JPS60140309A - Refractive index distribution type single lens - Google Patents

Abstract

PURPOSE:To correct satisfactorily a spherical aberration by constituting a titled lens so that a luminous flux incident surface and a luminous flux emitting surface of a single lens having a refractive index gradient in the direction vertical to an optical axis used by a reducing magnification form a plane and a convex surface, respectively. CONSTITUTION:In case a single lens having a refractive index gradient in the direction vertical to an optical axis is used by a reducing magnification, a surface of a luminous flux incident side and a surface of its luminous flux emitting side form a plane S1 and a convex surface S2 against an image field side, respectively, and a plano-convex lens is formed. A radius of the convex surface S2, a thickness of the single lens, and a focal distance are denoted as r2, (d), and (f), respectively, so as to satisfy a condition of -1.5<=r2/f<=-0.5, and 0.8<=d/f<=2.8. Also, it is desirable to satisfy 0.4<=¦r2/d¦<=0.6. In this way, a spherical aberration and a sine condition are corrected satisfactorily and easily, and this lens is suitable as a collimator lens and a pickup use objective lens of an optical disk.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a gradient index lens suitable for a collimator lens of a semiconductor laser, an objective lens for a pickup of an optical disk, and the like.

Selfoc lens (trade name) is well known as a so-called radial gradient index lens, which has a refractive index distribution in the direction perpendicular to the optical axis. It is used in copying machines, etc.

In recent years, attempts have been made to use this gradient index lens as a single lens as an objective lens for pickups of digital audio discs, etc.
topical meetington fradien
t-index optical imafinfs
) 'atem shows the use of a gradient index lens with a plano-convex shape. However, in the single lens shown here, only the correction of spherical aberration, which is an axial aberration, is considered. On the other hand, when actually used as a pickup objective lens or a collimator lens, not only axial aberrations but also off-axis aberrations must be well corrected.

In view of the above-mentioned points, it is an object of the present invention to provide a gradient index single lens in which spherical aberration and sine conditions are simultaneously well corrected.

In the single lens according to the present invention, the shape is a plano-convex lens, and when the single lens is used at a reduction magnification, the surface on the light incident side is If the thickness of the single lens is d and the focal length is f, then the above objective is achieved by satisfying the following conditions: -1,5≦r, /f≦-05 0,8≦d/f≦28 It is something. Therefore, when the single lens according to the present invention is used as an objective lens for an optical pickup, the convex surface faces the recording medium, and when used as a collimator lens for a semiconductor laser, the convex surface faces the semiconductor laser.

Furthermore, in the single lens according to the present invention, 0.4≦l r
By satisfying the condition t/d I≦0.6, better aberration correction is possible.

In addition, when the single lens according to the present invention is used with the imaging magnification being a reduction magnification, the above-mentioned setting of the lens means that a parallel light beam or a light beam close to a parallel light beam enters or exits a plane. It is something.

The present invention will be explained in detail below.

In order to correct the spherical aberration and the sine condition, it is necessary to reduce the values of the third-order spherical aberration coefficient and coma aberration coefficient.

For the distance r where the refractive index N is − from the optical axis, N(r)=
No +N, r” + Nzr' + N, r'
+ N4r@+ Song・-(1) (No, Ns, K
, Ns - N4 (constant)), the parameters that contribute to the value of the third-order aberration coefficient are No, N11N
2 and r, : radius of curvature of first surface r, : second surface 〃 d: thickness. Of these, the axial refractive index N0 is 1.4・-1
, 8 degrees, so if we consider it as No. 1.6, the parameters contributing to the third-order aberration coefficient are rl lr, +
It is thought that there are five: d + NI + Nt.

On the other hand, the following three conditions are required, so since one surface is flat, r,
Even if it is limited to , r2+d satisfies condition (2),
It is expected that there are many solutions for Nl, N.

From these many solutions, it is possible to select one that can correct high-order aberrations or one that has an appropriate working distance, depending on the conditions of use.

Of rt + d * N1 g N2, three contribute to the paraxial quantity: r, , d, N, and P, J, Jour, Opt, S by 5ands
oc, Am, + 60. pp. 1436-1443 (197
0 years), N has a linear relationship with each third-order aberration coefficient. Therefore, for a certain rt, d, N'+, and Nt that satisfy condition (2) can be determined by the following procedure.

■Give d arbitrarily.

Set N so that (-3(==constant).

■Set Nt so that l=o.

■■ Change d so that it becomes -0 and repeat ■~■ After determining the initial value of the parameter rt+lN1+N2 through these steps, change each parameter as in the case of conventional lens design, It is sufficient to balance each aberration.

In addition, the higher-order coefficients of the refractive index distribution NS t N41...
By introducing this, it is possible to further correct spherical aberration and achieve a large I'' diameter ratio.

The following facts have become clear from the above design process. First, for correction of spherical aberration and sine conditions, it is desirable that r, d satisfy the following conditions.

−1,5≦r＠/f≦−0゜5 ・・−・−・−(3−
1) 0.8≦d/f≦2.8 ・・・・・・・・・・・・
(3-2) When r2 exceeds the upper limit of conditional expression (3-1), it becomes difficult to correct spherical aberration, and when r2 exceeds the lower limit, the effect of coma aberration correction by the second surface cannot be obtained. O Also, if d exceeds the lower limit of conditional expression ('1-2).

In order to keep the focal length constant, the absolute value of N increases,
This makes manufacturing difficult and worsens spherical aberration. When d exceeds the upper limit, the working distance decreases.

For better correction of spherical aberration and sine conditions, it is desirable to further satisfy the following conditions: Q O14≦l r2/d l≦0.6...
...(3-3) That is, when 1r21 becomes thicker and the refractive power due to the second lens is in a small state, in order to keep the focal length constant throughout,
Although it is necessary to increase the refractive power of the refractive index distribution, spherical aberration and the sine condition can be well corrected by increasing d according to the relationship of conditional expression (3-3') and suppressing the increase in the refractive index gradient. can.

An embodiment of the present invention will be described below. Table 1 shows the lens data of the first to seventh embodiments of the single lens according to the present invention, and as shown in FIG. 1, r is the radius of curvature of the plane + rt is the radius of curvature of the convex surface. d is the thickness of the lens. Note that the radius of curvature r1 of the plane is infinite.
+Nr +Nt +Na +N4 is a constant that determines the refractive index distribution of a single lens, as shown in equation (1). Further, the lens data indicates a value when the focal length is normalized to 1. In this application, when the light beam advances from the flat surface to the convex surface of the threat lens when used at a reduction magnification as shown in Fig. 1, the light beam incident side of the single lens is the object world side, and the light beam exit side is the image side. Therefore, the value of the radius of curvature of the surface is positive when the center of curvature is located on the image field side of the surface, and negative when vice versa.

Table 1 Table 2 shows the back focus S'K + third-order spherical aberration coefficient 1. when the object is at infinity for each of the examples (floors 1 to 7) shown in Table 1. The values of the coma aberration coefficient (■), the astigmatism coefficient (■), the Petzval sum P, the distortion aberration coefficient V, and I rt/d l are shown.

Figure 2 (a) and (b) are diagrams showing the aberrations of the third embodiment (Ugly 3), where the solid line in Figure 2 (a) is the spherical aberration, and the broken line is the sine condition. The solid line in (b) represents the spherical field curvature.

Dashed lines indicate meridional field curvature. Figure 3 also shows the third
It is a diagram showing the refractive index distribution N(r) in the direction orthogonal to the optical axis of the lens shown in the example, where the vertical axis shows the refractive index N and the horizontal axis shows the distance from the optical axis (r-0).

The fourth embodiment (la4) has a particularly large aperture with an NA of about 0.5, and can be used as an objective lens for optical disk pickup.

As shown in Fig. 2 (a) and (b), each aberration is well corrected, and the aberrations of other lenses are also reduced to NA: 0.2 to 0.3.
．． It shows good performance with a half angle of view of about 3 degrees.

As can be seen from Table 2 for all of the first to seventh embodiments, the third-order spherical aberration coefficient and coma aberration coefficient are well corrected, and when increasing the aperture, higher-order Higher-order spherical aberrations can be corrected by controlling the coefficients of the refractive index distribution.

In addition, in the embodiment, the spherical aberration is corrected by the coefficient I Nt l N'31... of the refractive index distribution, but a similar effect can also be obtained by introducing an aspheric surface to the second surface. It will be done.

This is because for the third-order spherical aberration coefficient caused by the refractive index gradient, N2 is L x/h''(x) dx.

For the third-order coma aberration coefficient, Nt x/h26cl
It contributes in the form of h (x) dx. Here, h(x
) is the height of the paraxial ray at a point inside the heterogeneous medium,
h(x) is the height of the paraxial principal ray, and integration is performed in the optical axis direction of the heterogeneous medium. Therefore, these integral values are r, H
r2 , d HNo HN (determined only by the object and entrance pupil position, but if the entrance pupil is near the lens and the lens is not very long, then h(x) will be a small value, N2 has almost no effect on the coma aberration coefficient.That is, the value of the coma aberration coefficient is rt
It is determined only by + rt + d + No + N+ and the object distance.

It is easy to obtain the effect of correcting the spherical aberration due to N by the fourth-order aspherical coefficient of the second surface, but in that case, the fourth-order aspherical coefficient does not contribute to the coma aberration coefficient. At the stage where the spherical aberration is corrected, the coma aberration coefficient is not related to the entrance pupil position, so if the entrance pupil is located on the second surface, the contribution of the fourth-order aspherical coefficient to the coma aberration coefficient becomes 0.

This situation is basically the same for higher-order aberrations, so the coefficient N2. of the refractive index distribution. N.

... is based on the 4th and 6th order aspheric coefficients and aberration correction,
are almost equivalent.

FIG. 4 is a partial schematic diagram of the case where the single lens of the present invention is applied as an objective lens for pinking up an optical disk. In FIG. 4, l is the single lens in the present invention % 2
is the glass plate of the original disk. t is the thickness s of the glass plate, NG is the refractive index of the glass plate, and WD is the air distance between the single lens and the glass plate.

For the third garment, t=1.2, NO-2,52, \VD=0.
Table 4 shows an example of the lens data of the single lens 1 in the case of 87 dust. Table 4 shows the focal length f at this time, the air-equivalent back focus S'K + 3rd order aberration coefficients of the single lens.

and +rt/a+ values are shown.

Table 4 and FIGS. 5(a) and 5(b) show the aberrations of this single lens.

The solid lines and broken lines in FIG. 5(a) are the same as those in FIG. 2(a), and the solid lines and broken lines in FIG. 5(b) are the same as those in FIG. 2(b).

To apply such a single lens according to the present invention, select a lens with an appropriate back focus from Table 1, and attach it to the glass plate 2.
This can be easily achieved by simply correcting the deterioration of spherical aberration caused by.

Note that the aberration coefficients in Tables 2 and 4 and the aberration diagrams in Figures 2 and 5 are values when the object is at infinity and the entrance pupil is aligned with the principal point on the image side. .

In addition, in the present invention, as shown in Figure 3, it is possible to correct higher-order spherical aberrations by having a very weak negative or positive refractive index gradient near the optical axis of the lens and a strong positive refractive index gradient around the lens. preferred above. Such a refractive index distribution is Y, K
oike, Y.

0htsuka: ApPlied 01)ties
It can be formed by a photocopolymerization method as shown in K. + 22 + pp. 418-423 (1983).

In addition, in the ion exchange method, ions that have the effect of increasing the refractive index by short-term ion exchange, such as TL, are used.
+, Cs+, etc., by distributing them around the lens periphery.

As described above, the present invention has good performance even though one end surface is flat, and this not only greatly facilitates lens processing and inspection, but also significantly simplifies the structure of the lens barrel. be done.

For example, in the case of the optical disk pickup objective lens described in FIG. 4, an autofocus mechanism and an autotracking mechanism are usually required to deal with optical disk surface damage and eccentricity. For this reason, a method is used in which the objective lens is attached to an electromagnetically driven movable element called an actuator, and the objective lens is moved in the direction of the optical axis and in the direction perpendicular to the optical axis.

In such a case, in order to improve drive responsiveness, it is necessary to reduce the weight of the objective lens itself and the lens barrel that supports the lens.

In the present invention, the objective lens is a single lens and is lightweight, but since the first surface of the objective lens is flat, the mechanism for attaching it to a lens barrel or actuator is significantly simplified.

FIG. 6 shows an example of how to attach an optical disk pickup objective lens and an actuator according to the present invention.

1 is a single lens according to the present invention, 3 is a simplified movable part of the actuator, and it is only necessary to bond the first surface of the single lens 1, which is a flat surface, to the end surface of the movable part.

In this way, in the single lens of the present invention, since the first surface is flat, processing of the lens itself is extremely easy, and the structure of the lens barrel that supports the lens is also significantly simplified and lightened. 0 Even when a prism or the like is placed in front of the lens, by bonding the prism surface and the end face of the single lens of the present invention, it is possible to simplify the lens barrel structure and reduce surface reflection. You can also get

In the embodiments described above, the case where the object point exists at an infinite distance from the plane is illustrated as an example of using the reduction magnification. However, even if the object point is at a finite distance from the plane, the reduction magnification The performance of the intimidation lens is good if used in

In the present application, spherical aberration and sine conditions are corrected using a single lens, but such a single lens can also be effectively utilized as one element of a combination lens.

As described above, according to the gradient index single lens according to the present invention, it is possible to correct spherical aberration and sine conditions, and it can be used as a collimator lens or an objective lens for picking up an optical disk.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the shape of a single lens according to the present invention, FIGS. 2(a) and (b) are diagrams showing aberrations of an example of the single lens according to the present invention, and FIG. 3 is a diagram showing the shape of a single lens according to the present invention. A diagram showing the refractive index distribution of an example of a single lens, FIG. 4 is a schematic diagram when the single lens according to the present invention is used as a pickup lens for an optical disk, and FIGS. 5(a) and (b) are FIG. FIG. 6 is an aberration diagram of an embodiment of the single lens shown in FIG. 1... Gradient index single lens, 2... Glass plate, 3...
・Actuator moving part, rl+rt...radius of curvature, d...axial wall thickness, N, NQ...refractive index, W,
D...axial air gap, t...thickness of the glass plate. Applicant Canon Co., Ltd. No. 30 η4 Kasumi

Claims (1)

[Claims] (1) In a single lens having a refractive index distribution in a direction perpendicular to the optical axis, when the single lens is used at a reduction magnification, the surface on the incident side of the light beam is a flat surface, and the surface on the light beam exit side is also a flat surface. The side surface forms a convex surface, and the radius of curvature of the surface on the light beam output side is r, d.
When is the thickness of the single lens and f is the focal length of the single lens, -1,5≦r, /f≦-0,5 0,8≦d/f≦2.8. Refractive index distribution type! ns. (2) The gradient index single lens according to claim 1, wherein the relationship between d and r is as follows: 0.4≦l rz/d I≦0.6.