JPH01152408A - Objective lens for optical disk - Google Patents

Abstract

PURPOSE:To obtain a double-aspherical face single lens having good performance over a wide visual field by constituting the single lens which has a positive refracting power on a light source side and disk side and is constituted of aspherical faces in such as manner that the aspherical faces satisfy specific conditions. CONSTITUTION:This single lens has the positive refracting power on the light source side and disk side and is constituted of the aspherical faces which satisfy the conditions expressed by equation I. In equation Xk is the distance in the optical axis direction from the plane perpendicular to the optical axis at the intersected point of the k-th face with the optical axis; (h) is the distance in the direction perpendicular to the optical axis of the aspherical faces; Cok is the paraxial curvature (=1/rR) of the aspherical face of the k-th face; Kk is the circular cone constant of the k-th face; Aik is the aspherical face coefft. of the k-th face; beta is the imaging magnification of the single lens; K1 is the circular cone constant of the light source side aspherical face; A41 is the quartic aspherical face coefft. of the light source side aspherical face; (f) is the focal length of the single lens. The performance which is good over + or -2 deg. visual field is thereby obtd.

Description

[Detailed description of the invention] Matasato ■ Tsuzumi Tsubu! The present invention relates to an objective lens for optical discs composed of a single lens.

1" 11 stones 1" Such an objective lens for an optical disk is used, for example, in a pickup for extracting signals from a CD (compact disk) or a DAD (digital audio disk). This type of pickup uses a collimator lens to condense light generated from a semiconductor laser into parallel light, and then converges the parallel light onto a disk through an objective lens. However, when an optical system is formed by a combination of a collimator lens and an objective lens, there is an inherent difficulty in coordinating the light beams of these lens systems. Therefore, in recent years, integrated bi-, 2-qua, and 7-pu optical systems have been developed in which the collimator lens and objective lens are integrated to form a lens system, which directly receives and takes the diverging light from the laser, and forms an image on the optical disk. It's becoming mainstream.

Furthermore, with rapid progress in aspheric molding of small diameter lenses,
Cases in which this integrated pickup optical system is being replaced with a single aspherical lens (single lens) are becoming more prominent. 1 for a single ball with aspherical surfaces on both sides, for example, JP-A No. 60.120310 and JP-A No. 61-56310.
No. 6'l-2005!8, etc., but in all of these conventional examples, the aspherical surface contains at least a 10th-order aspherical coefficient, and it is difficult to use an aspherical mold. The drawback was that it was difficult to manufacture.

Purpose of the present invention The purpose of the present invention is to provide an objective lens for an optical disc consisting of a double aspherical single lens that has good performance over a wide field of view of ±2° using a high-order aspherical coefficient of 8th order or higher. shall be.

Summary of the Invention The objective lens for an optical disc of the present invention is a single lens having positive refractive power on the light source side and the disc side and composed of aspherical surfaces, and these aspherical surfaces satisfy the following conditions.

, (i=3.4,5....) ■-〇, 25〈β＜-0,15 ■ A□-0 (i≧8.=1.2) ■-10＜Kl
〈-3,5 ■-12〉 K , A, , f3 <-1' However, X is the distance in the optical axis direction from the plane perpendicular to the optical axis at the intersection of the first surface with the optical axis, h is the distance perpendicular to the optical axis of the aspherical surface, and Cod is the paraxial curvature of the aspherical surface (=1/r
i), the conic constant of the second surface.

Aik is the aspherical coefficient of the first surface, β is the imaging magnification of the single lens, and 1 is the conic constant of the aspherical surface on the light source side, A4. is the fourth-order aspherical coefficient of the aspherical surface on the light source side, and f is the focal length of the single lens.

If the lower limit of Equation 0 is exceeded, the distance from the light source to the disk cannot be sufficiently shortened, which is contrary to compactness. Also, since the NA on the disk side is about 0.45,
If the lower limit is exceeded, the NA on the light source side becomes small, making it impossible to capture a sufficient amount of light from the laser into a single beam. If the upper limit of Equation 0 is exceeded, coma aberration will occur, making it impossible to maintain good performance over a field of view of ±2°. Equation 0 indicates that high-order aspherical coefficients of 8th order or higher on the light source side and disk side surfaces do not contribute to layering the aspherical shape, which makes it easier to process the aspherical mold. Make it.

If the lower limit of Equation 0 is exceeded, the spherical aberration will cause undulation, making it impossible to obtain a good wavefront. If the upper limit is exceeded, it becomes impossible to correct the spherical aberration due to the light beam near the effective diameter with an aspherical coefficient that satisfies Equation 0.

Equation (2) is a combination of the conic constant of the aspherical surface on the light source side and the fourth-order aspherical coefficient value. Even if it exceeds the lower limit or the upper limit,
The value of astigmatism becomes too large, and good performance cannot be maintained over a field of view of ±2°. Furthermore, considering ease of processing, it is desirable that the following conditional expression be satisfied.

■A□=0 (=1.2 for i≧6.) ■-4,5<Kl<-3,5 Equation 0 shows that the aspherical surfaces on the light source side and disk side are composed of aspherical coefficients up to the fifth order. It was found that good performance can be obtained over a field of view of ±2° when combined with the 0 type.

If the lower limit of Equation 0 is exceeded, it becomes difficult to correct the waviness of spherical aberration, and if the upper limit is exceeded, it becomes impossible to correct the spherical aberration due to the light beam near the effective diameter with an aspherical coefficient that satisfies Equation 0.

Ikkori p Kaifunemasu Examples of the present invention will be shown below.

In each example, r9++ r, □ is the radius of curvature of the cover glass (g) of the semiconductor laser +
rl+r2 is the paraxial radius of curvature of the single lens (L) on the light source side and the disk side + rlil+rp2 is the radius of curvature of the disk (P). d9 is cover glass (g)
core thickness, do is from cover glass (g) to single lens (L)
dl is the axial core thickness of the single lens.

d2 is the air distance from the single lens (L) to the disk (P), and dp is the core thickness of the disk (P). N.

N, , Np are the refractive indices of the cover glass (g), single lens (L), and disk (P) at a wavelength of 780 nm, respectively. *mark indicates an aspherical surface.

Its shape is defined by the following formula described above.

(Positive = 1.2, 3...1m,...) Note that NA is the numerical aperture f on the optical disk side, and β is the focal length of the single lens (L).
is the projection magnification.

<Example 1> NA=0.45 f=1 β=-0
,20d90.0769n, 1.4900r9
2 o.

d, 2.5641 rp2 o.

Trigram 11 old [to r+* +-6,5 831- 0.33082 Xl0-' A4
1=0.884685X10゜8s+=0.889
87 XIO' A7+=0.24628 X
IO'r z" Kg = 0.6 A3□-10.16880 X10° A4□-0
,16509XIO”Asz=0.25204
xloo 11+z=0.16688 XIO“1
β=-0,20 A==0 (i≧8) KI=-6,5, 841 f3=-5,75 <Example 2> NA=0.45 f=1.0 β=- 0
, 20 rate radius Supervised Plate M Question Plate 1 Emperor rel
■ d, 0.0769 n, 1.4900dx
O,2564 C11200 Escape 11 old [drink r(*ni+ = 9.0000 1hl= -0,32339xlO-' 8n+
-0,12295xlO"As+=-0,14587x
lO” A?+=0.61624 Xl08r,
*Kt = -0,6000, 8, □--0.16119 X10° A4
□=0.15997 XIO"AS2= 0.2
4281 XIO” 8t;=0.17711
xio"1β=-0,20 A=-0(i≧8) Kl--9,0 to 18=+f 3= 11.25 <Example 3> NA=0.45 f=1.0 β =, 0
．． 20 rate radius supervision plate book plate plate folding rate rg
Io.

d-0,0769n 91.4900 raz C13 di 0.2564 rpl　　　　　ci.

dpO, 3077npl, 5722 rpH00 way 111 [drinkr, * Kl = 4.5000 A31 = 0.11543
0.45i543Xto”As+=0.3351
0xto.

r z * K t' = 0.600083z=
0.66998 Xl0-' 8,,=0.3
9856 XIO'A5□=-0,38772X10
0 β--0,20 A,=0 (i≧6) K,=-4,5 K rAa+ f'-2,05 <Example 4> NA=0.45 f = 1.0 β--0
,201 吋1 1d11 Plate Goko dz O,2564 rp2 ω Inarihokan r ＋＊に+ = 4.0000 As+= 0.13845 xlO-' AJl=
0.40944 xlO'As+= -0,28899
xlO8 rz*Kt= 0.6000 A32= 0.98843 Xl0-'
A<z"=0.83276 xto-'Asz=
0.14685 −
0,20111 Aikamizato MIN 1 refraction rate rpz 00 Kaya 111 [defeat r I” Kl = 4.0000＾31= 0
．． 21025 xto-'Aa+=0.5042
3
10183XIO' A42-0.24965X
l0-'A5□= 0.40151 xlO-'β
--0.20 A==0 (+≧6) K, --4,0 KI441 f' --2.02 <Example 6> NA=0.45 f = 1.0 β=-
0,20fpz o.

11 [defeat r, *Kl = -3,6000 A:++= 0.69666 xio-” A41
=0.39331 XIO'As+ = 0.264
36 X 100, * Kg = 1.0000 A32 = 0.34580 Xl0-'
Aai=0.13549 XIO'Asz=0.
81036 XIO bow β--0,20 Voyage = O (i≧6) K+ = 3.6 KIA41 f ” = 1.4 <Example 7> NA = 0.45 f = 1.0 β = -0
, 24dz O, 2564 rpl o.

d, 0.3077 n, 1.5722rp
z o.

Saratrapara r + * Kl = -6,700 111 + = 0.26355 Xl0-'
A41=0.89648 XIO'As+=-0,
93948xlO'A,=0.30484xl
O"ri" Kt = 0.600 Aa*= 0.10347 xlO'A<z'
=0.12659xlO”Asz”=0.189
58 XIO” A?z=0.25787 X20”
β=-0,24 A, Mi0 (i≧8) K+=6.7 KIA4. ry=~6. O FIG. 1 shows an optical path diagram of the lens configuration of the present invention, and FIGS. 2 to 8 show various aberrations of Examples 1 to 7 of the present invention, respectively.

In FIG. 1, (g) is a cover glass, (L) is a single lens, and (P) is a disk. Further, in FIGS. 2 to 8, a solid line 780 represents spherical aberration when the wavelength of the light beam is 780 nm. (DM) and (DS) indicate astigmatism on the meridional plane and the sagittal plane.

[Brief explanation of the drawing]

FIG. 1 is an optical path diagram of the objective lens for an optical disc of the present invention, and FIGS. 2, 3, 4, and 5. FIG. 6, FIG. 7, and FIG. 8 are aberration diagrams of each example.

Claims (2)

[Claims] (1) An objective lens for an optical disc, which is a single lens having positive refractive power on the light source side and the disc side and composed of an aspherical surface, the aspherical surface satisfying the following conditions. ▲There are mathematical formulas, chemical formulas, tables, etc.▼ (i=3, 4, 5,...) (k=1, 2) -0.25<β<-0.15 A_i_k=0(i≧8, k=1 , 2) -10<K_1<-3.5 -12<K_1A_4_1f^3<-1 However, X_k is the distance in the optical axis direction from the plane perpendicular to the optical axis at the intersection of the k-th surface with the optical axis, h is the distance perpendicular to the optical axis of the aspherical surface, and Cok is the paraxial curvature of the k-th aspherical surface (=1/r
_R), K_k is the conic constant of the k-th surface, A_i_k is the k-th
The aspherical coefficient of the surface, β is the imaging magnification of the single lens, K_1 is the conic constant of the aspherical surface on the light source side, and A_4_1 is the 4 of the aspherical surface on the light source side.
The next aspherical coefficient, f, is the focal length of the single lens. (2) Claim 1, wherein the A_i_k and K_1 are respectively A_i_k=0 (i≧6, k=1, 2) −4.5<K_1<−3.5 objective lens for optical discs.