JPS61132915A - Object lens for optical pickup - Google Patents

Abstract

PURPOSE:To make an object lens light-weight and easy to form and compensate aberrations with a high precision by using a single plano-convex lens, whose surface facing a recording medium is plane and an incidence surface is a convex aspherical surface, as the object lens and satisfying specific conditions. CONSTITUTION:The single plano-convex lens 1 is used as the object lens, and a surface r2 facing a disc surface 2 is plane, and a surface r1 on which a parallel light is made incident is a convex aspherical surface. Conditional equations I, II, III are satisfied where (r) is the radius of the curvature of the aspherical surface and A2-A10 are coefficients of the extension of second - tenth orders and (a) is the radius of the aperture of the lens and (n) is the refractive index of the lens and (f) is the focal length and (d) is the thickness of the lens. Thus, the object lens is constituted with one plano-convex lens and is made light- weight and easy to form, and the spherical aberration is compensated with a high precision.

Description

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

[Technical background and conventional technology]

Optical pickups used in compact disc players and the like are equipped with an objective lens that focuses a laser beam onto the recording surface of the disc. Fig. 1 schematically shows a player using this type of optical pickup. Reference numeral 2 in Fig. 1 is a disk, the center of which is supported on a spindle ll and driven to rotate. There is. In addition, there is an optical big 77 on the underside of the disc 2.
A tap 12 is provided. This optical pickup 12
is driven along the radial direction of the disk 2. This optical pickup 12 is provided with a lens barrel 10, and an objective lens is housed within this lens barrel 10. A laser beam emitted within the optical peak-up lz is focused on the recording surface of the disk 2 by this objective lens, and the pits on the recording surface are read and the recording is reproduced.

[Problems with conventional technology]

By the way, in the above optical pickup, the disc 2
It is necessary to reproduce very small signals recorded at a high density on the recording surface of a recording device, and a resolution of at least 1 μm is required. Therefore, it is necessary to focus a minute beam spot on the recording surface of the disk 2 with high precision.In addition, in order to accurately reproduce information, it is necessary to appropriately correct the spherical aberration of the objective lens and to Various conditions must be satisfied, such as the distance between the lens and the disk (operating pressure l1l) must be sufficiently long.

However, in order to construct an objective lens that satisfies such conditions, a highly accurate lens system is required. Conventionally,
To construct such a highly accurate lens system, it was necessary to combine multiple polished lenses. When a plurality of lenses are used, the weight of the lens barrel lO increases by the number of lenses, and the load when operating the lens barrel lO for correction increases. Furthermore, when using a plurality of lenses, there is a limit to the ability to obtain a sufficiently long working distance due to the focal length of the entire lens system.

Recently, it has also been considered to press mold an aspherical lens and use a single lens as an objective lens. However, in conventional aspherical lenses, aberrations are corrected by forming both surfaces into aspherical surfaces, so that the shapes of both surfaces of the lens are complicated. Furthermore, if both surfaces are aspherical, the axes of both surfaces must be aligned with high precision during press molding, which requires precise work for positioning the mold, etc.

C. Purpose of the Invention The present invention has been made by focusing on the above-mentioned conventional problems, and aims to apply it to mass production and reduce costs by using a lens with an easy-to-manufacture shape. It is an object of the present invention to provide an objective lens for an optical pickup that satisfies various conditions such as highly accurate correction and a sufficiently large working distance.

C. Configuration of the present invention] The objective lens for an optical pickup according to the present invention has a second
As shown in the figure, it consists of a single plano-convex lens 1 press-molded from a glass material, and this plano-convex lens l has a flat surface r2 facing the disk 2 and a convex non-convex surface rl on which parallel light enters. It is spherical.

In addition, as shown in FIG. 3, the above-mentioned surface rl on which the parallel light is incident is the meridian of a rectangular coordinate system in which the optical axis is the X axis, the radial direction of the lens is the y axis, and the apex 0 of the aspherical surface is the origin. In the plane, x= r (L −1−y2 /r2 ) ) +A2
y2+A4 Y' +Al, y6 +A978+Al
07IO・・・・・・(a) (However, r is the radius of curvature of the inscribed method at the apex of the aspheric surface, A2
, A4 , As , Ae , A@ are quadratic and 4, respectively.
These are the expansion coefficients of the 1st, 6th, 8th, and 10th orders. ) It has an axisymmetric general spherical shape expressed by the equation (2), and is formed so as to satisfy the following conditions (1) and (2).

■ 2.00<r/ (aX (n-1)) <2.
24 (However, a is the radius of the lens aperture, and n is the refractive index of the lens medium at the oil length of the incident light.) ■(n f-d) /n>2.5 (However, f is the focal length, d is the center thickness of the lens.) The above equation (a) is an equation in which the first term is a spherical surface. Further, the second term and the following are correction terms for correcting spherical aberration. This correction term is an expansion term, and is used up to the 10th order term.

Note that the correction term may be set to a necessary order depending on the lens characteristics.

Also, for ■ of the above conditions, the NA (numerical aperture) is 0.45.
These are the conditions for setting the value to 0.5. NA is 0.45~0
．． 5, it becomes possible to form a beam spot on the recording surface of the disk 2 under optimal conditions. Furthermore, if the values of a and r are selected so as to satisfy the condition (2), higher-order aberrations can be effectively corrected.

The above condition (2) is a condition for setting the WD (working distance) to 1.8 ms or more. If the WD is 1.8-1 or more, the lens will not hit the disk 2 during deformation of the disk 2 or correction operation of the objective lens, and the performance of the optical pickup can be improved.

[Example of the present invention]

Next, an example of the objective lens for an optical pickup described above will be shown (see FIG. 2 for each reference numeral).

(Example 1) rl is an aspherical surface in which r and A1 to AIo are the following values in formula (a).

rl3.75 A 2 = -0,497200X 1O-3A a
= -0,150430X 1(12A s = -0
, 108650X to-3A @ = + 0.37
0000X 10"A, = -0,702500X
10''rl is a plane.

Other figures are as follows.

dl! t, 5ool-W D = 2.470
■■L depth 1.200■■ nl = 1.88788 n3 = 1.55f =
L792 N A = 0.4117 (Example 2
) ro is an aspherical surface in which r and A1 to AIo are the following values in formula (a).

rl3.75 A 2= -0,23i3000X 1O-3A 4
= -0,148800X 1O-2As = -0,1
11100XlG'3A e = + 0.15000
0X 1G-7A 1o= -0, fi85000X
10'r2 is a plane.

Other figures are as follows.

d l -1,800■s W D = 2.
47Ls layer t = 1.200 ■■ n 1 = 1.89788 n3 = 1.55f
= 3.797 N A = 0.488 However, dl is the thickness of the plano-convex lens on the optical axis, nl, n3 are the refractive index for the input of the plano-convex lens l and disk 2 = 788.2 nm, f is the focal length, and WD is the The working distance, t, is the thickness of the optical disc.

If the lenses shown in each of the above embodiments are used, the aperture ratio NA
are 0.487 and 0.488, which are optimal values for an objective lens for an optical pickup.

Also, the working distance WD is 2.470■■ and 2,471w
m, which can be taken sufficiently large.

FIG. 4 shows the lateral aberration curve of the objective lens of (Example 1) among the above-mentioned examples. As can be seen from this diagram, the lateral aberration is extremely small, resulting in a lens with optical characteristics at the 1-diffraction limit.

[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 a single plano-convex lens, the weight of the lens barrel portion of the optical pickup is reduced. Therefore, it is possible to increase the drive sensitivity during focus servo, which always focuses the laser beam irradiated from the objective lens on the recording surface of the disk, and tracking servo, which causes the beam spot to follow the track on the recording surface. Furthermore, the sub-resonant frequency can be increased and the closed loop gain can be increased, so the servo becomes stable. In addition, since the objective lens is made up of one lens, compared to the case where multiple lenses are used,
Loss of light amount due to internal reflection or absorption of light within the lens is reduced.

(2) Since one surface of a plano-convex lens is flat and only one surface is aspherical, there is no need to consider the axis misalignment of the two surfaces. Therefore, when press molding a plano-convex lens, there is no need to align the two surfaces, and only the inclination of the surfaces needs to be taken into account, and the lens can be molded easily.

Therefore, mass production is possible and costs can be reduced.

Also, since one side is flat, there is the advantage that the lens can be easily positioned using this flat surface as a reference. Therefore, the task of assembling the entire objective lens is simple and the assembly precision is also high. Furthermore, it becomes easier to manufacture the lens barrel on the side that receives the lens.

(3) By configuring an aspherical surface that satisfies formula (a) and conditions (1) and (2) described in claim 2, spherical aberration can be minimized and a lens with optical characteristics of the 1-diffraction limit can be obtained. Obtainable. Furthermore, since the working distance is sufficiently large and the numerical aperture is an appropriate value, it is optimal as an objective lens for an optical pickup.

[Brief explanation of the drawings] Figure 1 is an explanatory diagram showing an overview of the optical pickup;
3 and 3 are cross-sectional views of an objective lens for an optical pickup according to an embodiment of the present invention, with the main axis oriented horizontally, and FIG. 4 is a sectional view of an objective lens for an optical pickup according to an embodiment of the present invention. It is a lateral aberration curve diagram. 1... Plano-convex lens, 2... Disc (recording medium),
10... lens barrel, 12... optical pickup, d,
...Thickness on the main axis of the lens, t...Thickness of the disk, nl+n3...Refractive index of the lens and disk relative to the wavelength of incident light, WD...Working distance.

Claims (3)

[Claims] (1) Consists of a single plano-convex lens; this plano-convex lens is
An objective lens for an optical pickup, in which the surface facing the recording medium is a flat surface, and the surface on which parallel light enters is a convex aspherical surface. (2) The surface on which parallel light enters is the meridian plane of a rectangular coordinate system in which the optical axis is the x-axis, the radial direction of the lens is the y-axis, and the apex of the aspherical surface is the origin, x=r{1-√[ 1-(y^2/r^2)]}+A_2
y^2+A_4y^4+A_6y^6+A_8y^8+
A_1_0y^1^0 (where r is the radius of curvature of the inscribed sphere at the apex of the aspheric surface, A_2, A_4, A_6, A_8, A
_1_0 are expansion coefficients of 2nd, 4th, 6th, 8th, and 10th orders, respectively. ) The objective for an optical pickup according to claim 1, which has an axisymmetric general aspherical shape expressed by the formula and satisfies the following conditions [1] and [2]. lens. [1] 2.00<r/{ax(n-1)}<2.24(
However, a is the radius of the lens aperture, and n is the refractive index of the lens medium at the wavelength of the incident light. ) [2] (nf-d)/n>2.5 (where f is the focal length and d is the center thickness of the lens.) (3) The objective lens for an optical pickup according to claim 2, wherein the plano-convex lens is a press-molded lens.