JP2004053291A - Optical wavelength variance space interference tomographic imaging apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical wavelength variance space interference tomographic imaging apparatus for freely and finely adjusting the incident angle of light waves to a wavelength variance element with a simple configuration by utilizing the image forming characteristics of a lens and the refraction characteristics of a refraction medium. <P>SOLUTION: The optical wavelength variance space interference tomographic imaging apparatus comprises a beam splitter 4 for dividing convergence light beams into signal light and reference light; a third lens 7 where reflection signal light from a specimen 5 enters through the beam splitter 4; a reflection prism 6 where the reference light enters; a refraction medium 8 that is arranged at the rear of the third lens 7 while the central position of the third lens 7 in which one portion of the reflection light from the reflection prism 6 enters the beam splitter 4 is installed at the middle position between the light axis of the signal light beam and that of the reference light beam; a wavelength variance element 9 that is arranged so that the reference light beam that is refracted by the refraction medium 8 for emission and the signal light beam cross on the surface; and an imaging apparatus 11 for imaging primary diffraction light that is diffracted by the wavelength variance element 9 for emission via an imaging lens 10. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention provides a light wavelength dispersion for irradiating an object, particularly a medium having a non-uniform structure, with a light beam and using the light reflected from the surface or inside of the object to perform optical image measurement of the object. The present invention relates to a spatial coherence tomographic imaging apparatus.
[0002]
[Prior art]
In recent years, among optical tomographic imaging techniques, attention has been paid to the low coherence in the time domain of a light source having a wide spectral width (also expressed as a short coherence length in a spatial domain). An optical coherence tomographic imaging method for detecting a reflected light wave from the inside of a non-uniform constituent material such as a living tissue with a distance resolution of the order of μm has attracted attention [for example, Naohiro Tanno, 'Optics', Vol. , 116 (1999)].
[0003]
In optical coherence tomography, for example, a Michelson interferometer divides a light beam from a low coherence light source into two using a translucent mirror. One light beam is applied to a mirror and its reflected light is used as reference light, and the other is applied to an object to be measured, and backscattered light from a deep layer of the object is used as signal light. Due to the low coherence property of the light source, the optical path length difference between the signal light and the reference light is within the coherent length of the order of μm of the light source, and the component having a phase correlation with the reference light wave, that is, only the coherent signal light component is selectively used as the reference light wave. Interfere with each other. Therefore, the reflected light profile can be detected by scanning the mirror position on the optical axis (Z-scan) and changing the reference optical path length. Further, two-dimensional tomographic imaging can be performed by scanning the incident light beam in the lateral direction.
[0004]
However, the two-dimensional tomographic image measurement method as described above requires a Z-scan and a lateral scan of a light beam, and thus has a limitation in reducing the measurement time. In order to further increase the speed of image measurement, a measurement method that spatially detects signal light in parallel seems to be effective. One such method is low coherence interferometry using an off-axis interferometer.
[0005]
In the off-axis interferometer shown in FIG. 4, when the plane wave signal light and the plane wave reference light enter the detection surface of the sensor array 113 from the left and right sides with respect to the center position O of the detection surface of the sensor array 113, respectively, The light intensity detected on the surface is calculated as follows.
[0006]
^{_{I (λ, z) = (}} 1/2) E r 2 + (1/2) E S 2 + E r E S × cos [(2π / λ) (Δl- 2zsinθ) ] ... (1)
Here, E _S and E _r is the amplitude of each signal light and reference light, lambda is the wavelength of light, .DELTA.l is the optical path length difference between the two light waves. In the case of a light source having a spectrum spread, Equation (1) may be integrated with respect to the wavelength distribution of the light source. Here, for convenience of calculation, if the wavelength distribution function of the light source is a top-hat type having a center λ ₀ and a width 2Δλ, interference The components are determined as follows.
[0007]
i (z) = E _{r S} Sinc [(πΔλ / λ ₀ ² ) (Δl-2z sin θ)] cos [(2π / λ ₀ ) (Δl-2z sin θ)] (2)
Equation (2) represents a sinc function modulated by a sine function having a period λ ₀ / sin θ, and the peak of the sinc function (Δl−2z sin θ = 0) corresponds to the optical path length difference Δl. That is, the reflected light wave from inside the subject locally interferes with the reference light wave on the detection surface, and the lateral direction of the detection surface corresponds to the depth direction of the subject.
[0008]
However, in the optical interference measurement represented by the equation (2), the interval between the interference fringes is narrow, and the sensor array 113 having a high spatial resolution is required to detect the interference fringes. In addition, detection of the envelope of interference fringes usually requires data arithmetic processing such as Fourier transform. It can be said that such a light detection and data arithmetic processing method is very complicated for application to optical tomographic image measurement. To overcome this, an optical coherence tomographic image measurement method using a chromatic dispersion imaging method has recently been reported [E. Umetsu, K .; P. Chan, N .; Tanno, "Optical Review", Vol. 9, 70 (2002)].
[0009]
FIG. 5 shows the measurement principle.
[0010]
5, a wavelength dispersion element (grating constant d) such as a diffraction grating 111 is placed on the detection surface shown in FIG. 4, and signal light and reference light enter from the left and right sides, respectively. The output angle β of the first-order diffracted light with respect to the incident angle θ is given as follows.
[0011]
sinβ + sinθ = λ / d (3)
Therefore, when setting so that θ becomes θ ₀ given by the following equation,
θ ₀ = sin ⁻¹ (λ ₀ / d) (4)
The center wavelength component is emitted at an angle β = 0, while the other wavelength components are emitted at an angle β = [(λ−λ ₀ ) / λ ₀ ] sin θ ₀ . Thus, the spatial frequency of the optical interference is down-shifted by the diffraction grating 111 so as to approach zero.
[0012]
Further, as shown in FIG. 5, when the first-order diffracted light from the diffraction grating 111 is imaged on the surface of the sensor array (CCD camera) 113 by using the lens 112, the interference component is obtained by the same analysis method as in the equation (2). Is determined as follows.
[0013]
i (z) = E _{r S} Sinc ｛πΔλ / λ ₀ ² [Δl + (2z sin θ ₀ ) / M]｝ cos [(2π / λ ₀ ) Δl] (5)
Here, M is a magnification of imaging. Equation (5) is a form in which the spatial interference signal represented by equation (2) is demodulated, which means that the envelope of the interference signal can be directly detected. Further, by detecting the peak position, the depth of the reflection point of the signal light can be specified.
[0014]
[Problems to be solved by the invention]
In the space interferometry using wavelength dispersion imaging method of the formula (5), the incident angle is varied to θ = θ _{0 +} Δθ, exit angle of the primary diffracted light having a center wavelength lambda ₀ is no longer beta = 0, It looks like this:
[0015]
sinβ = −Δθ√ [1- (λ ₀ / d) ² ] (6)
As a result, the interference light on the detection surface is again modulated by a sine function [period Λ = Mλ ₀ / (2 sin β)], and becomes as follows.
[0016]
i (z) = E _{r S} Sinc {πΔλ / λ ₀ ² [Δl + (2z sin θ / M)] {cos ｛2π / λ ₀ [Δl + (2z sin β / M)]} (7)
For example, in a measurement using an SLD (super luminescent diode) low coherence light source with λ ₀ = 800 nm and Δλ = 30 nm and a diffraction grating with d = 1.67 μm (600 lines / mm), β at θ = 28.6 ° = 0, but β = −1.8 ° with a change of Δθ = 2 °. From equation (6), it can be seen that the spatial interference signal has a plurality of interference fringes.
[0017]
As is clear from the above description, for example, in optical coherence tomographic imaging measurement using a wavelength dispersion element such as the diffraction grating 111, it is necessary to accurately adjust the incident angles θ of the reference light and the signal light to the diffraction grating 111. There is. In the conventional measuring apparatus as shown in FIG. 6, since the incident angles of the signal light and the reference light are adjusted using the mirror 107 and the mirror 110, respectively, the configuration becomes complicated and there is a difficulty in miniaturizing the apparatus. .
[0018]
6, 101 is an SLD (super luminescent diode) light source, 102, 103, 112 are lenses, 104 is a first beam splitter BS1, 105 is a second beam splitter BS2, 106 is a subject, 107, 109, 110 is a mirror, 108 is a prism, 111 is a diffraction grating, and 113 is a CCD camera (sensor array).
[0019]
In view of the above circumstances, the present invention can freely and finely adjust the incident angle of a light wave to a wavelength dispersion element with a simple configuration by utilizing the imaging characteristics of a lens and the refraction characteristics of a refraction medium. An object of the present invention is to provide an optical wavelength dispersion spatial coherence tomographic imaging apparatus.
[0020]
[Means for Solving the Problems]
The present invention, in order to achieve the above object,
[1] In a light wavelength dispersion spatial coherence tomographic imaging apparatus, a low coherence light source, a first lens that converts a light beam emitted from the light source into a parallel beam having an enlarged beam diameter, and a second lens that converges the parallel beam. Lens, a beam splitter that bisects the convergent light beam into a signal light and a reference light, a subject on which the signal light is incident, and a signal light reflected from the subject passes through the beam splitter. A third lens, the reflecting prism on which the reference light is incident, and a central position of the third lens, on which a part of the reflected light from the reflecting prism is reflected by the beam splitter and incident. It is installed at an intermediate position between the optical axis of the signal light beam and the optical axis of the reference light beam, and the reference light and the signal light that have passed through the third lens are opposite to the center position of the third lens. A refracting medium disposed behind the third lens that is incident from the optical disc, and a wavelength dispersive element that is disposed so that a reference light beam and a signal light beam that are refracted and emitted by the refracting medium intersect on a plane. And an image pickup device for forming an image of the first-order diffracted light of the reference light and the signal light diffracted by the wavelength dispersion element via an imaging lens.
[0021]
[2] The optical wavelength dispersion spatial coherence tomographic imaging apparatus according to the above [1], wherein the reflecting prism enables horizontal (X-axis) scanning.
[0022]
[3] The optical wavelength dispersion spatial coherence tomographic imaging apparatus according to [1], wherein the refraction medium is a biprism.
[0023]
[4] The optical wavelength dispersion spatial coherence tomographic imaging apparatus according to the above [1], wherein the wavelength dispersion element is a diffraction grating.
[0024]
[5] The optical wavelength dispersion spatial coherence tomographic imaging apparatus according to [1], wherein the imaging device is a CCD camera.
[0025]
[6] The optical wavelength dispersion spatial coherence tomographic imaging apparatus according to [1], wherein the subject is a medium having a non-uniform structure.
[0026]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail.
[0027]
FIG. 1 is a configuration diagram of an optical coherence tomographic image measurement apparatus according to a first embodiment of the present invention.
[0028]
In this figure, 1 is a low coherence light source, 2, 3, 7, 10 are lenses L1 to L4, 4 is a beam splitter BS, 5 is an object, 6 is a reflection prism RP, 8 is a biprism BP (refraction medium), 9 is a diffraction grating (wavelength dispersion element), and 11 is a sensor array (imaging device).
[0029]
As shown in FIG. 1, a light beam emitted from a low coherence light source 1 having a wide spectral width is converted into a parallel beam having an enlarged beam diameter by a lens L1 (2). The parallel beam is converged by the lens L2 (3) and enters, for example, a Michelson interferometer. The convergent light beam incident on the interferometer is split into signal light and reference light by the beam splitter BS (4).
[0030]
The signal light separated from the reference light enters, for example, the subject 5 made of a non-uniform constituent material, and the signal light reflected from the subject 5 passes through the beam splitter BS (4) and passes through the lens L3 (7). ). On the other hand, the reference light enters the reflection prism RP (6), and a part of the reflected light is reflected by the beam splitter BS (4) and propagates to the lens L3 (7).
[0031]
Although FIG. 1 is configured based on the principle of an off-axis optical interferometer, the present invention provides a different feature from the conventional off-axis optical interferometer by providing the following mechanism in the optical interferometer. And a spatial coherence tomographic imaging apparatus having the same.
[0032]
Here, a horizontal (X-axis direction) scanning mechanism of the reflecting prism RP (6) is provided so that the optical axis interval between the reference light and the signal light can be freely adjusted, and the center of the lens L3 (7) is provided. The position is set at an intermediate position between the optical axis of the signal light beam and the optical axis of the reference light beam. The reference light and the signal light having passed through the lens L3 (7) are located on opposite sides of the center position of the lens L3 (7), and enter a refraction medium such as a biprism BP (8). The diffraction grating (9) is arranged so that the reference light beam and the signal light beam that are refracted and emitted by the biprism BP (8) intersect on the surface of the diffraction grating (9). The first-order diffracted light of the reference light and the signal light that are diffracted and emitted is imaged by the imaging lens L4 (10) on the surface of a sensor array (11) such as a CCD (charge-coupled device) camera.
[0033]
Hereinafter, the principle of the optical tomographic image measurement according to the present invention will be described.
[0034]
FIG. 2 shows how the signal light beam and the reference light beam propagate in the off-axis interferometer shown in FIG. 1 due to the imaging property of the lens L3 (7) and the refraction property of the biprism BP (8). .
[0035]
The signal light and the reference light viewed from the lens L3 (7) are regarded as a spherical wave from a point S on the surface or inside the subject (5) and a point R inside the reflection prism RP (6), respectively. be able to. According to the present invention, the point S and the point R are symmetrical with respect to the center axis of the lens L3 (7), and the distance between the two points is 2r. Assuming that the point S and the point R are on the object plane of the lens L3 (7) as shown in FIG. 2, the signal light and the reference light transmitted through the lens L3 (7) have emission angles -α and α, respectively. . This α is obtained by the following equation.
[0036]
α = r / f (8)
Here, f is the focal length of the lens L3 (7), and from this equation (8), it can be seen that the value of α is proportional to r. Therefore, as shown in FIG. 1, α can be adjusted by scanning at least one of the reflection prism RP (6) and the lens L3 (7) in a direction perpendicular to the optical axis to change r. However, the effective value of α is limited by the numerical aperture of the lens, and it is generally difficult to approach θ ₀ given by equation (4). For example, if r = 3 mm and f = 25 mm, α = 7 ° is calculated. On the other hand, as in the above-described example, in an apparatus configuration using an SLD light source with λ ₀ = 800 nm and a diffraction grating (600 lines / mm) with d = 1.67 μm, θ ₀ = 28.6 ° is calculated. I have.
[0037]
The present invention is well aware that the use of the lens L3 (7) alone is not sufficient to satisfy the angle of incidence of the light wave on the diffraction grating (9) required in the optical coherence tomographic imaging by chromatic dispersion imaging. In addition, by disposing a refraction medium such as a biprism BP (8) behind the lens L3 (7), the incident angle of the light wave to the diffraction grating (9) is changed.
[0038]
As shown in FIG. 2, when the signal light and the reference light enter the biprism BP (8) at incident angles -α and α, respectively, they are refracted by the biprism BP (8) and exit at angles -β and β, respectively. . β is obtained by the following equation, where n is the refractive index of the biprism BP (8) and φ is the biprism angle.
[0039]
θ = sin ⁻¹ {nsin [φ + sin ⁻¹ (sin α / n)]} − φ (9)
For example, when the biprism BP (8) with n = 1.5 and φ = 30 ° is selected in the device configuration of α = 7 °, θ = 28.6 ° is calculated, and λ ₀ = 800 nm and d It can be seen that it is equal to θ ₀ that creates a measurement condition of β = 0 in the optical wavelength dispersion imaging of = 1.67 μm.
[0040]
It should be noted that the present invention is characterized in that α is finely adjusted by changing r, as shown in Expression (8), so that the angle θ given by Expression (9) can be finely adjusted. Is obvious. Therefore, it is also easy to create the measurement condition of β ≠ 0 represented by Expression (6) and generate the spatial interference fringe represented by Expression (7). Since β <θ, the period is longer than the period of the interference fringes observed in the conventional measurement represented by Expression (2), and the requirement for the spatial resolution for detecting the interference fringes is reduced. There is. The detection of the envelope of the interference fringe in which the spatial frequency has been downshifted in this manner can be performed using a conventional method such as Fourier transform.
(Example)
FIG. 3 is a view showing an embodiment in which a continuous output SLD is used as the low coherence light source in the optical measurement device of FIG. 1 according to the present invention.
[0041]
The coherence length Lc of the low coherence light source can be expressed as Lc ≒ λ ² / Δλ in inverse proportion to the wavelength spread Δλ of the light source. In the case of a commercially available near-infrared region SLD, Lc ≒ 50 μm, and in the case of a light emitting diode (LED), Lc ≒ 10 μm.
[0042]
Further, the embodiment of FIG. 3 is characterized in that the following two-dimensional optical tomographic image measurement is enabled by using a cylindrical lens for the lens L2 (23) and the lens L3 (27).
[0043]
Output light from the SLD light source 21 is converted into a parallel beam by the lens L1 (22), and is incident on the cylindrical lens L2 (23). Since the cylindrical lens L2 (23) converges the light wave in only one direction, the light beam incident on the interferometer is narrowed linearly in the horizontal direction (y-) perpendicular to the light propagation direction. The linearly narrowed incident light is split into signal light and reference light by the beam splitter BS (24), and propagates to the subject (25) and the reflecting prism RP (26), respectively.
[0044]
Part of the signal light reflected from the subject (25) passes through the beam splitter BS (24) and is collected by the cylindrical lens L3 (27). On the other hand, a part of the reference light reflected from the reflection prism RP (26) is reflected by the beam splitter BS (24) at 90 °, and then collected by the cylindrical lens L3 (27). The condensed signal light and reference light are converted into a parallel beam by the diverging property of the cylindrical lens L3 (27) in only one direction, and transmitted to the diffraction grating (29) via the biprism BP (28).
[0045]
Lens L4 of the diffraction grating 1-order diffracted light emitted from the (29) the focal length respectively _{f 1} and _{f 2,} L5 by (31, 33), for example, the detection plane in the two-dimensional sensor array such as a CCD camera (34) Is imaged. It is clear that the light interference signal detected in the z-direction of the CCD camera (34) corresponds to the depth information of the object (25) from the measuring principle according to the invention.
[0046]
On the other hand, the y-direction of the CCD camera (34) corresponds to the position in the horizontal direction (y-) of the signal light. Therefore, in this embodiment, the incident light to the subject (25) is narrowed linearly, and the interference light is detected by the two-dimensional sensor array (34), so that the depth of the subject (25) and the lateral direction are detected. Information can be obtained at the same time.
[0047]
It should be noted that the present invention is not limited to the above embodiment, and various modifications are possible based on the spirit of the present invention, and these are not excluded from the scope of the present invention.
[0048]
【The invention's effect】
As described above in detail, according to the present invention, in the optical wavelength dispersion spatial coherence tomographic imaging apparatus, the wavelength dispersion element (bi-prism) is utilized by utilizing the imaging characteristics of the lens and the refraction characteristics of the refraction medium (biprism). The angle of incidence of the light wave on the diffraction grating can be freely and finely adjusted with a simple configuration.
[0049]
This realizes a spatial coherence tomographic imaging apparatus that does not need to have a spatial frequency of zero. The incident angle can be finely adjusted by scanning the reflection prism in the horizontal direction.
[Brief description of the drawings]
FIG. 1 is a configuration diagram of an optical coherence tomographic image measurement apparatus according to a first embodiment of the present invention.
FIG. 2 is a diagram showing a propagation state of a signal light beam and a reference light beam due to an imaging property of a lens L3 and a refraction property of a biprism BP in the off-axis interferometer shown in FIG.
FIG. 3 is a diagram showing an embodiment in which a continuous output SLD is used as a low coherence light source in the optical measurement device of FIG. 1 according to the present invention.
FIG. 4 is a diagram showing a state in which a signal light wave and a reference light wave are incident on a sensor array detection surface in a conventional off-axis interferometer.
FIG. 5 is a diagram illustrating a state in which a signal light wave and a reference light wave are incident on a diffraction grating in an off-axis interferometer using a wavelength dispersion method.
FIG. 6 is an explanatory diagram of a measurement principle of a conventional optical coherence tomographic image measurement device using a chromatic dispersion method.
[Explanation of symbols]
1 Low coherence light source 2, 3, 7, 10 Lens L1 to L4
4,24 beam splitter BS
5,25 Subject 6,26 Reflective prism RP
8,28 Refractive medium (biprism BP)
9,29 wavelength dispersion element (diffraction grating)
11 Imaging device (sensor array)
21 SLD light source 22 Optical lens L1
23 cylindrical lens L2
27 cylindrical lens L3
31, 33 Lens L4, L5
32 aperture 34 imaging device (two-dimensional sensor array like CCD camera)

Claims (6)

(A) a low coherence light source;
(B) a first lens that converts a light beam emitted from the light source into a parallel beam having an enlarged beam diameter;
(C) a second lens that converges the parallel beam;
(D) a beam splitter for bisecting the convergent light beam into signal light and reference light;
(E) a subject on which the signal light is incident;
(F) a third lens into which signal light reflected from the subject enters via the beam splitter;
(G) a reflecting prism on which the reference light is incident;
(H) the center position of the third lens at which a part of the light reflected from the reflection prism is reflected by the beam splitter and enters the beam splitter, and the intermediate position between the optical axis of the signal light beam and the optical axis of the reference light beam. A refraction medium disposed behind the third lens, where the reference light and the signal light passing through the third lens are respectively incident from opposite directions with respect to the center position of the third lens;
(I) a wavelength dispersion element arranged so that a reference light beam and a signal light beam refracted and emitted by the refraction medium intersect on a plane;
(J) an image pickup device for forming an image of a first-order diffracted light of the reference light and the signal light diffracted by the wavelength dispersion element through an imaging lens, and an optical wavelength dispersion spatial coherence tomographic image. Device. 2. The optical wavelength dispersion spatial coherence tomographic imaging apparatus according to claim 1, wherein the reflection prism enables horizontal (X-axis) scanning. The optical wavelength dispersion spatial coherence tomographic imaging apparatus according to claim 1, wherein the refraction medium is a biprism. 2. The optical wavelength dispersion spatial coherence tomographic imaging apparatus according to claim 1, wherein said wavelength dispersion element is a diffraction grating. 2. The optical wavelength dispersion spatial coherence tomographic imaging apparatus according to claim 1, wherein the imaging device is a CCD camera. 2. The optical wavelength dispersion spatial coherence tomographic imaging apparatus according to claim 1, wherein the subject is a medium having a non-uniform structure.

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