Abstract:
A method of extracting a tomographic image of a layer within a body by optical coherence tomography, involves capturing three images, namely a non-interference background image, a first interference-fringe image of said layer, and a second interference-fringe image phase-shifted relative to the first interference-fringe image. The tomographic image is obtained by mathematically combining the three captured images. In a preferred embodiment random noise is removed by averaging and inter-layer effects are removed by applying a compensation function. This system is then used to extract the 2D cross-sectional images encoded in a multiple-layer information carrier.

Description:
FIELD OF THE INVENTION 
   This relates generally to the field of optical interference tomography, and in particular to a method and apparatus for performing full-field optical coherence tomography for multiple-layer structure imaging and information retrieval. 
   BACKGROUND OF THE INVENTION 
   Tomography is the art of creating an image of a sectional plane within a body. Optical coherence tomography (OCT) is a technique for high-resolution cross-sectional imaging of scattering media. The basic technique is described in Brett E. Bouma and Guillermo J. Tearney, Handbook of Optical Coherence Tomography. Marcel Dekker Inc., New York Basel, 2002, the contents of which are herein incorporated by reference. 
   Most OCT systems use 3-axis point-scanning based technology. However, this technique is slow and cumbersome. A few OCT systems work directly on a two-dimensional full field image using a sinusoidal-phase-modulation method. Examples of such systems are described in Arnaud Dubois, Laurent Vabre, Albert-Claude Boccara and Emmanuel Beaurepaire, “High-resolution full-field optical coherence tomography with a Linnik microscope”, Applied Optics, 41, 4, 805–812, 2002; and Arnaud Dubois, “Phase-map measurements by interferometry with sinusoidal phase modulation and four integrating buckets,” Journal of Optic Society of America, A, 18, 8, 1972–1979, 2001. The related electronics hardware and algorithm make such systems complex and expensive. 
   SUMMARY OF THE INVENTION 
   The present invention simplifies the full field OCT system and overcomes other limitations including the inter-layer effect compensation and phase noise removal. The invention is especially well-suited to the decoding of information from multi-layer carriers, although it can have other applications in the field of tomography. 
   According to the present invention there is provided a method of extracting a tomographic image of a target layer within a body by optical coherence tomography, comprising capturing a non-interference background image I d (x,y) of the body; capturing a first interference-fringe image of said target layer I 0 (x,y); capturing a second interference-fringe image I φ (x,y) of said target layer phase-shifted by an amount φ relative to said first interference-fringe image; and computing said tomographic image A(x,y) by mathematical manipulation of said non-interference image and said first and second interference-fringe images. 
   The method in accordance with the invention allows information to be encoded on multiple layers within a carrier and then quickly and conveniently retrieved by OCT. In a preferred embodiment random phase noise is removed by averaging images taken at different times. Image distortion resulting from inter-layer effects is compensated by applying a compensation operation. 
   In a further aspect the invention provides an apparatus for extracting a tomographic image of a target layer within a body by optical coherence tomography, comprising an interferometer for creating interference-fringe images of layers within said body; a camera for capturing images of said body including a non-interference background image I d (x,y); a computer for controlling said interferometer to enable said camera to capture a first interference-fringe image of said target layer I 0 (x,y) and a second interference-fringe image I φ (x,y) of said target layer phase-shifted by an amount φ relative to said first interference-fringe image; and said computer being programmed to compute said tomographic image A(x,y) by mathematical manipulation of said non-interference background image and said first and second interference-fringe images. 
   The interferometer is preferably a Michelson interferometer, although other types of interferometer may be employed. 
   In a still further aspect the invention provides a method of encoding information on a carrier, comprising providing a substrate having a solid background color; and providing a stack of multiple layers on said substrate, each having information printed thereon with a transparent ink. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described in more detail, by way of example only, with reference to the accompanying drawings, in which: 
       FIG. 1  is a simplified diagram showing an optical coherence tomography system; 
       FIG. 2  shows the structure of an info-chip; 
       FIG. 3  shows a direct reflection image of the info-chip; 
       FIG. 4  shows the interference image at the first layer; 
       FIG. 5  shows the interference image at the second layer; 
       FIG. 6  shows the interference-fringe-removed image of the first layer; 
       FIG. 7  shows the interference-fringe-removed image of the second layer; 
       FIG. 8  shows the inter-compensated image of the first layer; and 
       FIG. 9  shows the inter-compensated image of the second layer. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The invention relies on optical coherence tomography. This utilizes a partial coherence light source and interferometer, typically a Michelson interferometer, to extract the cross-sectional images at different depth or layer within a body. In the preferred embodiment the body is an information carrier having multiple layers having information printed on them with a transparent ink. 
   In a multi-layer body, the interference image generated by a selected layer can be written as
 
 I   0 ( x,y )= I   d ( x,y )+ A ( x,y )sin φ( x,y ),  (1)
 
where I 0 (x,y) is the received image, I d (x,y) is the direct-reflection image or background image, A(x,y) is the image at this layer, sin( ) represents the interference-fringe.
 
   To obtain the real image A(x,y) from Eq. (1), three images are needed. The first one is I d (x,y), which can be obtained as the non-interference background image. The second image, I 0 (x,y) is captured as the interference-fringe image defined by Eq. (1). The last one, I π/2 (x,y), is captured as that of the I 0 (x,y) with a phase difference of π/2 introduced to the beam, 
   
     
       
         
           
             
               
                 
                   
                     
                       
                         
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 I   φ ( x,y )= I   d ( x,y )+ A ( x,y )sin [φ( x,y )+φ], and  (7)
 
 D   2   =A ( x,y )sin[φ( x,y )+φ],
 
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 A={D   1   2 +[( D   2   −D   1  cos φ)/sin φ] 2 } 1/2 .  (8)
 
When φ=π/2, Eq. (5) and Eq. (7) are identical.
 
   To remove the random noise resulting from unsteady phase changes and vibrations, it is desirable to capture N images of I d (x,y) and I φ (x,y) at different times. These can be averaged in accordance with
 
I 0 ( x,y )=(1 /N )Σ i   I   0 ( x,y )| t=i   . i= 1, 2, 3, . . .  N 
 
and
 
 I   φ ( x,y)=( 1 /N )Σ i    I   φ ( x,y )| t=i   . i− 1, 2, 3, . . .  N   (9)
 
   The depth resolution of an OCT system is determined by coherence length of the light source. 
   The configuration of one practical embodiment of an optical system for implementing the above method is shown in  FIG. 1 . This consists of a modified Michelson interferometer  10  that incorporates a tilted cubic beam splitter  12  and a spatial filter mask 14 associated with an aperture stop  34  set in the back focal plane of the lens  16 . This tilt angle α should be below about 5° and in this example is about 1.7°. The interferometer  10  has as a light source a superluminescent diode  18 . As in any OCT system, the depth resolution is determined by the coherence length of the light source. 
   The output of the superluminescent diode  18  is collimated by lens  20 . The non-polarizing beam splitter  12  separates light into the reference arm  22  and sample arm  24  of the interferometer. A neutral density filter  26  is used to adjust the intensity of the reference beam reflected from the reference mirror  28  mounted on a translation stage  40  controlled by computer  38 . 
   The lens  16  images the sample, info-chip or information carrier  30  mounted on sample holder  36  on the CCD camera  32  connected to the computer  38 . The information carrier  30  consists of a substrate with a solid black background layer and a plurality of transparent information layers each bearing information printed with a transparent ink. 
   By performing a Fourier transform, the lens  16  focuses the strong light reflected from the surfaces of the beam splitter on the blocking area of the spatial filter mask  14 . This results in the minimization of the DC noise resulting from the beam splitter  12 . 
   The spatial filter mask  14  is a two-dimensional DC block function defined by 
                   f   ⁡     (     x   ,   y     )       =     {             0   ,           ⁢     ⋐       A   s     ⁡     (     x   ,   y     )                     1   ,           ⁢   otherwise           ,               (   10   )               
where A(x,y) is the DC spot formed by the reflective lights of BS surfaces passing through the lens  16 .
 
   The tilting angle α of the beam splitter  12  ensures the spatial filter mask  14  only removes the DC component coming from beam splitter  12  rather than from the reference mirror  28 . 
   The least thickness of each layer of the information carrier  30  is determined by the depth resolution of the OCT, which as noted above in turn depends on the coherence length of the light source. Both sides of the layer are coated with anti-reflective coating. The encoding procedure involves writing or printing the two dimensional information, image or text, on one side of a layer. The ink applied should be transparent and be distributed evenly. 
   The differences of phase and reflectance between the ink and surface of the layer make the two-dimensional information distinguishable. For the purpose of protection, a hard film with a near infra-red window is provided at the top of the chip. The information layers are all bonded together and mounted on a substrate that is solid and completely black.  FIG. 2  shows one example of an info chip  30 . Multiple information layers  52  are supported on substrate  54  and protected by a protective film  50 . 
   The optical system described with reference to  FIG. 1  can be used to extract the cross-sectional image at each layer. In this exemplary embodiment, the superluminescent diode  18  forming the light source has a central wavelength is 830 nm and longitudinal resolution of 20 μm. The information carrier  30  is placed on the sample holder  36  in the sample arm  24  and a suitable reference mirror  28  is mounted on the reference arm  22  of the interferometer. 
   The computerized translation stage  40  controls the position of the reference mirror  28  such that it can scan through the info-chip  30  by moving the reference mirror  28 . The CCD camera  32  captures the image from the appropriate information layer  52  and transfers the data to the computer  38 . 
   As discussed above, three images are needed to produce a tomography image. The first image, direct-reflection image I d ( x,y ), is obtained before the scanning starts, and with the optical path length of the reference arm set to be shorter than the length of sample arm. Subsequently, during the course of scanning the carrier, at each layer of the OCT system captures two interference images: a direct interference image, and a π/2-phase-difference image. These can be generated either by a positioner or by a phase retardation plate. From these images, The computer  38  obtains the cross-sectional image of the selected layer by solving Eq. (5) or (6). Equation (8) may be used to remove the random phase noise; this procedure requires capturing more images at the each position for I 0 (x,y) and I φ (x,y). 
   To remove the inter-layer effect, the computer  38  should preferably perform a compensation operation for the OCT image A(x,y) at each layer by applying a compensation operation
 
 F ( x,y )=[ A ( x,y )+ k·I   d ( x,y )] m ,  (11)
 
where k is a weighting factor, range 0˜1 and m is an index, 1˜3. F(x,y) is the fully compensated tomography image.
 
     FIGS. 3 to 9  show a set of images obtained during decoding of an information carrier. The information encoded on the first layer is “NRC”, and second layer is “OCT”.  FIG. 3  shows an IR image I d (x,y), the direct-reflection image of the carrier, on which “OCT” and “NRC” are overlapped and fused together.  FIGS. 4 and 5  show the interference images at the first and second layer, respectively. The tomography images of the first layer and the second layer are shown in  FIG. 6  and  FIG. 7 .  FIG. 8  and  FIG. 9  show the inter-compensated images of the first layer and second layer, respectively. 
   Though the above system is described using a Michelson interferometer, other types of interferometer are also applicable to the invention. It will be further understood by persons skilled in the art that numerous other embodiments may be envisaged without departing from the spirit and scope of the invention.