JPS6064318A - Objective lens for recording and reproducing light information - Google Patents

Abstract

PURPOSE:To constitute an objective lens which is advantageous to reproduction and recording while extending the working distance and increasing the aperture by providing specific conditions among focal lengths and radii of curvature of three lenses. CONSTITUTION:The objective lens system consists of a biconvex lens as the 1st group, a negtive meniscus lens having a concave surface on an object side as the 2nd group, and a positive meniscus lens having a convex surface on the object side as the 3rd group successively from the object side, and the conditions shown in a figure are satisfied. In the inequalities, (f) is the composite focal length of the whole system, f12 is the composite focal length of the 1st group and the 2nd group, and f2 is the focal length of the 2nd group; and r3 is the radius of curvature of the object-side surface of the 2nd lens group , and r6 is the radius of curvature of the image-side surface of the 3rd lens group. Consequently, the working distance is long, i.g. 0.4-0.48f and the numerical aperture is large, i.e. 0.55-0.60. This lens system is used suitably for the video reproduction and recording of a video disk.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an objective lens with a large numerical aperture (NA) used for recording and reproducing information on an optical information recording medium such as a video disc.

(Prior Art) In recent years, semiconductor lasers have come into use in place of e-Ne lasers, especially as light sources for information reproducing optical systems from optical information recording media such as video discs.
H, -N, NA is 0.45 as an objective for laser.
A lens that can obtain a relatively invisible and diffraction-limited spot (I) was used, but to obtain a spot of the same level as this, a lens with an NA of about 0.55 is required for semiconductor lasers. becomes.

On the other hand, the objective lens is driven for focusing and tracking, and for this purpose it is desirable that the objective lens K be small and one-piece. Lenses with a large NA as mentioned above are also known (for example, Japanese Patent Laid-Open No. 58
-42021 publication) It has a long overall length and consists of 4 pieces.
It cannot be said to be small in size, and a large burden is placed on the driving device of the objective lens.

(Objective of the invention) The present invention has a simple lens configuration, a small lens, a large NA of 0.55 to 0.60, a diffraction-limited O spot diameter, and a working distance. The objective is to obtain a large objective lens.

(Structure of the Invention) As shown in FIG. 1, the lens system of the present invention has, in order from the object side, a first group lens which is a biconvex lens, and a first lens group which is a negative meniscus lens with a concave surface facing the object side. 2nd group lens, 3rd F lens which is a positive meniscus lens with a convex surface facing the object side 2.8f<f, 2<3.4f...(1)-6f
<f2< -3f ... (2) -2,1i <rs
< ”.7f − (3) 1.2f < r6 < 1.
9. f ・(4) However, f The combined focal length of the two-combination system f, 2: The combined focal length of the first group lens and the second group lens f2: The focal length of the second group lens r3: The object side of the second group lens Surface curvature radius r6: 3rd
It is configured to satisfy each condition of the radius of curvature of the side surface of the lens group.

Condition (1) is a condition for securing the necessary working distance.If the working distance increases beyond the upper limit, it becomes easier to increase the working distance, but the power of the third group lens becomes stronger.
Negative spherical aberration is more likely to occur. Even if we try to forcefully correct this, it is not possible to increase υNA because the residual spherical aberration at the center of the curve is large. On the other hand, if it becomes smaller than the lower limit, it becomes impossible to obtain the required working distance.

Condition (2) is a condition regarding the power of the second P lens. If the upper limit is exceeded, the spherical aberration becomes excessive.
If the value exceeds the lower limit, the spherical aberration can be corrected well, but the shape of the second group lens O approaches the outlet link, making processing difficult.

Condition (3) is a condition for correcting spherical aberration.

When the curvature becomes steep beyond the upper limit, spherical aberration becomes overcorrected. When the curvature becomes loose beyond the lower limit, the spherical aberration becomes corrected. Condition (4) is a condition for keeping the sine condition good and obtaining the necessary angle of view. 0 When the curvature becomes loose beyond the upper limit, the sine condition becomes automatic. -, and conversely, if it becomes tighter than the lower limit, the sine condition becomes under.

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

Ri: Lens surface curvature at the 1st distance from the object side d1: Distance between the 12th lens surface from the object side n1: Refractive index of the glass at the 1st w7rth point from the object side νi; tVi from the object side
Eye ○ Glass O Atsube number WD: Working distance M: Iron magnification do; Axial thickness of cover glass (refractive index 1.51072
).

Example 1 f=1. o NA O,60M=O d,=0.3353 W,D,=0.4113f+2=
3.1870 f2=-5,5270 Example 2 f=1. o NA O,55M=O f+2= 3.2790 f2=-4,4212 Example 3 f=1. o NA O,55M=O f,2=2.9412 f2 =-3,5740 (Invention ■ Effect) The objective lens of this invention has a working distance of 0.4 to 048 f.
As is clear from the aberration diagrams of each example shown in Figs. 2 to 4, despite the simple structure of 3 large-mouth bowls with a long NA of 0.55 to 0.60, To,
This can be advantageously used for the purpose of reproducing and recording video discs and the like.

[Brief explanation of the drawing]

FIG. 1 is a sectional view of the first embodiment of the θ objective lens of the present invention.
sg2 to 4 are the first to third examples, respectively.
It is an aberration diagram of an example. Patent applicant Roku Konishi Photography Eimi Ni Company Applicant agent Patent attorney Fumio Sato (and one other person) Figure 3 Astigmatism Spherical aberration Sine condition astigmatism

Claims (1)

[Claims] In order from the object side, the first group lens is a biconvex lens, the second group lens is a negative meniscus lens with a concave surface facing the object side, and the second group lens is a positive meniscus lens with a convex surface facing the object side. From the 3rd group lens, 2.8f <f <3.4f 2 -6f <f2 <-3f -2,1f<r, <-1,7f 1.2f (r, s <L9f) Because of the composite focal length of the thread: Composite focal length of the first group lens and second group lens f2: Focal length of the second group lens r3: Curvature of the object side surface of the second group lens R6: Third
An objective lens for recording and reproducing optical information, characterized in that it satisfies various conditions regarding the radius of curvature of the edge side surface of the group lens.