JP2003215447A - Objective and optical device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an objective capable of excellently compensating both of axial aberration and off-axis aberration. <P>SOLUTION: The objective is provided with a 1st phase shifter equipped with an annular level difference part X centering an optical axis 4 on its one surface. The 1st phase shifter has a function for making a phase difference to reduce aberration at the time of reproducing a 1st optical disk for the light of 1st wavelength. Then, it is provided with a 2nd phase shifter equipped with an annular level difference part Y centering the optical axis 4 on the one surface, and the 2nd phase shifter has a function for making the phase difference to reduce the aberration at the time of reproducing a 2nd optical disk for the light of 2nd wavelength. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an objective lens and an optical device suitable for recording or reproducing on an optical disc such as a CD (compact disc) and a DVD (digital video disc) and having a diffraction limit performance. A CD is a recordable CD-R (compact disc recordable)
Shall also be included.

[0002]

2. Description of the Related Art Conventionally, there has been proposed an objective lens for reproducing both a DVD having a transparent substrate having a thickness of 0.6 mm and a CD having a transparent substrate having a thickness of 1.2 mm (Japanese Patent Laid-Open No. 10-29138). -255305, JP-A-11-1619
0, JP-A-11-2759).

In these conventional examples, on one surface of an objective lens having an aspherical surface, there is provided a phase shifter consisting of a concave portion or a convex portion in the shape of a ring centered on the optical axis. The basic shape of the objective lens is optimized so that the DVD can be reproduced well, and the aberration is corrected as small as possible by correcting the phase with the phase shifter when reproducing the CD. As a result, on both DVD and CD, axial aberration, particularly axial spherical aberration can be corrected well.

[0004]

However, in the above-mentioned conventional example, the off-axis coma aberration during reproduction of the CD cannot be corrected well, and the off-axis coma aberration is large, so that the light source, the objective lens, etc. When the optical device is assembled, the optical performance is greatly degraded when it is tilted from the optical axis or when an axis shift occurs.
High accuracy is required for positioning the objective lens and the like, and there is a problem that productivity is poor.

In addition, since the positioning of the objective lens is required to have high accuracy, the mechanism for moving the lens and the light source (moving mechanism) is worn or the like, so that the objective lens is tilted or deviated from the optical axis and the optical There was a problem that the performance deteriorates over time.

Further, since the off-axis coma aberration is large, when actually operating the optical device, the allowable range for positioning the objective lens by autofocus driving of the objective lens, especially for the axis deviation is narrowed, so that the optical performance is reduced. There was a problem of deterioration.

The present invention has been made to solve the above-mentioned drawbacks. For example, when recording or reproducing a plurality of types of optical disks such as DVD and CD, both axial aberrations and off-axis aberrations are recorded. Both aim to provide an objective lens and an optical device capable of excellent correction.

[0008]

According to the present invention, during recording or reproduction of a first optical disc, light of a first wavelength is focused on an information recording surface of the first optical disc to record information on the first optical disc. The reflected light from the surface is received by the light receiving element, and the second wavelength different from the first wavelength is used when recording or reproducing the second optical disk.
In an objective lens having an aspherical surface on both sides, which is used in an optical system in which light having a wavelength of 2 is condensed on the information recording surface of the second optical disk and reflected light from the information recording surface of the second optical disk is received by a light receiving element. On one surface of the objective lens, a first phase shifter having one or more ring-shaped step portions X centered on the optical axis is provided, and the first phase shifter is a first phase shifter for the light of the first wavelength. The second optical disc has a function of producing a phase difference that reduces aberration during recording or reproduction of the first optical disc, and further has one or more ring-shaped stepped portions Y centered on the optical axis on one side. An objective lens characterized in that a phase shifter is provided, and the second phase shifter has a function of causing a phase difference for reducing the aberration at the time of recording or reproducing of the second optical disk with respect to the light of the second wavelength. I will provide a.

A lens optical system which includes the objective lens and the auxiliary lens and is focused on the information recording surface of the optical disc, wherein the auxiliary lens and the aspherical surface including the apex of one surface of the objective lens. The combination of the surface including the remaining one-sided vertex of the objective lens with the aspherical surface is used for recording or reproducing the data on the information recording surface of the first optical disk, and is used for recording or reproducing the first optical disk at the first wavelength. The axial spherical aberration is set to an RMS value of 0.08λ _{1 to} 0.25λ ₁ with respect to the light of λ ₁ , and the data on the information recording surface of the second optical disc is recorded or reproduced. 0.08Ramuda ₂ in axial spherical aberration to the second wavelength lambda ₂ of light RMS value when
Provided is a lens optical system set to be ˜0.25λ ₂ .

Further, at the time of recording or reproducing on the first optical disk, the light of the first wavelength is condensed on the information recording surface of the first optical disk through the objective lens so as to be emitted from the information recording surface of the first optical disk. The reflected light is received by the light receiving element, and the light having the second wavelength different from the first wavelength is condensed on the information recording surface of the second optical disc through the objective lens when recording or reproducing the second optical disc. In the optical device for causing the light receiving element to receive the reflected light from the information recording surface of the second optical disc,
There is provided an optical device using the objective lens as the objective lens.

Further, at the time of recording or reproducing on the first optical disk, the light of the first wavelength is condensed on the information recording surface of the first optical disk through the objective lens so as to be emitted from the information recording surface of the first optical disk. The reflected light is received by the light receiving element, and the light having the second wavelength different from the first wavelength is condensed on the information recording surface of the second optical disc through the objective lens when recording or reproducing the second optical disc. In the optical device for causing the light receiving element to receive the reflected light from the information recording surface of the second optical disc,
An optical device using the lens optical system is provided.

[0012]

FIG. 1 is a sectional view through an optical axis 4 of an embodiment of the objective lens of the present invention. The surface without the stepped portion is the first surface (the surface on the light source side). 2 is a front view of the objective lens of FIG. 1 when viewed from the optical disk side.

The objective lens of the present invention collects the light from the light sources of different wavelengths on the information recording surface of each optical disc when recording or reproducing each of the two optical discs. It is used in an optical system in which the light receiving element receives the reflected light from the recording surface. In order to improve the accuracy of light collection, the objective lens of the present invention has aspherical surfaces on both sides. The first wavelength is used when recording or reproducing on the first optical disk, and the second wavelength is used when recording or reproducing on the second optical disk.

As described above, the objective lens of the present invention has aspherical surfaces on both sides, but it is not enough to correct aberrations by itself, so one or both sides of the objective lens have a ring-shaped step centered on the optical axis. A phase shifter including a section is provided.

In the objective lens shown in FIG. 1, annular step portions 21, 22, 23, 24, 25, 2 centered on the optical axis are used.
6, 27, 28, 29, 210 are provided on the second surface (surface on the optical disk side) of the objective lens. Step portion 21,
22, 23, 24, 25, 26, 27, 28, 29, 2
At least one selected from 10 is the step portion X having a function of performing a phase shift only for the first wavelength, and the step portions 21, 22, 23, 24, 25, 26, 27, 2
At least one selected from 8, 29, 210 is the second
The stepped portion Y has a function of shifting the phase only with respect to the wavelength.

The step portion W has a function of phase-shifting at least one of the step portions provided on the second surface with respect to the first wavelength and the second wavelength, and is provided as necessary. May be That is, the step portion having the function of phase shifting with respect to the first wavelength and the function of phase shifting with respect to the second wavelength is referred to as step portion W.

As described above, in the present invention, the step portion X having the function of shifting the phase with respect to the first wavelength is provided on one surface of the objective lens, and has the function of shifting the phase with respect to the second wavelength. The stepped portion Y is provided on the one surface of the objective lens. At least one of the plurality of step portions is the step portion X, and at least one of the plurality of step portions is the step portion Y. The number of stepped portions is not limited to that shown in FIG. 1 and can be changed arbitrarily.

By the phase shift of the light of the first wavelength, the aberration at the time of recording or reproducing of the first optical disk, especially the spherical aberration on the axis is improved. The phase shift of the light of the second wavelength causes the second
The aberration at the time of recording or reproducing of the optical disk, especially the spherical aberration on the axis is improved.

The objective lens shown in FIG. 1 is provided with a phase shifter having a ring-shaped step portion centered on the optical axis on one surface thereof. From the viewpoint of ease of processing the objective lens, the objective lens is provided. It is preferable to provide a phase shifter having a step portion on one surface of the above. However, the present invention is not limited to this, and a phase shifter having a step portion may be provided on both surfaces of the objective lens.

In FIG. 1, φ_{twenty one}, Φ_{twenty two}, Φ_{twenty three}, Φ_{twenty four},
φ_{twenty five}, Φ₂₆, Φ₂₇, Φ₂₈, Φ₂₉, Φ ₂₁₀Are each
Difference part 21, 22, 23, 24, 25, 26, 27, 2
Inner diameter (diameter) of 8, 29, 210, φ₂₁₁Is the second side
Effective diameter (diameter) of φ₁₁Is the effective diameter (diameter) of the first surface
It

The stepped portion X, the stepped portion Y or the stepped portion W may be a convex portion or a concave portion provided on the surface of the objective lens. The stepped portion is not limited to one having an angle of right angle in a sectional view passing through the optical axis, and may have a gentle inclination. In the following description, unless otherwise specified, the unit of dimensions such as distance, interval, length, thickness, etc. is mm.

In order for the step portion X to have a function of performing a phase shift with respect to the first wavelength λ ₁ , in order to shift the phase of the light of the first wavelength, the position of the light of the first wavelength is generated. When the phase difference is converted into a distance, 0.9λ _{2 to} 1.1
It is preferable that the size and shape of the stepped portion X be determined so as to be i times λ ₂ . However, λ ₂ is the second wavelength and i is a natural number. By setting the phase difference generated for the light of the first wavelength within this range, the aberration at the time of recording or reproducing of the first optical disk, particularly the spherical aberration on the axis is reduced. Further, a phase difference that is a natural number multiple of 360 degrees is regarded as not a phase difference. When the phase difference is represented by a multiple of the wavelength, the phase is converted into a distance.

In order to shift the phase of the light of the first wavelength, when the phase difference generated for the light of the first wavelength is converted into the distance, (i-0.1) λ ₂ to (i + 0.1) λ ₂ It is more preferable that the size and shape of the stepped portion X be determined so that

In order to make the phase difference generated for the light of the first wavelength i times 0.9λ _{2 to} 1.1λ ₂ , the drop γ ₁ and λ _{2 of} each ring-shaped stepped portion X. , And the refractive index n ₂ of the material of the objective lens 3 at the second wavelength needs to satisfy the following expression 1.

[0025]

[Equation 1]

When the phase difference generated for the light of the first wavelength is converted into the distance, (i-0.1) λ ₂ to (i + 0.
1) In order to obtain λ ₂ , it is necessary to satisfy the following Expression 2.

[0027]

[Equation 2]

When a ring-shaped aspherical surface is provided between the ring of the stepped portion X and the ring of another stepped portion or near the effective diameter, these aspherical surfaces are referred to as ring-shaped aspherical surfaces. To call it
The drop means that the optical axis 4 is defined by
The distance between the intersection of the extension line and the optical axis 4 and the apex of the surface of the objective lens provided with the ring-shaped aspherical surface is, for example, γ ₂₁ in FIG. ), And γ ₂₂ (fall of the step portion 22). This interval is along the optical axis 4. The concept of the drop is the same for the following equations 3 and 4.

Further, in order that the stepped portion Y has a function of phase-shifting with respect to the second wavelength λ ₂ , in order to shift the phase of the light of the second wavelength, When the generated phase difference is converted into a distance, 0.9λ ₁ ~
It is preferable that the size and shape of the stepped portion Y be determined so as to be j times 1.1λ ₁ . However, j is a natural number. By setting the phase difference generated for the light of the second wavelength within this range, the aberration at the time of recording or reproducing of the second optical disk, especially the spherical aberration on the axis is reduced.

In order to shift the phase of the light of the second wavelength, when the phase difference generated for the light of the second wavelength λ ₂ is converted into the distance, (j-0.1) λ ₁ to (j + 0.1) λ
At _1, it is more preferable that the size and shape of the step portion Y are determined.

The phase difference generated with respect to the light having a second wavelength is made to be a j times 0.9λ ₁ ~1.1λ ₁ includes a drop gamma ₂ having the stepped portion Y, and the lambda _1, the first It is necessary that the refractive index n ₁ of the material of the objective lens 3 at the wavelength of ₁ satisfies the following expression 3.

[0032]

[Equation 3]

When the phase difference generated for the light of the second wavelength λ ₂ is converted into the distance, (j-0.1) λ ₁ to (j +)
0.1) In order to obtain λ ₁ , it is necessary that the drop γ ₂ of the stepped portion Y, λ _1, and the refractive index n ₁ of the material of the objective lens 3 satisfy the following expression 4.

[0034]

[Equation 4]

[0035] NA ₁ between the numerical aperture NA ₁ of the objective lens in the case of recording or reproducing the first optical disk, and the numerical aperture NA ₂ of the objective lens in the case of recording or reproducing the second optical disk> NA ₂ when the holds, drop the inner diameter of the step portion (diameter) smallest (21 in FIG. 1) (e.g., gamma ₂₁ in FIG. 3) preferably satisfies the formula 4.

The step portion is a step portion having a function of producing a phase difference with respect to the light of the second wavelength (phase shift of the light of the second wavelength), and is the step portion Y. Aberration correction must be performed within the narrow range of NA ₂ for the second wavelength, and if the step difference does not satisfy Equation 4, recording or reproduction of the second optical disk is not possible. May not be good.

The step W has the first wavelength λ₁And the second wavelength
λ₂To have the function of phase shifting with respect to
In order to phase shift the light of the first wavelength,
Wavelength λ ₁When the phase difference caused by the light of
(I-0.1) λ₂~ (I + 0.1) λ₂To become
In addition, the size and shape of the stepped portion of the phase shifter has been determined.
Preferably, and phase shifts the light of the second wavelength.
To allow the second wavelength λ₂Phase of light
When the difference is converted into a distance, (j-0.1) λ₁~ (J +
0.1) λ₁Dimension of the stepped part of the phase shifter
And the shape is preferably determined.

The number N of steps of the stepped portion X_XAnd the ring of the step Y
Number of N_YAnd the number N of steps W _W(Providing a step W
N if not_WBetween 0 (zero) and N_X
+ N_Y+ N_WIt is preferable that there is a relationship of 8 to 13
Yes. However, N_XIs a natural number, N _YIs a natural number, N_WIs 0 (Z
B) or a natural number.

If the number is less than 8, it may not be possible to satisfactorily correct the axial spherical aberration. Further, considering that the effective diameters of the first surface and the second surface of the objective lens are usually 5.0 mm or less, it may be difficult to manufacture if each has more than 13 effective diameters. More preferably, N _X + N _Y + N _W is 9 to 13.

FIG. 4 is a block diagram showing an embodiment of the optical device of the present invention. In FIG. 4, 1 is a first light source, 2 is an optical medium having a reflection function, 3 is an objective lens, 5 is an auxiliary lens, 6 is a first optical disc, and 6a is a transparent substrate of the first optical disc 6 (hereinafter, referred to as a transparent substrate). The first transparent substrate), 6b is the information recording surface of the first optical disc 6 (hereinafter referred to as the first information recording surface), 7 is the second optical disc, and 7a is the transparent substrate of the second optical disc 7 (hereinafter , A second transparent substrate), 7b is an information recording surface of the second optical disk 7 (hereinafter,
A second information recording surface), 9 is a diaphragm, 10 is a light receiving element, and S ₁ is a surface from the light source 1 to the auxiliary lens 5 on the light source side (first
The optical axis distance to the surface), S ₂ is the optical axis distance from the optical disk side surface (second surface) of the auxiliary lens 5 to the first surface of the objective lens 3.

In the optical device shown in FIG. 4, the numerical aperture NA ₁ of the objective lens 3 when recording or reproducing the first optical disc using the first wavelength and the second optical disc using the second wavelength are used. And the numerical aperture NA ₂ of the objective lens 3 in the case of recording or reproducing the above satisfy NA ₁ > NA ₂ . In addition, FIG.
It is a block diagram which shows another Example different from FIG. In the optical device shown in FIG. 5, the light from the light source 1 is focused on the information recording surface of the optical disc only by the objective lens 3.

In FIG. 4, the light of the first wavelength from the light source 1 is sequentially guided to the first information recording surface 6b via the optical medium 2, the auxiliary lens 5 and the objective lens 3 and converges. In FIG. 5, since the auxiliary lens 5 is not provided, the light of the first wavelength is sequentially guided to the first information recording surface 6b via the optical medium 2 and the objective lens 3 and converges.

In FIG. 4, the light of the second wavelength from the light source 1 is sequentially guided to the second information recording surface 7b via the optical medium 2, the auxiliary lens 5 and the objective lens 3 and converges. Since the auxiliary lens 5 is not provided in FIG. 5, the light of the first wavelength is sequentially guided to the second information recording surface 7b via the optical medium 2 and the objective lens 3 and converges.

The light of the first wavelength reflected by the first information recording surface 6b and the light of the second wavelength reflected by the second information recording surface 7b return along the optical path that came together and the optical medium. The light is reflected by 2 and received by the light receiving element 10.

The optical system of the optical device shown in FIGS. 4 and 5 constitutes a finite optical system as a whole. The objective lens 3 in FIG. 4 is an infinite lens when the auxiliary lens is a collimator lens, and the objective lens 3 in FIG. 5 is a finite lens. This is because the light of the light source located in a finite distance range as viewed from the optical disc is focused on the information recording surface of the optical disc. Even if a finite system lens can be used as the objective lens 3, the finite system objective lens is designed as an infinite system and includes an objective lens that can also be used as a finite system.

Marks representing digital signals are recorded on the first information recording surface 6b and the second information recording surface 7b. The size of one pit represented by this mark is several μ
In the case of m or less, the optical system of the optical device of the present invention has a diffraction limit performance in order to perform accurate recording or reproduction.

The light source 1 may be, for example, a laser light source. The first optical disk 6 is a DVD and the second optical disk 7 is a DVD.
Assuming that the CD is a CD, a laser light source having a wavelength of 785 nm for a CD, a laser light source having a wavelength of 655 for a DVD, and
nm laser light source and the like. The wavelength of the light source is not limited to the above-mentioned 655 nm and 785 nm.

In the optical device shown in FIG. 4, with respect to the thickness t _{1 of} the first transparent substrate 6a, the auxiliary lens 5 is arranged so that the light of the first wavelength converges favorably on the first information recording surface 6b. And the objective lens 3 are optimized, and the light of the second wavelength converges favorably on the second information recording surface 7b with respect to the thickness t _{2 of} the second transparent substrate 7a. Thus, the combination of the auxiliary lens 5 and the objective lens 3 is optimized.

In other words, the combination of the auxiliary lens 5 and the objective lens 3 in FIG. 4 properly corrects the aberration with respect to the first wavelength, the object-image distance and the thickness t _{1 of} the first transparent substrate 6a. Therefore, the aberration characteristics of the optical system for recording or reproducing the first optical disc 6 using the light of the first wavelength are optimized both off-axis and on-axis.

Further, in the combination of the auxiliary lens 5 and the objective lens 3 in FIG. 4, the aberration is properly corrected with respect to the second wavelength, the object-image distance and the thickness t _{2 of} the second transparent substrate 7a. Therefore, the aberration characteristics of the optical system for recording or reproducing the second optical disk 7 using the light of the second wavelength are optimized both off-axis and on-axis. The auxiliary lens 5 is set so that the aberration is optimal when used in combination with the objective lens 3.

In the optical device shown in FIG. 5, with respect to the thickness t _{1 of} the first transparent substrate 6a, the objective lens 3 is arranged so that the light of the first wavelength converges favorably on the first information recording surface 6b. Is optimized, and the objective lens 3 is optimized so that the light of the second wavelength converges favorably on the second information recording surface 7b with respect to the thickness t _{2 of} the second transparent substrate 7a. Has been converted. In other words, the objective lens 3 in FIG.
Aberrations are properly corrected with respect to the object-image distance and the thickness t _{1 of} the first transparent substrate 6a, and the optical recording / reproducing of the first optical disc 6 is performed by using the light of the first wavelength. The aberration characteristics of the system are optimized both off-axis and on-axis.

Further, the objective lens 3 in FIG. 5 has the second wavelength, the object-image distance, and the thickness t of the second transparent substrate 7a.
_The aberration is properly corrected with respect to ₂ , and the aberration characteristics of the optical system for recording or reproducing the second optical disk 7 by using the light of the second wavelength are optimized both off-axis and on-axis. There is.

In this way, the first transparent substrate having a different thickness is used.
The recording or the reproduction on the optical disk 6 and the second optical disk 7 are improved. The aberration characteristics are affected not only by the thickness of the transparent substrate but also by the refractive index of the transparent substrate.

When the objective lens as shown in FIG. 1 is used in the optical device shown in FIG. 4, the auxiliary lens 5, the aspherical surface including the apex of one surface of the objective lens 3, and the objective lens 3 are used. When the data on the information recording surface of the first optical disc 6 is recorded or reproduced, the axial spherical aberration is 0.08 in RMS value in combination with the aspherical surface of the surface including the apex of the other one surface.
is set so that λ ₁ ~0.25λ _1, and,
When recording or reproducing data on the information recording surface of the second optical disk 7, the axial spherical aberration has an RMS value of 0.08λ ₂
It is preferably set to be 0.25λ ₂ .

When the axial spherical aberration is RMS value of 0.08λ ₁ or more and 0.08λ ₂ or more, off-axis coma aberration cannot be corrected, and 0.25λ ₁ or less, and
When it is 0.25λ ₂ or less, the number of stepped portions can be at least corrected for aberrations and the number of stepped portions does not increase excessively, and the objective lens 3 can be easily manufactured.

When the objective lens as shown in FIG. 1 is used in the optical device shown in FIG. 4, each ring-shaped aspherical surface provided on one surface of the objective lens 3 (for example, the ring-shaped aspherical surface 21a in FIG. 1). , 22a, 23a, 24a, 25a,
26a, 27a, 28a, 29a, 210a, 211
At least one selected from a), an aspherical surface of a surface including an apex on the opposite side of the objective lens 3 on which each zone-shaped aspherical surface is provided, and an optical system configured by the auxiliary lens 5 are When the data of the information recording surface 6b of the first optical disc 6 is recorded or reproduced by the light having the wavelength of, the off-axis coma aberration at the image height of 0.05 mm is 0.0 in RMS value.
The information recording surface 7b of the second optical disk 7 is set by the light of the second wavelength, which is set to 3 λ ₁ or less.
Image height of 0.05mm when recording or reproducing data
It is preferable that the off-axis coma aberration at is set to an RMS value of 0.03λ ₂ or less.

When the off-axis coma aberration at an image height of 0.05 mm is 0.03λ or less, compared with the case where the off-axis coma aberration is more than 0.03λ, the light source, the auxiliary lens, the objective lens, or the optical disk,
Allowable range for tilt or axis deviation from the optical axis is 0.1%
Expand more. A more preferable range of off-axis coma aberration at an image height of 0.05 mm is 0.01λ or less, and in this case, this allowable range is less than 0.01λ.
Expand by 5% or more.

When the objective lens as shown in FIG. 1 is used in the optical device shown in FIG. 5, the aspherical surface of the objective lens 3 including the apex of one surface and the other one surface of the objective lens 3 are formed. 0.08λ in RMS values axial spherical aberration when the combination of the aspheric surface including an apex, for recording or reproducing data of the information recording surface of the first optical disc 6 ₁ ～0.25Ramuda ₁
Is set so that the axial spherical aberration is 0.08λ _{2 to} 0.25λ _{2 in} RMS value when recording or reproducing data on the information recording surface of the second optical disc 7. It is preferably set.

When the axial spherical aberration is RMS value of 0.08λ ₁ or more and 0.08λ ₂ or more, off-axis coma aberration cannot be corrected, and 0.25λ ₁ or less, and
When it is 0.25λ ₂ or less, the number of stepped portions can be at least corrected for aberrations and the number of stepped portions does not increase excessively, and the objective lens 3 can be easily manufactured.

When the objective lens as shown in FIG. 1 is used in the optical device shown in FIG. 5, each ring-shaped aspherical surface (for example, the ring-shaped aspherical surface 21a in FIG. 1) provided on one surface of the objective lens 3 is used. , 22a, 23a, 24a, 25a,
26a, 27a, 28a, 29a, 210a, 211
The optical system configured by a combination of at least one selected from a) and the aspherical surface of the objective lens 3 including the apex on the opposite side where each annular aspherical surface is provided is controlled by the light of the first wavelength. When recording or reproducing data on the information recording surface 6b of the first optical disc 6, the off-axis coma aberration at an image height of 0.05 mm is set to be RMS value of 0.03λ ₁ or less, and The second by the light of the second wavelength
When recording or reproducing data on the information recording surface 7b of the optical disc 7, it is preferable that the off-axis coma aberration at the image height of 0.05 mm is set to be RMS value of 0.03λ ₂ or less.

In the case where the off-axis coma aberration at the image height of 0.05 mm is 0.03λ or less, as compared with the case where the off-axis coma aberration is more than 0.03λ, the light source, the objective lens, or the optical disk is tilted or decentered from the optical axis. The allowable range is expanded by 0.1% or more. A more preferable range of off-axis coma aberration at an image height of 0.05 mm is 0.01λ or less, and in this case 0.0
This allowable range is expanded by 0.5% or more as compared with the case of exceeding 1λ.

[0062] corresponding to the combination of the first light and the first transparent substrate 6a having a wavelength in the optical device shown in FIG. 4, the auxiliary lens 5 and the beta ₁ a lateral magnification of the combination of the objective lens 3, FIG. 5
Corresponding to a combination of the first light and the first transparent substrate 6a having a wavelength in the optical device shown in a lateral magnification of the combination of the objective lens 3 is also referred to as beta _1, the second in the optical device shown in FIG. 4 The lateral magnification of the combination of the auxiliary lens 5 and the objective lens 3 corresponding to the combination of the light of the wavelength and the second transparent substrate 7a is β ₂ , and the light of the second wavelength in the optical device shown in FIG. Objective lens 3 corresponding to the combination with the transparent substrate 7a of
When the lateral magnification of the combination with and is also referred to as β ₂ , it is preferable that both the following conditions (A) and (B) are satisfied. (A) 0.05 ≦ | β ₁ | ≦ 0.3, (B) 0.05 ≦ | β ₂ | ≦ 0.3.

0.05 ≦ | β ₁ | and 0.05 ≦ |
If β ₂ | is not satisfied, the object-image distance becomes too long, which makes it difficult to downsize the optical device, which is not preferable. Further, if | β ₁ | ≦ 0.3 and | β ₂ | ≦ 0.3 are not satisfied, it becomes difficult to correct aberrations, which is not preferable.

Further, in the optical device shown in FIG. 4, it is preferable that S ₁ which is the distance on the optical axis from the light source 1 to the surface of the auxiliary lens 5 on the light source 1 side is 8 mm ≦ S ₁ ≦ 25 mm. If S ₁ is less than 8 mm, it may be difficult to correct aberrations, and if S ₁ is more than 25 mm, it may be difficult to downsize the optical device.

Examples of the optical medium 2 include a beam splitter, a half mirror, a prism and the like. The optical medium 2 is provided as necessary. In the optical device shown in FIGS. 4 and 5, the objective medium 3 may be directly irradiated with the light from the light source 1 without providing the optical medium 2. The means for reading the data on the information recording surface of the optical disc into the light receiving element is not limited to the means shown in FIGS.

The diaphragm 9 has a function of changing the numerical aperture (NA). The reason for providing the diaphragm 9 is that when recording or reproducing,
This is because when the numerical aperture used for the first optical disc 6 and the numerical aperture used for the second optical disc 7 are different, the numerical aperture is adjusted by the diaphragm 9. If the numerical aperture used for the first optical disc 6 and the numerical aperture used for the second optical disc 7 are the same, the diaphragm 9 is usually unnecessary.
The diaphragm 9 includes a mechanical diaphragm and an optical diaphragm, and is not particularly limited.

[0067] The numerical aperture NA ₁ of the objective lens used for the first optical disc, between the aperture NA ₂ of the objective lens used for the second optical disc, when the NA _1> NA ₂ is satisfied,
A stepped portion for blocking transmission of the light of the second wavelength and narrowing down to the numerical aperture NA ₂ may be provided in the ring-shaped region centered on the optical axis of one surface or both surfaces of the objective lens, and may be used instead of the diaphragm 9. .

Although the auxiliary lens 5 is composed of one lens in FIG. 4, the invention is not limited to this, and the auxiliary lens 5 may be composed of a plurality of lens groups. The recording or reproducing of the two types of optical discs has been described above, but the present invention is not limited to this, and three or more types of optical discs having different transparent substrate thicknesses may be the targets of recording or reproduction. Further, the optical disc in the present invention is a DVD or a CD.
However, the optical disc is not limited to this and may be another type of optical disc.

In the optical device shown in FIGS. 4 and 5, one light source 1 emits light of the first wavelength and light of the second wavelength. However, the present invention is not limited to this, and the light sources of the light of the first wavelength and the light of the second wavelength may be separately separated.

A synthetic resin is usually used as a material for the auxiliary lens 5 and the objective lens 3. However, the material is not limited to the synthetic resin, and glass may be used. Further, the optical device of the present invention may include an autofocus drive of the objective lens.

[0071]

EXAMPLE As a first optical disc 6, a DVD (t ₁ =
0.60 mm) and a CD as the second optical disc 7.
(T ₂ = 1.20 mm) is used, CD, DV
An optical device shown in FIG. 4 was manufactured on the premise of recording or reproduction of D.

A wavelength of 655 nm for reproducing or recording a DVD
Laser light source, wavelength 785nm for CD reproduction or recording
The laser light source of was used. The optical medium 2 has a thickness of 3.00 m
m, a beam splitter made of BK7 was used. D
The VD transparent substrate was designed to have a refractive index of 1.580 at a wavelength of 655 nm, and the CD transparent substrate was designed to have a refractive index of 1.573 at a wavelength of 785 nm.

The shape of the aspherical surface of the objective lens 3, including each ring-shaped aspherical surface, was determined by the following expression 5. Equation 5
, I is 2, 4, 6, 8, 10, and j is 1,
2, h is the height from the optical axis, z _j is the distance from the tangent plane of the vertex of the j-th aspherical surface to the point of height h on that aspherical surface, and r _j , k _j , A _{i, j} are coefficients of the j-th surface.

[0074]

[Equation 5]

"Example 1 (Comparative Example)" The objective lens 3 was shaped as shown in FIG. For both the CD and the DVD, it was not designed to correct the axial spherical aberration, but was designed to favorably correct the off-axis coma aberration.

Specifications and various numerical values of the optical device and the objective lens of Example 1 are shown in the upper part of Table 1. In the upper part of Table 1, f ₁ is the focal length of the objective lens 3 at the wavelength of 655 nm, f ₂ is the focal length of the objective lens 3 at the wavelength of 785 nm, d ₁ is the center thickness of the objective lens 3, and n ₁ is the objective lens 3 at the wavelength of 655 nm. , N ₂ is the refractive index of the objective lens 3 at a wavelength of 785 nm.

The coefficients of the aspherical surface of the first surface of the objective lens in Example 1 are shown in the middle row of Table 1, and the coefficients of the aspherical surface of the second surface of the objective lens in Example 1 are shown in the lower row of Table 1. In the table below, E-1 means 10 ^-1 .

[0078]

[Table 1]

The auxiliary lens is a collimator lens, and the shape of the aspherical surface of the auxiliary lens is determined by equation (5).
Table 2 shows the coefficients of the aspherical surface of the auxiliary lens. In the upper part of Table 2, f _C1 is the focal length at a wavelength of 655 nm,
f _C2 is a focal length at a wavelength of 785 nm, d _C is a center thickness, n _C1 is a refractive index at a wavelength of 655 nm, and n _C2 is a refractive index at a wavelength of 785 nm. In addition, in Examples 2 to 4, the auxiliary lens shown in the upper part of Table 2 was used.

The specifications of the optical system including the auxiliary lens in the upper part of Table 2 and the objective lens in Table 1 are as shown in the lower part of Table 2.
In the lower part of Table 2, P ₁ is the distance (working distance) between the second surface of the objective lens and the surface of the first optical disc 6 on the objective lens side at a wavelength of 655 nm, and P ₂ is the second objective lens at a wavelength of 785 nm. This is the distance (working distance) between the surface and the surface of the second optical disk 7 on the objective lens side.

[0081]

[Table 2]

FIG. 7 shows the off-axis wavefront aberration characteristics of the CD optical system. In FIG. 7, the solid line shows the wavefront aberration including all types of aberrations. The broken line shows only the off-axis coma aberration among the off-axis wavefront aberrations. FIG. 8 shows the off-axis wavefront aberration characteristic of the DVD optical system. The meanings of the solid line and the broken line are the same as in FIG. 7. In the following aberration characteristic diagrams, the solid and broken lines have the same meanings as in FIG. All aberration values in the aberration characteristic diagrams and tables described below for each example are calculated values.

When the objective lens and the auxiliary lens of Example 1 were manufactured by injection-molding plastic, the optical device of Example 1 was manufactured, and DVD and CD were recorded or reproduced by this optical device. Recording / playback could not be performed.

"Example 2 (Comparative Example)" The objective lens was shaped like the lens shown in FIG. 9, and the specifications in the upper part of Table 1, which is a basic specification, were the same as in Example 1. The second lens of the objective lens is used to correct the spherical aberration on the DVD.
A phase shifter is provided on the surface. The shape of the first surface of the objective lens was the same as that of the first surface of the objective lens of Example 1. Hereinafter, the aspherical surface of the second surface of the objective lens will be described using the reference numerals in FIG.

The aspherical surface 31a of the surface including the apex of the second surface of the objective lens was the same as the aspherical surface of the surface including the apex of the second surface of the objective lens of Example 1. The first surface of the objective lens in FIG. 9 and the aspherical surface 31a satisfactorily correct off-axis coma aberration when recording or reproducing DVD and CD.

With the first surface of the objective lens of FIG. 9 and the ring-shaped aspherical surfaces 32a, 33a, 34a, 35a, DVD and CD
The off-axis coma aberration at the time of recording or reproducing is favorably corrected. The ring-shaped aspherical surface 32a and the ring-shaped aspherical surface 34a have the same head and the same coefficient of each aspherical surface.

The coefficients of the aspherical surfaces of the ring-shaped aspherical surfaces 32a and 34a are shown in the upper left part of Table 3, and the coefficients of the aspherical surfaces of the ring-shaped aspherical surfaces 33a and 35a are shown in the upper right part of the table. Each head is shown in the middle row of Table 3. Further, φ ₃₁ , φ ₃₂ , φ ₃₃ , φ ₃₄ , and φ ₃₅ are shown in the lower part of Table 3. Since the drop γ ₃₁ , the drop γ ₃₂ , the drop γ ₃₃ , and the drop γ ₃₄ shown in the upper part of Table 4 are calculated in Expression 1 with j being 0 (zero), respectively, the laser of wavelength 785 nm for CD is used. The light is not phase-shifted, but only the laser light having a wavelength of 655 nm for DVD is phase-shifted. Therefore, the DVD can be faithfully recorded and reproduced, but the CD cannot be faithfully recorded and reproduced.

[0088]

[Table 3]

FIG. 10 shows the off-axis wavefront aberration characteristics of the CD optical system. FIG. 11 shows the off-axis wavefront aberration characteristic of the DVD optical system. The objective lens 3 and the auxiliary lens 5 having the shape of Example 2 were manufactured by injection-molding plastic, and the optical device of Example 2 was manufactured. When recording or reproducing was performed on the DVD and the CD with this optical device, the DVD could be faithfully recorded and reproduced, but the CD could not be faithfully recorded and reproduced.

"Example 3 (Comparative Example)" The objective lens was shaped like the lens shown in FIG. 12, and the specifications in the upper part of Table 1, which is a basic specification, were the same as in Example 1. In order to correct the spherical aberration on the axis for CD, a phase shifter is provided on the second surface of the objective lens as shown in FIG. The first surface of the objective lens was the same as the first surface of the objective lens of Example 1.
Hereinafter, the aspherical surface of the second surface of the objective lens will be described using the reference numerals in FIG.

The aspherical surface 41a of the surface including the apex of the second surface of the objective lens was the same as the aspherical surface of the surface including the apex of the second surface of the objective lens of Example 1. The combination of the aspherical surface and the aspherical surface 41a on the first surface of the objective lens of the present example is not designed to correct axial spherical aberration for both CD and DVD, as in the objective lens of Example 1. The setting was made so that the off-axis coma aberration was well corrected for both the CD and the DVD.

Ring zone aspherical surfaces 42a, 43a, 44a, 45
The aspherical shapes of a and 46a were set so that the off-axis coma aberration for both CD and DVD was favorably corrected in combination with the aspherical surface of the first surface in consideration of the drop. The aspherical shape of the ring-shaped aspherical surface 47a was set so that the combination with the aspherical surface of the first surface would favorably correct off-axis coma aberration for both CD and DVD.

The steps 41 and 45 have the same head.
Therefore, the steps 42 and 44 have the same head. Aspherical surface
41a and the ring zone aspherical surface 47a have the same aspherical surface coefficient.
Yes (see Example 1). Ring zone aspherical surface 42a and ring zone aspherical surface 46
The coefficient of aspherical surface is the same as that of a. Ring zone aspheric surface 43a
And the annular aspherical surface 45a have the same aspherical surface coefficient.
Coefficients of the aspherical surfaces of the annular aspherical surface 42a and the annular aspherical surface 46a
Are shown in the upper left of Table 4, and the aspherical zone surface 43a and the aspherical zone surface 4
The coefficients of the aspherical surface of 5a are shown in the upper right part of Table 4,
The coefficients of the aspherical surface of 44a are shown in the lower left part of Table 4. further,
The drop γ at each annular step is shown in the upper part of Table 5. Well
Φ₄₁, Φ₄₂, Φ₄₃, Φ₄₄, Φ₄₅, Φ ₄₆, Φ₄₇About
Are shown in the lower part of Table 5.

Table 5 shows the head γ ₄₁ , the head γ ₄₂ ,
Since the drop γ ₄₃ , the drop γ ₄₄ , the drop γ ₄₅ , and the drop γ ₄₆ are calculated in Expression 1 with i being 0 (zero), the phase shift is not performed for the laser light of the wavelength 655 nm for DVD The phase shift is performed only for the laser beam having a wavelength of 785 nm for CD. Therefore, the CD can be faithfully recorded and reproduced, but the DVD cannot be faithfully recorded and reproduced.

[0095]

[Table 4]

[0096]

[Table 5]

FIG. 13 shows the off-axis wavefront aberration characteristics of the CD optical system. FIG. 14 shows the off-axis wavefront aberration characteristic of the DVD optical system. The objective lens and auxiliary lens having the shape of Example 3 were produced by injection molding of plastic, and the optical device of Example 3 was produced. When recording or reproducing was performed on a DVD or a CD with this optical device, the CD could be recorded and reproduced faithfully, but the DVD could not be recorded and reproduced faithfully.

"Example 4 (Example)" The objective lens was shaped like the lens shown in FIG. 1, and the specifications in the upper part of Table 1, which is a basic specification, were the same as in Example 1. The first surface of the objective lens was the same as the first surface of the objective lens in Example 1. Further, the drop γ of each ring-shaped step and the drop of the annular zone aspherical surface 210a are shown in the upper part of Table 6, and the diameter (diameter) of each ring-shaped step is shown in the lower left part of Table 6.

The relationship between the steps of Example 4 and the steps 2 and 3 is shown in the lower right part of Table 6. That is, the diameter of the step in Example 4 and the diameter of the step in Examples 2 and 3 are the same,
The lower part of Table 6 shows that the step of Example 4 is a combination of the step of Example 2 and the step of Example 3.

[0100]

[Table 6]

Ring-shaped aspherical surfaces 21a, 22a, 23a, 2
4a, 25a, 26a, 27a, 28a, 29a, 21
Table 7 shows the aspherical shapes of 0a and 211a.

[0102]

[Table 7]

FIG. 15 shows the off-axis wavefront aberration characteristics of the CD optical system. FIG. 16 shows the off-axis wavefront aberration characteristic of the DVD optical system. The objective lens and the auxiliary lens having the shape of Example 4 were manufactured by injection-molding plastic, and the optical device of Example 4 was manufactured. When recording or reproducing was performed on a CD or a DVD with this optical device, it was possible to faithfully record and reproduce.

[0104]

The objective lens of the present invention has a first surface on one side.
Since the phase shifter and the second phase shifter are provided, it is possible to satisfactorily correct both the axial aberration and the off-axis aberration when recording or reproducing the first optical disc and the second optical disc.

Therefore, the objective lens of the present invention has good axial aberration and off-axis aberration even when the light source or the objective lens is tilted or deviated from the optical axis due to aging. High accuracy is easy, and there is little possibility that optical performance deteriorates with age.

[Brief description of drawings]

FIG. 1 is a sectional view showing an embodiment of an objective lens of the present invention.

FIG. 2 is a front view of the objective lens of FIG. 1 when viewed from the optical disc side.

FIG. 3 is an enlarged cross-sectional view of the second surface of the objective lens in FIG. 1 near the optical axis.

FIG. 4 is a configuration diagram showing an embodiment of an optical device of the present invention.

FIG. 5 is a configuration diagram of an embodiment of an optical device different from that of FIG.

6 is a cross-sectional view showing an objective lens of Example 1. FIG.

7 is an off-axis wavefront aberration characteristic diagram of the CD optical system of Example 1. FIG.

8 is an off-axis wavefront aberration characteristic diagram of the DVD optical system of Example 1. FIG.

FIG. 9 is a sectional view showing an objective lens of Example 2;

FIG. 10 is an off-axis wavefront aberration characteristic diagram of the CD optical system of Example 2.

11 is an off-axis wavefront aberration characteristic diagram of the DVD optical system of Example 2. FIG.

FIG. 12 is a sectional view showing an objective lens of Example 3;

FIG. 13 is an off-axis wavefront aberration characteristic diagram of the CD optical system of Example 3.

14 is an off-axis wavefront aberration characteristic diagram of the DVD optical system of Example 3. FIG.

FIG. 15 is an off-axis wavefront aberration characteristic diagram of the CD optical system of Example 4.

16 is an off-axis wavefront aberration characteristic diagram of the DVD optical system of Example 4. FIG.

[Explanation of symbols]

1: first light source 2: optical medium 3: objective lens 5: auxiliary lens 6: first optical disc 6a: transparent substrate 6b of the first optical disc 6: information recording surface 7 of the first optical disc 6: second Optical disk 7a: Transparent substrate 7b of second optical disk 7: Information recording surface 9 of second optical disk 7: Aperture 10: Light receiving element S ₁ : Distance S on the optical axis from the light source 1 to the first surface of the auxiliary lens 5. ₂ : Distance on the optical axis from the second surface of the auxiliary lens 5 to the objective lens 21, 22, 23, 24, 25, 26, 27, 28, 2
9, 210: ring-shaped step portions 21a, 22a, 23a, 24a, 25a, 26a, 2 centered on the optical axis
7a, 28a, 29a, 210a, 211a: annular aspherical surfaces φ ₂₁ , φ ₂₂ , φ ₂₃ , φ ₂₄ , φ ₂₅ , φ ₂₆ , φ ₂₇ , φ ₂₈ , φ
₂₉ , φ ₂₁₀ : stepped portions 21, 22, 23, 24, respectively
Diameter (diameter) inside of 25, 26, 27, 28, 29, ₂₁₀ φ ₂₁₀ : Effective diameter (diameter) of the second surface φ ₁₁ : Effective diameter (diameter) of the first surface

   ─────────────────────────────────────────────────── ─── Continued front page    F term (reference) 2H087 KA13 LA01 LA25 PA01 PA02                       PA17 PB01 PB02 QA02 QA07                       QA14 QA21 QA34 QA41 RA00                       RA05 RA12 RA13 RA32 RA42                       RA46 UA01                 5D119 AA41 BA01 BB01 BB04 EC01                       EC04 EC45 EC47 FA08 JA44                 5D789 AA41 BA01 BB01 BB04 EC01                       EC04 EC45 EC47 FA08 JA44

Claims (9)

[Claims] 1. A light receiving element for collecting light reflected from the information recording surface of the first optical disk by condensing light of a first wavelength on the information recording surface of the first optical disk when recording or reproducing the first optical disk. To receive light, and when recording or reproducing the second optical disc, collects light of a second wavelength different from the first wavelength on the information recording surface of the second optical disc to emit light from the information recording surface of the second optical disc. In an objective lens having aspherical surfaces on both sides, which is used in an optical system for receiving reflected light in a light receiving element, a first step having one or more ring-shaped step portions X centered on the optical axis is provided on one surface of the objective lens. A phase shifter is provided, and the first phase shifter has a function of causing a phase difference that reduces the aberration at the time of recording or reproducing of the first optical disc with respect to the light of the first wavelength, and further, on one side of this. Is one or more rings around the optical axis A second phase shifter having a stepped portion Y is provided, and the second phase shifter has a function of causing a phase difference that reduces the aberration at the time of recording or reproducing of the second optical disc with respect to the light of the second wavelength. An objective lens having. 2. The objective lens according to claim 1, wherein a convex portion or a concave portion is provided on one surface of the objective lens to form part or all of the step portion X. 3. The objective lens according to claim 1, wherein a part or the whole of the stepped portion Y is formed by providing a convex portion or a concave portion on one surface of the objective lens. And the number N _X of wheel wherein step part X, between the number N _Y ring of the stepped portion Y, one of the _{8 ≦ N X + N Y ≦} 13 associated claims 1-3 The objective lens described in. However,
N _X is a natural number and N _Y is a natural number. 5. A light having a first wavelength and a light having a second wavelength are both condensed on an information recording surface of an optical disc through an objective lens, and a surface including a vertex of one surface of the objective lens is not reflected. The combination of the spherical surface and the aspherical surface of the surface including the apex of the other one surface of the objective lens is applied to the light of the first wavelength λ ₁ when recording or reproducing data on the information recording surface of the first optical disc. The axial spherical aberration is set so as to have an RMS value of 0.08λ _{1 to} 0.25λ ₁ , and the second wavelength λ is set when data is recorded or reproduced on the information recording surface of the second optical disc. axial spherical aberration with respect to the _second light is RMS value 0.08Ramuda ₂ to 0.25
The objective lens according to claim 1, wherein the objective lens is set to have λ ₂ . 6. A lens optical system including the objective lens according to any one of claims 1 to 4 and an auxiliary lens, which is focused on an information recording surface of an optical disc, wherein the auxiliary lens and one side of the objective lens. The combination of the aspherical surface of the surface including the apex of the objective lens and the aspherical surface of the surface including the apex of the remaining single side of the objective lens is used to record or reproduce data on the information recording surface of the first optical disk. The axial spherical aberration is set so that the RMS value is 0.08λ _{1 to} 0.25λ ₁ with respect to the light of the first wavelength λ ₁ used for recording or reproduction, and the second optical disc lens optics axial spherical aberration is set to be 0.08λ ₂ ~0.25λ ₂ in RMS value for the second wavelength lambda ₂ of light when recording or reproducing data of the information recording surface system. 7. The lens optical system according to claim 6, wherein the auxiliary lens is a collimator lens. 8. When recording or reproducing data on or from the first optical disc, the light of the first wavelength is focused on the information recording face of the first optical disc through an objective lens to emit light from the information recording face of the first optical disc. The reflected light is received by the light receiving element, and the light having the second wavelength different from the first wavelength is condensed on the information recording surface of the second optical disc through the objective lens when recording or reproducing the second optical disc. An optical device using the objective lens according to any one of claims 1 to 5 as the objective lens in an optical device that causes a light receiving element to receive the reflected light from the information recording surface of the second optical disc. 9. When recording or reproducing data on or from the first optical disk, the light of the first wavelength is condensed on the information recording surface of the first optical disk through an objective lens to allow the light to be emitted from the information recording surface of the first optical disk. The reflected light is received by the light receiving element, and the light having the second wavelength different from the first wavelength is condensed on the information recording surface of the second optical disc through the objective lens when recording or reproducing the second optical disc. An optical device for causing a light receiving element to receive the reflected light from the information recording surface of the second optical disc, wherein the lens optical system uses the lens optical system according to claim 6 or 7.