JP2004078166A - Diffraction optical element and optical system with same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve a problem in which a conventional stack type diffraction optical element is insufficient in suppression of flare light of unnecessary diffraction order. <P>SOLUTION: The diffraction optical element structured by stacking a plurality of diffraction gratings 8 and 9 made of materials differing in Abbe number ν<SB>d</SB>is characterized in that the partial dispersion ratio θ<SB>g,F</SB>of a material constituting at lest one of the plurality diffraction gratings to a (g) line and an F line is <(-1/600)ν<SB>d</SB>+0.55, where θ<SB>g, F</SB>=(n<SB>g</SB>-n<SB>F</SB>)/(n<SB>F</SB>-n<SB>C</SB>), ν<SB>d</SB>=(n<SB>d</SB>-1)/(n<SB>F</SB>-n<SB>C</SB>), and n<SB>g</SB>, n<SB>F</SB>, n<SB>F</SB>, and n<SB>C</SB>are refractive indexes to the (g) line, the F line, a (d) line, and a C line. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a diffractive optical element and an optical system having the same.
[0002]
[Prior art]
As a method for reducing chromatic aberration of the lens system, there is a method using a combination of glass materials, and there is a method of providing a diffractive optical element or a diffraction grating having a diffractive action on the surface of the lens or a part of the optical system.
[0003]
For example, it is proposed or disclosed in Non-Patent Document 1, Patent Documents 1 to 3, and the like.
[0004]
The method using a diffractive optical element utilizes a physical phenomenon in which chromatic aberration appears with respect to a light beam having a certain reference wavelength in the opposite direction between a refraction surface and a diffraction surface. In addition, the diffractive optical element can have the effect of an aspheric lens by appropriately changing the period of the periodic structure, so that it is effective in reducing aberrations other than chromatic aberration.
[0005]
In a lens system having a diffractive optical element, when a light beam in a used wavelength region is concentrated on diffracted light of one specific order (hereinafter, also referred to as “specific order” or “design order”), other diffractions are performed. The intensity of the diffracted light of the order is low, and when the intensity is 0, the diffracted light does not exist.
[0006]
However, when diffracted light of an order other than the design order exists and has a certain degree of intensity, an image is formed at a place different from the light of the design order, so that the light becomes flare light in the lens system.
[0007]
Therefore, in order to utilize the above-described aberration reducing effect of the diffractive optical element, it is necessary that the diffraction efficiency of the diffracted light of the design order is sufficiently high over the entire use wavelength range, and the spectral distribution of the diffraction efficiency at this design order is required. It is important to fully consider the behavior of diffracted light other than the design order.
[0008]
FIG. 16 shows a diffractive optical element (hereinafter, referred to as “single-layer DOE”) including a substrate 302 and a diffraction grating 301 formed on the substrate 302. FIG. 17 shows the characteristic of the diffraction efficiency with respect to a specific order when the light emitting device is formed as follows.
[0009]
In FIG. 17, the horizontal axis represents the wavelength of the incident light, and the vertical axis represents the diffraction efficiency. The value of the diffraction efficiency is a ratio of the amount of the diffracted light in each order to the amount of the total transmitted light flux, and the value of the reflected light at the lattice boundary is not considered because the description is complicated.
[0010]
As shown in FIG. 17, the single-layer DOE shown in FIG. 16 is designed to have the highest diffraction efficiency in the used wavelength region in the first diffraction order (shown by a thick solid line in the drawing). The design order is the first order. At this design order, the diffraction efficiency becomes highest at a certain wavelength (hereinafter, this wavelength is referred to as “design wavelength”), and gradually decreases at other wavelengths. The decrease in the diffraction efficiency at this design order becomes diffracted light of another order and becomes flare light. FIG. 17 also shows diffraction efficiencies of orders near the design order (0th order and 2nd order of 1 ± 1 order) as other orders.
[0011]
Various proposals have been made as a configuration for reducing the influence of flare light generated as described above.
[0012]
As shown in FIG. 18, the diffractive optical element proposed in Patent Document 4 optimally selects three types of different grating materials 306 to 308 and two types of different grating thicknesses d1 and d2, and makes a plurality of diffraction gratings equal. As shown in FIG. 19, the close contact arrangement with the pitch distribution achieves a somewhat high diffraction efficiency over the entire visible range in the design order.
[0013]
As shown in FIG. 13, a diffractive optical element proposed in Patent Document 5 has a structure in which element portions 202 and 203 each including a diffraction grating are brought close to each other via an air layer 210 (hereinafter, referred to as a diffractive optical element). The diffractive optical element having such a configuration is referred to as a “stacked DOE” 201, and by optimizing the refractive index, dispersion characteristics (Abbe number νd) of the material constituting each diffraction grating, and the grating thickness of each layer. As shown in FIG. 14, high diffraction efficiency is realized over the entire visible region in the design order.
[0014]
Further, by defining the Abbe number of the material constituting the diffraction grating, a high diffraction efficiency is realized with a grating thickness of 10 μm or less. Accordingly, the diffraction efficiency of the design order ± 1st order is more favorably suppressed as shown in FIG. 15 than the single-layer DOE of FIG.
[0015]
[Non-patent document 1]
SPIE @ Vol. 1354 \ International \ Lens \ Design \ Conference (1990)
[Patent Document 1]
JP-A-4-213421
[Patent Document 2]
JP-A-6-324262
[Patent Document 3]
U.S. Pat. No. 5,044,706
[Patent Document 4]
JP-A-9-127322
[Patent Document 5]
JP 2000-98118 A
[0016]
[Problems to be solved by the invention]
By using the diffractive optical elements proposed in Patent Documents 4 and 5, the diffraction efficiency of the design order is greatly improved as compared with the single-layer DOE, and is 94% or more and 450 nm over the entire wavelength range used. A high diffraction efficiency of 98% or more can be obtained in the main wavelength range from 650 nm to 650 nm. Further, the flare light of the unnecessary diffraction order is also approximately satisfactorily suppressed to 2% or less over the entire use wavelength range and 0.6% or less over the main wavelength range from 450 nm to 650 nm.
[0017]
For this reason, in an application to an optical system in which photographing (projection) conditions do not change (for example, a leader lens of a copying machine or a projection lens of a liquid crystal projector), the effect of flare is suppressed to a level that does not cause any problem by the stacked DOE.
[0018]
However, in an optical system of an optical device such as a still camera or a video camera that photographs various subjects under various conditions, a slight remaining flare may be a problem.
[0019]
For example, when a light source is present in a subject, generally, the photographing is not performed so that the light source has a proper exposure at the time of photographing, and the photographing is performed such that a subject other than the light source has a proper exposure.
[0020]
For this reason, the light source in the subject is photographed with an exposure equal to or more than the proper exposure. For example, if the light source is exposed at 500 times the proper exposure, the flare of the light source will be multiplied by 500, even if only 0.6% of the flare remains.
0.6 × 500 = 300%
And a flare that is three times the proper exposure, and will always appear in the captured image.
[0021]
As described above, when the laminated DOE is applied to the optical system of a still camera or a video camera, even a slight flare poses a problem.
[0022]
Accordingly, the present invention provides a diffractive optical element capable of obtaining high diffraction efficiency, particularly for light of a specific order, in a wide range of use wavelengths, and capable of particularly suppressing diffracted light of unnecessary diffraction orders, and an optical system using the diffractive optical element. It is intended to be.
[0023]
[Means for Solving the Problems]
In order to achieve the above object, according to the present invention, in a diffractive optical element having a structure in which a plurality of diffraction gratings made of materials having different Abbe numbers νd are superimposed on each other, at least one of the plurality of diffraction gratings is provided. Partial dispersion ratio θ of materials constituting two diffraction gratings to g-line and F-line_{g, F}But,
θ_{g, F}<(-1/600) ν_d+0.55 (1)
Where θ_{g, F}= (N_g-N_F) / (N_F-N_C)
ν_d= (N_d-1) / (n_F-N_C)
n_g, N_F, N_d, N_CIs the refractive index for g-line, F-line, d-line, and C-line, respectively.
To satisfy certain conditions.
[0024]
That is, a stack including at least one diffraction grating using a material whose partial dispersion ratio with respect to the g-line and the F-line is smaller than the value on the right side of the conditional expression (1) (the value indicated by the solid line in the graph of FIG. 6). -Type diffractive optical element, while increasing the diffraction efficiency of a specific order (design order) over the entire wavelength (operating wavelength) region of the incident light, and unnecessary diffraction orders that can become flare light when incorporated into an optical system. Is realized.
[0025]
Further, by setting the grating thickness of each of the plurality of diffraction gratings to 10 μm or less, high diffraction efficiency can be achieved with a thin diffraction grating shape, and unnecessary diffraction orders that can become flare even when incorporated in an optical system having a wider angle of view. A diffractive optical element capable of favorably suppressing this light can be realized.
[0026]
The Abbe number ν of a material satisfying the above conditional expression (1)_dIs preferably 30 or less.
[0027]
Furthermore, the Abbe number ν of at least one of the materials of the plurality of diffraction gratings other than the material satisfying conditional expression (1)._dIs preferably 40 or more, the range of selection of materials satisfying conditional expression (1) is increased, which is preferable.
[0028]
As a material satisfying the above conditional expression (1), for example, Abbe number ν_dHaving a particle size of 15 or less (TiO 2)₂The moldability of the diffraction grating is improved by using a mixture of a resin material (for example, an ultraviolet-curable resin) and a material such as ITO or the like, particularly having a particle diameter of 1/20 or less of the wavelength of the incident light. be able to.
[0029]
Further, by changing the direction of change of the grating thickness in the grating period direction of at least one of the plurality of diffraction gratings from the direction of change of the grating thickness of the other diffraction gratings, a high value over the entire wavelength region to be used is obtained. More effective in achieving diffraction efficiency.
[0030]
When each of the plurality of diffraction gratings has a grating pitch P and a grating thickness d,
d / P <1/6
By satisfying the following conditions, the workability of the diffraction grating can be improved.
[0031]
Further, by integrally forming the diffraction grating with the substrate using the same material as the (transparent) substrate, it becomes easy to manufacture an element portion (a portion corresponding to a single-layer DOE) composed of the diffraction grating and the substrate. It is also easy to manufacture a diffractive optical element made by overlapping element parts.
[0032]
BEST MODE FOR CARRYING OUT THE INVENTION
(1st Embodiment)
FIG. 1A is a front view of a diffractive optical element according to a first embodiment of the present invention, and FIG. 1B is a side view of the diffractive optical element. FIG. 2 is an enlarged view of a part of the cross-sectional shape when the diffractive optical element of FIG. 1 is cut along the line A-A ′. However, FIG. 2 is a diagram that is considerably deformed in the lattice depth direction.
[0033]
As shown in these figures, the diffractive optical element 1 includes a first element part 2 and a second element part 3 which are formed by a first diffraction grating 8 and a second diffraction grating formed in each element part. 9 are superposed so as to be close to each other with the air layer 10 interposed therebetween, and the first, second element sections 2 and 3 and the air layer 10 as a whole act as one diffractive optical element. Things.
[0034]
The first and second diffraction gratings 8 and 9 have a concentric grating shape, and have a lens function by changing the grating pitch in the radial direction. Further, the first diffraction grating 8 and the second diffraction grating 9 have substantially the same grating pitch distribution.
[0035]
As shown in FIG. 2, the first element section 2 includes a first transparent substrate 4, a lattice base section 6 provided on the first transparent substrate 4, and an integral formation with the lattice base section 6. And a first grating forming layer formed of the first diffraction grating 8, and a grating surface 8 a is formed at a boundary between the first diffraction grating 8 and the air layer 10.
[0036]
On the other hand, similarly to the first element section 2, the second element section 3 includes a second transparent substrate 5, a grid base section 7 provided on the second transparent substrate 5, and a A second grating forming layer made of an integrated second diffraction grating 9, and a grating surface 9 a is formed at the boundary between the second diffraction grating 9 and the air layer 10.
[0037]
The air layer 10 is set so that the thickness becomes D between the edges formed by the grating surfaces 8a and 9a of the two diffraction gratings 8 and 9 and the grating side surfaces.
[0038]
Here, regarding the dimensions of the first and second element portions 2 and 3, the lattice pitch is P₁, P₂(Μm), the grating thickness is d₁, D₂(Μm),
d₁/ P₁<1/6
d₂/ P₂<1/6
Is satisfied, there is an advantage that the lattice shape is easily machined with respect to the element parts 2 and 3 themselves and a mold for manufacturing (resin molding) these element parts 2 and 3.
[0039]
In the present embodiment, the wavelength region of light incident on the diffractive optical element 1, that is, the used wavelength region is a visible region, and the materials and grating thicknesses of the first and second diffraction gratings 8, 9 are in the visible region. It is selected so as to increase the diffraction efficiency of the first-order diffracted light as a whole.
[0040]
Next, the diffraction efficiency of the diffractive optical element 1 of the present embodiment will be described. In the ordinary single-layer DOE shown in FIG.₀In the case of (1), the condition under which the diffraction efficiency of the diffracted light of a certain order is maximized is that when the light beam is incident perpendicularly to the base surface of the diffraction grating (the surface indicated by a dotted line in FIG. 2), The optical path length difference of a valley (that is, the optical path length difference between light beams passing through each of a peak and a valley) is to be an integral multiple of the wavelength of a light flux.
(N₀₁-1) d = mλ₀… (2)
It becomes.
[0041]
Where n₀₁Is the wavelength λ₀Is the refractive index of the material of the diffraction grating for this light. Also, d is the grating thickness and m is the diffraction order.
[0042]
Since the above equation (2) includes a wavelength term, the equality can be established only at the design wavelength with the same order, and the diffraction efficiency decreases from the maximum value at wavelengths other than the design wavelength.
[0043]
The diffraction efficiency η (λ) at an arbitrary wavelength λ is
η (λ) = sinc²[Π ｛M- (n₁(Λ) -1) d / λ｝] (3)
Can be represented by
[0044]
In the above equation (3), M is the order of the diffracted light to be evaluated, n₁(Λ) is the refractive index of the material of the diffraction grating for the light of wavelength λ. Also, sinc²(X) = {sin (x) / x}²Is a function represented by
[0045]
The basic principle is the same for a diffractive optical element having a laminated structure of two or more layers as in the present embodiment. In order for the diffractive optical element to function as one diffractive optical element through all the layers, the material (air or the like) constituting each layer is also required. The optical path length difference between the peaks and valleys of the diffraction grating formed at the boundary of the diffraction grating is calculated, and the sum of the optical path length differences over the entire diffraction grating is set to be an integer multiple of the wavelength. Is determined.
[0046]
Therefore, in the diffractive optical element 1 shown in FIG.₀In the case of, the condition for maximizing the diffraction efficiency of the diffracted light of the diffraction order m is
± (n₀₁-1) d₁± (n₀₂-1) d₂= Mλ₀… (4)
It becomes.
[0047]
Here, in the above equation (4), n₀₁Is a refractive index of a material forming the first diffraction grating 8 in the first element section 2 with respect to light having a wavelength λ0, and n₀₂Is the wavelength λ of the material forming the second diffraction grating 9 in the second element section 3.₀Is the refractive index for the light. Also, d₁, D₂Is the grating thickness of the first diffraction grating 8 and the second diffraction grating 9, respectively.
[0048]
Assuming that the diffraction order of light diffracted downward from the 0th-order diffracted light in FIG. 2 is a positive diffraction order and the diffraction order of light diffracted upward from the 0th-order diffracted light is a negative diffraction order, each layer in the above formula (3) Is negative in the case of the first diffraction grating 8 having a grating shape in which the grating thickness decreases from top to bottom in the drawing, and conversely, in the second diffraction grating having a grating shape in which the grating thickness increases from top to bottom. Is positive in the case of the diffraction grating 9.
[0049]
In the configuration shown in FIG. 2, the diffraction efficiency η (λ) at a wavelength λ other than the design wavelength λ0 is
η (λ) = sinc²[Π ｛M- ｛± (n₁(Λ) -1) d₁± (n₂(Λ) -1) d₂｝ / Λ｝]
= Sinc²[Π {M-φ (λ) / λ}] (5)
It can be expressed by the following equation.
[0050]
Φ (λ) in the above equation (5) is
φ (λ) = ± (n₁(Λ) -1) d₁± (n₂(Λ) -1) d₂
It is. M is the order of the diffracted light to be evaluated, n₁(Λ) is the refractive index of the material forming the first diffraction grating 8 at the wavelength λ, n₂(Λ) is the refractive index of the material forming the second diffraction grating 9 at the wavelength λ, d₁, D₂Is the grating thickness of the first diffraction grating 8 and the second diffraction grating 9, respectively. Also sinc²(X) = {sin (x) / x}²Is a function represented by
[0051]
In the diffractive optical element 1 shown in FIG. 2, the grating surfaces 8a and 9a are formed on the boundary surface with the air layer 10, but the diffractive optical element of the present invention is not limited to this. As shown in FIG. 9, it is also possible to use a diffractive optical element composed of two diffraction gratings each having a grating surface formed on a boundary surface between two different materials (optical materials) different from air.
[0052]
FIG. 9A shows an example in which diffraction gratings 8 and 9 having different grating thicknesses are closely attached, and FIG. 9B shows an example in which diffraction gratings 8 and 9 having the same grating thickness are closely attached. Depending on the combination of materials constituting the diffraction grating, the grating thicknesses of the two diffraction gratings 8 and 9 can be made equal as shown in FIG. 9B.
[0053]
Next, conditions for obtaining high diffraction efficiency in the diffractive optical element 1 of the present embodiment will be described.
[0054]
In order to obtain a high diffraction efficiency over the entire use wavelength range, the value η (λ) expressed by the expression (5) should be close to 1 for all use wavelengths λ. In other words, it can be seen from the equation that φ (λ) / λ in equation (5) should be m in order to increase the diffraction efficiency at the design order m. For example, when the design order m is the first order, φ (λ) / λ only needs to approach 1.
[0055]
Further, it can be seen from the above relationship that the optical path length difference φ (λ) obtained from the grating shape needs to change linearly in proportion to the wavelength λ.
[0056]
To this end, a term that varies with the wavelength in the optical path length difference φ (λ),
± n₁(Λ) d₁± n₂(Λ) d₂
Need to have linearity. In other words, the ratio of the change in the refractive index due to the wavelength of the material forming the second diffraction grating 9 to the change in the refractive index due to the wavelength of the material forming the first diffraction grating 8 is constant over the entire wavelength range used. Is required.
[0057]
Expressing this in an equation,
n₁(Λ₁) -N₁(Λ₂): N₂(Λ₁) -N₂(Λ₂)
= N₁(Λ₃) -N₁(Λ₄): N₂(Λ₃) -N₂(Λ₄)… (6)
Where λ₁, Λ₂, Λ₃, Λ₄Indicates any wavelength used
It becomes.
[0058]
Here, as a configuration that almost satisfies the relationship of the above expression (6), a diffractive optical element 1 having a laminated structure shown in FIG. 2 will be described as an example. First, in order to obtain high diffraction efficiency, it is sufficient that at least two diffraction gratings exist, but the diffractive optical element 1 shown in FIG. 2 satisfies this.
[0059]
Further, in the diffractive optical element 1 shown in FIG. 2, the first diffraction grating 8 is made of a material (n_d= 1.5702, ν_d= 13.5) and the grating thickness is 5.6 μm. On the other hand, the second diffraction grating 9 is made of an ultraviolet curable resin C001 (n) manufactured by Dainippon Ink and Chemicals, Inc._d= 1.524, ν_d= 50.8) and the grating thickness is 7.2 μm.
[0060]
FIG. 3 shows the diffraction efficiency characteristics in the first order, which is the design order of the diffractive optical element 1, and FIG. 4 shows the diffraction efficiency characteristics in the design order ± 1st order (0th order and 2nd order). . As can be seen from these characteristic diagrams, the diffractive optical element 1 has improved diffraction efficiency of the design order as compared with the characteristics shown in FIGS. Flare light is less likely to occur.
[0061]
In addition, the diffraction efficiency of the design order is 99.7% or more in the entire visible region, and accordingly, the flare light of the unnecessary order is 0.05% or less in the visible region, and the diffractive optical element using the conventional material. Is reduced to about 1/10.
[0062]
Here, regarding the diffraction efficiency of the unnecessary order light, only the 0th order and the 2nd order, which are the design order ± 1st order, are considered. However, since the diffraction orders farther away from the design order contribute less to the flare, This is because if the zero-order and second-order flare lights are reduced, the influence of other flare lights can be similarly reduced.
[0063]
This means that the diffractive optical element designed to diffract light of a specific design order mainly has a tendency that the diffraction efficiency tends to decrease as the order moves away from the design order. This is due to the fact that the order farther away from the lens is, the larger the blur is on the image forming surface and the less noticeable a flare is.
[0064]
Next, FIG. 5 shows the refractive index characteristics in the visible wavelength region of the material described in Japanese Patent Application Laid-Open No. 2000-98118 (the aforementioned Patent Document 5) and the material that is a feature of the present embodiment. In FIG. 5, a material 1 is a material used for both the second diffraction grating 9 of the present embodiment and the conventional example, and a material 2 is a material constituting the first diffraction grating 8 of the present embodiment. It is. Material 3 is a material constituting the first diffraction grating described in JP-A-2000-98118.
[0065]
In this characteristic diagram, the material 1 and the material 2 used in the present embodiment appear to have different slopes in the graph, but the change in the refractive index with respect to the change in wavelength is almost constant.
[0066]
On the other hand, in the material 1 and the material 3 used in the diffractive optical element described in JP-A-2000-98118, the change in the refractive index with respect to the wavelength is almost constant in the material 1, whereas the material 3 is short. The characteristic has a large degree of change on the wavelength side.
[0067]
This is because the characteristic of the Abbe number νd proposed in JP-A-2000-98118 is
ν_d= (N_d-1) / (n_F-N_C)
Where n_F, N_d, N_CIs the refractive index for F-line, d-line, and C-line, respectively.
This is because it is only a value that defines the average gradient of the refractive index change near the d-line (wavelength 587 nm).
[0068]
This ν_dThe characteristics were evaluation characteristics suitable for improving the diffraction efficiency characteristics as compared with the single-layer DOE while keeping the grating thickness small with a diffractive optical element having a laminated structure. However, in order to further improve the diffraction efficiency characteristic aimed at by the present invention, a conventional ν representing an average refractive index change is used._dVarious studies have shown that rating scales alone are not enough.
[0069]
In order to clarify the difference between these two materials, that is, material 2 and material 3, one evaluation scale used as a characteristic of the optical glass material will be examined.
[0070]
FIG. 6 shows the partial dispersion ratio θ with respect to the g-line and the F-line as the evaluation scales._{g, F}Shows the characteristics of In FIG. 6, the horizontal axis is ν_d, The vertical axis is θ_{g, F}Is the value of θ_{g, F}Is
θ_{g, F}= (N_g-N_F) / (N_F-N_C)
Where n_g, N_F, N_d, N_CIs a value defined by the refractive index for the g-line, F-line, d-line, and C-line, respectively, and is an evaluation scale representing the ratio of the change in the refractive index on the short wavelength side to the change in the refractive index on the long wavelength side. .
[0071]
The material 2 in FIG. 6 is the material used for the first diffraction grating 8 in the present embodiment._{g, F}Shows a considerably small value of about 0.3.
[0072]
Material 3 is a material described in JP-A-2000-98118, and this material 3 belongs to a general optical material. Then, the material 2 of the present embodiment is the same as the general optical material including the material described in JP-A-2000-98118._{g, F}Θ greatly different from characteristics_{g, F}It can be understood from FIG. 6 that it has characteristics.
[0073]
FIGS. 7 and 8 show the diffraction efficiency of the diffractive optical element using the optical material shown as the material 4 in FIG. FIG. 7 shows the first-order diffraction efficiency characteristic, which is the design order, and FIG. 8 shows the zero-order and second-order diffraction efficiency characteristics, which are the design order ± 1 order. As the diffraction efficiency of the design order, a high diffraction efficiency of 97% or more over the entire use wavelength region and 99.5% or more over the main wavelength region of 450 nm to 650 nm is obtained.
[0074]
On the other hand, the flare light of the unnecessary diffraction order is 0.9% or less in the entire use wavelength range, and is 0.2% or less in the main wavelength range from 450 nm to 650 nm, which is about １ ／ of the conventional example, which is well suppressed.
[0075]
As can be seen from the above, θ_{g, F}Regarding the characteristics, even with the material 4, sufficient diffraction efficiency characteristics can be obtained.
[0076]
Therefore, in order to improve the diffraction efficiency, which is the object of the present invention, the angle θ is larger than the solid line shown in FIG._{g, F}Is small, that is,
θ_{g, F}<(-1/600) ν_d+0.55
An optical material that satisfies the above condition may be used.
[0077]
As a material exhibiting such optical characteristics, there is ITO (Indium-Tin Oxide) or the like. However, when it is difficult to form a lattice shape using ITO as it is, ITO is used as fine particles having a diameter of the order of nanometers as proposed in Japanese Patent Application Laid-Open No. 2001-74701. It is preferable to use a material such as a material 2 or a material 4 mixed with a resin material that easily forms a lattice shape. Material 2 is a fluororesin Cytop (asahi Glass n_d= 1.34, ν_d= 90) mixed with ITO fine particles. Material 4 is polyvinyl carbazole (n_d= 1.70, ν_d= 18) mixed with ITO fine particles.
[0078]
As the same material, Abbe number ν_dThe use of a material having a value of 30 or less is preferable because the grating thickness of the diffraction grating can be reduced.
[0079]
Similarly, as the optical material in which fine particles are mixed in the resin material, it is preferable that the Abbe number is 30 or less after the fine particles are mixed. For this reason, a fine particle material having an Abbe number of 15 or less is used as the fine particles. It is desirable to do. As a particulate material satisfying such conditions, for example, in addition to the above-mentioned ITO, TiO₂The use of is considered.
[0080]
Also, the size (diameter) of the fine particles used is preferably 1/20 or less of the used wavelength so that light is not scattered by the mixed fine particles.
[0081]
On the other hand, it is preferable to use a material having an Abbe number of 40 or more as the material for forming the second diffraction grating 9 because the thickness of the grating can be kept small.
[0082]
When optical glass is used as a material for forming the first diffraction grating 8 or a material for forming the second diffraction grating 9, the transparent substrates 4 and 5 shown in FIG. In this case, both can be manufactured integrally, and the number of parts is reduced, which is advantageous for cost reduction.
[0083]
When each of the diffraction gratings 8 and 9 has a grating pitch P and a grating thickness d,
d / P <1/6
It is preferable to have a grating shape that satisfies the condition, since it becomes easy to manufacture a mold for forming the diffraction gratings 8 and 9.
[0084]
In the first embodiment described above, the diffractive optical element in which the diffraction gratings (laminated DOEs) 8 and 9 are provided on the substrates 4 and 5 as flat plates has been described. However, curved surfaces such as convex and concave surfaces of the lens are described. The same effect as that described in the present embodiment can be obtained even if the diffractive portion is provided in the first embodiment.
[0085]
Further, in the present embodiment, the diffractive optical element using the so-called first-order diffracted light whose design order is 1 has been described, but the design order is not limited to 1 and is different from the 2nd or 3rd-order first order. Even in the case of diffracted light, the same effect as in the present embodiment can be obtained by setting the combined value of the optical path length differences in the diffraction gratings 8 and 9 so as to have a desired design order and a desired design wavelength.
[0086]
(2nd Embodiment)
In the first embodiment, two kinds of materials forming the two diffraction gratings 8 and 9 have been described for comparison with the conventional example. However, the embodiment of the present invention is not limited to this.
[0087]
For example, as shown in FIG. 10, the present invention is also applicable to a diffractive optical element having a three-layer structure in which three diffraction gratings 8, 9, and 11 are formed of three different materials (materials indicated by 8, 9, and 11). An invention can be provided.
[0088]
Even in this case, at least one material may be a material that satisfies the above expression (1). For example, in the diffractive optical element 1 ′ shown in FIG. 10, the third diffraction grating 11 provided between the second diffraction grating 9 and the air layer 10 so as to be in contact with the grating surface 9 a of the second diffraction grating 9. It is preferable to use a material satisfying the above formula (1).
[0089]
In addition, the meaning of the dimension of each part shown in FIG. 10 is the same as that of the first embodiment, and D1 and D2, which are not included in the first embodiment, are the difference between the grating surface 8a and the grating side surface of the first diffraction grating 8, respectively. The dimension from the edge formed to the boundary surface with the air layer 10 in the third diffraction grating 11 and the air layer 10 in the third diffraction grating 11 from the edge formed by the grating surface 9a of the second diffraction grating 9 and the side surface of the grating. Is the dimension up to the boundary surface.
[0090]
(Third embodiment)
FIG. 11 shows a configuration of a photographing (imaging) optical system of a camera (still camera, video camera, or the like) according to the third embodiment of the present invention. In this figure, reference numeral 101 denotes a photographing lens mostly constituted by a refractive optical element (for example, a normal lens element), and includes an aperture stop 102 and the diffractive optical element 1 described in the first embodiment.
[0091]
Reference numeral 103 denotes a photographing medium such as a film or a CCD disposed on the image plane. The diffractive optical element 1 is an element having a lens function, and corrects chromatic aberration generated by the refractive optical element in the taking lens 101.
[0092]
As described in the first embodiment, since the diffraction efficiency characteristic of the diffractive optical element 1 is greatly improved as compared with the conventional one, the flare light is small, the resolving power at a low frequency is high, and the diffractive optical element 1 is high. A photographing optical system having optical performance can be realized.
[0093]
Further, the diffractive optical element 1 can be manufactured by a simple method such as bonding each of the diffraction gratings at the peripheral portion after manufacturing the respective diffraction gratings, like the optical element having the air layer 10 shown in FIG. An inexpensive optical system which is excellent in mass productivity as a photographing optical system can be provided.
[0094]
In the present embodiment, the diffractive optical element 1 is provided on the flat glass surface arranged near the stop 102. However, the present invention is not limited to this. It may be provided on the concave or convex surface of the lens. Further, a plurality of diffractive optical elements 1 may be arranged in the taking lens 101.
[0095]
Further, in the present embodiment, the case where the diffractive optical element according to the present invention is used for the photographing lens of the camera has been described. However, the present invention is not limited to this, and a wide wavelength range such as an image scanner of an office machine or a leader lens of a digital copier is used. When the diffractive optical element of the present invention is used in the imaging optical system used in the above, the same effects as described above can be obtained.
[0096]
(Fourth embodiment)
FIG. 12 shows a configuration of an observation optical system of binoculars according to a fourth embodiment of the present invention. In this figure, 104 is an objective lens, 105 is a prism for erecting an inverted image, 106 is an eyepiece, and 107 is an evaluation plane (pupil plane). Reference numeral 1 denotes the diffractive optical element described in the first embodiment, which is provided for the purpose of correcting chromatic aberration and the like on the imaging surface 103 of the objective lens 104.
[0097]
As described in the first embodiment, this observation optical system has significantly improved diffraction efficiency characteristics as compared with the conventional one, so that there is little flare light, high resolution at low frequencies, and high optical performance. Having.
[0098]
Further, the diffractive optical element 1 can be manufactured by a simple method such as bonding each of the diffraction gratings at the peripheral portion after manufacturing the respective diffraction gratings, like the optical element having the air layer 10 shown in FIG. An inexpensive optical system having excellent mass productivity can be provided as (the objective lens unit of) the observation optical system.
[0099]
In this embodiment, the case where the diffractive optical element 1 is provided on the flat glass surface has been described. However, the present invention is not limited to this, and the diffractive optical element 1 may be provided on a concave or convex surface of a lens. Further, a plurality of diffractive optical elements 1 may be arranged in the observation optical system.
[0100]
In the present embodiment, the case where the diffractive optical element 1 is provided in the objective lens unit has been described. However, the present invention is not limited to this, and the diffractive optical element 1 may be provided on the surface of the prism 105 or the position in the eyepiece 106. The same effect as described in (1) can be obtained. However, providing the diffractive optical element 1 on the object side with respect to the imaging plane 103 has an effect of reducing chromatic aberration only with the objective lens unit. Therefore, in the case of a visual observation system, it is desirable to provide the diffractive optical element 1 at least in the objective lens unit.
[0101]
Further, in this embodiment, the observation optical system of the binoculars has been described. However, the diffractive optical element of the present invention can be applied to an observation optical system such as a terrestrial telescope or an astronomical observation telescope, and furthermore, a lens shutter camera. It can also be applied to an optical viewfinder such as a camera or a video camera, and the same effects as described above can be obtained.
[0102]
【The invention's effect】
As described above, according to the present invention, the laminated type including at least one diffraction grating using a material whose partial dispersion ratio with respect to the g-line and the F-line is smaller than the value on the right side of the conditional expression (1) is described. By using a diffractive optical element, while increasing the diffraction efficiency of a specific order (design order) over the entire wavelength (use wavelength) region of incident light, light of an unnecessary diffraction order that can become flare light when incorporated into an optical system. Can be realized.
[Brief description of the drawings]
FIG. 1 is a front view and a side view of a diffractive optical element according to a first embodiment of the present invention.
FIG. 2 is a partial cross-sectional view of the diffractive optical element according to the first embodiment.
FIG. 3 is a graph showing diffraction efficiency characteristics at a design order of the diffractive optical element according to the first embodiment.
FIG. 4 is a graph showing diffraction efficiency characteristics of the diffractive optical element according to the first embodiment at a design order ± 1st order.
FIG. 5 is a graph showing a refractive index characteristic (nd-λ characteristic) of a material constituting the diffractive optical element of the first embodiment.
FIG. 6 is a graph showing partial dispersion ratio characteristics (θg, F−νd characteristics) of a material constituting the diffractive optical element of the first embodiment.
FIG. 7 is a graph showing a diffraction efficiency characteristic of a design order when another material is used in the diffractive optical element of the first embodiment.
FIG. 8 is a graph showing the diffraction efficiency characteristic of the design order ± 1 order when another material is used in the diffractive optical element of the first embodiment.
FIG. 9 is a partial cross-sectional view of another mode of the diffractive optical element of the first embodiment.
FIG. 10 is a partial sectional view of a diffractive optical element according to a second embodiment of the present invention.
FIG. 11 is a configuration diagram of a photographic optical system according to a third embodiment of the present invention.
FIG. 12 is a configuration diagram of a photographic optical system according to a fourth embodiment of the present invention.
FIG. 13 is a partial sectional view of a conventional laminated diffractive optical element.
FIG. 14 is a graph showing diffraction efficiency characteristics at a design order of a conventional laminated diffractive optical element.
FIG. 15 is a graph showing diffraction efficiency characteristics of a conventional laminated diffractive optical element at a design order ± 1st order.
FIG. 16 is a partial cross-sectional view of a conventional single-layer diffractive optical element.
FIG. 17 is a graph showing diffraction efficiency characteristics at a design order of a conventional single-layer diffractive optical element.
FIG. 18 is a partial sectional view of a conventional laminated diffractive optical element.
FIG. 19 is a graph showing diffraction efficiency characteristics at a design order of a conventional laminated diffractive optical element.
[Explanation of symbols]
1 diffractive optical element
2 First element part
3 2nd element part
4 First substrate
5 Second substrate
6 First grid base
7 Second diffraction base
8 First diffraction grating
9 Second diffraction grating
10 air layer
11 3rd layer
101mm shooting lens
102 aperture
103 image plane
104 objective lens
105 ° prism
106 eyepiece
107 ° evaluation plane (pupil plane)

Claims (12)

A diffractive optical element having a structure in which a plurality of diffraction gratings made of materials having different Abbe numbers νd are stacked,
The partial dispersion ratio θ _{g, F of} the material constituting at least one of the plurality of diffraction gratings with respect to the g-line and the F-line is:
θ _{g, F} <(− 1/600) ν _d +0.55
_{However, θ g, F = (n} g -n F) / (n F -n C)
_{ν d = (n d -1)} / (n F -n C)
_{n g, n F, n d} , each of _{n C,} g-line, F-line, d line, the diffractive optical element which satisfies the condition that the refractive index for the C line. 2. The diffractive optical element according to claim 1, wherein each of the plurality of diffraction gratings has a grating thickness of 10 μm or less. The diffractive optical element according to claim 1, wherein the Abbe number ν _{d of the} material satisfying the condition is 30 or less. The diffractive optical element according to claim 1, wherein at least one of the materials of the plurality of diffraction gratings other than a material satisfying the condition has an Abbe number ν _d of 40 or more. Material satisfying the condition, the diffractive optical element according to claim 1, wherein the Abbe number [nu _d is a mixture of 15 or less particulate material in the resin material. The diffractive optical element according to claim 5, wherein a particle diameter of the fine particle material is 1/20 or less of a wavelength of incident light. The diffractive optical element according to claim 5, wherein the particulate material, characterized in that it is a TiO ₂ or ITO. The diffractive optical element according to claim 5, wherein the resin material is an ultraviolet curable resin. The diffractive optic according to claim 1, wherein a change direction of a grating thickness in a grating period direction of at least one of the plurality of diffraction gratings is different from a change direction of a grating thickness of another diffraction grating. element. When each of the plurality of diffraction gratings has a grating pitch P and a grating thickness d,
d / P <1/6
The diffractive optical element according to claim 1, wherein the following condition is satisfied. 3. The diffractive optical element according to claim 1, wherein the diffractive optical element has a diffractive effect on light in a visible wavelength range. An optical system comprising a refractive optical element and the diffractive optical element according to claim 1.

Images