JPH1027373A - Optical head and optical disk device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To converge luminous fluxes different in wavelengths by using a single objective lend and to record/reproduce an optical disk different in optimally reproducing wavelength for every substrate thickness by correcting a spherical avrratsion generated on a converged spot due to the difference of disk substrate thickness and wavelength. SOLUTION: Light beams emitted from a semiconductor laser 1a of 650nm wavelength pass through a wavelength selective aperture filter 8 and a wavefront correction plate 9 and are converged by a lens 10 to form a spot SPa on an optical disk 12a of 0.6mm substrate thickness. Similarly, light beams emitted from a semiconductor laser 1b of 780nm wavelength pass through the filter 8 and correction plate 9 and are converged by the lens 10 to form spot SPb on an optical disk of 1.2mm substrate thickness. By the correction plate 9 comprising two kinds of materials n1, n2, the wavefront of the transmission light is hardly affected at almost 560nm wavelength. At almost 780nm wavelength, a luminous flux surface emitted from the correction plate 9 offsets spherical aberration generated by the deviation of the substrate thickness of the optical disk 7.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical disk apparatus for recording information on an optical recording medium or optically reproducing information from the optical recording medium, and in particular, for obtaining a substrate thickness or an optimum reproduction signal. The present invention relates to an optical head that performs recording or reproduction on optical recording media having different wavelengths.

[0002]

2. Description of the Related Art Optical discs have been developed as large-capacity and medium-replaceable information recording media, but their recording methods and disc sizes are diversified, and their compatibility has become an important issue. Until now, CD (Compa
ct Disc) based on the CD-ROM (Compa
ct Disc-Read Only Memory), CD-R (Compact Disc)
-Rewitable) has become widespread, and there is a growing demand for ensuring compatibility with the above media when developing new optical discs.

At present, as a next-generation high-density optical disk, DV
D has been proposed. Here, the numerical aperture of the objective lens (N
By increasing the numerical aperture (NA) to 0.6 from 0.45 of the conventional CD, and shortening the wavelength of the light source to be used from 780 nm of the conventional CD to 650 nm, the aperture spot can be miniaturized. The recording density is improved.

[0004] When the NA of the objective lens is increased, the coma aberration generated when the disk is tilted sharply increases, so that there is a problem that the light condensing characteristic of the narrowed spot is easily deteriorated. Since the coma caused by the tilt of the disk is proportional to the thickness of the disk substrate, it is possible to suppress an increase in coma due to an increase in NA by reducing the thickness of the disk substrate. Based on this concept, the disk substrate thickness of DVD
It has been proposed to reduce the thickness of D from 1.2 mm to 0.6 mm.

On the other hand, if the disk substrate thickness is different, there is a problem that it is difficult to ensure disk compatibility. Usually, the objective lens is designed assuming a specific disk substrate thickness, and for disks that differ significantly from the design substrate thickness,
Spherical aberration occurs in the narrowed-down spot, and the light-collecting characteristics of the narrowed-down spot deteriorate. Conventional techniques for solving this problem include, for example, Optical Review Vol. 1, No. 1 (1994), pp. 27-29 (OPTICAL REVIEW, V).
ol.1, No.1, (1994) pp.27-29), and Mitsubishi Electric News Release Development No. 9507 (June 2, 1995
1). The former has a substrate thickness of 0.6 mm
A hologram is formed on the surface of the objective lens, and a CD is reproduced by diffracted light and a DVD is reproduced by transmitted light. Here, the hologram pattern is designed in advance so as to compensate for the spherical aberration that occurs when reproducing a CD. In the latter, both an objective lens for a substrate thickness of 0.6 mm and an objective lens for a 1.2 mm substrate are mounted on an optical head, and the two lenses are switched and used by an actuator.

Further, if the wavelength of the light source is different, there arises a problem that it is difficult to ensure the compatibility of the disk. For example, the CD-R is designed on the assumption that the wavelength of the light source is about 780 nm. When the wavelength of the light source becomes 650 nm, the reproduction signal is deteriorated due to the difference in the reflectance and the absorptivity of the recording medium. Conventional techniques for solving this problem include:
For example, a method described in the 43rd Lecture Meeting on Applied Physics, Lecture No. 28aF-7 (Preprint of the Lecture No. 3, p. 1084) or Japanese Patent Application Laid-Open No. 8-55363 has been proposed.
The former uses two light sources having wavelengths of 650 nm and 780 nm, and has a wavelength of 78 in the light flux before entering the objective lens.
By arranging a hologram element acting only on a light beam of 0 nm, spherical aberration due to a difference in substrate thickness is corrected. The latter is 650 nm and 780 nm like the former.
The use of two light sources having the following wavelengths and only slightly diverging light at a wavelength of 780 nm attempts to correct spherical aberration due to differences in substrate thickness.

[0007]

The above-mentioned prior art has some problems as before, such as CDs, CD-ROMs, and CDs.
It is not always sufficient to ensure compatibility between the DVD and the DR. That is, when a hologram element is used, sufficient characteristics are not necessarily obtained from the viewpoint of controllability of the shape, workability, or wavelength shift. When two objective lenses are used, it is difficult to cope with high-speed rotation of the disk due to an increase in the size and weight of the movable portion of the objective lens actuator, which is disadvantageous from the viewpoint of improving the transfer speed or shortening the access time. .
Further, when two different wavelengths are used and the light flux incident on one objective lens is substantially parallel light of one wavelength and divergent light of the other wavelength, it is extremely difficult to design the objective lens. Become. That is, for example, the principle of lens design by Max Belek (translated by Kazuo Miyake, Kodansha 1
970), pages 122-124,
In the case of a single lens used when lightness is required like an optical disc, when considering a case where a parallel light beam of one wavelength is incident, a lens shape that minimizes aberration is almost uniquely determined. Even if correction means such as an aspherical shape is taken, when divergent light of the other wavelength enters under the condition that the coma aberration and the astigmatism are corrected by the parallel light flux, both aberrations cannot be corrected simultaneously. Therefore, it is inevitable that the light-collecting characteristic of the narrowed-down spot is degraded by the luminous flux incident as the divergent light.

An object of the present invention is to solve the above problems,
It is an object of the present invention to realize an optical head capable of condensing light beams having different wavelengths using one objective lens and recording and reproducing characteristics with respect to optical disks having different wavelengths that are optimal for reproduction and thickness of a disk substrate.

[0009]

According to the present invention, a first light source having a first wavelength and a second light source having a second wavelength different from the first light source are provided. A light beam combining means for combining light beams from the two light sources, one objective lens for narrowing a light beam emitted from the light beam combining means onto an optical recording medium through a transparent substrate, and reflection from the optical recording medium. A light beam separating unit that separates light into light beams having only the first wavelength or the second wavelength, and a light detecting unit that receives each of the light beams separated by the light beam separating unit and performs photoelectric conversion. An optical head having at least a light wavefront compensating unit in which two kinds of materials having different wavelength dependences of refractive index are stuck on an optical axis between the light combining unit and the objective lens.

The bonding surface of the two types of materials in the optical wavefront compensating means is a substantially fourth-order curved surface, and the refractive indices of the two types of materials are substantially equal at one of the two wavelengths. Has become.

In addition, between the light wavefront compensating means and the two light sources, recording or reproduction is performed by a light beam having a first wavelength, and recording or reproduction is performed by a light beam having a second wavelength. Thus, a filter having wavelength selectivity for changing the effective numerical aperture of the objective lens is disposed.

Further, the bonding surface in the light wavefront compensating means corresponds to a smaller numerical aperture among a numerical aperture corresponding to the light beam having the first wavelength and a numerical aperture corresponding to the light beam having the second wavelength. Only the region of the luminous flux is formed so as to have a substantially fourth-order curved surface.

By adopting such a configuration, it is possible to compensate for the spherical aberration generated in the narrowed spot due to the difference in the thickness and wavelength of the disk substrate.

That is, 2 in the optical wavefront compensating means.
If the kind of material is selected so that the refractive index is substantially equal at the first wavelength for recording and reproducing the optical disc having the first substrate thickness, the light wavefront of the first wavelength is The light wavefront compensator has no effect. Therefore, if the objective lens is designed on the assumption of the first wavelength and the first substrate thickness, good focusing characteristics can be obtained for a light beam having the first wavelength. On the other hand, when recording / reproducing an optical disk having a second substrate thickness using the second wavelength using the objective lens, spherical aberration due to the substrate thickness deviation may occur unless the optical wavefront compensating means as in the present invention is used. The light-collecting characteristics of the generated light beam having the second wavelength are deteriorated. In general, it is known that distortion of an optical wavefront due to spherical aberration is a substantially fourth-order curved surface. Therefore, materials having different wavelength dependencies are selected so that the refractive indexes of the two materials in the optical wavefront compensating means have a predetermined value at the second wavelength, and the two materials are bonded together. If the surface shape is a substantially fourth-order curved surface, it is possible to compensate for distortion of an optical wavefront having a substantially fourth-order curved surface caused by spherical aberration. Thus, it is possible to realize an optical head capable of condensing light beams having different wavelengths using one objective lens and recording and reproducing characteristics with respect to an optical disk having a disk substrate thickness and a wavelength optimal for reproduction.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a first embodiment of the present invention will be described.

FIG. 1 (a) shows a wavelength 65 in the present invention.
FIG. 1B shows a case where the optical disk 12a having a substrate thickness of 0.6 mm is recorded / reproduced by a semiconductor laser 1a having a wavelength of 780 nm. Shows the case. In FIG. 1A, the wavelength is 650n.
The light emitted from the m semiconductor laser 1a is converted into a substantially parallel light beam by the collimating lens 2a, and the beam intensity distribution shaping optical system 3a, the beam splitter 4a, the wavelength filter 5, the mirror 7, the wavelength selective aperture filter 8, the light wavefront compensation. Passing through the plate 9, the substrate thickness is 0.6 m by the objective lens 10.
m is narrowed down to the optical disc 12a, and the spot SPa
To form The objective lens 10 is mounted on the actuator 11, and causes the objective lens 10 to follow up and down of the disk or eccentricity of the information track.

The wavelength-selective aperture filter 8 has a wavelength of 650 nm.
It is transparent to nearby wavelengths and transmits light without being blocked. On the other hand, the optical wavefront compensating plate 9 is formed by laminating two kinds of materials having different wavelength dependences of the refractive index, and the laminating surface has a substantially quadratic curved surface. The operation will be described later.
The types of materials are chosen such that their refractive indices are approximately equal at wavelengths around 650 nm, and therefore,
The wavefront of the transmitted light is hardly affected for wavelengths around 50 nm. The numerical aperture of the objective lens 10 is set to be 0.6 or more for a light beam having a wavelength near 650 nm.

The reflected light from the optical disk 12a passes through an objective lens 10, an optical wavefront compensator 9, a wavelength-selective aperture filter 8, a mirror 7, and a wavelength filter 5, is reflected by a beam splitter 4a, and is provided with a reproduction signal detection optical system. 13a
Incident on. In the reproduction signal detecting optical system 13a, a light point control signal such as automatic focus or track following and an information signal recorded on an optical disk are reproduced by various conventionally known optical systems.

In the above optical system, the beam intensity distribution shaping optical system 3a has a function of shaping a light beam having an elliptical intensity distribution emitted from the semiconductor laser 1a into a substantially circular intensity distribution. It is constituted by a conventionally known optical system such as one triangular prism, a laminated triangular prism, or two independently arranged triangular prisms. However, even if the shaping optical system 3a for beam intensity distribution is not used, the effect of the present invention is not impaired.

Next, according to FIG. 1B, the wavelength is 780 nm.
The case of recording / reproducing a disk 12b having a substrate thickness of 1.2 mm by the semiconductor laser 1b described above will be described. Wavelength 780n
The light emitted from the semiconductor laser 1b of m is converted into a substantially parallel light beam by the collimating lens 2b, passes through the shaping optical system 3b of the beam intensity distribution, the beam splitter 4b, and the mirror 6, and is emitted by the wavelength filter 5 to the light beam having the wavelength of 650 nm. After being combined, the light passes through a mirror 7, a wavelength-selective aperture filter 8, and an optical wavefront compensator 9, and is narrowed down by an objective lens 10 to an optical disk 12b having a substrate thickness of 1.2 mm to form a spot SPb.

The luminous flux near the wavelength of 780 nm is reduced by the wavelength-selective aperture filter 8 when the numerical aperture of the objective lens 10 is substantially smaller than that of the luminous flux having a wavelength of 650 nm. The beam diameter is restricted to be optimal. For example, wavelength 780n
CD, CD-ROM, CD-R
In consideration of recording or reproducing the data, it is preferable to set the substantial numerical aperture of the objective lens 10 to 0.45 or less.

The optical wavefront compensator 9 is formed by laminating two kinds of materials having different wavelength dependences of the refractive index, and the laminating surface has a substantially fourth-order curved surface, as described above. The two kinds of materials bonded to each other have different wavelength dispersions of the refractive index, and there is a difference in the refractive index at a wavelength near 780 nm. Although the details of the operation will be described later, the spherical aberration generated when a light beam having a wavelength of about 780 nm is narrowed down to the optical disk 12b having a substrate thickness of 1.2 mm is corrected by the above-described bonding surface shape and the refractive index difference between the two materials. . Thereby, the spot SPb can also obtain good light-collecting characteristics.

The reflected light from the optical disk 12b passes through the objective lens 10, the optical wavefront compensator 9, the wavelength-selective aperture filter 8, and the mirror 7, and is wavelength-separated by the wavelength filter 5. Thereafter, the light is reflected by the beam splitter 4b via the mirror 6, and enters the reproduction signal detection optical system 13b. In the reproduction signal detection optical system 13b, similarly to the reproduction signal detection optical system 13a, light point control signals such as automatic focus or track following and information signals recorded on the optical disk are reproduced by various conventionally known optical systems.

In the above optical system, the beam intensity distribution shaping optical system 3b has a function of shaping a light beam having an elliptical intensity distribution emitted from the semiconductor laser 1b into a substantially circular intensity distribution. Beam shaping optical system 3a
Similarly, one triangular prism, a laminated triangular prism or two independently arranged triangular prisms,
It is constituted by a conventionally known optical system. However, even if the shaping optical system 3b of the beam intensity distribution is not used, the effect of the present invention is not impaired.

In the optical system shown in FIG. 1, if there is a concern that the objective lens 10 may be affected by the eccentricity of the light beam incident on the objective lens 10 due to the eccentricity of the disk, , The optical wavefront compensator 9 and the wavelength-selective aperture filter 8 are integrated, and the actuator 1
Driving by 1 is effective.

Here, the operation of the wavelength-selective aperture filter 8 and the optical wavefront compensator 9 in the present invention will be described with reference to FIG. FIG. 2A shows a case where the optical disk 12a having a substrate thickness of 0.6 mm is recorded / reproduced by a light beam having a wavelength of 650 nm, and FIG. 2B shows an optical disk 12b having a substrate thickness of 1.2 mm using a light beam having a wavelength of 780 nm. Is shown for recording and playback.

In FIG. 2A, since the wavelength-selective aperture filter 8 has a transmittance for a wavelength near 650 nm, the light beam is transmitted without being shielded. As shown in the figure, the light wavefront compensator 9 is formed by laminating two kinds of materials having different wavelength dependences of the refractive index, and has a substantially quadratic curved surface. The refractive indices n1 and n2 of the above two materials are selected so as to be substantially equal at a wavelength near 650 nm, so that the light is transmitted at a wavelength near 650 nm incident on the optical wavefront compensator 9 as substantially parallel light. The light wavefront is hardly affected. The objective lens 10 is optimally designed for a case where the aperture is narrowed down to an optical disk having a substrate thickness of 0.6 mm at a wavelength around 650 nm, and has a numerical aperture corresponding to the light beam diameter around the wavelength of 650 nm (in the case of this embodiment, the numerical aperture is 0.1 mm). Light spot S narrowed down in 6)
Good light-collecting characteristics with respect to Pa are obtained.

On the other hand, in the case of FIG.
The luminous flux in the vicinity is adjusted by the wavelength-selective aperture filter 8 so that the numerical aperture of the objective lens 10 is substantially smaller than that of a luminous flux having a wavelength of 650 nm, and becomes optimal when recording / reproducing an optical disk having a substrate thickness of 1.2 mm. Restricted beam diameter. In the optical wavefront compensating plate 9, only a portion of the two surfaces bonded to each other where the light beam restricted by the wavelength-selective aperture filter 8 is incident is a substantially fourth-order curved surface. This is for compensating the light wavefront only for a light beam having a wavelength of around 780 nm. As described later, fluctuations and variations in wavelength caused by differences in the characteristics of semiconductor lasers, light emission output, and ambient environmental temperature are considered. The required configuration is taken into account.

As shown, the light wavefront compensator 9 is made of a material 9-1 having a thicker outer peripheral portion with respect to the central portion of the light beam.
The (refractive index n1) and the material 9-2 (refractive index n2) having a small thickness at the outer peripheral portion are bonded together, and the bonding surface shape is a substantially fourth-order curved surface as described above. Now, wavelength 78
In the vicinity of 0 nm, if a material is selected such that n1 is larger than n2 and the material 9-1 is arranged so as to be convex toward the objective lens as shown in FIG. The wavefront deviates slightly from the parallel plane wave as shown. The shift of the wavefront is substantially along the fourth-order curved surface, and is in a direction opposite to the spherical aberration (which causes distortion of the wavefront along the substantially fourth-order curved surface) caused by the substrate thickness shift of the optical disk. Therefore, if the material is selected so that the difference between the refractive indices n1 and n2 becomes a predetermined value, it is possible to cancel the spherical aberration caused by the substrate thickness deviation of the optical disk. As will be described later, it is possible to select a material that realizes the above contents. Therefore, even if an objective lens designed to support a disk with a substrate thickness of 0.6 mm near the wavelength of 680 nm is used, a wavelength of about 780 nm is used. Even when the luminous flux is narrowed down to an optical disk having a substrate thickness of 1.2 mm, good light-collecting characteristics can be obtained.

In the above description, the configuration of the optical wavefront compensating plate 9 for changing the wavefront in the same direction as that of FIG.
If the shape of the material having a larger refractive index near 80 nm is formed so that the thickness of the outer peripheral portion is thicker than the central portion of the light beam, the same material is arranged as shown in FIG. However, the effect of the present invention does not change even if it is arranged so as to be convex toward the wavelength selective aperture filter 8.

FIG. 3 shows the results of a principle study for confirming the effects of the present invention. The figure shows a substrate thickness of 0.6 mm at a wavelength of 588 nm.
In the case where a condensing optical system for narrowing down an optical disc with no aberration is set, the light condensing characteristics when the optical disc is narrowed down to an optical disc having a wavelength of 780 nm and a substrate thickness of 1.2 mm using the optical system is shown. NbFD13 and FD6 as optical wavefront compensators
The diameter of the condensed spot was calculated by using a bonding plate of No. 4 and changing the bonding surface to a quartic curved surface and changing the coefficient of the curved surface. According to the glass material property table manufactured by HOYA Corporation, the refractive index for each wavelength of NbFD13 and FD6 is
As shown in the table in the figure, there is almost the same difference at the wavelength of 588 nm and about 0.0085 at the wavelength of 780 nm. As can be seen from the graph, if the coefficient of the fourth-order curved surface is appropriately set, the substrate thickness is 1.2 mm at a wavelength of 780 nm.
Good light-collecting characteristics can be obtained even when the aperture is narrowed down to an optical disk.

FIG. 4 shows another embodiment of the optical wavefront compensator 9 in the present invention.

As described above, for a predetermined first wavelength, the refractive indices of the material 9-1 and the material 9-2 forming the optical wavefront compensator 9 are substantially equal to each other. Therefore, no matter what shape the boundary surface 9-3 has, the wavefront does not change for the light beam of the first wavelength. However, in an actual optical head, there is a possibility that a wavelength shift of about 10 to 20 nm may occur with respect to an originally intended wavelength due to a wavelength variation of a semiconductor laser light source or a temperature variation due to a temperature change. If the actual wavelength deviates from the target value in this way, a difference appears in the refractive index between the material 9-1 and the material 9-2, so that the light transmitted through the optical wavefront compensator 9 deforms the wavefront. This causes unnecessary wavefront aberration.

In order to minimize the wavefront aberration caused by the wavelength shift, in the embodiment shown in FIG. 4A, the light beam having the second wavelength different from the first wavelength passes outside the central area 9-4. In the annular zone 9-5 in
1 and the material 9-2 are made flat, and the plate thickness of the material 9-1 and the plate thickness of the material 9-2 in the annular zone 9-5 are respectively set to the material 9-1 in the central region 9-4. The shape of the boundary surface 9-3 is determined so as to substantially coincide with the average plate thickness of the material 9-2.

Further, in the embodiment of FIG.
The average plate thickness of the material 9-1 and the material 9-2 is substantially reduced over the entire regions 9-4 and 9-5 without forming an extreme inflection point on the boundary surface 9-3 as in the embodiment of FIG. In order to make the material constant, the material 9-1 is dug down by a predetermined depth d in the central region 9-4, and thereafter, the material 9-2 is inserted thereinto, and the material 9-2 is similarly deepened in the annular region 9-5. d
The structure is such that the material 9-1 is inserted after that. As described above, regarding the light beam of the second wavelength, the boundary surface of the portion that does not contribute to the deformation of the wavefront is made flat, and the average plate thickness of each material is made to match over the entire optical wavefront compensator, so that the first wavelength Therefore, even if there is a wavelength shift, unnecessary wavefront distortion can be prevented, and an increase in wavefront aberration can be minimized.

FIG. 5 shows an optical wavefront compensator 9 according to the present invention.
Another embodiment relating to the wavelength selective aperture filter 8 will be described.
FIG. 5A shows an objective lens 10 in which a wavelength-selective aperture filter 8 is directly formed on a light wavefront compensator 9 by a method such as vapor deposition.
And drive together. In FIG. 5B, the wavelength-selective aperture filter 8 is directly formed on the objective lens 10 by a method such as vapor deposition, and is driven integrally with the objective lens 10. In FIG. 5C, the objective lens 10 itself is formed as an optical wavefront compensator 9 with a laminated structure of two kinds of materials, and the wavelength-selective aperture filter 8 is directly formed on the lens surface. In any case, it is possible to suppress the positional deviation between the area which is limited by the wavelength-selective aperture filter 8 and the area where the wavefront is compensated by the optical wavefront compensator 9, and the objective lens 10 follows the eccentricity of the information recording track. Accordingly, the influence of the eclipse of the luminous flux can be reduced.

FIG. 6 shows another embodiment relating to the combination of the objective lens 10 and the optical wavefront compensator 9. Depending on how two kinds of materials are selected for the optical wavefront compensator 9, it may be considered that the refractive index n1 of the material 9-1 and the refractive index n2 of the material 9-2 cannot be set to be equal at one wavelength. In such a case, unnecessary wavefront aberration occurs in the light flux transmitted through the light wavefront compensator 9. FIG. 6A shows an example in which the shape of the objective lens 10 is slightly changed in order to correct the refractive index difference as described above. Considering both the objective lens 10 and the optical wavefront compensator 9, it is possible to design a condensing optical system that can obtain optimum condensing characteristics.
The shape correction may be performed using not only one surface of the objective lens 10 but also both the input and output surfaces as shown in FIG. Further, as shown in FIG. 6B, the end face of the optical wavefront compensating plate 9 can be corrected by, for example, making the end face slightly aspherical.

FIG. 7 shows various optical head configurations according to the present invention.

FIG. 7A shows that a beam shaping optical system 3a is inserted for a light beam emitted from a semiconductor laser 1a having a wavelength of 650 nm, and a quarter-wave plate 14 is used to produce circularly polarized light. This is an example in which a light beam is focused on the disk 12. The reflected light from the disk is again １ ／ wavelength plate 1
4, the polarization direction is rotated by 90 degrees with respect to the light beam emitted from the semiconductor laser 1a. If the beam splitter 4a is used as a polarization beam splitter, the light utilization of the optical system from the semiconductor laser 1a to the disk can be reduced. In addition to being enhanced, the amount of light guided to the signal detection system 13a can be increased. on the other hand,
The beam shaping is not performed on the light beam emitted from the semiconductor laser 1b having a wavelength of 780 nm, and the beam splitter 4b is inserted between the semiconductor laser 1b and the collimating lens 2b, so that the size of the optical system can be reduced. The optical system described above performs recording / reproduction of a DVD-RAM in which information can be rewritten at a wavelength of 650 nm, and a wavelength of 78 nm.
It is suitable for reproducing a CD, CD-ROM, CD-R or the like at 0 nm.

FIG. 7B shows a configuration in which the quarter-wave plate 14 is removed from FIG. 7A. In this case, for example, a non-polarized half beam splitter is used as the beam splitter 4a. Although the light utilization rate of the optical system from the semiconductor laser 1a to the disk is not as high as in FIG. 7A, inexpensive components such as a non-polarized half beam splitter can be used. This optical system reproduces, for example, a read-only DVD-ROM at a wavelength of 650 nm and a CD, CD-ROM, CD-R at a wavelength of 780 nm.
It is suitable for playing back etc.

FIG. 8A shows a case where beam shaping is not performed for a system of any wavelength, which is suitable for reducing the size and cost of an optical head.

FIG. 8B shows a configuration in which a beam splitter is disposed between the collimator lens and the semiconductor laser for any wavelength system, and is more suitable for miniaturization than the configuration of FIG. 8A. . Further, the semiconductor lasers 1a and 1b and the signal detection optical system 13 are formed using a conventionally known hologram element or the like.
If the module structures a and 13b are integrated, the size can be further reduced.

FIGS. 9A and 9B respectively show FIGS.
3A and 3B, the wavelength-selective aperture filter 8 and the optical wavefront compensator 9 are arranged apart from the objective lens 10. It is important to reduce the weight of the movable part of the actuator 11 including the objective lens 10 in order to reduce the access time and increase the disk rotation speed for improving the transfer speed. In the first embodiment of the present invention shown in FIG. 1, it has been described that it is preferable to drive the wavelength-selective aperture filter 8, the light wavefront compensator 9 and the objective lens 10 integrally. This is disadvantageous from the viewpoint of improving the transfer speed. Even when the wavelength-selective aperture filter 8 and the optical wavefront compensator 9 are arranged at a distance from the objective lens 10, even if the objective lens 10 moves following the eccentricity of the disk, the area where light flux is blocked or the aperture is limited. In order to suppress the deviation,
A so-called 2 is to move the entire optical head or only the objective lens actuator portion following the eccentricity of the disk.
What is necessary is just to employ a step servo system.

FIG. 10 shows a configuration in which a collimator lens is shared in the configuration of FIG. 8A, and is one measure for reducing the size of the optical head.

In the embodiments of the present invention described above, DVDs, CDs, CD-ROMs, CD-Rs and the like have been mainly described. However, the present invention is applicable to a magneto-optical disk. For example, a magneto-optical signal detection optical system may be provided as the reproduction signal detection optical system 13.

[0046]

According to the present invention, it is possible to compensate for the spherical aberration that occurs in the narrowed spot due to the difference in the thickness and wavelength of the disk substrate, so that one objective lens can be used to collect light beams having different wavelengths. It is possible to realize an optical head capable of recording and reproducing characteristics for optical disks having different disk substrate thicknesses and optimal wavelengths for reproduction.

[Brief description of the drawings]

FIG. 1 is an optical system block diagram showing one embodiment of the present invention;

FIG. 2 is a schematic diagram illustrating the operation of the optical wavefront compensator in the present invention;

FIG. 3 is a schematic diagram illustrating the effect of the optical wavefront compensator in the present invention;

FIG. 4 is a side view showing another embodiment of the optical wavefront compensator according to the present invention;

FIG. 5 is a schematic diagram illustrating an integrated configuration of an optical wavefront compensator, a wavelength-selective aperture filter, and an objective lens according to the present invention;

FIG. 6 is a schematic diagram showing a method for correcting the influence of the refractive index difference of the bonding material in the optical wavefront compensator,

FIG. 7 is an optical system block diagram showing another embodiment of the present invention.

FIG. 8 is an optical system block diagram showing another embodiment of the present invention.

FIG. 9 is an optical system block diagram showing another embodiment of the present invention.

FIG. 10 is an optical system block diagram showing another embodiment of the present invention.

[Explanation of symbols]

 1a, 1b: semiconductor laser, 2a, 2b: collimating lens, 3a, 3b: beam shaping optical system, 4a, 4b: beam splitter, 5: wavelength filter, 6, 7: mirror 8, wavelength selective aperture filter 9, light wave Surface compensator 10: Objective lens, 11: Actuator, 12a, 12b: Optical disk 13a, 13b: Reproduction signal detection optical system, 14: 1/4 wavelength plate.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Kuniichi Onishi, 292 Yoshida-cho, Totsuka-ku, Yokohama-shi, Kanagawa Prefecture Multimedia Systems Development Division, Hitachi, Ltd. (72) Masayuki Inoue Masayuki Inoue Yoshidacho, Totsuka-ku, Yokohama-shi, Kanagawa 292 Hitachi Ltd. Multimedia System Development Division

Claims (14)

[Claims] A first light source having a first wavelength; a second light source having a second wavelength different from the first light source;
Light beam combining means for combining light beams from the two light sources;
One objective lens for narrowing a light beam emitted from the light beam synthesizing means onto an optical recording medium through a transparent substrate, and a driving means for causing the objective lens to follow up and down and rotational eccentricity of the optical recording medium; A light beam separating unit that separates the reflected light from the optical recording medium into a light beam having only the first wavelength or the second wavelength; and a light beam separated by the light beam separating unit. An optical head having at least a light detecting means for converting, wherein two kinds of materials having different wavelength dependences of refractive index are stuck on an optical axis between the light combining means and the objective lens. An optical head comprising a compensation means. 2. The optical head according to claim 1, wherein:
An optical head characterized in that the thickness of a transparent substrate is different between an optical recording medium that performs recording or reproduction with a light beam having a wavelength of 2 and a optical recording medium that performs recording or reproduction with a light beam having a second wavelength. 3. An optical head according to claim 1, wherein said optical wavefront compensating means has a substantially quadratic curved surface formed by bonding two types of materials. 4. An optical head according to claim 1, wherein recording or reproduction is performed by a light beam having a first wavelength, and recording or reproduction is performed by a light beam having a second wavelength. An optical head comprising means for changing an effective numerical aperture of the objective lens when performing the above. 5. The optical head according to claim 4, wherein the means for changing the numerical aperture is a filter having wavelength selectivity. 6. The optical head according to claim 3, wherein the bonding surface of the optical wavefront compensating means has a numerical aperture corresponding to the light beam having the first wavelength and a light beam having the second wavelength. An optical head characterized in that only a region of a light beam corresponding to a small numerical aperture among corresponding numerical apertures has a substantially fourth-order curved surface. 7. The optical head according to claim 6, wherein a numerical aperture of a numerical aperture corresponding to the luminous flux having the first wavelength and a numerical aperture of a numerical aperture corresponding to the luminous flux having the second wavelength are larger. Is 0.6 or more. 8. The optical head according to claim 6, wherein a numerical aperture corresponding to a light flux having the first wavelength and a numerical aperture corresponding to a light flux having the second wavelength are smaller. Is 0.45 or less. 9. The optical head according to claim 1, wherein said objective lens, said light wavefront compensating means and said means for changing said numerical aperture are integrated into said optical head. An optical head mounted on a drive unit for following up and down deflection and rotation eccentricity of a recording medium. 10. An optical head according to claim 1, wherein the refractive index of two kinds of materials in said optical compensation means is one of the two kinds of wavelengths. An optical head characterized in that: 11. An optical head according to claim 1, wherein said objective lens has one wavelength and one of said two wavelengths and said two substrate thicknesses. An optical head characterized in that optimal light collecting characteristics are obtained when substrate thicknesses are combined. 12. An optical head according to claim 1, wherein said objective lens is constituted by said optical wavefront compensating means. 13. An optical head according to claim 1, wherein said means for changing the numerical aperture is formed on said optical wavefront compensating means. . 14. A first light source having a first wavelength and a first light source having a first wavelength.
A second light source having a second wavelength different from that of the first light source, a light beam combining means for combining light beams from the two light sources, and a light beam emitted from the light beam combining means on an optical recording medium through a transparent substrate. One objective lens, a driving unit for causing the objective lens to follow up-and-down swing and rotational eccentricity of the optical recording medium, and reflected light from the optical recording medium to the first wavelength or the second wavelength, respectively. A light beam separating unit that separates into a light beam having only the wavelength of the light beam, and an optical disk device having at least a light detecting unit that receives each of the light beams separated by the light beam separating unit and performs photoelectric conversion, An optical wavefront compensating means in which two kinds of materials having different wavelength dependences of the refractive index are laminated on an optical axis between the objective lens and the objective lens. The optical disk device.