Abstract:
A polarization-sensitive OCT apparatus includes an interference unit configured to split light emitted from a light source into measurement light and reference light and to generate interfered light by causing returning light of the measurement light that has irradiated a subject to interfere with the reference light that has traveled through a reference arm, a splitting unit configured to split the interfered light into different polarization components, a generation unit configured to detect the polarization components split by the splitting unit and to generate a signal, detection units configured to detect respective polarization states of the measurement light in a sample arm, the returning light of the measurement light that has passed through the interference unit, and the reference light that has passed through the interference unit, and polarization control units configured to control the respective polarization states on the basis of the respective polarization states that have been detected.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to polarization-sensitive optical coherence tomography (PS-OCT) apparatus and method for controlling PS-OCT apparatus. In particular, the present invention relates to a PS-OCT apparatus that is capable of obtaining polarization characteristic information of an eye to be examined and to a method for controlling such a PS-OCT apparatus. 
       BACKGROUND ART 
       [0002]    Currently, OCT apparatuses that utilize interference of low coherence light are in practical use. OCT apparatuses allow high-resolution tomographic images of subjects to be obtained noninvasively. Thus, especially in ophthalmic field, OCT apparatuses are becoming indispensable in order to obtain tomographic images of fundus of eye to be examined. In addition, OCT apparatuses are employed in fields other than ophthalmic field so as to carry out tomographic observation of skin or to capture tomographic images of walls of digestive organs or circulatory organs by forming OCT apparatuses as endoscopes or catheters. 
         [0003]    With ophthalmic OCT apparatuses, an effort is made to obtain, in addition to typical OCT images (also referred to as intensity images) in which images of the shape of fundus tissues are captured, functional OCT images in which images of the optical characteristics or the movements of fundus tissues are captured. In particular, PS-OCT apparatuses are being developed as functional OCT apparatuses that are capable of capturing images of birefringent nerve fiber layer or retina layer having depolarizing properties by obtaining signals with the use of polarization parameters of light, leading to the advancement of research on glaucoma, age-related macular degeneration, and so forth. 
         [0004]    A PS-OCT apparatus can form a PS-OCT image by using polarization parameters (retardation and orientation), which are part of the optical characteristics of fundus tissues, and can differentiate among fundus tissues or segment the fundus tissues. Typically, in a PS-OCT apparatus, the optical system includes a wave plate (e.g., a quarter-wave plate or a half-wave plate) and is thus able to change the polarization states of measurement light and reference light as desired in the PS-OCT apparatus. A PS-OCT image is formed by controlling the polarization of light emitted from a light source, by using light that has been modulated to have a predetermined polarization state as measurement light for observing a sample, by splitting interfered light into two linearly polarized light components that are polarized in directions orthogonal to each other, and by detecting the light components. 
         [0005]    As a method for controlling the polarization, a method in which reflected or scattered measurement light is detected and the polarization of the measurement light is controlled to a predetermined polarization state by using a wave plate or a polarization controller is being discussed (PIL1). Using such a method makes it possible to correct the polarization state even if the polarization state changes as the OCT apparatus is being used. 
         [0006]    In addition, a method in which the polarization state is modulated by using an electro-optical modulator (EOM) is also being discussed (PTL2). In this method, a single site is irradiated with a plurality of light rays having different polarization states, and thus a PS-OCT image can be generated on the basis of the polarization information obtained in the plurality of polarization states, which makes it possible to obtain a more accurate PS-OCT image. In addition, a polarization controller for controlling the polarization state is disposed in each of a sample arm, a reference arm, and an optical path through which interfered light travels toward a detector (hereinafter, referred to as an interfered light optical path), and thus the polarization state can be controlled independently in each of the optical paths. 
         [0007]    In existing PS-OCT apparatuses, polarization-maintaining (PM) fibers, wave plates, or EOMs are used in order to control the polarization. 
         [0008]    PTL1 discloses a PS-OCT apparatus in which the measurement light is reflected or scattered so as to detect the polarization state of the measurement light and the polarization is corrected by using a wave plate or a polarization controller so that the measurement light has a predetermined polarization state. This method, however, is limited to controlling the polarization of only the measurement light, and this configuration does not allow the polarization to be controlled in the reference arm. 
         [0009]    PTL2 discloses a PS-OCT apparatus that includes an EOM and a plurality of polarization controllers for controlling the polarization. PTL2, however, does not disclose how each of the polarization controllers is controlled, and the polarization state cannot be corrected, for example, in a case in which the polarization state changes as the PS-OCT apparatus is being used. 
       CITATION LIST 
     Patent Literature 
       [0010]    PTL1 Japanese Patent Laid-Open No. 2013-165961 
         [0011]    PTL2 Japanese Patent Laid-Open No. 2007-298461 
       SUMMARY OF INVENTION 
       [0012]    The present invention is directed to providing a PS-OCT apparatus that is capable of detecting a polarization state in each optical path and controlling the polarization in each optical path on the basis of detected polarization information. 
         [0013]    According to one aspect of the present invention, a polarization-sensitive OCT apparatus includes an interference unit configured to split light emitted from a light source into measurement light and reference light and to generate interfered light by causing returning light of the measurement light that has irradiated a subject to interfere with the reference light that has traveled through a reference arm, a splitting unit configured to split the interfered light into different polarization components, a generation unit configured to detect the polarization components split by the splitting unit and to generate a signal, detection units configured to detect respective polarization states of the measurement light in a sample arm, the returning light of the measurement light that has passed through the interference unit, and the reference light that has passed through the interference unit, and polarization control units configured to control the respective polarization states of the measurement light, the returning light of the measurement light, and the reference light on the basis of the respective polarization states that have been detected by the detection units. 
         [0014]    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 DRAWINGS 
         [0015]      FIG. 1  is a schematic diagram illustrating an overall configuration of a spectral-domain (SD) PS-OCT apparatus according to a first exemplary embodiment. 
           [0016]      FIG. 2  is a flowchart for describing a method for controlling a polarization state in the SD PS-OCT apparatus according to the first exemplary embodiment. 
           [0017]      FIG. 3  is a schematic diagram illustrating an overall configuration of a swept-source (SS) PS-OCT apparatus according to a second exemplary embodiment. 
           [0018]      FIG. 4  is a flowchart for describing a method for controlling a polarization state in the SS PS-OCT apparatus according to the second exemplary embodiment. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0019]    A first exemplary embodiment of the present invention will be described in detail with reference to the drawings. 
       First Exemplary Embodiment 
       [0020]    In the present exemplary embodiment, a configuration of a PS-OCT apparatus will be described with reference to  FIG. 1 . 
       Overall Configuration of Apparatus 
       [0021]      FIG. 1  is a schematic diagram illustrating an overall configuration of the PS-OCT apparatus according to the present exemplary embodiment. In the present exemplary embodiment, a configuration of an SD PS-OCT apparatus will be described. 
       Configuration of SD PS-OCT Apparatus  100   
       [0022]    A configuration of an SD PS-OCT apparatus  100  will be described. 
         [0023]    A light source  101  is a superluminescent diode (SLD) light source, which is a low coherence light source, and emits light, for example, at a central wavelength of 850 nm and with a bandwidth of 50 nm. Although an SLD light source is used as the light source  101 , any light sources, such as an amplified spontaneous emission (ASE) light source, that is capable of emitting low coherence light may be used. 
         [0024]    Light emitted from the light source  101  is guided to a beam splitter  106  via a single-mode (SM) fiber  102 , a polarization controller  103 , a connector  104 , and an SM fiber  105 , and is split into measurement light (also referred to as OCT measurement light) and reference light (also referred to reference light corresponding to OCT measurement light). The split ratio of the beam splitter  106  is 90:10 (reference light:measurement light). It is to be noted that the split ratio is not limited to these values, and can take other values. The beam splitter  106  is connected to SM fibers  105 ,  107 ,  117 , and  125  in the present exemplary embodiment. An advantage of connecting a beam splitter to SM fibers lies in that the polarization can be controlled in-line with ease by using a polarization controller. The aforementioned SM fibers  105 ,  107 ,  117 , and  125 , however, may be PM fibers. In a case in which the beam splitter  106  is connected to PM fibers, polarization controllers  108 ,  118 , and  126  do not need to be provided. Alternatively, wave plates may be provided in a sample arm and in a reference arm. For example, in place of the polarization controller  108 , a quarter-wave plate may be disposed between a collimator  109  and a galvanoscanner  110 , and in place of the polarization controller  118 , a quarter-wave plate may be disposed between a collimator  119  and a shutter  120 . 
         [0025]    The polarization controller  103  controls the polarization of light emitted from the light source  101  to a predetermined polarization state. The polarization controller  103 , for example, is a bulk polarization controller in which light is emitted to a space from a fiber and the polarization of the light is controlled by using a half-wave plate and a quarter-wave plate, a paddle fiber polarization controller in which paddles are formed by winding a fiber in a coil form and the polarization is controlled by rotating each paddle, and an in-line fiber polarization controller in which the polarization is controlled by pressurizing and rotating a fiber. 
         [0026]    In the present exemplary embodiment, light from the light source  101  is controlled to be linearly polarized by the polarization controller  103 . It is to be noted that, although the description is omitted in the present exemplary embodiment, if the degree of polarization of the light source  101  is not high, a polarizer may be disposed between the polarization controller  103  and the connector  104  so as to increase the degree of polarization of the light emitted from the light source  101 . In this case, the quantity of light passing through the polarizer may be controlled by controlling the polarization controller  103 . In addition, in place of disposing the polarization controller  103 , only a polarizer may be disposed on the SM fiber  102 . In this case, the polarization state of the light emitted from the light source  101  does not need to be controlled, and only the degree of polarization of the light can be increased. However, the quantity of light guided to an interferometer may be reduced depending on the polarization state of the light, and thus it is desirable to check whether a sufficient quantity of light is obtained. In one method for checking the quantity of light, for example, light that has passed through the polarizer and is emitted from the collimator  119  disposed in the reference arm or light that has passed through the polarizer and reaches a position corresponding to the pupil position in the sample arm may be measured with a power monitor, and a determination may be made as to whether the obtained result is equal to or greater than a preset quantity of light. In an alternative method, a determination is made as to whether a sufficient quantity of light is detected by a detector  131  or  133 . 
         [0027]    The split measurement light is emitted through the SM fiber  107  serving as a measurement light side fiber, and is collimated by the collimator  109 . In addition, the polarization controller  108  is disposed on the SM fiber  107  and can change the polarization state of the emitted measurement light as desired. In the present exemplary embodiment, the polarization controller  108  is controlled such that circularly polarized measurement light is incident on an eye  115  to be examined. 
         [0028]    In a case in which the polarization state of light incident on the eye  115  to be examined, or a subject, differs from the polarization state of light incident on a measurement light detector  116 , if the light is controlled to be circularly polarized when the light is incident on the eye  115  to be examined, the light becomes elliptically polarized when the light is incident on the measurement light detector  116 . The state of the elliptical polarization to be detected by the measurement light detector  116  in a case in which the light is circularly polarized when being incident on the eye  115  to be examined is uniquely determined, and thus the polarization controller  108  is controlled such that the light is circularly polarized when the light is incident on the eye  115  to be examined while the elliptically polarized light is detected by the measurement light detector  116 . Here, it is needless to say that the measurement light detector  116  and the eye  115  to be examined are disposed so as to be conjugate to each other. Aside from using a detector, such as a polarization measuring device, as the measurement light detector  116 , the polarization state may be determined by using an optical power meter and a polarizer, a wave plate, or the like. 
         [0029]    Alternatively, a polarization controller or a wave plate may be used so that the polarization state of the measurement light to be detected by the measurement light detector  116  becomes identical to the polarization state of the measurement light at the position of the eye  115  to be examined. For example, in a case in which the measurement light is to be made incident on the measurement light detector  116  by varying the angle of the galvanoscanner  110 , a wave plate may be disposed between the galvanoscanner  110  and the measurement light detector  116  in such a manner that the polarization state to be detected by the measurement light detector  116  becomes identical to the polarization state at the position of the eye  115  to be examined. 
         [0030]    When an image is actually to be captured, the collimated measurement light is incident on the eye  115  to be examined via the galvanoscanner  110 , which scans a fundus Er of the eye  115  to be examined with the measurement light, a scan lens  111 , and an objective lens  112 . Here, although the galvanoscanner  110  is illustrated as a single mirror, the galvanoscanner  110  may be formed by two galvanoscanners so as to carry out a raster scan of the fundus Er of the eye  115  to be examined. In addition, the objective lens  112  is fixed to a stage  113 , and as the stage  113  is moved in the direction of the optical axis, the diopter of the eye  115  to be examined can be adjusted. The galvanoscanner  110  and the stage  113  are controlled by a driving control unit  136 , and the galvanoscanner  110  can scan the fundus Er of the eye  115  to be examined within a predetermined range (also referred to as a tomographic image obtaining range, a tomographic image obtaining position, or a measurement light irradiation position) with the measurement light. 
         [0031]    The measurement light is made to be incident on the eye  115  to be examined via the objective lens  112  disposed on the stage  113  and is focused on the fundus Er. The measurement light that has irradiated the fundus Er is reflected or scattered by each retina layer and returns to the beam splitter  106  through the above-described optical path. 
         [0032]    Meanwhile, the reference light that has been split by the beam splitter  106  is emitted through the SM fiber  117  serving as a reference light side fiber, and is collimated by the collimator  119 . The polarization controller  118  is disposed on the SM fiber  117  and can change the polarization state of the emitted reference light as desired. In the present exemplary embodiment, the driving control unit  136  controls the polarization controller  118  in such a manner that the reference light that has been reflected by a mirror  123  is incident on a polarizing beam splitter  129  as linearly polarized light that is polarized at an angle of 45° relative to each of the two polarization axes that are orthogonal to each other. The reference light passes through a dispersion compensation glass  121  and an ND filter  122  and is reflected by the mirror  123  disposed on a coherence gate stage  124 , and the reflected reference light returns to the beam splitter  106 . The coherence gate stage  124  is controlled by the driving control unit  136  so as to accommodate to the differences in the axial length among the eyes of subjects. 
         [0033]    The measurement light and the reference light that have returned to the beam splitter  106  are combined to result in interfered light, and the resulting interfered light is incident on the polarization beam splitter  129  via the SM fiber  125  serving as a detection light side fiber, the polarization controller  126 , a connector  127 , and an SM fiber  128 . The interfered light is split into a vertical polarization component (hereinafter, V polarization component) and a horizontal polarization component (hereinafter, H polarization component) by the polarization beam splitter  129  in accordance with the two polarization axes that are orthogonal to each other. The V polarization component of the split interfered light is incident on the detector  131  via an SM fiber  130 . Meanwhile, the H polarization component of the interfered light is incident on the detector  133  via an SM fiber  132 . The light received by each of the detectors  131  and  133  is outputted to a signal processing unit  135  in the form of an electric signal in accordance with the intensity of the light. 
         [0034]    It is to be noted that, as the reference light is linearly polarized at an angle of 45° in the present exemplary embodiment, the reference light is split into the V polarization component and the H polarization component in the equal quantity. In addition, as the measurement light is circularly polarized in the present exemplary embodiment, data can be obtained simultaneously regardless of the directions of the cells and the fibers of the fundus Er of the eye  115  to be examined. As a result, data can be obtained at once in the entire polarization directions. Thus, it is not necessary to capture images of a single site in different polarization directions, and data can be obtained through a single instance of image capturing. 
         [0035]    In addition, in the present exemplary embodiment, a movable mirror  114  for reflecting the measurement light is disposed in the optical path of the measurement light prior to being incident on the eye  115  to be examined. The mirror  114  is controlled by the driving control unit  136 , and has a function of preventing the measurement light from being incident on the eye  115  to be examined when the polarization controller  126  is controlled by reflecting the measurement light and returning the measurement light to the beam splitter  106 . Therefore, the angle of the mirror  114  is adjusted such that, in a state in which the mirror  114  is disposed in the sample arm, the light that has been emitted through the SM fiber  107  is guided to the mirror  114  via the collimator  109 , the galvanoscanner  110 , the scan lens  111 , and the objective lens  112  and is reflected by the mirror  114 , and such that the reflected light returns to the beam splitter  106 . It is to be noted that, although a mirror is disposed in the present exemplary embodiment, any reflector that can reflect the measurement light can be employed. 
         [0036]    In addition, in the present exemplary embodiment, the shutter  120  for blocking the reference light is disposed next to the collimator  119 . The shutter  120  is controlled by the driving control unit  136 , and prevents the reference light from returning to the beam splitter  106  when the driving control unit  136  controls the polarization controller  126 . 
       Control Unit  134   
       [0037]    A control unit  134  for controlling the SD PS-OCT apparatus  100  as a whole will be described. 
         [0038]    The control unit  134  includes the signal processing unit  135 , the driving control unit  136 , and a display unit  137 . 
         [0039]    The signal processing unit  135  generates an image, analyzes the generated image, and generates visualized information of the result of the analysis on the basis of the signals outputted from the detectors  131  and  133 . The methods for generating and analyzing the image are well-known, and thus descriptions thereof will be omitted herein. 
         [0040]    The image generated by the signal processing unit  135  or the result of the analysis is displayed on a display screen of the display unit  137  (e.g., liquid crystal display or the like). The image data generated by the signal processing unit  135  may be transmitted to the display unit  137  through a cable or wirelessly. 
         [0041]    Although the display unit  137  is included in the control unit  134 , the present invention is not limited to such a configuration, and the display unit  137  may be provided separately from the control unit  134 . In that case, a touch panel function may be provided in the display unit  137 , and a user may be able to operate the touch panel so as to change the display position of the image, to enlarge or reduce the image, or to modify the displayed image. 
         [0042]    In addition, the signal processing unit  135  receives polarization information outputted from the measurement light detector  116  or the detectors  131  and  133 , and transmits information necessary for controlling the polarization to the driving control unit  136 . 
         [0043]    The driving control unit  136  drives the galvanoscanner  110  and the stage  113  as described above when an image of the eye  115  to be examined is to be captured. In addition, the driving control unit  136  drives the galvanoscanner  110 , the mirror  114 , the shutter  120 , and the polarization controllers  108 ,  118 , and  126  in accordance with the information received from the signal processing unit  135 . 
       Processing Operation 
       [0044]    Subsequently, a process of controlling the polarization state by the polarization controllers  108 ,  118 , and  126  will be described with reference to the flowchart illustrated in  FIG. 2 , and this process is a characteristic processing operation according to the present exemplary embodiment. 
         [0045]    First, a correction starts when, for example, an examiner presses a correction start button (not illustrated) displayed on the display unit  137  or presses a correction start button physically provided on the SD PS-OCT apparatus  100 . Alternatively, a timing of carrying out a correction may be preset as desired. For example, a correction may be set to be carried out when the SD PS-OCT apparatus  100  is started or immediately before the measurement starts. Alternatively, the temperature of the SD PS-OCT apparatus  100  may be monitored, and a correction may be set to be carried out when a variation in the temperature is large. 
         [0046]    After the correction starts, in step S 201 , the driving control unit  136  controls the angle of the galvanoscanner  110  so as to cause the measurement light to be incident on the measurement light detector  116 . In step S 202 , the measurement light is measured and determined whether it is circularly polarized. If the measurement light is not circularly polarized (NG in step S 202 ), the processing proceeds to step S 203 , and the driving control unit  136  controls the polarization controller  108  so as to control the polarization state of the measurement light to be detected by the measurement light detector  116 . In step S 204 , the polarization state of the controlled measurement light is determined, and if the measurement light is circularly polarized (OK in step S 204 ), the processing proceeds to step S 205 ; otherwise, the processing returns to step S 203 . Here, an example of the criteria for determining the polarization state is based on the ellipticity of the measurement light or the signal intensity of the measurement light that has passed through a polarizer. 
         [0047]    Thereafter, the polarization controller  126  is controlled. The polarization controller  126  is controlled by using only the measurement light. In step S 205 , the driving control unit  136  drives the mirror  114  so that the measurement light is reflected by the mirror  114  and the reflected measurement light returns to the beam splitter  106 . Then, in step S 206 , the shutter  120  is closed so that the reference light does not return to the beam splitter  106 . In step S 207 , the angle of the galvanoscanner  110  is controlled so that the measurement light is reflected by the mirror  114  and the reflected measurement light returns to the beam splitter  106 . Since the light to be incident on the mirror  114  is controlled to be circularly polarized, the measurement light that returns to the beam splitter  106  again becomes linearly polarized. The measurement light incident on the beam splitter  106  is then guided to the polarization beam splitter  129  via the SM fiber  125 , the polarization controller  126 , the connector  127 , and the SM fiber  128 . The measurement light is split into two polarization components, namely, the V polarization component and the H polarization component by the polarization beam splitter  129 . In step S 208 , the signal processing unit  135  determines whether the light is detected only with one of the detectors  131  and  133 . If the light is not detected with only one of the detectors  131  and  133  (NG in step S 208 ), in step S 209 , the driving control unit  136  controls the polarization controller  126  so that the light is detected only with one of the detectors  131  and  133 . Then, in step S 210 , as in step S 208 , the polarization state is determined, and if the determination result is NG, the processing returns to step S 209 . Meanwhile, if the determination result is OK, the processing proceeds to step S 211 . Here, an example of the criteria for determining that a signal is detected only with one of the detectors  131  and  133  is based on a case in which the ratio between the signal intensities of the two detectors is the highest. 
         [0048]    Lastly, the polarization controller  118  is controlled. The polarization controller  118  is controlled by using only the reference light signal. 
         [0049]    In step S 211 , the driving control unit  136  drives the galvanoscanner  110  so that the measurement light does not return to the beam splitter  106  and causes the measurement light to be incident on the measurement light detector  116 . In step S 212 , the mirror  114  is removed. Here, although the measurement light is made to be incident on the measurement light detector  116  in the present exemplary embodiment, the measurement light does not have to be made to be incident on the measurement light detector  116  as long as the measurement light does not return to the beam splitter  106 . Subsequently, in step S 213 , the shutter  120  is opened, and the block on the reference light is released. The reference light travels through the SM fiber  117 , the polarization controller  118 , the collimator  119 , the dispersion compensation glass  121 , and the ND filter  122 , and is reflected by the mirror  123 . The reflected light is then guided to the beam splitter  106 . The reference light to be emitted from the SM fiber  117  has been controlled to be linearly polarized by the polarization controller  103 . Therefore, it is obvious that, even in a case in which the reference light has been made to be elliptically polarized or circularly polarized by the polarization controller  118 , as the reference light is reflected by the mirror  123  and passes through the polarization controller  118  again, the reference light becomes linearly polarized. In step S 214 , the signal processing unit  135  determines whether the signal intensities of the light components detected by the respective detectors  131  and  133  are substantially equal to each other. If the signal intensities are not substantially equal to each other (NG in step S 214 ), in step S 215 , the driving control unit  136  controls the polarization controller  118  so that the signal intensities become substantially equal to each other. Then, in step S 216 , as in step S 214 , the polarization state is determined, and if the determination result is NG, the processing returns to step S 215 . Meanwhile, if the determination result is OK, the processing proceeds to step S 217 . Here, an example of the criteria for determining that the signal intensities of the light components detected by the two detectors  131  and  133  are substantially equal to each other is based on the ratio of the signal intensities obtained from the two detectors  131  and  133 . The reference light to be guided to the polarization beam splitter  129  in the end can be controlled to be linearly polarized light in which the ratio of the V polarization component and the H polarization component is 1:1, or in other words, linearly polarized light that is polarized at an angle of 45° relative to each of the two polarization axes that are orthogonal to each other. Then in step S 217 , the galvanoscanner  110  is driven so that the measurement light is directed in a direction in which the measurement light is incident on the eye  115  to be examined at the time of the measurement, and the measurement is continued. 
         [0050]    According to the configuration and the processing operation described thus far, the polarization state is controlled as appropriate by the polarization controllers provided in the respective optical paths in the interferometer in accordance with the detected polarization states. Therefore, even in a case in which the polarization state changes due to heat produced while the SD PS-OCT apparatus  100  is being used, the polarization state can be corrected. 
         [0051]    Although a polarizer is not disposed between the SM fiber  102  and the SM fiber  105  in the present exemplary embodiment, a polarizer may be disposed between the SM fiber  102  and the SM fiber  105  depending on the degree of polarization of the light source  101 . In that case, the connector  104  is disconnected from the SM fibers  102  and  105 , and the SM fiber  102  is connected to an input terminal of the polarizer. Meanwhile, an output terminal of the polarizer is connected to the SM fiber  105 , and thus the aforementioned configuration can be achieved. In addition, although a method for connecting the SM fibers  102  and  105  directly to the polarizer has been described above, the present exemplary embodiment is not limited to such a configuration. In a case in which a component formed by integrating optical fibers and a polarizer is to be added, the SM fiber  102  is disconnected from the connector  104 , and the optical fiber at an input side of the polarizer is connected to the SM fiber  102  by using another connector. In addition, the optical fiber at an output side of the polarizer is connected to the connector  104 , and thus the polarizer can be added. 
         [0052]    In addition, although the SD PS-OCT apparatus  100  in which the polarization controllers  108 ,  118 , and  126  are controlled on the basis of the result of detecting the polarization state with the measurement light detector  116  or the detectors  131  and  133  has been described in the present exemplary embodiment, such control may be carried out semi-automatically. Specifically, the result of detecting the polarization state with the measurement light detector  116  or the detectors  131  and  133  may be displayed on the display unit  137 , and the user may control each of the polarization controllers  108 ,  118 , and  126  as appropriate in accordance with the displayed result. 
         [0053]    In addition, it is obvious that the present invention can be applied not only to the case in which a PS-OCT is formed by fibers but also in a case in which a PS-OCT is formed by a space optical system. 
         [0054]    As described thus far, by capturing an image of an eye to be examined after each of the polarization controllers is controlled, an accurate PS-OCT image can be captured. 
       Second Embodiment 
       [0055]    While an example of SD-OCT has been illustrated in the first exemplary embodiment, the present invention is not limited thereto, and a PS-OCT image can also be obtained in a similar manner through a swept-source (SS) OCT that uses an SS-light source. In addition, while the SD PS-OCT apparatus  100  is formed by a Michelson interferometer in the first exemplary embodiment, a similar effect can be obtained by a PS-OCT apparatus formed by a Mach-Zehnder interferometer. 
         [0056]    In the present exemplary embodiment, as an example of PS-OCT having a different configuration, a configuration and a method for controlling the polarization in a case in which an SS PS-OCT apparatus is formed by a Mach-Zehnder interferometer will be described. A basic configuration of SS-OCT is well-known, and thus detailed description thereof will be omitted. 
       Configuration of SS PS-OCT Apparatus  300   
       [0057]    A configuration of an SS PS-OCT apparatus  300  will be described with reference to  FIG. 3 . It is to be noted that detailed descriptions of configurations that are similar to those of the first exemplary embodiment will be omitted. 
         [0058]    A light source  301  is formed by using an SS-light source of which the oscillation wavelength of the light varies periodically, and in the present exemplary embodiment, for example, the light source  301  emits light at a central wavelength of 1040 nm and with a bandwidth of 100 nm. 
         [0059]    Light emitted from the light source  301  is guided to a beam splitter  306  via an SM fiber  302 , a polarization controller  303 , a connector  304 , and an SM fiber  305 , and is split into measurement light and reference light. The split ratio of the beam splitter  306  is 90:10 (reference light:measurement light). It is to be noted that the split ratio is not limited to these values, and can take other values. The beam splitter  306  is connected to SM fibers  305 ,  307 ,  317 , and  327  in the present exemplary embodiment. Although a beam splitter  330  is connected to SM fibers  326 ,  329 ,  331 , and  336 , the aforementioned SM fibers may instead be PM fibers. In a case in which the beam splitters  306  and  330  are connected to PM fibers, polarization controllers  308 ,  318 ,  332 , and  337  do not need to be provided. Alternatively, wave plates may be disposed in the sample arm and in the reference arm. For example, in place of the polarization controller  308 , a wave plate may be disposed between a collimator  309  and a galvanoscanner  310 , and in place of the polarization controller  318 , a wave plate may be disposed between a collimator  319  and a shutter  320 . 
         [0060]    The polarization controller  303  can change the polarization of the light emitted from the light source  301  to a predetermined polarization state. In the present exemplary embodiment, the light from the light source  301  is controlled to be linearly polarized by the polarization controller  303 . It is to be noted that, although the description is omitted in the present exemplary embodiment, when the degree of polarization of the light source  301  is not high, a polarizer may be disposed between the polarization controller  303  and the connector  304  so as to increase the degree of polarization of the light emitted from the light source  301 . In that case, the quantity of light passing through the polarizer can be controlled by controlling the polarization controller  303 . In addition, in place of disposing the polarization controller  303 , only a polarizer may be disposed on the SM fiber  302 . In this case, the polarization state of the light emitted from the light source  301  does not need to be controlled, and only the degree of polarization of the light can be increased. However, the quantity of light guided to an interferometer may be reduced depending on the polarization state of the light, and thus it is necessary to determine whether a sufficient quantity of light is obtained. 
         [0061]    The split measurement light is emitted through the SM fiber  307  and is collimated by the collimator  309 . In addition, the polarization controller  308  is disposed on the SM fiber  307  and can change the polarization state of the emitted measurement light as desired. In the present exemplary embodiment, the polarization controller  308  is controlled such that circularly polarized light is incident on an eye  315  to be examined. 
         [0062]    In a case in which the polarization state of light incident on the eye  315  to be examined, or a subject, differs from the polarization state of light incident on a measurement light detector  316 , if the light is controlled to be circularly polarized when the light is incident on the eye  315  to be examined, the light becomes elliptically polarized when the light is incident on the measurement light detector  316 . The state of the elliptical polarization to be detected by the measurement light detector  316  as the light is circularly polarized when being incident on the eye  315  to be examined is uniquely determined, and thus the polarization controller  308  is controlled such that the light is circularly polarized when the light is incident on the eye  315  to be examined while the elliptically polarized light is detected by the measurement light detector  316 . 
         [0063]    It is to be noted that aside from using a detector, such as a polarization measuring device, as the measurement light detector  316 , the polarization state may be determined by using an optical power meter and a polarizer, a wave plate, or the like. 
         [0064]    Alternatively, a polarization controller or a wave plate may be used so that the polarization state of the measurement light to be detected by the measurement light detector  316  becomes identical to the polarization state of the measurement light at the position of the eye  315  to be examined. For example, in a case in which the measurement light is to be made incident on the measurement light detector  316  by varying the angle of the galvanoscanner  310 , a wave plate may be disposed between the galvanoscanner  310  and the measurement light detector  316  in such a manner that the polarization state to be detected by the measurement light detector  316  becomes identical to the polarization state at the position of the eye  315  to be examined. 
         [0065]    The collimated measurement light is incident on the eye  315  to be examined via the galvanoscanner  310 , which scans the fundus Er of the eye  315  to be examined with the measurement light, a scan lens  311 , and an objective lens  312 . Here, although the galvanoscanner  310  is illustrated as a single mirror, the galvanoscanner  310  may be formed by two galvanoscanners so as to carry out a raster scan of the fundus Er of the eye  315  to be examined. In addition, the objective lens  312  is fixed to a stage  313 , and as the stage  313  is moved in the direction of the optical axis, the diopter of the eye  315  to be examined can be adjusted. The galvanoscanner  310  and the stage  313  are controlled by a driving control unit  349 , and the galvanoscanner  310  can scan the fundus Er of the eye  315  to be examined within a predetermined range (also referred to as a tomographic image obtaining range, a tomographic image obtaining position, or a measurement light irradiation position) with the measurement light. 
         [0066]    The measurement light is made to be incident on the eye  315  to be examined through the objective lens  312  disposed on the stage  313  and is focused on the fundus Er. The measurement light that has irradiated the fundus Er is reflected or scattered by each retina layer and returns to the beam splitter  306  through the above-described optical path. 
         [0067]    The reference light that has been split by the beam splitter  306  is emitted through the SM fiber  317  and is collimated by the collimator  319 . The polarization controller  318  is disposed on the SM fiber  317  and can change the polarization state of the emitted reference light as desired. In the present exemplary embodiment, the polarization controller  318  controls the polarization state of the reference light that is to be reflected by mirrors  323 - a  and  323 - b  and that is to be incident on polarization beam splitters  335  and  340  to become linearly polarized at an angle of 45° relative to each of the two polarization axes that are orthogonal to each other. 
         [0068]    The reference light travels through a dispersion compensation glass  321  and an ND filter  322  and is then reflected by the mirrors  323 - a  and  323 - b  disposed on a coherence gate stage  324 . The reflected reference light is incident on the beam splitter  330  via a collimator  325  and the SM fiber  326 . 
         [0069]    At the beam splitter  330 , the returning light of the measurement light that is incident on the beam splitter  330  via the beam splitter  306 , the SM fiber  327 , a connector  328 , and the SM fiber  329  is combined with the reference light that is incident on the beam splitter  330  via the SM fiber  326 , resulting in interfered light, and the resulting interfered light is split into two components by the beam splitter  330 . The split components of the interfered light have phases that are inverted relative to each other (hereinafter, referred to as a positive component and a negative component). The split positive interfered light is then guided to the polarization beam splitter  335  via the SM fiber  331 , the polarization controller  332 , a connector  333 , and an SM fiber  334 . Here, the interfered light is split along the two polarization axes that are orthogonal to each other, and is split into a positive H polarization component and a positive V polarization component. In a similar manner, the negative interfered light is guided to the polarization beam splitter  340  via the SM fiber  336 , the polarization controller  337 , a connector  338 , and an SM fiber  339 , and is then split into a negative H polarization component and a negative V polarization component. 
         [0070]    The positive H polarization component generated at the polarization beam splitter  335  and the negative H polarization component generated at the polarization beam splitter  340  are guided to a detector  346  via SM fibers  342  and  344 , respectively, and are detected by the detector  346 . Meanwhile, the positive V polarization component generated at the polarization beam splitter  335  and the negative V polarization component generated at the polarization beam splitter  340  are guided to a detector  345  via SM fibers  341  and  343 , respectively. 
         [0071]    Interference signals detected by the detectors  345  and  346  are converted to electric signals, and the electric signals are then transmitted to a signal processing unit  348 . The signal processing unit  348  generates a PS-OCT image on the basis of the information from each of the detectors  345  and  346 . The method for generating a PS-OCT image is well-known, and thus description thereof will be omitted. 
         [0072]    It is to be noted that as the reference light is linearly polarized at an angle of 45° in the present exemplary embodiment, the reference light is split into the V polarization component and the H polarization component in the equal quantity. In addition, the measurement light is circularly polarized in the present exemplary embodiment, and thus data can be obtained simultaneously regardless of the directions of the cells and the fibers of the fundus Er of the eye  315  to be examined. As a result, data can be obtained at once in the entire polarization directions. Thus, it is not necessary to capture images of a single site in different polarization directions, and data can be obtained through a single instance of image capturing. 
         [0073]    In addition, in the present exemplary embodiment, a movable mirror  314  for reflecting the measurement light is disposed in the optical path of the measurement light prior to being incident on the eye  315  to be examined. The mirror  314  is controlled by the driving control unit  349 , and has a function of preventing the measurement light from being incident on the eye  315  to be examined when the polarization controllers  332  and  337  are controlled by reflecting the measurement light and returning the measurement light to the beam splitter  306 . Therefore, the angle of the mirror  314  is adjusted such that, in a state in which the mirror  314  is disposed in the sample arm, the light that has been emitted through the SM fiber  307  is guided to the mirror  314  via the collimator  309 , the galvanoscanner  310 , the scan lens  311 , and the objective lens  312  and is reflected by the mirror  314 , and such that the reflected light returns to the beam splitter  306 . It is to be noted that, although a mirror is disposed in the present exemplary embodiment, any reflector that can reflect the measurement light can be employed. 
         [0074]    In addition, in the present exemplary embodiment, the shutter  320  for blocking the reference light is disposed next to the collimator  319 . The shutter  320  is controlled by the driving control unit  349 , and the shutter  320  prevents the reference light from being incident on the beam splitter  330  when the driving control unit  349  controls the polarization controllers  332  and  337 . 
       Control Unit  347   
       [0075]    A control unit  347  for controlling the SS PS-OCT apparatus  300  as a whole will be described. 
         [0076]    The control unit  347  includes the signal processing unit  348 , the driving control unit  349 , and a display unit  350 . 
         [0077]    The signal processing unit  348  generates an image, analyzes the generated image, and generates visualized information of the result of the analysis on the basis of the signals outputted from the detectors  345  and  346 . The methods for generating and analyzing the image are well-known, and thus descriptions thereof will be omitted. 
         [0078]    The image generated by the signal processing unit  348  or the result of the analysis is displayed on a display screen of the display unit  350  (e.g., liquid crystal display or the like). The image data generated by the signal processing unit  348  may be transmitted to the display unit  350  through a cable or wirelessly. 
         [0079]    Although the display unit  350  is included in the control unit  347 , the present invention is not limited to such a configuration, and the display unit  350  may be provided separately from the control unit  347 . In that case, a touch panel function may be provided in the display unit  350 , and a user may be able to operate the touch panel so as to change the display position of the image, to enlarge or reduce the image, or to modify the displayed image. 
         [0080]    In addition, the signal processing unit  348  receives polarization information outputted from the measurement light detector  316  or the detectors  345  and  346 , and transmits information necessary for controlling the polarization to the driving control unit  349 . 
         [0081]    The driving control unit  349  drives the galvanoscanner  310  and the stage  313  as described above when an image of the eye  315  to be examined is to be captured. In addition, the driving control unit  349  drives the galvanoscanner  310 , the mirror  314 , the shutter  320 , and the polarization controllers  308 ,  318 ,  332 , and  337  in accordance with the information received from the signal processing unit  348 . 
       Processing Operation 
       [0082]    In the present exemplary embodiment, a process of controlling the polarization in a system by controlling the polarization controllers  308 ,  318 ,  332 , and  337  will be described with reference to  FIGS. 3 and 4 . 
         [0083]    First, a correction starts when, for example, an examiner presses a correction start button (not illustrated) displayed on the display unit  350  or presses a correction start button physically provided on the SS PS-OCT apparatus  300 . 
         [0084]    After the correction starts, in step S 401 , the driving control unit  349  controls the angle of the galvanoscanner  310  so as to cause the measurement light to be incident on the measurement light detector  316 . In step S 402 , it is determined whether the measurement light is circularly polarized. If the measurement light detected by the measurement light detector  316  is not circularly polarized (NG in step S 402 ), the processing proceeds to step S 403 , and the driving control unit  349  controls the polarization controller  308  so as to control the measurement light to become circularly polarized. In step S 404 , the polarization state of the controlled measurement light is determined, and if the measurement light is circularly polarized (OK in step S 404 ), the processing proceeds to step S 405 ; otherwise, the processing returns to step S 403 . Here, an example of the criteria for determining the circularly polarized light is based on the ellipticity of the measurement light or the signal intensity of the measurement light that has passed through a polarizer. 
         [0085]    Thereafter, the polarization controllers  332  and  337  are controlled. The polarization controllers  332  and  337  are controlled by using only the measurement light. In step S 405 , the driving control unit  349  drives the mirror  314  so that the measurement light is reflected by the mirror  314  and the reflected measurement light returns to the beam splitter  306 . Then, in step S 406 , the shutter  320  is closed so that the reference light is not incident on the beam splitter  330 . In step S 407 , the angle of the galvanoscanner  310  is controlled so that the measurement light is incident on the eye  315  to be examined. Since the light to be incident on the mirror  314  has been controlled to be circularly polarized, the light that returns to the beam splitter  306  becomes linearly polarized. The measurement light that is incident on the beam splitter  306  is emitted to the SM fiber  327  and is incident on the beam splitter  330  via the connector  328  and the SM fiber  329 . The light is split by the beam splitter  330  into two components that are in a positive/negative inverted phase relationship. One of the components is incident on the polarization beam splitter  335  via the SM fiber  331 , the polarization controller  332 , the connector  333 , and the SM fiber  334 , and the other component is incident on the polarization beam splitter  340  via the SM fiber  336 , the polarization controller  337 , the connector  338 , and the SM fiber  339 . Then, the components are each split into two polarization components, namely, the V polarization component and the H polarization component by the respective polarization beam splitters  335  and  340 . The V polarization components are guided to the detector  345 , and the H polarization components are guided to the detector  346 . 
         [0086]    In step S 408 , the signal processing unit  348  determines whether the light is detected only with one of the detectors  345  and  346 . If the light is not detected with only one of the detectors  345  and  346  (NG in step S 408 ), in step S 409 , the driving control unit  349  controls the polarization controllers  332  and  337  so that the light is detected only with one of the detectors  345  and  346 . Then, in step S 410 , as in step S 408 , the polarization state is determined, and if the determination result is NG, the processing returns to step S 409 . Meanwhile, if the determination result is OK, the processing proceeds to step S 411 . Here, an example of the criteria for determining that a signal is detected only with one of the detectors  345  and  346  is based on the ratio between the signal intensities of the two detectors  345  and  346 . 
         [0087]    Lastly, the polarization controller  318  is controlled. The polarization controller  318  is controlled by using only the reference light signal. 
         [0088]    In step S 411 , the driving control unit  349  drives the galvanoscanner  310  so that the measurement light does not return to the beam splitter  306  and causes the measurement light to be incident on the measurement light detector  316 . In step S 412 , the mirror  314  is removed. Here, although the measurement light is made incident on the measurement light detector  316  in the present exemplary embodiment, the measurement light does not have to be made incident on the measurement light detector  316  as long as the measurement light does not return to the beam splitter  306 . 
         [0089]    Subsequently, in step S 413 , the shutter  320  is opened, and the block on the reference light is released. The reference light travels through the SM fiber  317 , the polarization controller  318 , the collimator  319 , the dispersion compensation glass  321 , and the ND filter  322 , and is reflected by the mirrors  323 - a  and  323 - b  disposed on the coherence gate stage  324 . The reflected reference light is then incident on the beam splitter  330  via the collimator  325  and the SM fiber  326 . The light incident on the beam splitter  330  is guided to the detectors  345  and  346  as described above. 
         [0090]    In step S 414 , the signal processing unit  348  determines whether the signal intensities of the light detected by the respective detectors  345  and  346  are substantially equal to each other. If the signal intensities are not substantially equal to each other (NG in step S 414 ), in step S 415 , the driving control unit  349  controls the polarization controller  318  so that the signal intensities becomes substantially equal to each other. Then, in step S 416 , as in step S 414 , the polarization state is determined, and if the determination result is NG, the processing returns to step S 415 . Meanwhile, if the determination result is OK, the processing proceeds to step S 417 . Here, an example of the criteria for determining that the signal intensities of the light detected by the two detectors  345  and  346  are substantially equal to each other is based on the ratio of the signal intensities from the two detectors  345  and  346 . The reference light guided to the polarization beam splitters  335  and  340  in the end can be controlled to be linearly polarized light in which the ratio of the V polarization component and the H polarization component is 1:1, or in other words, linearly polarized light that is polarized at an angle of 45° relative to each of the two polarization axes that are orthogonal to each other. Then in step S 417 , the galvanoscanner  310  is driven so that the measurement light is directed in a direction in which the measurement light is incident on the eye  315  to be examined at the time of the measurement, and the measurement is continued. 
         [0091]    According to the configuration and the processing operation described thus far, even in the SS PS-OCT apparatus  300 , the polarization state is controlled as appropriate by the polarization controllers provided in the respective optical paths in the interferometer in accordance with the detected polarization states. Therefore, even in a case in which the polarization state changes due to heat produced while the SS PS-OCT apparatus  300  is being used, the polarization state can be corrected. 
       OTHER EMBODIMENTS 
       [0092]    Embodiment(s) of the present invention can also be realized by a computer of a system or apparatus that reads out and executes computer executable instructions (e.g., one or more programs) recorded on a storage medium (which may also be referred to more fully as a ‘non-transitory computer-readable storage medium’) to perform the functions of one or more of the above-described embodiment(s) and/or that includes one or more circuits (e.g., application specific integrated circuit (ASIC)) for performing the functions of one or more of the above-described embodiment(s), 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) and/or controlling the one or more circuits to perform the functions of one or more of the above-described embodiment(s). The computer may comprise one or more processors (e.g., central processing unit (CPU), micro processing unit (MPU)) and may include a network of separate computers or separate processors to read out and execute the computer executable instructions. 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. 
         [0093]    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. 
         [0094]    This application claims the benefit of Japanese Patent Application No. 2014-034553, filed Feb. 25, 2014, which is hereby incorporated by reference herein in its entirety.