KR100224886B1 - Optical pickup apparatus - Google Patents

Abstract

A compact optical pickup apparatus for recording and reproducing information on a recording medium is disclosed.

The optical pick-up apparatus comprises a light source, an optical path changing means for changing the path of the incident light according to the polarization of the incident light, and an optical path changing means for shifting the incident light, A wavelength plate for changing the polarization characteristic of the incident light by means of the optical path changing means, and a wavelength plate for changing the polarization characteristic of the incident light to be incident on the light source side, And a polarized light hologram reflector that reflects the light toward the objective lens and diffracts and reflects the light incident from the recording medium toward the photodetector.

Description

[0001] The present invention relates to an optical pickup apparatus,

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical pickup apparatus for recording and reproducing information on a recording medium, and more particularly to a miniaturized optical pickup apparatus.

2. Description of the Related Art In general, an optical pick-up apparatus irradiates light emitted from a light source to record / reproduce images, sound or various information on a recording medium. There are many types of recording media such as a card, a tape, and a disk. At present, a disk type well known as a compact disk (CD) or a digital video disk (DVD), for example, is mainstream.

A disc-shaped recording medium has a transparent medium made of plastic or glass with a predetermined thickness, and a recording surface formed on the transparent medium to record information. The light converged by the objective lens is refracted in the transparent medium of the recording medium, and is then reflected on the recording surface, reflected again, and returned to the optical pickup.

In order to increase the recording density of the disc-shaped recording medium, it is necessary to reduce the size of the light spot on the recording surface as much as possible. For this purpose, the optical pickup apparatus generally includes an objective lens having a large numerical aperture (NA) Light emitting this short light is used.

On the other hand, development of a DVD-RAM (read and memory) capable of recording and reproducing the information at will, with a large capacity of video information storage capability in consideration of the convenience of a magnetic recording and reproducing apparatus free from recording and reproduction, is being attempted. This DVD-RAM is mainly used as an auxiliary storage device of a computer. Since an optical pickup device for this requires high optical power for recording, it is necessary to employ a high output laser diode. This is because the disk power needs to be maintained at 10 mW or more for recording while the optical disk power is read at about 10 mW for reproducing power. Therefore, in the optical pickup for the DVD-RAM, it is important to maximize the light efficiency while increasing the output of the laser diode. In order to maximize the light efficiency, a method of using a polarizing beam splitter and a wave plate has been generalized. In this case, the weight of the polarizing beam splitter is increased as compared with an optical pickup apparatus for a compact disc for ordinary reproduction only . On the other hand, since the auxiliary storage device requires a high-speed access and a search function, it is necessary to make the optical pickup device for DVD-RAM compact and lightweight by reducing the size and the quantity of optical parts.

1, the conventional optical pick-up apparatus includes a laser diode 1 for emitting laser light, a collimating lens 4 for converting incident light into parallel light, a reflector 5, a polarization hologram lens An objective lens 8 for focusing the incident light to form a light spot on the disk 9 and a photodetector 2 for receiving the light reflected from the disk 9, (3).

Here, the polarization hologram lens 6 is composed of a grating 6a having a diffraction pattern engraved thereon and an exchange region 6b formed in the grating 6a. The grating 6a is made of lithium niobate (LiNbO3) as a birefringent material. The incident light is divided into an ordinary ray parallel to the optical axis of the grating 6a and an extraordinary ray not parallel to the optical axis of the grating 6a It is released. The exchange region 6b is filled with a proton whose refractive index is similar to that of an ordinary ray. Therefore, the diffraction efficiency according to the polarization of the incident light can be different.

As described above, the conventional optical pickup apparatus is miniaturized by using a polarization hologram lens using the characteristics of a birefringent medium, and its operation is as follows.

In the laser diode 1, linearly polarized light is emitted. The linearly polarized light is collimated by the collimating lens 4 and is then incident on the objective lens 8 via the reflecting mirror 5, the polarization hologram lens 6 and the wave plate 7, / 4 wave plate 7, the polarized wave surface is rotated by 90 degrees and converted into circularly polarized light. The circularly polarized light is focused by the objective lens 8 to form a light spot on the disk 9 and is reflected and transmitted again through the objective lens 8 to transmit the quarter- The angle is changed into light of linearly polarized light rotated by 90 degrees further. The polarization plane of the returning linearly polarized light at this time is rotated by 180 degrees with respect to the incident linearly polarized light and is received by the optical detectors 2 and 3 by the polarization hologram lens 6, respectively.

Thus, conventionally, the polarization hologram lens is used so that the photodetector can be disposed near the laser diode, thereby miniaturizing the optical pickup apparatus by reducing the volume by optimizing the arrangement thereof. However, in the conventional miniaturized optical pick-up apparatus, the polarization hologram lens uses an ion exchange region formed on the birefringent medium, and thus the production cost increases, and the manufacturing process is complicated.

SUMMARY OF THE INVENTION Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and it is an object of the present invention to provide a miniaturized optical pick-up apparatus which is inexpensive and simple to manufacture.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic view showing an optical arrangement of a conventional optical pickup device. FIG.

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to an optical pickup apparatus,

3 is a schematic diagram showing an optical arrangement of an optical pickup device according to a second embodiment of the present invention.

4 is a graph showing the diffraction efficiency according to the groove depth of the polarized light hologram reflector of the optical pick-up apparatus according to the first and second embodiments of the present invention.

FIG. 5 and FIG. 6 are schematic structural views showing the optical arrangement of the optical pickup device according to the third embodiment of the present invention. FIG.

7 is a graph showing the diffraction efficiency according to the groove depth of the polarization hologram reflector of the optical pickup device according to the third embodiment of the present invention.

DESCRIPTION OF THE REFERENCE NUMERALS

11 ... light source 12 ... photodetector

13 ... collimating lens 15 ... wavelength plate

16 ... objective lens 17 ... optical disc

18 ... Detection lens 20 ... Polarized hologram reflector

21, 23 ... grating 22, 25 ... reflective thin film

24 ... prism

According to an aspect of the present invention, An optical path changing means for changing the traveling path in accordance with the polarization of the incident light; An objective lens disposed on an optical path between the optical path changing means and the recording medium for converging incident light to form a light spot on the recording medium; And an optical detector for detecting an error signal and an information signal by receiving the light reflected by the recording medium and passing through the objective lens and the optical path changing means, Wherein the optical path changing means comprises a wave plate for changing incident linearly polarized light into circularly polarized light and circularly polarized light into linearly polarized light; A polarized light hologram reflector for reflecting the light incident from the light source toward the objective lens and diffracting and reflecting the light reflected by the recording medium toward the photodetector; And the like.

Hereinafter, an optical pickup apparatus according to embodiments of the present invention will be described in detail with reference to the accompanying drawings.

2, the optical pickup apparatus according to the first embodiment of the present invention includes a light source 11, a photodetector 12, a polarization hologram reflector 20, a wave plate 15, and an objective lens 16 ). Here, the reference numeral 17 denotes an optical disc.

The light source 11 emits linearly polarized laser light having a predetermined wavelength with a laser diode. The photodetector 12 is disposed adjacent to the light source 11 for receiving the light reflected from the optical disc 17. The polarization hologram reflector 20 is disposed on the optical path between the light source 11 and the objective lens 16 so that the light incident from the light source 11 is directed to the optical disc 17 And the light reflected by the optical disk 17 is diffracted and reflected so that the traveling direction of the light is directed to the photodetector 12.

The polarization hologram reflector 20 is a birefringent material and has a grating 21 having a diffraction pattern formed on its surface and a reflection thin film 22 deposited on the grating 21 to reflect incident light. The reflective thin film 22 is composed of a metal thin film or a dielectric thin film.

The diffraction pattern period of the polarization hologram reflector 20 is shorter than the wavelength of the light emitted from the light source 11. That is, when the period of the diffraction grating is smaller than the wavelength, a form birefringence is generated and the diffraction efficiency varies depending on the polarization direction. Such a diffraction grating is referred to as a surface relief grating.

Here, the polarization hologram reflector 20 is preferably composed of a birefringent material having a reflective surface. In the case of the birefringent material, the diffraction angle can be kept small, so that the light source 11 and the photodetector 12 can be constituted by a single unit.

The wave plate 15 is a 1/4 wave plate, and serves as a polarization wave rotator. That is, the plane of polarization of incident linearly polarized light is rotated by 90 ° to make circularly polarized light, and the returned polarized plane of the circularly polarized light is further rotated by 90 ° to produce linearly polarized light whose plane of polarization is rotated 180 ° with respect to the linearly polarized light .

The objective lens 16 focuses the circularly polarized light incident on the recording surface of the optical disk 17 so that a light spot is formed.

In addition, the present invention may further include a collimating lens 13 on the optical path between the light source 11 and the polarization hologram reflector 20. The collimating lens 13 condenses the divergent light incident from the light source 11 into parallel light and converges the collimated light incident from the optical disc 17 to be incident on the optical detector 12.

3, the optical pick-up apparatus according to the second embodiment of the present invention includes a polarization hologram reflection plate 20 and a photodetector 12 that converges the light diffracted and reflected by the photodetector 12, And further comprises a lens 18. This is advantageous for optimizing the diffraction efficiency when the diffraction angle of the polarization hologram reflector 20 is large, and the optical arrangement of the photodetector 12 is easy.

4 is a graph showing the diffraction efficiency of the surface etching diffraction grating according to the first and second embodiments of the present invention. As shown in the figure, the P-polarized light exhibits a diffraction efficiency of 5% or less at a groove depth of 0.7 mu m on the optical disk 17, and the S-polarized light exhibits a diffraction efficiency of 95% or more. That is, when the incident linearly polarized light is P-polarized light, the light is incident on the objective lens 16 with little diffraction from the polarization hologram reflector 20, and when the incident linearly polarized light is S-polarized light, Is diffracted by 95% or more in the polarization hologram reflector 20 and is received by the photodetector 12 with almost no loss. Therefore, it is possible to record and reproduce information while maintaining a sufficiently high optical efficiency.

As shown in FIGS. 5 and 6, the optical pickup apparatus according to the third embodiment of the present invention includes a grating 23 for diffracting and transmitting incident light to the polarization hologram reflector 20, And a prism 24 for reflecting the incident light. The grating 23 is a plate-like birefringent material, for example, LiNbO3, and a rotation pattern is formed on the surface thereof. Further, the reflection thin film 25 deposited on the grating 23 may be further provided to increase the reflection efficiency of the grating 23. This reflective thin film 25 is composed of a metal thin film or a dielectric thin film.

5, the optical axis of the prism 24 is orthogonal to the optical axis of the light source 11 and the optical axis of the objective lens 16, and the inclined surface, to which the grating 23 is coupled, Lt; / RTI > When the angle of incidence of the light incident on the inclined surface is maintained at 45 °, the grating 23 has an incident angle of 17.5 °. Thus, when the reflective film is deposited, the light can be totally reflected.

6, the prism 24 may be an acute-angled prism having an optical axis toward the light source 11 and an optical axis toward the objective lens 16 at approximately 81 degrees. In the case where the inclined surface to which the grating of the prism 24 is fitted is inclined by 49.6 DEG with respect to the incident light, a total reflection angle of 30 DEG can be achieved in the grating 23. Thus, when the angle of the prism 24 is adjusted in this way, the deposition of the reflective thin film 25 is substantially unnecessary.

7 is a graph showing the diffraction efficiency of the 0-order light according to the groove depth of the grating according to the third embodiment of the present invention. As shown in the figure, the P-wave shows a diffraction efficiency of 75% or more at a groove depth of 1.86 μm, and the S-wave shows a diffraction efficiency of 1% or less. That is, when the incident beam is a P-plane, at least 75% of light is incident on the objective lens 16 without diffraction, and a light spot is formed on the disk 17. Further, the light reflected by the disk 17 and passing through the wave plate 15 becomes an S wave, and the diffraction efficiency of the first-order light is 75% or more while the diffraction efficiency of the 0-order light is 1% or less. Therefore, the optical path of the light is converted and directed to the photodetector 12 while maintaining a high optical efficiency.

The compact optical pickup device according to the embodiments of the present invention operates as follows.

The light emitted from the light source 11 is collimated by the collimating lens 13 and is reflected by the polarization hologram reflector 20 and is incident on the objective lens 16 via the wave plate 15, And is focused on the recording surface of the optical disc 17 by the objective lens 16. The next light is reflected by the optical disk 17 and is incident on the polarized light hologram reflector 20 via the objective lens 16 and the wave plate 15 and is reflected and diffracted by the polarized light hologram reflector 20, And then converged on the photodetector 12 located around the light source 11 by the collimating lens 13. [

Here, the light emitted from the light source 11 is converted into linearly polarized light by the wave plate 15 as linearly polarized light. The light of the circularly polarized light is reflected by the optical disk 17 and passes through the wave plate 15 again becomes linearly polarized light in which the polarization plane is rotated. For example, when the initial linearly polarized light is referred to as P polarized light, the reflected light becomes S polarized light. Therefore, the characteristic diffracted by the polarization hologram reflector 20 is changed and diffracted in a direction different from the initial incidence direction.

According to the present invention, in constituting the optical pickup capable of recording and reproduction, it is possible to reduce the weight by optimizing the arrangement of optical components and reducing the number of optical components. Further, by using the reflection type polarization hologram element, the production is facilitated, and the productivity can be improved.

Claims (7)

A light source; An optical path changing means for changing the traveling path in accordance with the polarization of the incident light; An objective lens disposed on an optical path between the optical path changing means and the recording medium so as to converge incident light on the recording medium; And an optical detector for detecting an error signal and an information signal by receiving the light reflected by the recording medium and passed through the objective lens and the optical path changing means, Wherein the photodetector is disposed around the light source, The optical path changing means comprises: A wave plate for converting incident linearly polarized light into circularly polarized light and circularly polarized light into linearly polarized light; A polarized light hologram reflector for reflecting the light incident from the light source toward the objective lens and diffracting and reflecting the light reflected from the recording medium toward the photodetector; Wherein the optical pickup device comprises: The apparatus of claim 1, wherein the polarization hologram reflector comprises: A grating made of a birefringent material and having a diffraction pattern formed on its surface; and a reflective thin film deposited on the grating and reflecting the incident light. The optical pickup apparatus of claim 2, wherein the reflective thin film is formed of a metal thin film or a dielectric thin film. 3. The optical pickup apparatus of claim 2, wherein the diffraction pattern period of the grating is shorter than the wavelength of the light emitted from the light source. The prism of claim 1, wherein the polarizing hologram reflector is a flat plate type birefringent material for diffracting incident light and has a diffraction pattern formed on a surface thereof, and a prism coupled with the grating and reflecting incident light . The optical pickup apparatus of claim 5, wherein the polarization hologram reflector further comprises a reflective thin film deposited on the grating to reflect incident light. The optical pickup apparatus of claim 6, wherein the reflective thin film is formed of a metal thin film or a dielectric thin film.