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Optical element and optical system

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Optical element and optical system


The optical element includes a base member configured to have an optical surface, a concave-convex structure configured to have an average pitch smaller than a shortest wavelength of a use wavelength range, and an intermediate layer formed between the optical surface and the concave-convex structure, made of a material different from that of the concave-convex structure, and having a refractive index between those of the base member and the material of the concave-convex structure. The optical surface is formed into a shape having a rotational symmetry axis. A thickness of the intermediate layer or each of thicknesses of the intermediate layer and the concave-convex structure varies so as to increase as a distance from the rotational symmetry axis increases. The optical element has good anti-reflection performance not only at a central part of the optical surface having a small curvature radius but also at a peripheral part thereof.

Browse recent Canon Kabushiki Kaisha patents - Tokyo, JP
Inventors: Kazue Uchida, Takeharu Okuno, Daisuke Sano
USPTO Applicaton #: #20120262794 - Class: 359601 (USPTO) - 10/18/12 - Class 359 


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The Patent Description & Claims data below is from USPTO Patent Application 20120262794, Optical element and optical system.

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CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No. 12/548,754 filed Aug. 27, 2009, which claims priority to Japanese Patent Application Nos. 2009-184144, filed on Aug. 7, 2009 and 2008-222899, filed on Aug. 29, 2008, each of which is hereby incorporated by reference herein in its entirety.

BACKGROUND OF THE INVENTION

The present invention relates to an optical element having a reflection suppressing function (anti-reflection function).

Optical elements such as lenses used in optical systems are manufactured by using a transparent base material such as optical glass or optical plastic. Such a transparent base material has a large refractive index, so that a reflectance thereof is high. A high reflectance of the transparent base material reduces an amount of effective light reaching an image surface, and generates unnecessary reflection which causes ghost or flare. Therefore, it is necessary to provide an anti-reflection function to the optical element manufactured by using the transparent base material.

To provide the anti-reflection function to the optical element, an anti-reflection film is generally formed on a surface of the optical element (transparent base material). The anti-reflection film is formed by laminating thin film layers on the surface of the transparent base material, based on a general optical interference theory.

Methods for forming anti-reflection films include a dry method (vacuum film forming method) and a wet method (wet film forming method). The dry method coats a surface of the transparent base material with metal fluoride or metal oxide having a low refractive index, as with a vapor deposition method and a sputtering method. Further, the wet method applies coating liquid containing a low refractive index material to the surface of the transparent base material by a dipping method, a spin coat method or the like, and then dries or fires it.

A thickness (hereinafter also referred to “film thickness”) of the anti-reflection film is frequently designed such that an anti-reflection effect becomes largest at a central part of the optical element (hereinafter referred to as lens), that is, at a portion where an incident angle of a light ray is 0 degree. The anti-reflection film designed as above has a uniform film thickness over the entire lens surface.

However, when a light ray vertically enters the central part of the lens on which the anti-reflection film is formed and light rays enter a peripheral part of the lens in parallel with the above light ray, incident angles of the light rays become large at the peripheral part. Therefore, anti-reflection performance at the peripheral part becomes lower than that of the central part of the lens.

Japanese Patent Laid-open No. 2004-333908 discloses an anti-reflection film having an optical film thickness which makes its reflectance lowest for an entering/emerging light ray at arbitrary positions on a lens surface. According to a general optical interference theory, an optical path length difference between reflected light at a surface of the anti-reflection film and reflected light at a boundary surface between the anti-reflection film and the transparent base material is an odd-numbered times of one-half of a wavelength of the light. Accordingly, these reflected lights interfere with each other to be mutually weakened. Japanese Patent Laid-open No. 2004-333908 utilizes this theory. According to the theory, it is necessary to increase the thickness of the anti-reflection film from the central part of the lens toward the peripheral part thereof.

However, the anti-reflection effect of the anti-reflection film manufactured based on the general optical interference theory depends on the thickness thereof, so that a difference between an actual film thickness and a designed film thickness makes it impossible to obtain a satisfactory anti-reflection effect. Therefore, a highly accurate film thickness control is required for forming the anti-reflection film.

Further, another method for providing the anti-reflection function to the optical element forms a structure having a concave-convex shape finer than a wavelength of entering light (hereinafter referred to as “use wavelength”) on the surface of the transparent base material.

In the concave-convex structure finer than the use wavelength, the entering light behaves as if the structure is a uniform medium since the entering light cannot recognize the concave-convex shape. The concave-convex structure has a refractive index according to a volume ratio of a material forming the concave-convex shape, thereby showing a low refractive index that cannot be obtained by normal materials. Consequently, the use of such a concave-convex structure can achieve a higher anti-reflection performance than that of anti-reflection films made of low refractive index materials.

Methods for forming the above described concave-convex structure include a method applying a film in which fine particles having a particle diameter smaller than a use wavelength are dispersed onto a surface of a transparent base material (refer to Japanese Patent No. 3135944), and a method forming a periodic concave-convex structure by pattern formation by using a microfabrication apparatus (refer to Japanese Patent Laid-open No. 50-70040). Further, the methods include a method forming a concave-convex structure of petal-shaped alumina by using a sol-gel method (refer to Japanese Patent Laid-open No. 09-202649).

However, formation of such a concave-convex structure requires complicated processes. Further, the concave-convex structure is formed by using some limited materials, which reduces a degree of freedom in design of the refractive index. Therefore, there is a problem that high anti-reflection performance of the concave-convex structure can be obtained only when using transparent base materials having limited refractive indexes.

Japanese Patent Laid-open No. 2005-275372 discloses a method providing, between a concave-convex structure and a transparent base material, a thin film layer (intermediate layer) formed of a material having a refractive index between those of a material forming the concave-convex structure and the transparent base material. This disclosed method changes the refractive index gradually from the concave-convex structure to the transparent base material, which can reduce reflection at the boundary surface of the transparent base material. Further, selection of a material forming the thin film layer can provide a lot of options in selecting the transparent base material.

The dry method described above such as the sputtering method and the vapor deposition method arranges a vapor deposition source such that it faces the central part of the lens to form a thin film layer. This arrangement enables formation of an anti-reflection film whose anti-reflection performance becomes the maximum at the central part of the lens as designed.

However, when anti-reflection film formation is performed by the dry method on a lens surface having a small curvature radius, an incident angle of a vapor deposition material increases toward the peripheral part, thus reducing the film thickness toward the peripheral part. Generally, when a film thickness at an incident angle of 0 degree is defined as D, a film thickness at an incident angle of 60 degrees is about D×cos(60°), which is about half of the film thickness at the incident angle of 0 degree. Consequently, it is difficult to make the film thickness at the peripheral part larger than that at the central part of the lens based on the optical interference theory by using the dry method. A method disposing a mask can make the film thickness at the peripheral part than that at the central part of the lens, which, however, requires huge equipment.

On the other hand, the wet method described above such as the dipping method and the spin coat method has a low controllability of the film thickness, which makes it difficult to achieve a highly accurate film thickness control.

Further, the anti-reflection structure disclosed in Japanese Patent Laid-open No. 2005-275372 in which the thin film layer is disposed between the concave-convex structure and the transparent base material has better antireflection characteristics in a large wavelength band in comparison with the anti-reflection structure constituted only by the anti-reflection film formed based on the interference theory or the concave-convex structure. However, in the disclosed anti-reflection structure, small reflection occurring at the peripheral part of the lens surface having a small curvature radius generates ghost and flare. Thus, further development in anti-reflection performance is desired.

SUMMARY

OF THE INVENTION

The present invention provides an optical element having good anti-reflection performance not only at a central part of an optical surface having a small curvature radius, but also at the peripheral part thereof. Further, the present invention provides an optical system using the optical element and an optical apparatus including the optical system.

The present invention provides as an aspect thereof an optical element including a base member configured to have an optical surface, a concave-convex structure configured to have an average pitch smaller than a shortest wavelength of a use wavelength range, and an intermediate layer formed between the optical surface and the concave-convex structure, made of a material different from that of the concave-convex structure, and having a refractive index between those of the base member and the material of the concave-convex structure. The optical surface is formed into a shape having a rotational symmetry axis. A thickness of the intermediate layer or each of thicknesses of the intermediate layer and the concave-convex structure varies so as to increase as a distance from the rotational symmetry axis increases.

The present invention provides as another aspect thereof an optical system using the above described optical element and an optical apparatus using the optical system.

Other aspects of the present invention will become apparent from the following description and the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing a basic configuration of an optical element that is an embodiment of the present invention.

FIG. 2 shows reflectance characteristics of each of first and second embodiments of the present invention and those of each of first through third comparative examples.

FIG. 3 shows reflectance characteristics of each of third and fourth embodiments of the present invention and those of a fourth comparative example.

FIG. 4 is a cross-sectional view showing a configuration of an image-pickup optical system that is a fifth embodiment of the present invention.

DESCRIPTION OF THE EMBODIMENTS

Exemplary embodiments of the present invention will hereinafter be described with reference to the accompanying drawings.

Before describing specific embodiments, characteristics common to optical elements of the embodiments will be described. FIG. 1 shows a basic configuration of the optical element. FIG. 1 schematically shows a concave-convex structure and an intermediate layer.

The optical element of each embodiment includes a lens 11 as a transparent base material (base member) having a concave lens surface as an optical surface, a thin film layer 12 as an intermediate layer formed on the optical surface, and a concave-convex structure layer 13 as a concave-convex structure including a fine concave-convex shape formed on the thin film layer 12.

The thin film layer 12 is made of a material different from that of the concave-convex structure layer 13, and disposed between the lens surface and the concave-convex structure layer 13. The thin film layer 12 may have a single layer structure or a multi-layer structure in which two or more thin film layers made of mutually different materials are laminated. In other words, it is sufficient that at least one thin film layer is formed between the lens surface and the concave-convex structure layer 13. In a case where the thin film layer 12 is made into a single structure, the thin film layer 12 is made of a material different from that of the concave-convex structure layer 13 and has a refractive index between those of the lens 11 and the material forming the concave-convex structure layer 13. In a case where the thin film layer 12 is made into a multi-layer structure, it is sufficient that at least one of the two or more thin film layers is made of a material different from that of the concave-convex structure layer 13 and has a refractive index between those of the lens 11 and the material forming the concave-convex structure layer 13.

Further, the lens surface of the lens 11 has a shape having a rotational symmetry axis, that is, the lens surface has a rotationally symmetric shape.

Moreover, in the optical element of each embodiment, a thickness (hereinafter also referred to as “film thickness”) of the thin film layer 12 or each of thicknesses of the thin film layer 12 and the concave-convex structure layer 13 varies so as to increase as a distance from the rotational symmetry axis increases.

In FIG. 1, Dc denotes a film thickness of the thin film layer 12 at a center of the lens 11, that is, at a position of the rotational symmetry axis (hereinafter referred to as “optical axis”) of the lens 11. The position of the rotational symmetry axis of the lens 11 is a position (hereinafter referred to as “optical axis position”) where the optical axis crosses the lens surface. In a case where the thin film layer 12 has a multi-layer structure, Dc represents a film thickness of each thin film layer in the multi-layer structure.

Further, h denotes a distance in a direction orthogonal to the optical axis (or a direction along the lens surface) from the optical axis position. D(h) denotes a film thickness of the thin film layer 12 at a position of the distance h from the optical axis position (hereinafter referred to as “h-position”).

Further, φ(h) denotes an angle that a light ray reaching the thin film layer 12 at the h-position in parallel with the optical axis forms in a most superficial surface side outside of the thin film layer 12 (that is, in a concave-convex structure layer side outside of the thin film layer 12) with a normal to a most superficial surface of the thin film layer 12 at the h-position. The “most superficial surface” of the thin film layer 12 is a concave-convex structure-side superficial surface thereof, in other words, an uppermost surface closest to the concave-convex structure layer 13.

Moreover, θ(h) denotes an angle that a light ray reaching the thin film layer 12 at the h-position in parallel with the optical axis forms in the thin film layer 12 (in the intermediate layer) with the normal to the most superficial surface of the thin film layer 12 at the h-position.

When n denotes a refractive index of the thin film layer 12, φ(h) and θ(h) satisfy the following Snell\'s equation:

sin(φ(h))=n×sin(θ(h)).

As described above, in a peripheral part of the lens 11 (hereinafter also referred to as “lens peripheral part”) having a curvature, incident angles of light rays increase, and therefore anti-reflection performance is deteriorated in comparison with that at a central part of the lens 11 (hereinafter also referred to as “lens central part”). Changing a film thickness of an anti-reflection film according to a position on a lens surface such that a typical optical interference theory is satisfied makes it possible to provide good anti-reflection performance. However, the anti-reflection performance of the anti-reflection film based on the typical optical interference theory largely depends on the film thickness, so that even a small difference of an actual film thickness from a designed film thickness deteriorates the anti-reflection performance.

Accordingly, in each embodiment, the thin film layer 12 is provided between the concave-convex structure layer 13 and the lens 11 (lens surface).

The concave-convex structure layer 13 is formed by a concave-convex structure whose average pitch is sufficiently smaller than that of a shortest wavelength in a use wavelength range that is a wavelength range of light entering the optical element. The pitch means, when one convex portion and one concave portion adjacent thereto are defined as one set of concave and convex portions, a distance between two sets of concave and convex portions adjacent to each other.

Further, a configuration (shape) of the concave-convex structure layer 13 varies in its thickness direction. Specifically, a width of the convex portion increases from a light entrance side on which light reaches the optical element toward a thin film layer 12 side (or a lens 11 side), a width of the concave portion decreases from the thin film layer 12 side toward the light entrance side. Therefore, the concave-convex structure layer 13 can be considered as a structure whose refractive index varies continuously in its thickness direction. The continuous variation of the refractive index generates in the structure innumerable reflected lights whose amplitudes are small, and the reflected lights interfere with each other to be mutually reduced. Reflected lights generated at a boundary surface between the thin film layer 12 and the lens 11 mutually interfere innumerably to be mutually reduced in the structure, so that the anti-reflection performance has a little dependency on film thickness accuracy.



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stats Patent Info
Application #
US 20120262794 A1
Publish Date
10/18/2012
Document #
13529942
File Date
06/21/2012
USPTO Class
359601
Other USPTO Classes
International Class
02B5/00
Drawings
3



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