Optical measurement system and optical measurement method

Abstract

An optical measurement system according to the present invention includes: a light source that generates coherent light; a beam splitter that splits the coherent light into illumination light and reference light; a synthesis unit that optically superimposes the reference light and object light generated by illuminating a sample with the illumination light; an objective lens of an infinity-corrected optical system, the objective lens being disposed between the sample and the synthesis unit; an image sensor that records a hologram generated in the synthesis unit; and a processing device that processes the hologram recorded by the image sensor. The processing device calculates a first complex amplitude distribution on the recording surface of the image sensor on the basis of the hologram, calculates a second complex amplitude distribution on the Fourier surface of the objective lens by calculating diffraction of the first complex amplitude distribution for a first distance between the recording surface of the image sensor and the Fourier surface of the objective lens, and performs coordinate transformation to transform the second complex amplitude distribution into a Fourier spectrum on spatial frequency spectrum coordinates.

Description

Optical measurement system and optical measurement method

[0001] The present invention relates to an optical measurement system and method based on digital holography.

[0002] Digital holography has been proposed and put into practical use as a method for measuring the shape of a sample with higher accuracy.

[0003] An optical system that combines digital holography and an objective lens is known. For example, Non-Patent Document 1 discloses an optical system that includes a microscope objective lens with a high numerical aperture (NA).

[0004] Japanese Patent No. 7157505

[0005] N. Verrier, D. Donnarumma, G. Tessier, and M. Gross, High numerical aperture holographic microscopy reconstruction with extended range, Applied Optics Vol. 54, No. 32, pp. 9540-9547.

[0006] Although the use of an objective lens can increase the numerical aperture, it poses the problem of reduced measurement accuracy due to aberrations and phase distortions caused by the objective lens. On the other hand, lensless digital holography, which does not use an objective lens, does not suffer from the reduction in measurement accuracy caused by the objective lens, but it does have the problem of making it difficult to increase the numerical aperture.

[0007] The present invention provides an optical measurement system and an optical measurement method that can use an objective lens while maintaining the advantages of lensless digital holography to the fullest extent.

[0008] An optical measurement system according to one aspect of the present invention includes a light source that generates coherent light, a beam splitter that splits the coherent light into illumination light and reference light, a combiner that optically superimposes the object light and the reference light generated by illuminating a sample with the illumination light, an objective lens of an infinity-corrected optical system arranged between the sample and the combiner, an image sensor that records a hologram generated in the combiner, and a processing device that processes the hologram recorded by the image sensor. The processing device calculates a first complex amplitude distribution on a recording surface of the image sensor based on the hologram, calculates a second complex amplitude distribution on the Fourier plane of the objective lens by performing diffraction calculation on the first complex amplitude distribution over a first distance between the recording surface of the image sensor and the Fourier plane of the objective lens, and performs coordinate transformation of the second complex amplitude distribution into a Fourier spectrum on spatial frequency spectral coordinates.

[0009] The processing device may treat the Fourier spectrum as a result of a Fourier transform of the third complex amplitude distribution in the focal plane of the objective lens.

[0010] The processing device may calculate a fourth complex amplitude distribution at the sample observation plane by using the Fourier spectrum to perform diffraction calculation of a third complex amplitude distribution over a second distance between the focal plane of the objective lens and the sample observation plane.

[0011] The processing device may reconstruct an image at the focal plane of the objective lens based on the fourth complex amplitude distribution.

[0012] The processing device may apply spatial frequency filtering to the hologram to calculate a complex amplitude hologram, and calculate a first complex amplitude distribution based on the calculated complex amplitude hologram and information on the light wave distribution of the reference light.

[0013] An optical measurement method according to another aspect of the present invention includes the steps of: configuring an optical system including a light source that generates coherent light; a beam splitter that splits the coherent light into illumination light and reference light; a combiner that optically superimposes object light and reference light generated by illuminating a sample with the illumination light; an objective lens of an infinity-corrected optical system that is arranged between the sample and the combiner; and an image sensor that records a hologram generated in the combiner; calculating a first complex amplitude distribution on a recording surface of the image sensor based on the hologram recorded in the image sensor; calculating a second complex amplitude distribution on the Fourier plane of the objective lens by performing diffraction calculation of the first complex amplitude distribution over a first distance between the recording surface of the image sensor and the Fourier plane of the objective lens; and coordinate transforming the second complex amplitude distribution into a Fourier spectrum on spatial frequency spectrum coordinates.

[0014] According to an embodiment of the present invention, an optical measurement system and an optical measurement method that can use an objective lens can be realized while maximally maintaining the advantages of lensless digital holography.

[0015] FIG. 1 is a schematic diagram showing an example of a measurement optical system of an optical measurement system according to the present embodiment. FIG. 2 is a schematic diagram showing an example of a hologram recording optical system of the optical measurement system according to the present embodiment. FIG. 3 is a schematic diagram showing an example of a configuration of a profile generation unit of the optical measurement system according to the present embodiment. FIG. 4 is a schematic diagram showing an example of a hardware configuration of a processing device included in the optical measurement system according to the present embodiment. FIG. 5 is a flowchart showing a processing procedure for calculating a complex conjugate of a light wave distribution of reference light in the optical measurement system according to the present embodiment. FIG. 6 is a flowchart showing a processing procedure for measurement processing in the optical measurement system according to the present embodiment. FIG. 7 is a diagram showing an example of measurement of a sample by the optical measurement system according to the present embodiment. FIG. 8 is a diagram showing an example of measurement of a sample by the optical measurement system according to the present embodiment. FIG. 9 is a schematic diagram showing an example of an optical measurement system according to the present embodiment.

[0016] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail with reference to the accompanying drawings, in which the same or corresponding parts in the drawings are designated by the same reference numerals and the description thereof will not be repeated.

[0017] <A. Overview> Optical systems that combine digital holography and objective lenses are known. Using an objective lens makes it easy to record a magnified image of a tiny object. However, using an objective lens can cause problems, such as (1) the occurrence of aberrations, (2) phase distortion, and (3) limitations on three-dimensional recording. (1) While some correction for aberrations is possible, not all aberrations can be corrected. (2) Regarding phase distortion, the phase distribution of the recorded magnified image does not match the phase distribution of the image before magnification. (3) Regarding the limitations on three-dimensional recording, objective lenses function as designed only in the designed planes (e.g., the focal plane and Fourier plane), and errors may occur in other planes.

[0018] The magnification of the magnified image can be easily calculated based on the design value of the objective lens, but it is not easy to calculate the phase distribution of the magnified image, which varies depending on the sample placement and illumination method.

[0019] The above-mentioned problem is caused by the mechanism of the objective lens, which maps a predetermined focal plane to a predetermined Fourier plane in two dimensions. The focal plane and Fourier plane of the objective lens are determined at the time of design, and aberrations of images that are shifted from the focal plane or Fourier plane are not corrected. The focal plane corresponds to a reference plane for correcting aberrations.

[0020] As a result of extensive research into the causes of errors caused by objective lenses, the inventors of the present application have discovered that by combining an objective lens of an infinity-corrected optical system with digital holography, it is possible to realize an optical system in which an objective lens can be used while maximizing the advantages of lensless digital holography.

[0021] When light from a sample passes through an objective lens with infinity corrected optical system, it is converted into parallel light at infinity. In other words, light from the sample is not imaged by an objective lens with infinity corrected optical system. An objective lens with infinity corrected optical system converts any point on the focal plane into a plane wave. Conversion to a plane wave means that an image is formed at infinity. Due to the Fourier transform effect of the lens, a Fourier spectrum, which is the Fourier transform of the complex amplitude distribution on the focal plane, appears on the Fourier plane of the objective lens.

[0022] By regarding the objective lens of the infinity-corrected optical system as an accurate Fourier transform element, the complex amplitude distribution of the Fourier plane of the objective lens is acquired, and the acquired complex amplitude distribution is then subjected to an inverse Fourier transform. This makes it possible to reconstruct an accurate image on the focal plane. Generally, an objective lens is designed for a predetermined focal plane, so it can be considered as an accurate Fourier transform element at least on the focal plane.

[0023] If diffraction calculations are performed on the enlarged image after it has passed through the objective lens, the correct results will not be obtained. However, by reconstructing the image before it passes through the objective lens and then performing diffraction calculations, the correct results can be obtained without being affected by the objective lens.

[0024] The optical measurement system and optical measurement method according to this embodiment employ an optical system that combines an objective lens of an infinity-corrected optical system with digital holography, thereby providing an optical measurement system and optical measurement method that can use an objective lens while maximizing the advantages of lensless digital holography.

[0025] <B. Optical System of Optical Measurement System> The optical measurement system according to this embodiment utilizes digital holography, which uses divergent light such as a point light source as reference light.

[0026] 1 is a schematic diagram showing an example of a measurement optical system of an optical measurement system 1 according to the present embodiment. The optical measurement system 1 records a light wave distribution indicating the shape of a sample S.

[0027] 1, the optical measurement system 1 includes a light source 10, a beam expander BE, beam splitters BS1 and BS2, mirrors M1 and M2, condenser lenses L1 and L2, an objective lens MO, and an image sensor D.

[0028] The light source 10 is composed of a laser or the like and generates coherent light. The beam expander BE expands the cross-sectional diameter of the light from the light source 10 to a predetermined size.

[0029] Beam splitter BS1 splits the coherent light expanded by beam expander BE into two beams. One beam split by beam splitter BS1 (light on the reflection side of beam splitter BS1) corresponds to illumination light Q, and the other beam (light on the transmission side of beam splitter BS1) corresponds to reference light R. Therefore, illumination light Q and reference light R are mutually coherent.

[0030] In the measurement optical system shown in Fig. 1, an object beam hologram is generated by modulating object beam O obtained by illuminating a sample S with illumination beam Q using reference beam R disposed off-axis. The generated object beam hologram is recorded by an image sensor D.

[0031] The illumination light Q is reflected by the mirror M1, and its propagation direction is changed, and then passes through the condenser lens L1. After passing through the condenser lens L1, the illumination light Q is condensed in the vicinity of the sample S.

[0032] Illumination light Q passes through sample S to generate object light O. The object light O passes through objective lens MO and enters beam splitter BS2. The object light O passes through half mirror HM2 of beam splitter BS2 and enters image sensor D.

[0033] The objective lens MO is an objective lens of an infinity corrected optical system and is disposed between the sample S and the beam splitter BS2.

[0034] The reference light R is reflected by mirror M2, changes its propagation direction, and then is focused by condenser lens L2. The point of focus by condenser lens L2 corresponds to the position of the point light source. In other words, the reference light R can be considered as light emitted from a point light source. Finally, the reference light R is reflected by half mirror HM2 of beam splitter BS2 and enters image sensor D.

[0035] In the half mirror HM2, object light O obtained by illuminating a sample S with illumination light Q is modulated by reference light R. An object light hologram generated by the modulation is recorded by an image sensor D. The half mirror HM2 (or beam splitter BS2) is a combining unit that optically superimposes the object light O and reference light R generated by illuminating a sample S with illumination light Q.

[0036] The image sensor D may be, for example, a CMOS image sensor. The image sensor D records a hologram that occurs at the beam splitter BS2 (half mirror HM2).

[0037] The processing device 100 processes the hologram recorded by the image sensor D. More specifically, the processing device 100 calculates information necessary for image reconstruction of the sample S based on the object beam hologram recorded by the image sensor D.

[0038] In the following description, the optical axis direction is the z-axis, and the two axes perpendicular to the z-axis are the x-axis and y-axis. The light-receiving surface of the image sensor D is the "recording surface," and the observation cross section of the sample S is the "sample observation surface." The recording surface 20, the Fourier plane 22, the focal plane 24 of the objective lens MO, and the sample observation surface 26 are all perpendicular to the z-axis. Unless otherwise specified, the diffraction calculations described below are performed along the z-axis.

[0039] The intersection of the recording surface 20 and the central optical axis of the beam splitter BS2 is defined as the origin. The central optical axis of the beam splitter BS2 may be defined as the z-axis. The plane formed by the x-axis and y-axis is parallel to the recording surface 20 of the image sensor D. For convenience of explanation, two adjacent sides of the image sensor D are assumed to be parallel to the x-axis and y-axis, respectively.

[0040] The image sensor D records the complex amplitude distribution on the recording surface 20. The complex amplitude distribution on the recording surface 20 of the image sensor D is represented as an object beam hologram U Σ The object beam hologram U Σ may be the object beam hologram U itself recorded by the image sensor D, or the result of an interpolation process as described below may be used.

[0041] In the following description, the object beam hologram at the focal plane 24 of the objective lens MO is referred to as a focal plane hologram U'. The object beam hologram at the Fourier plane 22 of the objective lens MO is referred to as a Fourier plane hologram U. ｌ In addition, the Fourier plane hologram U ｌ is equal to the Fourier spectrum F[U'] of the focal plane hologram U'. The object beam hologram at the sample observation plane is converted into the observation plane hologram U' ｄ Let's say.

[0042] The distance from the recording plane 20 to the Fourier plane 22 is defined as distance 1, and the distance from the focal plane 24 of the objective lens MO to the sample observation plane is defined as distance d. Note that distances 1 and d are known, accurate values.

[0043] FIG. 1 shows an example of a configuration that employs a cube-shaped beam splitter BS2, but the refractive index of the beam splitter BS2 may be relatively increased or the cube shape may be modified to any other shape in order to increase the numerical aperture or ensure the working distance.

[0044] The beam splitter used to record a hologram may have any shape as long as the boundary surface between different media can be considered flat. For example, a plate-shaped beam splitter may be used instead of a cube-shaped one.

[0045] The beam splitter BS2 may be tilted with respect to the wavefront of the object beam O. Such tilting can suppress stray light.

[0046] Figure 1 shows an example configuration in which a sample S and an image sensor D are arranged opposite each other with a beam splitter BS2 in between, but it is also possible to adopt an example configuration in which a focusing lens L1 and an image sensor D are arranged opposite each other with a beam splitter BS2 in between.

[0047] A mask may be placed between the mirror M1 and the condenser lens L1 to limit the cross section of the reference beam R. By placing such a mask, it is possible to suppress the generation of noise due to unnecessary interference.

[0048] <C. Measurement Process> Next, a measurement process of the sample S by the optical measurement system 1 according to the present embodiment will be described.

[0049] In the optical system shown in FIG. 1, the light wave distributions of the object light O and the reference light R recorded by the image sensor D are expressed by the following general formulas (1) and (2).

[0050]

[0051] The object beam O and the reference beam R are mutually coherent beams having an angular frequency ω. For convenience of explanation, the coordinates (x, y) may be omitted as appropriate in the following equations.

[0052] The image sensor D records a hologram I ＯＲ is calculated according to the following equation (3) as the light intensity of the composite light of the light expressed by equation (1) and the light expressed by equation (2).

[0053]

[0054] By applying spatial frequency filtering to equation (3), the complex amplitude hologram J ＯＲ is calculated according to the following formula (4).

[0055]

[0056] The complex conjugate R of the light wave distribution of the reference light R ＊ (=R ０ exp(-iφ Ｒ ) (the calculation method will be described later) to obtain the complex amplitude hologram J ＯＲBy dividing by , the object beam hologram U is calculated according to the following equation (5).

[0057]

[0058] The object light hologram U shown in equation (5) corresponds to the light wave distribution of the object light O on the recording surface 20 of the image sensor D minus the time term (−ωt). Therefore, by using a diffraction calculation that does not use approximations such as plane wave expansion, accurate image reconstruction without aberrations can be performed.

[0059] If the object light hologram U contains frequency components that do not satisfy the sampling theorem, the complex conjugate R of the light wave distribution of the reference light R ＊ Before dividing by, the complex amplitude hologram J is obtained by interpolation. ＯＲ After increasing the number of samples, the increased complex amplitude hologram J ＯＲ is divided into a grid and the divided grids are superimposed to form a complex amplitude hologram J ＯＲ The hologram may be reduced in size. Note that the size of the grating is preferably larger than the size of the image reconstructed from the hologram. By increasing the number of samples and superimposing them in this way, the increase in the amount of calculations can be suppressed.

[0060] Furthermore, the properties of the Fourier transform may be utilized to realize processing equivalent to increasing the number of sampling points and superimposing them on the Fourier spectrum.

[0061] The complex amplitude distribution obtained by increasing the number of samples and superposing them is called the object beam hologram U Σ However, if there is no need to increase the number of samples or to superimpose, the object light hologram U can be used as it is as the object light hologram U Σ Treat as.

[0062] First, the object beam hologram U Σ By performing a diffraction calculation using a plane wave expansion for the object beam hologram U, the light wave distribution on the Fourier plane 22 can be reconstructed. Σ The Fourier plane hologram U is a light wave distribution propagated by a distance l. ｌ(x, y) is calculated according to the following equation (6).

[0063]

[0064] Next, the light wave distribution in the Fourier plane 22 corresponds to the Fourier spectrum F[U'](u, v) of the focal plane hologram U' in the focal plane 24 of the objective lens MO, which is regarded as a Fourier transform element. ｌ (x, y) is coordinate-transformed into a Fourier spectrum F[U'](u, v) on the coordinates (u, v) of the spatial frequency spectrum. That is, the Fourier spectrum F[U'(x, y)](u, v) is calculated according to the following equation (8).

[0065]

[0066] Next, the Fourier spectrum F[U'](u, v) is subjected to an inverse Fourier transform to calculate a focal plane hologram U', which is the image before passing through the objective lens MO. The focal plane hologram U' is the object light distribution at the focal plane 24 of the objective lens MO, which is the reference plane for correcting aberrations.

[0067] If the focal plane hologram U' contains aberrations or distortions, the aberrations or distortions may be corrected before performing diffraction calculations toward the sample observation surface.

[0068] Observation plane hologram U', which is the light wave distribution propagated by a distance d through the focal plane hologram U' by plane wave expansion. ｄ (x, y) is calculated according to the following equation (9).

[0069]

[0070] Observation surface hologram U' ｄ is a light wave distribution that indicates the shape of the sample S. For example, the observation surface hologram U' ｄ By extracting and imaging the amplitude component from the signal, an image equivalent to the bright field of an optical microscope can be obtained.

[0071] <D. Information on the light wave distribution of the reference light R> Next, information on the light wave distribution of the reference light R (complex conjugate R of the light wave distribution) used in the measurement process of the sample S by the optical measurement system 1 according to this embodiment will be described. ＊ ) calculation process will be described.

[0072] Fig. 2 is a schematic diagram showing an example of a hologram recording optical system of an optical measurement system 1 according to this embodiment. The hologram recording optical system shown in Fig. 2 is the measurement optical system shown in Fig. 1 with the condenser lens L2, objective lens MO, and sample S removed, and with a calibration unit 50 installed. Note that the configuration related to the reference beam R (mirror M1 and condenser lens L1) remains the same as in Fig. 1.

[0073] 2 is used to generate a known light wave distribution P, thereby obtaining information on the light wave distribution of the reference light R. The information on the light wave distribution of the reference light R is obtained by calculating the complex conjugate R of the light wave distribution of the reference light R. ＊ Includes:

[0074] In this specification, the term "known light wave distribution" refers to a light wave distribution that can be identified with the accuracy required for the required measurement performance. Therefore, the known light wave distribution may be approximated with the required accuracy. Note that the light wave distribution itself does not change with translation.

[0075] The calibration unit 50 includes a profile generating section 60 and a mask FA. The profile generating section 60 is an optical system that generates a known light wave distribution. The profile generating section 60 may be configured to be able to spatially move the known light wave distribution. For example, the profile generating section 60 may have a function that allows the known light wave distribution to be translated (shifted) in the x-axis direction, y-axis direction, and z-axis direction. The profile generating section 60 may employ any optical system as long as it can generate a known light wave distribution. Several configuration examples of the profile generating section 60 will be described.

[0076] 3 is a schematic diagram showing an example of the configuration of the profile generating unit 60 of the optical measurement system 1 according to the present embodiment. The optical systems shown in FIGS. 3A to 3D generate a known light wave distribution P when illumination light Q, which is a plane wave, is incident thereon.

[0077] 3A is an optical system that uses a pinhole and includes a light-shielding plate 61 that can move parallel to three axes. A pinhole 62 is provided within the illumination range of illumination light Q of the light-shielding plate 61. The beam diameter of illumination light Q is set to a size that can sufficiently illuminate the range including the pinhole 62.

[0078] The profile generator 60B shown in FIG. 3B is an optical system that uses a pinhole and a condenser lens, and includes a light-shielding plate 63 and a light-shielding plate 64 that can move parallel to three axes. The light-shielding plate 63 and the light-shielding plate 64 are connected to each other and move parallel to each other as a unit. A condenser lens 65 is provided within the illumination range of the illumination light Q of the light-shielding plate 63, and a pinhole 66 is provided within the illumination range of the illumination light Q that has passed through the light-shielding plate 64. The beam diameter of the illumination light Q is set to a size that allows a sufficient amount of light to pass through the condenser lens 65 and the pinhole 66.

[0079] The profile generator 60C shown in Fig. 3C is an optical system that does not use a pinhole but uses a condenser lens with known aberration. The profile generator 60C includes a light-shielding plate 61 that can move parallel to each of three axes. A condenser lens 67 is provided within the illumination range of the illumination light Q of the light-shielding plate 61. The beam diameter of the illumination light Q is set to a size that can sufficiently illuminate the range including the condenser lens 67.

[0080] The profile generator 60D shown in Fig. 3(D) is an optical system that does not use a pinhole but uses an objective lens with known aberrations. The profile generator 60D includes a light-shielding plate 61 that can move parallel to each of three axes. An objective lens 68 is provided within the illumination range of the illumination light Q of the light-shielding plate 61. The beam diameter of the illumination light Q is set to a size that can sufficiently illuminate the range including the objective lens 68.

[0081] It should be noted that the present invention is not limited to the configuration examples shown in FIGS. 3A to 3D, and any optical system capable of generating a known light wave distribution can be employed.

[0082] 2 again, the mask FA limits the field of view recordable by the image sensor D to an appropriate range. For example, the field of view of the mask FA is preferably equal to or larger than the pupil diameter of the objective lens MO.

[0083] Using the calibration unit 50, the known light wave distribution P is calculated by an arbitrary offset coordinate (x ｐｉ , y ｐｉ , z ｐｉ ) (i=0 to N−1), one or more light wave distributions P ｉ For example, by arbitrarily moving the calibration unit 50 in one or more directions among the x-axis direction, the y-axis direction, and the z-axis direction, one or more light wave distributions P ｉ can be generated.

[0084] Light wave distribution P ｉ is obtained by translating the known light wave distribution P along each axis direction. Therefore, the light wave distribution P on the recording surface 20 (z=0) of the image sensor D is ｉ is the light wave distribution P (known) on the recording surface 20 (z=0) of the image sensor D and the offset coordinate (x ｐｉ , y ｐｉ , z ｐｉ ) can be expressed as the following equation (10).

[0085]

[0086] In the hologram recording optical system shown in FIG. 2, the image sensor D records a hologram I in one image capture. ＰｉＲ can be expressed as the following equation (11): By filtering the third term of equation (11), the complex amplitude hologram J ＰｉＲ is calculated according to the following equation (12).

[0087]

[0088] Offset coordinate (x ｐｉ , y ｐｉ , z ｐｉ ) is known, the complex conjugate R of the light wave distribution of the reference light R ＊ (=R ０ exp(-iφ Ｒ )) is calculated according to the following equation (13).

[0089]

[0090] In this case, the known offset coordinate (x ｐ０ , y ｐ０ , z ｐ０ ) corresponding to the light wave distribution P ０ from the complex conjugate R of the light wave distribution of the reference light R ＊ can be calculated.

[0091] On the other hand, the offset coordinate (x ｐｉ , y ｐｉ , z ｐｉ ) is not known or the offset coordinate (x ｐｉ , y ｐｉ , z ｐｉ If multiple offset coordinates (x ｐｉ , y ｐｉ , z ｐｉ ) a plurality of holograms I recorded in ＰｉＲ By parameter fitting, the complex conjugate R of the light wave distribution of the reference light R is obtained using ＊ Specifically, the calibration process disclosed in Japanese Patent No. 7157505 (Patent Document 1) can be used.

[0092] <E. Processing Device 100> Next, an example of the hardware configuration of the processing device 100 included in the optical measurement system 1 will be described.

[0093] 4 is a schematic diagram showing an example of the hardware configuration of processing device 100 included in optical measurement system 1 according to the present embodiment. Referring to FIG. 4, processing device 100 includes, as main hardware elements, a processor 102, a main memory 104, an input unit 106, a display unit 108, a storage 110, an interface 120, a network interface 122, and a media drive 124.

[0094] The processor 102 is typically an arithmetic processing unit such as a CPU (Central Processing Unit) or a GPU (Graphics Processing Unit), and reads one or more programs stored in the storage 110 into the main memory 104 and executes them. The main memory 104 is a volatile memory such as a DRAM (Dynamic Random Access Memory) or an SRAM (Static Random Access Memory), and functions as a working memory for the processor 102 to execute the programs.

[0095] The input unit 106 includes a keyboard, a mouse, etc., and receives operations from a user. The display unit 108 outputs results of program execution by the processor 102 to the user.

[0096] The storage 110 is a non-volatile memory such as a hard disk or flash memory, and stores various programs and data. As an example, the storage 110 holds an operating system (OS) 111, a measurement program 112, calibration information 113, hologram data 114, and measurement results 115.

[0097] The operating system 111 provides an environment in which the processor 102 executes programs. The measurement program 112 is executed by the processor 102 to realize measurement processing and the like according to this embodiment. The calibration information 113 includes information on the light wave distribution of the reference light R calculated by the hologram recording optical system. The hologram data 114 includes image data recorded by the image sensor D. The measurement results 115 include measurement results (such as a reconstructed image) obtained by executing the measurement program 112.

[0098] The interface 120 mediates data transmission between the processing device 100 and the image sensor D. The network interface 122 mediates data transmission between the processing device 100 and an external server device.

[0099] The media drive 124 reads necessary data from a recording medium 126 (e.g., an optical disk) that stores programs to be executed by the processor 102, and stores the data in the storage 110. The measurement program 112 to be executed by the processing device 100 may be installed via the recording medium 126 or downloaded from a server device via the network interface 122.

[0100] The measurement program 112 may execute processing by calling necessary modules from among the program modules provided as part of the operating system 111 in a predetermined order and at a predetermined timing. In such a case, a measurement program 112 that does not include such modules is also included in the technical scope of the present invention. The measurement program 112 may be provided as part of another program.

[0101] Note that all or part of the functions provided by the processor 102 of the processing device 100 executing a program may be realized by a hardwired logic circuit (e.g., an FPGA (Field-Programmable Gate Array) or an ASIC (Application Specific Integrated Circuit)).

[0102] <F. Processing Procedure> Next, an example of a processing procedure executed in optical measurement system 1 according to the present embodiment will be described.

[0103] (f1: complex conjugate R of the light wave distribution of the reference light R ＊ 5 is a calculation process of the complex conjugate R of the light wave distribution of the reference light R in the optical measurement system 1 according to the present embodiment. ＊ 5 may be realized by the processor 102 of the processing device 100 executing the measurement program 112.

[0104] 5, the optical system shown in FIG. 2 is configured, and the calibration unit 50 is disposed (step S100). At this time, the offset coordinates (xｐ０ , y ｐ０ , z ｐ０ ) is accurately set (step S102).

[0105] The processing device 100 detects a hologram I recorded by the image sensor D while the light source 10 is emitting coherent light. Ｐ０Ｒ is acquired (step S104).

[0106] The processing device 100 uses the known light wave distribution P and offset coordinates (x ｐ０ , y ｐ０ , z ｐ０ ) to obtain the light wave distribution P ０ is calculated (step S106) (see the above-mentioned formula (10)), and the calculated light wave distribution P ０ and the acquired hologram I Ｐ０Ｒ Using the complex conjugate R of the light wave distribution of the reference light R ＊ (Step S108) (see the above-mentioned formulas (11) to (13)). The processing device 100 calculates the complex conjugate R of the light wave distribution of the calculated reference light R. ＊ The calibration information 113 including the above is output (step S110).

[0107] As long as the optical arrangement between the measurement optical system shown in FIG. 1 and the hologram recording optical system shown in FIG. 2 can be considered to be substantially the same, the complex conjugate R of the light wave distribution of the reference light R calculated using the hologram recording optical system shown in FIG. ＊ can be used as is.

[0108] (f2: Measurement Processing) Fig. 6 is a flowchart showing the procedure of the measurement processing in the optical measurement system 1 according to the present embodiment. The steps executed by the processing device 100 shown in Fig. 6 may be realized by the processor 102 of the processing device 100 executing the measurement program 112.

[0109] Referring to FIG. 6, the measurement optical system shown in FIG. 1 is configured, and a sample S is placed (step S200).

[0110] The processing device 100 processes the hologram I ＯＲ Based on this, an object light hologram U, which is a complex amplitude distribution on the recording surface 20 of the image sensor D, is obtained. ΣThe processing device 100 calculates the first complex amplitude distribution (the first complex amplitude distribution). ＯＲ Applying spatial frequency filtering to the complex amplitude hologram J ＯＲ and the calculated complex amplitude hologram J ＯＲ and the information on the light wave distribution of the reference light R, the object light hologram U Σ Calculate.

[0111] More specifically, the processing device 100 detects a hologram I recorded by the image sensor D in a state where the light source 10 generates coherent light. ＯＲ (Step S202). The processing device 100 acquires the acquired hologram I ＯＲ By applying spatial frequency filtering to the complex amplitude hologram J ＯＲ (step S204) (see the above equations (3) and (4)). The processing device 100 calculates the calculated complex amplitude hologram J ＯＲ is the complex conjugate R of the light wave distribution of the reference light R ＊ The object beam hologram U is calculated by dividing by (step S206) (see equation (5) above).

[0112] The processing device 100 increases the number of sampling points and performs superposition processing on the object light hologram U, thereby Σ In the case where the increase in the number of sampling points and the superposition process are omitted, the object light hologram U may be directly calculated as the object light hologram U Σ Treat as.

[0113] Next, the processing device 100 calculates the object beam hologram U by the distance l (first distance) between the recording surface 20 of the image sensor D and the Fourier plane 22 of the objective lens MO. Σ By performing diffraction calculation, a Fourier plane hologram U ｌ (second complex amplitude distribution) is calculated.

[0114] More specifically, the processing device 100 calculates the object beam hologram U Σ By calculating the diffraction of the Fourier plane hologram U ｌ(x, y) is calculated (step S208) (see equation (6) above).

[0115] Next, the processing device 100 calculates the Fourier plane hologram U ｌ is transformed into a Fourier spectrum on the spatial frequency spectrum coordinate.

[0116] More specifically, the processing device 100 processes the Fourier plane hologram U ｌ (x, y) is coordinate-transformed into a Fourier spectrum F[U'(x, y)](u, v) on the coordinates (u, v) of the spatial frequency spectrum (step S210) (see equations (7) and (8) above). The Fourier spectrum F[U'(x, y)](u, v) is the Fourier transform of the focal plane hologram U'(x, y).

[0117] That is, the processing device 100 treats the Fourier spectrum F[U'(x, y)](u, v) as the result of Fourier transforming the focal plane hologram U' (third complex amplitude distribution), which is the complex amplitude distribution at the focal plane 24 of the objective lens MO.

[0118] Next, the processing device 100 uses the Fourier spectrum to perform diffraction calculations on the focal plane hologram U'(x, y) over the distance d between the focal plane 24 of the objective lens MO and the sample observation plane, thereby obtaining an observation plane hologram U', which is a complex amplitude distribution on the sample observation plane. ｄ (fourth complex amplitude distribution) is calculated.

[0119] More specifically, the processing device 100 calculates the focal plane hologram U'(x, y) by diffraction calculation over a distance d to obtain the observation plane hologram U' ｄ (x, y) is calculated (step S212) (see equation (9) above).

[0120] Finally, the processing device 100 calculates the observation plane hologram U'. ｄ The processing device 100 generates a measurement result using part or all of the information of the observation plane hologram U' (step S214). ｄ For example, an observation plane hologram U' is generated based on the observation plane hologram U'. ｄBy extracting and imaging the amplitude component from the signal, an image equivalent to the bright field of an optical microscope is generated.

[0121] 6 is executed for each sample S. Note that the observation plane holograms U′ corresponding to the multiple samples S are ｄ may be recorded in advance, and a process for generating the measurement results may be executed afterwards.

[0122] <G. Measurement Example> Next, a measurement example of the sample S using the optical measurement system 1 according to the present embodiment will be described.

[0123] 7 and 8 are diagrams showing an example of measurement of a sample S by the optical measurement system 1 according to this embodiment. FIGS. 7 and 8 show an example of measurement of a sample S using the measurement optical system shown in FIG. 1. A USAF 1951 resolution test target was used as the sample S. A laser with a wavelength of 532 nm was used as the light source 10. The numerical aperture of the objective lens MO was 0.9.

[0124] FIG. 7 shows the result of placing the sample S as the subject on the focal plane 24 (aberration correction plane) of the objective lens MO and reconstructing the image of the sample according to the measurement process shown in FIG. 6 (distance d=0).

[0125] FIG. 7A shows the entire image of the reconstructed field of view, and FIG. 7B shows an enlarged image of a part of FIG. 7A.

[0126] The theoretical resolution obtained with a numerical aperture (NA) of 0.9 is 0.61λ / NA=361 nm according to the Rayleigh standard. The pattern in the USAF 1951 resolution test target that can be confirmed with this theoretical resolution is pattern 10-3. As shown in Figure 7, pattern 10-3 appears clearly, and it can be seen that the image is accurately reproduced.

[0127] FIG. 8 shows the results of reconstructing an image of a sample S, which is the subject, according to the measurement process shown in FIG. 6, with the sample S positioned 80 μm from the focal plane 24 (aberration correction surface) of the objective lens MO (distance d=80 μm).

[0128] The theoretical resolution obtained with a numerical aperture (NA) of 0.9 is 0.61λ / NA=361 nm according to the Rayleigh standard. The pattern in the USAF 1951 resolution test target that can be confirmed with this theoretical resolution is pattern 10-3. As shown in Figure 8, pattern 10-3 appears clearly, and it can be seen that the image is accurately reproduced.

[0129] As described above, it can be seen that the optical measurement system 1 according to this embodiment can accurately reconstruct an image not only when the sample S is placed on the focal plane 24 of the objective lens MO, but also when the sample S is placed at a position away from the focal plane 24 of the objective lens MO. In other words, it is possible to achieve numerical focus changes without being restricted by the focal depth of the objective lens MO.

[0130] <H. Application Examples> Next, configuration examples in which optical measurement system 1 according to the present embodiment is combined with other optical systems will be described.

[0131] Fig. 9 is a schematic diagram showing an example of an optical measurement system 2 according to the present embodiment. Fig. 9 shows an optical system in which a microscope optical system is combined with the measurement optical system (optical measurement system 1) shown in Fig. 1.

[0132] The optical measurement system 2 shown in FIG. 9 is obtained by adding a beam splitter BS3, an imaging lens 30, and an image sensor 40 to the optical measurement system 1 shown in FIG.

[0133] The beam splitter BS3 splits the object light O into two beams. One beam split by the beam splitter BS3 enters the beam splitter BS2, and the other beam split by the beam splitter BS3 enters the imaging lens 30.

[0134] The imaging lens 30 may be, for example, a tube lens. The object light O that passes through the imaging lens 30 is focused on the image sensor 40. The image sensor 40 captures an image of the object light O that has passed through the sample S. The image sensor 40 may be, for example, a CMOS image sensor.

[0135] According to the optical measurement system 2 shown in FIG. 9, it is possible to reconstruct an image of the sample S by digital holography and also to capture an enlarged image of the sample S.

[0136] The beam splitter BS3 may reflect only light in a specific wavelength band (for example, the visible range). By employing such a beam splitter BS3, the image sensor 40 can capture an image of the sample S at wavelengths in the specific wavelength band.

[0137] An optical system for visual observation may be provided instead of the image sensor 40. By providing an optical system for visual observation, the user can observe the state of the sample S in parallel with image reconstruction by digital holography.

[0138] For observation using a general microscope optical system, a mechanism for moving the sample S and the objective lens MO may be provided.

[0139] As described above, the optical measurement system 1 according to this embodiment can increase the numerical aperture by using the objective lens MO, and therefore can be combined with various other optical systems.

[0140] <I. Modifications> The optical system described above is one example, and any optically equivalent modification can be made depending on the required specifications, space constraints, etc. For example, a single lens may be changed to a compound lens, or any reflective member may be used instead of a mirror.

[0141] In the above description, an implementation example in which the processing device 100 executes arithmetic processing related to the measurement of the sample S has been exemplified, but the present invention is not limited to this and any implementation form can be adopted. For example, a part or all of the processing performed by the processing device 100 may be performed using computing resources on the cloud.

[0142] J. Advantages The optical measurement system according to this embodiment provides a mechanism that can perform a wide range of numerical focus changes using digital holography, which handles complex amplitude distributions, and the Fourier transform action of a lens. The image sensor records the complex amplitude distribution of the Fourier spectrum generated by the Fourier transform action of the lens. By performing an inverse Fourier transform on the obtained Fourier spectrum, an accurate light wave distribution before passing through the lens can be obtained. By setting appropriate parameters for the obtained light wave distribution and then performing diffraction calculations, numerical focus changes can be achieved without being restricted by the focal depth of the lens.

[0143] In this way, an objective lens can be used while maintaining the maximum benefits of lensless digital holography.

[0144] The embodiments disclosed herein should be considered to be illustrative in all respects and not restrictive. The scope of the present invention is defined by the claims, not by the above description, and is intended to include all modifications within the meaning and scope of the claims.

[0145] 1, 2 Optical measurement system, 10 Light source, 20 Recording surface, 22 Fourier plane, 24 Focal plane, 26 Sample observation surface, 30 Imaging lens, 40, D Image sensor, 50 Calibration unit, 60, 60A, 60B, 60C, 60D Profile generation unit, 61, 63, 64 Light shielding plate, 62, 66 Pinhole, 65, 67, L1, L2 Condenser lens, 68, MO Objective lens, 100 Processing device, 102 Processor, 104 Main memory, 106 Input unit, 108 Display unit, 110 Storage, 111 Operating system, 112 Measurement program, 113 Calibration information, 114 Hologram data, 115 Measurement results, 120 Interface, 122 Network interface, 124 Media drive, 126 Recording medium, FA Mask, BE Beam expander, BS1, BS2, BS3 beam splitters, M1 mirror, O object beam, Q illumination beam, R reference beam, S sample.

Claims

1. An optical measurement system comprising: a light source that generates coherent light; a beam splitter that splits the coherent light into illumination light and reference light; a combiner that optically superimposes the object light and the reference light generated by illuminating a sample with the illumination light; an objective lens of an infinity corrected optical system arranged between the sample and the combiner; an image sensor that records a hologram generated in the combiner; and a processing device that processes the hologram recorded by the image sensor, wherein the processing device calculates a first complex amplitude distribution on a recording surface of the image sensor based on the hologram, calculates a second complex amplitude distribution on the Fourier plane of the objective lens by performing diffraction calculation of the first complex amplitude distribution over a first distance between the recording surface of the image sensor and the Fourier plane of the objective lens, and coordinate transforms the second complex amplitude distribution into a Fourier spectrum on spatial frequency spectral coordinates.

2. The optical measurement system of claim 1, wherein the processing device treats the Fourier spectrum as a result of Fourier transforming a third complex amplitude distribution in the focal plane of the objective lens.

3. The optical measurement system of claim 2, wherein the processing device calculates a fourth complex amplitude distribution at the sample observation plane by using the Fourier spectrum to perform diffraction calculation of the third complex amplitude distribution over a second distance between the focal plane of the objective lens and the sample observation plane.

4. The optical measurement system of claim 3, wherein the processing device reconstructs an image at a focal plane of the objective lens based on the fourth complex amplitude distribution.

5. An optical measurement system according to any one of claims 1 to 4, wherein the processing device calculates a complex amplitude hologram by applying spatial frequency filtering to the hologram, and calculates the first complex amplitude distribution based on the calculated complex amplitude hologram and information on the light wave distribution of the reference light.

6. An optical measurement method comprising the steps of: constructing an optical system including a light source that generates coherent light; a beam splitter that splits the coherent light into illumination light and reference light; a combining unit that optically superimposes the object light and the reference light generated by illuminating a sample with the illumination light; an objective lens of an infinity corrected optical system that is arranged between the sample and the combining unit; and an image sensor that records a hologram generated in the combining unit; calculating a first complex amplitude distribution on a recording surface of the image sensor based on the hologram recorded by the image sensor; calculating a second complex amplitude distribution on the Fourier plane of the objective lens by performing diffraction calculation of the first complex amplitude distribution over a first distance between the recording surface of the image sensor and the Fourier plane of the objective lens; and coordinate transforming the second complex amplitude distribution into a Fourier spectrum on spatial frequency spectrum coordinates.

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