JP6873602B2 - Diffractive optical elements, optical systems, and optical equipment - Google Patents

Description

The present invention relates to a diffractive optical element that reduces the generation of unnecessary light.

In the diffraction optical element used for the lens of the optical system, it is possible to obtain high diffraction efficiency in a wide wavelength band by arranging two diffraction gratings in close contact with each other and appropriately setting the material and lattice height constituting each diffraction grating. Are known. When a luminous flux is incident on a diffractive optical element having a lattice surface and a lattice wall surface, unnecessary light (flare) is generated due to the influence of the lattice wall surface even in an ideal diffractive optical element calculated by scalar diffraction theory. ..

Patent Document 1 uses a diffraction optical element that utilizes strict coupling wave analysis (RCWA: Regulus Coupled Wave Analysis) and provides an optical waveguide on the lattice wall surface to improve the diffraction efficiency of the design order of the incident luminous flux at the design incident angle. Is disclosed. In Patent Document 2, a thin film is provided on the wall surface of the lattice to improve the diffraction efficiency of the design order of the incident luminous flux at the design incident angle, reduce the diffraction efficiency of the design order ± 1st order, and obliquely incident angle (out-of-screen light). A diffractive optical element that reduces unnecessary light that causes an incident light flux at an incident angle) to reach an imaging surface is disclosed.

International Publication No. 2011/099550 Pamphlet Japanese Unexamined Patent Publication No. 2014-170109

The diffractive optical element of Patent Document 1 improves the diffraction efficiency of the design order of the luminous flux incident at the design incident angle, but it is not possible to reduce unnecessary light reaching the image plane due to the luminous flux incident at the oblique incident angle. Have difficulty. The diffractive optical element of Patent Document 2 improves the diffraction efficiency of the design order of the luminous flux incident at the design incident angle, reduces the diffraction efficiency of the design order ± 1st order, and forms an image plane due to the luminous flux incident at the oblique incident angle. It is possible to reduce unnecessary light that reaches. However, since the diffractive optical element of Patent Document 2 has high wavelength dependence and polarization dependence, it is difficult to configure the diffractive optical element so as to have desired wavelength characteristics and polarization characteristics.

Therefore, the present invention provides diffractive optical elements, optical systems, and optical devices that reduce wavelength dependence and polarization dependence and have desired wavelength characteristics and polarization characteristics.

とするとき、
ｎ_１＞ｎ_２＞ｎ_３
０．００５＜Δ＜０．０４５
０．５≦Ｗ／Ｗ_Ｃ≦２．０
なる条件式を満たし、前記薄膜の材料のアッべ数は、前記第２の回折格子の材料のアッべ数よりも大きい。 The diffraction optical element as one aspect of the present invention includes a first diffraction grating having a first grating surface and a first lattice wall surface, and a second diffraction grating having a second lattice surface and a second lattice wall surface. It has a diffraction grating and a thin film in contact with both the first and second lattice wall surfaces, and the extinction coefficient of the thin film with respect to the wavelength λ (μm) in the wavelength band used is 0.0005 or less. The refractive index of each material of the first diffraction grating and the second diffraction grating with respect to the wavelength λ is n ₁ , n ₂ , and n ₃ , and the width of the thin film is W.

When
n ₁ > n ₂ > n ₃
0.005 <Δ <0.045
0.5 ≦ _W / W _C ≦ 2.0
The conditional expression is satisfied, and the number of materials of the thin film is larger than the number of materials of the second diffraction grating.

Optical system as another aspect of the present invention includes a pre-Symbol diffractive optical element, and a diaphragm.

An optical device as another aspect of the present invention includes the optical system and an image pickup device .

Other objects and features of the present invention will be described in the following embodiments.

According to the present invention, it is possible to provide a diffractive optical element, an optical system, and an optical device having desired wavelength characteristics and polarization characteristics by reducing wavelength dependence and polarization dependence.

It is the schematic of the diffraction optical element in this embodiment. It is an enlarged sectional view of the diffraction optical element in this embodiment. It is an enlarged sectional view of the diffraction optical part in this embodiment. It is the schematic of the optical system which has a diffractive optical element in this embodiment. It is an enlarged cross-sectional view of the diffraction optical part in Example 1. FIG. In Example 1, it is explanatory drawing of the influence of unnecessary light of the design incident angle (photographing light incident angle) about the optical system of FIG. It is a graph of the diffraction efficiency of the + 1st order diffracted light, the 0th order diffracted light, and the + 2nd order diffracted light with respect to the design incident angle luminous flux of the diffractive optical element in Example 1. Design of a Diffractive Optical Element as a Comparative Example It is a graph of the diffraction efficiency of the + 1st order diffracted light, the 0th order diffracted light, and the + 2nd order diffracted light with respect to the incident angle luminous flux. In Example 1, it is explanatory drawing of the influence of unnecessary light of the oblique incident angle (out-of-screen light incident angle) with respect to the optical system of FIG. Wavelength of the diffractive optical element in Example 1 with respect to the extrascreen incident + 10 degree luminous flux . 400 μm , 0. 550 μm , 0. It is a graph of the diffraction efficiency of 700 μm. As a comparative example, the wavelength of the diffractive optical element with respect to the extrascreen incident + 10 degree luminous flux is 0. 400 μm , 0. 550 μm , 0. It is a graph of the diffraction efficiency of 700 μm. Wavelength of the diffractive optical element in Example 1 with respect to the extrascreen incident −10 degree luminous flux . 400 μm , 0. 550 μm , 0. It is a graph of the diffraction efficiency of 700 μm. As a comparative example, the wavelength of the diffractive optical element with respect to the off-screen incident -10 degree luminous flux is 0. 400 μm , 0. 550 μm , 0. It is a graph of the diffraction efficiency of 700 μm. It is a graph of the diffraction efficiency of the + 1st order, 0th order, + 2nd order diffracted light with respect to the design incident angle luminous flux of the diffractive optical element in Example 2. Wavelength of the diffractive optical element in Example 2 with respect to the extrascreen incident + 10 degree luminous flux . 400 μm , 0. 550 μm , 0. It is a graph of the diffraction efficiency of 700 μm. It is a graph of the diffraction efficiency of the + 1st order, 0th order, + 2nd order diffracted light with respect to the design incident angle luminous flux of the diffractive optical element in Example 3. Wavelength of the diffractive optical element in Example 3 with respect to the extrascreen incident + 10 degree luminous flux . 400 μm , 0. 550 μm , 0. It is a graph of the diffraction efficiency of 700 μm. It is a graph of the diffraction efficiency of the +1st order diffracted light with respect to the design incident angle luminous flux of the diffractive optical element as a comparative example. As a comparative example, the wavelength of the diffractive optical element with respect to the extrascreen incident + 10 degree luminous flux is 0. 400 μm , 0. 550 μm , 0. It is a graph of the diffraction efficiency of 700 μm. It is a graph of the diffraction efficiency of the +1st order diffracted light with respect to the design incident angle luminous flux of the diffractive optical element as a comparative example. As a comparative example, the wavelength of the diffractive optical element with respect to the extrascreen incident + 10 degree luminous flux is 0. 400 μm , 0. 550 μm , 0. It is a graph of the diffraction efficiency of 700 μm. As a comparative example, the wavelength of the diffractive optical element with respect to the extrascreen incident + 10 degree luminous flux is 0. It is a graph of the diffraction efficiency of 550 μm. It is a graph of the diffraction efficiency of the +1st order diffracted light with respect to the design incident angle luminous flux of the diffractive optical element in Examples 1 to 3. It is a graph of the diffraction efficiency of the + 1st order, 0th order, + 2nd order diffracted light with respect to the design incident angle luminous flux of the diffractive optical element in Example 4. Wavelength of the diffractive optical element in Example 4 with respect to the extrascreen incident + 10 degree luminous flux . 400 μm , 0. 550 μm , 0. It is a graph of the diffraction efficiency of 700 μm. It is a graph of the diffraction efficiency of the + 1st order, 0th order, + 2nd order diffracted light with respect to the design incident angle luminous flux of the diffractive optical element in Example 5. Wavelength of the diffractive optical element in Example 5 with respect to the extrascreen incident + 10 degree luminous flux . 400 μm , 0. 550 μm , 0. It is a graph of the diffraction efficiency of 700 μm. It is an enlarged cross-sectional view of the diffraction optical part as a modification as a modification in this embodiment. It is a graph of the diffraction efficiency of the + 1st order diffracted light, the 0th order diffracted light, and the + 2nd order diffracted light with respect to the design incident angle luminous flux of the diffractive optical element in Example 6. Design of a Diffractive Optical Element as a Comparative Example It is a graph of the diffraction efficiency of the + 1st order diffracted light, the 0th order diffracted light, and the + 2nd order diffracted light with respect to the incident angle luminous flux. Wavelength of the diffractive optical element in Example 6 with respect to the extrascreen incident + 10 degree luminous flux . 400 μm , 0. 550 μm , 0. It is a graph of the diffraction efficiency of 700 μm. As a comparative example, the wavelength of the diffractive optical element with respect to the extrascreen incident + 10 degree luminous flux is 0. 400 μm , 0. 550 μm , 0. It is a graph of the diffraction efficiency of 700 μm. It is a graph of the diffraction efficiency of the +1st order diffracted light with respect to the design incident angle luminous flux of the diffractive optical element in Example 6.

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

First, the diffractive optical element in this embodiment will be described. FIG. 1 is a schematic view (front view and side view) of the diffractive optical element (DOE) in the present embodiment. The diffractive optical element 1 is configured to increase the diffraction efficiency of diffracted light of a specific order (specific order or design order) in the used wavelength region over the entire visible wavelength body. The diffraction optical element 1 has a pair of transparent substrate lenses 2 and 3 and a diffraction grating portion 10 arranged between the substrate lenses 2 and 3. Each of the substrate lenses 2 and 3 has a flat plate shape or a shape that acts as a lens. In this embodiment, both sides of the substrate lenses 2 and 3 are curved surfaces. The diffraction grating portion 10 has a concentric diffraction grating shape centered on the optical axis O, and has a lens function.

FIG. 2 is an enlarged cross-sectional view of the diffraction optical element 1 enlarged by cutting the line AA'in FIG. In order to make the grid shape easy to understand, FIG. 2 is a diagram deformed in the grid depth direction. Also, the number of grids is drawn less than it actually is. The same applies to the cross-sectional views described below. FIG. 3 is an enlarged cross-sectional view of the diffraction optical unit 10 of FIG. In FIGS. 2 and 3, the incident luminous flux a is a luminous flux incident at an incident angle of 0 degrees, which is the design incident angle of the diffractive optical element 1. The incident luminous flux b is a luminous flux incident downward at an oblique incident angle (out-of-screen light incident angle). The incident luminous flux c is a luminous flux incident upward at an oblique incident angle (out-of-screen light incident angle).

As shown in FIGS. 2 and 3, the diffraction grating portion 10 has a diffraction grating 21 (first diffraction grating), a diffraction grating 31 (second diffraction grating), and a thin film 11. The diffraction grating 21 and the diffraction grating 31 are formed in close contact with each other in the optical axis direction (direction along the optical axis O). The thin film 11 is provided between the lattice wall surface 21b (first lattice wall surface) of the diffraction grating 21 and the lattice wall surface 31b (second lattice wall surface) of the diffraction grating 31, and is in contact with both the lattice wall surfaces 21b and 31b. There is.

Further, the thin film 11 is transparent to light having a wavelength λ (arbitrary wavelength) in the wavelength band used (for example, the visible wavelength band). Specifically, if the extinction coefficient of the thin film 11 with respect to light (wavelength λ) in the used wavelength band is 0.0005 or less, it can be said that the thin film 11 is substantially transparent. If the extinction coefficient of the thin film 11 exceeds 0.0005, the thin film 11 has absorption characteristics. Further, when light is obliquely incident on the diffraction optical element, reflected light is generated at the interface between each diffraction grating and the thin film 11 due to the difference between the extinction coefficient of each diffraction grating and the extinction coefficient of the thin film 11. It ends up. Therefore, the extinction coefficient of the thin film 11 is preferably 0.0005 or less. More preferably, the extinction coefficient of the thin film 11 with respect to light in the wavelength band used is 0.0003 or less.

The diffraction grating 21 may be integrated with or separate from the substrate lens 2. Similarly, the diffraction grating 31 may be integrated with or separate from the substrate lens 3. In the present embodiment, the diffraction gratings 21 and 31 are in close contact with each other in the optical axis direction, but the thin film 11 interposed between the diffraction gratings 21 and 31 is provided over the entire boundary surface between the two as will be described later. In some cases. Therefore, the diffraction gratings 21 and 31 may be laminated in the optical axis direction.

The diffraction grating 21 has a concentric blaze structure composed of a lattice surface 21a (first lattice surface) and a lattice wall surface 21b (first lattice wall surface). Similarly, the diffraction grating 31 has a concentric blaze structure composed of a lattice surface 31a (second lattice surface) and a lattice wall surface 31b (second lattice wall surface). Each of the diffraction gratings 21 and 31 realizes a lens action (astringent action and a divergent action of light) by gradually changing the lattice pitch as the distance from the optical axis O (approaches the outer peripheral portion). The lattice planes 21a and 31a are in contact with each other without a gap, and the diffraction gratings 21 and 31 act as one diffraction grating portion 10 as a whole. Further, by forming the diffraction gratings 21 and 31 into a blaze structure, respectively, the incident light incident on the diffraction optical element 1 has a specific diffraction order (with respect to the 0th order diffraction direction transmitted without being diffracted by the diffraction grating portion 10). In FIGS. 2 and 3, the diffraction is concentrated in the direction of +1).

The wavelength region used for the diffractive optical element 1 of the present embodiment is the visible region. Therefore, the materials and lattice heights constituting the diffraction gratings 21 and 31 are selected according to the scalar diffraction theory so that the diffraction efficiency of the diffracted light of the design order is high in the entire visible region. That is, the maximum optical path length difference (maximum value of the optical path length difference between the peak and valley of the diffraction portion) of the light passing through the plurality of diffraction gratings (diffraction gratings 21 and 31) is within the wavelength range used and is near an integral multiple of the wavelength. The material and lattice height of each diffraction grating are determined so as to be. By appropriately setting the materials and shapes of the diffraction gratings 21 and 31 in this way, high diffraction efficiency can be obtained over the entire wavelength used.

Generally, the lattice height of a diffraction grating is defined by the height of the lattice tip and the lattice groove in the direction perpendicular to the lattice periodic direction (plane normal direction). In addition, when the lattice wall surface is shifted from the surface normal direction or when the lattice tip is deformed, the lattice height is defined by the distance between the extension line of the lattice surface and the intersection of the surface normal lines. Will be done. In this embodiment, the material and height of the diffraction grating are not limited.

In this embodiment, the diffraction gratings 21 and 31 are made of different materials. For example, the diffraction grating 31 is made of a low refractive index dispersion material, and the diffraction grating 21 is made of a high refractive index dispersion material having a higher refractive index than the diffraction grating 31. In this embodiment, high diffraction efficiency can be obtained by satisfying the following equations (1) to (3).

νd2> 35 ... (1)
νd3 <25 ... (2)
0.960 ≤ (n _2- n ₃ ) x d / (m x λ) ≤ 1.040 ... (3)
In the formula (1) to (3), the refractive index of the material respectively _n 2, _{n 3} constituting the diffraction grating 21 and 31 at the wavelength lambda, the Abbe number vd2, and vd3. Further, the height of each of the diffraction gratings 21 and 31 is d, and the design order is m. The visible wavelength range in this embodiment mainly has a wavelength of 0. 400 μm or more and 0. It is a band of 700 μm or less.

Further, in order to obtain high diffraction efficiency in the entire visible wavelength region, the number of abbreviations of the high refractive index low dispersion material (diffraction grating 21) is made larger than 35, and the number of the low refractive index high dispersion material (diffraction grating 31) is increased. It is preferable that the total number is smaller than 25. Further, it is preferable to use a material having a partial dispersion ratio θgF smaller than that of a normal material (linear anomalous dispersibility). In order to obtain this linear dispersion characteristic, a method of dispersing ITO fine particles in fine particles and mixing them with the base resin material can be used. Unlike other inorganic oxides, ITO changes its refractive index due to free carriers generated by doping with tin and vacancies of oxygen, in addition to changes in the refractive index due to electronic transitions. Due to the influence of electronic transitions and free carriers, it has a very strong linear dispersion characteristic. Therefore, SnO2 and ATO (Antimony-doped SnO2), which are affected by free carriers as well as ITO, can also be used.

The resin material in which the fine particles are dispersed is an ultraviolet curable resin and includes, but is not limited to, an acrylic-based, fluorine-based, vinyl-based, or epoxy-based organic resin. The average particle size of the fine particle material is preferably 1/4 or less of the wavelength (wavelength used or design wavelength) of the incident light on the diffractive optical element. If the particle size is larger than this, Rayleigh scattering may increase when the fine particle material is mixed with the resin material. The lattice height d is preferably set to 15 μm or less. As a result, it is possible to reduce the decrease in diffraction efficiency when obliquely incident light is incident.

The thin film 11 has a substantially uniform thickness along the wall surface of the grating and is arranged at least a part of the boundary surface of the diffraction gratings 21 and 31. In the present embodiment, the thin film 11 is provided on the lattice wall surfaces 21b and 31b. By providing the thin film 11, the luminous flux incident on the vicinity of the lattice wall surface is confined inside the thin film 11 and becomes an optical waveguide.

In the diffraction optical element 1 of the present embodiment, the refractive indexes of the materials of the thin film 11, the diffraction grating 21, and the diffraction grating 31 are set to n ₁ , n ₂ , and n ₃ , respectively, with respect to the wavelength λ of the wavelength band used. Further, the difference in the specific refractive index between the thin film 11 and the diffraction grating 21 is defined as Δ. At this time, by satisfying the following equations (4) and (5), the diffraction efficiency of the design order of the luminous flux incident at the design incident angle is improved, the diffraction efficiency of the design order ± 1st order is reduced, and the oblique incident is oblique. Of the unnecessary light due to the luminous flux incident at an angle, the unnecessary light that reaches the imaging surface can be reduced. Further, by satisfying the equations (4) and (5), it is possible to reduce the wavelength dependence and the polarization dependence of the diffractive optical element 1 and to have a desired wavelength characteristic and polarization characteristic.

n ₁ > n ₂ > n ₃ ... (4)
0.005 <Δ <0.045 ... (5)
In the formula (5), the specific refractive index difference Δ is obtained by the following formula (6).

Further, it is preferable that the formula (5) satisfies the following formula (5a).

0.007 <Δ <0.042 ... (5a)
By satisfying the lower limit of the formula (5) or the formula (5a), it is possible to reduce the unnecessary light that reaches the image plane among the unnecessary light due to the luminous flux incident at the oblique incident angle. Further, the polarization dependence can be reduced by satisfying the upper limit of the formula (5) or the formula (5a).

Further, since the materials constituting the thin film 11 and the diffraction gratings 21 and 31 are asymmetric three-layer flat film waveguides, it is known that the waveguide mode satisfies the following eigenvalue equation. Equations (7) and (8) relate to TE-polarized light and TM-polarized light, respectively.

In equations (7) and (8), k ₀ is a value defined as in equation (9) below.

Further, regarding the cutoff width in which the single mode of the asymmetric three-layer flat plate waveguide occurs, the cutoff widths WC _{and TE} of TE polarized light are expressed by the following equations (10), and the cutoff widths WC _{and TM} of TM polarized light are expressed by equations. It is expressed as (11).

In equations (10) and (11), κ _c and δ _c are values defined as in equation (12) below.

In the present embodiment, the width W (film width) of the thin film 11 and the average Wc of the cutoff width generated by the single mode of TE polarization and TM polarization of the following formula (14) satisfy the following formula (13). As a result, the diffractive optical element 1 can achieve a desired effect. That is, the diffractive optical element 1 improves the diffraction efficiency of the design order of the luminous flux incident at the design incident angle, reduces the diffraction efficiency of the design order ± 1st order, and forms an image of unnecessary light due to the luminous flux incident at the oblique incident angle. It is possible to reduce unnecessary light that reaches the surface. Further, the diffractive optical element 1 can reduce the wavelength dependence and the polarization dependence and have desired wavelength characteristics and polarization characteristics. Here, the width W of the thin film 11 is the width of the thin film 11 (distance between the lattice wall surface 21b and the lattice wall surface 31b) between the lattice wall surface 21b and the lattice wall surface 31b.

0.5 ≤ W / Wc ≤ 2.0 ... (13)

In the present embodiment, the formula (13) preferably satisfies the following formula (13a).

0.75 ≤ W / Wc ≤ 1.75 ... (13a)
Further, regarding the cutoff width at which the primary mode, which is the single mode condition of the asymmetric three-layer flat plate waveguide, occurs, the cutoff width W _TE0 of TE polarized light is expressed by the following equation (15), the cutoff width W _{TM0 of TM polarized light.} Is expressed as the following equation (16).

In equations (15) and (16), a'is a value defined by the following equation (17).

Also relates to the equivalent refractive index is the refractive index of the guided mode feel, TE polarization equivalent refractive index _{n eq, TE} and TM polarization equivalent refractive index _{n eq, TM} has the following formula (18), (19) Each is represented as.

In equations (18) and (19), β _TE and β _TM are propagation constants of TE-polarized light and TM-polarized light, respectively.

When the waveguide width of the asymmetric three-layer flat plate waveguide is less than the cutoff width of the single mode of the equation (10) for TE polarization and the equation (11) for TM polarization, it becomes the radiation mode, so that the equation (7) ) And (8) have no solution. On the other hand, when the cutoff width of the single mode is larger than that, the waveguide mode is generated, so that the equations (7) and (8) have solutions. Further, when the cutoff width of the primary mode of the equation (15) is smaller than that of the equation (15) for TE polarization and the cutoff width of the primary mode of the equation (16) is for TM polarization, a single mode condition is obtained. It is known to have only a solution. Under this condition, the equivalent refractive index of the formulas (18) and (19) can be obtained one by one for each of TE-polarized light and TM-polarized light. This condition is satisfied when the width W (film thickness) of the thin film 11 satisfies the following equation (20).

Strictly speaking, since W _TE0 <W _TM0 always holds, it is necessary to satisfy the following equation (21) in order for both TE polarized light and TM polarized light to have one solution.

W <W _TE0 ... (21)
Guided mode feels formula asymmetric 3-layer plate waveguide (18), TE polarized light and TM (equivalent refractive index n _eq of the propagation light propagating inside of the thin film ₁₁₎ average polarization equivalent refractive index of the (19) , Is expressed as the following equation (23). In the present embodiment, the phase difference between the refractive index n ₂ of the equivalent refractive index n _eq and diffraction grating 21 is a high refractive index material is less so as to satisfy the following equation (22).

0 ≦ (n _eq −n ₂ ) × d / λ <0.3… (22)

As a result, the light that would have been unnecessary light when the thin film 11 is not provided can be confined in the optical waveguide, the waveguide mode and the diffraction grating can be phase-matched, and the diffraction efficiency of the design order can be improved. Is possible. In addition, the diffraction efficiency of the design order is improved, and at the same time, the diffraction efficiency of the design order ± 1st order is reduced, and the unnecessary light due to the luminous flux incident at the oblique incident angle (out-of-screen light incident angle) reaches the imaging surface. Unnecessary light can be reduced.

In the present embodiment, the formula (22) preferably satisfies the following formula (22a).

0 ≦ (n _eq −n ₂ ) × d / λ <0.2… (22a)
At this time, even if the phase difference in the equation (22) is N × λ (N is an integer of 1 or more), the phase matching condition is satisfied, but under this condition, the wavelength dependence is large and it is difficult to satisfy the entire visible wavelength range. , Not preferable. In the present embodiment, the specific refractive index difference Δ is smaller on the short wavelength side than on the long wavelength side in the used wavelength band (the specific refractive index difference with respect to the first wavelength (λ1) in the used wavelength band is larger than the first wavelength. Less than the differential index difference for the long second wavelength (λ2> λ1)). Thereby, the wavelength dependence can be reduced more effectively. Further, the wavelength dependence can be reduced by satisfying the following equation (24) for the specific refractive index difference Δ and the wavelength λ of the wavelength used.

0.01 <Δ / λ <0.08… (24)
Further, in the present embodiment, the wavelength dependence can be reduced by increasing the number of materials of the thin film 11 to be larger than the number of materials of the diffraction grating 31 (second diffraction grating).

The effect of this embodiment can be obtained by appropriately setting the material of the thin film 11 and the film width W satisfying the above relationship.

Further, the refractive index n ₁ of the thin film 11 preferably satisfies the following formula (25).

1.64 <n ₁ <1.75 ... (25)
By satisfying the formula (25), as described in Examples 1 to 6 below, the selectivity of the material of the thin film and the diffraction grating constituting the diffractive optical element with reduced wavelength dependence and polarization dependence can be determined. Can be expanded. If the lower limit of the formula (25) is not satisfied, both the refractive index of the thin film and the refractive index of the diffraction grating become small. In this case, the selectivity of the thin film material is limited, or the cost of the thin film material increases. Further, the selectivity of the material for obtaining a high diffraction grating in the entire visible wavelength band according to the thin film is narrowed.

If the upper limit of the formula (25) is not satisfied, it becomes difficult to reduce the wavelength characteristics. In addition, both the refractive index of the thin film and the refractive index of the diffraction grating increase. In this case, the selectivity of the material for obtaining a high diffraction grating in the entire visible wavelength band is also narrowed. More preferably, the following formula (26) is satisfied.

1.65 <n ₁ <1.70 ... (26)
The material of the thin film 11 is not particularly limited. Examples of the material of the thin film 11 include oxides such as Al ₂ O ₃ , ZrO ₂ , La ₂ O ₃ , Y ₂ O ₃ , HfO ₂ , Ta ₂ O ₅ , Nb ₂ O ₅ , TiO ₂ , and SiO ₂ . Fluoride such as LaF ₃ , NdF ₃ , CeF ₃ , and MgF ₂ can be adopted. Further, as the material of the thin film 11, a compound such as ZnS, CdS, ZnSe, ZnTe, or a mixture or compound of the above materials can be adopted. Further, as the material of the thin film 11, an organic resin such as acrylic, fluorine, vinyl, or epoxy, or a material in which fine particles are dispersed in the organic resin may be adopted.

Similarly, the method for producing the thin film 11 is not particularly limited. For example, the diffraction grating 31 can be manufactured, and then the thin film 11 can be selectively formed. Specifically, a method in which the material constituting the thin film 11 is deposited into a thin film shape by a physical vapor deposition method such as vacuum deposition or a spin coating method, and then patterned by a lithography method or a nanoimprint method and selectively formed by an etching method. Can be used. Further, a method of selectively forming by a vapor deposition method using a mask pattern or the like can be used. Further, the thin film 11 may be provided over the entire boundary surface between the two as described later. In this case, it is not necessary to selectively form the thin film 11 only on the grid wall surface portion. After that, the diffraction optical element 1 can be manufactured by forming the diffraction grating 21. Further, the width or shape of the thin film 11 may be changed (controlled) for each ring band of the diffractive optical element 1.

FIG. 28 is an enlarged cross-sectional view of the diffraction optical unit 10a as a modification of the present embodiment. As shown in FIG. 28, the thin film 11 may be provided not only on the wall surface of the grating but also on the entire boundary surface of the diffraction gratings 21 and 31. That is, the thin film 11 is continuously provided from between the lattice wall surface 21b and the lattice wall surface 31b to between the lattice surface 21a and the lattice surface 31a. In this case, it is sufficient that the lattice wall surface portion satisfies the above-mentioned relationship and the lattice surface portion has an antireflection function. Further, the refractive index and the film width of the thin film on the lattice surface may be different from those on the lattice wall surface. Since the thin film is formed over the entire boundary surface, the diffractive optical element can be manufactured easily and inexpensively. For example, after manufacturing the diffraction grating 21, a thin film may be formed from the lattice surface to the entire surface of the lattice wall surface by a physical vapor deposition method such as vacuum deposition or a spin coating method, and then the diffraction grating 31 may be formed. However, this embodiment is not limited to this. Further, by providing a thin film over the entire boundary, the adhesion of the diffraction gratings 21 and 31 to each other can be improved. Since the refractive index and the film thickness of the lattice surface and the lattice wall surface may be different, the antireflection function of the lattice surface and the flare reduction function of the lattice wall surface can be arbitrarily designed according to the manufacturing method.

In this embodiment, the case where _{the refractive index n 3} of the material constituting the diffraction grating 31 _{is smaller than the refractive index n 2 of the} material constituting the diffraction grating 21 _{(n 2} > n ₃ ) is described as an example. .. However, this embodiment is not limited to this. When n ₃ > n ₂ , the direction of the lattice shape of the diffraction grating is only reversed, so that the influence of unnecessary light on the lattice wall surface is the same.

FIG. 4 is a schematic view of an optical system having a diffractive optical element 1 in this embodiment. The optical system of FIG. 4 is a telescopic type photographing optical system provided with a diffractive optical element 1 that can be applied to an imaging device such as a camera, and a diffractive surface is provided on a second surface. The optical system of FIG. 4 has a diaphragm 40 and a diffractive optical element 1 inside. The diaphragm 40 is arranged on the exit side opposite to the incident side where the light is incident on the diffractive optical element 1, that is, on the rear side (image plane side) of the diffractive optical element 1. Reference numeral 41 denotes a film as an image plane or a photoelectric conversion element (imaging element) such as a CCD or CMOS.

When the diffractive optical element 1 of the present embodiment is applied to such an optical system, the unnecessary light of the photographing light is reduced, and the unnecessary light reaching the image plane when the light beam is incident from outside the screen is reduced. Therefore, a shooting lens with less flare can be obtained. In FIG. 4, the diffractive optical element 1 is provided on the bonding surface of the front lens, but the present embodiment is not limited to this. The diffractive optical element 1 may be arranged inside the optical system or may be provided on the lens surface. Further, a plurality of diffractive optical elements 1 may be provided in the photographing lens. The optical system to which the diffractive optical element 1 can be applied is not limited to the photographing optical system shown in FIG. The optical system of this embodiment is an imaging optical system used in a wide wavelength range such as a photographing lens of a video camera, a leader lens of an image scanner or a copying machine, an observation optical system such as binoculars or a telescope, or an optical finder. It is also applicable to. Further, the device to which the optical system including the diffractive optical element 1 can be applied is not limited to the image pickup device, and can be widely applied to the optical device. Hereinafter, specific examples of the diffractive optical element 1 of the present embodiment will be described in Examples 1 to 6.

First, the diffractive optical element according to the first embodiment of the present invention will be described. In this embodiment, the diffraction grating 21 is composed of _{an acrylic ultraviolet curable resin mixed with ZrO 2} fine particles, and the diffraction grating 31 is composed of an acrylic ultraviolet curable resin mixed with ITO fine particles. The lattice height d is 10.79 μm, and the design order is +1 order. The thin film 11 is _{composed of a thin film of Al 2} O ₃ , and has a thickness or width W of 0. It is 360 μm. Further, the wavelength of the thin film 11 is 0. From 400 μm to 0. The extinction coefficient in the 700 μm band is 0.0003 or less. Specifically, the extinction coefficient of the thin film 11 has a wavelength of 0. It is maximum for 400 μm light, and its value is 0.0002.

Table 1 shows each parameter and the numerical value of each formula for each wavelength λ (μm ) of the diffractive optical element in this embodiment. n ₁ , n ₂ , and n ₃ are the refractive indexes of the materials constituting the thin film 11 for each wavelength λ, the refractive indexes of the materials constituting the diffraction grating 21, and the refractive indexes of the materials constituting the diffraction grating 31, respectively. is there. d (μm) is the lattice height, W ( μm ) is the film width of the thin film 11, Δ is the specific refractive index difference represented by the formula (6), and a'is the value represented by the formula (17). WC _{, TE} ( μm ), WC _{, TE} ( μm ) are the cutoff width of TE polarized light and the cutoff width of TM polarized light represented by the formulas (10) and (11), respectively. W _C ([mu] m) is the average of the cut-off width of the TE polarization cutoff width and TM polarized light represented by the formula (14). W / _{W C} is a value represented by the formula (13). n _{eq, TE} , n _{eq, and TM} are the equivalent refractive index of TE polarized light and the equivalent refractive index of TM polarized light represented by the formulas (18) and (19), respectively. n _eq is the equivalent refractive index as an average of the equivalent refractive index of TE polarized light and the equivalent refractive index of TM polarized light represented by the formula (23). (N _eq −n ₂ ) d / λ is the phase difference between _{the average equivalent refractive index n eq} represented by the equation (22) and _{the refractive index n 2} of the high refractive index material of the diffraction grating 21.

FIG. 5 is an enlarged cross-sectional view of the diffraction optical unit in this embodiment. FIG. 6 is an explanatory diagram of the influence of unnecessary light on the design incident angle (shooting light incident angle) with respect to the optical system of FIG. 4 in this embodiment. In FIGS. 5 and 6, the photographing luminous fluxes A and A'incident on the optical axis O are the m-th and m-th, respectively, counting upward from the optical axis O after passing through the substrate lens 2. It is incident on the second diffraction grating, the m-grating and the m'grating, respectively. The incident angles of the photographing luminous fluxes A and A'with respect to the m grid and the m'grid are in the direction of the center of gravity ray. Further, the directions of the lattice wall surfaces 1b and 1b'are equal to the direction of the center of gravity ray.

In FIG. 6, the + 1st-order refracted light emitted from the m-lattice of the photographing luminous flux A is Am1, the 0th-order diffracted light is Am0, the + 2nd-order diffracted light is Am2, and the + 1st-order diffracted light emitted from the m'lattice of the photographing luminous flux A'is A'm1. The 0th-order diffracted light is shown as A'm0, and the + 2nd-order diffracted light is shown as A'm2. The + 1st-order diffracted light Am1 and A'm1, which are design orders, are imaged on the image plane 41. On the other hand, the 0th-order diffracted light Am0 and A'm0, which are the design order-1st order, are imaged on the image side of the image plane 41. The + second-order diffracted light Am2 and A'm2, which are the design order + 1st order, form an image on the object side of the image plane 41. The farther the spot size on the image plane is from the design order, the more blurred it becomes, so that unnecessary light becomes less noticeable. That is, the diffraction efficiency of the diffracted light of the design order ± 1st order is most affected by the unnecessary light at the design incident angle (shooting light incident angle).

FIG. 7 is a graph of the diffraction efficiency of the + 1st-order diffracted light, the 0th-order diffracted light, and the + 2nd-order diffracted light with respect to the design incident angle luminous flux of the diffractive optical element in this embodiment. That is, FIG. 7 is an RCWA calculation at an incident angle of 0 degrees and a lattice pitch of 100 μm, assuming the incident luminous flux a which is the design incident angle (photographed light incident angle) shown in FIG. 3 and the incident luminous flux A of FIGS. 5 and 6. The result is shown. 7 (a) to 7 (c) show the diffraction efficiencies of TE-polarized light and TM-polarized light of the + 1st-order diffracted light, the 0th-order diffracted light, and the + 2nd-order diffracted light, which are the design orders, respectively. As for the diffraction angle, the downward direction in FIG. 3 is the positive direction.

FIG. 8 is a graph of the diffraction efficiency of the + 1st-order diffracted light, the 0th-order diffracted light, and the + 2nd-order diffracted light with respect to the design incident angle luminous flux of the diffractive optical element as a comparative example. That is, FIG. 8 is a graph as a comparative example corresponding to FIG. 7 when a diffractive optical element (DOE) having the same configuration as that of FIG. 1 is used except that the thin film 11 is not provided. Compared with a diffraction grating without a thin film, the diffraction efficiency of the + 1st-order diffracted light of TE-polarized light and TM-polarized light is improved, and the diffraction efficiency of the 0th-order diffracted light and the + 2nd-order diffracted light is reduced in the entire visible wavelength band. Although the numerical values of the diffraction efficiencies of the 0th-order diffracted light and the + 2nd-order diffracted light are low values, they are affected as unnecessary light during shooting such as a high-luminance light source, a small aperture, and a long exposure, so the effect of this embodiment is obtained. Is big.

FIG. 9 is an explanatory diagram of the influence of unnecessary light on the oblique incident angle (out-of-screen light incident angle) with respect to the optical system of FIG. In FIG. 5, the angles of incidence of the off-screen luminous fluxes B and B'on the m grid and the m'grid are angles ωi and ωi'with respect to the direction of the center of gravity ray, respectively. FIG. 10 shows the wavelength of the diffractive optical element with respect to the extrascreen incident + 10 degree luminous flux . 400 μm , 0. 550 μm , 0. It is a graph of the diffraction efficiency of 700 μm. That is, FIG. 10 shows the RCWA calculation results at an incident angle of +10 degrees and a grid pitch of 100 μm, assuming the off-screen incident luminous flux b shown in FIG. 3 and the incident luminous flux B shown in FIGS. 5 and 9. As for the incident angle, the downward direction in FIG. 3 is the positive direction.

FIG. 10 is a result of enlarging the portion of the vertical axis having low diffraction efficiency and displaying the high diffraction angle range by changing the horizontal axis from the diffraction order to the diffraction angle. The reason why the numerical value near +10 degrees exceeds the display range is that the numerical value of the diffraction efficiency is high because it is the diffraction efficiency near the + 1st order diffracted light which is the design order. The diffraction efficiency of the + 1st-order diffracted light, which is the design order, is concentrated, but the effect is small because the + 1st-order diffracted light does not reach the image plane. 10 (a) to 10 (c) show wavelength 0. 400 μm , 0. 550 μm , 0. It is the result of TE polarized light and TM polarized light of 700 μm respectively. As shown in FIG. 10, the unwanted light propagates as unwanted light having a peak in a specific angle direction. This unnecessary light has a peak in the direction of approximately -10 degrees, and this propagation direction is substantially equal to the direction of emission of -10 degrees in which the components of the off-screen incident angle + 10 degrees light flux incident on the lattice wall surface are totally reflected and propagated. .. FIG. 11 shows the wavelength of the diffractive optical element as a comparative example with respect to the extrascreen incident + 10 degree luminous flux . 400 μm , 0. 550 μm , 0. It is a graph of the diffraction efficiency of 700 μm. FIG. 11 shows a graph as a comparative example corresponding to FIG. 10 when a diffractive optical element having the same configuration as that of FIG. 3 is used except that the thin film 11 is not provided.

Of the unnecessary light incident on the screen outside light +10 degrees, the unnecessary light emitted near the diffraction angle +0.19 degrees of the +1st-order diffracted light of the design incident angle reaches the image plane (Bm in FIG. 9). Depending on the optical system in the subsequent stage of the diffractive optical element, the diffraction order and the diffraction angle at which the unnecessary light of the off-screen incident light reaches the image plane differ (Bm to Bm + in FIG. 9). However, in any optical system, the diffracted light of unnecessary light due to the off-screen light that substantially matches the diffraction angle propagated by the design diffraction order at least at the design incident angle reaches the image plane, which causes deterioration of the image performance. The unnecessary light peak angle in the −10 degree direction shown in FIG. 10 is substantially the same as that in FIG. However, the spread of unnecessary light differs between FIGS. 10 and 11, and the diffraction efficiency in the vicinity of the diffraction angle +0.19 degrees in FIG. 10 is higher in the entire visible wavelength band as compared with the diffraction grating without the thin film. Both TE polarization and TM polarization are reduced. In this embodiment, in the unnecessary light, a part of the luminous flux b incident near the lattice wall surface is confined inside the thin film 11 and propagates like an optical waveguide, and as a result of these luminous fluxes interfering with each other after emission, the image plane. It is considered that the luminous flux reaching to is less than that in the comparative example.

FIG. 12 shows a wavelength of 0. 400 μm , 0. 550 μm , 0. It is a graph of the diffraction efficiency of 700 μm. That is, FIG. 12 shows the RCWA calculation results at an incident angle of -10 degrees and a lattice pitch of 100 μm, assuming the incident luminous flux c shown in FIG. 3 and the incident luminous flux B'shown in FIGS. 5 and 9. The angle of incidence is positive in the downward direction in FIG. 3 (upward is the positive direction in the m'lattice in FIG. 5). FIG. 12 is a result showing the high diffraction angle range by enlarging the portion of the vertical axis having low diffraction efficiency and changing the horizontal axis from the diffraction order to the diffraction angle. The numerical value near -10 degrees exceeds the display range because the numerical value of the diffraction efficiency near the +1st order diffracted light, which is the design order, is high. The diffraction efficiency of the + 1st-order diffracted light, which is the design order, is concentrated, but the effect is small because the + 1st-order diffracted light does not reach the image plane. 12 (a) to 12 (c) show wavelength 0. 400 μm , 0. 550 μm , 0. It is the result of TE polarized light and TM polarized light of 700 μm respectively.

FIG. 13 shows a wavelength of 0. 400 μm , 0. 550 μm , 0. It is a graph of the diffraction efficiency of 700 μm. FIG. 13 is a graph as a comparative example corresponding to FIG. 12 when a diffractive optical element having the same configuration as that of FIG. 3 is used except that the thin film 11 is not provided. As shown in FIG. 12, the unwanted light propagates as unwanted light having a peak in a specific angle direction. Compared with FIG. 13, the peak of unnecessary light in the + direction is increased, and the peak of unnecessary light in the-direction is decreased. This is because the thin film provided on the lattice wall surface reflects a part of the luminous flux incident on the lattice wall surface from the low refractive index medium side, so that unnecessary light in the + direction increases, and unnecessary light due to transmission in the-direction increases. It means that it is decreasing. In the optical system shown in FIGS. 4 and 9, the diffracted light of unnecessary light due to the off-screen light substantially matching the diffraction angle +0.19 degrees propagated by the design diffraction order at the design incident angle is increased as compared with the comparative example. ing. However, since the numerical value of the diffraction efficiency is extremely small and the influence of the m-grid is larger, the influence on the deterioration of the image performance is small.

As described above, in the optical system to which the diffractive optical element of this embodiment is applied, the increase of unnecessary light of the m'lattice, which is less affected by unnecessary light, is suppressed to the extent that it is not affected, and the m lattice, which is greatly affected by unnecessary light, is unnecessary. Light can be significantly reduced. As a result, unnecessary light reaching the image plane is reduced, so that deterioration of image performance can be suppressed. In this embodiment, the lattice pitch is set to 100 μm. Further, in the ring zone having a wide lattice pitch, the contribution of the wall surface is small, so that the diffraction efficiency of the design order is relatively high and the diffraction efficiency of unnecessary light is relatively low. Further, although not shown, the propagation direction of unnecessary light does not depend on the lattice pitch, and the propagation direction is the same. Therefore, as one of the criteria, the diffraction efficiency with a lattice pitch of 100 μm is shown.

Further, in this embodiment, the incident angles of the off-screen luminous fluxes B and B'are assumed to be +10 degrees outside the screen (incident angle ω is +13.16 degrees with respect to the optical axis direction). At an angle smaller than this incident angle, ghosts due to reflection on the lens surface and the image plane surface and scattering due to the inside of the lens and surface minute irregularities are often scattered, so that unnecessary light from the diffractive optical element is relatively inconspicuous. On the other hand, at an angle larger than this incident angle, the influence of unnecessary light of the diffractive optical element is relatively small due to reflection on the front lens surface and shading by the lens barrel. Therefore, the incident light flux outside the screen has the greatest effect on the unnecessary light of the diffractive optical element at around +10 degrees, and in this embodiment, the incident angle of the incident light flux outside the screen is assumed to be approximately +10 degrees.

Next, the diffractive optical element according to the second embodiment of the present invention will be described. In the diffractive optical element of this embodiment, the material of the thin film 11 is the same as that of the first embodiment, and the width W of the thin film 11 is 0. It is 450 μm. Other configurations of the diffractive optical element are the same as in the first embodiment. Similar to Table 1, Table 2 shows each parameter and the numerical value of each formula for each wavelength λ (μm) of the diffractive optical element in this embodiment.

FIG. 14 is a graph of the diffraction efficiency of the + 1st, 0th, and + 2nd order diffracted light with respect to the design incident angle luminous flux of the diffractive optical element in this embodiment. That is, FIG. 14 shows the RCWA calculation results at an incident angle of 0 degrees and a lattice pitch of 100 μm. Compared to a diffraction grating without a thin film, the diffraction efficiency of the + 1st-order diffracted light for both TE-polarized light and TM-polarized light is improved, and the diffraction efficiency of the 0th-order diffracted light and the + 2nd-order diffracted light is reduced in the entire visible wavelength band. There is.

FIG. 15 shows the wavelength of the diffractive optical element in this embodiment with respect to the extrascreen incident + 10 degree luminous flux . 400 μm , 0. 550 μm , 0. It is a graph of the diffraction efficiency of 700 μm. That is, FIG. 15 shows the RCWA calculation results at an incident angle of +10 degrees and a lattice pitch of 100 μm. Diffraction efficiency near +0.19 degrees diffraction angle for both TE-polarized light and TM-polarized light is reduced over the entire visible wavelength band as compared to a diffraction grating without a thin film.

Next, the diffractive optical element according to the third embodiment of the present invention will be described. In the diffractive optical element of this embodiment, the material of the thin film 11 is the same as that of the first embodiment, and the width W of the thin film 11 is 0. It is 300 μm. Other configurations of the diffractive optical element are the same as in the first embodiment. Similar to Table 1, Table 3 shows each parameter and the numerical value of each formula for each wavelength λ (μm) of the diffractive optical element in this embodiment.

Wavelength 0. At 600 μm or more, the width W of the thin film 11 is less than the cutoff width of the single mode of the formula (10) in the case of TE polarized light and the formula (11) in the case of TM polarized light. Therefore, the equations (7) and (8) do not have a solution, and the equivalent refractive index cannot be obtained.

FIG. 16 is a graph of the diffraction efficiency of the + 1st, 0th, and + 2nd order diffracted light with respect to the design incident angle luminous flux of the diffractive optical element in this embodiment. That is, FIG. 16 shows the RCWA calculation results at an incident angle of 0 degrees and a lattice pitch of 100 μm. Compared to a diffraction grating without a thin film, the diffraction efficiency of the + 1st-order diffracted light for both TE-polarized light and TM-polarized light is improved, and the diffraction efficiency of the 0th-order diffracted light and the + 2nd-order diffracted light is reduced in the entire visible wavelength band. There is.

FIG. 17 shows the wavelength of the diffractive optical element in this embodiment with respect to the extrascreen incident + 10 degree luminous flux . 400 μm , 0. 550 μm , 0. It is a graph of the diffraction efficiency of 700 μm. That is, FIG. 17 shows the RCWA calculation results at an incident angle of +10 degrees and a lattice pitch of 100 μm. Diffraction efficiency near +0.19 degrees diffraction angle for both TE-polarized light and TM-polarized light is reduced over the entire visible wavelength band as compared to a diffraction grating without a thin film.

Subsequently, in order to show the effect of this example more clearly, it will be described with reference to a comparative example. FIG. 18 is a graph of the diffraction efficiency of the +1st order diffracted light with respect to the design incident angle luminous flux of the diffractive optical element as a comparative example. FIG. 18 corresponds to the diffraction optical element disclosed in Patent Document 2, and shows an incident angle of 0 degrees when the film width of the thin film is changed when the specific refractive index difference Δ of the equation (6) is 0.045. The RCWA calculation result of the diffraction efficiency of the +1st order diffracted light is shown. This diffractive optical element has a wavelength of 0. At 550 μm, it has the characteristics of n ₁ = 1.70135, n ₂ = 1.62298, n ₃ = 1.57243, and Δ / λ = 0.0818. FIG. 18 shows a lattice pitch of 100 μm and a wavelength of 0. 400 μm , 0. 550 μm , 0. The results for each of 700 μm are shown. The film width at which the diffraction efficiency of the +1st-order diffracted light is highest differs depending on both wavelength and polarization, and is highly wavelength-dependent and polarization-dependent. For example, wavelength 0. Film width of the peak of TM polarized light of 700 μm 0. 200-0 . 220 μm has a wavelength of 0. It is extremely low at 400 μm TE polarized light.

FIG. 19 shows a wavelength of 0. 400 μm , 0. 550 μm , 0. It is a graph of the diffraction efficiency of 700 μm. That is, in FIG. 19, when the specific refractive index difference Δ of the formula (6) is 0.045, the film width of the thin film is 0. The RCWA calculation result at an incident angle of +10 degrees at 120 μm (W / Wc = 0.99) is shown. Wavelength 0. 400 μm , 0. Wavelength 0. compared to 550 μm. Diffraction efficiency and wavelength dependence around a diffraction angle of 700 μm + 0.19 degrees are high, and the polarization dependence of TE-polarized light and TM-polarized light is also large. As a result, the flare is more colored.

FIG. 20 is a graph of the diffraction efficiency of the +1st order diffracted light with respect to the design incident angle luminous flux of the diffractive optical element as a comparative example. FIG. 20 corresponds to the diffraction optical element disclosed in Patent Document 1, and when the specific refractive index difference Δ of the formula (6) is 0.005, the incident angle is 0 degrees when the film width of the thin film is changed. The RCWA calculation result of the diffraction efficiency of the +1st order diffracted light of is shown. This diffractive optical element has a wavelength of 0. At 550 μm, it has the characteristics of n ₁ = 1.63116, n ₂ = 1.62298, n ₃ = 1.57243, and Δ / λ = 0.0091. FIG. 20 shows a lattice pitch of 100 μm and a wavelength of 0. 400 μm , 0. 550 μm , 0. The results for each of 700 μm are shown. Compared with FIG. 18, the polarization dependence is smaller. This is because the polarization dependence of the asymmetric three-layer flat plate waveguide depends on the specific refractive index difference Δ of the equation (6). Therefore, when the specific refractive index difference Δ is small, the polarization dependence of the equivalent refractive index in the waveguide mode is small, and the polarization dependence is also small when applied to a diffractive optical element.

FIG. 21 shows the wavelength of the diffractive optical element as a comparative example with respect to the extrascreen incident + 10 degree luminous flux . 400 μm , 0. 550 μm , 0. It is a graph of the diffraction efficiency of 700 μm. That is, in FIG. 21, when the specific refractive index difference Δ of the formula (6) is 0.005, the width of the thin film 11 is 0. The RCWA calculation result at an incident angle of +10 degrees at 700 μm (W / Wc = 1.09) is shown. Wavelength 0. 400 μm , 0. 550 μm , 0. In any case of 700 μm , the diffraction efficiency near the diffraction angle +0.19 degrees is not a sufficiently small value.

FIG. 22 shows a wavelength of 0. It is a graph of the diffraction efficiency of 550 μm. That is, FIG. 22 shows the RCWA calculation result at an incident angle of +10 degrees, a wavelength of 550 nm, and TE polarized light when the film width of the thin film is changed. When the specific refractive index difference Δ is small, a value having a small diffraction efficiency cannot be obtained even if the film width is changed. This is because when the specific refractive index difference Δ is small, the confinement coefficient of the waveguide becomes small, so the +1st order light, which is the design diffracted light, and the flare light reflected by the-lattice wall surface and propagating in the -10 deg direction are separated. It is thought that this is because it becomes impossible.

FIG. 23 is a graph of the diffraction efficiency of the +1st order diffracted light with respect to the design incident angle luminous flux of the diffractive optical element in Examples 1 to 3. That is, FIG. 23 shows the wavelength 0. At an incident angle of 0 degrees and a lattice pitch of 100 μm when the film width of the thin film is changed according to each of the refractive indexes and the lattice height of Examples 1 to 3. 400 μm , 0. 550 μm , 0. The RCWA calculation result of the diffraction efficiency of the + 1st order diffracted light of 700 μm is shown. Compared with FIG. 18, the wavelength dependence and the polarization dependence are reduced. Further, as shown in FIGS. 10, 15, and 17 of Examples 1 to 3, the result of the incident angle +10 degrees shows the diffraction efficiency near the diffraction angle +0.19 degrees as compared with FIGS. 21 and 22. Is a low value. Therefore, among the unnecessary light due to the oblique incident angle, the unnecessary light that reaches the image plane can be reduced, and the wavelength dependence and the polarization dependence can be reduced.

Next, the diffractive optical element according to the fourth embodiment of the present invention will be described. The diffractive optical element of this embodiment relates to a refractive index n ₁ and a width W of the thin film 11 is different from the diffractive optical element of Example 1-3. In the diffractive optical element of this embodiment, the thin film 11 is _{composed of a thin film of a mixed material of Al 2} O ₃ and Zr _{O 2} , and the thickness or width W in the direction perpendicular to the lattice wall surface, which is the laminated surface, is 0. It is 160 μm. Further, the wavelength of the thin film 11 is 0. From 400 μm to 0. The extinction coefficient in the 700 μm band is 0.0003 or less. Specifically, the extinction coefficient of the thin film 11 has a wavelength of 0. It is maximum for 400 μm light, and its value is 0.0002.
Other configurations of the diffractive optical element are the same as those of Examples 1 to 3. Similar to Table 1, Table 4 shows each parameter and the numerical value of each formula for each wavelength λ (μm) of the diffractive optical element in this embodiment.

Wavelength 0. At 650 μm or more, the width W of the thin film 11 is less than the cutoff width of the single mode of the formula (10) in the case of TE polarized light and the formula (11) in the case of TM polarized light. Therefore, the equations (7) and (8) do not have a solution, and the equivalent refractive index cannot be obtained.

FIG. 24 is a graph of the diffraction efficiency of the + 1st, 0th, and + 2nd order diffracted light with respect to the design incident angle luminous flux of the diffractive optical element in this embodiment. That is, FIG. 24 shows the RCWA calculation results at an incident angle of 0 degrees and a lattice pitch of 100 μm. Compared with the conventional diffractive optical element, the diffraction efficiency of the + 1st-order diffracted light for both TE-polarized light and the TM-polarized light is improved, and the diffraction efficiency of the 0th-order diffracted light and the + 2nd-order diffracted light is reduced in the entire visible wavelength band.

FIG. 25 shows the wavelength of the diffractive optical element in this embodiment with respect to the extrascreen incident + 10 degree luminous flux . 400 μm , 0. 550 μm , 0. It is a graph of the diffraction efficiency of 700 μm. That is, FIG. 25 shows the RCWA calculation results at an incident angle of +10 degrees and a lattice pitch of 100 μm. Diffraction efficiency near +0.19 degrees diffraction angle for both TE-polarized light and TM-polarized light is reduced over the entire visible wavelength band as compared with conventional diffraction optics.

Next, the diffractive optical element according to the fifth embodiment of the present invention will be described. The diffractive optical element of this embodiment relates to a refractive index n ₁ and a width W of the thin film 11 is different from the diffractive optical element of Example 1-4. In the diffractive optical element of this embodiment, the thin film 11 is _{composed of a thin film of Al 2} O ₃ , and the thickness or width W in the direction perpendicular to the lattice wall surface, which is the laminated surface, is 0. It is 400 μm. Further, the wavelength of the thin film 11 is 0. From 400 μm to 0. The extinction coefficient in the 700 μm band is 0.0003 or less. Specifically, the extinction coefficient of the thin film 11 has a wavelength of 0. It is maximum for 400 μm light, and its value is 0.0002. Other configurations of the diffractive optical element are the same as those in Examples 1 to 4. Similar to Table 1, Table 5 shows each parameter and the numerical value of each equation for each wavelength λ (μm) of the diffractive optical element in this embodiment.

FIG. 26 is a graph of the diffraction efficiency of the + 1st, 0th, and + 2nd order diffracted light with respect to the design incident angle luminous flux of the diffractive optical element in this embodiment. That is, FIG. 26 shows the RCWA calculation results at an incident angle of 0 degrees and a lattice pitch of 100 μm. Compared with the conventional diffractive optical element, the diffraction efficiency of the + 1st-order diffracted light for both TE-polarized light and the TM-polarized light is improved, and the diffraction efficiency of the 0th-order diffracted light and the + 2nd-order diffracted light is reduced in the entire visible wavelength band.

FIG. 27 shows the wavelength of the diffractive optical element in this embodiment with respect to the extrascreen incident + 10 degree luminous flux . 400 μm , 0. 550 μm , 0. It is a graph of the diffraction efficiency of 700 μm. That is, FIG. 27 shows the RCWA calculation results at an incident angle of +10 degrees and a lattice pitch of 100 μm. Diffraction efficiency near +0.19 degrees diffraction angle for both TE-polarized light and TM-polarized light is reduced over the entire visible wavelength band as compared with conventional diffraction optics.

Next, the diffractive optical element according to the sixth embodiment of the present invention will be described. The diffractive optical element of this example is different from the diffractive optical element of Examples 1 to 5 in terms of _{the refractive index n 1 and the width W of the diffraction grating and the thin film.}

In the diffraction optical element of this embodiment, the diffraction grating 21 is composed of _{an ultraviolet curable resin mixed with ZrO 2} fine particles, and the diffraction grating 31 is composed of an ultraviolet curable resin mixed with ITO fine particles. The refractive index of the material is higher than that of the materials of Examples 1 to 4, the lattice height d is 10.80 μm, which is the same as that of Examples 1 to 4, and the design order is +1 order. The thin film 11 is _{composed of a thin film of a compound material of Al 2} O ₃ and La ₂ O ₃ , and the thickness or width W in the direction perpendicular to the lattice wall surface, which is the laminated surface, is 0. It is 340 μm. This is a case where both the refractive index of the material of the diffraction grating and the refractive index of the material of the thin film are higher than those of Example 1, and Δ is the same. Further, the wavelength of the thin film 11 is 0. From 400 μm to 0. The extinction coefficient in the 700 μm band is 0.0003 or less. Specifically, the extinction coefficient of the thin film 11 has a wavelength of 0. It is maximum for 400 μm light and its value is 0.0003. Similar to Table 1, Table 6 shows each parameter and the numerical value of each formula for each wavelength λ (μm) of the diffractive optical element in this embodiment.

Wavelength 0. At 700 μm , the width W of the thin film 11 is less than the single mode cutoff width of equation (10) for TE polarized light and equation (11) for TM polarized light. Therefore, the equations (7) and (8) do not have a solution, and the equivalent refractive index cannot be obtained.

FIG. 29 is a graph of the diffraction efficiency of the + 1st, 0th, and + 2nd order diffracted light with respect to the design incident angle luminous flux of the diffractive optical element in this embodiment. That is, FIG. 29 shows the RCWA calculation results at an incident angle of 0 degrees and a lattice pitch of 100 μm. FIG. 30 is a graph of the diffraction efficiency of the + 1st, 0th, and + 2nd order diffracted light with respect to the design incident angle luminous flux of the diffractive optical element having the same configuration as that of FIG. 1 except that the thin film 11 is not provided as a comparative example. That is, it is a graph as a comparative example corresponding to FIG. 29. Compared to a diffraction grating without a thin film, the diffraction efficiency of the + 1st-order diffracted light for both TE-polarized light and TM-polarized light is improved, and the diffraction efficiency of the 0th-order diffracted light and the + 2nd-order diffracted light is reduced in the entire visible wavelength band. There is.

FIG. 31 shows the wavelength of the diffractive optical element in this embodiment with respect to the extrascreen incident + 10 degree luminous flux . 400 μm , 0. 550 μm , 0. It is a graph of the diffraction efficiency of 700 μm. That is, FIG. 31 shows the RCWA calculation results at an incident angle of +10 degrees and a lattice pitch of 100 μm. FIG. 32 shows an out-of-screen incident + 10 degree luminous flux of a diffractive optical element having the same configuration as that of FIG. 1 except that the thin film 11 is not provided as a comparative example . 400 μm , 0. 550 μm , 0. It is a graph of the diffraction efficiency of 700 μm. The spread of unnecessary light differs between FIGS. 31 and 32, and the diffraction efficiency near +0.19 degrees at the diffraction angle for each of TE-polarized light and TM-polarized light in FIG. 31 is shown in FIG. 32 (diffraction grating without a thin film). Diffraction efficiency) is reduced over the entire visible wavelength band.

FIG. 33 is a graph of the diffraction efficiency of the +1st order diffracted light with respect to the design incident angle luminous flux of the diffractive optical element in Example 6. That is, FIG. 33 shows a wavelength of 0 at an incident angle of 0 degrees and a lattice pitch of 100 μm when the film width of the thin film is changed according to each refractive index and lattice height of Example 6. 400 μm , 0. 550 μm , 0. The RCWA calculation result of the diffraction efficiency of the + 1st order diffracted light of 700 μm is shown. Similar to FIGS. 23 shown in Examples 1 to 3, the wavelength dependence and the polarization dependence are reduced.

It can be seen that Example 6 has the same relationship with Example 1 as in Example 1. In Example 6, both the refractive index of the material of the diffraction grating and the refractive index of the material of the thin film are higher than those of Example 1, and the specific refractive index difference Δ is the same. Therefore, it can be seen that the diffractive optical element that reduces the wavelength dependence and the polarization dependence depends not on the absolute values of the refractive index of the diffraction grating and the refractive index of the thin film, but on the specific refractive index difference Δ.

However, when FIG. 23 and FIG. 33 are compared, it can be seen that the wavelength dependence is lower in FIG. 23. Therefore, it can be seen that the combination of the thin film and the material of the diffraction grating having a lower refractive index has a lower wavelength dependence and is a more preferable configuration.

According to the diffractive optical element of each embodiment, the diffraction efficiency of the design order of the luminous flux incident at the design incident angle is improved, the diffraction efficiency of the design order ± 1st order is reduced, and the oblique incident angle (out-of-screen light incident angle). ), Among the unnecessary light due to the incident light beam, the unnecessary light that reaches the imaging surface can be reduced. Further, according to each embodiment, it is possible to provide a diffractive optical element, an optical system, and an optical device having desired wavelength characteristics and polarization characteristics by reducing wavelength dependence and polarization dependence.

Although preferred embodiments of the present invention have been described above, the present invention is not limited to these embodiments, and various modifications and modifications can be made within the scope of the gist thereof.

1 Diffractive optical element 11 Thin film 21 Diffraction grating (first diffraction grating)
31 Diffraction grating (second diffraction grating)

Claims (14)

A first diffraction grating having a first grating surface and a first lattice wall surface,
A second diffraction grating having a second grating surface and a second lattice wall surface,
It has a thin film in contact with both the first and second lattice walls, and has.
The extinction coefficient of the thin film with respect to the wavelength λ (μm ) in the wavelength band used is 0.0005 or less.
The refractive index of each material of the thin film, the first diffraction grating, and the second diffraction grating with respect to the wavelength λ is n ₁ , n ₂ , and n ₃ , and the width of the thin film is W.

When
n ₁ > n ₂ > n ₃
0.005 <Δ <0.045
0.5 ≦ _W / W _C ≦ 2.0
Satisfy the conditional expression
A diffractive optical element characterized in that the number of materials of the thin film is larger than the number of materials of the second diffraction grating. When the lattice heights of the first and second diffraction gratings are d (μm) and the propagation constants of TE polarized light and TM polarized light in the thin film are β _TE and β _TM , respectively.

In the range defined by

Satisfying the formula

When
0 ≦ (n _eq −n ₂ ) × d / λ <0.3
The diffractive optical element according to claim 1, wherein the diffractive optical element satisfies the conditional expression. Is the Δ for the first wavelength ([mu] m), the diffractive optical element according to claim 1 or 2, characterized in the smaller than Δ for second wavelength longer than the wavelength of said 1 ([mu] m). 0.01 <Δ / λ <0.08
The diffractive optical element according to any one of claims 1 to 3, wherein the diffractive optical element satisfies the conditional expression. The diffractive optical element according to any one of claims 1 to 4, wherein the thin film is composed of a single film. 1.64 <n ₁ <1.75
The diffractive optical element according to any one of claims 1 to 5, wherein the diffractive optical element satisfies the conditional expression. When the number of materials of the first and second diffraction gratings is νd2 and νd3, respectively, and the design order is m,
νd2> 35
νd3 <25
0.960 ≤ (n _2- n ₃ ) x d / (m x λ) ≤ 1.040
The diffractive optical element according to any one of claims 1 to 6, wherein the diffractive optical element satisfies the conditional expression. The diffraction optical element according to any one of claims 1 to 7, wherein each of the first and second diffraction gratings has a lattice height of 15 μm or less. The diffractive optical element according to any one of claims 1 to 8, wherein the design order is +1 or -1. Any one of claims 1 to 9, wherein the thin film is continuously provided between the first and second lattice wall surfaces and between the first and second lattice surfaces. The diffractive optical element according to the above. The diffractive optical element according to any one of claims 1 to 10, wherein the thin film is transparent with respect to the wavelength λ. An optical system comprising the diffractive optical element according to any one of claims 1 to 11 and a diaphragm. The optical system according to claim 12, wherein the aperture is arranged on the light emitting side of the diffractive optical element. An optical device comprising the optical system according to claim 12 or 13 and an image pickup device.

Images