JP2000195087A - Optical head and optical disk device using it - Google Patents

Abstract

PROBLEM TO BE SOLVED: To record information to optical disks with different base thicknesses and those with different corresponding wavelengths or to reproduce information from them by using an objective lens that has been designed to reduce the high-order aberration of a DVD and to ensure the jitter of a CD. SOLUTION: An optical head is composed of two light sources 1 and 4 for emitting nearly parallel luminous fluxes, an objective lens 12 for converging the two luminous fluxes for the information recording surface of a light- transmission flat plate according to the thickness of a base, and photo detectors 19 and 21 for outputting an electrical signal by detecting luminous fluxes being reflected from either information recording surface of the light-transmission flat plate. The objective lens 12 is provided with a central part and an outer- periphery part for surrounding the central part, and composes the central part and the outer-periphery part so that aberration becomes small when luminous fluxes through the light-transmission flat plate are focused for forming a point image, thus setting the numerical aperture of the central part of the objective lens 12 to 0.3 or more and 0.4 or less.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an optical head for optically recording or reproducing information on or from optical disks having different substrate thicknesses and corresponding wavelengths, and an optical disk device using the same.

[0002]

2. Description of the Related Art An optical disk has been developed as an information recording medium capable of medium exchange at a high density, and standards of the optical disk have been diversified, and securing compatibility thereof has become an important issue. Conventionally, 1.2mm thick CD, CD
-ROMs, CD-Rs, etc. are widely used, and it is strongly required to ensure compatibility with the above-mentioned media in the development of new optical discs. In recent years, DVDs have been standardized as high-density optical disks. In DVDs, the numerical aperture of an objective lens (hereinafter referred to as “NA”) has been increased to 0.6 from 0.45 of a conventional CD, and By shortening the wavelength of the light source used from 780 nm of the conventional CD to 650 nm, the spot is miniaturized to improve the recording density. In general, when the NA of the objective lens is increased, coma aberration increases when the disc is inclined, so that the spot convergence characteristics deteriorate. Since the coma caused by the tilt of the disk is proportional to the thickness of the disk base material, reducing the disk base thickness
Since it is possible to suppress an increase in coma due to the increase in the diameter, the thickness of the disk substrate is reduced from 1.2 mm of a conventional CD to 0.6 mm of a conventional DVD.

[0003] On the other hand, if the disk base material thickness is different, it is difficult to ensure disk compatibility. This is because the ordinary objective lens is designed in consideration of the thickness of the disk base material, and the convergence performance of a disk that is significantly different from the designed substrate thickness is deteriorated due to the occurrence of spherical aberration in the spot.

Further, it is difficult to ensure the compatibility of the optical disk even when the light source wavelength corresponding to the optical disk is different. For example, a CD-R is designed so that the wavelength of a light source is about 780 nm. At a wavelength of 650 nm, which is a light source used for a DVD, a reproduced signal is deteriorated due to a difference in reflectance and absorptivity of a recording medium, and it is difficult to perform recording and reproduction. Becomes

In order to solve the above problem, Japanese Patent Application Laid-Open
In Japanese Patent Application Laid-Open No. 208208/1995, a CD / DVD compatible objective lens is used to support optical disks having different substrate thicknesses.
An optical head that uses two light sources having different wavelengths and is compatible with optical disks having different wavelengths is disclosed.

FIG. 8 shows a CD / CD used in the optical head disclosed in Japanese Patent Laid-Open No. 10-208281.
FIGS. 9A and 9B show the shape of a DVD compatible objective lens. FIGS. 9A and 9B show a state in which information is recorded and reproduced on an optical disk having a wavelength of 650 nm and a substrate thickness of 0.6 mm and an optical disk having a wavelength of 780 nm and a substrate thickness of 1.2 mm. Is shown. In FIG. 8, the objective lens 112 has a base material thickness of 0.9 m for the light-transmitting flat plate at the center B1 where the numerical aperture NA is 0.45.
m, the aberration of the light spot is designed to be minimum, and when the base material thickness of the light transmitting flat plate is 0.6 mm at the outer peripheral portion B2 where the numerical aperture NA is 0.45 or more and 0.6 or less. It is designed so that the aberration of the light spot is minimized. The 650 nm light beam 107 for DVD is the objective lens 1
12 are transmitted through the area where the NA is 0.6, and 78 for CD.
Since the NA of the light beam 108 of 0 nm is limited to 0.45 by the wavelength filter 111, it can be transmitted only through the central portion B1 of the objective lens 112. In the conventional example, the numerical aperture NA of the central portion B1 of the objective lens is set to 0.45 in order to secure the resolution of the CD. When a normal DVD lens is used, a large aberration occurs when the substrate thickness is 1.2 mm. However, in FIG. 8, the optimal substrate thickness of the light-transmitting flat plate is set to 0.9 mm. Is larger than a conventional objective lens designed for a substrate thickness of 1.2 mm, but has a level that does not affect jitter.

In the conventional objective lens, the substrate thickness is 0.6.
mm DVD, the portion B1 is designed so that all the aberration degradation due to the optimum base material thickness of the central portion B1 being larger than the original value becomes a higher-order component. It is considered that aberration occurs but does not usually affect jitter.

[0008]

However, in the conventional configuration of the objective lens, in consideration of manufacturing variations, in the case of DVD, high-order aberrations increase due to processing errors, so that DVD jitter deteriorates. However, there arises a problem that the production yield is deteriorated.

That is, a conventional dual wavelength optical head uses a CD /
When a DVD compatible objective lens is used, it is necessary to set the NA to 0.45 in order to secure the resolution of the CD. , The jitter has deteriorated. In order to reduce the higher-order aberrations of the DVD, it is necessary to set the optimum base material thickness at the center to about 0.7 mm.
It was difficult to achieve D compatibility.

[0010] In order to solve the above-mentioned problems, the present invention uses an objective lens designed to reduce high-order aberrations of DVD and to ensure jitter of CD in consideration of manufacturing variations. It is an object of the present invention to provide an optical head for recording or reproducing information on or from an optical disk having a different thickness and an optical disk having a corresponding wavelength, and an optical disk device using the optical head.

[0011]

To achieve the above object, an optical head according to the present invention comprises a first light source for emitting a substantially parallel first light beam, and a substantially parallel first light source having a wavelength different from that of the first light source. A second light source for emitting two light beams and a first light source having a substrate thickness of t1.
An object that converges the first light beam on the information recording surface of the light-transmitting flat plate and converges the second light beam on the information recording surface of the second light-transmitting flat plate whose substrate thickness is t2 which is thicker than t1. A lens, a photodetector that detects a light beam reflected from one of the information recording surfaces of the first light-transmitting flat plate and the second light-transmitting flat plate and outputs an electric signal, and an objective lens having a central portion and a central portion. An outer peripheral portion surrounding the portion, wherein the outer peripheral portion and the first light flux transmitted through the first light-transmitting flat plate are converged to form an outer peripheral portion so that aberration is reduced when a point image is formed; The central portion is formed so that the second light flux transmitted through the second light transmitting flat plate converges to form a point image so that aberration is reduced, and the numerical aperture of the central portion of the objective lens is set to 0.3 or more and 0.4 or more. It is characterized in that it is set as follows.

With this configuration, it is possible to reduce higher-order aberrations of the DVD and to record or reproduce information on optical disks having different substrate thicknesses and corresponding wavelengths. Here, if the numerical aperture at the center of the objective lens is smaller than 0.3, the reduction in resolution becomes more pronounced than the effect of waveform equalization, and it becomes difficult to prevent jitter deterioration. On the other hand, a value larger than 0.4 is not preferable because the spherical aberration is large and the jitter becomes large.

Further, the optical head according to the present invention is a means for making the effective numerical aperture of the objective lens different between when recording or reproduction is performed by the first light beam and when recording or reproduction is performed by the second light beam. It is preferable to have This is for recording or reproducing information on optical disks having different substrate thicknesses and corresponding wavelengths.

Further, in the optical head according to the present invention, the wavelength of the light beam from the first light source is 600 nm or more and 700 nm or less, and the wavelength of the light beam from the second light source is 750 nm or more and 8 nm or less.
When it is 60 nm or less, it is preferable to set the numerical aperture of the outer peripheral portion of the objective lens to approximately 0.6. If the numerical aperture at the outer peripheral portion of the objective lens is around 0.6, a good reproduction signal can be obtained for the light beam from the first light source. Since the light beam from the second light source passes only through the central portion of the objective lens, it does not particularly affect the numerical aperture of the outer peripheral portion.

Further, in the optical head according to the present invention, in the means for making the effective numerical aperture of the objective lens differ,
It is preferable that the numerical aperture limit for the light beam is 0.3 or more and 0.4 or less. The means for limiting the numerical aperture is not limited, and as long as the numerical aperture limitation is satisfied by any method, it is possible to reduce the high-order aberrations of DVD and reduce the difference between optical disks having different substrate thicknesses and optical disks having different corresponding wavelengths. This is because an effect that information can be recorded or reproduced can be expected.

According to another aspect of the present invention, there is provided an optical disk apparatus including a waveform equalizing circuit for correcting an electric signal output from a photodetector. An optical head is used.

With this configuration, information is recorded on an optical disk having a different base material thickness and an optical disk having a different wavelength using an objective lens designed to reduce higher-order aberrations of a DVD and to assure jitter of a CD. Alternatively, it is possible to realize an optical disk device capable of reproducing.

[0018]

Embodiment 1 Hereinafter, an optical head according to Embodiment 1 of the present invention will be described with reference to the drawings. FIG. 1 is a configuration diagram of the optical head according to the first embodiment of the present invention. In FIG. 1, reference numeral 1 denotes a first semiconductor laser, 2 denotes a first deflection beam splitter, 3 denotes an optical path combining / separating unit, 4 denotes a second semiconductor laser, and 5 denotes a second semiconductor laser.
A polarizing beam splitter, 6 a condenser lens, 7 and 8
Is a light beam, 9 is a quarter-wave plate, 10 is a reflection mirror,
11 is a wavelength filter, 12 is an objective lens, 13 is an objective lens holder, 14 and 15 are light spots, 16
And 17 are reflected light beams, 18 is a first cylindrical lens, 19 is a first photodetector, 20 is a second cylindrical lens, 21 is a second photodetector, and 22 is a first waveform equalizing circuit. , 23 indicate a second waveform equalizing circuit, 24 and 25 indicate optical disks, and 26 indicates an objective lens driving device.

In FIG. 1, a light beam having a wavelength of 650 nm emitted from a first semiconductor laser 1 passes through a first polarizing beam splitter 2 and enters an optical path combining / separating means 3. The optical path combining / separating means 3 transmits a light beam having a wavelength of 650 nm,
It is configured to reflect a light beam having a wavelength of 780 nm.
For this reason, a light beam emitted from the first semiconductor laser 1 having a wavelength of 650 nm passes through the optical path combining / separating means 3 and becomes a substantially parallel light beam 7 by the condenser lens 6.

The light beam 7 is composed of a quarter-wave plate 9 and a reflection mirror 1.
The light passes through 0 and the wavelength filter 11 and enters the objective lens 12. The wavelength filter 11 is configured such that a light beam having a wavelength of 650 nm is transmitted therethrough, and a light beam having a wavelength of 780 nm is subjected to aperture restriction to have a numerical aperture of 0.35. Therefore, the luminous flux 7 having a wavelength of 650 nm transmits through the wavelength filter 11 and
The aperture of the objective lens 12 is limited by the objective lens holder 13, and the objective lens 12 converges to a light flux having a numerical aperture of 0.6, thereby forming a light spot 14 on the information recording surface of the optical disc 24 having a substrate thickness of 0.6 mm. Form.

On the other hand, the light beam emitted from the second semiconductor laser 4 having a wavelength of 780 nm is transmitted to the second polarization beam splitter 5.
And is reflected by the optical path combining / separating means 3 to become a substantially parallel light beam 8 by the condenser lens 6.

The light beam 8 includes a quarter-wave plate 9 and a reflection mirror 1.
The light passes through 0 and the wavelength filter 11 and enters the objective lens 12. The luminous flux 8 having the wavelength of 780 nm is applied to the wavelength filter 1.
1 limits the aperture of the objective lens 12 to substantially 0.35. The light beam 8 whose numerical aperture is limited to 0.35 is converged by the objective lens 12 to form a light spot 15 on the information recording surface of the optical disk 11 having a base material thickness of 1.2 mm.

FIG. 2 is an explanatory diagram of the objective lens of the optical head according to the first embodiment of the present invention. In FIG. 2, 1
4 and 15 are optical spots, 24 and 25 are optical disks,
A1 is the center of the objective lens 12, and A2 is the objective lens 1.
2 shows the outer peripheral portion.

In FIG. 2, the objective lens 12 is designed such that the aberration of the light spot is minimized when the base material thickness of the optical disc is 0.96 mm at the center A1 having a numerical aperture of 0.35. In the outer peripheral portion A2 having a numerical aperture of 0.35 or more and 0.6 or less, the thickness of the base material of the optical disk is 0.1 mm.
It is designed so that the aberration of the light spot is minimized at 6 mm. In the objective lens 12, the surface through which the light flux passes is a continuous and smooth curved surface.

Next, FIG. 3A shows a state in which information is recorded / reproduced on / from the optical disk 24 having a substrate thickness of 0.6 mm with a light beam having a wavelength of 650 nm. In this case, the wavelength is 650 nm
Of the objective lens 12 and the outer peripheral portion A2
Are transmitted together, so that the numerical aperture of the objective lens 12 is 0.6
Is limited to The light beam 16 reflected by the optical disk 24
Are condensed again by the objective lens 12, and the wavelength filter 1
1. The light passes through the reflecting mirror 10, the quarter-wave plate 9, and the optical path combining / separating means 3, is reflected by the first polarizing beam splitter 2, passes through the first cylindrical lens 18, and is received by the first photodetector 19. . The first photodetector 19 photoelectrically converts the received reflected light beam 16 and corrects the reproduced signal by the first waveform equalizing circuit 22 to obtain a good reproduced signal. In addition, a reproduction signal is formed, a focus control signal is formed by an astigmatism method, a tracking control signal is formed by a phase difference method and a push-pull method, and these signals are output.

FIG. 3B shows a state in which information is recorded / reproduced on / from an optical disk having a substrate thickness of 1.2 mm with a light beam having a wavelength of 780 nm. In this case, the luminous flux 8 having a wavelength of 780 nm passes only through the central portion A1 of the objective lens 12, so that the numerical aperture of the objective lens 12 is limited to 0.35. The reflected light beam 17 reflected by the optical disk 25 is again
, The wavelength filter 11, the reflection mirrors 10 and 1
After passing through the 波長 wavelength plate 9 and reflected by the optical path combining / separating means 3,
The light is reflected by the second polarization beam splitter 5, passes through the second cylindrical lens 20, and is received by the second photodetector 21. The second photodetector 21 can obtain a good reproduction signal by photoelectrically converting the received reflected light beam 17 and correcting the reproduction signal by the second waveform equalizing circuit 23. In addition, a reproduction signal is formed, a focus control signal is formed by an astigmatism method, a tracking control signal is formed by a phase difference method and a push-pull method, and these signals are output.

FIG. 4 shows that, in an infinite optical system having a wavelength of 780 nm, the entire lens area is optimally designed with a substrate thickness of 0.6 mm, and the aperture is limited to a normal DVD objective lens to obtain a substrate thickness of 1.2 mm.
4 shows the result of calculating the jitter when a disk having a diameter of mm is reproduced. When the NA is in the range of 0.33 or more and 0.41 or less, it is considered that the jitter is reduced because the spherical aberration is reduced by reducing the NA. Further, in the range where NA is smaller than 0.33, the waveform equalization coefficient increases because the resolution decreases as the NA decreases. If the NA is smaller than 0.3, the resolution is significantly reduced more than the effect of waveform equalization, so that it is considered that jitter deterioration cannot be prevented. When performing waveform equalization by this effect, the jitter can be reduced to 10% or less when the NA is in the range of 0.3 to 0.4.

The jitter calculated in FIG.
Lens at the center of the objective lens.
By setting the optimum base material thickness of the light-transmitting flat plate in the region where A is 0.3 or more and 0.4 or less to be greater than 0.6 mm, the jitter value can be reduced.

From these facts, when recording / reproducing a disk having a base material thickness of 1.2 mm using an objective lens for DVD in an infinite optical system, the NA at the center of the objective lens is required.
Is set to 0.3 or more and 0.4 or less, it is considered that the CD can be reproduced by performing the waveform equalization of the reproduction signal.

In a conventional objective lens, a DVD
Since the area at the center, which has been a cause of an increase in higher-order aberrations, can be made smaller than the conventional example, the area that deviates from the original design value becomes smaller.
Higher order aberrations of the DVD can be reduced as compared with the conventional example. Also, compared with the conventional DVD objective lens, D
VD tertiary spherical aberration can be suppressed to the same level.

As described above, according to the first embodiment, D
In the central region of the objective lens, which has been a cause of higher-order aberrations of VD, the conventional NA = 0.45 to NA = 0.
By reducing the numerical aperture to 35, the high-order aberration of the DVD is reduced, and the lack of resolution of the CD caused by reducing the numerical aperture NA at the center is improved by correcting the reproduced signal by the waveform equalizing circuit. Good recording and reproduction can be performed for both R and DVD. Although the numerical aperture NA at the center of the objective lens is 0.35, it can be changed as long as it is 0.3 or more and 0.4 or less.

In the first embodiment, the wavelength filter 11 is used as a means for limiting the aperture of the light beam 8.
The same effect can be obtained even if the aperture of the reflected light beam 17 is limited using a polarizing hologram.

Embodiment 2 Hereinafter, an optical head according to Embodiment 2 of the present invention will be described with reference to the drawings. FIG. 5 is a configuration diagram of the optical head according to the second embodiment of the present invention. In FIG. 5, reference numeral 1 denotes a first semiconductor laser, 2 denotes a first deflection beam splitter, 3 denotes an optical path combining / separating means, 4 denotes a second semiconductor laser, 5 denotes a second deflection beam splitter, and 6 denotes a condensing beam. Reference numerals 7 and 8 denote light beams, 9 denotes a quarter-wave plate, 10 denotes a reflection mirror, 12 denotes an objective lens, 13 denotes an objective lens holder, 14 and 1
5 is a light spot, 16 and 17 are reflected light fluxes, 18 is a first cylindrical lens, 19 is a first photodetector,
20 is a second cylindrical lens, 21 is a second photodetector, 22 is a first waveform equalization circuit, 23 is a second waveform equalization circuit, 24 and 25 are optical disks, and 26 is an objective lens driving device. Are respectively shown. The difference from the first embodiment is that the wavelength filter 11 is removed.

In FIG. 5, a light beam having a wavelength of 650 nm emitted from the first semiconductor laser 1 passes through the first polarizing beam splitter 2 and enters the optical path combining / separating means 3. The optical path combining / separating means 3 transmits a light beam having a wavelength of 650 nm,
It is configured to reflect a light beam having a wavelength of 780 nm.
For this reason, a light beam emitted from the first semiconductor laser 1 having a wavelength of 650 nm passes through the optical path combining / separating means 3 and becomes a substantially parallel light beam 7 by the condenser lens 6.

The light beam 7 is made up of a quarter-wave plate 9 and a reflection mirror 1.
0, the light enters the objective lens 12, the aperture of which is limited by the objective lens holder 13, and is converged to a light flux having a numerical aperture of 0.6 by the objective lens 12, on the information recording surface of the optical disc 24 having a substrate thickness of 0.6 mm. A light spot 14 is formed on the substrate.

In FIG. 2, the objective lens 12 is designed so that the aberration of the light spot is minimized when the base material thickness of the light-transmitting flat plate is 0.96 mm at the center A1 having a numerical aperture of 0.35. The outer peripheral portion A2 having a numerical aperture of 0.35 or more and 0.6 or less is designed so that aberration of a light spot is minimized when the base material thickness of the light transmitting flat plate is 0.6 mm. The surface of the objective lens 12 through which the light beam passes is a continuous and smooth curved surface.

The light beam 16 reflected by the optical disk 24
Are collected again by the objective lens 12, and are reflected by the reflection mirror 10,
The light passes through the 波長 wavelength plate 9 and the optical path combining / separating means 3, is reflected by the first polarizing beam splitter 2, passes through the first cylindrical lens 18, and is received by the first photodetector 19. The first photodetector 19 can obtain a good reproduced signal by photoelectrically converting the received reflected light beam 16 and correcting the reproduced signal by the second waveform equalizing circuit 22. In addition, a reproduction signal is formed, a focus control signal is formed by an astigmatism method, a tracking control signal is formed by a phase difference method and a push-pull method, and these signals are output.

On the other hand, the light beam emitted from the second semiconductor laser 4 having a wavelength of 780 nm is transmitted to the second polarization beam splitter 5.
And is reflected by the optical path combining / separating means 3 to become a substantially parallel light beam 8 by the condenser lens 6.

The luminous flux 8 includes a 波長 wavelength plate 9 and a reflection mirror 1
0, the light enters the objective lens 12, the aperture of which is limited by the objective lens holder 13, is converged to a light flux having a numerical aperture of 0.6 by the objective lens 12, and falls on the information recording surface of the optical disc 11 having a base material thickness of 1.2 mm. A light spot 15 is formed.

The light beam 17 reflected by the optical disk 25
Is again condensed by the objective lens 12, passes through the reflection mirror 10 and the 板 wavelength plate 9, is reflected by the optical path combining / separating means 3, is reflected by the second polarization beam splitter 5, and is reflected by the second cylindrical lens 20. , And is received by the second photodetector 21. The second photodetector 21 can obtain a good reproduction signal by photoelectrically converting the received reflected light beam 17 and correcting the reproduction signal by the second waveform equalizing circuit 23. In addition, a reproduction signal is formed, a focus control signal is formed by an astigmatism method, a tracking control signal is formed by a phase difference method and a push-pull method, and these signals are output.

Next, the reason why the numerical aperture at the center of the objective lens is set to 0.3 or more and 0.4 or less in the present invention will be described. First, FIGS. 6 and 7 show a case in which an infinite optical system having a wavelength of 780 nm is used to limit the aperture of a disk having a substrate thickness of 1.2 mm using a normal DVD objective lens whose entire area is optimally designed with a substrate thickness of 0.6 mm. None (NA = 0.6)
5 shows the results of calculation of jitter and wavefront distribution when defocusing is performed from the working distance when a disk having a base material thickness of 1.2 mm is reproduced at NA = 0.6.
FIG. 10 shows an infinite system having a wavelength of 780 nm and a substrate thickness of 1.2.
mm when playing back a disc of mm
FIG. 10 is a diagram illustrating a relationship between defocusing from a working distance when there is no aperture limit (NA = 0.6). The positive direction of defocus is a direction in which the objective lens approaches the disk.

FIG. 6 shows that the defocus is 15.8-20.
In the range of 8 μm, the change in jitter is small, and there is no difference in the wavefront in the region where the numerical aperture NA is 0.41 or more even in the range where the change in jitter is small as indicated by the hatched portion in FIG. Therefore, it is considered that the aberration in the area where NA is 0.41 or less affects the jitter. Also, from FIG. 10, 15.8 μm and 20.8 μm of defocus correspond to the case where the aperture is limited to about 0.4 and 0.3, respectively.

From the above, NA is 0.37 or more.
In the range of 41 or less, it is considered that the jitter is reduced because the spherical aberration is reduced by decreasing the NA. Further, in the range where NA is smaller than 0.37, the waveform equalization coefficient increases because the resolution decreases as the NA decreases, but the jitter is reduced by performing the waveform equalization up to NA of 0.31. is decreasing. NA
Is larger than 0.3, it is considered that the deterioration of the resolution becomes more remarkable than the effect of the waveform equalization, so that the jitter deterioration cannot be prevented. When waveform equalization is performed by this effect, the jitter hardly changes when the NA is in the range of 0.3 to 0.4.

The jitter value calculated in FIG.
This is the case of the D lens, and the jitter can be reduced by improving the aberration in the area where the NA at the center of the objective lens is 0.3 or more and 0.4 or less. That is, the jitter value can be reduced by setting the optimum base material thickness of the light transmitting flat plate at the center of the objective lens to be larger than 0.6 mm.

From these facts, when recording / reproducing a disk having a substrate thickness of 1.2 mm using an objective lens for DVD in an infinite optical system, the NA of the inner peripheral portion of the objective lens is required.
Is set to 0.3 or more and 0.4 or less, it is considered that the CD can be reproduced by performing the waveform equalization of the reproduction signal.

In the conventional objective lens, the central area, which has been a cause of an increase in DVD higher-order aberrations, can be made smaller than in the conventional example. Since the area becomes smaller, higher-order aberrations of the DVD can be reduced as compared with the conventional example. Further, even when compared with the conventional DVD objective lens, the third-order spherical aberration of the DVD can be suppressed to the same level.

As described above, according to the second embodiment, D
In the central region of the objective lens, which has been a cause of higher-order aberrations of VD, the conventional NA = 0.45 to NA = 0.
By reducing the diameter to 35, the high-order aberration of the DVD is reduced, and the insufficient resolution of the CD caused by reducing the NA at the center is improved by correcting the reproduced signal by the waveform equalizing circuit. Good recording and reproduction can be performed on both DVDs. Although the numerical aperture NA at the center of the objective lens is 0.35, it can be changed as long as it is 0.3 or more and 0.4 or less.

[0048]

As described above, according to the optical head of the present invention, the area of the center of the objective lens, which has caused high-order aberration of DVD, is reduced from the conventional NA = 0.45 to NA = 0.45.
By reducing the diameter to 0.35, higher-order aberrations of the DVD are reduced, and the lack of resolution of the CD caused by reducing the NA at the center is improved by correcting the reproduced signal by a waveform equalizing circuit. -Good recording and reproduction can be performed for both -R and DVD.

Also, in the optical head according to the present invention, the higher order aberration of the DVD is reduced by narrowing the region at the center of the objective lens and optimizing the thickness of the base material at the center.
Can be corrected by waveform equalization, and good recording and reproduction can be performed for both CD, CD-R and DVD.

[Brief description of the drawings]

FIG. 1 is an optical configuration diagram of an optical head according to a first embodiment of the present invention.

FIG. 2 is a sectional view and a plan view showing an objective lens in the optical head according to the present invention.

FIG. 3 is an explanatory diagram of a state of recording and reproducing optical disks having different substrate thicknesses by the optical head according to the present invention.

FIG. 4 is a characteristic diagram of jitter when aperture limiting is performed at each NA.

FIG. 5 is an optical configuration diagram of an optical head according to a second embodiment of the present invention;

FIG. 6 is a characteristic diagram of jitter when there is no aperture limit (NA = 0.6).

FIG. 7 is a characteristic diagram of a wavefront when there is no aperture restriction (NA = 0.6).

FIG. 8 is a sectional view and a plan view showing an objective lens in an optical head of a conventional example.

FIG. 9 is an explanatory diagram of a state in which recording / reproducing is performed on an optical disc having a different base material thickness by the conventional optical head.

FIG. 10 is a relationship diagram between working distance and defocus.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 first semiconductor laser 2 first polarization beam splitter 3 optical path combining / separating means 4 second semiconductor laser 5 second polarization beam splitter 6 condensing lens 7, 8, 107, 108 light flux 9 1/4 wavelength plate 11, 111 wavelength filter 12, 112 Objective lens 13, 113 Objective lens holder 14, 15, 114, 115 Light spot 16, 17, 116, 117 Reflected light beam 18 First cylindrical lens 19 First light detector 20 Second cylindrical lens 21 Second light detection Device 22 First Waveform Equalization Circuit 23 Second Waveform Equalization Circuit 24, 25, 124, 125 Optical Disk

 ──────────────────────────────────────────────────の Continuing on the front page (72) Inventor Hideki Hayashi 1006 Kazuma Kadoma, Osaka Pref. Matsushita Electric Industrial Co., Ltd. Terms (reference) 5D119 AA41 CA16 EC01 EC47 FA08 JA44 JA59 JB02 KA43

Claims (5)

[Claims] A first light source that emits a substantially parallel first light beam; a second light source that emits a substantially parallel second light beam having a wavelength different from that of the first light source; The first light flux is converged on the information recording surface of one light transmissive flat plate, and the second light flux is converged on the information recording surface of the second light transmissive flat plate whose substrate thickness is t2 which is thicker than t1. An objective lens; a photodetector that detects a light flux reflected from any of the information recording surfaces of the first light-transmitting flat plate and the second light-transmitting flat plate and outputs an electric signal; A central portion and an outer peripheral portion surrounding the central portion, wherein the first light flux transmitted through the outer peripheral portion and the first light-transmitting flat plate converges to form a point image so that aberration is reduced. An outer peripheral portion is formed, and the second light flux transmitted through the central portion and the second light transmitting flat plate converges. It constitutes the central portion so aberration is reduced when forming the point image, the optical head the numerical aperture of the objective lens center to and sets 0.3 to 0.4. 2. A means for making an effective numerical aperture of the objective lens different between a case where recording or reproduction is performed using the first light beam and a case where recording or reproduction is performed using the second light beam. Optical head as described. 3. The wavelength of the light beam from the first light source is 600.
2. The light according to claim 1, wherein when the wavelength of the light beam from the second light source is not less than 750 nm and not more than 860 nm, the numerical aperture of the outer peripheral portion of the objective lens is set to approximately 0.6. head. 4. The optical head according to claim 2, wherein the means for varying the effective numerical aperture of the objective lens has a numerical aperture limit for the second light flux of 0.3 or more and 0.4 or less. 5. An optical disc device comprising a waveform equalizing circuit for correcting an electric signal output from the photodetector,
An optical disk device using the optical head according to claim 1.