JPH04163510A - Object lens for optical disk - Google Patents

Abstract

PURPOSE:To realize an object lens with numerical aperture about 0.6-0.8 for an optical disk by an aspherical single lens having high optical performance without using an aspherical term of higher order by using a single lens which is aspherical at least on one side of the light source and the optical disk side and satisfying specific conditions. CONSTITUTION:In constitution of an object lens, the lens is a single lens which is aspherical at least on one side of the light source and the optical disk side, and the conditions of formulae I-III are satisfied. In the formulae I-III, r1 is a radius of curvature of the lens surface on the light source side at the position of a vertex, r2 is a radius of curvature of the lens on the optical disk side at the position of a vertex, (n) is an index of refraction of the lens at a used wavelength, d is a thickness of the single lens at its center, and f is a total length of the single lens. Accordingly, an object lens with numerical aperture about 0.6-0.8 for an optical disk is realized by an aspherical single lens with practically sufficient optical performance without using an aspherical term of higher order.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an objective lens suitable for an optical information recording/reproducing device.

(Prior Art) When using a semiconductor laser (usually wavelength of about 780 nm) as a light source, the objective lens used in the reproduction optical system of the optical information recording medium has a numerical aperture (NA) of 0.45 to 0.00 for a compact disc. 47, 0 for video discs.
It is necessary to have diffraction limited performance between 5 and 0.53. On the other hand, NAo of 5 to 0.6 is required for recording optical systems, DRAW optical systems, and magneto-optical recording optical systems.

However, in order to further increase the amount of information recorded on the optical information recording medium, it is necessary to increase the density by (1) shortening the wavelength of the light source light, or (2) increasing the numerical aperture of the lens. No.

Among objective lenses for optical discs composed of a single lens, examples of objective lenses with relatively large numerical apertures include the Japanese Patent Laid-Open No. 6
There is one described in Japanese Patent No. 1-200518, but the NAo is about 6.

In order to increase the numerical aperture to greater than O06 while maintaining the required optical performance, two or more lenses were required.

(Problems to be Solved by this Invention) This invention provides an objective lens for optical discs with a numerical aperture of about 0.6 to 0.8, which has high optical performance without using a high-order aspherical term. This is what we are trying to achieve with a single spherical lens.

(Means for Solving the Problem) In the present invention, the objective lens is configured as a single lens having an aspherical shape on at least one of the light source side and the optical disk side, and satisfies the following conditions.

1.5<n (1)0.8
3<d/f<1.2 (2) l r, /
r21 <0.7 (3) However, rd: Radius of curvature at the apex of the lens surface on the light source side r2: Radius of curvature at the apex of the lens surface on the optical disk side n: Refractive index of the lens at the wavelength used d: Core thickness of the single lens f : Focal length of a single lens There are various ways to express the shape of an aspheric surface, but the best method is to express it by adding a correction term expressed by an even hexadecimal series of heights from the optical axis to a term of a rotational quadratic surface. Common. In this expression method, in an orthogonal coordinate system with the origin at the vertex of the surface and the optical axis direction as the The spherical shape is expressed as r=f1℃T. In this expression method, Po is limited to 8 at most so as not to use high-order terms. -0,6<Δ2/f<O,O (4) However, Δ2: has an aspheric surface at the most peripheral effective diameter of the image side surface (position where the maximum NA ray enters) and apex radius of curvature r2 The difference in the optical axis direction from the reference spherical surface is defined as positive if the aspherical surface is displaced toward the light source as the distance from the optical axis increases.

Further, it is desirable that 0.5<r, /f<0.9 (5).

(Function) Condition (1) relates to the refractive index of the lens. If this condition is not met, it will be impossible to increase the numerical aperture while maintaining the required performance for short wavelength light source light. Become.

Condition (2) relates to the core thickness of the lens; if the upper limit is exceeded, the core thickness of the lens will increase and the lens will become larger. Also, the working distance (
It becomes difficult to increase the WD). If the lower limit is not met, it is advantageous for miniaturization, but the meridional field curvature will occur strongly in one direction, the optical axis will shift, the focal point of the light beam will be off the optical axis, and the image height will increase. When this occurs, the deterioration of wavefront aberration becomes significant.

Condition (3) is mainly intended to satisfactorily correct spherical aberration. If this condition is exceeded, high-order spherical aberration occurs, and even if the aberrations are balanced, the spherical aberration curve will become largely meandering, and the wavefront aberration will worsen. In order to correct these higher-order spherical aberrations, higher-order aspherical terms must be used, which is undesirable in terms of processing.

In particular, in order to obtain a large numerical aperture for short-wavelength source light, it is optimal to satisfy 0.4<l r,/r, I<0.55. Outside this range, even if the spherical aberration is corrected, the meridional curvature of field cannot be completely corrected. This becomes difficult when using short wavelength light source light, but
For light source light having the wavelength of a normal semiconductor laser (about 780 nm), a numerical aperture of 0.7 or more can be secured in this range.

Condition (4) relates to the amount of aspherical displacement on the image side; if it exceeds the upper limit, the spherical aberration will be over-corrected; conversely, if it falls below the lower limit, the spherical aberration will be under-corrected.Condition (5) is as follows: Regarding the radius of curvature at the apex of the surface on the light source side. The spherical aberration is
Although it can be corrected using an aspheric surface, it must be determined so as not to worsen the sine condition. When the upper limit is exceeded, the sine condition becomes over, and conversely, when it falls below the lower limit, the sine condition becomes under.

(Example) Examples of the objective lens of this invention will be shown below.

In the table, f is the focal length of the single lens, λ is the wavelength of the light source, m is the imaging magnification of the single lens, rl is the radius of curvature at the apex of the lens surface on the light source side, r2 is the radius of curvature at the apex of the lens surface on the optical disk side, n represents the refractive index of the lens, d represents the core thickness of the single lens, and C represents the Abbe number for the d-line of the single lens.

In addition, the aspherical shape is expressed by the above formula,
is a conic constant, A1 is an aspherical coefficient, and Pl is an aspherical surface.

Note that C represents a plane-parallel plate corresponding to the protective layer of the optical disc, which is arranged between the objective lens and the imaging point.

In Example 1, for light source light of λ=532 nm, NAo
, 7. The maximum image height at which the least root mean square value (RMS value) of wavefront aberration is within 0.07λ (Marechal's tolerance) is approximately 0.085 mm, despite the large numerical aperture and short source light wavelength. It has a sufficiently wide field of view.

Example 2 is for light source light of λ=532 nm.

NAo is 8. The maximum image height at which the RMS value of wavefront aberration is within 0.07λ is about 0.086 mm.

In Example 3, for light source light of λ=532 nm, NAo
, 6. In addition, plastic, which has relatively good temperature characteristics, is used as the glass material. RMS value of wavefront aberration is 0.0
The maximum image height within 7λ is 0°075 mm.

In Example 4, for light source light of λ=780 nm, NAo
, 7. The maximum image height at which the RMS value of wavefront aberration is within 0.07λ is 0.083 mm.

Example 5 is for light source light of λ=780 nm.

NAo is 65. RMS value of wavefront aberration is 0°07λ
The maximum image height within this range is 0.1 mm or more.

Furthermore, by using a glass material with a large Abbe number and low chromatic dispersion, it is possible to create an objective lens with a large numerical aperture and low chromatic aberration, which can be used even when the wavelength changes each time writing and erasing, such as with magneto-optical disks. It is. Example 6 is an example of such a case. In this example, wavelength 780
The axial chromatic aberration is approximately ±0.002 mm for wavelengths of 760 nm, 760 nm, and 800 nm, achieving an axial chromatic aberration/wavelength difference of ±0.0001 mm/nm.

The maximum image height at which the RMS value of wavefront aberration is within 0.07λ is 0.1 mm or more, and has a sufficiently wide field of view.

Example 1 f = 3.89 mn λ = 532 nm m = o
NA O,70r d n Aspheric coefficient/heavy number on the first surface = -1,28392 A1 = 6.23360X10-3P, = 4.0
000A, = 9.07906xlO-SP2=
6.0000A, = 8.05098X10-'
P3=8.000OA4=6.56022×1O-
7P4=10. ooooo.

On the second side = -2,34275x 10 A□ = 5.10396X10-3P, = 4.0
000A2= -4,97446X10-' P2=
6.0000A, = 9.48216X10-'
P, = 8.0000A4 = 1.33818
x10-' P4= 10.0000d/f=
0.925 1 rzl rzl = 0.473△2
=-0,1933r, / f =0.648 Example 2 f =3.89mm λ=532nm m=ONA
O, 80r d n νd 1] L2・5183・601・52141 64・52
”-5,3351,20 31C001,201,5190064,14”
ω Aspheric coefficient/power on the first surface = -1,31094 A□= 6.47833X10-' P□=
4.0000A2=9.65323XIP'
P2 = 6.0000A, = 2.17873X1
0-”P,=8.0000A4=1.2
6328xlO-” p4= 10.0000 2nd
On the surface = -2,51481X 10 A1 = 4.44446X10-” P, = 4
．． 0000A2=-3,67271X10-'P
2= 6.0000A3= 109482 X 1
O-sP3= 111.000OA4= 7.86
137X10-” P4= 10.0000d/
f =9.25 1 r□/ rzl=0.472△2
=-0,5606r, /f=0.647Example 3 f=3.6(Jwn λ=532r+m m
'=o NA O,60r d n to L 2.307 3.10 1.50
249 56.4□ 2n -4,6161,19 31(: oo 1.20 1.51900
64.14" ψ Aspheric coefficient/power on the first surface = -7,66450X 1O-1 A, = 2.88930 x 1O-3P,
= 4.0000A2= -9, (16490xlO
-' P2 = 6.0000A, = 1.376
20X10-SP, = 8.0000A4 = -3,
11170X10-' P4= 10.0000 2nd
On the surface = -2,44450X 10 A, = -1,49330X10-' P1 = 4
．． 0000A2=-1,18330X10-'P
2 = 6.0000A, = 1.05520X10
-' P, = 8,0000A, = 6.782
00XIO-7P4=10.0000d/f=
0.861 1 r, / rl= 0.500△2=
-0,2504r, / f = 0.641 Example 4 f = 3.69 + m λ = 780 nm m = O
NA O,70rd n I L 2.575 3.12 1.60910
36.32] -9,4711,20 Aspherical coefficient/power on the first surface = -1,29744 A1= 6.28096X10-" p, :=
4.0000A, = 1.05771X10-'
P2 = 6.000OA, = 9.39405X1
0-'P, = 8.000OA4 = 6.50
502X10-7P, = 10.0000 on the second side = -6,72012X10 A, = 4.96720X10-3P, = 4.0
000A, :=-4,89956XIO-4P2=
6.0000A, = 9.97738XIO-6P,
= 8.0000A,, = 1.35357xlO
-' P, = 10.0000d/f = 0
．． 846 l r1/ rzl=0.272△2=-
0,1186r, / f = 0.698 Example 5 f = 3.84wn λ = = 780nm m =
ONA 0.65r dy) L L 3,072 3.90 1.60910
36.321 -5.0861.20 Aspheric coefficient/power on the first surface = -2,83817X 10"1 A1 = -1,41538X10-' P, = 4
．． 0000A2=-2,73623X10-'P
2 = 6.0000A, = 1.05674xlo
-sP, = 8.0000A, = -5,7875
1X10-' P4 = 10.0000 on the second side = -2,05642X 10 80 = -4,17364xlO-3P□ = 4.00
00A2=-4,64061X10-4P2=6.
0000A 3= 7.52342 x 10-'
P, = 8.000OA4 = -2,7691
2xlO-'P, = 10.0000cl/f
=1.016 1 r1/ r21=0.604△2
=-0,0631r, / f =0.800 Example 6 f =3.80m+ λ=780nm m=o
NAo, 70rd n I L 2.451 3.60 1.51314
64.5] 2J -4,7921,15 Aspheric coefficient/heavy number 1st surface K = -1,27020 A□= 6,59776 X 1O-3P 1=
4.0000A2=1.27463X10-4
PJ = 6.0000A, = 2.71143xl
O-'P, = 8.0000A4 = 1.0
1747xlO-' P4 = 10.0000 on the second side = -2,25124X 10 A = 4.66178xlO-' P engineering = 4
．． 0000A2=-4,82678xlO-'P
2 = 6.000OA, = 1.21529X10
”5P3= 8.0000A4= 1.49282
X10-'P, = 10.0000d/f =
0.947 l r, / r, l=0.511△,=
-0,230 Or, / f = 0.645 (Effect of the invention) With this invention, as seen in each example and the aberration diagram, the numerical aperture is 0.6, including for light source light with a short wavelength.
It has become possible to realize an objective lens for an optical disk of about 0.8 with a single aspherical lens having practically sufficient optical performance without using a high-order aspherical term.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view including the parallel plane plate of the objective lens of the present invention, FIGS. 2 to 7 are aberration curve diagrams of the first to sixth embodiments, respectively, and FIGS. 8 to 13 are diagrams of wavefront aberrations, respectively. It is an image height characteristic diagram of RMS value.

Claims (1)

[Claims] An objective lens for an optical disc, characterized in that it is a single lens having an aspherical shape on at least one of the light source side and the optical disc side, and satisfies the following conditions: 1.5<n 0.83<d/f<1.2|r_1/r_2|<0.7 However, r_1: radius of curvature at the apex of the lens surface on the light source side r_2: radius of curvature at the apex of the lens surface on the optical disk side n: refractive index of the lens at the wavelength used d : Core thickness of single lens f: Focal length of single lens