JPH08179195A - Objective lens for optical disk - Google Patents

Abstract

PURPOSE: To improve a wave front aberration when the distance L between object images is 16-20mm so as to reduce the size of an optical pickup device by satisfying specific conditions in the objective lens for the optical disk which is arranged between a light source and the optical disk. CONSTITUTION: This objective lens 2 for the optical disk which is arranged between the light source 1 and optical disk 7 is aspherical on both the sides of the light source 1 and optical disk; and 16<=L<=20 and a1 p+b1 <= log (-1/') <=a2 p+b2 are satisfied, where L is the distance between object images (distance between the light source 1 and the pit surface 3 of the optical disk). Here, a1 =-0.0280L+0.287, b1 =0.0616L-0.262, a2 =0.0084L-0.404, and b2 =0.0178L+0.631, and the unit of the distance is mm. This objective lens 2 for the optical disk has large power (-1/β) and does not deteriorate in wave front aberration even when the distance between the object images is shortened from 16mm to 20mm, which contributes to the size reduction of the optical pickup device.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical disk objective lens having a finite specification capable of condensing a light beam from a light source on an optical disk surface without passing through a collimator lens.

[0002]

2. Description of the Related Art In an optical system for an optical disk, a laser beam from a light source is directly focused on the optical disk surface by a single lens without passing through a collimator lens. Since it has many advantages such as high productivity, it is widely and generally used. On the other hand, the demand for system compactification has further increased, and optical disc optical systems used in optical pickup devices have also been required to be smaller and lighter.

Conventionally, as a single lens used in such an optical system for an optical disk for the purpose of compactness and lightness, JP-A-64-
Those described in Japanese Patent No. 25113, etc. have been reported.

In the above publication, the distance between objects and images is 15.06 m.
Although the example of m is described, since the correction term from the spherical surface to the term proportional to the 18th power of the height from the optical axis is used as the aspherical shape, the shape of the lens becomes too complicated and the processing is difficult. There was a drawback.

[0005]

SUMMARY OF THE INVENTION The present invention has been made in order to eliminate such drawbacks, and its object is to achieve a short image-to-image distance of 20 mm or less and a good imaging performance.
An object of the present invention is to provide an objective lens for an optical disc that is excellent in workability.

[0006]

The present invention has been made to solve the above-mentioned problems. "In an optical disk objective lens arranged between a light source and an optical disk, both the light source side and the optical disk side have an aspherical shape. And the object-image distance is L, the working distance is P, and the lateral magnification is β, 16 ≦ L ≦
An objective lens for an optical disk, which satisfies 20 and satisfies the following conditions.

A ₁ P + b ₁ ≦ log (−1 / β) ≦ a ₂ P + b ₂ where a ₁ = −0.0280L + 0.287, b ₁ = 0.0616L−0.262, a ₂ = 0.0084L− 0.404, b ₂ = 0.0178L + 0.631, and the unit of distance is mm. "I will provide a.

The present invention also relates to an "optical disk used in an optical pickup device in which an optical source, an optical medium for a light receiving element or a transparent plate-like member, an objective lens for an optical disk, and an optical disk are arranged in this order on the optical axis. In the objective lens for use, both the light source side and the optical disc side have an aspherical shape,
When the object-to-image distance is L, the working distance is P, the lateral magnification is β, the thickness of the light receiving element optical medium or the transparent plate is d ₀ , and the refractive index is n ₀ , 16 ≦ L ≦ An objective lens for an optical disk, which satisfies 20 and satisfies the following conditions.

A ₁ P + b ₁ ≦ log (−1 / β) ≦ a ₂ P + b ₂ where L _c = L−d ₀ (n ₀ −1) / n ₀ , a ₁ = −0.0280L _c +0.287 , B ₁ = 0.0616L _c −0.262, a ₂ = 0.0084L _c −0.404, b ₂ = 0.0178L _c +0.631, and the unit of distance and thickness is mm. "I will provide a.

According to the present invention, the objective lens for an optical disk described above is characterized in that, when the full angle at half maximum of the luminous flux of the light source is θ and the image side NA (numerical aperture) is sin ∠B, the following conditions are satisfied.

(-/ Β) ≧ (sin ∠B) / sin
θ. "I will provide a.

The present invention also provides the above-mentioned optical disk objective lens, characterized in that the surface shapes on the light source side and the optical disk side are expressed by the equation (2).

[0013]

[Equation 2]

The objective lens of the present invention will be described below with reference to the drawings.

FIG. 1 is a side view showing a usage state of the lens of the present invention. Reference numeral 1 is a light source, that is, an object point, 2 is an objective lens for an optical disk of the present invention, and 3 is a pit surface of an optical disk (data is formed). Surface of the optical disk), that is, the image plane,
Reference numeral 4 is a protective layer of the optical disc, 5 is an optical medium, 6 is a light receiving element, L is a distance between object images, P is a working distance of the lens 2, d is a center thickness of the lens 2, t is a thickness of the optical disc protective layer, and θ ( The angle is the full angle at half maximum of the light flux of the light source 1.

Further, in FIG. 1, the object side NA (NA means NA) is sin∠A, and the image side NA is sin∠.
Each is represented by B. That is, the outer line represented by the angle A and the angle B is the light flux of the outermost ray passing through the lens 2. Further, the object-image distance L is the distance between the light source 1 and the pit surface 3 as shown in FIG.

In order to effectively use the light of the light source 1 close to a point light source, when the light source 1 is a laser or the like, the light source 1 is up to a double angle of the full angle at half maximum θ, that is, a divergence angle of 2θ. It can be effectively used as light. Therefore, when the angle A is larger than θ, the main light flux of the light source 1 passes only in the vicinity of the central portion of the lens 2, and the substantial image-side NA becomes small. Therefore, the practical NA satisfies about 0.45 which is the value of the image side NA which is required to secure the resolution for accurately reading the data on the pit surface 3 of the optical disc. The result is impossible.

Therefore, in order to secure a preferable resolution, it is necessary to set the substantial image side NA to approximately 0.45 or more. For that purpose, it is preferable that the angle A is smaller than θ, and from this condition, sin θ ≧ sin A is required.

Further, (−1 / β) = (sin ∠B) /
Since (sin ∠A) (where (−1 / β) is the magnification of the single lens and β is the lateral magnification), sin θ ≧ sin ∠
A is such that (−1 / β) ≧ (sin ∠B) / sin θ.

Therefore, the effective image-side NA is substantially 0.
To ensure that it is 45 or more, (-1 / β)
It is preferable that ≧ (sin ∠B) / sin θ.

In order to secure a further excellent resolution,
It is more desirable to effectively use a brighter light beam within the full angle at half maximum θ of the light beam of the light source 1. For that, the angle A
Is preferably smaller than θ / 2, and from this condition, (−1 / β) ≧ (sin ∠B) / sin (θ / 2) is more preferable.

In order to ensure a particularly excellent resolution, since a particularly bright light beam exists within 80% of the full-width half-maximum angle θ of the light source 1, it is particularly desirable to effectively use such a particularly bright light beam. For that, the angle A is (θ /
2) · 0.8, that is, smaller than 0.4 · θ, and from this condition, (−1 / β) ≧ (sin ∠B) / sin (0.4θ) is particularly desirable. .

As described above, in order to obtain a high image side NA of approximately 0.45 or more in order to secure the necessary resolution,
The magnification of the single lens (-1 / β) is (sin ∠B) / sin
It is preferable to set θ or more, but log (−1 / β) <
At a ₁ P + b ₁ , the occurrence of off-axis wavefront aberration, especially astigmatism, becomes large, so for a given object-image distance L,
The magnification of the single lens (-1 / β) is (sin ∠B) / sin
It will be difficult to set θ or more.

For log (-1 / β)> a ₂ P + b ₂ ,
The center thickness of the lens has to be made thin, it is difficult to secure the edge thickness of the lens, it becomes impossible to manufacture a practical lens, and the off-axis wavefront aberration becomes too large, which is not preferable.

The thickness d between the light source 1 and the lens 2 is
_When the optical medium 5 having a refractive index n ₀ is arranged, it is necessary to apparently correct the object-image distance L by subtracting d ₀ (n ₀ −1) / n ₀ from the object-image distance L. .

The optical medium 5 includes, for example, a beam splitter, an optical medium for a light receiving element such as a half mirror, a synthetic resin such as acryl, and a transparent plate-like body made of glass or the like.

[0027]

EXAMPLE An example will be described below with reference to FIG.
The following tables show various numerical values of each example and comparative example. In each table, f is the focal length, n is the refractive index of the lens, and "E-
"1" represents 10 " ¹ and" E-2 "represents 10 ^-2 . Examples,
In the comparative example, the protective layer of the optical disc has a refractive index of 1.5.
5, having a thickness of 1.20 mm was used. In addition, the aspherical shape is represented by Equation 3, and each coefficient is shown in the table.

[0028]

(Equation 3)

Wavefront aberrations of the lenses of Examples 1 to 8 and Comparative Examples 1 to 5 are sequentially shown in FIGS. 2 to 1, respectively.
4 shows. The horizontal axis of each figure is the image height, and the unit is mm. The vertical axis represents the RMS wavefront aberration value, the wavelength λ = 780 nm, and the unit is the wavelength.

As described in Embodiments 1 to 8, by properly selecting the lateral magnification β, the object-image distance L and the working distance P, the off-axis can be realized without using a higher-order aspherical surface of 4th order or higher. In addition, it is possible to obtain a lens with excellent image forming performance, excellent workability as compared with the conventional design using a high-order aspherical surface, and easy molding by injection molding.

By using a polymer having a water absorption of 0.01% or less and a glass transition temperature of 140 ° C. or more, a lens excellent in heat resistance and moisture resistance can be provided.

A comparison will be made below with a comparative example which does not satisfy the conditions.

In Example 1 (Table 1), (a ₂ P + b ₂ )
/ Log (-1 / β) is 1.05, and as shown in FIG. 2, the RMS wavefront aberration value is not a bad value, but Comparative Example 1
In (Table 9), (a ₂ P + b ₂ ) / log (−1 / β)
Is 0.95, and as shown in FIG. 10, when compared with Example 1, the off-axis RMS wavefront aberration value is a very bad value. Further, in Comparative Example 1, the center thickness of the lens is thin, and the shape of the lens is not practical.

In Example 2 (Table 2), (a ₁ P + b ₁ )
/ Log (−1 / β) is 0.95, and as shown in FIG. 3, the RMS wavefront aberration value is not a bad value, but Comparative Example 2
In Table 10, (a ₁ P + b ₁ ) / log (-1 /
β) is 1.05, and as shown in FIG. 11, the off-axis RMS wavefront aberration value is extremely poor as compared with Example 2.

In Example 7 (Table 7), (a ₂ P + b ₂ )
/ Log (−1 / β) is 1.05, and as shown in FIG. 8, the RMS wavefront aberration value is not a bad value, but Comparative Example 3
In Table 11, (a ₂ P + b ₂ ) / log (-1 /
β) is 0.95, and as shown in FIG. 12, the off-axis RMS wavefront aberration value is extremely poor as compared with Example 7.

In Example 8 (Table 8), (a ₁ P + b ₁ )
/ Log (-1 / β) is 0.95, and as shown in FIG. 9, the RMS wavefront aberration value is not a bad value, but Comparative Example 4
In Table 12, (a ₁ P + b ₁ ) / log (−1 /
β) is 1.05, and as shown in FIG. 13, the off-axis RMS wavefront aberration value is extremely poor as compared with Example 8.

Comparative Example 5 is an example in which the object-image distance L = 15 mm, but as shown in FIG. 14, the off-axis RMS wavefront aberration has a very bad value.

In the case of the lenses of Comparative Examples 1 to 5, since the off-axis aberration is severely deteriorated as described above, even if the lens is actually manufactured, the wavefront due to the mounting error of the lens in the optical pickup device is generated. The variation of the aberration characteristic becomes very large, which makes the lens impractical.

When the object-image distance L> 20 mm, it is against the object of the present invention to make the pickup compact.

[0040]

[Table 1]

[0041]

[Table 2]

[0042]

[Table 3]

[0043]

[Table 4]

[0044]

[Table 5]

[0045]

[Table 6]

[0046]

[Table 7]

[0047]

[Table 8]

[0048]

[Table 9]

[0049]

[Table 10]

[0050]

[Table 11]

[0051]

[Table 12]

[0052]

[Table 13]

[0053]

According to the present invention, there is provided an objective lens for an optical disk, which effectively utilizes laser light and can obtain a high numerical aperture of 0.45 or more.

The objective lens for an optical disc of the present invention has a large lens magnification (-1 / β) and an object-image distance of 16 m.
Even if the length is shortened from m to 20 mm, the wavefront aberration is not deteriorated, which can greatly contribute to downsizing of the optical pickup device for the optical disc.

[Brief description of drawings]

FIG. 1 is a side view showing a usage state of a lens of the present invention.

FIG. 2 is a wavefront aberration characteristic diagram of the lens of Example 1.

FIG. 3 is a wavefront aberration characteristic diagram of the lens of Example 2.

FIG. 4 is a wavefront aberration characteristic diagram of the lens of Example 3.

5 is a wavefront aberration characteristic diagram of the lens of Example 4. FIG.

FIG. 6 is a wavefront aberration characteristic diagram of the lens of Example 5.

FIG. 7 is a wavefront aberration characteristic diagram of the lens of Example 6.

FIG. 8 is a wavefront aberration characteristic diagram of the lens of Example 7.

FIG. 9 is a wavefront aberration characteristic diagram of the lens of Example 8.

10 is a wavefront aberration characteristic diagram of the lens of Comparative Example 1. FIG.

11 is a wavefront aberration characteristic diagram of the lens of Comparative Example 2. FIG.

FIG. 12 is a wavefront aberration characteristic diagram of the lens of Comparative Example 3.

13 is a wavefront aberration characteristic diagram of the lens of Comparative Example 4. FIG.

FIG. 14 is a wavefront aberration characteristic diagram of the lens of Comparative Example 5.

[Explanation of symbols]

 1: Light source 2: Objective lens for optical disc 3: Pit surface of optical disc 4: Protective layer of optical disc 5: Optical medium 6: Light receiving element L: Object-image distance P: Working distance d: Center thickness of objective lens for optical disc t: Optical disc protection layer thickness

Claims (4)

[Claims] 1. An optical disk objective lens arranged between a light source and an optical disk, wherein both the light source side and the optical disk side have an aspherical shape, the object-image distance is L, the working distance is P,
An objective lens for an optical disk, which satisfies 16 ≦ L ≦ 20 when a lateral magnification is β, and also satisfies the following conditions. a ₁ P + b ₁ ≦ log (−1 / β) ≦ a ₂ P + b ₂ where a ₁ = -0.0280L + 0.287, b ₁ = 0.0616L-0.262, a ₂ = 0.0084L-0.404 , B ₂ = 0.0178L + 0.631, and the unit of distance is mm. 2. An optical disk objective lens used in an optical pickup device, which comprises a light source, an optical medium for a light receiving element or a transparent plate-like member, an optical disk objective lens, and an optical disk arranged in this order on an optical axis. Both the light source side and the optical disk side have an aspherical shape, the object-image distance is L, the working distance is P, the lateral magnification is β, the thickness of the optical medium for the light receiving element or the transparent plate is d _0. An objective lens for an optical disk, which satisfies 16 ≦ L ≦ 20 when the refractive index is n ₀ and satisfies the following conditions. a ₁ P + b ₁ ≦ log (−1 / β) ≦ a ₂ P + b ₂ where L _c = L−d ₀ (n ₀ −1) / n ₀ , a ₁ = −0.0280L _c +0.287, b _{1 = 0.0616L c -0.262, a 2 =} 0.0084L c -0.404, and b ₂ = 0.0178L _c +0.631, distance and the unit of thickness and mm. 3. The objective lens for an optical disk according to claim 1 or 2, wherein the following conditions are satisfied when the full-width half-maximum angle of the light source is θ and the image-side NA (numerical aperture) is sin ∠B. . (− / Β) ≧ (sin ∠B) / sin θ. 4. The surface shape on the light source side and the optical disk side is the number 1
It is represented by
Objective lens for optical disks. [Equation 1]