JPS599619A - Large diameter condenser lens - Google Patents

Abstract

PURPOSE:To reduce eccentric error sensitivity even when an aspherical lens is molded out of plastics, by using two positive lenses whose 1st surface only is made aspherical, and specifying the condition of the aspherical surface. CONSTITUTION:The condenser lens 1 for an optical disk consists of the 1st lens 2 and the 2nd lens 3 which are both positive lenses; the 1st surface of the 1st lens 2 on a light source side is an aspherical surface 2a, and the 2nd surface and the 3rd and 4th surfaces of the 2nd lens 3 are spherical surfaces 2b, and 3a and 3b. The 1st and the 2nd lenses 2 and 3 differ in diameter slightly and are fitted fixedly in a lens barrel 4 in the same direction; and the center curvatures, refractive indexes, focal lengths, etc., are determined so that the 1st aspherical surface satisfies a conditional inequality.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a large-diameter aspherical lens whose aberrations are corrected to form a point image determined by the diffraction limit of light.
This invention relates to a condensing lens for optical disks.

In recent years, technology in the field of micro-optics has progressed rapidly, and the above-mentioned condensing lens has come to be widely used as an optical element in this field.

In particular, this type of condensing lens is used in laser pickups for video discs and digital audio discs.

This condensing lens for optical discs has an OT F as a resolution.
Since the cutoff frequency of 1,000 lines/mm or more is required, the NA must be 0.4 to 0.5, and the on-axis and off-axis (±1 to 2°) Aberrations must also be corrected so that they are within the diffraction limit (so-called Leley limit, within λ/4).

Traditionally, this type of condensing lens has used three or more glass lenses, but in this case, the lens itself and the lens barrel must be manufactured with extremely high precision. There was an unavoidable difficulty.

In order to solve this problem, a condensing lens using an aspherical lens and reducing the number of lenses has been proposed (Japanese Patent Application Laid-Open Nos. 156945-1984, 45084-1984, 45084-1980, and 57-1982). (See Publication No. 76512).

JP-A-50-156945 and JP-A-57-765
The condensing lens disclosed in Japanese Patent No. 12 uses aspheric surfaces on both surfaces, which poses problems in lens processing. In other words, a problem with machining aspherical molds made of plastic or hydrated glass is that when aspherical molds are subjected to NC turning, the mold surface is undulated due to the unavoidable "backlash" of the feed mechanism of the lathe. Submicron waviness also occurs in time-controlled polishing due to variations in the amount of polishing. Such waviness affects the optical performance of the condensing lens, for example, the wavefront aberration of 4λ/4 (λ=0.8μ
0.2μ or less). Incidentally, in the case of a spherical mold, there is no room for such waviness to occur due to processing characteristics.

Therefore, it is better to use as few aspherical surfaces as possible. Japanese Patent Laid-Open No. 55-45084 discloses a condensing lens that has only one aspherical surface and is composed of two positive lenses. However, the condensing lens disclosed in Japanese Patent Application Laid-Open No. 55-45084 has an NAo of 45 and a magnification of 1/20 with respect to a finite object point, and has an f2 of 8.4 mm.
As mentioned above, the focal length is long, and if the focal length is shortened to f24-5, which is suitable for the digital audio disc mentioned above, and the size is reduced, a sufficient working distance (W, D working distance) can be obtained. There are some drawbacks such as not being able to do so. Furthermore, the sensitivity to parallel eccentricity errors between the first and second surfaces is relatively large, and mass production with stable performance cannot be achieved unless sufficient precision is maintained during molding.

This invention is composed of two positive lenses, has one aspherical surface, and can secure a sufficient working distance even when the focal length is 5 am or less. Even when manufacturing by plastic molding, the sensitivity to eccentricity error can be reduced.
The purpose is to provide a single diameter condensing lens.

That is, the gist of the present invention is to provide a large-diameter condensing lens consisting of two positive lenses, with only the first surface being an aspherical surface, and the aspherical surface satisfying the following conditions. be.

tl) 0.1<(C1+2A1)(n-1)f<0
．． 6C1 Central curvature of aspherical surface nl Refractive index f for the second run = soon m Focal length of the entire system A Expansion formula of; Expansion coefficient of Y in .

First, when using only one aspherical surface in a large-diameter condensing lens using two positive lenses, the effect of using this as the first surface will be explained.

In conclusion, the parallel eccentricity error sensitivity between the first and second surfaces of the first lens is reduced, the design performance can be fully realized, and the sine condition is favorable, resulting in improved coma correction. The above effects can be seen by comparing the case where the first surface is an aspherical surface and the optimal aspherical shape is determined through trial and error, and the case where the second surface is an aspherical surface and the optimal aspherical shape is determined through trial and error. This became clear. That is,
Regarding the parallel eccentricity error sensitivity, the former case is almost half that of the latter case, and regarding the sine condition, it is possible for the 1j corrector to make further improvements over the best possible situation in the latter case. I understand.

Next, the above condition (1) will be explained. In condition (1), if the lower limit is exceeded, the sine condition cannot be satisfied,
Large off-axis frames occur. On the other hand, when the upper limit is exceeded, the sine condition is almost satisfied, but the parallel eccentricity error sensitivity of both surfaces of the second lun, which has an aspherical surface, becomes extremely large, reaching about 2 to 3 microns for eccentricity ±0.01+++i. . In such cases, mold alignment accuracy of ±0.005 m+l+ or less is required during plastic molding or molded glass molding, making molding difficult. 0.005+nm
The following precision is required, making it practically impossible to manufacture.

Furthermore, when implementing the present invention, it is desirable that the following conditions be satisfied, where f is the focal length of the entire system.

+2+ 0.1<d2/f<0.7 In the above condition (2), if the upper limit is exceeded, the working distance decreases, making it practically unusable as a single pick-up. On the other hand, if the lower limit is exceeded, the eccentricity error sensitivity tends to increase, making it impossible to provide a lens system with stable performance in mass production.

Regarding the above condition (2), in order to ensure a sufficient working distance, it is more desirable to set the range of d2/f as shown below.

+2) 0.1 <d2'/f< 0.6 In addition, the following conditions are further desirable to be satisfied in implementing the present invention.

However, k3 is the radius of curvature of the third surface, and n2 is the refractive index of the second lens. If the lower limit of condition (3) is exceeded, a sufficient working distance cannot be obtained, while if the upper limit is exceeded, the eccentricity error sensitivity increases.

The structure of the large-diameter condensing lens according to the present invention will be explained below. In FIG. 1, reference numeral 1 denotes a condensing lens for an optical disc, which is composed of a second lens 2 and a second lens 3, each of which is a positive lens. .

The first lens on the light source side has an aspherical surface 2a as a first surface and a spherical surface 2b as a second surface, and the second lens 3 has two spherical surfaces 3a as a third surface and a fourth surface.

3b. The first and second lenses 2 and 3 have slightly different apertures, and are fitted and fixed into the lens barrel 4 from the same direction. In this case, the fitting and fixing procedure is preferably as follows. First, the second lens 3 with a small diameter is inserted into the lens barrel 4 from the light source side, and is fitted into the small diameter fitting step 6 provided on the optical disk 5 side of the lens barrel 4. Next, the ring spring 8 is fitted into the ring groove 7 provided, and the second lens 3 is fixed with the fourth surface 3b of the second lens 3 in contact with the lens abutment 6a of the fitting step 6. . Next, Runs 2
is fitted into the lens barrel 4PL91 from the light source side, and fitted into the large-diameter opposing step 9 provided at the middle part of the lens barrel 4, and then the link spring 10 is fitted into the link groove 11 to form the lens barrel 4PL91. Fix 2.

In this way, by adopting a structure in which the second lens 2 and the second lens 3 can be inserted from the same direction, it is possible to simultaneously process the lens contact surfaces 5a, 9a of the lens barrel 4 and the fitting parts 6b, gb. Therefore, the machining centers can be matched. Therefore, the optical axes are easily aligned optically, and eccentricity is less likely to occur.

Furthermore, by bringing the fourth surface of the second lens 3, which is particularly sensitive to eccentricity error, into contact with the lens contact surface 6a of the fitting step 6, there is an advantage that the eccentricity error of the fourth surface can be reduced to almost zero. .

In other words, if the second lens 3 is held in this way, even if there is an assembly error of the second lens 3 with respect to the lens barrel 4 and the second lens 3 is tilted or decentered, the fourth surface is a spherical or flat surface. , eccentricity does not occur on the fourth surface itself. However, in this case, since the entire second lens is decentered, the third surface is decentered. However, there is no problem because the eccentricity error sensitivity of the third surface is small. Conversely, if you try to hold lens 2 by bringing the third surface into contact with the contact surface of the lens barrel, the third surface
Although the eccentricity of the surface is eliminated, the fourth surface is now eccentric, and as a whole, the optical performance deteriorates significantly due to the eccentricity of the second lens.

Next, explaining the aspheric surface of the first surface, the distance X from the tangential plane at the vertex of the first surface to the tangential plane at the incident height Y is expressed by the following general formula. Here, ε is a quadratic surface relation coefficient.

By appropriately selecting ε and Ai in the above equation, it is possible to correct spherical aberration, satisfy the sine condition, and reduce off-axis coma aberration.

Next, a general method of determining the shape when implementing the condensing lens of the present invention will be explained.

(410,7</□ #2<2.6 +51 0.8ψ × ψ3 × 3ψ However, fl and f2 are the focal lengths of the first and second lenses, respectively, ψ3 is the refractive power of the third surface, and ψ is the total It is the refractive power (-d) of the system.

First, the refractive power distribution of the @luns and the second lens is determined so as to satisfy condition (4). If the upper limit of condition (4) is exceeded, the refractive power of the second lens becomes too large, making it impossible to satisfy the sine condition. On the other hand, if the lower limit is exceeded, the refractive power of the aspherical lens becomes too large, and the sensitivity to eccentricity error increases.

In this way, the distribution of refractive power between the first lens and the $2 lens is determined, but the above-mentioned condition (1) determines how the refractive power of the first lens is distributed between the first and second surfaces. be.

On the other hand, condition (5) determines how the refractive power of the second lens is distributed between the third and fourth surfaces. In condition (5), if the lower limit is exceeded, the working distance will be shortened, while if the upper limit is exceeded, the eccentricity error sensitivity will increase and the balance between correction of spherical aberration and coma will deteriorate.

In addition, in order to satisfy the above-mentioned condition (3), the air distance d2 between the first lens and the second lens is determined in relation to the refractive power of the third surface.
decide. In determining the air gap d2, the condition (2) is also satisfied in relation to the focal length f of the entire system.

Specific design examples of the lens according to the present invention are shown below,
Aberration diagrams are shown in FIGS. 2 to 8.

[Example 1] NAo, 45. Angle of view 1", W, D 1.65 Aspheric coefficient ε=I A1=OA2=-0,15646X1
0-"A3-0.59111X10-', A4--O
A4967 Kari O- [Example 2] NAo, 45. Angle of view 1, W, D2. OR15,80
6 dll, 2 n1=1.63 R214,24 Aspheric coefficient ε=I AI=0. OA2=0.8
8700X10-”A3--0.26545 mowing 0-4.
A4 0.21850 mowing o6 [Example 3] NAo, 45 Angle of view 1° W, 1. ) 2.1 Aspheric coefficient ε=l A□=OA2=-0,19349xl
O-2A3=-0,87682 mowing 0-','A4=-0
．． 80132x105 [Example 4] NAo, 45 Angle of view 1° W, Dl, 64 Aspheric coefficient ε=I AI=0. OA2=-0,23481x1
0A3=-0,93051xlOA4=-0,5678
6X10=[Example 5] NAo, 45 Angle of view 1° W, D, 1.98 Aspheric coefficient ε=I AI=0.0 A2--0.165
54 mowing 0-2A3--0.14091×10-4 A4
Knee 0.80188 Mowing 0-5 [Example 6] NA Q, 45 Angle of view 1" W, Dl, 52 Aspherical coefficient ε21 AI=0.OA2-0.1276
5xlO-2A3--0.49010 0-', A
4=-0,33964xlO-5 The aspheric coefficient of the above embodiment may be as follows.

(1) ε=OAl”0.OA2=0.22277 mochi0-
8A3=-0,12806xlO-4,A4=-0,1
3715 mowing 0-5 (2) ε"OA1"'0.0
A22 0.24371 cutting 0-8A3 = 0.23839
X10-4 (3) ε--I AI=0.0 A2=0.172
48K0-2A3 = -0,54611xlO-4 That is, many combinations of coefficients of the same aspherical shape are possible.

[Example 7] NAo, 45 Angle of view 1° W, D2.2 Aspheric coefficient ε=1°OA1=O,o A2=-0,2012xl
O-2A3 supplementary, 14853 mowing 0-5. A4=-0,1
0072 mowing 0-4

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional explanatory diagram showing the configuration of a large-diameter condensing lens according to the present invention, and FIGS. 2, 3, 4, 5, 6, 7, and 8 are respectively Examples 1.2, 3, 4 of the present invention
, 5, 6.7 is an aberration diagram of the lens system. 2...No. 1 lens, 2a...Aspherical surface, 3...Second lens, 4... Lens barrel, 6a...Lens contact surface. Patent applicant: Minolta Camera Co., Ltd.
Buried patent attorney Aoyama Ao and two others 1st plan O

Claims (1)

[Scope of Claims] (1) A large-diameter condensing lens consisting of two positive lenses, with only the first surface being an aspherical surface, and the aspherical surface satisfying the following conditions; o. i < (C, +2A,)・(n-1)f < 0
．． 6C□ ... Central curvature of the aspherical surface nl ... Refractive index f for λ of the th lens: 800 nm ... Focal length of the entire system 80 - From the tangential plane at the apex of the aspherical surface to the tangential plane at the incident height Y Expansion formula of aspherical surface when the distance of is set to X; Expansion coefficient of Y in . (2. In the large-diameter condensing lens described in claim 1. The relationship between the air distance d2 between both lenses and the focal length f of the entire system is selected as follows for the large-diameter condensing lens = 0 .1 <d
2/f < 0.7 (3) In the large-diameter condensing lens according to claim 1 or 2, the air distance d2 between both lenses and the radius of curvature of the third surface is 3 and the second lens. A large-diameter condensing lens whose refractive index n2 is selected as follows: (4) In the large-diameter condenser lens according to claim 1, the fourth surface of the second lens is attached to the lens barrel. A large-diameter condensing lens that is characterized by being in contact with each other. (5) The large-diameter condenser lens according to claim 2, characterized in that both the first and second lenses can be inserted into the lens barrel from the same direction. .