JPH09152548A - Objective - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide constitution of NA>=0.55 and small wavefront aberration for an aspherical objective which is used for an optical information recording and reproducing device and has both its 1st and 2nd surfaces made aspherical. SOLUTION: The 1st and 2nd surfaces 2 and 3 both satisfy Z=(h<2> / R)/(1+(1-(K+1).(h/R)<2> )<1/2> )+ah<4> +bh<6> +ch<8> +dh<10> , where Z is the distance from the vertex of curvature R along the optical axis, K a cone constant, (h) the distance from the optical axis, (a) a quadratic aspherical surface coefficient, (b) a 6th-degree aspherical surface coefficient, (c) an 8th-degree aspherical surface coefficient, and (d) a 10th-degree aspherical surface coefficient. Then R, K, (a), (b), (c), and d(d) are limited as to the 1st and 2nd surfaces 2 and 3 as specified and the ocular is molded out of an optical material (PMMA: methyl polymetacrylic resin) with an about 1.485 refractive index.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an objective lens used in an optical information recording / reproducing apparatus, particularly a first surface on which parallel light of a predetermined wavelength is incident and a second surface on which the incident light is focused and emitted. And both relate to an aspheric infinite single lens.

[0002]

2. Description of the Related Art As an objective lens for an optical information recording / reproducing apparatus, an aspherical lens molded of plastic is generally used, and the NA (numerical aperture) of them is 0.5.
It was less than 5.

A conventional objective lens, for example, JP-A-2-26
Japanese Patent No. 4213 discloses an objective lens for an optical disc using a low hygroscopic plastic, particularly amorphous polyophylene, in order to reduce the performance change due to humidity and temperature changes. Such an objective lens maintains stable performance especially against humidity changes.

Another conventional objective lens, for example, JP-A-4-
Japanese Patent No. 143715 discloses that although the NA is about 0.5 and the imaging magnification is as large as about -0.3, the lens core thickness is thin, and the size and weight are small. There is disclosed an unused objective lens for optical discs. The lens has an optical material with a refractive index of 1.5 to
1.76 of glass or plastic such as polycarbonate, polystyrene, styrene-acrylonitrile polymer, and amorphous polyophylene is used.

[0005]

A conventional objective lens molded of plastic has a NA of 0.55 or more for use in writing / reading of recorded information in an optical information recording / reproducing apparatus, particularly a magneto-optical disk apparatus. However, there is a problem that the aberration becomes large beyond the allowable limit and the performance is impaired.

For example, the lens disclosed in Japanese Unexamined Patent Publication No. 2-264213 maintains stable performance against changes in humidity, but has an NA of 0.53. Furthermore, since amorphous polysilicon has a relatively large birefringence,
As a result, the optical characteristics are impaired.

The other conventional finite objective lens disclosed in Japanese Patent Laid-Open No. 4-143715 makes it possible to make it compact and lightweight, but the NA is 0.5 to 0.53. Furthermore, the Abbe number (57.0 or less) for that lens
Is low, and the birefringence is large depending on the optical material used, whereby the optical characteristics are impaired.

Further, the plastic aspherical lens is
Molded by injecting molten plastic into the mold, the conventional lens has a gate at the time of molding that opens in the lens body, and internal distortion generated in the lens near the gate improves the optical performance of the lens. There was also the problem of partial loss.

[0009]

SUMMARY OF THE INVENTION An object of the present invention is to provide an aspherical objective lens having an NA of 0.55 or more, particularly a wavelength of 780.
Used in an optical system that uses light source light of ± 20 nm or 680 ± 20 nm, has a small birefringence and an NA of 0.5.
The objective is to provide an objective lens having a number of 5 or more and a small wavefront aberration.

A first aspect of the present invention for achieving the above object is a single lens having an aspherical first surface and a second surface for an optical system having a light source wavelength of 680 nm ± 20 nm. Assuming a plane orthogonal to the optical axis passing through the vertices of the two surfaces and including the vertices of the first surface or the second surface, the distance from the plane to any point on the first surface or the second surface is Z, The distance from the arbitrary point to the optical axis is h, the radius of curvature of the apex of the first surface or the second surface is R, the conical constant of the first surface or the second surface is K, the first surface or the second surface. The fourth-order aspherical coefficient of the second surface is a, the sixth-order aspherical coefficient of the first surface or the second surface is b, the eighth-order aspherical coefficient of the first surface or the second surface is c, and When the tenth-order aspherical coefficient of the first surface or the second surface is d, the first surface and the second surface are expressed by the following formula 1
Represented by the formula,

[0011]

(Equation 1)

With respect to the first surface, the radius of curvature R ₁ is 1.64.
₁ to 1.643, the conic constant K ₁ is -0.4460 to-
0.4466, fourth-order aspherical coefficient a ₁ is -0.334
−0.335 × 10 ⁻² , 6th-order aspherical coefficient b ₁ is −0.
536 to −0.537 × 10 ⁻³ , 8th-order aspherical coefficient c ₁
0.274 to 0.276 × 10 ⁻³ , 10th-order aspherical coefficient d ₁ is −0.777 to −0.779 × 10 ⁻⁴ , and the radius of curvature R ₂ is −4 for the second surface. .710-4.7
12, the conic constant K ₂ is −2.194 to −2196,4
The following aspherical coefficient a ₂ is -0.2177 to -0.2179.
× 10 ⁻¹ , 6th-order aspherical coefficient b ₂ is −0.3440 to −
0.3442 × 10 ^-2 , 8th-order aspherical coefficient c ₂ is 0.5
081-0.5083 × 10 ⁻³ , 10th-order aspherical coefficient d
₂ is −0.7359 to −0.7361 × 10 ⁻⁴ ,
An optical material having a refractive index of 1.475 to 1.495 is used,
That is, the thickness is set to 1.3595 mm to 1.3795 mm.

A second aspect of the present invention that achieves the above object is that, for an optical system having a light source wavelength of 680 nm ± 20 nm, both the first surface and the second surface have an aspherical surface represented by the above formula (1). In the lens, the radius of curvature R ₁ is 1.804 to 1.806, and the conic constant K ₁ is −0.4 with respect to the first surface.
438 to −0.4440, the fourth-order aspherical coefficient a ₁ is −0.178 to −0.180 × 10 ⁻² , and the sixth-order aspherical coefficient b ₁ is −0.6146 to −0.6148. × 1
0 ^-3 , the aspherical coefficient c _{1 of the} 8th order is 0.2194 to 0.
2196 × 10 ⁻³ , the tenth-order aspherical surface coefficient d ₁ is −
0.9758 to −0.9760 × 10 ⁻⁴ , and the second
With respect to the surface, the radius of curvature R ₂ is −5.554 to −5.5.
56, the conical constant K ₂ is 0.8238 to 0.824
0, the fourth-order aspherical coefficient a ₂ is 0.1978 to 0.1
980 × 10 ⁻¹ , the sixth-order aspherical surface coefficient b ₂ is −0.3
767 to -0.3769 × 10 ^-2 , the eighth-order aspherical coefficient c ₂ is 0.4880 to 0.4882 × 10 ^-3 , and 1
The zero-order aspherical coefficient d ₂ is -0.3760 to -0.376.
A 2 × 10 ^-4, the refractive index using optical material from 1.486 to 1.488, the thickness 1.5244mm~1.54
It is 44 mm.

Further, in the present invention, the outer peripheral portion of the objective lens has a flange integrally formed with the lens body. Furthermore, in the said 1st this invention, refractive index is 1.486-1.
By using PMMA (polymethylmethacrylate resin) having a small birefringence as an optical material of 488 and limiting the shapes of the first surface and the second surface as described above, a wavelength of 780 nm ±
In an optical system using a 20 nm light source, NA is 0.
It is possible to provide 55 or more objective lenses.

Further, in the second aspect of the present invention, PMMA having a small birefringence is used as an optical material having a refractive index of 1.486 to 1.488, and the shapes of the first surface and the second surface are limited as described above. This makes it possible to provide an objective lens having an NA of 0.55 or more in an optical system using a light source having a wavelength of 680 nm ± 20 nm.

Further, in the objective lens of the present invention formed by molding plastic, the flange provided on the outer edge portion eliminates the optical disturbance near the gate by opening the molding gate in the flange.

[0017]

FIG. 1 is an explanatory view of an objective lens according to an embodiment of the present invention, FIG. 2 is an explanatory view of wavefront aberration in the first embodiment lens of the present invention, and FIG. 3 is an embodiment lens of the present invention. FIG. 4 is a perspective view showing the light intensity distribution at the in-focus point, and FIG. 4 is an explanatory diagram of wavefront aberration in the lens of the second example of the present invention.

In FIG. 1, a lens 1 having a thickness t ₁ has a first surface 2 on which parallel light 11 is incident and a second surface 3 on which the incident parallel light 11 is focused, both of which are convex outward. A flange 4 having a thickness t ₂ projects from the outer diameter portion.

The lens body of the lens 1 and the flange 4
Is integrally formed by molding a plastic having a refractive index of 1.475 to 1.495, and the first surface 2 and the second surface 3 are represented by the above-mentioned formula 1.

Therefore, in an optical system using a light source light having a wavelength of 780 ± 20 nm, the parallel light of the light source light is the first
In the first embodiment of the present invention which is incident on the surface 2, with respect to the first surface 2 which is an aspherical surface rotationally symmetric with respect to the optical axis 5, R ₁ = 1.641 to 1.643 K ₁ = −0. 4460 to −0.4466 a ₁ = −0.334 × 10 ⁻² to −0.335 × 10 ⁻² b ₁ = −0.536 × 10 ⁻³ to −0.537 × 10 ⁻³ c ₁ = 0 and ^{.274 × 10 -3 ~0.276 × 10 -3} d 1 = -0.777 × 10 -4 ~-0.779 × 10 -4, is an aspherical surface of rotational symmetry with respect to the optical axis 5 a 2 sides 3
_{Relates, R 2 = -4.710~-4.712 K} 2 = -2.194~-2.196 a 2 = -0.2177 × 10 -1 ~-0.2179 × 10
⁻¹ b ₂ = −0.3440 × 10 ⁻² to −0.3442 × 10
^-2 c ₂ = 0.5081 × 10 ^{−3 to} 0.5083 × 10 ⁻³ d ₂ = −0.7359 × 10 ⁻⁴ to −0.7361 × 10
^-4 .

The refractive index is 1.475 to 1.495.
In the first embodiment of the present invention, in which PMMA is used as the optical material, the thickness t ₁ of the lens ₁ is about 1.37.
mm, the thickness t ₂ of the flange 4 is 0.25 mm, and the parallel rays 7 incident from the first surface 2 are emitted from the second surface 3 and are focused on the point P.

The RMS (root mean square) wavefront aberration of such a lens is as shown in FIG. 2, that is, when the incident angle of (a) is 0 degree, it is about 0.0204λ (λ: wavelength), and the incident angle of (b) is. Is about 0.0178λ when the angle is 0.05 °, and about 0.0314λ when the incident angle of (c) is 0.1 °. The defocus value (deviation from the focus point) representing the wavefront aberration by the XY component is 0.00
00.

Further, in the lens 1 of the first embodiment, the light intensity distribution at the focal point P is extremely sharp as shown in FIG. 3 (a), and the spot diameter is 0.5 μm.
m.

Therefore, the lens 1 is arranged so that the back surface of the optical disk 6 faces the second surface 3, and when the refractive index of the optical disk 6 is 1.57 and the thickness t ₃ is 1.2 mm, the desired information is printed on the front surface 61. If the distance α between the back surface of the optical disc 6 on which is recorded and the second surface 3 is 1.2 mm ± 0.05 mm, the wavelength of 780 nm ± 2 transmitted through the lens 1 and the optical disc 6
The focus P ′ of the transmitted light of 0 nm is on the surface 61 of the optical disc 6.
And the information recorded or to be recorded on the surface 61 can be accurately recorded or read.

Next, a second embodiment of the present invention will be described with reference to FIGS. 1, 3 (b) and 4. Wavelength is 680 ± 20n
In the second embodiment of the present invention in which the parallel light of the light source light is incident on the first surface 2 in the optical system using the light source light of m,
Regarding the first surface 2, which is an aspherical surface rotationally symmetric with respect to the optical axis 5, R ₁ = 1.804 to 1.806 K ₁ = −0.4438 to −0.4440 a ₁ = −0.178 × 10 ^{-2 ~-0.180 × 10 -2 b} 1 = -0.6146 × 10 -3 ~-0.6148 × 10
^-3 c ₁ = 0.2194 × 10 ^{-3 to} 0.2196 × 10 ^-3 d ₁ = -0.9758 × 10 ^-4 to -0.9760 × 10
^-4, and the second surface 3 which is an aspheric surface rotationally symmetric with respect to the optical axis 5.
With respect to R ₂ = −5.554 to −5.556 K ₂ = 0.8238 to 0.8240 a ₂ = 0.1978 × 10 ^{−1 to} 0.1980 × 10 ⁻¹ b ₂ = −0.3767 × 10 ^-2 to -0.3769 x 10
^-2 c ₂ = 0.4880 × 10 ^{−3 to} 0.4882 × 10 ⁻³ d ₂ = −0.3760 × 10 ⁻⁴ to −0.3762 × 10
^-4 .

The refractive index is 1.486 to 1.488.
The second aspect of the present invention wherein PMMA is used as the optical material of
In this example, the thickness t ₁ of the lens ₁ is about 1.524.
4 mm to 1.5444 mm and the thickness t of the flange 4
₂ is 0.28 ± 0.05 mm.

The RMS (root mean square) wavefront aberration of such a lens is as shown in FIG. 4, that is, when the incident angle of (a) is 0 degree, it is about 0.0230λ (λ: wavelength), and the incident angle of (b). Is about 0.0197λ when the angle of incidence is 0.05 degree, and about 0.0299λ when the incident angle of (c) is 0.1 degree. The defocus value (deviation from the focus point) representing the wavefront aberration by the XY component is 0.00
00.

Further, in the lens 1 of the second embodiment of the present invention, the light intensity distribution at the focal point is shown in FIG. 3 (b).
As shown in, the spot diameter becomes extremely sharp and the spot diameter becomes 0.5 μm.

Therefore, the lens 1 is arranged so that the rear surface of the optical disk 6 faces the second surface 3, and when the refractive index of the optical disk 6 is 1.57 and the thickness t ₃ is 1.2 mm, the desired information is recorded on the front surface 61. If the distance α between the second surface 3 and the back surface of the optical disc 6 on which is recorded is 1.4 mm ± 0.05 mm, the wavelength 680 nm ± 2 transmitted through the lens 1 and the optical disc 6
The focus P ′ of the transmitted light of 0 nm is on the surface 61 of the optical disc 6.
And the information recorded or to be recorded on the surface 61 can be accurately recorded or read.

In the first and second embodiments of the present invention, the flange 4 is used for fixing and positioning of the lens 1, and the gate when manufacturing the lens 1 by molding is the outer peripheral surface of the flange 4. If you open it to
The optical disturbing portion generated in the vicinity of the gate due to molding is contained in the flange 4, and the lens body is not affected by the internal stress in the vicinity of the gate.

[0031]

As described above, the objective lens of the present invention has an NA of 0.55 or more, and by using PMMA, the birefringence is small and the wavefront aberration is small. Enables an optical system for a magnetic disk device.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of an objective lens according to an embodiment of the present invention.

FIG. 2 is an explanatory diagram of wavefront aberration in the lens of Example 1 of the present invention.

FIG. 3 is a perspective view showing a light intensity distribution at a focal point of an example lens of the present invention.

FIG. 4 is an explanatory diagram of wavefront aberration in the lens of the second embodiment of the present invention.

[Explanation of symbols]

 1 lens 2 1st surface 3 2nd surface 4 flange 5 optical axis 6 optical disk 61 optical disk surface

Claims (6)

[Claims] 1. A single lens in which both the first surface and the second surface are aspherical surfaces, the single lens being orthogonal to an optical axis passing through the apexes of the first surface and the second surface, and
Assuming a plane including the vertices of the surface or the second surface, the distance from the plane to an arbitrary point on the first surface or the second surface is Z, the distance from the arbitrary point to the optical axis is h, and The radius of curvature of the apex of the first surface or the second surface is R, the conical constant of the first surface or the second surface is K, the quaternary aspherical coefficient of the first surface or the second surface is a, the first The sixth-order aspherical coefficient of the surface or the second surface is b, the eighth-order aspherical coefficient of the first surface or the second surface is c, and the tenth-order aspherical coefficient of the first surface or the second surface is d. And the first and second surfaces are Z
= (H ² / R) / [1+ {1- (K + 1) ・ (h / R)
² } ^1/2 ] + ah ⁴ + bh ⁶ + ch ⁸ + dh ^{10 and} the radius of curvature R ₁ is 1.641 to 1.64 with respect to the first surface.
3, the conic constant K ₁ is -0.4460 to -0.4466,
Quaternary aspherical surface coefficient a ₁ is -0.334 to -0.335 ×
10 ⁻² , 6th-order aspherical coefficient b ₁ is −0.536 to −0.
537 × 10 ⁻³ , 8th-order aspherical coefficient c ₁ 0.274-
0.276 × 10 ⁻³ , 10th-order aspherical coefficient d ₁ is −0.
777 to −0.779 × 10 ⁻⁴ , and the radius of curvature R ₂ of the second surface is −4.710 to −4.
712, the conic constant K ₂ is −2.194 to −2196,
The fourth-order aspherical coefficient a ₂ is -0.2177 to -0.217.
9 × 10 ⁻¹ , 6th-order aspherical coefficient b ₂ is −0.3440
−0.3442 × 10 ⁻² , 8th-order aspherical coefficient c ₂ is 0.
5081 to 0.5083 × 10 ⁻³ , 10th-order aspherical coefficient d ₂ is −0.7359 to −0.7361 × 10 ⁻⁴ , and an optical material having a refractive index of about 1.485 is used. Is about 1.37 mm. 2. A single lens having an aspherical surface, wherein both the first surface and the second surface are represented by the formula of claim 1, wherein the radius of curvature R ₁ is 1.804. ~ 1.
806, the conical constant K ₁ is -0.4438 to -0.4
440, the fourth-order aspherical coefficient a ₁ is −0.178 to −
0.180 × 10 ⁻² , the aspherical coefficient b _{1 of the} 6th order is −
0.6146 to −0.6148 × 10 ⁻³ , the eighth-order aspherical coefficient c ₁ is 0.2194 to 0.2196 × 10 ⁻³ ,
The tenth-order aspherical surface coefficient d ₁ is −0.9758 to −0.
9760 × 10 ⁻⁴ , and with respect to the second surface, the radius of curvature R ₂ is −5.554 to −
5.556, the conical constant K ₂ is 0.8238 to 0.8
240, the fourth-order aspherical coefficient a ₂ is 0.1978 to
0.1980 × 10 ⁻¹ , the sixth-order aspherical surface coefficient b ₂ is −
0.3767 to -0.3769 × 10 ^-2 , the eighth-order aspherical coefficient c ₂ is 0.4880 to 0.4882 × 10 ^-3 ,
The tenth-order aspherical surface coefficient d ₂ is −0.3760 to −0.
An objective lens, which is 3762 × 10 ⁻⁴ , uses an optical material having a refractive index of about 1.487, and has a thickness of about 1.535 mm. 3. The objective lens according to claim 1, wherein a flange protruding from an outer edge portion is integrally formed. 4. The second surface according to claim 1, wherein parallel light having a wavelength of 780 nm ± 20 nm is incident on the first surface according to claim 1, and the incident light is focused and emitted. 5
7. It is arranged so as to face the back surface of an optical disk having a thickness of 1.2 mm and desired information recorded on the front surface at an interval of 1.2 ± 0.05 mm. The objective lens according to claim 1. 5. The second surface according to claim 2, wherein parallel light having a wavelength of 680 nm ± 20 nm is incident on the first surface according to claim 2, and the incident light is focused and emitted, and the second surface has a refractive index of 1. 5
7. It is arranged so as to face the back surface of the optical disk having a thickness of 1.2 mm and desired information recorded on the front surface at an interval of 1.4 ± 0.05 mm. The objective lens according to claim 2. 6. The objective lens according to claim 1 or 2, wherein the optical material according to claim 1 or 2 is polymethyl methacrylate (PMMA).