JPS6236623A - Objective lens for optica pickup - Google Patents

Abstract

PURPOSE:To easily attain press forming to obtain on objective lens corrected at its spherical aberation and since condition and optimized to a pickup by expressing sphere of an aspherical, convex and positive meniscus lens by a specific equation I and setting up a specific inequality II between the refractive index and center thickness of the lens. CONSTITUTION:The objective lens is constituted of a single positive meniscus lens 1 convexed on its aspherical side and its aspherical surface is expressed by the equation I when the optical axis is (x), the radium direction of the lens is the y axis, a development factor is An, and the vortex of the aspherical surface is the original point and the inequality II is set up between the refractive index (n) and the center thickness (d) of the lens. The spherical aberration is corrected by the 2nd term and after in the equation I, the higher-level aberration is corrected by (a) of the inequality II and the working distance is sufficiently increased and the center thickness (d) of the lens is increased up to a proper level on the basis of (2) in the inequality II to easily attain the press forming of the lens. Consequently, massproductivity and cost reduction can be attain and the objective lens proper to a pickup can be obtained by correcting its spherical aberration and since condition and increasing the working distance.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field] The present invention relates to an optical pickup used in compact disc players and the like, and in particular 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 11 and driven to rotate. There is. Further, an optical pickup 12 is disposed below the disk 2. 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 pickup 12 is focused on the recording surface of the disk 2 by this objective lens, and the beats on the recording surface are read to reproduce the recording.

[Problems with conventional technology]

By the way, with the optical pickup mentioned above.

It is necessary to reproduce a very small signal recorded at high density on the recording surface of the disk 2, and a resolution of at least IBm is required. Therefore, it is necessary to focus a minute beam spot on the recording surface of the disk 2 with a high-precision jet, and in order to reproduce information accurately, the spherical aberration of the objective lens must be appropriately corrected, and Various conditions must be satisfied, such as the distance between the objective lens and the disc (working distance) 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 using multiple lenses, attach the lens barrel 10 for the number of lenses.
The weight of the portion increases, and the load required to correct the mirror surface 10 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, with conventional aspheric lenses, it has been believed that aberrations cannot be sufficiently corrected unless both surfaces are formed into aspheric surfaces. A known example of this type of objective lens having aspherical surfaces on both sides is the one disclosed in Japanese Patent Application Laid-Open No. 57-201210. Such a lens having aspherical surfaces on both sides has the disadvantage that the shape of the mold becomes very complicated in order to form two aspherical surfaces. Furthermore, since the axes of both sides must be aligned with high precision, precise work is required for positioning the mold, etc.

[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. The object of the present invention is to provide an objective lens for an optical pickup that satisfies various conditions such as high accuracy and a sufficiently large working distance.

[Structure of the 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 positive meniscus lens l press-molded from a glass material, and the surface of this meniscus lens l that faces the disk 2 is a concave spherical surface with a radius of curvature R, and the surface on which parallel light enters. is a convex aspherical surface.

I did. As shown in Figure 3, the aspheric surface on the side where parallel light enters is 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 origin is roughly at the vertex 0 of the aspheric surface. In the meridian plane, x=r(1-yr)+A2y2+A4
y4 Ta6 V6+A8 V8+AloV1゜...
...(a) (However, r is the radius of curvature of the standard inscribed method at the apex of the aspheric surface,
A2, A4 Ta2, A8, AID are each 2
These are the expansion coefficients of the 4th, 6th, 8th, and 10th orders. ) It has an axially symmetric general aspherical shape expressed by the formula, and is formed so as to satisfy the following conditions (1) and (2).

■0.5<((n2-1)/n2)Xb/f)<0
．． 8 (where n is the refractive index of the lens medium at the wavelength of the incident light, and f is the focal length.), 2: (n - 1/n)X (d/r) < 1-
(2,8/f) (where d is the center thickness of the lens.)
Equation (a) in L is an equation in which the first term is a spherical surface. Further, the second term and the following are correction groups 1 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.

Further, among the conditions E, ■ is a condition for effectively correcting higher-order aberrations.

The above condition \false is a condition for setting the WD (working distance) sufficiently large. If enough WD can be secured,
When the disk 2 is deformed or the objective lens is corrected, the lens does not hit the disk 2, and the performance of the optical pickup can be improved. Furthermore, this condition (2) allows d to be increased to some extent, making it possible to press-form the lens.

As is clear from the above conditions, the objective lens according to the present invention has a basic shape of a positive meniscus lens with spherical surfaces on both sides, and one side of the objective lens has a positive meniscus lens with spherical surfaces. By making it a spherical surface, it is possible to appropriately correct higher-order aberrations compared to a meniscus lens whose both surfaces are spherical.

Furthermore, since the objective lens of the present invention is a meniscus lens, it can be said that it has an optimal shape for press molding. FIG. 7 shows a technique for press-molding lenses from glass materials, according to the type of lens shape. In this figure, (A) shows the inside of the cavity when pressing a biconvex lens, and (B) shows the inside of the cavity when pressing a plano-convex lens. This (A
In the case of molding convex lenses shown in ) and (B), when the upper and lower molds are pressurized, the softened glass flows toward the center of the cavity. Therefore, air tends to remain in the center of the cavity, and sink marks tend to occur on the molded lens. However, as shown in FIG. 7(C), when press-molding a meniscus lens, when the upper and lower molds are pressurized.

The glass material within the cavity tends to flow outward. Therefore, air is less likely to remain in the cavity and sink marks are less likely to occur on the molded lens. As described above, since the objective lens of the present invention is configured as a meniscus lens having an aspherical surface, it has sufficient advantages in press molding technology.

Further, the condition n≧1.7 for the refractive index of the lens medium shown in claim 3 is a condition for minimizing the spherical aberration and making it possible to construct a meniscus lens with this minimum aberration. It is. This condition will be explained below.

In the lens shown in FIG. 6, the radius of curvature of the surface on which parallel light enters is ra, and the radius of curvature of the surface on the opposite side is rb.

Note that since the present invention is an aspherical lens, ra may be considered as the radius of curvature of the inscribed circle of the aspherical surface.0 The ratio of ra and rb for minimizing the spherical aberration is:

rb/ra=na(2na+1)/(2na 2-
It is known that the condition is na -4) (where na is the refractive index of the lens medium with respect to the wavelength of the incident light). Since na is positive, the numerator is always positive. Therefore, the denominator is positive na
If ra and rb are both positive, a meniscus lens can be constructed.The condition for the 0 denominator to be positive is na>1.888.

Therefore, if the lens is constructed from a material with a refractive index of 1.888 or more, and the ratio of ra and rb is selected to match the above formula, a meniscus lens with minimum 1-spherical aberration can be constructed. .

The same thing can be said about coma aberration.

The ratio of ra and rb for minimizing coma aberration is r b/raw (na2-na-1)/na2
It is. n a>IJ18 If there is ra and rb
are positive, a meniscus lens can be formed, and r
Comatic aberration can be minimized by adjusting the ratio of a and rb to the equation shown in L.

From the above, if the refractive index of the medium of the objective lens with respect to the wavelength of the incident light is set to n≧1.7, it becomes possible to design a meniscus lens that minimizes spherical aberration and coma aberration.

Therefore, in the meniscus lens according to the present invention, the radius of curvature r of the inscribed sphere of the aspheric surface is made to correspond to the above ra, and the radius of curvature R of the spherical surface is made to correspond to the above rb, and the ratio of r and H is expressed by the above two equations. Try to match as much as possible, and make sure that n≧1.
By applying the condition 7, spherical aberration and comatic aberration can be suppressed to a minimum.

[Embodiments of the invention]

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

The aspherical surface has a shape in which r and A1 to Ale are the following values in formula (a).

r=3.75 A2=O A4= -1,150X 1O-3 A6=-2,100XIO" Day A = -1,570X 10'5 AIO='←8.OOQ The radius of curvature of the surface is R = 30.0 mm.

Other figures are as follows.

d = 2.273 am WD = 2.4
88 ma+n = 1.897 [i8 f = 4.802arm N A = 0.45
6 However, d is the thickness of the meniscus lens on the optical axis,
n is the refractive index of the lens medium with respect to the wave penetration of incident light = 788.2 mm, f is the focal length, and WD is the working distance.

If the lens shown in each example in I- is used, the aperture ratio N
A is 0.456, which is a value suitable for Pt for an objective lens for an optical/academic pickup.

Further, the working distance WD is 2.498 ff1m, which is sufficiently wide. Fig. 4 shows the lateral aberration curve of the objective lens of the embodiment described above. Further, FIG. 5 is a diagram showing the longitudinal aberration and the sine condition. As can be seen from this diagram, each aberration is extremely small, resulting in a lens with optical characteristics at the 1-diffraction limit.

〔Effect of the 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 meniscus 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. Also, the sub-resonant frequency can be increased.

1. Closed loop gain can be increased. The above servo becomes stable. Furthermore, since the objective lens is composed of a single lens, the amount of light is less likely to be lost due to internal reflection or absorption of light within the lens, compared to the case where a plurality of lenses are used.

(2) Since a meniscus lens has an aspherical surface on only one side, the lens can be press-molded more easily than a lens with aspherical surfaces on both sides.

(3) Formula and condition stated in claim 2 +4
),... If we construct an aspherical surface that satisfies the smell.

It is possible to make the spherical aberration extremely small and obtain a lens having optical characteristics at the 1-diffraction limit. Further, 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 drawings]

Figure 1 is an explanatory diagram showing the main points of an optical pickup, Figure 2
The figure and the in-3 diagram are cross-sectional views of an objective lens for an optical pickup according to the present invention, with the main axis oriented horizontally, and Fig. 4 is a cross-sectional view of an objective lens for an optical pickup according to an embodiment of the present invention. Figure 5 is a diagram showing the longitudinal aberration and sine conditions, Figure 6 is an explanatory diagram for determining the conditions for minimizing lens aberration, and Figure 7 is a diagram showing the lens being press-molded. It is an explanatory view showing a process of doing so. l... Meniscus lens, 2... Disk (recording medium), 10... Lens barrel, 12... Optical pickup, d... Thickness on the main axis of the lens, n... Lens thickness The refractive index of the medium relative to the wavelength of incident light, WD...working distance. Figure 1 Figure 3 Figure 5 -40,0-20,0020,040,0 Figure 7

Claims (3)

[Claims] (1) An objective lens for an optical pickup consisting of a single positive meniscus lens with a convex portion facing the side into which parallel light enters, and the convex surface of this meniscus lens is an aspherical surface. (2) The surface on which the parallel light is incident is the meridian plane of an orthogonal 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 reference inscribed sphere at the apex of the aspheric surface, A_2, A_4, A_6, A_8
, A_1_0 are 2nd, 4th, 6th, 8th, and 10th, respectively.
It is the following expansion coefficient. ) 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]0.5<{(n^2-1)/n^2)×(r/f
) < 0.6 (where n is the refractive index of the lens medium at the wavelength of the incident light, and f is the focal length.) [2] (n-1/n) x (d/r) < 1-(2 .8/f
) (However, d is the center thickness of the lens.) (3) The objective lens for an optical pickup according to claim 1, wherein the refractive index of the lens medium is n≧1.7.