JPH0463312A - Refractive index distribution type single lens - Google Patents

Abstract

PURPOSE:To offer a single lens whose spherical aberration and coma aberration are effectively corrected inspite of having a high NA by aspherically forming both sides or a single side, correcting a spherical aberration based upon the shape of an aspherical surface and a refractive index distribution coefficient and satisfying specific conditions. CONSTITUTION:A single lens having an aspherical face on the object side face and a plane on the image side face is used. When it is defined that the refractive index of a center is n0, the radius direction distance of a lens is (r), refractive index distribution n(r) is expressed by n(r)=n0(1-(gr)<2>+ h1(gr)<4> + h2(gr)<6> +...)<1/2>, the spherical aberration is corrected by the shape of the aspherical surface and the refractive index distribution coefficient and the conditions of 0.6<NA<0.85 and 0.5<d/(n0.f)<1.15 are satisfied. Provided that g, h. and h, are refractive index distribution coefficients, NA is the number of apertures of the lens, (d) is the center thickness of the lens, and (f) is the focal distance of the lens.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a gradient index single lens used in a pick amplifier for signal light of video discs, optical discs for computers, etc., and particularly relates to a gradient index single lens that can achieve a high aperture by using an aspheric surface. The present invention relates to a gradient index single lens that achieves a numerical aperture (hereinafter referred to as NA).

2. Description of the Related Art As an objective lens for an optical disk, there is a gradient index single lens. Lenses whose refractive index is distributed in the radial direction are disclosed in JP-A-61-102617 and JP-A-61-13.
This method has been proposed in Japanese Patent Publication No. 8223, Japanese Patent Application Laid-open No. 163310/1984, and the like.

Further, lenses in which the refractive index is distributed in the axial direction have been proposed in Japanese Patent Laid-Open Nos. 127706/1988 and 409/1989. All of these lenses have at least one surface processed into a spherical surface, and their NA
is suitable for conventional optical discs, from 0.45 to 0-,50
It was a moderate lens. In addition, a lens that combines a refractive index gradient lens with an aspherical surface was published in Japanese Patent Application Laid-Open No. 61-240214.
It is proposed in the Publication No. However, the effect of an aspheric surface is used to properly correct spherical aberration, coma aberration, and astigmatism while ensuring a working distance.

Problems to be Solved by the Invention However, when the NA of the lens is increased in order to increase the recording density of optical discs, the aberrations of conventional gradient index lenses become too large, resulting in insufficient performance. The problem was that the refractive index distribution coefficient had to be controlled, making it difficult to put it into practical use.

This invention is above! ! ! The object of the present invention is to provide a gradient index single lens in which spherical aberration and coma aberration are well corrected while having a very high NA.

Means for Solving the Problems In order to achieve the above object, the present invention sets the central refractive index to n0.
, the distance from the optical axis in the radial direction of the lens is r, the H refractive index distribution coefficient is g, h1, h2, NA is the numerical aperture of the lens, d
where is the central thickness of the lens and f is the focal length of the lens, the refractive index distribution n (r) is n(r) = n0 [1-(gr)2
+h1(gr)4+h2(gr)6+...]
1″ However, 0, 6 <NA<0.85 ・・・・・・
・(1) 0.5<d/(n0・r)<1.15-
- (In the gradient index single lens represented by 21, both surfaces or one surface are aspheric, and the spherical aberration is corrected by the shape of the aspheric surface and the refractive index distribution coefficient, and the sine condition and coma aberration are corrected. The thickness is selected under certain conditions.

Effect of the present invention With the above-described configuration, in the case of a high NA, the spherical aberration that cannot be corrected by the refractive index distribution alone is corrected by using an aspheric surface, and the sine condition is corrected by appropriately selecting the lens thickness. can also be corrected.

By processing one side of a lens having a refractive index distribution in the radial direction into an aspherical surface, it is possible to correct aberrations for a lens having an NA within the range of the above condition (1). If the lower limit of the range of condition (1) is exceeded, there is no need to make it aspherical, and if it is made aspherical, the processing cost will increase.

Furthermore, if the upper limit of condition (1) is exceeded, spherical aberration and sine condition cannot be sufficiently corrected, and condition (2)
is the 0 condition (
If 2) is not satisfied, the amount of unsatisfactory sine condition becomes large, and coma aberration correction becomes insufficient, resulting in a lens with a narrow angle of view.

EXAMPLE Hereinafter, a gradient index single lens according to an example of the present invention will be described with reference to the drawings. FIG. 1 is a sectional view of Examples 1 to 3 of the present invention. FIG. 2 is a sectional view of Examples 4 to 7 of the present invention. FIG. 3 is a sectional view of Examples 8 to 10 of the present invention. FIG. 4 is a sectional view of Examples 11 to 13 of the present invention. In FIGS. 1 to 4, parallel light 1 is incident on a gradient index single lens 2 according to the present invention, and is imaged on an image plane 3. Note that in all Examples, the side on which parallel light is incident is defined as the object side. In the examples, f: focal length of the lens NA: numerical aperture of the lens R1: radius of curvature of the object side surface R2: radius of curvature of the image side surface d: lens thickness WD: working distance of the lens Wl: on-axis Wavefront aberration (rmsλ) W2: Wavefront aberration (rmsλ) at 1 degree off-axis, and the wavelength was designed at 780 nm. Furthermore, the aspherical shape is jh2 ±ADjh4+AEjh6+AFjh8 ++AG J
h” +・・・・・・ However, h: Height from the optical axis curvature of the aspherical apex (=1/Rj) CCj: conic constants of the first surface ADj, AEj, AFj, AGj: 4th, 6th, and 8th orders of the J-th surface, respectively.

10th order aspheric coefficient It is.

Furthermore, in Examples 1 to 7, the refractive index distribution n (
r) is n(r)=n. (1-(gr)2 +h1(gr)40h2(gr)6+...)1/2, where r is the distance in the radial direction from the optical axis, no is the refractive index on the optical axis, g, h1 and h2 indicate refractive index distribution coefficients.

In Examples 8 to 13, the refractive index distribution n (z
) is expressed as n (zl=n0+a z+bz2+-...-=, Z is the distance in the optical axis direction from the object side surface, and no is the refractive index at the point where the object side surface of the lens and the optical axis intersect. ,a,b!,
tff indicates the refractive index distribution coefficient.

In addition, FIG. 5 to FIG. 17 respectively show Example 1 of the present invention.
- FIG. 12 is an aberration curve diagram showing each spherical aberration, sine condition, and astigmatism of Example 13. In the diagram of astigmatism, the solid line indicates sagittal curvature of field, and the number of points indicates meridional curvature of field.

Example 1 r = 1. d=2.15 WD=0.030R,=1
．． 5261 CC,= 0.119319 AD1= 8.76182X10 AE, = 4.37888X10ε AF, = 1.76869XIO name AC,--4,
41139X10”n, =1.89 g =0.43
W, =0.0051 W2=0.007
7 aberration curves are shown in FIG.

Example 2 f = 1.0 d = 0.995 R, = 0.73957 CC,--0,447123 AD, = 3.55154X10'2A E 1--
3.77097 x 1 (12AF, -1,2067
5X104 AC, -1,90754X104 n0=1.65 g =0.32W□ -0
,0035W, =0.0025The aberration curve is shown in FIG.

Implementation N3 f -1,0 d -1,4 NA = 0.75 NA = 0.8 WD = 0.429 WD = 0.255 R, = 0.941 CC, = -0, 265496 AD, = 2 .67076xtO'AE, =-
2,43105X102AP, = 3.8064
6x102AC,,=-1,16491xlOI n0=1.75 g =0.33W,
=0.0129 W2=0.0381 The aberration curve is shown in FIG.

Examples 4 to 7 are lenses that have a refractive index distribution in the radial direction and have aspheric surfaces on both sides.

In this example, 0.7<NA (3
)0.5<d/(no −f)<0.85−(4) must be satisfied. If the lower limit of condition (3) is exceeded, there is no need to correct aberrations by making both sides aspheric, and if both sides are made aspheric, the cost will be high, and condition (4) will not be satisfied. This is a condition for properly correcting the sine condition. If condition (4) is not satisfied,
The amount of dissatisfaction with the sine condition increases, and coma aberration correction becomes insufficient, resulting in a narrow lens.

Example 4 f = 1.0 d = 1.2 R, = 0.83 CC, = -0,451528 AD, = 4.71633X10°2AE, = -1
, 10164X102 AF, -8,7815x104 AC, --7,38374X10'' n0·1,75 W, -0,0063 The aberration curves are shown in FIG.

Example 5 r　　　　-t,.

d = 1.3 R, -1,2 CC, -0,512292 AD, = 2.5ss33xto* -0,85 -0,264 =0.79011 M=~28.317 =O NA=0.85 WD = 0.319 R2 = 4.2394 CC2 = -1,0038X102 AD2 =O AE2 E AF, =O AG2 = O g =0.33 W2 = 0.0397 NA WD C2 AD2 Angle of view AE, = 2.30392xl (L2A F
, =-6,61325xlO'AC,=-1,67
236xlO7n = 1.95 W, = 0.0110 The aberration curve is shown in the 9th article.

Example 6 f = 1. ONA d = 1.5 WD R, = 0.92 R2 CC, = -0,217076CC2 AD, = 6°19823 x 10 people AD2AE,
= 8.06998xlOJ AE2AF, =7
．． 0163xlO' AF2AG, =-1,31
957xlO2AG2n0=1.95 g W, = 0.0163 The W2 aberration curve is shown in FIG.

Example 7 f = 1.0 NA AE2 = O AF2 = O AC3 = O g = 0.56 W2 = 0.0712 = 0.85 = 0.1419 = 0.81466 = -0,712634 10101 =0 ==0 =0.28 =0.0466 d=1.0 WD =0.330R,
= 0.8 R2=5.38576CC=
-0,133694CC2=-8,13983X102
AD, = 2.17717xlO2AD2=OAE,
-1,14804x104 AE2=OA F,
=-1,89727x101 A F 2=OAC
,= 2.20507x104 AG2=On0=
1.55 g = 0.56W, =
0.0061 W2=0.0097 aberration curve is shown in FIG.

Examples 8 to 10 have a refractive index distribution in the optical axis direction of the lens, and the object side surface is aspheric and the image side surface is flat. In this example, 0,6 <NA<0.85
・・・・・・(5) 1.6<no<1.
9 ・・・・・・(6)0.5<d/
It is necessary to satisfy the condition of (no −f) <0.8 5---- force. If the lower limit of condition (5) is exceeded, there is no need to correct aberrations by using an aspherical surface, and if an aspherical surface is used, the lens will be expensive. If the lower limit of condition (6) is exceeded, spherical aberration cannot be sufficiently corrected, and if the upper limit of condition (6) is exceeded, the thickness of the lens becomes too thick to satisfy the sine condition. It becomes impossible to secure the working distance.

Furthermore, condition (7) is a condition for properly correcting the sine condition.

If condition (7) is not satisfied, the amount of unsatisfactory sine condition becomes large, and the correction of coma aberration becomes insufficient, resulting in a lens with a narrow angle of view.

Example date f = 1.0 NA = 0.75d
=1.65 WD=0.204R, =1. I CC,=-0,242718 AD, =-1,3q6o6x1o2 AE, =-1,58581x102 AF, =-t,5sts7xto4AC,=-1
, 5459xlo2 n0 = 2.1 a = -3, 23525X10' b = 2.
38305xlO'W =0.0028
The W2=0.0383 aberration curve is shown in FIG.

Example 9 f = 1.0 d = 1.0 R = 0.8 CC, = -0,532372 AD, = 4.26674xlO2 AE, = 1.0957 xlO°2AF, = 2°04995 x 102AC, = - 5,1907XIO
' n, = 1.8 a = -1,46549X10 [b = 7.44
17x104]W, =0.0010 W2=
The 0.0358 aberration curve is shown in FIG.

Example 10 f = 1.0 d = 1.4 R, = 0.95 CC, = -0,537807 AD, = 3.48593xlO' NA = 0.75 NA = 0.75 W D = 0. 420 W D -0,241 AE = 1.0701 Xl02A F,
= 8.03507x104AC, = -2,653
06x 102n = 1.95 a = -1,48644X10' b =
7.44094XIO4]W = 0.000
9 W2=0.0222 The aberration curve is shown in FIG.

Examples 11 to 13 have a refractive index distribution in the optical axis direction, and both surfaces are aspherical. In this example, 0.7<NA (8
)1.55<n, <1.85...
・(9)0.3<d/ (no・f)<0.7 --
-----It is necessary to satisfy the condition (10). (8)
If the lower limit of the condition is exceeded, there is no need to use an aspherical surface to correct aberrations, and if an aspherical surface is used, the lens will be expensive. If the lower limit of condition (9) is exceeded, it becomes impossible to satisfactorily correct spherical aberration. Moreover, if the upper limit of condition (9) is exceeded, the thickness of the lens to satisfy the sine condition becomes too thick, making it impossible to secure the working distance. Furthermore, condition (7) is a condition for properly correcting the sine condition.

If condition (7) is not satisfied, the amount of unsatisfactory sine condition becomes large, and the correction of coma aberration becomes insufficient, resulting in a lens with a narrow angle of view.

Example 11 f = 1. d=0.868 WD=0.449
R=0.8225 R2=-4,426CC,
=-0,109053CC2=3.71765AD,
= 1.11453x104 AD2=1.6033x
lO'IAE, ==1.07919x10[AE2=
-3,20477x10 [A F, --1,850
97xlO(AF2=-8,68995xlOIA
G, = 2.20507xlO (AG2=2.391
68n0" 1.80 b = -1,2
0392X10'a = -5,87957X10[
W2=0.0444W, = 0.0304 The aberration curve is shown in FIG.

Example 12 f Sophistry 1. d=0.6 WDR,=0.6
5 R2 CC,=-0,597894CC2 AD,=1.61476xlOIAD2AE,
= 1.11746xlOIAE2AF, = 1.
89828xlOIAF2AC,, = 2.69894
xlOIAG2no=1.62 ha=-0,32W2 W1=0.0166 The aberration curve is shown in FIG.

Example 13 f = 1. ONA d = 0.671 WD R = (1,668R2 CC1 = -0,512225CC2 AD, = 4.70818xlO°2 AD2AE,
= 1.87428x102 AE2A F , =
3.75224x104 A F2A G , =-
6,29078 =0 =0 =O =0 n, -1,68b =-6,6022XIO'
a=-0,1144W2=0.0423W,
=0.0017 The aberration curve is shown in FIG.

Note that if the angle of view of the lens is less than 1 degree, problems such as difficulty in alignment will occur, making it impractical to
The wavefront aberration for light rays from off-axis is 1/14λr
If it exceeds ms, it is difficult to actually use it because the performance exceeds the diffraction limit. Therefore, as shown in each of the above embodiments, W2 needs to be 1/14λrms or less.

In addition, in the above embodiment, the gradient index single lens of the present invention, which is designed with the thickness of the protective layer of the optical disc as 0, is a high NA lens, and when the thickness of the protective layer of the optical disc is thick, the lens This is because the comatic aberration that occurs when the lens is tilted with respect to the disk is large. Therefore, the thickness of the protective layer of the optical disc is thinner than the 1.2 to 1.25 mm thick used in ordinary optical discs, and preferably 1 mm or less. However, similar performance can be obtained by adjusting the aspheric coefficient or refractive index distribution coefficient.

Effects of the Invention As is clear from the above embodiments, according to the present invention, (1) by making one or both surfaces of a lens with a refractive index distribution aspherical, a lens with a high NA can be realized; Since spherical aberration and coma aberration are well corrected, high performance within the diffraction limit can be ensured both on-axis and off-axis.

(2) Lenses with a flat surface on one side are easier to process. In addition, in a lens with a refractive index distribution in the axial direction,
Processing is facilitated because there is no need to align the optical axis of the aspherical surface with the optical axis of the refractive index distribution.

(3) Make the thickness of the disc's protective layer thinner than before, or
Alternatively, by setting it to 0, it is possible to reduce or eliminate the occurrence of coma aberration, which becomes a problem when the NA of the lens is increased.

(4) The gradient index single lens of the present invention does not require controlling higher-order terms of the index distribution coefficient, and slight design changes to the aspherical coefficient can be made to reverse manufacturing errors of the gradient index lens. This is easy to implement because it can be absorbed by Therefore, it is possible to provide a gradient index single lens that has a very high numerical aperture and is sufficiently corrected for spherical aberration and comatic aberration.

[Brief explanation of drawings]

FIGS. 1 to 4 are cross-sectional views of a gradient index single lens according to an embodiment of the present invention, and FIGS. 5 to 17 are aberration curve diagrams of an embodiment of the present invention, respectively. . ■...Parallel light, 2...Gradient index single lens, 3...Image surface. Name of agent: Patent attorney Shigetaka Awano Haka1/-m
-Parallel t 2---1! Digit science can! Destruction of Fuzu-103 = Regret Figure 17 NARIO8 ~^O08 IN-0,01”15 -Ht oat IF!'llI Feast

Claims (1)

[Scope of Claims] (1) A single lens whose object side surface is an aspherical surface and whose image side surface is a flat surface, where the refractive index of the center is n_0 and the distance in the radial direction of the lens from the optical axis is r. When, refractive index distribution n(
r) is n(r)=n_0[1-(gr)^2+h_1(gr)
It is expressed as ^4+h_2(gr)^6+...]^1^/^2, and the spherical aberration is corrected by the shape of the aspherical surface and the refractive index distribution coefficient, and 0.6<NA<0.85 0.5<d A gradient index single lens that satisfies the following condition: /(n_0・f)<1.15. However, g, h_1, h_2 are refractive index distribution coefficients, NA is the numerical aperture of the lens, d is the center thickness of the lens, and f is the focal length of the lens. (2) A single lens with two aspherical surfaces, where the refractive index at the center is n_0 and the distance in the radial direction of the lens 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^/^2, and the spherical aberration is corrected by the shape of the aspherical surface and the refractive index distribution coefficient, and 0.7<NA 0.5<d/(n_0・f)< A gradient index single lens that satisfies the condition of 0.85. However, g, h_1, h_2 are refractive index distribution coefficients, NA is the numerical aperture of the lens, d is the center thickness of the lens, and f is the focal length of the lens. (3) A single lens whose object-side surface is aspheric and whose image-side surface is flat; the refractive index at the point where the object-side surface intersects with the optical axis is n_0, and the refractive index from the object-side surface is n_0. The distance in the optical axis direction is z
Then, the refractive index distribution n(z) is expressed as n(z)=n_0+az+bz^2+..., and the spherical aberration is corrected by the shape of the aspherical surface and the refractive index distribution coefficient, and 0.6<NA<0. 85 A gradient index single lens that satisfies the following conditions: 1.6<n_0<1.9 0.5<d/(n_0・f)<0.85. Here, a and b are refractive index distribution coefficients, NA is the numerical aperture of the lens, d is the center thickness of the lens, and f is the focal length of the lens. (4) A single lens with aspheric surfaces on both the object side and the image side, where the refractive index at the point where the object side surface and the optical axis intersect is n.
_0, when the pole in the optical axis direction from the object side surface is z,
The refractive index distribution n(z) is expressed as n(z)=n_0+az+bz^2+..., and the spherical aberration is corrected by the shape of the aspheric surface and the refractive index distribution coefficient, and 0.7<NA 1.55<n_0<1 .85 A gradient index single lens that satisfies the following condition: 0.3<d/(n_0・f)<0.7. Here, a and b are refractive index distribution coefficients, NA is the numerical aperture of the lens, d is the center thickness of the lens, and f is the focal length of the lens. (5) When the wavelength of the light beam is λ, the wavefront aberration between the on-axis light beam and the off-axis light beam with an angle of view of 1 degree is 1/14
Claim (1), characterized in that it is less than or equal to λrms.
2), the gradient index single lens according to any one of (3) and (4). (6) Forms an image through a transparent parallel plate that is a protective layer,
The gradient index single lens according to claim 1, wherein the parallel plate has a thickness of 1 mm or less.