JPS62108217A - Objective lens for optical disk - Google Patents

Abstract

PURPOSE:To directly receive the light form a semiconductor laser without requiring a collimator lens by forming a single lens of which the face on a light source side is formed as an aspherical face having positive refracting power and the face on a disk side has positive refracting power in such a manner as to satisfy prescribed conditions. CONSTITUTION:The face on the light source side of this single L is constituted of the aspherical face having positive refracting power and the face on the disk side has the positive refracting power. Said lens is used within the -1/2<beta<1/8 range of projection magnification beta and the lens is so formed as to satisfy the various conditions expressed by formula I - formula III. In formulas, N1: the refractive index of the single lens L, r1: the radius of paraxial curvature of the face on the light source side of the single lens L, f: the focal length of the single lens L, d1: the core thickness of the single lens.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an objective lens of a pickup optical system used for reproducing information from a disk such as a DAD.

Conventionally, this type of pickup optical system has a collimator lens (C) in addition to the objective lens in order to convert the incident light beam into a parallel light beam, even if the objective lens is composed of a single lens as shown in Fig. 3. required. Alternatively, if a collimator lens is not required, the objective lens is composed of four lenses, as shown in Figure 4. In Figure 3, the collimator lens is used in a fixed state, but the objective lens is in a movable state. Since this type of optical system is used, two lens barrels are required, and it is difficult to adjust the optical axis of the entire optical system. If the objective lens is a single lens, this lens is usually an aspherical lens, so the performance deteriorates significantly with respect to eccentricity, and the adjustment of the optical axis requires strict precision. On the other hand, when the objective lens is composed of a plurality of lenses as shown in FIG. 4, strict accuracy of the lens barrel is required to avoid performance deterioration. In addition, compared to a single lens, the area of the lens barrel must be larger, and the distortion of the lens barrel due to temperature changes causes eccentricity of the lens, resulting in a decrease in performance.The present invention solves these problems. In order to reduce this, the present invention specifically aims to provide a single lens that directly receives light from a semiconductor laser and focuses it on a disk without requiring a collimator lens.

The present invention will be explained in more detail below.

Since the objective lens according to the present invention does not use a collimator lens, aberrations are well corrected by a concave circle in which the projection magnification β is one peak'<β<<-. If the magnification increases beyond one lens, the distance from the light source to the disk cannot be sufficiently shortened, and the optical system cannot be made more compact. Furthermore, if the magnification becomes smaller than m-, there will not be enough room to control the optical system such as a beam splitter, and if the objective lens is mounted at an angle, the image height range necessary to properly correct aberrations will be reduced. I can't get enough. Therefore,
The objective lens according to the present invention is used within the range of -H<β<--H.

Here, we use a conventional objective lens with aberrations corrected in the β-0 state as it is as an objective lens for a pickup optical system that does not require a collimator lens, with a magnification of -7 and β<-
If used within the range of −H, the numerical aperture NA decreases, so 1
It is not possible to obtain μm resolution. In other words, the numerical aperture when β−〇 is NAω, and the numerical aperture when β×O is NAω.
If β is N A p -N Aoo/ (1-β),
holds true, but if the objective lens with NAω-0,45 is set to β=-
7 gives NAβ-0.36, resulting in β-
-7, it is not possible to obtain a resolution of 1 μm.

Therefore, in order to increase NAβ for finite object points,
When the aperture of the lens is enlarged, spherical aberration significantly occurs on the axis, and comatic aberration occurs significantly off-axis, making it difficult to correct these aberrations.

Therefore, the objective lens according to the present invention has a magnification equal to or higher than the value determined by the numerical aperture on the light source side in order to directly receive light from a semiconductor laser with a single lens, and has In order to correct significant aberrations and comatic aberrations, the following configuration is used. In other words, this objective lens is composed of an aspherical surface whose surface on the light source side has positive refractive power, and a single lens whose surface on the disk side has positive refractive power, and the projection magnification β is 11≦β<−7. This is an objective lens for an optical disc, which is used within the scope of the present invention and is characterized by satisfying the following conditions.

(130,9≦(Nl-1) rl/f, ri3.9<2
) 1.4≦dt rl/72<4.0(3) 1
．． 4≦d12/f2 (Nl −1)≦4.0, where f: focal length of a single lens d1=core thickness of a single lens.

Here, conditions (1) to (3) are conditions for well correcting aberrations.

If the radius of curvature of the surface on the light source side becomes smaller than the lower limit of condition (1), it becomes difficult to correct comatic aberration. , and also the condition (
1) If the refractive index of the single lens decreases beyond the lower limit, in order to secure a prescribed value of focal length, the absolute value of the radius of curvature of the surface on the light source side or the radius of curvature of the surface on the disk side must be increased. It is necessary to reduce the aberration, but either of these methods makes it difficult to correct coma aberration. In this way, either the radius of curvature of the surface on the light source side or the refractive index of the single lens is the condition (1
), it becomes difficult to correct coma aberration. On the other hand, in the lens of the present invention, when the radius of curvature of the surface on the light source side becomes large, significant coma flare occurs around the pupil. This can be corrected by using high refractive index glass, but under the condition that the combination of the radius of curvature and the refractive index of the surface on the light source side (if the value exceeds the upper limit of υ, astigmatism will occur and the axial If the core thickness becomes smaller beyond the lower limit of condition (2), or the radius of curvature of the light source side surface becomes smaller, it becomes impossible to maintain good external performance.
In order to maintain a predetermined focal length, it is necessary to increase the absolute value of the radius of curvature of the disk side surface, but as a result, it becomes difficult to correct astigmatism, and on the other hand, if the upper limit of condition (2) is exceeded,
As the core thickness increases or the radius of curvature of the surface on the light source side increases, the absolute value of the radius of curvature of the surface on the disk side must be reduced in order to maintain a predetermined focal length, but this results in an aspherical effect. However, it becomes difficult to correct coma aberration.

If the core thickness decreases or the refractive index increases beyond the lower limit of condition (3), the absolute value of the radius of curvature of the surface on the light source side or the disk side must be increased. here,
If you increase the absolute value of the radius of curvature of the surface on the disk side, it becomes difficult to correct astigmatism, and if you increase the radius of curvature of the surface on the light source side, coma flare occurs, and even if you use the aspherical effect, coma aberration correction becomes difficult. On the other hand, if the core thickness increases beyond the upper limit of condition (3) or the refractive index decreases, the absolute value of the radius of curvature of the round surface must be decreased, and as a result, coma aberration occurs. If it can be corrected even with aspherical effects.

Note that in order to satisfactorily correct coma aberration, it is desirable to satisfy the following conditions.

(4) O<f(N12-Nt-1)/l r21
<1.2, where f: focal length of the single lens r2: radius of curvature N1 of the disk-side surface of the single lens: refractive index of the single lens.

When the refractive index becomes smaller than the lower limit of condition (4), the absolute value of the radius of curvature of the surface on the light source side or the disk side must be reduced, and as a result, coma cannot be corrected, and furthermore, the radius of curvature of the surface on the light source side or the disk side must be reduced. If the surface is aspheric, the absolute value of the radius of curvature of this surface becomes smaller, and the absolute value of the paraxial radius of curvature becomes smaller, so the amount of displacement from the tangent plane at the apex of the aspheric surface increases significantly. Another problem is that it becomes difficult to achieve the necessary precision in the mold manufacturing and molding process. On the other hand, under the condition (the absolute value of the radius of curvature of the disk-side surface becomes smaller than the upper limit), it becomes difficult to correct comatic aberration.

Furthermore, when the disk side surface is spherical and only the light source side surface is aspherical, in order to better correct coma aberration,
It is desirable to use glass with a higher refractive index. Therefore, it is desirable to satisfy the following conditions.

(5) 1.4<(Nt-1)rt/f≦3.9(6
) 1.6≦dsrt/f2<4, O where f: focal length of a single lens.

By satisfying the above conditions, it is possible to obtain a single lens which directly receives light from the semiconductor laser and whose aberrations are well corrected even off-axis. This single lens is composed of aspheric surfaces on both sides or a combination of an aspheric surface and a spherical surface, and as long as the optical axes of each surface match, the precision of the outer diameter of the single lens can be relaxed.
The need for core removal is reduced. Further, the requirements for precision of the lens barrel are low, and if the lens barrel that holds the single lens is provided with an adjustment mechanism for parallelism, inclination, and eccentricity, it is possible to maintain the performance of the single lens. As a result, cost reduction can be achieved. Note that the aspherical surface of this single lens can also be achieved by forming an aspherical resin layer on a spherical glass. Examples of the present invention will be shown below.

In the examples, rgl and 1g2 are the radius of curvature of the cover glass (g) of the semiconductor laser, rl and r2 are the single lens (
radius of curvature of the surface counted from the light source side of L), rpl, r
p2 is the radius of curvature of the disk (P). dgx,
dgz.

(do), dl, d2. dp indicates the axial surface spacing counted in order from the light source side, and Nge (No)? N
1*NP is a cover glass (g), (resin layer (a)), and a single lens (respectively) at a wavelength of 800 nm.
L) is the refractive index of the disk (p). * indicates an aspherical surface, and its shape is defined by the following formula.

However, X is the distance in the optical axis direction from the plane perpendicular to the optical axis at the intersection with the optical axis, S is the distance in the direction perpendicular to the optical axis, CO is the paraxial curvature (-1/r) at the intersection, ε is the quadratic surface radius, and Ci is the aspheric coefficient. Note that NA is numerical aperture, f
is the focal length of the single lens (L), and β is the projection magnification.

(Left below) [Example 1] Figure 1 NA-0,47f-1,0β=-0,17 radius of curvature
On-axis spacing Refractive index aspheric coefficient r12ε; 10 CI=O, OC2=-0, 62371X10-1Ca=
-0,35855xlO-'C4=-0.34573X
10-'Cs--0,10152XIO-2r2"
ε=1,0 C mini 0. OC2-0, 94476X10-1C3-0
,31391C4==-0,34388xlO-'
C5=0.26302XLO-'(N1-1) r
1/f-1,40d1r1/f2=1.73 ch2/f2cN1-1)-1,68 f(N12-NZ-1)lr21-0.36 [Example 2
] Figure 2 NA-0,47f-1,0β--0,17 radius of curvature
On-axis spacing Refractive index aspheric coefficient ra ε=1.0 C1-Q, OC2-0, 20762C3-0, 1317
3C4--0,64485xlO-' Cs-0,
36803X10-2(Nl-1)rl/,? ”=1.
89 dlrl, #2=2.36 d12/f''(N1-1)-1,77 f(Nl2-Nt-t)/l rzl-o, 73 [Example 3] Figure 1 NA-0, 471-1,0β--0,17 radius of curvature
On-axis spacing Refractive index aspheric coefficient r 14F C! 1.0 Ct=Q, Q C2=-0,96:367X10-'
Ca=-0,48541Kari0-IC4=-0,5
7170xlO-' C5=-Q, 34961X1
0-”(N1-1) r 1/f-1,89dtrt
/f2-2.36 ch2/f2cN1-1 )-1,77f (N12-
N1-1)/Irl=0.73 [Example 4
] Figure 1 NA-0,471=1. Oβ--0,17 radius of curvature
On-axis spacing Refractive index aspheric coefficient r12 ε=1.0 C1-0,0C2=-0,51488X10-1C3-
0,26625xlO-1C4=-0,11890x1
0-' C5-0,69239X10-3(Nl-
1) r 1/f-2,43 dIrl/f2-2.97 d12/f2(N1-1)=2.08f(N12-N
l-1)/l rz l -0,79 [Example 5]
Fig. 1 NA-Q, 47 f-1,0β--0,22 radius of curvature axial surface spacing refractive index aspherical coefficient rl ε=1.0 C1-0,0,C2-0,71690X10-1Catan-〇 ,29855Kari0-IC4--0,50256xl
O"" C5=-0,16435 mowing o-2r2
ε-1,0 CI-Q, OC2-0,13636C3=-0,32
847C4=0.57220X10””C5-0
,36914X10-'(Nt-1) 11/f-1
, 31 dirt/f2=1.66 d12/f2(Nl-1)-1,68 f(Nl2-Nx-1)/giant moon-〇, 39 [Example 6
] Figure 2 NA, Q. 47 f-1,0β--0,22 radius of curvature axis spacing refractive index aspherical coefficient ra" ε=1.0 CI=0.0 C2=-0, 14089C3=-0,
57431X10-”C4--0,56260 mowing 0-”
C3=-0, 18309 mowing O-2r2” ε=
1.0 CI=O, OC2=0.14537 C3=
-0,31199C4=0.95588x10-3Cs
=0.40854X10-'(N1-1)rx/f=1
．． 25 dlr1/f2=1.62 d12/f2(Nt-1)=1.68 t(N12-N11)/Giant i'l-o, 3s [Example 7] FIG. 1 NA=0. 47 f-1,0β--0,38 Radius of curvature Axial spacing Refractive index rp2 ■ Aspheric coefficient r12 ε=1.0 Ct=0. □ C2--0, 10852Ca--〇,
16503xlO-”C4--0,58946xlO-
"Cs--0,35616X10-2r2" ε
Cod 1.0 CI-0, OC2=0.15569 C3=
-0,18428C4-0,19846X10-'
C5=0.56733xlO-3(Nl-1) rl
/f-1,20 dirl/72-t63 d12/f2(Nt-1)-1,75 f (N12-Nt-1)/I r21-0.44 [Example 8] Figure 1 NA, -Q , 47 f-1,0β--0,45 radius of curvature axial distance refractive index dp O,2
67NP 1.57147rp2 00 Aspheric coefficient r 11) ε Self 1°O CI-0,0C2--0,13844C3--0,12
271X10-'C4=-0.83257X10-1C
5-0,58614X10-”r2”ε-1,0 C1-Q,OC2-0,15549C3-0,1705
1C4-0,49442XlO-I C5-0,1
6965 Example 9] Figure 1 NA=0,49 f-1β--0,17 radius of curvature
On-axis spacing Refractive index aspherical coefficient rl" ξ, 1.0 C1-Q, OC2="0.871129Kari0-1Ca=
-0,316098xlQ-'C4--0.57106
5xlO-' Cs--0,349594XIO-2
(N1-1) rt/7-1.83 dtrl/f2-2.14 ds/f (Nt-1)-1,55 f (N12-Nl-1)/l r21=0.66 [Example 10 ] Fig. 1 NA-0,47f ” 1 β--0,17 radius of curvature Axial surface spacing Refractive index aspheric coefficient r12 ε〒1.O Cs -0,0C2--0,906604X10-"
Ca gu-0,633411X10-”C4-0,14
9674x10-1Cs-0, 177734X10-3
(Nl-1) rl/f-1,51 dlrl/72-1.82 dx2/f2cNt-1)-1,54 f(Nl2-Nt-1)/I r21-0.53 [Example 11] Figure 1 NA=0.47 f-1,0β--0,22 Radius of curvature Axial spacing Refractive index aspheric coefficient rl ε=l, 0 Ct-Q, OC2=-0,702213X10-'
C3-0,397707X10-1C4-0,2565
11xlO-” C5-Q, 810222X10-2
r2 ε electric 1. OCI-Q, OC2=0.571793 C3
--1,278405C4=0.286478x10-
2Cs=0.974928X10-'(Nl-1) r
t/f-1,49 dlrl/72g2.68 ch2/f2(Nl-1)=3.43 f(N12-Nt-1)/I r2 I =0.9
1

[Brief explanation of drawings]

Figure 1 shows Examples 1, 3.4.5, 7.8, 9.10.1
1, FIG. 2 is a lens configuration diagram of Examples 21 and 6, and FIGS. 3 and 4 are conventional lens configuration diagrams. 5 to 15 show aberration diagrams of Examples 1 to 11, respectively. Applicant: Minolta Camera Co., Ltd. Figure 2 L r Figures 14 to 75 Distortion; ≦ Distortion %

Claims (1)

[Claims] 1. A single lens whose surface on the light source side is composed of an aspherical surface having positive refractive power and whose surface on the disk side has positive refractive power, and whose projection magnification β is -(1/ 2) An objective lens for optical discs that is used within the range of <β<1/8 and satisfies the following conditions: 0.9≦(N_1-1) r1/f≦3.9 1.4 ≦d1r1/f^2≦4.0 1.4≦d_1^2/f^2 (N_1-1)≦4.0, where, N_1: refractive index of single lens, r1: light source side of single lens paraxial radius of curvature of the surface, f: focal length of the single lens, d1: core thickness of the single lens. 2. The objective lens for an optical disk according to claim 1, wherein the disk-side surface of the single lens has an aspherical shape.