JP2014206433A - Optical tomographic image-capturing device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical tomographic image- capturing device in which components to be used are reduced, adjustment is facilitated and further, cost can be reduced, by simplifying a structure of a polarization-sensitive OCT (optical coherence tomography), and which can be mass-produced and is also capable of implementing normal OCT measurement in spite of the polarization-sensitive OCT.SOLUTION: Polarized beam splitters used for a polarization dependent delay line 133 are collected into one, thereby eliminating angle deviation. By employing an inline-type polarized beam splitter for a polarization-sensitive detection arm 136, adjustment is facilitated and cost can be reduced by simplifying a configuration by reducing collimator lens parts of an output section into two. Further, by providing switching means using an optical switch or shielding means using a shield plate in a sample arm part, an optical tomographic image-capturing device is configured to be available not only as a polarization-sensitive OCT but also as a normal OCT.

Description

The present invention relates to an optical tomographic imaging apparatus, and more particularly to an optical tomographic imaging apparatus that takes a tomographic image by OCT used in ophthalmic medicine or the like.

Optical coherence tomography (OCT) is a method widely used as a means for acquiring high-resolution tomographic images of living tissue in ophthalmology because it can be measured non-invasively and non-contactly.

In optical coherence tomography (OCT), a time domain method called a time domain method, a time domain OCT that acquires a tomographic image while mechanically changing the optical path length of reference light by moving a mirror, and a spectroscope called a Fourier domain method are used. There are spectral domain OCT that uses spectral information to detect tomographic images, and optical frequency swept OCT that detects spectral interference signals using a wavelength scanning light source to acquire tomographic images.

Birefringence that changes the polarization state occurs in tissues in which molecules are arranged in a certain direction. In the retina at the fundus, there is strong birefringence in the retinal nerve fiber layer, retinal pigment epithelium layer, blood vessel wall, sclera, and phloem. In recent years, the development of various polarization-sensitive OCTs has been attempted for polarization-sensitive OCT (PS-OCT), which is one of functional OCTs, in order to visualize these tissues by forming a birefringent tomography.

Polarization-sensitive OCT (PS-OCT) employs a configuration in which circularly polarized light or polarization-modulated light is used as measurement light for observing a sample, and interference light is detected as two orthogonal linearly polarized lights.

  Patent Document 1 discloses an example of polarization-sensitive OCT (PS-OCT). At the same time (synchronously) with the B-scan, a polarized beam from a light source (a beam linearly polarized by a polarizer) is continuously modulated by an EO modulator (polarization modulator, electro-optic modulator). Then, the polarized light beam whose polarization is continuously modulated is divided, one of the samples is irradiated, and the reflected light is obtained, and the other is used as the reference light, and OCT measurement is performed by spectral interference between the two. Of these, the Joan matrix representing the polarization characteristics of the sample can be obtained by measuring the vertical polarization component and the horizontal polarization component simultaneously with two photodetectors.

However, since EO modulators (polarization modulators, electro-optic modulators) are very expensive, attempts have been made to use no EO modulator. Non-Patent Document 1 discloses a configuration of a polarization-sensitive OCT (PS-OCT) that obtains a Joan matrix representing the polarization characteristics of a sample without using an EO modulator.

In this document (Non-Patent Document 1), instead of using an EO modulator, a sample arm (an optical system for irradiating a sample with a polarized beam and obtaining the reflected light) before irradiating the sample from a light source. By providing a polarization-dependent delay line consisting of a polarizer, two polarizing beam splitters, and two dove prisms, and controlling the delay between the two incident polarization states of vertical and horizontal polarization incident on the sample The Joan matrix representing the polarization characteristics of the sample can be obtained without using an EO modulator.

The detection arm is composed of one cube-type non-polarization beam splitter and two cube-type polarization beam splitters, and interference waves in two polarization states are detected by two balanced photodetectors.

Japanese Patent No. 434429

"Passive component based multifunctional Jones matrix swept source optical coherence to doppler and polarization immobilization" OPTICSL. 37, no. 11, June 1, 2012, p1958

However, the configuration described in Non-Patent Document 1 has the following problems, and mass production of polarization-sensitive OCT (PS-OCT) using this configuration is difficult.

First, since two polarization beam splitters are used for the polarization-dependent delay line, it is unavoidable that the delay line itself becomes large, and the optical adjustment of the delay line, in particular, the angles of the two polarization beam splitters are matched. In order to achieve this, strict adjustment was necessary.

Next, since the detection arm employs a cube-type polarization beam splitter, four output side collimator lens portions are required, the structure of the detection arm itself is complicated, and adjustment is difficult.

Furthermore, the polarization-sensitive OCT of Non-Patent Document 1 using a polarization-dependent delay line delays two different polarized waves in terms of time, so the depth measurement range is as shallow as 1/2 of normal OCT. Therefore, when it is desired to see a deeper portion, it is necessary to separately measure by normal OCT.

The present invention solves the above-mentioned problems, and by simplifying the structure of the polarization-dependent delay line, detection arm, etc., the number of parts used is reduced, adjustment is easy, and cost reduction is possible. An object of the present invention is to provide an optical tomographic imaging apparatus that can be mass-produced and that is capable of performing normal OCT measurement while being polarization-sensitive OCT.

First, an optical tomographic imaging apparatus according to the present invention includes:
A branching means for branching into a sample arm for irradiating the object to be inspected with light from the light source and a reference arm for irradiating the reference object;
In the sample arm, the light (sample light) applied to the object to be inspected is divided into first light and second light having different polarization directions, and the divided first light and second light are divided. Sample light splitting delay means for delaying the light at different times,
An interference signal composed of a combination of the first light reflected from the inspection object of the sample arm and the light from the reference arm, and the second light reflected from the inspection object of the sample arm. And an acquisition means for acquiring a tomographic image indicating polarization information of the object to be inspected based on an interference signal formed by multiplexing light from the reference arm,
The sample light splitting delay means includes one polarization beam splitter and two total reflection prisms, and the sample light is incident on a position away from the center of the polarization beam splitter by a predetermined distance.

According to the apparatus having the above configuration,
Since the first light and the second light having different polarization directions can be delayed at different times by one polarization beam splitter, the reflection surface angles of the two polarization beam splitters are matched as in Non-Patent Document 1. Therefore, the adjustment becomes easy and the reflection surface angle deviation itself is eliminated, and a good tomographic image can be acquired.
Further, by simplifying the configuration, the number of parts is reduced, and the size and cost of the apparatus can be reduced.

Secondly, an optical tomographic imaging apparatus according to the present invention includes:
Means for combining the first light reflected from the inspection object of the sample arm and the light from the reference arm, and the second light reflected from the inspection object of the sample arm and the reference A cube-type or plate-type non-polarizing beam splitter and an in-line type polarizing beam splitter are used as means for multiplexing the light from the arm.

According to the apparatus having the above configuration,
The first light reflected from the sample arm inspection object and the light from the reference arm are combined, and the second light reflected from the sample arm inspection object and the light from the reference arm are combined. By using an in-line type polarization beam splitter as the means, it is possible to reduce the collimator lens part of the output part to two places, making adjustment and easy, and cost reduction by reducing parts.

Thirdly, an optical tomographic imaging apparatus according to the present invention includes:
An in-line type polarization controller is provided before the sample light enters the sample light splitting delay means in the sample arm branched by the branch means, and the polarization angle is controlled in the direction of 45 degrees, and in front of the polarization beam splitter. Control means for controlling the polarization angle of the arranged polarizer to 45 degrees is also provided.

According to the apparatus having the above configuration,
By controlling the polarization angle of the in-line type polarization controller and the polarization angle of the polarization element to 45 degrees, it becomes possible to efficiently extract vertically polarized waves and horizontally polarized waves.

Fourth, an optical tomographic imaging apparatus according to the present invention is
A branching means for branching into a sample arm for irradiating the object to be inspected with light from the light source and a reference arm for irradiating the reference object;
Within the sample arm, the light irradiated to the object to be inspected is divided into first light and second light having different polarization directions, and the divided first light and second light are mutually separated. Sample light split delay means for delaying at different times;
An interference signal composed of a combination of the first light reflected from the inspection object of the sample arm and the light from the reference arm, and the second light reflected from the inspection object of the sample arm. And an acquisition means for acquiring a tomographic image indicating polarization information of the object to be inspected based on an interference signal formed by multiplexing light from the reference arm,
The sample arm is provided with a second optical path that does not pass through the sample light splitting delay means after the light from the light source is split into the sample arm by the splitting means, and the first optical path through the sample light splitting delay means Switching means for switching between the second optical path is provided.

According to the apparatus having the above configuration,
Since the switching means can switch between the polarization-sensitive OCT (PS-OCT) and the normal OCT, it is possible to acquire tomographic images by both the polarization-sensitive OCT and the normal OCT without replacing the apparatus. By switching to, the depth measurement range is doubled compared to the polarization sensitive OCT.

Fifth, an optical tomographic imaging apparatus according to the present invention is
A branching means for branching into a sample arm for irradiating the object to be inspected with light from the light source and a reference arm for irradiating the reference object;
Within the sample arm, the light irradiated to the object to be inspected is divided into first light and second light having different polarization directions, and the divided first light and second light are mutually separated. Sample light split delay means for delaying at different times;
An interference signal composed of a combination of the first light reflected from the inspection object of the sample arm and the light from the reference arm;
Acquisition means for acquiring a tomographic image indicating polarization information of the inspection object based on an interference signal formed by combining the second light reflected from the inspection object of the sample arm and the light from the reference arm With
The sample light splitting delay means includes a blocking means for blocking any one of the first light and the second light.

According to the apparatus having the above configuration,
By blocking any one of the first light and the second light having different polarization directions in the polarization-dependent delay line, the tomographic image by OCT of the polarization state of one of the light that is not blocked is blocked. It can be obtained in the measurement range twice as deep as before.

Sixthly, the optical tomography apparatus according to the present invention includes means for selecting whether the light to be blocked is the first light or the second light as the blocking means in the sample light splitting delay means. It is characterized by that.

According to the apparatus having the above configuration,
The selection means can obtain a measurement range twice as deep as before the tomographic image by OCT in the polarization state which is the polarization direction the examiner wants to see is cut off.

Seventh, an optical tomographic imaging apparatus according to the present invention provides:
With an in-line type polarization controller before the sample light is incident on the sample light splitting delay means in the sample arm branching by said branching means,
Without being blocked by the blocking means in the sample light splitting delay means, the polarization angle of the polarization direction of any one of the first light and the second light that irradiates the inspection object, Control means is provided for controlling the polarization angle controlled by the inline polarization controller and the polarization angle of the polarizer disposed in front of the polarization beam splitter so as to coincide with each other.

According to the apparatus having the above configuration,
In order to control the birefringence axis and phase delay amount of the in-line polarization controller and the polarization angle of the polarizing element so that the intensity of light irradiating the inspection object is optimized, the polarized wave irradiating the inspection object is efficiently extracted. It becomes possible.

According to the present invention, a Joan matrix representing a polarization characteristic of a sample can be obtained by simultaneously measuring a vertical polarization component and a horizontal polarization component without using a very expensive EO modulator (polarization modulator, electro-optic modulator). Therefore, an optical tomographic imaging apparatus capable of performing normal OCT measurement while being polarization-sensitive OCT can be provided at low cost.

It is the figure which showed the detail of an example of the optical tomographic image acquisition part by this invention. It is the figure which showed the structure of the optical tomography apparatus. It is the figure which showed the switching means of polarization-sensitive OCT and normal OCT. It is the figure which showed the method of interrupting any one of the measurement light of two polarization states of a perpendicular direction and a horizontal direction in a polarization dependence delay line. It is the figure which showed another Example of the switching means of polarization-sensitive OCT and normal OCT.

An optical tomographic imaging apparatus according to an embodiment of the present invention will be described below with reference to the drawings. FIG. 1 shows a detailed configuration of the tomographic image acquisition unit 100.

  As shown in FIG. 1, the tomographic image acquisition unit 100 shoots a two-dimensional or / and three-dimensional tomographic image of the sample 117 by irradiating the sample (inspection object) 117 with measurement light. In this embodiment, a Fourier domain (optical frequency sweep) method using a wavelength scanning light source 101 that scans while changing the wavelength with time is employed.

  That is, the light output from the wavelength scanning light source 101 is input to the fiber coupler 102 through the optical fiber, and is split into the reference light and the measurement light at the ratio of, for example, 5:95, and is respectively referred to. It is output to the arm 160 and the sample arm 150. Of these, the reference light output to the reference arm 160 is input to the optical circulator 120 through the optical fiber, is then input to the collimator lens 121, and is incident on the reference mirror 122. The reference mirror 122 can be moved and controlled on the optical path axis to adjust the optical path length for aligning the reference optical path with the surface position of the sample, and the measurement optical path length and the reference optical path length are matched before measuring the OCT tomographic image.

Then, the reference light reflected by the reference mirror 122 passes through the optical fiber from the collimator lens 121, the optical path is changed by the optical circulator 120, passes through the polarization controller 119, enters the collimator lens 123, and enters the polarization sensitive detection arm 136. The

On the other hand, the measurement light output from the fiber coupler 102 to the sample arm 150 is input to the collimator lens 104 of the polarization dependent delay line 133 through the optical fiber via the polarization controller 103 and then passes through the polarizer 105. In this embodiment, the polarization angle of the polarizer 105 is set to 45 degrees. Furthermore, the polarization angle immediately before passing through the polarization controller 103 and entering the collimator lens 104 is also controlled to 45 degrees, and the polarization controller 103 and the polarizer 105 are adjusted and adjusted so that measurement light polarized at 45 degrees can be efficiently extracted. It is controlled.

The measurement light polarized at 45 degrees passes through the polarization beam splitter 106 in the polarization-dependent delay line 133 and is split into light in two linear polarization states (vertical direction and horizontal direction) orthogonal to each other. The divided measurement lights are reflected by different total reflection prisms 107 and 108, respectively, and propagate in two different optical paths. Here, the movement between at least one of the total reflection prisms 107 and 108 is controlled to cause a delay between two different polarization states (vertical direction and horizontal direction). In FIG. 1, the total reflection prism 107 is shown to be movable. However, as a matter of course, the total reflection prism 108 may be movable, or both the total reflection prisms 107 and 108 may be movable. Good.

Here, by setting the incident measurement light to be incident on a position deviated from the center of the polarization beam splitter 106, light of two different polarization states is generated with only one polarization beam splitter (106), Reflected by different total reflection prisms 107 and 108, measurement light having a certain delay between two different polarization states (vertical and horizontal directions) is generated, and the optical path is changed by the reflection mirror 110. Thereafter, an optical fiber is connected by a collimator lens 109.

  After the measurement light passing through the optical fiber passes through the polarization controller 111, the optical path is changed by the optical circulator 112, enters the collimator lens 113, is reflected by the galvanometer mirrors 114 and 115, is condensed by the lens 116, It enters the sample (inspection object) 117.

  The galvanometer mirrors 114 and 115 are for scanning the measurement light. By controlling the galvanometer mirrors 114 and 115, the measurement light is scanned in the horizontal direction and the vertical direction on the surface of the sample 117. Yes. Thereby, a two-dimensional tomographic image and a three-dimensional tomographic image of the sample 117 can be acquired.

  The measurement light reflected by the sample (inspection object) 117 passes through the lens 116 and the galvanometer mirrors 115 and 114, and is input to the collimator lens 113. Then, the optical path of the measurement light is changed by the optical circulator 112 through the optical fiber, and after passing through the polarization controller 118, is input to the collimator lens 125 and input to the polarization sensitive detection arm 136.

  The reference light that is input from the collimator lens 123 to the polarization sensitive detection arm 136 and is polarized by the polarizer 124 and the measurement light that is reflected by the sample (inspection object) 117 are combined using a non-polarizing beam splitter 132, Divided. The split light is then input to collimator lenses 126 and 127 and divided into two orthogonal polarization states by two in-line polarization beam splitters 128 and 129.

  Here, in order to equalize the power of the linearly polarized light in the vertical and horizontal directions of the reference light after the inline polarization beam splitters 128 and 129, the polarization angle of the polarizer 124 is adjusted to 45 degrees and the efficiency is increased. Therefore, the polarization controller 119 passing in advance is controlled so that the polarization angle immediately before entering the polarizer 124 is approximately 45 degrees.

  The interference between the two polarization states is detected by the two balanced photodetectors 130 and 131. The detected interference signals of two polarization states in the vertical direction and the horizontal direction are subjected to processing such as Fourier transform for each interference signal in the arithmetic unit 202 provided in the control device 200 shown in FIG. A tomographic image along is acquired. The acquired tomographic image is stored in the storage unit 203.

  Instead of using the EO modulator as described above, a polarization dependent delay line comprising a configuration of one polarization beam splitter and two total reflection prisms is used, so that Jones representing the polarization characteristics of the sample (inspected object) 117 is obtained. You can get a matrix.

  Next, a method for switching between the polarization-sensitive OCT and the normal OCT will be described with reference to FIG.

  FIG. 3 is a diagram showing an embodiment of a method for switching between polarization-sensitive OCT and normal OCT, and uses optical switches 301 and 302. Between the optical switch 301 and the optical switch 302, there are provided two optical paths: an optical path through the polarization-dependent delay line 133 for the polarization-sensitive OCT and an optical path through the optical fiber 303 not through the polarization-dependent delay line 133. Any one of the optical paths can be switched by the optical switches 301 and 302.

  When the measurement light passes through the polarization-dependent delay line 133 by switching the optical switches 301 and 302, the tomographic image acquisition unit 100 functions as a polarization-sensitive OCT, and thus the vertical polarization component and the horizontal polarization component of the sample 117 are used. Thus, a Joan matrix representing the polarization characteristic of the sample 117 can be obtained.

  When the measurement light does not pass through the polarization-dependent delay line 133 but passes through the optical fiber 303, the tomographic image acquisition unit 100 acts as a normal OCT, so that the measurement light enters the sample 117 without being polarized, As a result, the control unit 200 acquires normal OCT and stores it in the storage unit 203. At this time, the acquired tomographic image can be acquired in a depth measurement range twice as large as that in the polarization-sensitive OCT.

  FIG. 4 shows a method of blocking any one of the measurement light in two polarization states in the vertical direction and the horizontal direction in the polarization dependent delay line 133. As shown in FIG. 4, the shielding plate 401 can move in the rotation direction, and can select and block one of the measurement lights in two polarization states in the vertical direction and the horizontal direction by a control signal. This makes it possible to acquire a tomographic image having a measurement range twice as deep in the depth direction as compared with the case where a tomographic image of one polarization state that the examiner wants to see is acquired simultaneously as described above. It becomes possible.

  At this time, if a tomographic image is acquired by selecting a vertical polarization state, the direction of polarized light incident on the polarization beam splitter 106 using the polarization controller 103 and the polarizer 105 is set to the vertical direction (90 degrees). By controlling to, the polarized wave can be taken out efficiently. Conversely, when acquiring a tomographic image by selecting the polarization state in the horizontal direction, the polarization wave is efficiently extracted by controlling the polarization state in the horizontal direction (0 degree) using the polarization controller 103 and the polarizer 105. It goes without saying that it can be done.

  In FIG. 4, the shielding plate 401 is movable in the rotational direction, but it is naturally not limited to this, and the examiner may manually place the shielding plate, or two in the vertical and horizontal directions. A configuration in which any one of the measurement light in the polarization state is limited and shielded may be used. Of course, the shielding method is not limited to the above-described configuration, and any configuration that can shield any one of the measurement lights in the two polarization states in the vertical direction and the horizontal direction may be used.

  Further, as shown in FIG. 5, reflection mirrors 501, 502, and 503 can be arranged in the polarization dependent delay line 133. In this case, the optical switches 301 and 302 and the optical fiber 303 as shown in FIG.

  In the above embodiment, Fourier domain (optical frequency sweep) type OCT using a wavelength scanning light source is adopted, but it is needless to say that the present invention is not limited to this type. Time domain OCT, which is called the time domain method, acquires tomographic images while mechanically changing the optical path length of the reference light by moving a mirror, or the same Fourier domain method as in the present embodiment. It goes without saying that the effect of the present invention can be obtained with the same configuration even in the spectral domain OCT that detects information and acquires a tomographic image.

  The embodiments of the present invention have been described in detail above. However, these are merely examples, and the present invention is not construed as being limited by specific descriptions in the embodiments. The present invention can be carried out in a mode in which various changes, modifications, improvements, etc. are added based on the knowledge, and such a mode is within the scope of the present invention as long as it does not depart from the gist of the present invention. It should be understood that it is included in.

As described above, according to the present embodiment, the vertical polarization component and the horizontal polarization component are measured simultaneously without using a very expensive EO modulator (polarization modulator, electro-optic modulator), and the polarization characteristics of the sample are measured. It is possible to provide an optical tomographic imaging apparatus capable of obtaining a Jones matrix to be expressed and capable of performing normal OCT measurement while being polarization-sensitive OCT at low cost.

100 .. Tomographic image acquisition unit 101. Wavelength scanning light source 102. Fiber coupler 106. Polarization beam splitter 112, 120. Optical circulators 128, 129. Inline polarization beam splitter 130, 131. Balanced light detection. ... Non-polarizing beam splitter 133.. Polarization dependent delay line 201.. AD board 202.

Claims (7)

A branching means for branching into a sample arm for irradiating the object to be inspected with light from the light source and a reference arm for irradiating the reference object;
In the sample arm, the light (sample light) applied to the object to be inspected is divided into first light and second light having different polarization directions, and the divided first light and second light are divided. Sample light splitting delay means for delaying the light at different times,
An interference signal composed of a combination of the first light reflected from the inspection object of the sample arm and the light from the reference arm;
Acquisition means for acquiring a tomographic image indicating polarization information of the inspection object based on an interference signal formed by combining the second light reflected from the inspection object of the sample arm and the light from the reference arm An optical tomographic imaging apparatus comprising:
The sample light splitting delay means includes one polarization beam splitter and two total reflection prisms, and the sample light is incident at a position away from the center of the polarization beam splitter by a predetermined distance. Tomographic imaging device. Means for combining the first light reflected from the inspection object of the sample arm and the light from the reference arm, and the second light reflected from the inspection object of the sample arm and the reference 2. The optical tomographic imaging apparatus according to claim 1, wherein a cube-type or plate-type non-polarization beam splitter and an in-line polarization beam splitter are used as means for multiplexing the light from the arm. A branching means for branching into a sample arm for irradiating the object to be inspected with light from the light source and a reference arm for irradiating the reference object;
Within the sample arm, the light irradiated to the object to be inspected is divided into first light and second light having different polarization directions, and the divided first light and second light are mutually separated. Sample light split delay means for delaying at different times;
An interference signal composed of a combination of the first light reflected from the inspection object of the sample arm and the light from the reference arm;
Acquisition means for acquiring a tomographic image indicating polarization information of the inspection object based on an interference signal formed by combining the second light reflected from the inspection object of the sample arm and the light from the reference arm An optical tomographic imaging apparatus comprising:
The sample arm further includes a second optical path that does not pass through the sample light split delay means after the light from the light source is split into the sample arm by the branch means, and the first light passes through the sample light split delay means. An optical tomographic imaging apparatus comprising switching means for switching between an optical path and the second optical path. A branching means for branching into a sample arm for irradiating the object to be inspected with light from the light source and a reference arm for irradiating the reference object;
Within the sample arm, the light irradiated to the object to be inspected is divided into first light and second light having different polarization directions, and the divided first light and second light are mutually separated. Sample light split delay means for delaying at different times;
An interference signal composed of a combination of the first light reflected from the inspection object of the sample arm and the light from the reference arm;
Acquisition means for acquiring a tomographic image indicating polarization information of the inspection object based on an interference signal formed by combining the second light reflected from the inspection object of the sample arm and the light from the reference arm An optical tomographic imaging apparatus comprising:
The optical tomographic imaging apparatus according to claim 1, further comprising: a blocking means for blocking any one of the first light and the second light in the sample light splitting delay means. 5. The optical tomograph according to claim 4, wherein the blocking means in the sample light splitting delay means comprises selection means capable of selecting whether the light to be blocked is the first light or the second light. Image shooting device. An in-line type polarization controller is provided before the sample light enters the sample light splitting delay means in the sample arm branched by the branch means, and the polarization angle is controlled in the direction of 45 degrees, and in front of the polarization beam splitter. 3. The optical tomographic imaging apparatus according to claim 1, further comprising a control unit that controls the polarization angle of the arranged polarizer to 45 degrees. An inline polarization controller is provided before the sample light is incident on the sample light split delay means in the sample arm branched by the branch means,
Without being blocked by the blocking means in the sample light splitting delay means, the polarization angle of the polarization direction of any one of the first light and the second light that irradiates the inspection object, 6. A control means for controlling the polarization angle controlled by an in-line polarization controller to match the polarization angle of a polarizer disposed in front of the polarization beam splitter. An optical tomographic imaging apparatus described in 1.