JP2018189863A - Diffractive optical element and optical system using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve such a problem that in a conventional adhered two-layer DOE, refractive index sensitivity of a material constituting a diffraction grating is high, resulting in fluctuations in wavelength characteristics of diffraction efficiency including a behavior of a grating wall surface.SOLUTION: A diffractive optical element of the present invention can give uniform wavelength characteristics of diffraction efficiency by controlling a film width of a thin film even when the refractive index of the refractive optical element fluctuates.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a diffractive optical element and an optical system having the same.

  Conventionally, a method of reducing chromatic aberration of a lens system by providing a diffractive optical element having a diffractive action on the surface of the lens or a part of the lens system is known as opposed to a method of reducing chromatic aberration of a lens system by combining glass materials. . This method using a diffractive optical element utilizes the physical phenomenon that the deflection direction with respect to a light beam having a certain reference wavelength is reversed between the refracting surface and the diffractive surface in the optical system. In addition, the diffractive optical element can have an effect of an aspheric lens by appropriately changing the period of the periodic structure, and thus is effective in reducing various aberrations other than chromatic aberration. In general, a diffractive optical element has a blazed structure composed of a grating surface and a grating side surface. Such a diffractive optical element having a blazed structure can diffract light with high efficiency with respect to a specific one order (hereinafter also referred to as “specific order” or “design order”) and a specific wavelength. On the other hand, there is known a diffractive optical element configuration for obtaining this specific order diffraction efficiency sufficiently high over the entire visible wavelength band.

  Specifically, as disclosed in Patent Document 1, two diffraction gratings are arranged in close contact with each other, and a low refractive index high dispersion material and a high refractive index low dispersion material are used as materials constituting each diffraction grating, High diffraction efficiency can be obtained in a wide wavelength band with respect to diffracted light of a specific order by appropriately setting the height of the diffraction grating (hereinafter, such a diffractive optical element is referred to as “adherent two-layer DOE”). The diffraction efficiency is defined as the ratio of the amount of diffracted light of each order to the amount of total transmitted light flux.

  Further, as disclosed in Patent Document 2, in order to obtain a high diffraction efficiency of 99% or more in the entire visible wavelength range, the partial dispersion ratio θgF has a smaller value (linear anomalous dispersion) than that of a normal material. It is known to use materials.

Japanese Patent No. 3717555 JP 2008-241734 A

  However, the conventional two-layer DOE has a problem that the refractive index sensitivity of the material constituting the diffraction grating is high, and the wavelength characteristics of the diffraction efficiency including the behavior of the grating wall surface vary.

  The present invention has been made in view of such problems, and an object of the present invention is to provide a diffractive optical element that reduces variations in wavelength characteristics of diffraction efficiency in diffracted light of a designed order including the behavior of a grating wall surface. To do.

In order to achieve the above object, the diffractive optical element according to the present invention includes:
Laminating first and second diffraction gratings made of two materials having different dispersions;
The first and second diffraction gratings are in close contact with each other,
A thin film on the first and second diffraction grating walls;
The material constituting the thin film has an average extinction coefficient k _{B in} the blue wavelength band and an average extinction coefficient k _{R in} the red wavelength band satisfying the following expressions.

0.1 <k _B −k _R <0.5

  According to the present invention, it is possible to provide a diffractive optical element that reduces variation in wavelength characteristics of diffraction efficiency in diffracted light of a designed order including the behavior of a grating wall surface in an adhesive two-layer DOE, and an optical system having the diffractive optical element. .

Schematic diagram of essential parts of diffractive optical element Schematic diagram showing the element structure of the diffractive optical element of this example Graph of diffraction efficiency of the diffractive optical element of Example 1 Graph of diffraction efficiency of diffractive optical element of Example 2 Graph of diffraction efficiency of diffractive optical element of Example 3 Schematic diagram showing the element structure of the diffractive optical element of the comparative example Graph of diffraction efficiency of the diffractive optical element of Comparative Example 1 Graph of diffraction efficiency of the diffractive optical element of Comparative Example 2 Graph of diffraction efficiency of the diffractive optical element of Comparative Example 3 Sectional drawing which shows the structure of the imaging optical system which is Example 4 of this invention.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

[Example 1]
FIG. 1 is a front view and a side view of a diffractive optical element of the present invention.

  A diffractive optical element 10 is formed on the surfaces of substrate lenses 20 and 30 made of flat plates or lenses. The surfaces of the substrate lenses 20 and 30 on which the diffractive optical element 10 is formed are curved surfaces. The diffractive optical element 10 has a concentric diffraction grating shape centered on the optical axis O and has a lens action. FIG. 2 shows a cross-sectional shape of the diffractive optical element 10 shown in FIG. FIG. 2 is a considerably deformed view for easy understanding of the lattice shape. In FIG. 2, the diffractive optical element 10 has a contact DOE structure in which a first diffraction grating 1 and a second diffraction grating 2 are closely disposed (laminated). The first and second diffraction gratings have a concentric blazed grating shape, and a lens action (convergence action or diverging action) is obtained by gradually changing the grating pitch from the center (optical axis) to the periphery. ing. Each diffraction grating acts as one diffractive optical element 10 through all layers. Further, by using a blazed structure, incident light incident on the diffractive optical element 10 is concentrated and diffracted in a specific diffraction order (+ 1st order in the figure) direction.

  In addition, the operating wavelength region of the diffractive optical element of the present invention is the visible region. For this reason, the material and grating height which comprise the 1st diffraction grating 1 and the 2nd diffraction grating 2 are selected so that the diffraction efficiency of the diffracted light of the design order may become high in the whole visible region. That is, the maximum optical path length difference (the maximum value of the optical path length difference between the peaks and valleys of the diffraction part) of light passing through a plurality of diffraction gratings (diffraction gratings 1 and 2) is in the operating wavelength range, and near an integral multiple of that wavelength. The material and grating height of each diffraction grating are determined so that Thus, by appropriately setting the material and shape of the diffraction grating, high diffraction efficiency can be obtained over the entire wavelength range. In general, the grating height of the diffraction grating is defined by the height of the grating tip and the grating groove in the direction (plane normal direction) perpendicular to the grating period direction.

  The diffraction grating portion 10 is formed by closely attaching the diffraction grating (first diffraction grating) 1 and the diffraction grating (second diffraction grating) 2 in the optical axis direction. A thin film 3 is provided.

  Specifically, the material constituting the diffractive optical element is an ultraviolet curable resin in which ITO fine particles are mixed with an acrylic ultraviolet curable resin as a first diffraction grating material, and ZrO2 in an acrylic ultraviolet curable resin as a second diffraction grating material. An ultraviolet curable resin mixed with fine particles is used. Further, the refractive index is adjusted, and the materials shown in Table 1 are used. The grating height d is 11.00 μm, the design order is + 1st order, the incident angle is 0 deg, and the grating pitch is 100 μm.

  The resin material in which the fine particles are dispersed is an ultraviolet curable resin, and includes any organic resin of acrylic, fluorine, vinyl, and epoxy, but is not particularly limited. In this embodiment, the design order is + 1st order, but the same effect can be obtained even if the design order is other than + 1st order, and the design order is not limited to the design order.

  The fine particles include, but are not particularly limited to, oxides, metals, ceramics, composites, and mixtures. The average particle diameter of the fine particle material is preferably ¼ or less of the wavelength (use wavelength or design wavelength) of light incident on the diffractive optical element. When the particle diameter is larger than this, there is a possibility that Rayleigh scattering becomes large when the fine particle material is mixed with the resin material.

  Instead of the resin material in which the fine particles are dispersed, an organic material such as a resin material, a glass material, an optical crystal material, a ceramic material, or the like may be used.

As a material constituting the thin film 3, a material in which the wavelength characteristic of the extinction coefficient rapidly changes in the visible wavelength band as represented by a colored composition or the like is known. In particular, a material is provided in which the average extinction coefficient k _{B in} the blue wavelength band and the average extinction coefficient k _{R in} the red wavelength band satisfy the following expressions. The film thickness w is 80 nm.

1 <k _B −k _R <0.5
If this lower limit is not satisfied, it is difficult to make the wavelength characteristics of diffraction efficiency uniform, which is not preferable. If the upper limit is not satisfied, reflection occurs at the interface with the thin film, which is not preferable.

In the material constituting the thin film, the average extinction coefficient k _{B in} the blue wavelength band, the average extinction coefficient k _{G in} the green wavelength band, and the average extinction coefficient k _{R in} the red wavelength band satisfy the following expressions.

k _R <k _G <k _B
If this equation is not satisfied, it is difficult to make the wavelength characteristics of diffraction efficiency uniform, which is not preferable.

  This can be derived from the transmittance and film thickness disclosed in, for example, Japanese Patent Application Laid-Open No. 2007-147738.

  The colored composition of the absorption layer is composed of a resin composition in which a dye or pigment is dispersed, and can obtain desired characteristics. The dye or pigment can be selected in consideration of spectral transmittance characteristics, heat resistance, light resistance, dispersibility in resin, and stability thereof. Specifically, there are various monoazo materials, diazo materials, condensed diazo materials, phthalocyanine materials, anthraquinone materials, lake materials, and the like, and a desired material can be obtained by mixing them. The fine particle diameter of the pigment can be selected in consideration of spectral transmittance characteristics, dispersibility, uniformity, and stability. In general, a resin composition in which a pigment is dispersed has a higher durability and is more preferable.

  The results of evaluating the diffraction efficiency are shown in FIG. FIG. 3 shows the wavelength characteristics of the diffraction efficiency of the designed order + 1 order diffracted light. With respect to the visible wavelength band of 400 to 700 nm, the wavelength characteristic of diffraction efficiency considering the behavior of the wall surface is almost uniform.

[Example 2]
Example 2 is an example where the refractive index of the material of Example 1 varies. As a material for the first diffraction grating, an ultraviolet curable resin in which ITO fine particles are mixed with an acrylic ultraviolet curable resin, and as a material for the second diffraction grating, an ultraviolet curable resin in which ZrO2 fine particles are mixed in an acrylic ultraviolet curable resin are used. Yes. As shown in Table 2, a material in which the material of the first diffraction grating varies +0.00050 as compared with the material of Example 1 is used. The grating height d is 11.00 μm, the design order is + 1st order, the incident angle is 0 deg, and the grating pitch is 100 μm. The material constituting the thin film 3 is the same as that in Example 1, and the film thickness w is 50 nm.

  The results of evaluating the diffraction efficiency are shown in FIG. FIG. 4 shows the wavelength characteristics of the diffraction efficiency of the designed order + 1 order diffracted light. With respect to the visible wavelength band of 400 to 700 nm, the wavelength characteristic of diffraction efficiency considering the behavior of the wall surface is almost uniform. Compared with the comparative example shown later, even when the refractive index of the material varies in the direction in which the difference in refractive index between the two materials is large, the wavelength characteristic of the diffraction efficiency can be made substantially uniform by changing the film width of the thin film. it can.

[Example 3]
Example 3 is an example when the refractive index of the material of Example 1 varies. As a material for the first diffraction grating, an ultraviolet curable resin in which ITO fine particles are mixed with an acrylic ultraviolet curable resin, and as a material for the second diffraction grating, an ultraviolet curable resin in which ZrO2 fine particles are mixed in an acrylic ultraviolet curable resin are used. Yes. As shown in Table 3, a material in which the material of the first diffraction grating is -0.00050, which is different from the material of Example 1, is used. The grating height d is 11.00 μm, the design order is + 1st order, the incident angle is 0 deg, and the grating pitch is 100 μm. The material constituting the thin film 3 is the same as in Example 1, and the film thickness w is 100 nm.

  The result of evaluating the diffraction efficiency is shown in FIG. FIG. 5 shows the wavelength characteristics of the diffraction efficiency of the designed order + 1st order diffracted light. With respect to the visible wavelength band of 400 to 700 nm, the wavelength characteristic of diffraction efficiency considering the behavior of the wall surface is almost uniform. Compared with the comparative example shown later, even when the refractive index of the material varies in the direction where the difference in refractive index between the two materials is small, the wavelength characteristic of the diffraction efficiency can be made substantially uniform by changing the film width of the thin film. it can.

  Even when the refractive index of the diffractive optical element varies from Examples 1 to 3, the wavelength characteristics of the diffraction efficiency can be made substantially uniform by adjusting the film width of the thin film.

  In the diffractive optical element of the present invention, the entire width w and grating pitch P of the thin film satisfy the following expressions.

w / P <0.01
If this equation is not satisfied, the absolute value of the diffraction efficiency of the designed order is lowered, which is not preferable.

  The refractive index of the material constituting the thin film is preferably higher than the refractive index of the material having a low refractive index among the materials constituting the first diffraction grating or the second diffraction grating. If this is not satisfied, reflection occurs at the interface between the thin film and the diffraction grating, which is not preferable.

  Further, the d-line refractive index nd3 of the material constituting the thin film and the d-line refractive index nd2 of the material having a high refractive index among the materials constituting the first diffraction grating or the second diffraction grating are: The following formula is preferably satisfied.

| Nd3-nd2 | <0.2
If this is not satisfied, reflection occurs at the interface between the thin film and the diffraction grating, which is not preferable.

[Comparative Example 1]
Comparative example 1 is shown in order to clarify the effect of the present invention. Comparative Example 1 is different from Examples 1 to 3 in that no thin film is provided as shown in FIG. As a material for the first diffraction grating, an ultraviolet curable resin in which ITO fine particles are mixed with an acrylic ultraviolet curable resin, and as a material for the second diffraction grating, an ultraviolet curable resin in which ZrO2 fine particles are mixed in an acrylic ultraviolet curable resin are used. Yes. Further, the refractive index is adjusted, and the materials shown in Table 4 are used. The grating height d is 11.00 μm, the design order is + 1st order, the incident angle is 0 deg, and the grating pitch is 100 μm.

  The result of evaluating the diffraction efficiency is shown in FIG. FIG. 7 shows the wavelength characteristics of the diffraction efficiency of the designed order + 1 order diffracted light. With respect to the visible wavelength band of 400 to 700 nm, the wavelength characteristic of diffraction efficiency considering the behavior of the wall surface is almost uniform.

[Comparative Example 2]
Comparative Example 2 is an example where the refractive index of the material of Comparative Example 1 varies. As a material for the first diffraction grating, an ultraviolet curable resin in which ITO fine particles are mixed with an acrylic ultraviolet curable resin, and as a material for the second diffraction grating, an ultraviolet curable resin in which ZrO2 fine particles are mixed in an acrylic ultraviolet curable resin are used. Yes. As shown in Table 5, a material in which the material of the first diffraction grating varies +0.00050 as compared with the material of Comparative Example 1 is used. The grating height d is 11.00 μm, the design order is + 1st order, the incident angle is 0 deg, and the grating pitch is 100 μm.

  The results of evaluating the diffraction efficiency are shown in FIG. FIG. 8 shows the wavelength characteristics of the diffraction efficiency of the designed order + 1 order diffracted light. With respect to the visible wavelength band of 400 to 700 nm, the wavelength characteristic of diffraction efficiency considering the behavior of the wall surface is lower on the short wavelength side.

[Comparative Example 3]
Comparative Example 3 is an example where the refractive index of the material of Comparative Example 1 varies. As a material for the first diffraction grating, an ultraviolet curable resin in which ITO fine particles are mixed with an acrylic ultraviolet curable resin, and as a material for the second diffraction grating, an ultraviolet curable resin in which ZrO2 fine particles are mixed in an acrylic ultraviolet curable resin are used. Yes. As shown in Table 6, a material in which the material of the first diffraction grating is -0.00050 as compared with the material of Comparative Example 1 is used. The grating height d is 11.00 μm, the design order is + 1st order, the incident angle is 0 deg, and the grating pitch is 100 μm.

  The result of evaluating the diffraction efficiency is shown in FIG. FIG. 9 shows the wavelength characteristics of the diffraction efficiency of the designed order + 1 order diffracted light. With respect to the visible wavelength band of 400 to 700 nm, the wavelength characteristic of the diffraction efficiency considering the behavior of the wall surface is higher on the short wavelength side.

  From Comparative Examples 1 to 3, it can be seen that when the refractive index of the diffractive optical element varies, the wavelength characteristic of the diffraction efficiency varies.

[Example 4]
A schematic diagram of Example 4 of the present invention is shown in FIG. FIG. 10 shows a cross section of a photographic optical system such as a camera. In FIG. 10, reference numeral 101 denotes a photographic lens having an aperture 40 and the diffractive optical element 10 of each of the above-described embodiments of the present invention. Yes. Reference numeral 41 denotes an image forming surface of a photoelectric conversion element such as a film or a CCD. The diffractive optical element 10 corrects chromatic aberration of the taking lens 101. When the diffractive optical element of the present invention is applied, the wavelength characteristics are good and high diffraction efficiency is made possible, so that a high-performance photographic lens with less flare can be obtained. In FIG. 10, the diffractive optical element 10 is provided on the surface of the front lens, but the present invention is not limited to this, and a plurality of diffractive optical elements may be used in the photographing optical system. In this embodiment, the case of a camera optical system is shown. However, the present invention is not limited to this, and a wide wavelength such as a video camera photographing lens, an office image scanner, or a digital copier reader lens. The same effect can be obtained even if it is used in an optical instrument used in the area.

10 diffractive optical element, 1st diffraction grating, 2nd diffraction grating,
20, 30 Substrate lens, 40 Aperture, 41 Imaging surface, 101 Shooting lens

Claims (6)

Laminating first and second diffraction gratings made of two materials having different dispersions;
The first and second diffraction gratings are in close contact with each other,
A thin film on the first and second diffraction grating walls;
A diffractive optical element characterized in that an average extinction coefficient k _{B in} a blue wavelength band and an average extinction coefficient k _{R in} a red wavelength band of the material constituting the thin film satisfy the following expressions.
1 <k _B −k _R <0.5 The material constituting the thin film has an average extinction coefficient k _{B in} the blue wavelength band, an average extinction coefficient k _{G in} the green wavelength band, and an average extinction coefficient k _{R in} the red wavelength band satisfying the following expressions: The diffractive optical element according to claim 1.
k _R <k _G <k _B The diffractive optical element according to claim 1, wherein the entire width w and the grating pitch P of the thin film satisfy the following expression.
w / P <0.01   The refractive index of the material constituting the thin film is higher than the refractive index of a material having a low refractive index among the materials constituting the first diffraction grating or the second diffraction grating. The diffractive optical element according to claim 3. The refractive index nd3 of the d-line of the material constituting the thin film and the refractive index nd2 of the d-line of the material having a high refractive index among the materials constituting the first diffraction grating or the second diffraction grating are: The diffractive optical element according to claim 1, wherein the following expression is satisfied.
| Nd3-nd2 | <0.2   An optical system comprising the diffractive optical element according to any one of claims 1 to 5.