KR20100073703A - Optical coherence tomography system and sample measurements method using the same - Google Patents

Abstract

PURPOSE: A sample measurements method using optical coherence tomography system is provided to measure the thickness and refractive index of a sample. CONSTITUTION: A sample measurements method using optical coherence tomography system comprises: a step of obtaining a plurality of tomographic images expressed a light visibility function along a longitudinal direction of a sample by using the interferometer optic(S100); a step of obtaining a cross-part view tomographic image by using tomographic images(S110); a step of setting the reference surface from cross-portal view tomographic images(S120); a step of obtaining the thickness value of the sample classified by the reference surface(S130); a step of obtaining the refractive index of the sample from the thickness value of the sample.

Description

Optical coherence tomography system and sample measurements method using the same

The present invention relates to an optical interferometer and a method for measuring a sample using the same to enable measuring the thickness and refractive index of the sample.

Conventionally, a technique for measuring thickness and refractive index of a sample using an optical interferometer acquires a graph of intensity of interference light for all pixel values of a two-dimensional charged coupled device (CCD) for detecting interference light generated from an optical interferometer, and each pixel The phase value is measured by Fourier transform of the intensity graph of, and the resultant value is obtained after applying the optimization technique to the error function.

In the conventional technique of measuring the thickness and refractive index of a sample using an optical interferometer, since the intensity graph of the interference light is obtained by using a CCD, when the sample to be measured is thick, the amount of data to be measured becomes too large and the amount of calculation becomes very large. There are disadvantages.

For example, Korean Patent Publication No. 10-0290086, "Three-dimensional thickness shape measurement and refractive index measurement method of a transparent thin film layer using a white light scanning interference method and a recording medium thereof" has a two-dimensional CCD for a thickness of about 1 [mu] m. The data value of each pixel is set to 256. Then, the average light intensity value known as zero padding is added by adding 256 "0" s.

Therefore, in the case of 1 μm, about 500 data values are required for each pixel. When the sample thickness increases, for example, in the case of a sample having a thickness of 1 mm, 500,000 data values are output from each pixel of the 2D CCD. Due to such a large amount of data, there is a problem that a significant load is applied to the calculation when the phase value is obtained by Fourier transform or when the error function is optimized.

The present invention provides an optical interferometer and a sample measuring method using the same to obtain a tomographic image of the surface or the internal structure of the sample without cutting or contacting the sample to measure the thickness and refractive index of the sample.

In the sample measuring method using an optical interferometer, a plurality of tomographic images expressed as optical visibility functions are obtained from an interference fringe image of a sample using an optical interferometer according to a depth direction of a sample, and a longitudinal tomographic image is obtained using the tomographic images. A reference plane is set from the longitudinal cross-sectional tomographic image, and a thickness value of the sample divided by the reference plane is obtained.

The refractive index of the sample can be obtained from the thickness value of the sample.

One tomographic image may be obtained by measuring a measurement value of the optical visibility function obtained from a plurality of the interference fringe images in pixel units of a photodetector, and arranging the measurement values in the order of the positions of the pixels.

The reference surface may be a sample stage made of a reflector that reflects light.

The sample may be a liquid state that can transmit light, and a holder for storing the sample may be provided on the reflector to maintain the shape of the sample in the liquid state.

The sample may be in a solid state capable of transmitting light.

The optical interferometer may be a white light interferometer or a global light interferometer.

The optical interferometer includes a reference stage including a light source, a light splitter for dividing the light guided by the light source, a reference light distributed by the light splitter, and a reference mirror for reflecting the reference light and a reference mirror driver for driving the reference mirror. And a sample stage including a sample stage made of a reflector and a sample stage driver for driving the sample stage, wherein the sample light distributed by the optical splitter travels, and a photo detector for detecting light reflected from and interfering with the reference stage and the sample stage. And a control unit for controlling the operation of the reference mirror driver and the sample stand driver and measuring the sample from the interference fringe signal detected by the photodetector.

The controller may calculate a thickness value of the sample, and calculate a refractive index of the sample according to the thickness value.

The sample may be provided in a liquid state capable of transmitting light, and a holder for storing the sample on the sample stage to maintain the shape of the sample in the liquid state.

The sample may be in a solid state that can transmit light (same as the previous page).

An optical fiber bundle may be provided between the light source and the optical splitter to guide the light emitted from the light source to the optical splitter.

An optical interferometer for measuring a sample according to the present embodiment and a method for measuring a sample using the optical interferometer obtain a surface shape image and an internal tomographic image of a sample from an optical visibility image and simultaneously use the same to measure the thickness and refractive index of the sample. It is possible to measure the thickness and refractive index of the sample, and to allow the study of the material properties of various samples without the limitation that only solid or liquid samples can be measured. Like research, it can be used medically.

Hereinafter, an optical interferometer and a sample measuring method using the same according to the present embodiment will be described.

The embodiments described below describe a method of measuring using a phase-shifting interferometry (PSI), which is a type of full-field optical coherence tomography. However, the present embodiment may not be limited to the phase shift interference method but may be implemented by white-light scanning interferometry.

1 is a diagram illustrating a full-field optical coherence tomography system according to the present embodiment.

As shown in FIG. 1, the optical interferometer includes a light source 100. The light source 100 may be a halogen illuminator that is a broadband light source. In addition, an optical fiber bundle 110 is connected to the light source 100 to guide the light of the light source 100. The first lens 120 is provided at the position where the light is emitted from the optical fiber bundle 110. After the first lens 120, a broadband beam splitter 130 for dividing light into reference light and sample light is positioned.

A reference arm 150 through which the reference light split from the broadband optical splitter 130 proceeds includes a ND filter 151, a first objective lens 152, and a reference mirror 153. Reference mirrors are placed consecutively. The reference mirror 153 may be a silver coated mirror (silver coated mirror).

A reference mirror driver 154 is provided below the reference mirror 153 to enable repetitive phase shift control of the reference light reflected from the reference mirror 153. The reference mirror driver 154 may be implemented as a piezo actuator (PZT actuator).

 On the other hand, a glass plate for compensating for dispersion mismatch by the ND filter 151 of the reference stage 150 to the sample stage 160 (sample arm) through which the sample light divided by the broadband optical splitter 130 proceeds. (161) (Glass plate), the second objective lens 162 having the same numerical aperture as the first objective lens 152 and the sample stage 163 (specimen) on which the sample is placed are continuously located .

The sample stage 163 is installed at the sample stage driver 164 which is a high performance motorized vertical stage having a high resolution of about 0.1 μm that can change the focal plane of the sample stage 163.

The second lens 171 and the photodetector 170 having a focal length of 250 mm are sequentially positioned in the projected arm on which the light returned from the reference stage 150 and the sample stage 160 is projected. In the present embodiment, the photodetector 170 may be implemented as a CCD camera having a 12-bit, 1024 × 1024 pixel array.

The control unit 140 controls the photodetector 170, the reference mirror driver 154, and the sample table driver 164 to process and store information on the interference fringe. The controller 140 includes a digital / analog converter for generating a sinusoidal signal for the reference mirror driver 154 which is synchronized with the pulse signal of the image detected by the photodetector 170.

Hereinafter, a method for measuring thickness and refractive index of a sample using the optical interferometer according to the present embodiment will be described.

2 is a flowchart of a sample measuring method using an optical interferometer according to the present embodiment.

As shown in FIG. 2, the sample measuring method using the optical interferometer according to the present embodiment includes a plurality of cross-sectional tomographic images represented as optical visibility functions from the interference fringe image of the sample using the optical interferometer along the depth direction of the sample. Obtain (S100), obtain a longitudinal cross-sectional tomographic image using the tomographic images (S110), set a reference plane from the longitudinal cross-sectional tomography image (S120), obtain a thickness value of the sample divided by the reference plane (S130), a series of equations It is made by a method for calculating the refractive index of the sample using.

Hereinafter, the sample measuring method using the optical interferometer for this embodiment in more detail.

First, a sample stand 163 made of a flat reflector that can cause reflection, such as a slide glass, is placed on the sample stand driver 164 of the sample stage 160, and a sample to be measured is placed on the sample stand 163. The sample may be any liquid transparent material, and may be various materials such as biomaterials, light transmitting solid state materials, semiconductor devices, and the like.

Thereafter, a process of obtaining an interference fringe image is performed. The process of obtaining the interference fringe image is described in detail as follows.

When light (or light power) emitted from the broadband light source 100 is transmitted to the first lens 120 through the optical fiber bundle 110, the first lens 120 transmits the light to the entire sample and the reference mirror 153. Adjust the focus position to be irradiated. The light passing through the first lens 120 is split into a reference light traveling to the reference stage 150 and a sample light traveling to the sample stage 160 in the broadband optical splitter 130, respectively.

The sample light reflected from the sample stage 163 on which the sample is seated (hereinafter, the sample light may be referred to as a signal light loaded with a specific signal for the sample) and the reference light reflected from the reference mirror 153 respectively Is then recombined in the optical splitter 130.

In this case, when the optical path difference between the sample light and the reference light is shorter than the coherence length of the light source 100, interference occurs. The interference is detected in the form of an interference fringe image by the photodetector 170.

Next, while finely transferring the reference mirror 153 of the reference stage 150 to the reference mirror driver 154, information and data on at least two or more interference images are obtained in the same manner as above.

" 의 변화로 나타난다. 수학식 1은 광검출기(170) 2 차원 픽셀 어레이 상에서 "(x, y)"에 위치하는 특정 픽셀에 가해지는 백색광 간섭 신호에 대한 수학식이다.In this process, when the reference mirror 153 attached to the reference mirror driver 154 is moved within the interference length of the light, the interference pattern of the interference fringe image is changed.

Equation 1 is an equation for a white light interference signal applied to a specific pixel located at "(x, y)" on the photodetector 170 two-dimensional pixel array.

"는 백색광 간섭신호, "

"는 간섭 신호와 관계가 없는 평균 광 강도 값, "

"는 샘플의 신호 강도에 비례하는 가시도 함수, "

"는 간섭 위상값으로 시료의 깊이에 관계한다. 그리고 "

"는 간섭 신호를 광검출기(170)를 이용하여 획득하였을 때 광검출기(170)의 픽셀 어레이 평면상에 위치한 특정 픽셀의 좌표값이다.In Equation 1

Is the white light interference signal,

"Is the average light intensity value independent of the interfering signal,"

"Is a visibility function that is proportional to the signal strength of the sample,"

"Is the interference phase value and is related to the depth of the sample."

Is the coordinate value of a particular pixel located on the pixel array plane of the photodetector 170 when the interference signal was obtained using the photodetector 170.

Therefore, after the interference signal for the first interference pattern image is obtained by Equation 1, the reference mirror 153 attached to the reference mirror driver 154 is transferred to the two or more different interference pattern images for phase change of a predetermined size. Get an interference signal for

", "

", "

,", "

" 로 명명하는 제 1, 제 2, 제 3, 제 4 간섭무늬 이미지들에 대한 간섭신호를 얻는다. 다음으로 획득된 간섭 이미지의 간섭신호들을 일련의 연산과정을 통하여 광 가시도 함수로 표현 되는 단층 이미지(xy)를 얻는다.In this embodiment, four interference fringe images are obtained. However, more or fewer interference fringe images may be obtained and used as necessary. As in this embodiment, each "

","

","

, ","

Obtain an interference signal for the first, second, third, and fourth interference fringe images named ". Next, the interference signals of the acquired interference image are expressed as a light visibility function through a series of calculations. Get the image xy.

"를 다음 수학식 2를 이용하여 얻는다.This process is performed by the optical visibility function described in Equation 1 when the interference fringe images are obtained by changing the phase by 90 degrees in the above-described steps.

"Is obtained using the following equation.

"는 광검출기(170)인 2차원 CCD 평면의 특정 위치 "

"에 위치한 한 화소에 대한 광 가시도 함수이다. 따라서 모든 화소에 대한 측정값을 화소의 위치 순서대로 배열하면 시료의 특정 깊이에 위치한 한 면의 단층 이미지를 얻을 수 있게 된다.In equation (2)

&Quot; is a specific position of the two-dimensional CCD plane which is the photodetector 170 "

"Is a light visibility function for one pixel at". "By arranging the measurements for all pixels in the order of the pixel positions, we can obtain a tomographic image of one side at a certain depth of the sample.

Next, while transporting the sample in the depth direction (z-axis) of the sample using the sample-to-driver 164, repeatedly extracting and storing the tomographic image, and these tomographic images are stacked to realize a three-dimensional image.

On the other hand, the sample may be a solid state, but may also be a liquid state. Hereinafter, an embodiment of a sample in a liquid state will be described.

3 are tomographic images according to the depth of the liquid sample obtained according to the present embodiment.

As shown in FIG. 3, the small amount of liquid polymer administered to the sample stage 163 is positioned on the surface of the sample stage 163 in the form of water droplets due to the surface tension. The height of the center of the liquid sample is higher than the periphery.

"A1" of FIG. 3 is a measured tomographic image of the liquid polymer corresponding to the highest point position from the surface of the sample stage 163 in the tomographic image of the liquid polymer sample. In the remaining drawings of FIG. 3, the tomographic images obtained by moving the sample table driver 164 from the highest point of the sample to the inside and depth directions of the sample are successively shown from left to right and top to bottom on the diagram of FIG. 3.

As shown in these drawings, since the second objective lens 162 is attached to the sample stage 160, when the sample stage 163 is transferred to the sample stage driver 164, a single layer corresponding to a cross section located at a different depth of the sample is provided. You can get a video.

On the other hand, "A2" of Figure 3 is a tomographic image obtained when the focus on the surface of the sample stage 163 while moving the sample stage driver 164 of the sample stage 160 from the highest point of the sample. Therefore, this tomographic image "A2" is used as a reference plane.

In this embodiment, the tomographic image of the sample can be obtained not only when the focal point is located above the sample stage 163 but also when the focal point is located below the sample stage 163 where the sample cannot be physically present. have. These tomographic images are shown on the right and below the lines of "A2" in FIG.

The reason for obtaining such tomographic images is due to the refractive index of the liquid polymer used as the sample. If the refractive index of the sample is large, more images can be continuously obtained after the surface image of the sample stage 163. "A3" of FIG. 3 is one of the tomographic images obtained due to the refractive index of the sample, and is a tomographic image corresponding to the lowest part of the reference plane of the droplet-shaped liquid polymer sample.

FIG. 4 is a side image of a liquid sample obtained by reconstructing 8 standard extracts from the tomographic images obtained in FIG. 3, and FIG. 5 is a reconstructed image of the 2D tomographic images obtained in FIG. 3 into a 3D image.

In the present embodiment, when the tomographic images of FIG. 3 are sequentially stacked in order to derive a refractive index value, the 3D image as shown in FIG. 4 may be generated. FIG. 4 is a view briefly illustrating a process of selecting only eight images from a total of 60 tomographic images obtained through experiments and configuring them into a 3D image, and a substantial image is illustrated in FIG. 5. Accordingly, as illustrated in FIG. 5, tomographic images of arbitrary planes in two dimensions may be obtained using data included in the three-dimensional image.

Subsequently, when a three-dimensional image of the sample is obtained, an "xz" or "yz" image is extracted from the three-dimensional image. Thereafter, a step of setting the tomographic image xy obtained from the sample stage 163 of the sample stage 160 as the reference plane from the extracted tomographic image xz or yz is performed.

6 is a tomographic image of the xz plane of a sample used to illustrate the operation of the present invention.

The distance t ₁ from the surface signal of the sample to the surface signal of the sample stage 163 corresponding to the reference plane A2 and the distance from the surface of the sample stage 163 to the signal obtained due to the refractive index of the subsequent signal. The refractive index n may be expressed by Equation 3 using (t ₂ ).

Therefore, the physical thickness of the sample on the surface of the sample stage 163 can be directly obtained by using the sample stage driver 164, which is a high performance feeder. In this embodiment, the thickness is determined by the distance t ₁ to the surface of the sample stage. Corresponding.

On the other hand, Figure 7 is a graph showing the thickness of the sample measured from the "xz" planar tomographic image with respect to the x-axis. The lines L1, L2, and L3 shown in FIG. 7 are obtained by performing a 2nd order polynomial fitting after a software thinning.

The surface signal of the measurement sample was changed from the reference line L2 corresponding to the surface of the sample stage 163 to about 18 μm to 24 μm, and the line L3 of the bottom signal of the sample generated by the refractive index of the sample was represented by the reference line ( Since it is located under L2), it is indicated by a line having a negative value.

8 is a refractive index graph of a sample obtained after applying Equation 3 using the data values shown in the graph of FIG. 7, and FIG. 9 is a shape and plane of a measurement sample formed when a drop of a liquid sample is dropped onto a slide glass plane. This is a view showing the refraction of the incident beam caused by the shape of the sample rather than.

Since the sample used consists of a uniform medium, the expected index of refraction of the sample should be a constant value for the "x" axis. However, in this embodiment, while dropping the liquid sample on the sample stage 163 has a shape as shown in Figure 9 by the surface tension. This is because the sample is a shape that can act as a lens, it is necessary to correct the effect of the lens effect of the sample to the value "t ₁ " obtained in the equation (3). Of course, some solid samples do not have this phenomenon, and thus a process of correcting the lens effect is unnecessary.

"의 굴절각으로 꺾이게 되는데 굴절된 빔이 기준면인 시료대(163)의 바닥에 도달할 때까지의 거리 " t₁ ^'"는 입사점 위치 "x"에서의 수직 시료 두께 "t₁"과 수학식 4와 같이 관계된다. Analysis to compensate for the lens effect generated by the sample is shown in FIG. 9 that the beam incident vertically at the left and right positions "x" at the center point of the sample is "

"T ₁ ^' " until the refracted beam reaches the bottom of the sample stage 163, which is the reference plane, is the vertical sample thickness "t ₁ " at the incidence point position "x" It is related as four.

"로 인하여 그 출사점의 위치가 입사점의 위치 보다"

"만큼 변하게 되고, 따라서 반사광의 광경로가 입사광 보다 광경로 의 차이 "

" 만큼 차이가 나게 된다. Therefore, the angle of refraction is reflected when the incident light is reflected from the bottom of the slide glass and comes out to the surface of the sample again.

"Due to the location of the exit point,

", So that the light path of the reflected light is different from the incident light

"As far as the difference.

"가 작아 광경로의 차이 "

"를 무시할 수 있게 된다. 따라서 시료 형상의 렌즈 효과에 의한 보정은 시료의 두께 "t₁"을 " t₁ ^'"으로 수학식 4와 같이 치환해 주는 것으로 충분하다. However, if the surface curvature of the sample is gentle and the measuring section is located at the center of the sample, the angle of refraction "

"Smaller light path differences"

"To be able to be ignored. Therefore, the correction according to the lens effect of the sample shape is the thickness of the sample", "the" t ₁ to t ₁ ^' "is sufficient that substituted as shown in Equation 4.

" 까지를 포함하는 연산을 필요로 하는데 이는 단순한 수식을 첨가하는 것으로 본 실시예의 기술적 범주에 포함된다. However, when measuring the edges of a large curvature sample, or when a more precise measurement is desired,

Operation is required to include simple formulas, which are included in the technical scope of the present embodiment.

Meanwhile, the refractive index distribution line R2 of FIG. 8 may be obtained by compensating for the lens influence due to the shape of the sample using Equation 4. Compared with the refractive index distribution line R1 before compensation, it can be seen that the refractive index shows a relatively constant value with respect to the x-axis. As expected, the measured value slightly changed at the edge of the sample. When the measured refractive index values were averaged over the measured "x-axis" range, 1.4055 ㅁ 0.0002 was obtained.

10 illustrates a holder in which a liquid sample is stored. As shown in FIG. 10, in order to remove the influence of the shape of the liquid sample in the process of obtaining the refractive index, the process of using Equation 4 may be eliminated by using the holder 180.

The optical interferometer for measuring a sample according to the present embodiment described above and a method for measuring a sample using the optical interferometer obtain a surface shape image and an internal tomographic image of the sample from the optical visibility image, and use the thickness of the sample And refractive index can be measured simultaneously.

In addition, the present embodiment allows the conventional equipment for measuring the thickness or refractive index of a sample to study the material properties of various samples, without limiting the measurement of only solid or liquid samples, and further, the characteristics of biological samples such as cells. It can also be used medically, like research.

In addition, the conventional optical microscope can position the focal length of the imaging optical system at the corresponding depth in order to obtain a tomographic image located at a certain depth, but only a specific tomographic image can be extracted because the images of the cross-sections located at different focal lengths are also superimposed. On the other hand, since the optical interferometer of the present embodiment uses only the sample end signal that caused interference with the reference end, it is possible to exclude an image generated at an out-of-focus position.

1 is a view showing a global optical interferometer system according to the present embodiment.

2 is a flowchart of a sample measuring method using an optical interferometer according to the present embodiment.

3 are tomographic images according to the depth of the liquid sample obtained according to the present embodiment.

FIG. 4 is a "xy" planar tomographic image of a liquid sample obtained by reconstructing eight samples from the tomographic images obtained in FIG. 3.

5 is a reconstructed image of the two-dimensional tomography (lateral side) images obtained in FIG. 3 into a three-dimensional image.

Figure 6 is a tomographic image of the "xz" plane of the sample used to illustrate the operation of the present invention.

 FIG. 7 is a graph showing the thickness of the sample measured from the "xz" planar tomographic image with respect to the "x" axis.

FIG. 8 is a graph of refractive index of a sample obtained after applying Equation 3 using data values shown in the graph of FIG. 7.

FIG. 9 is a view illustrating the refraction of an incident beam generated due to the shape of a measurement sample and a shape of a sample rather than a plane when a liquid sample is dropped on a slide glass plane.

10 is a view showing a holder in which a liquid sample is stored.

Claims (13)

Using an optical interferometer, a plurality of tomographic images, expressed as optical visibility functions, from an interference fringe image of a sample are obtained along the depth direction of the sample, Obtain a longitudinal cross-sectional tomographic image using the tomographic images, Set a reference plane from the longitudinal cross-sectional tomographic image, A sample measuring method using an optical interferometer, characterized in that to obtain a thickness value of the sample divided by the reference plane. The sample measuring method using an optical interferometer according to claim 1, wherein a refractive index of said sample is obtained from a thickness value of said sample. The method of claim 1, wherein the one tomography image measures a measurement value of the optical visibility function obtained from a plurality of the interference fringe images in pixel units of a photodetector, and measures the measurement values in the order of the positions of the pixels. Sample measurement method using an optical interferometer, characterized in that obtained by arranging. The sample measuring method using an optical interferometer according to claim 1, wherein the reference plane is a sample stage made of a reflector reflecting light. The sample using an optical interferometer according to claim 4, wherein the sample is in a liquid state that can transmit light, and a holder for storing the sample is provided on the sample stage to maintain the shape of the sample in the liquid state. How to measure. The sample measuring method using an optical interferometer according to claim 4, wherein the sample is in a solid state capable of transmitting light. The method of claim 1, wherein the optical interferometer is a white light scanning interferometer. The method of claim 1, wherein the optical interferometer is a global light interferometer. Light source; A light splitter that splits the light guided by the light source; A reference stage including a reference mirror driving the reference light distributed by the light distributor and reflecting the reference light and the reference mirror driver; A sample stage including a sample stage made of a reflector and a sample stage driver for driving the sample stage distributed by the light distributor; A photodetector for detecting light reflected from the reference stage and the sample stage and interfering with each other; And a control unit for controlling the operation of the reference mirror driver and the sample stand driver and measuring the sample from the interference fringe signal detected by the photodetector. The optical interferometer according to claim 9, wherein the controller calculates a thickness value of the sample and calculates a refractive index of the sample according to the thickness value. 10. The optical interferometer of claim 9, wherein the sample is in a liquid state through which light is transmitted and includes a holder on which the sample is stored on the sample stage to maintain the shape of the sample in the liquid state. 10. The optical interferometer of claim 9, wherein the sample is in a solid state through which light is transmitted. 10. The optical interferometer of claim 9, wherein an optical fiber bundle is provided between the light source and the optical splitter to guide light emitted from the light source to the optical splitter.

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