JP2012159725A - Diffractive optical element and optical system having the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a diffractive optical element which suppresses light generated from grating vertical surfaces and a light quantity of multiple reflection light reaching an image surface (evaluation surface) while suppressing deterioration in diffraction efficiency of design order.SOLUTION: In a diffractive optical element where a plurality of diffraction gratings are laminated, an area where a phase change in the diffraction gratings of each layer is inconsistent occupies equal to or larger than 30% of the area of a unit period and also the phase change in the whole diffraction gratings is substantially consistent in a unit periodic structure.

Description

  The present invention relates to a diffractive optical element and an optical system having the diffractive optical element, and is suitable for an optical system used in an optical apparatus such as a video camera, a digital camera, and a television camera.

  As a method for reducing chromatic aberration of an optical system (lens system), a method using a combination of different glass materials is known. In addition, a method of providing a diffractive optical element having a diffractive action in a part of the optical system is known.

  It is known that a diffractive optical element can have an aspherical lens effect by appropriately changing the period of its periodic structure in addition to correcting chromatic aberration.

  Here, in refraction, one light beam is a single light beam after refraction, whereas in diffraction, light is divided into each order. Therefore, when a diffractive optical element is used as the lens system, it is necessary to determine the grating structure so that the light flux in the used wavelength region is concentrated in one specific order (hereinafter also referred to as a design order).

  Therefore, a diffractive optical element generally used as a diffractive lens is a diffractive optical element having a blazed structure including a grating surface 34 and a grating vertical surface 25 as shown in FIG. Such a blazed diffractive optical element can diffract light with high efficiency for a specific diffraction order and a specific wavelength.

  FIG. 12 is a cross-sectional view of a main part of the diffractive optical element disclosed in Patent Document 1. The diffractive optical element disclosed in Patent Document 1 includes a structure 201 (hereinafter referred to as “laminated DOE”) 201 in which element portions 202 and 203 each including a diffraction grating are close to each other via an air layer 210. Yes.

  By optimizing the refractive index, the dispersion characteristic (Abbe number νd) of the material constituting each diffraction grating, the grating thickness of the grating portion of each layer, and the like, as shown in FIG. Has gained efficiency.

  However, on the other hand, the light beam incident on the grating vertical surface 25 behaves differently from the grating surface 34 such as reflection and refraction on the grating vertical surface, which is undesirable for the diffractive lens because it becomes unnecessary light.

  The writer calculated a diffractive optical element in a laminated DOE using strict coupled wave analysis (hereinafter, RCWA) among strict wave calculations. As a result, in the multilayer DOE, it was found that unnecessary light generated from the vertical plane of the grating hardly reaches the image plane and does not easily flare.

  However, in the case of the stacked DOE as shown in FIG. 12, the multiple reflected light as shown in FIG. 7 may reach the image plane and cause flare.

JP 2000-98118 A

  The conventional configuration does not sufficiently suppress the light incident on the grating vertical plane from reaching the image plane (evaluation plane). Further, it is not a diffraction grating in which flare due to multiple reflected light is considered.

  Therefore, in the present invention, while suppressing deterioration of the diffraction efficiency of the design order, the amount of light reaching the image plane (evaluation plane) after entering the grating vertical plane and reaching the image plane (evaluation plane) by multiple reflection. The object is to achieve a diffractive optical element in which the amount of light is suppressed.

  In order to achieve the above object, the diffractive optical element of the present invention is a diffraction grating in which a plurality of diffraction gratings are stacked, and in the unit periodic structure, there is at least a region in which the phase change in the diffraction grating of each layer is mismatched. It occupies an area of 30% or more of the unit period, and the phase change of the entire diffraction grating is roughly matched.

  According to the present invention, it is possible to obtain a diffractive optical element that has high diffraction efficiency of the designed order and that suppresses unnecessary light reaching the image plane due to a grating vertical plane or multiple reflection.

The front view and side view of the diffractive optical element which are embodiment of this invention. 1 is a partial cross-sectional view of a diffractive optical element showing a first embodiment of the present invention. The figure which shows a mode that light injects into the perpendicular surface of the diffraction grating of 1st Embodiment of this invention. The figure which shows a mode that light injects into the perpendicular | vertical surface of the diffraction grating of the comparative example 2. FIG. FIG. 6 is a partial cross-sectional view of a diffractive optical element of Comparative Example 3. The fragmentary sectional view of the diffractive optical element which shows 2nd Embodiment of this invention. The figure which shows the behavior of multiple reflected light. The block diagram of the imaging optical system using the diffractive optical element of this invention. The block diagram of the imaging optical system using the diffractive optical element of this invention. 1 is a schematic diagram of a main part of an imaging apparatus of the present invention. Conventional diffractive optical element. Conventional diffractive optical element. Diffraction efficiency in conventional diffractive optical elements.

(First embodiment)
FIG. 1A is a front view of the diffractive optical element according to the first embodiment of the present invention, and FIG. 1B is a side view of the diffractive optical element. Further, FIG. 2 shows an enlarged part of a cross-sectional shape when the diffractive optical element of FIG. 1 is cut along the line AA ′. However, FIG. 2 is a figure deformed considerably in the lattice depth direction.

  As shown in these drawings, the diffractive optical element 1 includes a first element part 2 and a second element part 3, a first diffraction grating 8 and a second diffraction grating formed in each element part. 9 and the intermediate layer 10 so as to be close to each other with the intermediate layer 10 interposed therebetween. The first and second element portions 2 and 3 and the intermediate layer 10 as a whole function as one diffractive optical element.

  The first and second diffraction gratings 8 and 9 have concentric grating shapes, and have a lens action by changing the grating pitch in the radial direction. Further, the first diffraction grating 8 and the second diffraction grating 9 have substantially the same grating pitch distribution.

  As shown in FIG. 2, the first element portion 2 is formed integrally with the first transparent substrate 4, the lattice base portion 6 provided on the first transparent substrate 4, and the lattice base portion 6. And a first grating forming layer made of the first diffraction grating 8.

  In addition, grating surfaces 8a and 8b are formed at the boundary between the first diffraction grating 8 and the intermediate layer 10, and the grating surfaces 8a and 8b have different phase modulation effects.

  On the other hand, similarly to the first element unit 2, the second element unit 3 includes a second transparent substrate 5, a lattice base unit 7 provided on the second transparent substrate 5, and the lattice base unit 7. And a second grating forming layer composed of the second diffraction grating 9 formed integrally.

  Furthermore, grating surfaces 9a and 9b are formed at the boundary between the second diffraction grating 9 and the intermediate layer 10, and the grating surfaces 9a and 9b have different phase modulation effects.

  The intermediate layer 10 is set to have a thickness D between the edges formed by the grating surfaces 8b and 9b of the diffraction gratings 8 and 9 and the grating side surfaces.

  In the present embodiment, the wavelength region of light incident on the diffractive optical element 1, that is, the used wavelength region is the visible region, and the material and the grating thickness constituting the first and second diffraction gratings 8 and 9 are the visible region. The total is selected so as to increase the diffraction efficiency of the diffracted light when incident at 0 °.

In the diffractive optical element 1 shown in FIG. 2, a resin (nd = 1.567, νd = 47...) In which the first diffraction grating 8 and the second diffraction grating 9 are mixed with ZrO ₂ fine particles in an acrylic resin. 0, θgF = 0.569) was used.

  On the other hand, a resin (nd = 1.504, νd = 16.3, θgF = 0.390) in which ITO fine particles are mixed with fluorine resin is used for the intermediate layer 10.

  The grating thickness d1 of the diffraction grating 8 and the grating thickness d2 of the diffraction grating 9 are both 9.29 μm, the diffraction pitch P is 200 μm, and the pitches of the grating surfaces 8a and 8b, Pa and Pb are 100 μm. The edge distance D is 1.5 μm.

  Next, the behavior of transmitted light when a light beam used for imaging is incident on the diffraction grating of this embodiment will be described.

In this embodiment, the optical path length difference φ of the entire optical system is such that the refractive index of the first diffraction grating is N1, the refractive index of the intermediate layer is N2, and the refractive index of the second diffraction grating is N2. in the case of,
φ = (N1-N2) d1- (N2-N3) d2 (1)
When φ is an integral multiple of the wavelength, high diffraction efficiency can be obtained.

  Here, the signs of d1 and d2 assume positive values when the lattice thickness increases from top to bottom in the figure.

  In the light beams passing through the diffraction gratings 8a and 9a, d1 = 0 and d2 = 9.29 μm, so the optical path length difference φa is 0.585. In the light beams passing through the diffraction gratings 8b and 9b, d1 = 9.29 μm and d2 = 0 μm, so the optical path length difference φb is 0.585, similar to φa.

  That is, it can be seen that the phase is matched between the pitch Pa region and the pitch Pb region. Therefore, high diffraction efficiency can be obtained as a whole of the diffractive optical element 1.

  Next, the behavior of unnecessary light when light is incident on the vertical plane in the diffraction grating of this embodiment will be described. As shown in FIG. 3, when a light beam having an angle of −10 ° is incident on the diffraction grating of the present embodiment, and as a comparative example, a diffractive optical element having a conventional configuration as shown in FIG. The amount of emitted light was calculated using RCWA when light having an angle of.

  In the configuration of Comparative Example 1 shown in FIG. 4, the material forming the first diffraction grating 8 is the same as the material of the first diffraction grating 8 in Example 1, and the second diffraction grating 9 of Comparative Example 1 is the same. The material to be formed is the same as the material of the intermediate layer 10 in the first embodiment. The lattice thickness d1 of Comparative Example 1 is 18.57 μm.

  Both Example 1 and Comparative Example 1 have substantially the same phase modulation effect, and both have a high diffraction efficiency. The diffractive optical elements having the configurations of Example 1 and Comparative Example 1 are arranged in an optical system in which light having an emission angle of about 0 ° reaches the image plane.

  Comparing the diffraction efficiencies around the emission angle of 0 ° (unnecessary light reaching the image plane) in the RCWA calculation results, it is 0.0014% in Comparative Example 1 and 0.0005% in Example 1. , You can see that it has improved significantly.

  This is because, by using the stacked DOE as in the present embodiment, light incident on the vertical plane of the first diffraction grating is scattered and reduced by the grating plane and vertical plane of the second diffraction grating. It is.

  Next, the behavior of unnecessary light due to multiple reflection will be described. As a comparative example 2, a diffraction grating having a configuration as shown in FIG. In Comparative Example 2, the material configuration is the same as that of Example 1, and the diffractive surfaces 8 and 9 are continuous. Even in the configuration as in Comparative Example 2, there is an effect of suppressing the light incident on the grating vertical plane from reaching the image plane. However, in such a stacked DOE, unnecessary light is easily generated due to multiple reflection.

  Therefore, by changing the grating thicknesses of the diffraction grating 8 and the diffraction grating 9 depending on the region as in this embodiment, it is possible to disperse only the light by multiple reflection without changing the optical path length of the transmitted light.

  In the two regions Pa and Pb of Example 1, when the light is incident at 0 °, + 10 °, and −10 °, and the emission angles of the transmitted light and the multiple reflected light in the comparative example 2 are geometric traces. The calculated results are shown in Table 1.

  As can be seen from Table 1, it can be seen that in Example 1, the transmitted light emission angle is in phase matching in the regions Pa and Pb because the emission angle does not change much. Moreover, the emission angles of the multiple reflected light in Example 1 are different in the regions Pa and Pb, and it can be seen that the multiple reflected light is dispersed.

  On the other hand, in Comparative Example 2, the multiple reflected light has the same emission angle in the entire region, so that the intensity of the multiple reflected light is strengthened, and it can be seen that it is not preferable as unnecessary light.

  When the intensity of multiple reflected light at −10 ° incidence is calculated by RCWA calculation, it is 0.0005% in Example 1 and 0.0007% in Comparative Example 2. It can be seen that the reflected light is reduced.

The present invention is directed to a diffraction grating having high diffraction efficiency. For this purpose, in the minimum period unit of the diffraction grating, the refractive index is N1, N2,..., N (i + 1) in order from the layer on the light incident side, and the grating thickness is d1 in order from the diffraction grating on the light incident side. , D2, ..., d (i),
0.85 <| (Σ (N (i + 1) −N (i)) × d (i)) / λ | <1.15 (2)
If this condition is satisfied, a diffraction grating having high diffraction efficiency can be obtained.

  Here, d (i) takes a positive value when the grating thickness increases with increasing distance from the optical axis, and takes a negative value when the grating thickness decreases with increasing distance from the optical axis. In the present embodiment, the first diffraction grating and the second diffraction grating are made of the same material.

  In the present invention, in order to obtain a diffraction grating having high diffraction efficiency and suppressing multiple reflected light, it is preferable that the refractive indexes of at least two grating materials are close to each other. This is because the closer the refractive index of the diffraction grating is, the greater the degree of freedom of the shape of the grating surface and the easier it is to disperse the multiple reflected light.

  For example, when the difference in refractive index between the first diffraction grating and the second diffraction grating in Example 1 increases, the deterioration in diffraction efficiency increases when the grating plane shapes of the region Pa and the region Pb are changed. . In particular, it becomes difficult to obtain high diffraction efficiency over the entire visible range.

  Therefore, it is desirable that there are at least one combination of materials satisfying the following conditional expression for the refractive index difference ΔN at the d-line among the materials of the stacked DOE.

ΔN <0.1 (3)
More desirably, the following range is desirable.

ΔN <0.05 (3b)
(Second Embodiment)
FIG. 1A is a front view of a diffractive optical element according to a second embodiment of the present invention, and FIG. 1B is a side view of the diffractive optical element. FIG. 6 shows an enlarged part of a cross-sectional shape when the diffractive optical element of FIG. 1 is cut along the line AA ′. However, FIG. 6 is a figure considerably deformed in the lattice depth direction.

  As shown in these drawings, the diffractive optical element 1 includes a first element part 2 and a second element part 3, a first diffraction grating 8 and a second diffraction grating formed in each element part. 9 are superposed so as to be close to each other with the air layer 10 in between. The first and second element portions 2 and 3 and the air layer 10 as a whole function as one diffractive optical element.

  The first and second diffraction gratings 8 and 9 have concentric grating shapes, and have a lens action by changing the grating pitch in the radial direction. Further, the first diffraction grating 8 and the second diffraction grating 9 have substantially the same grating pitch distribution.

  As shown in FIG. 6, the first element portion 2 is integrally formed with the first transparent substrate 4, the lattice base portion 6 provided on the first transparent substrate 4, and the lattice base portion 6. And a first grating forming layer made of the first diffraction grating 8.

  In addition, grating surfaces 8a and 8b are formed at the boundary between the first diffraction grating 8 and the air layer 10, and the grating surfaces 8a and 8b have different phase modulation effects.

  On the other hand, similarly to the first element unit 2, the second element unit 3 includes a second transparent substrate 5, a lattice base unit 7 provided on the second transparent substrate 5, and the lattice base unit 7. And a second grating forming layer composed of the second diffraction grating 9 formed integrally.

  Furthermore, grating surfaces 9a and 9b are formed at the boundary between the second diffraction grating 9 and the air layer 10, and the grating surfaces 9a and 9b have different phase modulation effects.

  The air layer 10 is set to have a thickness D between the edges formed by the grating surfaces 8b and 9b of the diffraction gratings 8 and 9 and the grating side surfaces.

  In the diffractive optical element 1 shown in FIG. 6, the first diffraction grating 8 has a resin in which ITO fine particles are mixed with an acrylic resin (nd = 1.568, νd = 22.7, θgF = 0.435). Was used.

  On the other hand, an acrylic resin (nd = 1.524, νd = 51.6, θgF = 0.576) was used for the second diffraction grating 9.

  The grating thickness d1a of the area Pa of the diffraction grating 8 is 6.86 μm, the grating thickness d1b of the area Pb is 5.94 μm, the grating thickness d2a of the area Pa of the diffraction grating 9 is 7.00 μm, and the grating thickness of the area Pb. d2b is 8.00 μm. The diffraction pitch P is 100 μm, the pitches of the grating surfaces 8a and 8b, Pa and Pb are 50 μm, respectively. The edge distance D is 1.5 μm.

  Similar to Example 1, Table 2 shows the results of calculating the outgoing angles of transmitted light and multiple reflected light using geometric traces in Example 2.

  As Comparative Example 3, the result of a configuration in which d1a = d1b = 6.4 μm and d2a = d2b = 7.5 μm with the same configuration as in the second embodiment is shown.

  As can be seen from Table 2, it can be seen that in Example 2, the transmitted light emission angle is in phase matching in the regions Pa and Pb because the emission angle does not change much.

  Moreover, the emission angles of the multiple reflected light in Example 2 are different in the regions Pa and Pb, and it can be seen that the multiple reflected light is dispersed.

  On the other hand, in Comparative Example 3, since the multiple reflected light has the same emission angle in the entire region, it can be seen that the intensity of the multiple reflected light is strengthened and is not preferable as unnecessary light.

  When the intensity of the multiple reflected light at 0 ° incidence is calculated by RCWA calculation, it is 0.116% in the second embodiment and 0.095% in the second comparative example. It can be seen that light is reduced.

  In the present invention, the diffraction grating is configured by deforming the grating shape of a half region of the unit period. However, the present invention is not limited to this.

  Desirably, the multiple reflected light can be sufficiently dispersed by changing the lattice shape in an area of 30% or more of the unit period.

  FIG. 8 shows a configuration of a photographing (imaging) optical system of a camera (such as a still camera or a video camera) using the diffractive optical element of the present invention.

  In this figure, reference numeral 101 denotes a photographic lens composed mostly of refractive optical elements (for example, ordinary lens elements), and has an aperture stop 102 and the diffractive optical element 1 described in the first embodiment. Reference numeral 103 denotes a photographing medium such as a film or a CCD disposed on the image plane.

  The diffractive optical element 1 is an element having a lens function. The diffractive optical element 1 has chromatic aberration generated by the refractive optical element in the taking lens 101. It is suppressed. As a result, it is possible to realize a photographing optical system with little flare light and high resolving power at low frequencies and high optical performance.

  In the present embodiment, the diffractive optical element 1 is provided on a flat glass surface disposed in the vicinity of the stop 102. However, the present invention is not limited to this, and the diffractive optical element 1 is a concave surface or convex surface of a lens. It may be provided above. Furthermore, a plurality of diffractive optical elements 1 may be arranged in the taking lens 101.

  FIG. 9 shows the configuration of an observation optical system of binoculars that is the fourth embodiment of the present invention. In this figure, 104 is an objective lens, 105 is a prism for erecting an inverted image, 106 is an eyepiece, and 107 is an evaluation surface (pupil surface). Reference numeral 1 denotes a diffractive optical element described in the first and second embodiments, and is provided for the purpose of correcting chromatic aberration and the like on the imaging surface 103 of the objective lens 104.

  As described in the first embodiment, this observation optical system has greatly improved diffraction efficiency characteristics compared to the conventional one, so that there is little flare light and high resolution at low frequencies, and high optical performance. Have

  In this embodiment, the case where the diffractive optical element 1 is provided on the flat glass surface has been described. However, the present invention is not limited to this, and the diffractive optical element 1 may be provided on the concave surface or convex surface of the lens. Furthermore, a plurality of diffractive optical elements 1 may be arranged in the observation optical system.

  In the present embodiment, the case where the diffractive optical element 1 is provided in the objective lens unit has been described. However, the present invention is not limited to this, and the diffractive optical element 1 may be provided on the surface of the prism 105 or a position in the eyepiece lens 106. The same effect as described above can be obtained.

  However, providing the diffractive optical element 1 on the object side with respect to the imaging plane 103 has an effect of reducing chromatic aberration only by the objective lens unit. Therefore, in the case of the naked eye observation system, it is desirable to provide at least the objective lens unit.

  In this embodiment, the observation optical system for binoculars has been described. However, the diffractive optical element of the present invention can also be applied to an observation optical system such as a terrestrial telescope or an astronomical observation telescope. Furthermore, it can be applied to an optical viewfinder such as a lens shutter camera or a video camera, and the same effect as described above can be obtained.

  Next, an embodiment of a digital still camera (imaging device) using the optical system having the diffractive optical element of the present invention as a photographing optical system will be described with reference to FIG. An imaging optical system 22 constituted by the optical system of the present invention is a solid-state imaging device (photoelectric conversion device) such as a CCD sensor or a CMOS sensor that is incorporated in the camera body and receives an object image formed by the imaging optical system 21. is there.

  Reference numeral 23 denotes a memory for recording information corresponding to the subject image photoelectrically converted by the image sensor 22, and reference numeral 24 denotes a finder configured by a liquid crystal display panel or the like for observing the subject image formed on the solid-state image sensor 22. .

  As described above, by applying the optical system of the present invention to an image pickup device such as a digital still camera, an image pickup apparatus with little flare, sufficient transparency, and high optical performance is realized.

DESCRIPTION OF SYMBOLS 1 Diffractive optical element 2 1st element part 3 2nd element part 4 1st board | substrate 5 2nd board | substrate 6 1st grating | lattice base part 7 2nd diffractive base part 8 1st diffraction grating 9 2nd Diffraction grating 10 Layer 25 made of intermediate medium Grating vertical plane 34 Grating plane 101 Shooting lens 102 Aperture 103 Imaging plane 104 Objective lens 105 Prism 106 Eyepiece 107 Evaluation plane (pupil plane)

Claims (6)

  A diffraction grating in which a plurality of diffraction gratings are stacked, and in the unit periodic structure, the region where the phase change in the diffraction grating of each layer is mismatched occupies at least 30% of the unit period, and the entire diffraction grating A diffractive optical element characterized in that the phase change is roughly matched. In the minimum period unit of the diffraction grating, the refractive index is N1, N2,..., N (i + 1) in order from the layer on the light incident side, and the grating thickness is d1, d2, in order from the diffraction grating on the light incident side.・ ・ When d (i)
0.85 <| (Σ (N (i + 1) −N (i)) × d (i)) / λ | <1.15
The diffraction grating according to claim 1, wherein:
Here, d (i) takes a positive value when the lattice thickness increases with increasing distance from the optical axis,
A negative value is assumed when the lattice thickness decreases as the distance from the optical axis increases. 3. The diffractive optical element according to claim 1, wherein the refractive index difference ΔN at the d-line includes at least one combination of materials satisfying the following conditional expression. 4.
ΔN <0.1   An optical system comprising a refractive optical element and a diffractive optical element according to any one of claims 1 to 3.   The optical system according to claim 4, wherein the optical system forms an image on a solid-state imaging device.   6. An optical apparatus comprising: the optical system according to claim 4; and a solid-state imaging device that receives an image formed by the optical system.