JPS6233567B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an objective lens for reading high-density signals on an optical disk. Since the objective lens used to read the optical disk must read information recorded in high density, it is required to have a resolution of at least 1 μ. Therefore, the numerical aperture (NA) of the objective lens is required to be approximately 0.5. Moreover, this objective lens is often movable for tracking and focusing, so it is required to be small and lightweight, and the distance between the disk surface and the objective lens (working distance) is larger than a certain level. If not, there may be a practical problem that the objective lens and the disk surface will come into contact with each other, so this working distance is required to be as large as possible. The present invention satisfies the above conditions, provides an objective lens for optical disks with a sufficiently long numerical aperture of 0.5, a sufficiently long working distance of 0.40 to 0.55 times the focal length, and a short overall length that satisfies various aberrations. It is something to do. First, to explain the lens structure of the present invention, it is composed of three lenses in three groups, the first lens is a plano-convex or biconvex positive lens, and the second lens has a concave surface facing the light source side with a positive focal length. The third lens is composed of a positive meniscus lens with a convex surface facing the light source _{side .} is the focal length of the second lens, F ₃ is the focal length of the third lens, Σd is the total length of the lens, ri is the radius of curvature of the i-th lens, di is the thickness of the i-th lens or the distance between the lenses, Nj is the j-th lens distance, This objective lens for optical disks has a short overall length and satisfies the following conditions when the refractive index of the lens is taken as the refractive index. (1) 1.0＜F ₁ ／F ₃ ＜1.2 (2) 20.0＜F ₂ ／F ₃ ＜30.0 (3) ｜r ₂ ｜＞10F, r ₂ ＜0 (4) 0.86F＜Σd＜1.18F Next Each of the above conditions will be explained. Condition (1) is a condition for determining the focal length distribution of the positive lenses, ie, the first lens and the third lens, constituting the lens of the present invention, and for reducing spherical aberration. It is desirable to set the focal length of the first lens and the focal length of the second lens to be approximately the same, and in the lens of the present invention, the height of the ray of light incident on the third lens is higher than the height of the ray of light incident on the first lens. Therefore, the focal length of the third lens is set smaller than the focal length of the first lens to reduce spherical aberration. Now, when F ₁ /F ₃ is smaller than the lower limit of 1.0, the focal length of the first lens becomes too small and spherical aberration becomes large. Also the upper limit
When it is larger than 1.2, the focal length of the third lens becomes too small and spherical aberration becomes large. Condition (2) is a condition for determining the focal length of the second lens, regulating the amount of positive spherical aberration generated in the second lens, and properly correcting the spherical aberration. now,
When F ₂ /F ₃ is smaller than the lower limit of 20.0, the focal length of the second lens becomes too small, the amount of positive spherical aberration generated in the second lens becomes small, and the amount of positive spherical aberration generated in the first and second lenses decreases. It becomes difficult to correct negative spherical aberration. On the other hand, when the upper limit is greater than 30.0, the focal length of the second lens becomes too large, and the amount of positive spherical aberration generated by the second lens becomes large, resulting in an increase in spherical aberration. Condition (3) is a condition for good correction of spherical aberration. In the lens of the present invention, the overall length of the lens is shortened, and the radius of curvature of _r3 is extremely small in order to obtain an appropriate working distance. Therefore, the amount of positive spherical aberration generated at _r3 increases. The condition for correcting this is (3). Now, when |r ₂ | is smaller than the lower limit of 10F, the amount of negative spherical aberration generated at r ₂ is too large, and the spherical aberration cannot be corrected well. Furthermore, when r ₂ is positive, the amount of positive spherical aberration generated at r ₃ is too large, resulting in an overcorrected state of spherical aberration, which is undesirable. Condition (4) is a condition for keeping the lens system small. If Σd is smaller than the lower limit of 0.68F, the lens thickness and edge thickness will become too small, making it difficult to manufacture, which is not preferable. Also, the upper limit
If it is larger than 1.18F, there will be no manufacturing problems, but the lens will become larger and heavier, which is not preferable. Based on the meaning of each of the conditions explained above, the present invention is configured to satisfy each of these conditions, thereby achieving a short total length light beam that has a sufficiently long working distance and that satisfactorily corrects various aberrations, especially spherical aberration. An objective lens for a disk can be obtained. The numerical values of Example 1, Example 2, and Example 3 of the objective lens of the present invention are shown below. However, Wt is the air distance (working distance) between the objective lens and the disk cover glass, t is the thickness of the cover glass, and Nt is the refractive index of the cover glass. Note that all refractive indices of glass are values at 790 nm.

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【table】 [Brief explanation of the drawing]

FIGS. 1, 3, and 5 are lens configuration diagrams of Examples 1, 2, and 3 of the present invention, respectively, and FIGS. 2, 4, and 6 are examples of the present invention including a cover glass, respectively. 1, 2, and 3 are comatic aberration and wavefront aberration diagrams.

Claims (1)

[Claims] 1. Consisting of a lens consisting of three elements in three groups, the first lens is a plano-convex or biconvex positive lens, the second lens is a negative meniscus lens with a positive focal length and a concave surface facing the light source side, The third lens is a positive meniscus lens with a convex surface facing the light source, and is an objective lens for optical disks with a short overall length that satisfies the following conditions. (1) 1.0＜F ₁ ／F ₃ ＜1.2 (2) 20.0＜F ₂ ／F ₃ ＜30.0 (3) ｜r ₂ ｜＞10F, r ₂ ＜0 (4) 0.86F＜Σd＜1.18F Here F is the combined focal length of the entire lens system, F ₁ is the focal length of the first lens, F ₂ is the focal length of the second lens, F ₃ is the focal length of the third lens, and r ₂ is the second lens of the first lens. The side radius of curvature, Σd, is the total length of the lens.