JP2001324674A - Optical system and optical equipment - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an optical system capable of always obtaining excellent diffraction efficiency over an entire image plane. SOLUTION: In this optical system where a diffraction grating rotationally symmetric with respect to an optical axis is provided on a lens surface having curvature, the sign of the curvature of the lens surface on which the diffraction grating is provided is the same as the sign of a focal distance in the designed wavelength of a synthetic system from the surface of the optical system nearest to an object side to the surface thereof just before the lens surface on which the diffraction grating is provided, and is different from the sign of the distance of a position where the center light beam of off-axis luminous flux is made incident on the lens surface on which the diffraction grating is provided from the optical axis.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical system having a diffractive optical element. The present invention relates to an optical system suitable for an optical device such as a projector.

[0002]

2. Description of the Related Art Conventionally, as one method of correcting chromatic aberration of an optical system, there is a method of combining two glass materials (lenses) having different dispersions.

In contrast to the conventional method of reducing chromatic aberration by combining glass materials, SPIE is a method of reducing chromatic aberration by providing a diffractive optical element such as a diffraction grating having a diffractive effect on a lens surface or a part of an optical system. Vol. 1354 Inter
It is disclosed in documents such as national Lens DesignConference (1990), JP-A-4-213421, JP-A-6-324262, and US Pat. No. 5,044,706.

[0004] This utilizes a physical phenomenon in which chromatic aberration for a light beam having a certain reference wavelength appears in the opposite direction between a refraction surface and a diffraction surface in an optical system. Further, such a diffractive optical element can have an effect like an aspheric lens by changing the period of the periodic structure, and has a great effect on reduction of aberration.

Here, in refraction, one light beam is one light beam after refraction, whereas in diffraction, light is divided into each order. Therefore, when a diffraction grating is used in the optical system, the grating structure is determined so that the luminous flux in the used wavelength region is concentrated on one specific order (hereinafter also referred to as a design order), and good diffraction efficiency is obtained over the entire screen. Needs to be obtained.

Japanese Patent Application Laid-Open No. H10-268115 proposes an optical system which uses a blazed diffraction grating as a diffraction grating and aims at uniformity of diffraction efficiency over the entire observation screen.

Japanese Patent Laid-Open No. 10-268115 discloses a blazed diffraction grating in which the height of the grating in the peripheral region is lower than the depth of the grating (grating portion) in the central region centered on the optical axis. A Kepler-type finder optical system configured so that the diffraction efficiency is almost the same in the region and the peripheral region, and a non-effective surface in the center region centered on the optical axis. (Corresponding to the side surface) is almost a part of a cylindrical surface, and a blazed diffraction grating is used in which the non-effective surface in the peripheral region is a part of a conical surface to prevent vignetting of light rays on the non-effective surface in the peripheral region. A Kepler-type finder optical system is disclosed.

[0008]

Problems to be Solved by the Invention Japanese Patent Application Laid-Open No. Hei 10-2681
In the Kepler-type finder optical system proposed in Japanese Patent Publication No. 15, each light beam is separated at a position where the on-axis light beam and the most off-axis light beam pass through a diffractive optical surface provided at a position relatively far from the stop. The configuration of the publication has a property that the effect of uniformity of diffraction efficiency including vignetting of the light beam cannot be obtained unless the light beam is in such a passing state.

That is, in the photographing optical system assumed as the optical system of the present invention, as can be seen from FIGS. 1, 2 and 3 which will be described in the embodiments described later, the on-axis light flux inside the optical system will be described. And the off-axis luminous flux are not completely separated, so that even on the lens surface away from the stop, the passing regions of the on-axis luminous flux and the off-axis luminous flux tend to overlap.

Therefore, it is difficult to obtain good diffraction efficiency both on-axis and off-axis at the same time, even if the configuration of this publication is applied to a photographic lens as it is.

In particular, in the case of a photographing lens having a relatively large aperture, the respective regions of the incident light beam on the lens surface provided with the diffraction grating of the on-axis light beam and the off-axis light beam largely overlap each other.
The configuration as it is further disadvantageous.

It is an object of the present invention to provide achromatization by combining a diffractive optical element and a refractive optical element, even if the light beams reaching each position on the screen greatly overlap each other on the diffractive optical surface. It is an object of the present invention to provide an optical system which has good diffraction efficiency over a wide range and has high optical performance.

[0013]

According to a first aspect of the present invention, there is provided an optical system in which a diffraction grating rotationally symmetric with respect to an optical axis is provided on a lens surface having a curvature, and the lens provided with the diffraction grating is provided. The sign of the curvature of the surface is the same sign as the sign of the focal length at the design wavelength of the synthesis system from the most object side surface of the optical system to the surface immediately before the lens surface provided with the diffraction grating, and The sign of the distance from the optical axis at the position where the center ray of the off-axis light beam enters the lens surface on which the diffraction grating is provided is different in sign.

According to a second aspect of the present invention, in the first aspect of the present invention, a vertex of a virtual cone formed when at least a part of the non-effective surface of the diffraction grating is extended is a lens surface provided with the diffraction grating. Is characterized by being located near the center of curvature.

According to a third aspect of the present invention, there is provided an optical system having a diffractive optical surface in which a concentric diffraction grating rotationally symmetric with respect to an optical axis is provided on a lens surface having a curvature. An apex of a virtual cone formed when at least a part of the effective surface is extended is located near the center of curvature of the lens surface on which the diffraction grating is provided.

According to a fourth aspect of the present invention, in the second or third aspect, the distance from the vertex of the virtual cone to the center of curvature of the lens surface on which the diffraction grating is provided is DL, and the diffraction grating is provided. When a radius of curvature of the lens surface is R, | DL / R | <0.3 is satisfied.

The invention of claim 5 is the invention of claim 1, 2, 3 or 4.
The focus at the design wavelength of the combining system from the center of curvature of the lens surface provided with the diffraction grating to the surface immediately before the lens surface provided with the diffraction grating from the most object side surface of the optical system. Where D is the distance to the lens and R is the radius of curvature of the lens surface on which the diffraction grating is provided, and | D / R | <5.

According to a sixth aspect of the present invention, in any one of the first to fifth aspects, the refractive power of the lens surface on which the diffraction grating is provided is P, and the height in the direction perpendicular to the optical axis is P. Y, design wavelength is λ, phase coefficient is C _i (i = 1,
2, 3,...), The phase shape of the diffraction grating is φ (Y) = (2π / λ ₀ ) (C ₁ Y ² + C ₂ Y ⁴ + C ₃ Y ⁶ +
..), Is characterized by satisfying a condition of C ₁ · P <0.

According to a seventh aspect of the present invention, in any one of the first to sixth aspects, the diffraction grating is a stacked diffraction grating.

The invention of claim 8 is characterized in that, in the invention of claim 7, the stacked diffraction grating is an adjacent stacked diffraction grating in which two diffraction gratings are arranged adjacent to each other with an air layer therebetween.

According to a ninth aspect of the present invention, in the invention of the eighth aspect, the adjacent laminated diffraction grating is provided between two adjacent lens surfaces having substantially the same curvature, and the first layer, the first layer, It has a three-layer structure of a second layer and a third layer.
It is characterized in that the layer is an air layer.

A tenth aspect of the present invention is characterized in that, in the eighth aspect of the invention, each of the two diffraction gratings of the adjacent stacked type diffraction grating is made of an ultraviolet-curable resin.

According to an eleventh aspect of the present invention, in any one of the first to seventh aspects of the present invention, a plurality of the diffractive optical surfaces are provided.

According to a twelfth aspect of the present invention, in any one of the first to sixth aspects, the diffraction grating is a blazed diffraction grating.

The optical apparatus according to the thirteenth aspect of the present invention is the first aspect of the present invention.
13. The optical system according to any one of items 1 to 12, wherein

[0026]

FIG. 1, FIG. 2, and FIG. 3 are cross-sectional views of a main part of an optical system OL according to embodiments 1, 2, and 3 of the present invention. 1 is a telephoto type, FIG. 2 is a Gaussian type, FIG.
Is an example in which the present invention is applied to a reverse telephoto type photographing optical system OL. In the drawing, DO denotes a diffractive optical surface having a diffraction grating provided on a lens surface, and each of the diffraction gratings is a concentric circle rotationally symmetric with respect to the optical axis, and has a blaze-shaped cross section. SP is an aperture stop that determines the brightness of the optical system, and IP is the image plane.

The sign of the curvature of the lens surface is positive when the center of curvature is on the light exit side (image side) with respect to the lens surface, and negative when the center of curvature is on the light incident side (object side). Therefore, the lens surface convex toward the object side (concave toward the image side) has a positive sign, and the lens surface concave toward the object side (convex toward the image side) has a negative sign. On the other hand, the sign of the distance from the optical axis at the position where the central ray of the off-axis light beam enters the lens surface provided with the diffraction grating is when the incident position of the light ray is on the opposite side to the incident direction with respect to the optical axis. Is positive, and when it is on the same side as the incident direction, it is negative. Therefore, if the light beam crosses the optical axis, the sign is negative, and if it crosses the optical axis, the sign is positive.

FIGS. 4 and 5 are explanatory diagrams of the diffractive optical surface according to the present invention, and show a state in which a blazed diffraction grating 2 is provided on one lens surface 1a of the lens 1. FIG. In the drawing, reference numeral 4 denotes an effective surface of the diffraction grating which performs a desired diffraction operation, and 3 denotes a side surface which is not involved in the diffraction operation when the blazed diffraction grating 2 is formed on the lens surface 1a (hereinafter referred to as "ineffective surface"). It is.)

These ineffective surfaces 3 intersect the optical axis 5 at one point when at least a part thereof is extended, and are a part of a conical surface having the intersection as a vertex. In FIG. 4, the vertex of this virtual cone is shown as DOP, and the center of curvature of the lens surface 1a is shown as RC.

In this embodiment, the vertex DOP is the center of curvature RC.
In the vicinity. Here, the term “near” means that the distance from the vertex DOP of the cone to the center of curvature RC of the lens surface 1a of the base on which the diffraction grating is provided is DL, and the radius of curvature of the lens surface 1a is R, | DL / R | <0.3 is satisfied.

In this embodiment, each axial ray (on-axis luminous flux) from an object point to the lens surface of the base on which the diffraction grating is provided.
The direction of incidence is approximately the same as the direction of each normal at the point of intersection of the rays with the lens surface on which the diffraction grating is provided, for example, by setting the height of each orbicular zone of the diffraction grating to the optical axis. The height (the height at which the optical path length is an integral multiple of the design wavelength) at which the diffraction efficiency of the light beam that matches with is 100%,
It is set uniformly in the normal direction of the lens surface on which the diffraction grating is provided. As a result, the diffraction efficiency of the on-axis light beam is reduced by almost 1
00%.

As a result, vignetting of the entire on-axis light beam is eliminated, and high diffraction efficiency is obtained.

First, the diffraction efficiency of the on-axis light beam among the light beams diffracted by the diffractive optical surface will be described.

In each embodiment, the sign of the curvature Ra (1 / R) of the lens surface on which the diffraction grating is provided (the surface with the concave surface facing the image surface side (light exit side) is a positive sign, and the opposite sign is a negative sign). Is)
The sign of the focal length fa at the design wavelength of the combining system from the surface closest to the object (light incident side) of the optical system to the surface immediately before the lens surface provided with the diffraction grating is set to be the same. In the first embodiment of FIG. 1, Ra and fa are both positive.
In Embodiments 2 and 3 of FIG. 3, Ra and fa are both negative signs.

At this time, when the light rays constituting the axial light flux reaching the center of the screen are incident on the lens surface provided with the diffraction grating, the incident directions thereof intersect with the lens surface provided with the diffraction grating. The direction is almost the same as the direction of each normal line at the intersection. In this state, the vertex DOP of the cone in which the ineffective surface of the diffraction grating forms a part is set so as to be located near the center of curvature RC (the position where each normal is collected) RC of the lens surface on which the diffraction grating is provided. . This reduces vignetting of each axial ray on the ineffective surface of the diffraction grating, thereby preventing a reduction in diffraction efficiency due to vignetting.

Next, the diffraction efficiency of an off-axis light beam among the light beams diffracted by the diffractive optical surface will be described.

In each of the embodiments, the sign of the distance Ha from the optical axis at the position where the center ray of the off-axis light beam enters the lens surface provided with the diffraction grating (the incident position is opposite to the incident direction with respect to the optical axis). The positive sign and the same direction as the incident direction are referred to as negative signs) are different signs from the sign of the curvature Ra of the lens surface provided with the diffraction grating.

In the first embodiment shown in FIG. 1, the central ray LPa of the off-axis light beam is incident on the lens surface DO on which the diffraction grating is provided. At this time, the incident position DOa is set in the incident direction of the central ray LPa with respect to the optical axis La. And the distance Ha <0. The curvature Ra of the lens surface DO is Ra> 0.

In the second and third embodiments shown in FIGS. 2 and 3, the center ray LPa of the off-axis light beam is incident on the lens surface DO on which the diffraction grating is provided.
Since the direction is opposite to the incident direction of a, the distance Ha> 0.
The curvature Ra of the lens surface DO is Ra <0.

At this time, the direction of incidence of each ray constituting each off-axis light beam reaching the periphery of the screen on the lens surface provided with the diffraction grating is the same as that of the on-axis light beam. The direction is substantially the same as the direction of each normal line at the intersection that intersects the lens surface provided with.

Therefore, as in the case of the on-axis light flux, the vertex DOP of the cone in which the ineffective surface 3 of the diffraction grating forms a part is located at the center of curvature of the lens surface on which the diffraction grating is provided (each normal line is gathered). (Position) Near the RC, vignetting of each ray of each off-axis light beam reaching the vicinity of the screen on the ineffective surface of the diffraction grating can be reduced, and a decrease in diffraction efficiency can be prevented.

The direction of incidence of each ray constituting each off-axis luminous flux reaching the periphery of the screen on the lens surface provided with the diffraction grating is determined at each intersection point at which the ray intersects the lens surface provided with the diffraction grating. Since the direction is almost the same as the direction of the normal line, high diffraction efficiency can be obtained even if the height of the diffraction grating previously determined by the on-axis light flux remains.

According to the present invention, by virtue of the above construction, vignetting can be reduced over the entire screen and high diffraction efficiency can be obtained. In particular, the influence of color flare caused by unnecessary diffracted light of each color light when photographing a high-luminance object is obtained. It has eased.

In the present invention, an optical system having good optical performance over the entire screen is achieved by specifying each element as described above. More preferably, at least one of the following structures is used. Good to be satisfied.

According to this, it is possible to further reduce the vignetting on the ineffective surface of the diffraction grating of the light beam that arrives near the center of the screen where the image quality is relatively important. The diffraction efficiency can be further improved.

(A-1) The following conditional expression must be satisfied.

| D / R | <5 where D: the diffraction grating is provided from the center of curvature of the lens surface on which the diffraction grating is provided and from the surface closest to the object in the entire optical system. Distance to the focal position at the design wavelength of the combining system up to the surface immediately before the lens surface R: radius of curvature of the lens surface on which the diffraction grating is provided If the conditional expression is not satisfied, the diffraction efficiency particularly near the center of the screen is deteriorated. Not so good. In the present invention, it is more preferable that the numerical range of the conditional expression be as follows, whereby the diffraction efficiency can be further improved. That is, | D / R | <3 ... '.

(A-2) The following conditional expression must be satisfied.

C ₁ · P <0 where P is the refractive power of the lens surface on which the diffraction grating is provided, and C ₁ is the phase shape of the diffraction surface given by the following equation:
This is the phase coefficient of the second order term.

Φ (Y) = (2π / λ ₀ ) (C ₁ Y ² + C ₂ Y ⁴ + C ₃ Y ⁶ ...) (A) where, Y: height in the direction perpendicular to the optical axis λ ₀ : design wavelength C _i : phase coefficient (i = 1, 2, 3,...)

The conditional expression is mainly for producing the diffraction grating according to the present invention with high accuracy.

Hereinafter, the technical meaning of the conditional expression will be described.

The optical power (corresponding to the refractive power and represented by the reciprocal of the focal length) φ _D (λ, m) of the diffraction surface for an arbitrary wavelength λ and an arbitrary diffraction order m is the phase of the above equation (a). it can be expressed as follows using the coefficients C _1.

Φ _D (λ, m) = − 2 C ₁ mλ / m ₀ λ ₀ (b) The optical power of the diffraction surface at the design wavelength and design order is φ _D (λ ₀ , m ₀ ) = − 2C ₁ ... (C).

That is, the conditional expression means that the optical power φ _D (λ ₀ , m ₀ ) of the diffraction grating at the design wavelength and the design order is provided so as to have the same sign as the refractive power of the lens surface on which the diffraction grating is provided. are doing.

[0056] Further, this is the meaning of play in the present invention, FIG. 4 of the two blazed with different codes of C _1,
This will be described with reference to FIG.

FIGS. 4 and 5 are schematic sectional views of a diffraction grating obtained by converting the phase shape of the equation into a blazed shape having a different sign of the phase coefficient C ₁ . As the sign of the refractive power P of the lens surface 1a on which the diffraction grating 2 is provided, a curvature sign that satisfies the respective equations is selected. FIG. 4 shows that C ₁ <0
FIG. 5 shows the case where C ₁ > 0 (the optical power of the diffraction grating is a negative value) and P <0, and FIG. 5 shows the case where P <0. As described above, in the figure, 1
Denotes a lens on which a diffraction grating is provided, 2 denotes a diffraction grating, 3 denotes an ineffective surface of the diffraction grating, and a vertex DOP of a virtual cone formed by the ineffective surface 3 is a curvature of each lens surface 1a on which the diffraction grating 2 is provided. It is a conical surface near the center position RC.

As a method for manufacturing such a blazed diffraction grating, for example, a method in which glass is melted at a high temperature and press-molded with a mold or the like, or an ultraviolet-curable plastic resin or the like on the surface of a glass substrate or the like is used. Is press-molded in a mold and cured by irradiating ultraviolet rays, or a method of molding the plastic resin itself in a mold together with a lens is applicable. A method of forming a diffraction grating by directly cutting glass or a method of forming a fine step-like diffraction grating by wet etching or dry etching of SiO ₂ or the like is also applicable.

As is clear from FIGS. 4 and 5, since the ineffective surface 3 of the diffraction grating 2 is a part of a conical surface, it is assumed that the refractive power P of the lens surface on which the diffraction grating is provided and the optical power of the diffraction grating. It is not very good to set φ _D to have the opposite sign and C ₁ · P> 0. For example, in mold processing, it is difficult to process the mold, and when transferring the mold, it is difficult for the molten glass or resin to flow in the depth direction of the lattice, and transferability may be deteriorated. Furthermore, at the time of mold release, especially in the method of molding by melting glass and the method of molding with a plastic resin lens together with the lens, the mold cannot be released at the worst, and the tip of the grating part is damaged, resulting in diffraction efficiency. May be worsened. In the method of transferring an ultraviolet-curable plastic resin to a glass substrate or the like, the resin may have a relatively small viscosity, so it may be able to be released from the mold, but the vicinity of the tip of the lattice portion is deformed by the stress at the time of release, Again, the diffraction efficiency may deteriorate.

That is, when C ₁ · P ₁ > 0, the transferability and releasability of the mold are not good in any of the molding methods using the mold, and the desired diffraction efficiency is obtained. Is difficult to obtain, so it is better to satisfy the conditional expression if possible. In numerical example 1 described later, C ₁ · P> 0,
In Numerical Examples 2 and 3, C ₁ · P <0.

(A-3) The diffraction grating is a stacked diffraction grating.

(A-4) The stacked diffraction grating is an adjacent stacked diffraction grating having at least one thin air layer, and the diffraction gratings are arranged adjacent to each other with the air layer interposed therebetween.

(A-5) The adjacent laminated diffraction grating is provided between two adjacent lens surfaces having substantially the same curvature, and the first, second, and third layers are arranged in order from the object side. It has a three-layer structure, and the second layer is a thin air layer.

(A-6) First of the adjacent stacked diffraction gratings
The second layer and the second layer are formed of an ultraviolet curable resin.

In the structures (A-3) to (A-6), a grating structure for increasing the diffraction efficiency over the entire use wavelength range is specified. Next, these configurations will be described.

As a method for increasing the absolute value of the diffraction efficiency over the entire wavelength range to be used, a plurality of blazed diffraction gratings are arranged in close contact with or adjacent to each other, and the refractive index, Abbe number, 2. Description of the Related Art A stacked diffraction grating in which the depth of the grating is appropriately set is known.

FIG. 7 shows the wavelength dependence of the diffraction efficiency of the first-order diffracted light when a light beam is vertically incident on the single-layer blazed diffraction grating shown in FIG. As shown in FIG. 6, the actual structure of the diffraction grating is such that an ultraviolet-curing resin is applied to the surface of the base material 11 and the resin portion has a grating thickness d such that the diffraction efficiency of the first-order diffraction light at a wavelength of 530 nm is 100%. A grid 12 is formed. As is clear from FIG. 7, the diffraction efficiency at the design order decreases as the distance from the optimized wavelength of 530 nm increases, and conversely, the 0th-order and second-order diffracted lights near the design order increase. This increase in diffracted light other than the design order causes a flare, which leads to a decrease in the resolution of the optical system.

On the other hand, FIG. 9 shows the wavelength dependence of the diffraction efficiency of the first-order diffracted light when a light beam is vertically incident on the close-stacked diffraction grating shown in FIG. As a specific configuration, as shown in FIG. 8, a first diffraction grating 13 made of an ultraviolet curable resin (nd = 1.499, νd = 54) is formed on a base material 11, and another ultraviolet curable resin is formed thereon. Resin (nd = 1.598, ν
d = 28) is formed. In this combination of materials, the grating thickness d1 of the first diffraction grating portion 13 is d1 = 13.8 μm, and the grating thickness d2 of the second diffraction grating portion 14 is d = 10.5 μm. As can be seen from FIG. 9, the diffraction efficiency of the design order has a high diffraction efficiency of 95% or more over the entire use wavelength range by using the diffraction grating having the laminated structure.

As described above, by using a diffraction grating having a laminated structure as the diffraction grating of the embodiment of the present invention, the optical performance is further improved. At the same time, by configuring as described above, the wavelength dependence of the entire screen is improved. , A good diffraction efficiency can be obtained. For this reason, it is desirable to use a laminated type.

Next, among the stacked diffraction gratings, the non-effective surface of the diffraction grating affects the diffraction efficiency in the close-stacking type in which the grating portions are in close contact with each other and in the adjacent stacking type in which the thin air layer is disposed adjacent to the adjacent stack type. The effect will be described.

When these two types of diffraction gratings are made of substantially the same material, the depth of the grating portion for obtaining the required diffracted light is determined by the absolute value of the difference in the refractive index between the incident side and the emission side of the grating portion. Therefore, the adjacent stacked type having an air layer can make the total depth of the entire lattice portion smaller than that of the close stacked type.

As a result, the adjacent lamination type has less vignetting on the non-effective surface than the close lamination type.
This is advantageous in improving diffraction efficiency.

Therefore, it is preferable to form a diffraction grating of an adjacent lamination type with a thin air layer separated.

At this time, one lens constituting the optical system is divided into two such that the lens surfaces have substantially the same curvature, and an adjacent laminated diffraction grating is provided between the divided lens surfaces, and the adjacent diffraction grating is provided from the object side. If a three-layer structure of a first layer, a second layer, and a third layer is used in order, and the second layer is a thin air layer, it does not affect various aberrations of the entire optical system with a relatively simple configuration. Further better diffraction efficiency can be obtained.

When one lens is divided into two so that the lens surface has substantially the same curvature, the curvature of the divided lens surface is set to be substantially the same and set to any value even if the curvature is set to any value. Is hard to contribute. Therefore, it is possible to arbitrarily set the curvature specified for optimizing the diffraction efficiency near the center of the screen and near the periphery. Therefore, it is preferable to use such an adjacent laminated type diffraction grating.

At this time, the glass materials of the divided lenses need not be the same, and may be changed as necessary. Furthermore,
Before the introduction of the diffraction grating, the two lens surfaces provided with the grating portion originally exist in the optical system due to the aberration correction structure such as chromatic aberration correction, spherical aberration correction, or coma aberration correction, and the curvature conditions and the like are different. If the cemented surface satisfies the configuration of the present invention, it is not necessary to newly split the lens, and a diffraction grating may be provided there.

FIG. 10 is an enlarged schematic view of a part of the adjacent laminated diffraction grating applied to the optical system of the first embodiment. In the figure,
Reference numerals 21 and 22 denote lenses divided at substantially the same curvature, and reference numerals 23 and 24 denote first layers (nd = 1.6, respectively).
685, νd = 19.7), third layer (nd = 1.524)
0, vd = 50.8) diffraction grating, 25 is a second layer which is an air layer, and O is an optical axis of the optical system. P and Q are light rays incident on the diffraction grating 23, V is a normal line at the intersection of the light ray P (Q) and the lens surface 21a provided with the diffraction surface, and θ is a light ray P
FIG. 1 shows an angle formed between (Q) and a normal line V of the lens surface 21a.
At 0, clockwise is a positive angle. Further, d1, d
2 is the lattice thickness of the first and third layers, d1 = 5 μm, d
2 = 7.5 μm. Pi is the lattice pitch of the first layer (the lattice pitch of the first layer and the third layer in the i-th annular zone is set to be equal), and the minimum pitch is 155th of the outermost peripheral portion. In the orbicular zone, it is 156 μm.

As the shape of the adjacent laminated diffraction grating, the shape proposed by the present applicant in the above-mentioned Japanese Patent Application No. 11-213374 can be applied.

FIG. 11 shows the values of θ of the central ray and the marginal rays (upper and lower lines) of the meridional rays of the on-axis light flux and the most off-axis light flux of the optical system of the first embodiment. FIG. 12 shows θ = 0 ° and ±± of the adjacent stacked diffraction grating shown in FIG.
The diffraction efficiency of the first-order diffracted light including the vignetting on the ineffective surface of the diffraction grating with respect to the incident light at 10 ° is 450 nm at a wavelength of 450 nm.
This is the dependency of the grating pitch at 0 nm and 650 nm.

In general, the smaller the grating pitch, the greater the converted optical path length of the light beam passing through the grating portion with respect to the change in the incident angle of the light beam, so that the diffraction efficiency is greatly deteriorated. The diffraction efficiency of the adjacent stacked diffraction grating applied to the optical system of Embodiment 1 shown in FIG.
It can be seen that it has good diffraction efficiency.

As shown in FIG. 11, the light incident angle θ of the optical system of the first embodiment is the maximum angle of view (half angle of view 3.16).
°) marginal ray (underline) has the largest absolute value,
θ = + 2.5 °. The marginal ray passing position is the outermost peripheral portion of the grating portion, and the pitch of the grating portion is the minimum value Pi = 156 μm (i = 15 μm) at this position as described above.
5). Although the diffraction efficiency of the light beam passing through this position is the lowest, it can be seen from FIG. 12 that a high efficiency equivalent to θ = 0 ° incidence can be maintained.

Next, a description will be given of a method of forming a laminated diffraction grating, a degree of freedom in designing aberration correction and diffraction efficiency.

One purpose of introducing a diffraction grating into an optical system having a refractive member such as a lens is to cancel chromatic aberration generated in the refractive optical system by the diffraction grating.

Therefore, when the diffraction grating is provided on the lens surface, the lens provided with the diffraction grating shares the necessary chromatic aberration as a refractive optical system in the chromatic aberration sharing with the diffraction grating. It is difficult to choose arbitrarily.

In other words, in a method in which glass is press-molded with a lens or the like while melting glass at a high temperature, or a method in which a glass is molded with a lens using a plastic resin, the lens and the diffraction grating are made of the same material. Therefore, it is difficult to achieve both the chromatic aberration and the improvement of the wavelength dependence of the diffraction efficiency of the laminated grating.

Therefore, if a molding method using an ultraviolet-curable plastic resin or the like is used, for example, in which the materials of the lens and the diffraction grating can be independently selected, an optical system having good chromatic aberration and good diffraction efficiency can be obtained. That is, it is preferable to adopt such a molding method.

The optical system of the present invention is widely used in image pickup devices such as film cameras, video cameras, digital cameras, etc., observation devices such as telescopes and binoculars, steppers for manufacturing semiconductor devices, and various optical measuring instruments. Can be applied.

Here, one example in which the optical system shown in the first to third embodiments is applied to an optical apparatus will be described with reference to FIG.

FIG. 13 is a schematic view of a main part of a single-lens reflex camera. In FIG. 13, reference numeral 10 denotes a photographing lens having any one of the optical systems 1 according to the first to third embodiments. The optical system 1 is held by a lens barrel 2 which is a holding member. Reference numeral 20 denotes a camera body, which includes a quick return mirror 3 for reflecting a light beam from the photographing lens 10 upward, a focusing plate 4 disposed at an image forming position of the photographing lens 10, and an upright image formed on the focusing plate 4. A penta roof prism 5 for converting the image into an image, and an eyepiece 6 for observing the erect image.
7 is a film surface. At the time of photographing, the quick return mirror 3 is retracted from the optical path, a shutter (not shown) is opened, and an image is formed on the film surface 7 by the photographing lens 10.

The advantages described in the first to third embodiments can be effectively obtained in the optical apparatus disclosed in the present embodiment.

Next, numerical examples of the optical systems according to the first to third embodiments of the present invention will be described.

In each numerical example, f is the focal length, fn
o is the f-number, ri is the radius of curvature of the i-th surface in order from the object side, and di is the i-th surface and (i) in order from the object side.
The (+1) -th surface interval, ni and νi are the refractive index and Abbe number of the i-th optical member in order from the object side.

For the aspherical shape, the optical axis direction is the X axis, the height in the direction perpendicular to the optical axis is H, the intersection point of the lens and the X axis is the origin, and r is the paraxial radius of curvature of the lens surface. A, B, C,
When D and E are each aspheric coefficients,

[0094]

(Equation 1)

This is represented by the following equation.

The respective coefficients C ₁ , C ₂ .
Is based on the above equation (a). Also, "D-0
"X" means 10- ^X . Table 1 shows the relationship between the above-described conditional expressions and various numerical values in numerical examples.

Table 1 shows the relationship between the above-mentioned conditional expressions and the numerical examples.

[0098]

[Outside 1]

[0099]

[Outside 2]

[0100]

[Outside 3]

[0101]

[Table 1]

[0102]

According to the present invention, by specifying each of the above elements, when performing achromatism by combining a diffractive optical element and a refractive optical element, each light beam reaching each position on the screen is on the diffractive optical surface. However, even if they overlap greatly, good diffraction efficiency can be obtained over the entire screen, and an optical system having high optical performance can be achieved.

[Brief description of the drawings]

FIG. 1 is a sectional view of a main part of a first embodiment of the present invention.

FIG. 2 is a sectional view of a main part according to a second embodiment of the present invention.

FIG. 3 is a sectional view of a main part of a third embodiment of the present invention.

FIG. 4 is a schematic diagram of a blaze shape when the refractive power of the diffraction grating and the lens surface is positive.

FIG. 5 is a schematic diagram of a blaze shape when the refractive power of the diffraction grating and the lens surface is negative.

FIG. 6 is a schematic diagram of a single-layer diffraction grating.

FIG. 7 is a graph showing the wavelength dependence of diffraction efficiency of the diffraction grating of FIG. 6;

FIG. 8 is a schematic diagram of an example of a laminated diffraction grating (contact type).

FIG. 9 is a graph showing the diffraction efficiency and wavelength dependence of the diffraction grating of FIG. 8;

FIG. 10 is a schematic view of an example of a laminated diffraction grating (adjacent type with an air layer interposed) applied to the first embodiment of the present invention.

FIG. 11 shows angles of incidence (θ) of a central ray (principal ray) and a marginal ray (upper and lower lines) of a meridional ray of an on-axis ray and an off-axis ray according to the first embodiment of the present invention. Illustration

FIG. 12 shows wavelengths of 450 nm and 550 n of first-order diffraction efficiency including vignetting on a non-effective surface with respect to θ = 0 ° and ± 10 ° incident light of the adjacent stacked diffraction grating of the first embodiment of the present invention.
Explanatory diagram showing grating pitch dependence characteristics at m and 650 nm

FIG. 13 is a schematic diagram of a main part of an embodiment of an optical apparatus (single-lens reflex camera) of the present invention.

[Explanation of symbols]

 OL optical system DO diffractive optical surface SP stop LP image surface 1 lens 1a lens surface 2 diffraction grating 3 non-lens surface 4 lens surface 5 optical axis DOP vertex of cone RC center of curvature 11 substrate 12, 13, 14, 23, 24 diffraction grating 21,22 lens 25 air layer

Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat II (reference) G02B 13/18 G02B 13/18 F term (reference) 2H049 AA04 AA18 AA37 AA39 AA43 AA45 AA53 AA55 AA65 2H087 KA02 KA03 LA02 LA03 LA06 NA14 PA06 PA09 PA12 PA16 PA18 PB07 PB10 PB16 QA02 QA06 QA07 QA12 QA17 QA21 QA22 QA25 QA26 QA32 QA34 QA41 QA45 QA46 RA05 RA13 RA46 UA01

Claims (13)

[Claims] In an optical system in which a diffraction grating rotationally symmetric with respect to the optical axis is provided on a lens surface having a curvature, the sign of the curvature of the lens surface provided with the diffraction grating is the closest to the object side of the optical system. Lens having the same sign as the sign of the focal length at the design wavelength of the combining system from the surface to the surface immediately before the lens surface provided with the diffraction grating, and the center ray of the off-axis light beam is provided with the diffraction grating. An optical system characterized by having a sign different from the sign of the distance from the optical axis at the position of incidence on the surface. 2. The method according to claim 1, wherein a vertex of a virtual cone formed when at least a part of the ineffective surface of the diffraction grating is extended is located near a center of curvature of a lens surface provided with the diffraction grating. The optical system according to claim 1, wherein 3. An optical system having a diffractive optical surface in which a concentric diffraction grating rotationally symmetric with respect to an optical axis is provided on a lens surface having a curvature, at least a part of an ineffective surface of the diffraction grating is extended. An apex of an imaginary cone formed when the optical system is located near the center of curvature of a lens surface on which the diffraction grating is provided. 4. When the distance from the vertex of the virtual cone to the center of curvature of the lens surface on which the diffraction grating is provided is DL, and the radius of curvature of the lens surface on which the diffraction grating is provided is R. 4. The optical system according to claim 2, wherein a condition of R | <0.3 is satisfied. 5. A design wavelength of a combining system from a center of curvature of a lens surface on which the diffraction grating is provided to a surface immediately before the lens surface on which the diffraction grating is provided from a surface closest to the object of the optical system. 4. A lens according to claim 1, wherein a distance to a focal point is D, and a radius of curvature of a lens surface on which the diffraction grating is provided is R. | D / R | <5
Or the optical system of 4. 6. The refractive power of the lens surface on which the diffraction grating is provided is P, the height in the direction perpendicular to the optical axis is Y, the design wavelength is λ ₀ , and the phase coefficient is C _i (i = 1, Two, three
..), The phase shape of the diffraction grating is φ (Y) = (2π / λ ₀ ) (C ₁ Y ² + C ₂ Y ⁴ + C ₃ Y ⁶ +
The optical system according to any one of claims 1 to 5, wherein the following condition is satisfied: C ₁ · P <0. 7. The optical system according to claim 1, wherein said diffraction grating is a stacked diffraction grating. 8. The multi-layer diffraction grating according to claim 1, further comprising:
The optical system according to claim 7, wherein the two diffraction gratings are adjacent stacked diffraction gratings arranged adjacent to each other. 9. The adjacent laminated diffraction grating is provided between two adjacent lens surfaces having substantially the same curvature, and has a three-layer structure of a first layer, a second layer, and a third layer in order from the object side. The optical system according to claim 8, wherein the second layer is an air layer. 10. The optical system according to claim 8, wherein each of the two diffraction gratings of the adjacent stacked diffraction grating is made of an ultraviolet curable resin. 11. The optical system according to claim 1, comprising a plurality of said diffractive optical surfaces. 12. The optical system according to claim 1, wherein said diffraction grating is a blazed diffraction grating. 13. An optical apparatus having the optical system according to claim 1. Description: