JP2003005067A - 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 numerical apertures. SOLUTION: The objective lens for 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 concave 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 convex on the light source side, in this order from the light source side. The fourth surface 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 / or writing information from / to an information recording medium (optical disc or the like) is used as an objective lens for an optical pickup. There is. 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 invention condenses a light beam from a light source on an information recording medium to read and / or write information. An objective lens for an optical pickup that does this, in order from the light source side, a first lens having a concave first surface on the light source side, a concave second surface on the light source side, and a convex third surface on the light source side. It is characterized in that it is composed of two lenses, a second lens composed of a convex fourth surface on the light source side, and has an aspheric surface on the fourth surface.

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 expression (1).

Fourth surface 0.31 ≦ α4max-α4min ≦ 0.35 (1) 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 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 order aspherical coefficient of aspherical surface, dZ (h) / dh: non The derivative of the spherical shape with respect to the incident height, SQRT: square root, is the maximum value of α when the x-th surface having an aspherical surface is carved at 0.1 pitch as a ratio from 0 to hmax
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 (2).

-2.3≤R2 / f≤-1.6 ... (2) However, R2: radius of curvature of the side of the first lens opposite the light source, f: focal length of the objective lens, Is.

An objective lens for an optical pickup according to a fourth invention is characterized in that the second lens satisfies the following conditional expression (3).

0.55≤R3 / f≤0.60 (3) However, R3: radius of curvature of the light source side surface of the second lens, f: objective lens focal length, Is.

[0012]

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 3 are lens configuration diagrams respectively corresponding to the optical pickup objective lenses of the first to third 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 optical pickup objective lenses are optical pickup objective lenses that perform at least one of reading and writing of information by converging a light flux from a light source on an information recording medium. In order from the light source side, a first lens having a concave 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.
Surface and a second lens having a convex fourth surface on the light source side and two positive lenses, and the parallel plate (PL) located on the image side of each optical pickup objective lens corresponds to the disk substrate. . Although FIG. 1 to FIG. 3 show the optical paths of parallel light incident, each objective lens for optical pickup can be used not only as an infinite system but also as a finite system.

In order to achieve high density of optical disks,
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 embodiment, a high NA is secured by having an aspherical surface on the fourth surface. Therefore,
If two lenses (G1, G2) with such a characteristic shape are used, conventional CDs and DVs can be achieved with high performance and high NA.
It is possible to realize a compact optical pickup objective lens with a small diameter while ensuring the back focus and working distance in a focal length range shorter than the focal length range of an optical pickup objective lens for D (digital video disc). Becomes

Further, in order to improve the performance, it is more preferable that the aspherical surface satisfies the following conditional expression (1) as in each embodiment. Conditional expression (1) defines a condition for achieving high performance in a small objective lens for an optical pickup having a high NA. If the range of the conditional expression (1) is 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.

Fourth surface 0.31 ≦ α4max-α4min ≦ 0.35 (1) 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 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 order aspherical coefficient of aspherical surface, dZ (h) / dh: non SQRT: Square root, which is the derivative of the spherical shape with respect to the incident height, and the maximum value of α when the 0th to hmax of the x-th surface that has an aspherical surface is divided by 0.1 pitch and the maximum value is αxm
ax, the minimum value is αxmini.

It is desirable that the first lens (G1) satisfy the following conditional expression (2). Conditional expression (2) defines a desirable condition for securing the back focus while maintaining high NA and maintaining compactness. If the lower limit of conditional expression (2) is exceeded, the total lens length will increase and the lens diameter will also increase when aiming for a higher NA, making it difficult to make the lens compact. Conversely, the upper limit of conditional expression (2) If it exceeds, it becomes difficult to secure the back focus.

-2.3≤R2 / f≤-1.6 (2) However, R2: radius of curvature of the side of the first lens opposite the light source, f: focal length of the objective lens, Is.

It is desirable that the second lens (G2) satisfy the following conditional expression (3). Conditional expression (3) defines a desirable condition for the shape of the second lens (G2). If the lower limit of conditional expression (3) is exceeded, the radius of curvature of the lens becomes too small, making it difficult to secure the lens edge, which is not desirable in manufacturing, and lens holding becomes difficult. Conversely, conditional expression (3)
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.

0.55 ≦ R3 / f ≦ 0.60 (3) 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 has no aspherical surface. If the first surface has an aspherical surface, desired aberration cannot be removed.

It is desirable that the second surface has no aspherical surface. If the second surface has an aspherical surface, the mold is concave because it is a convex surface, so the locus of the grindstone becomes shorter than the size of the aspherical surface.
Precision is poor because it is difficult to perform delicate control during manufacturing.

It is desirable that the third surface has no aspherical surface. If the third surface has an aspherical surface, one lens has a plurality of aspherical surfaces, and decentering between the aspherical surfaces becomes severe.
Further, if the surface is convex, the mold becomes concave, the locus of the grindstone becomes shorter than the size of the aspherical surface, and it becomes difficult to perform delicate control during manufacturing, resulting in poor accuracy.

[0024]

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. It should be noted that Examples 1 to 3 shown in Tables 1 to 6 listed below correspond to the above-described first to third embodiments, respectively, and are lens configurations representing the first to third embodiments. The drawings (FIGS. 1 to 3) show corresponding Embodiments 1 to 1.
3 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 7 to 9 show the values corresponding to the conditional expressions of the examples. 4 to 6 are aberration diagrams corresponding to Examples 1 to 3, respectively.
(A) is the wavefront aberration (TANGENTIA
L) 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, the image height =
The wavefront aberration at 0 is shown.

[0026]

[Table 1]

[0027]

[Table 2]

[0028]

[Table 3]

[0029]

[Table 4]

[0030]

[Table 5]

[0031]

[Table 6]

[0032]

[Table 7]

[0033]

[Table 8]

[0034]

[Table 9]

[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 an aberration diagram of Example 1.

FIG. 5 is an aberration diagram of Example 2.

FIG. 6 is an aberration diagram of Example 3.

[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                       QA03 QA06 QA12 QA21 QA32                       QA41 RA05 RA12 RA42                 5D119 AA01 AA11 AA22 BA01 DA01                       DA05 JA44 JB01 JB02 JB06

Claims (4)

[Claims] 1. An objective lens for an optical pickup for reading and / or writing information by condensing a light beam from a light source on an information recording medium, the concave lens having a concave surface on the light source side in order from the light source side. The first lens has one surface, the second lens has a concave second surface on the light source side, and the second lens has a third surface having a convex surface on the light source side and a second lens having a convex fourth surface on the light source side. An objective lens for an optical pickup, which has aspherical surfaces on four surfaces. 2. The objective lens for an optical pickup according to claim 1, wherein the aspherical surface satisfies the following conditional expression (1): Fourth surface 0.31 ≦ α4max-α4min ≦ 0.35 ... (1) However, α (h) ≡dz (h) / dh-h / (r * SQRT (1- (h / r) ^ 2)) h: From the optical axis of the axial ray incident on the aspherical surface Incident height, hmax: Incident height from the optical axis of the axial marginal ray incident on the aspherical surface, Z (h): Aspherical shape (direction from the apex of the aspherical surface at each height along the optical axis Distance Z (h) ＝ r- (r ^ 2-ε ・ h ^ 2) ^ 1/2 + (A4 ・ H ^ 4 + A6 ・ H ^ 6 + A8 ・ H ^ 8 +…) r: Aspherical surface Paraxial radius of curvature of ε, ε: elliptic coefficient, Ai: aspherical coefficient of i-th degree of aspherical surface, dZ (h) / dh: differential value with respect to incident height of aspherical shape, SQRT: square root, with an aspherical surface as The maximum value of α when engraved at 0.1 pitch with 0 to hmax of the x-th surface as a ratio is 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 (2); −2.3 ≦ R2 / f ≦ −1.6 ... ( 2) where R1 is the radius of curvature of the side of the first lens opposite to the light source, 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 (3): 0.55 ≦ R3 / f ≦ 0.60 (3) 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.