JP2014006280A - Diffraction optical element, optical system and optical equipment - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a diffraction optical element in which wavelength characteristics of diffraction efficiency of a grating including influences of a grating side face are controlled to be approximately uniform.SOLUTION: A diffraction optical element 10 comprises layers of a plurality of diffraction gratings 1, 2 formed of different materials. Each material for the plurality of gratings shows such a property that in a range 2/3 or more of the use wavelength band of the diffraction optical element, ΔΦ(λ) defined by the following formula increases with the shorter wavelength. In the formula, λ represents a wavelength included in the use wavelength band; ni represents a refractive index at the wavelength λ of the material for an i-th grating in the plurality of gratings; di represents a grating height of the i-th grating; and m represents a diffraction order used.

Description

  The present invention relates to a diffractive optical element, and more particularly to a diffractive optical element configured by stacking a plurality of diffraction gratings.

  As a method of reducing chromatic aberration in an optical system including a lens, a method of providing a diffractive optical element on the surface of the lens or a part of the optical system is known. Such a method using a diffractive optical element utilizes a physical phenomenon that the deflection direction with respect to a light beam having a certain reference wavelength is reversed between the refracting surface and the diffractive surface in the optical system. In addition, the diffractive optical element can have an optical action like an aspherical lens by appropriately changing the period of the periodic structure, and thus is effective in reducing various aberrations other than chromatic aberration.

  In general, a diffractive optical element has a blaze structure formed by a grating surface that faces obliquely with respect to a light incident direction and a grating side surface (grating wall surface) that connects adjacent grating surfaces. Such a blazed diffractive optical element can diffract light with high diffraction efficiency for a specific one order (hereinafter referred to as “specific order” or “used diffraction order”) and a specific wavelength. . On the other hand, Patent Document 1 discloses a configuration of a diffractive optical element for sufficiently increasing the diffraction efficiency of a specific order over the entire visible wavelength band. The diffractive optical element disclosed in Patent Document 1 is configured by closely adhering two diffraction gratings formed using a low refractive index and high dispersion material and a high refractive index and low dispersion material. Is set appropriately. Hereinafter, such a diffractive optical element is referred to as “adherent two-layer DOE”. The close-contact two-layer DOE has high diffraction efficiency over a wide wavelength band with respect to diffracted light of a specific order. Here, the diffraction efficiency is the ratio of the amount of diffracted light of each order to the amount of all transmitted light.

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

JP-A-9-127321 JP 2008-241734 A

  However, in the conventional diffractive optical element, since the diffraction efficiency including the influence on the diffraction of the grating side surface is not taken into consideration, the wavelength characteristic of the diffraction efficiency in the diffracted light of the used diffraction order becomes lower on the long wavelength side. There was a problem. This is because when a diffractive optical element is used in the optical system, the diffraction efficiency in the blue wavelength band is higher than the diffraction efficiency in the red wavelength band, and red is noticeable in unnecessary light. Furthermore, if the intensity of the red wavelength band is increased using an antireflection film, image processing, or the like in order to balance the diffracted light of the used diffraction order, red as unnecessary light is further emphasized.

  The present invention provides a diffractive optical element that can uniformly approximate the wavelength characteristics of diffraction efficiency including the influence of the grating side surface of the diffraction grating, and an optical system and optical apparatus using the diffractive optical element.

A diffractive optical element according to one aspect of the present invention is configured by stacking a plurality of diffraction gratings formed of different materials. Each of the materials of the plurality of diffraction gratings is a material in which ΔΦ (λ) shown below becomes larger toward the shorter wavelength side in a range of 2/3 or more of the wavelength band used for the diffractive optical element.

Here, λ is a wavelength included in the used wavelength band, ni is a refractive index with respect to the wavelength λ of the material of the i-th diffraction grating among a plurality of diffraction gratings, and di is a grating height of the i-th diffraction grating. M is the diffraction order used.

  An optical system including the diffractive optical element and an optical apparatus using the optical system also constitute another aspect of the present invention.

  According to the present invention, the wavelength characteristics of the diffraction efficiency of the diffractive optical element including the influence of the grating side surface of each diffraction grating can be made close to each other by specifying the material of the plurality of diffraction gratings to be stacked. And by using this for an optical system or an optical apparatus, it is possible to realize an optical system or an optical apparatus having high optical performance such that chromatic aberration is satisfactorily reduced.

1 is a front view and a side cross-sectional view of a lens element including a diffractive optical element that is Embodiment 1 of the present invention. FIG. The schematic diagram which shows the structure of the said diffractive optical element. The graph which shows the diffraction efficiency of the diffractive optical element which is the numerical example 1 of this invention. The graph which shows the diffraction efficiency of the diffractive optical element which is the numerical example 2 of this invention. The graph which shows the diffraction efficiency of the diffractive optical element which is the numerical example 3 of this invention. The graph which shows the diffraction efficiency of the diffractive optical element which is the numerical example 4 of this invention. Sectional drawing which shows the structure of the imaging optical system which is Example 2 of this invention. The graph which shows the diffraction efficiency of the diffractive optical element which is the comparative example 1 with respect to this invention. The graph which shows the diffraction efficiency of the diffractive optical element which is the comparative example 2 with respect to this invention. The graph which shows the diffraction efficiency of the diffractive optical element which is the comparative example 3 with respect to this invention.

  Embodiments of the present invention will be described below with reference to the drawings.

  FIG. 1 shows a lens element including a diffractive optical element that is Embodiment 1 of the present invention. Reference numerals 20 and 30 denote substrate lenses, and the diffractive optical element 10 is disposed between the substrate lenses 20 and 30. Specifically, the diffractive optical element 10 is formed on the inner surface of each of the substrate lenses 20 and 30. The surface of each substrate lens on which the diffractive optical element 10 is formed is a curved surface. However, the substrates on both sides of the diffractive optical element 10 may be flat plates.

  As shown in FIG. 2, the diffractive optical element 10 includes a first diffraction grating 1 and a second diffraction grating 2 each having a plurality of concentric gratings each having an optical axis O as a center. Are laminated in the direction in which the lens extends (hereinafter referred to as the optical axis direction), and has a lens action. FIG. 2 is a diagram showing a cross section taken along the line AA ′ of the diffractive optical element 10 shown in FIG. 1 in order to make the shape of the grating easy to understand. A direction in which a plurality of gratings are formed (radial direction in the front view of FIG. 1) is also referred to as a grating arrangement direction in the following description.

  As shown in FIG. 2, the diffractive optical element 10 includes a first diffractive grating 1 and a second diffractive grating 2 that are stacked so as to be in close contact with each other, thereby forming one diffractive optical element (DOE). It is a functioning two-layer DOE. The first and second diffraction gratings 1 and 2 have a blazed structure in which each grating is formed by a grating surface A and a grating side surface (also referred to as a grating wall surface) B. The lattice plane A is an inclined surface that is inclined with respect to the optical axis direction or the light incident direction, and the lattice side surface B is connected to the adjacent lattice planes A (so as to extend in the optical axis direction or the light incident direction). ) The formed surface.

  Further, the grating pitch of the first and second diffraction gratings 1 and 2 gradually changes (decreases or increases) from the center through which the optical axis O passes to the periphery, thereby converging as the lens action described above. Has an action or divergent action. In addition, with the blazed structure, the light incident on the diffractive optical element 10 is diffracted in a specific direction as diffracted light of the used diffraction order (+ 1st order in FIG. 2). The used diffraction order is the diffraction order of the diffracted light used, and is also referred to as the design order.

  The working wavelength region, which is the wavelength region of light incident on the diffractive optical element 10 in this embodiment, is a visible wavelength region (for example, 400 to 700 nm). For this reason, the materials and shapes (particularly the grating height) constituting the first and second diffraction gratings 1 and 2 are selected so that the diffraction efficiency of the diffracted light of the used diffraction order is increased over the entire visible wavelength band. Specifically, the maximum optical path length difference of light passing through the first and second diffraction gratings 1 and 2 (that is, the maximum value of the optical optical path length difference between the crest and trough of the grating) is within the used wavelength band. The material and shape of each diffraction grating are determined so as to be an integral multiple of the wavelength of the light. The term “integer multiple” here includes not only a perfect integer multiple but also a case where it deviates from a perfect integer multiple to the extent that it can be optically regarded as an integer multiple, and is synonymous with “integer multiple or its vicinity”. It is.

  By appropriately setting the diffraction grating material and the grating height as described above, a high diffraction efficiency can be obtained over the entire wavelength band used. As shown in FIG. 2, the lattice height d is the height between the tip (vertex) of the lattice and the bottom of the groove in the normal direction orthogonal to the lattice arrangement direction.

First, the diffraction efficiency using the conventional scalar diffraction theory will be described. In the diffractive optical element in which a plurality of diffraction gratings are stacked, the conditions under which the diffraction efficiency becomes maximum at a certain diffraction order having the wavelength λ are as follows. The condition is that the optical path length difference between the peaks and troughs of the grating (for example, the difference in the optical path lengths of the light beams passing through the peak of the grating peaks and the bottom of the valley) is added over the entire diffraction grating. Is an integral multiple of. Therefore, the condition that the diffraction efficiency at the diffraction order m is maximized when the light of wavelength λ is incident perpendicular to the base surface of the diffraction grating (the surface extending in the grating arrangement direction and indicated by the dotted line in FIG. 2). Is represented by the following formula (1).

  In Formula (1), λ is any wavelength included in the used wavelength band, and ni is the refractive index at the wavelength λ of the material forming the i-th diffraction grating among the plurality of stacked diffraction gratings. . di is the grating height of the i-th diffraction grating, and m is the diffraction order.

  The sign of di is that when a light beam diffracted downward from the 0th order diffracted light as shown in FIG. 2 is a positive diffraction order light beam, the grating height of the diffraction grating increases from the bottom to the top in FIG. The case is negative, and the case where the grating height of the diffraction grating decreases from bottom to top is positive.

The diffraction efficiency η (λ) at the wavelength λ and the diffraction order m of the diffractive optical element in which a plurality of diffraction gratings are stacked is expressed by the following equation (2).

Here, ΔΦ (λ) is defined as the following expression (3), where m is the diffraction order used.

  When ΔΦ (λ) defined by the equation (3) becomes 0, the diffraction efficiency η (λ) shown by the equation (2) becomes 100%.

The close-contact two-layer DOE such as the diffractive optical element 10 of this embodiment shown in FIGS. 1 and 2 has a structure in which two diffraction gratings are stacked and the grating surfaces of the two diffraction gratings are in close contact with each other. Have In this configuration, the condition that maximizes the diffraction efficiency at the diffraction order m when light of wavelength λ is incident on the base surface of the diffraction grating perpendicularly is expressed by the following equation (4).

  In Expression (4), n1 is the refractive index with respect to the wavelength λ of the material forming the first diffraction grating 1, and n2 is the refractive index with respect to the wavelength λ of the material forming the second diffraction grating 2. d is the grating height of the first and second diffraction gratings 1 and 2.

The diffraction efficiency η (λ) at the wavelength λ and the diffraction order m of the diffractive optical element (adherent two-layer DOE) 10 of the present embodiment is expressed by the following equation (5).

  In this embodiment, in order to obtain high diffraction efficiency in a wide wavelength band in the diffractive optical element 10, a low refractive index and high dispersion material is used for the first diffraction grating 1, and a high refractive index is used for the second diffraction grating 2. And a low dispersion material. In addition, in order to obtain a diffraction efficiency of 99% or more in the entire visible wavelength band, it is known to use a material having a linear dispersion characteristic with a partial dispersion ratio θgF smaller than that of a normal material for a low refractive index and high dispersion material. Yes. In order to obtain this linear dispersion characteristic, a method of dispersing and mixing ITO fine particles in a base resin material is also known.

Further, ΔΦ (λ) is defined as the following formula (6).

  When ΔΦ defined by the equation (6) is 0, the diffraction efficiency η represented by the equation (5) is 100%.

  In the diffractive optical elements disclosed in Patent Documents 1 and 2 described above, the diffraction efficiency is calculated using the scalar diffraction theory, and the design evaluation is performed. It is known that scalar diffraction theory can be calculated with high accuracy when the grating pitch of the diffraction grating is sufficiently larger than the wavelength of incident light. However, this scalar diffraction theory describes only the diffraction phenomenon due to the grating surface (slope) of the diffraction grating, and does not consider the influence of the grating side surface. That is, the diffraction efficiency disclosed in Patent Documents 1 and 2 does not consider the influence of the grating side surface. Since an actual diffractive optical element naturally has not only a grating surface but also a grating side surface, the influence of the grating side surface should be considered.

  Next, diffraction efficiency using strict electromagnetic field calculation, which is a feature of this embodiment, will be described. By using this exact electromagnetic field calculation, it is possible to calculate the diffraction efficiency considering not only the grating plane but also the grating side face. In the exact electromagnetic field calculation, the diffraction efficiency at each diffraction order of the transmitted diffracted light and the reflected diffracted light with respect to a structure of an arbitrary shape can be strictly calculated by solving the Maxwell equation numerically. Conventionally, strict electromagnetic field calculation is often used when the accuracy of scalar theory is reduced and the grating pitch is smaller than the wavelength of incident light. However, even when the grating pitch is sufficiently larger than the wavelength of incident light, it is possible to obtain strict diffraction efficiency using strict electromagnetic field calculation.

  Here, the diffraction efficiency of the diffractive optical elements shown in Numerical Examples 1 to 4 below was evaluated using rigorous coupled wave analysis (RCWA) among strict electromagnetic field calculations.

  In the conventional scalar diffraction theory, the materials of the first and second diffraction gratings are combined so that ΔΦ (λ) defined by Expression (3) (and Expression (6)) becomes zero.

  On the other hand, in the present embodiment (each numerical example), ΔΦ (λ) defined by Expression (3) (and Expression (6)) is a short wavelength in the range of 2/3 or more of the used wavelength band. A combination of materials of the first and second diffraction gratings that increase toward the side is used. As a result, the wavelength characteristics of the diffraction efficiency at the used diffraction order including the influence of the grating side surface can be made closer to each other. In other words, unless this condition is satisfied within a range of 2/3 or more of the used wavelength band, the wavelength characteristics of diffraction efficiency will not be uniform.

Moreover, it is preferable that the diffractive optical element satisfies the relationship represented by the following formula (7).

In Equation (7), both λν1 and λν2 are wavelengths (nm) in the visible wavelength band. Further, λ ₅₅₀ has a wavelength of 550 nm, and E−n is × 10 ⁻ⁿ .

Furthermore, it is preferable that the diffractive optical element satisfies the relationship represented by the following formula (8).

However, Ramudanyu is the wavelength of the visible wavelength range (nm), λ ₅₅₀ is the wavelength 550 nm. Further, ^En is ^x10 -n.

  If the values of these formulas (7) and (8) are below the lower limit, they approach the scalar diffraction theory, and if they exceed the upper limit, they are worsened. As described above, unless the relations of the expressions (7) and (8) are satisfied, the wavelength characteristics of the diffraction efficiency in the used diffraction order including the influence of the grating side surface are not uniform, which is not preferable.

  Moreover, it is preferable that the used diffraction order is + 1st order or −1st order. If the other diffraction orders are used, the grating height increases, and the diffraction efficiency when light is incident on the diffractive optical element obliquely may be reduced.

Furthermore, the grating pitch of each diffraction grating is preferably 80 μm or more. When the grating pitch is smaller than 80 μm, the influence on the diffraction phenomenon on the grating side surface is increased, so that the diffraction efficiency of the used diffraction order may be lowered.
[Numerical example 1]
In Numerical Example 1, as the material of the first diffraction grating 1, an ultraviolet curable resin in which ITO fine particles are mixed with an acrylic ultraviolet curable resin is used. Further, as the material of the second diffraction grating 2, an ultraviolet curable resin in which ZrO ₂ fine particles were mixed with an acrylic ultraviolet curable resin was used. Further, the refractive index of each material was adjusted so that ΔΦ (λ) shown in Table 1 was obtained. ΔΦ (λ) is larger toward the shorter wavelength side in the entire use wavelength band (range of 2/3 or more). In this numerical example, the grating height d is 11.00 μm, the used diffraction order is + 1st order, the incident angle of incident light with respect to the base surface is 0 deg (perpendicular incidence), and the grating pitch is 100 μm.

  In this numerical example, the results of evaluating the diffraction efficiency using RCWA are shown in FIG. FIG. 3A shows the wavelength characteristics of diffraction efficiency at the used diffraction order + 1 order. FIG. 3A shows that the diffraction efficiency considering the influence of the grating side surface is substantially uniform in the visible wavelength band of 400 to 700 nm. Moreover, it turns out that the wavelength characteristic is improving compared with the comparative example 1 shown later.

  FIG. 3B shows an enlarged view of the lower part of the diffraction efficiency at high diffraction orders at wavelengths of 400 nm, 550 nm and 700 nm. From FIG. 3 (b), the diffraction efficiency peak at the diffraction order of unnecessary diffraction light in the vicinity of the −60th to −40th order (hereinafter referred to as unnecessary diffraction order) is longer than the wavelength of 400 nm on the short wavelength side. The wavelength 700 nm on the wavelength side is higher. This is a diffraction phenomenon that occurs on the side surface of the grating even when incident light is perpendicularly incident on the diffractive optical element (base surface), and the peak of the diffraction efficiency is approximately the same as the wavelength. At a relatively low diffraction order (approximately −10th to 0th orders), the peak of diffraction efficiency is higher at 400 nm than at the wavelength of 700 nm. By balancing the unnecessary diffraction light of the high diffraction order and the low diffraction order, the wavelength characteristics of the diffraction efficiency of the used diffraction order are almost uniform.

  Unnecessary diffracted light having a low diffraction order on the short wavelength side is relatively inconspicuous, so there is no problem even if it exists. In addition, the absorption on the short wavelength side tends to increase due to the digitization of recent optical equipment and the increase in the number of lenses in the photographic optical system accompanying the increase in image quality required when printing large images. Unnecessary diffracted light having a low diffraction order on the short wavelength side tends to be less problematic.

  The material of this numerical example is a material in which a base resin material and fine particles are mixed. However, in order to adjust the refractive index, a different resin or fine particles may be mixed. Further, the refractive index of the material of each diffraction grating may be adjusted by changing the process for curing the diffraction grating with ultraviolet rays.

Table 7 shows the values of Equations (7) and (8) in this numerical example.
[Numerical example 2]
In Numerical Example 2, as in Numerical Example 1, an ultraviolet curable resin in which ITO fine particles are mixed with an acrylic ultraviolet curable resin is used as the material of the first diffraction grating 1. Further, as the material of the second diffraction grating 2, an ultraviolet curable resin in which ZrO ₂ fine particles were mixed with an acrylic ultraviolet curable resin was used. However, these materials are different from those of Numerical Example 1. Furthermore, the refractive index of each material was adjusted so that ΔΦ (λ) shown in Table 2 was obtained. ΔΦ (λ) is larger toward the shorter wavelength side in the entire use wavelength band (range of 2/3 or more). In this numerical example, the grating height d is 11.00 μm, the used diffraction order is + 1st order, the incident angle of incident light with respect to the base surface is 0 deg (perpendicular incidence), and the grating pitch is 100 μm.

  In this numerical example, the results of evaluation of diffraction efficiency using RCWA are shown in FIG. FIG. 4A shows the wavelength characteristics of the diffraction efficiency at the used diffraction order + 1 order. From FIG. 4A, it can be seen that the diffraction efficiency considering the influence of the grating side surface is almost uniform in the visible wavelength band of 400 to 700 nm. Moreover, it turns out that the wavelength characteristic is improving compared with the comparative example 1 shown later.

  FIG. 4B shows an enlarged view of the lower part of the diffraction efficiency at high diffraction orders at wavelengths of 400 nm, 550 nm and 700 nm. Similar to Numerical Example 1, the wavelength characteristics of the diffraction efficiency of the used diffraction orders are substantially uniform by balancing the unnecessary diffraction light of the high diffraction order and the low diffraction order.

Table 7 shows the values of Equations (7) and (8) in this numerical example.
[Comparative Example 1]
In order to clarify the features of Example 1 (Numerical Examples 1 and 2), Comparative Example 1 is shown. This comparative example is a design example using scalar diffraction theory. Also in this comparative example, an ultraviolet curable resin obtained by mixing ITO fine particles with an acrylic ultraviolet curable resin was used as the material of the first diffraction grating. Further, as the material of the second diffraction grating, an ultraviolet curable resin in which ZrO ₂ fine particles were mixed with an acrylic ultraviolet curable resin was used. Further, the refractive index of each material was adjusted so that ΔΦ (λ) shown in Table 3 was obtained. In this comparative example, the grating height d is 11.00 μm, the used diffraction order is + 1st order, the incident angle of incident light with respect to the base surface is 0 deg (perpendicular incidence), and the grating pitch is 100 μm.

  As can be seen from Table 3, ΔΦ (λ) is 0 in the entire wavelength band used, and the diffraction efficiency is 100% in the scalar diffraction theory of Equation (3) (and Equation (6)).

  On the other hand, in this comparative example, the result of having evaluated diffraction efficiency using RCWA is shown in FIG. FIG. 8A shows the wavelength characteristics of the diffraction efficiency in the + 1st order which is the used diffraction order. From FIG. 8A, the diffraction efficiency in consideration of the influence of the grating side surface is 400 nm to 700 nm which is a visible wavelength band, and the long wavelength side is lower than the short wavelength side, and the wavelength characteristics are not preferable. I understand.

  FIG. 8B shows an enlarged view of the lower part of the diffraction efficiency at high diffraction orders at wavelengths of 400 nm, 550 nm and 700 nm. From FIG. 8B, the peak of the diffraction efficiency at the unnecessary diffraction order is higher at the wavelength of 700 nm than at the wavelength of 400 nm, as in Numerical Examples 1 and 2. As described in Numerical Example 1, this is a diffraction phenomenon that occurs on the grating side surface even when incident light is perpendicularly incident on the diffractive optical element (base surface), and the peak of diffraction efficiency is almost the same as the wavelength. It is a ratio. Even at a low diffraction order, the peak of diffraction efficiency is higher at 700 nm than at a wavelength of 400 nm. For this reason, the diffraction efficiency at the used diffraction order is lower on the long wavelength side than on the short wavelength side.

Thus, in the diffractive optical element designed based on the conventional scalar diffraction theory that does not consider the influence of the grating side surface of the diffraction grating, good characteristics cannot be obtained when the grating side surface is considered.
[Numerical example 3]
In Numerical Example 3, as in Numerical Examples 1 and 2, an ultraviolet curable resin in which ITO fine particles are mixed with an acrylic ultraviolet curable resin is used as the material of the first diffraction grating 1. Further, as the material of the second diffraction grating 2, an ultraviolet curable resin in which ZrO ₂ fine particles were mixed with an acrylic ultraviolet curable resin was used. However, these materials are different from those of Numerical Examples 1 and 2. Further, the refractive index of each material was adjusted so that ΔΦ (λ) shown in Table 4 was obtained. ΔΦ (λ) is larger toward the shorter wavelength side in the entire use wavelength band (range of 2/3 or more). In this numerical example, the grating height d is 11.00 μm, the used diffraction order is + 1st order, the incident angle of the incident light with respect to the base surface is 0 deg (perpendicular incidence), and the grating pitch is different from the numerical examples 1 and 2, 200 μm.

  In this numerical example, the results of evaluation of diffraction efficiency using RCWA are shown in FIG. FIG. 5A is a wavelength characteristic of diffraction efficiency at the used diffraction order + 1 order. From FIG. 4A, it can be seen that the diffraction efficiency considering the influence of the grating side surface is almost uniform in the visible wavelength band of 400 to 700 nm. Moreover, it turns out that the wavelength characteristic is improving compared with the comparative example 2 shown later.

  FIG. 5B shows an enlarged view of the lower part of the diffraction efficiency at high diffraction orders at wavelengths of 400 nm, 550 nm and 700 nm. From FIG. 5B, the peak of diffraction efficiency at the unnecessary diffraction orders near −120th to −80th is higher at the wavelength of 700 nm than at the wavelength of 400 nm. Compared with Numerical Examples 1 and 2 and Comparative Example 1 in which the grating pitch is smaller than this numerical example, the peak of diffraction efficiency is lower. This is because the influence of the grating side face on the entire diffraction grating on the diffraction phenomenon is reduced by the large grating pitch in this numerical example. On the other hand, the diffraction efficiency at a relatively low diffraction order (approximately −20th to 0th orders) is higher at a wavelength of 400 nm than at a wavelength of 700 nm. As described above, even if the grating pitch is different, the wavelength characteristics of the diffraction efficiency of the used diffraction order are substantially uniform by balancing the unnecessary diffraction light of the high diffraction order and the low diffraction order.

Table 7 shows the values of Equations (7) and (8) in this numerical example.
[Comparative Example 2]
In order to clarify the characteristics of Example 1 (Numerical Example 3), Comparative Example 2 is shown. This comparative example is a design example using the scalar diffraction theory, and the grating pitch is the same as the numerical example 3. Further, the refractive index of the material of the diffraction grating in this comparative example is the same as that of Comparative Example 1 (Table 3) because the scalar diffraction theory does not depend on the grating pitch as can be seen from Equation (5). The grating height d is 11.00 μm, the used diffraction order is + 1st order, the incident angle of the incident light with respect to the base surface is 0 deg (perpendicular incidence), the incident angle is 0 deg, and the grating pitch is 100 μm.

  In this comparative example, the result of evaluating the diffraction efficiency using RCWA is shown in FIG. FIG. 9A shows the wavelength characteristics of the diffraction efficiency in the + 1st order which is the used diffraction order. From FIG. 9A, for the visible wavelength band of 400 to 700 nm, the diffraction efficiency considering the influence of the grating side surface is lower on the long wavelength side than on the short wavelength side, and the wavelength characteristics are not preferable. I understand.

  FIG. 9B shows an enlarged view of the lower part of the diffraction efficiency at high diffraction orders at wavelengths of 400 nm, 550 nm and 700 nm. From FIG. 9B, the peak of the diffraction efficiency at the unnecessary diffraction order is higher at the wavelength of 700 nm than at the wavelength of 400 nm. Moreover, the peak of diffraction efficiency is low compared with the comparative example 1 with a small grating | lattice pitch. This is because, as described in Numerical Example 3, the influence on the diffraction phenomenon of the grating side surface with respect to the entire diffraction grating is reduced due to the large grating pitch.

In this comparative example, although the grating pitch is different from that of the comparative example 1, as in the comparative example 1, the diffraction efficiency at the used diffraction order is lower on the long wavelength side than on the short wavelength side. As described above, even if the grating pitch is different, the diffractive optical element designed based on the conventional scalar diffraction theory that does not consider the influence of the grating side surface of the diffraction grating has good characteristics when the grating side surface is taken into consideration. I can't.
[Numerical example 4]
In Numerical Example 4, as in Numerical Examples 1 to 3, an ultraviolet curable resin obtained by mixing ITO fine particles with a thioacrylic ultraviolet curable resin was used as the material of the first diffraction grating 1. On the other hand, low melting point glass was used as the material of the second diffraction grating 2. Further, the refractive index of each material was adjusted so that ΔΦ (λ) shown in Table 5 was obtained. ΔΦ (λ) is larger toward the shorter wavelength side in the entire use wavelength band (range of 2/3 or more). Note that the grid height d in this numerical example is 5.00 μm, unlike the numerical examples 1 to 3. The diffraction order used is + 1st order, the incident angle of incident light with respect to the base surface is 0 deg (perpendicular incidence), and the grating pitch is 100 μm.

  In this numerical example, the results of evaluation of diffraction efficiency using RCWA are shown in FIG. FIG. 6A shows the wavelength characteristics of the diffraction efficiency at the used diffraction order + 1 order. From FIG. 6A, it is understood that the diffraction efficiency considering the influence of the grating side surface is almost uniform in the visible wavelength band of 400 to 700 nm. Moreover, it turns out that the wavelength characteristic is improving compared with the comparative example 3 shown later.

  FIG. 6B shows an enlarged view of the lower part of the diffraction efficiency at high diffraction orders at wavelengths of 400 nm, 550 nm and 700 nm. From FIG. 6B, the peak of the diffraction efficiency at the unnecessary diffraction orders in the vicinity of the −90th order to the −70th order is higher at the wavelength of 700 nm than the wavelength of 400 nm, and is almost the same as the wavelength even if the grating height is different. It is a ratio.

Compared to Numerical Examples 1 and 2 and Comparative Example 1 in which the grating height is higher than this numerical example, the peak of diffraction efficiency is low. This is because the diffraction phenomenon on the side surface of the grating is reduced due to the low grating height in this numerical example. The diffraction efficiency at a relatively low diffraction order (approximately −10th to 0th orders) is higher at a wavelength of 400 nm than at a wavelength of 700 nm. As described above, even if the grating heights are different, the wavelength characteristics of the diffraction efficiency of the used diffraction orders are substantially uniform by balancing the unnecessary diffraction light of the high diffraction order and the low diffraction order.
[Comparative Example 3]
In order to clarify the characteristics of Example 1 (Numerical Example 4), Comparative Example 3 is shown. This comparative example is a design example using the scalar diffraction theory, and the grating height is the same as the numerical example 4. As in Numerical Example 4, an ultraviolet curable resin obtained by mixing ITO fine particles with a thioacrylic ultraviolet curable resin was used as the first diffraction grating material, and low-melting glass was used as the second diffraction grating material.

  Further, the refractive index of each material was adjusted so that ΔΦ (λ) shown in Table 6 was obtained. In this comparative example, the grating height d is 5.00 μm, the used diffraction order is + 1st order, the incident angle of incident light with respect to the base surface is 0 deg (normal incidence), and the grating pitch is 100 μm.

  As can be seen from Table 6, ΔΦ (λ) is 0 in the entire wavelength band used, and the diffraction efficiency is 100% in the scalar diffraction theory of Equation (3) (and Equation (6)).

  In this comparative example, the result of evaluating the diffraction efficiency using RCWA is shown in FIG. FIG. 10A shows the wavelength characteristics of the diffraction efficiency in the + 1st order which is the used diffraction order. From FIG. 8A, the diffraction efficiency in consideration of the influence of the grating side surface is 400 nm to 700 nm which is a visible wavelength band, and the long wavelength side is lower than the short wavelength side, and the wavelength characteristics are not preferable. I understand.

  FIG. 10B shows an enlarged view of the lower part of the diffraction efficiency at high diffraction orders at wavelengths of 400 nm, 550 nm and 700 nm. From FIG. 10B, the peak of the diffraction efficiency at the unnecessary diffraction order is higher at the wavelength of 700 nm than at the wavelength of 400 nm. This is a diffraction phenomenon that occurs on the side surface of the grating even when the grating height is different, as in Comparative Example 1, even when incident light is perpendicularly incident on the diffractive optical element (base surface). The peak is almost the same as the wavelength. Even at a low diffraction order, the peak of diffraction efficiency is higher at 700 nm than at a wavelength of 400 nm. For this reason, the diffraction efficiency at the used diffraction order is lower on the long wavelength side.

  As described above, even when the grating height is different, the diffractive optical element designed based on the conventional scalar diffraction theory that does not consider the influence of the grating side surface of the diffraction grating has good characteristics when the grating side surface is taken into consideration. I can't get it.

  Table 7 summarizes the values of ΔΦ (λ) and the values of equations (7) and (8) in Numerical Examples 1 to 4 described above. In equations (7) and (8), λν1 is 400 nm and λν2 is 700 nm.

  From Table 7, it can be seen that all of Numerical Examples 1 to 4 satisfy the relationships represented by the equations (7) and (8). In Comparative Examples 1 to 3, since ΔΦ (λ) is 0, it is clear that the relations expressed by the equations (7) and (8) are not satisfied. Therefore, by satisfying the relations of the expressions (7) and (8), it is possible to obtain a diffractive optical element in which the wavelength characteristic of the diffraction efficiency at the used diffraction order including the influence of the grating side surface is uniform.

  FIG. 7 shows the configuration of a photographing optical system that is Embodiment 2 of the present invention. This photographing optical system is used for an imaging device such as a digital still camera or a video camera, or an interchangeable lens.

  Reference numeral 101 denotes a photographing optical system (photographing lens), which includes an aperture 40 and the diffractive optical element 10 described in the first embodiment. The diffractive optical element 10 is provided on a bonding surface of lenses (substrate lenses) 20 and 30 constituting a lens unit closest to the object side in the photographing optical system 101.

  Reference numeral 102 denotes an image pickup apparatus (optical device), and reference numeral 41 denotes an image pickup element constituted by a CCD sensor or the like. The imaging optical system 101 forms a subject image on the imaging surface of the imaging element 41. The diffractive optical element 10 has a role of correcting chromatic aberration of the photographing optical system 101. The image sensor 41 photoelectrically converts the subject image and outputs an imaging signal. An image processing circuit 103 in the imaging apparatus 102 performs various processes on the imaging signal to generate an image, which is output for display on a monitor (not shown) or stored in a storage medium (not shown). To do.

  Since the diffractive optical element 10 of Example 1 has good wavelength characteristics and can provide high diffraction efficiency, a high-performance photographic optical system with less flare can be realized.

  When the diffractive optical element 10 is used in the photographic optical system, the arrangement position of the diffractive optical element 10 is not limited to the position shown in FIG. 7, and a plurality of diffractive optical elements may be provided in the photographic optical system. Further, in this embodiment, the photographic optical system has been described as an example of an optical system including a diffractive optical element. However, as an optical system of various optical devices used in a wide wavelength region such as an image scanner or a reading optical system of a copying machine. Can be used.

  Each embodiment described above is only a representative example, and various modifications and changes can be made to each embodiment in carrying out the present invention.

A diffractive optical element having good wavelength characteristics and high diffraction efficiency can be provided, whereby a high-performance optical system and optical apparatus can be provided.

10 diffractive optical element 1 first diffraction grating 2 second diffraction grating

Claims (10)

A diffractive optical element formed by laminating a plurality of diffraction gratings formed of different materials,
Each of the materials of the plurality of diffraction gratings is a material in which ΔΦ (λ) shown below becomes larger toward the shorter wavelength side in a range of 2/3 or more of the wavelength band used for the diffractive optical element. Diffractive optical element.

Here, λ is any wavelength included in the use wavelength band, ni is a refractive index with respect to the wavelength λ of the material of the i-th diffraction grating among the plurality of diffraction gratings, and di is the i-th diffraction Is the grating height of the grating, and m is the diffraction order used.   2. The diffractive optical element according to claim 1, wherein each of the materials of the plurality of diffraction gratings is a material in which ΔΦ (λ) increases toward the shorter wavelength side in the entire use wavelength band. The diffractive optical element is configured by laminating a first diffraction grating and a second diffraction grating formed of different materials,
The diffractive optical element according to claim 1 or 2, wherein the grating surfaces of the first and second diffraction gratings are in close contact with each other.   The diffractive optical element according to claim 1, wherein the used wavelength band is a visible wavelength band. The diffractive optical element according to claim 1, wherein both of the following two relationships are satisfied.

However, Ramudanyu1 and λν2 are both wavelengths in the visible wavelength range (nm), λ ₅₅₀ is the wavelength 550 nm, E-n is × ^{10 -n.} The diffractive optical element according to claim 1, wherein the following relationship is satisfied.

However, Ramudanyu is the wavelength of the visible wavelength range (nm), λ ₅₅₀ is the wavelength 550 nm,
^En is ^x10 -n.   7. The diffractive optical element according to claim 1, wherein the diffractive optical element used has a + 1st order or −1st order diffraction order.   The diffractive optical element according to claim 1, wherein a grating pitch of the diffractive optical element is 80 μm or more.   An optical system using the diffractive optical element according to claim 1.   An optical apparatus using the optical system according to claim 9.