JPS60247213A - Objective lens for optical pickup - Google Patents

Abstract

PURPOSE:To minimize the number of lens elements and to reduce the size and weight over the entire part of the system by the constitution in which the 1st lens having an aspherical face on the 2nd lens side and a convex spherical face on the side opposite therefrom and the 2nd lens of a positive meniscus lens having a convex shape on the 1st lens side are coaxially disposed. CONSTITUTION:The objective lens for an optical pickup to be used for a compact disk player, etc. is reduced in size and weight by disposing coaxially the 1st lens 1 and the 2nd lens 2. The 1st lens has the aspherical face 1a on the 2nd lens side and the convex spherical face on the side opposite therefrom. The 2nd lens 2 is the positive meniscus lens having the convex shape on the 1st lens side. The objective lens is constituted only of two elements of the lenses 1, 2 i the above-mentioned manner and therefore the reduction in the size over the entire part is possible. An aberration is thoroughly corrected even in the case of forming both of the lens 1 and the lens 2 by press molding of a plastic material. The use of such lens as the objective lens having high accuracy is possible.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field] The present invention relates to an optical pickup objective lens used in a compact disc player or the like to focus a laser beam or the like onto a recording surface of a disc.

[Technical background and conventional technology]

An optical pickup used in a compact disc player or the like is equipped with an objective lens that focuses a laser beam or the like onto the recording surface of the disc. FIG. 3 schematically shows a player using this type of optical pickup. Reference numeral 3 in the figure is a disk, the center of which is supported on a spindle 11 and driven to rotate. Further, an optical pickup 12 is provided below the disk 3. This optical pickup 12 is the disk 3
It is driven along the radial direction. This optical pickup 12 is provided with a lens barrel lO, and an objective lens is housed within this lens barrel lO. A laser beam emitted within the optical pickup 12 is focused onto the recording surface of the disk 3 by this objective lens, and the pits on the recording surface are read and the recording is reproduced.

[Problems with the Prior Art] Incidentally, in the optical pickup described above, it is necessary to reproduce a very small signal recorded at high density on the recording surface of the disk 3, and a resolution of at least 1 gm is required. Therefore, it is necessary to focus a minute beam spot on the recording surface of the disk 3 with high precision. Also,
In order to accurately reproduce information, it is necessary to provide appropriate correction for the spherical aberration of the objective lens, and also to make the distance between the lens closest to the disk and the disk (working distance) sufficiently long. The following conditions must be met.

However, in order to construct an objective lens that satisfies such conditions, a highly accurate lens system is required. Conventional objective lenses mainly used polished convex lenses and meniscus lenses, which required a large number of lenses, and each lens required a high degree of polishing, making processing complicated. This made the system as a whole expensive.

[Object of the present invention]

The present invention has been made by focusing on the above-mentioned conventional problems, and aims to reduce the size and weight of the entire system by minimizing the number of lenses used.In addition, it can be pressed using plastic materials, simplifying the molding process. The purpose of the present invention is to provide an objective lens for optical pickup that can perform

[Configuration of the present invention]

As shown in FIG. 1, the objective lens for optical pickup according to the present invention has a first lens (1) and a second lens arranged coaxially.
The second lens (1) has an aspherical surface (1a) on the second lens side and a convex spherical surface on the opposite side, and the second lens (2) has an aspherical surface (1a) on the second lens side. It is characterized by being a positive meniscus lens with a convex shape. Since the objective lens is constituted by only the two lenses (1) and (2) in this way, the entire system can be downsized. In addition, the first lens (1) and the second lens (2)
Even if both are press-molded from plastic materials, aberrations are sufficiently corrected and it can be used as a high-precision objective lens.

In particular, if the first lens and the second lens satisfy the following conditions (1) and (2), they are optimal as objective lenses.

■The aspherical surface of the lunes, in the meridian plane of the x, y, z orthogonal coordinate system with the optical axis as the X axis and the apex of the aspherical surface as the origin, x = (Cy/ (1+ 1-KCy)) y'
・・・・・・・・・・・・(A) However, K: Eccentricity C: Curvature of the inscribed sphere at the vertex A4: Forms an axisymmetric general aspherical shape expressed by the fourth-order expansion coefficient. However, when the radius of curvature of the inscribed sphere is r2, this r2 satisfies the following.

-5,6≦((n2-1)/n2)(r2/f)≦-5
,5・・・・・・・・・(b) However, n: refractive index of the medium of the first lens at the wavelength of the incident light f: focal length of the entire system The focal length f2 satisfies the following.

0.8<f/f2<0.7...
(c) In E, by selecting the radius of curvature r2 of the inscribed sphere that conforms to equation (b),
Low-order aberrations in the paraxial region are corrected. By satisfying this condition, off-axis aberrations are corrected, and even if there is a tilt of the incident light beam with respect to the objective lens, the aberrations can be minimized. Also, when r2 is adapted to (b),
This means that the aspherical surface (la) expressed by equation (A) can be formed with a small asphericity.

Further, equation (c) is a condition for obtaining the optimum working distance WD.

C. Embodiments of the Present Invention] Next, embodiments of the optical pickup objective lens described above will be shown (see FIG. 1 for each reference numeral).

r l=1.141 nt =1.486d, =0.
357 d2 =0.285r3 = -0,530
n2=1.486r4=-1,711d3=0.
214 f=1, N A = 0.45, WD = 0.485
t = 0.17, f2 = 1.41112 Further, the coefficients that determine the shape of the aspherical surface of the second lens are as follows.

u = 1, r2 = -10, 1f (5A 4 =
0.3825
refractive index for 768.5 nm, n2 is the refractive index for second lens entrance = 788° 5 nm, f is the focal length of the entire system, NA is the aperture ratio, WD is the working distance, t is the thickness of the optical disk, f2 is the This is the focal length of the two lenses.

If the lens shown in Example L is used, the aperture ratio NA will be 0.45, which is an optimal value for an objective lens for optical pickup. Further, the working distance WD can also be made sufficiently large.

Furthermore, when the objective lens according to the above embodiment is constructed (f
= 7.01174+nm) are shown in the table below.

Table 1 (Aberrations in Examples) Table 2 (Transverse aberrations when the incident light beam is tilted by -0.9") The refraction state of the light beam incident on this optical pickup objective lens is shown in Fig. 2. This figure As shown in , it can be seen that the light is uniformly refracted by the first lens (1) and the second lens (2) and then enters the disc.

[Effects of the present invention]

As described above, according to the present invention, the following effects can be achieved.

(1) Since the objective lens is composed of only two lenses: the first lens, which is an aspherical lens, and the second lens, which is a meniscus lens, the lens holder can be made smaller.

Furthermore, since the number of objective lenses is small, the weight of the entire system is also light, so correction operations in the focusing and tracking directions can be easily performed, and the driving force for this correction can be small.

(2) By configuring the first lens and the second lens based on the conditions (2) and (2) described in claim 2, it becomes possible to obtain the optimum correction of spherical aberration and the working distance, and the optical It will be able to fully demonstrate its function as a pickup objective lens.

In particular, the lens that satisfies the condition (2) has a small aspherical surface and has good characteristics, making lens processing easier. Therefore, as described in claim 3, the lens can be easily pressed using a plastic material, and the manufacturing cost of the lens can be extremely reduced.

[Brief explanation of the drawing]

FIG. 1 shows an optical pickup objective lens according to the present invention.
FIG. 2 is an explanatory diagram showing the state of refraction of the light beam incident on the optical pickup objective lens of the present invention, and FIG. 3 is a schematic diagram of the optical pickup. . (1)...Luns, (!a)...Aspherical surface, (2)
...Second lens, (3)...Optical disc, 10...
- Objective lens, 12... optical pickup. 2

Claims (3)

[Claims] (1) Consisting of a second lens and a second lens arranged coaxially, the second lens is an aspherical lens with an aspherical surface on the second lens side, and the second lens has an aspherical surface on the second lens side. An objective lens for optical pickup that is a positive meniscus lens with a convex shape. (2) The objective lens for optical pickup according to claim 1, wherein the first lens and the second lens satisfy the following conditions (1) and (2). ■In the meridian plane of the x, y, z orthogonal rectangular system with the optical axis as the X axis and the apex of the aspheric surface as the origin, 10 A4 y
'However, K: Eccentricity C: Curvature of the inscribed sphere at the vertex A4: Forms an axisymmetric general aspherical shape expressed by a fourth-order expansion coefficient, and when the radius of curvature of the inscribed sphere is rz , this rz satisfies the following. =5.6≦((n2-1)/n2)(rz/f)≦-
5,5 where n: refractive index of the medium of the second lens at the wavelength of the incident light f: focal length of the entire system (1) f and the focal length f2 of the second lens satisfy the following. 0, El < f/ f2 < 0.7 (3) The objective lens for optical pickup according to claim 1 or 2, wherein both the first lens and the second lens are plastic lenses.