JPH02195317A - Objective lens for optical disk - Google Patents

Abstract

PURPOSE:To make the objective lens lightweight and to reduce error sensitivity to the axis shift of a single lens by composing the objective lens of the single lens which has aspherical surfaces and positive refracting power on the light- source side and optical-disk side, and satisfying specific conditions. CONSTITUTION:The objective lens consists of the single lens which has the positive refracting power and the aspherical surfaces on the light-source side and optical-disk side and satisfies the specific conditions. Namely, 1.5<n<1.52 as a condition I, 0<A21f<1.15 as a 2nd condition II, and 0.8<d/f<0.85 as a condition 3 hold, where (f) is the focal length, (d) the core thickness, (n) the refractive index, and A21 the aspherical surface coefficient of the light-source side aspherical surface when the aspherical surface is expressed by the equation I. The condition I is for the weight reduction of the single lens, and even if an over spherical aberration which exceeds the lower limit of the condition II is generated and the upper limit is exceeded, an under spherical aberration is generated. When the condition III is satisfied, a single lens which has small parallel deviation error sensitivity is obtained. Consequently, the lightweight objective lens which has small error sensitivity to the axis shift of the single lens is formed.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an objective lens for optical discs, and particularly for recording code information and image information on optical discs for computer document files.

This invention relates to an objective lens for optical discs that performs erasing, reproducing, etc. by focusing a laser beam.

Conventional q and sub-optical discs include optical discs for CDs (compact discs) in which information is recorded with holes of several micrometers and used only for reproduction, and discs for both recording and reproduction, such as magneto-optical recording systems. There are some things that can do this. In the latter method, called the magneto-optical recording method, an amorphous alloy is used for the optical disk, and when light is irradiated, the amorphous alloy is perpendicularly magnetized by a rise in temperature and the action of an external magnetic field, and information is recorded. When weak light with a slightly larger spot diameter than the recording laser beam is applied to the recording section, the plane of polarization rotates slightly. Reproduction is performed using this phenomenon. In this method, the beam emitted from the semiconductor laser must be narrowed down to a spot diameter approximately at the diffraction limit by an objective lens on the optical disk, as in the case of playback with a playback-only optical disk device.In addition, information must be recorded on the optical disk. In this case, it is necessary to make the spot diameter smaller than in the case of reproduction. The size of the spot diameter is expressed as 0.8×λ/NA, where NA is the numerical aperture and λ is the wavelength. Therefore, while an optical disk for playback only requires an NA of 0.45 to 0.47, a magneto-optical recording system for both recording and playback requires a NA of 0.53 to 0.55.
degree required. However, in order to increase NA,
In order to realize such an objective lens with a large NA with a single lens, the objective lens must have a large aperture or a short focal length, which is subject to severe manufacturing constraints. This is because the effective area of the aspheric surface in accordance with the design value increases, or at least the refractive power of one side of the single lens increases, and the error sensitivity of the surface increases. Therefore, it is important to design a single lens that is as easy to manufacture as possible.
In other words, it is necessary to consider a configuration in which the error sensitivity is reduced with respect to the relative parallel eccentricity of both side surfaces of the single lens.

On the other hand, when considering high-speed access in optical disk devices, it is also desirable to reduce the weight of the objective lens.

o < ゛ し -= Therefore, in order to overcome the above problems, the present invention provides an objective lens for optical discs that is lighter than conventional single lenses and has less error sensitivity to axis deviation of the single lens. The purpose is to provide.

In order to achieve the above object, the first objective lens for an optical disc according to the present invention is a single lens having positive refractive power on the light source side and the optical disc side, and furthermore, the light source side and the optical disc side both have positive refractive power. It has an aspherical surface and is configured to satisfy the following conditions.

1.5 < n < 1.52 ・
...■0 <Al f <0.15
・・・・・・■0.8 < d / f <
0.85 ・・・・・・■ However, f: Focal length of a single lens d: Core thickness of a single lens n: Refractive index of a single lens A2: When an aspherical surface is represented by 2 below, the aspherical surface on the light source side This is a second-order aspheric coefficient.

Here, X: Distance from the tangential plane on the optical axis of the aspherical surface to the aspherical surface y: Height from the optical axis C: Reciprocal of the paraxial radius of curvature (ro) of the aspherical surface (=1/r'') K: This is the conic constant of the aspheric surface on the light source side.

In a single lens used as an objective lens for an optical disk, the aberrations required to obtain good performance up to an angle of view of about ±1° off-axis are spherical aberration and a sine condition. In addition, considering the axis misalignment of a single lens during the manufacturing stage, it is desirable to have a single lens shape that minimizes the deterioration of wavefront aberration due to the relative parallel eccentricity of both sides. It is important to reduce the weight of the One possible method for reducing the weight of a single lens is to use a glass material with low specific gravity for the single lens.

Glasses having a refractive index within the range of condition (2) often have particularly low specific gravity, and are therefore effective in reducing the weight of a single lens. Therefore, outside the range of condition (2), it becomes difficult to reduce the weight of a single lens.

Condition ■ indicates the relationship between the second-order aspheric coefficient of the light source side aspheric surface and the focal length, and a single lens having a refractive index within the range of condition ■ satisfies the range of condition ■. If the following aspherical coefficient values exist, high-order spherical aberrations can be well corrected despite the large NA.If the lower limit of the 0 condition (2) is exceeded, excessive spherical aberration will occur. Even if correction is made using the aspherical coefficients of Unacceptable waviness remains.

Furthermore, condition (■) indicates the relationship between the lens core thickness and the focal length of the lens. If this condition is satisfied, a single lens with low parallel decentration error sensitivity can be obtained.If the lower limit of condition (1) is exceeded, minute Even when the axis is misaligned, on-axis coma occurs significantly, and the wavefront aberration collapses. Moreover, if the upper limit is exceeded, the core thickness of the single lens becomes large, making it difficult to reduce the weight of the lens.

Moreover, the second objective lens for optical disc according to the present invention is
The lens is a single lens having a positive refractive index on the light source side and the optical disk side, and has an aspherical surface on both the light source side and the optical disk side, and is configured to satisfy the following conditions.

1.6 < n < 1.69 ・
...■rt However, f: focal length of a single lens d: core thickness of a single lens n: refractive index of a single lens r,: paraxial radius of curvature on the light source side of a single lens rz = optical disk side of a single lens When the aspherical surface is expressed by the following formula, the conic constant of the aspherical surface on the light source side is where: C: Reciprocal of the paraxial radius of curvature (r") of the aspherical surface (=1/r") ^,: Aspherical coefficient.

Condition (2) indicates the appropriate range of the refractive index of the second objective lens for optical disks of the present invention, and by constructing a single lens with glass having a refractive index within the range of condition (2), the refractive index can be reduced to 1.
．． A glass material having a smaller specific gravity than that of a single lens having a diameter of 7 to 1.8 can be used, and the weight of the lens can be reduced. Further, since the single lens according to the present invention has a large NA of about 0.55, if the lower limit of condition (2) is exceeded, it becomes difficult to correct spherical aberration well. Condition (■) shows the relationship between the core thickness, focal length, and refractive index of a single lens; if the upper limit of condition (■) is exceeded, the core thickness becomes too thick, which goes against weight reduction, and if the lower limit is exceeded, off-axis flare occurs. Condition (2) represents a shape that minimizes performance deterioration due to axis misalignment of a single lens, based on the core thickness and refractive index that satisfy the zero condition (2) and condition (2) in which the sine condition is broken. The 0 condition ■ indicates the ratio of the refractive power of the side surface of the disk to the refractive power of a single lens, and if the upper limit is exceeded, when axial misalignment occurs, under-axial coma aberration will occur, and the axial wavefront aberration will become severe. deteriorates to.

In addition, if the lower limit is exceeded, excessive axial comatic aberration occurs due to axial misalignment, leading to deterioration of wavefront aberration. is responsible for the correction of

Furthermore, by maintaining the conic constant of the aspherical surface on the light source side and the curvature radius ratio of both side surfaces of the single lens as shown in condition (2), it is possible to reduce the occurrence of significant on-axis coma due to axis misalignment.

In order to further correct the sine condition in a single ball shape given by the conditions (1) to (2), it is desirable to satisfy the following condition (2).

0<A, f snow<0.13...
...■ The above equation defines the range of the third-order aspherical coefficient on the optical disk side. When the upper limit is exceeded, the sine condition falls to the over side, coma aberration occurs off-axis, and the off-axis wavefront Aberrations deteriorate. Furthermore, axial coma aberration due to axial misalignment also increases, making error sensitivity extremely severe.

When the lower limit is exceeded, the sine condition falls to the under side, and coma aberration in the opposite direction to the above-mentioned coma aberration occurs off-axis, and off-axis wavefront aberration deteriorates.

IL members Below, examples of the present invention will be shown. Here, the first objective lens for optical disc corresponds to Example 1, and the second objective lens for optical disc corresponds to Examples 2 to 7.

In addition, the performance when the single lens of the example has an axis misalignment is evaluated by changing the magnitude of the axis misalignment, S, 1. R value and wavefront aberration
, M, and S, were investigated by measuring the values. The results are shown in Table 1, where S.

! , the value represents the degree of deterioration of the center intensity of the spot on the optical disk, and the value in the diffraction limit state is taken as 100%.

Further, R, M, S, and wavefront are the least root mean square values of wavefront aberration.

Note that the refractive index in the examples is a value for a wavelength (λ) of 825 nm, and the magnification is 0.

<Example 1> NA-0,55f bulk 3.6 all-0,0 Rei proverb--8,0264956 n -1,51029 Ad 0.06363 d/f corridor 0.822 <Example 2> NA -0,55f -3,6 Aspherical coefficient r, *L-0,0 Aspherical coefficient g=-8,0132863 Z-7,9890113 rz rl =1.60901 Child 1.60901 <Example 3><Example4> NA-0,55 f = 3.6 NA-0,55 f = 3.6 Aspherical coefficient Aspherical coefficient Kl = 0.0, = 0.0 A3-0.119880X10-” As -0.139523 ,=-0,469302X10-3 x,--7,9872831 Kg-7,9896146 -r! -r value=1.643 -1.660 <Example 5><Example6> NA=0.55 f =3.6 NA=0.55 f =3.6 Aspherical coefficient Aspherical coefficient, =0.0 L=0.0 A, =0.591386X10 bow As=-0,836470x10 8, =-0, 379934X10-" 8, = -0, 439613X10-' *= 6.9999685 (1-n) f 1 layer = 0.079 f completed box = 0.0 -rχ r*" K!-2,0 ^ z=0.185180X10-" - "Tenth=1.68694 (!) ^sf" =0.01996 <Example 7> N A = 0.55 f = 3.6 -1.68694 (proverb) ^sf" =0.02398 Aspheric coefficient r, *L=0.0 From the results shown in Table 1, even if an axis deviation of about 20 μm occurs, S, I, and R, M, and S values are 80% or more.

It can be seen that the wavefront value is λ/14 or less, and the wavefront aberration satisfies Marshall's standard.

Next, FIG. 1 shows Example 1. FIG. 2 shows examples 2 and 3. FIG. 3 shows examples 4 and 5. FIG. 4 shows the schematic structure and the optical path of the light beam incident on the objective lens (L) corresponding to Examples 6 and 7, respectively.

Further, (P) is a flat plate representing an optical disc.

5 to 11 represent aberration diagrams corresponding to Examples 1 to 7, respectively.

Further, according to the present invention, it is possible to realize an objective lens for an optical disc that is lighter than a conventional single lens and has less error sensitivity to axis deviation of the single lens.

[Brief explanation of the drawing]

FIG. 1, FIG. 2, FIG. 3, and FIG. 4 each illustrate Example 1 of the present invention. Examples 2 and 3. Examples 4 and 5. FIG. 5, FIG. 6, FIG. 7, and FIG.
FIG. 8, FIG. 9, FIG. 1θ, and FIG. 11 are aberration diagrams corresponding to Examples 1 to 7, respectively.

Claims (1)

[Scope of Claims] (1) An objective lens for an optical disc, characterized in that both the light source side and the optical disc side are composed of a single lens having an aspherical surface and positive refractive power, and the objective lens satisfies the following conditions: 1.5<n <1.52 0<▲There are mathematical formulas, chemical formulas, tables, etc.▼<0.15 0.8<d/f<0.85 However, f: Focal length of a single lens d: Core thickness of a single lens n: Single lens Refractive index of the lens ▲ There are mathematical formulas, chemical formulas, tables, etc. ▼: When the aspherical surface is expressed by the following formula, the second-order aspherical coefficient of the aspherical surface on the light source side ▲ There are mathematical formulas, chemical formulas, tables, etc. ▼ Here, : Distance y from the tangential plane on the optical axis of the aspherical surface to the aspherical surface: Height from the optical axis C: Reciprocal of the paraxial radius of curvature of the aspherical surface K: Conic constant of the aspherical surface on the light source side. (2) An objective lens for an optical disc, characterized in that both the light source side and the optical disc side are composed of a single lens having an aspherical surface and a positive refractive power, and the objective lens satisfies the following conditions: 1.6<n<1.69 0. 07<[(1-n)f]/r_2<0.290.0
≦r_1/-r_2・√(|K_1|)<0.020.
7<d/[f(n-1)]<1.35, where f: focal length of a single lens d: core thickness of a single lens n: refractive index of a single lens r_1: paraxial curvature on the light source side of a single lens Radius r_2: Paraxial radius of curvature on the optical disk side of a single lens K_1: When an aspheric surface is expressed by the following formula, the conic constant of the aspheric surface on the light source side ▲ There are mathematical formulas, chemical formulas, tables, etc. ▼ Where, X: Light of the aspheric surface Distance y from the tangential plane on the axis to the aspherical surface: Height from the optical axis C: Reciprocal of the paraxial radius of curvature of the aspherical surface A_i: Aspherical coefficient. (3) The objective lens for an optical disc according to claim 2, which satisfies the condition represented by 0<▲ mathematical formula, chemical formula, table, etc. in the formula representing the aspherical surface ▼<0.13; however, ▲ mathematical formula, chemical formula, etc. , tables, etc. ▼: This is the third-order aspherical coefficient of the aspherical surface on the optical disc side.