Diffractive optical element, optical system, optical apparatus, and method for manufacturing diffractive optical element
阅读说明：本技术 衍射光学元件、光学系统、光学设备以及衍射光学元件的制造方法 (Diffractive optical element, optical system, optical apparatus, and method for manufacturing diffractive optical element ) 是由 籔本洋 于 2018-12-27 设计创作，主要内容包括：本发明提供在将衍射效率的降低抑制为最小限度的同时眩光着色小、不影响光学性能的优秀的衍射光学元件、使用了该衍射光学元件的光学系统以及光学设备。用于作为光学设备的相机(1)的光学系统OL且通过具有衍射光栅而作为透镜发挥作用的衍射光学元件GD构成为,周边区域Ap中的衍射光栅的光栅高度hmax比以光轴Z为中心的中央区域Ac中的衍射光栅的光栅高度h0高。(The invention provides a diffractive optical element which has the advantages of minimal reduction of diffraction efficiency, small glare coloring and no influence on optical performance, and an optical system and an optical device using the diffractive optical element. A diffractive optical element GD having a diffraction grating and functioning as a lens is used in an optical system OL of a camera (1) as an optical device, and is configured such that the grating height hmax of the diffraction grating in a peripheral region Ap is higher than the grating height h0 of the diffraction grating in a central region Ac around an optical axis Z.)
1. A diffractive optical element having a diffraction grating and functioning as a lens, wherein,
the grating height of the diffraction grating in the peripheral region is higher than the grating height of the diffraction grating in the central region centered on the optical axis.
2. The diffractive optical element according to claim 1,
the diffraction grating is configured to have a blazed shape.
3. The diffractive optical element according to claim 1 or 2,
the following condition is satisfied:
0.50<rc<0.98
0.01％<Δh/h0<10.00％
wherein the content of the first and second substances,
r: normalized radius (a value obtained by dividing the height of the optical axis in a direction perpendicular to the optical axis by the effective diameter)
h 0: average value of grating height of the diffraction grating at 0.00< r <0.50
hmax: average value of grating height of the diffraction grating at 0.98< r <1.00
Δ h: difference between hmax and h0
rc: the grating height is (h0+ hmax)/2 normalized radius.
4. The diffractive optical element according to any one of claims 1 to 3,
a diffraction grating having a grating height h0 is formed in the central region,
a diffraction grating having a grating height hmax is formed in the peripheral region,
and satisfies the following conditions:
Se/(Sc+Sp)≤50％
and (C) Sc: area of the central region
Sp: area of the peripheral region
Se: the area of the region in which the diffraction grating having the grating height other than the grating height h0 and the grating height hmax is formed.
5. A diffractive optical element functioning as a lens by a diffraction grating having a blazed shape,
having two or more regions with different diffraction efficiencies for each wavelength,
the two or more regions include a central region that is a region centered on the optical axis and located inward of a predetermined standardized radius and a peripheral region located outward of the predetermined standardized radius,
and satisfies the following conditions:
E_in<E_out
0.50<rs<0.98
E_in>90％
E_out>90％
wherein the content of the first and second substances,
rs: the predetermined normalized radius (a value obtained by dividing a height in a direction perpendicular to the optical axis from the optical axis by an effective diameter)
E _ in: diffraction efficiency at line C of the diffraction grating in the central region
E _ out: diffraction efficiency at line C of the diffraction grating in the peripheral region.
6. The diffractive optical element according to claim 5,
the following condition is satisfied:
F_out/F_in<0.50
wherein the content of the first and second substances,
f _ in: glare ratio at line C of diffraction grating in the central region
F _ out: a glare ratio at a line C of the diffraction grating in the peripheral region.
7. A diffractive optical element functioning as a lens by a diffraction grating having a blazed shape,
having two or more regions with different diffraction efficiencies for each wavelength,
the two or more regions include a central region that is a region centered on the optical axis and located inward of a predetermined standardized radius and a peripheral region located outward of the predetermined standardized radius,
and satisfies the following conditions:
W_in<W_out
0.50<ra<0.98
5nm<W_out-W_in<50nm
wherein the content of the first and second substances,
ra: the predetermined normalized radius (a value obtained by dividing a height in a direction perpendicular to the optical axis from the optical axis by an effective diameter)
W _ in: the wavelength having the highest diffraction efficiency among the diffraction gratings in the central region
W _ out: a wavelength having the highest diffraction efficiency among the diffraction gratings in the peripheral region.
8. The diffractive optical element according to any one of claims 1 to 7,
in any region within the effective diameter, the number of design steps is the same,
the design order is the diffraction order of the diffracted light having the highest intensity among the n-, n ± 1-, and n ± 2-order … … diffraction orders diffracted by the diffraction grating.
9. The diffractive optical element according to any one of claims 1 to 8,
the diffractive optical element is formed of two optical materials having different refractive indices, and the diffraction grating is formed on a boundary surface of the optical materials.
10. An optical system having the diffractive optical element according to any one of claims 1 to 9.
11. An optical system includes, in order from an object side:
a 1 st lens group having positive power; and
a 2 nd lens group having negative power,
the 1 st lens group has the diffractive optical element according to any one of claims 1 to 9,
and satisfies the following conditions:
0.001<f1/fpf<0.030
wherein the content of the first and second substances,
f 1: focal length of the 1 st lens group
fpf: a focal length of the diffractive optical element.
12. An optical device having the optical system of claim 10 or 11.
13. A method for manufacturing a diffractive optical element having a diffraction grating and functioning as a lens, wherein,
the diffraction grating is formed such that the grating height of the diffraction grating in the peripheral region is higher than the grating height of the diffraction grating in the central region centered on the optical axis.
Technical Field
The invention relates to a diffractive optical element, an optical system, an optical apparatus, and a method of manufacturing a diffractive optical element.
Background
Conventionally, as a type of lens suitable for a long-focus photographing optical system, there is disclosed a method of reducing chromatic aberration using a diffractive optical element in which a diffraction grating having a diffractive action is provided on a lens surface or a part of an optical system in an optical system including, in order from an object side, a 1 st lens group having positive refractive power and a 2 nd lens group having negative refractive power, that is, a so-called telephoto lens (for example, see patent document 1). However,
Disclosure of Invention
The diffractive optical element according to the first aspect of the present invention has a diffraction grating having a grating height in a peripheral region higher than a grating height of a diffraction grating in a central region around an optical axis and functions as a lens.
A diffractive optical element according to a second aspect of the present invention is a diffractive optical element that functions as a lens by having a blazed diffraction grating, wherein the diffractive optical element has two or more regions having different diffraction efficiencies for each wavelength, the two or more regions include a central region that is a region centered on an optical axis and located inward of a predetermined normalized radius and a peripheral region located outward of the predetermined normalized radius, and satisfy a condition of the following equation:
0.50<rs<0.98
E_in<E_out
E_in>90％
E_out>90％
wherein the content of the first and second substances,
rs: a predetermined normalized radius (a value obtained by dividing a height in a direction perpendicular to the optical axis from the optical axis by an effective diameter)
E _ in: diffraction efficiency at C-line of diffraction grating in central region
E _ out: diffraction efficiency at line C of the diffraction grating in the peripheral region.
A diffractive optical element according to a third aspect of the present invention is a diffractive optical element that functions as a lens by having a blazed diffraction grating, wherein the diffractive optical element has two or more regions having different diffraction efficiencies for each wavelength, the two or more regions include a central region that is a region centered on an optical axis and located inward of a predetermined normalized radius and a peripheral region located outward of the predetermined normalized radius, and satisfy a condition of the following equation:
0.50<rs<0.98
W_in<W_out
5nm<W_out-W_in<50nm
wherein the content of the first and second substances,
rs: a predetermined normalized radius (a value obtained by dividing a height in a direction perpendicular to the optical axis from the optical axis by an effective diameter)
W _ in: the wavelength having the highest diffraction efficiency in the diffraction grating of the central region
W _ out: the wavelength having the highest diffraction efficiency among the diffraction gratings in the peripheral region.
A method of manufacturing a diffractive optical element according to a first aspect of the present invention is a method of manufacturing a diffractive optical element that functions as a lens by having a diffraction grating, wherein the diffraction grating is formed such that a grating height of the diffraction grating in a peripheral region is higher than a grating height of the diffraction grating in a central region centered on an optical axis.
Drawings
Fig. 1 is a schematic diagram showing a cross section of a diffractive optical element of
Fig. 2 is a schematic diagram showing a cross section of the diffractive optical element of
Fig. 3 is an explanatory diagram showing a relationship between a normalized radius and a grating height in the diffractive optical element according to
Fig. 4 is an explanatory diagram showing the grating height in the intermediate region of the diffractive optical element of
Fig. 5 is an explanatory view showing a relationship between a wavelength of each grating height and a glare ratio in the diffractive optical element of the 3 rd and 4 th embodiments.
Fig. 6 is an explanatory view showing the grating height when the diffractive optical element of the present embodiment is formed on the lens surface.
Fig. 7 is an explanatory view showing a flare image of the diffractive optical element according to
Fig. 8 is an explanatory view showing a flare image of the diffractive optical element according to
Fig. 9 is a sectional view showing a lens structure in an infinity focus state of the optical system of
Fig. 10 is an aberration diagram in the infinity focus state of the optical system of
Fig. 11 is a sectional view showing a lens structure in an infinity focus state of the optical system of
Fig. 12 is an aberration diagram in the infinity focusing state of the optical system of
Fig. 13 is a sectional view showing a lens structure in an infinity focus state of the optical system of
Fig. 14 is an aberration diagram in the infinity focus state of the optical system of
Fig. 15 is a sectional view showing a lens structure in an infinity focus state of the optical system of
Fig. 16 is an aberration diagram in the infinity focus state of the optical system according to
Fig. 17 is a sectional view showing a lens structure in an infinity focus state of the optical system of
Fig. 18 is an aberration diagram in the infinity focus state of the optical system of the
Fig. 19 is a sectional view of a camera on which the optical system is mounted.
Fig. 20 is a flowchart illustrating a method of manufacturing the diffractive optical element.
Detailed Description
Hereinafter, preferred embodiments will be described with reference to the drawings.
The diffractive optical element is an optical element manufactured to have a grating structure of several slits or steps or saw-blade shapes (blazed shapes) arranged at a specific interval on the left and right sides at a minute interval (1mm), and has a property of generating a diffracted light beam in a direction determined by the interval (interval) of the slits or blazes and the wavelength of light when the light is incident. Such a diffractive optical element is used in various optical systems, and for example, a technique of converging diffracted light of a specific order at a single point and using the converged diffracted light as a lens is recently known.
By using such a diffractive optical element, a telephoto type optical system (telephoto lens) having a small telephoto ratio (short total length of the lens) and high optical performance, which can satisfactorily correct various aberrations such as chromatic aberration, can be realized. In particular, in the axial chromatic aberration, apochromatic correction in which axial chromatic aberration is corrected with a wavelength of 2 in general can be realized by using a diffractive optical element, and apochromatic correction in which axial chromatic aberration is corrected with a wavelength of 3.
Light incident on the diffractive optical surface of the diffractive optical element is divided into diffracted lights of multiple orders. In general, in a diffractive optical element, a light beam in a wavelength region to be used is concentrated in a specific order (hereinafter, referred to as a "design order"), and a grating structure thereof is determined so that the diffraction efficiency of diffracted light in the design order is optimized at a design wavelength (λ 0). However, since the diffraction efficiency of diffracted light shows wavelength dependence, the diffraction efficiency is highest at a wavelength near the design wavelength (λ 0), and the diffraction efficiency decreases as the deviation from the design wavelength (λ 0) increases. Therefore, when the wavelength of the light used is in a wide band, diffracted light of orders other than the designed order (hereinafter referred to as "unwanted diffracted light") has intensity at wavelengths other than the designed wavelength (λ 0). The design order is the order of diffraction of the diffracted light having the highest intensity among the n-, n ± 1-, and n ± 2-orders … … diffracted by the diffraction grating.
In a telephoto lens using a diffractive optical element, an optical system is designed so that axial chromatic aberration is corrected for diffracted light of the designed order. However, since the axial chromatic aberration cannot be corrected in addition to the difference between the imaging position of the unwanted diffracted light and the diffracted light of the design order, there is a problem that glare having a large spot diameter occurs as the wavelength becomes longer on the imaging surface (hereinafter, color shift caused by the difference in spot diameter of the glare for each wavelength is referred to as "flare coloring").
Fig. 1 shows a close-contact multilayer diffractive optical element GD composed of a 1 st diffractive optical element G1 and a 2 nd diffractive optical element G2, wherein the 1 st diffractive optical element G1 is composed of a 1 st optical material having a low refractive index and a high dispersion, the 2 nd diffractive optical element G2 is composed of a 2 nd optical material having a high refractive index and a low dispersion, and a blazed structure Pr (relief pattern) forming a blazed (sawtooth-shaped) diffraction grating is provided between the 1 st diffractive optical element G1 and the 2 nd diffractive optical element G2 and is brought into close contact therewith. In addition, although fig. 1 shows a case where a diffraction grating (blazed structure) is formed along a plane for the sake of simplicity of explanation, a diffraction grating (blazed structure) may be formed on a curved surface of a lens. Further, a separate multilayer type diffractive optical element in which an air space is provided between the 1 st diffractive optical element G1 and the 2 nd diffractive optical element G2 to form a diffraction grating (blazed structure) in each diffractive optical element, or a single layer type diffractive optical element in which the 2 nd diffractive optical element G2 is not provided but air may be used.
In the blazed diffraction grating forming the diffractive optical surface of the diffractive optical element, the design wavelength is determined by the refractive index of the material (in fig. 1, as described above, the 1 st diffractive optical element G1 and the 2 nd diffractive optical element G2) before and after the blazed structure of the diffraction grating and the grating height of the blazed structure of the diffraction grating. As the grating height increases, the design wavelength increases, and the diffraction efficiency on the long wavelength side increases and the diffraction efficiency on the short wavelength side decreases. That is, when considering the hue of the glare, the higher the grating height is, the more the red component of the glare is reduced and the more the green and blue components are enhanced. In glare coloration, the grating height can be reduced by locally optimizing the grating height for red glare located outside of the green and blue glare spots.
(embodiment 1)
As shown in fig. 1, the diffractive optical element GD according to the present embodiment is configured such that the grating height of the diffraction grating in the peripheral region Ap is higher than the grating height of the diffraction grating in the central region Ac centered on the optical axis Z in order to reduce the occurrence of the glare (particularly, red glare).
Specifically, the diffractive optical element GD according to the present embodiment preferably satisfies the following conditional expression (1) and conditional expression (2).
0.50<rc<0.98 (1)
0.01％<Δh/h0<10.00％ (2)
Wherein the content of the first and second substances,
r: normalized radius (a value obtained by dividing the height in the direction orthogonal to the optical axis Z from the optical axis Z by the effective diameter)
h 0: average value of grating height of diffraction grating at 0.00< r <0.50
hmax: average value of grating height of diffraction grating at 0.98< r <1.00
Δ h: difference between hmax and h0
rc: normalized radius with grating height of (h0+ hmax)/2
The conditional expression (1) defines a normalized radius of a boundary between the central region Ac and the peripheral region Ap. If the ratio is less than the lower limit of the conditional expression (1), it is not preferable that the flare intensity of green and blue on the inside of the flare spot is increased in a portion where the red flare affecting the longer wavelength and the short-wavelength flare are overlapped with each other (less coloring) in the spot diameter, which increases the grating height. In order to reliably obtain the effect of the conditional expression (1), the lower limit of the conditional expression (1) is more preferably 0.65, and still more preferably 0.70, 0.75, or 0.80. If the upper limit of conditional expression (1) is exceeded, most of the red glare light having a long wavelength and the maximum flare diameter of the glare light will not reach the region where the effect of changing the grating height is not achieved, and therefore the effect of reducing the glare coloration will be reduced, which is not preferable. In order to reliably obtain the effect of the conditional expression (1), it is more preferable that the upper limit value of the conditional expression (1) is 0.978, and further 0.975, 0.973, and 0.971.
Conditional expression (2) specifies the difference between the grating height of the region 0.00< r <0.50 in the central region Ac and the grating height of the region 0.98< r <1.00 in the peripheral region Ap. If the lower limit of conditional expression (2) is exceeded, the change in diffraction efficiency in the long wavelength region (red) is insufficient, and therefore the effect of reducing red glare on the outer side (peripheral region) is not preferable. In order to reliably obtain the effect of the conditional expression (2), the lower limit of the conditional expression (2) is more preferably set to 0.05%, further preferably set to 0.10%, 0.15%, 0.20%, 0.25%, or 0.30%. When the upper limit of the conditional expression (2) is exceeded, the design wavelength is excessively shifted to the longer wavelength side, and the diffraction efficiency in the short wavelength region (green and blue) is lowered, so that the green glare on the inner side (central region) becomes excessively strong, which is not preferable. In order to reliably obtain the effect of conditional expression (2), it is more preferable that the upper limit value of conditional expression (2) is 9.00%, and further 8.00%, 7.00%, 6.00%, and 5.00%.
(embodiment 2)
In the above-described configuration, the case where the normalized radius is 0.00 to 0.50, the central region Ac, the average value of the grating heights in this region is h0, the normalized radius is 0.98 to 1.00, and the peripheral region Ap, the average value of the grating heights in this region is hmax, has been described, but as shown in fig. 2, when the diffraction grating is configured such that the grating height of the central region Ac is h0, and the grating height of the peripheral region Ap is hmax, the diffractive optical element GD according to the present embodiment preferably satisfies the following conditional expression (3).
Se/(Sc+Sp)≤50％ (3)
And (C) Sc: area of the central region Ac
Sp: area of peripheral region Ap
Se: area of region Ae in which diffraction grating is formed with grating height h0 and a grating height other than grating height hmax
The conditional expression (3) defines a ratio of an area Ae other than the central region Ac and the peripheral region Ap (a region (intermediate region) having a grating height h0 and a grating height hmax) to an area obtained by summing the central region Ac and the peripheral region Ap. As shown in fig. 2, the intermediate region Ae connecting the central region Ac and the peripheral region Ap preferably has a grating height of the peripheral region Ap that is increased in a stepwise manner from the grating height of the central region Ac in the intermediate region Ae. If the upper limit value of the conditional expression (3) is exceeded, the change in color tone in the intermediate region Ae is too gradual, and the effect of reducing glare coloration is not preferable. In order to reliably obtain the effect of the conditional expression (3), it is more preferable that the upper limit value of the conditional expression (3) is 45%, and further 40%, 35%, 30%, 25%, 20%, 15%, 10%.
The intermediate area Ae may be configured to be gently changed from the grating height h0 to the grating height hmax as shown in C1 in fig. 3 and 4, or may be configured to be gradually changed from the grating height h0 to the grating height hmax as shown in C2 in fig. 4 without providing the intermediate area Ae.
(embodiment 3)
The diffractive optical element GD according to the present embodiment is configured to have two or more regions having different diffraction efficiencies for each wavelength, including a central region Ac that is a region centered on the optical axis and located inward of a predetermined normalized radius and a peripheral region Ap located outward of the predetermined normalized radius, in order to reduce the occurrence of the glare (particularly, red glare).
Here, the glare ratio F (λ) is defined as the ratio of 0 order diffracted light and 2 order diffracted light to 1 order diffracted light in the light having the wavelength λ diffracted on the diffractive optical surface (diffraction grating) of the diffractive optical element GD, as shown in the following formula (a).
F(λ)＝(E0(λ)+E2(λ))/E1(λ) (a)
Wherein the content of the first and second substances,
E0(λ): diffraction efficiency of 0 th order diffracted light
E1(λ): diffraction efficiency of 1 st order diffracted light
E2(λ): diffraction efficiency of 2 nd order diffracted light
Fig. 5 shows a glare ratio of a diffraction grating having a low grating height and a glare ratio of a diffraction grating having a high grating height for a wavelength of light incident to the diffraction grating. The diffractive optical element GD of the present embodiment preferably satisfies conditional expressions (4) to (7) shown below.
E_in<E_out (4)
0.50<rs<0.98 (5)
E_in>90％ (6)
E_out>90％ (7)
Wherein the content of the first and second substances,
rs: a predetermined normalized radius (a value obtained by dividing a height in a direction perpendicular to the optical axis from the optical axis by an effective diameter)
E _ in: diffraction efficiency at line C of diffraction grating in central region Ac
E _ out: diffraction efficiency at C-line of diffraction grating in peripheral area Ap
The diffraction efficiencies E (E _ in, E _ out) at the C-line are defined as the following equation (b) using the glare ratios F (F _ in, F _ out) at the C-line shown in fig. 5.
E＝1-F (b)
As shown in conditional expression (4), the diffractive optical element GD according to the present embodiment is configured such that the diffraction efficiency at the C-line in the peripheral region Ap is higher than the diffraction efficiency at the C-line of the diffraction grating in the central region Ac.
The conditional expression (5) specifies the normalized radius of the boundary between the central region Ac and the peripheral region Ap. If the ratio is less than the lower limit of the conditional expression (5), it is not preferable that the flare intensity of green and blue on the inner side of the flare spot is increased in a portion where the influence of the increased grating height on red flare to the longer wavelength and the flare diameters of green and blue flare to the shorter wavelength overlap (the coloring is less). In order to reliably obtain the effect of conditional expression (5), the lower limit of conditional expression (5) is more preferably 0.55, and still more preferably 0.60, 0.65, 0.70, 0.75, or 0.80. When the upper limit of the conditional expression (5) is exceeded, the red glare light having a long wavelength with the largest flare diameter of the glare becomes a region where the effect of changing the grating height is not achieved, and therefore the effect of reducing the glare coloration is not preferable. In order to reliably obtain the effect of conditional expression (5), it is more preferable that the upper limit value of conditional expression (5) is 0.978, and further 0.975, 0.973, and 0.971.
The conditional expressions (6) and (7) define diffraction efficiencies at the C-line of the diffraction grating in the central region Ac and the peripheral region Ap.
When the lower limit of conditional expression (6) is lower, the red color on the inner side of the glare is strong, which is not preferable. In addition, diffraction efficiency of long wavelength is deteriorated, and thus glare intensity becomes strong, contrast is lowered, and optical performance is deteriorated. In order to reliably obtain the effect of conditional expression (6), it is more preferable that the lower limit of conditional expression (6) is 91%, and further 93%, 95%, and 97%.
When the lower limit value of the conditional expression (7) is exceeded, the red color on the outer side of the glare is strong, and is excessively red as an absolute value even if a difference is provided between the central region and the peripheral region. In addition, the glare intensity becomes strong, and the optical performance deteriorates. In order to reliably obtain the effect of conditional expression (7), the lower limit of conditional expression (7) is more preferably 91%, and still more preferably 93%, 95%, and 97%.
The diffractive optical element GD according to the present embodiment preferably satisfies conditional expression (8) shown below.
F_out/F_in<0.50 (8)
Wherein the content of the first and second substances,
f _ in: glare ratio at line C of diffraction grating in central region Ac
F _ out: glare ratio at line C of diffraction grating in peripheral area Ap
The conditional expression (8) specifies the ratio of the glare rating at the C-line of the diffraction grating in the peripheral region Ap to the glare rating at the C-line of the diffraction grating in the central region Ac, and indicates that the glare rating at the C-line of the peripheral region Ap is preferably equal to or less than half of the glare rating at the C-line of the central region Ac. If the upper limit value of the conditional expression (8) is exceeded, the red color reduction effect is deteriorated, which is not preferable. In order to reliably obtain the effect of conditional expression (8), it is more preferable that the upper limit value of conditional expression (8) is 0.45, and further 0.40, 0.35, 0.30, and 0.25.
The diffractive optical element GD according to the present embodiment is preferably used in an optical system having a diffraction efficiency of 90% or more at 450nm to 650 nm.
(embodiment 4)
The diffractive optical element GD according to the present embodiment is configured to have two or more regions having different diffraction efficiencies for each wavelength, in order to reduce the occurrence of the glare (particularly, red glare), and the two or more regions include a central region Ac that is a region centered on the optical axis and located inward of a predetermined normalized radius and a peripheral region Ap located outward of the predetermined normalized radius.
The diffractive optical element GD of the present embodiment preferably satisfies conditional expressions (9) to (11) shown below.
W_in<W_out (9)
0.50<ra<0.98 (10)
5nm<W_out-W_in<50nm (11)
Wherein the content of the first and second substances,
ra: a predetermined normalized radius (a value obtained by dividing a height in a direction perpendicular to the optical axis from the optical axis by an effective diameter)
W _ in: the wavelength having the highest diffraction efficiency in the diffraction grating of the central region Ac
W _ out: the wavelength having the highest diffraction efficiency in the diffraction grating of the peripheral area Ap
When there are a plurality of wavelengths at which the diffraction efficiency is highest, the minimum value is set from the visible light to the shortest wavelength side in the near-infrared light.
As shown in conditional expression (9), the diffractive optical element GD according to the present embodiment is configured such that the wavelength having the highest diffraction efficiency in the peripheral region Ap is longer than the wavelength having the highest diffraction efficiency in the diffraction grating in the central region Acc. As shown in fig. 5 and formula (b), the wavelength having the highest diffraction efficiency is the wavelength having the lowest glare ratio.
The conditional expression (10) specifies the normalized radius of the boundary between the central region Ac and the peripheral region Ap. If the optical grating height is less than the lower limit of the conditional expression (10), it is not preferable that the grating height be increased in a portion where the spot diameters of red glare which reaches the long wavelength and of green and blue glare which reaches the short wavelength overlap (the coloring is small), because the glare intensities of green and blue glare on the inner side of the glare spot are increased. In order to reliably obtain the effect of conditional expression (10), the lower limit of conditional expression (10) is more preferably 0.55, and still more preferably 0.60, 0.65, 0.70, 0.75, or 0.80. When the upper limit of conditional expression (10) is exceeded, most of the red glare light having a long wavelength and the maximum flare diameter of the glare light becomes a region where the effect of changing the grating height is not achieved, and therefore the effect of reducing the glare coloration is not preferable. In order to more reliably obtain the effect of conditional expression (5), it is more preferable that the upper limit value of conditional expression (10) is 0.978, and further 0.975, 0.973, and 0.971.
The conditional expression (11) specifies the difference between the wavelength having the highest diffraction efficiency in the diffraction grating in the central region Ac and the wavelength having the highest diffraction efficiency in the diffraction grating in the peripheral region Ap. If the optical axis length is less than the lower limit of the conditional expression (11), the portion where the grating height is increased and the spot diameter of the red glare which affects the long wavelength and the short wavelength green and blue glare overlaps (is less colored), the glare intensity of green and blue on the inner side of the glare spot becomes high, and the coloring reduction effect becomes poor, which is not preferable. In order to reliably obtain the effect of conditional expression (11), it is more preferable that the lower limit of conditional expression (11) is 10nm, and further 12nm or 15 nm. When the upper limit of conditional expression (11) is exceeded, most of the red glare of long wavelength with the maximum flare diameter of the glare becomes an area where the effect of changing the grating height is not achieved, and the intermediate portion of the glare is colored green in addition to a large decrease in diffraction efficiency, which is not preferable. In order to reliably obtain the effect of conditional expression (11), it is more preferable that the upper limit of conditional expression (11) is 45nm, and further 40nm, 35nm, and 30 nm.
The diffractive optical element GD according to the present embodiment is preferably used in an optical system having a diffraction efficiency of 90% or more at 450nm to 650 nm.
In addition, in the diffractive optical element GD according to the present embodiment, the number of design steps is preferably the same in any region within the effective diameter.
The diffractive optical element GD according to the present embodiment is preferably a so-called multilayer type diffractive optical element that is formed of two optical materials having different refractive indices and has the diffraction grating formed on a boundary surface between the two optical materials. With this structure, the diffraction efficiency is improved over the entire visible light range.
As shown in fig. 6, the grating height h when the diffractive optical element GD is formed along the lens surface (curved surface) of the lens L is the distance between lines S1 and S2 (the distance in the direction perpendicular to the lines S1 and S2) connecting the peaks and valleys of the diffraction grating (blazed structure).
The diffractive optical element GD according to the present embodiment can be used in an optical system. Specifically, in the optical system OL including, in order from the object side, the 1 st lens group G1 having positive refractive power and the 2 nd lens group G2 having negative refractive power, by providing the diffractive optical element GD in the 1 st lens group G1, an optical system having excellent optical performance and minimal glare coloration while minimizing a decrease in diffraction efficiency can be provided.
Here, the optical system OL preferably satisfies the conditional expression (12) shown below.
0.001<f1/fpf<0.030 (12)
Wherein the content of the first and second substances,
f 1: focal length of the 1 st lens group G1
fpf: focal length of diffractive optical element GD
The conditional expression (12) specifies the ratio of the focal length of the diffractive optical element GD to the focal length of the 1 st lens group G1. By satisfying the conditional expression (12), chromatic aberration in the axial direction and magnification can be corrected satisfactorily. If the lower limit value of the conditional expression (12) is exceeded, the focal length of the diffractive optical element GD becomes long (the focal power becomes small), and although ± 1 st order diffracted light hardly diffuses, the glare coloration is not noticeable, but if the grating height is changed halfway, the adverse effect of the decrease in the overall diffraction efficiency becomes large, which is not preferable. In order to reliably obtain the effect of conditional expression (12), it is more preferable that the lower limit of conditional expression (12) is 0.003, and further 0.005, 0.008, and 0.010. When the upper limit value of the conditional expression (12) is exceeded, the focal length of the diffractive optical element GD becomes short (the focal power becomes large), and the ± 1 st order diffracted light is diffused too much and becomes thin, and the intensity is inherently weak, and becomes inconspicuous, but when the grating height is changed halfway, the adverse effect of the decrease in the overall diffraction efficiency becomes large, which is not preferable. In order to reliably obtain the effect of conditional expression (12), it is more preferable that the upper limit of conditional expression (12) is 0.025, and further 0.020, 0.018, and 0.015.
The above-described conditions and configurations exhibit the above-described effects, respectively, and are not limited to satisfying all of the conditions and configurations, and the above-described effects can be obtained even if any one of the conditions or configurations is satisfied or any combination of the conditions and configurations is satisfied.
An outline of the method for manufacturing the diffractive optical element GD according to the present embodiment will be described below with reference to fig. 20. First, diffractive optical elements constituting the diffractive optical element GD are prepared (step S100). Here, the diffractive optical elements include, for example, the 1 st diffractive optical element G1 and the 2 nd diffractive optical element G2 in the case of the structure shown in fig. 1. Then, the diffraction grating is formed such that the grating height of the diffraction grating in the peripheral region is higher than the grating height of the diffraction grating in the central region around the optical axis (step S200).
As described above, according to the present embodiment, it is possible to provide a diffractive optical element having excellent glare coloration with minimal decrease in diffraction efficiency and no influence on optical performance, and an optical system and an optical apparatus using the diffractive optical element.