Astigmatic lens for use in detecting a focussing error in an optical pickup system

• US 5,546,235 A
• Filed: 11/04/1994
• Issued: 08/13/1996
• Est. Priority Date: 11/05/1993
• Status: Expired due to Fees

• Current Assignee: Daewoo Electronics Company Limited (Dayou Holdings Co.,Ltd.)
• Original Assignee: Daewoo Electronics Company Limited (Dayou Holdings Co.,Ltd.)
• Inventors: Park, Chan-Kyu
• Primary Examiner(s): Epps, Georgia Y.
• Assistant Examiner(s): Lester, Evelyn A.
• Application Number:
  US08/334,288
• Time in Patent Office: 648 Days
• Field of Search: 359/742, 359/571
• US Class Current: 359/742
• CPC Class Codes: G02B 3/08 with discontinuous faces, e.g. Fresnel lens diffractive Fresnel lenses G02B5/1876
• CPC Class Codes: G02B 5/1876 Diffractive Fresnel lenses; Zone plates; Kinoforms G02B5/1842, G02B5/1847 take precedence; optical systems having diffractive correction means G02B27/0037; Fresnel lenses operating by refraction G02B3/08
• CPC Class Codes: G11B 7/0909 by astigmatic methods
• CPC Class Codes: G11B 7/1372 Lenses

Abstract

A cylindrical astigmatic lens is used conventionally in detecting focussing errors in an optical pickup system. The inventive astigmatic lens is comprised of an orthorhombic base, made of a refractive material, provided with two parallel surfaces and a centerplane perpendicular thereto, and two sets of M pentahedrons, made of the same refractive material, placed on one of the two parallel surfaces thereof in such a way that each set of M pentahedrons is symmetric with respect to the centerplane. Each of the pentahedrons consists of three rectangular surfaces and two right angled triangular side surfaces, each of the triangular side surfaces being defined by a height H_m, a base R_m and an inclination. Since the height is the same in all of the triangular side surfaces in the M pentahedrons, the dimension of each pentahedron in the inventive astigmatic lens is defined by R_m, R_m being expressed as ##EQU1## wherein f represents the focus length and λ is the wavelength of the light source with the inventive astigmatic lens, it is possible to improve the alignment accuracy of the optical pickup system, since the inventive astigmatic lens being planar.

Claims

1. A convex astigmatic lens for use in an optical pickup system comprising:

an orthorhombic base, made of a refractive material, having a first and a second surfaces, and provided with a vertically directional centerplane, wherein the first and second surfaces are parallel to each other, each of the surfaces being provided with a pair of aerie sides and a pair of lengthwise sides, and the centerplane is perpendicular to the first and second surfaces; and

two sets of M pentahedrons placed on the first surface of said orthorhombic base, each of the pentahedrons in each set being made of said refractive material and having a rectangular base surface, a rectangular inclined surface, a rectangular normal surface and a pair of right angled triangular parallel side surfaces, each of the three rectangular surfaces being provided with a pair of lengthwise sides and a pair of aerie sides and each of the triangular side surfaces having a base R_m, a height H_m, and an inclination, m being a natural number indicating the number of the order, said base, height and inclination of the pair of triangular side surfaces of each pentahedron being defined by the pair of aerie sides of the rectangular base, rectangular normal and rectangular inclined surfaces of the corresponding pentahedron, respectively, the height H_m of the triangular side surfaces in each of the M pentahedrons being equal, each of the pentahedrons being formed by joining one of the lengthwise sides of the rectangular inclined surface with one of the lengthwise sides of the rectangular normal surface at an angle Θ

_m, thereby forming a first edge, one of the lengthwise sides of the rectangular base surface with the remaining lengthwise side of the rectangular inclined surface at a right angle to thereby form a second edge, and the remaining lengthwise side of the rectangular base surface with the remaining lengthwise side of the rectangular inclined surface at an angle (Θ

_m <

90°

), thereby forming a third edge, the first, second and third edges of the M pentahedrons being parallel to each other, the second edge of each of the M pentahedrons being collinear to the third edge of a neighboring pentahedron with the exception of the pentahedrons having the triangular side surfaces with the largest R_m and shortest R_m, the rectangular surfaces of the pentahedron having the triangular side surfaces with the largest base R_m from each set sharing the same rectangular surface, the third edge of the pentahedrons having the triangular side surfaces with the shortest R_m from each set being collinear to the lengthwise sides of the first surface, respectively, and the rectangular base surfaces of the M pentahedrons from the two sets and the first surface of the orthorhombic base being also coplanar, thereby allowing the two sets of M pentahedrons to be symmetrical with respect to the centerplane, the angle Θ

_m for each of the M pentahedrons being further determined by the R_m of the triangular side surfaces of the corresponding pentahedron, the base R_m being mathematically expressed as ##EQU7## wherein f represents the focus length and λ

, the wavelength of the light source.

3. The astigmatic lens of claims 1 or 2, wherein the refractive material is made of a poly methyl meta acryllrate(PMMA).

4. The astigmatic lens of claims 1 or 2, wherein the refractive material is made of a poly carbonate(PC).

5. The astigmatic lens of claims 1 or 2, wherein the refractive material is made of a poly styrate(PS).

2. A concave astigmatic lens for use in an optical pickup system comprising:

an orthorhombic base, made of a refractive material, having a first, a second, a third and a fourth surfaces, and provided with a vertically directional centerplane, wherein the first and second surfaces are parallel to each other, the third and fourth surfaces also being parallel to each other, each of the surfaces being provided with a pair of aerie sides and a pair of lengthwise sides, and the centerplane is perpendicular to the first and the second surfaces; and

two sets of M pentahedrons, placed on the first surface of said orthorhombic base, each of the pentahedrons in each set being made of said refractive material and having a rectangular base surface, a rectangular inclined surface, a rectangular normal surface and a pair of right angled triangular parallel side surfaces, each of the three rectangular surfaces being provided with a pair of lengthwise sides and a pair of aerie sides and each of the triangular side surfaces having a base R_m, a height H_m, and an inclination, m being a natural number indicating the number of the order, said base, height and inclination of the pair of triangular side surfaces of each pentahedron being defined by the pair of aerie sides of the rectangular base, rectangular normal and rectangular inclined surfaces of the corresponding pentahedron, respectively, the height H_m of the triangular side surfaces in each of the M pentahedrons being equal, each of the pentahedrons being formed by joining one of the lengthwise sides of the rectangular inclined surface with one of the lengthwise sides of the rectangular normal surface at an angle Θ

_m, thereby forming a first edge, one of the lengthwise sides of the rectangular base surface with the remaining lengthwise side of the rectangular inclined surface at a right angle to thereby form a second edge, and the remaining lengthwise side of the rectangular base surface with the remaining lengthwise side of the rectangular normal surface at an angle (Θ

_m <

90°

) thereby forming a third edge, the first, second and third edges of the M pentahedrons being parallel to each other, the second edge of each of the M pentahedrons being collinear with the third edge of neighboring pentahedrons with the exception of the pentahedron having the triangular side surfaces with the largest R_m and the shortest base R_m from each set, the pentahedrons having the triangular side surfaces with the largest R_m from each set sharing the third edge, the rectangular surfaces of pentahedrons having the triangular side surfaces with the shortest R_m being on the same plane as the third and fourth surfaces of the orthorhombic base, respectively, and rectangular bases of the pentahedrons from the two sets and the first surface of the orthorhombic base being also coplanar, thereby allowing the two sets of M pentahedrons to be symmetrical with respect to the centerplane, the angle Θ

_m for each of the M pentahedrons being further determined by the base R_m of the triangular side surfaces of the corresponding pentahedron, the base R_m being mathematically expressed as ##EQU8## wherein f represents the focal length and λ

, the wavelength of the light source.