JPH0143290B2 - - Google Patents

Description

[Detailed description of the invention]

a Technical field The present invention relates to an optical disk (high-density information recording medium)
This relates to lenses for use in b. Prior art and its problems Lenses used to read high-density information recorded on optical disks must read high-density signals, so their resolution is required to be about 1 μm. In the past, many optical disk lenses have been invented to meet this requirement, but most of them only correct aberrations at a single wavelength. In cases where the light source (laser light in most cases) is stable, or in optical systems (products) where changes in lens imaging performance due to slight wavelength fluctuations can be ignored, aberration correction at a single wavelength is sufficient. However, in an optical system where the light source is unstable and the wavelength fluctuates, and the resulting fluctuations in aberrations cannot be ignored, the above requirements cannot be satisfied only by aberration correction at a single wavelength. c. Purpose The present invention aims to provide an optical disk lens that corrects aberrations not only at a single wavelength but also at multiple wavelengths and minimizes changes in aberrations due to fluctuations in the wavelength of a light source. d Means for solving the problem The optical disk lens of the present invention has the following advantages:
a first group lens of a cemented lens of a negative lens and a positive lens or a cemented lens of a positive lens and a negative lens;
The second negative meniscus lens with its concave surface facing the light source.
The lens is composed of four lenses in three groups, consisting of a group lens and a third group lens which is a positive meniscus lens with its convex surface facing the light source side, and is characterized by satisfying the following conditions. (1) 1.5f＜f _I <3.5f (2) ｜ν ₊ −ν _- ｜＞20 (3) ｜r ₂ ｜＜f Here, f is the composite focal length of the entire lens system, and f _I is the first The focal lengths of the group lenses, ν ₊ and ν ₋ are the Abbe numbers of the positive and negative lenses in the first group lens, and r ₂ is the radius of curvature of the bonding surface of the first group lens. e Effect Next, each of the above conditions will be explained. Condition (1) is a condition for good correction of spherical aberration, and when f _I is smaller than the lower limit, the first
The amount of negative spherical aberration generated in the group lens becomes too large, making it difficult to correct the spherical aberration. On the other hand, when it is larger than the upper limit, the amount of negative spherical aberration generated in the first group lens becomes small, but the burden on the third group lens increases, and as a result, it becomes difficult to correct the spherical aberration, which is not preferable. Condition (2) is also related to condition (3) and is a condition for good correction of aberrations at multiple wavelengths, that is, chromatic aberration, which is a feature of the present invention _.
When ν _- | is smaller than the lower limit, the effect on correction of chromatic aberration is too small, making it difficult to satisfactorily correct aberrations at a plurality of target wavelengths. Condition (3), like condition (2), is a condition for good correction of chromatic aberration, and when |r ₂ | is larger than the upper limit, the positive lens and negative lens in the first group lens As a result, the achromatic effect is reduced, chromatic aberration correction becomes insufficient, and it becomes difficult to correct aberrations at a plurality of target wavelengths. f Example Hereinafter, numerical values of Example 1 to Example 4 in the present invention will be described. Here, f is the focal length,
NA is the numerical aperture, ω is the half angle of view, f _B is the back focus, r is the radius of curvature of each lens surface, d is the lens thickness or distance between lenses, N is the refractive index of each lens, and ν is the Abbe number of each lens. .

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[Table] g Effect As explained above, the present invention is a lens composed of 4 elements in 3 groups, and by satisfying each of the conditions (1) to (3) above, various aberrations (especially chromatic aberration) can be suppressed. It is possible to provide an optical disk lens that can be corrected to 1μ, has a resolution of about 1μ, and has very little performance change due to wavelength fluctuations.

[Brief explanation of drawings]

Figure 1 is a lens configuration diagram of Example 1 of the present invention, Figure 2 is a diagram of various aberration curves of Example 1, Figure 3 is a lens configuration diagram of Example 2 of the invention, and Figure 4 is Example 2. 5 is a diagram of the lens configuration of Example 3 of the present invention, FIG. 6 is a diagram of various aberration curves of Example 3, and FIG. 7 is a diagram of the lens configuration of Example 4 of the present invention. FIG. 8 is a diagram showing various aberration curves of Example 4.

Claims (1)

[Claims] 1. From the light source side, a first lens group of a cemented lens of a negative lens and a positive lens or a cemented lens of a positive lens and a negative lens, and a second lens group of a negative meniscus lens with its concave surface facing the light source side. A lens for an optical disk, which is a lens consisting of four elements in three groups, consisting of a group lens and a third group lens which is a positive meniscus lens with its convex surface facing the light source side, and which satisfies the following conditions. (1) 1.5f＜f _I <3.5f (2) ｜ν ₊ −ν _- ｜＞20 (3) ｜r ₂ ｜＜f Here, f is the composite focal length of the entire lens system, and f _I is the first The focal lengths of the group lenses, ν ₊ and ν ₋ are the Abbe numbers of the positive and negative lenses in the first group lens, and r ₂ is the radius of curvature of the bonding surface of the first group lens.