JP2000242954A - Near-field optical system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a near-field optical system with a simple optical system which shows good imaging performance and which can be easily assembled and adjusted for recording and reproducing of a high-density optical information recording medium. SOLUTION: In this near-field optical system, an objective lens L1 and the recording face of an optical information recording medium are arranged with the distance between them smaller than the wavelength of the used light source. The objective lens L1 at least partly consists of a spheroid formed by rotating an ellipse around its principal axis. The feature of the optical system is that a light beam emitted from the light source disposed at the position of a first focal point F1 which is one of the two focal points on the principal axis of the spheroid is reflected by the curved face of the spheroid and focused at the position of the other second focal point F2.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a near-field optical system, and more particularly, to an optical system for recording / reproducing information recorded on an optical information recording medium at a high density. The present invention relates to a so-called near-field optical system in which a distance from a surface is equal to or less than a wavelength of light.

[0002]

2. Description of the Related Art Conventionally, as a so-called near-field optical system in which an interval between an objective lens and a recording surface of an optical disk is smaller than or equal to a wavelength of light, a high-density recording / reproducing apparatus using an optical disk or the like as an optical information recording medium has been used. An optical system using a field optical system is known. For example, US Pat.
No. 50 discloses the technology. This technical content is an optical system having a large numerical aperture NA using two objective lenses having a uniform refractive index and separated, and in particular, using a hemispherical spherical lens as a lens on the optical disk side. The numerical aperture NA
Is a value given by NA = n · sinU as the refractive index n of the medium on the object side with an aperture angle of 2U.

[0003]

However, in the above-mentioned near-field optical system, since a precise optical system having a large numerical aperture NA is constituted by two lenses, it is often used for assembling adjustment such as centering of the lens. In addition, it takes a long time, and there is a problem that the cost is increased if the microlenses are produced by polishing or the like.

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and it is an object of the present invention to provide a recording method for a high-density optical information recording medium having a simple optical system, having good imaging performance, and easy assembly adjustment. It is to provide a near-field optical system for reproduction.

[0005]

The above object can be achieved by any of the following.

(1) A near-field optical system in which a distance between an objective lens and a recording surface of an optical information recording medium is arranged to be equal to or less than a wavelength of a light source to be used. The light flux emitted from the light source provided at the position of one of the first focal points of the two focal points located on the main axis of the spheroid is formed of the spheroid. A near-field optical system that reflects light on a curved surface and forms an image at the position of the other second focal point.

(2) A near-field optical system in which the distance between the objective lens and the recording surface of the optical information recording medium is arranged to be equal to or less than the wavelength of the light source used, wherein the objective lens is rotated at least partially about an elliptical principal axis. The light flux from the external light source is once converged to the position of one of the first focal points, among the two focal points located on the main axis of the spheroid, and An objective lens is provided, in which a position of one focal point is a secondary light source, and a light beam emitted from the secondary light source is reflected by the curved surface of the spheroid and forms an image at the position of the other second focal point. Near-field optics.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An example of a near-field optical system according to an embodiment of the present invention will be described with reference to the drawings, but the present invention is not limited to the following embodiment. The reference numerals used in this embodiment are as follows.

R _X : radius of curvature of the center on axis X which is perpendicular to the main axis R _Y : radius of curvature of the center on axis Y which is perpendicular to the main axis k _X : conic constant k _Y in the cross section of the axis X k _Y : Conic constant at axis Y cross section d: Surface interval n: Refractive index f: Focal length NA: Numerical aperture h: Height of principal ray from axis Z in axis Y direction h _O : Axis of principal ray from axis Z _O Here, the height in the Y direction Here, the spheroidal surface of the spheroid with the axis Y as the rotation axis (in the optical design, a biological-surfac
The function of the plane (also called e) is “Equation 1”.

[0010]

(Equation 1)

The first focus F1 and the second focus F2 are on the axis Y.
The major axis of the ellipse that cuts the main axis (axis Y, axis Z) is a,
Assuming that the minor axis is b, the distance from the center O of the coordinate origin is “Equation 2”.

[0012]

(Equation 2)

(First Embodiment) FIG. 1 is a perspective view of a near-field optical system according to a first embodiment, and axes Y and Z according to the first embodiment.
FIG. 2 shows a cross-sectional view of the near-field optical system in a plane including. Further, Table 1 shows lens data of the first example.

[0014]

[Table 1]

As shown in FIGS. 1 and 2, the objective lens L1 of the near-field optical system is a semi-spheroid having a shape obtained by cutting a spheroid having the axis Y as a rotation axis in half along the axis Y. The objective lens L1 has two focal points, the first focal point F1 is a light source as an object, and the second focal point F2 is an image forming point.
In addition, 11 is a part of the recording surface of the optical disk. Also,
The curved surface of the surface of the spheroid is mirror-deposited C, and the inner surface of the surface of the spheroid is a reflection surface. Also,
The refractive index n of the near-field optical system is n> 1.4 or more. Note that the solid light beam shown in FIG. 2 is the principal light beam shown in Table 1.

The function of the lens will now be described. A divergent light beam of a spherical wave is emitted from the light source at the first focal point F1, reflected inside the surface of the spheroid, becomes convergent light, and forms an image at the second focal point F2. I do.

As described above, by placing an object (point object) or a light source (point light source) at the first of the two focal points in the spheroid, the divergent light flux of the spherical wave emitted from the object or the light source is obtained. Since the light is reflected by the inner surface of the spheroid and becomes convergent light and forms an image at the second focal point F2 in the plane section cut along the axis Y, the plane section and the recording surface of the optical disk are not more than the wavelength order. , It is possible to substantially maintain the state of the point image formed on the flat portion even on the recording surface of the optical disc.

Since the objective lens L1 is made of a material having a refractive index of n> 1.4 or more, such as glass or plastic, a lens having a numerical aperture NA close to 1.0 or a lens having a numerical aperture NA exceeding 1.0 is used. The optical system can be obtained without aberration.

The reason that this optical system has no aberration is that the first focal point F is an ellipse in a planar system and a spheroid in a three-dimensional system.
The light reflected at an arbitrary point P on the circumference of the ellipse or spheroid starting from 1 always passes through the second focal point F2. At this time, the total value of the line segment F1P and the line segment F2P is constant irrespective of the position of the point P, and when this is light, the total optical path length of the optical path length F1P and the optical path length F2P is constant. Since a light beam having a constant optical path length is equivalent to image formation with no aberration, the light source at the first focal point of the spheroid forms an image with no aberration at the second focal point.

The reason why the numerical aperture NA can be increased is that, even if the aperture angle 2U increases, an image is formed with no aberration as described above, and the numerical aperture NA = n · sinU and the refractive index n
In particular, when an optical material having a high refractive index is used, the numerical aperture NA can be increased.

(Second Embodiment) FIG. 3 is a sectional view of a near-field optical system in a plane including the axis Y and the axis Z according to the second embodiment. Table 2 shows lens data of the second example.

[0022]

[Table 2]

This near-field optical system is a modification of the first embodiment. The objective lens L2 is a semi-spheroid having a shape obtained by cutting a spheroid having the axis Y as a rotation axis in half along the axis Y. In addition, the lens is obtained by cutting out a part L2a of the surface near the intersection of the axis Y and the spheroidal surface. The first focus F1 is a light source position, and the second focus is an imaging position. The curved surface of the surface of the spheroid is mirror-deposited C, and the inner surface of the surface of the spheroid is a reflection surface. The solid-line light beam shown in FIG. 3 is the principal light beam shown in Table 2. In addition, 11 is a part of the recording surface of the optical disk.

In the second embodiment, the numerical aperture N is increased by using an optical material having a higher refractive index as compared with the first embodiment.
A can be increased. It is desirable to appropriately configure the surface of the portion or the optical system so that the light leaking from the portion does not enter the objective lens again depending on the cut shape.

(Third Embodiment) FIG. 4 is a perspective view of a near-field optical system according to a third embodiment, and axes Y and Z according to the third embodiment.
FIG. 5 shows a cross-sectional view of the near-field optical system in a plane including. Further, Table 3 shows lens data of the third example.

[0026]

[Table 3]

The objective lens L3 is a semi-spheroid obtained by cutting a spheroid having the axis Y as a rotation axis in half along the axis Y, and is provided at an intersection between the axis Y and the surface of the spheroid. A semi-spheroidal lens L31 having a semi-cylindrical end face L3a
And a spherical lens L32 having a hemispherical shape. Note that the end face portion L3a is not limited to a semicircular column, and may be, for example, a prism.

The semi-spheroidal lens L31 has a first focal point F1.
Is a second light source, and the second focal point F2 is an image forming position. The curved surface of the surface of the semi-spheroid is subjected to mirror surface deposition C, and the inner surface of the surface of the semi-spheroid lens L31 is a reflection surface. The spherical lens L32 has a center on the axis Z _O parallel to the axis Z, and the spherical surface R is set so that the convergent ray is perpendicularly incident on the spherical surface. S is the axis Z and the axis Z
Indicates the interval of _O. 5 are the principal rays shown in Table 3. In addition, 11 is a part of the recording surface of the optical disk.

Here, the function of the lens will be described. The convergent ray of the spherical wave is made to enter the spherical lens L32 perpendicularly to the spherical surface from the external optical system, and the incident light beam is converted to the spherical lens L3.
In 2, an image is formed at the center position of the spherical surface as it is. And
It again becomes a secondary light source and changes to a divergent light beam of a spherical wave. This secondary light source position is made to coincide with the first focal point of the spheroid. The light beam emitted from the secondary light source is reflected by the inner surface of the surface of the spheroid, and the other second focal point F2 of the spheroid is obtained.
Then, an image is formed without aberration.

(Fourth Embodiment) FIG. 6 is a perspective view of a near-field optical system according to a fourth embodiment, and axes Y and Z according to the fourth embodiment.
FIG. 7 shows a cross-sectional view of the near-field optical system in a plane including. Further, Table 4 shows lens data of the fourth example.

[0031]

[Table 4]

As shown in FIGS. 6 and 7, the objective lens L4 is
A semi-spheroid having a shape obtained by cutting a spheroid having the axis Y as a rotation axis in half along the axis Y, and a semi-spheroid lens L41 having one side cut off perpendicularly to the axis Y; A quarter of
This is a lens in which a spherical lens L42 having a shape is integrally combined. In the semi-spheroidal lens L41, the first focal point F1 is a second light source, and the second focal point F2 is an imaging position. The curved surface of the surface of the spheroid is mirror-deposited C, and the inner surface of the surface of the spheroid is a reflection surface.
Also, the spherical lens L42 has its center on an axis Z _O parallel to the axis Z, and a convergent ray is perpendicularly incident on the spherical surface.
A spherical radius R is set. S indicates the interval between the axis Z and the axis Z _O. The solid-line light beam shown in FIG.
Reference numeral 11 denotes a part of the recording surface of the optical disk, and reference numeral 10 denotes a reflection optical member.

Here, the function of the lens will be described. The luminous flux of the spherical wave perpendicularly incident on the spherical surface of the spherical lens L42 from the external optical system forms an image as it is at the center position of the spherical sphere. At this image forming position, the light beam is reflected by the reflection optical member 10.
, And again becomes a secondary light source and changes into a divergent light beam of a spherical wave. The position of the secondary light source is made to coincide with the first focal point F1. Using this as a secondary light source, a light beam emitted from the secondary light source is reflected by the inner surface of the surface of the spheroid and forms an image at the second focal point F2 of the spheroid without aberration.

In the embodiment, the position of the secondary light source and the position of the final image are not necessarily required to be on the same plane, and a portion where the distance between the objective lens and the optical recording surface of the disk is very small. Is only around the final imaging position.
A portion corresponding to the secondary light source may be provided with a step so as to have a sufficient working interval, and a secondary light source may be formed at a position corresponding to the focal point of the spheroid, including the reflection surface. In the third and fourth embodiments, it is preferable that the integrally combined lens is formed by integral molding.

[0035]

According to the above configuration, the following effects can be obtained. According to the first and second aspects of the present invention, a near-field optical system for recording / reproducing a high-density optical information recording medium which has good imaging performance with a simple optical system and easy assembly and adjustment is provided. .

According to the third aspect of the present invention, the positioning of the objective lens is facilitated, and the molding process is facilitated.

According to the fourth aspect of the present invention, the numerical aperture NA can be increased by using an optical material having a high refractive index.

According to the fifth aspect of the present invention, the numerical aperture N
A is a lens close to 1.0, or a numerical aperture NA is 1.0
Can be obtained with no aberration.

According to the sixth aspect of the present invention, since the surface of the semi-spheroid of the objective lens is mirror-deposited, the divergent light at the first focal point can be converged on the second focal point.

According to the seventh aspect of the present invention, since the light emitted from the external light source is perpendicularly incident on the spherical lens, it is possible to converge on the first focal point without aberration and form an image.

According to the eighth aspect of the present invention, since the spherical lens and the half-spheroid are integrated optical components, there is no need to adjust between the two optical components, and the cost is reduced.

[Brief description of the drawings]

FIG. 1 is a perspective view of a near-field optical system according to a first embodiment.

FIG. 2 is a cross-sectional view of a near-field optical system on a plane including an axis Y and an axis Z according to the first embodiment.

FIG. 3 is a sectional view of a near-field optical system in a plane including an axis Y and an axis Z according to the second embodiment.

FIG. 4 is a perspective view of a near-field optical system according to a third embodiment.

FIG. 5 is a sectional view of a near-field optical system in a plane including an axis Y and an axis Z according to a third embodiment.

FIG. 6 is a perspective view of a near-field optical system according to a fourth embodiment.

FIG. 7 is a sectional view of a near-field optical system in a plane including an axis Y and an axis Z according to a fourth embodiment.

[Explanation of symbols]

L1, L2, L3, L4 objective lens L32, L42 spherical lens C specular deposited O center X, Y, Z, Z _O axis S length F1 first focal point F2 second focal point

Claims (8)

[Claims] 1. A near-field optical system in which a distance between an objective lens and a recording surface of an optical information recording medium is arranged to be equal to or less than a wavelength of a light source to be used, wherein the objective lens rotates at least partially about an elliptical main axis. Of two spheroids formed on the main axis of the spheroid, a light beam emitted from a light source provided at one of the first focal points is a curved surface of the spheroid. A near-field optical system that reflects light at a second focal point and forms an image at the position of the other second focal point. 2. A near-field optical system in which an interval between an objective lens and a recording surface of an optical information recording medium is arranged to be equal to or less than a wavelength of a light source to be used. The luminous flux from the external light source is once converged to the position of one first focal point among the two focal points located on the main axis of the spheroid and formed by the first spheroid. With the position of the focal point as a secondary light source, the light flux emitted from the secondary light source is reflected by the curved surface of the spheroid,
A near-field optical system having an objective lens that forms an image at the position of the other second focal point. 3. The near-field optical system according to claim 1, wherein the objective lens has a substantially half shape cut by a plane passing through the main axis of the spheroid. 4. The object lens has a shape obtained by cutting a part of the surface of the spheroid or a part of the surface of the spheroid near the intersection of the main axis and the surface of the spheroid. The near-field optical system according to claim 1, wherein the near-field optical system has a shape obtained by adding an end face to the shape portion. 5. The near-field optical system according to claim 1, wherein the following condition is satisfied, where n is a refractive index of the objective lens with respect to a light source wavelength to be used. . n> 1.4 6. The objective lens according to claim 1, wherein a surface of the spheroid is mirror-deposited, and an inner surface of the surface is a reflection surface. Near-field optics. 7. The near-field optical system according to claim 2, wherein a spherical lens is arranged so as to converge a light beam from an external light source at the position of the first focal point and form an image. 8. The near-field optical system according to claim 7, wherein the spherical lens and the spheroid are integrated optical components.