JP2003005060A - Objective lens for optical pickup - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high-performance and small objective lens for an optical pickup securing a working distance while having large numerical apertures. SOLUTION: The objective lens or the optical pickup that carries out read/ write of information by condensing luminous flux from a light source on an information recording medium comprises two lenses consisting of a first lens having a first surface convex on the light source side and a second surface concave on the light source side, and a second lens having a third surface convex on the light source side and a fourth surface flat on the light source side, in order from the light source side. Each of the second and third surfaces has an aspherical surface.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an objective lens for an optical pickup, for example, a numerical aperture (NA: NA) used as an objective lens in an optical pickup mounted on an optical information recording device, a magneto-optical recording device, or the like. numerical ape
The objective lens for an optical pickup having a large rture).

[0002]

2. Description of the Related Art In a conventionally known optical information recording apparatus, an objective lens for reading and writing information which is shifted with respect to an information recording medium (optical disc or the like) is used as an objective lens for an optical pickup. ing. There are various types of objective lenses for optical pickups. For example, as an objective lens for an optical pickup for a blue laser, an objective lens having a two-element structure having a positive refractive power is disclosed.
No. 00-75107, Japanese Patent Laid-Open No. 2000-206404.
Japanese Patent Laid-Open No. 2000-180717 and the like.

[0003]

[Patent Document 1] Japanese Patent Application Laid-Open No. 2000-751
In the objective lens for an optical pickup proposed in JP-A-2007-206404 and JP-A-2000-206404, the numerical aperture is increased by using a second lens (lens on the information recording medium side) having a substantially hemispherical / plano-convex shape. ing. But first
Since the lens (lens on the light source side) has a biconvex shape,
It is difficult to secure the back focus of the objective lens for the optical pickup, and therefore the thickness of the disc substrate cannot be increased. JP-A-2000-180717
In the objective lens for an optical pickup proposed in the publication, the shape of the second lens is a meniscus shape convex toward the light source side. Therefore, it is difficult to secure the space between the lens peripheral portion and the disk substrate even if the back focus is increased, and thus the working distance (working distance) is increased.
distance) is not desirable.

The present invention has been made in view of the above circumstances, and an object thereof is to provide a high-performance and compact objective lens for an optical pickup while ensuring a working distance with a large numerical aperture. .

[0005]

In order to achieve the above object, the objective lens for an optical pickup according to the first aspect of the present invention collects a light beam from a light source on an information recording medium so as to detect the information. An objective lens for an optical pickup that is at least readable and writable, in order from the light source side, a first lens having a convex first surface on the light source side and a concave second surface on the light source side, and a third convex surface on the light source side. It is characterized in that it is composed of two lenses, a surface and a second lens composed of a flat fourth surface, and has aspherical surfaces on the second and third surfaces.

The objective lens for an optical pickup of the second invention is characterized in that, in the constitution of the first invention, the aspherical surface satisfies the following conditional expressions (1) and (2).

Second surface 0.15 ≦ α2max-α2min ≦ 1.0 (1) Third surface 0.15 ≦ α3max-α3min ≦ 1.0 (2) where α (h) ≡dz (h) / dh-h / (r * SQRT (1- (h / r) ^ 2)) h: incident height from the optical axis of the axial ray incident on the aspherical surface, hmax: axial marginal incident on the aspherical surface Incident height of the ray from the optical axis, Z (h): Aspherical shape (distance in the direction along the optical axis from the apex of the aspherical surface at each height, Z (h) = r- (r ^ 2 -ε ・ h ^ 2) ^ 1/2 + (A4 ・ H ^ 4 + A6 ・ H ^ 6 + A8 ・ H ^ 8 +…) r: paraxial radius of curvature of aspherical surface, ε: elliptic coefficient, Ai: I-th degree aspherical coefficient of aspherical surface, dZ (h) / dh: differential value with respect to incident height of aspherical shape, SQRT: 0.1 from 0 to hmax of x-th surface having aspherical surface as square root The maximum value of α when carved in pitch is αxm
ax, the minimum value is αxmini.

An objective lens for an optical pickup according to a third invention is characterized in that the first lens satisfies the following conditional expression (3). 0.8 ≦ R1 / f ≦ 25.0 (3) where R1 is the radius of curvature of the light source side surface of the first lens, and f is the focal length of the objective lens.

An objective lens for an optical pickup according to a fourth invention is characterized in that the second lens satisfies the following conditional expression (4).

0.25≤R3 / f≤25.0 (4) However, R3: radius of curvature of the light source side surface of the second lens, f: objective lens focal length, Is.

[0011]

DETAILED DESCRIPTION OF THE INVENTION An objective lens for an optical pickup according to the present invention will be described below with reference to the drawings. 1 to 4 are lens configuration diagrams respectively corresponding to the optical pickup objective lenses of the first to fourth embodiments,
In each lens configuration diagram, the surface marked with si (i = 1,2, ...) is the i-th surface counted from the light source side, and the surface marked with * is an aspherical surface. . These objective lenses for optical pickups are objective lenses for optical pickups in which reading and writing of information is performed by converging light flux from a light source on an information recording medium. Side, in order from the light source side, a first lens having a convex first surface on the light source side, a concave second surface on the light source side, and a third convex surface on the light source side.
The parallel plate (PL) located on the image side of each optical pickup objective lens corresponds to a disk substrate.
1 to 4 show the optical paths of incident parallel light,
The objective lens for each optical pickup can be used not only as an infinite system but also as a finite system.

To achieve high density of optical discs,
It is effective to increase the NA of the objective lens for the optical pickup and shift the design wavelength to the blue side (that is, the short wavelength side). When increasing the NA of an objective lens for an optical pickup, if the focal length range is the same as that of a conventional objective lens for an optical pickup for a CD (compact disc), it becomes difficult to achieve a compact system due to an increase in the lens diameter. It becomes difficult to reduce the weight of the optical head due to the increase in the lens weight. In order to reduce the lens diameter in the shorter focal length range than the conventional one, a working distance similar to the conventional one is required. If the disk substrate used is made thin, it is possible to gain a margin for the tilt of the optical disk. However, if the disc substrate is too thin, problems occur in substrate strength and productivity. Therefore, it can be said that it is desirable to secure the thickness of the optical disk to some extent. Also,
When the laser wavelength is shifted to the blue side, the wavefront error must be considered with a short wavelength, so that the optical performance required of the objective lens for the optical pickup becomes higher than that of the conventional one.

Therefore, in each of the embodiments, a high NA is ensured by having aspherical surfaces on the second surface and the third surface. Therefore, the two lenses (G
1, G2) If used, it has high performance and high NA
To realize a compact optical pickup objective lens having a small diameter while ensuring a back focus and a working distance in a focal length range shorter than a focal length range of an optical pickup objective lens for D or DVD (digital video disc). Is possible.

Further, in order to improve the performance, it is more preferable that each aspherical surface satisfies the following conditional expressions (1) and (2) as in each embodiment. Conditional expressions (1) and (2) specify conditions for achieving high performance in a small objective lens for an optical pickup having a high NA. If the ranges of the conditional expressions (1) and (2) are exceeded, high-order aberrations due to the aspherical surface occur and it becomes difficult to correct the aberrations, which is not desirable for achieving high performance. On the other hand, when the value goes below the lower limit, the effect of correcting the aberration due to the aspherical surface becomes small, and it becomes difficult to correct the spherical aberration in particular, which makes it difficult to achieve high performance, which is not desirable.

Second surface 0.15 ≦ α2max-α2min ≦ 1.0 (1) Third surface 0.15 ≦ α3max-α3min ≦ 1.0 (2) where α (h) ≡dz (h) / dh-h / (r * SQRT (1- (h / r) ^ 2)) h: incident height from the optical axis of the axial ray incident on the aspherical surface, hmax: axial marginal incident on the aspherical surface Incident height of the ray from the optical axis, Z (h): Aspherical shape (distance in the direction along the optical axis from the apex of the aspherical surface at each height, Z (h) = r- (r ^ 2 -ε ・ h ^ 2) ^ 1/2 + (A4 ・ H ^ 4 + A6 ・ H ^ 6 + A8 ・ H ^ 8 +…) r: paraxial radius of curvature of aspherical surface, ε: elliptic coefficient, Ai: The i-th order aspherical coefficient of the aspherical surface, dZ (h) / dh: differential value with respect to the incident height of the aspherical surface, SQRT: square root, and the ratio from 0 to hmax of the x-th surface having the aspherical surface as 0. The maximum value of α when carved at 1 pitch is αxm
ax, the minimum value is αxmini.

It is desirable that the first lens (G1) satisfy the following conditional expression (3). Conditional expression (3) defines a desirable condition for securing a back focus while maintaining a high NA and maintaining compactness. If the lower limit of conditional expression (3) is exceeded, it will be difficult to secure the back focus, and conversely, if the upper limit of conditional expression (3) is exceeded, the total lens length will increase and the high NA will increase.
When the size of the lens is increased, the diameter of the lens also becomes large, which makes it difficult to make the lens compact.

0.8 ≦ R1 / f ≦ 25.0 (3) However, R1: radius of curvature of the light source side surface of the first lens, f: focal length of the objective lens, Is.

It is desirable that the second lens (G2) satisfy the following conditional expression (4). Conditional expression (4) defines a desirable condition for the shape of the second lens (G2). If the lower limit of conditional expression (4) is exceeded, the radius of curvature of the lens becomes too small, which makes it difficult to secure the lens edge, which is not desirable in manufacturing and lens holding becomes difficult. Conversely, conditional expression (4)
If the upper limit of is exceeded, the radius of curvature of the lens becomes large, and the aberration generated on this surface becomes large, which is not desirable for higher NA.

0.25≤R3 / f≤25.0 (4) However, R3: radius of curvature of the light source side surface of the second lens, f: objective lens focal length, Is.

It is desirable that the first surface and the fourth surface do not have an aspherical surface. If the first surface or the fourth surface has an aspherical surface, one lens becomes an aspherical surface on both sides, and decentering between the aspherical surfaces becomes severe.

[0021]

EXAMPLES Hereinafter, the objective lens for an optical pickup according to the present invention will be described more specifically with reference to construction data and the like. In addition, Examples 1 to 4 shown in Tables 1 to 8 listed below correspond to the above-described first to fourth embodiments, respectively, and are lenses that represent the first to fourth embodiments. The configuration diagrams (FIGS. 1 to 4) are corresponding to Embodiments 1 to 1.
4 shows the respective lens configurations.

In the construction data of each embodiment, si (i = 1,2, ...) is the i-th surface counted from the light source side, ri
(i = 1,2, ...) is the radius of curvature (mm) of the surface si, and di (i = 1,2, ...) is the i-th axial upper surface distance (core thickness, mm) from the light source side. ), And Ni (i = 1,2,3) and νi (i = 1,2,3) are i counted from the light source side.
The refractive index (Nd) and the Abbe's number (νd) of the third optical element with respect to each light of wavelength 405 nm and d-line are shown. The surface si marked with * indicates that the surface is an aspherical surface, and is defined by the above formula (Z (H) = ...) That represents the surface shape of the aspherical surface. The wavelength (λ) of the light beam used, the numerical aperture (NA), and the aspherical surface data of each aspherical surface are shown together with other data.
Tables 9 to 12 show the values corresponding to the conditional expressions of the examples. 5 to 8
FIG. 4A is an aberration diagram corresponding to Example 1 to Example 4, respectively, and (A) shows a wavefront aberration (TANGENT
IAL) and (B) show the wavefront aberration (SAGITTAL) in the sagittal light flux (λ = 405.0 nm). Since the wavefront aberration is important in the objective lens for the optical pickup, each aberration diagram shows the wavefront aberration at the image height = 0.

[0023]

[Table 1]

[0024]

[Table 2]

[0025]

[Table 3]

[0026]

[Table 4]

[0027]

[Table 5]

[0028]

[Table 6]

[0029]

[Table 7]

[0030]

[Table 8]

[0031]

[Table 9]

[0032]

[Table 10]

[0033]

[Table 11]

[0034]

[Table 12]

[0035]

As described above, according to the present invention, it is possible to realize a high-performance and small-sized objective lens for an optical pickup while securing a working distance with a large numerical aperture. When the objective lens for an optical pickup according to the present invention is used in an optical pickup device (optical information recording device, magneto-optical recording device, etc.), it is possible to contribute to increasing the density of optical discs and the like.

[Brief description of drawings]

FIG. 1 is a lens configuration diagram of a first embodiment (Example 1).

FIG. 2 is a lens configuration diagram of a second embodiment (Example 2).

FIG. 3 is a lens configuration diagram of a third embodiment (Example 3).

FIG. 4 is a lens configuration diagram of a fourth embodiment (Example 4).

FIG. 5 is an aberration diagram of Example 1.

FIG. 6 is an aberration diagram of Example 2.

FIG. 7 is an aberration diagram of Example 3.

FIG. 8 is an aberration diagram of Example 4.

[Explanation of symbols]

G1 ... 1st lens G2 ... Second lens PL: Parallel plate (disk substrate of information recording medium)

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Hideki Nagata             2-3-3 Azuchi-cho, Chuo-ku, Osaka-shi, Osaka Prefecture               Osaka International Building Minolta Co., Ltd. F term (reference) 2H087 KA13 LA01 PA02 PA17 PB02                       QA02 QA05 QA14 QA21 QA33                       QA41 RA05 RA12 RA13 RA42                 5D119 AA01 AA11 AA22 BA01 DA01                       DA05 JA44 JB01 JB02 JB06

Claims (4)

[Claims] 1. An objective lens for an optical pickup that at least reads and writes information, which is shifted by condensing a light beam from a light source on an information recording medium, the convex lens having a convex surface on the light source side in order from the light source side. The first surface is a first lens having a concave second surface on the light source side, and the second lens is a second lens having a convex third surface and a flat fourth surface on the light source side. And an objective lens for an optical pickup, which has an aspherical surface on the third surface. 2. The objective lens for an optical pickup according to claim 1, wherein the aspherical surface satisfies the following conditional expressions (1) and (2): Second surface 0.15 ≦ α2max-α2min ≦ 1.0… (1) Third surface 0.15 ≦ α3max-α3min ≦ 1.0… (2) where α (h) ≡dz (h) / dh-h / (r * SQRT (1- (h / r) ^ 2)) h: incident height from the optical axis of the axial ray incident on the aspherical surface, hmax: incident height from the optical axis of the axial marginal ray incident on the aspherical surface, Z (h) : Aspherical shape (distance in the direction along the optical axis from the apex of the aspherical surface at each height, Z (h) ＝ r- (r ^ 2-ε ・ h ^ 2) ^ 1/2 + (A4・ H ^ 4 + A6 ・ H ^ 6 + A8 ・ H ^ 8 + ...) r: paraxial radius of curvature of the aspherical surface, ε: elliptic coefficient, Ai: i-th order aspherical coefficient of the aspherical surface, dZ (h) / dh: differential value with respect to the incident height of the aspherical surface, SQRT: square root, and the maximum value of α when the 0th to hmax of the x-th surface having an aspherical surface is divided by 0.1 pitch as a ratio αxm
ax, the minimum value is αxmini. 3. The objective lens for an optical pickup according to claim 1, wherein the first lens satisfies the following conditional expression (3): 0.8 ≦ R1 / f ≦ 25.0 (3) Where R1 is the radius of curvature of the light source side surface of the first lens, and f is the focal length of the objective lens. 4. The objective lens for an optical pickup according to claim 1, wherein the second lens satisfies the following conditional expression (4): 0.25 ≦ R3 / f ≦ 25.0 (4) Where R3 is the radius of curvature of the light source side surface of the second lens, and f is the focal length of the objective lens.