JPS62251709A - Objective lens for optical disk - Google Patents

Abstract

PURPOSE:To obtain a single lens which directly receives light from a semiconductor laser and whose aberration is well corrected to the outside of its optical axis, by constituting the lens of the combination of an aspheric and spheric surfaces and making the lens to satisfy specific conditions. CONSTITUTION:The objective lens for optical disk of this invention has an aspheric surface of positive in refracting power on the light source side and a single lens L having a positive refracting power on the disk side. The projecting magnification beta is used within a range of -1/2<beta<-1/8 and the lens satisfies conditions of (1) 0.9<(N-1)r1/f<3.9, (2) 0.6<n/(-r2).(N-1)<1.7, and (3) 0.5<epsilon<1.5, where N is the refractive index of the objective lens, r1 is the paraxial radius of curvature of the surface of the objective lens which is closest to the light source, r2 is the radius of curvature of the surface of the objective lens on the disk side, and epsilon is a conical constant. When the paraxial radius of curvature r1 of the surface of the single lens L which is closest to the light source becomes smaller than the lower limit of Condition (1), correction of coma becomes difficult. Condition (2) is a condition for well correcting the out-of axis aberration and Condition (3) is a condition for well correcting the on-the-axis aberration.

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 (q Or, if a collimator lens is not required, as shown in Figure 9,
The objective lens was composed of four lenses. In Figure 8, the collimator lens is used in a fixed state, but the objective lens is used in a movable state, so two lens barrels are required, and it is difficult to adjust the optical axis of the entire optical system. be. 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 severe precision. On the other hand, when the objective lens is composed of a plurality of lenses as shown in FIG. 9, strict precision of the lens barrel is required to avoid performance deterioration. Furthermore, compared to a single lens, the area of the lens barrel must be larger, and distortion of the lens barrel due to temperature changes causes decentering of the lens, resulting in a problem of degraded performance.

In order to alleviate these problems, it is an object of the present invention to specifically 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 within the range where the projection magnification β is - and β<-. If the magnification exceeds m-, the distance from the light source to the optical disk cannot be sufficiently shortened, and the optical system cannot be made more compact. Also, the magnification is .

If it becomes smaller than m-, there will not be enough room to dominate the optical system such as a beam splitter, and if the objective lens is tilted, the image height range necessary to properly correct aberrations will be insufficient. I can't get enough. Therefore, the objective lens according to the present invention is used within the range of -1<β<-1.

Here, a conventional objective lens that has been aberration-corrected in the β-0 state is used as an objective lens for a pickup optical system that does not require a collimator lens, and the magnification is - β × m.
If used within the range of -, the numerical aperture NA will decrease, so 1μ
m resolution cannot be obtained. In other words, the numerical aperture when -=O is N A c.

And if the numerical aperture when β is O is NAβ, then NAβ=
NA■/(1-β) holds true, but a resolution of 1 μm cannot be obtained with an objective lens of NAω=0.45 and β=−1.

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 greater 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 the significant off-axis distortion that inevitably occurs at this time. In order to correct aberrations and coma, the following configuration is used. In other words, as shown in Figures 1 and 2.3, this objective lens has an aspherical surface with positive refractive power on the light source side surface, and a single lens (L) with positive refractive power on the disk side surface. ) is characterized in that it is used within the range of projection magnification m- β x m- and satisfies the following conditions.

(1) 0.9<(N-1) r 1/f<3.9(
2) 0.6 <□・(N-1)<1.7r2 (330,5<ε<1.5 where, N: refractive index of the objective lens rB paraxial radius of curvature of the surface of the objective lens closest to the light source rl: radius of curvature ξ of the disk-side surface of the objective lens; biconic constant;

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

If the lower limit of condition (υ is exceeded and the paraxial radius of curvature (rl) of the surface closest to the light source of the single lens (L) becomes small, it becomes difficult to correct comatic aberration. Also, if the lower limit of condition (1) is exceeded, , when the refractive index (N) of the single lens (L) becomes smaller, in order to ensure a predetermined focal length, the paraxial radius of curvature (rl) of the surface closest to the light source or the radius of curvature of the surface closest to the disk must be increased. (
It is necessary to reduce the absolute value of rl), but either of these methods makes it difficult to correct comatic aberration. in this way,
The paraxial radius of curvature (rl) of the surface on the light source side or the single lens (
If either of the refractive indexes (N) of L) becomes smaller than the lower limit of condition (1), it becomes difficult to correct comatic aberration.On the other hand, in the lens of the present invention, the lens closest to the light source side When the axial radius of curvature (rl) becomes sharp, significant coma flare occurs around the pupil. This can be corrected by using high refractive index glass, but the paraxial radius of curvature (rl) of the surface closest to the light source The combination of and refractive index (N) is the condition (1
), astigmatism occurs and it becomes impossible to maintain good off-axis performance.

Condition (2) is a condition for good correction of off-axis aberrations.In a single lens (L), if the refractive index exceeds the upper limit of condition (2) at a predetermined focal length, The absolute value of the radius of curvature (rl) of the side surface of the disk increases,
Astigmatism occurs, and the ratio of the paraxial radius of curvature (rl) of the surface on the light source side to the absolute value of the radius of curvature (rl) of the surface on the disk side increases beyond the upper limit of condition (2). , the absolute value of the radius of curvature (rl) of the disk-side surface becomes too small, resulting in coma aberration that is difficult to correct well off-axis. On the other hand, if the refractive index of the objective lens (L) decreases beyond the lower limit of condition (2), the absolute value of the radius of curvature (rl) of the disk-side surface decreases, causing coma aberration. Furthermore, if the lower limit of condition (2) is exceeded and the ratio of the radius of curvature (rl) of the surface on the light source side to the absolute value of the radius of curvature of the surface on the disk side (r2) becomes small, then the radius of curvature of the surface on the disk side (r2) becomes smaller. The absolute value of (rl) becomes too large and astigmatism occurs, making it impossible to maintain good performance over a desired image height.

Condition (3) is a formula for properly correcting axial aberrations.If the conic constant increases beyond the upper limit of condition (3), under-spherical aberrations will occur especially around the periphery of the light beam, making correction difficult. On the other hand, if the conic constant becomes smaller than the lower limit of condition (3), excessive spherical aberration will occur and correction will become difficult.

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 centering 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. As for the processing method for the aspherical surface, if it is made of glass, a molding method or the like can be applied. It is also possible to bond an aspherical resin layer or plastic material to the spherical glass.

Examples of the present invention will be shown below.

In the examples, rgt and rgz are the radius of curvature of the cover glass (g) of the semiconductor laser on the light source side and the disk side, rl
(r+') and rl are the paraxial radii of curvature of the light source side and disk side surfaces of the single lens (L), and rpl and rp2 are the curvature radii of the light source side and disk side of the disk (P).

dg+ is the core thickness of the cover glass (g), dgz is the axial air distance from the cover glass (g) to the single lens (L) 1,
dl is the core thickness of the single lens, dl is the axial air distance from the single lens (L) to the disc (P) 4, and dP is the axial distance between the discs. However, when an aspherical surface is bonded onto a spherical glass, drM is the axial distance of the resin layer or plastic material, and d+' is the core thickness of the spherical glass. Ng
, NJ (Nt'), and NP each have a wavelength of 780 nm.
cover glass (g), single lens (L).

It is the refractive index of the disk (P). However, when an aspherical surface is bonded onto a spherical glass surface, NIM is the refractive index of the resin layer or plastic material. Yellow indicates an aspherical surface, and its shape is defined by the formula below.

(i=t 1213m) However, X is the distance in the optical axis direction from the plane perpendicular to the optical axis in the distance from the optical axis, and CO is the paraxial curvature (=1/r)
, Ci are aspheric coefficients. Note that NA is the numerical aperture of the surface on the optical disk side, f is the focal length of the single lens (L), and β is the projection magnification.

FIG. 1 and FIG. 2 each show the cylinder 1 of the present invention. A cross-sectional view of the optical f-isk objective lens of the second embodiment, FIG. Fourth
It is a lens sectional view of the objective lens for optical disks of an Example. 4 to 7 are lens aberration diagrams of the first to fourth embodiments. FIGS. 8 and 9 are lens configuration diagrams of conventional objective lenses for optical disks.

[Example 1] NA-0,45f, 1. C
a=-0,484883 mowing0-1C4=-0,3301
97X10-' C3=-0,443189 mowing 0-2
(N-1) rl/7=3.09 [Example 2] NA-Q, 45 /-1,0β Sense-0,20 Radius of curvature Axial spacing Refractive index Aspherical coefficient rl” (=1.4 Ct =*0.OC2=-0, 230057Ca=-0,
192504C4=-0, 134990C5=-0,3
87242X10-2(N-1)71/7=0.92 □・(N-1)=0.62 - "2 [Example 3j NA-0,45f-1,0βHatake-0,20 radius of curvature
On-axis spacing Refractive index aspheric coefficient rl” ξ=0.55 C+=0.0 C2=-0, 292694C3=-0
,228172C4=-0,223349C3=-0,
351381 mowing 0-2(N-1)rl/7=0.93 □・(N-1)=0.67 r2 ε=0.55 [Example 4] NA-Q, 45/-1,0β- -0,20 radius of curvature Axis spacing Refractive index aspherical coefficient r-ε=1.0 CI=0.0 C2=-0,345502C3=-0
, 243398C4=-0, 378201C5=-0,
665623xlO-2(N-1) rt/7=1.0
3 □・(N-1)=0.79 r2 ξ=1.0

[Brief explanation of drawings]

FIG. 1 and FIG. 2 respectively show the first aspect of the present invention. FIG. 3 is a cross-sectional view of the optical optical objective lens of the second embodiment. Fourth
FIG. 3 is a lens cross-sectional view of an objective lens for an optical lens according to an example. 4 to 7 are lens aberration diagrams of the first to fourth embodiments. FIGS. 8 and 9 are lens configuration diagrams of conventional objective lenses for optical disks. Single lens... L Applicant: Minolta Camera Co., Ltd. / Figure 20 Figure 3 Figure 4 Spherical aberration I, double difference. Figure S: Irohara aberration Figure 6: 711 One-point aberration

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 an optical disc that is used within the range of <β<-1/8 and satisfies the following conditions: 0.9<(N-1)r1/f<3.9 0.6<(r1/-r2)・(N-1)<1.70.5
<ε<1.5 However, N: refractive index of the objective lens r1: paraxial radius of curvature of the surface of the objective lens closest to the light source r2: radius of curvature of the surface of the objective lens closest to the disk ε: conic constant