JP2001324610A - Diffraction optical device and optical lens using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a diffraction optical device in which flare light caused by an incomplete form such as a chipped part of the diffraction grating can be suppressed to the minimum and therefore, deterioration in an image can be prevented, and to provide an optical lens using this device. SOLUTION: The diffraction optical device 2 is formed from an optical material such as a resin and glass by using a die in which the grating plane pattern is formed, and the device is provided with a diaphragm 3 which shields the incomplete form part of the diffraction plane of the diffraction optical device 2 from light.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a diffractive optical element formed of an optical material such as resin or glass using a mold and a diffractive optical element suitable for use in a photographic camera, an image reading device, and the like. It is related to the optical lens that was used.

[0002]

2. Description of the Related Art As is well known, various aberrations are present in an optical system, and various optical elements are assembled so as to correct these aberrations. Conventionally, chromatic aberration among various aberrations generated in an optical system has been reduced by combining glass materials having different dispersion characteristics. For example, in an objective lens such as a telescope, a glass material with a small dispersion is used as a positive lens, and a glass material with a large dispersion is used as a negative lens. By combining these, chromatic aberration appearing on the optical axis is eliminated. For this reason, when the number of constituent lenses is limited or when the usable glass material is limited, chromatic aberration cannot be sufficiently corrected.

As a conventional method of reducing chromatic aberration by a combination of glass materials, for example, SPIE is to reduce chromatic aberration by providing a diffractive optical element having a diffractive effect on a lens surface or a part of an optical system. Vol.
1354 International Lens Design Conference
(1990), JP-A-4-213421, JP-A-6-324262, US Pat.
No. 4,706 and the like. This utilizes a physical phenomenon in which chromatic aberration appears with respect to a light beam having a certain reference wavelength in the opposite direction between the refraction surface and the diffraction surface in the optical system.

[0004] This will be briefly described with reference to FIG.

The diffractive optical element 11 is placed in air having a refractive index of 1 and is arranged perpendicular to the optical axis 13. Here, assuming that the periodic pitch of the diffraction grating 12 is P, a light ray A parallel to the optical axis 13 generates diffracted light in the direction of Psin θ = mλ (1). Where θ is the diffraction angle, m
Is the diffraction order, and λ is the wavelength of light ray A.

In FIG. 1, the periodic structure is shown only in one direction. However, such a periodic structure is configured to be rotationally symmetric about a certain axis such as an optical axis, and the periodic pitch P of the diffraction grating 12 is changed. With a gradual change, an annular structure having this periodic structure acts as a lens. by the way,
In a lens using such a diffraction grating 12, (1)
As is easily understood from the equation, the diffraction angle θ (0 <θ <(π / 2)) increases as the wavelength λ becomes longer at a certain order. One that has the power in the same direction (such a lens
(The shorter the wavelength, the larger the refraction angle). In the above-mentioned known documents, aberrations (chromatic aberrations) are corrected mainly using this principle.

[0007] In refraction, one ray is one after refraction.
In the case of diffraction, light is split into a plurality of light beams for each order, whereas the light beam is a book. So, as a lens system,
When a diffraction grating is used, the diffraction grating structure is determined so that the luminous flux in the used wavelength region is concentrated on a specific order (hereinafter, referred to as “design order”). When the light intensity is concentrated at a specific order, the directions of the other diffracted lights are also determined by equation (1), but the intensity of the light beam is low, and when the intensity is 0, The diffracted light does not exist.

In order to increase the diffraction efficiency of the m-th order diffracted light, a structure for providing a phase difference Δλ has the following disadvantages.
If a phase difference of 2πm is given to each light ray in the diffraction direction, each light ray will interfere and be strengthened. Therefore,
For example, in a transmission type diffraction grating in which the grating height is d and the refractive index of the material is n, in order to give a phase difference Δλ of 2πm to the light of order m, Δλ = 2πm = 2πd ( n-1)... (2). When the condition of the expression (2) is satisfied between the pitches, the diffraction efficiency becomes highest.

The specific structure of the diffractive optical element for obtaining this diffractive action is called "kinoform", which has a continuous shape while giving a phase difference of 2π, and a continuous phase difference. "Binary structure" that approximates the distribution stepwise
And those formed by approximating the minute periodic structure to a “triangular wave shape” are known. Such a structure can be formed on the surface of a flat plate in an optical system,
The diffraction effect can be generated by forming it on the lens surface. Such a diffractive optical element can be manufactured by a semiconductor element process such as lithography, cutting, or the like, and can also be manufactured by molding resin, glass, or the like.

Further, such a diffractive optical element is particularly effective in correcting chromatic aberration generated on a refraction surface due to dispersion of glass, but the aspherical lens is obtained by changing the period of the periodic structure. Effect can be provided, and there is a great effect on reduction of aberration.

In the conventional example, various aberrations, particularly chromatic aberration, are reduced by the effect of diffraction, and the effect of incorporating the diffractive optical element into the optical system can be confirmed on an aberration diagram or the like. However, if the diffraction efficiency of the diffracted light that has contributed to the reduction of the aberration is not high, the light does not actually exist, and therefore it is necessary to sufficiently increase the diffraction efficiency of the light that reduces the aberration. Also, if there is a ray having a diffraction order other than the “design order”,
An image is formed at a place different from the light beam of the “design order” to form a flare or a ghost, which causes a decrease in image contrast.
Therefore, in the optical system using the diffraction effect, it is important to sufficiently study the distribution state of the diffraction efficiency and the behavior of light rays other than the “design order”.

FIG. 6 shows the spectral transmission characteristics of a general optical system. In the figure, the horizontal axis represents wavelength, and the vertical axis represents spectral transmittance. This spectral transmission characteristic is determined by the absorption of light by the glass and the reflection of light on the refraction surface. This optical system is required to have a spectral transmission characteristic suitable for an evaluation object in a used wavelength range.

FIG. 7 shows the characteristics of the diffraction efficiency for a specific diffraction order when the diffractive optical element is formed on a certain surface. Also in this figure, the horizontal axis shows the wavelength and the vertical axis shows the diffraction efficiency. This diffractive optical element has a first-order diffraction order (solid line in the figure).
It is designed to have the highest diffraction efficiency in the used wavelength region. That is, the “design order” is the first order. Further, FIG. 2 also shows the diffraction order near the “design order” and the diffraction efficiency of the (1 ± 1) -order light. As shown in the figure,
For diffracted light of the "design order", the diffraction efficiency is highest at a certain wavelength (hereinafter, referred to as "design wavelength"), and gradually decreases at other wavelengths. The cause is as follows.
Equation (2) shows the grating thickness for the phase difference to be 2π. However, when the grating thickness is set to satisfy this condition at the “design wavelength”, the condition deviates slightly from the condition at other wavelengths. As a result, the diffraction efficiency is reduced.

For example, as shown in FIG. 8, when the minute structure of the diffraction grating 12 constituting the diffractive optical element 11 is formed by an eight-step “binary structure”, the primary light of the diffractive optical element 11 If the “design wavelength” with respect to is 530 nm, the actual grating structure is given by
The thickness 143.7 nm obtained by dividing the thickness d = 1150 nm by 8 when 1, λ = 530 nm and n = 1.461 corresponds to the thickness of each step. At this time, the diffraction efficiency with the diffracted light of the “design wavelength” becomes 95%. Similarly, wavelength 4
The diffraction efficiency of the primary light at 00 nm is approximately 67%, and the wavelength 6
The diffraction efficiency for the primary light at 50 nm is about 85%. Therefore, in an optical system using the diffraction effect,
It is necessary to take measures such as setting the “design wavelength” near the center of the wavelength region where the optical system is used.
If only the diffraction efficiency in the “design order” is considered, it must be considered similarly to the spectral transmission characteristics.

ΗLEN is the spectral transmission characteristic of the optical system including the diffractive optical element 11 excluding the diffractive surface expressed as a function of wavelength.
S, when the diffraction efficiency of the diffractive optical element 11 is ηDOE, the spectral transmission characteristic η at the “design order” of the entire optical system
(Λ) can be expressed as the following equation η (λ) = ηLENS (λ) · ηDOE (λ) (3) Therefore, when a diffraction surface having a diffraction efficiency as shown in FIG. 7 is added to the optical system having the spectral transmission characteristic shown in FIG. 6, the spectral transmission characteristic at the “design order” is obtained from the equation (3). It looks like 9. This figure 9
As can be seen from the above, in order to obtain good spectral transmission characteristics in the wavelength region to be used in the entire optical system, it is desirable to keep the diffraction efficiency of the “design order” diffracted light high in that wavelength region.

Next, the effect of diffracted light at orders other than the "design order" will be described. Diffracted light of orders other than the “design order” will “run” while being defocused on the evaluation surface. This will be described briefly. Here, it is assumed that the “design order” is the first order and the power of the lens having the diffraction effect is positive. The diffracted light of a higher order (second order, third order,...) Than the “design order” has a larger diffraction angle θ from Expression (1), and forms an image before the image forming position of the first order light. This diffraction position increases as the diffraction order moves away from the “design order”. On the other hand, diffracted light of orders lower than the "design order" (0th order, -1st order, ...) forms an image behind the image forming position of the primary order light. Since the evaluation surface is placed at the image formation position of the “design order” diffracted light, the diffracted light other than the “design order” is “covered” in a defocused state on the image formation surface.

Of these, the diffracted light of the order away from the "design order" is considerably blurred on the evaluation surface and does not contribute to the image formation, but is added to the entire surface in a state like flare light. On the other hand, diffracted light whose diffraction order in the vicinity of the “design order” is (1 ± 1) is not resolved in the spatial frequency region where the imaging performance is evaluated, but is not completely blurred. Resolution occurs in the low spatial frequency range. For this reason, if the diffracted light of this diffraction order is large, the spot becomes a spot having a considerably large side lobe around the designed diffracted light, and the optical performance deteriorates. However, as shown in FIG.
The diffraction efficiency of the diffracted light of the diffraction orders (0th and 2nd orders) near the “design order” is almost 0 at the “design wavelength”, and has a diffraction efficiency of several% only at wavelengths far from the “design wavelength”. are doing. Therefore, in the light quantity integrated in the working wavelength range,
About 2%, depending on the type of photosensitive material placed on the evaluation surface,
This is a slight light amount of about 0.5%. Further, since this light amount is blurred on the evaluation surface, the light amount per unit area decreases and is not normally detected as a side lobe.

However, when an optical system utilizing this diffraction effect is applied to a camera lens (photographing system), special conditions need to be considered. For example, in the case of a camera, a film or a CCD is used for an evaluation surface,
Various conditions occur for the shooting conditions (subjects, exposure conditions). When a high-luminance light source exists in a part of the subject, the high-luminance light source section is saturated by appropriate exposure of a film or a CCD, and is adjusted to an appropriate exposure by another subject and photographed. There are cases. In this case, since the light source section has several times the proper exposure, the diffracted light of the diffraction order near the “design order” also becomes several times. For this reason, side lobes may be seen around the light source unit as if the afterglow was applied.

Therefore, when a diffractive optical element is used in an optical system having various exposure conditions such as a camera, it is desirable to reduce the diffraction efficiency of diffracted light in an order near this “design order”. As shown in FIGS. 10 to 12, at least two diffraction gratings 12 </ b> A and 12 </ b> B made of materials having different Abbe numbers (dispersion) are superimposed and configured as the diffractive optical element 11 so that the “design order” can be improved. Diffraction efficiency is high throughout the wavelength range of use, and the diffraction efficiency of diffracted light in orders near the design diffraction order can be reduced.In addition, it can be used for photographic cameras, video cameras, binoculars, projectors, telescopes, copiers, etc. When applied to an optical system, various aberrations such as chromatic aberration can be satisfactorily corrected. A diffractive optical element suitable for use in an optical system and an optical system using the same are disclosed in JP-A-10-133149 and JP-A-10-133149. 11
Japanese Unexamined Patent Publication (Kokai) No. 223717 has been proposed.

[0020]

In order to reduce the cost of the apparatus disclosed in the above publication, for example, a diffractive optical element is formed by molding using a mold 6 as shown in FIG. Generally, the element 11 'is manufactured. However, as shown in FIG.
When the 'is released from the mold 6, as shown in FIG. The presence of the notch D causes a reduction in diffraction efficiency of the diffracted light passing through that portion, and furthermore, the flare light generated in the notch D portion causes a serious problem of image deterioration.

In particular, in the case of a laminated type diffractive optical element in which two diffraction gratings made of materials having different Abbe numbers (dispersion) are joined to face each other on the grating surface, the "design order"
If the diffraction efficiency is set to be higher in the visible light range, the grating height d needs to be several times higher than that of the single-layer type diffractive optical element. And chipping D are more likely to occur. Further, as shown in FIG. 14, the chipping D of the diffraction grating does not occur over the entire diffractive optical element 11 but appears remarkably particularly in the mold release direction.

For example, FIG. 15A is a sectional view of a laminated type diffractive optical element. This diffractive optical element has a grating pitch P = 143.2 μm and a refractive index n _d = 1.521.
9. Abbe number ν _d = 51.0, lattice height d = 10.2
A first diffraction grating (RC8922 manufactured by Dai Nippon Ink Co., Ltd.) having a thickness of 95 μm and a refractive index n _d = 1.63.
63, Abbe number ν _d = 23.0, grating height d = 7.4
A second diffraction grating (UV1000 manufactured by Mitsubishi Chemical Corporation) 12B having a size of 32 μm and a vertex interval of the grating of 1.0
This is a configuration in which they are joined at a distance of μm. On the other hand, FIG. 2B is a cross-sectional view showing a state in which the apex portion of the laminated type diffractive optical element of FIG.

Further, FIGS. 16A and 16B show FIG.
5A and 5B show diffraction efficiencies of the diffractive optical elements shown in FIGS. According to FIGS. 16A and 16B, diffracted light (0-order, second-order, third-order,..., M-order light) other than the first-order light (shown by a solid line in FIG. ) All become flare light, which adversely affects the imaging performance. In particular, when there is a chipping D in the grating, for example, in FIG. 16B, the diffraction efficiency of the primary light is low over the entire visible light range, and it is effective to prevent the generation of flare light due to the chipping D. The development of means has been awaited.

In view of the above-mentioned drawbacks, the present invention can minimize the occurrence of flare light due to an incompletely shaped portion such as a chipped portion of a diffraction grating, thereby preventing image deterioration. It is an object of the present invention to provide a diffractive optical element capable of performing the same and an optical lens using the same.

[0025]

That is, the invention according to claim 1 is a diffractive optical element formed of an optical material such as resin or glass using a mold having a lattice surface formed in a cavity. And a light shielding means for shielding an incompletely shaped portion of the grating surface portion of the diffractive optical element.

The diffractive optical element according to the second aspect is the first aspect.
In the invention according to the invention, the diffractive optical element is a laminated type diffractive optical element in which the grating surfaces are bonded to face each other, and the optical materials forming the diffraction grating are formed to have different Abbe numbers. It is a feature.

According to a third aspect of the present invention, there is provided a diffractive optical element.
In the invention according to the second aspect, the grating shape in at least one cross section including the optical axis of the diffractive optical element is asymmetric.

The diffractive optical element according to the fourth aspect is the first aspect.
In the invention according to any one of (1) to (3), a marking for confirming a release direction when the diffractive optical element which is a molded part is released from the mold is removed from a part of the diffractive optical element outside the effective light beam area. It is characterized by having been provided.

According to a fifth aspect of the present invention, there is provided a diffractive optical element.
In the invention according to any one of the first to fourth aspects, the marking is formed together with the molding of the diffractive optical element by a mold used for molding the diffractive optical element.

In the diffractive optical element according to the sixth aspect of the present invention, a marking for confirming the direction of release when the diffractive optical element, which is a molded product, is released from the mold is placed on a portion of the diffractive optical element outside the effective beam area. A diffractive optical element is provided.

An optical lens using the diffractive optical element according to the seventh aspect is characterized by using the diffractive optical element according to any one of the first to sixth aspects.

The mold according to the invention of claim 8 is characterized in that, in the mold having a lattice surface shape formed in the cavity, the lattice shape in at least one sectional view including the optical axis of the diffraction grating is asymmetric. Mold.

In a ninth aspect of the present invention, there is provided a mold according to the eighth aspect, wherein the diffraction grating is formed concentrically, and a wall portion thereof is symmetric with respect to an optical axis.

According to a tenth aspect of the present invention, there is provided a diffractive optical element characterized by being formed from an optical material such as resin or glass using the mold according to the eighth or ninth aspect.

The optical lens according to claim 11 of the present invention.
An optical lens characterized by using the above diffractive optical element.

[0036]

Embodiments of the present invention will be described below with reference to the accompanying drawings. In this embodiment, the same parts as those of the prior art are denoted by the same reference numerals, and redundant description will be avoided.

FIG. 1 is a cross-sectional view of an imaging lens 1 using a diffractive optical element according to the present invention when cut in the main scanning direction. A reading light emitted from a light source (not shown) and reflected by a document surface set on a document table is incident on the imaging lens 1. The light is condensed by the light source, and the image is formed on the line sensor 2 so that the original image is read.

The imaging lens 1 is provided with a diffractive optical element 2 and a stop 3 as a light shielding means in the vicinity of the diffractive optical element 2 in addition to a lens group composed of a plurality of lenses. The axial chromatic aberration of the lens 1 is corrected.

The diffractive optical element 2 is a laminated type diffraction grating in which first and second diffraction gratings 21 and 22 are arranged and joined so as to face each other. These first and second diffraction gratings 21 and 22 are formed of optical materials (materials) such as resin and glass having different Abbe numbers (dispersion), and as shown in FIG. 2A or 2B. Each of the diffraction gratings is formed concentrically.

When the diffractive optical element 2 is manufactured using a mold, when the molded article is released from the mold, the diffraction grating has an incomplete shape in the direction along the release direction. Since a defect such as a portion D (see FIG. 2) easily occurs, for example, as shown in FIGS. 3 (A) to 3 (D), the mold release direction is confirmed outside the effective light area of the diffractive optical element 2. Marking M is formed. The marking M is for easily performing relative positioning with the diaphragm 3 described later, and can be accurately assembled to a lens barrel (not shown).

As the marking M, for example, FIG.
As shown in FIG. 3A, a part (peripheral portion) of each of the diffraction gratings 21 and 22 may be deleted or cut out as shown in FIG. In order to transfer and mold such a marking, such a marking forming portion may be provided in the mold itself in advance. Further, as the marking M, for example, as shown in FIG. 3C, ink or the like may be printed in a spot shape on a goba portion, or a concave depression or a convex projection may be provided in a spot shape. Is also good. Further, in order to transfer and form a projection as shown in FIG. 3D as the marking M, such a marking forming portion may be provided in the mold itself in advance.

The stop 3 blocks light passing through a defective portion such as a notch D generated in the diffraction grating at the time of mold release,
It cuts off flare light that does not contribute to image formation, and transmits light that passes through other normal grating portions to the line sensor 4.
It is opened in a non-arc shape in order to pass through. The diaphragm 3 has an opening such as a flat diaphragm as shown in FIG. 2 (A) or a deformed diaphragm which blocks only the chipped D as shown in FIG. 2 (B).

In the first embodiment, the light transmitted through the notch D of the diffraction grating is blocked by the stop 4, but the present invention is not limited to this. That is, when the chipping D of the diffraction grating is small, the flare light generated by the chipping is small. In such a case, as described above, the marking M is provided on the diffraction gratings 21 and 22 so that the chipped D portion is easily visually observed or touched with a finger or the like to generate the chipped D portion. Can be identified. In other words, since the direction and area in which flare light is generated can be specified, for example, the image read by the line sensor 4 can be subjected to image processing later to minimize the deterioration of the image. Become like

Further, since the line sensor 4 described above is usually used in the image reading apparatus, the incident angle of the reading light is different between the main scanning direction and the sub-scanning direction. The diffraction grating can be assembled to the lens barrel under the condition of the incident angle that minimizes the light.

Next, a second embodiment of the present invention will be described.

FIG. 4 is a sectional view of the diffractive optical element 2 according to the second embodiment. In this diffractive optical element 2, the optical axis 5
The lattice shape is formed asymmetrically on both the left and right sides with. Also in this diffractive optical element 2, as described above with reference to FIG. 14, one end of the diffractive optical element 2 is released from the mold 6 by beating out or pushing up with an appropriate jig. The mold release method is adopted.

In the second embodiment, the left and right diffraction gratings 2
A, 2B, with respect to the left diffraction grating 2B, the wall portion 24 of the grating portion is inclined by an angle α with respect to the optical axis 5 to form a molded component that becomes a diffractive optical element when releasing the mold. The lattice surface is prevented from coming into contact with a mold (not shown), and chipping hardly occurs. The angle α is uniquely determined from factors such as the grating pitch P, the grating height, the mold release speed, and the viscosity of the molding material.

Also in the second embodiment, a marking is provided on the diffractive optical element 2 so that the releasing direction can be recognized.
By combining this diffractive optical element 2 with a non-circular stop, flare light can be further prevented. Also,
By adjusting the position at which the diffractive optical element 2 is assembled to the lens barrel or by performing image processing on the obtained image, it is possible to prevent flare light and suppress image deterioration.

[0049]

As described above, according to the present invention, there is provided a diffractive optical element formed of an optical material such as resin or glass using a mold having a lattice surface shape,
The diffractive optical element has light-shielding means for shielding an incompletely shaped portion of the grating surface portion, and does not contribute to image formation by blocking light transmitted through the incompletely shaped portion with the light-shielding means. Since the flare light can be cut, it is possible to suppress image deterioration due to chipping in the diffraction grating and the like.

Further, according to the present invention, the grating shape in at least one cross section including the optical axis of the diffractive optical element is asymmetric, and among the diffractive optical elements which are molded products, conventionally, a mold is used. When releasing the formed product from the center of the diffraction surface that was easy to contact with the mold to one side part,
It is possible to make the mold difficult to contact. As a result, it is possible to prevent inconveniences such as breakage due to contact with the mold at the time of mold release, and occurrence of molding defects due to such causes can be effectively suppressed, so that the yield is also reduced. Improve, and in turn, reduce costs.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram showing an imaging lens and the like provided with a diffractive optical element according to Embodiment 1 of the present invention.

FIG. 2A is an explanatory diagram showing a relative positional relationship between an imaging lens and a flat stop, and FIG. 2B is an explanatory diagram showing a relative positional relationship between an imaging lens and a deformed stop.

FIGS. 3A to 3D are explanatory diagrams each showing a marking provided on an imaging lens; FIGS.

FIG. 4 is a sectional view of a principal part showing a diffractive optical element according to Embodiment 2 of the present invention.

FIG. 5 is an explanatory diagram of a diffraction action of a diffraction grating.

FIG. 6 is a graph showing a relationship between transmittance and wavelength of a general lens.

FIG. 7 is a graph showing the relationship between the diffraction efficiency and wavelength of a conventional diffraction grating.

FIG. 8 is a sectional view of a main part showing a conventional diffraction grating.

FIG. 9 is a graph showing the relationship between the combined transmittance, which is the product of the transmittance and the diffraction efficiency, and the wavelength.

FIG. 10 is an explanatory view showing a laminated type diffraction grating.

FIG. 11 is an explanatory view showing another laminated type diffraction grating.

FIG. 12 is an explanatory view showing a stacked type diffraction grating.

FIGS. 13A and 13B are explanatory views showing a method for forming and releasing a diffractive optical element using a mold.

FIG. 14 is an explanatory diagram showing a chipped portion generated in the diffractive optical element.

FIG. 15A is a cross-sectional view showing a normal laminated type diffractive optical element, and FIG. 15B is a cross-sectional view showing a laminated type diffractive optical element having a chipped portion.

FIGS. 16A and 16B respectively show FIGS.
6 is a graph showing a relationship between a diffraction efficiency and a wavelength in the diffractive optical elements of (A) and (B).

[Explanation of symbols]

 Reference Signs List 1 imaging lens 2 diffractive optical element 2A diffraction grating 2B diffraction grating 21 first diffraction grating 22 second diffraction grating 24 wall of grating portion 3 stop 4 line sensor 5 optical axis 6 mold d grating height D chipping m Order (of diffracted light) n Refractive index M Marking P Grid pitch θ Diffraction angle λ Wavelength ν Abbe number

Claims (11)

[Claims] 1. A diffractive optical element formed of an optical material such as resin or glass using a mold having a lattice surface shape formed in a cavity, wherein the diffractive optical element has an incomplete lattice surface portion. A diffractive optical element, comprising: a light-shielding unit that shields a light-shape portion. 2. A laminated type diffractive optical element in which two diffraction gratings are joined with their respective grating surfaces facing each other, wherein each diffraction grating is formed of an optical material having a different Abbe number. The diffractive optical element according to claim 1. 3. At least one element including an optical axis of the diffractive optical element.
3. The diffractive optical element according to claim 1, wherein the grating shape at the two cut surfaces is asymmetric. 4. A marking outside the effective light area of the diffractive optical element is provided with a marking for confirming a releasing direction when the diffractive optical element, which is a molded product, is released from the mold by releasing. Item 4. The diffractive optical element according to any one of items 1 to 3. 5. The diffractive optical element according to claim 1, wherein the marking is formed together with the molding of the diffractive optical element by a mold used for molding the diffractive optical element. 6. A diffractive optical element characterized in that a marking for confirming a releasing direction when a diffractive optical element which is a molded product is released from a mold is provided on a portion of the diffractive optical element outside a beam effective area. Optical element. 7. An optical lens using the diffractive optical element according to any one of claims 1 to 6. 8. A mold in which a lattice surface shape is formed in a cavity, wherein the lattice shape in at least one sectional view including the optical axis of the diffraction grating is asymmetric. 9. The mold according to claim 8, wherein said diffraction grating is formed concentrically, and a wall thereof is symmetric with respect to an optical axis. 10. A diffractive optical element formed by using the mold according to claim 8 or 9 with an optical material such as resin or glass. 11. An optical lens using the diffractive optical element according to claim 10.