JPS61177407A - Objective lens for recording and reproducing of optical information recording medium - Google Patents

Abstract

PURPOSE:To obtain an objective lens which is used most suitably when the distance between a light source and an information recording surface is short and consists of a small number of constituting lenses and uses a minimum number of aspherical surfaces, by constituting the objective lens with the first lens which has the convex directed to the light source side and has a positive refracting power and the second lens which has the convex directed to the light source side and has a positive refracting power arranging the first and the second lenses in order from the light source side and forming the surface in the side opposite to the light source side of the second lens to an aspherical surface and satisfying specific conditions. CONSTITUTION:The objective lens consists of the first lens which has the convex directed to the light source side and has a positive refracting power and the second lens which has the convex directed to the light source side and has a positive refracting power, and the first and the second lenses are arranged in order from the light source side, and the surface in the side opposite to the light source side of the second lens is aspherical, and conditions of formulas are satisfied. Though the first lens is a spherical lens which converts a divergent light to a convergent light, the negative spherical aberration generated by this conversion must be reduced as much as possible. A condition (1) is given for the purpose of reducing this aberration. The objective lens consists of two lenses and is realized with a minimum number of aspherical surfaces and contributes to the reduction of the cost.

Description

[Detailed Description of the Invention] [Industrial Application Field] This invention is particularly directed to a recording/reproducing system comprising a two-lens configuration suitable for focusing light source light directly onto the information recording surface of an optical information recording medium such as an optical disk. Regarding the objective lens for use.

[Conventional technology]

The most common optical system used in recording and reproducing devices for information recording media such as optical disks in recent years is as shown in FIG. The lens 2 focuses the light onto the information recording surface 1. In this optical system, 7-ocusing is performed by moving the objective lens 2 in the optical axis direction in response to surface wobbling of an optical disk or the like.

This method has the advantage that the performance of the optical system remains unchanged even if the objective lens 2 is moved;
Since two lenses, including the collimator lens 3 and the collimator lens 3, are required, there is a problem that the optical system becomes expensive.

Therefore, in order to reduce the cost of the optical system, a collimator lens is not used and the light source 4 is
A method of concentrating the light directly onto the information recording surface 1 using the objective lens 2 is being actively researched.

#49 The one shown in Figure 7 is the objective lens 2.
However, since the numerical aperture and performance of the objective lens 2 change due to the movement, the imaging magnification cannot be increased very much, and the standard imaging magnification is about -1740 to -178.

In recent years, optical systems for compact disc playback have
(1) A compact optical system is required.

(2) As the quality of compact discs has improved, even if the possible range of 7-orcasing is narrow, there is no longer any practical problem.

As a result of reviewing the above optical system for the following reasons, it has become clear that the optical system shown in FIG. 9 can be used at a standard imaging magnification of about -174.

On the other hand, the one shown in FIG. 10 performs 7 orsing by moving the unit 5 of the entire optical system including the light source 4 and objective lens 2, and there is no change in numerical aperture or performance deterioration due to 7 orsing. In order to make the unit 5 as light as possible, it is important to reduce the distance between the light source 4 and the information recording surface 1 while ensuring the necessary working distance. Therefore, the imaging magnification must be set to -176 to -172, which is larger than that of the optical system shown in FIG.

The most suitable lens for use as the objective lens in the optical system shown in Figures 9 and 10 is JP-A-59-86018.
There is something described in the publication. However, the above-mentioned known example had a four-layer configuration and was expensive.

In addition, in recent years, attempts have been made to reduce the number of lens components and reduce costs by correcting the spherical aberration inherent in spherical lenses by making the refractive surfaces of lenses aspheric.

Among these, the one with two sheets is JP-A-55-4
5084, JP 58-219511, JP 59-7917, JP 59-9619, JP 59-48724, JP 59-49
There are those described in JP-A-512 and JP-A-59-49513. However, these lenses were designed to be used as objective lenses in the optical system shown in FIG.

Therefore, it is difficult to use it when the distance between the light source and the information recording surface is small, and there is a need to develop an objective lens with good performance that can be used in the optical system shown in Figures 9 and 10. There is.

[Problem that the invention seeks to solve]

This invention has a two-element configuration that is optimally used when the distance between the light source and the information recording surface is short, and a smaller number of elements than conventional ones.
Another object of the present invention is to provide an objective lens for optical disks that uses the minimum number of aspherical surfaces.

[Means for solving problems]

In this invention, the objective lens is composed of, in order from the light source side, an @ lens with a convex surface facing the light source side and having a positive refractive power, and a $2 lens with a convex surface facing the light source side and having a positive refractive power. , the surface of the second lens opposite to the light source side is an aspherical surface, and the following condition (1) 0.4# <1 <
0.07. #(2) 0.32 <-□<0.5
2f However, r: Composite focal length of the entire system r2: Radius of curvature of the surface of the first lens opposite to the light source side r,: Radius of curvature of the surface of the second lens on the light source side n2: Refractive index of the second lens The present invention provides an objective lens for recording and reproducing an optical information recording medium, which satisfies the following.

[Effect]

In the objective lens of the present invention, it is necessary to convert divergent light with a large numerical aperture into convergent light using only two single lenses.

The first lens is a spherical lens that has the function of converting diverging light into convergent light, but the negative spherical aberration that occurs at this time must be minimized, and the 1° condition (1) is a condition for this purpose. If the upper limit is exceeded and the surface of the first lens opposite to the light source becomes concave and tight, the surface of the first lens on the light source side becomes convex and tight in order to maintain the necessary refractive power.

Therefore, the angle of incidence of the outermost ray with respect to the axial object point becomes large on each surface of the lens, and a large amount of negative high-order aberration occurs. Conversely, if the surface of the first lens opposite to the light source side is convex and stiff, the refraction angle on this surface becomes large, and similarly, large negative higher-order aberrations occur. In order to correct this with the aspherical surface of the second lens on the side opposite to the light source, the amount of deformation of the aspherical surface becomes considerably large, making it difficult to process and manufacture the aspherical surface with high precision.

Furthermore, if the refractive power of the first lens is weakened, the negative spherical aberration that occurs even if it is out of the range of condition (1) can be reduced, but the burden on the refractive power of the second lens increases, and the second lens It is not possible to correct the sine condition by simply making the light source side surface of the lens aspheric. Condition (2) relates to the apex radius of curvature r of the light source side surface of the second lens, which is an aspheric surface.

In this invention, since only one aspherical surface can be used, in order to maintain a good sine condition, the second lens should be a bar, and if the lower limit is exceeded, it becomes under.

Furthermore, in this invention, the focal length f1 of the first lens is
It is desirable that the value be within the following range.

1.5f < f, > 3f
(3) If the focal length of the first lens becomes longer than the upper limit, the sine condition cannot be corrected simply by making the light source side surface of the second lens aspheric.

If the focal length of the first lens exceeds the lower limit and the focal length of the first lens becomes shorter, the height of the rays on the surface of the second lens opposite to the light source becomes smaller, so the amount of deformation of the aspherical surface becomes larger, and the sine condition also increases when the numerical aperture is large. By the way, it is under. If this is forcibly corrected, the spherical aberration will be undersized in the intermediate annular zone.

〔Example〕

Examples of the objective lens of this invention will be shown below.

The symbols in the table are: ri: Vertex radius of curvature of the first lens surface from the light source side di: First lens surface from the light source side FWJWIni:
Refractive index stain of the first lens material from the light source side: Abbe number for the d-line of the first lens material from the light source side.Also, for an aspherical shape, the vertex of the surface is the origin, and the optical axis direction is
In the orthogonal coordinate system with the axis as the axis, the apex curvature is C1, the conic constant is K, the aspherical coefficient is Ai, and Pi is the power of the aspherical surface (P
It is expressed when i>2.0>.

In addition, the value of cover plus G is also shown in the table.

Example 1 r=I N〇, 45 m=-1/4ri
di ni V il 1
．． 66304 0.4500 1.91180
27.92-153.23898 0.1940
3 0.61233 0.3600 1.48595
55.0 Aspherical coefficient/power to the 4th surface = 6.503) OD-03 1/rz = -0,0002 f, = 1.8068 square to white Example 2 r = I NA 0045 m = -174ri
di ni v il 1
．． 52575 0.4500 1.70214 29.
52-10.21540 0.1940 3 0.60197 0.3600 1,48595
55.04 1.63809 0.5341 Aspheric coefficient/power on the 4th surface = 1.682720-02 ^1 = 6.610790-01 P1 = 4.
0OOQ^2= 8.17174D-02P2=
6.0000^3= 9,72686D-01P3=
8,0000^4= 3.241850-01
P4= 10,00001/r2= 0.0979 f, =1.9210 Example 3 f=IN〇, 45 m= -174ri
Di ni! 'il 1.
37453 0,4000 1,51072 64.1
2 -4.77074 0.1940 3 0.59758 0,4000 1,48595
55.04 1.93466 0.5423 Aspheric coefficient/power number on the 4th surface = 3.000960-02 ^1 = 8,49615D-01P1 = 4,00
00^2= 1.480910-01 P2=
6,0000^3= 9.936640-01 P
3 = 8,0000^4 = 3.287450-0
1 P4= 10.00001/rz =-0,20
96 Example 4 f=I NA0.45 ts=-1/44
1.23371 0.5119 c[: 0,3333'', 55000 30.
0 Aspheric coefficient/power number 4th surface K = -2,899060-02 ^3 = 9.920120-01 P3 = 8.
0000A4= 3,28234D-01P4= 1
0.00001/r2=-0,0326 -=0.4126 2f f, =2.0170 Example 5 f=I NAo, 45 ta=1/4ri
di ni j/il
1.75355 0.4000 1.70214 29
．． 52-27.30120 0.1940 3 0.55998 0,4500 1.48595
55.04 1.68011 0.3889 Aspheric coefficient/power number on the 4th surface = 5,88373D-02 ^1 = 9.53574D-01P1 = 4,00
00^2= 9.93779D-02P2= 6,
0000^3= 9.72577D-01P3=
8,0000^4= 3.240060-01 P
4 = 10.000017r2 = 0.0366 f, =2.3601 Example 6 f = ISA 0.45 m = -178r
I di ni Will
1.40973 0.4500 1.76204 49
．． 62 20.00000 0.1940 3 0.57G92 0,4000 1.58252
B1.04 0.89049 0.3482 Aspheric coefficient/power number 4th surface K = 3.524150-01 ^1 = 6.585100-01 P1 = 4.
0000^2= 5,21845D-01P2=
6.0000^3= 1.031640+ 00
P3= 8,0000^4= 3.312290-
01 P4= 10,00001/rz=O,'
[Effects of the Invention] FIGS. 2 to 7 are diagrams of various aberrations of Examples 1 to 6. As is clear from these figures, aberrations have been sufficiently corrected as an objective lens for optical disks.

In Examples 1 to 5, aberrations were corrected by setting the imaging magnification to 1174 times, but these examples can be used as objective lenses for both optical disk optical systems shown in FIGS. 9 and 10. This is possible, and the fact that such a lens could be realized with a two-element lens as shown in the cross-sectional view shown in Figure 1, and with a minimum number of aspherical surfaces, will greatly reduce the cost of optical disk optical systems. It can be said that it contributes. Furthermore, since there are no restrictions on aberration correction for the lens constituent materials, it is possible to further reduce costs by using plastic injection molding technology or the like.

Note that in optical systems for optical discs, an optical element such as a polarizing beam splitter is often disposed on the light source side of the objective lens, but this can be accommodated by slight design changes to the above embodiments.

[Brief explanation of drawings]

Figure tJIJ1 is a sectional view including a cover glass of the objective lens of the present invention. FIG. 2, FIG. 3, FIG. 4, FIG. 5, FIG. 6, and FIG. 7 are aberration diagrams of the first to sixth embodiments, respectively. Figure w&8 is a layout diagram of a conventional optical disk optical system. 9 and 10 are optical layout diagrams of an optical system using the objective lens of the present invention. Optical disk (optical information recording medium) 2: Objective lens 3: Collimator lens 4: Light source 5: Light source unit Applicant Konishiroku Photo Industry Co., Ltd.
Figure 2, Figure 3, Figure 4, Figure 5 r*mJ4 Matakago positive 5mu condition!
Ftη again difference - Figure 6 Spherical convergence Positive # License suspension Tamae convergence Figure 7

Claims (1)

[Scope of Claims] Consisting of, in order from the light source side, a first lens having a positive refractive power with a convex surface facing the light source side, and a second lens having a positive refractive power with a convex surface facing the light source side. The surface of the lens opposite to the light source side is an aspherical surface, and the following condition (1) -0.4/f<1/r_2<0.07/f(2)
0.32<r_3/n_2f<0.5 where f: Combined focal length of the entire system r_2: Radius of curvature of the surface of the first lens opposite to the light source side r_3: Radius of curvature of the surface of the second lens on the light source side n_2: An objective lens for recording and reproducing an optical information recording medium, which satisfies the refractive index of the second lens.