JPS6123109A - Objective lens for optical information recording and reproduction - Google Patents

Abstract

PURPOSE:To compensate aberrations excellently and to reduce the size and weight of the lens with a small number of lens elements by allowing the lens consisting of four groups in a specific shape to meet various specific requirements. CONSTITUTION:The 1st lens L1, the 2nd lens L2, the 3rd lens L3, and the 4th lens L4 are arranged successively from the side of a laser diode 1 across a beam splitter 2. The 1st lens L1 is a positive lens having the small-curvature convex surface opposite the beam splitter 2 and the 2nd lens L2 is a positive lens having the large-curvature convex surface opposite the 1st lens L1; and the 3rd lens L3 is a negative meniscus lens having the large-curvature concave surface opposite the 2nd lens L2 and the 4th lens L4 is a positive lens having the large- curvature convex surface opposite the 3rd lens L3. When focal lengths of said respective lenses are denoted as f1-f4, radii of curvature are denoted as r1-r8, and the focal length of the whole length is denoted as F, the focal lengths and radii of curvature of the respective lens and the focal length of the whole system are so determined as to satisfy conditions shown by inequalities.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an objective lens for optical information recording and reproduction.

(Prior art) For example, an optical pickup optical system for optical reproduction and recording of a digital audio system is configured such that a semiconductor laser is used as a light source, and the laser oscillation side is followed by a collimating system, a beam splitter, and an objective lens. A beam splitter, a collimating system, and an objective lens are arranged in this order, and lens aberrations are corrected independently for each of the collimating system, beam splitter, and objective lens.

(Problems to be Solved by the Invention) When correcting aberrations for each element constituting the optical system for optical pickup, the number of components of the optical system increases. It will be difficult to achieve this goal.

Furthermore, since the working distance must be increased, the focal length of the lens system cannot be shortened, which also hinders the reduction in size and weight of the optical system for picking up light.

(Means for solving the problem) The collimating system of the optical pickup optical system and the objective lens are considered to be one objective lens system, and the aberrations are corrected by minimizing the number of lenses. conduct.

The objective lens of the present invention, as shown in FIG.
Lens L2, second lens L2, third lens L3 and fourth lens
The lenses are arranged in the order of L4, and the third lens is a positive lens with a weakly convex surface facing the beam splitter 2, the second lens is a positive lens with a strongly convex surface facing toward the second lens, and the third lens is a positive lens with a strongly convex surface facing toward the beam splitter 2.
The lens is composed of a negative meniscus lens with a strongly concave surface facing toward the second lens, and the fourth lens is a positive lens with a strongly convex surface facing toward the third lens.

Note that numeral 3 represents a photodetector, and numeral 4 represents a disk.

The focal length of each lens above is f1 to f4, and the radius of curvature is ri.
~r8, When the focal length of the entire system is F, the focal length of each lens, the radius of curvature, and the focal length of the entire system are determined so as to satisfy the following conditions.

(1) 0.4F< f1/ f4<0.7F
(2) 0.3 < r 5/ f 3 < 0.5
(3) 0.8F<rl <], 3F Each of the above conditions will be explained below.

(1) 0.4F< f 1/ f 4 < 0.7F This condition determines the power of the objective lens for optical information recording and reproducing of the present invention, and is particularly related to spherical aberration and working distance. Therefore, when f L / f 4 exceeds the upper limit of 0.7F, the spherical aberration decreases but the off-axis coma aberration increases, and when it falls below the lower limit of 0.4, the absolute amount of spherical aberration increases and the working distance decreases. becomes smaller, making it difficult to secure a sufficient distance from the disk.

(2) 0.3<r5/f3<0.5.

This condition is for good correction of spherical aberration and comatic aberration, and in a state where r5/f3 exceeds the upper limit of 0.5, the rim part of spherical aberration ( Properly correcting the spherical aberration (the outermost edge of the spherical aberration) will increase the comatic aberration, so if it is smaller than the lower limit of 0.3, the rim of the spherical aberration should be properly corrected using other constituent groups. If this is done, the absolute value of the spherical aberration at the zonal portion (position at 70% of the distance from the center to the rim portion) of the spherical aberration increases.

(3) 0.8F < r 7 < 1.3F This condition is for correcting spherical aberration, and rl is 0.3
If the value exceeds F, spherical aberration will be insufficiently corrected, and the lower limit will be 0.8F.
If it becomes smaller, over-correction will occur, and the processability of the lens will deteriorate.

(Example) rl = 127.900 di = 1.26 nl = 1.76201r2=
9.380 d2=0.20 r3= 7.760 d3 =1.13 n2 =1.76201r4
=-55,830 d4 =Q, 4Q r5 = 6.670 d5 =1.21 n3 =1.78571r6
=-14,960 d6 =0.52 r7= 3.830 d7 =1.50 n4 =1.76201r8
= 101.639 to=5.3. t1=1.2. Nto=1
．． 5111.8. Ntl=1.51118 However,
to = thickness of beam splitter tl: thickness of disk Nto: refractive index of beam splitter Ntl: refractive index of disk (Example 2) r L = 130.0OO di = 1.30 nl = 1.79266r2
= -9,500 d2=0.20 r3 = 8.000 d3 =1.16 n2 =1.79266r4
= -57,600 d4 =0.4+ r5 = -6,880 d5 =1.25 n3 =1.78571r6
= -15,430 d6 =0.54 rl = 3.830 d7 =1.54 n4 =1.79266r8
= 42.800 t □ = 5.3. t 1 = 1.2 (Effects of the Invention) According to the present invention, it is possible to obtain a small and lightweight objective lens for optical information recording and reproducing, which has a small number of lens elements and has a large working distance, in which aberrations are well corrected. be able to.

[Brief explanation of the drawing]

FIG. 1 is a configuration diagram of an objective lens for optical information recording and reproduction showing an embodiment of the present invention, and FIG. 2 shows spherical aberration of embodiment 1,
Diagrams showing astigmatism and coma aberration, FIG. 3 is a three-dimensional diagram of wavefront aberration at the center of Example 1, FIG. 4 is a three-dimensional diagram of wavefront aberration at the periphery of Example 1, and FIG. Example 2
Figure 6 is a three-dimensional diagram of wavefront aberration at the center of Example 2, and Figure 7 is a three-dimensional diagram of wavefront aberration at the periphery of Example 2. . ;〉1teshii [;Gchufumaqnuleft NA-U, f Ryo GtD Ryo? 2

Claims (1)

[Scope of Claims] An optical information recording/reproducing objective lens consisting of four elements in four groups with a beam splitter interposed between the laser diode light source and the first group in order from the laser diode light source side. The second group is a biconvex lens with a convex surface with a small radius of curvature facing the light source.The third group is a negative meniscus lens with a concave surface with a small radius of curvature facing the light source. The fourth group is composed of a positive lens with a convex surface with a small radius of curvature facing the light source side, and is an objective lens for optical information recording and reproducing that satisfies the following conditions. (1) 0.4F<f1/f4<0.7F (2) 0.3<r5/f3<0.5 (3) 0.8F<r7<1.3F where fi is the focus of the i-th group lens The distance ri is the radius of curvature F of the i-th surface is the composite focal length of the entire system.