Abstract:
A method of forming a three-dimensional internal image of an object includes illuminating the object with terahertz (THz) radiation and detecting THz radiation that is either transmitted through, reflected from or backscattered from the object. The detected radiation is used to form a series of two-dimensional images of the object at different angles or positions. The recorded two-dimensional images are electronically processed using computer aided tomography (CAT) algorithms to form the three-dimensional image of the object.

Description:
PRIORITY  
       [0001]     This application is a continuation-in-part of U.S. patent application Ser. No. 11/085,859, filed Mar. 22, 2005, and is also a continuation-in-part of U.S. patent application Ser. No. 11/231,079, filed Sep. 20, 2005. This application claims priority to U.S. Provisional Application Ser. No. 60/814,771, filed Jun. 19, 2006, the disclosure of which is incorporated by reference. 
     
    
     TECHNICAL FIELD OF THE INVENTION  
       [0002]     The present invention relates in general to terahertz (THz) or submillimeter imaging systems. The invention relates in particular to THz imaging systems using heterodyne detection to generate three dimensional images of the interior of an object.  
       DISCUSSION OF BACKGROUND ART  
       [0003]     The terahertz frequency 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 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) or as sub-millimeter waves.  
         [0004]     Many materials that are opaque to wavelengths shorter then 30 micrometers are either transparent or semi-transparent in the terahertz region. 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 because of their longer wavelengths. Further, such materials do not have much spectral content, i.e., characteristic absorption lines, in these longer wavelength regions that would allow one material to be easily distinguished from another.  
         [0005]     Terahertz radiation is not an ionizing radiation, so it does not have the potential to damage biological tissues 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.  
         [0006]     Based on these advantages, researchers have explored the application of THz radiation in direct detection laser systems to probe and image the inside of plastic, textiles, paper cardboard, wood, ceramic, opaque glasses, etc. packages and packaged semiconductor chips. Direct detection THz laser radiation systems have also been used to detect compositions of gas, drugs, and biological agents, and the like. Astronomers have developed THz heterodyne detection systems for earth, planetary, and space science applications. The biological and biomedical researchers have also begun to pursue THz technology.  
         [0007]     The following patent references illustrates some of the applications of THz radiation utilizing direct detection and time domain systems, each of which is incorporated herein by reference.: U.S. Pat. No. 6,525,862; and U.S. Patent Application Publication Nos. 2004/0065831 and 2003/0178584.  
         [0008]     Researchers have also started to explore the 3-dimensional imaging potential of THz radiation using direct detection THz laser systems coupled with well known computer aided tomography (CAT) techniques extensively utilized in 3-D x-ray medical imaging systems. Such systems are also being considered for homeland security applications, for examining the interior of luggage or packages, or examining the interior defects in plastic, wood, ceramic, etc. packages or structural materials. The following references provide examples of such time domain and direct detection THz 3-D imaging applications and implementation approaches each of which is incorporated herein by reference:  
         [0009]      Pulsed Terahertz Tomography  by S. Wang and X-C. Zhang; Journal of Physics D: Applied Physics 37 (2004) R1-R36.  
         [0010]      Three - Dimensional Terahertz Wave Imaging  by X-C. Zhang; Phil. Trans. Royal Society of London A(2004) 362 PPS. 283-299.  
         [0011]      Three - Dimensional Imaging With A Terahertz Quantum Cascade Laser;  Optics Express (20 Mar. 2006), Vol. 14, No. 6 PPS 2123-2129.  
         [0012]     In many industrial, scientific research, or medical applications, it is necessary to determine the distribution of some physical property (e.g., density, absorption, scattering, etc. variations) internal to the object/sample under investigation. The value of strip integrals of such a distribution within the object/sample can, in certain cases, be deduced from appropriate physical measurements and the set of line strip integrals corresponding to a particular angle of view known as a projection of the object. Obtaining a number of such projections at different angles of view, an estimation of the corresponding distribution within the object can be obtained. By the practitioners of the art, this process is called image reconstruction from projections. Computed x-ray tomography is undoubtedly the most significant application to-date of image reconstruction from projections.  
         [0013]     In computed x-ray tomography, an x-ray beam is passed through the portion of a person or object which is to be imaged. The amount of the beam that is transmitted is detected and the data stored in memory. The x-ray beam is rotated 180 degrees so a set of data on the amount of x-rays transmitted along strips of the object as a function of angle is obtained and stored. The beam is then moved to an adjacent location and the process repeated until the object has been completely irradiated and all the data as a function of angle and lateral displacement is stored. All the collected strip data is then processed by the appropriate software reconstruction algorithms that are now well known to those experienced in the state of the art of computed aided tomography (CAT). In this lay-man explanation of the CAT process, the x-ray beam transmission was used as an example, but the process can also work by detecting, storing, and then processing the transmitted or the back scattered radiation throughout the electromagnetic spectrum as a function of angle and lateral movement of the beam of radiation.  
         [0014]     In the x-ray CAT example above, one can easily visualize the replacement of the x-ray beam with a terahertz laser beam and the x-ray detector replaced with a terahertz direct detection receiver, e.g., to form a direct detection terahertz computed tomography (CT) systems. The references cited above discuss in detail various implementation of direct detection terahertz computed tomography systems. The Wang article ( Pulsed Terahertz Tomography ) points out that the complex phase of the terahertz signal can be used to reconstruct the THz-computed tomography (CT) image in the same way as in the x-ray CT. This means that the same reconstruction algorithm can be used in THz-CT systems. In THz-CT, the reconstructed object function is the complex refractive index function of the object. Consequently properly constructed THz-CT systems can offer amplitude and phase variation information from the radiation transmitted through or back scattered from an object.  
         [0015]     The same properties that make THz radiation attractive-namely the high absorption and emission from many gaseous species, liquids, and solids—make THz waves extremely difficult for obtaining significant penetration or propagation of THz radiation in the atmosphere and in many objects (e.g., especially if they have a H 2 O content). This attenuation severally limits the use of THz radiation in imaging, radar, CAT, and communication applications. This is especially true for direct detection or time domain THz systems.  
         [0016]     Researchers have recognized that a need exists for a THz transceiver system that has increased dynamic range and measurement capability over the direct detection systems. Specifically, a need exist for a THZ trans-receiver system that can detect weak THz signals through samples that have high loss. As pointed out in U.S. Patent Application Publication No. 2006/0016997 (the disclosures of which is incorporated by reference), continuous wave (CW) heterodyne imaging systems provide extremely large dynamic range and high signal-to-noise ratio advantages while maintaining fast data acquisition, stable magnitude and phase measurements, reasonable frequency flexibility and millimeter-scale penetration through wet tissues as well as other biological materials. In addition, heterodyning systems offer the capability of obtaining phase information from either the transmitted radiation propagated through the object or from the back scattered radiation from the object.  
         [0017]     To date we are not aware of anyone that has conceived of a heterodyne THz computer aided tomography system to obtain superior sensitivity in obtaining internal images of objects. This is the subject of this patent disclosure.  
       SUMMARY OF THE INVENTION  
       [0018]     In one aspect, a method in accordance with the present invention for forming a three-dimensional internal image of an object, comprises illuminating the object with terahertz radiation and detecting, using a heterodyne receiver, terahertz radiation that is transmitted through the object, reflected from the object, or backscattered from the object. A series of two-dimensional images of the object at a plurality of different angles, or a plurality of different positions is recorded using the detected radiation. The two-dimensional images are electronically processing using computer aided tomography (CAT) algorithms to form the three-dimensional image of the object.  
         [0019]     One embodiment of the present invention utilizes a THz transmitter and a RF frequency off-set THz laser local oscillator from the transmitter&#39;s output frequency to form a coherent (i.e., a heterodyne) detection computer aided tomography system for obtaining 3-D images of the interior of objects by detecting the amplitude variations of either the transmitted or the back scattered radiation. Another embodiment of the invention is to obtain tomographic images of an object by detecting amplitude and the phase changes of either the transmitted or the back scattered THz radiation. It would be advantageous to exploit the additional information that a 3-D imaging system would provide from such CAT THz systems in security examination of luggage, or packages for detecting concealed objects or substances such as explosives, drugs, biological agents, and the like. Such CAT THz systems would also be useful in imaging internal composition variations, such as defects, etc. within parts made from plastics, ceramics, concrete, composite materials, wood, paper, opaque glasses, etc. Since THz radiation is not an ionizing radiation, it does not have the potential to present health problems as would x-rays for such systems. It also will not damage biological samples. Consequently, THz CAT systems would have advantages over x-ray CAT systems. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]      FIG. 1  is a schematic diagram illustrating one embodiment of a terahertz heterodyne system employing computer aided tomography techniques for generating a three dimensional image of the interior of an object.  
         [0021]      FIG. 2  is a schematic diagram similar to  FIG. 1  except that the terahertz signal is derived from reflection rather than transmission.  
         [0022]      FIG. 3  is a schematic diagram similar to  FIG. 2  and including parabolic collecting optics.  
         [0023]      FIG. 4  is a schematic diagram illustrating another embodiment of a terahertz heterodyne system capable of measuring both amplitude and phase and employing computer aided tomography techniques for generating a three dimensional image of the interior of an object based on both measurements.  
         [0024]      FIG. 5  is a schematic diagram of the processing electronics used in the  FIG. 4  embodiment.  
         [0025]      FIG. 6  is a schematic diagram similar to  FIG. 4  except that the terahertz signal is derived from reflection rather than transmission.  
         [0026]      FIG. 7  is a schematic diagram illustrating a modification for improving the performance of the embodiments shown in  FIGS. 4 and 6 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0027]     Referring now to the drawings, wherein like components are designated by like reference numerals,  FIG. 1  schematically illustrates one preferred embodiment  10  of a heterodyne THz computer aided tomography imaging apparatus in accordance with the present invention. In  FIG. 1 , and in other drawings referred to herein below, 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 the open arrowheads. Electronic connections are depicted by double-weighted solid lines with the direction of electronic communication indicated, where appropriate, with a solid arrowhead.  
         [0028]     Apparatus  10  includes two sources  12  and  14  of THz radiation. Here each of the sources is a THz-laser. One serves as a local oscillator  14  and the other as a transmitter  12 . A preferred THz laser for the invention is an optically pumped THz-laser in which a gaseous gain-medium is pumped by radiation from a CO 2  laser. The output of the THz laser can be modulated (e.g., turned off and on) by modulating the output of the CO 2  pump laser by pulsing the RF power supply of the CO2 laser. This can conveniently be accomplished by turning the RF power supply energizing the CO 2  laser on and off. A THz-laser may have different nominal frequencies depending on the gaseous THz gain-medium contained within it. Any particular gain-medium has different discrete lasing frequencies about some nominal frequency characteristic of that gain-medium.  
         [0029]     Accordingly, it is possible to select an output frequency ν 0  from many different THz frequencies between about 0.3 THz and 10.0 THz, by selecting a particular gain-medium and adjusting a diffraction grating within the THz resonator. Such CO 2  laser-pumped THz-lasers are commercially available. One such commercially-available THz-laser is a SIFIR-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.  
         [0030]     CO 2  laser-pumped THz lasers are preferred for CAT imaging applications, such as for apparatus  10  because of advantages including a wide range of available THz frequencies, relatively high power output, room temperature operations, and reliability. Those skilled in the art, however, know that in theory at least, other THz radiation sources both laser and electronic in nature 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 a backward-wave oscillator. Such an oscillator can emit up to 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 optically pumped THz-lasers and may not be as reliable as commercially available THz-lasers.  
         [0031]     Other possible THz-lasers include Quantum Cascade semiconductors 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 THz. QCL lasers, however, must be operated at cryogenic temperatures in order to achieve milliwatts of power output. For most applications, operation at cryogenic temperature is a serious disadvantage.  
         [0032]     Another possible THz source is the use of tunable solid state lasers to drive a photomixer. Such a source can provide tunable radiation over the entire THz spectrum at room temperature operation range but with output power limited to tens of nanowatts.  
         [0033]     Continuing with reference to  FIG. 1 , in apparatus  10 , THz-radiation source  12  provides a beam  24  of radiation (the signal beam), having a frequency ν 0 , which will be propagated through an object  26  to provide data for computing a series of strip integrals to obtain an image reconstruction from projection as is done in x-ray CAT to reconstruct a 3-D image of that object. The object  26 , shown only as an example in  FIG. 1 , is an aerospace part constructed from composite materials (say a blade for either a jet engine or an aircraft&#39;s propeller, or a helicopter rotor blade). The disclosed THz CAT system would be useful in detecting delaminated layers within such composite structures. The occurrence of delaminated layers in such airborne structures would be highly dangerous to flight if not detected. Apparatus  10  is a heterodyne imaging system for which THZ-radiation source  14  functions as local oscillator (LO) and 12 functions as the transmitter. A beam  28  of radiation from THz-radiation source  14  is required to have a frequency that is offset from the frequency ν 0  of the signal beam  24  by a RF frequency f 0 . Frequency f 0  is one preferred frequency of an electronic signal that contains data that will be electronically processed to provide a reconstructed 3-D image of the object being scanned by rotation and translation of the object and storing the variations of the THz radiation transmitted through the object.  
         [0034]     For a frequency offset f 0  between about 0.5 MHz and 15 MHz, lasers  12  and  14  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. (Note, one can also get frequency offsets in the GHz region by using different laser lines for the transmitter and the local oscillator if this is desirable). This frequency offsetting method for gas lasers, and circuits therefore, 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. Pat. No. 7,199,330, assigned to the assignee of the present invention, and the complete disclosure of which is hereby incorporated by reference.  
         [0035]     The gain-medium of a THz laser typically consists of large, heavy gas molecules, for example, methanol (CH 3 OH) or difluoromethane (CH 2 F 2 ). Because of these heavy molecules there are many possible laser transitions for any gas, which can be spectrally very closely spaced. Accordingly, values for f 0  using this frequency offsetting method are typically in the above referenced MHz range. For larger values of f 0 , say between about 500 MHz and 200 GHz, lasers  12  and  14  preferably have different gain-media.  
         [0036]     Continuing with reference to  FIG. 1 , beam  24  of frequency ν 0  from laser  12  is redirected by mirror  40  to irradiate the desired object  26  of which a 3-D tomography image is desired. In the preferred arrangement of  FIG. 1 , the laser beam is passed through the object. The radiation  24 A transmitted through the object is redirected by mirror  51 , to mirror  53 , to mirror  41  and to partly reflecting mirror  48 . Mirror  48  redirects the laser radiation transmitted through the object onto the coherent detector (or receiver—RCVR)  50 . The output beam  28  from the THz local oscillator  14  of frequency ν 0 ±f 0  is redirected to mirror  48  by mirror  30 . Most of the beam  28  is reflected into the beam stop  49  by mirror  48  because only tens of milliwatts or less are needed from the local oscillator to perform the optimum heterodyne detection of beams  24 . The high reflectivity (greater than ˜90%) of mirror  48  is desirable for redirecting most of the laser radiation  24 A from the target onto the heterodyne detector  50 .  
         [0037]     Due to the heterodyne detection process caused by the mixing of part of the beam  28  and most of the beam  24 A on the detector  50 , the detector produces a RF signal f 0  which is amplified by amplifier  52  and fed to a processor  54  that contains the 3-D tomography image algorithms used to generate the desired image. The amplitude “A” of the signal f 0  (e.g., the IF frequency) varies with time “t” as the laser beam  24  moves over the object. A[f 0  (t, φ)] is detected and stored as the object is rotated and translated with time.  
         [0038]     The object  26  is rotated as a function of time (Θ(t)) by a suitable motor  59 . While the object is rotated, it is also move laterally as a function of time (x(t)) by a suitable motor not shown. This process is continued until the entire object is scanned. Information regarding Θ(t) and x(t) and the amplitude variation of the signal is provided to the data processor which stores the data and computes from the stored A[f 0 (t,φ)], and x(t) signals the tomographic images by the use of 3-D tomography algorithms well known to those experienced in the art. See for example, Gabor T. Herman, Image Reconstruction from Projections, The Fundamentals of Computerized Tomography, Academic Press, Inc., Orlando Fla. (1980). The derivations found in the latter reference concentrate on X-Ray tomography and amplitude-only detection and images, but the equations derived are general enough to support the extension to fully-coherent (amplitude and phase data) imagery. Examples of THz CT image calculation techniques are also found in  Pulsed Terahertz Tomography  by S. Wang and X-C. Zhang, cited above.  
         [0039]     The processor provides signals to an imaging system  58 , displaying a tomographic image  56  of the object. The processor allows the image to be rotated on the display screen for detailed examination from numerous aspect angles by the viewer as in x-ray tomographic images.  
         [0040]     The object  26  in  FIG. 1  shown only as one example is a composite aerospace structure (i.e., a turbine engine, propeller, or helicopter blade, or other composite structure). Actually, it can be any object constructed from a material that will transmit a reasonably detectable amount of THz radiation. The high detection sensitivity of the heterodyne receiver approach disclosed allows the tomographic imaging of objects that have orders of magnitudes higher THz wave attenuation than is possible with direct detection THz systems. The object could be constructed from one or a combination of glass, ceramic, plastic, wood, paper, card-board, etc. type materials or of biological material.  
         [0041]     Improvements can be made to the basic system illustrated in  FIG. 1  by adding more optical components in the  24  and  24 A beam paths. For example, a focusing system can be added to focus the THz radiation within the object for increased resolution and for enhanced signal to noise. This change would also require moving the focal spot in the vertical direction Y(t) by moving the focusing lens to obtain a higher resolution image of a given plane within the object. Such an improvement would also require a wider angle radiation collection optical system to collect the radiation transmitted through the object and an additional optical system to re-collimate the radiation transmitted through the object to fill the surface of the detector  50 . The addition of these optical components is well known to those experienced in the art.  FIG. 3 , discussed below, illustrates an example of these type of extra optics.  
         [0042]     The THz detector  50  is preferably a Schottky-diode detector as schematically depicted in  FIG. 1 . Such detectors are commercially available, for example, from Virginia Diode, Inc., of Charlottesville, Va.  
         [0043]     For a given power in beam  28 , the transmission of beam splitter  48  for radiation having one of the frequencies ν 0     —   +f 0  is selected to allow sufficient power to be incident on detector  50  to optimize its heterodyne performance. The wave fronts of the portions of beams  24 A and  28  incident on the detector are preferably aligned to be parallel. The diameter of the two beams portion are also preferably arranged to be equal. The beams of one of the selected frequencies ν 0 ±f 0  and ν 0  interfere in the detector to provide a signal having the offset RF frequency f 0 . This signal varies in amplitude according to the instantaneous intensity of the transmitted beam  24 A, through the object  26 . The amplitude of this signal is dependent on the transmitted properties of the beam through the object and as a function of the motion Θ(t) and x(t) of the object. The phase of signal f 0  varies as the radiation passes though various portions of the object. The phase change occurs due to the changes in the distribution of the object&#39;s refractive index through which the beam propagates. In  FIGS. 4 and 6 , discussed below, systems are presented for also utilizing the phase change in f 0  as a function of x(t) and 0(t). This phase change information is processed by a processing electronics subsystem to obtain different image information then available from the amplitude variations information.  
         [0044]     Another preferred embodiment of the 3-D THz tomography system using heterodyned detection is illustrated by  FIG. 2 . In the  FIG. 2  system  20 , the variations of the back scattered radiation from the object are detected as a function of the time varying parameters Θ(t) and x(t) instead of detecting the variation of the transmitted radiation through the object as shown in  FIG. 1 . A THz laser transmitter  12  and a local oscillator  14  are again utilized by the system  20  of  FIG. 2 . The transmitter laser beam  24  of frequency ν 0  is passed through a partially reflecting mirror  40  onto the object  26 . Mirror  40  has ˜50% reflectivity so that fifty percent of the transmitter power is reflected into the radiation stop (e.g., radiation absorber)  41 A, and the other fifty percent is propagated to the rotating and laterally translating object  26 .  
         [0045]     Back scattered radiation occurs from the non-uniformities residing within the object. The imaging of such non-uniformities within the object is a purpose of systems shown in herein. One half of the back scattered radiation  24 R is reflected by mirror  40  toward the partially reflecting mirror  48 . Mirror  48  typically has a reflectivity greater than ninety percent so that most of the back scattered radiation  24 R reaches the RCVR heterodyne detector  50 . As in  FIG. 1 , the output beam  28  of the laser local oscillator  14  having a frequency of ν 0 ±f 0 , is redirected by mirror  30  to the partially reflecting mirror  48 . Most of the local oscillator beam is reflected by mirror  48  into the radiation absorber  49 .  
         [0046]     The adjustment of mirrors  30  and  48  again allow for aligning the wave fronts of the combined radiation to be parallel when irradiating the detectors surface. The power of the local oscillator beam irradiating the detector is adjusted to optimize the detector&#39;s heterodyne performance.  
         [0047]     The interference (i.e., mixing) of the radiation from beam  28  and back scattered radiation from beam  24 R again cause an amplitude variation of the radiation from which the detector generates an RF frequency signal f 0  output. The amplitude of signal f 0  is dependent on the amount of radiation back-scattered from the target. Again as in the system of  FIG. 1 , the data processing of the amplitude or phase information will enhance image quality over non-heterodyned THz 3-D imaging system. One commonly used direct detection system utilizes ultra-short pulses from mode-locked lasers transmitters. The signal f 0  is again amplified by amplifier  52  and provided to a digital processor  54  as in system  10  of  FIG. 1 .  
         [0048]     The radiation passing through the object is absorbed by the radiation stop  41 B in the system  20  of  FIG. 2 .  
         [0049]     The object is again rotated as a function of time by well known means (i.e., a variable speed motor  59 ) and a signal Θ(t) representing the motor&#39;s rotation with time is provided to the processor. In addition the object/rotating motor combination is moved laterally as a function of time by any one of numerous mechanical means not shown in  FIG. 2 . A signal representing this lateral motion with time x(t) is also provided to the processor as also described in  FIG. 1 . With the use of well known algorithms in the computer aided tomography state of the art, the process computes an image from the stored f 0 (φ,t), Θ(t), and x(t) data streams.  
         [0050]     Signal enhancement improvements can also be made to the basic back-scattering THz heterodyne 3-D tomography system  20  of  FIG. 2  as stated for the system of  FIG. 1 . The system  30  of  FIG. 3  illustrates one such possible improvement. It uses two parabolic mirrors for signal enhancement purposes. Parabolic mirror  60  has a small hole  60   a  to allow passage of the transmitter beam  24  onto the target as shown. Parabolic mirror  60  collects and collimates most of the back-scattered radiation  24 R from the target and redirects the radiation to parabolic mirror  61 . Mirror  61  brings the back scattered radiation  24 R to a focus and lens  62  re-collimates the radiation  24 R. Mirror  41  redirects the re-collimated beam  24 R from lens  62  to the detector  50 . The description for the rest of the system of  FIG. 3  is identical as for  FIG. 2  and will therefore not be repeated.  
         [0051]     The heterodyne systems of  FIGS. 1, 2 , and  3  provide an image of the interior of an object by processing the amplitude variations of the radiation either transmitted through or back reflected from the object as a function of the angle of the object&#39;s rotation and of its translation. The variations in the phase of the THz radiation either transmitted through or back reflected from the object as it is rotated and translated can also provide imaging information of the interior of an object. Since the phase variations of the detected radiation depends on the changes in the velocity of propagation within the material distributed throughout the interior of the object, and not from the attenuation of the radiation by either absorption or reflection within the object, different details should be observed when the images obtained from either the amplitude variations in the attenuation of the transmitted beam or in the amplitude variation of the related beam are compared with the images obtained from detecting the phase change of either beam.  
         [0052]      FIG. 4  illustrates a coherent detection THz tomography system  40  that senses both the phase and amplitude of the transmitted radiation through an object. It consists of a laser transmitter having a frequency ν 0  and a local oscillator having one of the frequencies ν 0 ±f 0 . By means of partially reflecting mirrors PM 1  and PM 2 , the transmitter and the superimposed local oscillator beams are made to illuminate the reference heterodyne detector  70  with their phase fronts parallel with each other. Angular adjustment of mirrors PM 1  and PM 2  are used to obtain the desired parallel phase fronts from the two beams. The transmitter beam is the solid line and the local oscillator beam is represented by the dashed line in  FIG. 4 . Under the described conditions the detector emits an RF signal f 0  as is well known in the state of the art. This reference RF signal f 0  is represented by the solid darker line in  FIG. 4 . The RF signal f 0  is fed to a processing electronics sub-system  72  which is shown in  FIG. 5  and will be discussed later.  
         [0053]     Partially reflecting mirror PM 1  has a low reflectivity (say ≦10%), so most of the transmitter beam will impinge upon total reflecting mirror M 1  and be directed to and through the object  26  to be examined. Partially reflecting mirror PM 2  also has low reflectivity (say ≦10%), so most of the local oscillator beam is propagated through PM 2  and directed to partially reflecting mirror PM 3 . Mirror PM 3  has a low reflectivity (again, say about ≦10%) so most of the local oscillating beam irradiating PM 3  is passed through to the beam stop  74 . The remaining portion of the local oscillator beam is redirected to the signal heterodyne detector  76 . Since PM 3  has a low reflectivity, most of the transmitter beam propagated through the object also illuminates the signal heterodyne detector  76 . Again the phase fronts of the two beams illuminating the detector are made parallel to each other by adjustments to the positioning of mirrors M 1  and PM 3 . The signal heterodyne detector  76  emits an RF signal f 0  resulting from the mixing of the two beams. The phase φ of this IF frequency signal differs from the fixed phase of the reference IF frequency f 0  because the phase of the beam propagated through the object is changed by the variations it encounters in the object&#39;s refractive index as the object is slowly rotated and then repeatedly stepped laterally to repeat the process until the entire object has been scanned. The time varying phase of the IF frequency, f 0 [φ(t)], is also provided to the processing electronic subsystem  72 . Subsystem  72  provides an electrical signal to the Tomographic Image Processor (TIP) subsystem  78  which utilizes well known algorithms to provide a tomographic image of the interior of the object by processing the electrical video signal and the time varying electrical signals θ(t) and x(t) produced by the sensors converting rotation (θ) and linear translation motion (x) of the object as a function of time (t), respectfully into electrical signals θ(t) and x(t). The rotation and translation electrical signals are denoted as cross-hatched heavy lines in  FIG. 4 .  
         [0054]     The systems illustrated by  FIGS. 1 through 4  illustrate only as an example, means of mechanically rotating and translating the object to obtain a tomographic image of the object. We believe these means to be more cost effective approach over other approaches, such as the use of scanning mirrors to scan the object and obtain the θ(t) and X(t) signals. The use of other means of illuminating the object as a function of time should not circumvent the basic of this invention which is to use heterodyne detection techniques to obtain tomographic images. Similarly, the use of various beam splitters to combine portions of the beams at the detectors  70  and  76  is merely for illustration only as there are many well know optical designs for combining radiation.  
         [0055]      FIG. 5  provides some details of the processing electronic subsystem  72  of  FIGS. 4 and 6 . The subsystem  72  utilizes an RF oscillator  79  generating a convenient frequency f 1  which is split between two RF detectors  80 ,  82  by a RF splitter  84 . The mixing of the f 1  signal with the reference IF signal f 0  of  FIG. 4  produces upper (f 0 +f 1 ) and lower (f 0 −f 1 ) sideband signals. As an example, let us assume that we select f 0 −f 1  to pass through the bandpass filter  85  while the filter is designed to stop the f 0 +f 1  signal. The referenced f 0 −f 1  signal is amplified and fed to either a high speed lock-in amplifier module or an in-phase quadrature demodulator module  88  discussed below.  
         [0056]     The mixing of the other half of the f 1  signal is passed through an RF isolator  90  and illuminates detector  82 . Detector  82  mixes the f 0 [φ(t)]IF signal from the signal heterodyne detector of  FIG. 4  with the f 1  fixed signal from oscillator  79  of to produce an upper f 0 [φ(t)+f 1 ] lower RF side-bands f 0 [φ(t)]−f 1 . We will again assume, as an example, to select the lower side band signal f 0 [φ(t)]−f 1  to pass through the band pass filter  92  while the filter is designed to stop the upper side band signal. This reference f 0 [φ(t)]−f 1  signal is amplified and fed to either a high speed lock-in amplifier module or an in-phase quadrature demodulator module  88 . These two modules are well known alternate electronic means of doing the same job which is to provide in-phase and quadrature (I and Q) voltage signals V[φ(t)] which can then be converted by the processor into amplitude and phase changes of the f 0 [φ(t)] signal as a function of θ and x. The amplitude and phase changes information is then provided to the tomographic image processor (TIP) subsystem shown in  FIG. 4  for display.  
         [0057]      FIG. 6  illustrates a heterodyne detection THz tomography system that senses the phase of the back-reflected radiation from throughout the object. The system is essentially the same as the system of  FIG. 4  except for the need for additional optics for collecting and recollimating the back scattered radiation. An inverse telescope lens arrangement is also needed to reduce the diameter of the signal beam to match the diameter of the referenced local oscillator beam before both beams illuminate the signal heterodyne detector. This is shown as an example in  FIG. 6  with a pair of parabolic collimating mirrors  102  and  104  and a two lens beam reducing telescope  106 . This arrangement is close to the same approach utilized in  FIG. 3 .  
         [0058]     There is a difficulty with the simplified systems shown in  FIGS. 4 and 6  that is easily corrected as per  FIG. 7 . The difficulty arises from the fact that the transmitter beam used to illuminate the reference heterodyne detector  70  is reflected from partially reflecting PM 2  and also redirected by PM 3  to illuminate the signal heterodyne detector  76 . Consequently there are signals ν 0 ±f 0 , ν 0 [φ(t)] and ν 0  illuminating the signal detector  76  which is undesirable because signal ν 0  confuses the processing subsystem.  
         [0059]     One preferred approach to solving this problem is to add another partially reflecting mirror PM 4 , another totally reflecting mirror M 2  and a second beam stop  110  as illustrated in  FIG. 7 . This arrangement prevents the transmitter beam from reflecting off of PM 2  and being collimated with the local oscillator beam and both beams being directed toward PM 3  as occurred  FIGS. 4 and 6 . Partially reflecting mirror PM 4  is used to redirect the local oscillator beam to totally reflecting mirror M 2  and then to PM 2 . The adjustment of these mirrors enable the superposition of the transmitter and local oscillator beams illuminating the reference detector  70  to have the parallel wave fronts required for efficient heterodyne detection.  
         [0060]     Additional information can found in U.S. Patent Application Publication Nos. 2006/0214107 and 2007/0114418 as well as U.S. patent application Ser. No. 11/231,079, filed Sep. 20, 2005, the disclosures of which are incorporated by reference.  
         [0061]     While the subject invention has been described with reference to the preferred embodiments, various changes and modifications could be made therein, by one skilled in the art, without varying from the scope and spirit of the subject invention as defined by the appended claims.