JPH11190818A - Objective lens, optical head, and recording and / or reproducing apparatus - Google Patents

Abstract

(57) [Problem] To provide an objective lens having a large numerical aperture NA and capable of suppressing the occurrence of wavefront aberration.
An optical head and a recording and / or reproducing apparatus using such an objective lens are provided. Further, it is possible to further increase the recording density and the capacity of the information recording medium. SOLUTION: An objective lens for an optical head used when performing recording and / or reproduction on an information recording medium,
It is composed of at least two or more lenses. The numerical aperture NA is set to 0.7 or more, the diameter of the entrance pupil is set to 1 mm or more and 5 mm or less, and the distance between the tip of the lens and the information recording medium during recording and / or reproduction (that is, the working distance W
D) is set to be 3.5 μm or more and 50 μm or less.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to an objective lens for an optical head used when recording and / or reproducing information on or from an information recording medium such as an optical disk. Further, the present invention relates to an optical head using such an objective lens and a recording and / or reproducing apparatus.

[0002]

2. Description of the Related Art Information recording media such as read-only optical disks, phase-change optical disks, magneto-optical disks, and optical cards are widely used to store data such as video information, audio information, and computer programs. I have. In recent years, demands for higher recording density and higher capacity of these information recording media have been increasing.

In order to increase the recording density of such an information recording medium, the numerical aperture N of an objective lens mounted on an optical head must be increased.
It is effective to increase A to reduce the spot diameter of light condensed by the objective lens. Therefore, for example, in the case of CD (trademark), which was put into practical use relatively early as a digital optical disk, the numerical aperture NA of the objective lens is equal to 0.1.
In contrast to 45, the numerical aperture NA of the objective lens is set to 0.6 in DVD (trademark), which is a digital optical disk having a higher recording density than CD.

As described above, it is desired that the information recording medium has a higher recording density and a larger capacity. To this end, it is desired that the numerical aperture NA is further increased from 0.6. I have.

[0005] However, it is not easy to increase the numerical aperture NA. For example, for an objective lens mounted on a conventional optical head, an aspherical single lens that is usually press-molded with glass or plastic mold using two aspherical molds is used,
Such an aspherical single lens can be manufactured with sufficient accuracy only when the numerical aperture NA is about 0.7.

Usually, an aspheric mold is processed by a diamond tool. However, if the curvature of the lens surface is made tight to increase the numerical aperture NA, the mold processing by the diamond tool can be performed accurately. Because it will be gone. Therefore, in the current aspherical surface processing technology, the maximum value of the numerical aperture NA obtained by the aspherical single lens is up to about 0.7, and the numerical aperture NA larger than that is about 0.7.
It is very difficult to accurately produce an aspherical single lens having the following. In other words, an aspherical single lens with a numerical aperture NA of 0.7 or more is not practical because there is almost no tolerance for manufacturing errors.

In order to cope with such a problem, an objective lens having a numerical aperture NA of 0.7 or more as a whole lens by combining two or more lenses has been devised. When two or more lenses are combined,
Even if the numerical aperture NA of the entire lens is increased,
Since the curvature of each lens surface can be kept low,
Even with the current aspherical surface processing technology, it is possible to relatively easily manufacture a device having a numerical aperture NA of 0.7 or more.

[0008]

However, when an objective lens is constructed by combining two or more lenses, it is difficult to combine those lenses with high accuracy.

For example, as shown in FIG. 25, it is assumed that an objective lens 101 for condensing light on a recording surface 100a of an information recording medium 100 is composed of two lenses 102 and 103. Then, at this time, as shown in FIG. 26, a so-called spacing error occurs, and the spacing d between the lenses 102 and 103 is increased.
May deviate from a predetermined value. In addition, as shown in FIG. 27, a so-called inter-plane tilt may occur, and the center axis 103a of the other lens 103 may be inclined with respect to the center axis of the other lens 102. FIG. 28
As shown in (1), a so-called eccentric error may occur, and the center axis 103a of the other lens 103 may be shifted from the center axis of the other lens 102. 26 to 28, the optical path when the two lenses 102 and 103 are ideally combined is indicated by a chain line as shown in FIG.

Manufacturing errors such as those shown in FIGS. 26 to 28 cause wavefront aberration. If the wavefront aberration is large, even if the numerical aperture NA is increased, it is possible to increase the recording density of the information recording medium. Large capacity cannot be realized. Therefore, in an objective lens that combines two or more lenses, while maintaining a manufacturing tolerance at a level that enables mass production,
It is required that the generation of the wavefront aberration caused by the manufacturing error at the time of lens combination can be sufficiently suppressed.

The present invention has been proposed in view of the above-mentioned conventional circumstances, and has a large numerical aperture NA and can suppress the occurrence of wavefront aberration while ensuring a sufficient manufacturing tolerance. An object of the present invention is to provide an objective lens, an optical head and a recording and / or reproducing apparatus using such an objective lens, and to further increase the recording density and capacity of an information recording medium. I have.

[0012]

An objective lens according to the present invention is an objective lens for an optical head used when recording and / or reproducing information on / from an information recording medium. And at least two or more lenses, having a numerical aperture NA of 0.7 or more, an entrance pupil diameter of 1 mm or more,
The distance between the tip of the lens and the information recording medium during recording and / or reproduction is set to be 3.5 μm or more and 50 μm or less.

An optical head according to the present invention is an optical head used for recording and / or reproducing information on and from an information recording medium, and has a numerical aperture of at least two lenses. Has an objective lens having an entrance pupil diameter of 0.7 mm or more and a diameter of 1 mm or more. Then, the distance between the tip of the objective lens and the information recording medium during recording and / or reproduction is set to be 3.5 μm or more and 50 μm or less.

A recording and / or reproducing apparatus according to the present invention is a recording and / or reproducing apparatus for performing recording and / or reproduction on an information recording medium using an optical head. The optical head of the recording and / or reproducing apparatus has an objective lens composed of at least two or more lenses. The objective lens has a numerical aperture NA of 0.7 or more and an entrance pupil diameter of 0.7 or more. The diameter is 1 mm or more. And
In this recording and / or reproducing apparatus, the distance between the tip of the objective lens and the information recording medium during recording and / or reproduction is 3.5
It is characterized in that it is not less than μm and not more than 50 μm.

In the objective lens, the optical head, and the recording and / or reproducing apparatus according to the present invention, since the objective lens is composed of at least two or more lenses, even if the numerical aperture NA is 0.7 or more, the lens is There is no need to make the curvature of the surface tight, and the production is easy. Moreover, in the present invention, the diameter of the entrance pupil of the objective lens is 1 mm or more, and the distance between the tip of the lens and the information recording medium during recording and / or reproduction (hereinafter, referred to as a working distance WD) is 50 μm. Because of the following, the manufacturing tolerance of the objective lens can be sufficiently ensured.

That is, the manufacturing tolerance of the lens generally tends to decrease in proportion to the diameter of the entrance pupil as the diameter of the entrance pupil decreases, but in the present invention, the diameter of the entrance pupil of the objective lens is reduced to 1 mm in diameter. As described above, the manufacturing tolerance of the objective lens can be made large.

In addition, an objective lens having a large working distance WD generally generates a large high-order spherical aberration. Therefore, if spherical aberration near the axis is corrected, the maximum annular aberration becomes an unacceptable spherical aberration. . If the higher-order spherical aberration is corrected by a higher-order aspherical surface, the absolute value of the higher-order aspherical coefficient increases, resulting in an interval error, an inter-surface tilt, and an eccentricity as shown in FIGS. Manufacturing tolerances for errors and the like become extremely small. In other words, the working distance WD
Since the absolute value of the higher-order aspherical coefficient needs to be increased in order to correct the higher-order spherical aberration, the manufacturing tolerance of the objective lens having a larger value needs to be extremely small, and the manufacturing is very difficult.

In the conventional objective lens for an optical head, the working distance WD is usually 1 mm or more, and is about 0.5 mm even if the working distance WD is set relatively small. In contrast, in the present invention,
The working distance WD of the objective lens is very small,
m or less. Therefore, in the present invention, the generation amount of high-order spherical aberration is small, and it is not necessary to increase the high-order aspherical coefficient too much. Therefore, according to the present invention, the manufacturing tolerance of the objective lens can be made large.

It is effective to reduce the working distance WD in order to obtain a large manufacturing tolerance.
If D is too small, there is a problem that the tip of the lens and the information recording medium tend to collide. However,
In the present invention, such a problem is avoided because the working distance WD of the objective lens is set to 3.5 μm or more.

In the objective lens, the optical head and the recording and / or reproducing apparatus according to the present invention, the diameter of the entrance pupil of the objective lens is preferably 5 mm or less. To increase the manufacturing tolerance, it is effective to increase the diameter of the entrance pupil of the objective lens. However, if the diameter of the entrance pupil is too large, the optical system becomes large. However, if the entrance pupil diameter of the objective lens is 5 mm or less, the optical system can be downsized to a practically sufficient degree. That is, by reducing the diameter of the entrance pupil to 5 mm or less, it is possible to reduce the size of the objective lens and the optical head equipped with the objective lens, and further reduce the size of the recording and / or reproducing apparatus equipped with the optical head. Can be achieved.

In the objective lens, the optical head, and the recording and / or reproducing apparatus according to the present invention, at least one of the two or more lenses constituting the objective lens has an aspherical surface. Is preferred. By making at least one lens surface aspherical, it is possible to further suppress the occurrence of wavefront aberration including spherical aberration.

[0022]

Embodiments of the present invention will be described below in detail with reference to the drawings.

<Objective Lens> FIG. 1 shows an example of the objective lens according to the present invention. The objective lens 1 is an objective lens for an optical head used when performing recording and / or reproduction on an information recording medium 2 such as a read-only optical disk, a phase-change optical disk, a magneto-optical disk, or an optical card. is there.

The objective lens 1 is a so-called infinite lens in which an object point is located at infinity, and is a lens 3 (hereinafter, referred to as an object-side lens 3) disposed on a light source side (not shown). And a lens 4 (hereinafter, referred to as an image-side lens 4) disposed on the information recording medium 2 side.

Although the objective lens 1 is composed of two lenses 3 and 4 in the example of FIG. 1, the objective lens according to the present invention may be composed of three or more lenses.
Here, an example of an infinite system will be described, but the present invention can also be applied to a finite system. However, in the case of a finite system,
Since the larger the numerical aperture NA, the more difficult it is to handle, an infinite system is practically preferable.

The objective lens 1 has a numerical aperture NA of 0.7 or more as a whole of the two lenses 3 and 4. In the objective lens 1, since the numerical aperture NA is very large, a very small spot diameter can be obtained. Therefore, by using the objective lens 1, it is possible to increase the recording density and the capacity of the information recording medium 2. In addition, since the objective lens 1 is configured by combining a plurality of lenses 3 and 4, even if the numerical aperture NA is 0.7 or more, each lens surface s1, s2, s3, s
It is not necessary to make the curvature of 4 so tight, and it can be easily manufactured. From the standpoint of miniaturization, it is preferable that the numerical aperture NA is larger.
In consideration of manufacturability, various optical conditions, and the like, it is actually preferable to set the thickness to about 0.95 or less.

In the objective lens 1 as described above, the lens surfaces s1,
s2, s3, s4, that is, the lens surface s1 of the object side lens 3 on the light source side, the lens surface s2 of the object side lens 3 facing the image side lens 4, and the object side lens 3 of the image side lens 4
It is preferable that at least one of the lens surface s3 on the side facing the optical recording medium 2 and the lens surface s4 on the information recording medium 2 side of the image-side lens 4 be aspherical. By making at least one of the lens surfaces s1, s2, s3, and s4 aspherical, it is possible to suppress the occurrence of wavefront aberration including spherical aberration.

However, it is preferable that the lens surface s4 of the image side lens 4 on the side of the information recording medium 2 is a flat surface or a gentle curved surface very close to the flat surface. In this objective lens 1,
Since the numerical aperture NA is set large, this lens surface s
4, the incident angle of the light beam incident on the information recording medium 2 becomes very large. Therefore, in the case where the lens surface s4 is greatly convex with respect to the information recording medium 2, the light incident on the information recording medium 2 is totally reflected, or the reflectance is extremely large. And the fill condition gets worse. Conversely, if the shape is concave with respect to the information recording medium 2, the working distance WD can hardly be obtained in the outer peripheral portion of the lens. Therefore, in this objective lens 1, the lens surface s of the image-side lens 4 on the side of the information recording medium 2
The surface shape of 4 is preferably a flat surface or a gentle curved surface very close to the flat surface. As described above, the working distance WD refers to the distance between the tip of the objective lens 1 and the information recording medium 2 during recording and / or reproduction.

When recording and / or reproduction is performed on the information recording medium 2 using the objective lens 1, a light beam from a light source is incident on the object side lens 3 through the aperture stop 5. The light incident on the object side lens 3 is condensed by the object side lens 3 and further condensed by the image side lens 4 to focus on the recording surface 2 a of the information recording medium 2. The light incident on the recording surface 2a of the information recording medium 2 is reflected by the recording surface 2a to be returned light, and is incident on the objective lens 1 again. And this return light is
After passing through the objective lens 1, it is detected by a photodetector as described later.

By the way, as described above, the plurality of lenses 3, 4
When the objective lens 1 is configured as shown in FIGS.
As shown in (1), manufacturing errors such as an interval error, an inter-surface tilt, and an eccentricity error occur, and these manufacturing errors cause aberration. And among the aberrations due to these manufacturing errors,
In particular, a problem in terms of manufacturing accuracy is coma due to decentering errors. Specifically, in order to achieve a level that can be mass-produced by the current lens manufacturing technology, it is necessary to secure a manufacturing tolerance of about 10 μm as an eccentricity error (that is, a shift amount of the center axis of each lens surface). In other words, the objective lens 1 composed of the plurality of lenses 3 and 4 is required to sufficiently suppress the generation of aberration even if there is an eccentricity error of about 10 μm.

Here, regarding the wavefront aberration generated when the objective lens 1 composed of the two lenses 3 and 4 has an eccentric error of 10 μm as shown in FIG.
FIG. 2 shows the relationship between D and the entrance pupil diameter. In FIG. 2, the relationship between the amount of generation of wavefront aberration and the working distance WD when the numerical aperture NA is 0.85 and there is an eccentric error of 10 μm is shown in FIG. Is 1.0 mm, the entrance pupil diameter is 2.0 mm, and the entrance pupil diameter is 3.0 mm. Here, the critical criterion of Marechal, which is the limit of good imaging, is 0.07λrms (λ is the wavelength of light),
The objective lens 1 used in the optical head needs to suppress the wavefront aberration to 0.07λrms or less.

As shown in FIG. 2, the aberration caused by the eccentric error increases as the entrance pupil diameter decreases, and the working distance W
It becomes larger as D is larger. As can be seen from FIG. 2, in order to suppress the generation amount of the wavefront aberration to be equal to or less than Marechal's criterion, the diameter of the entrance pupil may be set to 1.0 mm or more and the working distance WD may be set to 50 μm or less. Therefore, in the objective lens 1 to which the present invention is applied, the entrance pupil diameter is 1 m.
m and the working distance WD is set to 50 μm or less.

The diameter of the entrance pupil here corresponds to the diameter of the light beam incident on the object-side lens 3, in other words, the diameter of the aperture stop 5 disposed in front of the objective lens 1. I do. As can be seen from FIG. 2, the aberration that occurs when an eccentricity error occurs decreases as the entrance pupil diameter decreases.
It increases almost in proportion to the entrance pupil diameter. That is, the lens manufacturing tolerance tends to decrease in proportion to the entrance pupil diameter as the entrance pupil diameter decreases.

The objective lens 1 to which the present invention is applied has an entrance pupil diameter of 1 mm or more. Therefore, 1
If it is up to about 0 μm, even if an eccentricity error occurs, the generation amount of the aberration can be suppressed below Marechal's criterion. That is, in the objective lens 1 to which the present invention is applied, since the entrance pupil diameter is 1 mm or more, a sufficient manufacturing tolerance can be ensured.

The working distance WD here is the distance between the tip of the lens and the information recording medium 2 during recording and / or reproduction. As can be seen from FIG. 2, the aberration that occurs when an eccentricity error occurs increases in proportion to the working distance WD as the working distance WD increases. That is, as the working distance WD increases, the manufacturing tolerance of the lens tends to decrease substantially in proportion to the working distance WD.

In the objective lens 1 to which the present invention is applied, the working distance WD is set to 50 μm or less. Therefore, up to about 10 μm, even if an eccentricity error occurs, the amount of aberration can be suppressed to less than Marechal's criterion. That is, the objective lens 1 to which the present invention is applied
Since the working distance WD is set to 50 μm or less, a sufficient manufacturing tolerance can be secured.

From the viewpoint of lens manufacturing tolerance alone, it is preferable that the entrance pupil diameter is larger. However, if the entrance pupil diameter is too large, there arises a problem that the optical system becomes large. That is, the objective lens 1 used for the optical head can be downsized so that the objective lens 1 and the optical head equipped with the objective lens 1 and the recording and / or reproducing apparatus equipped with the optical head can be miniaturized. A smaller entrance pupil diameter is preferred.

Usually, the objective lens 1 is mounted on a biaxial actuator for performing focus servo and tracking servo. However, if the weight of the objective lens 1 becomes too heavy, the objective lens 1 is driven by driving the biaxial actuator. These servos cannot be performed with high accuracy. Therefore, in order to perform the servo with high accuracy, the objective lens 1 is downsized and its weight is reduced by two.
It is desired to suppress the amount to about 00 mg or less.

In consideration of the above points, it is desirable that the entrance pupil diameter of the objective lens 1 is about 5 mm or less. If the entrance pupil diameter of the objective lens 1 is 5 mm or less, the optical system can be miniaturized to a practically sufficient level while a sufficient manufacturing tolerance of the lens is secured, and the height is sufficiently high for a practical level. Accurate servo can be performed.

As described above, the working distance WD is preferably small from the viewpoint of the lens manufacturing tolerance only. However, if the working distance WD is too small, the tip of the lens and the information recording medium 2 may collide. There is a problem that it becomes easier. If the tip of the lens collides with the information recording medium 2 and the tip of the lens or the information recording medium 2 is scratched or stained, the focus servo may be disengaged or a signal reading error may occur. Therefore, it is desirable to secure a sufficient working distance WD to such an extent that such a collision does not easily occur.

Such a collision mainly occurs when the information recording medium 2 is tilted during recording and / or reproduction as shown in FIG. The maximum tolerance of such a gradient θ is set to about ± 0.4 ° in the standards of CD, DVD and the like. It is very difficult to reduce such a tolerance of the inclination θ because a large remodeling of the manufacturing equipment is required. Therefore, even if the recording density and capacity of the information recording medium 2 are increased, it is desired that the tolerance of the inclination θ be as high as that of a conventional CD or DVD.

Incidentally, the objective lens 1 to which the present invention is applied
In this case, it is preferable that the entrance pupil diameter of the objective lens 1 be about 1 mm to 5 mm as described above. When the diameter of the entrance pupil is about 1 mm to 5 mm, it is practical that the diameter of the image-side lens 4 is about 1 mm. In the objective lens 1 to which the present invention is applied,
As described above, the surface shape of the lens surface s4 of the image side lens 4 on the side of the information recording medium 2 is preferably a flat surface or a gentle curved surface very close to the flat surface.

Therefore, the diameter t1 of the image side lens 4 is 1 mm
And the surface shape of the lens surface s4 of the image side lens 4 on the side of the information recording medium 2 is assumed to be substantially flat. In order to prevent the tip of the lens from colliding with the information recording medium 2 even if a tilt of 0.4 ° occurs at this time,
The working distance WD may satisfy the following expression (1).

WD> (1000/2) × tan 0.4 ° ≒ 3.5 μm (1) As can be seen from the above equation (1), the working distance WD is 3.5.
μm or more, while ensuring sufficient tolerance,
Collision between the tip of the lens and the information recording medium 2 can be avoided. Therefore, in the objective lens 1 to which the present invention is applied,
The working distance WD is set to 3.5 μm or more. As a result, the tilt θ causing the collision between the tip of the lens and the information recording medium 2 can be determined by using a CD or D
The same tolerance as VD is secured. In other words, in the objective lens 1 according to the present invention, the tip of the lens collides with the information recording medium 2 even if the inclination θ occurs, within the range allowed for a CD, a DVD, or the like. There is no such thing.

By the way, as shown in FIG. 4, if dust 6 adheres to the surface of the information recording medium 2, the dust 6 may collide with the tip of the lens. Such a collision also causes a deviation of the focus servo and a signal reading error. Therefore, the objective lens 1 according to the present invention
When using the information recording medium, it is preferable to take measures against dust as much as possible, for example, by storing the information recording medium 2 in a cartridge so that the information recording medium 2 is not exposed to the outside when not in use.

However, even if the information recording medium 2 is dust-proofed by being housed in a cartridge, etc.
Usually, it is very difficult to prevent even dust having a diameter of about 1 to 2 μm or less from entering. Therefore, in the objective lens 1 to which the present invention is applied, the working distance WD is 3.5 μm in order to secure the tolerance for the inclination θ as described above.
It is preferable that the thickness is not less than 5 μm in consideration of the presence of the dust 6 as well as the length of not less than m.

<Optical Head> FIG. 5 shows an example of an optical head to which the present invention is applied. This optical head 10
Is an optical head used when recording / reproducing on / from the phase-change optical disk 11;
It has.

Here, the phase change type optical disk 11
The present invention is widely applicable to an optical head having an objective lens, and an information recording medium to be recorded and / or reproduced includes: It may be a read-only optical disk, a magneto-optical disk, an optical card, or the like.

The phase-change type optical disk 11 on which recording and reproduction are performed by the optical head 10 has, for example, a layer on which an information signal is recorded by a phase change on a substrate 12 having a thickness t2 of 1.2 mm or 0.6 mm. And a protective layer 1 having a thickness t3 of, for example, 0.1 mm on the recording layer.
3 is formed. The phase-change type optical disk 11 performs recording and reproduction by allowing light not to enter from the side of the substrate 12 but to enter from the side of the protective layer 13 which is much thinner than the substrate 12.

As described above, by causing light to enter from the side of the protective layer 13 having a small film thickness, the occurrence of aberration can be suppressed. The capacity can be increased. However, needless to say, the optical head according to the present invention is also applicable to an information recording medium in which recording and / or reproduction is performed by irradiating light from the substrate side.

The optical head 10 used for recording / reproducing on / from the phase-change optical disk 11 includes a light source, a diffraction grating, and a collimator lens (not shown).
Here, the light source is, for example, a semiconductor laser that emits linearly polarized laser light having a wavelength λ of 635 nm. This light source emits a laser beam having a constant output when reproducing an information signal from the phase-change optical disc 11, and emits a laser beam according to a signal to be recorded when recording an information signal on the phase-change optical disc 11. Modulate the intensity of the laser light to be emitted.

In the present invention, the wavelength λ of the laser light emitted from the light source is not particularly limited.
For example, when a semiconductor laser that emits laser light having a wavelength shorter than 635 nm is put to practical use, using a laser with a shorter wavelength is preferable for further increasing the recording density and increasing the capacity. is there.

The laser light emitted from the light source is
The light is diffracted by the diffraction grating and divided into zero-order light and ± first-order light, and the zero-order light and ± first-order light (hereinafter collectively referred to as “incident laser light”) are collimated by a collimator lens. It is considered as parallel light.

As shown in FIG. 5, the optical head 10 includes a polarizing beam splitter 14 and a 波長 wavelength plate 1.
5, the objective lens 1 mounted on a biaxial actuator 16, a focusing lens 17, a multi-lens 18, and a photodetector 19. Then, the incident laser light collimated by the collimator lens as described above passes through the polarization beam splitter 14, the quarter-wave plate 15, and the objective lens 1, and then enters the phase-change optical disc 1
No. 1 is incident on the recording surface 11a. At this time, when the incident laser light is transmitted through the quarter-wave plate 15, the incident laser light is converted into a circularly polarized state.

The incident light that has entered the recording surface 11a of the phase-change optical disk 11 is reflected by the recording surface 11a and becomes return light. The return light follows the original optical path, passes through the objective lens 1, and then enters the quarter-wave plate 15. Then, this return light is transmitted through the 板 wavelength plate 15 so that
Return light that has been converted into linearly polarized light that has been rotated by 90 degrees with respect to the outgoing polarization direction, and then has been converted into linearly polarized light that has been rotated by 90 degrees with respect to the outgoing polarization direction, is a polarization beam splitter 14.
Is reflected by

The return light reflected by the polarizing beam splitter 14 passes through a focusing lens 17 and a multi-lens 18 and is detected by a photodetector 19. Here, the multi-lens 18 has a cylindrical incident surface,
The lens has a concave exit surface. The multi-lens 18 imparts astigmatism to the incident light beam so that a focus servo signal can be detected by a so-called astigmatism method.

The photodetector 19 has six photodiodes, outputs electric signals corresponding to the light intensities of the return light incident on the respective photodiodes, and outputs a predetermined signal to the electric signals. To generate and output a focus servo signal, a tracking servo signal, and the like.

More specifically, the photodetector 19 detects return light to which astigmatism has been given by the multi-lens 18, and generates and outputs a focus servo signal by a so-called astigmatism method. The optical head 10 performs focus servo by driving the biaxial actuator 16 based on the focus servo signal.

The photodetector 19 detects return lights of the zero-order light and ± first-order lights diffracted by the diffraction grating, respectively, and generates and outputs a tracking servo signal by a so-called three-beam method. I do. And
The optical head 10 performs tracking servo by driving the biaxial actuator 16 based on the tracking servo signal.

Further, when the information signal is reproduced from the phase-change optical disk 11, the photodetector 19 performs a predetermined arithmetic processing on the electric signal corresponding to the light intensity of the return light incident on each photodiode. Thus, a reproduction signal from the phase change optical disk 11 is generated and output.

In the optical head 10 described above, since the objective lens 1 is composed of two lenses 3 and 4,
Even when the numerical aperture NA is 0.7 or more, it is not necessary to make the curvature of the lens surface tight, and the manufacturing is easy. Moreover, since the entrance pupil diameter of the objective lens 1 is 1 mm or more and the working distance WD is 50 μm or less, the objective lens 1
Manufacturing tolerance can be sufficiently ensured. Further, since the working distance WD of the objective lens 1 is set to 3.5 μm or more, the phase change optical disc 1
Even if the tilt of 1 occurs, recording / reproduction can be performed without collision of the tip of the objective lens 1 with the phase change optical disc 11.

<Recording and / or Reproducing Apparatus> FIG. 6 shows an example of a recording and reproducing apparatus to which the present invention is applied. This recording / reproducing apparatus 30 is a recording / reproducing apparatus for recording / reproducing on / from the above-mentioned phase-change optical disk 11, and includes the above-mentioned optical head 10.

In this case, the phase change type optical disk 11
The present invention is widely applicable to a recording and / or reproducing apparatus equipped with an optical head having an objective lens, and performs recording and / or reproducing. The information recording medium to be played back is
It may be a read-only optical disk, a magneto-optical disk, an optical card, or the like.

The recording / reproducing apparatus 30 includes a spindle motor 31 for rotatingly driving the phase-change optical disk 11, the optical head 10 used for recording / reproducing information signals, and a feeder for moving the optical head 10. Motor 32
A modulation / demodulation circuit 33 for performing predetermined modulation / demodulation processing, a servo control circuit 34 for performing servo control and the like of the optical head 10,
A system controller 35 for controlling the entire system.

The spindle motor 31 is driven and controlled by a servo control circuit 34, and is driven to rotate at a predetermined rotation speed. That is, the phase-change optical disc 11 to be recorded / reproduced is chucked by the spindle motor 31,
The spindle motor 31 driven and controlled by the servo control circuit 34 is driven to rotate at a predetermined rotation speed.

When recording and reproducing an information signal, the optical head 10 irradiates the laser beam to the phase-change optical disk 11 which is driven to rotate as described above, and detects the return light. This optical head 10 is connected to a modulation / demodulation circuit 33. When recording an information signal, a signal input from the external circuit 36 and subjected to predetermined modulation processing by the modulation / demodulation circuit 33 is supplied to the optical head 10, and the optical head 10 receives the signal supplied from the modulation / demodulation circuit 33. Is applied to the phase-change type optical disk 11 based on the laser light subjected to the light intensity modulation. When reproducing the information signal, the optical head 10 irradiates the laser light of a constant output to the phase-change optical disk 11 that is driven to rotate, generates a reproduction signal from the return light, and generates the reproduction signal. Is supplied to the modulation / demodulation circuit 33.

The optical head 10 is also connected to a servo control circuit 34. Then, at the time of recording / reproduction of the information signal, the focus servo signal and the tracking servo signal are generated from the return light reflected and returned by the phase-change optical disk 11 which is driven to rotate as described above, and these servo signals are generated. It is supplied to the servo control circuit 34.

The modulation / demodulation circuit 33 is connected to a system controller 35 and an external circuit 36. The modulation / demodulation circuit 33 transmits the information signal to the phase-change optical disk 11.
When recording on the optical disc, a signal to be recorded on the phase-change optical disc 11 is received from the external circuit 36 under the control of the system controller 35, and the signal is subjected to a predetermined modulation process. Then, the signal modulated by the modulation / demodulation circuit 33 is supplied to the optical head 10. When the information signal is reproduced from the phase-change optical disk 11, the modulation / demodulation circuit 33 receives the reproduction signal reproduced from the phase-change optical disk 11 from the optical head 10 under the control of the system controller 35, The reproduction signal is subjected to a predetermined demodulation process. The signal demodulated by the modulation / demodulation circuit 33 is output from the modulation / demodulation circuit 33 to the external circuit 36.

The feed motor 32 is for sending the optical head 10 to a predetermined position in the radial direction of the phase-change optical disk 11 when recording and reproducing the information signal. It is driven based on. That is, the feed motor 32 is connected to the servo control circuit 34 and is controlled by the servo control circuit 34.

Under the control of the system controller 35, the servo control circuit 34 controls the feed motor 32 so that the optical head 10 is sent to a predetermined position facing the phase-change optical disk 11. The servo control circuit 34 is also connected to the spindle motor 31, and controls the operation of the spindle motor 31 under the control of the system controller 35. That is, the servo control circuit 34 controls the spindle motor 31 so that the phase change optical disk 11 is driven to rotate at a predetermined rotation speed when recording and reproducing the information signal. Also, the servo control circuit 3
Numeral 4 is also connected to the optical head 10, receives a servo signal from the optical head 10 when recording and reproducing an information signal, and performs focus servo by a two-axis actuator 16 mounted on the optical head 10 based on the servo signal. And controls the tracking servo.

In the recording / reproducing apparatus 30 described above, since the objective lens 1 mounted on the optical head 10 is composed of two lenses 3 and 4, the numerical aperture NA is set to 0.7.
Even with the above, it is not necessary to make the curvature of the lens surface tight, and the manufacture is easy. In addition, the entrance pupil diameter of the objective lens 1 is 1 mm or more, and the working distance WD is 50 μm.
As described below, the manufacturing tolerance of the objective lens 1 can be sufficiently ensured. Further, since the working distance WD of the objective lens 1 is set to be 3.5 μm or more, if the working distance WD is up to about ± 0.4 °, the tip of the objective lens 1 and the phase change Optical disk 1
1 can be recorded and reproduced without colliding.

[0072]

EXAMPLES Specific examples of the objective lens according to the present invention will be described below. The objective lenses listed below are shown in FIG.
Is an objective lens composed of an object-side lens 3 and an image-side lens 4 like the objective lens 1 shown in FIG. Therefore,
In the following description, the same reference numerals as those of the objective lens 1 are used.

<Embodiment 1> FIG. 7 shows the configuration of the objective lens of this embodiment, and Table 1 shows the design values of this objective lens.
Shown in

[0074]

[Table 1]

In Table 1 and Tables 2 to 5 described later, an object point located at infinity is OBJ,
The aperture stop 5 is STO, the lens surface of the object side lens 3 on the light source side is s1, the lens surface of the object side lens 3 on the side facing the image side lens 4 is s2, and the object side lens 3 of the image side lens 4 is S3, the lens surface of the image side lens 4 on the side of the information recording medium 2 is s4, the surface of the information recording medium 2 is s5, the recording surface of the information recording medium 2 is s6, The imaging plane is shown as IMG.

FIG. 8A shows the spherical aberration of the objective lens.
FIG. 8B shows astigmatism, and FIG. 8C shows distortion.
FIG. 9A shows the lateral aberration at an angle of view of 0.5 °, and FIG. 9B shows the on-axis lateral aberration.

As shown in Table 1, in this objective lens,
The distance between the lens surface s4 and the surface s5 of the information recording medium 2 is 0.05 mm. That is, this objective lens has a working distance WD of 50 μm. As can be seen from FIGS. 8 and 9, this objective lens is a good lens with little aberration. Moreover, as shown in Table 1, the objective lens has a relatively small absolute value of a higher-order aspherical coefficient, and is easy to manufacture.

<Embodiment 2> FIG. 10 shows the configuration of the objective lens of this embodiment, and Table 2 shows the design values of this objective lens.

[0079]

[Table 2]

FIG. 11A shows the spherical aberration of this objective lens.
FIG. 11B shows astigmatism, and FIG.
FIG. 12A shows lateral aberrations at an angle of view of 0.5 ° in FIG.
FIG. 12B shows the lateral aberration on the axis.

As shown in Table 2, in this objective lens,
The distance between the lens surface s4 and the surface s5 of the information recording medium 2 is 0.03 mm. That is, the working distance WD of this objective lens is 30 μm. And, as can be seen from FIGS. 11 and 12, this objective lens
It is a good lens with little aberration. Moreover, as shown in Table 2, the objective lens has a relatively small absolute value of a higher-order aspherical coefficient, and is easy to manufacture.

<Embodiment 3> FIG. 13 shows the configuration of the objective lens of this embodiment, and Table 3 shows the design values of the objective lens.

[0083]

[Table 3]

FIG. 14A shows the spherical aberration of the objective lens.
FIG. 14B shows astigmatism, and FIG.
FIG. 15C shows the lateral aberration at an angle of view of 0.5 ° in FIG.
FIG. 15B shows the on-axis lateral aberration.

As shown in Table 3, in this objective lens,
The distance between the lens surface s4 and the surface s5 of the information recording medium 2 is 0.01 mm. That is, this objective lens has a working distance WD of 10 μm. And as can be seen from FIGS. 14 and 15,
It is a good lens with little aberration. Moreover, as shown in Table 3, this objective lens has a relatively small absolute value of a higher order aspherical coefficient, and is easy to manufacture.

<Embodiment 4> FIG. 16 shows the structure of the objective lens of this embodiment, and Table 4 shows the design values of the objective lens.

[0087]

[Table 4]

FIG. 17A shows the spherical aberration of this objective lens.
FIG. 17B shows astigmatism, and FIG.
FIG. 18C shows the lateral aberration at an angle of view of 0.5 ° in FIG.
FIG. 18B shows the lateral aberration on the axis.

As shown in Table 4, in this objective lens,
The distance between the lens surface s4 and the surface s5 of the information recording medium 2 is 0.04 mm. That is, this objective lens has a working distance WD of 40 μm. And as can be seen from FIGS. 17 and 18, this objective lens
It is a good lens with little aberration. Moreover, as shown in Table 4, the objective lens has a relatively small absolute value of a higher-order aspherical coefficient, and is easy to manufacture.

<Embodiment 5> FIG. 19 shows the structure of an objective lens of this embodiment, and Table 5 shows design values of the objective lens.

[0091]

[Table 5]

The spherical aberration of this objective lens is shown in FIG.
FIG. 20B shows astigmatism, and FIG.
FIG. 21C shows lateral aberrations at an angle of view of 0.5 ° in FIG.
FIG. 21B shows the on-axis lateral aberration.

As shown in Table 5, in this objective lens,
The distance between the lens surface s4 and the surface s5 of the information recording medium 2 is 0.03 mm. That is, the working distance WD of this objective lens is 30 μm. And as can be seen from FIGS. 20 and 21,
It is a good lens with little aberration. Moreover, as shown in Table 5, the objective lens has a relatively small absolute value of a higher order aspherical coefficient, and is easy to manufacture.

<Embodiment 6> FIG. 22 shows the structure of an objective lens of this embodiment, and Table 6 shows design values of the objective lens.

[0095]

[Table 6]

FIG. 23 (a) shows the spherical aberration of this objective lens.
FIG. 23B shows astigmatism, and FIG.
FIG. 24C shows the lateral aberration at an angle of view of 0.5 ° in FIG.
FIG. 24B shows the lateral aberration on the axis.

As shown in Table 6, in this objective lens,
The distance between the lens surface s4 and the surface s5 of the information recording medium 2 is 0.02 mm. That is, this objective lens has a working distance WD of 20 μm. And as can be seen from FIGS. 23 and 24, this objective lens
It is a good lens with little aberration. Moreover, as shown in Table 6, the objective lens has a relatively small absolute value of a higher-order aspherical coefficient, and is easy to manufacture.

[0098]

As is apparent from the above description, according to the present invention, an objective lens having a large numerical aperture NA and capable of suppressing the occurrence of wavefront aberration while securing a sufficient manufacturing tolerance is provided. Further, it is possible to provide an optical head and a recording and / or reproducing apparatus using such an objective lens. Therefore, according to the present invention, it is possible to further increase the recording density and the capacity of the information recording medium.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating an example of an objective lens to which the present invention is applied.

FIG. 2 shows entrance pupil diameters of 1.0 mm, 2.0 mm, and 3.0 m.
m, and wavefront aberration and working distance WD
FIG.

FIG. 3 is a diagram illustrating a state in which the tip of the lens collides with the information recording medium when the information recording medium is tilted.

FIG. 4 is a diagram showing a state where dust adheres to the surface of the information recording medium.

FIG. 5 is a diagram showing an example of an optical head to which the present invention has been applied.

FIG. 6 is a diagram showing an example of a recording / reproducing apparatus to which the present invention is applied.

FIG. 7 is a diagram showing an objective lens of Example 1.

FIG. 8 is a diagram illustrating aberrations of the objective lens of Example 1.
(A) is a figure which shows spherical aberration, (b) is an astigmatism, (c) is a figure which shows distortion.

FIG. 9 is a diagram illustrating aberrations of the objective lens of Example 1.
(A) is a figure which shows the transverse aberration at the angle of view of 0.5 degree, (b) is a figure which shows the transverse aberration on an axis.

FIG. 10 is a diagram illustrating an objective lens according to a second embodiment.

11A and 11B are diagrams illustrating aberrations of the objective lens according to the second embodiment, in which FIG. 11A illustrates spherical aberration, FIG. 11B illustrates astigmatism, and FIG. 11C illustrates distortion.

12A and 12B are diagrams illustrating aberrations of the objective lens according to the second embodiment, in which FIG. 12A is a diagram illustrating lateral aberrations at an angle of view of 0.5 °, and FIG. 12B is a diagram illustrating on-axis transverse aberrations.

FIG. 13 is a diagram illustrating an objective lens according to a third embodiment.

14A and 14B are diagrams illustrating aberrations of the objective lens according to the third embodiment, in which FIG. 14A illustrates spherical aberration, FIG. 14B illustrates astigmatism, and FIG. 14C illustrates distortion.

15A and 15B are diagrams illustrating aberrations of the objective lens according to the third embodiment, in which FIG. 15A is a diagram illustrating lateral aberrations at an angle of view of 0.5 °, and FIG.

FIG. 16 is a diagram illustrating an objective lens according to a fourth embodiment.

17A and 17B are diagrams illustrating aberrations of the objective lens of Example 4, in which FIG. 17A illustrates spherical aberration, FIG. 17B illustrates astigmatism, and FIG. 17C illustrates distortion.

18A and 18B are diagrams illustrating aberrations of the objective lens according to the fourth embodiment. FIG. 18A is a diagram illustrating lateral aberration at an angle of view of 0.5 °, and FIG.

FIG. 19 is a diagram showing an objective lens of Example 5;

20A and 20B are diagrams illustrating aberrations of the objective lens according to the fifth example, in which FIG. 20A illustrates spherical aberration, FIG. 20B illustrates astigmatism, and FIG. 20C illustrates distortion.

FIGS. 21A and 21B are diagrams illustrating aberrations of the objective lens according to the fifth example, in which FIG. 21A is a diagram illustrating lateral aberrations at an angle of view of 0.5 °, and FIG.

FIG. 22 is a diagram showing an objective lens of Example 6;

23A and 23B are diagrams illustrating aberrations of the objective lens according to the sixth example, in which FIG. 23A illustrates spherical aberration, FIG. 23B illustrates astigmatism, and FIG. 23C illustrates distortion.

24A and 24B are diagrams illustrating aberrations of the objective lens according to the sixth embodiment. FIG. 24A is a diagram illustrating lateral aberrations at an angle of view of 0.5 °, and FIG.

FIG. 25 is a diagram illustrating an example of an objective lens including two lenses.

26 is a diagram showing a state in which an interval error has occurred in the objective lens shown in FIG.

FIG. 27 is a diagram showing a state in which an inter-surface tilt has occurred in the objective lens shown in FIG. 25;

FIG. 28 is a diagram showing a state where an eccentric error has occurred in the objective lens shown in FIG. 25;

[Explanation of symbols]

1 Objective lens, 2 Information recording medium, 3 Object side lens, 4 Image side lens, 5 Aperture stop, WD Working distance

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Toshio Watanabe 6-7-35 Kita-Shinagawa, Shinagawa-ku, Tokyo Inside Sony Corporation (72) Inventor Akira Suzuki 6-7-35 Kita-Shinagawa, Shinagawa-ku, Tokyo Inside Sony Corporation (72) Inventor Kiyoshi Osato 6-35 Kita Shinagawa, Shinagawa-ku, Tokyo Inside Sony Corporation

Claims (9)

[Claims] 1. An objective lens for an optical head used when recording and / or reproducing information on and from an information recording medium, comprising at least two lenses and having a numerical aperture NA of 0.7. An objective lens having an entrance pupil diameter of at least 1 mm and a distance between the tip of the lens and the information recording medium of 3.5 μm or more and 50 μm or less during recording and / or reproduction. 2. The objective lens according to claim 1, wherein the diameter of the entrance pupil is 5 mm or less. 3. The objective lens according to claim 1, wherein at least one of the two or more lenses has an aspheric surface. 4. An optical head used when performing recording and / or reproduction on an information recording medium, comprising at least two or more lenses, and having a numerical aperture NA.
Has an objective lens with an entrance pupil diameter of at least 1 mm and a distance of at least 3.5 μm and at most 50 μm during recording and / or reproduction. An optical head characterized in that: 5. The optical head according to claim 4, wherein said entrance pupil diameter is 5 mm or less. 6. The optical head according to claim 4, wherein at least one of the two or more lenses constituting the objective lens has an aspherical surface. 7. A recording and / or reproducing apparatus for performing recording and / or reproduction on an information recording medium using an optical head, wherein the optical head is constituted by at least two or more lenses. The objective lens has a numerical aperture NA of 0.7 or more and an entrance pupil diameter of 1 mm or more, and the distance between the tip of the objective lens and the information recording medium during recording and / or reproduction is 3. A recording and / or reproducing apparatus characterized in that the thickness is 5 μm or more and 50 μm or less. 8. The recording and / or reproducing apparatus according to claim 7, wherein the diameter of the entrance pupil is 5 mm or less. 9. The recording and / or reproducing apparatus according to claim 7, wherein at least one of the two or more lenses constituting the objective lens has an aspherical surface. .