produced in the sample cell <b>12</b> and send the response to a processor <b>26</b>. The processor <b>26</b> can then determine the identify of the sample that is present in the sample cell <b>12</b>. The processor <b>26</b> knows the frequency of the ray of light <b>16</b> that reach the sample cell <b>12</b> to produce the photo acoustic effect at any given moment. Therefore, as each of a plurality of frequencies of light reach the sample cell <b>12</b>, the processor <b>26</b> can create a spectrum for the sample in the sample cell <b>12</b>. Comparing the spectrum to known spectrums, the processor <b>26</b> can then determine the identity of the sample in the sample cell <b>12</b>. Quantification of the sample may also be performed due to the fact that the greater concentration of sample particles the greater the photo acoustic effect detected by the microphones <b>22</b>, <b>24</b>.

[0009] It would be highly desirable to be able to use such a system from a standoff distance to determine the substances in a gaseous cloud or region without enclosing that cloud or region in a sample cell. This would allow for determination of a gaseous substance from a distance without requiring high powered lasers or other devices.

SUMMARY OF THE INVENTION

[0010] The present invention is directed to a micro-doppler ladar comprising a tunable laser to identify an unknown constituent of a gaseous cloud or region. The tunable laser produces a laser beam tunable over a range of different frequencies. Each frequency produces a different vibrational response, creating a vibrational signature, in the constituent in the gaseous cloud. A micro-doppler ladar system then interprets a back scattered beam to determine the induced signature. The determined induced signature can then be compared to known signatures to identify the constituent in the cloud.

[0011] A first preferred embodiment of the present invention provides a apparatus to identify a substance in a gaseous cloud at a distance from the gaseous cloud. The apparatus comprises a laser which produces a laser beam tunable to one of a plurality of physical characteristics of laser light such that the substance in the gaseous cloud reacts. The apparatus also includes a micro-doppler system which receives a beam of light reflected back from the gaseous cloud, wherein the reflected beam includes a sample signature formed in the reflected beam produced by the reaction. A data storage device is used for storing known signatures of a plurality of substances. A processor then determines whether the sample signature substantially matches any of the stored known signatures. A display system is used to display the determination made by the processor.

[0012] A second preferred embodiment of the present invention comprises a ladar system for determining a constituent of a gas. The ladar system comprises a tunable laser beam tunable to a plurality of substantially discrete physical characteristics. The system further comprises a micro-doppler ladar system which senses a laser beam after the laser beam has been reflected from the constituent. The tunable laser beam provokes a plurality of discrete sample constituent characteristics in the constituent, wherein each discrete sample constituent characteristic is provoked by each discrete physical characteristic of the tunable laser beam. A processing system is incorporated into the system that comprises a memory system having a look-up table of a plurality of known constituent characteristics, and a processor. The processor can determine the constituent in the gas by comparing stored constituent characteristics in the look up table with the provoked response to find a match.

[0013] The present invention also provides for a new method of determining a constituent in a gaseous cloud from a safe distance. The method comprises providing a laser which is capable of producing a laser light beam which is tunable to a plurality of selectable physical characteristics. The laser light beam, comprising a selectable set of physical characteristics, is directed at a gaseous cloud including at least one unknown substance. The laser light beam produces a sample physical response in the substance due to the selectable set of physical characteristics. A look-up table of known physical responses and associated known substances is provided. Comparing the sample physical response to the look-up table of stored physical responses allows one to determine an identity of the substance in the gaseous cloud.

[0014] Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:

[0016]FIG. 1 is a diagrammatic view of a prior art photo-acoustic response spectroscopy system;

[0017]FIG. 2 is a diagrammatic view of a photo-accoustic micro-doppler ladar system according to a preferred embodiment of the present invention; and

[0018]FIG. 3 is a detailed block diagram of the photo-accoustic micro-doppler ladar system in FIG. 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses.

[0020] With reference to FIG. 2, a photo-accoustic micro-doppler ladar system <b>100</b> in accordance with a preferred embodiment of the present invention is shown. The system <b>100</b> is used to identify an unknown constituent, such as a chemical and biological species or substance, which is present in a gaseous cloud or area <b>300</b> from a safe distance.

[0021] The system <b>100</b> includes a tunable laser <b>102</b> which produces a tunable laser beam <b>104</b> having a plurality of selectable and discrete physical characteristics, such as a plurality of different and selectable wavelengths of light. The tunable laser beam <b>104</b> is focused through a lens <b>106</b> before the tunable laser beam <b>104</b> reaches the cloud <b>300</b>. After reaching the cloud <b>300</b>, a photo-accoustic response is created in the cloud <b>300</b>. The system <b>100</b> also includes a micro-doppler ladar <b>200</b>. The micro-doppler ladar <b>200</b> produces a highly coherent laser beam <b>202</b> which is also known as a sample laser beam, produced by a sample laser <b>212</b> (illustrated in FIG. 3). The sample laser beam <b>202</b> is focused through a lens <b>204</b> before the sample laser beam <b>202</b> reaches the cloud <b>300</b>. The sample laser beam <b>202</b> is the sample retrieval laser beam. After the sample laser beam <b>202</b> has engaged the cloud <b>300</b>, it reflected or back scattered and forms the backscattered or reflected laser beam <b>206</b>. The back scattered laser beam <b>206</b> is reflected back through the focusing lens <b>204</b> to engage the micro-doppler ladar <b>200</b>.

[0022] The reflected laser beam <b>206</b> is first processed in the micro-doppler ladar <b>200</b>. A signal is generated by the micro-doppler ladar <b>200</b> which is interpreted by a processor system <b>208</b>. Finally, a determination from the processor system <b>208</b> is sent to a display unit <b>210</b>, via a signal, to be displayed. It will be understood that the display unit <b>210</b> may comprise any appropriate display unit. For example, the display unit <b>210</b> may include an LED display to indicate a safe or unsafe gaseous cloud <b>300</b>, more complex CRT or LCD systems, which would give a description or probability of chemical and biological species present.

[0023] With reference to FIG. 3, the photo-accoustic micro-doppler ladar system <b>100</b> is illustrated in greater detail. The tunable laser <b>102</b> may be any appropriate tunable laser generally known in the art. As a brief description, not meant to be technically exhaustive, the tunable laser <b>102</b> generally includes a non-linear amplification material <b>116</b> which is pumped with a short wave laser or short wave radiation source <b>118</b>. The short wave source <b>118</b> pumps the amplification material <b>116</b> which then emits a light beam including photons of two different wavelengths. The beam emitted by the amplification material <b>116</b> is the tunable laser beam <b>104</b>. It will be understood, however, that the lens <b>106</b> may include prisms and gratings to columnate the emitted photons to increase the efficiency of the tunable laser beam <b>104</b>. The energy of the two emitted photons is always equal to the energy of the pumping laser radiation source <b>118</b>. The tunable laser beam <b>104</b> is then emitted through lens <b>106</b>.

[0024] The tunable laser beam <b>104</b> is tuned by altering the amplification material <b>116</b>. The amplification material may be any appropriate material or tunable parametric oscillator, such as lithium niobate (LINBO₃). Appropriate materials exhibit non-linear optical properties wherein a single photon will cause the emission of two others. It will be understood that other appropriate lasers include dye and diode lasers. The non-linear properties of the amplification material <b>116</b> allow that a change in orientation or temperature of the amplification material <b>116</b> will alter the frequencies of the two emitted photons. Therefore, altering or changing the temperature and/or orientation of the amplification material <b>116</b> allows the tunable laser beam <b>104</b> to be tuned to a plurality of frequencies. Also, by altering the amplification material <b>116</b> to a known temperature and orientation, a known frequency of light will be emitted from the amplification material <b>116</b>. In this way, tunable laser beam <b>104</b> having a known frequency is produced.

[0025] The orientation and temperature of the amplification medium <b>116</b> is controlled by the processor system <b>208</b>. Therefore, the signal received by the micro-doppler ladar <b>200</b> can be associated with a known frequency of the tunable laser beam <b>104</b>.

[0026] The tunable laser beam <b>104</b> reaches the cloud <b>300</b> and heats a portion of the cloud <b>300</b>. As the tunable laser beam <b>104</b> is tuned over a range of frequencies, different amounts of expansion will occur depending upon the constituent that is present in the cloud <b>300</b>. Also, as the frequency of the tunable laser beam <b>104</b> changes from one frequency to the next, the response produced by the first frequency will change as the tunable laser beam <b>104</b> goes to the second frequency, thereby allowing a relaxation or condensation of the substance due to being illuminated at the first frequency. By tuning the tunable laser beam <b>104</b> from one frequency to another, the tunable laser beam <b>104</b> is effectively chopped. As described above in relation to the PAS apparatus <b>10</b>, the vibrational response in the sample is induced by an expansion and contraction of the sample due to illumination by the intermittent beam. The constituents in the cloud <b>300</b>, likewise, expand at a certain photon frequency and then contract as that photon frequency is eliminated. The constituents in the cloud <b>300</b> expand and contract creating a acoustic effect or vibration induced by the photons of the tunable laser beam <b>104</b>, this is the photo acoustic effect. This photo acoustic effect creates a doppler effect in the reflected beam <b>206</b>. The reflected beam <b>206</b> is then received by the micro-doppler ladar <b>200</b>.

[0027] Once the tunable laser beam <b>104</b> has been used to excite and heat the gaseous cloud <b>300</b>, so as to produce the photo acoustic effect, the reflected beam <b>206</b> is received by the micro-doppler ladar <b>200</b>. The micro-doppler ladar <b>200</b> may be one similar to that described in U.S. Pat. No. 5,867,257 to Rice et al. entitled “Battlefield Personnel Threat Detection System and Operating Method Therefore”, incorporated herein by reference. Generally, the micro-doppler ladar <b>200</b> comprises a laser <b>212</b> which produces the sample laser beam <b>202</b>. The sample laser beam <b>202</b> reaches the cloud <b>300</b> and is then reflected back as reflected laser beam <b>206</b>. The micro-doppler ladar <b>200</b> receives the reflected beam <b>206</b>, which will be out of phase from the sample laser beam <b>202</b> depending upon the vibrations of the constituents in the cloud <b>300</b>. The reflected beam <b>206</b> is received through the lens <b>204</b>. FIG. 3 illustrates that both the sample laser beam <b>202</b> is emitted and the reflected beam <b>206</b> is received through a single lens <b>204</b>. It will be understood, however, that the single lens <b>204</b> may be separated so that the sample beam <b>202</b> and the reflected beam <b>206</b> each have their own lens. It will be further understood that with the proper design, the lens <b>106</b>, for the tunable laser beam <b>104</b>, and the lens <b>204</b>, for the micro-doppler ladar <b>200</b>, may also be a single lens unit. The lens <b>204</b> focuses the reflected beam <b>206</b> as it enters the micro-doppler ladar <b>200</b>. An amplification module <b>214</b> may be provided to amplify the reflected beam <b>206</b> to make the signature, as discussed herein, more easily detectable and to thereby facilitate analysis of the reflected beam.

[0028] The phase shift in the reflected beam <b>206</b> can be used to determine the velocity or vibration frequency produced in the gaseous cloud <b>300</b>. As will be appreciated by those skilled in the art, the reflected beam <b>206</b> has a phase which varies due to the vibrations and distance of the cloud <b>300</b>. This phase characteristic is the important characteristic which is relayed by the reflected beam <b>206</b> back to the system <b>100</b>.

[0029] After reflected beam <b>206</b> has been amplified by the amplification module <b>214</b>, the signal can be further processed if necessary. The processing of a reflected beam in a micro-doppler ladar <b>200</b> is well known in the art. One appropriate method and apparatus is discussed in U.S. Pat. No. 5,867,257 incorporated above. Briefly, the signal, produced by the amplifier <b>214</b>, is communicated to the micro-doppler base band module <b>216</b> and to an integrator/low pass filter (I/LPF) <b>218</b>. The micro-doppler base band module <b>216</b> determines the distance of the cloud <b>300</b> from the system <b>100</b>. The vibrational characteristics of the constituents in the cloud <b>300</b> are determined in the I/LPF <b>218</b>. The distance determined by the micro-doppler base band module <b>216</b> and the vibrational characteristics determined by the I/LPF <b>218</b> are then transferred to a processor <b>220</b> in the processor system <b>208</b>.

[0030] The processor <b>220</b> then determines a sample “signature” for the constituent which is in the gaseous cloud <b>300</b>. The processor <b>220</b> first determines the vibrational rate of the constituent in the cloud <b>300</b> for each of the frequencies of the tunable laser beam <b>104</b>. Combining the vibrational rates for each frequency produces the signature for each of the constituents of the gaseous cloud <b>300</b> which is then compared to known signatures. The known signatures are stored in a memory core <b>220</b> in the processor system <b>208</b>. The processor <b>220</b> then determines, within an appropriate confidence interval, the identity of the constituent of the gaseous cloud <b>300</b>. It will be understood that the memory core <b>220</b> may also store the sample signatures so that the comparison need not occur in real time. That is, the comparison need not occur only as each vibrational rate of the sample signature is determined. Once the determination is made, the display unit <b>210</b> displays the identity of the constituents of the gaseous cloud <b>300</b>.

[0031] Signatures for different constituents are created by inducing a different photo acoustic effect or a different magnitude of effect for each frequency of the tunable laser beam <b>104</b>. The signature is a “graph” of a measurement of the photo acoustic effect at several photon wavelengths. If a laser other than tunable laser <b>102</b> were used, then the photo acoustic effect would be singular. Simply, a non-tunable laser would provoke only one photo acoustic effect for each constituent. It is likely, however, that more than one constituent would have an identical photo acoustic effect for any given photon frequency of the laser beam. Therefore, since the tunable laser beam <b>104</b> is produced by a tunable laser <b>102</b>, a different photo acoustic effect can be induced for each frequency of the tunable laser beam <b>104</b>. This plurality of unique photo acoustic effects provides a signature or fingerprint for each constituent. It is unlikely that any two constituents will have an identical or substantially similar signature. In addition, only known harmful chemicals or biological agents need be stored in the memory core <b>220</b>. Furthermore, the signatures can be limited to only a few predetermined photon frequencies. These predetermined frequencies can be selected for scanning. Thus, only the predetermined frequencies must be produced by the tunable laser <b>102</b> in the tunable laser beam <b>104</b>. This will reduce the number of frequencies that must be processed by the processor <b>220</b> to only those necessary to provide an appropriate confidence level to identify different constituents.

[0032] Using the present invention allows identification of unknown chemical and/or biological constituents in a cloud or plume from a safe standoff distance. It will be understood, however, that any appropriate tunable laser and micro-doppler system may be used in the present invention. It will also be understood that various tunable lasers using different amplification materials <b>116</b> can be used depending upon the possible constituent(s) of the gaseous cloud <b>300</b>. For example, biological constituents, such as anthrax spores, are more likely to produce photo acoustic effects with an ultraviolet laser. However, organic compounds, such as nerve gases, are more likely to produce photo acoustic effects with mid to long wavelength infrared light. Finally, nuclear weapons or nuclear material will emit or create iodine isotopes, such as <b>1</b>¹³¹, that may be determined by creating a photo acoustic effect with a tunable laser <b>102</b> tuned through a very narrow bandwidth. Therefore, a micro-doppler ladar system <b>100</b> may include one or more than one tunable laser <b>102</b> to produce more than one tunable laser beam <b>104</b>, depending upon the range of detection desired or necessary. Therefore, the description of a micro-doppler ladar system <b>100</b> with only a single tunable laser <b>102</b> is not meant to limit the scope of the present a invention.

[0033] The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.