Abstract:
This invention provides a device and a method for THz imaging to obtain real 3D image of sample and achieve high resolution, by combining THz technology and amplitude-division interference technology.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to an imaging technology, in particular, relates to a device and a method for Terahertz (THz) imaging combining THz technology and amplitude-division interference technology. 
       BACKGROUND OF THE INVENTION 
       [0002]    THz radiation can penetrate most of non-polarity dielectric, so the THz radiation can be acted as information carrier in imaging technology. In THz imaging technology, time waveform of THz pulse of each measure point of sample is recorded, then spectrum information of the measure point is obtained from Fourier transform of the time waveform. Therefore, not only contour but also components of the sample can be obtained from THz imaging technology. Moreover, refractive index distribution (depth distribution) can be calculated from time interval (phase change) of THz pulse. 
         [0003]    Transmission mode or reflection mode is used in the THz imaging technology of prior art, and 2D image of sample can be obtained. Furthermore, 3D structure information of the sample can be obtained from time interval of the reflected THz pulse or by computer-aided tomography. But the THz imaging technology of prior art has not intrinsic 3D imaging ability. 
         [0004]    Although real 3D image of sample by utilizing THz technology is desired, this technical difficulty is not solved so far. 
         [0005]    This invention intends to provide a device and a method for THz imaging to solve the problem above. 
       SUMMARY OF THE INVENTION 
       [0006]    The object of the present invention is to provide a device and a method for THz imaging to obtain real 3D image of samples and achieve high resolution, by combining THz technology and amplitude-division interference technology. 
         [0007]    To accomplish this object, the present invention is characterized by a device for THz imaging. The device comprises: a THz pulse emitter emitting THz pulses; an amplitude-division interferometer receiving the THz pulses, splitting each THz pulse into a reference beam and a signal beam, and making an optical path difference between the reference path and the signal path being less than a length of wave train, and a detector. The signal beam is used for scanning sample; the reference beam and the signal beam interfere with each other, then THz pulse amplitude-division interference signals containing sample information are generated; the detector receives the THz pulse amplitude-division interference signals and generates electrical signals containing the sample information. 
         [0008]    The invention is also characterized by a method for THz imaging. The method comprises: emitting THz pulses; splitting each THz pulse into a reference beam and a signal beam, and making an optical path difference between the reference path and the signal path being less than a length of wave train; generating THz pulse amplitude-division interference signals containing sample information; receiving the THz pulse amplitude-division interference signals; and generating electrical signals containing the sample information. 
         [0009]    The THz imaging device has intrinsic 3D imaging ability and high resolution, by combining THz technology and amplitude-division interference technology. So the invention solves the technical difficulty and has unexpected technical effect. The invention can be regard as a pioneer invention, compared with the THz imaging technology of prior art. 
         [0010]    Furthermore, the THz imaging device has high SNR, by modulating phase of the reference beam. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    Example embodiments of the invention with refinements will be explained hereafter. Examples set forth do not constitute a restriction of the invention. In particular, the size ratios are purely schematic. In the drawings: 
           [0012]      FIG. 1  shows a schematic illustration of a THz imaging device. 
           [0013]      FIG. 2  shows a preferred embodiment of the THz imaging device of  FIG. 1 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0014]    In describing example embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this patent specification is not intended to be limited to the specific terminology so selected and it is to be understood that each specific element includes all technical equivalents that operate in a similar manner. 
         [0015]      FIG. 1  shows a schematic illustration of a THz imaging device, and  FIG. 2  shows a preferred embodiment of the THz imaging device of  FIG. 1 . The THz imaging device  100  includes a THz pulse emitter  13 , an amplitude-division interferometer  15 , and a detector  17 . 
         [0016]    The THz pulse emitter  13  is an optical rectification emitter, and includes a laser part, a time-delay device  131 , and an emitter  132 . The laser includes a femto-second laser  111 , a first beam splitter  112 , a first reflector  113 , and a second reflector  115 . The amplitude-division interferometer  15  includes a second beam splitter  151 , a reference mirror  152 , and a sample platform (not shown). The sample platform is used for carrying the sample  153 . The detector  17  is an electro-optical detector  171  (electro-optical crystal), such as zine telluride, gallium arsenide, and so on. 
         [0017]    The first beam splitter  112  splits each femto-second beam emitted from the femto-second laser  111  into a pump beam and a probe beam. The pump beam  201  transmits through the time-delay device  131  and reaches the emitter  132 . The emitter  132  is used for emitting THz pulses. The probe beam  202  is reflected by the first reflector  113  and the second reflector  115 , then reaches the electro-optical detector  171 . The time-delay device  131  is used for changing relative delay time between the pump beam  201  and the probe beam  202 . 
         [0018]    In alternative embodiment, the time-delay device  131  is placed between the first beam splitter  112  and the electro-optical detector  171 . In this case, the time-delay device  131  is used for changing relative delay time between the probe beam  202  and the pump beam  201 . 
         [0019]    The reference mirror  152  can be a plane mirror. The reference mirror  152  is moved along propagation direction of the reference beam  301  under the control of an electro-motor (not shown). Doppler shift can be obtained by moving the reference mirror  152 . The THz pulse emitted from the emitter  132  reaches the second beam splitter  151 , and the second beam splitter  151  splits the THz pulse into a reference beam  301  and a signal beam  302 . The reference beam  301  is reflected by the reference mirror  152 , then transmits through the second beam splitter  151  and propagates towards the electro-optical detector  171 . The signal beam  302  is reflected by the sample  153 , then reaches the second beam splitter  151  and is reflected by the second beam splitter  151 . The signal beam  302  also propagates towards the electro-optical detector  171 . When an optical path difference between the reference beam  301  and the signal beam  302  is less than a length of wave train, the reference beam  301  and the signal beam  302  interfere with each other, and THz pulse amplitude-division interference signals (interference patterns) are generated correspondingly. The electro-optical detector  171  receives the THz pulse amplitude-division interference signals. The THz pulse amplitude-division interference signals contain sample information, such as 3D structure information of the sample, spectrum information of the sample, and so on. 
         [0020]    In alternative embodiment, the amplitude-division interferometer  15  further includes phase modulator  303 , the phase modulator  303  is placed between the second beam splitter  151  and the reference mirror  152 . The phase modulator  303  is used for modulating phase of the reference beam  301 . The sample information loaded on specific frequencies keeps from being submerged in noise by modulating the reference beam  301 . In this embodiment, the THz imaging device  100  has high SNR. 
         [0021]    Electrical field intensity of the reference beam  301  can be expressed as follows: 
         [0000]        E   R   =A   R   e   i[ωt+φ     R     (t)]   (1) 
         [0022]    Where A R  is amplitude of the reference beam  301 , ω is circular frequency of the reference beam  301 , and φ R (t) is modulated phase. 
         [0023]    Electrical field intensity of the signal beam  302  can be expressed as follows: 
         [0000]        E   S   =A   S   e   i[(ω+Δω)t+φ   s   ]   (2) 
         [0024]    Where A S  is amplitude of the signal beam  302 , ω is circular frequency of the signal beam  302 , Δω is frequency excursion of the signal beam  302  after being reflected by the sample  153 , φ S  is phase of the signal beam  302 . 
         [0025]    Electrical field intensity of the THz pulse amplitude-division interference signal can be expressed as follows: 
         [0000]        E=E   R   +E   S    (3) 
         [0000]        E*=E   R   *+E   S *   (4) 
         [0026]    Where E*, E R *, and E S * are conjugate electrical field intensity of E, E R , and E S , respectively. 
         [0027]    According to theory of physics, luminous intensity is merely determined by electrical field component of electromagnetic wave. Therefore, luminous intensity of the THz pulse amplitude-splittine interference signal can be expressed as follows: 
         [0000]    
       
         
           
             
               
                 
                   I 
                   = 
                   
                     
                       ( 
                       
                         
                           E 
                           R 
                         
                         + 
                         
                           E 
                           S 
                         
                       
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                      
                     
                       ( 
                       
                         
                           E 
                           R 
                           * 
                         
                         + 
                         
                           E 
                           S 
                           * 
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
             
               
                 
                   I 
                   = 
                   
                     
                       
                         1 
                         2 
                       
                        
                       
                         
                            
                           
                             A 
                             R 
                           
                            
                         
                         2 
                       
                     
                     + 
                     
                       
                         1 
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                        
                       
                         
                            
                           
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                         A 
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                         cos 
                          
                         
                           [ 
                           
                             
                               Δ 
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                                
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                                
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                             - 
                             
                               
                                 φ 
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                                
                               
                                 ( 
                                 t 
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                   ( 
                   6 
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         [0028]    Where 
         [0000]    
       
         
           
             
               1 
               2 
             
              
             
               
                  
                 
                   A 
                   R 
                 
                  
               
               2 
             
           
         
       
     
         [0000]    is direct-current component of the modulated reference beam  301 , 
         [0000]    
       
         
           
             
               1 
               2 
             
              
             
               
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                   A 
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               2 
             
           
         
       
     
         [0000]    is direct-current component of the modulated signal beam  302 , and 2A R A S  cos [Δωt+φ S −φ R (t)] is real part of the interference signal. The real part of the interference signal is merely determined by Δω, φ S , and φ R (t). The real part of the interference signal contains the sample information. 
         [0029]    The electro-optical detector  171  is used for detecting the THz pulse amplitude-division interference signals according to birefrigent effect of electro-optical crystal. When the THz pulse amplitude-division interference signal transmits through the electro-optical detector  171 , instantaneous birefrigent effect of the electro-optical detector  171  occurs. Polarization of the probe beam  202  changes from linear polarization to elliptic polarization when the probe beam  202  transmits through the electro-optical detector  171 . Electrical field intensity of the THz pulse amplitude-division interference signal is obtained by measuring degree of elliptic polarization of the probe beam  202 . Therefore, the electro-optical detector  171  is used for transforming optical signals into electrical signals. 
         [0030]    In alternative embodiment, the THz imaging device  100  further includes a Fresnel lens  304 . The Fresnel lens  304  is placed between the second beam splitter  151  and the sample  153 . NA of the Fresnel lens  304  is greater than or equal to 0.7. In this embodiment, the THz imaging device  100  has sub-millimeter lateral resolution. 
         [0031]    The THz imaging device  100  further includes a signal processing circuit  19 . The signal processing circuit  19  is used for processing (amplifying, filtering, demodulating, and so on) the electrical signals output from the electro-optical detector  171  and outputting amplitude and phase information of the THz pulse amplitude-division interference signals. 
         [0032]    A method of demodulating signals is as follows. 
         [0033]    Let a signal is f(t), its Hilbert&#39;s transform is: 
         [0000]    
       
         
           
             
               
                 
                   
                     
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                               ∞ 
                             
                             
                               + 
                               ∞ 
                             
                           
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                                   τ 
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                   ( 
                   7 
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         [0034]    Where H is Hilbert&#39;s transform, f(t) and {tilde over (f)}(t) are orthogonal. 
         [0035]    Let a complex signal is z(t)=f(t)+j{tilde over (f)}(t), and let f(t)=a(t)cos θ(t), an orthogonal component of f(t) is {tilde over (f)}(t)=a(t)sin θ(t), the complex signal is z(t)=a(t)cos θ(t)+ja(t)sin θ(t)=a(t)e jθ(t) . Amplitude and phase of f(t) can be calculated: 
         [0000]    
       
         
           
             
               
                 
                   
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                       t 
                       ) 
                     
                   
                   = 
                   
                     
                       
                         
                           f 
                           2 
                         
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                           ( 
                           t 
                           ) 
                         
                       
                       + 
                       
                         
                           
                             f 
                             ~ 
                           
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                          
                         
                           ( 
                           t 
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                   ( 
                   8 
                   ) 
                 
               
             
             
               
                 
                   
                     θ 
                      
                     
                       ( 
                       t 
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                   = 
                   
                     
                       arctan 
                        
                       
                         { 
                         
                           
                             Im 
                              
                             
                               [ 
                               
                                 z 
                                  
                                 
                                   ( 
                                   t 
                                   ) 
                                 
                               
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                             Re 
                              
                             
                               [ 
                               
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                        
                       
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                   ( 
                   9 
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         [0036]    Visual images can be obtained by digital processing the output of the signal processing circuit. The digital processing includes resampling, quantification, pseudo-color staining, and so on. The digital processing can be implemented by a computer. 
         [0037]    In the THz imaging device  100 , axial scan is achieved by moving the reference mirror  152 , and axial structure information of the sample  153  is obtained by axial scan. Lateral scan consists of a series of axial scans. 3D structure information of the sample  153  is obtained by combining the axial scans and the lateral scans. 
         [0038]    In alternative embodiment, the amplitude-division interferometer  15  can be replaced by a Michelson&#39;s interferometer or other optical instrument realizing amplitude-division interference. 
         [0039]    In alternative embodiment, the THz pulse emitter  13  can be a photoconductive emitter. 
         [0040]    In alternative embodiment, the detector  17  can be a photoconductive detector. 
         [0041]    Atomic radiation is not a simple harmonic wave, but a series of wave trains. Phase difference between any two wave trains is not constant. Therefore, interference occurs merely when two sub-wave-trains belonging to the same wave train meet at a superposition point. Because the wave train of the THz pulse is short, interference occurs merely when the optical path difference between the reference beam  301  and the signal beam  302  is smaller than a length of wave train. Thus, the THz imaging device  100  has high spatial location. 
         [0042]    The amplitude-division interferometer  15  can be regarded as a Michelson&#39;s interferometer, therefore, the THz imaging device  100  has micron axial resolution and sub-millimeter lateral resolution. The THz imaging device  100  ingeniously utilizes amplitude-division interference principle of the Michelson&#39;s interferometer and special signal modulating technology. Therefore, detecting sample with non-destructive, non-contact, and long working distance can be achieved. Imaging with real time, non-destructive, and original position can also be achieved. 
         [0043]    The THz imaging device  100  can be applied in many fields, such as medical testing, safety inspection, jade identification, mining, archeology identification, and so on. 
         [0044]    Although only limited example embodiments are described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the present invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.