JPH1021574A - Optical information recording/reproducing device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize a small-sized, light-weighted and inexpensive optical information recording/reproducing device capable of recording/reproducing optical disk media of plural standards with different disk thickness and recording density in high performance. SOLUTION: This device consists of a finite system objective lens 3a of NA 0.6 designed for the disk thickness 0.6mm, an aperture limit means 5 switching an effective size of laser light incident on the finite system objective lens, a beam splitter 6 branching the reflection light from an information medium and a photodetector 10, etc., converting return light to an electric signal, and deals with two kinds of information media with different disk thickness by switching a lens effective size by the aperture limit means 5. Further, the opening part shape of the aperture limit means 5 is made an elliptic shape opening that a lens track moving direction becomes the minor axis.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical information recording / reproducing apparatus for writing and reading optical information on various optical disks having different thicknesses.

[0002]

2. Description of the Related Art For example, in order to record / reproduce both a high-density optical information medium (DVD, etc.) having a disk thickness of 0.6 mm and a conventional density information medium (CD, etc.) having a disk thickness of 1.2 mm by one optical reproducing apparatus. There are several methods proposed.
This is achieved by a so-called twin lens system in which two objective lenses optimally designed for each disk thickness are driven by a single lens actuator, and a hologram by two.
There are a bifocal system for creating two focal points, an aperture limiting system for switching the aperture of a lens, and the like.

The present invention is an improvement and a development of the aperture limiting method. Therefore, the prior art will be described focusing on the aperture limiting method among the above various methods.

FIG. 5 is a schematic diagram illustrating a conventional aperture limiting system. In order to accurately reproduce the information recorded on the disc and to record high-density information on the disc,
It is important to form a minute spot without aberration on the disc information surface. Therefore, in the description of the conventional example, only the outward path in the conventional information recording / reproducing apparatus will be described.

FIG. 5A shows a recording and reproducing state of a high-density optical information medium. 1 is a laser diode (hereinafter referred to as LD), 1a is a laser beam emitted from the LD, 11
Is a collimator lens, 3 is an infinite objective lens, 4b is a high-density optical information medium (hereinafter referred to as a high-density disk) having a thickness of, for example, 0.6 mm, and the objective lens 3 optimally records the high-density disk. Designed to be playable.
Now, the laser beam 1a emitted from the LD 1 is converted into parallel light by the collimator lens 11 and enters the infinite system objective lens 3.
At this time, the aperture diameter of the parallel light beam incident on the objective lens is limited by a lens stop (not shown). For example, during reproduction of a high-density disc, the NA is limited to 0.6. The laser condensed by the lens 3 forms a small spot with little aberration on the information surface 4b of the high-density disk.

Next, in FIG. 5B, for example, a thickness of 1.2 mm
Normal density optical information medium (hereinafter referred to as normal density disc)
The recording and reproduction states of the above will be described. 2B, the aperture limiting means 5 is inserted between the collimator lens 11 and the infinite system objective lens 3. Laser light 1 emitted from LD 1 and converted into parallel light by collimator lens 11
The light beam a is restricted by the aperture 5a of the aperture limiting means 5 and is incident on the objective lens 3. At this time, the limited opening diameter is a circular shape and the optimal opening for normal density discs,
For example, NA is set to 0.45. As a result, a spot with little aberration is formed on the information surface of the normal density disk.

Usually, the aperture limiting means 5 and the infinite objective lens 3 are simultaneously held by a lens actuator (not shown).
The recording and reproduction of a normal density disc are enabled by one infinite system objective lens 3 designed for a high density disc depending on whether or not the aperture limiting means 5 is inserted. Several methods have been proposed for the aperture limiting means 5. There are a deflection filter using a laser deflection direction, a wavelength filter using a difference in laser wavelength, a mechanical shutter system, and the like.

The optical information recording / reproducing apparatus shown in FIG. 5 has a problem in that the length of the optical path is increased due to the presence of the collimator lens 11, making it difficult to reduce the size and thickness of the entire apparatus. For this purpose, a configuration based on a finite system of the objective lens as shown in FIGS. 6A and 6B has been considered. Finite objective lens 3
“a” is designed to obtain the optimum characteristics for recording and reproduction of the high-density disk 4b. In FIG. 6a, LD1
The laser beam 1a which emits light is set to an aperture of, for example, NA 0.6 by a lens stop (not shown) and the finite objective lens 3
a. The laser light condensed by the finite objective lens 3a forms a small spot with little aberration on the information surface of the high-density disk. Next, recording on the normal density disk 4a,
At the time of reproduction, the aperture limiting means 5 is inserted between the objective lens 3a and the LD 1 like the infinite optical system. The laser beam 1a emitted from the LD 1 has its aperture diameter restricted by the aperture 5a of the aperture restricting means 5, and enters the finite objective lens 3a. The limited opening diameter is set to a circular opening which is optimal for a normal density disk, for example, NA 0.45. As a result, a spot with little aberration is formed on the information surface of the normal density disk. At this time, the finite objective lens 3a and the aperture limiting means 5
Are simultaneously held and driven by a lens actuator (not shown).

[0009]

In the case of an optical system using this finite objective lens, the size and thickness of the entire apparatus can be reduced, but the off-axis reproduction characteristics of a normal density disk are particularly deteriorated, and the optical system is practically used. Had the following major problems.

In general, a finite optical system is inferior in image height characteristics and optical axis movement characteristics of a lens as compared with an infinite optical system. However, in a CD or the like, a finite optical system is mainly used due to a system design of the entire apparatus. Therefore, adoption of a finite optical system for a high-density optical disk such as a DVD is naturally studied. A finite objective lens designed for high-density discs has the same on-axis performance as an infinite objective lens, but the off-axis performance is inferior to the infinite optical system in the system design of the entire device. It will be. However, this finite optical system is shown in FIG.
When an application is made to an optical information recording / reproducing apparatus capable of recording and reproducing a plurality of standard media of (a) and (b), the off-axis reproduction performance of a normal density disc is too large to be supported by the system design of the entire apparatus. Performance degradation occurs.

Next, the problem will be described with reference to FIGS. FIG. 4A is a graph showing a change in jitter of a normal density disk reproduction signal when various finite objective lenses are moved in the disk track direction. A in the graph is a characteristic of a reproducing apparatus including an optical system of a finite objective lens designed exclusively for reproducing a normal density disk (CD). Even if the lens moves and an image height is generated, a stable reproduction performance is obtained with little deterioration of jitter. B in the graph is FIG.
5 shows jitter characteristics when a normal density disc is reproduced by the optical system including the finite objective lens for high density disc and the aperture limiting means shown in FIG. When the lens moves and an image height is generated, the jitter rapidly deteriorates. C is a characteristic of the present invention described later.

These phenomena can be explained by the image height characteristics of the finite objective lens wavefront aberration shown in FIG. A, B, and C in the graph correspond to A, B, and C in FIG. In the case of the finite objective lens for a high-density disk and the optical system B using the aperture limiting means, the axial aberration can be reduced to almost zero, but the degree of deterioration of the wavefront aberration due to the lens movement is large compared to the objective lens A for the normal density disk Then, a practical problem arises.

An object of the present invention has been proposed in view of such a conventional situation, and is an inexpensive and compact optical information recording / reproducing apparatus capable of efficiently reproducing and recording information on an optical disk medium of a plurality of standards. It is to provide.

[0014]

SUMMARY OF THE INVENTION An optical information recording / reproducing apparatus according to the present invention has been proposed to achieve the above-mentioned object, and has an NA of 0.6, for example, designed for a disk thickness of 0.6 mm. A finite system objective lens, a laser diode that emits a laser, an aperture limiting means for switching the effective diameter of laser light incident on the finite system objective lens, and an information recording medium that simultaneously holds the finite system objective lens and the aperture limiting means. Including a lens actuator that drives in the focus direction and the track direction, a beam splitter that branches reflected light from the information medium to the photodetector side, and a photodetector that converts return light into an electric signal. By switching the effective diameter of the lens, two types of information media having different disk thicknesses can be recorded and reproduced. Characterized by an elliptical shape whose direction the minor axis.

In the present invention, the use of the aperture limiting means having an elliptical aperture shape reduces the NA in the track direction and can reduce the deterioration of wavefront aberration. The optical disk can be recorded and reproduced stably.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, an embodiment of the optical information recording / reproducing apparatus of the present invention will be described, and the configuration and operation of the present invention will be clarified.

First, the shape of the opening of the opening limiting means of the present invention will be described with reference to FIG. FIG. 2A shows a conventional opening shape, and the opening 5a has a perfect circular shape. In this case, the inner diameter a of the opening is set so that the numerical aperture NA is optimum for normal density disc reproduction. On the other hand, the aperture shape of the aperture limiting means of the present invention has an elliptical aperture 5a having a single axis b in the tracking movement direction of the lens (the direction of the arrow in the figure) as shown in FIG. 2B. . The axial length a on the long axis side is the same as the conventional opening inner diameter a.

Reducing the aperture diameter in the track direction as in the present invention decreases the lens numerical aperture NA in the track direction, which deviates from the original optimum design value. The spot shape on the disk narrowed by the elliptical aperture also becomes an elliptical shape long in the track direction as shown in FIG. 4B, and crosstalk between tracks deteriorates. However, as shown in the graph C of FIG. 4B, the degree of wavefront aberration degradation when the lens moves can be greatly improved as compared with the case of the conventional aperture shape B, as the NA becomes smaller. Along with this, the jitter characteristic can be greatly improved as shown in the graph C of FIG. Although the initial aberration and the jitter performance on the axis are deteriorated as compared with the conventional example, they do not pose a practical problem. The optimum aperture shape including the crosstalk can be obtained from the calculation and the experimental data.

As a specific method of the aperture limiting means 5, several methods can be considered corresponding to the method of the conventional aperture limiting means. Shape can be applied.

FIG. 3A shows a wavelength filter film 5 that transmits a specific wavelength band and blocks a predetermined wavelength band on a transparent glass substrate.
1 shows an aperture limiting means formed with 1; For example, when recording and reproducing a high-density disk, an LD of, for example, 635 nm is used. In this case, the laser light of 635 nm passes through the entire area of the elliptical opening 5a without the filter film and the entire area of the wavelength filter film 51, and travels toward the finite objective lens. Next, record the normal density,
At the time of reproduction, for example, an 780 nm LD is used. The 780 nm laser light passes through the elliptical aperture 5a without a filter and is reflected by the wavelength filter surface 51. As a result, the 780 nm laser light is restricted to a narrow elliptical light beam in the track direction and travels toward the finite objective lens. In addition, the structure of (a) can also be realized by a deflection filter. By switching the linear deflection direction of the laser beam incident on the aperture limiting means 5 by 90 degrees by a method not shown, it is possible to limit the aperture diameter as described above.

FIG. 3B shows an example of an elliptical aperture limiting means using a hologram. In the conventional aperture shape, a concentric hologram is formed on a transparent substrate, and the first-order diffracted light is incident on an objective lens with an optimum numerical aperture for normal density disc reproduction. The embodiment in which the elliptical aperture of the present invention is applied to the hologram can be realized by removing the peripheral portion in the track direction of the conventional concentric hologram region 20a formed on the transparent substrate 20 to a predetermined size 20b.

FIG. 3C shows the configuration of the apparatus when the elliptical aperture limiting means shown in FIG. 3B is used, and the laser beam 1a emitted from the LD 1 is incident on the hologram type elliptical aperture limiting means. The hologram area 20a has a sufficient area for producing a low NA light beam used for normal density disc reproduction, and the hologram area 2a is used for recording and reproduction of a high density disc.
All the zero-order transmitted light of 1a including the zero-order transmitted light of 0a enters the finite objective lens 3a. The laser light condensed by the finite objective lens forms a small spot with little aberration on the high-density disc information surface 4b. Next, when recording / reproducing a normal density disk, the first-order diffracted light 1b of the elliptical hologram area 20a is used. The first-order diffracted light 1b of 20a enters the finite objective lens 3 as an elliptical light beam narrow in the track direction by the action of the holographic region 20a having the elliptical aperture shape. The laser beam condensed by the finite objective lens 3 forms a spot with little aberration on the normal density disk information surface 4a.

Next, an embodiment of an optical information recording / reproducing apparatus using an elliptical aperture limiting means using a wavelength filter will be described with reference to FIG.

The embodiment shown in FIG. 1 has a short wavelength (635 nm).
Laser 1, 780nm laser unit 2 with built-in detection system
And NA0.6 finite objective lens 3 for high density disc playback,
Optical disc 4, wavelength filter type elliptical aperture limiting means 5,
It comprises a non-deflection beam splitter (NPBS) 6, a wavelength selection prism 7, a focus error detection lens 8, an internal reflection prism 9, a photodiode 10, and the like. The wavelength filter 5 having an elliptical aperture shape is shown in FIGS. 2B and 3A.

Short-wavelength laser light 1 emitted from 635LD1
a is an NPBS 6, of which about 50% is reflected and enters the wavelength selection prism 7. The wavelength selection prism 7 efficiently transmits 635 nm (for example, 95% or more) and efficiently reflects 780 nm. Therefore, most of the 635 laser 1a passes through the selection prism, is reflected by the internal reflection prism 9, and travels to the wavelength filter 5. 635 laser 1a is wavelength filter 5
Through the entire surface (including the opening), the objective lens 3 forms a minute spot on the information recording surface 4b of the high-density disk (DVD or the like). The reflected light of the disc returns to the laser side in the same path,
The light passes through the NPBS 6 and the focus detection lens 8 to focus on the photodiode 10 and is converted into an electric signal.

The 780 nm laser beam is emitted from the 780 nm laser unit 2 and is efficiently reflected by the wavelength selection prism 7 and the internal reflection prism 9 to enter the wavelength filter 5. Since the wavelength filter 5 restricts the transmission aperture of the 780 nm laser light by the elliptical aperture, only the 780 nm laser light 2a incident on the elliptical aperture is condensed by the objective lens and spotted on the information recording surface 4a of a normal density disk (CD or the like). tie. At this time 7
By setting the position of the laser emission point on the 80 nm side in the optical axis direction at a predetermined position, it is possible to arbitrarily set the correction amount of the spherical aberration due to the difference in the disk side focal length and the disk thickness. 7
The 80 nm disk reflected light also returns to the laser unit 2 along the same path, and is converted into an electric signal by a detection system in the unit. In addition, the internal reflection prism 9 described in the present embodiment employs an internal reflection method having no wavelength dependency in order to satisfy a high reflectance at a low cost in a wide wavelength band from 635 nm to 780 nm. However, the surface reflection mirror method can be applied. Needless to say.

The shape of the aperture limiting means in some of the present invention described so far is an elliptical shape which forms a single axis in the track moving direction of the finite objective lens. It works in combination with the finite objective lens designed as described above to exhibit its effect.

[0028]

As described above, according to the present invention, the use of the aperture limiting means having an elliptical aperture shape can reduce the NA in the track direction and reduce the deterioration of the wavefront aberration. High-NA finite objective lens enables stable recording and reproduction of optical discs of different standard media with different thicknesses and recording densities, resulting in an inexpensive, thin, compact and lightweight optical information recording / reproducing apparatus. be able to.

[Brief description of the drawings]

FIG. 1 is a diagram showing an embodiment of the present invention.

FIGS. 2A and 2B are views for explaining an elliptical aperture limiting means according to the present invention, wherein FIG. 2A shows a conventional shape, and FIG.

FIG. 3 is a diagram showing an example of the configuration of an aperture limiting means according to the present invention;
(A) shows an example using a wavelength filter and a deflection filter, (b) shows an example of a hologram method, and (c) is a device configuration diagram when a hologram method is used.

4A and 4B are diagrams showing characteristics of a finite objective lens, where FIG. 4A is an image height characteristic diagram and FIG. 4B is a jitter characteristic diagram.

5A and 5B are diagrams showing a configuration of a conventional technique, wherein FIG. 5A shows a case of a high-density disk, and FIG. 5B shows a case of a normal-density disk.

6A and 6B are diagrams showing a configuration of another conventional technique applied to a finite optical system, wherein FIG. 6A shows a case of a high-density disk, and FIG. 6B shows a case of a normal-density disk.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Short wavelength semiconductor laser 2 780 nm laser unit 3, 3a Object lens for high-density disk 4 Optical disk 4a Normal-density optical disk information surface 4b High-density optical disk information surface 5 Aperture restricting means 5a Aperture restriction opening 6 NPBS 7 Wavelength selection prism 8 Focus Error detection lens 9 Internal reflection prism 10 Photodetector (photodiode) 11 Collimator lens 20 Holographic aperture limiting means 20a Elliptical hologram area

Claims (6)

[Claims] 1. An optical information recording / reproducing apparatus for writing / reading information media having different disk thicknesses through an objective lens by using a laser beam, wherein an aperture is controlled in front of the objective lens in accordance with the information medium. An optical information recording / reproducing apparatus comprising a control means, wherein the opening has an elliptical shape in which a lens track moving direction is a short axis. 2. The optical information recording / reproducing apparatus according to claim 1, wherein laser beams having different wavelengths are used according to the information medium, and the aperture of said aperture control means changes according to the wavelength. 3. An optical information recording / reproducing apparatus according to claim 2, wherein said aperture control means comprises a wavelength filter film. 4. The optical information recording / reproducing apparatus according to claim 2, wherein said aperture control means comprises a deflection filter. 5. The optical information recording / reproducing apparatus according to claim 2, wherein said aperture control means comprises a base having a hologram area. 6. A finite objective lens having a high numerical aperture, a laser diode for emitting a laser, aperture limiting means for switching an effective diameter of laser light incident on the finite objective lens, and a reflected light from the information medium. And a photodetector for converting return light into an electric signal, and recording two types of information media having different disk thicknesses by switching the effective diameter of the lens by the aperture limiting means. In an optical information recording / reproducing apparatus for reproducing,
An optical information recording / reproducing apparatus, wherein the opening of the opening limiting means is an elliptical opening having a short axis in a lens track moving direction.