Patent Publication Number: US-9427149-B2

Title: Optical tomographic imaging apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an optical tomographic imaging apparatus for use in an ophthalmologic medical care and the like. 
     2. Description of the Related Art 
     Currently, various ophthalmologic apparatuses using optical apparatuses are known. For example, various apparatuses such as an anterior eye portion imaging apparatus, a fundus camera, and a confocal scanning laser ophthalmoscope (SLO) are used as optical apparatuses for observing a subject&#39;s eye. Among them, an optical tomographic imaging apparatus based on optical coherence tomography (OCT) utilizing multi-wavelength lightwave interference is an apparatus that can acquire a tomographic image of a sample at a high resolution, and is becoming an apparatus essential for clinics specialized in retinas as an ophthalmologic apparatus. Hereinafter, this apparatus will be referred to as an OCT apparatus. 
     The OCT apparatus emits measurement light, which is low-coherent light, to the sample, and can measure backscattering light from this sample at a high sensitivity by using an interference system or an interference optical system. The low-coherent light is characterized in that a tomographic image can be acquired at a high resolution by increasing a wavelength width thereof. Further, the OCT apparatus can acquire a tomographic image at a high resolution by scanning the measurement light on the sample. Therefore, the OCT apparatus can acquire a tomographic image of a retina on a fundus of a subject&#39;s eye, and is widely used in an ophthalmologic diagnosis and the like of a retina. 
     On the other hand, generally, the OCT apparatus as an ophthalmologic apparatus is provided with a fundus observation optical system, an anterior eye observation optical system, and the like for an alignment adjustment between the apparatus and the subject&#39;s eye. The OCT apparatus is constructed by using light beams having different wavelengths in the respective optical systems and separating the wavelengths with use of a wavelength separation unit such as a dichroic mirror, to allow the OCT apparatus to be used together with these optical systems. 
     Now, suppose that a light source for OCT emits light having a central wavelength of 855 nm, and a wavelength band from approximately 805 nm to approximately 905 nm with a wavelength bandwidth of approximately 100 nm. On the other hand, a light source for OCT discussed in Japanese Patent Application Laid-Open No. 2011-11052 emits light having a central wavelength of 840 nm, and a wavelength band from approximately 815 nm to approximately 865 nm with a wavelength bandwidth of approximately 50 nm. Further, suppose that a light source configured to produce light having a wavelength of 780 nm is used as a light source of an SLO. In this case, an interval between the wavelength of the light source of the SLO and an end of the wavelength band of the light source for OCT is approximately 35 nm (815 nm-780 nm) in the technique discussed in Japanese Patent Application Laid-Open No. 2011-11052. On the other hand, in the case where the wavelength bandwidth is approximately 100 nm, this interval is approximately 25 nm (805 nm-780 nm). In this manner, in the case of the wavelength bandwidth of approximately 100 nm, the wavelength bandwidth of the light source for OCT is wider than that of the technique discussed in Japanese Patent Application Laid-Open No. 2011-11052, thereby leading to a reduction in the interval between the wavelength of the light source of the SLO and the end of the wavelength band of the light source for OCT. 
     SUMMARY OF THE INVENTION 
     According to an aspect of the present invention, an optical tomographic imaging apparatus, which is configured to acquire a tomographic image of an object to be examined based on light generated by combining return light from the object to be examined irradiated by measurement light via a first lens and reference light corresponding to the measurement light. The optical tomographic imaging apparatus includes a scanning unit disposed on an optical path of the measurement light and configured to scan the measurement light on the object to be examined, a second lens disposed on the optical path of the measurement light between the scanning unit and the first lens, and an optical path branching unit disposed between the first lens and the second lens and configured to branch the optical path of the measurement light to form an observation optical path for observing the object to be examined therefrom. The second lens and the scanning unit are disposed in such a manner that an angle at which the measurement light scanned by the scanning unit is incident on the optical path branching unit satisfies a wavelength separation characteristic of 90% or higher at a wavelength of observation light of the object to be examined in a wavelength band of the measurement light. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an outline of a configuration of an optical tomographic imaging apparatus according to an exemplary embodiment of the present invention. 
         FIG. 2  illustrates exemplary optical paths of a light flux of light incident on a pupil of a subject&#39;s eye, in an optical tomographic imaging apparatus according to the exemplary embodiment of the present invention. 
         FIG. 3  illustrates an example of how measurement light is scanned on a subject&#39;s eye by an X scanner in an X direction in the optical tomographic imaging apparatus. 
         FIG. 4  illustrates exemplary representations of an image of an anterior eye, a two-dimensional image of a fundus, and a B-scan image displayed on a monitor in the optical tomographic imaging apparatus. 
         FIGS. 5A, 5B, and 5C  illustrate what kind of characteristic a wavelength transmission characteristic of a dichroic mirror is in the optical tomographic imaging apparatus according to the exemplary embodiment of the present invention. 
         FIG. 6  illustrates an angle range Δθ at which the measurement light is incident on the dichroic mirror when a scanning unit is located offset from a focal position of a lens by a distance ΔX in the optical tomographic imaging apparatus. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     A change in an incident angle of measurement light to a dichroic mirror typically leads to a change in a wavelength separation characteristic (a change in a wavelength band of light transmittable through the dichroic mirror). Therefore, if a low-coherence light source configured to emit light having a wider wavelength bandwidth than that of a conventional technique is used as a light source for OCT, the accuracy for wavelength separation should be further improved compared to the conventional technique because of a shorter interval between a wavelength of a light source of an SLO and an end of the wavelength band of the light source for OCT. 
     According to an exemplary embodiment of the present invention, an optical tomographic imaging apparatus includes a scanning unit disposed on an optical path of measurement light with which an object to be examined is irradiated via a first lens, and a second lens disposed between the scanning unit and the first lens. Then, the second lens and the scanning unit are disposed in such a manner that an angle at which the measurement light scanned by the scanning unit is incident on an optical path branching unit is maintained substantially unchanged during scanning. To that end, the scanning unit is disposed at a focal position of the second lens. As a result, even when the measurement light is scanned by the scanning unit, it is possible to reduce a change in a wavelength separation characteristic of the optical path branching unit. Therefore, even if a low-coherent light source configured to emit light having a wider wavelength bandwidth than that of the conventional technique is used as the light source for OCT, it is possible to improve the accuracy of wavelength separation for separating the wavelength of the light source for OCT and the wavelength of the light source of the SLO. For example, it is possible to reduce a variation in a transmittance (or a reflectance) of a predetermined wavelength to be separated by the dichroic mirror, which occurs due to a difference in the angle at which the measurement light is incident on the dichroic mirror. Wavelength separation characteristic refers to, for example, a ratio of wavelength transmission to wavelength reflection achieved by the dichroic mirror. 
     Now, it is diagnostically desirable that an OCT tomographic image of a fundus of a subject&#39;s eye allows well observation of a thinner membrane than a layer such as a junction between photoreceptor inner and outer segments (IS/OS), and an external limiting membrane (ELM). For realizing this, an axial resolution of 3 μm should be achieved in the OCT tomographic image. Then, the axial resolution depends on the wavelength bandwidth of the OCT light source, and the wavelength bandwidth of the OCT light source should be approximately 100 nm to achieve the axial resolution of 3 μm. 
     As illustrated in  FIG. 5A , the second lens and the scanning unit should be disposed in such a manner that the angle at which the measurement light scanned by the scanning unit is incident on the optical path branching unit can satisfy a wavelength separation characteristic of 90% or higher at a wavelength in the wavelength band of the measurement light that is closest to a wavelength of observation light of the object to be examined, to maintain approximately 100 nm as the wavelength bandwidth of the OCT light source.  FIGS. 5A, 5B, and 5C  illustrate wavelength transmission characteristics of the dichroic mirror. 
     For example, suppose that, as illustrate in  FIG. 5C , the second lens and the scanning unit are disposed in such a manner that the angle at which the measurement light is incident on the dichroic mirror is largely deviated from an angle of 45 degrees, when the wavelength band of the OCT light source as an example of the light source of the measurement light is from approximately 805 nm to approximately 905 nm. In this case, suppose that this angle is large to a degree at which the wavelength transmission characteristic (a transmittance) of the dichroic mirror falls below 90% at approximately 805 nm, which is close to a wavelength 780 nm of the SLO light source as an example of the wavelength of the observation light of the object to be examined. In this case, the wavelength band of the OCT light source is narrowed to, for example, from approximately 810 nm to approximately 905 nm, whereby the wavelength bandwidth thereof falls below approximately 100 nm. As a result, the axial resolution also falls below 3 μm (the value thereof increases), whereby the image quality of the OCT tomographic image is also deteriorated. Therefore, the second lens and the scanning unit should be disposed in such a manner that the wavelength transmission characteristic of 90% or higher can be satisfied at approximately 805 nm. 
     Further, as illustrated in  FIG. 5B , it is desirable that the second lens and the scanning unit are disposed in such a manner that a wavelength separation characteristic of 8% or lower is satisfied at the wavelength of the observation light of the object to be examined. For example, suppose that the wavelength transmission characteristic (the transmittance) of the dichroic mirror exceeds 8% at the wavelength 780 nm of the SLO light source, which is an example of the light source of the observation light of the object to be examined. In this case, return light for the SLO is introduced into an OCT optical system by a light amount large to a degree at which this light is observed in an OCT tomographic image as a noise. Therefore, it is desirable that the second lens and the scanning unit are disposed in such a manner that the wavelength transmission characteristic of 8% or lower is satisfied at approximately 780 nm. 
     To meet the above-described conditions, as illustrated in  FIG. 6 , it is desirable that the second lens and the scanning unit are disposed in such a manner that the angle at which the measurement light is incident on the optical path branching unit can become equal to or larger than 44 degrees and smaller than 46 degrees (Δθ=45 degrees±1 degrees), although this also depends on features of the optical path branching unit.  FIG. 6  illustrates the angle Δθ with which the measurement light is incident on the dichroic mirror when the scanning unit is located offset from a focal position of the lens by a distance ΔX. Further, examples of the features of the optical path branching unit include a material of a film of the dichroic mirror, the number of films of the dichroic mirror, and a thickness of the film of the dichroic mirror. In this case, it is desirable that the second lens  101 - 3  and a central position  127  of the scanning unit are disposed in such a manner that the position of the scanning unit is located within a range of ±f/10 mm (±ΔX) from the focal position of the second lens in an optical axis direction, where f represents a focal length of the second lens. 
     Next, an optical path desirable for an anterior eye observation optical system, and an optical path desirable for a fundus observation optical system will be described. 
     &lt;Optical Path Desirable for Anterior Eye Observation Optical System&gt; 
     First, an optical path desirable for the anterior eye observation optical system will be described. 
     As described above, the fundus observation optical system and the OCT optical system are configured in such a manner that return light generated by a return of the measurement light scanned by the scanning unit from a subject&#39;s eye is incident on the second lens at an angle of approximately 45 degrees via the dichroic mirror. On the other hand, in the anterior eye observation optical system, light from an anterior eye portion of the subject&#39;s eye enters a fourth lens (for example, a lens  141  illustrated in  FIG. 1 ), which is a lens of the anterior eye portion optical system, while being scattered via the dichroic mirror. Therefore, in the anterior eye portion optical system, as the distance from the first lens (for example, a lens  101 - 1  illustrated in  FIG. 1 ) to the fourth lens increases, the light is scattered more widely, thereby leading to an increase in size of the fourth lens. Constructing a relay lens to reduce the scattering results in an increase in the number of optical members, thereby leading to an increase in the size of the anterior eye observation optical system. 
     For this reason, it is desirable that the anterior eye observation optical system is disposed on an optical path close to the first lens. For example, as illustrated in  FIG. 1 , it is desirable that the anterior eye observation optical system is disposed on an optical path L3, which is a transmission optical path of a first dichroic mirror  102 . 
     &lt;Optical Path Desirable for Fundus Observation Optical System&gt; 
     Next, an optical path desirable for the fundus observation optical system will be described. 
     First, it is desirable that the measurement light for the SLO in the fundus observation optical system has a larger beam diameter when the measurement light is incident on the anterior eye portion, than the measurement light for OCT in the OCT optical system. The reason thereof is as follows. In the SLO optical system, the return light from the subject&#39;s eye is detected by a single detector  116  after being formed into a ring shape by a prism  118  such as a holed mirror. In the SLO optical system, a light amount reduces according to the formation of the return light into a ring shape, whereby it is desirable to increase the beam diameter as much as possible. On the other hand, the OCT optical system acquires a tomographic image with use of interference light at a position close to a coherence gate. Therefore, when a tomographic image of a fundus is captured, the tomographic image does not contain an image of normal reflection on the anterior eye portion away from a range where the tomographic image is acquired. Therefore, the return light from the subject&#39;s eye does not have to be formed into a ring shape in the OCT optical system, unlike the SLO optical system, whereby the measurement light for OCT can have a smaller beam diameter than the measurement light for the SLO. The SLO optical system may be configured to form the measurement light before application to the subject&#39;s eye into a ring shape. 
     In this case, as the beam diameter increases, a range where the lens is irradiated by the beam increases in a radial direction of the lens, whereby the beam is scattered more widely. Therefore, the measurement light for the SLO is more largely affected by an astigmatism on a transmission optical path of the dichroic mirror than the measurement light for OCT. For this reason, it is desirable that the fundus observation optical system is disposed on a reflection optical path of the dichroic mirror. For example, as illustrated in  FIG. 1 , it is desirable that the fundus observation optical system is disposed on an optical path L2, which is a reflection optical path of the first dichroic mirror  102  and a reflection optical path of a second dichroic mirror  103 . 
     In this manner, as illustrated in  FIG. 1 , it is desirable that the anterior eye observation optical system, the fundus observation optical system, and the OCT optical system are disposed on the optical path L3, the optical path L2, and an optical path L1, respectively. 
     In the following description, the present exemplary embodiment will be described with reference to the accompanying drawings. The same elements are identified with the same reference numerals throughout the present disclosure. 
     &lt;Configuration of Apparatus&gt; 
     A configuration of the optical tomographic imaging apparatus (OCT apparatus) according to the present exemplary embodiment will be described with reference to  FIG. 1 . The optical tomographic imaging apparatus includes an optical head  900  and a spectrometer  180 . The optical tomographic imaging apparatus acquires a tomographic image of the object to be examined based on light generated by combining the return light from the object to be examined irradiated by the measurement light via the scanning unit, and reference light corresponding to this measurement light. 
     First, an internal configuration of the optical head  900  will be described. The optical head  900  includes measurement optical systems for capturing an image of an anterior eye of a subject&#39;s eye  100 , a two-dimensional image of a fundus of the subject&#39;s eye  100 , and a tomographic image of the subject&#39;s eye  100 . The lens  101 - 1 , which is an objective lens and an example of the first lens, is disposed so as to face the subject&#39;s eye  100 . Further, an optical path is branched by the first dichroic mirror  102  and the second dichroic mirror  103 , which are the optical path branching unit. Herein, the first dichroic mirror  102  and the second dichroic mirror  103  are non-limiting examples of an optical path branching unit. Other examples of the optical path branching unit may include a prism, a half-silvered mirror, a half-wave plate, or the like. Regardless of how the optical path branching unit is implemented, the optical path is branched into the measurement optical path L1 of the OCT optical system, the fundus observation optical path and fixation lamp optical path L2, and the anterior eye portion observation optical path L3. These optical paths L1, L2 and L3 are branched according to each wavelength band. 
     &lt;Optical Path L1: Measurement Optical Path of OCT Optical System&gt; 
     The optical path L1 is the measurement optical path of the OCT optical system, which is separated according to the wavelength from the light of the anterior eye observation optical system on the optical path L3 by the first dichroic mirror  102 , and is further separated according to the wavelength from the measurement light of the fundus observation optical system on the optical path L2 by the second dichroic mirror  103 . The OCT optical system is used to capture a tomographic image of the fundus of the subject&#39;s eye  100 . More specifically, the optical path L1 is used to acquire an interference signal for forming a tomographic image. A lens  101 - 3 , which is an example of the second lens, a mirror  121 , and the scanning unit are disposed on the optical path L1. The scanning unit includes an X scanner  122 - 1 , which is an example of a first scanning unit, and a Y scanner  122 - 2 , which is an example of a second scanning unit. A non-limiting example of the X scanner  122 - 1  and Y scanner  122 - 2  includes a scanning galvanometer. One-dimensional (1D) or two-dimensional (2D) galvanometer optical scanners may be used. Other non-limiting examples may include 1D or 2D MEMS (micro-electromechanical mirrors) scanning mirrors. Regardless of the implementation, the X scanner  122 - 1  and the Y scanner  122 - 2  scan the light on the fundus of the subject&#39;s eye  100  in an X direction (a main scanning direction also referred to as a first direction), and a Y direction (a sub scanning direction also referred to as a second direction intersecting with the first direction). In  FIG. 1 , an optical path between the X scanner  122 - 1  and the Y scanner  122 - 2  is formed in a direction in parallel with the plane of the sheet of  FIG. 1 , but actually, this optical path is formed in a direction perpendicularly to the plane of the sheet of  FIG. 1 . 
     &lt;Optical Path L2: Optical Path of Fundus Observation Optical System&gt; 
     The optical path L2 is the optical path of the fundus observation optical system, which is separated according to the wavelength from the measurement light of the OCT optical system on the optical path L1 by the second dichroic mirror  103 . Among a lens  101 - 2 , a focusing lens  111 , and a lens  112 , the focusing lens  111  is driven by a motor (not-illustrated) for a focusing adjustment for a fixation lamp and fundus observation (not-illustrated). 
     First, on the optical path L2, a light source  115  (the light source of the SLO) for fundus observation generates light having a wavelength of 780 nm. Further, an X scanner  117 - 1 , which is an example of a first observation scanning unit, and a Y scanner  117 - 2 , which is an example of a second observation scanning unit, are disposed on the optical path L2 to scan the light emitted from the light source  115  for fundus observation on the fundus of the subject&#39;s eye  100 . The lens  101 - 2 , which is an example of a third lens, is disposed so as to have a focal position around a central position between the X scanner  117 - 1  and the Y scanner  117 - 2 . The X scanner  117 - 1  includes a polygon mirror to scan the light in the X direction at a high speed. Further, the X scanner  117 - 1  may include a resonant mirror. Further, the single detector  116  includes an avalanche photodiode (APD), and detects the light scattered by and reflected from the fundus. The prism  118  is a prism to which a holed mirror or a hollow mirror is evaporated, and separates the illumination light from the light source  115  for fundus observation and the return light from the fundus. 
     Further, a dichroic mirror (not-illustrated) may be further provided, and a light-emitting diode (LED) or the like may be further provided as a light source of the fixation lamp (not-illustrated). In this case, the light source of the fixation lamp is disposed on the SLO light source side relative to the scanning unit for observation. Due to this arrangement, the scanning unit for observation is also used as a scanning unit for visual fixation, by which a scanning fixation lamp can be formed. In this case, this scanning fixation lamp can work well by using a control unit (not-illustrated) that performs control in such a manner that the light source of the fixation lamp is turned on when light from the light source of the fixation lamp is scanned at a position desired by an examiner. Turning on and turning off the light source of the fixation lamp may be replaced with opening and closing a shutter disposed on this optical path. 
     The optical path L2 may be a line scanning SLO (a line SLO) that scans a line beam in a single direction by using a cylindrical lens or the like, instead of the above-described point scanning SLO that two-dimensionally scans a spot to acquire a two-dimensional image of the fundus. Further, the optical path L2 may be configured to perform infrared observation by using a two-dimensional charge coupled device (CCD) sensor, instead of using the scanning unit. More specifically, the optical path L2 may be configured to include a CCD sensor for fundus observation, instead of the X scanner  117 - 1  and the Y scanner  117 - 2 , to acquire a two-dimensional image of the fundus of the subject&#39;s eye  100 . In this case, the two-dimensional CCD sensor is configured to detect a wavelength of the illumination light (not-illustrated) for fundus observation, in particular, around 780 nm. 
     Further, the fixation lamp on the optical path L2 may be configured in such a manner that the examiner prompts visual fixation of an examinee to a desired position by generating visual light by a display for visual fixation such as a liquid-crystal display, and changing a lighting position on the display for visual fixation. In this case, the display for visual fixation is disposed closer to a third dichroic mirror  104  relative to the scanning unit for observation. 
     &lt;Optical Path L3: Optical Path of Anterior Eye Observation Optical System&gt; 
     The optical path L3 is the optical path of the anterior eye observation optical system where the lens  141 , and an infrared CCD sensor  142  for anterior eye observation are disposed. The infrared CCD sensor  142  has a sensitivity to a wavelength of the illumination light (not-illustrated) for anterior eye observation, in particular, around 970 nm. 
     &lt;Position Optically Conjugate with Anterior Eye Portion: Substantially Central Position Between X and Y Scanners Coincides with Focal Position of Lens&gt; 
     Now, conjugate relationships between the eye position and the optical path L1 and the optical path L2, and a light flux of the eye will be described with reference to  FIG. 2 . The optical tomographic imaging apparatus is configured in such a manner that a position conjugate with a predetermined portion such as the anterior eye portion of the subject&#39;s eye  100  is located between the first scanning unit and the second scanning unit. The present exemplary embodiment can be realized as long as at least one of the optical path L1 and the optical path L2 is configured in this manner. 
     First, referring back to  FIG. 2 , on the optical path L1, a scanner central position  127  between the X scanner  122 - 1  and the Y scanner  122 - 2 , and a pupil position  128  (the anterior eye portion) of the subject&#39;s eye  100  are in an optically conjugate relationship. More specifically, the optical system of the optical head  900  is designed in such a manner that the X and Y scanners  122 - 1  and  122 - 2  configured to scan the measurement light for OCT in the X and Y directions and the anterior eye portion of the subject&#39;s eye are set in an optically conjugate relationship, when the optical head  900  and the subject&#39;s eye  100  are aligned with each other. As a result, it is possible to reduce vignetting of the measurement light on the anterior eye portion of the subject&#39;s eye  100 . 
     Further, the lens  101 - 1 , the lens  101 - 3 , and the X scanner  122 - 1  and the Y scanner  122 - 2  (or the scanner central position  127 ) are disposed in such a manner that a light flux of the measurement light scanned by the scanning unit is substantially collimated between the lens  101 - 1  and the lens  101 - 3 . According to this configuration, an optical path for which a measurement light deflection unit is set as an object point is substantially collimated between the lens  101 - 1  and the lens  101 - 3 . Then, the scanner central position  127  coincides with a focal position of the lens  101 - 3 . Due to this configuration, it is possible to substantially maintain angles at which the measurement light is incident on the first dichroic mirror  102  and the second dichroic mirror  103 , even when the X scanner  122 - 1  and the Y scanner  122 - 2  scan the measurement light. As a result, even when the measurement light for OCT is scanned by the X and Y scanners  122 - 1  and  122 - 2 , it is possible to reduce changes in the wavelength separation characteristics of the dichroic mirrors  102  and  103 , whereby it is possible to improve the accuracy of wavelength separation by the dichroic mirrors  102  and  103 . 
     Further, on the optical path L2, a scanner central position  119  between the X scanner  117 - 1  and the Y scanner  117 - 2 , and the pupil position  128  of the subject&#39;s eye  100  are also in a conjugate relationship. Further, the lens  101 - 2  and the scanner central position  119  (the X scanner  117 - 1  and the Y scanner  117 - 1 ) are disposed in such a manner that a light flux is substantially collimated between the lens  101 - 1  and the lens  101 - 2 . According to this configuration, an optical path for which a measurement light deflection unit is set as an object point is substantially collimated between the lens  101 - 1  and the lens  101 - 2 . Then, the scanner central position  119  coincides with a focal position of the lens  101 - 2 . Due to this configuration, it is possible to substantially maintain angles with which the measurement light is incident on the first dichroic mirror  102  and the second dichroic mirror  103 , even when the X scanner  117 - 1  and the Y scanner  117 - 2  scan the measurement light. As a result, even when the measurement light for the SLO is scanned by the X and Y scanners  117 - 1  and  117 - 2 , it is possible to reduce changes in the wavelength separation characteristics of the dichroic mirrors  102  and  103 , whereby it is possible to improve the accuracy of wavelength separation by the dichroic mirrors  102  and  103 . 
     Further, the optical path L1 and the optical path L2 are configured to share the lens  101 - 1 , and it is desirable that the lens  101 - 2  and the lens  101 - 3  are configured by lenses having similar shapes and made of similar materials. As a result, it is possible to establish matching optical systems from the subject&#39;s eye  100  to the respective X and Y scanners  122 - 1 ,  122 - 2 ,  117 - 1 , and  117 - 2  on the optical path L1 and the optical path L2, whereby it is possible to uniform optical characteristics on the optical paths L1 and L2. Therefore, it becomes possible to reduce an error in a measurement. 
     Now, as illustrated in  FIG. 2 , assume that 0 represents an angle formed by the light flux of the pupil to the pupil of the subject&#39;s eye  100 , θ1 represents an angle formed by the light flux of the pupil to the scanner central position  127 , and θ2 represents an angle formed by the light flux of the pupil to the scanner central position  119 . In other words, the optical tomographic imaging apparatus is configured to provide the angles θ1 and θ2 to the light beams with use of the scanners respectively to acquire the angle θ formed by the light flux of the pupil on both the optical path L1 and the optical path  12 . 
     Further, as one of the optical characteristics, an optical magnification of the scanner central position  119  to the pupil position  128  and an optical magnification of the scanner central position  127  to the pupil position  128  can be also uniformed on the optical path L1 and the optical path L2. As a result, relationships between scan angles of the X and Y scanners  122 - 1 ,  122 - 2 ,  117 - 1 , and  117 - 2  on the respective optical paths L1 and L2, and illumination positions on the fundus of the subject&#39;s eye  100  can be uniformed on the optical paths L1 and L2, whereby the angles θ1 and θ2 can become equal to each other. Due to this arrangement, it becomes possible to reduce an error between the respective scanning positions. 
     &lt;Position Optically Conjugate with Fundus: Focusing Adjustment&gt; 
     Further, the optical system of the optical head  900  is designed in such a manner that a fiber end  126  for introducing the measurement light to the measurement optical path and the fundus of the subject&#39;s eye  100  are set into an optically conjugate relationship by performing a focusing adjustment, when the X and Y scanners  122 - 1  and  122 - 2  and the anterior eye portion are in an optically conjugate relationship. The focusing lens  123  and a lens  124  are provided adjacent to the fiber end  126 , and the focusing lens  123 , which is one of them, is driven in directions indicated by a double-headed arrow by a motor (not-illustrated) to perform a focusing adjustment. The focusing adjustment is performed by making an adjustment in such a manner that light emitted from the measurement light source  126 , which is the fiber end, is imaged on the fundus. The focusing lens  123 , which is an example of a focusing unit, is disposed between the measurement light source  126 , and the X scanner  122 - 1  and the Y scanner  122 - 2 , which are the measurement light deflection unit. This configuration eliminates the necessity of moving the larger lens  101 - 3  and a fiber  125 - 2  connected to the measurement light source  126 . 
     Now, for example, U.S. Pat. No. 5,537,162 discusses a configuration that maintains a constant angle as an incident angle at which a beam is incident on a dichroic mirror even when the beam is scanned by placing a beam scanner on a back focal plane of a lens (lens corresponding to the lens  101 - 3  in the present exemplary embodiment). Further, U.S. Pat. No. 5,537,162 discusses that the beam scanner and the lens are integrally driven during execution of a focusing adjustment for a fundus of a subject&#39;s eye. In this case, the lens (lens corresponding to the lens  101 - 3  in the present exemplary embodiment) with the beam scanner placed on the back focal plane thereof tends to have a large size to introduce scanning light of the beam scanner. Therefore, a driving mechanism therefor is complicated, because the beam scanner and the large-sized lens should be integrally moved. Further, since they are integrally moved, a measurement light source in an optically conjugate relationship with a fundus position should be moved at the same time. If this measurement light source is an optical fiber end, an optical fiber should be moved, whereby a change may occur in a polarized state. Therefore, according to the present exemplary embodiment, as described above, the focusing lens  123  is disposed between the X and Y scanners  122 - 1  and  122 - 2  that scan the measurement light for OCT in the X and Y directions, and the fiber end  126  that emits the measurement light for OCT (or an optical coupler  125  that branches light into the measurement light and the reference light). If the focusing position is changed by moving the lens  101 - 1  in the optical axis direction, this also causes a change in the optically conjugate relationship between the X and Y scanners  122 - 1  and  122 - 2  and the anterior eye portion, whereby vignetting of the measurement light may occur on an iris of the anterior eye portion and the like. 
     With this focusing adjustment, an image of the measurement light source  126  can be formed on the fundus of the subject&#39;s eye  100 , and the return light from the fundus of the subject&#39;s eye  100  can be efficiently returned to the fiber  125 - 2  via the measurement light source  126 . Further, a focusing adjustment can be performed with use of a focusing lens  111  on the optical path L2 in a similar manner. 
     &lt;Configuration of OCT Optical System&gt; 
     Next, configurations of an optical path of light emitted from a light source  130  illustrated in  FIG. 1 , a reference optical system, and the spectrometer  180  will be described. A Michelson interference system is formed by the light source  130 , a mirror  153 , a dispersion compensation glass  152 , the optical coupler  125 , optical fibers  125 - 1  to  125 - 4 , a lens  151 , and the spectrometer  180 . The optical fibers  125 - 1  to  125 - 4  form a single-mode optical fiber by being connected to the optical coupler  125  to be integrated all together. 
     The light emitted from the light source  130  is transmitted to the optical coupler  125  via the optical fiber  125 - 1 , and is divided into the measurement light emitted to the optical fiber  125 - 2  and the reference light emitted to the optical fiber  125 - 3  via the optical coupler  125 . The fundus of the subject&#39;s eye  100  is irradiated with the measurement light, which is an observation target, via the above-described optical path of the OCT optical system, and reaches the optical coupler  125  via the same optical path by being reflected or scattered by a retina. 
     On the other hand, the reference light reaches the mirror  153  and is reflected thereby after being transmitted via the optical fiber  125 - 3 , the lens  151 , and the dispersion compensation glass  152  inserted to match dispersion of the measurement light and dispersion of the reference light. Then, the reference light reaches the optical coupler  125  by returning through the same optical path. 
     The measurement light and the reference light are combined by the optical coupler  125 , thereby producing interference light. Interference occurs when an optical path length of the measurement light and an optical path length of the reference light become substantially equal. The mirror  153  is held in such a manner that its position can be adjusted in the optical axis direction by a motor and driving mechanism (not-illustrated), and can match the optical path length of the reference light to the optical path length of the measurement light, which varies depending on the subject&#39;s eye  100 . The interference light is guided to the spectrometer  180  via the optical fiber  125 - 4 . 
     The spectrometer  180  includes a lens  181 , a diffraction grating  182 , a lens  183 , and a line sensor  184 . The interference light emitted from the optical fiber  125 - 4  is dispersed by the diffraction grating  182  after being substantially collimated via the lens  181 , and is imaged on the line sensor  184  by the lens  183 . 
     Next, the light source  130  will be described. The light source  130  is a super luminescent diode (SLD), which is a representative low-coherent light source. The central wavelength is 855 nm, and the wavelength bandwidth is approximately 100 nm. The wavelength bandwidth is an important parameter, because it affects a resolution of an acquired tomographic image in the optical axis direction. Further, the SLD is selected in the present example as the type of the light source, but the light source  130  may be any light source that can emit low-coherent light and can be also realized by amplified spontaneous emission (ASE) and the like. A suitable central wavelength is near infrared light in consideration of the fact that the optical tomographic imaging apparatus is used to measure a subject&#39;s eye. Further, it is desirable that the central wavelength is as a small wavelength as possible, because it affects a lateral resolution of an acquired tomographic image. For both reasons, 855 nm is selected as the central wavelength. 
     The present exemplary embodiment uses a Michelson interferometer as the interferometer, but may use a Mach-Zehnder interferometer. It is desirable to, according to a difference in light amount between the measurement light and the reference light, use a Mach-Zehnder interferometer if their light amounts are largely different and to use a Michelson interferometer if their light amounts are relatively slightly different. 
     &lt;Method for Capturing Tomographic Image&gt; 
     The optical tomographic imaging apparatus can capture a tomographic image of a desired portion on the fundus of the subject&#39;s eye  100  by controlling the X scanner  122 - 1  and the Y scanner  122 - 2 . 
       FIG. 3  illustrates how the subject&#39;s eye  100  is irradiated with measurement light  201 , and the measurement light  201  is scanned on a fundus  202  in the X direction. Information corresponding to a predetermined number of times of imaging is captured from an imaging range on the fundus  202  in the X direction by the line sensor  184 . Luminance distribution on the line sensor  184  that is acquired at a certain position in the X direction is transformed by fast Fourier transformation (FFT). Linear luminance distribution acquired by FFT is converted into density or color information to be displayed on a monitor, and this converted image is referred to as an A-scan image. Further, a two-dimensional image formed by arranging a plurality of A-scan images is referred to as a B-scan image. A plurality of B-scan images can be acquired by capturing a plurality of A-scan images to construct a single B-scan image, and then moving a scanning position in the Y direction and performing scanning in the X direction again. The plurality of B-scan images or a three-dimensional tomographic image constructed from the plurality of B-scan images is displayed on the monitor, whereby the examiner can use the image for a diagnosis of the subject&#39;s eye  100 . 
       FIG. 4  illustrates examples of an anterior eye image  210 , a fundus two-dimensional image  211 , and a B-scan image  212 , which is a tomographic image, displayed on a monitor  200 . The anterior eye image  210  is an image processed and displayed from an output of the infrared CCD sensor  142 . The fundus two-dimensional image  211  is an image processed and displayed from an output of a CCD  114 . Then, the B-scan image  212  is an image constructed by performing the above-described processing from an output of the line sensor  184 . 
     As described above, according to the present exemplary embodiment, in the optical tomographic imaging apparatus, the focusing unit (the focusing lens  123  and the driving mechanism (not-illustrated)) configured to perform a focusing adjustment that targets the subject&#39;s eye  100  is disposed between the measurement light deflection unit (the X and Y scanners  122 - 1  and  122 - 2 ) configured to deflect the measurement light, and the measurement light source  126 . Further, the first lens (the lens  101 - 1 ) and the second lens (the lens  101 - 3 ) are disposed on the measurement optical path between the measurement light deflection unit (the X and Y scanners  122 - 1  and  122 - 2 ) and the subject&#39;s eye  100 , and the optical path branching unit (the first dichroic mirror  102  and the second dichroic mirror  103 ) is disposed between the first lens and the second lens. 
     In other words, the focusing lens  123  is disposed between the fiber end  126  as the measurement light source and the X and Y scanners  122 - 1  and  122 - 2  as the measurement light deflection unit, which eliminates the necessity of moving the large lens  101 - 3 , the fiber  125 - 2  connected to the measurement light source  126 , and the like, leading to simplification of the driving mechanism. Further, because the fiber end  126  does not have to be moved, it is possible to provide an optical tomographic imaging apparatus capable of maintaining a polarized state. Further, according to the present exemplary embodiment, in the optical tomographic imaging apparatus, the first lens (the lens  101 - 1 ) and the second lens (the lens  101 - 3 ), and the measurement light deflection unit (the X and Y scanners  122 - 1  and  122 - 2 ) are positionally adjusted and arranged in such a manner that the light is collimated on the measurement optical path between the first lens (the lens  101 - 1 ) and the second lens (the lens  101 - 3 ). As a result, it is possible to maintain constant angles as the incident angles with which the beam is incident on the first and second dichroic mirrors  102  and  103 , thereby improving the accuracy of wavelength separation. 
     The present exemplary embodiment has been described, targeting a subject&#39;s eye. However, the present invention may scan light on not only a subject&#39;s eye but also another object to be examined, like a human body such as skin and an internal organ, and can be employed for not only an ophthalmologic apparatus but also an imaging apparatus such as an endoscope. 
     OTHER EMBODIMENTS 
     Embodiments of the present invention can also be realized by a computer of a system or apparatus that reads out and executes computer executable instructions recorded on a storage medium (e.g., non-transitory computer-readable storage medium) to perform the functions of one or more of the above-described embodiment(s) of the present invention, and by a method performed by the computer of the system or apparatus by, for example, reading out and executing the computer executable instructions from the storage medium to perform the functions of one or more of the above-described embodiment(s). The computer may comprise one or more of a central processing unit (CPU), micro processing unit (MPU), or other circuitry, and may include a network of separate computers or separate computer processors. The computer executable instructions may be provided to the computer, for example, from a network or the storage medium. The storage medium may include, for example, one or more of a hard disk, a random-access memory (RAM), a read only memory (ROM), a storage of distributed computing systems, an optical disk (such as a compact disc (CD), digital versatile disc (DVD), or Blu-ray Disc (BD)™), a flash memory device, a memory card, and the like. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2013-095622 filed Apr. 30, 2013, which is hereby incorporated by reference herein in its entirety.