JPH10268115A - Diffraction optical element and optical system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a diffraction optical element having the same diffraction efficiency in the central region and peripheral region and generating no vignetting of light in the peripheral region by setting the grating height of a diffraction grating in the peripheral region lower than the grating height of the diffraction grating in the central region centering on the optical axis. SOLUTION: A diffraction optical element is arranged at the position relatively apart from the aperture position of an optical system or the position where the main light crosses the optical axis. The diffraction optical element has a blaze-shaped diffraction grating and acts as a lens. A pair of circular curved surfaces centering on the optical axis (a blaze-shaped transmission face provided to transmit diffracted light and a wall face having no lens action) are formed from the central region to the peripheral region of the diffraction grating. The grating height h2 of the diffraction grating in the peripheral region is set lower than the grating height h1 of the diffraction grating in the central region.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a diffractive optical element and an optical system, and more particularly, to a diffractive optical element having a blazed diffraction grating and acting as a lens and an optical system having the same. Things.

[0002]

2. Description of the Related Art In a diffractive optical element which functions as a lens by having a diffraction grating, it is necessary to increase the power in a peripheral region rather than the power in a central region around the optical axis. In order to increase the power in the peripheral region, it is necessary to reduce the lattice spacing in the peripheral region, but it is difficult to manufacture a diffraction grating with a small lattice spacing. In order to increase the power without reducing the grating interval, a diffractive optical element in which the design order of the grating is increased is proposed in European Patent No. 0427175. In this diffractive optical element, in order to increase the design order of the grating, the grating height of the diffraction grating in the peripheral region is higher than the grating height of the diffraction grating in the central region around the optical axis.

[0003]

However, European Patent No. 0
The diffractive optical element proposed in Japanese Patent No. 412751 has a problem that the diffraction efficiency in the peripheral region is reduced and a problem that vignetting occurs in the peripheral region. In Japanese Patent Publication No. 7-50206, a grating shape (bi-level grating height, bi-level grating height,
(Inclination to form lattice convex part and refractive index difference between convex part and concave part)
Japanese Patent Application Laid-Open No. 5-150108 proposes a grating shape (a shape in which an optical path length in a thickness direction of a diffraction grating is changed in a predetermined region) to obtain a desired phase shift performance. However, these grating shapes do not act as lenses. Therefore, even if these grating shapes are adopted, it is not possible to prevent the reduction in the diffraction efficiency and the occurrence of vignetting.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and a first object of the present invention is to act as a lens and to provide a central region and a peripheral region centered on an optical axis. Another object of the present invention is to provide a diffractive optical element having the same diffraction efficiency. The second purpose is to act as a lens, and
It is an object of the present invention to provide a diffractive optical element in which vignetting of light rays does not occur in a peripheral region around an optical axis.

[0005]

To achieve the above object, a diffractive optical element according to a first aspect of the present invention is a diffractive optical element having a blazed diffraction grating and acting as a lens. The grating height of the diffraction grating in the peripheral region is lower than the grating height of the diffraction grating in the central region as the center.

An optical system according to a second aspect of the invention has a configuration in which the diffractive optical element according to the first aspect of the invention is disposed at a position relatively far from a stop position or a position where a principal ray and an optical axis intersect. .

A diffractive optical element according to a third aspect of the present invention is a diffractive optical element having a blazed diffraction grating and acting as a lens. The diffractive optical element has a blazed transmission surface provided for transmitting diffracted light and a lens. On the non-working wall surface, each blazed shape is configured, the blazed wall surface in the central area around the optical axis is almost a cylindrical surface, and the blazed wall surface in the peripheral area around the optical axis is a conical surface. Is characterized by being a part of

An optical system according to a fourth aspect of the invention has a configuration in which the diffractive optical element according to the third aspect of the invention is arranged at a position relatively far from a stop position or a position where the principal ray and the optical axis intersect. .

A diffractive optical element according to a fifth aspect of the present invention is the diffractive optical element according to the third aspect of the present invention, wherein the light incident side surface is made of the blazed diffraction grating, and the light exit angle is larger than the light incident angle. It is a diffractive optical surface having a high power.

According to a sixth aspect of the present invention, in the diffractive optical element according to the third aspect of the present invention, the light emitting side surface is formed of the blazed diffraction grating, and the light emitting angle is smaller than the light incident angle. It is a diffractive optical surface having a high power.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a diffractive optical element embodying the present invention and an optical system having the same will be described. In general, the grating height h of a blazed diffraction grating is determined by the following equation (1). h = m0 × λ0 / (n0 × cosθ−n′0 × cosθ ′) (1) where, m0: design order, λ0: design wavelength, n0: refractive index on the light incident side at the design wavelength, n′0: Refractive index on the light emission side at the design wavelength, θ: light incident angle, θ ′: light emission angle.

The diffraction efficiency of a ray satisfying the above equation (1) is 1 (ie, 100%). In general, the incident angle of a light beam on an arbitrary portion of the diffractive optical element is uniformly distributed at positive and negative angles around 0 °, and is represented by a light incident angle θ = 0 °. Further, since the power of the diffraction grating is weak, the light emission angle θ ′ ≒ 0 ° can be set. Therefore, the grating height h of the diffraction grating can be generally represented by the following equation (2). h = m0 × λ0 / (n0−n'0)… (2)

However, if the diffractive optical element is arranged at a position relatively far from the stop position of the optical system or the position where the principal ray intersects the optical axis, the incident angle of the light beam to an arbitrary position of the diffractive optical element (particularly, the optical axis Is not distributed uniformly at positive and negative angles about 0 °. Therefore, in consideration of this, in order for the diffraction efficiency to be 1 also with respect to the light beam incident at an angle with respect to the peripheral region of the diffractive optical element, the diffraction height is larger than the grating height of the diffraction grating in the central region.
It is necessary to lower the grating height of the diffraction grating in the peripheral region.

Table 1 shows e-line, various incident angles θ of the ray at the diffraction grating designed for the incident ray of e-line, θ = 0 °.
Are shown together with the calculation results of the diffraction efficiency when the grating height is changed from the designed grating height.
The “grating height ratio” in Table 1 indicates that the light incident angle θ = 0.
This is the grating height when the grating height h at which the diffraction efficiency becomes 1 at ° is 1.

[0015]

[Table 1]

As can be seen from Table 1, when the incident angle θ of the light beam is large, the diffraction efficiency can be improved by lowering the grating height. As described above, in the diffractive optical element which acts as a lens by having the blazed diffraction grating, the grating height of the diffraction grating in the peripheral region is larger than the grating height of the diffraction grating in the central region centered on the optical axis. , The diffraction efficiency can be made the same in the central region and the peripheral region.

When a lens function is realized by a blazed diffraction grating, the blazed shape is a pair of curved surfaces forming a circular shape around the optical axis (that is, a blazed shape provided for transmitting diffracted light). (A transmission surface and a wall surface which does not act as a lens) are formed from the central region to the peripheral region. One of the pair of curved surfaces
(Wall surfaces that do not act as lenses) are often composed of curved surfaces (substantially cylindrical surfaces) containing straight lines that are almost parallel to the optical axis, but the grating height h of the diffraction grating is generally about 1 micron. Therefore, the effect of vignetting of the light beam on the curved surface is very small. However, in the case of a light beam having an angle, vignetting occurs due to the blaze shape. In particular, when the design order is increased or when there is a diffraction effect at the interface between two optical materials,
Since the diffraction grating height h is relatively large, vignetting due to the blaze shape is increased.

Therefore, in consideration of the occurrence of vignetting, it is desirable that the blaze shape in the peripheral region where the light beam has an angle does not have a curved surface including a straight line substantially parallel to the optical axis. In other words, the blaze shape of the central region centered on the optical axis is partially formed by a curved surface including a straight line substantially parallel to the optical axis (that is, one of the pair of curved surfaces), If the blaze shape of the peripheral region around the axis is such that the curved surface corresponding to the curved surface includes only a straight line that forms a larger angle with the optical axis than the straight line
(Walls that do not act as lenses form part of a conical surface.) Also, vignetting due to the blaze shape should not occur for light rays incident at an angle to the peripheral area of the diffractive optical element. Can be.

If the light incident side surface is a diffractive optical surface made of the blazed diffraction grating and having a power (eg, a positive power) such that the light exit angle is larger than the light incident angle, Vignetting due to the blaze shape does not occur even for a light beam incident with the angle. Further, if the light emitting side surface is a diffractive optical surface having a power (for example, negative power) such that the blazed diffraction grating is formed of the blazed diffraction grating and the light emitting angle is smaller than the light incident angle. Vignetting due to the blaze shape does not occur even for a light beam that is incident upon being carried.

[0020]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a diffractive optical element embodying the present invention and an optical system using the same will be described more specifically with reference to construction data, drawings and the like.

Embodiments 1 and 2 (FIGS. 1 to 4) Embodiment 1 shown in FIG. 1 is a Kepler-type finder optical system using the diffractive optical element according to the first invention. This corresponds to the optical system according to the invention. Embodiment 2 shown in FIG. 3 is a Keplerian finder optical system using the diffractive optical element according to the third invention, and corresponds to the optical system according to the fourth invention. Tables 2 and 4 show construction data {surface, radius of curvature, axial spacing, refractive index (e-line), Abbe number} of Examples 1 and 2, and Tables 3 and 5 show Examples 1 and 2. 2 shows the aspherical surface data and the diffractive optical surface data.

In each construction data, Si
(i = 1,2,3, ...) is the i-th lens surface counted from the object side, and Pi is (i = 1,2,3, ...) is the i-th lens surface counted from the object side. Prism surface. The surface Si marked with [ASP] indicates that the surface is constituted by an aspheric surface, and is defined by the following equation (AS) representing the surface shape of the aspheric surface. Also, the surface Si marked with [DOE] indicates that the surface is composed of a diffractive optical surface, and is defined by the following formula (DS) representing the phase shape of the pitch of the diffractive optical surface. I do.

Z (h) = c · h ² / {1+ (1-ε · c ² · h ² ) ^1/2 } + ΣAi · h ⁱ (AS) where Z ( h): displacement amount from the reference plane in the optical axis direction, h: height in the direction perpendicular to the optical axis, c: paraxial curvature, ε: quadratic surface parameter, Ai: i-th order aspherical coefficient is there.

Φ (h) = (2π / λ) · (ΣCi · h ²ⁱ ) (DS) In the equation (DS), φ (h) is a phase function of the diffractive optical surface, and Ci is a diffractive optical surface. i-th phase function coefficient, h: height in the direction perpendicular to the optical axis (grating height), λ: design wavelength (= 546 nm).

[0025]

[Table 2]

[0026]

[Table 3]

[0027]

[Table 4]

[0028]

[Table 5]

The Kepler-type finder optical systems of Embodiments 1 and 2 are composed of four positive, negative, negative and positive objective lenses, a stop (E0) between the objective lenses, four inversion system prisms, and an inversion system. A field frame arranged at a primary image plane (I) position between the prisms;
It consists of two eyepieces. In both Examples 1 and 2,
The lens closest to the object is the diffractive optical element. In the first embodiment, the image side surface S2 is the diffractive optical surface [DOE], and in the second embodiment, the object side surface S1 is the diffractive optical surface [DOE]. In both Examples 1 and 2, the off-axis object height K is -1412.4 mm and the object distance is -3.
000mm, finder magnification is -0.399, diopter is -1.001.

FIG. 2 is a sectional view showing the diffractive optical surface (S
2A shows an enlarged cross-sectional shape of (A) a central region and a cross-sectional shape of (B) a peripheral region. In FIG. 2, ax indicates an axis parallel to the optical axis AX, h1 indicates the grating height of the diffraction grating in the central region, and h2 indicates the grating height of the diffraction grating in the peripheral region.

In the first embodiment, since the diffractive optical surface [DOE] is arranged at a position far from the entrance pupil (E1), a light ray enters the peripheral area centered on the optical axis AX at an angle.
Therefore, as described above, when the diffraction grating height h is designed in consideration of the incident angle with respect to the diffraction optical surface, the grating height h2 in the peripheral region is larger than the grating height h1 in the central region. Lower.

FIG. 4 is a sectional view showing the diffractive optical surface (S
1A is an enlarged view of (A) the cross-sectional shape of the central region and (B) the cross-sectional shape of the peripheral region. 4, ax indicates an axis parallel to the optical axis AX, and V1 in the central region is a curved surface including a straight line substantially parallel to the optical axis AX {the straight line appearing in the cross section of FIG. 4A}. V2 in the peripheral region (substantially cylindrical surface) and a curved surface corresponding to the curved surface V1 (part of a conical surface). Α is a curved surface
The straight line included in V2 {the straight line appearing in the cross section of FIG. 4B} is the axis ax
Is the angle made with respect to

In the second embodiment, since the diffractive optical surface [DOE] is arranged at a position far from the entrance pupil (E1), a light ray enters the peripheral area around the optical axis AX at an angle.
Therefore, as described above, when the blazed shape of the diffraction grating is designed in consideration of the incident angle with respect to the diffractive optical surface, the straight line included in the blazed curved surface V2 in the peripheral region {FIG.
Angle between the optical axis AX and the straight line appearing in the cross section of FIG.
Is larger than the straight line {the straight line appearing in the cross section of FIG. 4A} included in the curved surface V1.

<< Embodiment 3 (FIG. 5) >> The cross-sectional view of FIG. 5 shows an enlarged cross-sectional shape of the peripheral area of the diffractive optical surface D3 of Embodiment 3. In FIG. 5, ax is an axis parallel to the optical axis AX, L is a ray incident on the diffractive optical surface D3, L 'is a ray emitted from the diffractive optical surface D3, θ is a ray incident angle, and θ' is a ray exit Angle, Δ is θ
And θ ′ are shown. In the third embodiment, the diffractive optical surface D3 is used on the light incident side of the lens. The diffractive optical surface D3 has a positive power such that the light exit angle θ ′ is larger than the light incident angle θ. I have.

<< Embodiment 4 (FIG. 6) >> The cross-sectional view of FIG. 6 shows an enlarged cross-sectional shape of the peripheral area of the diffractive optical surface D4 of Embodiment 4. In FIG. 6, ax is an axis parallel to the optical axis AX, L is a light beam incident on the diffractive optical surface D3, L 'is a light beam emitted from the diffractive optical surface D3, θ is a light incident angle, and θ' is a light beam emission Angle, Δ is θ
And θ ′ are shown. In the fourth embodiment, the diffractive optical surface D4 is used on the light exit side of the lens, and the diffractive optical surface D4 has a negative power such that the light exit angle θ ′ is smaller than the light incident angle θ. I have.

[0036]

According to the first aspect of the present invention, as described above, a diffractive optical element which functions as a lens and has the same diffraction efficiency in the central region and the peripheral region around the optical axis is realized. be able to. According to the second aspect, the effect of equalizing the diffraction efficiency is more effectively achieved.

According to the third, fifth, and sixth aspects of the invention,
It is possible to realize a diffractive optical element that acts as a lens and does not cause vignetting of light rays in a peripheral region around the optical axis. According to the fourth aspect, the occurrence of the vignetting is more effectively achieved.

[Brief description of the drawings]

FIG. 1 is an optical configuration diagram showing a lens configuration and an optical path of a Kepler type finder optical system according to a first embodiment.

FIG. 2 is an enlarged sectional view showing shapes of a central area and a peripheral area of a diffractive optical surface used in the first embodiment.

FIG. 3 is an optical configuration diagram illustrating a lens configuration and an optical path of a Kepler-type finder optical system according to a second embodiment.

FIG. 4 is an enlarged sectional view showing the shapes of a central area and a peripheral area of a diffractive optical surface used in Example 2.

FIG. 5 is an enlarged sectional view showing a shape of a peripheral area of a diffractive optical surface used in a third embodiment.

FIG. 6 is an enlarged sectional view showing a shape of a peripheral area of a diffractive optical surface used in Example 4.

[Explanation of symbols]

 DOE… diffractive optical surface ASP… aspherical surface E0… stop E1… entrance pupil E2… exit pupil I… primary image plane

Claims (6)

[Claims] 1. A diffractive optical element having a blazed diffraction grating and acting as a lens, wherein the height of the diffraction grating in a peripheral region is higher than the height of the diffraction grating in a central region centered on the optical axis. A diffractive optical element characterized by having a lower value. 2. An optical system comprising the diffractive optical element according to claim 1 arranged at a stop position or a position relatively distant from a position where a principal ray and an optical axis intersect. 3. A diffractive optical element which acts as a lens by having a blazed diffraction grating, wherein a blazed transmission surface provided for transmitting diffracted light and a wall surface which does not act as a lens have individual elements. The blazed shape is configured, the blazed wall in the central area around the optical axis is almost cylindrical, and the blazed wall in the peripheral area around the optical axis is part of a conical surface. Diffractive optical element. 4. An optical system wherein the diffractive optical element according to claim 3 is disposed at a position relatively far from a stop position or a position where a principal ray and an optical axis intersect. 5. The light-incident side surface is a diffractive optical surface made of the blazed diffraction grating and having a power such that a light-emitting angle is larger than a light-incident angle. 4. The diffractive optical element according to 1. 6. A light diffusing optical surface having a power such that a light exit angle is smaller than a light incident angle, wherein the light exit side surface is made of the blazed diffraction grating. 4. The diffractive optical element according to 1.