JPH10104506A - Converging lens - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a lens which is less degraded in the light quantity in its peripheral part and has the low possibility of the occurrence of a coma aberration by eccentricity at the time of assembly and adjustment by constituting the converging lens of a front group G1 of a single lens which satisfy specific conditions and has weak power and a rear group which has a convergence effect and making the front group exchangeable for the purpose of spherical aberration adjustment. SOLUTION: This converging lens comprises, successively from an incident side of parallel luminous fluxes, the front group G1 of the single lens having the weak power and the rear group G2 having the convergence effect. The front group G1 is exchangeable for the purpose of the spherical aberration adjustment. The power of the front group G1 is determined in a range satisfying the conditions |f/f1|<0.100 when the focal length over the entire part of the converging lens is defined as (f) and the focal length of the front group G1 as fl. A parallel plane plate of which the exit side of the parallel luminous fluxes is a plane and the incident side surface is a plane, a planoconvex lens of a convex face and a planoconcave lens of a concave face are usable for the front group G1. The correction of the spherical aberrations by exchanging only the front group G1 is made possible without moving the rear group G2 having the convergence effect according to this constitution.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a convergent lens for forming a diffraction-limited minute spot, such as an objective lens of an apparatus for manufacturing a master of a high-density optical disk.

[0002]

2. Description of the Related Art In order to narrow a spot to the diffraction limit, the occurrence of spherical aberration must be minimized. However, even if the spherical aberration is sufficiently corrected by design, the spherical aberration may occur due to a manufacturing error or an error at the time of assembly. In order to correct the spherical aberration generated due to a manufacturing error or the like, a method of adjusting the distance between the lenses constituting the convergent lens is generally used. Since the distance is high, coma is likely to occur due to eccentricity when the lens interval is changed for correcting spherical aberration.

Japanese Patent Application Laid-Open No. 4-274207 discloses a convergent lens composed of a convergent lens group and a plane-parallel plate arranged in order from the parallel light beam incident side. Here, it is described that spherical aberration can be corrected by changing the thickness of a plane-parallel plate disposed in convergent light according to the generated spherical aberration.

[0004]

However, in the conventional configuration described in the above-mentioned publication, for example, when the NA is 0.
In the 90 lens, the angle of the marginal light incident on the plane-parallel plate reaches 64 degrees, so that the reflectivity on the surface of the plane-parallel plate becomes higher in the peripheral part than in the paraxial light, and the amount of peripheral light decreases significantly. Problem. Further, since the parallel flat plate is provided in the convergent light, coma aberration occurs when the parallel flat plate is tilted during replacement.

[0005] The present invention has been made in view of the above-mentioned problems of the prior art, and there is little decrease in the amount of light in the peripheral portion.
It is an object of the present invention to provide a convergent lens which is less likely to generate coma due to eccentricity during assembly and adjustment.

[0006]

In order to achieve the above object, a converging lens according to the present invention comprises, in order from the incident side of a parallel light beam, a front group of a single lens having a low power satisfying the following conditions; And a rear group having the following arrangement, wherein the front group is interchangeable for spherical aberration adjustment. | F / f1 | <0.100 where f is the focal length of the entire converging lens, and f1 is the focal length of the front group. Further, the front group can use a plane-parallel plate having at least a plane on the exit side of the parallel light beam and a plane on the incident side, a convex-planar lens having a convex surface, and a concave-plano lens having a concave surface.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a converging lens according to the present invention will be described below. The convergent lens according to the embodiment includes, for example, as shown in FIG. 1, a front group G of a single lens having a low power in order from the incident side of the parallel light beam on the left side in the figure.
1 and a rear group G2 having a convergence function, and the front group G1 is exchangeable for adjusting spherical aberration. Front group G1
Is defined as a range satisfying the following condition (1), where f is the focal length of the entire converging lens and f1 is the focal length of the front group. | F / f1 | <0.100 (1)

In general, in a lens in which spherical aberration and coma are corrected, the spherical aberration changes depending on the degree of convergence of the light beam incident on the lens (the radius of curvature of the wavefront). Negative (under) spherical aberration occurs when divergent light is incident on a lens that converges a parallel light beam without spherical aberration, and positive (over) spherical aberration occurs when convergent light is incident. Therefore, when a converging lens whose spherical aberration is corrected by design is constituted by the front group having weak power and the rear group having converging action as described above, a positive spherical aberration occurs due to a manufacturing error or an assembly error. When it is done, by design (hereinafter referred to as the reference state)
If the front group is replaced with a lens that has a function of diverging the luminous flux more than the front group, and if negative spherical aberration occurs,
By replacing the front group with a lens having a function of converging the light flux more than the front group in the design reference state, the generated spherical aberration can be canceled.

According to the above arrangement, since spherical aberration can be corrected only by exchanging the front group without moving the rear group having a convergence function, it is possible to prevent the occurrence of new aberration due to adjustment. In addition, since the front group to be exchanged is disposed in the parallel light beam, the incident angle on the front group becomes substantially uniform from the center to the periphery, so that a decrease in the amount of light in the periphery can be prevented. Moreover, the front group is arranged in a parallel light beam,
Since the power is extremely weak, even when the front unit is decentered when the front unit is replaced to correct spherical aberration, the amount of generation of coma due to decentering can be suppressed to a small value.

[0010]

Next, two specific examples that satisfy the conditions of the above-described embodiment will be presented. Each of the embodiments is a convergent lens for forming a diffraction-limited spot using ultraviolet rays, and is used as an objective lens of a digital video disc master making apparatus having a higher recording density than a compact disc. In the ultraviolet region, lens materials capable of ensuring a sufficient transmittance are limited to quartz glass and fluorite, but fluorite is practically limited to quartz glass because of its low hardness and difficulty in processing. For this reason, in the embodiment, all the lenses are made of quartz glass.

[0011]

Embodiment 1 FIG. 1 shows a lens configuration of a convergent lens according to Embodiment 1 in a reference state. Table 1 shows the numerical configuration of the first embodiment in the reference state. In the table, NA is the numerical aperture,
f is the focal length of the entire system (unit: mm), ω is the half angle of view, fB is the back focus (unit: mm), r is the radius of curvature of each lens surface (unit: mm), d is the lens thickness or lens interval. (Unit: mm), n is the refractive index of each lens at a wavelength of 266 nm.

[0012]

[Table 1]

In the first embodiment, the front group G1 is constituted by a parallel plane plate which is a single lens having no power represented by the first surface and the second surface in the reference state as described above.
Is a meniscus negative lens having a concave surface facing the entrance side represented by the third and fourth surfaces, a biconvex lens represented by the fifth and sixth surfaces, and a biconvex lens represented by the seventh and eighth surfaces. A concave lens, a biconvex lens represented by the ninth and tenth surfaces, a biconvex lens represented by the eleventh and twelfth surfaces, and a meniscus positive lens having a convex surface directed to the incident side represented by the thirteenth and fourteenth surfaces. Lens, 15th surface, 1st
A meniscus positive lens having a convex surface facing the incident side represented by six surfaces is arranged in order from the incident side.

When a negative spherical aberration occurs due to a manufacturing error or the like, a positive lens having a weak power is inserted as a front group instead of a parallel plane plate, and when a positive spherical aberration occurs,
A negative lens having a weak power is inserted as a front group in place of the plane-parallel plate. The power of the inserted lens is determined according to the generated spherical aberration, that is, the magnitude of the spherical aberration to be corrected. When spherical aberration does not occur due to a manufacturing error or the like, a parallel flat plate is used as a front group in a reference state.

FIG. 2A shows the spherical aberration SA and the sine condition violation amount OSC of the lens of Example 1 in the reference state.
(b) is astigmatism (S: sagittal, M: meridional)
Is shown. The unit of the horizontal axis indicating the amount of aberration is mm. FIG.
(a), (b), (c), and (d) show image heights of 0.00 mm and 0.03, respectively.
9 shows the wavefront aberration of the meridional section of the lens of Example 1 in the reference state at mm, 0.05 mm, and 0.07 mm. The unit of the vertical axis of the wavefront aberration diagram is the wavelength λ. It can be seen that all aberrations are well corrected.

Next, the performance when the lens according to the first embodiment is in a state different from the reference state will be described. Front group G
The radius of curvature of the first surface is -24000 instead of a parallel plane plate as 1.
If a concave lens having an extremely weak negative power and having a flat second surface is 000 mm, the light incident on the rear group G2 becomes a light beam slightly diverging from the parallel light, and negative spherical aberration occurs. FIGS. 4A and 4B respectively show the spherical aberration, the sine condition violation amount, and the astigmatism when the front unit is a concave flat lens. It can be understood that only the spherical aberration changes to the under side without changing other aberrations.

That is, if the same amount of spherical aberration as shown in FIG. 4A occurs in the reference state due to a manufacturing error or the like on the over side in the direction opposite to the under side shown in FIG. Is inserted in place of the parallel plane plate which is the front group in the reference state, the spherical aberration can be changed to the under side without changing other aberrations, which is almost the same as that shown in FIG. The spherical aberration can be reduced to the same state.

FIG. 5 shows the wavefront aberration at each image height when the front unit G1 is a concave lens having a curvature radius of the first surface of -24000.000 mm. As compared with FIG. 3, it can be seen that the wavefront of about 0.2λ changes at the peripheral portion. Also, FIG.
The wavefront aberration when the concave flat lens is tilted by 1 degree is shown. Compared with FIG. 5, it can be seen that the wavefront hardly changes. That is, even when the concave flat lens is inclined, a large coma aberration is not generated unlike the related art.

[0019]

Second Embodiment FIG. 7 shows a lens configuration of a convergent lens according to a second embodiment in a reference state. Table 2 shows the numerical configuration of the second embodiment in the reference state. Also in this example, all the lenses are made of quartz, and the values of the refractive indexes are all common.

[0020]

[Table 2]

In the second embodiment, the front group G1 is composed of a concave flat lens in the reference state as described above, and the rear group G2 is composed of the same seven lenses as in the first embodiment. That is,
In the reference state, divergent light is incident on the rear group G2, and the positive power of the rear group is designed to be larger than that of the first embodiment. When a negative spherical aberration occurs due to a manufacturing error or the like, a negative lens having a weaker negative power than the reference state is inserted as a front group, and when a positive spherical aberration occurs, a negative power stronger than the reference state is generated. Insert a negative lens having as the front group.

FIG. 8A shows the spherical aberration and the sine condition violation amount OSC of the lens of Example 2 in the reference state, and FIG.
Also indicates astigmatism. 9 (a), 9 (b), 9 (c), 9 (d)
Indicates the wavefront aberration of the meridional section in the reference state of the lens of Example 2 at image heights of 0.00 mm, 0.03 mm, 0.05 mm, and 0.07 mm, respectively. It can be seen that all aberrations are well corrected.

Next, the performance when the lens of the second embodiment is in a state different from the reference state will be described. Front group G
When a concave lens with a radius of curvature of -198.320 mm on the first surface is inserted instead of the concave lens with a radius of curvature of -200.000 mm on the first surface as 1, the divergence of light incident on the rear group G2 is slightly larger. And negative spherical aberration occurs. FIGS. 10A and 10B respectively show the spherical aberration, the sine condition violation amount, and the astigmatism when the front unit is a concave flat lens whose first surface has a radius of curvature of -198.320 mm. It can be understood that only the spherical aberration changes to the under side without changing other aberrations.

That is, if the same amount of spherical aberration as shown in FIG. 10A occurs in the reference state due to a manufacturing error or the like in the opposite over side instead of the under side as shown in FIG.
The concave flat lens of the front group G1 has a curvature radius of the first surface of -200.000mm
The radius of curvature of the first surface from the element in the reference state is -198.320
By changing to an element that is mm, spherical aberration can be changed to the under side without changing other aberrations,
The spherical aberration can be reduced to a state almost similar to that shown in FIG.

FIG. 11 shows the wavefront aberration at each image height when the front unit G1 is a concave lens having a curvature radius of the first surface of -198.320 mm. It can be seen from the comparison with FIG. 9 that the wavefront of about 0.2λ changes at the peripheral portion. FIG.
Shows the wavefront aberration when the concave flat lens in the reference state is tilted by 1 degree. As compared with FIG. 9, the wavefront slightly changes at an image height of 0.07 mm. This change is due to the occurrence of astigmatic difference.

FIG. 13 shows the first group G1 in the front group G1 in the reference state.
FIG. 1 is a wavefront aberration diagram when a concave flat lens having a surface curvature radius of -200.000 mm is decentered by 0.20 mm in a direction perpendicular to the optical axis.
4 is a wavefront aberration diagram when the front group G1 in the reference state moves 1.0 mm away from the rear group along the optical axis. By comparing each with FIG. 9, it can be understood that the wavefront is hardly affected by the eccentricity and slightly affected by the movement in the optical axis direction. However, since the sensitivity of the front group G1 to the aberration with respect to the eccentricity and the movement in the optical axis direction is extremely small, any error within the normal mounting accuracy does not degrade the performance of the entire lens.

In the first embodiment, since the front group G1 in the reference state is a parallel plane plate, the power of the front group G1 used in place of the parallel plane plate is extremely weak when spherical aberration occurs due to a manufacturing error or the like. As a result, even when the front group is tilted, almost no astigmatic difference and coma aberration occur. However,
When used in the reference state, the first surface, the second surface
When reflection occurs on the surface, the ghost light due to the reflection and the light reflected from the medium and emitted from the converging lens are both collimated light and condensed at the signal detection position to generate noise in the signal. To prevent this, it is necessary to apply an antireflection film to the first surface.

On the other hand, in the second embodiment, since the first surface of the front group G1 in the reference state is concave, the light reflected on the first surface and the second surface does not become parallel light but is reflected from the medium. Then, the light is not condensed at the converging position of the parallel light emitted from the converging lens, that is, the signal detecting position. Therefore, generation of noise due to ghost light can be prevented. From this viewpoint, the power of the front group in the reference state is such that the power range of the front group required to correct the spherical aberration in the expected range is “0”.
That is, it is desirable that the front group is not formed into a parallel plane plate when the spherical aberration to be corrected takes any value within the range.

For example, when a concave flat lens is used as the front group, when a negative spherical aberration occurs due to a manufacturing error or the like,
In order to correct this by changing it to the over side, it is necessary to increase the radius of curvature of the concave first surface in order to weaken the diverging power of the front group. At this time, even when the maximum possible negative spherical aberration occurs, the shape of the first surface for correcting the negative spherical aberration has a concave surface, and has an allowable width so as not to be a flat surface or a convex surface. It is necessary to determine the radius of curvature of the concave surface in the state. However, the smaller the absolute value of the radius of curvature of the first surface, the larger the astigmatic difference when the front group is tilted. Therefore, in order to suppress the occurrence of astigmatic difference to a level that does not pose a practical problem, the power of the front group is reduced. Is limited to a range that satisfies the above condition (1). If the condition (1) is satisfied, the occurrence of astigmatic difference can be suppressed to a level that does not pose a practical problem, assuming that the inclination of the front group is suppressed to about 0.1 degrees or less. In the second embodiment, a concave flat lens is used as the front group G1 in the reference state. However, if the positive power of the rear group is reduced, a convex flat lens can be used as the front group.

When the converging lens of the embodiment is actually assembled to the lens barrel, if the front group in the reference state is a parallel flat plate as in the first embodiment, spherical aberration in the reference state is generated only by the rear group. Therefore, only the rear group is assembled without the front group, and the spherical aberration is measured by the interferometer. When spherical aberration does not occur, a parallel plane plate is mounted as a front group as in the reference state, and when spherical aberration occurs, a lens having a radius of curvature corresponding to the amount of generation is mounted as a front group. When the front group in the reference state has power as in the second embodiment, the front group in the reference state is temporarily fixed after the rear group is assembled, and spherical aberration is measured by an interferometer. When no spherical aberration occurs, the front group in the reference state is fixed, and when spherical aberration occurs, a lens having a radius of curvature according to the amount of generation is attached as the front group.

[0031]

As described above, according to the present invention, the converging lens is divided into a front group having a weak power and a rear group having a positive power, so that the spherical aberration can be easily reduced by exchanging the front group. Correction can be performed with high accuracy. Since the front group is located in the parallel light, there is little decrease in the amount of light in the peripheral portion.
Further, since the sensitivity of the performance change of the whole lens such as the eccentric coma aberration to the eccentricity and inclination of the front group is extremely small, high accuracy is not required for assembling the front group, and replacement and assembling are easy.

[Brief description of the drawings]

FIG. 1 is a lens diagram of a convergent lens according to a first embodiment in a reference state.

FIG. 2 is a graph showing the spherical aberration, the sine condition violation amount, and the astigmatism in the reference state of the converging lens of Example 1.

FIG. 3 is a graph showing a wavefront aberration of a meridional section in a reference state of the converging lens of the first embodiment.

FIG. 4 is a graph showing the spherical aberration, the sine condition violation amount, and the astigmatism when the front group of the converging lens of Example 1 is replaced.

FIG. 5 is a graph showing the wavefront aberration of the meridional section when the front group of the converging lens of Example 1 is replaced.

FIG. 6 is a graph showing the wavefront aberration of the meridional section when the front group in which the converging lens of Example 1 is replaced is tilted by 1 degree.

FIG. 7 is a lens diagram of a convergent lens according to a second embodiment in a reference state.

FIG. 8 is a graph showing the spherical aberration, the sine condition violation amount, and the astigmatism in the reference state of the converging lens of the second embodiment.

FIG. 9 is a graph showing a wavefront aberration of a meridional section in a reference state of the converging lens of the second embodiment.

FIG. 10 is a graph showing the spherical aberration, the sine condition violation amount, and the astigmatism when the front group of the converging lens of Example 2 is replaced.

FIG. 11 is a graph showing the wavefront aberration of the meridional section when the front group of the converging lens of Example 2 is replaced.

FIG. 12 is a graph showing a wavefront aberration of a meridional section when the front unit is tilted by 1 degree from the reference state of the converging lens of the second embodiment.

FIG. 13 is a graph showing the wavefront aberration of the meridional section when the front unit is decentered by 0.2 mm from the reference state of the converging lens of the second embodiment.

FIG. 14 is a graph showing the wavefront aberration of the meridional section when the front lens group moves 1.0 mm away from the rear lens group from the reference state of the converging lens of the second embodiment.

[Explanation of symbols]

 G1 front group G2 rear group

Claims (3)

[Claims] 1. A lens for converging a parallel light beam,
In order from the incident side of the parallel light beam, the front lens group includes a front group of a single lens having a weak power satisfying the following conditions, and a rear group having a converging action, and the front group is interchangeable for adjusting spherical aberration. A convergent lens characterized by the following. | F / f1 | <0.100 where f is the focal length of the entire converging lens, and f1 is the focal length of the front group. 2. The converging lens according to claim 1, wherein the front group has a curved surface on the incident side of the parallel light beam and a flat surface on the exit side. 3. The converging lens according to claim 1, wherein all the lens elements of the front group and the rear group are formed of quartz glass, and converge a parallel light beam in an ultraviolet region.