JPH02106709A - Aspherical objective lens - Google Patents

Abstract

PURPOSE:To improve heat resistance and moisture resistance by using a single lens constituted of an aspherical face the 1st face of which has a positive refracting power and the 2nd face of which has a negative refracting power and forming the lens so as to satisfy specific optical conditions. CONSTITUTION:This objective lens is the positive meniscus single lens the 1st face of which is constituted of the aspherical face having the positive refracting power and approximated by the elliptic face constituting a convex face of a curvature C1 and the 2nd face of which is constituted of the aspherical face having the negative refracting power and approximated by the hyperbolic face constituting the concave face of a curvature C2. The lens is so constituted as to satisfy the conditions 1.80<n<1.96, 5.0<C1/C2<21.0, 0.30<D/f<0.78, where n denotes the refractive index of the lens glass material; C1 denotes the curvature of the 1st face; C2 denotes the curvature of the 2nd face; D denotes the thickness of the lens; f denotes a focal length. The objective lens which has the excellent heat resistance and moisture resistance, is stable in optical characteristics and is adequate for optical disks is obtd. in this way.

Description

Detailed Description of the Invention [Object of the Invention] (Industrial Application Field) The present invention relates to an objective lens for an optical disk reproduction optical system, particularly an aspheric objective lens suitable for being directly driven for focusing and tracking. Regarding lenses.

(Prior Art) An optical head for an optical disk irradiates the optical disk with a laser beam and photoelectrically converts the reflected light from the disk into signal light to obtain a signal current.

Optical discs have pits (dents), and signals are discriminated by using the difference in the amount of reflected light (approximately 1:10) due to the difference in reflection diffraction index from the optical disc with and without pits. are doing.

Therefore, as an objective lens for an optical head, it is bright (
It is required that the lens has a numerical aperture of 100 mm), and is also small and lightweight for focusing and tracking purposes.

If an objective lens is constructed of a spherical glass lens in order to satisfy such requirements, it will be a combination of a large number of lenses, and it will be impossible to achieve a reduction in size and weight.

In order to overcome these drawbacks of glass lenses, the objective lens has been constructed of a single plastic aspherical lens, as disclosed in, for example, Japanese Patent Application Laid-open No. 76512/1983.

2. Description of the Related Art Recently, in addition to conventional read-only optical discs, write-once optical discs and rewritable optical discs have become popular. These latter two recording optical heads must focus high-energy laser light into a minute spot in order to perform high-density recording. Therefore, the numerical aperture of the objective lens is as high as 0.5 to 0.6, and it is necessary to shape the elliptical beam of a high-power semiconductor laser device of 20 mW to 30 mW into a circular shape. On the other hand,
The objective lens of the reproduction-only optical head has a numerical aperture of 0.40.
~0.50, and the output of the semiconductor laser device is 5 m
w is sufficient.

However, many conventional plastic aspherical single lenses are designed as read-only objective lenses, and have the disadvantage that they are not suitable as write-once type or rewritable type objective lenses. Moreover, since there are fewer types of plastic lenses compared to glass lenses, there are restrictions on the refractive index range that can be selected, as well as temperature and
It has the disadvantage that its optical properties, especially its refractive index, change sensitively due to changes in humidity.

(Problems to be Solved by the Invention) The present invention has been made in consideration of the above circumstances, and has the versatility to be applied to optical heads for write-once and rewritable optical disks as well as read-only optical disks. The purpose is to provide an aspheric objective lens.

[Structure of the invention]

(Means and effects for solving the problem) A single lens constituted by an aspherical surface whose first surface has a positive refractive power and whose second surface has a negative refractive power, and which satisfies the three optical conditions described below. By satisfying the above criteria, it is possible to obtain an objective lens that has heat resistance and moisture resistance and is optimal for use in an optical head for an optical disk.

(Example) Hereinafter, one embodiment of the present invention will be described in detail with reference to the drawings.

FIG. 1 shows the aspheric objective lens of this embodiment. This lens is obtained by hot-molding a lens material using a mold. The first and second surfaces (1) and (2), which are side refractive surfaces of this lens, form a positive meniscus single lens having positive and negative refractive powers, respectively. By the way, generally speaking, an aspherical shape has first and second surfaces (IL (
2) vertices (X, Y, Z with 3L (4) as the apex
In the orthogonal coordinate system, it is expressed by the following formula (■).

Tatami, h: Optical axis (5) Karano height (-vX2+y2)
C: Curvature of aspherical apex (-r, l': radius of curvature) k: Conic constant de, f, g: 4th, 6th, and 8th order, respectively.

10th order aspheric coefficient Such an aspherical objective lens meets the following conditions ■.

It is characterized by satisfying the formulas ■ and ■.

1.80 < n < 1.96 ・・・・・・■
5.0<C1/C2<210 ・・・・・・・・・■0
．． 30<D/f<, 0.78...
...■ However, n: refractive index of the lens glass material C1: curvature of the first surface C2: curvature of the second surface D: thickness of the lens r: focal length where condition 0 is an aberration correction characteristic lens This is a condition required of lens materials in terms of tolerance to manufacturing errors (refractive index, lens thickness, axis misalignment, inclination of two planes, etc.). Glass materials that meet this condition include 5FSL, 5FSOI, 5F
There are heavy flint glasses such as SO2. Within this range, desired aberration correction characteristics and lens manufacturing error characteristics can be obtained.

Next, condition 0 formula is based on condition 0 formula, and a lens spherical system that satisfies good aberration correction within a constant field of view and the sine condition, which was required in the old days for objective lenses for optical disc devices. This is a specified condition, and if it deviates from this range, excessive spherical aberration will remain, and even if you try to correct it by using higher-order aspherical coefficients, it will have little effect, and it will not be possible to correct it by using higher-order aspherical coefficients. It is not possible to satisfy the aberration limit value at .

Furthermore, condition 0 is the original condition for the material of condition 0, and if the upper limit of condition 0 is exceeded, the working distance (Wl) required for the objective lens for the optical tisf device will be You won't be satisfied. Conversely, if the lower limit of condition 0 is exceeded, the lens manufacturing error characteristics will deteriorate.

Furthermore, in order to satisfactorily correct wavefront aberration both on-axis and off-axis, the first and second surfaces (j) of the lens.

(2) is preferably an aspherical surface, and the first surface (
1) is approximated by a convex ellipsoid (-1<k<O),
The second surface (2) is preferably an aspheric surface approximated by a concave hyperboloid (k<-1).

A specific example is shown below.

Specific example 1 (see Figures 2 and 3) Wavelength λ = = 780 nm, ( = 4.5. Numerical aperture N8-0, 45C, = 0.280140. n = 2.0O
n=1.8590C2=0.028931. CL/
C2=9.68. D/f=0.44 cover glass (
6) Thickness t=1.2 (refractive index IJ5), working distance W, D=
2.498 [First surface] [Second surface] k=-0,510355k=-181,194892d
= -8,96582X 10'-'! = −8,
32445X 10'-'t --1,89670xl
O' Specific example 2 (see Figures 4 and 5) Wavelength λ = 780 nm, f = 4.5. Numerical aperture NA
=0.45C,=0.273322. D = 2
．． 42. n = 1.8900°"t"' 0.
034326, Ct/C,=7.96, D/f
” 0.54.

Cover glass (6) thickness t=12 (refractive index 155), working distance WD, =z, zs.

[First surface] [Second surface] k=-0,481262k=-18,814951d
= 8.26416X10 d = 2.670
00X10-'e= 6.42962X10-'e
-=-5,87363xlO-'t=-3,675
93xlO-11t = -2,39786xlO-'
""" 3.61130xlO-7g = 1.
78995X10-6 Specific Example 3 (See Figures 6 and 7) Wavelength λ = 830 nm, f = 4.2. Numerical aperture N A = 0.55C, = 0.279454.
n=3.05. n=1.9500゜C2=O,Q
49299. C+/C2= 5.67, D/(=
0.73 cover crow (6) Ji g= 1.20 (
JrE refractive index 1.57), working distance W]), = 1
510 [Side 1] [Side 2] k = -0,694320k = -10,71,19
60=4 d=8.49957X10 d=8.
68878xlO-'e = 4.721.79X1
0 e = 7.26991X10-5g=-2
,78047x1. Ot = −1,,33894xl
O-'g=-8,18537X10 g'=1
．． 94513X10-3 Specific Example 4 (See Figures 8 and 9) Wavelength λ = 830 nm, g = 4.2. Numerical aperture NA
=0.55C+ = 0.287454. D =
2.95. n=1. ．． 8900°C2=0.0
331812. C+/C2=8.66, D/f=
0.70 cover glass (6) thickness 7 = 1.20 (refractive index 1
．． 57), working distance WD, = 1.642 [first surface]
[Second surface] k = -0,690881k = -12,398301
d = 8.69301X10 d =
1.16428X10””3e=4.04108X
1. Oe--8,42256X10-5t =-1,7
6379x1. Ot--7,96067xlO-5g=
-2,58664X10 g= 1.163
98
Numerical aperture NA=0.55C,=0.298963.
D = 3.12. n = 1.8350°C2-
0,014390,C1/C2-2078,D/f-0
76 cover glass (6) thickness t = 1.20 (refractive index 1.5
7) , Working distance WD, -1,,570 [First surface]
[Second surface] k=-0.683256 k= 759.83
44.1-8d = 9.26784X10d
= 1.40429X10”e=5.408
88X10 e=-2, 20017X10't=
8.26405X10 g=-1,098
60X10-5g=-9,64510xlOg=-2,
88399xlO' Among the above embodiments, Specific Examples 1 and 2 are reproduction-only optical microscope objective lenses, and
Specific examples 3 and 4.5 are objective lenses for write-once type or rewritable type optical disks. Here, FIGS. 2 to 5 are as follows:
They are characteristic diagrams each showing aberration performance in Specific Example 1.2 of the present invention, and show spherical aberration, sine condition, and astigmatism. These have a fundamental wavelength of λ=780 nm. Further, FIGS. 6 to 11 are characteristic diagrams showing aberration performance in specific examples 3.4.5 of the present invention, respectively,
Similarly, spherical aberration and sine conditions.

It shows astigmatism. These are λ-830nm
is the fundamental wavelength. Diagram of astigmatism (Figure 3).

5, FIG. 7, FIG. 9, FIG. 11), solid lines indicate zadicle field curvature, and dotted lines indicate meridional field curvature. As shown in FIGS. 2, 4, 6, 8, and 10, the lenses of the examples have small absolute amounts of each aberration in each NA range, and as shown in FIG. As shown in Fig. 5, Fig. 7, Fig. 9, and Fig. 11, on-axis and off-axis aberrations are well corrected, making this lens suitable for use in optical disc devices. It can be seen that it is.

〔Effect of the invention〕

The aspheric objective lens of the present invention has the following remarkable effects. That is, (1) it has a single lens configuration and the overall length can be shortened, so the weight including the lens barrel of the optical head for optical tisf can be significantly reduced. ■A relatively long working distance can be obtained compared to the focal length f (WD/f > 0.35
). Therefore, even if a tilt surface runout or a spindle mounting surface error occurs, a sufficient distance can be maintained to prevent damage to the tilt lens or the lens due to collision between the tilt lens and the lens. (2) Axial and off-axis (±1°) aberrations are also well corrected. ■Manufacturing errors (refractive index error, lens thickness error, axis misalignment).

It is resistant to surface tilt) and easy to manufacture. ■Compared to plastic lenses, it has superior heat resistance and moisture resistance, and stable optical properties, making it ideal for objective lenses for optical disks that require high product quality and high reliability.

[Brief explanation of drawings]

FIG. 1 is a diagram showing an aspheric objective lens according to an embodiment of the present invention, and FIGS. 2 to 11 are diagrams for explaining the optical characteristics. (1)...First page. (2)...Second side. Agent Patent Attorney Nori Ken Ken Yudo Matsuyama Hikaruyuki NA0.45 2nd Figure 1 Figure 3 Figure NAo, +5 Handa Daikyo N△055 1st Kyodai Sen i1/A0.55 Suta Daime N△055 Φ Eyes and mouth

Claims (1)

[Claims] The first surface is constituted by an aspherical surface approximated by a convex ellipsoidal surface having a positive refractive power and a curvature C_1, and the second surface is a concave surface having a negative refractive power and a curvature C_2. A positive meniscus single lens constituted by an aspheric surface approximated by a hyperboloid with the following: 1.80<n<1.96 5.0<C_1/C_1<21.0 0.30<D/f< 0.78 However, n: refractive index of the lens C_1: curvature of the first surface C_2: curvature of the second surface D: thickness of the lens f: an aspherical surface that satisfies each condition of the focal length of the lens objective lens.