Angle selection filter

Abstract

To provide an angle selection filter which is a small optical system, can accurately remove unnecessary light from a diffraction optical element, and can obtain a desired optical pattern.SOLUTION: The angle selection filter of the present invention has a periodic fine structure. The filter has a two-dimensional angle distribution response which removes at least one of zero-order light propagating at an angle which corresponds to one of an incident angle or a reflection angle of incident light into a diffraction optical element and high-order diffraction light of the diffraction optical element, when the angle selection filter is arranged on a subsequent stage of the diffraction optical element.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to an angle selection filter, and more particularly to an angle selection filter used together with a diffractive optical element.

Background Art

[0002] A diffractive optical element that spatially modulates at least one physical quantity of the amplitude, phase, and polarization of light and uses the diffraction phenomenon of light to generate a desired light pattern is used in various fields of optical devices and optical technologies, including laser processing, holographic display, cameras, spectrometers, microscopes, lidar, and optical communication. Diffractive optical elements include static diffractive optical elements and dynamic diffractive optical elements. A static diffractive optical element is obtained by subjecting a dielectric material (glass, liquid crystal material, resin material) or a metal material to processing based on lithography, etching, nanoimprinting, laser processing, lathe processing, or multiphoton absorption, photoalignment, or photolithographic exposure. From the change in the physical quantity of light depending on the processed shape, a single light pattern is generated based on the diffraction phenomenon. A dynamic diffractive optical element is a spatial light modulator, a digital micromirror device, etc., which can modulate the physical quantity of light using liquid crystal molecules, MEMS (Micro Electro Mechanical Systems), magneto-optical effects, etc., and make the light pattern generated by the diffraction phenomenon variable.

[0003] The light pattern generated from an actual diffractive optical element generates, as unnecessary light in addition to the desired light pattern, zero-order light and higher-order diffracted light.

[0004] Regarding the zero-order light, the main cause is the processing error of the diffractive optical element. In particular, in the case of a liquid crystal type spatial light modulator, it is caused by the existence of non-modulated light waves due to a low fill factor, and the cross-talk between liquid crystal molecules and voltage. Although it also depends on the distribution of the desired light pattern, the light wave on the Fourier transform plane of the diffractive optical element generally has the strongest intensity of the zero-order light. Depending on the application, the existence of the zero-order light may be negligible, but especially in the fields of laser processing and holographic display, it is necessary to remove the zero-order light. Also, when modulating the complex amplitude distribution of light based on holography with a diffractive optical element (that is, when generating a desired complex amplitude distribution by utilizing interference and diffraction phenomena from the modulation of one physical quantity of the amplitude distribution or the phase distribution), in many hologram calculation methods, the DC light component is generated as the zero-order light, and these zero-order lights must be removed necessarily.

[0005] Regarding the high-order diffracted light, it necessarily occurs when the modulation surface of the diffractive optical element has a periodic structure, for example, when having periodicity such as a square lattice, a zone plate, a hexagonal lattice, etc. Also, similar to the case of the zero-order light, when modulating the complex amplitude distribution of light based on holography with a diffractive optical element, in many hologram calculation methods, not only high-order but also low-order unnecessary diffracted light components (for example, minus first-order light) are generated.

[0006] When it is necessary to remove the above-mentioned zero-order light and higher-order diffracted light, spatial frequency filtering has been performed using a 4f optical system in most conventional applications. FIG. 16 is a conceptual diagram of a 4f optical system. The diffractive optical element 1 generates a predetermined light pattern from the incident light based on the diffraction phenomenon. The light pattern generated by the actual diffractive optical element 1 contains unnecessary light in addition to the desired light pattern. The first lens 2 performs a Fourier transform on the light from the diffractive optical element 1 to obtain a spatial frequency distribution, that is, an angular spectrum distribution. A desired filter 3 (for example, a filter having a desired aperture) is arranged on this plane, and by removing the zero-order light generated near the optical axis, the high-frequency components in the region exceeding a certain spatial frequency band (for example, exceeding the Nyquist size), and the light in a predetermined spectral band, only the light pattern that is purely desired to be generated can be accurately obtained. When it is desired to use the light pattern in the real space plane, a Fourier transform is performed again by the second lens 4.

[0007] Also, in the field of holographic displays, it has been proposed to use a dielectric multilayer film as an angular selection filter (Non-Patent Document 1). A dielectric multilayer film is generally used as a wavelength filter and is a laminate of two or more homogeneous optical thin films having different dielectric constants. The dielectric multilayer film has wavelength selectivity and at the same time has angular selectivity. By utilizing this function, the above-mentioned unnecessary diffracted light components can be removed without using spatial frequency filtering based on a 4f optical system.

Prior Art Documents

Non-Patent Documents

[0008]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0009] As described above, in applications using diffractive optical elements, in order to remove unnecessary zero-order light and higher-order diffracted light, it has been proposed to use spatial frequency filtering based on a 4f optical system or a dielectric multilayer film.

[0010] However, spatial frequency filtering based on a 4f optical system has a problem that the optical system becomes very large. On the other hand, an angle selection filter using a dielectric multilayer film enables miniaturization of the optical system. However, since the dielectric multilayer film is a laminate, the change in dielectric constant occurs only in the uniaxial direction, and the degree of freedom in designing the filter transmittance distribution as an angle filter is extremely low. As shown in Non-Patent Document 1, while removing unnecessary diffracted light components, a part of the desired light pattern is also removed. Furthermore, the shape of the effective spatial frequency band (two-dimensional angular distribution) of the diffractive optical element is usually rectangular, but the shape of the spatial frequency band (two-dimensional angular distribution) when using a dielectric multilayer film as an angle selection filter becomes circular. In this case, since a mismatch occurs between the rectangular and circular shapes, a choice has to be made between passing some of the unnecessary diffracted light or removing some of the desired light pattern, making it difficult to precisely generate the desired light pattern.

[0011] That is, in the above-described method of spatial frequency filtering based on a 4f optical system and the conventional method using a dielectric multilayer film, there is a trade-off relationship between the size of the optical system and the accuracy of the generated light pattern.

[0012] Therefore, an object of the present invention made in view of the above problems is to provide an angle selection filter that can accurately remove unnecessary light from a diffractive optical element and obtain a desired light pattern with a small optical system.

Means for Solving the Problems

[0013] In order to solve the above problems, the angle selection filter according to the present invention is (1) An angle selection filter having a periodic fine structure, which, when disposed downstream of a diffractive optical element, has a two-dimensional angular distribution response for removing at least one of the zero-order light propagating at an angle matching the incident angle or the reflection angle of the incident light on the diffractive optical element and the higher-order diffracted light of the diffractive optical element.

[0014] (2) In the angle selection filter of (1) above, the two-dimensional angular distribution response for removing the higher-order diffracted light preferably has a response that allows light within a predetermined angular range set by these parameters to pass through and attenuates light exceeding this angular range, where p is the pitch of the square lattice of the diffractive optical element, λ is the wavelength of the light source, and n is the diffraction order.

[0015] (3) In the angle selection filter of (1) or (2) above, it is preferable to stack an angle selection filter for removing the zero-order light propagating at an angle matching the incident angle or the reflection angle of the incident light on the diffractive optical element and an angle selection filter for removing the higher-order diffracted light of the diffractive optical element to remove the zero-order light and the higher-order diffracted light.

[0016] (4) In any of the angle selection filters of (1) to (3) above, it is preferable that the angle selection filter for removing the higher-order diffracted light of the diffractive optical element is configured by stacking four angle selection filters each having an angular distribution response for attenuating light exceeding a predetermined angle in a uniaxial angular direction and rotating them by 90 degrees each.

[0017] (5) In any of the angle selection filters of (1) to (3) above, it is preferable that the angle selection filter for removing the higher-order diffracted light of the diffractive optical element is an angle selection filter having an angular distribution response for attenuating light in a specific angular region in a uniaxial angular direction, and is configured by stacking a plurality of angle selection filters in which the angular regions are shifted from each other.

[0018] (6) Any of the angular selection filters of (1) to (5) above preferably further has a passband in the uniaxial angular direction reduced to half or less.

[0019] (7) Any of the angular selection filters of (1) to (6) above preferably further removes the zeroth-order light of the cross generated along the angular axis.

[0020] (8) Any of the angular selection filters of (1) to (7) above preferably comprises a one-dimensional or two-dimensional periodic dielectric or metal microstructure smaller than the wavelength of the incident light on one or more dielectric layers.

[0021] (9) Any of the angular selection filters of (1) to (8) above is preferably arranged in close contact with the diffractive optical element.

Advantages of the Invention

[0022] According to the angular selection filter of the present invention, unnecessary light from a diffractive optical element can be accurately removed in a small optical system, and a desired light pattern can be generated.

Brief Description of the Drawings

[0023]

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Embodiments for Carrying Out the Invention

[0024] Hereinafter, embodiments of the present invention will be described with reference to the drawings.

[0025] FIG. 1 is a conceptual diagram of an optical system using the angle selection filter of the present invention. In the optical system of FIG. 1, the diffractive optical element 1 generates a predetermined optical pattern from the incident light. The generated optical pattern contains unwanted light in addition to the desired optical pattern. In the optical system, the angle selection filter 10 is arranged at the subsequent stage of the diffractive optical element 1. For the light wave modulated by the diffractive optical element 1, the angle selection filter 10 having a periodic fine structure of a dielectric or metal with a size on the order of wavelength removes the unwanted light components. Thus, the desired optical pattern is obtained after the application of the angle selection filter 10. The angle selection filter 10 may be arranged at a position arbitrarily distant from the diffractive optical element 1, or may be in close contact with the diffractive optical element 1 itself. FIG. 1 shows a conceptual diagram in the case where the angle selection filter 10 of the present invention is introduced into the transmissive diffractive optical element 1, but it can also be applied when the diffractive optical element 1 is reflective.

[0026] Figures 2A and 2B are conceptual diagrams of an optical system using the angle selection filter 10 of the present invention when the diffractive optical element 1 is of the reflective type. In Figure 2A, the optical system has a beam splitter 5 disposed between the diffractive optical element 1 and the angle selection filter 10. The component of the incident light reflected by the beam splitter 5 travels toward the diffractive optical element 1. The diffractive optical element 1 generates reflected light having a predetermined light pattern based on the diffraction phenomenon. This reflected light contains unwanted light in addition to the desired light pattern. The reflected light passes through the beam splitter 5 and enters the angle selection filter 10 having a periodic fine structure. The angle selection filter 10 removes the unwanted light components, and a desired light pattern is obtained after transmission through the angle selection filter 10.

[0027] Figure 2B shows an optical system in the case where light propagates in the order of the beam splitter 5, the angle selection filter 10, and the diffractive optical element 1. In this optical system, in order to prevent the incident light from being removed by the angle selection filter 10, the incident angle of the incident light is deliberately shifted to enter each optical element. The incident light enters the beam splitter 5 at a predetermined angle, and the component reflected by the beam splitter 5 travels toward the angle selection filter 10. Since the angle selection filter 10 is disposed at an inclination with respect to the incident light, the incident light passes through the angle selection filter 10 and travels toward the diffractive optical element 1. The diffractive optical element 1 generates reflected light having a predetermined light pattern based on the diffraction phenomenon and reflects the light at an angle in the direction opposite to the incident angle. This reflected light contains unwanted light in addition to the desired light pattern. In Figure 2B, the reflected light is set to be incident perpendicularly to the main surface of the angle selection filter 10 disposed at an inclination, and the angle selection filter 10 removes the unwanted light. The light pattern from which the unwanted light has been removed passes through the beam splitter 5 and is output.

[0028] In addition, in Figures 2A and 2B, an example is illustrated in which the light propagating from the diffractive optical element 1 enters from the normal direction of the angle selection filter 10. However, as will be described later, in order to change the response of the angle selection filter 10, the angle selection filter 10 may be actively disposed at an inclination (see Figure 7B).

[0029] The angular selection filter 10 can be realized by a metasurface, a plasmonic crystal, or a photonic crystal, and can be fabricated by a method based on lithography, etching, nanoimprinting, laser processing, multiphoton absorption, or optical interference exposure. FIGS. 3A to 3D show conceptual diagrams of the structure of the angular selection filter 10 used in the present invention.

[0030] The angular selection filter 10 of the present invention is composed of a periodic microstructure 13 of a dielectric or metal having a size on the order of wavelength, formed on a substrate 11. The material of the substrate 11 may be any optically transparent material. As the material of the microstructure 13, as the dielectric, SiO2, Si, TiO2, SiN, GaN, resin materials, etc. are applicable, and as the metal material, plasmonic materials such as Au, Ag, and Al can be applied. Also, a layer 12 of a dielectric material may be provided between the microstructure 13 and the substrate 11.

[0031] FIG. 3A shows an angular selection filter 101 having a one-dimensional periodic microstructure 13 on a substrate 11, and FIG. 3B shows an angular selection filter 102 having a two-dimensional periodic microstructure 13 on a substrate 11. Also, FIG. 3C shows an angular selection filter 103 having a one-dimensional periodic microstructure 13 via a layer 12 of a dielectric material on a substrate 11, and FIG. 3D shows an angular selection filter 104 having a two-dimensional periodic microstructure 13 via a layer 12 of a dielectric material on a substrate 11. Note that the substrate 11 may be regarded as a single dielectric layer, and the layer 12 of the dielectric material may be a plurality of layers. Also, in FIGS. 3B and 3D, an example in which cubic microstructures 13 are periodically arranged is described, but triangular prisms, triangular pyramids, or free-form microstructures 13 derived by optimization calculations may also be used.

[0032] The dimensions and shape of the microstructure 13 for obtaining the response of the desired angle selection filter 10 are not obtained by analytical calculations, but are obtained by calculating the electromagnetic wave response by the time-domain finite-difference method, the rigorous coupled-wave theory, or the finite element method. Further, the appropriate dimensions and shape of the microstructure 13 change according to the refractive index, absorption rate, thickness, film quality of the material, and the arrangement interval of the microstructure 13 in the plane. Since the edge portion of the microstructure 13 also has a vertical wall surface, a tapered shape, etc., there are innumerable options for the dimensions and shape of the microstructure 13 for obtaining the response of the desired angle selection filter 10. Therefore, in realizing the present invention, the microstructure 13 is not limited to a specific dimension and shape. The essential point as the angle selection filter 10 in the present invention is that it is formed of a periodic microstructure of a dielectric or a metal having a size on the order of wavelength (smaller than the wavelength of incident light), and has a characteristic spatial frequency response (two-dimensional angle distribution response) as described later.

[0033] FIG. 4A is an example of the two-dimensional angle distribution response of the angle selection filter 10 of the present invention. This angle selection filter 10 is designed corresponding to the diffractive optical element 1 shown in FIG. 4B. That is, when determining the passband of the two-dimensional angle distribution response of FIG. 4A, the parameters of the diffractive optical element (arranged in front of the angle selection filter 10) in FIG. 4B are referred to. When the pitch of the diffractive optical element 1 having the periodicity of a square lattice is p and the wavelength of the incident light modulated by the diffractive optical element 1 is λ, the angle selection filter 10 is

[0034]

Equation

[0035] It allows light within a certain range to pass through and blocks light beyond that area. In FIG. 4A, the white area represents the range with a transmittance of 1, and the black area represents the range with a transmittance of 0. n is the diffraction order. When n = 1, this range corresponds to the Nyquist region of the diffractive optical element and can remove unnecessary higher-order light components resulting from the periodicity of the square lattice. When n = 2, it corresponds to twice the Nyquist region and can more efficiently generate light with a higher spatial frequency. However, as the diffraction order n increases, the noise components increase. The value of n can be changed and set by the user according to the application. In addition, when the diffractive optical element 1 has the periodicity of a rectangular lattice, the size of the passband can be set by changing the size of p in each of the x and y directions.

[0036] Furthermore, in order to remove the zero-order light component that propagates at an angle matching the incident angle of the light incident on the diffractive optical element 1 (in the case of a reflective diffractive optical element, the reflection angle with the opposite sign to the incident angle), θ x 、θ y suppresses light near 0. Therefore, when arranged downstream of the diffractive optical element 1, the angle selection filter 10 having the two-dimensional angular distribution response in FIG. 4A removes the higher-order diffracted light of the diffractive optical element 1 and the zero-order light that propagates at an angle matching the incident angle or the reflection angle of the light incident on the diffractive optical element 1. Note that in FIG. 4A, an ideal response with a two-tone transmittance is illustrated, but the value of each transmittance may vary, and the angular distribution response may not change stepwise but may change continuously. Removing unnecessary light in the present invention may include attenuating the unnecessary light to such an extent that it does not adversely affect the desired light pattern.

[0037] The two-dimensional angle distribution response of Fig. 4A does not necessarily need to be realized by a single angle selection filter, and in order to reduce the difficulty of producing an angle selection filter, the two-dimensional angle distribution response may be decomposed, angle selection filters having each response may be produced, and these may be laminated. For example, a desired angle selection filter may be produced by laminating an angle selection filter that removes zero-order light propagating at an angle that matches the incident angle or reflection angle of the incident light to the diffractive optical element, and an angle selection filter that removes higher-order diffracted light of the diffractive optical element.

[0038] FIG. 5 shows an example of a configuration in which the two-dimensional angular distribution response of FIG. 4A is realized by stacking multiple angle-selective filters. Specifically, the two-dimensional angular distribution response of FIG. 4A is a two-dimensional angular distribution response (a) that removes or attenuates the zero-order light, and a two-dimensional angular distribution response (b) that removes or attenuates the zero-order light, and x Or θ y direction (called "angle direction of one axis") -1 This is an example of decomposing into four two-dimensional angular distribution responses (b) to (e) that remove or attenuate light in the range exceeding (λ / 2p). When angle selection filters 21 to 25 corresponding to the two-dimensional angular distribution responses (a) to (e) are superimposed, the filter characteristics become multiplication, and only the common area with transmittance 1 in all filters becomes the transmission area of ​​the superimposed filters.

[0039] 5(b) to (e) are two-dimensional angle distribution responses with rotation angles differing by 90 degrees, so if an angle-selective filter having one of the responses is fabricated, the desired four responses can be obtained by rotating it by 90 degrees. When stacking multiple angle-selective filters 21 to 25, all the angle-selective filters may be in contact with each other or may be spaced apart.

[0040] In addition, at least one of the angle selection filters shown in FIG. 5 may be fabricated and applied to various applications. If there is no practical problem in not removing all of the zeroth order light and the higher order diffracted light, it is possible to remove only one of the unnecessary lights. For example, it is possible to remove only the zeroth order light, or to remove θ x It may also be used to remove only high-order diffracted light in a direction.

[0041] Any one of the two-dimensional angular distribution responses (b) to (e) in FIG. 5 may be further divided and composed of a plurality of angular selection filters. FIG. 6 is a configuration example when one of the two-dimensional angular distribution responses in FIG. 5 is realized by stacking a plurality of angular selection filters. Specifically, the two-dimensional angular distribution response (b) in FIG. 5 is decomposed into two-dimensional angular distribution responses (a) to (n) in which a specific angular region of θ x is set as a narrow-band light blocking region (attenuation region), and the angular regions are slightly shifted from each other, and the angular selection filters 221 to 224 for realizing the two-dimensional angular distribution responses (a) to (n) are stacked to realize the angular selection filter 22 in FIG. 5.

[0042] Note that the angular selection filters 221 to 224 for the responses (a) to (n) in FIG. 6 can also be realized by arranging one angular selection filter at an inclination. FIG. 7A is the two-dimensional angular distribution response of the angular selection filter that removes light near θ x = 0. The angular selection filter having the two-dimensional angular distribution response in FIG. 7A is arranged at an inclination of θ x1 from the normal direction of the incident light as shown in FIG. 7B. Due to the arrangement in FIG. 7B, an angular distribution response having a narrow-band blocking region (attenuation region) near θ x = θ x1 can be realized.

[0043] FIG. 8 is another configuration example when the two-dimensional angular distribution response in FIG. 4A is realized by stacking a plurality of angular selection filters. The two-dimensional angular distribution response in FIG. 4A includes a two-dimensional angular distribution response (a) that removes or attenuates the zero-order light, and θ x or θ yIt can be decomposed into two-dimensional angular distribution responses (b) to (g) that remove or attenuate light in a specific angular region of the direction. Each of the angular filters 31 to 33 having the angular distribution responses of (b) to (d) in FIG. 8 has an angular distribution response symmetric with respect to the x direction with respect to the origin. By rotating each of the angular filters 31 to 33 having the angular distribution responses of (b) to (d) in FIG. 8 by 90 degrees, angular filters 34 to 36 having the angular distribution responses of (e) to (g) symmetric with respect to the y direction are obtained. By stacking these, the two-dimensional angular distribution response of FIG. 4A can be realized.

[0044] FIG. 9 is an example of a two-dimensional angular distribution response when holography is implemented with a diffractive optical element. When it is desired to obtain a target complex amplitude distribution by modulating only the amplitude or phase with a pattern encoded based on holography for the diffractive optical element 1 and removing unnecessary light, an angular selection filter 10 having response characteristics as shown in FIG. 9 is used. θ x Or θ y One of the passbands is reduced to less than half. By removing light in at least half of the quadrants, conjugate images that are generated in principle by holography and unnecessary diffracted light components (for example, minus first-order light, etc.) can be removed.

[0045] In the filter of FIG. 9, unnecessary zero-order light and higher-order diffracted light can be removed. However, when the intensity of the zero-order light generated from the diffractive optical element is high, on the focal plane, the zero-order light is not point-like, but θ x axis and θ y It occurs in a cross shape along the angular axes of the axis. If the influence of this cross-shaped unnecessary diffracted light cannot be ignored and it is desired to remove it, for example, as shown in FIG. 10, at θ x An angular response characteristic of removing a width of a predetermined angle from 0° (all directions of θ y direction) may be added to the angular response characteristic of the angular selection filter of FIG. 9 described above. Note that since the second and third quadrants of the two-dimensional angular distribution response in FIG. 9 are light removal regions, θ y Below 0° in the direction has already been removed. Furthermore, if the removal region is expanded by a predetermined angle in the positive direction of θ y the unnecessary diffracted light in the cross shape can be completely removed.

[0046] The two-dimensional angular distribution responses of all the angle selection filters 10 described above are all settings for effectively utilizing the spatial bandwidth product of the diffractive optical element 1, and the two-dimensional angular distribution response may be narrowed or deformed according to the application and the difficulty of processing.

[0047] (Example 1) The structure of one period of the angle selection filter 10 composed of a fine rectangular structure in the present invention is shown in FIG. 11. FIG. 11 corresponds to the cross-sectional view of the one-dimensional periodic angle selection filter 103 in FIG. 3C. In FIG. 11, the substrate 11 (medium 1) is made of synthetic quartz, the layer 12 (medium 2) and the fine structure 13 (medium 3) are made of SiN, and the parameters of each structure are set as follows.

[0048] [Table 1]

[0049] When the structure of the angle selection filter 10 is set as Condition 1, the angular response characteristic in the x-axis direction is shown in FIG. 12A. By using this angle selection filter 10, light with an incident angle near 0° (in this example, -2° ≤ θ ≤ 2°) can be cut. That is, the 0th-order light from the diffractive optical element 1 can be suppressed. Note that the angular response characteristic of this filter in the y-axis direction is generally constant, resulting in a two-dimensional angular response distribution as shown in FIG. 10. Also, by setting the structure in FIG. 11 as the two-dimensional periodic structure shown in FIG. 3D, angular selectivity can also be realized in the y-axis direction, and the two-dimensional angular response distribution (a) in FIG. 5 can be realized.

[0050] (Example 2) In the structure of one period of the angle selection filter 10 shown in FIG. 11, the materials of the substrate 11 (medium 1), the layer 12 (medium 2), and the fine structure 13 (medium 3) are the same as those in Example 1, and the parameters of each structure are set as follows.

[0051] [Table 2]

[0052] When the structure of the angle selection filter 10 is set as condition 2, the angle response characteristic in the x-axis direction is as shown in FIG. 12B. This filter can cut incident light near ±6°. The angle response characteristic of this filter in the y-axis direction is generally constant, resulting in a two-dimensional angle response distribution as shown in FIG. 8(b). Also, by preparing a plurality of these elements and arranging them while slightly changing the arrangement angle as shown in FIG. 7B, incident light within a specific incident angle range can be cut.

[0053] Note that the angle response characteristics in FIGS. 12A and 12B are just examples, and various angle response characteristics can be realized by optimizing the parameters, materials, shapes, etc. in FIG. 11.

[0054] (Verification experiment) With the application in a holographic display in mind, the optical system shown in FIG. 13 was realized by simulation to verify the effect of the present invention. The optical system in FIG. 13 includes a light source 6, a spatial filter 7, a lens 2, a beam splitter 5, a diffractive optical element 1, and an angle selection filter 10. All calculations in the verification experiment utilized propagation calculations based on the angular spectrum method.

[0055] The light source 1 is a coherent laser with a wavelength of 633 nm. From this laser light, a plane wave was generated by the spatial filter 7 and the lens 2. The plane wave is incident on the beam splitter 5 and reflected, then incident on the diffractive optical element 1.

[0056] The diffractive optical element 1 is composed of an amplitude modulation type spatial light modulator and displays the pattern of a computer-generated hologram. The pixel pitch of the spatial light modulator is 8 μm, and the number of pixels for displaying the computer-generated hologram is set to 512 × 512 pixels. Fig. 14 shows the computer-generated hologram displayed on the spatial light modulator. This computer-generated hologram is an amplitude modulation type, that is, a hologram in which the information of the complex amplitude distribution is compressed into the amplitude distribution, and is created by off-axis interference between the object light and the reference light. When a plane wave is irradiated onto the spatial light modulator that displays this computer-generated hologram, a light wave having the information of the hologram is reflected by diffraction. The reflected light passes through the beam splitter 5 and enters the angle selection filter 10.

[0057] The angle selection filter 10 was assumed to have the ideal two-dimensional angular distribution response characteristics shown in Fig. 9. The ideal angle selection filter 10 was applied to the reflected light, and the generated light pattern was photographed.

[0058] Figs. 15A to 15C are images (light patterns) of the results obtained in this verification experiment. These are obtained by acquiring intensity images while shifting the light detection surface in the optical axis direction, and the target light pattern is clearly obtained three-dimensionally according to the distance in the optical axis direction as "1", "2", "3". Note that from Figs. 15A to 15C, the image is shifted to the upper left as a whole, which is because the linear phase of the reference light is superimposed during the production of the computer-generated hologram and is not due to the present invention. Also, this shift of the image is caused simply by the propagation direction being oblique, so it can be avoided by adjusting the optical system.

[0059] For comparison, the results of obtaining intensity images with an optical system from which the angle selection filter 10 was removed (the present invention was not applied) are shown in FIGS. 15D to 15F. Since the zero-order light and conjugate images derived from holographic encoding could not be removed, the contrast is very low and a low-quality image is obtained. From the above, it was confirmed that with the angle selection filter of the present invention, unnecessary light can be removed with a simple and compact optical system, and a desired light pattern can be obtained. In particular, in the application of a holographic display, a high-quality reproduced image can be obtained with a small optical system.

[0060] Here, a verification experiment assuming application in a holographic display was conducted, but the present invention can be utilized in all applications introducing diffractive optical elements.

[0061] Although the above-described embodiments have been described as representative examples, it is obvious to those skilled in the art that many changes and substitutions can be made within the spirit and scope of the present invention. Therefore, the present invention should not be construed as being limited by the above-described embodiments, and various modifications or changes are possible without departing from the scope of the claims.

Explanation of Reference Numerals

[0062] 1 Diffractive optical element 2 Lens 3 Filter 4 Lens 5 Beam splitter 6 Light source 7 Spatial filter 10 Angle selection filter 11 Substrate 12 Dielectric material layer 13 Microstructure 21 to 25 Angle selection filters 31 to 36 Angle selection filters

Claims

1. An angle selection filter having a periodic fine structure, which, when disposed downstream of a diffractive optical element, has a two-dimensional angular distribution response for removing at least one of the zero-order light propagating at an angle matching the incident angle or the reflection angle of the incident light on the diffractive optical element and the higher-order diffracted light of the diffractive optical element. An angle selection filter having a two-dimensional angular distribution response for removing at least one of the zero-order light propagating at an angle matching the incident angle or the reflection angle of the incident light on the diffractive optical element and the higher-order diffracted light of the diffractive optical element.

2. The angle selection filter according to claim 1, wherein the two-dimensional angular distribution response for removing the higher-order diffracted light allows light within an angular range of 【Number 1】 to pass through and attenuates light outside this angular range, where p is the pitch of the square lattice of the diffractive optical element, λ is the wavelength of the light source, and n is the diffraction order.

3. The angle selection filter according to claim 1 or claim 2, wherein the angle selection filter for removing the zero-order light propagating at an angle matching the incident angle or the reflection angle of the incident light on the diffractive optical element and the angle selection filter for removing the higher-order diffracted light of the diffractive optical element are laminated to remove the zero-order light and the higher-order diffracted light.

4. The angle selection filter according to claim 1 or claim 2, wherein the angle selection filter for removing the higher-order diffracted light of the diffractive optical element is configured by rotating and laminating four angle selection filters each having an angular distribution response for attenuating light exceeding a predetermined angle in a uniaxial angular direction by 90 degrees each.

5. The angle selection filter according to claim 1 or claim 2, wherein the angle selection filter for removing the higher-order diffracted light of the diffractive optical element is an angle selection filter having an angular distribution response for attenuating light in a specific angular region in a uniaxial angular direction, and is configured by laminating a plurality of angle selection filters in which the angular regions are shifted from each other.

6. The angle selection filter according to claim 1 or claim 2, further comprising an angle selection filter having a passband in a uniaxial angular direction reduced to half or less.

7. The angle selection filter according to claim 1 or claim 2, further comprising an angle selection filter for removing the cross-shaped zero-order light generated along the angular axis.

8. The angle selection filter according to claim 1 or claim 2, comprising a one-dimensional or two-dimensional periodic dielectric or metal fine structure smaller than the wavelength of the incident light on a layer of one or more dielectric layers.

9. In the angular selection filter according to claim 1 or claim 2, An angular selection filter disposed in close contact with the diffractive optical element.

Images