JP2011022319A - Diffraction optical element, optical system and optical apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a diffraction optical element which has good wavelength characteristics, while preventing reflections. <P>SOLUTION: The diffraction optical element (110) includes two sheets of optical members (11, 12), which are laminated by arranging diffraction optical surfaces with lattice structure formed at a first pitch (Pd) to face each other. Furthermore, the diffraction optical element (110) has, on one surface thereof, concavo-convex structure (80) formed at a second pitch (Ps) which is shorter than a reference wavelength of light which transmits through the diffraction optical element (110). <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a diffractive optical element, an optical system using the diffractive optical element, and an optical apparatus.

  A diffractive optical element is an element that diffracts transmitted light or reflected light with a fine grating structure and performs operations such as spectroscopy, branching, or coupling of incident light. In addition, the diffractive optical member can realize operations such as condensing, diverging, or imaging by modulating the grating structure of the diffractive optical element into various shapes.

  The diffractive optical element disclosed in Patent Document 1 has a grating structure on one surface of a transparent flat plate, and an antireflection layer is formed on the other surface. In particular, the transmission type diffractive optical element has an antireflection film formed on one side in order to improve the transmittance. If the wavelength of light transmitted through the diffractive optical element is, for example, 400 nm to 750 nm in the visible region, many layers of films can be formed by an ordinary vacuum deposition method using an expensive vacuum apparatus to prevent reflection for these wide wavelengths. It is formed.

  On the other hand, the diffractive optical element disclosed in Patent Document 2 prevents a reflection in a wavelength region to be prevented from being reflected by forming a fine uneven pattern called “Mothey” on one side of the diffractive optical element.

JP 2004-206039 A JP 2004-145064 A

  However, as shown in Patent Document 1 or Patent Document 2, a single-layer type diffractive optical element cannot maintain high diffraction efficiency in almost the entire desired wide wavelength region (for example, visible light region).

  The present invention has been made in view of the above problems, and proposes a diffractive optical element that prevents reflection in a wide wavelength region and has good wavelength characteristics.

  The diffractive optical element in the first aspect is a diffractive optical element composed of two optical members in which diffractive optical surfaces having a grating structure formed at a first pitch are stacked facing each other. In addition, a concavo-convex structure having a second pitch shorter than a reference wavelength of light transmitted through the diffractive optical element is provided on one surface of the diffractive optical element.

The optical system in the second aspect is an optical system that has a plurality of optical elements, guides the light beam, and acts on the light beam. In the optical system, at least one of the optical elements is a diffractive optical element according to the first aspect.
The optical device according to the third aspect is an optical device having the diffractive optical element according to the first aspect.

  The present invention provides a diffractive optical element that prevents reflection and has good wavelength characteristics.

It is the schematic of the diffractive optical element 110 of 1st Embodiment. 6 is a graph comparing the diffraction efficiencies of a multilayer diffractive optical element and a single layer diffractive optical element with respect to different light wavelengths. (A) is a partial perspective view of SWS80A which is a pyramid structure. (B) is a layout diagram of the SWS 80B having a conical structure. (C) is another arrangement | positioning figure of SWS80B which is a cone structure. It is the graph which showed the reflectance of the air interface. 3 is a graph showing diffraction efficiency of a first diffraction grating 11 and a second diffraction grating 12 by a multilayer diffractive optical element. It is the graph which showed the transmission efficiency at the time of entering the diffractive optical element 110 of a 1st example, and + 1st-order diffracted light radiate | emitted the diffractive optical element 110. It is the graph which showed the reflectance of the air interface. 3 is a graph showing diffraction efficiency of a first diffraction grating 11 and a second diffraction grating 12 by a multilayer diffractive optical element. It is the graph which showed the transmission efficiency at the time of entering the diffractive optical element 110 of the 2nd example, and + 1st order diffracted light radiate | emitted the diffractive optical element 110. It is the graph which showed the reflectance of the air interface 3 is a graph showing diffraction efficiency of a first diffraction grating 11 and a second diffraction grating 12 by a multilayer diffractive optical element. It is the graph which showed the transmission efficiency at the time of entering the diffractive optical element 110 of the 3rd example, and + 1st order diffracted light radiate | emitted the diffractive optical element 110. It is a conceptual sectional view showing diffractive optical element 120 stuck on an optical member. It is a conceptual sectional view showing diffractive optical element 130 stuck on an optical member. It is a conceptual sectional view showing diffractive optical element 140 stuck on an optical member. It is the figure which showed the modification of the 1st diffraction grating 11 of 1st Embodiment, and the 2nd diffraction grating 12. FIG. (A) is an optical system 200 in which the diffractive optical element 110 is disposed on the image plane side from the stop DP. (B) is an optical system 210 in which the diffractive optical element 110 is disposed on the object side of the stop DP.

(First embodiment)
<Configuration of Diffraction Optical Element 110>
The configuration of the diffractive optical element 110 according to the first embodiment will be described with reference to FIGS. FIG. 1 is a schematic diagram of a diffractive optical element 110 according to the first embodiment. A direction in which a sub-wavelength structured surface (SWS) 80 protrudes is defined as a + Z-axis direction, and directions perpendicular to the Z-axis are defined as an X-axis direction and a Y-axis direction. As shown in FIG. 1, the diffractive optical element 110 is formed by sequentially joining the first diffraction grating 11, the second diffraction grating 12, and the sub-wavelength plane 80. The second diffraction grating 12 and the sub-wavelength surface 80 may be formed at the same time.

  The first diffraction grating 11 includes a wall 17 extending in the Z-axis direction and a slope 18 formed obliquely from the Z-axis direction, and is made of a glass material or a resin material having a refractive index of N1. The wall 17 and the slope 18 of the first diffraction grating 11 are connected to each other, and the surface on the + Z side of the first diffraction grating 11 is formed in a continuous sawtooth shape. The second diffraction grating 12 has a wall 17 extending in the Z-axis direction and a slope 18 formed obliquely from the Z-axis direction, and is made of a material having a refractive index of N2. The wall 17 and the slope 18 are connected to each other so that the surface on the −Z side of the second diffraction grating 12 has a continuous sawtooth shape. The walls 17 and the inclined surfaces 18 of the first diffraction grating 11 and the second diffraction grating 12 are joined to each other without a gap.

  The refractive index N1 of the first diffraction grating 11 and the refractive index N2 of the second diffraction grating 12 may be different from each other. For example, the refractive index N2 may be higher than the refractive index N1. The second diffraction grating 12 is arranged on the object side regardless of the high refractive index or the low refractive index.

  Further, in the first diffraction grating 11 or the second diffraction grating 12, the height of the wall 17 is H, the pitch of the inclined surface 18 in the Y-axis direction is Pd, and the height of the first diffraction grating 11 in the Z-axis direction is In DL, the height of the second diffraction grating 12 in the Z-axis direction is DH. The pitch Pd of the inclined surface 18 of the first diffraction grating 11 or the second diffraction grating 12 is longer than the incident light (reference wavelength).

  The sub-wavelength surface 80 formed on the surface of the second diffraction grating 12 is a surface in contact with outside air such as air or a vacuum. The sub-wavelength surface 80 is made of a glass material or a resin material having a refractive index of Ns. The sub-wavelength surface 80 is composed of a plurality of uneven structures. The height of each sub-wavelength surface 80 is h, and the pitch in the Y-axis direction of each sub-wavelength surface 80 is Ps. The pitch Ps of the sub-wavelength surface 80 is shorter than the incident light (reference wavelength). The refractive index N2 of the second diffraction grating 12 and the refractive index Ns of the sub-wavelength surface 80 may be the same. The sub-wavelength surface 80 will be described with reference to FIG.

  In the first embodiment, a multi-layered diffractive optical element 110 composed of a first diffraction grating 11 and a second diffraction grating 12 is used. This is because the diffraction efficiency of the multilayer diffractive optical element is superior to that of the single layer diffractive optical element including only the first diffraction grating 11 or the second diffraction grating 12. Details will be described with reference to FIG.

FIG. 2 is a graph comparing the diffraction efficiencies of the multilayer diffractive optical element and the single layer diffractive optical element with respect to the wavelength of light. The vertical axis represents the diffraction efficiency and the horizontal axis represents the wavelength in the visible light region.
The horizontal axis of FIG. 2 shows g line (435.835 nm), F line (486.133 nm), e line (546.074 nm), d line (587.562 nm), and C line (656.273 nm). Yes. As shown in FIG. 2, the diffraction efficiency of the multilayer diffractive optical element indicated by the solid line is 95% or more for light of all wavelengths, and is close to 100% for the d-line. On the other hand, the diffraction efficiency of the single-layer diffractive optical element indicated by the dotted line is 95% or more for the e-line, d-line, and c-line, but about 60% for the g-line and about F-line. About 85%. That is, it is understood that the diffraction efficiency of the single layer type diffractive optical element is remarkably lowered for the g-line and the F-line on the short wavelength side. In particular, the multilayer diffractive optical element 110 is preferable for an optical system used in an optical apparatus used for wavelengths in the visible light region.

  FIG. 3A is a partial perspective view of the sub-wavelength surface 80A having a quadrangular pyramid structure (or pyramid structure). FIG. 3B is a layout diagram of the sub-wavelength surface 80B having a conical structure. FIG. 3C is another layout diagram of the sub-wavelength surface 80B having a conical structure.

  As shown in FIG. 3, the sub-wavelength surface 80 includes a protrusion 82 having a quadrangular pyramid structure and a protrusion 84 having a conical structure. Although not shown, the sub-wavelength surface 80 may have a structure including a plurality of projections of a polygonal pyramid such as a triangular pyramid.

  As shown in FIG. 3A, the height of the projection 82 having a quadrangular pyramid structure in the Z-axis direction is h, and the bottom surface of each projection 82 is a square whose one side (pitch) is Ps. The pitch between the individual protrusions 82 is also Ps. The sub-wavelength surface 80A has an excellent reflection effect in a wide wavelength range and a wide incident angle. In addition, since the surface structure of the sub-wavelength surface 80A is a two-dimensional grating, the refractive index characteristics do not depend on the polarization direction of light incident on the surface of the sub-wavelength surface 80A compared to a general antireflection film. Further, when the bottom surface of each protrusion 82 is square as shown in FIG. 3A, the second diffraction grating 12 (see FIG. 1) is hardly exposed to the air, so that the reflection loss from the second diffraction grating 12 is lost. Disappears.

  The height of the conical structure projections 84 in the Z-axis direction is h, and the pitch between the individual projections 84 is Ps. The conical protrusions 84 may be arranged in a square array as shown in FIG. Further, the conical protrusions 84 may be arranged in a hexagonal close-packed arrangement as shown in FIG. The conical structure projection 84 forms a smooth refractive index distribution as compared with the quadrangular pyramid structure projection 82, and is thus effective in preventing reflection. However, the area where the second diffraction grating 12 is exposed increases.

  As shown in FIG. 3B, the conical structure projections 84 are arranged in a square, and the conical structure projections 84 are arranged in a hexagonal close-packed manner as shown in FIG. 3C. In comparison, the area of the second diffraction grating 12 exposed to the air is smaller when the conical protrusions 84 are arranged in a hexagonal close-packed manner. For this reason, the arrangement | positioning of FIG.3 (c) has a more favorable antireflection effect. However, the difference in the antireflection effect between the square array in FIG. 3B and the hexagonal close-packed array in FIG. 3C is small.

  The sub-wavelength surface 80 shown in FIGS. 1 and 3 has ten or more individual protrusions 82 or protrusions 84, but in fact, thousands or tens of thousands of protrusions 82 or protrusions 84 are illustrated. It is made up of.

<Outline of Diffraction Optical Element 110>
The diffractive optical element 110 preferably has the following conditions.
0.02 <ΔNd <0.45 (1)
0.1 <λd / Ns <1.0 (2)

  Formula (1) is a condition for a difference in refractive index between the refractive index N1 of the first diffraction grating 11 and the refractive index N1 of the second diffraction grating 12 on the inclined surface 18. That is, the refractive index N1 of the first diffraction grating 11 and the refractive index N2 of the second diffraction grating 12 are composed of a combination of a relatively high refractive index and a low refractive index. This refractive index difference is one of the important conditions for determining the diffraction efficiency of the contact type diffractive optical element. Formula (1) defines the refractive index difference ΔNd (= | N1−N2 |) between the first diffraction grating 11 and the second diffraction grating 12 in the d-line. In order to reduce manufacturing error sensitivity, the refractive index difference ΔNd is preferably 0.45 or less, and more preferably 0.2 or less. If the upper limit of Equation (1) is exceeded, the refractive index difference ΔNd becomes too large, and the manufacturing error sensitivity of the diffractive optical element 110 becomes too large.

On the other hand, if the lower limit of Equation (1) is not reached, the refractive index difference ΔNd becomes too small, and the height of the diffraction grating wall 17 must be increased in order to produce the necessary diffraction. For this reason, if it falls below the lower limit of Expression (1), the first diffraction grating 11 and the second diffraction grating 12 are disadvantageous in manufacturing, and the wall 17 causes a shadow on the incident light. In addition, the diffraction efficiency of blazed light is reduced, and stray light due to scattering and reflection by incident light incident on the wall 17 is increased.
In addition, in order to fully demonstrate the effect of Numerical formula (1), it is more preferable to set a lower limit to 0.15.

Equation (2) is a condition for sufficiently reducing the reflectance at the interface between the diffractive optical element 110 and air by using the sub-wavelength surface 80. The refractive index Ns of the sub-wavelength surface 80 is defined with respect to the wavelength λ of the incident light. As shown in Equation (2), if the value obtained by dividing the wavelength λ of the light by the refractive index Ns is smaller than 1, the diffracted light is not generated, and the incident light passes through as 0th order light. It becomes low and the antireflection function is fully exhibited. In addition, in order to exhibit the effect of an antireflection function more, it is preferable that the upper limit of Formula (2) is 0.8 or less.
Further, when the value is below the lower limit of the mathematical formula (2), in the case of a general optical material (glass, resin, etc.), the internal absorption increases and a problem that the transmittance decreases is likely to occur. In addition, in order to fully exhibit the effect of Formula (2), it is more preferable to set the lower limit value to 0.15.

The diffraction grating and the sub-wavelength surface of the diffractive optical element 110 preferably have the following conditions.
(DH × Ps) / (DL × Pd) <5.0 (3)
Formula (3) is a configuration in which a diffraction grating having a high refractive index is arranged closer to the entrance pupil of the entire optical system as will be described later, and a diffraction grating having a low refractive index is arranged farther away. For example, in FIG. 1, the entrance pupil of the entire optical system is arranged on the right side of the diffractive optical element 110, and becomes a convex power grating as a whole. For example, in FIG. 1, when the entrance pupil of the entire optical system is arranged on the right side of the diffractive optical element 110, the refractive index N2 of the second diffraction grating 12 is higher than the refractive index N1 of the first diffraction grating 11. In this case, the maximum thickness of the second diffraction grating 12 made of a high refractive index material is DH, the maximum thickness of the first diffraction grating 11 made of a low refractive index material is DL, the maximum pitch of the sub-wavelength surface 80 is Ps, Let Pd be the minimum pitch of the diffraction grating formed on the slope 18.

  If the upper limit of Expression (3) is exceeded, the pitch Ps of the sub-wavelength surface 80 becomes large, and a sufficient antireflection effect cannot be obtained. Further, the minimum pitch Pd of the diffraction grating formed on the inclined surface 18 becomes small, and the diffraction efficiency is impaired. In order to sufficiently exhibit the effect of the antireflection function, it is preferable that the upper limit of Equation (3) is 2.0 or less.

Furthermore, it is preferable that the diffraction grating and the sub-wavelength surface of the diffractive optical element 110 have the following conditions.
0.01 <h / H <5.0 (4)
Equation (4) shows the relationship between the sub-wavelength surface 80 when the maximum height is h and the maximum grating height of the walls 17 of the first diffraction grating 11 and the second diffraction grating 12 is H.

  If the upper limit of Expression (4) is exceeded, the maximum height of the sub-wavelength surface 80 becomes too large and it becomes difficult to manufacture. Furthermore, the aspect ratio of the sub-wavelength surface 80 (the ratio between the height h and the side Ps in FIG. 3A) is increased, resulting in a disadvantage that the environmental performance or durability is lowered. When the upper limit of Expression (4) is 2.0 or less, the sub-wavelength surface 80 can be easily manufactured.

  If the lower limit of Expression (4) is not reached, the antireflection function of the sub-wavelength surface 80 will drop. Furthermore, the grating height H becomes too large, making it difficult to manufacture the first diffraction grating 11 and the second diffraction grating 12. Furthermore, since the walls 17 of the first diffraction grating 11 and the second diffraction grating 12 are long, the angle characteristic with respect to the incident light is deteriorated, and the incident light is reflected by the wall 17 and stray light is likely to be generated. When the lower limit of the mathematical formula (4) is 0.015 or more, problems such as the production of a grating or stray light are reduced.

Moreover, it is preferable that the diffraction grating and the sub-wavelength surface of the diffractive optical element 110 have the following conditions.
((Eg + Ed + EC) / 3) / Rsd> 10.0 (5)
Here, Eg is the diffraction efficiency of g-line (435.8 nm), Ed is the diffraction efficiency of d-line (587.6 nm), and EC is the diffraction efficiency of C-line (656.3 nm). Rsd is the d-line reflectance at the interface between the sub-wavelength surface 80 and air (normal incidence, 100% = 1.0).

  This equation (5) is a condition regarding the diffraction efficiency of the first diffraction grating 11 and the second diffraction grating 12 and the reflectance at the reference wavelength of the sub-wavelength surface 80 at each spectral line. If the lower limit of Equation (5) is not reached, the reflectance is not sufficiently reduced, and flare due to reflected light increases and stray light increases. In order to sufficiently prevent stray light, the lower limit is preferably set to 100.0 or more.

Furthermore, in order for the diffractive optical element 110 to achieve better performance, it is preferable to satisfy the following mathematical formula.
(Eg + EC) / (2 × Ed)> 0.9 (6)
−20.0 <ΔNd / Δ (NF−Nc) <− 2.0 (7)
1.0 <h / Ps <10.0 (8)

  Equation (6) defines the balance of the first diffraction grating 11 and the second diffraction grating 12 with respect to incident light. In particular, this is a condition for improving the balance of diffraction efficiency in a broadband visible light region. When the left side of Equation (6) is less than the right side, the diffraction efficiency is reduced at either the short wavelength or the long wavelength, the diffraction flare becomes large, and stray light is generated. In order to prevent stray light in a broadband visible light region, the lower limit is desirably 0.95 or more. Moreover, in order to fully demonstrate the effect of Numerical formula (6), it is preferable to make an upper limit into 1.1. When the upper limit of the formula (6) exceeds 1.1, Eg and EC rise and Ed falls due to the balance of Eg, EC, and Ed, so that the green diffraction flare becomes strong and the inconvenience of impairing the image quality is likely to occur. .

  Equation (7) defines the distribution of refractive index and dispersion between the first diffraction grating 11 and the second diffraction grating 12. Δ (NF−Nc) represents a difference between the main dispersion (NF−Nc) of the first diffraction grating 11 and the main dispersion (NF−Nc) of the second diffraction grating 12. Moreover, NF shows the refractive index with respect to F line (486.1 nm), and Nc shows the refractive index with respect to C line (656.3 nm). ΔNd indicates a difference in refractive index between the first diffraction grating 11 and the second diffraction grating 12 in the d-line.

  The condition of Equation (7) is a preferable condition for the diffractive optical element 110 to obtain sufficiently high diffraction efficiency over a wide band of incident light wavelength. When the inequality sign in Expression (7) is reversed, the diffractive optical element 110 cannot obtain a sufficiently high diffraction efficiency. In order for the diffractive optical element 110 to obtain sufficiently high diffraction efficiency over a wide wavelength band, it is preferable that the upper limit is −3.0 or less and the lower limit is −10.0 or more.

  Equation (8) defines the conditions for the sub-wavelength plane 80. That is, Expression (8) defines the aspect ratio of the sub-wavelength surface 80 (the ratio between the height h and one side (pitch) Ps in FIG. 3A). If the lower limit of Expression (8) is not reached, the transition region of the refractive index from the air layer to the surface of the second diffraction grating 12 becomes too small, and the reflection prevention becomes insufficient. If the upper limit of Expression (8) is exceeded, the aspect ratio becomes too large, and the sub-wavelength surface 80 becomes difficult to manufacture. In addition, in order to fully demonstrate the effect of Numerical formula (8), it is more preferable to set the lower limit to 2.0.

Examples of the diffractive optical element 110 will be described below with reference to FIGS.
<First example>
The diffractive optical element 110 of the first example configured with the condition values shown in Table 1 will be described with reference to FIGS. 4A to 4C.

In the diffractive optical element 110 of the first example, the refractive index N2 of the second diffraction grating 12 and the refractive index Ns of the sub-wavelength surface 80 are made of the same material. 4A to 4C are based on calculations based on scalar theory. The blaze order is +1. The reflectance is based on an electromagnetic field analysis technique (RCWA). The sub-wavelength surface 80 is a projection 82 having a quadrangular pyramid structure shown in FIG.
Table 1:
When the reference wavelength is d line (587.6 nm), the pitch Pd of the inclined surface 18 is longer than the reference wavelength, and the pitch Ps of the sub-wavelength surface 80 is shorter than the reference wavelength.

According to the condition values in Table 1, the calculation results of Expression (1) to Expression (8) are as shown in Table 2. As shown in Table 2, the diffractive optical element 110 of the first example satisfies the conditions of Expressions (1) to (8).
Table 2:

  FIG. 4A is a graph showing the reflectance of the air interface. The horizontal axis represents the wavelength of incident light, and the vertical axis represents the reflectance. A solid line indicates the reflectance in the diffractive optical element 110 of the first example. A dotted line indicates the reflectance of the multilayer diffractive optical element including only the first diffraction grating 11 and the second diffraction grating 12 without the sub-wavelength surface 80.

As shown in FIG. 4A, the diffractive optical element 110 having the sub-wavelength surface 80 has a significantly reduced reflectance in a wide visible light region (440 nm to 800 nm). Specifically, the sub-wavelength surface 80 has a reflectivity reduced to about 1/40.
In particular, Equation (2), Equation (3), Equation (4), Equation (5), and Equation (8) contribute to the reduction of reflectance by the sub-wavelength surface 80.

FIG. 4B is a graph showing the diffraction efficiency of the first diffraction grating 11 and the second diffraction grating 12 by the multilayer diffractive optical element. The horizontal axis represents the wavelength of incident light, and the vertical axis represents diffraction efficiency. A solid line (1T) indicates + 1st order diffracted light.
As shown in FIG. 4B, the multilayer diffractive optical element efficiently forms + 1st order diffracted light. The diffraction efficiency of other diffracted light is extremely small.
In particular, Formula (1), Formula (6), and Formula (7) contribute to improvement of the diffraction efficiency of the first-order diffracted light by the multilayer diffractive optical element.

  FIG. 4C is a graph showing the transmission efficiency when + 1st-order diffracted light exits the diffractive optical element 110 after entering the diffractive optical element 110 of the first example. The horizontal axis represents the wavelength of incident light, and the vertical axis represents transmission efficiency. The solid line shows the transmission efficiency in the diffractive optical element 110 of the first example. The dotted line indicates the transmission efficiency of the multilayer diffractive optical element including only the first diffraction grating 11 and the second diffraction grating 12 without the sub-wavelength surface 80.

  As shown in FIG. 4C, the diffractive optical element 110 having the sub-wavelength surface 80 has a multi-layered diffractive optical element that has a transmission efficiency of the first-order diffracted light in a wide visible light region (400 nm to 800 nm) and does not have the sub-wavelength surface 80. Compared to the element, it is improved by about 5%.

<Second example>
The diffractive optical element 110 of the second example configured with the condition values shown in Table 3 will be described with reference to FIGS. 5A to 5C.

The diffractive optical element 110 of the second example is also a case where the refractive index N2 of the second diffraction grating 12 and the refractive index Ns of the sub-wavelength surface 80 are made of the same material. 5A to 5C are based on calculations based on scalar theory. The blaze order is +1. The reflectance is based on an electromagnetic field analysis technique (RCWA). The sub-wavelength surface 80 is a projection 82 having a quadrangular pyramid structure shown in FIG.
Table 3:

According to the condition values in Table 3, the calculation results of Expressions (1) to (8) are as shown in Table 4. As shown in Table 4, the diffractive optical element 110 of the second example satisfies the conditions of Expressions (1) to (8).
Table 4:

  FIG. 5A is a graph showing the reflectance of the air interface. The horizontal axis represents the wavelength of incident light, and the vertical axis represents the reflectance. A solid line indicates the reflectance in the diffractive optical element 110 of the first example. A dotted line indicates the reflectance of the multilayer diffractive optical element including only the first diffraction grating 11 and the second diffraction grating 12 without the sub-wavelength surface 80.

As shown in FIG. 5A, the diffractive optical element 110 having the sub-wavelength surface 80 has a reduced reflectance in a wide visible light region (440 nm to 800 nm). However, the reflectance is poor near the wavelength of 500 nm and near the wavelength of 570 nm.
In particular, the reason why the reflectance is poor near the wavelength of 500 nm and near the wavelength of 570 nm is that the value of Equation (2) is larger and the value of Equation (3) is smaller than in the case of FIG. 4A. Seems to be the cause. However, in other wavelength regions, the sub-wavelength surface 80 reduces the reflectance.

FIG. 5B is a graph showing the diffraction efficiency of the first diffraction grating 11 and the second diffraction grating 12 by the multilayer diffractive optical element. The horizontal axis represents the wavelength of incident light, and the vertical axis represents diffraction efficiency. A solid line (1T) indicates + 1st order diffracted light.
As shown in FIG. 5B, the multilayer diffractive optical element efficiently forms + 1st order diffracted light. The diffraction efficiency of other diffracted light is extremely small.
In particular, Formula (1), Formula (6), and Formula (7) contribute to improvement of the diffraction efficiency of the first-order diffracted light by the multilayer diffractive optical element.

  FIG. 5C is a graph showing the transmission efficiency when + 1st-order diffracted light exits the diffractive optical element 110 after entering the diffractive optical element 110 of the second example. The horizontal axis represents the wavelength of incident light, and the vertical axis represents transmission efficiency. The solid line shows the transmission efficiency in the diffractive optical element 110 of the second example. The dotted line indicates the transmission efficiency of the multilayer diffractive optical element including only the first diffraction grating 11 and the second diffraction grating 12 without the sub-wavelength surface 80.

  As shown in FIG. 5C, the diffractive optical element 110 having the sub-wavelength surface 80 has a transmission efficiency of the first-order diffracted light in a wide visible light region (400 nm to 800 nm) except for the wavelength near 500 nm and the wavelength near 570 nm. Compared to a multilayer diffractive optical element having no wavelength plane 80, the improvement is about 1 to 5 percent.

<Third example>
A diffractive optical element 110 of a third example configured with the condition values shown in Table 5 will be described with reference to FIGS. 6A to 6C.

The diffractive optical element 110 of the third example is also a case where the refractive index N2 of the second diffraction grating 12 and the refractive index Ns of the sub-wavelength surface 80 are made of the same material. The diffraction efficiencies of FIGS. 6A to 6C are based on calculations based on scalar theory. The blaze order is +1. The reflectance is based on an electromagnetic field analysis technique (RCWA). The sub-wavelength surface 80 is a projection 82 having a quadrangular pyramid structure shown in FIG.
Table 5:

According to the condition values in Table 5, the calculation results of Expression (1) to Expression (8) are as shown in Table 6. The diffractive optical element 110 of the third example satisfies the conditions of Expressions (1) to (8) as shown in Table 6.
Table 6:

  FIG. 6A is a graph showing the reflectance of the air interface. The horizontal axis represents the wavelength of incident light, and the vertical axis represents the reflectance. A solid line indicates the reflectance in the diffractive optical element 110 of the first example. A dotted line indicates the reflectance of the multilayer diffractive optical element including only the first diffraction grating 11 and the second diffraction grating 12 without the sub-wavelength surface 80.

As shown in FIG. 6A, the diffractive optical element 110 having the sub-wavelength surface 80 has a significantly reduced reflectance in a wide visible light region (440 nm to 800 nm). Specifically, the sub-wavelength surface 80 has a reflectivity reduced to about 1/40.
In particular, Equation (2), Equation (3), Equation (4), Equation (5), and Equation (8) contribute to the reduction of reflectance by the sub-wavelength surface 80.

FIG. 6B is a graph showing the diffraction efficiency of the first diffraction grating 11 and the second diffraction grating 12 by the multilayer diffractive optical element. The horizontal axis represents the wavelength of incident light, and the vertical axis represents diffraction efficiency. A solid line (1T) indicates + 1st order diffracted light.
As shown in FIG. 6B, the multilayer diffractive optical element efficiently forms + 1st order diffracted light. The diffraction efficiency of other diffracted light is extremely small.
In particular, Formula (1), Formula (6), and Formula (7) contribute to improvement of the diffraction efficiency of the first-order diffracted light by the multilayer diffractive optical element.

  FIG. 6C is a graph showing the transmission efficiency when + 1st-order diffracted light exits the diffractive optical element 110 after entering the diffractive optical element 110 of the third example. The horizontal axis represents the wavelength of incident light, and the vertical axis represents transmission efficiency. The solid line shows the transmission efficiency in the diffractive optical element 110 of the third example. The dotted line indicates the transmission efficiency of the multilayer diffractive optical element including only the first diffraction grating 11 and the second diffraction grating 12 without the sub-wavelength surface 80.

  As shown in FIG. 6C, the diffractive optical element 110 having the sub-wavelength surface 80 has a multi-layered diffractive optical element having a transmission efficiency of first-order diffracted light in a wide visible light region (400 nm to 800 nm) and no sub-wavelength surface 80. Compared to the device, it is improved by about 4%.

(Second Embodiment)
<Configuration of Diffraction Optical Element 120>
FIG. 7 is a conceptual cross-sectional view showing the diffractive optical element 120 attached to the optical member.
Unlike the first embodiment, one surface of the third diffraction grating 21 is attached to a flat optical member 90. Further, the third diffraction grating 21 and the fourth diffraction grating 22 have walls 27 and inclined surfaces 28.

  The multilayer diffractive optical element including the third diffraction grating 21 and the fourth diffraction grating 22 is a flat plate in the Z-axis direction. The refractive index N1 of the third diffraction grating 21 is a low refractive index, and the refractive index N2 of the fourth diffraction grating 22 is a high refractive index. The slope 28 of the fourth diffraction grating 22 is a curved surface bulging in a convex shape. The wall 27 is straight in the Z-axis direction. The pitch Pdv of the diffraction grating formed on the inclined surface 28 is gradually narrowed from the center of the XY plane toward the periphery. For this reason, the multilayer diffractive optical element has the same positive power as a general convex lens in the −Z-axis direction.

A sub-wavelength surface 80 is formed on one surface of the fourth diffraction grating 22 opposite to the inclined surface 28. The sub-wavelength surface 80 is in contact with a gas such as air or a vacuum.
In formula (3) in the first embodiment, the pitch Pdv of the diffractive optical element 120 may be calculated by replacing it with the pitch Pd.

(Third embodiment)
<Configuration of Diffractive Optical Element 130>
FIG. 8 is a conceptual cross-sectional view showing the diffractive optical element 130 attached to the optical member.
Unlike the first embodiment, one surface of the fifth diffraction grating 31 is attached to a flat optical member 90. Further, the fifth diffraction grating 31 and the sixth diffraction grating 32 have a wall 37 and a slope 38.

  The multilayer diffractive optical element including the fifth diffraction grating 31 and the sixth diffraction grating 32 is a flat plate in the Z-axis direction. The refractive index N1 of the fifth diffraction grating 31 is a high refractive index, and the refractive index N2 of the sixth diffraction grating 32 is a low refractive index. The slope 38 of the fifth diffraction grating 31 is a curved surface bulging in a convex shape. The wall 37 is straight in the Z-axis direction. The pitch Pdv of the diffraction grating formed on the inclined surface 38 is gradually narrowed from the center of the XY plane toward the periphery. For this reason, the multilayer diffractive optical element has the same positive power as the convex lens in the + Z-axis direction. Note that when the diffractive optical element 120 or the diffractive optical element 130 has a negative (concave) power, the low refractive index and the high refractive index may be reversed.

A sub-wavelength surface 80 is formed on one surface of the sixth diffraction grating 32 opposite to the inclined surface 38. The sub-wavelength surface 80 is in contact with a gas such as air or a vacuum.
In Formula (3) in the first embodiment, the pitch Pdv of the diffractive optical element 130 may be calculated by replacing it with the pitch Pd.

(Fourth embodiment)
<Configuration of Diffraction Optical Element 140>
FIG. 9 is a conceptual cross-sectional view showing the diffractive optical element 140 attached to the optical member.
Unlike the third embodiment, one surface of the seventh diffraction grating 41 is attached to the optical member 95 of a convex lens. Further, the seventh diffraction grating 41 and the eighth diffraction grating 42 have walls 47 and inclined surfaces 48.

  The multi-layered diffractive optical element including the seventh diffraction grating 41 and the eighth diffraction grating 42 has a curved surface along the optical member 95 of the convex lens, and the thickness in the Z-axis direction is almost uniform over the entire surface. The refractive index N1 of the seventh diffraction grating 41 is a high refractive index, and the refractive index N2 of the eighth diffraction grating 42 is a low refractive index. The slope 48 of the seventh diffraction grating 41 is a curved surface that bulges in a convex shape. The wall 47 is straight in the Z-axis direction. Further, the pitch Pdv of the diffraction grating formed on the slope 48 is gradually narrowed from the center of the XY plane toward the periphery. Therefore, like the convex lens optical member 95, the multilayer diffractive optical element has the same positive power as the convex lens in the + Z-axis direction.

A sub-wavelength surface 80 is formed on one surface of the eighth diffraction grating 42 opposite to the inclined surface 48. The sub-wavelength surface 80 is in contact with a gas such as air or a vacuum.
In Formula (3) in the first embodiment, the pitch Pdv of the diffractive optical element 140 may be calculated by replacing it with the pitch Pd.

(Other embodiments)
<Wall of the multilayer diffractive optical element>
FIG. 10 is a diagram showing a modification of the first diffraction grating 11 and the second diffraction grating 12 of the first embodiment. The same reference numerals are given to the same members in the first embodiment.

  As described in Expression (4), when the height (length) H of the wall 17 is increased, the angle characteristic with respect to the incident light is deteriorated, and the incident light is reflected by the wall 17 and stray light is likely to be generated. . Further, when the height (length) H of the wall 17 increases, the diffraction efficiency tends to decrease. To solve these problems, you can do the following.

  The wall 57 shown in FIG. 10A is formed to be inclined with respect to the Z-axis direction. This tilt angle is preferably tilted according to the chief ray. In other words, the wall 57 is directed toward the pupil (incidence pupil or exit pupil).

  The wall 67 shown in FIG. 10B has a stepped step portion. By forming the stepped portion, the entire wall 67 is inclined with respect to the Z-axis direction. Incident light that has entered the first diffraction grating 11 and the second diffraction grating 12 with a predetermined angle is reflected twice by the step portions (a surface in the Z-axis direction and a surface in the XY-axis direction). It will return to the opposite direction in the same direction. As a result, the incident light incident on the wall 67 does not reach the imaging direction, and therefore does not become flare light.

  The wall 77 shown in FIG. 10C has a curved cross-sectional shape. When the cross-sectional shape of the wall 77 is a curve, reflected or transmitted light is diverged, and unnecessary high-order diffracted light, flare, and ghost can be reduced.

  In the first to fourth embodiments and other embodiments, the multilayer diffractive optical element and the sub-wavelength surface 80 are formed of glass or resin. However, it is preferable to use a curable resin because of its complicated and fine shape. Furthermore, it is preferable in terms of production efficiency that both the multilayer diffractive optical element and the sub-wavelength surface 80 are ultraviolet curable resins. Furthermore, in order to reduce the size and weight, the optical material constituting the diffractive optical element is preferably a resin material having a specific gravity of 2.0 or less. Since the specific gravity of resin is smaller than that of glass, it is effective for reducing the weight of the optical system. And in order to exhibit an effect further, it is preferable that specific gravity is 1.6 or less.

<Optical system using diffractive optical element>
FIG. 11 is a conceptual cross-sectional view of the optical system 200 or the optical system 210 that uses the diffractive optical element 110 of the first embodiment. FIG. 11A shows an optical system 200 in which the diffractive optical element 110 is arranged on the image plane side from the stop DP. FIG. 11B shows an optical system 210 in which the diffractive optical element 110 is arranged on the object side with respect to the stop DP.

  In the optical system 200 shown in FIG. 11A, the left side of FIG. 11A is an object (not shown) and the right side is an imaging plane IP. The optical system 200 includes an imaging lens 211, a diaphragm DP (pupil plane), and a diffractive optical element 110 in order from the left side. The optical system 200 is used as a photographing lens of a camera, for example.

  The sub-wavelength surface 80 of the diffractive optical element 110 prevents reflection of broadband visible light that has passed through the imaging lens 211 and the stop DP, as shown in FIG. 4A, FIG. 5A, or FIG. 6A. it can. Since the diffractive optical element 110 has high transmission efficiency of the first-order diffracted light as shown in FIG. 4C, FIG. 5C, or FIG. 6C, a lot of light can be guided to the imaging plane IP. Further, as shown in FIG. 2, FIG. 4B, FIG. 5B, or FIG. 6B, the first diffraction grating 11 and the second diffraction grating 12 have high diffraction efficiency with respect to broadband visible light. Chromatic aberration can be suppressed.

  In the optical system 210 shown in FIG. 11B, the left side of FIG. 11B is an object (not shown) and the right side is an imaging plane IP. The optical system 210 includes an imaging lens 212, a diffractive optical element 110, a diaphragm DP (pupil plane), and an imaging lens 213 in order from the left side. The optical system 210 is also used as a camera taking lens.

  The sub-wavelength surface 80 of the diffractive optical element 110 can prevent reflection of broadband visible light that has passed through the imaging lens 212. Since the diffractive optical element 110 has high transmission efficiency of the first-order diffracted light, a lot of light can be guided to the stop DP side. Further, since the first diffraction grating 11 and the second diffraction grating 12 have high diffraction efficiency with respect to broadband visible light, chromatic aberration and the like can be suppressed.

  In FIG. 11B, the imaging lens 212 and the diffractive optical element 110 are separate bodies. However, the present invention is not limited to this, and the diffractive optical element 140 may be provided in the imaging lens 212 as shown in FIG.

  The optical system 200 or the optical system 210 shown in FIG. 11 has a high transmittance, and there is little decrease in the diffraction efficiency partially in the used wavelength region. Therefore, the optical system 200 or the optical system 210 is provided as a high-performance photographic lens having high resolution.

  Further, although the optical system 200 or the optical system 210 is shown as a camera taking lens, the present invention is not limited to this, but a video camera taking lens, an office image scanner, a digital copier reader lens, binoculars, a microscope, etc. The same effect can be obtained even when used in an imaging optical system such as an optical system of an observation apparatus.

The optimum embodiment of the present invention has been described above, but as will be apparent to those skilled in the art, the present invention can be implemented with various modifications within the technical scope thereof.
For example, as an optimum embodiment, a broadband visible light (for example, 400 nm to 800 nm) has been described. However, the present invention can also be applied to a diffractive optical element, a diffractive optical member, and an optical system in which narrow-band visible light, narrow-band or broadband infrared light or ultraviolet light is incident. Further, the present invention can be applied not only to a transmission type but also to a reflection type optical system.

DESCRIPTION OF SYMBOLS 11 ... 1st diffraction grating 12 ... 2nd diffraction grating 21 ... 3rd diffraction grating 22 ... 4th diffraction grating 31 ... 5th diffraction grating 32 ... 6th diffraction grating 41 ... 7th diffraction grating 42 ... 8th diffraction grating 17, 27, 37, 47, 57, 67, 77 ... Walls 18, 28, 38, 48 ... Slope 80 (80A, 80B) ... Sub-wavelength surfaces 82, 84 ... Protrusions 90, 95 ... Optical members 110, 120, 130, 140 ... Diffraction optical element 200, 210 ... Optical system 211 ... Imaging lens DH, DL, H, h ... Height DP ... Aperture IP ... Imaging surface N1 ... Refractive index N2 ... Refractive index Ns ... Refractive index Pd, Pdv, Ps … Pitch

Claims (13)

In a diffractive optical element composed of two optical members in which diffractive optical surfaces having a grating structure formed at a first pitch are stacked facing each other,
A diffractive optical element having a concavo-convex structure having a second pitch shorter than a reference wavelength of light transmitted through the diffractive optical element on one surface of the diffractive optical element. When the reference wavelength is d-line (587.6 nm), the wavelength is λd, the refractive index difference between the d-line of the two optical members is ΔNd, and the refractive index of the optical member on the one surface side is Ns, The diffractive optical element according to claim 1, which satisfies the following condition.
0.02 <ΔNd <0.45 (1)
0.1 <λd / Ns <1.0 (2) One of the two optical members is composed of a relatively high refractive index material and the other is composed of a relatively low refractive index material,
When the maximum thickness of the lattice structure of the high refractive index material is DH, the maximum thickness of the lattice structure of the low refractive index material is DL, the minimum pitch of the first pitch is Pd, and the maximum pitch of the second pitch is Ps. The diffractive optical element according to claim 1, wherein the following condition is satisfied.
(DH × Ps) / (DL × Pd) <5.0 (3) When the maximum height of the lattice structure is H and the maximum height of the uneven structure is h,
The diffractive optical element according to any one of claims 1 to 3, wherein the following condition is satisfied.
0.008 <h / H <2.0 (4) The diffraction efficiency of the d-line (587.6 nm) is Ed, the diffraction efficiency of the g-line (435.8 nm) is Eg, the diffraction efficiency of the C-line (656.3 nm) is EC, and the uneven structure is formed. 5. The diffractive optical element according to claim 1, wherein the d-line reflectivity (normal incidence, 100% = 1.0) of the diffractive optical element is Rsd, and the following condition is satisfied. element.
However, it does.
((Ed + EC + Eg) / 3) / Rsd> 10.0 (5) The diffraction efficiency of d-line (587.6 nm) is Ed, the diffraction efficiency of g-line (435.8 nm) is Eg, the diffraction efficiency of C-line (656.3 nm) is EC, and the following equation is satisfied: The diffractive optical element according to claim 5.
(EC + Eg) / (2 × Ed)> 0.9 (6) When the difference in refractive index of d-line between the two optical members is ΔNd and the difference in main dispersion (NF−Nc) between the two optical members is Δ (NF−Nc), the following formula is satisfied. The diffractive optical element according to any one of claims 1 to 6.
−20.0 <ΔNd / Δ (NF−Nc) <− 2.0 (7) The diffractive optical element according to claim 1, wherein h is a maximum height of the concavo-convex structure and Ps is a maximum pitch of the second pitch.
1.0 <h / Ps <10.0 (8)   The diffractive optical element according to any one of claims 1 to 8, wherein a wall of the diffractive optical surface is inclined with respect to an optical axis of light transmitted through the diffractive optical element.   The diffractive optical element according to claim 1, wherein the diffractive optical surface has a positive refractive power.   The diffractive optical element according to any one of claims 1 to 10, wherein a material of the two optical members is an ultraviolet curable resin. An optical system having a plurality of optical elements, guiding light and acting on the light,
An optical system, wherein at least one of the optical elements is a diffractive optical element according to any one of claims 1 to 11.   An optical apparatus comprising the diffractive optical element according to any one of claims 1 to 11.