JPS61240215A - Lens for optical disk - Google Patents

Abstract

PURPOSE:To constitute the titled lens so that the sensitivity of an aberration change against a parallel eccentricity is small, also the out-of-axis performance is good, and it is strengthened against an inclination of the lens, by making both the first surface and the second surface have a shape of a rotation symmetry against the optical axis, and also aspherical so that a specified surface shape is shown. CONSTITUTION:The titled lens consists of one piece of biconvex lens, and both its first surface and second surface have a shape of a rotation symmetry against the optical axis. Also, it is made aspherical so that its surface is shown by an expression. Moreover, the following three conditions, namely, 0.1<=K1<0.21, 0.74<(n1-1)f/r1<0.80, and 0.70<d1/f<0.75 are satisfied. In this regard, K1, r1, n1, (f), and d1 denote K of the first surface, (r) of the first surface, a refractive index of the lens, a focal distance of the lens, and an interval between the first surface and the second surface, respectively. In this way, this lens has a good out-of-axis performance by which it can be used against an incident angle of + or -1.9 deg., and also the allowance against a parallel shift of the first and the second surface of the time of forming becomes large enough.

Description

Detailed Description of the Invention a. Technical Field The present invention relates to a compact and high-performance optical disk lens, and more specifically, a large-diameter aspherical lens 1 whose both surfaces are composed of aspherical surfaces with positive refractive power. This invention relates to a lens for optical disks consisting of a plurality of lenses.

b. Prior art and its problems The lens used for the optical disc has a numerical aperture NA of 0.4.
It must have a large aperture ratio of ~0.5, and it must also be corrected so that residual aberrations are within the diffraction limit. Also, in actual use of the lens, off-axis (±
1 to 2 degrees) must also be corrected so that it falls within the diffraction limit.

Conventionally, lenses for this purpose were composed of three to five spherical glass lenses, but due to the demand for lower cost, smaller, and lighter lenses, we developed a lens of this type with aspherical surfaces on both sides. A lens for optical disks made up of two lenses has been proposed (Japanese Patent Application Laid-open No. 57-76512, No. 58-68).
711. 59-23313. 59-26714).

However, lenses that use aspherical surfaces generally have a wide angle of view, as the optical performance deteriorates significantly due to eccentricity of the surface.
It has been difficult to create a lens that has a sufficiently large working distance and also has a large tolerance for parallel decentering between the first and second surfaces.

C1 Purpose The present invention is an aspherical single lens having positive refractive power on both the first and second surfaces, which has low sensitivity to aberration changes with respect to parallel eccentricity of the first and second surfaces, and which has off-axis refractive power. The object of the present invention is to provide a lens for an optical disk that has good performance and is resistant to lens tilt.

Solution to Problem d1 The optical disk lens of the present invention is composed of one biconvex lens, and both the first and second surfaces have shapes that are rotationally symmetrical with respect to the optical axis, and moreover, the lens has the following formula: It is an aspherical surface whose surface shape is expressed by, where x(h): Length of a perpendicular drawn from a point on the aspherical surface at a height h from the optical axis to a plane tangent to the apex of the aspherical surface h: From the optical axis height r: radius of curvature near the apex of the aspherical surface: conic constant A2i: 21st order (i is an integer from 2 to 5) aspherical coefficient and satisfies the following conditions (1) to (3): It is characterized by

0, 11 Kl (0, 21 (1) Q, 70(di/f(0,75(3) however 1: K on the first side rl: r of the first side nl: refractive index of lens f: Lens focal length dl: Distance between the first and second surfaces (lens thickness) 00 action Each of the above conditions will be explained below.

Condition (1) is a condition that defines the aspheric amount of the first surface, but when Kl is smaller than the lower limit, off-axis performance cannot be satisfied when reducing the sensitivity of aberration fluctuation to surface eccentricity. . On the other hand, when 1 is larger than the upper limit, the off-axis performance cannot be improved because the sine condition deteriorates. Furthermore, if the aberrations are corrected forcibly using high-order aspherical coefficients, aberration fluctuations due to eccentricity of the surface will increase, which is contrary to the purpose of the present invention.

Condition (2) is a condition that defines the balance between rl and r2 (r2 is r of the second surface), and if lrl becomes larger than the lower limit of condition (2), the change in aberration due to eccentricity should be reduced. However, it is not practical if the working distance is small or the tolerance for disk warping is small. On the other hand, if r is smaller than the upper limit of condition (2), the spherical aberration without the aspheric surface will be too strong, and the spherical aberration cannot be corrected unless the aspheric amount is increased. This will lead to deterioration of off-axis performance.

Condition (3) is a condition that defines the lens thickness.
), astigmatism cannot be corrected well and the effective angle of view becomes narrower. On the other hand, anything exceeding the upper limit is inappropriate because the working distance is too short.

f. Examples Examples of the present invention are shown below. Here, NA is numerical aperture,
f is the focal length, r is the radius of curvature of each surface, and d is the distance between the first surface and the (
i+1 ) surface spacing, ν is the refractive index, WD is the working distance, K1 is the conic constant of the first surface, K2 is the conic constant of the second surface, A 1.2 i is the 21st-order aspheric surface of the first surface The coefficient A 2,2 i is the 21st order aspheric coefficient of the second surface.

[Example 1] NA=0.45 f=4.500 Usable incident angle ±1.9°r d n (wavelength 800n
m) '2.853 3.298
1.48400-5.734 2.003 (
WD)oo 1.200
1.50000 (disk■
Protective layer) Aspheric coefficient K 10.110 K 2 1.340AI
4 0.406X10” A24 0.914X
10 "Al6-0.420X10-" Azs -
0,968X10-” At s O,148X10-
'A2 g 0.140X10-”Ax to
O,132Xl0-' A41G 0.390X
10' [Example 2] NA=0.45 f=4.507 Usable incident angle ±1.9° r d n (wavelength 800 nm
)2.828 3.265 1.48400-
-5.946 2.010 (WD)oo
1.200 1.50000 (disc protective layer) Aspheric coefficient to 10.11, 1.320 A14-0.397X10-2A24 0.906X
10-2A11o0.200X10-' A2 t
. 0.374X10-' [Example 3] NA = 0.45 f = 4.50
6 Usable incident angle ±1.9°r d n (wavelength 800n
m) 2.849 3.289 1.48400
-5.799 2.009 (WD)oo
1.200 1.50000 (disc protective layer) Aspheric coefficient 10.195 K22.200A□, -0
,445X10-2A240.104X10"-"A. -0,439X10-3A2 a O,141
Xl0-2A□s O,435X10-' A
2 g 0.195X10-”A1□.-〇, 14
7X10' A2to 0.942X10-
'g8 Effect As explained above, the optical disk lens according to the present invention is a double-convex aspherical single lens, and is configured to satisfy each of the above conditions, thereby achieving an incident angle of ±1.9°. It has good off-axis performance that can be used even in the case of molding, and has a sufficient tolerance for parallel misalignment of the first and second surfaces during molding.

[Brief explanation of drawings]

FIG. 1 is a lens configuration diagram of Example 1 of the present invention, FIGS. 2 and 3 are aberration diagrams of Example 1, FIG. 4 is a lens configuration diagram of Example 2 of the present invention, and FIGS. Figure 6 is an aberration diagram of Example 2, Figure 7 is a lens configuration diagram of Example 3 of the present invention, Figures 8 and 9.
The figure is an aberration diagram of Example 2. The unit of the horizontal axis in FIGS. 2 and 5.8 is the cylinder, and the unit of the vertical axis in FIGS. 6 and 9 is λ (wavelength). Further, Y is image height, DS is sagittal astigmatism, and DM is meridional astigmatism (unit: mm). hjpH diagram Figure 2 Sine condition Figure 3 5F plane aberration Figure 4 Figure 5 Positive arc condition 6 (!I Noyama convergence Figure 7 Figure 8 Positive ■ Appetizer Figure 9 *1 Aberration procedure correction Book 11, April 1986) Guhibire

Claims (1)

[Claims] 1. Consisting of one biconvex lens, both the first and second surfaces thereof have a rotationally symmetrical shape with respect to the optical axis, and the surface shape is expressed by the following equation. X(h)=(h^2/r)/{1+√[1-(i+K)
h^2/r^2]}+Σ^5_i_=_2A_2_ih
^2^i However, X(h): Length of a perpendicular drawn from a point on the aspherical surface at a height h from the optical axis to the tangent plane of the aspherical apex h: Height from the optical axis r: Near the aspherical apex radius of curvature K: conic constant A_2_i: 2i-th order (i is an integer from 2 to 5) aspheric coefficient, and an optical disc lens that satisfies the following conditions (1) to (3): . 0.1≦K_1<0.21(1) 0.74<[(n_1-1)/r_1]f<0.80(
2) 0.70<d_1/f<0.75 (3) where K_1: K of the first surface r_1: r of the first surface n_1: Refractive index of the lens f: Focal length of the lens d_1: K of the first surface and the Distance between two surfaces (lens thickness)