JP2002156579A - Objective lens for optical disk - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an objective lens for an optical disk whose best image surface chromatic aberration characteristic is made excellent, whose off-axis aberration characteristic is made excellent and which has gentle eccentric tolerance. SOLUTION: This objective lens for an optical disk is a single lens whose numerical aperture is >=0.7 and whose both surfaces are aspherical, and the center thickness of the lens is larger than a focal distance.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an objective lens having a high numerical aperture (NA) for realizing a large-capacity optical disk.

[0002]

2. Description of the Related Art Conventionally, a CD disk has a numerical aperture of 0.4.
Reading or writing is performed with a laser beam having a wavelength of about 780 nm using an objective lens of 5 to 0.5. A DVD disk is read or written with a laser beam having a wavelength of about 650 nm using an objective lens having a numerical aperture of about 0.6.

On the other hand, in order to increase the capacity of an optical disk,
A next-generation optical disc pickup system using a laser beam having a shorter wavelength and a lens having a higher numerical aperture has been developed.

As a laser having a shorter wavelength, a so-called blue laser having a wavelength of 400 nm has been considered.

As an objective lens having a higher numerical aperture, a system using a single lens with a numerical aperture of 0.7 or a system using a two-group lens with a numerical aperture of 0.85 has been reported. .

The former is disclosed in Jpn. J. Appl. Phys. Vol. 39 (20
00) pp.978-979 M. Itonaga et al. “Optical Disk Sys
tem Using a High-Numerical Aperture Single Objecti
veLens and a Blue LD ”.

The latter is disclosed in Jpn. J. Appl. Phys. Vol. 39 (20
00) pp.937-942 I. Ichimura et al. “Optical Disk R
ecording Using a GaN Blue-Violet Laser Diode ”.

However, the system using the two-group lens requires an assembling process and requires two lenses, so that the mass production is inferior and the cost is high.

Therefore, as the next-generation system, an optical pickup using a single lens having a simpler configuration is desired. Here, in an optical pickup using a single lens, an objective lens having a numerical aperture larger than 0.7 is desired.

[0010]

Problems to be solved for putting a single lens having a high numerical aperture into practical use are that manufacturing tolerances become strict and design performance deteriorates.

More specifically, the manufacturing tolerance is a distance tolerance between the entrance and exit surfaces of the double-sided asymmetric lens, or
It means the tolerance of the distance between the geometric centers of the two surfaces (eccentricity tolerance) or the tolerance of the inclination between the two surfaces. However, these manufacturing tolerances can be addressed by improving and improving manufacturing techniques. That is, it is possible to manufacture with a tolerance of about several microns to several tens of microns.

On the other hand, the deterioration of the design performance refers to the deterioration of the performance of the lens design. More specifically, the generation of aberration with respect to off-axis rays (hereinafter abbreviated as off-axis aberration) and a plurality of wavelengths are considered. Spherical aberration at the best image plane at each wavelength with respect to the on-axis ray (hereinafter, abbreviated as best image plane chromatic aberration)
Means Here, the on-axis ray is a ray incident parallel to the optical axis of the lens, and the off-axis ray is a ray incident obliquely with respect to the optical axis of the lens. That is, it is possible to design such that spherical aberration does not occur with respect to an axial ray having a design reference wavelength, but the aberration is obtained as a better value than a conventional CD or DVD objective lens. It is difficult.

The off-axis aberration and the best image plane chromatic aberration will be described in detail below.

The off-axis aberration is generally inferior to the conventional one even when designing without considering the manufacturing tolerance. This is because, when the numerical aperture increases, a light beam having a large inclination angle with respect to the optical axis enters. It is known that the off-axis aberration is further worsened in consideration of manufacturing tolerances. More specifically, the most important tolerance among the manufacturing tolerances is the eccentricity tolerance, but in order to increase the eccentricity tolerance, it is necessary to sacrifice the on-axis aberration and the off-axis aberration. . That is, by designing a lens to have a certain degree of on-axis aberration and off-axis aberration, it is possible to realize a lens that can substantially maintain lens performance even if eccentricity occurs. Here, the axial aberration is only slightly deteriorated, but in a high numerical aperture lens having a numerical aperture exceeding 0.6, it is possible to manufacture the lens on the micron order, which can be manufactured without considerably sacrificing the off-axis aberration. Eccentricity tolerance cannot be secured.

This is compared with the case of a DVD lens as follows. For example, in a DVD lens having a focal length of 3.3 mm and a thickness of 2 mm, if a lens having, for example, an eccentricity tolerance of 5 μm is designed, a performance with an off-axis aberration of 0.03λ or less with respect to incident light of 0.5 degrees is obtained. The lens can be easily manufactured.

However, it is difficult to manufacture such a lens with a high numerical aperture lens using a short wavelength laser beam.

On the other hand, the best image plane chromatic aberration is as described above.
This is spherical aberration that occurs when the wavelength of the laser light is shifted from the design wavelength of the lens and is evaluated on the best image plane for the laser wavelength. The details are as follows.

FIG. 8 is a diagram showing longitudinal aberrations generated for light of 400 nm and 410 nm when aberration is compensated for light of 405 nm. If the line representing the longitudinal aberration is bent, spherical aberration is present.

In FIG. 8, for example, when the laser wavelength is shifted from the design reference wavelength of 405 nm to 410 nm, the best image plane for the laser wavelength is x = 0.
From the position x to the position x = + a. In the 410 nm longitudinal aberration diagram in which the position fluctuates, a ray having a high ray height (that is, a large value of y) intersects with the optical axis at a position different from the principal ray to generate spherical aberration as shown in the figure.

FIG. 9 shows the relationship between the best image plane chromatic aberration and the wavelength when the aberration is evaluated as the amount of wavefront aberration (hereinafter, this relation is referred to as the best image plane chromatic aberration characteristic).

As shown in FIG. 9, the best image plane chromatic aberration characteristic has the lowest value at the lens design reference wavelength λ0.
It has a larger value as it deviates from the design reference wavelength. Therefore, the wavelength range λ ± (maximum wavelength λ +, minimum wavelength λ−) that can be used by the lens is determined from the best image plane chromatic aberration characteristic in FIG.

The best image plane chromatic aberration for a DVD lens is as follows.

For example, the focal length is 3.3 mm and the thickness is 2 m
If a lens having a eccentricity tolerance of 5 μm is designed for a DVD lens of m, the wavelength range in which the best image plane chromatic aberration when a wavelength change occurs can be suppressed to 0.02λ or less is 615n.
Very wide, from m to 700 nm.

However, in the wavelength region of the blue laser, the chromatic aberration characteristics of the best image plane become severe, and it is difficult to obtain a wide wavelength range.

The reason why the best image plane chromatic aberration characteristic becomes severe with respect to light having a short wavelength is that, for example, wavelength fluctuation of the refractive index of glass is large. Also, this is because the aberration increases in inverse proportion to the wavelength. Therefore, when the wavelength becomes 450 nm, the wavelength becomes 7 compared with 650 nm used in DVD.
0%, resulting in an accuracy tolerance of 70%. When the numerical aperture increases, aberrations due to this increase.

An object of the present invention is to overcome the above-mentioned problems, and to provide an objective lens for an optical disc having excellent chromatic aberration characteristics of the best image plane, excellent off-axis aberration characteristics, and gradual eccentricity tolerance. is there.

[0027]

An objective lens for an optical disc according to the present invention is a double-sided aspheric single lens having a numerical aperture of 0.7 or more, and is characterized in that the center thickness of the lens is longer than the focal length.

According to this lens, the deflection angle at the time of refraction on the first surface of the lens can be reduced. This means
This means that the radius of curvature of the first surface can be reduced, and the angle between the normal line of the first surface and the optical axis can be reduced. Therefore, it is possible to minimize the change in the refraction angle when the wavelength changes, thereby suppressing the occurrence of spherical aberration. That is, chromatic aberration at the best image plane can be improved. Regarding the aberration with respect to the off-axis ray, the influence of the change in the direction of the incident light on the change in the refraction angle after exiting the first surface is reduced, and the off-axis aberration can be minimized.

It is preferable that the objective lens for an optical disk has an imaging magnification of 0 at a design reference wavelength. Here, the design reference wavelength is a wavelength adopted as a reference when designing the lens, and the lens emits light of the design reference wavelength in the same image plane including off-axis rays and on-axis rays. The sharpest convergence.

By setting the imaging magnification to 0 times as described above, it is possible to easily measure the performance of a single lens using an interferometer, and to perform a high quality control.

Preferably, the objective lens for an optical disc has the design reference wavelength shorter than 0.45 μm.

The objective lens for an optical disk also has a focal length shorter than 4.0 mm and t = d /
It is preferably longer than n + 0.9 (mm). Here, d is the thickness of the optical disk, and n is the refractive index of the optical disk.

By making the focal length longer than t, the working distance (the distance between the lens tip and the disk surface) becomes 0.3 mm.
The above can be secured. More specifically, by securing the working distance of 0.25 mm or more, the possibility of collision between the lens and the disk can be reduced. That is, a disc made of plastic has warpage. The amount of this warpage also depends on the diameter of the disk.
The run-out of a next-generation system disk having a size of 20 mm is considered to be about ± 0.2 mm. Therefore 0.25mm
With the above working distance, it is possible to reduce the risk of collision between the disk and the lens to a necessary and sufficient degree, in addition to the device on the control circuit side (for example, control to avoid the lens when there is a defect). Of course, by applying other technologies such as servo technology, it is guaranteed that the disk does not collide with the lens, or when using a disk system that allows collision or when using a smaller-diameter disk (for example, a movie) Alternatively, a lens with a shorter focal length can be used.

By designing the focal length to be 4.0 mm or less, the luminous flux diameter can be reduced to 5.6 mm or less even when the numerical aperture is 0.7 or more, and the miniaturization of the pickup can be maintained. . In addition, the lens itself can be reduced in size or weight, and the wide range characteristics of the actuator used for the focus servo or the tracking servo can be maintained, and the required servo characteristics in a wide range can be obtained.

[0035]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below in detail with reference to FIGS.

FIG. 1 shows an optical disc objective lens 21 according to a first embodiment 21 of the present invention and an optical disc 23 used with the objective lens.

The optical disk objective lens 21 of the first embodiment generally has a design reference wavelength shorter than 450 nm, a numerical aperture of 0.7 or more, and a center thickness D of the lens. It is a double-sided aspheric single lens longer than the distance.

More specifically, the design reference wavelength of the objective lens 21 is set to 405 nm.

The numerical aperture (NA) of this objective lens is designed to be 0.75.

The focal length of this objective lens is 2.5 m
m, and the imaging magnification at the design reference wavelength of 405 nm is 0.

The eccentricity δ between the first surface S1 and the second surface S2 (the distance between the geometric center axis a1 of the surface S1 and the geometric center axis a2 of the surface S2) is 5 μm. Is designed so that the aberration (eccentricity characteristic) at the time of is not more than 0.03λ.

The glass types of the lenses are as follows.

NbF1 (refractive index nd = 1.7433,
(Abbe number νd = 49.22) The optical disk 23 is designed as follows.

Cover glass thickness: 0.11 mm (polycarbonate 0.1 mm + acrylic 0.01 mm) This lens 21 has an aberration of about 0.003λ (rms) with respect to rays parallel to the axis, that is, on-axis aberration. Designed to have. Here, rms means root mean square. Λ is a design reference wavelength, which is 405 nm in this embodiment.

FIG. 2 shows how the off-axis aberration changes in the lens according to the first embodiment in accordance with the change in the thickness D of the lens.

Here, the off-axis aberration is, as already described,
When a light beam enters at an angle inclined with respect to the optical axis of the lens, it means aberration that occurs on the focal plane. In FIG. 2, it is assumed that the light beam enters at an angle of 0.5 degrees with respect to the optical axis. FIG. 2 shows values calculated for the objective lens 21 by the ray tracing method.

As understood from FIG. 2, the lens thickness D
Is larger than the focal length (2.5 mm), the aberration is smaller than 0.04λ (rms), and a good converged image is obtained.

FIG. 3 shows a lens according to the first embodiment.
The following describes how the best image plane chromatic aberration changes according to the change in the lens thickness D.

More specifically, the spherical aberration (rm) of a ray having a wavelength of 410 nm deviated from the design reference wavelength of 405 nm.
7 shows how s) changes according to the change in the lens thickness D.

As can be understood from FIG.
It is understood that the spherical aberration (rms) at m also becomes a sufficiently small value (less than 0.02λ in FIG. 3) when the lens thickness D is larger than the focal length of 2.5 mm.

Therefore, according to the first embodiment, by setting the center thickness D of the lens to be longer than the focal length, it is possible to obtain excellent best image plane chromatic aberration characteristics and off-axis aberration characteristics. Therefore, according to the lens of the first embodiment,
By making the lens thickness D larger than the focal length of 2.5 mm, the usable laser wavelength range can be widened.

The values of the off-axis aberration and the best image plane chromatic aberration shown in FIGS. 2 and 3 differ depending on the numerical aperture of the lens, the type of glass forming the lens, and other design differences. It also depends on the specifications of the lens. For example, if the focal length is shortened, the characteristic relating to aberration is naturally improved. However, the wavelength of light is shorter than 450 nm,
Further, in a double-sided asymmetric single lens having a numerical aperture of 0.7 or more, if the center thickness of the lens is longer than the focal length, an objective lens for an optical disc having good chromatic aberration characteristics and off-axis aberration characteristics can be manufactured. Can be generalized.

Therefore, when the design reference wavelength is shorter than 0.45 μm and the numerical aperture is 0.7 or more, the best image plane chromatic aberration can be obtained by making the center thickness D of the lens longer than the focal length. Characteristics and off-axis aberration characteristics can be obtained.

In the first embodiment, by setting the imaging magnification at the design reference wavelength to 0, it is possible to easily measure the performance of a single lens using an interferometer, and to perform advanced quality control. Becomes possible.

If there is an increase in spherical aberration due to a lens manufacturing error, a disc thickness error, a temperature change, or the like, the parallelism of light incident on the objective lens is changed to generate a spherical aberration in the opposite direction. The generated spherical aberration can be compensated by the reverse spherical aberration. In the case where the spherical aberration occurs due to the lens manufacturing error, the imaging magnification is deviated from 0 times.

FIG. 4 shows a second embodiment 31 of the optical disk objective lens of the present invention and an optical disk 33 used with this objective lens.

The lens specifications of the lens 31 are as shown in Table 1.

[0058]

[Table 1] The lens design values of this lens are as shown in Table 2.

[0059]

[Table 2] Here, the third and fourth surfaces indicate design values of the optical disc 33. The refractive indexes of the glasses in Table 2 are as shown in Table 3.

[0060]

[Table 3] Table 4 shows the aspheric coefficients of the first surface and the second surface of the lens 31.
It is as shown in Table 5. Note that the distance X from the tangent plane of the aspherical vertex of the coordinate point on the aspheric surface where the height of the optical axis is Y is:
The curvature (1 / r) at the vertex of the aspheric surface is represented by C, the conic coefficient (conic constant) is represented by K, and the fourth to twelfth aspherical coefficients are represented by A4 to A12.

[0061]

X = CY ² / [1 + √ ｛1- (1 + K) C ² Y
² ｝] + A4Y ⁴ + A6Y ⁶ + A8Y ⁸ + A10Y ¹⁰
+ A12Y ¹²

[Table 4]

[Table 5] FIG. 5 is a view showing the structure of the objective lens according to the second embodiment.
It is a longitudinal aberration figure in three wavelengths of 0, 405, and 410 nm.

The best field aberration (rms) of this lens is:
It is as shown in Table 6.

[0063]

[Table 6] Therefore, according to the second embodiment, it is possible to realize an objective lens for an optical disk having an excellent image plane chromatic aberration characteristic.

In this lens, when the inter-plane eccentricity is 5 μm, the aberration is 0.025λ (rms).
In this lens, the working distance is 0.60 mm
It is.

FIG. 6 shows a third embodiment 41 of the objective lens for an optical disk of the present invention and an optical disk 43 used with this objective lens.

The lens specifications of the lens 41 are as shown in Table 7.

[Table 7] Table 8 shows the lens design values of this lens.

[0067]

[Table 8] Here, the third and fourth surfaces indicate design values of the optical disk 43. The refractive indexes of the glasses in Table 8 are as shown in Table 3. The aspherical coefficients of the first surface and the second surface of the lens 31 are as shown in Tables 9 and 10.

[0068]

[Table 9]

[Table 10] FIG. 7 shows the structure of the objective lens of the third embodiment.
It is a longitudinal aberration figure in three wavelengths of 0, 405, and 410 nm.

The best field aberration (rms) of this lens is
It is as shown in Table 11.

[0070]

[Table 11] Therefore, according to the third embodiment, it is possible to realize an objective lens for an optical disk which is excellent in the best image plane chromatic aberration characteristics.

In this lens, when the inter-plane eccentricity is 5 μm, the aberration is 0.027λ (rms).
In this lens, the working distance is 0.50 mm
It is.

[0072]

As described above, according to the present invention, it is possible to realize an objective lens for an optical disk having excellent chromatic aberration characteristics of the best image plane and excellent off-axis aberration characteristics.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of a first embodiment of an objective lens for an optical disk of the present invention.

FIG. 2 is a diagram illustrating a relationship between a lens thickness D and an aberration when a light beam having an inclination angle of 0.5 degrees with respect to the optical axis of the objective lens according to the first embodiment is incident. is there.

FIG. 3 is a view showing a structure of the objective lens according to the first embodiment;
FIG. 5 is a diagram illustrating a relationship between a lens thickness D and a best image surface aberration (rms) when a laser beam having a wavelength of 0 nm is incident.

FIG. 4 is a diagram showing a second embodiment of the objective lens of the present invention.

FIG. 5 is a diagram illustrating 400n of an objective lens according to a second embodiment.
It is a figure which shows the longitudinal aberration with respect to the laser beam of m, 405 nm, and 410 nm.

FIG. 6 is a diagram showing a third embodiment of the objective lens for an optical disk of the present invention.

FIG. 7 is a perspective view showing an objective lens according to the third embodiment;
It is a figure which shows the longitudinal aberration of each incident light in case laser light of 0 nm, 405 nm, and 410 nm enters.

FIG. 8 is an explanatory diagram for explaining the best image plane chromatic aberration.

FIG. 9 shows the variation of the best image plane chromatic aberration when laser light having a wavelength deviated from the design reference wavelength is incident on the lens, and also shows the usable range of the lens in relation to the best image plane chromatic aberration. FIG. 3 is an explanatory diagram for explaining the method.

Claims (4)

[Claims] 1. An objective lens for an optical disk, comprising: a double-sided aspheric single lens having a numerical aperture of 0.7 or more, wherein a center thickness of the lens is longer than a focal length. 2. The objective lens for an optical disk according to claim 1, wherein an imaging magnification at a design reference wavelength is 0. 3. The objective lens for an optical disk according to claim 1, wherein the design reference wavelength is shorter than 0.45 μm. 4. The objective lens for an optical disk according to claim 1, wherein the focal length is shorter than 4.0 mm and longer than t represented by the following equation. t = d / n + 0.9 (mm) Here, d is the thickness of the optical disk, and n is the refractive index of the optical disk.