JP2021184033A - Optical modulator and phase measuring device - Google Patents

Abstract

To provide an optical modulator capable of realizing a function that has been conventionally achieved by a combination of plural optical elements by a single element.SOLUTION: In an optical modulator, 16 areas (4×4) are set to be a repletion unit, four phase shifters are allocated to each area for each of four kinds of phase shifters in which rotational angles differ by 45 degrees, and the repetition unit is disposed periodically in an in-plane direction. A phase measuring device is composed of the optical modulator, a linear polarizer, and an imaging element.SELECTED DRAWING: Figure 1

Description

The present invention relates to a light modulation element and a phase measuring device, and more particularly to an optical element that spatially modulates a coherent or incoherent electromagnetic wave (particularly light), and a phase measuring device such as an interferometer.

If an electromagnetic wave that is reflected, diffracted, or transmitted by an object and propagated, especially the phase distribution of light, can be detected, it will be possible to measure the three-dimensional shape, three-dimensional position, refractive index distribution, etc. of the object. .. For this reason, the development of practical phase measurement technology is required in each of the industrial, medical, and video fields. While various phase measurement technologies have been proposed, interference measurement technology or digital holography technology that utilizes the phenomenon of interference / diffraction, which is the nature of waves, is steadily practical because it enables highly accurate phase measurement. It is becoming more and more popular and is being introduced in various fields.

In interference measurement or digital holography, the interference fringes between the object light, which is the electromagnetic wave (light) propagating from the object, and the reference light are photographed by the image pickup element, and the calculation is applied to the interference fringes using a computer. Can measure the phase distribution. Further, by applying an operation based on propagation to this phase distribution, it is possible to reconstruct the three-dimensional information and the refractive index distribution of the object. Generally, in order to measure the phase distribution with high accuracy, the phase of the object light or the reference light is shifted, and interference fringes in which the light and dark positions of three or four fringes are different are photographed, and a plurality of these fringes are photographed. It is necessary to perform an operation based on the phase shift algorithm for the interference fringes of. Many interference systems have been proposed in which the phase is sequentially shifted to photograph interference fringes, but it is difficult to photograph dynamic phenomena / objects by this method.

In order to solve this problem, a single-shot phase shift method has been proposed that can simultaneously capture a plurality of interference fringes required for phase measurement. Patent Documents 1 and 2 and Non-Patent Document 1 propose a single-shot phase shift method using a polarizing element array. For example, as described in Patent Documents 3 and 4, the polarizing element array has a pixel structure corresponding to the pixel size of the image pickup device, and the transmission axis is 0 degrees for each adjacent pixel. Four types of polarizing elements having different degrees of 45 degrees, 90 degrees, and 135 degrees are arranged. This polarizing element array is mainly used for measuring the polarization state, but it can also be applied to phase measurement as described below. By setting the polarization states of the object light and the reference light to circularly polarized light orthogonal to each other and causing them to enter the polarizing element array, four types of interference fringes having different phase shift amounts corresponding to the transmission axes of the substituents can be obtained. By applying the demosaicing process to these interference fringes, four interference fringes having different phase shift amounts can be obtained. Patent Document 1 and Non-Patent Document 1 describe an interference system when coherent light is used as a light source, and Patent Document 2 describes an interference system when coherent light is used as a light source.

For example, Non-Patent Document 1 proposes a single-shot phase shift method using a single checkered diffraction grating, a four-region split phase shifter, and a linear polarizing element. By combining three specially designed optical elements, light using coherent light as a light source is divided into four directions, and it is possible to form four interference fringes having different phase shift amounts.

Further, Patent Document 5 proposes a single-shot phase shift method using two checkered diffraction gratings. In this technique, by appropriately shifting the two checkered diffraction gratings in the in-plane direction, it is possible to obtain four interference fringes to which four different phase shift amounts are given. Therefore, as in Patent Documents 1 and 2, it is possible to obtain a plurality of interference fringes in one shooting.

Japanese Patent No. 4294526 Japanese Patent No. 6245551 International Publication No. 2004/008196 Japanese Unexamined Patent Publication No. 2009-156712 Japanese Unexamined Patent Publication No. 2019-144520

N. Brock, J. Hayes, B. Kimbrough, J. Millerd, M. North-Morris, M. Novak, JC Wyant, “Dynamic interferometry,” Novel Optical System Design and Optimization VIII, vol. 5875, pp. 101- 110, (2005) B. Wang, F. Dong, Q. Li, D. Yang, C. Sun, J. Chen, Z. Song, L. Xu, W. Chu, Y. Xiao, Q. Gong, and Y. Li, “ Visible-frequency dielectric metasurfaces for multiwavelength achromatic and highly dispersive holograms, ”Nano. Lett., vol. 16, pp. 5235-5240, (2016)

In the prior art using the polarizing element arrays of Patent Documents 1 and 2, the phase shift method can be applied by considering that the amount of change in the wavefront of light from the object to be measured is sufficiently small between adjacent pixels. It is possible to obtain the phase information necessary for reconstructing the stereoscopic information in one shooting. However, in this method, since the amount of change in the wavefront of light between adjacent pixels needs to be small, the resolution of the interference fringes to be photographed is lower than the resolution of the image sensor, and as a result, the resolution of the reconstructed image. Decreases. For example, when four types of phase shift amounts of 0, π / 2, π, and 3 / 2π [rad] are given, the resolution of the interference fringes is about 1/4 times the resolution of the image pickup element (1/2 in the vertical direction). × It becomes 1/2) in the horizontal direction. In addition, since it is necessary for each pixel of the image sensor to play the role of a linear polarizing element, it is not only difficult to achieve both finer pixel pitch and improvement of light utilization efficiency, but also applicable image sensor specifications. There are restrictions on.

Further, in the technique of Non-Patent Document 1, a method using three elements of a checkered diffraction grating, a four-region split phase shifter, and a linear polarizing element has been proposed, but in order to obtain four interference fringes. At least three elements are required, which causes the optical system to become large. Further, as the number of elements increases, when light is incident on each element, light loss and absorption due to Fresnel reflection always occur, and the light utilization efficiency is lowered. Further, complicated aberrations are generated in each element according to the incident angle of light, which lowers the phase measurement accuracy.

Further, in the conventional technique using the two checkered diffraction gratings of Patent Document 5, a highly accurate alignment technique is required for arranging the two elements, and the difficulty of constructing the optical system is extremely high. High. Furthermore, since it is necessary to use two diffractive optical elements, it is basically necessary to use a two-optical path interferometer, which is easily affected by air fluctuations and vibrations during interference fringe photography, and the detection accuracy of phase information is high. There is a problem to be reduced. Further, the technique of Patent Document 5 can be used only when incoherent light is used.

Therefore, an object of the present invention made in view of the above-mentioned problems is to provide a light modulation element capable of realizing a function conventionally realized by a combination of a plurality of optical elements with a single element. It is in. Further, by reducing the number of optical elements, it is possible to suppress the generation of light reflection / absorption / aberration and the like, and it is an object of the present invention to provide a highly accurate and compact phase measuring device. Here, the phase measuring device is an arbitrary device that detects and measures the phase, including an interferometer and a hologram (interference fringe) photographing device.

In order to solve the above problems, the light modulation element according to the present invention has 16 regions of 4 × 4 as repeating units, and four types of phase shifters having different rotation angles of 45 degrees are assigned to each region. The repeating unit is periodically arranged in the in-plane direction.

Further, in the light modulation element, when the four types of phase shifters are A, B, C, and D, the first row of the 4 × 4 region is A, B, C, D, and the second row is B. It is desirable to assign the phase shifters so that the third column is C, D, A, B and the fourth column is D, C, B, A.

Further, in the light modulation element, the ratio of the lengths of the regions in one direction in the in-plane direction of 4 × 4 is 1: 1: 1: 1, and the ratio of the lengths of the regions in the direction orthogonal to the direction is 1: 1: 1: 1. It is desirable that it is 1: 3: 1: 3.

Further, it is desirable that the light modulation element is configured such that the phase shifter is formed by periodically arranging an anisotropically shaped structure made of a plasmonic metal or a dielectric as a material.

Further, in the light modulation element, the plasmonic metal is one of gold, silver, and aluminum, and the dielectric is one of silicon, SiO ₂ , TiO ₂ , and amorphous silicon. Is desirable.

In order to solve the above problems, the phase measuring device according to the present invention captures an image of the light modulation element, a linear polarizing element through which the light modulated by the light modulation element is transmitted, and light transmitted through the linear polarizing element. It is characterized by being provided with an image pickup element for taking an image.

Further, it is desirable that the phase measuring device causes the object light and the reference light due to the coherent light to be incident on the light modulation element.

Further, the phase measuring device divides the incoherent light into the first divided light and the second divided light, imparts different phase distributions to the first divided light and the second divided light, and gives the first divided light different phases to each other. It is desirable that the light and the second divided light be incident on the light modulation element.

Further, it is desirable that the phase measuring device extracts four interference fringes from the image data captured by the image pickup element and obtains a complex amplitude distribution from the four interference fringes by a phase shift method.

According to the light modulation element of the present invention, a function conventionally realized by a combination of a plurality of optical elements can be realized by a single element. Further, according to the phase measuring apparatus of the present invention, it is possible to suppress the generation of light reflection / absorption / aberration and the like, and it is possible to realize highly accurate phase detection and miniaturization.

It is a figure which shows an example of the light modulation element of this invention. It is a conceptual diagram of the phase measuring apparatus of this invention. It is a figure which shows the example of the structure for realizing the phase shift of a light modulation element. It is an example of a phase shifter of each region when the structure of FIG. 3A is used. It is a conceptual diagram of the repeating unit of a light modulation element. It is an example of a phase shifter of each region when the structure of FIG. 3B is used. It is another example of the phase shifter of each region in the case of using the structure of FIG. 3 (b). This is an example of an optical system of a phase measuring device using a coherent light and a transmissive light modulation element. This is another example of an optical system of a phase measuring device using a coherent light and a transmissive light modulation element. This is an example of an optical system of a phase measuring device using a coherent light and a reflection type light modulation element. This is an example of an optical system of a phase measuring device using incoherent light and a transmissive light modulation element. It is a figure which shows the example of the parameter of the structure of a rectangular parallelepiped. It is a figure which shows the object to be measured. It is an intensity image taken by an image sensor. The obtained amplitude distribution and phase distribution. This is the result of reconstruction of the target object.

Hereinafter, embodiments of the present invention will be described.

FIG. 1 is a diagram showing an example of the light modulation element of the present invention. The light modulation element 1 has a phase shifter array structure in which phase shifters are periodically arranged. The right side of FIG. 1 is an overall view of the light modulation element, and the left side is an enlarged view of a part thereof, showing a region of 16 phase shifters of 4 × 4, which is a repeating unit.

The light modulation element 1 is composed of four types of phase shifters having different polarization conversion characteristics, and when the four types of phase shifters are A, B, C, and D, one row in a 4 × 4 region. Moved so that the eyes are A, B, C, D, the second column is B, A, D, C, the third column is C, D, A, B, and the fourth column is D, C, B, A. Assign Aiko. The size of one side of each phase shifter region depends on the wavelength of the electromagnetic wave used, but is about several hundred nm to several tens of μm.

The polarization conversion characteristics of each phase shifter are 4 of the Jones matrix of equations (2) to (4) in which the Jones matrix is shifted by 45 degrees with respect to the phase shifter represented by the following equation (1). It is desirable to configure it so that it has the characteristics of the type. In addition, the equation (1) is obtained by multiplying the phase shift plate of phase Δ by the rotation matrix of φ before and after.

Here, α is a complex number constant determined by the thickness, refractive index, absorption coefficient, etc. of the element, i is an imaginary number, φ is an arbitrary rotation angle as a reference, and Δ is a phase shift amount at the wavelength λ. For example, the characteristics of A, B, C, and D in FIG. 1 correspond to J _{φ + 0} , J _{φ + 45} , J _{φ + 90} , and J _{φ + 135} , respectively. Since it is sufficient to assign phase shifters whose rotation angles are deviated by 45 degrees in the order of A → B → C → D, A, B, C, and D are assigned to J _{φ + 135} and J _{φ + 90, respectively.} , J _{φ + 45} , J _{φ + 0} , J _{φ + 90} , J _{φ + 135} , J _{φ + 180} (= J _{φ + 0} ), J _{φ + 135} (= J _{φ + 45} ). ..

The size of each region in FIG. 1 in the x-direction and the y-direction can be arbitrarily set, but in the present embodiment, the size in the x-direction is fixed by Lx, and the size in the y-direction is Ly, Repeat 3 Ly. The x-direction and the y-direction may be transposed. That is, the ratio of the lengths of the regions in one direction in the in-plane direction of 4 × 4 is 1: 1: 1: 1, and the ratio of the lengths of the regions orthogonal to this is 1: 3: 1: 3. Is. In this setting, two types of checkered phase shift plates (optical elements in which the regions that give phase changes are arranged in a checkered pattern) are shifted by 1/2 of 1 square in the x direction and 1/4 of 1 square in the y direction. It is equivalent to a stacked structure.

With the above settings, the light modulation element 1 evenly divides the two incident orthogonal polarized lights in four directions, and further, the two polarized lights have phase shifts opposite to each other (for example, + 45 ° and −45 ° C.). By giving °), it is possible to give four types of phase shift amounts shifted by π / 2 to the split light in four directions. Even if the setting deviates from the setting of the present embodiment, it is possible to divide into four types of phase shift amounts, but the phase shift amount deviates from π / 2 or the division direction is different. Since it may not be even, the set value of this embodiment is desirable in order to efficiently perform the subsequent analysis.

Materials that realize phase shifters include metals often used in the field of plasmonics such as gold, silver, and aluminum (called plasmonic metals), or _{dielectrics such as silicon, SiO 2} , TiO ₂ , and amorphous silicon. Alternatively, an organic material such as a liquid crystal molecule is used. When a metal material is used, the reflection type light modulation element 1 can be manufactured. Further, when a dielectric or an organic material is used, the transmissive light modulation element 1 can be manufactured.

Specifically, each substance has no symmetry between the x-direction and the y-direction, such as an ellipse, a rectangle, and a V-shape, that is, it has an anisotropic shape (not a circle, a square, or an octagon). The structure is formed by a lithography technique such as nanoimprint and electron beam lithography, and a phase shifter is produced as a structural birefringence or a metasurface. In the case of an organic polymer material such as a liquid crystal display, the film pressure is appropriately adjusted by aligning the orientation direction, and the object is manufactured as a polarizing diffraction optical element. The relationship between the polarization conversion characteristics of the structure and the phase shifter is known as described in, for example, Non-Patent Document 2.

From the above, each phase shifter _{of J φ + 0} , J _{φ + 45} , J _{φ + 90} , and J _{φ + 135 can be realized.} From the viewpoint of light utilization efficiency, it is desirable to select materials and design the structure so that Δ = π + 2nπ (n is an integer).

The light modulation element 1 in which the above-mentioned four types of phase shifters are periodically arranged provides a function of dividing two polarized lights in four directions with four types of phase shift amounts, which was conventionally used by a plurality of optical elements. It can be realized with one element.

FIG. 2 is a conceptual diagram of the phase measuring device of the present invention using the light modulation element 1 of FIG. The linear polarizing element 2 is arranged behind the light modulation element 1 in which the phase shifters are periodically arranged (on the rear side with respect to the traveling direction of light). These elements may or may not be in close contact with each other. It is also possible to bond the light modulation element 1 and the linear polarizing element 2 and handle them as an integrated element. From the viewpoint of high light efficiency, it is desirable to use an element that is adhered and adhered. The linear polarizing element 2 transmits the light modulated by the light modulation element 1. The phase measuring device further includes an image pickup device 3 that captures an image (phase difference image) of light transmitted through the linear polarizing element 2. In addition, the arithmetic unit 4 that analyzes the received image data may be further provided.

When a coherent light source is used, the object light 5 to be measured and the reference light 6 are incident on the light modulation element 1. It is desirable that the polarization states of the object light 5 and the reference light 6 are orthogonal to each other. After passing through the above-mentioned optical elements 1 and 2, the object light 5 and the reference light 6 are divided and propagated in each of the four directions, and the phase shift amount is 0, π / 2, π, on the image pickup surface of the image pickup element 3. Four interference fringes of 3π / 2 [rad] are formed (note that in FIG. 2, the light is divided in two directions in the vertical direction, but in reality, it is also divided in the horizontal direction and four. An image is produced. See FIG. 14). This is taken as one image by the image pickup device 3, and the interference fringes are input to the arithmetic unit 4. The phase distribution can be measured by extracting four interference fringes from one captured image and applying an operation based on the algorithm of the four-step phase shift method. If you want to reconstruct the 3D information of the object to be photographed, you can apply the propagation calculation.

When an incoherent light source is used, if the above-mentioned object light 5 and reference light 6 are replaced with the first divided light and the second divided light used for self-interference, respectively, the three-dimensional information of the object is reconstructed. The phase distribution required for this can be detected by the same procedure as described above.

Unlike the prior arts of Patent Documents 1 and 2, the phase measuring apparatus of the present invention can acquire interference fringes with the resolution of an image pickup element and can reconstruct a stereoscopic image with a relatively high resolution. In addition, since a polarizing element array is not required, an image sensor that meets desired specifications can be freely set according to the purpose and shooting target, such as spatial resolution priority, frame rate priority, and gradation number priority. It can be easily selected and changed.

Further, unlike the prior arts of Patent Document 5 and Non-Patent Document 1, the phase measuring device of the present invention is an optical system using one light modulation element 1 and one polarizing element 2 in which phase shifters are periodically arranged. It is possible to improve the light utilization efficiency by suppressing the occurrence of aberrations and the suppression of reflection / absorption in the optical element, as well as being effective for miniaturization.

From the above, the present invention can provide a practical phase measuring device by the light modulation element 1 in which the phase shifters are periodically arranged.

Next, an element structure for carrying out the light modulation element 1 in which the phase shifters of the present invention are periodically arranged will be described.

FIG. 3 is an example of a structure for realizing a phase shifter of the light modulation element 1. Each structure is a fine structure formed in a size smaller than the wavelength of light. There are various structures that realize the Jones matrix of Eq. (1), such as those having periodic grooves as shown in FIG. 3 (a) and rectangular structures as shown in FIG. 3 (b). Some are oval as shown in FIG. 3 (c), some are L-shaped or V-shaped as shown in FIG. 3 (d), and some are composed of two rectangles as shown in FIG. 3 (e). .. The specific size of the structure varies depending on the wavelength used, the material, and the processing process. As the material, a plasmonic metal such as gold, silver, or aluminum, _{a dielectric such as silicon, SiO 2} , TiO ₂ , or amorphous silicon, or an organic polymer material such as a liquid crystal molecule is used.

It is desirable to be able to process each structure as designed from the viewpoint of light utilization efficiency, but due to the problem of processing accuracy, the reproducibility of the corners of the structure may be poor and the aspect ratio may deviate from the design value. .. However, the error in the shape of the structure does not impair the function of the light modulation element in which the phase shifters of the present invention are periodically arranged, and if an anisotropic shape can be processed in the in-plane direction, phase detection is performed. It can be applied to technology.

FIG. 4 shows an example of a phase shifter in each region of A, B, C, and D in FIG. 1 when the structure of FIG. 3A is used. It can be seen that the structure rotates at intervals of 45 degrees in each region. With this structure, a phase shifter whose rotation angle is deviated by 45 degrees is realized in the order of A → B → C → D.

Further, FIG. 5 is a conceptual diagram of a 4 × 4 region (repeating unit) when the light modulation element 1 of FIG. 1 is configured by using the phase shifters A, B, C, and D of FIG. be.

FIG. 6 shows an example of a phase shifter in each region of A, B, C, and D in FIG. 1 when the structure of FIG. 3 (b) is used. The structures are arranged in a square grid as shown in FIG. 6, and the structures are rotated by 45 degrees between each region to form A, B, C, and D phase shifters.

Further, FIG. 7 shows another example of the phase shifter in each region of A, B, C, and D when the structure of FIG. 3 (b) is used. The structures may be arranged in a hexagonal grid as shown in FIG. 7, and the phases A, B, C, and D may be configured by rotating the structure by 45 degrees between each region.

Even when the structures of FIGS. 3 (c), 3 (d), and 3 (e) are used, they can be used as four types of phase shifters by arranging them as shown in FIGS. 6 or 7. can.

An example of a configuration of an optical system of the phase measuring device of the present invention using the light modulation element 1 will be described. 8 and 9 show an example of an optical system of a phase measuring device using coherent light and a transmissive light modulation element. In addition to the light modulation element 1, the linear polarizing element (hereinafter, simply referred to as “polarizer”) 2, and the image pickup element 3, the optical system includes a laser light source 7, a spatial filter 8, a lens 9, and a polarizing beam splitter 10, 4. It includes a 1/1 wave plate 11 and a mirror 13.

In the optical system of FIG. 8, the polarization state of the laser light output from the laser light source 7 is linearly modulated at 45 ° by the polarizing element 2, and this is converted into a plane wave by the spatial filter 8 and the lens 9. The plane wave is split into two by the polarizing beam splitter 10. This results in linearly polarized light in which the polarization states of the object light and the reference light are orthogonal to each other. The object light is reflected by the polarizing beam splitter 10, passes through the quarter wave plate 11, and travels toward the object to be imaged 12. In FIG. 8, the object light is reflected by the object 12 to be photographed, obtains the phase information of the object, passes through the quarter wave plate 11 again, and is incident on the polarizing beam splitter 10. On the other hand, the reference light passes through the polarizing beam splitter 10, passes through the quarter wave plate 11, is reflected by the mirror 13, passes through the quarter wave plate 11 again, and enters the polarizing beam splitter 10. do. The object light from the object 12 is transmitted through the polarizing beam splitter 10, the reference light from the mirror 13 is reflected by the polarizing beam splitter 10, and both are combined to form a light modulation element (an element in which phase shifters are periodically arranged). ) Proceed to 1. The object light and the reference light are divided into four directions by the light modulation element 1, and are formed into a set of light waves having different phase differences in each direction. Further, the polarization directions are aligned by the polarizing element 2, and the image pickup device 3 is used. Four interference fringes are generated on the image pickup surface.

In the example of FIG. 8, the light wave passes through the quarter wave plate 11 twice, and as a result, the light wave functions as the half wave plate, and the linearly polarized light of the object light and the reference light is 90. ° Rotate. As a result, unnecessary transmission and reflection of light are reduced when the light is combined by the polarizing beam splitter 10, and the loss of light energy is reduced.

In the optical system of FIG. 9, the polarization state of the laser light output from the laser light source 7 is linearly modulated at 45 ° by the polarizing element 2, and this is converted into a plane wave by the spatial filter 8 and the lens 9. The plane wave is split into two by the first polarization beam splitter 10. This results in linearly polarized light in which the polarization states of the object light and the reference light are orthogonal to each other. The object light is reflected by the first polarization beam splitter 10, reflected by the mirror 13, passes through the object to be imaged 12, obtains the phase information of the object, and is incident on the second polarization beam splitter 10. On the other hand, the reference light passes through the first polarizing beam splitter 10, is reflected by the mirror 13, and is incident on the second polarizing beam splitter 10. The object light from the object 12 is reflected by the second polarization beam splitter 10, the reference light from the mirror 13 is transmitted through the second polarization beam splitter 10, and both are combined and advanced to the light modulation element 1. Finally, by modulating the object light and the reference light with the light modulation element (element in which the phase shifters are periodically arranged) 1 and the polarizing element 2, the object light and the reference light in four directions having different phase differences, respectively. Is generated, and four interference fringes are formed on the image pickup surface of the image pickup element 3. By analyzing these interference fringes, the phase distribution can be measured.

FIG. 10 shows an example of an optical system of a phase measuring device using coherent light and using a reflection type light modulation element 1. Compared with the optical system of FIG. 9, a beam splitter 15 (not a polarization beam splitter 10 but a polarization-independent one) is further arranged, and the light modulation element 1 is configured as a reflection type.

The laser light output from the laser light source 7 passes through the splitter 2, the spatial filter 8, and the lens 9, is split by the first polarization beam splitter 10, passes through the mirror 13, and passes through the object 12 to be imaged. It is the same as FIG. 9 until the object light and the reference light are incident on the second polarization beam splitter 10. The object light and the reference light combined with the second polarization beam splitter 10 pass through the polarization-independent beam splitter 15 and are incident on the reflection type light modulation element 1 in which the phase shifters are periodically arranged. ,reflect. At this time, the object light and the reference light are divided into four directions by the light modulation element 1 and reflected as a set of light waves having different phase differences in each direction, and the direction of the image sensor 3 is reflected by the beam splitter 15. Further, the polarization directions are aligned by the splitter 2, and four interference fringes are generated on the image pickup surface of the image pickup element 3. In this way, even when the light is modulated by the reflection type light modulation element 1, the interference fringes can be detected by the image pickup device 3 by the reflected light wave.

FIG. 11 is an example of an optical system of a phase measuring device using incoherent light and using a transmissive light modulation element 1. In this optical system, unlike the examples of FIGS. 8 and 9, there is no distinction between object light and reference light. The optical system includes a lens 9, a bandpass filter 16, a polarizing element 2, a polarizing diffraction optical element 17, a light modulation element 1, a polarizing element 2, and an image pickup element 3. Incoherent light such as reflected light, transmitted light, or diffracted light from the object 12 to be photographed passes through the lens 9 and the bandpass filter 16. The bandpass filter 16 transmits only light in a specific band of incoherent light waves to improve temporal coherence. The wavelength width of the transmitted light is, for example, 1 nm to 100 nm, and it is desirable that the transmitted wavelength width is narrow. The light transmitted through the bandpass filter 16 is linearly polarized by the polarizing element 2 and is incident on the polarization diffractive optical element 17.

The polarized diffraction optical element 17 is an optical element that divides incident linearly polarized light into first-divided light and second-divided light, and is called a “polarized direct flat lens” or a “polarized light diffractive lens” in the present embodiment. To use. Polarization diffraction optical element 17 is a lens the focal length varies depending on the polarization state of the incident light, by incident linearly polarized light, the focal length polarization state in circularly polarized light converging spherical wave of f _d, A divergent spherical wave having a _{focal length of −f d} can be simultaneously generated by circularly polarized light in the opposite direction to this light wave. The light whose polarization state is clockwise circularly polarized light is referred to as the first split light L1, and the light whose polarization state is counterclockwise circularly polarized light is referred to as the second split light L2. The polarization diffractive optical element 17 imparts spherical phases having opposite curvatures to the first divided light L1 and the second divided light L2, respectively. Therefore, the first divided light L1 and the second divided light L2 have different phase distributions from each other. These light waves (L1, L2) are incident on the light modulation element 1.

The first divided light L1 and the second divided light L2 are each divided into four directions by the light modulation element (element in which the phase shifters are periodically arranged) 1, and a set of light waves having different phase differences in each direction. Further, by further modulating with the polarizing element 2, four interference fringes are generated on the image pickup surface of the image pickup element 3. By analyzing these interference fringes, the phase distribution can be measured.

As for the optical system when the incoherent light source is used and the reflection type light modulation element 1 is used, the optical system can be constructed by using the beam splitter 15 as in FIG. 10.

As described above, even if the light source and the optical system used are different, the arrangement of the light modulation element 1, the polarizing element 2, and the image pickup element 3 in which the phase shifters are periodically arranged is fixed, and the present invention can be applied. The optical system is not limited to the above. The present invention can be applied to any interference-based phase measuring device.

(Image reconstruction process and its verification)
Based on the optical system of the incoherent light shown in FIG. 11, the results of photographing the interference fringes by the phase measuring apparatus of the present invention and reconstructing the image are shown below. The arithmetic unit 4 of FIG. 2 performs image reconstruction processing based on the image data of the interference fringes obtained by the image pickup device 3.

The central wavelength of the light source was 633 nm, and the structure for realizing the phase shift was a rectangular parallelepiped structure described in Non-Patent Document 2. FIG. 12 shows an example of parameters (design values) of a rectangular parallelepiped structure. A rectangular parallelepiped of 105 nm × 145 nm × 320 nm was formed in a region of 370 nm × 370 nm of the material of SiO _2. This structure can be manufactured by an electron beam drawing apparatus or the like. By arranging this structure in a square grid as shown in FIG. 6, the light modulation element 1 in which the phase shifters of FIG. 1 are periodically arranged was manufactured. Lx = 12 μm and Ly = 3 μm.

FIG. 13 shows the objects “1”, “2”, and “3” as the objects to be measured and their positional relationships. The sizes of the objects "1", "2", and "3" are 90 μm × 150 μm, 108 μm × 150 μm, and 108 μm × 150 μm, respectively. Assuming that the object of "1" is the reference (origin), the center positions of the objects of "2" and "3" in the in-plane direction are (x = -336 μm, y = 306 μm) and (x = 66 μm, y =, respectively). 336 μm). Further, the distance between the objects "1" and "2" and between the objects "2" and "3" in the depth (z) direction is 30 mm, respectively. It should be noted that each screen of FIG. 13 is conceptual, and the objects "1", "2", and "3" reflect or emit light at the above positions, and are shown in other regions (black). ) May be translucent. These were photographed and measured by the phase measuring device of the present invention as a three-dimensional object. Below, the images of each figure were created by simulation.

FIG. 14 shows image data (intensity image) taken by the image sensor 3. From FIG. 14, it can be seen that four interference fringes (holograms) having different bright and dark positions are formed. Four interference fringes are extracted from the intensity image of FIG. 14 from the arithmetic unit 4. At this time, it is necessary to accurately extract the four interference fringes so as not to cause in-plane deviation. Therefore, in order to accurately grasp the positional relationship of the interference fringes, the phase-limited correlation calculation is used. Of the four interference fringes in FIG. 14, any one interference fringe is cut out so as not to cause eclipse, and this is used as a template image t (x, y). Assuming that the intensity image of FIG. 14 is i (x, y), the calculation of the phase-limited correlation is given by the following equation (5).

Here, I (u, v) and T (u, v) are Fourier spectra of i (x, y) and t (x, y), respectively, and * indicates a complex conjugate. FT [...] is a Fourier transform operator. In addition, in the equation (5), even if the inverse Fourier transform is performed, it is substantially the same. As a result of the calculation, a clear peak is generated at the center position of each of the four interference fringes in p (x, y). The relative position information of the four interference fringes is acquired from the peak position of p (x, y). By referring to this position information, in-plane deviation can be sufficiently suppressed, and four individual interference fringes can be accurately extracted.

The four interference fringes obtained are images of interference fringes (holograms) having phase shift amounts of 0, π / 2, π, and 3π / 2 [rad], respectively. In the subsequent image processing, for example, the processing is performed based on the (x, y) coordinates of each aligned image with the center of each image as the origin.

The complex amplitude distribution U (x, y) is obtained by applying the following equation (6), which is an algorithm of the four-step phase shift method, to the interference pattern intensity I of these interference fringes.

Here, I ₀ , I _{π / 2} , I _π , and I 3 _{π / 2} are interference fringes having phase shift amounts of 0, π / 2, π, and 3π / 2 [rad], respectively. U (x, y) contains a phase distribution necessary for reconstructing the three-dimensional information of the object to be photographed 12.

FIG. 15 shows the obtained U (x, y) (a) amplitude distribution and (b) phase distribution, respectively. By applying the propagation calculation to this U (x, y), the light distribution in any z-plane can be reconstructed. The propagation calculation is given by the following equation (7).

Here, FT ^-1 [...] is an inverse Fourier transform operator. λ is the wavelength of the light source. When the placement position z _{s in} the depth direction of the object to be photographed is known, in order to obtain an image in focus on the object to be photographed, z _r of Eq. (7) is set according to the following Eq. (8). do it.

Here, f _d is the focal length of the polarized diffraction optical element (polarized diffraction lens), z _h is the distance between the polarized diffraction optical element and the image pickup element, and z _d is given by the following equation (9).

f ₀ is the focal length of the lens 9, and d is the distance between the lens 9 and the polarizing diffraction optical element 17. In this embodiment using the optical system of FIG. 11, f ₀ = 200 [mm], d = 0 [mm], f _d = 200 [mm], and z _h = 300 [mm]. FIG. 16 shows the result of reconstructing the target object by applying the equation (7) with reference to the information _{of the arrangement positions z s} = 270, 300, 330 [mm] of “1”, “2”, and “3”. show.

With reference to FIG. 16, it can be seen that the focus position has been successfully adjusted to each of the objects “1”, “2”, and “3”. When the focus position is adjusted to the object "1", the images of the object "2" and the object "3" are blurred. Similarly, when the focus position is adjusted to each of the object "2" and the object "3", the image of the object arranged on the surface different from the focus position is blurred. As described above, according to the present invention, it is possible to realize an optical system with fewer optical elements than before, and it is possible to capture and reconstruct three-dimensional information in one shooting.

In the above embodiment, the configuration and operation of the phase measuring device have been described, but the present invention is not limited to this, and may be configured as a phase measuring method and an image reconstruction method.

A computer can be suitably used to function as the arithmetic unit 4 of the above-mentioned phase measuring device, and such a computer has a program describing processing contents for realizing each arithmetic procedure of the arithmetic unit 4. This can be realized by storing the program in the storage unit of the computer and reading and executing this program by the CPU of the computer. This program can be recorded on a computer-readable recording medium.

Although the above embodiments have been described as representative examples, it will be apparent to those skilled in the art that many modifications and substitutions can be made within the spirit and scope of the invention. Therefore, the present invention should not be construed as being limited by the above-described embodiments, and various modifications and modifications can be made without departing from the scope of claims. For example, it is possible to combine the plurality of components described in the embodiment into one, or to divide one component.

The present invention can be used as a camera for stereoscopic images, and can be applied to interference measurement / analysis devices such as a fluorescent three-dimensional microscope.

1 Optical modulation element 2 Linear splitter 3 Image pickup element 4 Computing device 5 Object light 6 Reference light 7 Laser light source 8 Spatial filter 9 Lens 10 Polarization beam splitter 11 Quarter wave plate 12 Object to be photographed 13 Mirror 15 Beam splitter 16 Bandpass filter 17 Polarization diffractive optical element

Claims (9)

It is characterized in that 16 regions of 4 × 4 are used as repeating units, four types of phase shifters having different rotation angles of 45 degrees are assigned to each region, and the repeating units are periodically arranged in the in-plane direction. An optical modulation element. In the light modulation element according to claim 1, when the four types of phase shifters are A, B, C, and D, the first row of the 4 × 4 region is the A, B, C, D, and the second row. Is B, A, D, C, the third column is C, D, A, B, and the fourth column is D, C, B, A. .. In the light modulation element according to claim 1 or 2, the ratio of the lengths of the regions in one direction in the in-plane direction of 4 × 4 is 1: 1: 1: 1, and the region in the direction orthogonal to the direction is A light modulation element characterized by a length ratio of 1: 3: 1: 3. In the light modulation element according to any one of claims 1 to 3, the phase shifter periodically arranges an anisotropically shaped structure made of a plasmonic metal or a dielectric. An optical modulation element characterized by being configured. In the light modulation element according to claim 4, the plasmonic metal is any one of gold, silver, and aluminum, and the dielectric is any one of silicon, SiO ₂ , TiO ₂ , and amorphous silicon. A light modulation element characterized by being. The light modulation element according to any one of claims 1 to 5, a linear polarizing element through which light modulated by the light modulation element is transmitted, and an image pickup device that captures an image of light transmitted through the linear polarizing element. A phase measuring device characterized by being provided with. The phase measuring device according to claim 6, wherein an object light and a reference light generated by coherent light are incident on the light modulation element. In the phase measuring device according to claim 6, the incoherent light is divided into a first divided light and a second divided light, and the first divided light and the second divided light are given different phase distributions to each other. A phase measuring device, characterized in that the first divided light and the second divided light are incident on the optical modulation element. In the phase measuring apparatus according to any one of claims 6 to 8, four interference fringes are extracted from the image data captured by the image pickup element, and the complex amplitude is obtained from the four interference fringes by a phase shift method. A phase measuring device characterized by finding a distribution.

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