JPH1083560A - Optical head device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical head device small in size, small in number of components and capable of recording/reproducing plural recording media different in substrate thickness in an optical head device used for an optical information recording/reproducing device, etc. SOLUTION: A liquid crystal optical rotator element 41 and a hologram element 5 are provided in an optical system. A condenser lens 61 is designed so as to optimally converge a light beam for the thickness of the substrate of a disk 7 and the polarizing hologram element 5 acts as a transparent plate without applying a voltage on the liquid crystal optical rotator element 41 for the disk having this substrate thickness. By applying a voltage on the liquid crystal optical rotator element 41 for a disk 7 having a different substrate thickness, the polarizing hologram element 51 acts as a hologram plate for compensating aberration caused by the difference of the substrate thickness. Consequently, plural recording media having different substrate thickness are recorded/reproduced by one condenser lens.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical head device used for an optical information recording / reproducing apparatus and the like.

[0002]

2. Description of the Related Art In recent years, the performance of an optical head of an optical information recording / reproducing apparatus for recording / reproducing information on / from an optical recording medium such as a compact disk and an optical disk has been improved, and the size and size of the optical head have been reduced.
Optical heads are being researched and developed for the purpose of weight reduction and the like.
Recently, short wavelength technology of semiconductor laser, high NA of objective lens
Advances in optical disc technology have made it possible to increase the density of optical discs.

[0003] However, if the current thickness of the base material of a disk, such as a compact disk, remains unchanged, the aberration generated due to the tilt of the disk increases due to the shorter wavelength of the semiconductor laser and the higher NA of the objective lens. Discs with a reduced base material thickness have been proposed.

[0004] In order to make such a thin disk having a thin base material compatible with a compact disk or an optical disk which is currently in wide use, an optical head device having a condenser lens suitable for each disk is required.

A conventional optical head device will be described below. FIG. 6 is a schematic diagram showing an example of an optical system of a conventional optical head.

[0006] The laser light source 1 that oscillates at a normal TE ₀₀ mode
A light beam (horizontally polarized light) emitted from the light beam is converted into a parallel beam by a collimating lens 2, passes through a beam splitter 3 (for example, a beam splitter having a transmission / reflection ratio of 7: 3), and is transmitted to a condenser lens system 4. The part is selectively incident. The beam is converged to a spot of approximately 1 μm by the condenser lens system 6, reaches the optical storage medium surface 7, and irradiates the pit pattern 8. The light beam reflected and diffracted by the medium surface 7 travels again through the condensing lens system 6, partially passes through the beam splitter 3 selectively, and is split into two directions by the prism half mirror 9. One reflected light is a detection lens 10 having astigmatism.
Through the photodetector 11 and converted into a focus control signal. The other transmitted light enters the two-part photodetector 12 for tracking control detection as it is in the far field pattern, and is converted into a tracking control signal.

[0007]

However, in the optical system of the above-mentioned conventional optical head device, a plurality of recording media having different substrate thicknesses (for example, the substrate thickness of a CD disk is 1.2 mm,
When a single optical head optical system is used for recording / reproducing a DVD disc with a base material thickness of 0.6 mm, the spherical aberration due to the base material thickness of the recording medium different from the designed thickness increases, and the warpage and surface of the recording medium are increased. Spot distortion due to run-out etc. increases,
Recording and playback cannot be performed normally. Therefore, in order to perform recording / reproduction on a plurality of recording media having different substrate thicknesses, a plurality of optical head optical systems or focus optical systems are required. For example, an optical head device of a type in which two lenses are mechanically switched has been proposed, but has problems in terms of miniaturization, low cost, mass productivity, and high reliability.

[0008] Common reasons for these problems are:
First, combining a large number of parts and mechanically switching the optical system according to the thickness of the base material of the storage medium requires a lot of time and complicated inspection and measurement equipment for assembly and adjustment. Since there is a limit to miniaturization of the optical system, there is also a great restriction on miniaturization of all optical systems.

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above-mentioned conventional problems, and to provide an optical head device which is small in size, has a small number of components, and can record and reproduce a plurality of recording media having different thicknesses.

[0010]

In order to achieve this object, the present invention provides a radiation source, an element for changing the polarization direction of a beam of the radiation source between optical paths of an optical storage medium, and a polarization direction of the beam. A hologram element whose diffraction efficiency changes due to the arrangement of the hologram element, the aberration that occurs when recording and reproducing a plurality of recording media having different substrate thicknesses, by changing the polarization direction of the optical system according to the recording medium, An optical head device for correcting an aberration generated in the focusing optical system by a polarization hologram element.

According to the present invention, downsizing, cost reduction,
Mass production and high reliability can be obtained. Further, since the distance between the objective lens and the radiation light source or the reflected light receiving surface does not change even when the substrate thickness of the recording medium is different, the degree of freedom in the arrangement and design of optical components can be increased.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The invention according to claim 1 of the present invention provides a radiation light source, a condensing optical system for converging the radiation light source to a minute spot, and a polarization direction of the radiation light source disposed in the optical path. This is an optical head device in which an element to be changed and a hologram element whose diffraction efficiency changes according to the polarization direction of the radiation light source are arranged. With this configuration, aberrations that occur when recording and reproducing a plurality of recording media having different substrate thicknesses with a single focus system can be reduced by changing the polarization direction of the optical system to reduce aberrations that occur in the focus optical system with the polarization hologram element. It has the effect of correcting.

According to a second aspect of the present invention, the element for changing the polarization direction of the light source emitted from the radiation light source is a liquid crystal rotation element, and the polarization direction of the liquid crystal rotation element is controlled by an electric signal. The optical head device according to the above, having a function of changing the polarization direction by giving an electric signal to the liquid crystal rotation element according to the recording medium, and performing the aberration correction according to the substrate thickness of the recording medium by the polarization hologram element. .

According to a third aspect of the present invention, the element for changing the polarization direction of the light source emitted from the radiation light source is a Faraday rotator, and the polarization direction of the element is controlled by the amount of magnetism in the element. The optical head device according to the item, having a function of changing the polarization direction by applying magnetism to the Faraday rotation element according to the recording medium, and performing the aberration correction according to the substrate thickness of the recording medium by the polarization hologram element. .

According to a fourth aspect of the present invention, the element for changing the polarization direction of the light source emitted from the radiation light source is a wavelength plate having birefringence, and the polarization direction of the wavelength plate is controlled by the amount of insertion of the wavelength plate. 2. The optical head device according to claim 1, wherein the polarization direction is changed by inserting or removing a wave plate into or from an optical path according to the recording medium, and the polarization hologram performs aberration correction according to the substrate thickness of the recording medium. It has the effect of performing by the element.

According to a fifth aspect of the present invention, there is provided an optical head device according to the first aspect, wherein a hologram element whose diffraction efficiency changes according to the polarization direction of the light source emitted from the radiation light source is formed on the condenser lens. Since the aberration correction according to the thickness of the base material of the recording medium is constituted by the same element as the light-collecting optical system, it has the effect of reducing the size and the number of parts.

Hereinafter, an embodiment of the present invention will be described with reference to FIG.
This will be described with reference to FIG. (Embodiment 1) FIG. 1 shows a schematic configuration of an optical system of an optical head device according to an embodiment of the present invention. In Figure 1, 1 is a laser which emits a horizontal polarization of the TE ₀₀ mode (for example, a semiconductor laser that oscillates at a wavelength of 650 nm), 2 is a collimating lens, 3 is the ratio of reflectance and the beam splitter (e.g. transmittance 7: 3 And 41 indicates that when a voltage is applied to the transparent electrodes stretched on both surfaces, the optical rotation angle in the polarization direction becomes 0.
This is a liquid crystal optical rotation element having the property of changing from 90 degrees to 90 degrees. Reference numeral 5 denotes a polarization hologram element designed so as to transmit the maximum with a horizontally polarized beam and to diffract the maximum with a vertically polarized beam. For the substrate of the polarization hologram element, for example, LiNO ₃ which is an anisotropic crystal can be used. The refractive index of the hologram portion is converted by proton exchange, and the proton exchange portion is slightly removed to perform phase matching. It is preferable that the hologram groove is blazed in order to improve the amount of transmitted light contributing to light collection. The specific example is 30 as shown in FIG.
Dry etching is performed at intervals of μm using a Ta2O5 mask on n steps, proton exchange is performed, and a blazed hologram formed in the opposite phase is formed. The lens of the hologram element 6 can perform beam correction so as to minimize aberration that occurs when, for example, an optical disk having a base material thickness of 1.2 mm is focused using a lens optimized for a disk having a base material thickness of 0.6 mm, The opening is designed to change to an appropriate size. Specifically, for example, the radius of curvature is 15.83.
A hologram having the effect of a convex lens with an aspheric coefficient of 8 mm and an aspheric coefficient of k = -4.34839 × 10 ⁻³ is designed. Reference numeral 6 denotes a condensing lens, for example, a lens designed to minimize aberration when condensing light on a disc having a base material thickness of 0.6 mm. Reference numeral 7 denotes a storage medium (optical disc) having a base material thickness of, for example.
Use 0.6mm optical disk and 1.2mm optical disk.

When the substrate thickness of the storage medium 7 is 0.6 mm, if no voltage is applied to the liquid crystal film 41, the light source 1
The collimated lens 2 converts the horizontally polarized beam into a parallel beam, and 70% of the amount of the light passes through the liquid crystal rotation element 41 as a horizontal polarization plane at the beam splitter 3 and enters the polarization hologram element 5. Since the polarization hologram 5 transmits the maximum with respect to the incidence of the horizontally polarized wave, the parallel beam is directly incident on the condenser lens 6 and narrowed down to a spot of about 1 μm, reaches the optical storage medium surface 7 and reaches the pit. The pattern 8 is irradiated. The light beam reflected and diffracted by the medium surface 7 travels again through the condensing lens system 6 and enters the polarization hologram element 5, but the reflected and diffracted light beam remains almost horizontally polarized and thus travels in the forward direction. As in the case of (1), almost all of the light is transmitted as a parallel beam, and is incident on the beam splitter 3 via the liquid crystal rotation element 41. Then, 30% of the beam is selectively passed by the beam splitter 3 and split by the prism half mirror 9 in two directions. One reflected light passes through the detection lens 10 provided with astigmatism and is incident on the four-divided detector 11.
It is converted into a focus control signal. The other transmitted light remains in the far-field pattern and is used for tracking control detection.
The light enters the divided photodetector 12 and is converted into a tracking control signal.

Next, when the substrate thickness of the storage medium 7 is 1.2 mm, by applying a voltage to the liquid crystal rotation element 41, the horizontally polarized beam emitted from the light source 1 becomes a parallel beam by the collimating lens 2, The beam splitter 3 uses
A portion transmits through the liquid crystal optical rotation element 41. When the parallel beam is transmitted through the liquid crystal rotation element 41, the plane of polarization is rotated by 90 degrees to be vertically polarized, and is incident on the polarization hologram element 5. Since the polarization hologram 5 is maximally diffracted with respect to the incidence of the vertically polarized light, the parallel beam is narrowed down to a spot of approximately 1 μm by the effect of the hologram element and the two lenses of the condensing lens 6 which is transmitted next. The light reaches the storage medium surface 7 and irradiates the pit pattern 8. The light beam reflected / diffracted by the medium surface 7 travels again through the condenser lens system 6 and enters the polarization hologram element 5, but the reflected / diffracted light beam remains almost in the horizontal polarization. As in the case of the outward path, most of the light is diffracted to the maximum and the distortion of the parallel light is corrected to be a parallel beam. The light is horizontally polarized through the liquid crystal rotation element 41 and then enters the beam splitter 3. Then, 30% of the beam is selectively passed by the beam splitter 3 and passed by the prism half mirror 9 to 2%.
Divided into directions. One of the reflected lights enters the four-divided detector 11 through the detection lens 10 provided with astigmatism, and is converted into a focus control signal. The other transmitted light enters the two-part photodetector 12 for tracking control detection while being in the far-field pattern, and is converted into a tracking control signal.

(Embodiment 2) FIG. 2 is a conceptual diagram showing another embodiment of the present invention. In the first embodiment, the liquid crystal rotation element is used as an element for changing the polarization direction of the beam in the optical path of the optical head device.
Uses a Faraday rotator 42 having the property that when a magnetic field is applied by a magnetic field generator (not shown), the optical rotation angle in the polarization direction changes from 0 degree to 90 degrees.
The operation is the same as in the first embodiment.

(Embodiment 3) FIG. 3 is a conceptual diagram showing another embodiment of the present invention. In the first embodiment, a liquid crystal optical rotation element is used as an element for changing the polarization direction of a beam in the optical path of the optical head device. On the other hand, in the present embodiment, insertion and removal from the optical system according to the recording medium 7 are performed. Half-wave plate (1 / 2λ) having a mechanism (not shown) capable of
Plate 43 is used, and the operation is the same as that of the first embodiment.

(Embodiment 4) FIG. 4 is a conceptual diagram showing another embodiment of the present invention. In the first embodiment, the condensing optical system is constituted by the condensing lens and the polarization hologram, whereas in the present embodiment, the condensing lens is constituted by the polarization hologram on the condensing lens 62. Is the same.

In each of the above-described embodiments, a liquid crystal optical rotation element, a liquid crystal rotation element,
Although the case of the Faraday rotator and the wave plate having birefringence have been described, the present invention is not limited to this.

[0024]

As described above, according to the present invention, a plurality of storage media having different substrate thicknesses can be recorded / reproduced by one optical head device, and the advantageous effects of miniaturization and simplification can be obtained. Can be

[Brief description of the drawings]

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

FIG. 2 is a schematic configuration diagram showing another embodiment of the invention.

FIG. 3 is a schematic configuration diagram showing another embodiment of the present invention.

FIG. 4 is a schematic configuration diagram showing another embodiment of the present invention.

FIG. 5 is a schematic view showing an embodiment of blazing in a polarization hologram;

FIG. 6 is a schematic configuration diagram showing a configuration example of an optical system of a conventional optical head device.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Laser light source 2 Collimating lens 3 Beam splitter 5 Polarization hologram element 7 Recording medium 8 Pit 9 Prism half mirror 10 Detection lens 11 Four division detector 12 Two division detector 41 Liquid crystal rotation element 42 Faraday rotation element 43 1 / 2λ wavelength plate 61 Condensing Lens 62 Condensing lens having polarization hologram element

Continuation of front page (72) Inventor Tetsuo Hosomi 1006 Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd.

Claims (5)

[Claims] 1. A radiation light source, a condensing optical system for converging a beam emitted from the radiation light source to a minute spot, and a polarization direction of the radiation light source disposed between the radiation light source and the condensing optical system And an hologram element having a diffraction efficiency that changes according to the polarization direction of the beam passing through the polarization direction changing element. 2. The optical head device according to claim 1, wherein the element for changing the polarization direction of the light source emitted from the radiation light source is a liquid crystal rotation element, and the polarization direction of the liquid crystal rotation element is controlled by an electric signal. . 3. The Faraday rotation element is an element for changing the polarization direction of the light source emitted from the radiation light source, and the polarization direction of the Faraday rotation element is controlled by a magnetic quantity in the Faraday rotation element. 2. The optical head device according to 1. 4. An element for changing the polarization direction of a light source emitted from a radiation light source is a wave plate having birefringence, and the polarization direction of the wave plate is controlled by inserting and removing the wave plate. Item 2. The optical head device according to item 1. 5. The optical head device according to claim 1, wherein a hologram element whose diffraction efficiency changes according to the polarization direction of the light source emitted from the radiation light source is formed on the condenser lens.