JPS6233566B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an objective lens with a large NA for recording and reproducing information on a magneto-optical disk surface. Conventionally, objective lenses for magneto-optical disks are
Those proposed in Japanese Patent Publication No. 52-44209 and Japanese Patent Application Laid-Open No. 54-127339 are known. However, with these known objective lenses,
The problem is that the NA is low at around 0.35 and the brightness is not sufficient. Furthermore, these objective lenses
The working distance is short, and as the NA increases, this working distance becomes even shorter, making it difficult to control autofocus and the like. The purpose of the present invention is to provide an objective lens for a magneto-optical disk that can have a large NA of 0.8 to 0.9, has a long working distance of 1 to 1.5 mm, can be sufficiently controlled, and is small and lightweight. be. Such an objective lens enables high-density recording and reproduction on magneto-optical disks such as video disks and digital auto disks. The present invention will be explained below. The objective lens for magneto-optical disks of the present invention has two
Group composition. The first group is composed of two lenses and is arranged on the object side. The second group is composed of four lenses and is arranged on the image side. One of the two lenses constituting the first group is a positive meniscus lens, and the other is a negative meniscus lens. The positive meniscus lens is placed on the object side, and the negative meniscus lens is placed on the image side of the positive meniscus lens with its concave surface facing the object side. Two of the four lenses constituting the second group constitute a convex lens cemented with a convex face toward the object, and the other two
The lens is a positive meniscus lens. The lenses constituting the second group are arranged in the order of a convex lens, a positive meniscus lens, and a positive meniscus lens from the object side to the image side. All positive meniscus lenses are arranged with their convex surfaces facing the object side.
The above convex lens cemented with a convex surface facing the object side is a cemented convex lens in which a biconvex lens with a convex surface facing the object side and a negative lens with a concave surface facing the object side are cemented in order from the object side. Now, in such an objective lens, the focal length of the first group is f ₁ , the focal length of the second group is f ₂ , the focal length of the entire system is f, and the 5th, 6th, and 10th lens surfaces from the object side are The radius of curvature is r ₅ , r ₆ , r ₁₀ , third from the object side,
When the refractive index of the fourth lens for light with a wavelength of 8200 Å is n ₃ and n ₄ , this objective lens satisfies the following four conditions. (i) 0.35＜｜r ₆ ｜ / r ₅ ＜0.65 (ii) 0.65＜r ₁₀ /f＜1.0 (iii) 0.86＜n ₃ /n ₄ ＜0.96 (iv) 0.05＜f ₂ /｜f ₁ ｜＜ 0.15, f ₁ <0 Here, r ₆ corresponds to the radius of curvature of the cemented surface of the cemented convex lens in the second group, and
The fourth and fourth lenses are lenses that are cemented together to form a convex lens. Condition (i) is a condition for maintaining a good sine condition; if the upper limit is exceeded, the positive limit condition will be undercorrected, and if the lower limit is exceeded, the positive limit condition will be overcorrected. Condition (ii) is a condition for maintaining good spherical aberration; when the upper limit is exceeded, the spherical aberration is under-corrected, and when the lower limit is exceeded, the spherical aberration is over-corrected. Condition (iii) is a condition for maintaining good off-axis aberrations and AS (astigmatism); if the upper limit is exceeded, these amounts will be insufficiently corrected, and if the lower limit is exceeded, the correction will be overcorrected. . Condition (iv) is a condition for maintaining a large working distance. If the upper limit of condition (iv) is exceeded, the load on the rear group becomes large and it becomes difficult to correct each aberration. Moreover, when the lower limit is exceeded, the working distance becomes small, making it difficult to control operations such as autofocus. By satisfying the above conditions, as shown in the example, the desired NA is extremely large at 0.8 to 0.9, the working distance is 1 to 1.5 mm, the size is 9 to 10φ, the length is 17 mm, and the weight is 1 g. An objective lens was obtained. FIG. 1 shows a first embodiment. In the figure, the symbol L ₁ ,
L ₂ , L ₃ , L ₄ , L ₅ , and L ₆ are lenses, G is a cover glass, and D is a magneto-optical disk.
Further, the symbol LA indicates the optical axis. Lens L ₁ , L ₂
constitutes the first group, and lenses L ₃ , L ₄ , L ₅ , and L ₆ constitute the second group. The left side of the figure is the object side. The symbols r ₁ to r ₁₁ are the radius of curvature of each lens surface of the objective lens, the symbols r ₁₂ and r ₁₃ are the radii of curvature of the front and back surfaces of the cover glass G, and the symbols d ₁ to d ₁₀ are the distance between the lens surfaces of the objective lens, Symbol d ₁₂ is the distance between the front and back surfaces of the cover glass G, and symbol d ₁₁ is the optical axis LA between the image-side lens surface of the lens L ₆ and the surface of the cover glass G.
It shows the distance above. Specific numerical values of these amounts are as follows.

[Table] Here, n ₁ to n ₆ are the refractive index of each lens L ₁ to L _L , n _G is the refractive index of the cover glass, and υ ₁
to υ ₆ indicates the Atsube number of each lens. A second embodiment is shown in FIG. 2, with symbols L' ₁ to L' ₆
indicates a lens, the other symbols mean:
It is the same as in FIG. The specific numerical values in this example are as follows.

【table】

[Table] Fig. 3 shows the third embodiment, and the symbols L″ ₁ to
L″ ₆ indicates a lens, the meanings of other symbols are the same as in FIG.

[Table] FIG. 4 shows an aberration diagram for the first embodiment, FIG. 5 shows an aberration diagram for the second embodiment, and FIG. 6 shows an aberration diagram for the third embodiment. Also, FIG. 7 shows a contour map of wavefront aberration for the first example, FIG. 8 shows a contour map of wavefront aberration for the second example, and FIG. 9 shows a contour map of wavefront aberration for the third example. . Among these diagrams related to wavefront aberration, the left diagram shows the image height h=0, that is, on the optical axis, and the right diagram shows the wavefront aberration in the peripheral area as a contour diagram of the pupil position.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing a first embodiment of the invention, FIG. 2 is a diagram showing a second embodiment of the invention, FIG. 3 is a diagram showing a third embodiment of the invention, and FIGS. FIG. 6 is an aberration diagram, and FIGS. 7 to 9 are contour diagrams of wavefront aberration. L ₁ , L ₂ , ..., L ₆ ... lens, G ... cover glass, D ... magneto-optical disk.

Claims (1)

[Claims] 1. A first group is arranged on the object side and a second group is arranged on the image side. The first group includes a positive meniscus lens on the object side and a negative lens on the image side with the concave surface facing the object side. Consisting of two meniscus lenses, the second group consists of a double convex lens with a convex surface facing the object side, a negative lens with a concave surface facing the object side, and a cemented convex lens with a concave surface facing the object side. It is constructed by arranging four lenses, a convex positive meniscus lens and a convex positive meniscus lens on the object side, in the above order from the object side to the image side, and the focal length of the first group is f ₁ and the focal length of the second group is f 1 . focal length of
f ₂ , the focal length of the entire system is f, the radius of curvature of the 5th, 6th, and 10th lens surfaces from the object side is r ₅ , r ₆ , r ₁₀ , the refraction of the 3rd and 4th lenses from the object side rate (wavelength
(for 8200 Å light) as n ₃ and n ₄ , () 0.35<|r ₆ |/r ₅ <0.65 () 0.65<r ₁₀ /f<1.0 () 0.86<n ₃ /n ₄ <0.96 () 0.05 An objective lens for a magneto-optical disk, characterized in that it satisfies the following conditions: <f ₂ /|f ₁ |<0.15, f ₁ <0.