JP2007298461A - Instrument for measuring polarized light light-receiving image - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To measure polarization data (birefringence distribution) of a sample 17 with a single B-scanning, for continuously modulating the polarization state of an incident beam, using an EO modulator (polarization modulator), for solving the problem, wherein an instrument for measuring conventional polarized light light-receiving needs a plurality of tomographic images to be acquired by respectively measuring the tomographic images at each different polarization state. <P>SOLUTION: The light coming out of a light source 2 is linearly polarized by a polarizer 3, and the polarized light is continuously modulated in synchronism with B-scanning by the EO modulator 4. This continuously modulated beam is used to perform B-scanning of the sample 17 (specimen) by a galvanomirror 16, while a beam wherein reference light and the matter light from a sample arm 7 are superimposed on each other is spectrally diffracted by the diffraction lattice 19 of a spectrometer 8. Of the vertical and horizontal polarized components in this spectrum, interfering component are simultaneously detected by two line CCD cameras 22 and 23, and a Jones vector, showing the polarization characteristics of the sample 17, is obtained. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to optical coherence tomography (OCT), and in particular, continuously modulates the polarization state of a linearly polarized beam simultaneously with B-scan to capture polarization information of a sample (test object), The present invention relates to a polarization-sensitive optical coherence tomography device (polarization-sensitive light-receiving image measuring device) capable of measuring a finer structure and refractive index anisotropy.

  Conventionally, OCT has been used to capture internal information of an object, that is, a differential structure of a refractive index distribution, in a nondestructive and high resolution manner.

  One of the non-destructive tomographic techniques used in the medical field or the like is an optical tomographic imaging method “optical coherence tomography” (OCT) (see Patent Document 1). Since OCT uses light as a measurement probe, it has the advantage that it can measure the refractive index distribution, spectral information, polarization information (birefringence distribution), etc. of the measured object.

  The basic OCT 43 is based on a Michelson interferometer, and its principle will be described with reference to FIG. The light emitted from the light source 44 is collimated by the collimator lens 45 and then divided into reference light and object light by the beam splitter 46. The object light is condensed on the measurement object 48 by the objective lens 47 in the object arm, scattered and reflected there, and then returns to the objective lens 47 and the beam splitter 46 again.

  On the other hand, the reference light passes through the objective lens 49 in the reference arm, is reflected by the reference mirror 50, and returns to the beam splitter 46 through the objective lens 49 again. The object light and the reference light that have returned to the beam splitter 46 in this way are incident on the condensing lens 51 together with the object light and are collected on the photodetector 52 (photodiode or the like).

  The light source 44 of the OCT uses a light source of light having low temporal coherence (light emitted from the light source at different times is extremely difficult to interfere with each other). In a Michelson interferometer using temporally low coherence light as a light source, an interference signal appears only when the distance between the reference arm and the object arm is approximately equal. As a result, when the intensity of the interference signal is measured by the photodetector 52 while changing the optical path length difference (τ) between the reference arm and the object arm, an interference signal (interferogram) for the optical path length difference is obtained.

  The shape of the interferogram shows the reflectance distribution in the depth direction of the measurement object 48, and the structure in the depth direction of the measurement object 48 can be obtained by one-dimensional axial scanning. Thus, in the OCT 43, the structure in the depth direction of the measurement object 48 can be measured by optical path length scanning.

  In addition to the scanning in the axial direction, a two-dimensional cross-sectional image of the object to be measured can be obtained by performing a two-dimensional scanning by adding a horizontal mechanical scanning. The scanning device that performs the horizontal scanning includes a configuration in which the object to be measured is directly moved, a configuration in which the objective lens is shifted while the object is fixed, and a pupil of the objective lens while the object to be measured and the objective lens are fixed. The structure etc. which rotate the angle of the galvanometer mirror in the surface vicinity are used.

  As a development of the above basic OCT, a wavelength scanning OCT (Swept Source OCT, abbreviated as “SS-OCT” for short) that scans the wavelength of a light source to obtain a spectrum interference signal, and a spectroscope are used. There is a spectral domain OCT for obtaining a spectral signal. The latter includes Fourier domain OCT (Fourier Domain OCT, abbreviated as “FD-OCT”; see Patent Document 2), and polarization-sensitive OCT (Polarization-Sensitive OCT, abbreviated as “PS”. -OCT "(see Patent Document 3).

  The wavelength scanning type OCT obtains a three-dimensional optical tomographic image by changing the wavelength of a light source with a high-speed wavelength scanning laser, rearranging interference signals using a light source scanning signal acquired in synchronization with a spectrum signal, and applying signal processing Is. As a means for changing the wavelength of the light source, a device using a monochromator can be used as the wavelength scanning OCT.

  In the Fourier domain OCT, the wavelength spectrum of the reflected light from the object to be measured is acquired with a spectrometer (spectrum spectrometer), and Fourier transform is performed on this spectrum intensity distribution, so that the real space (OCT signal space) is obtained. This Fourier domain OCT does not need to scan in the depth direction, and can measure the cross-sectional structure of the object to be measured by scanning in the x-axis direction.

  Like the Fourier domain OCT, the polarization-sensitive OCT acquires the wavelength spectrum of the reflected light from the object to be measured with a spectrum spectrometer. For example, horizontal linearly polarized light, vertical linearly polarized light, 45 ° linearly polarized light, and circularly polarized light passed through a four-wavelength plate, etc. Only the horizontally polarized component is incident on the spectrum spectrometer to cause interference, and only the component having a specific polarization state of the object light is extracted and subjected to Fourier transform. This polarization sensitive OCT also does not need to be scanned in the depth direction.

  Doppler OCT is a method for obtaining the velocity of blood flow and the like by utilizing the fact that the amount of phase change obtained by Fourier transform of spectral interference information corresponds to the moving speed of the subject as a Doppler signal. It can be summarized in the Fourier domain OCT or the like (see Non-Patent Document 1).

  The outline of the conventional OCT has been described above, but the differential structure of the refractive index distribution of the object can be grasped with nondestructive and high resolution, but the polarization dependence inherent in the object itself cannot be sufficiently grasped. .

  In particular, when considering the application of OCT to biological measurement, in the measurement of biological samples having polarization dependence due to birefringence due to differences in fibrous structure (e.g. fiber elongation direction) and tooth enamel, As the resolution decreases, problems such as inability to capture the structure arise.

In order to solve this problem, a low-coherence interferometer such as OCT has already caused the scattered light component from a specific part to interfere with the reference light in a certain polarization state, and the interference component has strong polarization characteristics. As a result, there has been proposed a polarization-sensitive light-receiving image measuring device that captures polarization information of a specific portion having a cross section in the depth direction (see Patent Document 3).
JP 2002-310897 A JP 11-325849 A JP 2004-028970 A BR White et al., Optics Express, 11:25 (2003) 3490

  However, in the conventionally proposed polarization-sensitive light-receiving image measuring apparatus, in order to acquire polarization information, it is necessary to measure tomographic images for each different polarization state and obtain a plurality of tomographic images. was there.

  An object of the present invention is to solve such a conventional problem. The polarization state of incident beam light used for measurement is continuously measured using an EO modulator (polarization modulator, electro-optic modulator). Thus, a polarization-sensitive received-light image measuring device is realized that measures the polarization information (birefringence distribution) of the sample by one B scan.

  In order to solve the above-mentioned problems, the present invention is a polarization-sensitive optical coherence tomography device including a light source, a polarizer, an EO modulator, a coupler, a reference arm, a sample arm, and a spectrometer, The beam from the light source is linearly polarized, and the EO modulator continuously modulates the polarization state of the linearly polarized beam simultaneously with a unidirectional scan perpendicular to the depth direction of the sample. The continually modulated beam is scanned in the unidirectional direction of the sample with a galvanometer mirror, the spectrometer comprises a diffraction grating and two photodetectors, the diffraction grating from the reference arm The interference light in which the reference light and the object light from the sample arm are superimposed is dispersed, and the two photodetectors are a vertical polarization component and a horizontal polarization among the spectral interference components dispersed by the diffraction grating. Provided is a polarization-sensitive optical coherence tomography device characterized by measuring components simultaneously.

  It is preferable that the light source, the polarizer, the EO modulator, and the coupler are sequentially connected by a fiber, and a reference arm, a sample arm, and a spectroscope are respectively connected to the coupler.

  The reference arm and the sample arm are each preferably provided with a polarization controller.

  It is preferable that a Jones vector representing a polarization characteristic of the sample is obtained from the vertical polarization component and the horizontal polarization component.

  According to the polarization-sensitive light-receiving image measuring device according to the present invention, as in the conventional polarization-sensitive light-receiving image measuring device, in order to acquire polarization information, a tomographic image is measured for each different polarization state, and a plurality of images are obtained. There is no need to obtain a tomographic image of an incident beam, and the polarization state of incident beam light used for measurement is continuously modulated using an EO modulator (polarization modulator, electro-optic modulator), and one B scan is performed. The polarization information (birefringence distribution) of the sample can be measured.

  The best mode for carrying out a polarization-sensitive optical coherence tomography apparatus (polarization-sensitive light-receiving image measuring apparatus) according to the present invention will be described with reference to the drawings.

(overall structure)
As described in the “Background Art” section, in the optical coherence tomography apparatus (OCT), the beam from the light source is sent separately to the reference arm and the sample arm, and in the sample arm, the depth direction (A The sample is irradiated in a direction perpendicular to (direction) (B-scan), and an AB image is obtained from the interference spectrum between the reflected light and the reference light reflected from the reference arm (performs OCT measurement). The present invention is applied to FD-OCT (Fourier domain OCT) and the like.

  The polarization-sensitive light-receiving image measuring apparatus according to the present invention converts a polarization beam (a beam linearly polarized by a polarizer) from a light source into an EO modulator (polarization modulator, electro-optic modulation) simultaneously (synchronously) with a B-scan. The polarized light beam is continuously modulated by the device, and the polarized light beam whose polarization is continuously modulated is divided, and one of the beams is scanned as an incident beam to irradiate the sample, and the reflected light (object light) is obtained. As light, OCT measurement is performed by spectral interference between the two.

  Of the spectral interference components, the vertical polarization component (H) and the horizontal polarization component (V) are simultaneously measured by two photodetectors, thereby obtaining a Jones vector representing the polarization characteristics of the sample (H image and V). Image) structure.

  FIG. 1 is a diagram showing an overall configuration of an optical system of a polarized light-sensitive received image measuring device according to the present invention. 1 includes optical elements such as a light source 2, a polarizer 3, an EO modulator 4, a fiber coupler (optical coupler) 5, a reference arm 6, a sample arm 7, a spectroscope 8, and the like. ing. The optical system of this polarization-sensitive received light image measuring apparatus 1 may have a structure of a type (free space type) in which optical elements are coupled to each other by a fiber 9 but not coupled to a fiber.

  The light source 2 uses a super luminescent diode (SLD) having a broadband spectrum. The light source 2 may be a pulse laser. The light source 2 includes a collimator lens 11, a polarizer 3 that linearly polarizes light from the light source 2, an EO modulator (polarization modulator, electro-optic modulator) 4 whose fast axis is set in a 45 ° direction, The condenser lens 13 and the fiber coupler 5 are sequentially connected.

  The EO modulator 4 fixes the fast axis in a 45 ° direction and modulates the voltage applied to the EO modulator 4 sinusoidally, so that the phase between the fast axis and the slow axis orthogonal thereto is obtained. The phase difference (retardation) is continuously changed. As a result, when light that has been emitted from the light source 2 and changed into (longitudinal) linearly polarized light by the polarizer 3 is incident on the EO modulator 4, the light is linearly changed at the above-described modulation period. Polarized light → elliptical polarized light → linearly polarized light, etc. As the EO modulator 4, a commercially available EO modulator may be used.

  A reference arm 6 and a sample arm 7 are connected to the fiber coupler 5 via a branching fiber 9. The reference arm 6 is sequentially provided with a polarization controller 10, a collimating lens 11, a polarizer 12, a condenser lens 13, and a reference mirror (fixed mirror) 14. The polarizer 12 of the reference arm 6 is used to select a direction in which the intensity of light returning from the reference arm 6 does not change even when the polarization state is modulated as described above. Adjustment of the direction of the polarizer 12 (polarization direction of linearly polarized light) is performed as a set with the polarization controller 10.

  In the sample arm 7, a polarization controller 15, a collimator lens 11, a fixed mirror 24, a galvano mirror 16, and a condenser lens 13 are sequentially provided, and an incident beam from the fiber coupler 5 is scanned by a biaxial galvano mirror 16. The sample 17 is irradiated. The reflected light from the sample 17 returns to the fiber coupler 5 again as object light, is superimposed on the reference light, and is sent to the spectrometer 8 as an interference beam.

  The spectroscope 8 includes a polarization controller 18, a collimator lens 11, a (polarization-sensitive volume phase holographic) diffraction grating 19, a Fourier transform lens 20, a polarization beam splitter 21, and two photodetectors 22 and 23 that are sequentially connected. I have. In this embodiment, a line CCD camera (one-dimensional CCD camera) is used as the photodetectors 22 and 23. The interference beam sent from the fiber coupler 5 is collimated by the collimating lens 11 and dispersed into an interference spectrum by the diffraction grating 19.

  The interference spectrum beam dispersed by the diffraction grating 19 is Fourier transformed by a Fourier transform lens 20 and divided into horizontal and vertical components by a polarization beam splitter 21 and detected by two line CCD cameras (photodetectors) 22 and 23, respectively. The Since the two line CCD cameras 22 and 23 are used to detect the phase information of both the horizontal and vertical polarization signals, the two line CCD cameras 22 and 23 do not contribute to the formation of the same spectrometer. must not.

  The light source 2, the reference arm 6, the sample arm 7, and the spectrometer 8 are provided with polarization controllers 10, 15, and 18, respectively. The initial polarization state of each beam sent to the device 8 is adjusted so that the polarization state continuously modulated by the EO modulator 4 has a constant amplitude and a constant relative polarization state in the reference light and the object light. The relationship is maintained, and the spectroscope 8 connected to the fiber coupler 5 is controlled so as to maintain a constant amplitude and a constant relative polarization state.

  Further, when the spectroscope 8 including the two line CCD cameras 22 and 23 is calibrated, the EO modulator 4 is stopped. The reference light is blocked, and the slide glass and the reflecting mirror are placed on the sample arm 7. This arrangement ensures that the positions of the peaks of the horizontal and vertical polarization components are the same. The OCT signals from the rear surface of the slide glass and the reflecting mirror are detected by the two spectrometers 8. The phase difference of the peak of the OCT signal is monitored.

  This phase difference should be zero at all optical axis depths. The signal is then windowed and inverse Fourier transformed to obtain a complex spectrum with a spectrometer 8 that includes two line CCD cameras 22,23. Since this phase difference should be zero at all frequencies, by monitoring these values, the physical positions of the two line CCD cameras 22, 23 are aligned so that the phase difference is minimized.

(principle)
The features of the present invention are as follows. The light from the light source 2 is linearly polarized, and the linearly polarized beam is continuously modulated in the polarization state by the EO modulator 4. That is, the EO modulator 4 fixes the fast axis in a 45 ° direction and modulates the voltage applied to the EO modulator 4 in a sine manner, so that the fast axis and the slow axis orthogonal to the fast axis are obtained. The phase difference (polarization angle: retardation) is continuously changed. When light that has been emitted from the light source 2 and converted into (longitudinal) linearly polarized light by the linear polarizer enters the EO modulator 4, the above modulation is performed. In this period, the light is modulated as linearly polarized light → elliptical polarized light → linearly polarized light.

  The linearly polarized polarized beam is continuously modulated in the polarization state by the EO modulator 4, and at the same time, the B scan is performed in synchronization. That is, during one B scan, the EO modulator 4 continuously modulates the polarization for a plurality of periods. Here, one period is a period during which the polarization angle (retardation) φ changes from 0 to 2π. In short, during this one cycle, the polarization of light from the polarizer is continuously modulated as linearly polarized light (vertical polarized light) → elliptical polarized light → linearly polarized light (horizontal polarized light).

  In this way, while continuously modulating the polarization of the polarized beam, the sample arm 7 scans the incident beam on the sample 17 by the galvano mirror 16 to perform the B scan, and the spectroscope 8 performs the object light as the reflected light. As for the interference spectrum of the reference light, the horizontal polarization component and the vertical polarization component are detected by the two line CCD cameras 22 and 23. Thus, two AB scan images corresponding to the horizontal polarization component and the vertical polarization component are obtained by one B scan.

  As described above, during one B-scan, continuous modulation of the polarization of the polarized beam is performed for a plurality of periods, but the two line CCD cameras 22 and 23 are continuously modulated during each period (one period). The polarization information of each of the horizontal polarization component and the vertical polarization component detected in step 1 becomes polarization information for one pixel. Polarization information is synchronized with the detection timing signal by two line CCD cameras 22 and 23 during one period of continuous modulation, and the number of detections (number of acquisitions) is four times, eight times, etc. What is necessary is just to decide suitably.

  Two-dimensional AB scan image data obtained during one B scan in this way is subjected to a one-dimensional Fourier transform in the B scan direction. Then, 0th, 1st and −1st order peaks appear. Here, when 0th-order peaks are extracted and inverse Fourier transform is performed using only the data, H0 and V0 images are obtained. Similarly, H1 and V1 images are obtained by extracting primary peaks and performing inverse Fourier transform using only the data.

  From the H0 and H1 images, J (1,1) and J (1,2) can be obtained from the Jones matrix components of the equation (see equation (18) below) which is the polarization characteristic of the sample 17. From the V0 and V1 images, J (2,1) and J (2,2) among the components of the Jones matrix of the expression (1), which is the polarization characteristic of the sample 17, can be obtained.

  In this way, information including four polarization characteristics can be obtained for one B-scan. Then, when these four pieces of information are Fourier-transformed in the A-scan direction in the same manner as in ordinary FD-OCT, the primary peak has information in the depth direction of the sample 17, and each 4 corresponds to the polarization characteristic. A sheet of AB image is obtained.

  The above principle will be further described using a complex expression. Here, XYZ is orthogonal coordinates, X and Y are parallel and perpendicular to the one-dimensional CCD cameras 22 and 23, and Z is the height. θ is the tilt of the one-dimensional CCD camera, and α is the tilt of the XZ plane.

When the horizontal polarization component of the interference light intensity of the reference arm 6 and the sample arm 7 is expressed by I _h (x, ω), this is expressed by Expression (1).

  The third term of equation (1) is inverse Fourier transformed to obtain the complex OCT signal of equation (2).

  When the direction of the fast axis of the EO modulator 4 is π / 4, the Jones matrix of the EO modulator 4 is expressed by Expression (3).

  The Jones vector from the sample 17 toward the line CCD cameras 22 and 23 is expressed by Expression (4), and the Jones matrix of one round is expressed by Expression (5).

In equation (5), (H _i , V _i ) ^T is a Jones vector before the fiber coupler 5 after exiting the EO modulator. Since the input light to the EO modulator 4 is vertically polarized light, Expression (6) is established.

Jones vector (H _sam , V _sam ) ^T is given by equation (7).

  The Jones vector of the reference light on the spectroscope 8 is as shown in Equation (8).

  The sensitivity of the line CCD cameras 22 and 23 is flat and does not depend on the phase delay of the EO modulator 4.

  From the above, the Fourier transform for x in Equation (2) is as shown in Equation (9) and Equation (10).

  Here, from the relationship between the following equations (11) and (12), the above equation (10) can be rewritten as the following equation (13).

  Furthermore, the first term and the second term of the formula (10) can be said to be as in the formula (14) and the formula (15).

  J (1,1) and J (1,2) can be calculated by equations (16) and (17).

In the same procedure, V _γ J ^* (2, 1) and V _γ J ^* (2, 2) can also be calculated from the vertical polarization components of the spectrum I _ν (x, ω). Since | H _γ | and | V _γ | are equal, and γ≡arg (H _γ ) −arg (V _γ ) includes the birefringence of the fiber, H _γ and V _γ can be eliminated. Eventually, the matrix of equation (18) is obtained.

  An embodiment of the polarization-sensitive received light image measuring apparatus 1 according to the present invention shown in FIG. 1 will be described. The light source 2 is an SLD having a central wavelength of 840 nm and a bandwidth of 50 nm, and the axial resolution is 6.2 μm. The state of the incident polarized light to the interferometer is continuously changed by making the linearly polarized light by the polarizer 3 and applying a sinusoidally changing voltage to the EO modulator 4 whose fast axis is fixed in the direction of 45 °. Modulate to The light whose polarization state is continuously modulated is put into a fiber coupler 5 that can demultiplex the incident light into an intensity ratio of 30 to 70. The fiber coupler 5 sends the split incident light to the reference arm 6 at an intensity ratio of 30 and to the sample arm 7 at an intensity ratio of 70.

  In the sample arm 7, an incident beam focused to a diameter of 1.5 mm by a collimator lens 11 having a focal length of 50 mm is scanned by a biaxial galvanometer mirror 16. The object beam and the reference beam superimposed by the fiber coupler 5 are collimated as an interference beam by the collimator lens 11 having a focal length of 120 mm in the spectroscope 8, and then by a polarization sensitive volume phase holographic diffraction grating 19 of 1200 lp / mm. Spectroscopic (dispersed).

  The split interference spectrum beam is Fourier-transformed by a Fourier transform lens 20 having a focal length of 250 mm, divided into horizontal and vertical polarization components by a polarizing beam splitter 21, and two line CCD cameras 22 and 23 having 2048 pixels and a pixel size of 14 μm. Is detected.

  The line triggers of the two line CCD cameras 22 and 23 operating at 27.7 kHz are made by a DAQ board (data recording board) and synchronized with the galvanometer mirror 16 and the EO modulator 4 of the sample arm 7. The waveform for driving the galvanometer mirror 16 is a sawtooth wave, and the EO modulator 4 is driven by a sine wave. Data from both line CCD cameras 22, 23 is collected by a frame grabber. The sensitivity of the system of the polarization-sensitive light-receiving image measuring device 1 is 101.4 dB as the sum of the horizontal and vertical channels. The intensity of the probe light is 700 μW and the phase stability is 0.17 °.

  The best mode for carrying out the present invention has been described above based on the embodiments. However, the present invention is not limited to such embodiments, and the technical matters described in the claims are not limited. It goes without saying that there are various embodiments within the scope.

  The polarization-sensitive light-receiving image measuring apparatus according to the present invention continuously modulates the polarization state of the probe light using an EO modulator (polarization modulator, electro-optic modulator) simultaneously with the B scan, thereby polarizing the sample. Since an AB image including characteristics can be easily obtained, it is optimal for medical fields that require detailed examinations such as visualization of the retina in ophthalmology and examination of enamel in dentistry.

It is a figure which shows the whole structure of the polarization sensitive light reception image measuring device which concerns on this invention. It is a figure explaining the conventional OCT.

Explanation of symbols

1 Polarized light receiving image measuring device
2 Light source
3, 12 Polarizer
4 EO modulator (polarization modulator, electro-optic modulator)
5 Fiber coupler (optical coupler)
6 Reference arm
7 Sample arm
8 Spectrometer
9 Fiber
10, 15, 18 Polarization controller
11 Collimating lens
13 Condensing lens
14 Reference mirror (fixed mirror)
16 Galvano mirror
17 samples
19 Diffraction grating
20 Fourier transform lens
21 Polarizing beam splitter
22, 23 Photodetector (Line CCD camera)
24 Fixed mirror

Claims (4)

A polarization-sensitive optical coherence tomography device comprising a light source, a polarizer, an EO modulator, a coupler, a reference arm, a sample arm, and a spectrometer,
The polarizer linearly polarizes the beam from the light source,
The EO modulator continuously modulates the polarization state of the linearly polarized beam simultaneously with a unidirectional scan perpendicular to the depth direction of the sample;
The sample arm scans the sample in one direction with the galvano mirror on the continuously modulated beam,
The spectrometer includes a diffraction grating and two photodetectors,
The diffraction grating splits interference light in which the reference light from the reference arm and the object light from the sample arm are superimposed,
The two light detectors simultaneously measure a vertical polarization component and a horizontal polarization component among spectral interference components dispersed by the diffraction grating, respectively, and a polarization-sensitive optical coherence tomography device.   2. The polarization sensitive device according to claim 1, wherein the light source, the polarizer, the EO modulator, and the coupler are sequentially connected by a fiber, and a reference arm, a sample arm, and a spectroscope are connected to the coupler. Type optical coherence tomography system.   The polarization-sensitive optical coherence tomography apparatus according to claim 2, wherein a polarization controller is provided for each of the reference arm and the sample arm.   4. The polarization-sensitive optical coherence tomography apparatus according to claim 1, wherein a Jones vector representing a polarization characteristic of the sample is obtained from the vertical polarization component and the horizontal polarization component.

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