JPH04355419A - Objective lens - Google Patents

Abstract

PURPOSE:To provide an objective lens capable of correcting easily rotational asymmetrical aberration to the optical axis. CONSTITUTION:An objective lens is constituted of a converging group 10 having mainly a converging function, an aberration correcting group 6 to correct rotational asymmetrical aberration mainly to the optical axis of the converging group and mirror frames 20 and 21 to support the converging group 10 and the aberration correcting group 6 so as to be rotatable relatively and freely around the optical axis, and has a characteristic that by setting power of the converging group 10 as (phia) and by setting power in the largest absolute value direction of the aberration correcting group 6 as (phib), ¦phia/phib¦>50 can be satisfied.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-performance objective lens used in an optical disc device, and more particularly to an objective lens capable of correcting rotationally asymmetric aberrations with respect to the optical axis.

0002

2. Description of the Related Art Hitherto, Japanese Patent Application No. 1-224177 discloses a lens having a parallel plane closest to the condensing side as means for correcting rotationally symmetrical aberrations with respect to the optical axis, such as spherical aberrations.

According to this configuration, it is possible to secure a long working distance while suppressing the occurrence of aberrations,
Furthermore, it is possible to provide a highly versatile lens system that can be easily applied to different types of media.

0004

[Problems to be Solved by the Invention] However, the lens disclosed in Japanese Patent Application No. 1-224177 suffers from eccentric coma aberration caused by manufacturing errors and astigmatism caused by the fact that the curvature of the lens surface differs depending on the direction. It is not possible to correct aberrations that are rotationally asymmetric with respect to the optical axis, such as aberrations.

Therefore, when assembling a lens with a very large numerical aperture (N.A.), that is, a lens that requires severe aberration correction, rotationally asymmetric aberrations that are greater than the required level remain, and the diffraction limit is exceeded. It was difficult to bring out the best performance.

[0006]

OBJECTS OF THE INVENTION The present invention has been made in view of the above problems, and it is an object of the present invention to provide an objective lens that can easily correct rotationally asymmetric aberrations with respect to the optical axis.

[0007]

[Means for Solving the Problems] In order to achieve the above object, the objective lens according to the present invention has a focusing group that mainly has a light focusing function and an aberration that is rotationally asymmetric with respect to the optical axis of the focusing group. It consists of an aberration correction group that corrects the aberration correction group, and a lens frame that supports the condensing group and the aberration correction group relatively rotatably around the optical axis. Letting the power in the direction where the value is large be φb, |φa/
It is characterized by satisfying φb|>50.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be explained below based on the drawings. 1 and 2 show an embodiment of the present invention.

The illustrated objective lens has a condensing group consisting of first to fifth lenses 1, 2, 3, 4, and 5, from the light source side on the left side to the condensing point side on the right side of the figure. 10 and
Prisms 6 as an aberration correction group are arranged in order.

The condensing group 10 is supported by a front lens barrel 20, and the prism 6 is supported by a rear lens barrel 21 that fits into a stepped portion of the front lens barrel 20 on the converging point side. The rear lens barrel 21 is rotatably provided around the optical axis of the condensing group 10 with respect to the front lens barrel 20, and is fixed by a retaining ring 22 that is screwed onto the front lens barrel 20 from the condensing point side. has been done.

In this example, the prism 6 is used as the aberration correction group.
By using this, it is possible to correct rotationally asymmetric aberrations such as eccentric coma aberration. As an aberration correction group, in addition to a prism, a parallel plane plate tilted with respect to the optical axis can be used to correct decentered comatic aberration, and a parallel plane plate tilted with respect to the optical axis can be used to correct eccentric comatic aberration. can use optical elements such as cylinder lenses and toric lenses that have different powers depending on the direction.

During assembly, each lens of the condensing group 10 is housed in the front lens barrel 20, aberrations are measured, and the type of optical element used as the aberration correction group is selected depending on the type of aberration. In addition, the inclination angle of the parallel plane plate, the apex angle of the prism, and the radius of curvature of the cylinder lens and toric lens are determined according to the amount of aberration.

Next, after assembling the determined optical elements of the aberration correction group into the rear lens barrel 21, the rear lens barrel 21 is fitted into the front lens barrel 20, and the front lens barrel 20 and the rear lens barrel 21 The directionality of the aberration and the directionality of the aberration correction group 6 are made to coincide with each other by rotating them relatively. After matching the direction, press the holding ring 22
to fix the rear lens barrel 21.

The objective lens according to the present invention satisfies the following condition: |φa/φb|>50, where the power of the condensing group 10 is φa, and the power of the aberration correction group 6 in the direction of the largest absolute value is φb. ing.

If this condition is not met, the power of the aberration correction group 20 becomes excessive, and the tolerance for errors in the shape of the aberration correction group 20 and errors in the distance between the condensing group 10 and the aberration correction group 20 becomes too large. This makes it difficult to manufacture and assemble the aberration correction group.

FIG. 3 shows a numerical example of the objective lens according to the present invention. The specific numerical structure is shown in Table 1.
is shown. FIG. 4 shows spherical aberration and astigmatism due to this configuration, and FIG. 5 shows wavefront aberration. Note that the wavefront aberration in FIG. 5 shows meridional on the left and sagittal on the right, and the unit of the vertical axis representing the amount of aberration is wavelength λ.

In the table, NA is the numerical aperture, f is the focal length, ω is the half angle of view, fB is the back focus, λ is the wavelength used, r is the radius of curvature, d is the interplanar distance, and N is the d-line ( 58
8 nm), and Nλ is the refractive index at the wavelength used.

[0018]

[Table 1]

When the above lens is processed and assembled as designed, it can provide sufficient performance as shown in FIGS. 4 and 5, but if the performance deteriorates due to processing errors, eccentricity, etc. In this case, by tilting the plane-parallel plates constituting the 13th and 14th surfaces or replacing them with members such as prisms, it is possible to function as an aberration correction group and correct aberrations.

For example, when the fifth lens is decentered by 0.01 mm, the wavefront aberration exhibits comatic aberration as shown in FIG. In order to correct this, the wavefront difference is improved as shown in FIG. 7 by tilting the parallel plane plate by 0.34°. In this case, |φa/φb|=∞.

When a parallel plane plate is used as the aberration correction group, it is easy to obtain surface accuracy, so errors occur less;
Further, since the power of the aberration correction group is 0, no aberration change occurs depending on the distance between the condensing group and the aberration correction group.

When the parallel plane plate is replaced with a prism, the wavefront aberration is improved as shown in FIG. 8 by setting the apex angle to 0.060°. In this case also |φa
/φb|=∞.

Next, when the first surface of the first lens has an asymmetric component equivalent to 0.5 Newtonian fringes, the wavefront aberration exhibits astigmatism as shown in FIG. When correcting this astigmatism, the wavefront aberration is improved as shown in FIG. 10 by replacing the parallel plate with a cylinder lens having a radius of curvature of -8000 mm. In this case |φa/φb|=37
It becomes 12.

In addition, a radius of curvature of −4 is provided on the front side of the first lens.
By adding a 4000 mm BK7, T=1.20 mm cylinder lens, wavefront aberration is improved as shown in FIG. 11. In this case |φa/φb|=204
It becomes 00.

[0025]

[Effect] As explained above, according to the present invention, by separating the lens group that has the main light focusing function and the aberration correction group that corrects aberrations that occur during manufacturing, the lenses that are more sensitive to aberrations can be moved. Rotationally asymmetric aberrations can be easily corrected without any problems.

[Brief explanation of the drawing]

FIG. 1 is a partially cutaway exploded side view of an objective lens according to an example.

2(A) is a partially cutaway side view of the objective lens shown in FIG. 1 in an assembled state, and FIG. 2(B) is a front view seen from the aberration correction group side.

FIG. 3 is a cross-sectional view of a lens in a numerical example.

FIG. 4 is a diagram of spherical aberration and astigmatism at design values of numerical examples.

FIG. 5 is a wavefront aberration diagram at design values of a numerical example.

FIG. 6 is a wavefront aberration diagram when the fifth lens of the numerical example is decentered.

7 is a wavefront aberration diagram after aberrations are corrected by tilting the parallel plane plate from the state shown in FIG. 6. FIG.

8 is a wavefront aberration diagram after aberrations are corrected by replacing the parallel plane plate with a prism from the state shown in FIG. 6. FIG.

FIG. 9 is a wavefront aberration diagram when the first surface of the first lens in the numerical example has astigmatism.

10 is a wavefront aberration diagram after aberrations are corrected by replacing the parallel plane plate with a cylindrical lens from the state shown in FIG. 9. FIG.

11 is a wavefront aberration diagram after aberrations are corrected by inserting a cylinder lens on the light source side of the condensing group from the state of FIG. 9; FIG.

[Explanation of symbols]

10...Focusing group 6... Prism (aberration correction group) 20...Front lens barrel 21... Rear lens barrel 22...Retaining ring

Claims (7)

[Claims] 1. A condensing group that mainly has a condensing effect; an aberration correction group that mainly corrects aberrations that are rotationally asymmetric with respect to the optical axis of the condensing group; and the condensing group and the aberration correction group. It is composed of a lens frame that is relatively rotatably supported around the optical axis, and where the power of the condensing group is φa, and the power of the aberration correction group in the direction of the largest absolute value is φb, |φa/
An objective lens that satisfies φb|>50. 2. The objective lens according to claim 1, wherein the condensing group and the aberration correction group are arranged in order from the light source side. 3. The objective lens according to claim 1, wherein the aberration correction group and the condensing group are arranged in order from the light source side. 4. The objective lens according to claim 1, wherein the aberration correction group is a parallel plane plate tilted with respect to the optical axis. 5. The objective lens according to claim 1, wherein the aberration correction group is a prism. 6. The objective lens according to claim 1, wherein the aberration correction group is a cylinder lens. 7. The objective lens according to claim 1, wherein the aberration correction group is a toric lens.