Abstract:
A method for inspecting a package to identify an object concealed in the package includes passing two beams of THz-radiation through the package. The frequency of THz radiation in one beam is different from that in the other, and the beams are at an angle to each other. Each of the transmitted beams is used to form an image of the package and the object. The absorption coefficient of the object is determined from the two images. The material of the object is determined from the absorption coefficients at the two frequencies. The method is useful for detecting explosive material concealed in baggage.

Description:
TECHNICAL FIELD OF THE INVENTION  
       [0001]     The present invention relates in general to terahertz (THz) lasers. The invention. relates in particular to identifying objects in a container by transmitting radiation from two THz-lasers through the container.  
       DISCUSSION OF BACKGROUND ART  
       [0002]     The terahertz frequency spectral range is a relatively underdeveloped band of the electromagnetic spectrum. The terahertz band is bordered by the infrared on the short-wavelength side and millimeter-waves on the long-wave length side. The terahertz band encompasses radiation having a frequency range of 0.3 to 10.0 THz and wavelengths between about 30 micrometers (μm) and 1 millimeter (mm). The terahertz band is sometimes referred to by practitioners of the art as the far infrared (FIR).  
         [0003]     Many materials that are opaque to wavelengths shorter then 30 micrometers are either transparent or semi-transparent in the terahertz band. Such materials include plastic, textiles, paper, cardboard, wood, ceramics, opaque glasses, semiconductors, and the like. Radiation at longer wavelengths, for example, millimeter waves have better transmissivity than terahertz radiation in these materials but the longer wavelengths are unsuitable for use in high resolution imaging systems. Further, such materials do not have much spectral content, i.e., characteristic absorption lines, in these longer wavelength regions that would allow one to be easily distinguished from another.  
         [0004]     Terahertz radiation is not an ionizing radiation, so it does not have the potential to present health problems as would, for example, X-radiation (X-Rays). Terahertz radiation can be propagated for much longer distances in the atmosphere than X-rays, for example, several meters, and does not cause damage to electronic devices and unexposed film. In addition to offering a higher potential resolution in imaging than millimeter waves, terahertz radiation also offers a potential to provide sharper differentiation between different materials superimposed on one another and, accordingly provide higher contrast images than would be possible with millimeter waves.  
         [0005]     It would be advantageous to exploit the imaging potential of terahertz radiation in security apparatus for examining luggage or packages for concealed objects or substances. Substances could include explosives, drugs, biological agents, and the like.  
       SUMMARY OF THE INVENTION  
       [0006]     The present invention is directed to a method identifying a material of an object concealed in a package. In one aspect, the method comprises recording a plurality of images of the package and the object therein by detecting THz-radiation transmitted through the package. At least two of the images are recorded at different frequencies of said THz-radiation and at least two of the images are recorded at different transmission directions of the THz radiation. The absorption coefficient of an object at the different frequencies is estimated from the recorded images. The estimated absorption coefficients are compared with a table of known absorption coefficients of known materials to determine which of those materials may be the material of the object.  
         [0007]     In one preferred embodiment only two images are recorded. One image is recorded at one transmission direction and one frequency and the image is recorded at another direction and another frequency. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]     The accompanying drawings, which are incorporated in and constitute a part of the specification, schematically illustrate a preferred embodiment of the present invention, and together with the general description given above and the detailed description of the preferred embodiment given below, serve to explain the principles of the present invention.  
         [0009]      FIG. 1  schematically illustrates one preferred embodiment of a apparatus in accordance with the present invention for identifying materials in a non-metallic container, including two THz transmitters each transmitting beams of different THz frequency through the container at an angle to each other, two corresponding THz receivers for receiving the transmitted beams and a processor arranged to compare outputs of the two receivers to identify materials.  
         [0010]      FIG. 2  schematically illustrates one preferred example of a THz transmitter and a THz receiver in a preferred implementation of the apparatus of  FIG. 1 .  
         [0011]      FIG. 3  schematically illustrates another preferred embodiment of a apparatus in accordance with the present invention for identifying materials in a non-metallic container, including two THz transmitters each transmitting beams of different THz frequency collinearly through the container, one diplexer for combining the two beams from the transmitters before the beams pass through the container, another diplexer for separating the two beams after the combined beams have passed through the container, two corresponding THz receivers for receiving the separated beams, and a processor arranged to compare outputs of the two receivers to identify materials.  
         [0012]      FIG. 4  schematically illustrates one preferred example of the two THz transmitters in a preferred implementation of the apparatus of  FIG. 3  illustrating preferred locations for the combining and separating diplexers. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0013]     The object identification method of the present invention relies on the fact that the transparency (transmission) of many dielectric or organic materials to THz radiation decreases with both increasing density of the material and increasing frequency of the radiation. Accordingly, by determining the thickness of an object, and determining THz transmission of an object at two or more THz frequencies, an absorption coefficient of the object can be calculated at each of the different frequencies. The absorption coefficients so determined can be compared with catalog (look-up table) data of absorption coefficient versus frequency for various materials or various material densities to make an approximation of the density of the object. This can provide an indication of a material included in the object.  
         [0014]     Referring now to the drawings, wherein like components are designated by like reference numerals,  FIG. 1  schematically illustrates one preferred embodiment  10  of scanning THz imaging apparatus in accordance with the present invention for estimating the density an object concealed in a non-metallic package. In  FIG. 1 , and in other drawings referred to hereinbelow, the path of optical (THz) radiation is depicted by single-weight lines, either solid or dashed depending on frequency. The direction of propagation of the radiation is indicated by open arrowheads. Electronic connections are depicted by double-weight solid lines with the direction or directions of electronic communication indicated, where appropriate, by one or more solid arrowheads.  
         [0015]     Apparatus  10  includes THz radiation transmitters  12  and  14 . Transmitter  12  delivers a beam  16  of THz radiation  16  at a frequency ν 0 . Transmitter  14  delivers a beam  18  of THz radiation at a frequency ν 1 , different from frequency ν 0 . A package  20  to be examined, here containing a volume  22  of interest (an object) is placed on a platform  24 , which is rotatable about an axis  26  as indicated by arrow φ. Beams  16  and  18  are directed at an angle θ to each other such that the beams intersect within package  24 . Angle θ is preferably between about 5° and 180°. Provision is made to scan package  20  with respect to beams  16  and  18  in orthogonal x and y-axes. This can be achieved by scanning beams  16  and  18  over a fixed position of the package, or by scanning the package, i.e., platform  24  with respect to fixed directions of the beams.  
         [0016]     Beams  16  and  18  are intercepted by THz receivers  30  and  32  respectively after being transmitted through package  20 . The output of each of the THz receivers  30  and  32  is connected to a processor (controller)  36 . Scan signals representative of the x, y, and φ positions of the beams with respect to the package are also transmitted to the processor. This allows two two-dimensional electronic “images”, one at each of the THz frequencies, to be captured by the processor, each image comprising values of transmission T (brightness) as a function of x and y coordinates (T νφ (x,y)) for a particular value of φ.  
         [0017]     Electronic images recorded in the processor may be displayed individually or as a composite (for example, a ratio of the two) on a display screen  38 . As the two images are taken at different angles the composite image can be in effect “stereoscopic”, i.e., providing some shape information on an object or objects therein.  
         [0018]     In one example of operation of apparatus  10 , a scan at the two different frequencies, with the images being taken at different angles, is performed to provide two different images of the entire package, from which can be constructed an overall ratio-image of the entire package. In any image, the outline of the package and the outlines of objects therein would be visible from density (absorption coefficient) discontinuities at edges of the package and the objects.  
         [0019]     The distance in an object traversed by radiation at one of the frequencies can be simply determined from the image at the other frequency, and a knowledge of the angle between the beams forming the images. Providing an angle of 90° between the beams forming the images simplifies the thickness determination. The brightness of an object at each frequency can be estimated at one or more positions in the object. The absorption coefficient of the object at each frequency can then be estimated from the brightness and the thickness (distance traveled in the object) estimate.  
         [0020]     If, for example, if was determined that an object had a thickness of about 10 millimeters (mm) and transmission of the object was about 71% at a frequency of 1.4 THz and 21% at a frequency of about 3.1 THz the object could be estimated to have absorption coefficients of 0.29 cm −1  at 1.4 THz and 0.79 cm −1  at 3.1 THz. If, in the look-up table, the two closest stored sets of absorption coefficient values 0.32 cm −1  for a material A (density A) and 0.55 cm −1  for a material B (density B) at the 1.4 THz frequency, and 0.81 cm −1  for a material A and 0.91 cm −1  for a material B at the 3.1 THz frequency, then it could be assumed that the object was more likely of material A than of material B.  
         [0021]     Clearly a thickness estimate, i.e., an estimated of the distance traveled in the object by a beam, can be made if the images at different angles are taken at the same frequency. In this case, however, at least a third image must be recorded at another frequency to allow the absorption coefficient to be estimated at both frequencies.  
         [0022]     Specific absorption coefficient ranges representing specific materials of interest may be identified by a color code or the like to aid recognition. If an object or volume of interest is identified in the initial “overall” image, the apparatus may be “zoomed-in” on the object by performing another scan within bounding x and y-coordinates of the volume of interest with the same number of scan steps but with shorter distances therebetween. The area to be zoomed can be defined by mouse-controlled movement of a cursor  40  or the like, around the overall image in display  38 . Acquisition and zooming of the object area may also be done automatically by suitable programming of the image processing software. Further shape-information on an object may be obtained processing individual-frequency images or ratio-images at different values of φ. Further shape-information and transmission on an object may also be obtained by arranging apparatus  10  such that angle between beams  16  and  18  can be varied and processing images taken at different values of θ.  
         [0023]     The above presented image processing schemes are but examples of schemes that may be used in apparatus in accordance with the present invention. One skilled in the image processing art, having appreciated principles of the present invention described herein, may devise other image processing or display methods for identifying objects or materials thereof without departing from the spirit and scope of the present invention. Those skilled in the art to which the present invention pertains will also recognize that while apparatus  10  is described as including only two THz transmitters providing two beams at two different frequencies, three or more THz transmitters may be arranged to provide three or more beams at three or more different frequencies. Provision of additional transmitters and different probe frequencies may provide an added degree of certainty in identification of materials, but at the expense of cost and complexity of the apparatus and the necessary image processing.  
         [0024]     THz transmitters for apparatus  10  must most preferably deliver CW radiation at a power of at least about 1.0 milliwatts (mW). This is achievable with certain types of THz lasers. One preferred THz laser for the inventive method is an optically pumped THz-laser in which a gaseous gain-medium is pumped by radiation from a CO 2  laser. A THz-laser may have different nominal frequencies depending on the gaseous THz gain-medium. Any particular gain-medium has different discrete lasing frequencies about some nominal frequency characteristic of that gain-medium. Accordingly, it is possible to select output frequencies ν 0  and ν 1  from many different THz frequencies between about 0.3 THz and 10.0 THz, by selecting a particular gain-medium. Such CO 2  laser-pumped THz-lasers are commercially available. One such commercially-available THz-laser is a SIFIR-50 THz-laser available from Coherent Inc. of Santa Clara, Calif. This laser has excellent spatial mode quality and can emit between about 50 milliwatts (mW) and 100 mW of continuous wave (CW) power.  
         [0025]     CO 2  laser-pumped THz lasers are preferred for apparatus  10  because of advantages including a wide range of available THz frequencies, relatively high power output, and reliability. Those skilled in the art, however, will recognize that, in theory at least, other THz radiation sources both laser and electronic may be used without departing from the spirit and scope of the present invention. By way of example, one possible electronic source of THz radiation is backward-wave oscillator. Such an oscillator can emit greater than 1.0 mW of CW power at (discrete) frequencies up to about 1.5 THz. THz backward-wave oscillators are at a less mature stage of development than THz-lasers and may not be as reliable as commercially available THz-lasers.  
         [0026]     Other possible THz-lasers include Quantum cascade semiconductor lasers (QCL). These have an advantage of being relatively small by comparison with CO 2  laser-pumped THz lasers. Another advantage is that continuous tuning is possible over frequencies up to about 10 GHz. QCL lasers, however, must be operated at liquid Helium temperatures in order to achieve milliwatts of power output. In room temperature operation, output power is presently limited to only a few nanowatts (nW).  
         [0027]     One THz source that is not suitable for use in the present invention is a laser-triggered gallium arsenide (GaAs) photomixer. While such a source can provide radiation over the entire THz range and accordingly be used to generate an entire THz spectrum of a material, output power is limited to tens of nanowatts which is too low to penetrate packages encountered in typical inspection situations.  
         [0028]      FIG. 2  schematically illustrates a preferred implementation  10 A of apparatus  10  including a preferred example of transmitter and receiver scheme and a scanner. The scheme is described with reference to only one of the THz transmitters and the corresponding receiver, here, transmitter  12  and  30 . One skilled in the art will recognize from the depiction of apparatus  10  in  FIG. 1  how a similarly configured transmitter  14  and receiver  32  would be incorporated in the apparatus.  
         [0029]     In the apparatus of  FIG. 2 , THz-radiation source (THz)  12  provides beam  16  (depicted here by only a single solid line) of THz radiation (a signal beam), having a frequency ν 0 . Beam  16  is directed by mirrors  40  and  42  to an x-y scanner  44 . Scanner  44  scans the beam in a raster fashion in x and y-axes perpendicular to each other and perpendicular to the general direction of propagation of the beam or system axis. As two-axis beam scanners are well-known in the art, a detailed description of the scanner is not presented herein.  
         [0030]     The beam from the scanner passes through package  20  at locations corresponding to the instant beam position. Extreme positions of the scanned beam are indicated by lines  16  and  16 ′. The scanned beam is incident on a concave mirror  46  and is reflected thereby to a lens  48 . The mirror and the lens are configured and arranged such that the scanned beam leaves lens  48  in the same direction, whatever the instant scan angle in the total range of scan angles. After leaving lens  48  the scanned beam is directed by another mirror  50  into receiver  30 .  
         [0031]     Receiver  30  is a heterodyne receiver and includes a THz laser  52 , which functions as a local oscillator (LO). A beam  54  of radiation from THz-laser  52  is required to have a frequency that is offset from the frequency ν 0  of the signal beam  16  by a frequency f 0 . Frequency f 0  is one preferred frequency of an electronic signal including image data that will be electronically processed to provide an image of package  20  being scanned including an object or objects therein  
         [0032]     For a frequency offset f 0  between about 0.5 MHz and 15.0 MHz, THz-lasers  12  and  52  preferably have the same gain medium with laser  12  having an output frequency ν 0  near the peak of the gain curve and laser  14  electronically tuned to output radiation at a frequency ν 0 +f 0  or ν 0 −f 0  where these frequencies are frequencies of transitions of the gain medium adjacent the transition of peak gain. This frequency-offsetting method for gas lasers, and circuits therefor, are well known in the art and a detailed description thereof is not necessary for understanding principles of the present invention. A detailed description is included in U.S. patent application Ser. No. 11/085859, filed Mar. 22, 2005, assigned to the assignee of the present invention, and the complete disclosure of which is hereby incorporated by reference.  
         [0033]     The gain-medium of a THz laser typically consists of larger heavy gas molecules, for example, methanol (CH 3 OH) or difluoromethane (CH 2 F 2 ). Because of this, there are many possible laser transitions for any gas, spectrally very closely spaced. Accordingly, values for f 0  using this frequency offsetting method are typically in the above referenced MHz range. For values of f 0  between about 500 MHz and 200 GHz, lasers  12  and  52  preferably have different gain-media.  
         [0034]     Continuing with reference to  FIG. 2 , on entering receiver  30 , signal beam  16  is transmitted through a beamsplitter  56  onto a THz-detector or mixer (MIXR)  58 . Detector  58  is preferably a corner-cube mounted Schottky-diode detector. Such detectors are commercially available, for example, from Virginia Diode Inc. of Charlottesville, Va. Beam  54  from laser  52  is directed by a mirror  60  to beamsplitter  56 . Beamsplitter  56  reflects a portion of beam  54  on detector  58 . A transmitted portion of beam  54  is incident on a beam dump  62 . Preferably beamsplitter  56  has a reflectivity less than 50% to maximize the portion of beam  16  that is incident on detector  58 . For a given power in beam  16 , the reflection of beamsplitter  56  for radiation in beam  54  having frequency ν 0 ±f 0  is selected to allow sufficient power to be incident on detector  58  to “swamp” out other noise sources of the detector.  
         [0035]     The wave fronts of the portions of beams  16  and  54  ( 16 T and  54 R) incident on the detector are preferably aligned to be parallel. The diameter of the two beam portions are also preferably arranged to be equal. The beam portions, beam portion  54 R having a frequency of either ν 0 +f 0  or ν 0 −f 0 , and beam portion  16 T having a frequency ν 0  interfere on the surface of the detector to provide a signal from the detector having the offset frequency f 0 . The signal varies in amplitude according to the instant intensity of transmitted beam portion  16 T, which, in turn, is dependent on the transmission through package  20  at corresponding instant x and y-coordinates of beam  16 . The x and y-coordinates are a finction of time.  
         [0036]     A signal generated by detector  58  is amplified by an amplifier  64 . The amplified signal is supplied to processor/controller  54 . Signals from scanner  44  representative of the instant x and y-coordinates of the scanner are also transmitted to the processor. Accordingly, the processor is able to generate a two-dimensional matrix of amplitude values that can be used in image processing as described above. Although not shown in  FIG. 2 , the arrangement of transmitter  14  and receiver  32  of may be configured in a similar manner to transmitter  12  and receiver  32 . A separate x-y scanner would be required. Output of the Schottky diode-detector of receiver  32  would be communicated via another amplifier  66  to processor/controller  36 .  
         [0037]     In the foregoing description, the present invention is described in terms of directing the two THz beams at an angle to each other through a package being inspected.  FIG. 3  schematically illustrates another preferred embodiment  70  of apparatus in accordance with the present invention similar to the apparatus of  FIG. 1 , but wherein the two THz-radiation beams are transmitted collinearly through the package and rotated simultaneously through a range of transmission directions. In apparatus  70  THz transmitters  12  and  14  are stacked one above the other. Beam  18  from transmitter  14  is directed by a turning mirror  72  to a frequency diplexer  74 . Diplexer  72  is preferably a silicon (Si) etalon having a thickness selected such that, at the angles of incidence of beams  16  and  18  on the etalon (here, about 45°), there is a transmission peak (reflection minimum) at frequency vo and a transmission minimum (reflection maximum) at frequency ν 1 . Transmitters  12  and  14 , turning mirror  74  and diplexer  76  are further arranged such that the beams transmitted and reflected by the diplexer proceed collinearly as a combined beam ( 16  &amp;  18 ).  
         [0038]     Beams  16  and  18  pass collinearly through the package  20  and are intercepted by another silicon frequency-diplexer  76 . Diplexer  76  is configured similarly to diplexer  74 , transmitting frequency vo and reflecting frequency ν 1  and thereby separating beams  16  and  18  from the combined beam after the combined beam has passed. Separated beam  18  is directed by a turning mirror  78  to receiver  32 .  
         [0039]      FIG. 4  schematically illustrates a preferred implementation  70 A of the apparatus of  FIG. 3 . Apparatus  70 A is similar to apparatus  10 A of  FIG. 2  with exceptions as follows. In apparatus  70 A transmitter  14  is located below transmitter  12  diplexer  74  of  FIG. 3  (transmissive for frequency vo and reflective for frequency ν 1 ) is arranged such that beams  16  and  18  from transmitters  12  and  14  are combined collinearly into a single beam ( 16  &amp;  18 ). It is this combined beam that is scanned by scanner  44 . Extreme scan positions (angles) of the combined beam are designated  16  &amp;  18 , and  16 ′ &amp;  18 ′. These extreme scan positions are brought into proximity and made parallel to each other by lens  48  as described above with reference to  FIG. 2 . Diplexer  76  is located after turning mirror  50  and separates beams  18  and  18 ′ from beams  16  and  16 ′, with beams  18  and  18 ′ being directed to receiver  32  (not shown). Locating the diplexer in this position provides that all positions if the scanned beams are incident at the same angle on the diplexer.  
         [0040]     This collinear arrangement of the inventive apparatus has an potential advantage compared with the non-collinear apparatus of  FIGS. 1 and 2 , inasmuch as the two THz-beams have exactly the same path length in any object through which the beams pass. This can provide that detected difference in brightness (transmission) at the two frequencies are due primarily to the density of the object and the frequency difference. There are, however, potential disadvantages of the arrangement. One such potential disadvantage is that the above discussed opportunity for deriving shape information from two beams at an angle to each other and accordingly from two images simultaneously recorded. Shape information can still be obtained, however, from a series of images made sequentially over time at different values of φ as a result of the rotation of platform  24 . A further potential disadvantage is that performance of receivers  30  and  32  may be adversely affected if separation of beams  16  and  18  from the combined beam by diplexer  76  is incomplete. This may be mitigated to some extent by electronic filtering in the receiver electronics or by additional optical filtering in paths of the separated beams.  
         [0041]     In summary, the present invention provides a means for estimating the material composition of an object or material from images or transmission measurements of the object or material made at two different THz frequencies. The present invention is described above in terms of a preferred and other embodiments. The invention is not limited, however, to the embodiments described and depicted. Rather, the invention is limited only by the claims appended hereto.