JPH11203712A - Lens capable of generating evanescent wave - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lens in a shape for hardly receiving the influence of spherical aberration and not being out of focus by determining the shape of the lens so as to make the coordinates of a lens surface satisfy specified relation at the time of defining the vertex of the lens as the origin of the coordinates, setting an X axis in a direction to be in contact with the vertex and setting a Z axis in the optical axis direction of the lens. SOLUTION: The shape of this lens is determined so as to satisfy the relation of an expression. The coordinates of the lens surface is defined as (X, Z), the radius of the incident range of light is defined as X0 , a numerical aperture is defined as NA, the refractive index of the lens is defined as (n) and a distance on an optical axis from the surface of the lens to a focus position is defined as T. Parallel light is made incident on an immersion lens 1 for which an objective lens and the lens capable of generating evanescent waves are integrated, however, since it is formed of the medium of a light refractive index higher than that of air, the incident light is refracted on the surface of the lens 1 and image-formed on a lens bottom surface. A recording medium is provided near the bottom surface of the lens 1.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a lens capable of generating an evanescent wave.

[0002]

2. Description of the Related Art An optical recording apparatus and an optical reproducing apparatus for recording / reproducing a signal using an evanescent wave generated by total reflection of light in a lens capable of generating an evanescent wave have been proposed. The evanescent wave generation lens can sufficiently reduce the diameter of a light spot at a light image forming position, and thus can increase the recording density of a recording medium such as an optical disk.

FIG. 4 is a diagram showing a schematic configuration of a conventional optical recording / reproducing apparatus using a lens capable of generating an evanescent wave. A spherical SIL (Solid Immersion Lens) is used as a lens capable of generating an evanescent wave. An example of use is shown.

[0004] In FIG. 4, light incident on an objective lens 11 is condensed by the objective lens 11, then incident on the SIL 12, further condensed by the SIL 12, and forms an image on the bottom surface of the SIL 12. If the recording surface of the optical disc 13 is arranged near the bottom surface of the SIL 12, a signal can be recorded at substantially the same resolution as the bottom surface of the SIL 12 due to the evanescent wave leaking from the bottom surface of the SIL 12.

[0005]

The SIL 12 used in the apparatus shown in FIG. 4 has a generally spherical shape, and the objective lens 1 is required to maintain a constant recording / reproducing accuracy.
It is necessary to keep the relative position between SIL 1 and SIL 12 constant. For this reason, SIL1
When 2 moves up and down, the objective lens 11 must also be moved in accordance with that, and there is a problem that the configuration of a control system for moving the objective lens 11 becomes complicated.

On the other hand, there has been proposed an example in which the objective lens 11 and the SIL 12 are integrated in order to make the movement control of the objective lens 11 unnecessary (for example, US Pat. No. 5,497,359). This publication discloses an example in which a light beam is incident on an aspheric lens in which an objective lens and an SIL are integrated. However, only the shape of the lens is described as being an aspherical shape, but the specific shape is not disclosed.

However, if the shape of the lens is not accurately determined, the focus will be defocused due to spherical aberration or the like, and it cannot be used for a recording / reproducing apparatus having a high recording density such as an optical disk.

The present invention has been made in view of such a point, and an object of the present invention is to provide a lens capable of generating an evanescent wave having a lens shape that is hardly affected by spherical aberration and the like and is not defocused. Is to provide.

[0009]

In order to solve the above-mentioned problems, the invention according to claim 1 is formed of a medium having a refractive index higher than that of air. A lens capable of generating an evanescent wave from this image forming surface, wherein the radius of the light incident range is x0, the numerical aperture is NA (where NA = sin θ, and θ is the end ray incident on the lens) Incident angle), the refractive index of the lens is n, the vertex of the lens is the origin of coordinates, the X axis is set in a direction contacting the vertex, and the Z axis is set in the optical axis direction of the lens. The lens shape is determined so that the coordinate (x, z) satisfies the relationship of the above equation (1).

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A lens capable of generating an evanescent wave according to the present invention will be specifically described below with reference to the drawings. The present embodiment is directed to a lens (hereinafter, referred to as an immersion lens) 1 in which an objective lens and a lens capable of generating an evanescent wave are integrated.
The lens of the present embodiment is used for recording / reproducing information on a recording medium such as an optical disk.

FIG. 1 shows an immersion lens 1 according to an embodiment.
FIG. As shown, parallel light enters the immersion lens 1. Immersion lens 1
Is formed of a medium having a higher light refractive index than air, the incident light is refracted on the surface of the immersion lens 1 and forms an image on the bottom surface of the lens. Near the bottom of the lens,
A recording medium is provided in the same manner as that shown in the conventional example of FIG.

FIG. 2 is a diagram specifically illustrating the shape of the immersion lens 1 of FIG. In FIG. 2, the vertex of the immersion lens 1 is set as the origin of the coordinates, the X axis is set in a direction in contact with the vertex, and the Z axis is set in the optical axis direction. The refractive index n1 of the immersion lens 1 is, as described above,
n1> 1.

In general, when parallel light is incident on a lens, the light is condensed in the lens without causing any aberration at all, by using a substantially ellipsoidal immersion lens 1 as shown in FIGS. It is generally known that what is needed is. In this case, the sagittal z of the quadratic surface (the sagitter refers to the distance z from the plane contacting the vertex of the lens to the lens surface as shown in FIG. 1) in a predetermined direction in this plane in FIG. Such an X-axis is set, the distance in the X-axis direction is x, the curvature is c, and the conic constant is k (where eccentricity is e, k = −e ² ).
Then, it is expressed by equation (3).

[0014]

(Equation 3) In FIG. 2, the beam diameter of the incident light is (2 × x0), the refractive index of the immersion lens 1 is n 1, and the numerical aperture in the immersion lens 1 is NA (NA = sinψ). If the three parameters can express the sagittar expressed by equation (3), the shape of the immersion lens 1 can be specified.

First, the numerical aperture NA will be discussed. According to Snell's law, the relationship of equation (4) holds. However,
θ0 is the incident angle, θ1 is the refraction angle, n0 is the refractive index of air (n0 =
1), n1 is the refractive index in the lens.

N0 sin θ0 = n1 sin θ1 (4) From equation (4), it can be seen that the larger the incident angle, the larger the outgoing angle. Therefore, when parallel light is incident on and condensed on the ellipsoidal lens 1 having the refractive index n1, the numerical aperture N of the light beam (light beam passing through xmax in FIG. 2) in contact with the edge of the lens
A is the largest, and the maximum numerical aperture NAmax of the lens is
NAmax = cosθ1max. Similarly, when the beam diameter of the incident light is (2 × x0), the numerical aperture NA of the lens
Becomes NA = cos θ1.

Here, in equation (4), n0 = 1 and θ0 = 90 °
By substituting, the equation (5) is obtained.

1 = n1sin θ1 (5) Using the expression (5), the maximum value of the numerical aperture NA is as shown in the expression (6).

[0019]

(Equation 4) Next, a method of calculating the cone coefficient k will be described. First, from FIG. 2, the relationship of equation (7) holds.

[0020]

(Equation 5) Also, from FIG. 2, the relationship of equation (8) holds.

Θ0 = 90 ° −α (8) When the equation (8) is substituted into the equation (4), the relation of the equation (9) is obtained.

[0022]

(Equation 6) Also, from FIG. 2, the following equations (10) to (12) hold.

Ξ = α + θ1 (10) zT = z0 + x0 · tanξ (11)

[0024]

(Equation 7) Here, since the optical path length of the light beam passing through the optical axis (x = 0) is equal to the optical path length of the light beam passing through other than the optical axis (x = x0), the relationship of the expression (13) is established.

N1 · zT = z0 + n1 · L (13) By substituting the equations (7) to (12) into the equation (13) and rearranging the equations, the relation of the equation (14) is obtained.

[0026]

(Equation 8) Next, a method of calculating the focal position zT in the lens will be described.
As described above, the light beam of x = xmax comes into contact with the immersion lens 1, so that when x = xmax, (15)
The relationship of the expression holds.

Dz / dx = ∞ (15) By differentiating the equation (3) and transforming the equation so that the relation of the equation (15) holds, the relation of the equation (16) is obtained.

[0028]

(Equation 9) By substituting equation (16) into equation (3), equation (17) is obtained.

[0029]

(Equation 10) From the above equations (5), (16) and (17), the focal position zT on the optical axis is as shown in equation (18).

ZT = zmax + xmax · tan θ1 = n / {c (n−1)} (18) Next, a method of calculating the curvature c of the ellipsoidal lens 1 will be described.
As described above, since NA = cos θ1, the following (1)
The relationship of the expression 9) is established.

[0031] By substituting the equations (5) and (7) for α and θ1 in the equation (19), the relation of the equation (20) is obtained.

[0032]

[Equation 11] Substituting equations (14) and (20) into equation (3) gives
As in Expression (21), the sagittal z at the x coordinate is represented by the numerical aperture NA of the lens, the refractive index n of the lens, and the radius x0 of the incident light.

[0033]

(Equation 12) Here, the second term on the right side of the equation (21) indicates an allowable error range of the lens shape. "100" in the denominator of the second term
Are approximate values, and the larger the value, the more accurate the lens shape, but the more severe the manufacturing conditions.

On the other hand, the distance T (= zT) on the optical axis from the surface of the immersion lens 1 to the focal position in FIG.
Substituting equations (14) and (20) into equation 8),
It is expressed by equation (22).

[0035]

(Equation 13) As an example, the refractive index n of the immersion lens 1 is set to n = 1.
Assuming that 8943, the numerical aperture NA is NA = 0.70, and the coordinate x is x0 = 10, the shape of the lens surface is as shown in equation (23).

[0036]

[Equation 14] In addition, the thickness of the central portion of the lens is T = 18.8518 ± 0.10.

Actually, as shown in FIG. 3, a film 2 for preventing the lens from being damaged is formed near the bottom surface of the immersion lens 1 in FIG. This film 2 is formed of, for example, polycarbonate. More precisely, membrane 2 is
A film that protects the immersion lens 1 itself or a protective film on a recording medium that performs recording / reproduction is conceivable, and air between the immersion lens 1 and the recording medium may be treated as a part of the film. In this case, if the thickness of the film 2 in the optical axis direction is sufficiently small, or if the refractive index of the film 2 is almost the same as the refractive index of the material most frequently used for the immersion lens 1, the shape of the lens surface becomes (20), (2)
3) It is expressed by an equation similar to the equation.

When two or more films 2 as shown in FIG. 3 are provided in front of the image forming position, the thickness of these media is sufficiently small or the refractive index of these media is most frequently used for the immersion lens 1. If the refractive index of the material is almost the same, the shape of the surface of the lens is expressed by the same equation as the above-mentioned equations (20) and (23).

As described above, in the present embodiment, when substantially parallel light is incident on the immersion lens 1, the lens shape is determined so that the incident light is condensed at one point, so that the influence of spherical aberration and the like can be avoided. A minute light spot can be formed at the focal position of the lens. Further, since the error range of the lens shape is set in advance, it is possible to suppress a difference in lens characteristics due to manufacturing variations within a certain range.

When an optical recording / reproducing apparatus is configured using the immersion lens 1 having a shape satisfying the above-mentioned relations (21) and (22), an optical disk is placed near the bottom of the immersion lens 1. In this case, the optical signal corresponding to the recording data may be incident on the immersion lens 1 in parallel. As a result, a minute light spot is formed on the bottom surface of the immersion lens 1, and a pit row can be formed on the optical disk by the evanescent wave leaking from the immersion lens 1.

By adopting such a configuration, it is not necessary to separately provide an objective lens in the optical recording / reproducing apparatus, and the configuration of the optical system can be simplified.

[0042]

As described above in detail, according to the present invention, the objective lens and the lens capable of generating an evanescent wave are integrated, and the lens shape is determined to a predetermined shape. A lens that is hard to receive light and is not out of focus is obtained.

[Brief description of the drawings]

FIG. 1 is a diagram showing an outer shape of an immersion lens 1 according to an embodiment.

FIG. 2 is a diagram specifically illustrating the shape of the immersion lens 1 of FIG.

FIG. 3 is a diagram in which a film for preventing damage to the lens is formed near the bottom surface of the immersion lens.

FIG. 4 is a diagram showing a schematic configuration of a conventional optical recording / reproducing apparatus using a lens capable of generating an evanescent wave.

[Explanation of symbols]

1 Lens capable of generating evanescent waves (immersion lens) 2 Film

Claims (2)

[Claims] (1) The medium is formed of a medium having a higher refractive index than air.
A lens capable of emitting light to this medium to form an image in the medium and generating an evanescent wave from the image forming surface, wherein a radius of a light incident range is x0, a numerical aperture is NA, and a refractive index of the lens. Let n be the vertex of the lens and the X axis set in the tangential direction passing through the vertex, and the Z axis set in the optical axis direction of the lens.
A lens capable of generating an evanescent wave, wherein the lens shape is determined so as to satisfy the relationship of equation (1). (Equation 1) 2. A lens capable of generating an evanescent wave according to claim 1, wherein the lens shape is determined so that a distance T between a vertex of the lens and a bottom surface of the lens is expressed by the following equation (2). . (Equation 2)