JPH08334686A - Objective lens for recording and reproducing optical information recording medium - Google Patents

Abstract

PURPOSE: To make the amt. of the occurrence of aberrations virtually negligible in spite of a fluctuation in an inter-object distance by specifying the conditions of a biconvex single lens of an aspherical shape on both of a light source side and an optical information recording medium side. CONSTITUTION: This objective lens for recording and reproducing of the optical information recording medium of finite system specifications is 0.5<NA<0.65 (where NA is the numerical aperture on an image side) and is the biconvex single lens of the aspherical shape on both the light source side and the optical information recording medium side. The objective lens satisfies the conditions expressed by 0.05<=|m|<=0.135, 0.75<r1 / (n-1)f√(1+|m|)}<1.5, equations I, II. In the equation I, (m) is an imaging magnification; r1 is the radius of curvature at the vertex on the light source side face; (n) is the refractive index of the lens; (f) is the focal length of the lens; δ1 (δ2 ) is a difference in the optical axis direction between the aspherical face on the extreme periphery of the effective diameter on the light source (image) side face and the aspherical lens in the position on the light source (image) side face where the rays of NA 0.45 are made incident; the case where the aspherical face on the extreme periphery of the effective diameter is displaced nearer the image side than the aspherical phase at NA 0.45 is defined positive; b(m) is the function of the imaging magnification (m) and is expressed by b(m)=|m|+0.33.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a large-diameter objective lens for an optical disc, and more particularly to a finite conjugate type double-sided aspherical biconvex single lens suitable for use when the distance between a light source and an information recording surface is relatively small. .

[0002]

2. Description of the Related Art In a recording / reproducing apparatus for an information recording medium such as an optical disk, an optical system for directly focusing light from a light source such as a semiconductor laser without using a collimator lens is effective for simplifying the entire reproducing apparatus. Is. In the field such as CD (Compact Disc), CD-ROM, etc., in which an objective lens having an image-side NA of about 0.45 is required, the finite conjugate type single lens has already been widely used. Disc), DVD (digital video disc),
In fields such as MO (magneto-optical) where NA on the image side exceeds 0.5, an infinite conjugate objective lens that generally requires a collimator lens is often used. . However, in this method, the number of parts is larger than that of the finite-conjugate type optical system and the configuration is complicated, so that the cost is inevitably high.

[0003]

In a finite conjugate type optical system which does not use a collimator lens as shown in FIG.
Distance from light source 3 to optical information recording surface 1 (distance between object images) U
Is determined by design, but in reality, a deviation ΔU occurs in this distance U due to the deflection of the optical information medium or the like. Currently, in this type of apparatus, when a deviation ΔU occurs within a usable magnification range, the objective lens 2 moves along the optical axis and is controlled so as to focus on the optical information recording surface. The device is being used. However, this causes aberration. This aberration is mainly spherical aberration, and is 4 of NA.
It is proportional to the power and also to ΔU. Further, the absolute value | m | of the image forming magnification of the optical system is proportional to a variable function. △
Amount of aberration caused by U is WU, absolute value of imaging magnification | m
Let α (| m |) be a function whose | is a variable, then α (| m
|) Is a monotonically increasing function of the variable | m |, and WU can be expressed by the following equation. WU = α (| m |) ・ (NA) ⁴・ △ U

This aberration occurs even at an NA of about 0.45, but does not reach a level at which it causes a problem. However, it cannot be ignored in the case of the high NA targeted by the present invention. A finite conjugate type single lens having a relatively high NA has been proposed in JP-A-1-152410, JP-A-1-167717, JP-A-2-223906, and the like, but any of these proposals are concluded. The image magnification is as large as | m | ≧ 0.2. If the objective lens and the light source are integrated and autofocusing is performed, the above problem is eliminated, but the peripheral devices become large-scale and the cost is increased. The present invention relates to a high NA objective lens having a numerical aperture exceeding 0.5, and is of a finite conjugate type in which the amount of aberration is practically negligible even if the object-image distance varies due to deflection of the optical information medium. It is intended to realize an objective lens.

[0005]

The objective lens for recording and reproducing of the optical information recording medium of the finite system specification of the present invention is 0.5 <NA <0.65 (1), and FIG. 2 includes the cover glass. As shown in the sectional view, the light source side and the optical information recording medium side are both biconvex single lenses having an aspherical shape, and are characterized by satisfying the following conditions. 0.05 ≦ | m | ≦ 0.135 (2) 0.75 <r ₁ /{(n−1)f√(1+|m|)}<1.5 (3)

(Equation 3) However, NA: numerical aperture on the image side m: imaging magnification r ₁ : radius of curvature of the vertex of the surface on the light source side n: refractive index of the lens f: focal length of the lens

[Equation 4] δ ₁ : a non-spherical surface at the periphery of the effective diameter of the surface on the light source side (the position on the surface on the light source side where the marginal ray of NA is incident) and a position on the surface on the light source side where the ray of NA 0.45 is incident The case where the aspherical surface at the outermost periphery of the effective diameter is displaced toward the image side with respect to the aspherical surface at NA 0.45 due to the difference in the optical axis direction from the spherical surface is positive. δ ₂ : The difference in the optical axis direction between the aspherical surface at the outermost periphery of the effective diameter of the image side and the aspherical surface at the position on the image side where the ray of NA 0.45 exits, and the aspherical surface at the outermost periphery of the effective diameter. Is positive when it is displaced toward the image side from the aspherical surface at NA 0.45. b (m): a function of the imaging magnification m, b (m) = | m
It is represented by | +0.33.

More preferably,

(Equation 5) However,

(Equation 6) Δ ₁ : Difference in the optical axis direction between the aspherical surface position at the position where a light ray having a NA of 0.45 is incident on the light source side and the reference spherical surface position having a vertex curvature radius r ₁ , and the aspherical surface is the light source as the distance from the optical axis increases. Positive when it is displaced to the side. Δ ₂ : The difference in the optical axis direction between the aspherical surface position at the position where a ray of NA 0.45 on the image side exits and the reference spherical surface position having the vertex curvature radius r ₂ , and the further the aspherical surface becomes from the light source, the further the aspherical surface becomes. Positive when it is displaced to the side. a (m): a function of the imaging magnification m, a (m) = 2.
It is represented by 5 | m | ² +0.11.

[0007]

The condition (1) is a precondition for controlling the numerical aperture (NA) of the finite high NA objective lens of the present invention. N
When A is 0.5 or less, the high NA targeted by the present invention
Below the range of, and exceeds the upper limit, the aberration caused by the variation of the object-image distance becomes large even if the present invention is used,
It becomes unsuitable for practical use. The condition (2) is a condition for suppressing the aberration generation amount to be small and providing a finite high NA objective lens suitable for practical use within the range of the condition (1). If it is less than the lower limit, it is advantageous for suppressing the amount of aberration generation to a small extent, but in order to secure the necessary working distance, the focal length must be lengthened, and therefore the distance between the light source and the information recording surface. Will be long. In this case, in order to reduce the size of the entire optical system, it is necessary to dispose a mirror or prism between the light source and the objective lens to bend the optical path, and even a finite optical system that does not require a collimator lens There arises a problem of increasing costs. On the other hand, if the upper limit is exceeded, it will be difficult to suppress the aberration sufficiently small.

The condition (3) relates to the apex curvature radius r ₁ of the surface on the light source side. Since both sides of the lens of the present invention are aspherical, it is possible to satisfactorily correct spherical aberration and sine conditions. However, by optimally selecting r ₁ , it is possible to reduce the occurrence of spherical aberration and coma as much as possible, and it is possible to correct with a simple aspherical surface with a small amount of aspherical surface. That is, the above-mentioned lens shape with a small amount of aberration generation is a biconvex lens with a surface having a strong curvature facing the light source side when the imaging magnification is zero and the refractive index of the lens is relatively small. It is well known that when the ratio is high, it is a similar convex meniscus lens. On the other hand, when the imaging magnification is -1, the lens becomes a symmetrical biconvex lens. The condition (3) is that the radius of curvature of the vertex of the surface on the light source side is r _{1 as the} imaging magnification increases.
It shows the above-mentioned relationship that the optimum value of becomes loose. When the value exceeds the upper limit and becomes large, the sine condition becomes over, and when the value becomes smaller than the lower limit and becomes small, the sine condition becomes under. Become. Conditions (4) and (5) are conditions relating to the amount of aspherical surface for favorably correcting spherical aberration. Assuming that the imaging magnification is constant, the image-side numerical aperture (N
The larger A), the smaller the refractive index (n) of the lens, and the longer the focal length (f), the larger the amount of aspherical surface for favorably correcting spherical aberration. Therefore, it is necessary to normalize the aspherical surface quantities δ ₁ and δ ₂ with NA, n, and f, and Δ ₁ and Δ ₂ with n and f. Specifically, δ ₁ , δ ₂
Is the amount normalized by (NA) ⁶ , 1-/ (n-1), f

(Equation 7) And

(Equation 8) Is positive and large,

[Equation 9] The more negative and smaller, the greater the effect of making spherical aberration under, so to correct spherical aberration

[Equation 10] Must be within a certain range. Also, △ ₁ , △ ₂
Is the amount normalized by 1 / (n-1) ³ , f

[Equation 11] And

(Equation 12) Is positive and large,

(Equation 13) The more negative and smaller the value is, the greater the effect of overcoming the spherical aberration becomes. Therefore, in order to correct the spherical aberration,

[Equation 14] Is

(Equation 15) It is necessary to be within a certain range as well.

Next, when the magnification is changed, the functions b (m) and a (m) whose variables are the imaging magnification m are introduced in consideration of the influence of the change in the imaging magnification on the wavefront aberration. ,

[Equation 16] Must be within some range. Conditions (4) and (5) are the position where the light ray passes through the periphery of the effective diameter and NA in this range.
It is defined at a position where a ray of 0.45 passes, and when the lower limit of condition (4) and the upper limit of condition (5) are exceeded, spherical aberration is overcorrected, and the upper limit of condition (4) and condition (5) are satisfied. If the lower limit of is exceeded, spherical aberration will be undercorrected.

[0010]

EXAMPLES Examples of the objective lens of the present invention will be shown below. The symbols in the table are in addition to those mentioned above. DL: axial thickness of lens dc: axial thickness of cover glass inserted on the image side nc: refractive index of cover glass inserted on the image side wD: working distance Show. The aspherical shape of the surface on the light source side and the surface on the image side is defined by
Let κ be the conic constant, Ai be the aspherical coefficient, and Pi be the power of the aspherical surface.

[Equation 17] It is represented by. H ₁ and H ₂ are the heights at which the image-side NA 0.45 ray passes through the light source side surface and the image side surface, respectively. When the aspherical surface shape is expressed as above, the aspherical surface quantities Δ ₁ and Δ ₂ are Δ ₁ = Xsp, j−Xas, j, (j = 1,2)

(Equation 18) Cj = 1 / r ₁ κj: Conical constant of j-plane Ai (j): Aspherical surface coefficient of j-plane Pi (j): Exponent of aspherical surface of j-plane. If H ₁ 'and H ₂ ' are the heights of the marginal rays on the surface on the light source side and the surface on the image side, the aspherical surface quantities δ ₁ and δ ₂ are as shown in the above-mentioned case. Is δj = Xas, j'-Xas, j, (j = 1,2) where

[Formula 19] Is.

[0011] Example 1 f = 1.00 NA = 0.64 m = -1 / 20 dc = 0.2222 WD = 0.5276 nc = 1.58 r 1 = 0.75500 dL = 0.70 n = 1.7000 r ₂ = −5.994064 Aspherical coefficient, power number 1st surface κ ₁ = −0.32219 A ₁ (1) = −0.20620 × 10 ⁻¹ P ₁ (1) = 4.0 A ₂ (1) = -0.555576 x 10 ^-1 P ₂ (1) = 6.0 A ₃ (1) = -0.25564 P ₃ (1) = 8.0 A ₄ (1) = -0. 78063 × 10 ⁻¹ P ₄ (1) = 10.0 Second surface κ ₂ = −0.54318 × 10 ² A ₁ (2) = + 0.43201 P ₁ (2) = 4.0 A ₂ (2) = -0.18900 × 10 P ₂ (2) = 6.0 A ₃ (2) = + 0.38530 × 10 P ₃ (2) = 8.0 A ₄ (2) = −0.30053 × 10 P ₄ (2) = 10.0 r ₁ /(n−1)f√(1+|m|)=1.0526

(Equation 20)

[0012] Example 2 f = 1.00 NA = 0.63 m = -1 / 7.5 dc = 0.2222 WD = 0.6337 nc = 1.58 r 1 = 0.80300 dL = 0.70 n = 1.7000 r ₂ = −3.498840 Aspheric coefficient, power number First surface κ ₁ = −0.35823 A ₁ (1) = −0.43155 × 10 ⁻¹ P ₁ (1) = 4. 0 A ₂ (1) = -0.67105 x 10 ^-1 P ₂ (1) = 6.0 A ₃ (1) = -0.13428 P ₃ (1) = 8.0 A ₄ (1) =- 0.10425 P ₄ (1) = 10.0 Second surface κ ₂ = −0.54193 × 10 ² A ₁ (2) = + 0.19311 P ₁ (2) = 4.0 A ₂ (2) = -0.70191 P ₂ (2) = 6.0 A ₃ (2) = + 0.98817 P ₃ (2 ) = 8.0 A ₄ (2) = −0.54963 P ₄ (2) = 10.0 r ₁ /(n−1)f√(1+|m|)=1.0776

[Equation 21]

[0013] Example 3 f = 1.00 NA = 0.60 m = -1 / 20 dc = 0.2222 WD = 0.5382 nc = 1.58 r 1 = 0.62013 dL = 0.70 n = 1.4899 r ₂ = −1.466699 aspherical coefficient, power number 1st surface κ ₁ = −0.78016 A ₁ (1) = + 0.13928 P ₁ (1) = 4.0 A ₂ (1) = −0.10146 P ₂ (1) = 6.0 A ₃ (1) = + 0.17257 P ₃ (1) = 8.0 A ₄ (1) = −0.42971 P ₄ (1) = 10.0 Second surface κ ₂ = −0.27250 × 10 ² A ₁ (2) = -0.45045 x 10 ^-1 P ₁ (2) = 4.0 A ₂ (2) = +0.26170 x 10 ^-1 P ₂ (2) = 6.0 A ₃ (2) = −0.45967 × 10 ⁻¹ P ₃ (2) = 8.0 A ₄ (2) = + 0.23605 × 10 ⁻¹ P ₄ (2) = 10.0 r ₁ / (n−1) f√ (1+ | m |) = 1.2353

[Equation 22]

[0014] Example 4 f = 1.00 NA = 0.564 m = -1 / 7.5 dc = 0.2222 WD = 0.6366 nc = 1.58 r 1 = 0.64647 dL = 0.70 n = 1.4899 r ₂ = −1.30257 aspherical coefficient, power number 1st surface κ ₁ = −0.93181 A ₁ (1) = + 0.13297 P ₁ (1) = 4.0 A ₂ (1 ) = − 0.75937 × 10 ⁻¹ P ₂ (1) = 6.0 A ₃ (1) = + 0.41641 × 10 ⁻¹ P ₃ (1) = 8.0 A ₄ (1) = −0. 29086 P ₄ (1) = 10.0 Second surface κ ₂ = −0.14879 × 10 ² A ₁ (2) = + 0.51309 × 10 ^-2 P ₁ (2) = 4.0 A ₂ (2) = -0.15536 P ₂ (2) = 6.0 A ₃ (2) = + 0.20191 _{P 3 (2) = 8.0 A} 4 (2) = -0.14246 P 4 (2) = 10.0 r 1 /(n-1)f√(1+|m|)=1.240

(Equation 23)

[0015] Example 5 f = 1.00 NA = 0.510 m = -1 / 10.0 dc = 0.2222 WD = 0.5964 nc = 1.58 r 1 = 0.63422 dL = 0.70 n = 1.4899 r ₂ = −1.37159 aspherical coefficient, power number 1st surface κ ₁ = −0.89511 A ₁ (1) = + 0.13998 P ₁ (1) = 4.0 A ₂ (1 _{^{) = -0.21030 × 10 -1 P 2}} (1) = 6.0 A 3 (1) = + 0.51236 × 10 -1 P 3 (1) = 8.0 A 4 (1) = -0. 18622 P ₄ (1) = 10.0 Second surface κ ₂ = −0.16854 × 10 ² A ₁ (2) = + 0.46240 × 10 ^-1 P ₁ (2) = 4.0 A ₂ (2) = -0.15527 P ₂ (2) = 6.0 A ₃ (2) = + 0.19709 _{P 3 (2) = 8.0 A} 4 (2) = -0.11607 P 4 (2) = 10.0 r 1 /(n-1)f√(1+|m|)=1.234

[Equation 24]

[0016]

The objective lens for recording / reproducing of the optical information recording medium of the present invention is a finite conjugate type objective lens with a high NA exceeding 0.5, as seen from the examples and the drawings. Not only the spherical aberration but also the sine condition and astigmatism are good, and it is possible to realize an objective lens in which the amount of aberration generated due to the variation of the distance between the object images due to the deflection of the optical information medium is practically negligible. done.

[Brief description of drawings]

FIG. 1 is an optical layout diagram of an example of an optical system using an objective lens of the present invention.

FIG. 2 is a sectional view including a cover glass of an objective lens of the present invention.

FIG. 3 is an aberration curve diagram of Example 1 of the objective lens of the present invention.

FIG. 4 is an aberration curve diagram of Example 2 of the objective lens of the present invention.

FIG. 5 is an aberration curve diagram of Example 3 of the objective lens of the present invention.

FIG. 6 is an aberration curve diagram of Example 4 of the objective lens according to the present invention.

FIG. 7 is an aberration curve diagram of Example 5 of the objective lens of the present invention.

[Explanation of symbols]

1 Optical information recording surface 2 Objective lens 3
light source

Claims (1)

[Claims] 1. A biconvex single lens in which both the light source side and the optical information recording medium side have an aspherical shape and 0.5 <NA <0.65, and which satisfies the following conditions: Objective lens for recording / reproduction of optical information recording medium with finite system specification 0.05 ≦ | m | ≦ 0.135 0.75 <r ₁ / {(n−1) f√ (1+ | m |)}
<1.5 [Equation 1] Where NA is the numerical aperture on the image side, m is the imaging magnification, r _{1 is the} radius of curvature of the vertex of the surface on the light source side, n is the refractive index of the lens, and f is the focal length of the lens. δ ₁ : a non-spherical surface at the periphery of the effective diameter of the surface on the light source side (the position on the surface on the light source side where the marginal ray of NA is incident) and a position on the surface on the light source side where the ray of NA 0.45 is incident The case where the aspherical surface at the outermost periphery of the effective diameter is displaced toward the image side with respect to the aspherical surface at NA 0.45 due to the difference in the optical axis direction from the spherical surface is positive. δ ₂ : The difference in the optical axis direction between the aspherical surface at the outermost periphery of the effective diameter of the image side and the aspherical surface at the position on the image side where the ray of NA 0.45 exits, and the aspherical surface at the outermost periphery of the effective diameter. Is positive when it is displaced toward the image side from the aspherical surface at NA 0.45. b (m): a function of the imaging magnification m, b (m) = | m
It is represented by | +0.33.