Abstract:
An optical tomographic image acquiring device which can suppress the occurrence of an artifact, and which can obtain an exact optical tomographic image of a measurement object includes a light source, a detector, an analysis unit, a circulator, a coupler, condensing lenses, optical fibers, and a reference mirror. Let Δk be a maximum value of intervals in wavenumber of lights received by adjacent two light receiving elements in the detector, an optical path length L 0ref  from the coupler to the detector via the reference mirror and an optical path length L 0obj  from the coupler to the detector via the measurement object satisfy |L 0obj −L 0ref |&lt;π/δk, and an optical path length L 1ref  from the coupler to the detector via the condensing lens and an optical path length L 1obj  from the coupler to the detector via the condensing lens satisfy |L 1obj −L 1ref |&gt;&gt;π/δk.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to an optical tomographic image acquiring device. 
       BACKGROUND ART 
       [0002]    The optical tomographic image acquisition technology on the basis of Optical Coherence Tomography (OCT) is able to measure a reflection intensity distribution in the direction of depth of a measurement object by utilizing optical interference. The optical tomographic image acquisition technology has recently been applied to biological measurement because of its capability of imaging an internal structure of the measurement object in a non-invasive manner with a high spatial resolution. 
         [0003]    In an optical tomographic image acquiring device on the basis of OCT, light output from a light source is branched into two beams of reference light and measurement light. Reflected light generated from a reference mirror upon the reference mirror being irradiated with the reference light and diffusely-reflected light generated from a measurement object upon the measurement object being irradiated with the measurement light are caused to interfere with each other. Resulting interference light is detected by a detector. A reflection information distribution (i.e., a one-dimensional optical tomographic image) in the direction of depth of the measurement object is obtained by analyzing the detection result. Furthermore, a two- or three-dimensional optical tomographic image can be obtained by scanning a position of the measurement object where it is irradiated with the light. 
         [0004]    An optical tomographic image acquiring device disclosed in US2011/0299091A includes a first coupler, a first circulator, a second circulator, a second coupler, and a detector. The first coupler branches light output from a light source into two beams of reference light and measurement light. The first circulator receives the reference light output from the coupler and outputs the reference light to a reference mirror. The second circulator receives the measurement light output from the coupler and outputs the measurement light to a measurement object. The second coupler combines reflected light generated from the reference mirror and obtained through the first circulator with object light generated from the measurement object and obtained through the second circulator, thus causing the reflected light and the object light to interfere with each other. The detector detects the interference light output from the second coupler. In addition, the disclosed optical tomographic image acquiring device employs an optical fiber in not only a part of a reference optical system, but also in a part of a measurement optical system. 
         [0005]    The optical tomographic image acquiring device detects interference light (referred to as “signal interference light” hereinafter) resulting from the interference between the reflected light from the reference mirror and the object light from the measurement object. On that occasion, when light is reflected at the circulators and respective end surfaces of the optical fibers in each of the reference optical system and the measurement optical system, the optical tomographic image acquiring device detects interference light (referred to as “noise interference light” hereinafter) resulting from the interference between those reflected lights as well. In other words, the detector of the optical tomographic image acquiring device detects the signal interference light superimposed with the noise interference light, analyzes the detection result, and obtains an optical tomographic image of the measurement object. In the optical tomographic image obtained in such a case, an artifact attributable to the noise interference light is superimposed as noise on the optical tomographic image of the measurement object. 
         [0006]    On the other hand, in an optical tomographic image acquiring device disclosed in JP2012-24551A, aiming to reduce the noise interference light, light incident and emergent surfaces of optical components included in a reference optical system and a measurement optical system are inclined such that reflected lights from the light incident and emergent surfaces of the optical components will not reach the detector. However, the light incident and emergent surfaces cannot be inclined in some of optical components. In that case, the artifact attributable to the reflected lights from those optical components may occur. 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0007]    An object of the present invention is to provide an optical tomographic image acquiring device, which can suppress the occurrence of an artifact, and which can obtain an exact optical tomographic image of a measurement object. 
       Solution to Problem 
       [0008]    To achieve the above object, the present invention provides an optical tomographic image acquiring device including (1) a light source that outputs light, (2) a branching member that branches the light output from the light source into two beams of reference light and measurement light, (3) a reference optical system including a first optical fiber, a first condensing lens, and a reference mirror, and constituted such that the reference light output from the branching member is guided to propagate through the first optical fiber to be incident on the reference mirror via the first condensing lens, and that reflected light generated from the reference mirror upon the incidence of the reference light is guided to propagate through the first optical fiber via the first condensing lens, (4) a measurement optical system including a second optical fiber and a second condensing lens, and constituted such that the measurement light output from the branching member is guided to propagate through the second optical fiber to be applied to the measurement object for irradiation via the second condensing lens, and that reflected light generated from the measurement object upon the irradiation with the measurement light is guided to propagate as object light through the second optical fiber via the second condensing lens, (5) a detector that receives interference light resulting from interference between the reflected light output from the reference optical system and the object light output from the measurement optical system, and that detects a spectrum of the interference light by a spectrometer including a plurality of light receiving elements set in array, and (6) an analysis unit that obtains an optical tomographic image of the measurement object based on a result detected by the detector. 
         [0009]    In the above-described measuring apparatus, let δk be a maximum value of intervals in wavenumber of lights received by adjacent two of the plural light receiving elements in the spectrometer, an optical path length L 0ref  from the branching member to the detector through a path going to and returned from the reference mirror and an optical path length L 0obj  from the branching member to the detector through a path going to and returned from the measurement object satisfy 
         [0000]    
       
      
       |L 
       0obj 
       −L 
       0ref 
       |&lt;π/δk  
      
     
         [0000]    and, an optical path length L 1ref  from the branching member to the detector through a path going to and returned from the first condensing lens and an optical path length L 1obj  from the branching member to the detector through a path going to and returned from the second condensing lens satisfy: 
         [0000]    
       
      
       |L 
       1obj 
       −L 
       1ref 
       |&gt;π/δk.  
      
     
         [0010]    According to a second aspect, the present invention provides an optical tomographic image acquiring device including (1) a wavelength-variable light source that outputs light, (2) a branching member that branches the light output from the light source into two beams of reference light and measurement light, (3) a reference optical system including a first optical fiber, a first condensing lens, and a reference mirror, and constituted such that the reference light output from the branching member is guided to propagate through the first optical fiber to be incident on the reference mirror via the first condensing lens, and that reflected light generated from the reference mirror upon the incidence of the reference light is guided to propagate through the first optical fiber via the first condensing lens, (4) a measurement optical system including a second optical fiber and a second condensing lens, and constituted such that the measurement light output from the branching member is guided to propagate through the second optical fiber to be applied to the measurement object for irradiation via the second condensing lens, and that reflected light generated from the measurement object upon the irradiation with the measurement light is guided to propagate as object light through the second optical fiber via the second condensing lens, (5) a detector that receives interference light resulting from interference between the reflected light output from the reference optical system and the object light output from the measurement optical system, and that detects intensity of the interference light at each wavelength of the light output from the wavelength-variable light source, and (6) an analysis unit that obtains an optical tomographic image of the measurement object based on a result detected by the detector. 
         [0011]    In the above-described measuring apparatus, let δk be a maximum value of intervals in wavenumber of light when the intensity of the interference light is detected by the detector, an optical path length L 0ref  from the branching member to the detector through a path going to and returned from the reference mirror and an optical path length L 0obj  from the branching member to the detector through a path going to and returned from the measurement object satisfy 
         [0000]    
       
      
       |L 
       0obj 
       −L 
       0ref 
       |&lt;π/δk  
      
     
         [0000]    and, an optical path length L 1ref  from the branching member to the detector through a path going to and returned from the first condensing lens and an optical path length L 1obj  from the branching member to the detector through a path going to and returned from the second condensing lens satisfy: 
         [0000]    
       
      
       |L 
       1obj 
       −L 
       1ref 
       |&gt;π/δk.  
      
     
         [0012]    In the optical tomographic image acquiring device according to the present invention, when the reference optical system includes a first optical component disposed midway the first optical fiber and the measurement optical system includes a second optical component disposed midway the second optical fiber, an optical path length L 2ref  from the branching member to the detector through a path going to and returned from the first optical component and an optical path length L 2obj  from the branching member to the detector through a path going to and returned from the second optical component satisfy: 
         [0000]    
       
      
       |L 
       2obj 
       −L 
       2ref 
       |&gt;π/δk.  
      
     
         [0013]    In the optical tomographic image acquiring device according to the present invention, when the reference optical system includes a first circulator disposed midway the first optical fiber and branching the reflected light generated from the reference mirror toward the detector, and a first optical component disposed between the first circulator and the first condensing lens, and the measurement optical system includes a second circulator disposed midway the second optical fiber and branching the object light generated from the measurement object toward the detector, and a second optical component disposed between the second circulator and the second condensing lens, an optical path length L 3ref  from the branching member to the detector through a path going to and returned from the first optical component and an optical path length L 3obj  from the branching member to the detector through a path going to and returned from the second optical component satisfy: 
         [0000]    
       
      
       |L 
       3obj 
       −L 
       3ref 
       |&gt;π/δk.  
      
     
       Advantageous Effects of Invention 
       [0014]    According to the present invention, the occurrence of an artifact can be suppressed, and an exact optical tomographic image of the measurement object can be obtained. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0015]      FIG. 1  is a conceptual view of an optical tomographic image acquiring device  1 A of a first comparative example. 
           [0016]      FIG. 2  is a conceptual view of an optical tomographic image acquiring device  1  according to a first embodiment of the present invention. 
           [0017]      FIG. 3  is a graph plotting the relationship among a measurement range width z max , an optical path length difference ΔL, and an optical path length difference Δd. 
           [0018]      FIG. 4  is a conceptual view of an optical tomographic image acquiring device  2 A of a second comparative example. 
           [0019]      FIG. 5  is a conceptual view of an optical tomographic image acquiring device  2  according to a second embodiment of the present invention. 
           [0020]      FIG. 6  is a conceptual view of an optical tomographic image acquiring device  3 A of a third comparative example. 
           [0021]      FIG. 7  is a conceptual view of an optical tomographic image acquiring device  3  according to a third embodiment of the present invention. 
           [0022]      FIG. 8  is a conceptual view of an optical tomographic image acquiring device  4 A of a fourth comparative example. 
           [0023]      FIG. 9  is a conceptual view of an optical tomographic image acquiring device  4  according to a fourth embodiment of the present invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0024]    Embodiments for carrying out the present invention will be described in detail below with reference to the attached drawings. It is to be noted that the same elements in the drawings are denoted by the identical reference signs and duplicate description of those elements is omitted. The embodiments are described in comparison with corresponding comparative examples. 
       First Comparative Example, First Embodiment 
       [0025]      FIG. 1  is a conceptual view of an optical tomographic image acquiring device  1 A of a first comparative example. The optical tomographic image acquiring device  1 A includes a light source  11 , a detector  12 , an analysis unit  13 , a circulator  20 , a coupler  30 , a first condensing lens  41 , a second condensing lens  42 , a first optical fiber  51 , a second optical fiber  52 , and a reference mirror  91 . The optical tomographic image acquiring device  1 A obtains an optical tomographic image of a measurement object  92  with those components. 
         [0026]    The light source  11  outputs light. The circulator  20  receives the light output from the light source  11  and reaching there, and outputs the received light to the coupler  30 . The coupler  30  serving as a branching member receives the light output from the light source  11  and reaching there through the circulator  20 , and branches the received light into two beams of reference light and measurement light. The coupler  30  outputs the reference light to the first optical fiber  51  and the measurement light to the second optical fiber  52 . 
         [0027]    A reference optical system includes the first condensing lens  41 , the first optical fiber  51 , and the reference mirror  91 . The optical fiber  51  receives at its one end the reference light output from the coupler  30  and outputs the reference light from the other end after guiding the reference light to propagate therethrough. The condensing lens  41  collimates the reference light output from the optical fiber  51  to be incident on the reference mirror  91 . Furthermore, the condensing lens  41  receives reflected light generated from the reference mirror  91  upon the incidence of the reference light, and condenses the reflected light to the end surface of the optical fiber  51 . The optical fiber  51  outputs the reflected light to the coupler  30  after guiding the reflected light to propagate therethrough. 
         [0028]    A measurement optical system includes the second condensing lens  42  and the second optical fiber  52 . The optical fiber  52  receives at its one end the measurement light output from the coupler  30  and outputs the measurement light from the other end after guiding the measurement light to propagate therethrough. The condensing lens  42  condenses the measurement light output from the optical fiber  52  to be applied to the measurement object  92  for irradiation. Furthermore, the condensing lens  42  receives light (object light) reflected from the measurement object  92  upon the irradiation with the measurement light, and condenses the object light to the end surface of the optical fiber  52 . The optical fiber  52  outputs the object light to the coupler  30  after guiding the object light to propagate therethrough. 
         [0029]    The coupler  30  receives not only the reflected light output from the optical fiber  51  and reaching there, but also the object light output from the optical fiber  52  and reaching there. The coupler  30  outputs interference light, resulting from interference between both the received lights, to the circulator  20 . The circulator  20  receives the interference light output from the coupler  30  and reaching there, and outputs the interference light to the detector  12 . The detector  12  receives the interference light output from the circulator  20  and reaching there, and detects the interference light. The analysis unit  13  obtains an optical tomographic image of the measurement object  92  based on the result detected by the detector  12 . 
         [0030]    In Spectrum Domain OCT (SD-OCT), a wide-range light source is used as the light source  11 . The detector  12  detects the spectrum of the interference light by a spectrometer including a plurality of light receiving elements set in array. 
         [0031]    In Swept-Source OCT (SS-OCT), a wavelength-variable light source is used as the light source  11 , and a single light receiving element is used as the detector  12 . The detector  12  detects the intensity of the interference light at each wavelength of light output from the wavelength-variable light source  11 . 
         [0032]    In SD-OCT and SS-OCT, a measurement range in the direction of depth of the measurement object  92  is limited by the Nyquist frequency in discrete Fourier transform that is used in an analysis executed by the analysis unit  13 . A measurement range width z max  in air is expressed by the following formula (1): 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         
                           z 
                           max 
                         
                         = 
                           
                          
                         
                           π 
                            
                           
                             / 
                           
                            
                           2 
                            
                           
                               
                           
                            
                           δ 
                            
                           
                               
                           
                            
                           k 
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         
                           π 
                            
                           
                               
                           
                            
                           N 
                            
                           
                             / 
                           
                            
                           2 
                            
                           
                               
                           
                            
                           Δ 
                            
                           
                               
                           
                            
                           k 
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         
                           π 
                            
                           
                               
                           
                            
                           N 
                            
                           
                             / 
                           
                            
                           2 
                            
                           
                             ( 
                             
                               
                                 2 
                                  
                                 
                                     
                                 
                                  
                                 
                                   π 
                                   / 
                                   
                                     λ 
                                     1 
                                   
                                 
                               
                               - 
                               
                                 2 
                                  
                                 
                                     
                                 
                                  
                                 π 
                                  
                                 
                                   / 
                                 
                                  
                                 
                                   λ 
                                   2 
                                 
                               
                             
                             ) 
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         
                           N 
                            
                           
                               
                           
                            
                           
                             λ 
                             1 
                           
                            
                           
                             
                               λ 
                               2 
                             
                             / 
                             4 
                           
                            
                           
                               
                           
                            
                           Δ 
                            
                           
                               
                           
                            
                           λ 
                         
                       
                     
                   
                   
                     
                       
                         ≈ 
                           
                          
                         
                           N 
                            
                           
                               
                           
                            
                           
                             
                               λ 
                               0 
                               2 
                             
                             / 
                             4 
                           
                            
                           
                               
                           
                            
                           Δ 
                            
                           
                               
                           
                            
                           
                             λ 
                             . 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0033]    Here, Δk is a band width of the spectrometer or a wave-number variable width of the wavelength variable light source. Δλ is a band width of the spectrometer or the wavelength variable width of the wavelength variable light source. δk is a unit of wave number in the wavelength range of the spectrometer or in the variable range of the wavelength variable light source. λ 1 , λ 2  and λ 0  are respectively the shortest wavelength, the longest wavelength, and the center wavelength (=(λ 1 +λ 2 )/2) in the wavelength range of the spectrometer or in the variable range of the wavelength variable light source. N is the number of spectrum samplings. Assuming λ 0 =1310 nm, Δλ=90 nm, and N=1024, for example, the measurement range width z max  in air is estimated to be 4.9 mm (=1024×1265×1355)/4×90) nm). 
         [0034]    In the first comparative example, it is assumed that reflected lights are generated from the condensing lenses  41  and  42 . Those reflected lights may also reach the detector  12  through the optical fibers  51  and  52 , the coupler  30 , and the circulator  20 . 
         [0035]    The distance along an optical path between the coupler  30  and the emergent end of the optical fiber  51  is denoted by Lr2. The distance along an optical path between the coupler  30  and the emergent end of the optical fiber  52  is denoted by Ls2. The distance along an optical path between the emergent end of the optical fiber  51  and the reference mirror  91  is denoted by Lr1. The distance along an optical path between the emergent end of the optical fiber  52  and the measurement object  92  is denoted by Ls1. The distance along an optical path between the emergent end of the optical fiber  51  and an arbitrary reflecting surface associated with the condensing lens  41  is denoted by dr. The distance along an optical path between the emergent end of the optical fiber  52  and an arbitrary reflecting surface associated with the condensing lens  42  is denoted by ds. The effective refractive index of the optical fibers  51  and  52  is denoted by n. 
         [0036]    On those assumptions, a difference ΔL between an optical path length L 0ref  from the coupler  30  to the detector  12  through a path going to and returned from the reference mirror  91  and an optical path length L 0obj  from the coupler  30  to the detector  12  through a path going to and returned from the measurement object  92  is expressed by the following formula (2a): 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         
                           Δ 
                            
                           
                               
                           
                            
                           L 
                         
                         = 
                           
                          
                         
                            
                           
                             
                               L 
                               
                                 0 
                                  
                                 
                                     
                                 
                                  
                                 obj 
                               
                             
                             - 
                             
                               L 
                               
                                 0 
                                  
                                 
                                     
                                 
                                  
                                 ref 
                               
                             
                           
                            
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         
                           2 
                            
                           
                             
                                
                               
                                 
                                   n 
                                    
                                   
                                     ( 
                                     
                                       
                                         Ls 
                                          
                                         
                                             
                                         
                                          
                                         2 
                                       
                                       - 
                                       
                                         Lr 
                                          
                                         
                                             
                                         
                                          
                                         2 
                                       
                                     
                                     ) 
                                   
                                 
                                 + 
                                 
                                   ( 
                                   
                                     
                                       Ls 
                                        
                                       
                                           
                                       
                                        
                                       1 
                                     
                                     - 
                                     
                                       Ls 
                                        
                                       
                                           
                                       
                                        
                                       1 
                                     
                                   
                                   ) 
                                 
                               
                                
                             
                             . 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     2 
                      
                     a 
                   
                   ) 
                 
               
             
           
         
       
     
         [0000]    Furthermore, a difference Δd between an optical path length L 1ref  from the coupler  30  to the detector  12  through a path going to and returned from the reflecting surface associated with the condensing lens  41  and an optical path length L 1obj  from the coupler  30  to the detector  12  through a path going to and returned from the reflecting surface associated with the condensing lens  42  is expressed by the following formula (2b): 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         
                           Δ 
                            
                           
                               
                           
                            
                           d 
                         
                         = 
                           
                          
                         
                            
                           
                             
                               L 
                               
                                 1 
                                  
                                 
                                     
                                 
                                  
                                 obj 
                               
                             
                             - 
                             
                               L 
                               
                                 1 
                                  
                                 
                                     
                                 
                                  
                                 ref 
                               
                             
                           
                            
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         
                           2 
                            
                           
                             
                                
                               
                                 
                                   n 
                                    
                                   
                                     ( 
                                     
                                       
                                         Ls 
                                          
                                         
                                             
                                         
                                          
                                         2 
                                       
                                       - 
                                       
                                         Lr 
                                          
                                         
                                             
                                         
                                          
                                         2 
                                       
                                     
                                     ) 
                                   
                                 
                                 + 
                                 
                                   ( 
                                   
                                     ds 
                                     - 
                                     dr 
                                   
                                   ) 
                                 
                               
                                
                             
                             . 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     2 
                      
                     
                         
                     
                      
                     b 
                   
                   ) 
                 
               
             
           
         
       
     
         [0037]    In the first comparative example, as expressed by the following formulae (3a), 
         [0000]      Δ L/ 2 z   max  and Δ d/ 2&lt; z   max,   (3a)
 
         [0000]    ΔL/2 and Δd/2 are both smaller than the measurement range width z max . By applying the formula (1), the formula (3a) can be rewritten to the following formulae (3b): 
         [0000]      Δ L&lt;π/δk  and Δ d&lt;π/δk.   (3)
 
         [0000]    In such a case, an artifact attributable to the reflected lights generated from the condensing lenses  41  and  42  is superimposed as noise on an optical tomographic image of the measurement object  92  (see  FIG. 3(   a )). 
         [0038]      FIG. 2  is a conceptual view of an optical tomographic image acquiring device  1  according to a first embodiment. The optical tomographic image acquiring device  1  is different from the optical tomographic image acquiring device  1 A of the first comparative example in that the distance Lr1 along the optical path between the emergent end of the optical fiber  51  and the reference mirror  91  in the reference optical system is increased, and that the length Ls2 of the optical fiber  52  in the measurement optical system is also increased. When respective changes of the optical path lengths resulting from increases of Lr1 and Ls2 are equal to other, ΔL expressed by the above formula (2a) is not changed and the optical tomographic image of the measurement object  92  is obtained in the first embodiment at the same position as that obtained in the first comparative example. 
         [0039]    Furthermore, in the first embodiment, as expressed by the following formulae (4a), 
         [0000]      Δ L/ 2&lt; z   max  and Δ d/ 2&gt; z   max,   (4a)
 
         [0000]    ΔL/2 remains smaller than the measurement range width z max , but Δd/2 is larger than the measurement range width z max . By applying the formula (1), the formula (4a) can be rewritten to the following formulae (4b): 
         [0000]      Δ L&lt;π/δk  and Δ d&gt;π/δk.   (4b)
 
         [0000]    In such a case, the artifact attributable to the reflected lights generated from the condensing lenses  41  and  42  is not superimposed on the optical tomographic image of the measurement object  92  (see  FIG. 3(   b )). 
         [0040]    In the first embodiment, because the optical fiber  51  in the reference optical system and the optical fiber  52  in the measurement optical system have different lengths from each other, the influences of dispersions in the optical fibers  51  and  52  are apt to appear in the optical tomographic image. To cope with that problem, a dispersion compensation element  61  is preferably inserted in the optical path between the condensing lens  41  and the reference mirror  91  in the reference optical system. As an alternative, it is also preferable in SD-OCT or SS-OCT to multiply the interference spectrum by a phase component reversed to that of the dispersion. 
       Second Comparative Example, Second Embodiment 
       [0041]      FIG. 4  is a conceptual view of an optical tomographic image acquiring device  2 A of a second comparative example. The optical tomographic image acquiring device  2 A of the second comparative example is different from the optical tomographic image acquiring device  1 A of the first comparative example in that the reference optical system includes an optical component  71  disposed midway the optical fiber  51 , and that the measurement optical system includes an optical component  72  disposed midway the optical fiber  52 . The optical components  71  and  72  are each, e.g., a polarization controller or an attenuator. 
         [0042]    In the second comparative example, it is assumed that reflected lights are generated from the optical components  71  and  72 . Those reflected lights may also reach the detector  12  through the optical fibers  51  and  52 , the coupler  30 , and the circulator  20 . 
         [0043]    The distance along an optical path between the coupler  30  and the emergent end of the optical fiber  51  is denoted by Lr2. The distance along an optical path between the coupler  30  and the emergent end of the optical fiber  52  is denoted by Ls2. The distance along an optical path between the emergent end of the optical fiber  51  and the reference mirror  91  is denoted by Lr1. The distance along an optical path between the emergent end of the optical fiber  52  and the measurement object  92  is denoted by Ls1. The distance along an optical path between the coupler  30  and the optical component  71  is denoted by dr. The distance along an optical path between the coupler  30  and the optical component  72  is denoted by ds. The effective refractive index of the optical fibers  51  and  52  is denoted by n. 
         [0044]    On those assumptions, a difference ΔL between an optical path length L 0ref  from the coupler  30  to the detector  12  through a path going to and returned from the reference mirror  91  and an optical path length L 0obj  from the coupler  30  to the detector  12  through a path going to and returned from the measurement object  92  is expressed by the following formula (5a): 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         
                           Δ 
                            
                           
                               
                           
                            
                           L 
                         
                         = 
                           
                          
                         
                            
                           
                             
                               L 
                               
                                 0 
                                  
                                 
                                     
                                 
                                  
                                 obj 
                               
                             
                             - 
                             
                               L 
                               
                                 0 
                                  
                                 
                                     
                                 
                                  
                                 ref 
                               
                             
                           
                            
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         
                           2 
                            
                           
                             
                                
                               
                                 
                                   n 
                                    
                                   
                                     ( 
                                     
                                       
                                         Ls 
                                          
                                         
                                             
                                         
                                          
                                         2 
                                       
                                       - 
                                       
                                         Lr 
                                          
                                         
                                             
                                         
                                          
                                         2 
                                       
                                     
                                     ) 
                                   
                                 
                                 + 
                                 
                                   ( 
                                   
                                     
                                       Ls 
                                        
                                       
                                           
                                       
                                        
                                       1 
                                     
                                     - 
                                     
                                       Lr 
                                        
                                       
                                           
                                       
                                        
                                       1 
                                     
                                   
                                   ) 
                                 
                               
                                
                             
                             . 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     5 
                      
                     a 
                   
                   ) 
                 
               
             
           
         
       
     
         [0000]    Furthermore, a difference Δd between an optical path length L 2ref  from the coupler  30  to the detector  12  through a path going to and returned from the optical component  71  and an optical path length L 2obj  from the coupler  30  to the detector  12  through a path going to and returned from the optical component  72  is expressed by the following formula (5b): 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         
                           Δ 
                            
                           
                               
                           
                            
                           d 
                         
                         = 
                           
                          
                         
                            
                           
                             
                               L 
                               
                                 2 
                                  
                                 
                                     
                                 
                                  
                                 obj 
                               
                             
                             - 
                             
                               L 
                               
                                 2 
                                  
                                 
                                     
                                 
                                  
                                 ref 
                               
                             
                           
                            
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         
                           2 
                            
                           
                             
                                
                               
                                 n 
                                  
                                 
                                   ( 
                                   
                                     ds 
                                     - 
                                     dr 
                                   
                                   ) 
                                 
                               
                                
                             
                             . 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     5 
                      
                     
                         
                     
                      
                     b 
                   
                   ) 
                 
               
             
           
         
       
     
         [0045]    In the second comparative example, as expressed by the above formula (3), ΔL/2 and Δd/2 are both smaller than the measurement range width z max . Thus, an artifact attributable to the reflected lights generated from the optical components  71  and  72  is superimposed as noise on an optical tomographic image of the measurement object  92  (see  FIG. 3(   a )). 
         [0046]      FIG. 5  is a conceptual view of an optical tomographic image acquiring device  2  according to a second embodiment. The optical tomographic image acquiring device  2  of the second embodiment is different from the optical tomographic image acquiring device  2 A of the second comparative example in that a length (Lr2−dr) of a portion of the optical fiber  51  in the reference optical system between the optical component  71  and the emergent end of the optical fiber  51  is increased, and that a length ds of a portion of the optical fiber  52  in the measurement optical system between the coupler  30  and the optical component  72  is also increased. When respective changes of the optical path lengths resulting from increases of (Lr2−dr) and ds are equal to other, ΔL expressed by the above formula (5a) is not changed and the optical tomographic image of the measurement object  92  is obtained in the second embodiment at the same position as that obtained in the second comparative example. 
         [0047]    Furthermore, in the second embodiment, as expressed by the above formula (4), ΔL/2 remains smaller than the measurement range width z max , but Δd/2 is larger than the measurement range width z max . In such a case, the artifact attributable to the reflected lights generated from the optical components  71  and  72  is not superimposed on the optical tomographic image of the measurement object  92  (see  FIG. 3(   b )). 
       Third Comparative Example, Third Embodiment 
       [0048]      FIG. 6  is a conceptual view of an optical tomographic image acquiring device  3 A of a third comparative example. The optical tomographic image acquiring device  3 A of the third comparative example includes a light source  11 , a detector  12 , an analysis unit  13 , circulators  21  and  22 , couplers  31  and  32 , a first condensing lens  41 , a second condensing lens  42 , a first optical fiber  51   1  to  51   3 , a second optical fiber  52   1  to  52   3 , and a reference mirror  91 . The optical tomographic image acquiring device  3 A obtains an optical tomographic image of a measurement object  92  with those components. 
         [0049]    The coupler  31  receives light output from the light source  11  and reaching there, and branches the received light into two beams of reference light and measurement light. The coupler  31  outputs the reference light to the optical fiber  511  and the measurement light to the optical fiber  52   1 . 
         [0050]    The circulator  21  receives the reference light output from the coupler  31  and reaching there after being guided to propagate through the optical fiber  51   1 , and outputs the reference light to the optical fiber  51   2 . The optical fiber  51   2  receives at its one end the reference light output from the circulator  21  and outputs the reference light from the other end after guiding the reference light to propagate therethrough. The condensing lens  41  collimates the reference light output from the optical fiber  51   2  to be incident on the reference mirror  91 . Furthermore, the condensing lens  41  receives reflected light generated from the reference mirror  91  upon the incidence of the reference light, and condenses the reflected light to the end surface of the optical fiber  51   2 . The optical fiber  51   2  outputs the reflected light to the circulator  21  after guiding the reflected light to propagate therethrough. The circulator  21  receives the reflected light output from the optical fiber  51   2  and outputs the reflected light to the optical fiber  51   3 . 
         [0051]    The circulator  22  receives the measurement light output from the coupler  31  and reaching there after being guided to propagate through the optical fiber  52   1 , and outputs the measurement light to the optical fiber  52   2 . The optical fiber  52   2  receives at its one end the measurement light output from the circulator  22  and outputs the measurement light from the other end after guiding the measurement light to propagate therethrough. The condensing lens  42  condenses the measurement light output from the optical fiber  52   2  to be applied to the measurement object  92  for irradiation. Furthermore, the condensing lens  42  receives light (object light) reflected from the measurement object  92  upon the irradiation with the measurement light, and condenses the object light to the end surface of the optical fiber  52   2 . The optical fiber  52   2  outputs the object light to the circulator  22  after guiding the object light to propagate therethrough. The circulator  22  receives the object light output from the optical fiber  52   2  and outputs the object light to the optical fiber  52   3 . 
         [0052]    The coupler  32  receives not only the reflected light output from the circulator  21  and reaching there after being guided to propagate through the optical fiber  51   3 , but also the object light output from the circulator  22  and reaching there after being guided to propagate through the optical fiber  52   3 . The coupler  32  outputs interference light, resulting from interference between both the received lights, to the detector  12 . The detector  12  receives the interference light output from the coupler  32  and reaching there, and detects the interference light. The analysis unit  13  obtains an optical tomographic image of the measurement object  92  based on the result detected by the detector  12 . 
         [0053]    In the third comparative example, it is assumed that reflected lights are generated from the condensing lenses  41  and  42 . Those reflected lights may also reach the detector  12  through the optical fibers  51   2  and  52   2 , the circulators  21  and  22 , the optical fibers  51   3  and  52   3 , and the coupler  32 . 
         [0054]    The distance along an optical path between the coupler  31  and the circulator  21  is denoted by Lri. The distance along an optical path between the coupler  31  and the circulator  22  is denoted by Lsi. The distance along an optical path between the circulator  21  and the emergent end of the optical fiber  51   2  is denoted by Lr2. The distance along an optical path between the circulator  22  and the emergent end of the optical fiber  52   2  is denoted by Ls2. The distance along an optical path between the emergent end of the optical fiber  51   2  and the reference mirror  91  is denoted by Lr1. The distance along an optical path between the emergent end of the optical fiber  52   2  and the measurement object  92  is denoted by Ls1. The distance along an optical path between the circulator  21  and the coupler  32  is denoted by Lro. The distance along an optical path between the circulator  22  and the coupler  32  is denoted by Lso. The distance along an optical path between the emergent end of the optical fiber  51   2  and an arbitrary reflecting surface associated with the condensing lens  41  is denoted by dr. The distance along an optical path between the emergent end of the optical fiber  52   2  and an arbitrary reflecting surface associated with the condensing lens  42  is denoted by ds. The effective refractive index of the optical fibers  51  and  52  is denoted by n. 
         [0055]    On those assumptions, a difference ΔL between an optical path length L 0ref  from the coupler  31  to the detector  12  through a path going to and returned from the reference mirror  91  and an optical path length Lo obj  from the coupler  31  to the detector  12  through a path going to and returned from the measurement object  92  is expressed by the following formula (6a): 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         
                           Δ 
                            
                           
                               
                           
                            
                           L 
                         
                         = 
                           
                          
                         
                            
                           
                             
                               L 
                               
                                 0 
                                  
                                 
                                     
                                 
                                  
                                 obj 
                               
                             
                             - 
                             
                               L 
                               
                                 0 
                                  
                                 
                                     
                                 
                                  
                                 ref 
                               
                             
                           
                            
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         
                           
                              
                             
                               
                                 n 
                                  
                                 
                                   ( 
                                   
                                     Lsi 
                                     + 
                                     
                                       2 
                                        
                                       
                                           
                                       
                                        
                                       Ls 
                                        
                                       
                                           
                                       
                                        
                                       2 
                                     
                                     + 
                                     Lso 
                                     - 
                                     Lri 
                                     - 
                                     
                                       2 
                                        
                                       
                                           
                                       
                                        
                                       Lr 
                                        
                                       
                                           
                                       
                                        
                                       2 
                                     
                                     - 
                                     Lro 
                                   
                                   ) 
                                 
                               
                               + 
                               
                                 2 
                                  
                                 
                                   ( 
                                   
                                     
                                       Ls 
                                        
                                       
                                           
                                       
                                        
                                       1 
                                     
                                     - 
                                     
                                       Ls 
                                        
                                       
                                           
                                       
                                        
                                       1 
                                     
                                   
                                   ) 
                                 
                               
                             
                              
                           
                           . 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     6 
                      
                     a 
                   
                   ) 
                 
               
             
           
         
       
     
         [0000]    Furthermore, a difference Δd between an optical path length L 1ref  from the coupler  31  to the detector  12  through a path going to and returned from the reflecting surface associated with the condensing lens  41  and an optical path length L 1obj  from the coupler  31  to the detector  12  through a path going to and returned from the reflecting surface associated with the condensing lens  42  is expressed by the following formula (6b): 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         
                           Δ 
                            
                           
                               
                           
                            
                           d 
                         
                         = 
                           
                          
                         
                            
                           
                             
                               L 
                               
                                 1 
                                  
                                 
                                     
                                 
                                  
                                 obj 
                               
                             
                             - 
                             
                               L 
                               
                                 1 
                                  
                                 
                                     
                                 
                                  
                                 ref 
                               
                             
                           
                            
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         
                           
                              
                             
                               
                                 n 
                                  
                                 
                                   ( 
                                   
                                     Lsi 
                                     + 
                                     
                                       2 
                                        
                                       
                                           
                                       
                                        
                                       Ls 
                                        
                                       
                                           
                                       
                                        
                                       2 
                                     
                                     + 
                                     Lso 
                                     - 
                                     Lri 
                                     - 
                                     
                                       2 
                                        
                                       
                                           
                                       
                                        
                                       Lr 
                                        
                                       
                                           
                                       
                                        
                                       2 
                                     
                                     - 
                                     Lro 
                                   
                                   ) 
                                 
                               
                               + 
                               
                                 2 
                                  
                                 
                                   ( 
                                   
                                     ds 
                                     - 
                                     dr 
                                   
                                   ) 
                                 
                               
                             
                              
                           
                           . 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     6 
                      
                     
                         
                     
                      
                     b 
                   
                   ) 
                 
               
             
           
         
       
     
         [0056]    In the third comparative example, as expressed by the above formula (3), ΔL/2 and Δd/2 are both smaller than the measurement range width z max , and an artifact attributable to the reflected lights generated from the condensing lenses  41  and  42  is superimposed as noise on an optical tomographic image of the measurement object  92  (see  FIG. 3(   a )). 
         [0057]      FIG. 7  is a conceptual view of an optical tomographic image acquiring device  3  according to a third embodiment. The optical tomographic image acquiring device  3  of the third embodiment is different from the optical tomographic image acquiring device  3 A of the third comparative example in that the distance Lr1 along the optical path between the emergent end of the optical fiber  51   2  and the reference mirror  91  in the reference optical system is increased, and that the length Lso of the optical fiber  52   3  in the measurement optical system is also increased. When respective changes of the optical path lengths resulting from increases of Lr1 and Lso are equal to other, ΔL expressed by the above formula (6a) is not changed and the optical tomographic image of the measurement object  92  is obtained in the third embodiment at the same position as that obtained in the third comparative example. 
         [0058]    Furthermore, in the third embodiment, as expressed by the above formula (4a), ΔL/2 remains smaller than the measurement range width z max , but Δd/2 is larger than the measurement range width z max . In such a case, the artifact attributable to the reflected lights generated from the condensing lenses  41  and  42  is not superimposed on the optical tomographic image of the measurement object  92  (see  FIG. 3(   b )). 
         [0059]    In the third embodiment, because the optical fiber  51  in the reference optical system and the optical fiber  52  in the measurement optical system have different lengths from each other, the influences of dispersions in the optical fibers  51  and  52  are apt to appear in the optical tomographic image. To cope with that problem, a dispersion compensation element  61  is preferably inserted in the optical path between the condensing lens  41  and the reference mirror  91  in the reference optical system. As an alternative, it is also preferable in SD-OCT or SS-OCT to multiply the interference spectrum by a phase component reversed to that of the dispersion. 
       Fourth Comparative Example, Fourth Embodiment 
       [0060]      FIG. 8  is a conceptual view of an optical tomographic image acquiring device  4 A of a fourth comparative example. The optical tomographic image acquiring device  4 A of the fourth comparative example is different from the optical tomographic image acquiring device  3 A of the third comparative example in that the reference optical system includes an optical component  71  disposed midway the optical fiber  51   2 , and that the measurement optical system includes an optical component  72  disposed midway the optical fiber  52   2 . The optical components  71  and  72  are each, e.g., a polarization controller or an attenuator. 
         [0061]    In the fourth comparative example, it is assumed that reflected lights are generated from the optical components  71  and  72 . Those reflected lights may also reach the detector  12  through the optical fibers  51   2  and  52   2 , the circulators  21  and  22 , the optical fibers  51   3  and  52   3 , and the coupler  32 . 
         [0062]    The distance along an optical path between the coupler  31  and the circulator  21  is denoted by Lri. The distance along an optical path between the coupler  31  and the circulator  22  is denoted by Lsi. The distance along an optical path between the circulator  21  and the emergent end of the optical fiber  51   2  is denoted by Lr2. The distance along an optical path between the circulator  22  and the emergent end of the optical fiber  52   2  is denoted by Ls2. The distance along an optical path between the emergent end of the optical fiber  51   2  and the reference mirror  91  is denoted by Lr1. The distance along an optical path between the emergent end of the optical fiber  52   2  and the measurement object  92  is denoted by Ls1. The distance along an optical path between the circulator  21  and the coupler  32  is denoted by Lro. The distance along an optical path between the circulator  22  and the coupler  32  is denoted by Lso. The distance along an optical path between the circulator  21  and the optical component  71  is denoted by dr. The distance along an optical path between the circulator  22  and the optical component  72  is denoted by ds. The effective refractive index of the optical fibers  51  and  52  is denoted by n. 
         [0063]    On those assumptions, a difference ΔL between an optical path length L 0ref  from the coupler  31  to the detector  12  through a path going to and returned from the reference mirror  91  and an optical path length L 0obj  from the coupler  31  to the detector  12  through a path going to and returned from the measurement object  92  is expressed by the following formula (7a): 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         
                           Δ 
                            
                           
                               
                           
                            
                           L 
                         
                         = 
                           
                          
                         
                            
                           
                             
                               L 
                               
                                 0 
                                  
                                 
                                     
                                 
                                  
                                 obj 
                               
                             
                             - 
                             
                               L 
                               
                                 0 
                                  
                                 
                                     
                                 
                                  
                                 ref 
                               
                             
                           
                            
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         
                           
                              
                             
                               
                                 n 
                                  
                                 
                                   ( 
                                   
                                     Lsi 
                                     + 
                                     
                                       2 
                                        
                                       
                                           
                                       
                                        
                                       Ls 
                                        
                                       
                                           
                                       
                                        
                                       2 
                                     
                                     + 
                                     Lso 
                                     - 
                                     Lri 
                                     - 
                                     
                                       2 
                                        
                                       
                                           
                                       
                                        
                                       Lr 
                                        
                                       
                                           
                                       
                                        
                                       2 
                                     
                                     - 
                                     Lro 
                                   
                                   ) 
                                 
                               
                               + 
                               
                                 2 
                                  
                                 
                                   ( 
                                   
                                     
                                       Ls 
                                        
                                       
                                           
                                       
                                        
                                       1 
                                     
                                     - 
                                     
                                       Ls 
                                        
                                       
                                           
                                       
                                        
                                       1 
                                     
                                   
                                   ) 
                                 
                               
                             
                              
                           
                           . 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     7 
                      
                     a 
                   
                   ) 
                 
               
             
           
         
       
     
         [0064]    Furthermore, a difference Δd between an optical path length L 3ref  from the coupler  31  to the detector  12  through a path going to and returned from the optical component  71  and an optical path length L 3obj  from the coupler  31  to the detector  12  through a path going to and returned from the optical component  72  is expressed by the following formula (7b): 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         
                           Δ 
                            
                           
                               
                           
                            
                           d 
                         
                         = 
                           
                          
                         
                            
                           
                             
                               L 
                               
                                 3 
                                  
                                 
                                     
                                 
                                  
                                 obj 
                               
                             
                             - 
                             
                               L 
                               
                                 3 
                                  
                                 
                                     
                                 
                                  
                                 ref 
                               
                             
                           
                            
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         
                           
                              
                             
                               
                                 n 
                                  
                                 
                                   ( 
                                   
                                     Lsi 
                                     + 
                                     Lso 
                                     - 
                                     Lri 
                                     - 
                                     Lro 
                                   
                                   ) 
                                 
                               
                               + 
                               
                                 2 
                                  
                                 
                                   ( 
                                   
                                     ds 
                                     - 
                                     dr 
                                   
                                   ) 
                                 
                               
                             
                              
                           
                           . 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     7 
                      
                     
                         
                     
                      
                     b 
                   
                   ) 
                 
               
             
           
         
       
     
         [0065]    In the fourth comparative example, as expressed by the above formula (3), ΔL/2 and Δd/2 are both smaller than the measurement range width z max , and an artifact attributable to the reflected lights generated from the optical components  71  and  72  is superimposed as noise on an optical tomographic image of the measurement object  92  (see  FIG. 3(   a )). 
         [0066]      FIG. 9  is a conceptual view of an optical tomographic image acquiring device  4  according to a fourth embodiment. The optical tomographic image acquiring device  4  is different from the optical tomographic image acquiring device  4 A of the fourth comparative example in that a length (Lr2−dr) of a portion of the optical fiber  51   2  in the reference optical system between the optical component  71  and the emergent end of the optical fiber  51   2  is increased, and that a length Lso of the optical fiber  52   3  in the measurement optical system is also increased. When respective changes of the optical path lengths resulting from increases of (Lr2−dr) and Lso are equal to other, ΔL expressed by the above formula (7a) is not changed and the optical tomographic image of the measurement object  92  is obtained in the fourth embodiment at the same position as that obtained in the fourth comparative example. 
         [0067]    Furthermore, in the fourth embodiment, as expressed by the above formula (4), ΔL/2 remains smaller than the measurement range width z max , but Δd/2 is larger than the measurement range width z max . In such a case, the artifact attributable to the reflected lights generated from the optical components  71  and  72  is not superimposed on the optical tomographic image of the measurement object  92  (see  FIG. 3(   b )). 
       Modifications 
       [0068]    The present invention is not limited to the above-described embodiments and can be variously modified. In the present invention, it is just required to set the optical path lengths of the reference optical system and the measurement optical system and to set the optical path lengths between the positions where reflected lights causing the artifact are generated (e.g., the condensing lenses or other optical components) and each of the light source and the detector such that the above-mentioned formula (4) is satisfied. Accordingly, there are various ways in adjusting lengths of optical path lengths in which portions of the reference optical system and the measurement optical system. 
       INDUSTRIAL APPLICABILITY 
       [0069]    The optical tomographic image acquiring device is used as an instrument for use in ophthalmology and for observing the lumen of a bored body.