Abstract:
The present invention relates to vibration-insensitive point-diffraction interferometry. For the purpose of obtaining high immunity to vibration, a single-mode optical fiber is used to generate the reference wave, by means of point diffraction, directly from the measurement wave reflected from test objects. The capability of vibration desensitization is further strengthened by adding a spatial phase-shift devise that enables to obtain four interferograms of different amounts of phase shift simultaneously with no time delay between interferograms. The present invention may be effectively used in the design of measuring systems for in-line applications where measurements need to be performed in the presence of significant level of vibration.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to interferometry. More precisely, the present invention relates to methods and apparatus for a vibration-insensitive point-diffraction interferometer. The methods and apparatus of the present invention may be implemented in measuring systems that measure various parameters of test objects by effectively removing the effect of vibration.  
         [0003]     2. Description of the Related Art  
         [0004]     Interferometry is a well-established method of measuring various parameters of test objects. Interferometry requires generating two waves; one is generally named the reference wave and the other the measurement wave. The reference wave is generally formed by either a plane wave using the beam of light reflected from a flat surface or a spherical wave using the beam of light generated by means of point diffraction using a pinhole or optical fiber. The measurement wave is generated by either transmitting light through test objects or having light reflected from test objects.  
         [0005]     A practical problem encountered in performing interferometry is the presence of vibration, which causes unwanted fluctuation in interference fringes obtained between the reference and measurement waves. The vibration effect is generally considered the main cause of deteriorating measurement accuracy. In order to cope with the vibration problem, several vibration-desensitization methods have been proposed as results of previous work. A method is adopting an active means of monitoring the fluctuation of interference fringes due to vibration using a sensor with subsequent fast moving the reference surface, thus stabilizing the interference fringe (see T. Yoshino et al., Opt. Lett., 23, p. 1576). Another method is using a spatial phase-shift method to capture interference fringes in a very short period time, minimizing vibration fluctuation (see R. A. Smythe et al., Opt. Eng., 23, p. 361). Another method is using a diffraction grating in combination with a pinhole to obtain three spatially phase-shifted interference fringes at the same time (see Osuk Y. Kwon et al., Opt. Lett., 12, p. 855). Another method is using a wave-splitting element that splits the interference wavefront into a plurality of sub-waves with different phase shifts (see U.S. Pat. No. 6,304,330).  
       SUMMARY OF THE INVENTION  
       [0006]     The present invention is to provide a new more effective way of vibration desensitization in interferometer design. The present invention involves two key features; one is generating the reference wave by means of point diffraction using a single-mode optical fiber, and the other is simultaneous capturing of a plurality of phase-shifted interferograms using a spatial phase-shift device.  
         [0007]     The first key feature of the present invention is summarized as follows: The reference wave is generated by transmitting the measurement wave, which is reflected from the test object under the influence of vibration, through a single-mode optical fiber. The single mode fiber provides a function of spatial filtering that permits only the lowest spatial mode of the measurement wave front to be transmitted to become the reference wave. The presence of vibration usually affects only the lowest spatial mode of the measurement wave front, so it also appears in the resulting reference wave. The single-mode fiber also provides a means of point diffraction at its exit end, so the wave front emitted from the fiber becomes a near-perfect spherical wave (see H. Kihm et al, Opt. Lett. 29, p. 2366). The reference wave emitted from the fiber is therefore of near perfect spherical wave front and has the same vibration effect as the measurement wave. The vibration effect is consequently cancelled out in the process of interference between the reference and measurement waves.  
         [0008]     The second key feature of the present invention is a spatial phase-shift device that allows obtaining a plurality of phase-shifted interferograms simultaneously without time delay between the obtained interferograms. The spatial phase-shift device is a special embodiment of the spatial phase-shift method previously proposed by R. A. Smythe (see R. A. Smythe et al., Opt. Eng., 23, p. 361). This spatial phase-shift device strengthens vibration immunity, in combination with the first key feature of using a single-mode optical fiber, that allows freezing any vibration fluctuation remaining in the interferograms generated by the interference between the reference and measurement waves.  
         [0009]     The above-described two features of the present invention are capable of providing a high level of vibration desensitization in various interferometric measurements of physical parameters from measurement objects. The two features may be used separately or in combination, depending on the level of vibration encountered.  
         [0010]     In accordance with the present invention, the point-diffraction interferometer further includes an optical path delay line that can be used for selection of a suitable pair of the reference and measurement waves from a plurality of reflected waves from a measurement object. The delay line allows measuring various physical parameters of transparent test objects, such as thickness profile or refractive index, whose information resides within the objects or on the top and/or bottom surfaces. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     The above and other objects, features and advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:  
         [0012]      FIG. 1  is a view showing a vibration-insensitive interferometer according to an embodiment of the present invention;  
         [0013]      FIG. 2  is a view illustrating the construction and phase shift principle of a spatial phase-shift device used in the interferometer of  FIG. 1 ;  
         [0014]      FIG. 3  is a view showing a vibration-insensitive interferometer according to another embodiment of the present invention;  
         [0015]      FIG. 4  is a schematic view illustrating interference patterns and optical paths that are obtained by the spatial phase-shift device of  FIG. 2 ;  
         [0016]      FIG. 5  is a view illustrating the optical path compensation block of the spatial phase-shift device according to an embodiment of the present invention;  
         [0017]      FIG. 6  is a view showing a state where an anti-distortion block or the phase-shift device of  FIG. 5  is constructed;  
         [0018]      FIG. 7  is a view showing four interference patterns that are obtained by the spatial phase-shift device according to the present invention;  
         [0019]      FIG. 8  is a schematic diagram illustrating interference patterns and optical paths that are obtained by the spatial phase-shift device of the present invention;  
         [0020]      FIG. 9   a  is a view showing the states of light reflected from the front and back of the glass flat plate of  FIG. 3 ;  
         [0021]      FIG. 9   b  is a view showing the principle in which the measurement wave and reference wave are interfered with through the scanning of a corner cube; and  
         [0022]      FIGS. 10   a  to  10   f  are views showing the polarization states of light on the optical paths of the interferometer of  FIG. 1 . 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0023]     The present invention will now be described in detail in connection with preferred embodiments with reference to FIGS.  1  to  5 .  
         [0024]      FIG. 1  is a view showing a vibration-insensitive point-diffraction interferometer according to an embodiment of the present invention.  FIGS. 10   a  to  10   f  are views showing the polarization states of light on the optical paths of the interferometer of  FIG. 1 .  
         [0025]     Referring to  FIG. 1 , a beam of linearly polarized light is emitted from a light source  101  and passed through an optical isolator  102 , and converged onto a spatial filter  104  by a lens  103 . The optical isolator  102  serves to prevent any light returning from the interferometer optics from going back to the light source  101 . The light gone through the spatial filter  104  is collimated by a lens  105 . The state of light having progressed so far is represented as light  501 , which has a polarization state shown in  FIG. 10   a.  The collimated light is then passed through a half-wave plate  106 , which is represented as light  502  having the polarization state of  FIG. 10b . The half-wave-plate  106  serves to rotate the polarization direction of light of light  501 . Light  502  is then passed through a polarizing beam splitter (PBS)  107 . The light passed through the PBS  107  is passed through a quarter-wave plate  108 , and then is reflected toward the test object  111  by an intensity beam splitter (BS)  109  through a lens  110 . One part of the light reflected from the test object  111  is transmitted by the BS  109  and then made to propagate to generate the measurement wave. At the same time, another part of the light reflected from the measurement object  111  is reflected by the BS  109  and is made to propagate back to the PBS  107  to generate the reference wave.  
         [0026]     The beam of light reflected from the BS  109  (hereinafter referred to as “the reference wave”) is again passed through the quarter-wave plate  108 . It is then reflected by the PBS  107 , and made incident on an optical fiber  114 . The beam of light passed through the optical fiber  114  becomes a near-perfect spherical wave by means of point diffraction at the exit end of the optical fiber  114 . The light passed through the optical fiber  114  becomes outgoing light  503 , i.e., P-polarized light as shown in  FIG. 10   b.  Since the loss of light caused by the optical fiber  114  is high, the BS  109  is given a high reflectance in comparison with its transmittance.  
         [0027]     The total amount of light going into both the reference and measurement waves is adjusted by rotating the transmission polarization direction of the half-wave plate  106 . The intensity ratio between the reference wave and the measurement wave is determined by the reflectance of the BS  109 . The reflectance of BS  109  is generally given about 90% considering the light loss occurring when focusing light into the single-mode optical fiber  114 . This allows obtaining a good fringe visibility in the interference between the reference wave and the measurement wave.  
         [0028]     Meanwhile, the light transmitted through the BS  109  (hereinafter referred to as “the measurement wave”) becomes outgoing light  504 , i.e., S-polarized light, while going through the quarter-wave plate  112 , as shown in  FIG. 10   c.  The optical path of the measurement wave is controlled by a corner cube  113  and is then incident on a polarizing beam splitter (PBS)  115 .  
         [0029]     At the PBS  115 , the reference wave, which has been passed through the optical fiber  114 , and the measurement wave, which has been reflected from the measurement object  111  and then passed through the corner cube  113 , meet each other. As shown in  FIG. 10   d,  at both the entrances to the PBS  115 , the reference wave  505  and the measurement wave  506  have polarization states that are perpendicular to each other. The reference wave  506  and the measurement wave  505  are therefore combined through the PBS  115  and then emitted. This is shown in  FIG. 10   e,  where the reference wave is represented by light  507 , while the measurement wave by light  508 .  
         [0030]     The polarization state of the reference wave transmitted through the PBS  115  is rotated by 45 degrees by a half-wave plate  116 . The polarization state of the measurement wave reflected by the PBS  115  is also rotated by  45  degrees at the same half-wave plate  116 . This is shown in  FIG. 10   f.  Accordingly, the two beams of light are incident on a spatial phase-shift device  117 . The interference pattern between the two beams of light appears as four interference patterns whose phases are shifted by 0, 90, 180, 270 degrees, respectively, through the spatial phase-shift device  117 . That is, the four generated interference signals  119 ,  120 ,  121  and  122  are detected on the surface of a photodetector  118  at the same time.  
         [0031]      FIG. 2  is a view illustrating the construction and phase shift principle of the spatial phase-shift device used in the interferometer of  FIG. 1 .  
         [0032]     Referring to  FIG. 2 , the spatial phase-shift device includes a beam splitter  201 , a quarter-wave plate  203 , a flat plate  202  for adjusting thickness, a polarizing beam splitter  204  and a prism mirror  205 . Incoming light, which is an 45-degree rotated combination of the reference and measurement waves, is divided into four separate beams with each beam differently phase-shifted between the reference wave and the measurement wave through the spatial phase-shift device, so that four interference patterns with different amounts of phase shift can be obtained.  
         [0033]     That is, the incoming light is reflected by the beam splitter  201  or is transmitted therethrough. The light reflected from the beam splitter  201  is phase-shifted by  90  degrees by the quarter-wave plate  203  and is then incident on the polarizing beam splitter  204 . The light incident on the polarization splitter  204  is reflected or is transmitted therethrough. The reflected light is emitted as outgoing light  207 . The transmitted light is reflected by the mirror  205  and is then emitted as outgoing light  208 . At this time, the outgoing light  207  reflected by the polarizing beam splitter  204  is phase-shifted additionally by  180  degrees. Accordingly, light  207  has a phase shift of  270  degrees and light  208  has a  90  degrees phase shift.  
         [0034]     Meanwhile, the light transmitted through the beam splitter  201  goes through the flat plate  202  without phase shift, and is then incident on the polarizing beam splitter  204 . The light incident on the polarizing beam splitter  204  is reflected or is transmitted therethrough. The reflected light is emitted as outgoing light  206 . The transmitted light is reflected by the mirror  205  and then is emitted as outgoing light  209 . The outgoing light  209  transmitted through the polarizing beam splitter  204  has no phase shift, while the reflected light  206  is phase-shifted by 180 degrees. Accordingly, the four beams of outgoing lights,  206 ,  207 ,  208  and  209  are phase-shifted by 0, 90, 180, and 270 degrees, respectively, so that fringe analysis of four interference patterns can be made using one photodetector.  
         [0035]      FIG. 3  is a schematic showing a vibration-insensitive interferometer according to another embodiment of the present invention. The overall construction of the interferometer system of  FIG. 3  is almost the same as that of  FIG. 1  except the insertion of a collimating lens  311 . The interferometer of  FIG. 3  is configured with an intention of measuring the thickness profile or refractive index of a transparent test object  312  such as a glass plate. In this case, the reference wave is generated from the wave reflected from the top surface of the test object, which is denoted as light  402  in  FIG. 9a . On the other hand, the measurement wave is selected the wave reflected from the bottom surface of the test object, which is shown as light  403  in  FIG. 9b . (Note that the opposite selection of waves, i.e., light  402  for the measurement wave and light  403  for the reference wave, is also possible.) This separate selection is made by adjusting the optical path delay line  314 . At the same time, the light source  301  is a low-coherence source such as a white light, a diode laser, or a short pulse laser. In this case, the optical path of the reference wave becomes different from that of the measurement wave, but the optical path offset is adjusted by moving the corner cube  314  so that the two waves from different surfaces interfere with each other as illustrated in  FIG. 9   b.  In  FIG. 9   b,  pulses  404  and  406  are reflected from the bottom surface of the transparent object  401 , while pulses  405  and  407  are from the top surface.  
         [0036]     The individual optical paths for four interference patterns at the exit of the spatial phase-shift device,  117  in  FIG. 1  (or  318  in  FIG. 3 ), are described below with reference to  FIG. 4 :  
         [0037]     As shown in  FIG. 4 , outgoing lights  206 ,  207 ,  208 , and  209  experience different optical paths within the spatial phase-shift device  117 .  
         [0038]      FIG. 5  is a view illustrating the optical path compensation blocks of the spatial phase-shift device  117  according to an embodiment of the present invention. Referring to  FIG. 5 , the optical path compensation block assembly  330  of the present embodiment is constructed by adding glass blocks  332 ,  333  and  334 , so that the optical path differences in lights  206 ,  207 ,  208 , and  209 , are made to be identical.  
         [0039]      FIG. 6  is a view showing an anti-distortion block assembly  340  for the phase-shift device  117 , which comprises a set of optical blocks made of fiber bundles,  345 ,  347 , and  348 . Using the optical path compensation block assembly  330  of  FIG. 5  along with the anti-distortion block assembly  340  of  FIG. 6 , the detector  118  in  FIG. 1  is capable of capturing four interference patterns of lights  206 ,  207 ,  208 , and  209 , in an identical image size with an identical level of image distortion. This situation is explained geometrically in  FIG. 8  with an exemplary set of interference patterns shown in  FIG. 7 . This embodiment allows accurate matching of image pixels among four interference patterns, which leads to accurate implement of fringe analysis.  
         [0040]     As described above, the present invention is capable of stabilizing interference patterns through a common path-type interferometer using a single mode optical fiber. Furthermore, using the spatial phase-shift device of the present invention, the influence of vibration can be reduced through real-time analyses of interference patterns. Furthermore, more accurate measurement results can be obtained based on the generation of a perfect reference wave front through a single mode optical fiber, so that the usability of the interferometer is very high in the presence of high level of vibration.  
         [0041]     Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible without departing from the scope and spirit of the invention as disclosed in the accompanying claims.