JP2001100015A - Diffracting optical element and optical system composed of same diffracting optical element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a diffracting optical element which can form a polygonal shape capable of maintaining high diffraction efficiency by a small number of straight lines when forming the polygonal shape similar to an ideal shape of a relief type diffracting optical element and can effectively suppress a flare, etc., when built in an optical system and the optical system composed of the diffracting optical element. SOLUTION: The diffracting optical element has a laminate structure wherein at least two or more layers of diffraction gratings are stacked; and the sectional phase of the diffraction grating is a relief type shape derived from a shape shift function by the polygonal shape obtained by connecting one or more straight lines in order and the sectional shapes of the diffraction gratings which are put one over another are similar.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a diffractive optical element, and more particularly to a diffractive optical element used for a plurality of wavelengths or bands of light and an optical system using the same.

[0002]

2. Description of the Related Art Hitherto, a method of reducing chromatic aberration by combining glass materials has been used. On the other hand, a method of providing a diffractive optical element (hereinafter also referred to as a diffraction grating) having a diffractive action on a lens surface or a part of an optical system to reduce chromatic aberration is described in SPIE Vol. 1354 International
onal Lens Design Conferen
ce (1990), JP-A-4-213421, JP-A-6-324262, U.S. Pat.
No. 44706 and the like. This utilizes a phenomenon in which chromatic aberration with respect to a light beam having a certain reference wavelength appears in a reverse direction on a refraction surface and a diffraction surface arranged in an optical system. Further, such a diffractive optical element can have the effect of an aspheric lens by changing the period of the periodic structure, and has a great effect in reducing aberration.

In refraction, one ray is one ray even after refraction, but in diffraction, rays are divided into each order.
In order to use a diffractive optical element as a lens system, it is necessary to determine a grating structure such that light beams in a used wavelength region concentrate on a specific order (hereinafter, also referred to as a design order). When light is concentrated at a specific order, the light intensity of the other diffracted light is low, and when the intensity is 0, the diffracted light does not exist. Therefore, in order to effectively use the feature, it is necessary that the diffraction efficiency of the light of the design order is sufficiently high over the entire wavelength range of use. Here, when a light beam having a diffraction order other than the design order exists, an image is formed at a different position from the light beam of the design order, so that flare light is generated. Therefore,
In an optical system using the diffraction effect, it is important to consider the spectral distribution of diffraction efficiency at the design order and the behavior of light rays other than the design order.

FIG. 11 shows the diffraction efficiency at a specific diffraction order when a diffractive optical element as shown in FIG. 10 is formed on a certain surface. The horizontal axis represents wavelength, and the vertical axis represents diffraction efficiency. This diffractive optical element is designed so as to have the highest diffraction efficiency in the used wavelength region at the first diffraction order. In this case, the design order is the first order. Furthermore, ± 1 of the design order
The following diffraction efficiency is also shown. As shown in FIG. 11, the diffraction efficiency becomes highest at a certain wavelength of the design order (hereinafter, referred to as the diffraction efficiency).
Design wavelength). At other wavelengths, the diffraction efficiency gradually decreases. The decrease in the diffraction efficiency in the design order becomes diffracted light in another order and appears as a flare. In particular, when a plurality of diffractive optical elements are used, a decrease in diffraction efficiency at a wavelength other than the design wavelength leads to a decrease in transmittance.

[0005] JP-A-9-127321, JP-A-9-127321
Japanese Unexamined Patent Publication (Kokai) No. 127322 discloses a configuration capable of reducing this reduction in diffraction efficiency. Japanese Patent Application Laid-Open No. 9-127321 has a sectional shape superimposed on two layers.
No. 127322 has a lattice structure superimposed on three layers. This diffractive optical element achieves high diffraction efficiency by optimizing the refractive index difference between the materials before and after the boundary and the depth of the grating groove in the diffractive optical element formed on the boundary surface of each material.

In the above embodiment, since it is necessary to set the wavelength characteristic of the difference between the refractive indices of the materials before and after the grating region to a desired value, the refractive index of a conventional diffractive optical element having one side made of air is higher. The difference cannot be made large, so that the grating thickness becomes considerably thick. In Japanese Patent Application Laid-Open No. 9-127321, the thickness is about 10 μm, and in Japanese Patent Application Laid-Open No. 9-127322, the number of types of materials and the number of lattices are increased to two.
At least one grating thickness has a grating thickness of 7 μm or more.
This can be said to be a considerably deeper grating shape as compared with a normal one-layer diffraction grating having a grating thickness of about 1 μm.

With the recent demand for smaller and lighter optical systems, relief type diffractive optical elements have attracted attention, and the cross-sectional shape of the relief type diffractive optical element has a shape derived from a phase shift function (hereinafter, an ideal shape). It is desirable to make the cross section an ideal shape, but there are various problems such as processing technology, processing time, and time required for inspection. For example, in order to obtain an ideal shape by cutting, it is necessary to perform processing at the tip of the cutting tool, and it is difficult to obtain a good surface roughness as compared with the case of performing processing at the side surface of the cutting tool. Therefore, when processing this relief type diffractive optical element, a method of forming a polygonal shape by connecting a shape approximate to a curved shape as an ideal shape with a plurality of straight lines by cutting is disclosed in JP-A-10-293205. And Japanese Unexamined Patent Application Publication No. 10-332918.

[0008]

However, when the cross-sectional shape of the relief type diffractive optical element is formed by connecting a plurality of straight lines to form a polygonal shape, and the shape approximate to the ideal shape is formed, the shape of the relief type diffractive optical element may be changed to the ideal shape. Since the diffraction efficiency decreases due to the deviation, it is necessary to reduce the deviation from the ideal shape. As described above, in a diffractive optical element composed of a plurality of layers, when the grating thickness becomes deep, the curvature of the ideal shape also becomes large. Therefore, when trying to obtain a shape close to the ideal shape, when cutting into a polygonal shape, the number of straight lines for forming the polygonal shape increases, and the processing time increases due to an increase in the number of times of processing. It becomes longer, and the merits of forming a shape that approximates a curved shape as an ideal shape by connecting the cross-sectional shapes with a plurality of straight lines and forming a polygonal shape are reduced.

In view of the above, the present invention has been made to solve the above-mentioned problems, and when forming a polygonal shape approximate to an ideal shape in a relief type diffractive optical element, a polygonal shape capable of maintaining high diffraction efficiency is formed with a small number of straight lines. It is an object of the present invention to provide a diffractive optical element capable of effectively suppressing flare and the like when incorporated in an optical system, and an optical system constituted by the diffractive optical element.

[0010]

According to the present invention, in order to achieve the above object, a diffractive optical element and an optical system constituted by the diffractive optical element are configured as in the following (1) to (12). It is characterized by the following. (1) A diffractive optical element according to the present invention is a diffractive optical element having a laminated structure in which at least two or more diffraction gratings overlap, wherein the cross-sectional shape of the diffraction grating is a polygon in which one straight line or a plurality of straight lines are sequentially connected. Depending on the shape, it has a shape similar to a relief type shape derived from a phase shift function, and the cross-sectional shape of the overlapping diffraction grating is similar. (2) The diffractive optical element of the present invention is characterized in that the diffraction grating is made of at least two kinds of materials having different dispersions. (3) The diffractive optical element of the present invention is characterized in that an air layer is provided between the diffraction gratings made of the materials having different dispersions. (4) The diffractive optical element of the present invention is characterized in that the polygonal shape in which the plurality of straight lines are sequentially connected is configured as a convex shape. (5) The diffractive optical element of the present invention is characterized in that the polygonal shape in which the plurality of straight lines are sequentially connected is formed in a lattice shape in which one layer is convex and two layers are concave. (6) The diffractive optical element of the present invention is characterized in that adjacent diffraction gratings have different shapes. (7) The diffractive optical element according to the present invention is characterized in that adjacent diffraction gratings have different numbers of a plurality of straight lines for forming the polygonal shape by being sequentially connected. (8) An optical system having a diffractive optical element according to the present invention is characterized in that the diffractive optical element is constituted by any one of the above-described diffractive optical elements according to the present invention. (9) The optical system according to the present invention is an imaging optical system. (10) In the image forming optical system of the present invention, the flat glass surface, the lens curved surface, or the inside of the taking lens arranged near the stop is constituted by any one of the diffractive optical elements of the present invention described above. I have. (11) The optical system according to the present invention is an observation optical system. (12) In the observation optical system of the present invention, the objective lens section, the prism surface, or the inside of the eyepiece is constituted by any one of the above-described diffractive optical elements of the present invention.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS As described above, by processing a plurality of overlapping diffraction gratings into similar shapes, in a diffractive optical element having a laminated structure, the number of divisions of each diffraction grating by straight lines is reduced. And the workability of the lattice shape at the time of cutting or the like is greatly improved. In addition, since the number of divisions of the grating is reduced, it is easy to obtain good surface roughness, and a good diffractive optical element can be obtained. Therefore, even when incorporated in an optical system, high diffraction efficiency can be maintained. Further, by using the diffractive optical element of the present invention, a high-performance photographing lens or a high-performance observation optical system can be provided.

[0012]

Embodiments of the present invention will be described below. Embodiment 1 FIG. 1 is a front view of a diffractive optical element according to Embodiment 1 of the present invention. The diffractive optical element 1 has a configuration in which a diffraction grating 3 is formed on the surface of a substrate 2. FIG. 2 shows FIG.
Is a part of the cross-sectional shape taken along the line AA ′ of FIG. FIG.
Is a diagram exaggerated in the depth direction. Although the sectional shape of the entire diffractive optical element is not shown in FIG. 2, the first layer region 4, the second layer region 5,
The structure has an air space 6 between the first layer region and the second layer region. Further, the first layer region and the second layer region are made of different materials, and function as one diffractive optical element throughout all layers.

Next, the diffraction efficiency of the diffractive optical element of the present invention will be described. In a single-layer transmission type diffraction grating as shown in FIG. 10, the condition at which the diffraction efficiency is maximized at the design wavelength λ ₀ is that when the light beam is perpendicularly incident on the grating, the peaks and valleys of the diffraction grating are It is sufficient that the optical path length difference is an integral multiple of the wavelength, and (n ₀₁₁ −n ₀₂₁ ) d = mλ ₀ (1). Here, n ₀₁₁ is the refractive index of the material on the incident side at the wavelength λ ₀ , and n ₀₂₁ is the refractive index of the material on the emitting side at the wavelength λ ₀ .
d is the grating thickness and m is the diffraction order.

A diffractive optical element having a structure of two or more layers
Is basically the same, as one diffraction grating through all layers
In order to work, the diffraction grating formed at the boundary of each material
Obtain the optical path length difference between the peak and valley of the child and spread it over all layers.
Is determined so that the sum is an integer multiple of the wavelength
You. Therefore, the conditional expression for the ideal shape shown in FIG.₀₁₁-N₀₂₁) D ₁± (n₀₂₂-N₀₁₂) D_Two= Mλ₀ (2) Where n₀₁₁Is the material on the incident side of the first diffraction grating
Quality wavelength λ₀Index of refraction at n₀₂₁Is the emission of the first diffraction grating
Wavelength of material on the side₀Index of refraction at n₀₁₂Is the second diffraction grating
Wavelength λ of the material on the incident side of₀Index of refraction at n₀₂₂Is the second time
The wavelength λ of the material on the exit side of the folded grating₀Index of refraction at d₁, D_Two
Are the grating thicknesses of the first diffraction grating and the second diffraction grating, respectively.
is there. Here, the diffraction direction is shifted leftward from the 0th-order diffracted light in FIG.
Assuming that the diffraction order is a positive diffraction order, each layer in the equation (2)
The sign of the addition and subtraction is the lattice whose lattice thickness decreases from left to right in the figure.
In the case of a shape, the lattice thickness increases from left to right.
Is negative in the case of a lattice shape. In FIG. 2, each diffraction grating surface is
It is formed at the boundary surface with air, but it is clear from equation (2).
It is not limited to this as it seems, but two different
Having a diffraction grating on the boundary surface of the material
May be.

Next, a diffraction grating formed on the diffractive optical element will be described. The phase function representing the diffraction grating 3 formed on the diffractive optical element 1 is: Becomes Here, r is the distance from the center of the substrate. A structure that gives 2π as a phase difference is one cycle, and the pitch is P
given that, Than Becomes

A specific example will be described below to explain the difference between the configuration of the present embodiment and the conventional configuration. In order to clarify the difference in diffraction efficiency due to the combination of the shapes of the overlapping diffraction gratings, the shape shown in FIG. 2 is assumed to be an ideal shape, and as an extreme example, both diffraction grating shapes are formed by straight lines (similar shape). FIG. 3 and FIG. 4 show two combinations in which one is constituted by a straight line. The shape of the diffraction grating in this embodiment is not limited as long as the vertically overlapping gratings have similar shapes. Also, the shapes of adjacent diffraction gratings need not be the same. Further, the laminated diffraction grating of this embodiment has a two-layer structure shown in FIG. Here, the material and grid thickness are shown in FIG.
4 to Dainippon Ink and Chemicals ultraviolet curing resin C001 of _{(n d = 1.524, ν d} = 50.8), ultraviolet curable resin 5 in FIG. 2 2 (n _d = 1.635, ν _d = 23.
0) as an example. The grating thickness of the diffraction grating 4 in FIG. 2 is 9.5.
2, the grating thickness of the diffraction grating 5 in FIG. 2 is 6.9 μm, and the distance between the two gratings (6 in FIG. 2) is 1.0 μm. Also,
The grating shape of the diffraction grating in this embodiment is as follows: C ₁ = −4.012057 × 10 ⁻⁵ and C ₂ = − in equation (4).
The sum up to the second term obtained as 4.823176 × 10 ⁻¹⁰ is substituted into the equation (3), and λ ₀ = 587.56 in the equation (3).
According to the phase function φ (r) obtained as [nm].

FIG. 5 shows the diffraction efficiency of the diffraction grating shown in FIGS. 2, 3 and 4 when the tenth orbicular zone is taken as an example. In the diffraction efficiency graph shown in FIG. 5, the maximum diffraction efficiency in the ideal shape is 100.0%. The difference between the diffraction grating formed in a similar shape and the ideal shape is 0.032 at the maximum.
%, The difference between the one formed in the different shape and the ideal shape was 2.581% at the maximum. Accordingly, in the present invention, since the diffraction grating can be constituted by a straight line while maintaining a high diffraction efficiency in the diffractive optical element, the diffractive optical element can be easily manufactured.

Embodiment 2 FIG. 6 shows a cross-sectional shape of a diffraction grating of Embodiment 2. In the first embodiment, in order to clearly show the effect of the present invention, a blazed diffraction grating in which an ideal shape is formed by one straight line is used. In the present embodiment, the first layer and the second layer are used. Is composed of a convex diffraction grating obtained by approximating an ideal shape to a polygon by a plurality of straight lines.
The thick solid line in FIG. 6 indicates the actual lattice shape, and the thin solid line indicates the ideal shape. The overlapping diffraction gratings of the first layer and the second layer have similar shapes. Since the lattice shape approximates the ideal shape to a polygon with a plurality of straight lines, a diffraction efficiency close to the ideal shape can be obtained. In addition, in the diffraction grating of the laminated structure, since the overlapping diffraction gratings have a similar shape,
High diffraction efficiency can be maintained as in the above-described embodiment.

Further, since the ideal shape is processed by a straight line, a diffraction grating can be easily manufactured with high accuracy. In the second embodiment, the ideal shape is divided by a plurality of straight lines, but if the overlapping diffraction gratings have similar shapes,
The number of divisions of adjacent diffraction gratings may be different.

Third Embodiment FIG. 7 shows a sectional shape of a diffraction grating of a third embodiment. In the second embodiment, the first diffraction grating is a convex diffraction grating obtained by approximating a diffraction grating having an ideal shape with a plurality of straight lines.
Although the first layer and the second layer are configured in the present embodiment, the diffraction grating of the first layer and the second layer is configured in a lattice shape obtained by approximating the ideal shape to a polygon with a plurality of straight lines. The convex shape and the second layer have a concave shape. The thick solid line in FIG. 7 indicates the actual lattice shape, and the thin solid line indicates the ideal shape. The overlapping diffraction gratings of the first layer and the second layer have similar shapes. Also in this lattice shape, since the ideal shape is approximated by a polygon with a plurality of straight lines, diffraction efficiency close to the ideal shape can be obtained. In addition, in the diffraction grating having the laminated structure, since the overlapping diffraction gratings have similar shapes, high diffraction efficiency can be maintained as in the above-described embodiment.

Further, since the ideal shape is processed by a straight line, the diffraction grating can be easily manufactured with high accuracy. In the third embodiment, the ideal shape is divided by a plurality of straight lines, but if the overlapping diffraction gratings have similar shapes,
The number of divisions of adjacent diffraction gratings may be different.

[Embodiment 4] FIG. 8 shows the configuration of Embodiment 4 of the present invention. FIG. 8 shows a cross section of a photographing optical system such as a camera. In the figure, reference numeral 7 denotes a photographing lens, which has an aperture stop 8 and the diffractive optical element 1 described in each of the above embodiments. Reference numeral 9 in the figure denotes an image forming surface, which is a film or a CCD. Since the overlapping gratings have the same shape and maintain high diffraction efficiency, it is possible to obtain a high-performance photographic lens with less flare and high resolution.

In this embodiment, the diffractive optical element described in each of the above embodiments is provided on the flat glass surface disposed near the stop. However, the present invention is not limited to this. A plurality of diffraction gratings may be provided in the lens.

In this embodiment, the taking lens of the camera has been described. However, the present invention is not limited to this, and may be used for an imaging optical system such as a taking lens of a video camera, an image scanner of an office machine, and a leader lens of a digital copier. The same effect can be obtained even if the same is performed.

[Embodiment 5] FIG. 9 shows the configuration of Embodiment 5 of the present invention. FIG. 9 is a cross section of an observation optical system such as binoculars, in which 10 is an objective lens, 11 is a prism for establishing an image, 12 is an eyepiece, and 13 is a pupil plane (evaluation plane). Denotes a diffractive optical element described in each of the above embodiments. Reference numeral 1 is formed to correct chromatic aberration and the like on the image forming surface 12 of the objective lens. Since the overlapping gratings have the same shape and maintain high diffraction efficiency, a high-performance photographic lens with less flare and high resolution can be obtained.

In this embodiment, the case where the diffractive optical element is formed in the objective lens portion has been described. However, the present invention is not limited to this, and the same effect can be obtained even on the prism surface or in the eyepiece. However, since it is effective to reduce chromatic aberration only with the objective lens by providing it on the object side from the imaging plane, it is desirable to provide it on the objective lens side in the case of a macroscopic observation system. In the present embodiment, the case of binoculars has been described, but the present invention is not limited to this, and may be a terrestrial telescope or a telescope for astronomical observation, or may be an optical viewfinder such as a lens shutter camera or a video camera. Similar effects can be obtained.

[0027]

As described above, according to the present invention, when forming a polygonal shape approximate to an ideal shape in a relief type diffractive optical element, a polygonal shape capable of maintaining high diffraction efficiency is reduced to a straight line with a small number of straight lines. A diffractive optical element which can be formed by a number and can effectively suppress flare and the like when incorporated in an optical system, and an optical system constituted by the diffractive optical element can be realized.

[Brief description of the drawings]

FIG. 1 is a front view of a diffractive optical element according to a first embodiment of the present invention.

FIG. 2 is a sectional view of the diffractive optical element taken along the line A-A 'in FIG.

FIG. 3 is a diagram illustrating a cross-sectional shape of a diffraction grating having a similar upper and lower shape for explaining a difference in diffraction efficiency in the first embodiment.

FIG. 4 is a diagram illustrating a cross-sectional shape of a vertically differently shaped diffraction grating for explaining a difference in diffraction efficiency in the first embodiment.

FIG. 5 is a diagram showing diffraction efficiency for explaining a difference from an ideal shape of the diffractive optical element in the first embodiment.

FIG. 6 is a diagram illustrating a cross-sectional shape of a diffraction grating according to a second embodiment of the present invention.

FIG. 7 is a diagram illustrating a cross-sectional shape of a diffraction grating according to a third embodiment of the present invention.

FIG. 8 is a diagram illustrating a configuration of a photographic optical system according to a fourth embodiment of the present invention.

FIG. 9 is a diagram illustrating a configuration of an observation optical system according to a fifth embodiment of the present invention.

FIG. 10 is a diagram showing a grating shape (blaze shape) in a conventional diffraction grating.

FIG. 11 is a diagram showing diffraction efficiency of a conventional example.

[Explanation of symbols]

 1: Diffractive optical element 2: Element substrate 3: Diffraction grating 4: Area of first layer 5: Area of second layer 6: Air layer between first and second layers 7: Photographing lens 8: Aperture 9: Imaging surface 10: Objective lens 11: Prism 12: Eyepiece 13: Pupil surface (evaluation surface)

Claims (12)

[Claims] 1. A diffractive optical element having a laminated structure in which at least two or more diffraction gratings overlap each other, wherein a cross-sectional shape of the diffraction grating is a phase shift function by a polygonal shape in which one straight line or a plurality of straight lines are sequentially connected. A diffractive optical element having a shape similar to a relief type shape derived from the above, wherein the cross-sectional shape of the overlapping diffraction grating is similar. 2. The method of claim 2, wherein said overlapping diffraction gratings have at least two diffraction gratings.
The diffractive optical element according to claim 1, wherein the diffractive optical element is made of materials having different kinds of dispersion. 3. The diffractive optical element according to claim 2, wherein an air layer is provided between the diffraction gratings made of the materials having different dispersions. 4. A polygonal shape in which said plurality of straight lines are sequentially connected is constituted by a convex shape.
4. The diffractive optical element according to any one of claims 1 to 3. 5. A polygonal shape in which said plurality of straight lines are sequentially connected is formed in a lattice shape with one layer being convex and two layers being concave. Item 10. A diffractive optical element according to item 1. 6. The diffractive optical element according to claim 1, wherein adjacent ones of the diffraction gratings have different shapes. 7. The diffraction grating according to claim 1, wherein adjacent diffraction gratings have different numbers of a plurality of straight lines for forming the polygonal shape by being sequentially connected.
The diffractive optical element according to any one of the above. 8. An optical system having a diffractive optical element, wherein the diffractive optical element is constituted by the diffractive optical element according to any one of claims 1 to 7. 9. The optical system according to claim 8, wherein said optical system is an imaging optical system. 10. The diffractive optical element according to any one of claims 1 to 7, wherein the diffractive optical element according to any one of claims 1 to 7 is provided on a flat glass surface, a lens curved surface, or a taking lens disposed near the stop in the imaging optical system. The optical system according to claim 9, wherein: 11. The optical system according to claim 8, wherein said optical system is an observation optical system. 12. A diffractive optical element according to claim 1, wherein said observation optical system is provided with an objective lens portion, a prism surface, or an eyepiece. Item 12. The optical system according to Item 11.