JP2003114383A - Objective for optical disk, optical pickup device, optical disk recording and reproducing device, and optical disk reproducing device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lens for an optical disk which is composed of a single lens of >=0.78 in numeric aperture and adaptive to an optical disk having a thin reproduction transmission layer of about <=0.3 mm and has characteristics (i) to (iv) to a light having a wavelength of about 400 nm; (i) the eccentricity tolerance between both the surfaces of the lens being within a manufacturable range, (ii) an excellent on-axis aberration characteristic being obtained, and (iii) deterioration in off-axis aberration characteristic being small. SOLUTION: This objective is a single lens which has at least one aspherical surface and a >=0.78 numerical aperture and an objective for an optical disk which meets the specific conditions. Namely, (1) d/f>1.2, (2) 0.65<R1/f<0.9, and (3) |R1/R2|<0.6, where (f) is the focal length of the lens, (d) is the center thickness of the lens, R1 is the radius of curvature of the lens at the vertex on a light source side, and R2 is the radius of curvature of the lens at the vertex on an optical disk side.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an objective lens having a high numerical aperture (NA) for realizing a large-capacity optical disc, an optical pickup device using the lens, an optical disc recording / reproducing device, and an optical disc reproducing device.

[0002]

2. Description of the Related Art A conventional CD disc has a numerical aperture of 0.45.
Reading or writing is performed with a laser beam having a wavelength of about 780 nm using an objective lens having a wavelength of ˜0.5. A DVD disc uses an objective lens having a numerical aperture of about 0.6 and is read or written with laser light having a wavelength of about 650 nm.

In order to increase the capacity of an optical disk, a next-generation optical disk pickup system using a laser beam having a shorter wavelength and a lens having a higher numerical aperture is under development.

A so-called blue laser having a wavelength of about 400 nm is considered as a laser having a shorter wavelength.

As the objective lens having the high numerical aperture, for example, the following systems have been reported.

(1) Jpn. J. Appl. Phys. Vol. 39 (2
000) pp.978-979 M. Itonaga et al. “Optical Disk
System Using a High-Numerical Aperture Single Obje
ctive Lens and a Blue LD ”.

(2) Jpn. J. Appl. Phys. Vol. 39 (2
000) pp.937-942 I. Ichimura et al. “Optical Disk
Recording Using a GaN Blue-Violet Laser Diode ”.

Here, (1) reports a system using a single lens whose numerical aperture is NA = 0.7, and (2) reports NA =
We report a system using a 0.85 two-group lens.

Although the system using the second group lens of (2) has a larger numerical aperture than that of (1), it is inferior in mass productivity because it requires an assembly process and two lenses. The cost is also high.

Therefore, as the next-generation system, an objective lens for an optical disk having a single lens with a numerical aperture of 0.7 or more is desired.

Japanese Patent Application Laid-Open No. 4-163510 describes an objective lens using a single lens having a numerical aperture of about 0.6 to 0.8.

More specifically, according to this document, it is possible to provide an objective lens having a numerical aperture of about 0.8 for a wavelength larger than 532 nm.

However, the objective lens of this document cannot exhibit practically usable characteristics for laser light having a wavelength of about 400 nm. Further, the objective lens according to this document cannot support a thin disc reproducing / transmitting layer suitable for a next-generation system.

More specifically, in the next-generation system, the thickness of the disk is set to about 0.3 mm or less in order to prevent the performance margin from being lowered by increasing the numerical aperture and to increase the margin as the system. Is desired. Here, the decrease in the performance margin means, for example, a decrease in the margin with respect to the tilt between the disc and the pickup. However, in the prior art (JP-A-4-163510), the transparent layer is set to about 1.2 mm,
If the thickness is less than this, good performance cannot be exhibited.

[0015]

Generally, the problems in putting a single lens having a high numerical aperture or a large numerical aperture to practical use are (1) the manufacturing tolerance becomes strict, and (2) the design performance. It's a bad point.

Here, (1) the manufacturing tolerance is the spacing tolerance between the entrance and exit surfaces of the double-sided asymmetric lens, the spacing tolerance between the geometric centers of the entrance and exit surfaces (eccentricity tolerance), or the entrance and exit surfaces. It means the tolerance of the inclination between. For example, the eccentricity tolerance is determined based on the amount of increase in wavefront aberration when there is eccentricity. However, these manufacturing tolerances can be met by improving manufacturing technology. That is, it is possible to manufacture with a tolerance within the range of several μm to several tens of μm.

On the other hand, (2) Deterioration of design performance means deterioration of performance in lens design, and more specifically, occurrence of aberration with respect to an off-axis ray (hereinafter abbreviated as off-axis aberration) and a plurality of wavelengths. Spherical aberration at the best image plane at each wavelength for an axial ray having the following (hereinafter abbreviated as best image plane chromatic aberration)
Means Here, the on-axis light ray means a light ray that is incident parallel to the optical axis of the lens, and the off-axis light ray means a light ray that is incident at an angle with respect to the optical axis of the lens. That is, it is possible to design so that spherical aberration does not occur with respect to an axial ray having a design reference wavelength. However, regarding the off-axis aberration and the best image plane chromatic aberration, the conventional CD or DVD is used. It is difficult to obtain a good value as compared with the objective lens.

The problem of the off-axis aberration will be described in more detail below.

The off-axis aberrations are generally inferior to conventional ones even when designing without considering the manufacturing tolerances. This is because a light beam having a large inclination angle with respect to the optical axis enters when the numerical aperture increases.

Further, the off-axis aberration becomes worse when manufacturing tolerances are taken into consideration. The details are as follows.
The most important tolerance among the manufacturing tolerances is the eccentricity tolerance. That is, in the case of a mold lens, the eccentricity between the lens surfaces is due to the mounting accuracy of the upper and lower molds, the mounting backlash (the mold moves during molding but the sliding margin at that time, the contraction due to the temperature change during molding). (Margin) etc. This eccentricity may cause an inclination between the surfaces. However, tilt and decentering have a fairly similar effect on aberrations, and the amount handled is μ
Since it is quite small on the order of m, it is usually treated collectively as eccentricity. This tolerance is an essential value in manufacturing.
With a conventional lens having a low NA, for example, a lens for DVD, even if there is a decentering of about 10 μm by design, it is possible to design to suppress the increase of aberration to 0.02λ or less. In addition, a method for suppressing eccentricity to 10 microns has been established. Furthermore, due to recent improvements in construction methods, it is possible to obtain an accuracy of, for example, about 5 μm or less. But,
Considering the sliding margin, etc., this is 1 to 2μ.
It is quite difficult to make it less than m.

Therefore, it is necessary to secure a certain degree of eccentricity tolerance in the lens design. For this purpose, it is necessary to sacrifice the on-axis aberration and the off-axis aberration. That is, it is necessary to realize a lens capable of substantially maintaining the lens performance as a result even if decentering occurs by designing the lens to have a certain amount of on-axis aberration and off-axis aberration. In this case, the on-axis aberration only slightly deteriorates. However, in a large numerical aperture lens with a numerical aperture exceeding 0.6, it can be manufactured without sacrificing off-axis aberrations considerably. Eccentricity tolerance cannot be secured.

Further, as described above, when the second group lens is adopted in order to increase the NA, the risk of the lens colliding with the disk remarkably increases due to the narrow working distance. . In the case of a plastic disc used as an optical disc, warping of the disc is inevitable. The amount of this sled is 0.3 m in the case of DVD
There are about m. This value is improved by half compared to 0.6 mm in the case of CD, but since it is an amount due to the characteristics of the material, further improvement is severe. On the other hand, the working distance of the second group lens is 0.13 mm as described above.
This distance varies depending on the lens design, but if the focal length of the lens is set in a range that does not cause the pickup to be large, it is difficult to set it to about 0.2 mm or more. That is, when the lens is at the position where the lens focuses on the disc, that is, the position where the recording / reproducing operation is performed, the lens collides with the disc unless the focus servo is operating. That is, if the focus servo shifts due to an accident caused by, for example, a disc defect or disturbance vibration, it may collide with the disc.

Another article (C) Jpn. J. Appl. Phy
s. Vol. 41 (2002) pp. 1804-1807 G. Hashimoto et al.
“Miniature Two-Axis Actuator for High-Data-Transf
er-Rate Optical Strorage Sytem ”., it is a lens of NA = 0.85 with a two-group structure and a focal length of 0.
A lens as small as 88 mm has been reported. By using this lens, it is possible to achieve miniaturization and high speed of the actuator and the pickup. However,
The working distance of the described lens is as narrow as 0.1 mm, and there is a problem that the above-mentioned danger is further increased.

An object of the present invention is to overcome the above-mentioned problems, and it can be applied to an optical disc which is composed of a single lens having a numerical aperture of 0.78 or more and has a thin reproduction transmitting layer of 0.3 mm or less. , An objective lens for an optical disc having the following characteristics (i) to (iv) with respect to light having a wavelength of about 400 nm.

(I) The eccentricity tolerance between both surfaces of the lens is within a manufacturable range.

(Ii) It has a good axial aberration characteristic.

(Iii) Less deterioration of off-axis aberration characteristics.

(Iv) The working distance is wide.

[0029]

The objective lens according to the present invention is a single lens having at least one aspherical surface and a numerical aperture of 0.78 or more, and satisfies the following conditions. Is an objective lens for an optical disc.

[0030] (1) d / f> 1.2 (2) 0.65 <R1 / f <0.95 (3) | R1 / R2 | <0.7, (4) n> 1.65.

Where f is the focal length of the lens,
d is the center thickness of the lens, R1 is the radius of curvature at the light source side vertex of the lens, and R2 is the radius of curvature at the optical disc side vertex of the lens.

The working distance of the objective lens is 0.3.
It is preferably at least mm.

The thickness of the transmission layer of the optical disk used with this objective lens is preferably 0.3 mm or less.

In order to solve the above-mentioned problems, it is preferable that the optical pickup device according to the present invention has a lens having at least one of the above characteristics, a laser light source, and a photodetector.

It is preferable that the working distance of the lens has the following relationship according to the diameter of the optical disc to be irradiated with the laser light emitted from the laser light source. Working distance> 0.005 x optical disc radius

The optical disk recording / reproducing apparatus according to the present invention is
It is preferable to have the optical pickup device and a recording / reproducing device for recording / reproducing information on / from an optical disc using the optical pickup device.

The optical disk reproducing apparatus according to the present invention preferably has the optical pickup apparatus and a reproducing means for reproducing information recorded on the optical disk by using the optical pickup apparatus.

[0038]

BEST MODE FOR CARRYING OUT THE INVENTION The embodiment of the present invention was invented by the following consideration.

That is, in order to improve the axial aberration, the lens may be designed so as to correct spherical aberration, for example. To improve the off-axis aberration, for example, the lens may be designed so as to satisfy the Abbe's sine condition. The double-sided aspherical lens can simultaneously satisfy these two conditions. That is, it is possible to design a lens that satisfies the above two conditions at the same time by using an aspherical lens for the incident surface and the exit surface.

However, such a lens has a numerical aperture of 0.
When it is 6 or more, it is difficult to secure the eccentricity tolerance. That is, when considering the eccentricity tolerance, the axial aberration or the off-axis aberration deteriorates from the off-axis aberration or the axial aberration when the eccentricity tolerance is not taken into consideration.

Therefore, in order to secure a large eccentricity tolerance, it is necessary to have an aspherical lens shape that does not greatly increase the above-mentioned aberrations even when the entrance surface and the exit surface have eccentricity. In other words, it is necessary to design an objective lens having a good balance that can appropriately deteriorate the on-axis aberration and the off-axis aberration and secure the decentering tolerance.

This objective lens according to the above consideration is a single lens having an aspherical surface on at least one surface on the light source side and the optical disk side and having a numerical aperture of 0.78 or more, and satisfies the following conditions.

[0043] (1) d / f> 1.2, (2) 0.65 <R1 / f <0.95, (3) | R1 / R2 | <0.7, (4) n> 1.65.

Here, f is the focal length of the objective lens, and d is the center thickness of the lens (FIG. 1). Further, as shown in FIG. 1, R1 is the radius of curvature at the vertex 21a on the light source side of the objective lens 21, and R2 is the radius of curvature at the vertex 21b on the optical disk 23 side of the lens.

According to this objective lens, it is possible to simultaneously satisfy the axial aberration characteristic, the off-axis aberration characteristic, and the eccentricity tolerance (which suppresses the increase in aberration).

More specifically, the axial aberration (wavefront aberration) is
Can be approximately 0.015λ or less, and the off-axis aberration (wavefront aberration) is, for example, 0.
It can be 1 λ or less. The eccentricity tolerance is 0.04 for the eccentricity δ (FIG. 1) of 5 μm.
It can be λ or less.

As will be described later, for example, t = 0.1 m
It is possible to ensure a working distance of at least 0.2 mm or more, preferably 0.4 mm or more, for a disc read layer thickness of m.

The details are as follows.

According to the lens satisfying the condition (1) (d / f> 1.2), it is possible to secure the eccentricity tolerance while suppressing the axial aberration and the off-axis aberration. The reason for this is that the thicker the core thickness of the lens, the larger the radius of the first surface (incident surface) of the lens can be made. More specifically, when the radius of curvature of the first surface increases, the angle of incidence θ of the light ray L (FIG. 1) passing through the outer end of the lens on the lens.
(The angle between the normal line n of the lens surface and the light beam) becomes smaller,
This is because the refraction effect as a non-linear phenomenon is reduced.

The d / f is preferably 1.5 or less.

As a result, the off-axis aberration characteristic can be maintained well. More specifically, when d is relatively small, the working distance can be secured even if R2 is relatively large. Therefore, the sine condition can be satisfied and the off-axis aberration can be suppressed relatively easily.

The condition (2) (0.65 <R1 /
According to the lens satisfying f <0.95), the correction of the sine condition becomes particularly easy, and the deterioration of the off-axis aberration can be suppressed.

More specifically, by setting R1 / f to 0.95 or less, the amount of violation of the sine condition can be suppressed and the off-axis aberration can be favorably maintained.

The details are as follows.

As described above, it is necessary to suppress the on-axis aberration and the off-axis aberration while ensuring the eccentricity tolerance. In this case, the value of the radius of curvature R1 of the first surface should be set large to form a biconvex lens. Is preferred. Here, when the focal length is constant, by setting R1 in the above range, the value of R2 can also be kept relatively small, and as a result, the amount of violation of the sine condition can be easily suppressed and the off-axis aberration can be improved. Can be held at For example, in the case of a lens having a focal length of 2 mm, by satisfying the above condition, the off-axis aberration (wavefront aberration) can be suppressed to 0.07λ or less for incident light having an incident angle of 0.5 degree. .

By setting R1 / f larger than 0.65, a large working distance a (FIG. 1) of the objective lens 21 with respect to the optical disc 23 can be secured.

More specifically, generally, when a single lens is used, the working distance a of the optical pickup is expressed as follows when there is an optical disk having a thickness t and a refractive index N.

[0058] a = f- (f / R1) d (n-1) / nt-N

Here, n is the refractive index of the objective lens. Therefore, a large working distance can be secured by setting R1 / f large as described above. For example, t =
It is possible to secure a working distance of 0.2 mm or more, preferably 0.4 mm or more for a disk read layer of 0.1. More specifically, for example, n = 1.75, f = 2 mm,
When d = 2.6 mm, t = 0.1 mm, N = 1.6,
0.22mm (when R1 / f is larger than 0.65)
The above working distance can be secured.

Further, the condition (3) (| R1 / R2 | <
According to the objective lens satisfying 0.7), spherical aberration (wavefront aberration) can be suppressed to be small as described above.

More specifically, a combination of radii that minimizes spherical aberration in a double-sided spherical lens is known, and such a lens is called a best-form lens.
By setting R1 and R2 so as to satisfy the above condition, it is possible to reduce the deviation from the best-form lens and reduce the spherical aberration.

Further, in the objective lens for the optical disk of this embodiment, it is preferable that | R1 / R2 | <0.3.

This makes it possible to correct spherical aberration more easily and maintain a good balance among the axial aberration, the off-axis aberration, and the eccentricity tolerance.

By satisfying the condition (4) n> 1.65, a relatively shallow spherical surface (the angle θ between the optical axis and the normal direction of the lens surface at the outermost circumference of the lens (Fig. With 1) a small spherical surface, a large numerical aperture (for example, 0.78 or more) can be easily realized.

More specifically, by satisfying the condition (4), (i) the aberration characteristic of the off-axis ray and (ii) the suppression of the increase of the aberration when there is decentering between the surfaces are simultaneously satisfied. be able to. Qualitatively, when the refractive index is within the range of the condition (4), the incident angle at the periphery of the first surface of the lens is small, and even when decentered, the influence on the second surface is small. Therefore, (i) the aberration characteristic of the off-axis ray and (ii) the suppression of the increase of the aberration when there is decentering between the surfaces can be satisfied at the same time.

It is more preferable that the refractive index n is 1.7 or more. As a result, it is possible to realize a required numerical aperture with an objective lens having a shallower spherical surface.

It is preferable that the objective lens of this embodiment further satisfies the following condition (5).

(5) -0.6 <d / R2 <0

As a result, on-axis aberrations and off-axis aberrations can be suppressed small as described above, and the eccentricity tolerance can be secured as described above.

The details are as follows.

The fact that d / R2 is negative means that R2 is negative, which means that the objective lens is a biconvex lens. Thereby, the eccentricity tolerance can be increased as described in the explanation of the condition (2). Further, by setting d / R2 to be larger than -0.6, it is possible to reduce the deviation from the perfect aplanate form, suppress the off-axis aberration to a small extent, and balance the aberrations.

The d / R2 is more preferably -0.5 or more.

In this case, better axial aberration characteristics, off-axis aberration characteristics, and decentering tolerance characteristics can be realized.

Next, a detailed description of matters relating to the optical pickup, recording device, and reproducing device using the above-mentioned lens will be given.

First, regarding the working distance required for the lens, it is desired that at least the working distance is larger than the maximum value of the surface wobbling of the disk.

The reason is that even if the focus servo shifts due to an accident caused by a disc defect or disturbance vibration, the possibility of colliding with the disc can be suppressed to a low level. Note that when the focus servo is not operating, it is possible to take collision avoidance measures such as retracting the lens in the direction away from the disc. I can say.

At this time, assuming that the angle of the warp of the disk is α, and the disk is along a simple bowl shape,
The amount L of surface deviation of the disk is L = R · t at the radius R.
an (α).

The warp angle of the disk is defined by the standard of the disk, but is 0.6 degrees for CD and 0.3 degrees for DVD. In the case of the above-mentioned sled shape, the surface runout of the disk becomes maximum at the outermost circumference, so
mm disc, 0.3 mm or
A surface runout of 0.6 mm can occur.

By the way, even in the higher density system, the disk material is plastic and
It is difficult to further improve the warp angle of the disk in the case of D. Also, the maximum disc runout is proportional to the radius. From this, when the maximum radius of the disk used in the optical pickup or the recording / reproducing apparatus is R, the surface deviation L of the disk is L = 0.005 · R.

Here, the working distance dw of the lens can be obtained by the following equation.

Dw = fb-d / nd

Here, d is the thickness of the optical disc, and n
d is the refractive index of the optical disk. fb is defined by the following equation. R1 is defined by the above equation.

Fb = f (1-t (n-1) / n / R1)

That is, as the lens becomes thicker, the working distance becomes shorter, but the working distance needs to be finite in order to be realized as a lens. Therefore, the upper limit of the lens thickness is in the range where the working distance is a finite value. This range is determined by the focal length of the lens, the thickness, and the thickness of the disc.

The thickness range of the lens can be set to, for example, 2 mm or more and 3.5 mm or less.

Here, preferably, the working distance dW
Is set to be larger than the maximum surface deviation L of the disk described above.

Dw> L > 0.005 ・ R

Therefore, for example, when the maximum radius of the disc handled by the recording / reproducing apparatus is 60 mm, the working distance is 0.3 m.
When m or more and 25 mm, the working distance is preferably 0.125 mm or more, and when 40 mm, the working distance is preferably 0.1 mm or more.

According to the present application, as described above, the working distance a can be made larger than that of the lens having the two-group structure.

An example of this embodiment will be shown below.

Example 1 The specifications of this objective lens are as shown in Table 1.

[0092]

[Table 1]

The design values of this objective lens are as shown in Table 2.

[0094]

[Table 2]

The third and fourth surfaces are the optical disk 23.
Means each surface of the transparent layer (see FIG. 1). Also, the radius,
The unit of thickness is mm.

The aspherical surface coefficients of the first surface and the second surface are as shown in Table 3 and Table 4, respectively.

[0097]

[Table 3]

Here, for example, e-6 means 10 ⁻⁶ .

[0099]

[Table 4]

FIG. 2 is a longitudinal aberration diagram for Example 1, and FIG.
[Fig. 4] is an astigmatism diagram.

According to the objective lens of Example 1, the on-axis wavefront aberration is as small as 0.01λ and can be said to be aberration-free in practical use. Further, the wavefront aberration with respect to an off-axis incident ray having an incident angle of 0.5 degree with respect to the optical axis is 0.056λ, and similarly good characteristics are exhibited. Further, regarding the decentering between the surfaces, the wavefront aberration is 0.030λ when the amount of decentering is 5 μm, and some increase in aberration can be seen, but there is no practical problem. That is, this objective lens has a manufacturing tolerance sufficient to endure mass production. The working distance is 0.71 mm, which is a sufficiently large value.

Example 2 The specifications of this objective lens are as shown in Table 5.

[0103]

[Table 5]

The design values of this objective lens are as shown in Table 6.

[0105]

[Table 6]

The third and fourth surfaces are the optical disk 23.
Means each surface of the permeable layer (Fig. 1). The unit of radius and thickness is mm.

Aspherical surface coefficients of the first surface and the second surface are as shown in Tables 7 and 8, respectively.

[0108]

[Table 7]

[0109]

[Table 8]

FIG. 4 is a longitudinal aberration diagram for Example 2, and FIG.
[Fig. 4] is an astigmatism diagram.

According to the objective lens of Example 2, the axial wavefront aberration is 0.006λ, which can be said to be substantially aberration-free. The off-axis wavefront aberration for an off-axis incident ray having an incident angle of 0.5 degree is 0.007λ, which is a good characteristic in practical use. The off-axis wavefront aberration is slightly larger than that of Example 1 because the numerical aperture (0.85) of Example 2 is larger than that (0.8) of Example 1.

Regarding the amount of eccentricity (decentering tolerance) between the surfaces, the wavefront aberration is 0.036λ when the amount of eccentricity is 5 μm. Therefore, this objective lens also has manufacturing tolerances that can withstand mass production. The working distance of this objective lens is 0.41m.
m, which is a sufficiently wide value for practical use.

Example 3 FIG. 6 is a sectional view of an objective lens of Example 3.

The light flux L incident on the objective lens 11 is
The light is refracted by the surface 1 and the second surface 2, passes through the third surface 3 and the transmission layer of the optical disc 21, and is condensed on the signal recording surface.

The lens specifications are as shown in Table 9.

[0116]

[Table 9]

Table 10 shows the design values of the lens.

[0118]

[Table 10]

Table 11 shows the aspherical surface coefficients of the first surface.

[0120]

[Table 11]

Table 12 shows the aspherical surface coefficients of the second surface.

[0122]

[Table 12]

In this lens, the on-axis wavefront aberration is as small as 0.002λ, which is practically an aberration-free value. The wavefront aberration for an incident ray of 0.5 degrees off-axis is
It shows a good characteristic of 0.008λ. Further, regarding the decentering between the surfaces, which is important in the manufacturing process, when the decentering is 3 μm, the wavefront aberration has a very good value of 0.037λ.

FIG. 7 is a longitudinal aberration diagram, FIG. 8 is a diagram showing a sine condition dissatisfaction amount, and FIG. 9 is an astigmatism diagram.

The working distance is 0.1735 mm, which is sufficiently wider than the preferred working distance of 0.125 mm when a disk having a radius of 25 mm is used.

Next, an embodiment of the optical pickup device is shown in FIG.
It will be explained with 0. The optical pickup device 30 includes a blue laser diode (LD) 31 which is a laser light source,
It has a beam splitter 32, an objective lens 33, a photodetector (PD), and a current-voltage converter (IV) 34.

The blue LD 31 emits blue light (laser light) of about 405 nm, for example. Beam splitter 32
Separates the light from the blue LD 32 toward the optical disk 35 and the light from the optical disk 35 toward the PD and the IV 34. As the objective lens 33, the one shown in the above-mentioned embodiment is used. The PD and the I-V 34 convert the incident light into a current, further convert the current into a voltage, and output the voltage.

The optical pickup device 30 includes the optical disc 3
It is possible to record a signal (information) at 5. That is,
The blue LD 31 emits blue light modulated by the input recording signal. This blue light is transmitted to the beam splitter 3
The light is focused on the optical disk 35 via 2 and the objective lens 33. In the optical disc 35, the optical pickup device 3
An information signal is recorded on the signal recording surface according to the intensity of the laser light emitted from 0. For example, a signal is recorded on the land or group of the optical disc 35 by pits or phase changes.

Further, the optical pickup device 30 can reproduce a signal from the optical disc 35. That is,
Light of a predetermined intensity emitted from the blue LD 31 is focused on the signal recording surface of the optical disc 35 via the beam splitter 32 and the objective lens 33. The reflected light from the optical disk 35 passes through the objective lens 33 and the beam splitter 32 to generate P
It is input to D and I-V34, and is converted into a voltage. Thus, for example, a signal recorded as a pit on a land or groove on the signal recording surface of the optical disc 35 is output as a voltage.

Next, an embodiment of the optical disk recording / reproducing apparatus or the optical disk reproducing apparatus will be described with reference to FIG.

The optical disc recording / reproducing apparatus has a PRML (Pa
rtial Response Maxim likelihood) block 50,
It has a controller block 60 and a recording compensation block 70. The optical disk recording / reproducing apparatus has the above-mentioned optical pickup device 30.

The PRML block 50 includes the A / D converter 5
1, a digital equalizer 52, and a tap coefficient controller 53
The phase shifter 54, the PLL 55, and the Viterbi detector 56. The controller block 60 is
It has a 1-7 RLL (Run Length Limited) processing unit 61. Further, in this example, the signal modulation method is 1-7RL.
L (Run Length Limit) is used.

The PRML block 50 receives a signal from the optical pickup 30 via the preamplifier, and outputs the PRML block 50.
Perform signal processing. The controller block 60 is a PRM
A signal is input from the Viterbi decoder 56 of the L block 50 and processed by the 1-7 RLL processing unit 61. A signal is input from the controller block 60 to the recording compensation block 70, and the recording compensation block 70 drives and controls the blue LD 31 of the optical pickup device 30 via the LD drive unit in accordance with the signal.

As described above, the optical disk recording / reproducing apparatus is
The signal read by the optical pickup device 30 from the optical disc 35 is reproduced by performing predetermined decoding, demodulating and outputting. In addition, the input signal is subjected to predetermined encoding and modulation, and is written on the optical disc 35 via the optical pickup 30 to be recorded. The optical disk recording / reproducing apparatus may be configured as an optical disk reproducing apparatus without the recording block. A polarization beam splitter can be used as an example of the above-mentioned beam splitter.

[0135]

As described above, according to the present invention, a single lens having a numerical aperture of 0.78 or more is used, and 0.
It can be applied to an optical disc having a thin reproduction transmission layer of 3 mm or less, has an eccentricity tolerance within a range in which light with a wavelength of about 400 nm can be manufactured, and has good on-axis aberration characteristics and off-axis aberration characteristics. It is possible to provide an objective lens having a wide working distance, an optical pickup device using the objective lens, an optical disc recording / reproducing device using the optical pickup device, and an optical disc reproducing device.

Furthermore, it is possible to provide an optical pickup or an optical disk recording / reproducing apparatus and a reproducing apparatus having a sufficiently wide working distance according to the maximum radius of the disk to be used.

[Brief description of drawings]

FIG. 1 is an explanatory diagram illustrating parameters of an objective lens according to an embodiment of the present invention.

FIG. 2 is a longitudinal aberration diagram for Example 1.

FIG. 3 is an astigmatism diagram of Example 1.

FIG. 4 is a longitudinal aberration diagram for Example 2.

5 is an astigmatism diagram of Example 2. FIG.

FIG. 6 is a sectional view of an objective lens according to a third embodiment.

7 is a longitudinal aberration diagram for Example 3. FIG.

FIG. 8 is a diagram illustrating a sine condition dissatisfaction amount according to the third embodiment.

FIG. 9 is a diagram showing astigmatism of Example 3;

FIG. 10 is a diagram showing an embodiment of an optical pickup device.

FIG. 11 is a diagram showing an embodiment of an optical disc recording / reproducing apparatus.

Continued front page    F term (reference) 2H087 KA13 LA01 PA01 PA17 PB01                       QA02 QA07 QA14 QA34 RA05                       RA12 RA13 RA45                 5D119 AA11 AA22 AA31 AA32 BA01                       BB01 BB02 BB03 DA01 DA05                       EB02 EC01 FA05 JA11 JA44                       JB01 JB02 JB03 JB06 JB10                 5D789 AA11 AA22 AA31 AA32 BA01                       BB01 BB02 BB03 DA01 DA05                       EB02 EC01 FA05 JA11 JA44                       JB01 JB02 JB03 JB06 JB10

Claims (7)

[Claims] 1. An objective lens for an optical disk, which is a single lens with at least one surface having an aspherical shape and a numerical aperture of 0.78 or more, and which satisfies the following conditions. (1) d / f> 1.2, (2) 0.65 <R1 / f <0.95, (3) | R1 / R2 | <0.7, (4) n> 1.65. Here, f is the focal length of the lens, d is the center thickness of the lens, R1 is the radius of curvature of the apex of the lens on the light source side, R2 is the radius of curvature of the apex of the lens on the optical disk side, and n is the radius. The refractive index of the lens. 2. The lens according to claim 1, which has a working distance of 0.3 mm or more. 3. The disk has a transparent layer thickness of 0.3 mm.
The lens according to claim 1 or 2, wherein: 4. An optical pickup device comprising the lens according to claim 1, a laser light source, and a photodetector. 5. The optical pickup device according to claim 4, wherein the working distance of the lens has the following relationship according to the diameter of the optical disc to be irradiated with the laser light emitted from the laser light source. Working distance> 0.005 x optical disc radius 6. An optical disc recording / reproducing apparatus, comprising: the optical pickup device according to claim 4; and a recording / reproducing device for recording / reproducing information on / from an optical disc using the optical pickup device. 7. An optical disc reproducing apparatus comprising: the optical pickup device according to claim 4 or 5, and a reproducing means for reproducing information recorded on an optical disc using the optical pickup device.