Diffraction optical element, optical system, imaging apparatus, and display unit

Abstract

To provide a diffraction optical element that has high optical performance despite its ease of manufacture.SOLUTION: A diffraction optical element (1) comprises a first diffraction grating (8) formed of a first material, a second diffraction grating (9) formed of a second material, and thin-film layers (10a, 10b). The first diffraction grating and the second diffraction grating are closely adhered to each other with the thin-film layers therebetween. The diffraction optical element has a plurality of orbicular zone areas different from each other in the arrangement pitch of orbicular zones in a radial direction. When the minimum value of the arrangement pitch is defined as P (μm), and for at least one orbicular zone of the plurality of orbicular zones, when the refractive index in the design wavelength of the first diffraction grating and the refractive index in the design wavelength of the second diffraction grating are defined as N1 and N2, respectively, the refractive index in the design wavelength of the thin-film layers as Nf, and the maximum value of the thickness of the thin-film layers as df (nm), conditional expressions of 0.4<|N1-N2|×P<6.0 and 0≤|N1+N2-2×Nf|×df<40 are satisfied.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a diffractive optical element, an optical system, an imaging device, and a display device. [Background technology]

[0002] Patent Document 1 discloses a diffractive optical element having high diffraction efficiency over the entire visible range by laminating diffraction gratings made of two different materials. Patent Document 2 discloses a diffractive optical element with improved diffraction efficiency by closely adhering a diffraction grating made of a different material to a diffraction grating made of an injection molding material. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2009-217139 A [Patent Document 2] International Publication No. 2010 / 032347 Summary of the Invention [Problem to be solved by the invention]

[0004] The diffractive optical element disclosed in Patent Document 1 is difficult to manufacture because it requires many molding operations to form the diffraction grating on a substrate using a mold. In the diffractive optical element disclosed in Patent Document 2, it is difficult to obtain sufficient diffraction efficiency due to melt penetration between the diffraction gratings.

[0005] SUMMARY OF THE PRESENT EMBODIMENTS An object of the present invention is to provide a diffractive optical element that is easy to manufacture and yet has high optical performance. [Means for solving the problem]

[0006] A diffractive optical element according to one aspect of the present invention is a diffractive optical element including a first diffraction grating made of a first material, a second diffraction grating made of a second material, and a thin film layer, the first diffraction grating and the second diffraction grating being in close contact with each other via the thin film layer, the diffractive optical element having a plurality of ring zone regions having different arrangement pitches of the ring zones in a radial direction, the minimum value of the arrangement pitch being P (μm), and with respect to at least one of the plurality of ring zone regions, the refractive indices of the first diffraction grating and the second diffraction grating at a design wavelength are N1 and N2, respectively, the refractive index of the thin film layer at a design wavelength is Nf, and the maximum thickness of the thin film layer is df (nm), 0.4<|N1-N2|×P<6.0 0≦|N1+N2-2×Nf|×df<40 The following condition is satisfied.

[0007] Other objects and features of the present invention are illustrated in the following examples. Effect of the Invention

[0008] According to the present invention, it is possible to provide a diffractive optical element that is easy to manufacture and has high optical performance. [Brief description of the drawings]

[0009] [Figure 1] 1A and 1B are a front view and a side view of a diffractive optical element in Examples 1 to 5. [Diagram 2] 1 is a partial cross-sectional view of a diffractive optical element in Example 1. [Diagram 3] 4 is an explanatory diagram of the relationship between the phase difference and the diffraction efficiency of the diffractive optical element in the first embodiment. FIG. [Figure 4] FIG. 4 is a diagram showing the diffraction efficiency and reflectance in Example 1. [Diagram 5] 3 is a partial cross-sectional view of the diffractive optical elements in Examples 2 to 5. FIG. [Figure 6] FIG. 11 is a diagram showing the diffraction efficiency in the second embodiment. [Figure 7] FIG. 11 is a diagram showing the diffraction efficiency and reflectance in Example 3. [Figure 8] FIG. 13 is a diagram showing the diffraction efficiency and reflectance in Example 4. [Figure 9] FIG. 13 is a diagram showing the diffraction efficiency and reflectance in Example 5. [Figure 10] 1 is a diagram illustrating the configuration of an optical system equipped with a diffractive optical element in each embodiment. [Figure 11] 1 is a schematic diagram of an imaging device including a diffractive optical element in each embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. EXAMPLES

[0011] First, a diffractive optical element 1 in a first embodiment of the present invention will be described with reference to Figs. 1(a), (b) and 2. Fig. 1(a) is a front view of the diffractive optical element 1. Fig. 1(b) is a side view of the diffractive optical element 1. Fig. 2 is a partial cross-sectional view of the diffractive optical element 1 taken along line A-A' in Fig. 1. However, Fig. 2 is a view deformed in the grating depth direction.

[0012] The diffractive optical element 1 has a sufficient thickness on the optical axis O and a second element portion 3 having optical power and a thin first element portion 2 arranged in close contact with each other, and a diffraction grating is formed between the first element portion 2 and the second element portion 3. The diffractive optical element 1 has a first diffraction grating 8 made of a first material, a second diffraction grating 9 made of a second material different from the first material, and dielectric thin films (thin film layers) 10a and 10b. The first diffraction grating 8 and the second diffraction grating 9 are layered in close contact with each other via the dielectric thin films 10a and 10b.

[0013] As shown in FIG. 2, the first element portion 2 has a first grating formation layer consisting of a grating base portion 6 and a first diffraction grating 8 integrally formed with the grating base portion 6. The second element portion 3 has a second grating formation layer consisting of a grating base portion 7 and a second diffraction grating 9 integrally formed with the grating base portion 7, similar to the first element portion 2. The first diffraction grating 8 and the second diffraction grating 9 are laminated in close contact with each other with a dielectric thin film 10a between the grating slope 8a of the first diffraction grating 8 and the grating slope 9a of the second diffraction grating 9, and with a dielectric thin film layer 10b between the grating wall surface 8b of the first diffraction grating 8 and the grating wall surface 9b of the second diffraction grating 9. In this embodiment, the first element portion 2 and the second element portion as a whole act as one diffractive optical element 1.

[0014] The first diffraction grating 8 and the second diffraction grating 9 each have a concentric grating shape, and have a lens effect due to the change in grating pitch in the radial direction. That is, the diffractive optical element 1 has a plurality of annular regions with different grating pitches in the radial direction. In this embodiment, the wavelength region of the light incident on the diffractive optical element 1, i.e., the wavelength region used, is the visible region, and the materials and grating thicknesses constituting the first diffraction grating 8 and the second diffraction grating 9 are selected so as to increase the diffraction efficiency of the first-order diffracted light over the entire visible region.

[0015] Next, a specific configuration of the diffractive optical element 1 will be described. In the diffractive optical element 1 of this embodiment, the first material forming the first diffraction grating 8 is an episulfide-based resin (Nd=1.6630, N55=1.6668, νd=36.8, θgF=0.583). The second material forming the second diffraction grating 9 is a polycarbonate-based thermoplastic resin (Nd=1.5880, N55=1.5924, νd=28.3, θgF=0.619). The dielectric thin films 10a and 10b are thin films of inorganic oxide of SiO2 (Nd=1.468, N55=1.470). The thickness of the dielectric thin film 10a on the grating slope is 40 nm, and the thickness of the dielectric thin film 10b on the grating wall surface is 10 nm.

[0016] In this embodiment, the Abbe number vd and the partial dispersion ratio θgF based on the d-line are defined as generally used. When the refractive indices of the Fraunhofer lines for the g-line, F-line, d-line, and C-line are Ng, NF, Nd, and NC, respectively, the Abbe number vd and the partial dispersion ratio θgF are expressed by the following formulas (a) and (b), respectively.

[0017] νd=(Nd-1) / (NF-NC) (a) θgF=(Ng-NF) / (NF-NC) ···(b) Here, N55 is the refractive index at a wavelength of 550 nm. The thickness of the first element portion (lens portion) 2 on the optical axis O is 3.5 mm, the outer diameter is 40 mm, the central radius of curvature of the interface with the second element portion (lens portion) is -115.1 mm, and the central radius of curvature of the lens surface facing the air is -40.1 mm. The thickness of the second element portion (lens portion) 3 on the optical axis O is 0.15 mm, the outer diameter is 38 mm, and the central radius of curvature of both the interface with the first element portion (lens portion) and the lens surface facing the air is -40.1 mm.

[0018] The diffraction pitch P of the diffractive optical element 1 is 45.6 to 1120 μm, the diffractive surface at the optical axis center has a positive refractive power, and the focal length is 1070 mm. The grating height d1 of the second diffraction grating 9 is 7.36 to 7.46 μm. When the angle between the grating wall surface and the surface normal of the envelope surface at the position where any grating wall surface comes into contact with the envelope surface connecting the apexes of the second diffraction grating 9 is θt, the wall surface angle θt in the diffractive optical element 1 of this embodiment is 4.0 to 8.9 degrees.

[0019] Next, the relationship between the phase difference and the diffraction efficiency of the diffractive optical element 1 of this embodiment will be described with reference to Fig. 3. Fig. 3 is an explanatory diagram of the relationship between the phase difference and the diffraction efficiency of the diffractive optical element 1, and shows a schematic diagram of a state in which the thickness of the dielectric thin film 10 in the diffractive optical element 1 is eliminated and the angle between the grating wall surface and the envelope surface connecting the grating vertices is a right angle.

[0020] In the diffractive optical element 1, in the case of a wavelength λ, the condition for maximizing the diffraction efficiency of diffracted light of diffraction order m is that the optical path length difference Φ(λ) satisfies the following formula (c).

[0021] Φ(λ)=―(n02-n01)×d1=mλ (c) Here, in formula (c), n02 is the refractive index for light of wavelength λ of the material forming second diffraction grating 9, and similarly, n01 is the refractive index for light of wavelength λ of the material forming first diffraction grating 8. d1 is the grating height (grating thickness) of first diffraction grating 8 and second diffraction grating 9.

[0022] The diffraction order of light diffracted downward from the zeroth-order diffracted light in Fig. 3 is defined as a negative diffraction order, and the diffraction order of light diffracted upward from the zeroth-order diffracted light is defined as a positive diffraction order. In this case, in the case of a diffraction grating having a grating shape in which the grating thickness of first diffraction grating 8 on the entrance side increases from bottom to top in Fig. 3 as shown in Fig. 3, the sign of the grating height d1 in formula (c) is positive.

[0023] Moreover, the diffraction efficiency η(λ) at an arbitrary wavelength λ is expressed by the following formula (d).

[0024] η(λ)=sinc 2 [π{m-Φ(λ) / λ}] ···(d) In formula (d), m is the order of the diffracted light to be evaluated, and Φ(λ) is the optical path length difference in one unit cell of the diffractive optical element for light of wavelength λ. Furthermore, sinc(x) is a function expressed as {sin(x) / x}. Furthermore, the design wavelength λd of the diffractive optical element 1 in this embodiment is 587.56 nm. This also applies to the following embodiments. Here, the design wavelength is a value near the average value of the wavelengths used by the diffractive optical element, and specifically, when the average value of the wavelengths used is λave(nm), it is the wavelength range expressed by the following formula (1).

[0025] 0.9<λd / λave<1.1 (1) The diffractive optical element 1 of this embodiment is used in the visible range, with wavelengths of 400 nm to 700 nm and λ ave of 550 nm. In the diffractive optical element 1 shown in Fig. 3, the diffraction efficiency is highest in the visible wavelength range when the grating height d1 = 7.36 µm.

[0026] The grating wall surface 8b of the first diffraction grating 8 does not need to be perpendicular to the envelope curve connecting the vertices of the first diffraction grating 8, and can be angled according to the light incidence angle. As shown in Fig. 2, when the grating wall surface 8b becomes a diffraction grating that is angled with respect to the envelope curve connecting the vertices of the first diffraction grating 8, the distance between the envelope curve connecting the vertices of the first diffraction grating 8 and the grating vertices is d1t.

[0027] In this embodiment, the first diffraction grating 8 and the second diffraction grating 9 are made of different materials. For example, the second diffraction grating 9 is made of a low refractive index, high dispersion material, and the first diffraction grating 8 is made of a high refractive index, low dispersion material having a higher refractive index than the second diffraction grating 9. It is preferable to satisfy the following conditional expression (2) to obtain high diffraction efficiency.

[0028] 1.0<(N1-N2) / (1 / ν2-1 / ν1)<20.0 ···(2) Here, the refractive indices of the materials constituting the first diffraction grating 8 and the second diffraction grating 9 at the d-line are N1 and N2, respectively, and the Abbe numbers of the materials constituting the diffraction gratings 8 and the diffraction gratings 9 based on the d-line are ν1 and ν2.

[0029] More preferably, the numerical range of conditional expression (2) is set as shown in the following conditional expression (2a).

[0030] 1.2<(N1-N2) / (1 / ν2-1 / ν1)<18.0 (2a) More preferably, the numerical range of conditional expression (2) is set as in the following conditional expression (2b).

[0031] 1.5<(N1-N2) / (1 / ν2-1 / ν1)<15.0 (2b) Next, the relationship between the minimum value P of the arrangement pitch and the refractive index of each material in the diffractive optical element 1 of this embodiment, and the meaning of having a thin film layer will be described. In the diffractive optical element 1 of this embodiment, the minimum value P of the arrangement pitch is preferably 80 μm or less. For example, when the diffractive optical element 1 is used in an observation optical system, the entire optical system cannot be made large due to space restrictions, so it is necessary to correct various aberrations with a small number of lenses. In that case, a small optical system can be realized by strengthening the power of the diffractive optical element 1 to correct chromatic aberration. Note that in the peripheral part of the diffractive optical element 1, a configuration in which the minimum value P of the arrangement pitch is smaller than 100 μm is possible. When the minimum value P of the arrangement pitch becomes narrow and the ratio of the grating height to the minimum value P becomes high, the influence of the wavefront disturbance on the wall surface becomes large, and the deterioration of the diffraction efficiency becomes noticeable.

[0032] Preferably, the grating height d (μm) and the minimum value P (μm) of the arrangement pitch satisfy the following conditional expression (3).

[0033] 0.10 <d / P<1.50 ···(3) More preferably, the numerical range of conditional expression (3) is set as in the following conditional expression (3a).

[0034] 0.11 <d / P<1.3 ···(3a) More preferably, the numerical range of conditional expression (3) is set as in the following conditional expression (3b).

[0035] 0.12 <d / P<1.0 ···(3b) When the diffractive optical element 1 is used in the visible range as in this embodiment, the design wavelength λ is often set to be near the d-line (587.56 nm). According to the above formulas (c) and (3), when the refractive index of the first diffraction grating 8 at the design wavelength λ is N1 and the refractive index of the second diffraction element 9 is N2, the following conditional formula (4) is satisfied.

[0036] 0.4<|N1-N2|×P<6.0 (4) If the lower limit of conditional expression (4) is not satisfied, the ratio of the grating height to the minimum arrangement pitch becomes high, which leads to a large deterioration in the diffraction efficiency, whereas if the upper limit of conditional expression (4) is exceeded, the minimum arrangement pitch becomes large, which makes it difficult to obtain the desired chromatic aberration correction effect, or the grating height becomes small, which makes it difficult to select the grating material, which is also undesirable.

[0037] Preferably, the numerical range of conditional expression (4) is set as in the following conditional expression (4a).

[0038] 0.5<|N1-N2|×P<5.0 (4a) More preferably, the numerical range of conditional expression (4) is set as in the following conditional expression (4b).

[0039] 0.6<|N1-N2|×P<4.5 (4b) Preferably, the minimum value P (μm) of the arrangement pitch satisfies the following conditional expression (5):

[0040] 10 <P<80 ···(5) If the lower limit of conditional expression (5) is not reached, the diffraction efficiency will deteriorate due to an increase in the grating height unless the difference between the refractive index of the first diffraction grating 8 and the refractive index of the second diffraction grating 9 is increased, which is not preferable. On the other hand, if the upper limit of conditional expression (5) is exceeded, the optical system will not be able to achieve a sufficient chromatic aberration correction effect, which is also not preferable.

[0041] More preferably, the numerical range of conditional expression (5) is set as in the following conditional expression (5a).

[0042] 12 <P<75 ···(5a) More preferably, the numerical range of conditional expression (5) is set as in the following conditional expression (5b).

[0043] 15 <P<70 ···(5b) The diffractive optical element 1 of this embodiment is intended to have a configuration that can be manufactured at low cost. For this reason, it is preferable to form the second element portion 3 having the thick second diffraction grating 9 by integral molding using a mold. Specifically, if the material for forming the second diffraction grating 9 is a thermoplastic material and the second element portion 3 is formed using an injection molding mold, the lens shape of the second diffraction grating 9 and the second element portion 3 can be obtained with high accuracy.

[0044] After forming the second element portion 3 having a diffractive surface (grating slope 9a), another resin material is applied to the diffractive surface (grating slope 9a) and cured to obtain a diffractive optical element 1 in which the first element portion 2 having the first diffraction grating 8 is closely laminated. In this case, by using an ultraviolet curable resin or the like as the first material for forming the first diffraction grating 8, it becomes easier to obtain a diffractive optical element whose shape after curing becomes a desired grating shape. However, as explained in the conditional expressions (3) and (4), in order to reduce the grating height d of each diffraction grating, it is necessary to increase the refractive index difference |N1-N2| between the two materials. In addition, as explained in the conditional expression (2), it is preferable to use materials that have a certain degree of dispersion difference for the two grating materials.

[0045] In order to obtain a material combination that satisfies the above conditions, for example, a polycarbonate-based resin or a polyester-based resin is used as the injection molding material for forming the second diffraction grating 9, thereby obtaining a resin with a low refractive index and high dispersion. In addition, by selecting an enethiol-based resin material or an episulfide-based resin material as the ultraviolet curing resin for forming the first diffraction grating 8, a resin with a high refractive index and low dispersion can be obtained, thereby obtaining high diffraction efficiency.

[0046] However, as a result of the inventor's investigation, it was found that when an ultraviolet curable resin is applied to a diffraction grating made of a polycarbonate-based resin or a polyester-based resin, migration of organic matter occurs between the resins in some resin combinations, and melting and penetration of the resin occurs. In particular, when an episulfide-based material with a higher refractive index is applied, melting and penetration of the resin into the diffraction grating made of a polycarbonate-based resin or a polyester-based resin occurs frequently.

[0047] Therefore, in the diffractive optical element 1 of this embodiment, a thin film layer (dielectric thin film 10a, 10b) containing an inorganic material is disposed between the second diffraction grating 9 made of a thermoplastic resin material and the first diffraction grating 8 made of an ultraviolet curable resin material. However, when providing the thin film layer between the two diffraction gratings, if the refractive index and film thickness of the thin film layer are not appropriately set, the increase in reflected light at the interface becomes significant, resulting in ghost light or flare light, which is undesirable as it deteriorates the image quality. Therefore, the diffractive optical element 1 of each embodiment satisfies the following conditional formula (6).

[0048] 0≦|N1+N2-2×Nf|×df<40 (6) Here, Nf is the refractive index (average refractive index) of the thin film layer at the design wavelength in at least one of the multiple annular zones, and df is the maximum thickness (maximum total thickness, maximum film thickness) (nm) of the thin film layer on the grating slope 10a. In conditional formula (6), |N1+N2-2×Nf| means the difference between the average value of the refractive index of the first diffraction grating 8 and the refractive index of the second diffraction grating 9 and the average refractive index of the thin film layer. By making the value of |N1+N2-2×Nf| closer to 0, the amount of reflected light on the grating surface can be suppressed. If the upper limit of conditional formula (6) is exceeded, the film thickness increases, the reflectance increases due to the increase in the refractive index difference between the thin film layer and the grating material, or problems such as peeling due to environmental resistance tend to occur, which is not preferable.

[0049] Preferably, the numerical range of conditional expression (6) is set as in the following conditional expression (6a).

[0050] 1<|N1+N2-2×Nf|×df<35 (6a) If the lower limit of conditional formula (6a) is not reached, the film thickness becomes too thin, making it difficult to suppress the melting and penetration of the resin. In addition, it becomes difficult to control the refractive index of the grating material and thin film material, resulting in a high-cost configuration.

[0051] More preferably, the numerical range of conditional expression (6) is set as in the following conditional expression (6b).

[0052] 3<|N1+N2-2×Nf|×df<30 (6b) The thin film layer is made of a material containing an inorganic material, which can effectively suppress the migration of organic components between the first diffraction grating 8 and the second diffraction grating 9. For example, aluminum oxide (Al2O3), silicon oxide (SiO2, SiO), titanium oxide (TiO x ), tantalum oxide (TaO x ), niobium oxide (NbO x ), chromium (Cr), etc. are suitable. The thin film layer does not have to be a layer of a single material, and may be formed of multiple layers having at least one layer containing an inorganic material. Methods for forming the thin film layer include, for example, vacuum deposition and sputter deposition, but it can also be formed by a spin coating method, etc.

[0053] In each embodiment, it is preferable to satisfy the following condition (7):

[0054] 0.001<|N1+N2-2×Nf|<0.600 ···(7) If the lower limit of condition (7) is exceeded, material selection becomes difficult, resulting in a high-cost configuration, which is undesirable, whereas if the upper limit of condition (7) is exceeded, the reflectance at the grating surface interface increases, which is undesirable.

[0055] More preferably, the numerical range of conditional expression (7) is set as in the following conditional expression (7a).

[0056] 0.005<|N1+N2-2×Nf|<0.500 (7a) More preferably, the numerical range of conditional expression (7) is set as in the following conditional expression (7b).

[0057] 0.010<|N1+N2-2×Nf|<0.400 ···(7b) In each embodiment, the maximum thickness df (nm) of the thin film layer preferably satisfies the following conditional expression (8):

[0058] 3 <df<200 ···(8) If the lower limit of condition (8) is not reached, it is difficult to suppress melting and penetration between the resins, which is undesirable, whereas if the upper limit of condition (8) is reached, the film stress increases after the thin film layer is formed, which leads to peeling between the film and the resin and surface deformation, which is undesirable.

[0059] More preferably, the numerical range of conditional expression (8) is set as in the following conditional expression (8a).

[0060] 5 <df<100 ···(8a) More preferably, the numerical range of conditional expression (8) is set as in the following conditional expression (8b).

[0061] 10 <df<90 ···(8b) Fig. 4(a) shows the diffraction efficiency of the annular zones of the diffractive optical element 1 of this embodiment, where the minimum array pitch P is 45.6 μm and the grating height d1 is 7.46 μm. In Fig. 4(a), the horizontal axis shows the wavelength (nm) and the vertical axis shows the diffraction efficiency (%). Fig. 4(b) shows the reflectance at the grating interface of the diffractive optical element 1 of this embodiment. In Fig. 4(b), the horizontal axis shows the wavelength (nm) and the vertical axis shows the reflectance (%).

[0062] The diffraction efficiency shown in Figure 4(a) is a value calculated using rigorous coupled wave analysis (RCWA), which is a rigorous wave calculation. As shown in Figure 4(a), the selection of the grating material is improved by having a thin film layer at the grating interface. In addition, by using a configuration that satisfies conditional formula (4), a high diffraction efficiency of 80% or more can be obtained in a wide wavelength range from 430 nm to 670 nm in the visible range, despite the narrow pitch configuration with the minimum array pitch P = 45.6 μm.

[0063] Furthermore, as shown in Figure 4(b), by appropriately setting the refractive indexes of the thin film layer, the first diffraction grating 8, and the second diffraction grating 9 and the thicknesses of the thin film layers, a diffractive optical element 1 having a low reflectance of 1% or less over a wide wavelength range from 430 nm to 670 nm in the visible range can be obtained.

[0064] As described above, the diffractive optical element 1 of this embodiment is configured to have a thin film layer between the grating surfaces of the first diffraction grating 8 and the second diffraction grating 9. This suppresses melting and penetration of the resin material and makes it easier to select the grating material, making it possible to realize a combination of materials that results in high diffraction efficiency. In addition, by using a thermoplastic resin material for the second diffraction grating 9 and integrally molding the diffraction grating surface, a low-cost diffractive optical element 1 can be obtained.

[0065] In this embodiment, as described above, the second element portion 3 having the second diffraction grating 9 is integrally formed by injection molding using a thermoplastic material, and then the first diffraction grating 8 is formed using an ultraviolet curable resin. In this case, making the thickness of the lens of the element portion 3 made of the injection molding material on the optical axis thicker to a certain extent can suppress deformation of the surface shape when the diffraction grating 8 is molded.

[0066] Regarding the thickness on the optical axis of the lens of the first element portion 2 having the first diffraction grating 8, if the lens is too thin, the shape of the diffraction grating is transferred to the surface during hardening, resulting in a deterioration in diffraction efficiency, which is undesirable. On the other hand, if the lens is too thick, the change in surface shape during hardening becomes large, resulting in an increase in aberration, which is undesirable.

[0067] Preferably, when the thickness on the optical axis of a lens (first lens) made of the same material as first diffraction grating 8 is L1, and the thickness on the optical axis of a lens (second lens) made of the same material as second diffraction grating 9 is L2, the following conditional formula (9) is satisfied.

[0068] 5 <L2 / L1<200 ···(9) More preferably, the numerical range of conditional expression (9) is set as in the following conditional expression (9a).

[0069] 7 <L2 / L1<150 ···(9a) More preferably, the numerical range of conditional expression (9) is set as in the following conditional expression (9b).

[0070] 10 <L2 / L1<100 ···(9b) As described above, in the diffractive optical element 1 of this embodiment, the second diffraction grating 9 made of an injection molding material is a low refractive index, high dispersion material. Conversely, it is theoretically possible to obtain high diffraction efficiency by selecting a high refractive index, low dispersion material as the material for forming the second diffraction grating 9 and using a low refractive index, high dispersion material as the material for forming the first diffraction grating 8. However, there are few options for high refractive index materials that are low dispersion injection molding materials, and there are also few options for low refractive index, high dispersion resins as ultraviolet curing resins, which results in high costs and is not preferable.

[0071] Therefore, it is preferable that the refractive index N2 of the second diffraction grating 9 made of an injection molding material is lower than the refractive index N1 of the first diffraction grating 8 made of an ultraviolet curing resin. It is preferable that the following conditional expression (10) is satisfied.

[0072] 0.02 <N1-N2<0.15 ···(10) If the lower limit of conditional expression (10) is exceeded, the grating height becomes large, which reduces the diffraction efficiency, which is not preferable, whereas if the upper limit of conditional expression (10) is exceeded, it becomes necessary to greatly separate the dispersions of the two materials, which makes it difficult to select the materials, resulting in a high-cost configuration, or makes it difficult to obtain high diffraction efficiency over the entire visible range, which is also not preferable.

[0073] More preferably, the numerical range of conditional expression (10) is set as in the following conditional expression (10a).

[0074] 0.02 <N1-N2<0.13 ···(10a) More preferably, the numerical range of condition (10) is set as in the following condition (10b).

[0075] 0.03 <N1-N2<0.10 ···(10b) In the diffractive optical element 1 of this embodiment, when a thin film layer is formed at the grating interface, it is preferable to form the thin film layer on both the grating slope and the grating wall surface, since melting and penetration between the two grating resins can be suppressed. However, in the case of the diffractive optical element 1 having a region with a narrow diffraction pitch as in this embodiment, if the thickness of the thin film layer on the wall surface is increased, the ratio of the slope portion that is the effective surface decreases, and the diffraction efficiency deteriorates. Therefore, it is preferable to make the thickness of the thin film layer on the grating wall surface thinner than the thickness of the thin film layer on the grating slope.

[0076] Furthermore, when forming a thin film layer, for example by deposition, it is preferable that the angle of the grating wall surface of the second diffraction grating 9 is at a certain angle with respect to the optical axis of the lens of the first element portion 2. If the angle of the wall surface approaches parallel to the optical axis, a film is not formed on the wall surface during deposition, and melting and penetration between the two resins occurs on the wall surface.

[0077] Specifically, when the angle of the wall surface portion of the second diffraction grating 9 with respect to the optical axis of the lens (first element portion 2) made of the same material as the first material of the first diffraction grating 8 is Ah (deg), it is preferable that the following conditional expression (11) is satisfied within the effective area. Here, the effective area is the area (effective diameter) through which effective light rays that contribute to image formation pass on the optical surface.

[0078] 2 <Ah<50 ···(11) More preferably, the numerical range of conditional expression (11) is set as in the following conditional expression (11a).

[0079] 2 <Ah<40 ···(11a) More preferably, the numerical range of condition (11) is set as in the following condition (11b).

[0080] 3 <Ah<30 ···(11b) EXAMPLES

[0081] Next, a description will be given of Example 2 of the present invention. In the diffractive optical element of Example 1, the thickness of the dielectric thin film (thin film layer) 10a on the grating slope is uniform, but in this example, the thickness of the dielectric thin film is not uniform.

[0082] In the diffractive optical element of this embodiment, the thin film layer is configured to include an inorganic material, and as described above, a film is formed on the grating surface of the second diffraction grating 9 made of an injection molding material by various deposition methods, spin coating methods, etc. Ideally, from the viewpoint of diffraction efficiency, it is preferable to form a film having the same shape as the grating surface 9a of the second diffraction grating 9 and a uniform thickness in the radial direction. However, in order to form a film with a uniform thickness, the deposition must be performed by planetary rotation deposition or mask deposition, and the film thickness distribution must be controlled with high precision. Even in the spin coating method, it is necessary to perform surface treatment of the second diffraction grating 9, which is the base material, and to strictly adjust the viscosity of the film material, so that both methods are high-cost manufacturing methods. Therefore, in the diffractive optical element of this embodiment, in order to obtain high-performance diffraction efficiency even with a low-cost manufacturing method, the thin film layer is configured to have little deterioration in diffraction efficiency even if the film thickness changes at the valley part (near the valley part) of the first diffraction grating 2. The diffractive optical element of this embodiment has the configuration shown in FIG. 1, and the lens shape and material of the first element part 2 and the second element part 3 are the same as those of Example 1.

[0083] Fig. 5 is a cross-sectional view of the diffractive optical element 1 of this embodiment taken along line A-A' in Fig. 1. However, Fig. 5 is a view deformed in the grating depth direction. As shown in Fig. 5, dielectric thin films (thin film layers) 10a, 10b are formed at the interface between the first diffraction grating 8 and the second diffraction grating 9 as in the first embodiment, but the thickness of the thin film layer 10a on the slanting surface of the grating is changed near the valley of the second diffraction grating 9.

[0084] In addition, the amount of change in film thickness is appropriately controlled to provide a diffraction grating that is both easy to manufacture and has high diffraction efficiency. More specifically, the reference thickness df of the thin film layer at the grating slope is 40 nm, and in a region with a width w of 4 μm from the valley of the second diffraction grating 9, the thickness of the thin film layer 10a gradually decreases toward the valley of the second diffraction grating 9. The minimum thickness (minimum film thickness) dfmn of the thin film layer 10a at the valley of the second diffraction grating 9 is 20 nm.

[0085] Preferably, when the minimum thickness of the thin film layer on the lattice slope is dfmn and the maximum film thickness (maximum total film thickness) is df, the following conditional formula (12) is satisfied:

[0086] 0.10 <dfmn / df<0.95 ···(12) If the lower limit of condition (12) is exceeded, the phase difference in the pitch direction due to the change in the film thickness of the thin film layer cannot be ignored, and the diffraction efficiency deteriorates, which is not preferable, whereas if the upper limit of condition (12) is exceeded, the cost of the configuration becomes high, which is not preferable.

[0087] More preferably, the numerical range of conditional expression (12) is set as in the following conditional expression (12a).

[0088] 0.15 <dfmn / df<0.93 ···(12a) More preferably, the numerical range of conditional expression (12) is set as in the following conditional expression (12b).

[0089] 0.20 <dfmn / df<0.90 ···(12b) In addition, when the thickness of the thin film layer varies, it is preferable to satisfy the following conditional expression (13), where dfs is the difference between the minimum thickness (minimum film thickness) dfmn and the maximum thickness (maximum film thickness) df of the thin film layer. This makes it possible to minimize the deterioration of the diffraction efficiency due to unevenness in the film thickness.

[0090] 0.2<|dfs×(N1+N2-2×Nf)|<30.0 ···(13) More preferably, condition (13) is set as in the following condition (13a):

[0091] 0.5<|dfs×(N1+N2-2×Nf)|<20.0 ···(13a) More preferably, condition (13) is set as in the following condition (13b).

[0092] 1.0<|dfs×(N1+N2-2×Nf)|<15.0 ···(13b) In addition, when the width in the pitch direction in which the film thickness changes on the grating inclined surface of the thin film layer is w (μm) and the grating height is d (μm), it is preferable that the following conditional formula (14) be satisfied in the annular zone having the smallest minimum value P (μm) of the arrangement pitch.

[0093] 0.002 <w / (P×d)<0.100 ···(14) Exceeding the upper limit of condition (14) is undesirable because it increases the deterioration of diffraction efficiency due to unevenness in the thickness of the thin film layer, while falling below the lower limit of condition (14) is undesirable because it requires the use of a costly manufacturing method to suppress the change in thickness.

[0094] More preferably, condition (14) is set as in the following condition (14a):

[0095] 0.003 <w / (P×d)<0.090 ···(14a) More preferably, condition (14) is set as in the following condition (14b):

[0096] 0.005 <w / (P×d)<0.080 ···(14b) FIG. 6 shows the diffraction efficiency calculated by RCWA in the annular zone with the minimum arrangement pitch P=45.6 μm and the grating height d1=7.46 μm of the diffractive optical element of this embodiment. In FIG. 6, the horizontal axis shows the wavelength (nm) and the vertical axis shows the diffraction efficiency (%). As shown in FIG. 6, by appropriately controlling the relationship between the refractive index of the thin film layer and the grating material, and the shape of the region where the film thickness of the thin film layer changes, a high diffraction efficiency of 80% or more is obtained in a wide wavelength range from 430 nm to 670 nm in the visible range. As can be seen by comparing the diffraction efficiencies in FIG. 6 and FIG. 4(a), by adopting a configuration that satisfies the conditional expressions (12), (13), and (14), the change in diffraction efficiency can be suppressed even if the film thickness of the thin film layer changes. EXAMPLES

[0097] Next, a third embodiment of the present invention will be described. In the diffractive optical elements of the first and second embodiments, the thin film layer 10a on the grating slope is made of a single material. On the other hand, in the diffractive optical element of this embodiment, the thin film layer is made of a plurality of materials.

[0098] The diffractive optical element of this embodiment has the configuration shown in Fig. 1, similar to that of embodiment 2. In this embodiment, the lens shapes of the first element portion 2 and the second element portion 3 are the same as those of embodiment 1, but the materials forming the lenses and the grating shapes are changed.

[0099] FIG. 5 is a cross-sectional view of the diffractive optical element of this embodiment taken along line A-A' in FIG. 1. However, FIG. 5 is a view deformed in the grating depth direction. As shown in FIG. 5, similar to the first embodiment, dielectric thin films (thin film layers) 10a and 10b are formed at the interface between the first diffraction grating 8 and the second diffraction grating 9, but the thickness of the thin film layer 10a on the grating slope is changed near the valley of the diffraction grating 9. Furthermore, the amount of film thickness change is appropriately controlled to achieve both ease of manufacture and high diffraction efficiency. In the diffractive optical element of this embodiment, the thin film layer 10a on the grating slope and the thin film layer 10b on the grating wall are each a three-layered thin film layer using three layers of material.

[0100] 5, the first material forming the first diffraction grating 8 is an episulfide resin (Nd=1.6630, N55=1.6668, νd=36.8, θgF=0.583). The second material forming the second diffraction grating 9 is a polyester thermoplastic resin (Nd=1.6079, N55=1.6126, νd=26.9, θgF=0.624).

[0101] The dielectric thin film 10 on the grating slope and grating wall surface has a three-layer structure consisting of a thin film layer made of SiO2, a thin film layer made of a mixed material of Ta2O5 and TiO2, and a thin film layer made of SiO2, in that order from the first diffraction grating 8 to the second diffraction grating 9. The thicknesses of the dielectric thin film 10a on the grating slope are 26.4 nm, 10 nm, and 25.4 nm from the first diffraction grating 8 to the second diffraction grating 9, respectively, and the total film thickness df is 61.8 nm. The refractive index of SiO2 is the same as in Example 1, and the refractive indexes of the inorganic oxide thin film layer made of a mixed material of Ta2O5 and TiO2 are Nd=2.1464 and N55=2.158. The average refractive index of the three thin film layers is Nd=1.5776. The total thickness of the dielectric thin film 10b on the grating wall surface is 10 nm. The grating height d1 of the second diffraction grating 9 is 10.18 to 10.43 μm. In addition, when the angle between any grating wall surface and the surface normal of the envelope surface at the position where the grating wall surface touches the envelope surface connecting the apexes of the second diffraction grating 9 is θt, the wall surface angle θt in the diffractive optical element of this embodiment is 4.0 to 8.9 degrees.

[0102] The thin film layer on the slope has a thickness change around the valley of the second diffraction grating 9. Specifically, the reference total thickness df of the thin film layer on the grating slope is 61.8 nm. In a region with a width w of 6 μm from the valley of the second diffraction grating 9, the thickness of the thin film layer 10a gradually decreases toward the valley of the second diffraction grating 9. The minimum thickness dfmn of the thin film layer 10a at the valley of the second diffraction grating 9 is 18.5 nm.

[0103] FIG. 7(a) shows the diffraction efficiency calculated by RCWA in the annular zone with the minimum arrangement pitch P=45.6 μm and the grating height d1=10.43 μm of the diffractive optical element of this embodiment. In FIG. 7(a), the horizontal axis shows the wavelength (nm) and the vertical axis shows the diffraction efficiency (%). FIG. 7(b) shows the reflectance at the grating interface of the diffractive optical element of this embodiment. In FIG. 7(b), the horizontal axis shows the wavelength (nm) and the vertical axis shows the reflectance (%). As shown in FIG. 7(a), the selection of the grating material is improved by having a thin film layer at the grating interface. In addition, by making the configuration satisfying the conditional formula (4), a high diffraction efficiency of 90% or more can be obtained in a wide wavelength range from 430 nm to 670 nm in the visible range, despite the narrow pitch configuration with the minimum arrangement pitch P=45.6 μm. 7(b), the refractive indexes of the thin film layer, the first diffraction grating 8, and the second diffraction grating 9, and the thicknesses of the thin film layer are appropriately set, and the thin film layer has a laminated structure, thereby obtaining a diffractive optical element having a low reflectance of 0.1% or less in a wide wavelength range from 430 nm to 670 nm in the visible range.

[0104] In the diffractive optical element of this embodiment, a thin film layer is provided between the grating surfaces of the first diffraction grating 8 and the second diffraction grating 9, which suppresses melting and penetration of the resin material and facilitates the selection of the grating material, thereby enabling a combination of materials that results in high diffraction efficiency. In addition, by using a thermoplastic resin material for the second diffraction grating 9 and integrally molding the diffraction grating surface, a low-cost diffractive optical element can be obtained. EXAMPLES

[0105] Next, a fourth embodiment of the present invention will be described. The diffractive optical element of this embodiment has the configuration shown in Fig. 1, similar to the second embodiment, except that the lens shapes of the first element portion 2 and the second element portion 3, and the materials forming the lenses and the lattice shape are changed.

[0106] 5 is a cross-sectional view of the diffractive optical element of this embodiment taken along line A-A' in FIG. 1. However, FIG. 5 is a deformed view in the grating depth direction. As shown in FIG. 5, a thin film layer is formed at the interface between the first diffraction grating 8 and the second diffraction grating 9 as in the first embodiment, but the thickness of the dielectric thin film (thin film layer) 10a on the grating slope is changed near the valley of the second diffraction grating 9. In addition, the amount of film thickness change is appropriately controlled to achieve both ease of manufacture and high diffraction efficiency.

[0107] 5, the first material forming the first diffraction grating 8 is an episulfide-based resin (Nd=1.6630, N55=1.6668, νd=36.8, θgF=0.583), while the second material forming the second diffraction grating 9 is a polycarbonate-based thermoplastic resin (Nd=1.6160, N55=1.6210, νd=25.8, θgF=0.623).

[0108] The thickness of the first element portion (lens portion) 2 on the optical axis O is 3.5 mm, the outer diameter is 40 mm, the central radius of curvature of the interface with the second element portion (lens portion) 3 is -133.9 mm, and the central radius of curvature of the lens surface facing the air is -41.5 mm. The thickness of the second element portion (lens portion) 3 on the optical axis O is 0.07 mm, the outer diameter is 37 mm, and the central radius of curvature of the interface with the first element portion (lens portion) 2 and the lens surface facing the air are both -41.5 mm. The minimum value P of the arrangement pitch of the diffractive optical element 1 is 19.4 to 820 μm, the diffractive surface at the center of the optical axis has positive refractive power, and the focal length is 571 mm. The grating height d1 of the diffraction grating 9 is 12.25 to 13.64 μm. In addition, when the angle between any grating wall surface and the surface normal of the envelope surface at the position where the grating wall surface is in contact with the envelope surface connecting the apexes of the second diffraction grating 3 is θt, the wall surface angle θt in the diffractive optical element of this embodiment is 4.0 to 16.5 degrees.

[0109] The dielectric thin films 10a and 10b on the grating slope and grating wall are made of inorganic oxide Al2O3 (Nd=1.5888, N55=1.5906). The maximum thickness of the dielectric thin film 10a on the grating slope is 80 nm, and the maximum thickness (total thickness) of the dielectric thin film 10b on the grating wall is 10 nm. The thickness of the dielectric thin film 10a on the grating slope changes around the valley of the second diffraction grating 9. Specifically, the reference thickness df of the dielectric thin film 10a on the grating slope is 80 nm, and the thickness of the dielectric thin film 10a gradually decreases from the valley of the second diffraction grating 9 to the region with a width w of 13 μm. The minimum thickness dfmn of the dielectric thin film 10a at the valley of the second diffraction grating 9 is 56 nm.

[0110] FIG. 8(a) shows the diffraction efficiency calculated by RCWA in the annular zone with the minimum arrangement pitch P=19.4 μm and the grating height d1=13.53 μm of the diffractive optical element of this embodiment. In FIG. 8(a), the horizontal axis shows the wavelength (nm) and the vertical axis shows the diffraction efficiency (%). FIG. 8(b) shows the reflectance at the grating interface of the diffractive optical element of this embodiment. In FIG. 8(b), the horizontal axis shows the wavelength (nm) and the vertical axis shows the reflectance (%). As shown in FIG. 8(a), the selection of the grating material is improved by having a thin film layer at the grating interface. In addition, by satisfying the conditional formula (4), a high diffraction efficiency of 85% or more can be obtained in a wide wavelength range from 430 nm to 670 nm in the visible range, despite the narrow pitch configuration with the minimum arrangement pitch P=19.4 μm. Furthermore, as shown in Figure 8(b), by appropriately setting the refractive indices of the thin film layer, first diffraction grating 8, and second diffraction grating 9, and the thickness of the thin film layer, a diffractive optical element having a low reflectance of 0.5% or less over a wide wavelength range from 430 nm to 670 nm in the visible range can be obtained.

[0111] According to this embodiment, by providing a thin film layer between the grating surfaces of the first diffraction grating 8 and the second diffraction grating 9, melting and penetration of the resin material is suppressed and the grating material can be easily selected, making it possible to realize a combination of materials that results in high diffraction efficiency. In addition, by using a thermoplastic resin material for the second diffraction grating 9 and integrally molding the diffraction grating surface, a low-cost diffractive optical element can be obtained. EXAMPLES

[0112] Next, a fifth embodiment of the present invention will be described. The diffractive optical element of this embodiment has the configuration shown in Fig. 1, similar to the fourth embodiment. The lens shape of the second element portion 3 is the same as that of the fourth embodiment, and the thickness on the optical axis of the first element portion 2, and the material and lattice shape forming each lens are changed. The thickness on the optical axis of the first element portion 2 is 0.2 mm.

[0113] 5 is a cross-sectional view of the diffractive optical element of this embodiment taken along line A-A' in FIG. 1. However, FIG. 5 is a deformed view in the grating depth direction. As shown in FIG. 5, similar to the first embodiment, dielectric thin films (thin film layers) 10a and 10b are formed at the interface between the first diffraction grating 8 and the second diffraction grating 9, but the thickness of the dielectric thin film 10a on the slanting grating surface is changed near the valley of the second diffraction grating 9. In addition, the amount of change in film thickness is appropriately controlled to achieve both ease of manufacture and high diffraction efficiency.

[0114] In the diffractive optical element 1 shown in FIG. 5, the first material forming the second diffraction grating 8 is an episulfide-based resin (Nd=1.6886, N55=1.6926, νd=35.9, θgF=0.584). On the other hand, the second material forming the second diffraction grating 9 is a polycarbonate-based thermoplastic resin (Nd=1.6447, N55=1.6926, νd=22.5, θgF=0.635). The dielectric thin films (thin film layers) 10a and 10b on the grating slope and grating wall are single-layer films made of SiO (Nd=1.521, N55=1.5248), which is an inorganic oxide. The maximum thickness of the dielectric thin film 10a on the grating slope is 70 nm, and the maximum thickness (total thickness) of the dielectric thin film 10b on the grating wall is 10 nm. The grating height d1 of the second diffraction grating 9 is 13.72 to 15.63 μm.

[0115] The thin film layer on the grating slope has a thickness change around the valley of the second diffraction grating 9. Specifically, the reference thickness df of the thin film layer on the grating slope is 70 nm, and the thickness of the dielectric thin film 10a gradually decreases from the valley of the second diffraction grating 9 to a region with a width w of 11 μm. The minimum thickness dfmn of the dielectric thin film 10a in the valley of the second diffraction grating 9 is 35 nm.

[0116] FIG. 9(a) shows the diffraction efficiency calculated by RCWA in the annular zone with the minimum arrangement pitch P=19.4 μm and the grating thickness d1=15.39 μm of the diffractive optical element of this embodiment. In FIG. 9(a), the horizontal axis shows the wavelength (nm) and the vertical axis shows the diffraction efficiency (%). FIG. 9(b) shows the reflectance at the grating interface of the diffractive optical element of this embodiment. In FIG. 9(b), the horizontal axis shows the wavelength (nm) and the vertical axis shows the reflectance (%). As shown in FIG. 9(a), the selection of the grating material is improved by having a thin film layer at the grating interface. In addition, by satisfying the conditional formula (4), a high diffraction efficiency of 90% or more can be obtained in a wide wavelength range from 430 nm to 670 nm in the visible range, despite the narrow pitch configuration with the minimum arrangement pitch P=19.4 μm. Furthermore, as shown in Figure 9(b), by appropriately setting the refractive indices of the thin film layer, first diffraction grating 8, and second diffraction grating 9, and the thickness of the thin film layer, a diffractive optical element having a low reflectance of 1% or less over a wide wavelength range from 430 nm to 670 nm in the visible range can be obtained.

[0117] According to the diffractive optical element of this embodiment, by having a thin film layer between the grating surfaces of the first diffraction grating 8 and the second diffraction grating 9, melting and penetration of the resin material is suppressed and the selection of the grating material is also made easy, so that a combination of materials that results in high diffraction efficiency can be realized. In addition, by using a thermoplastic resin material for the second diffraction grating 9 and integrally molding the diffraction grating surface, a low-cost diffractive optical element can be obtained.

[0118] Table 1 shows values ​​relating to each conditional expression of the optical systems of Examples 1 to 5.

[0119] [Table 1] EXAMPLES

[0120] Next, an optical system (observation optical system) 100 according to a sixth embodiment of the present invention will be described with reference to Fig. 10. Fig. 10 is a configuration diagram of the optical system 100. In Fig. 10, 101 is a display panel such as an LCD, 102 is an optical path branching means, 103 is a correction lens, and 105 is a pupil surface. 104 is a diffractive optical element according to any one of the first to fifth embodiments, and is provided to correct chromatic aberration and the like of the correction lens 103.

[0121] As described in each of the above-mentioned embodiments, the optical system 100 has a high diffraction efficiency, is easy to manufacture, and is low-cost. The optical system 100 can be applied to observation optical systems such as terrestrial telescopes or astronomical observation telescopes, observation optical systems such as head-mounted displays (HMDs), or optical finders such as lens-shutter cameras or video cameras, and can provide the same effects as described above. In this embodiment, one diffractive optical element is disposed in the optical system 100, but this is not limiting, and multiple diffractive optical elements may be disposed in the photographing lens. EXAMPLES

[0122] Next, an imaging device (video camera) 200 according to a seventh embodiment of the present invention will be described with reference to Fig. 11. Fig. 11 is a schematic diagram of the imaging device 200. In Fig. 11, 201 is a video camera body, 202 is an imaging optical system that forms a subject image on an imaging element (not shown), and 203 is a sound collecting microphone. 204 is an observation device (electronic viewfinder, display device) for observing the subject image (image) displayed on a display element (not shown) via an observation optical system (for example, the optical system 100 of the sixth embodiment). The display element is composed of a liquid crystal panel or the like, and the subject image formed by the imaging optical system 202 is displayed on the display element.

[0123] In this manner, the optical system 100 of the sixth embodiment can be applied to an imaging device 200 such as a video camera. This makes it possible to obtain an imaging device 200 having an eyepiece optical system (observation optical system) that satisfies a wide viewing angle and can sufficiently correct various aberrations such as field curvature and astigmatism while ensuring sufficient space for arranging an optical path branching means in the optical system 100. The eyepiece optical system of this embodiment is not limited to a video camera as shown in Fig. 11, but can also be applied to, for example, a lens-interchangeable mirrorless camera or an HMD.

[0124] According to each embodiment, a diffractive optical element having a simple configuration can obtain high diffraction efficiency over the entire visible range, and a high-performance diffractive optical element in which melting and penetration between resins is suppressed can be obtained. Furthermore, by using the diffractive optical element in an optical system, an optical system in which various aberrations such as chromatic aberration and flare are favorably reduced can be obtained. Therefore, according to each embodiment, a diffractive optical element, an optical system, an imaging device, and a display device that are easy to manufacture and have high optical performance can be provided.

[0125] The disclosure of each embodiment includes the following configuration.

[0126] (Configuration 1) A diffractive optical element comprising a first diffraction grating made of a first material, a second diffraction grating made of a second material, and a thin film layer, the first diffraction grating and the second diffraction grating are in close contact with each other via the thin film layer, the diffractive optical element has a plurality of ring zone regions, the arrangement pitches of which in a radial direction are different from one another; When the minimum value of the arrangement pitch is P (μm), the refractive indices of the first diffraction grating and the second diffraction grating at the design wavelength for at least one of the plurality of annular zones are N1 and N2, respectively, the refractive index of the thin film layer at the design wavelength is Nf, and the maximum thickness of the thin film layer is df (nm), 0.4<|N1-N2|×P<6.0 0≦|N1+N2-2×Nf|×df<40 A diffractive optical element characterized by satisfying the following conditional expression: (Configuration 2) 2. The diffractive optical element according to configuration 1, wherein the thin film layer contains an inorganic material. (Configuration 3) When the Abbe numbers of the first diffraction grating and the second diffraction grating based on the d line are ν1 and ν2, respectively, 1.0<(N1-N2) / (1 / ν2-1 / ν1)<20.0 3. A diffractive optical element according to configuration 1 or 2, which satisfies the following conditional expression: (Configuration 4) 10 <P<80 4. A diffractive optical element according to any one of configurations 1 to 3, which satisfies the following conditional expression: (Configuration 5) 5. The diffractive optical element according to any one of configurations 1 to 4, wherein the second material is a thermoplastic resin. (Configuration 6) 6. The diffractive optical element according to any one of configurations 1 to 5, wherein the first material is an ultraviolet curable resin. (Configuration 7) Let L1 be the thickness on the optical axis of a first lens made of the same material as the first diffraction grating, and L2 be the thickness on the optical axis of a second lens made of the same material as the second diffraction grating. 5 <L2 / L1<200 7. A diffractive optical element according to any one of configurations 1 to 6, which satisfies the following conditional expression: (Configuration 8) 0.001<|N1+N2-2×Nf|<0.600 8. A diffractive optical element according to any one of configurations 1 to 7, which satisfies the following conditional expression: (Configuration 9) 0.02 <N1-N2<0.15 9. A diffractive optical element according to any one of configurations 1 to 8, which satisfies the following conditional expression: (Configuration 10) 3 <df<200 10. A diffractive optical element according to any one of configurations 1 to 9, which satisfies the following conditional expression: (Configuration 11) 11. The diffractive optical element according to any one of configurations 1 to 10, wherein the thin film layer has a thickness that changes in the valleys of the first diffraction grating or the second diffraction grating. (Configuration 12) When the minimum thickness of the thin film layer is dfmn, 0.10 <dfmn / df<0.95 13. A diffractive optical element according to any one of configurations 1 to 12, which satisfies the following conditional expression: (Configuration 13) Let dfs be the difference between the minimum and maximum thicknesses of the thin film layer. 0.2<|dfs×(N1+N2-2×Nf)|<30.0 13. A diffractive optical element according to claim 12, which satisfies the following condition: (Configuration 14) When the width in the pitch direction in which the film thickness changes on the grating slope of the thin film layer is w (μm) and the grating height is d (μm), in the annular zone region with the smallest diffraction pitch P (μm), 0.002 <w / (P×d)<0.100 14. A diffractive optical element according to any one of configurations 1 to 13, which satisfies the following conditional expression: (Configuration 15) 15. The diffractive optical element according to any one of configurations 1 to 14, wherein the thin film layer has a smaller thickness on the grating wall surfaces than on the grating oblique surfaces. (Configuration 16) In the diffractive optical element, an angle of a grating wall surface of the second diffraction grating with respect to an optical axis of a lens formed of the same material as the first material of the first diffraction grating is Ah (deg). Within an effective area, 2 <Ah<50 16. A diffractive optical element according to any one of configurations 1 to 15, which satisfies the following conditional expression: (Configuration 17) 17. An optical system comprising the diffractive optical element according to any one of claims 1 to 16. (Configuration 18) 18. An imaging apparatus comprising: the optical system according to aspect 17; and an imaging element for receiving an image formed by the optical system. (Configuration 19) 18. A display device comprising: a display element for displaying an image; and the optical system according to claim 17 for guiding light from the display element.

[0127] Although the preferred embodiments of the present invention have been described above, the present invention is not limited to these embodiments, and various modifications and changes are possible within the scope of the gist of the present invention. [Explanation of symbols]

[0128] 1 Diffractive optical element 8 First diffraction grating 9 Second diffraction grating 10a, 10b Dielectric thin film (thin film layer)

Claims

1. A diffractive optical element comprising a first diffraction grating made of a first material, a second diffraction grating made of a second material, and a thin film layer, the first diffraction grating and the second diffraction grating are in close contact with each other via the thin film layer, the diffractive optical element has a plurality of annular zone regions, each of which includes a plurality of annular zones arranged in a radial direction; the arrangement pitches of the plurality of ring zones are different from one another among the plurality of ring zone regions, Let P (μm) be the minimum value of the arrangement pitch, N1 and N2 be the refractive indices at the design wavelength of the first diffraction grating and the second diffraction grating for at least one of the plurality of annular zones, Nf be the refractive index at the design wavelength of the thin film layer, and df be the maximum thickness of the thin film layer, 0.4<|N1-N2|×P<6.0 0≦|N1+N2-2×Nf|×df<40 A diffractive optical element characterized by satisfying the following conditional expression:

2. The diffractive optical element according to claim 1 , wherein the thin film layer contains an inorganic material.

3. When the Abbe numbers of the first diffraction grating and the second diffraction grating based on the d line are ν1 and ν2, respectively, 1.0<(N1-N2) / (1 / ν2-1 / ν1)<20.0 2. The diffractive optical element according to claim 1, which satisfies the following condition:

4. 10<P<80 2. The diffractive optical element according to claim 1, which satisfies the following condition:

5. The diffractive optical element according to claim 1 , wherein the second material is a thermoplastic resin.

6. The diffractive optical element according to claim 1 , wherein the first material is an ultraviolet curable resin.

7. When the thickness on the optical axis of a first lens made of the same material as the first diffraction grating is L1, and the thickness on the optical axis of a second lens made of the same material as the second diffraction grating is L2, 5<L2 / L1<200 2. The diffractive optical element according to claim 1, which satisfies the following condition:

8. 0.001<|N1+N2-2×Nf|<0.600 2. The diffractive optical element according to claim 1, which satisfies the following condition:

9. 0.02<N1-N2<0.15 2. The diffractive optical element according to claim 1, which satisfies the following condition:

10. 3<df<200 2. The diffractive optical element according to claim 1, which satisfies the following condition:

11. 2. The diffractive optical element according to claim 1, wherein the thin film layer has a thickness that varies in a valley portion of the first diffraction grating or the second diffraction grating.

12. When the minimum thickness of the thin film layer is dfmn, 0.10<dfmn / df<0.95 2. The diffractive optical element according to claim 1, which satisfies the following condition:

13. When the difference between the minimum thickness and the maximum thickness of the thin film layer is dfs, 0.2<|dfs×(N1+N2-2×Nf)|<30.0 13. The diffractive optical element according to claim 12, which satisfies the following condition:

14. When the width in the pitch direction in which the film thickness changes on the grating slope of the thin film layer is w (μm) and the grating height is d (μm), in the annular zone region with the smallest diffraction pitch P (μm), 0.002<w / (P×d)<0.100 2. The diffractive optical element according to claim 1, which satisfies the following condition:

15. 2. The diffractive optical element according to claim 1, wherein the thin film layer has a thickness on a grating wall surface that is thinner than a thickness on a grating oblique surface.

16. In the diffractive optical element, an angle of a grating wall surface of the second diffraction grating with respect to an optical axis of a lens formed of the same material as the first material of the first diffraction grating is Ah (deg). Within an effective area, 2<Ah<50 2. The diffractive optical element according to claim 1, which satisfies the following condition:

17. An optical system comprising a diffractive optical element according to claim 1 .

18. 18. An imaging apparatus comprising: the optical system according to claim 17; and an imaging element that receives an image formed by the optical system.

19. 20. A display device comprising: a display element for displaying an image; and the optical system according to claim 17 for guiding light from the display element.

Images