JPH04114112A - High numerical aperture lens - Google Patents

Abstract

PURPOSE:To obtain a high numerical aperture and to compensate the spherical aberration and comatic aberration excellently by cementing an aspherical lens made of a homogeneous medium and a lens with a refractive index distribution together. CONSTITUTION:The lens consists of two elements in one group is constituted of cementing the homogeneous medium lens 2 which has at least one aspherical surface and the lens 3 with the refractive index distribution together. The spherical aberration is therefore compensated by the aspherical surface of the homogeneous medium lens 2, which is cemented to the distributed index lens 3 to prevent the tilt angle of the aspherical surface from becoming too larger in a high numerical aperture (NA) state. Consequently, the high numerical aperture lens is obtained which have the spherical aberration and comatic aberration compensated excellently although the numerical aperture is extremely large.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a high numerical aperture lens used for optical pickup of video disks, optical memory disks for computers, etc., and particularly relates to a high numerical aperture lens used for optical pickup of video disks, optical memory disks for computers, etc. This invention relates to a high numerical aperture lens that achieves a high numerical aperture.

Conventional Technology A single lens is often used as an objective lens for an optical disk in order to pursue miniaturization and weight reduction to the utmost. Among homogeneous medium lenses, those that achieved performance by making the surface aspherical are disclosed in Japanese Patent Application Laid-Open Nos. 5776512 and 1983.
This method has been proposed in Japanese Patent Publication No. 201210 and Japanese Unexamined Patent Publication No. 59-26714. On the other hand, as a single lens based on a piezoelectric index distribution, Japanese Patent Laid-Open No. 61-102617 and Japanese Patent Laid-Open No. 6113
This method has been proposed in Japanese Patent Laid-Open No. 8223, Japanese Patent Laid-Open No. 163310/1983, and the like. All of these lenses have at least one surface processed into a spherical surface.

The above two types of lenses have numerical apertures of about 0.45 to 0.50, which are suitable for conventional optical discs.

Problems to be Solved by the Invention However, when increasing the numerical aperture of a lens in order to increase the recording density of an optical disk, the inclination angle of the lens surface near the effective diameter becomes too large for conventional aspheric single lenses made of homogeneous media. Therefore, there have been problems such as making processing difficult and increasing loss of incident light due to reflection. - In an ancient gradient index lens, even if both end surfaces are flat, the inclination angle of the lens surface will not become too large because it has refractive power, but if only one surface is made spherical, aberrations will become too large. Performance will be insufficient. Furthermore, it is necessary to control high-order refractive index distribution coefficients, making it difficult to put it into practical use.

In view of the above-mentioned problems, the present invention provides a high numerical aperture lens that has a very high numerical aperture and has spherical aberration and coma aberration well corrected.

Means for Solving the Problems In order to solve the above problems, the high numerical aperture lens of the present invention is a lens composed of two elements in one group, and includes a homogeneous medium lens in which at least one surface is an aspherical surface, and a refractive index. It is constructed by pasting together lenses with a distribution.

Effect of the Invention With the above-described configuration, the present invention corrects spherical aberration using the aspheric surface of the homogeneous medium lens, and further prevents the inclination angle of the aspheric surface from becoming too steep at high numerical apertures by laminating it with a gradient index lens. This is a high numerical aperture lens designed to prevent

EXAMPLE Hereinafter, a high numerical aperture lens according to an example of the present invention will be described with reference to the drawings. FIG. 1 is a configuration diagram of an embodiment to a fourth embodiment of the present invention. In FIG. 1, parallel light l enters a homogeneous medium lens 2, further passes through a gradient index lens 3, and forms an image on an image plane 4. In the example, f: focal length NA of the lens: numerical aperture R1 of the lens: radius of curvature of the first surface R1 radius of curvature of the second surface R3: radius of curvature n of the third surface: refractive index dI of the homogeneous medium lens: Thickness of the lens between the first and second surfaces d2: Thickness of the lens between the second and third surfaces WD: Working distance of the lens WI: On-axis wavefront aberration (rmsλ) W2: 1 degree off-axis shows the wavefront aberration (rmsλ) at . Further, the wavelength was designed to be 780 nm.

10AF j h8+AGj h”+... Curvature of the aspheric apex from the 5th surface CCj: Conic constants ADj, AEj, AFj, AC; be done.

Furthermore, in Examples 1 to 4, the refractive index distribution n (
r) is n(r)-no[1-(gr)2+hi-(gr)'+
h2 (g r)' +=..., , 11/-2, r is the radial distance from the optical axis, no is the refractive index on the optical axis, g, h, , h2 is the refractive index distribution coefficient shows.

Note that FIGS. 2(a) to (C) to FIGS. 5(a) to (C) show the spherical aberration, sine condition, and astigmatism of the embodiments of the present invention to Embodiment 4, respectively. . In the diagram of astigmatism, the solid line indicates sagittal curvature of field, and the dotted line indicates meridional curvature of field.

Example 1 f = 1.08 N A = 0.7d
, =0.6 WD=0.452R,
=0.8 1. =1.738639C
C--8,96882XIOI A D -1,00127X 10"A E
=2.92907x102A F --6,568
8XIO”'AG −-1,07976X102 d2=0.5 2-oO no=1.757813 W=0.00173λ Example 2 f=1.084 d+=0.45 R,=0.7974 CC=4 .98703 AD, =3.49909xlOI AE, --1,78655x101 AF, =1.25918x104 AG, =-9,68438X102d2=0.6
55 R2-SonoR3=C0 g =0.0753 W2=0.0332λ NA=0.7 W D =0.477 n , =1.674959 n ””1.75 W =0.0077λ Example 3 f = 0.9902 d, =0.40 R, =0.8616 CC--2,16117 AD, =3.17247X101 AE, =-1,72204X10' AF =1.09116X10' AG, --9,15065x102 d2- 0゜9651 R2-ω n0 = 1.75 W1 = 0゜0053λ Example 4 f = 1.000 d , = 0.5665 R, = 0.74115 CC, = -5,46737xl (11 g = 0, 33 W2 = 0.0349λ NA = 0.75 WD = 0.2587 n = 1.785691 R3 = (1) g = 0.33 W2 = 0.0304λ NA = 0.75 WD = 0.4232 n, =1.738639 AD, =1.40887X102 AE, =-1,63621x 102A F
, --3,61319x102AG =-7,
26287xlO2cl,, =0.3831 R2-■ R3-ωn 0=1.5
17347 g = 0.1035W,
=0.0304λ W2 =0.0437S In the above embodiment, the thickness of the protective layer of the disk was designed to be O. The high numerical aperture lens of the present invention has a high N
This is because, in the case of lens A, if the disk is thick, the coma aberration that occurs when the lens is tilted with respect to the disk is large. Therefore, the thickness of the disk is preferably thinner than the 1.2 mm to 1.25 mm used in ordinary optical disks, and less than 1.2 mm thick. Note that even if the thickness of the disk is not zero, similar performance can be obtained by adjusting the aspheric coefficient or the refractive index distribution coefficient.

Effects of the Invention As described above, the effects of the high numerical aperture lens of the present invention are as follows.

(1) Since the structure is made by laminating an aspherical lens with a homogeneous medium and a lens with a refractive index distribution, even if a lens with a high NA is achieved, the tilt angle of the lens near the effective system will not be very large. In addition, spherical aberration and coma aberration are well corrected, so both on-axis and off-axis,
High performance within analysis limits can be ensured.

(2) If both end surfaces of the gradient index lens are made flat, processing is easy.

(3) By making the thickness of the protective layer of the disk thinner than before or zero, it is possible to reduce or eliminate the occurrence of coma aberration, which becomes a problem when the NA of a lens is increased.

(4) In the high numerical aperture lens of the present invention, there is no need to control the high-order coefficients of the gradient index lens, and manufacturing errors of the gradient index lens can be reduced by making slight design changes to the aspherical coefficients. This is easy to implement because it can be absorbed by

(5) By creating an aspherical lens made of a homogeneous medium using a glass molding method, highly accurate aspherical lenses can be mass-produced at low cost.

(6) Because it has a laminated structure of two groups and one sheet,
No lens barrel is required and it can be handled like a single lens.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a high numerical aperture lens according to an embodiment of the present invention, and FIGS. 2 to 2 are aberration diagrams of each embodiment of the present invention. 1... Parallel light, 2... Homogeneous medium lens, 3... Gradient index lens, 4... Image plane. Name of agent: Patent attorney Yoshiaki Kodama and 2 others

Claims (4)

[Claims] (1) A high numerical aperture lens that is composed of two lenses in one group and is characterized by laminating a homogeneous medium lens whose at least one surface is an aspherical surface and a lens with a refractive index distribution. (2) When numerical aperture is NA, 0.6<NA<0.9
5. The high numerical aperture lens according to claim 1, which satisfies condition 5. (3) The lens according to claim 1, wherein the lens is configured in the order of a homogeneous medium lens and a gradient index lens from the object side, the surface closest to the object side is an aspherical surface, and both surfaces of the gradient index lens are flat. numerical aperture lens. (4) When the refractive index at the center of the gradient index lens is n_0 and the radial distance from the optical axis is r, the refractive index distribution n(
r) is n(r)=n_0[1-(gr)^2+h_1(
gr)^4+h_2(gr)^6+・・・・・・]^1
The high numerical aperture lens according to claim (1), expressed as ^/^2.