JPS633284B2 - - Google Patents

Description

[Detailed description of the invention]

Technical Field The present invention relates to a lens for optical disks (high-density information recording media). Prior art and its problems The objective lens used to read high-density information recorded on optical disks must read high-density signals, so its resolution is 1μ.
degree is required. Many objective lenses for optical disks have been invented in the past to satisfy this requirement, but most of them only correct aberrations at a single wavelength, and the light source (in most cases laser light)
For optical systems (products) where the light source is stable or where changes in lens imaging performance due to slight wavelength fluctuations can be ignored, aberration correction at a single wavelength may be sufficient. In an optical system where there is a fluctuation in wavelength and the resulting fluctuation in aberration cannot be ignored, it is not possible to satisfy the above requirements only by correcting the aberration of a single wavelength. 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. Means for Solving Problems The optical disk lens of the present invention is composed of 5 lenses in 4 groups.From the light source side, the first group is a cemented lens of a positive lens and a negative lens, and the second group has a concave surface facing the light source. The third group is a positive lens, and the fourth group is a positive meniscus lens with its convex surface facing the light source, and is characterized by satisfying the following conditions. (1) ｜ν ₁ −ν ₂ ｜≧20 (2) 0.6f＜｜r ₂ ｜＜1.1f (3) ｜r ₄ ｜＜2f, r ₄ ＜0 (4) n ₅ ＞1.7 Here, f is The combined focal length of the entire lens system, r _i is the radius of curvature of the i-th surface, d _i is the thickness or air gap of the i-th lens, n _j is the refractive index of the j-th lens,
ν _j is the Abbe number of the j-th lens. Effect Next, each of the above conditions will be explained. Condition (1) is a condition for appropriately correcting changes in aberration due to wavelength fluctuations, which is a feature of the present invention, that is, chromatic aberration. Now, when |ν ₁ −ν ₂ | is smaller than the lower limit, , the effect on correction of chromatic aberration is too small, making it impossible to reduce changes in aberration due to changes in the target wavelength. Condition (2) is related to condition (1) and is a condition for appropriate correction of chromatic aberration, and when |r ₂ | is larger than the upper limit, the achromatization by the positive and negative lenses of the first group is The effect becomes smaller, the correction of chromatic aberration becomes insufficient, and it becomes impossible to reduce the change in aberration due to the desired wavelength variation. Also, |r ₂ |
When is smaller than the lower limit, it is advantageous for correcting chromatic aberration, but |r ₂ | becomes too small, which not only makes it difficult to process, but also leads to overcorrection of spherical aberration. Condition (3) is a condition for good correction of spherical aberration, and when |r ₄ | is larger than the upper limit, the amount of positive spherical aberration generated at r ₄ becomes too small, and as a result, the spherical This results in insufficient correction of aberrations. Condition (4) is also a condition for good correction of spherical aberration, and when _n5 is smaller than the lower limit, the amount of negative spherical aberration generated in the fourth group lens increases, resulting in insufficient correction of spherical aberration. I end up. EXAMPLE Below, numerical values at the total system focal length f=1 of Example 1 and Example 2 of the present invention will be shown. Example 1

【table】

【table】 Example 2

[Table] Effects As explained above, the present invention satisfies the conditions (1) to (4) above in a lens composed of 5 elements in 4 groups, and thereby satisfies various aberrations (especially chromatic aberration). The present invention makes it possible to provide an optical disk lens that has a resolution of about 1 μm and exhibits very little change in performance due to wavelength fluctuations.

[Brief explanation of the drawing]

Figure 1 is a diagram of the lens configuration of Example 1, Figure 2 is a diagram of various aberration curves of Example 1, Figure 3 is a diagram of the lens configuration of Example 2, and Figure 4 is a diagram of various aberration curves of Example 2. be.

Claims (1)

[Claims] 1 Consists of 5 lenses in 4 groups, the 1st group is a cemented lens of a positive lens and a negative lens, the 2nd group is a negative meniscus lens with its concave surface facing the light source side, and the 3rd group is a positive lens. 1. A lens for an optical disk, characterized in that the fourth lens group is a positive meniscus lens with a convex surface facing the light source side, and satisfies the following conditions. (1) ｜ν ₁ −ν ₂ ｜≧20 (2) 0.6f＜｜r ₂ ｜＜1.1f (3) ｜r ₄ ｜＜2f, r ₄ ＜0 (4) n ₅ ＞1.7 Here, f is Combined focal length of the entire lens system, ν ₁ , ν ₂
is the Abbe number of the positive lens and negative lens of the first group lens, _r2 is the radius of curvature of the cemented surface of the first group lens, _r4 is the radius of curvature of the surface of the second group lens on the light source side, and _n5 is the radius of curvature of the surface of the second group lens on the light source side.
This is the refractive index of the group lens.