JPH09161306A - Optical disk device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To stably reproduce two kinds of optical disk different in thickness by providing a light shielding means and a phase shifter giving a part of laser light a phase difference arranged between a light source and an objective lens. SOLUTION: When a high density optical disk 13 is reproduced, a liquid crystal optical rotator 6 and a liquid crystal drive circuit 7 optically rotates luminous flux 4 so that no phase difference occurs in the luminous flux 9 by the phase shifter 10 and the luminous flux 9 becomes the polarization direction so as to transmit through a polarizing plate (luminous flux 25) as it is. On the other hand, when a CD 14 in thickness of 1.2mm is reproduced, the liquid crystal optical rotator 6 and the liquid crystal drive circuit 7 optically rotates the luminous flux 4 so that the phase difference occurs in the luminous flux 9 by the phase shifter 10, and the luminous flux 26 of the central part of the luminous flux 9 transmits through the polarizing plate 11, and the luminous flux of the peripheral part becomes the polarization direction so as to be light shielded by the polarizing plate 11. Then, only the luminous flux of the center part is made incident on the objective lens 12.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical disk device suitable for reproducing optical disks having different substrate thicknesses.

[0002]

2. Description of the Related Art At present, compact discs (hereinafter abbreviated as CDs) are widely used for music or computer data. This CD has a diameter of 120 mm, a track pitch of about 1.6 μm, and a minimum pit length of about 0.
9 μm. An optical disk device for reproducing this CD uses a semiconductor laser of near-infrared light of 770 to 800 nm as a light source, and the numerical aperture (NA) of an objective lens that focuses the laser light on the optical disk is generally about 0.45.

On the other hand, the progress of semiconductor lasers in recent years has been remarkable, and a semiconductor laser for red light having a wavelength of 630 to 650 nm has been put into practical use. By combining it with an objective lens having a numerical aperture (NA) of 0.6, it is A high-density optical disc having a recording density more than double the size has been proposed. (For example, 1993 Autumn Meeting of the Institute of Electronics, Information and Communication Engineers, Lecture No. C
-364 "High density CD using red laser pickup
-Study of ROM ") By the way, since the substrate of an optical disk such as a CD is generally manufactured by injection molding of polycarbonate resin, warping of the substrate is unavoidable.
It is customary that there is a warp of about 3 to 0.5 degrees. When the optical disk is tilted, coma aberration is generated, and when it is significant, signal reproduction becomes impossible. Since the amount of coma generated is proportional to the cube of the numerical aperture (NA) of the objective lens, for example, NA
In the 0.6 objective lens, coma aberration of about 2.4 times occurs as compared with the 0.45 NA objective lens. On the other hand, since the amount of coma generated is proportional to the substrate thickness of the optical disc,
Attempts have been made to suppress the amount of coma aberration as much as a CD even if the NA of the objective lens is increased by using an optical disk having a thin substrate, for example, 0.6 mm. However, the permissible value of the substrate thickness error of the optical disc (the substrate thickness of the optical disc to be reproduced with respect to the optimum substrate thickness for the objective lens) is inversely proportional to the fourth power of the numerical aperture of the objective lens. 0.1 mm or less, 0.03 mm for NA 0.6
The following is a problem that an optical disk device for a 0.6 mm substrate cannot reproduce a CD on a 1.2 mm substrate due to spherical aberration.

When the above-mentioned high-density optical disk is put to practical use in the future, it is naturally desirable that the optical disk device can also reproduce the conventional CD. An optical recording / reproducing apparatus disclosed in Japanese Patent Application Laid-Open No. 7-182690 has been proposed as a first conventional technique to meet this demand. this is,
The objective lens is compatible with a high-density disc, and its optimum substrate thickness is 0.6 mm. CD (substrate thickness 1.2m
Since a large spherical aberration occurs when reproducing m), the spherical aberration is corrected by inserting a conversion lens having a function of a concave lens in the optical path between the objective lens and the beam splitter. Further, as a second conventional technique, an optimum substrate thickness of 0.6 mm corresponding to a high-density disc and an NA of 0.6 are used as an objective lens, and the NA of the objective lens is limited to 0.35 when reproducing a CD. The method used is also proposed. (56th Academic Society of Applied Physics, Lecture No. 29a-ZA-6 "Examination of compatibility between two types of discs having different thicknesses") As described above, the spherical aberration generation amount is proportional to the fourth power of NA. It focuses on doing. Since the size of the light spot for reproducing the optical disk is inversely proportional to the NA of the objective lens and proportional to the wavelength of the light source, NA 0.35 to 0.37 is sufficient when reproducing a CD when using a light source having a wavelength of 635 nm. .
As a result, the spherical aberration generated becomes small, and the CD can be reproduced by some means even with the objective lens having the optimum substrate thickness of 0.6 mm.

[0005]

In the first prior art described above, a mechanism for moving the conversion lens in and out of the optical path is required depending on the type of the optical disk to be reproduced, which complicates the structure of the optical head. Further, there is a problem that the focus target point is displaced due to a slight inclination of the conversion lens. Further, the second conventional technique also requires a mechanism for moving the aperture for reducing the effective NA of the objective lens into and out of the optical path. Furthermore, even if the NA of the objective lens is limited to 0.35, C
The spherical aberration that occurs when D is reproduced is by no means small. When an aperture is inserted in the optical path and the objective lens moves in a direction orthogonal to the optical axis following the eccentricity of the optical disc, the reflected light from the optical disc is partially vignetted by the aperture and the entire reflected light is reflected. There is also the problem of not reaching the detector. When the aperture is placed in the optical path, it is desirable that the aperture move integrally with the objective lens.

An object of the present invention is to realize an optical disk device capable of stably reproducing two kinds of optical disks having different thicknesses with a simple structure.

[0007]

In order to solve the above-mentioned problems of the prior art, in the optical disk device of the present invention, a polarizing plate functioning as an aperture for limiting the NA of the objective lens, and a CD when reproducing the CD. A phase shifter having a polarization dependency that generates a phase difference to reduce the spherical aberration that occurs and an optical rotation means that rotates the polarization direction of the laser light generated by the light source by about 90 degrees are provided in the optical path. As the objective lens, a substrate having a substrate thickness of 0.6 mm and having an NA of 0.6 corresponding to a high-density optical disc is used. When reproducing the high-density optical disc, the polarizing plate does not function as an aperture and the phase shifter does not generate a phase difference. The polarization direction of the laser light generated by the light source is controlled by the optical rotator in various directions, and when reproducing a CD having a substrate thickness of 1.2 mm, the polarizing plate functions as an aperture and the effective NA of the objective lens is 0.3.
The phase shifter controls the polarization direction so that the phase shifter produces a phase difference that corrects spherical aberration. As a result, both the high density optical disc and the CD can be reproduced. By using a liquid crystal optical rotator as the optical rotator, no special mechanism is required, and both optical disks can be reproduced by only the electrical switching means. Further, by driving the polarizing plate and the phase shifter integrally with the objective lens, the reflected light is not eclipsed even if the optical disc is decentered.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described in detail below with reference to the drawings. FIG. 1 shows the configuration of an embodiment of an optical pickup which is a main part of the optical disk device of the present invention. A semiconductor laser 1, which is a laser light source, has a wavelength of about 650 nm and generates a substantially linearly polarized and divergent light beam 2. The light flux 2 is converted into a parallel light flux 4 by the collimator lens 3, passes through the beam splitter 5 and the liquid crystal optical rotator 6 as an optical rotator, and is reflected by the mirror 8.
The reflected light flux 9 has a phase shifter 10 having polarization dependence,
After passing through the polarizing plate 11, it is focused on the optical disk 13 or 14 by the objective lens 12 to form a light spot. The light beam reflected by the optical disk 13 or 14 follows the original optical path, is reflected by the reflecting surface of the beam splitter 5, passes through the detection lens 15 and the cylindrical lens 16, and enters the photodetector 17. The photodetector 17 has a plurality of light receiving regions, and detects the focus, tracking error signal, information signal, etc. by calculating the output signals of these regions. Here, the objective lens 12 is designed so that the high density optical disc 13 having a thickness of 0.6 mm can be reproduced. When reproducing the high-density optical disk 13, the liquid crystal optical rotator 6 and the liquid crystal drive circuit 7 do not cause a phase difference in the light beam 9 by the phase shifter 10 and the light beam 9 passes through the polarizing plate as it is (light beam 25). The light beam 4 is rotated (the polarization direction is rotated) so as to have such a polarization direction. On the other hand, when reproducing a CD 14 having a thickness of 1.2 mm, the liquid crystal optical rotator 6 and the liquid crystal drive circuit 7 cause a phase difference in the light beam 9 by the phase shifter 10, and the light beam 26 at the center of the light beam 9 passes through the polarizing plate 11. , The luminous flux of the peripheral part is the polarizing plate 1
The light beam 4 is rotated so that the polarization direction is blocked by 1. Then, only the light beam 26 in the central portion is the objective lens 1
2 is incident. The phase shifter 10 and the polarizing plate 11 are fixed integrally with the objective lens 12 to the movable portion of the actuator as described later.

Next, the operation of the liquid crystal optical rotator 6 will be described. 2 and 3 show that light can be transmitted or shielded by a combination of a liquid crystal optical rotator and a polarizing plate. The liquid crystal 6a enclosed in the liquid crystal optical rotator 6 is a twisted nematic (TN) liquid crystal. Transparent electrodes 6b and 6c are formed on both sides of the liquid crystal 6a. Transparent electrodes 6b, 6c
Is supplied with a voltage from the voltage source 7a of the liquid crystal drive circuit 7 via the switch 7b. As shown in FIG. 2, the orientation of the liquid crystal 6a is twisted by 90 degrees when no electric field is applied,
It has an optical rotation of 90 degrees. Therefore, the polarization 2 of the incident light 20
1 is rotated by 90 degrees and becomes polarized light 22. On the other hand, the polarizing plate 23
The light is transmitted when the transmission axis 23a of is aligned with the direction of the polarized light 22. Next, when a voltage is applied to the transparent electrodes 6b and 6c, the liquid crystal 6a is oriented along the electric field as shown in FIG. 3, so that the optical rotatory power disappears. At this time, the polarization of the light transmitted through the liquid crystal 22
Does not transmit light because is orthogonal to the transmission axis 23a of the polarizing plate 23. FIG. 4 shows an example of operating characteristics of the liquid crystal optical rotator 6,
The relationship between the applied voltage and the optical rotation angle is shown. The operation threshold value is about 4V, and the optical activity disappears when a voltage of 6V or more is applied.

Next, the function of the polarizing plate 11 used in the present invention will be described. FIG. 5 shows the luminous flux that passes through the polarizing plate 11 when reproducing the high density optical disc 13. Polarization direction a of light beam 9
Corresponds to the direction of the transmission axis of the polarizing film 11a formed around the upper surface of the polarizing plate 11. Therefore, the entire light flux 9 passes through the polarizing plate 11, and the transmitted light flux 25 enters the objective lens 12. FIG. 6 shows the case of reproducing the CD 14. The light flux 9 is rotated by the liquid crystal optical rotator 6 so as to have a polarization direction b orthogonal to the polarization direction a in FIG. Therefore, the light around the light beam 9 is blocked by the polarizing plate 11, and only the light beam 26 in the central portion where the polarizing film 11a is not formed is incident on the objective lens 12. In this embodiment, the polarizing plate 11 is arranged so as to be orthogonal to the light beam 9, but in order to reduce the incident angle dependency and the wavelength dependency of the polarizing film, it is inclined by 5 to 10 degrees or more with respect to the light beam. You may arrange.

FIG. 7 shows a second embodiment of the optical rotation means 1 /
It is a figure for demonstrating operation | movement of a two-wave plate, It uses instead of the liquid crystal optical rotator 6 of FIG. When the optical axis of the half-wave plate and the polarization direction of the incident light 32 are made to coincide with each other, the outgoing light 33 is transmitted without being rotated. Here, the half-wave plate is indicated by the arrow 31
As shown in (4), when the light is rotated 45 degrees around the optical axis, the polarization direction of the emitted light 34 is rotated 90 degrees, whereby the same operation as the liquid crystal optical rotator 6 can be performed.

Next, the principle and operation of the phase shifter will be described. FIG. 8 shows how spherical aberration generated when reproducing the CD 14 is reduced by the phase difference generated by the phase shifter. The wavefront W (ρ) of the third-order spherical aberration at the best image point of the light spot on the CD 14 is

[0013]

[Equation 1]

The cross section of the wavefront is represented by (1) in FIG. The position where ρ is ± 1 corresponds to the outer diameter of the light beam 26. From (Equation 1)

[0015]

(Equation 2)

Therefore, in order to reduce the maximum value of the wavefront aberration by half, an annular phase difference as shown in (2) of FIG. 8 may be given. The wavefront aberration obtained by combining the spherical aberration and the phase difference is shown in (3) of FIG. From (Equation 1)

[0017]

(Equation 3)

Let δ be the phase difference generated by the phase shifter 10.

[0019]

(Equation 4)

An embodiment of the phase shifter is shown in FIGS. FIG. 9 shows an annular dielectric film 41 on a glass substrate 40.
Is formed. Let the thickness of the dielectric film 41 be d,
In order to generate the above phase difference δ,

[0021]

(Equation 5)

Since aberrations outside ρo in (3) of FIG. 8 occupy a very small proportion in the whole aberration, the dielectric film 41 may be omitted only in the central portion as shown in FIG. Since the phase shifters shown in FIGS. 9 and 10 have no polarization dependence, it is necessary to remove them from the optical path when reproducing the high density optical disk 13, and the mechanism as in the conventional example is required.

FIG. 11 shows another embodiment of the phase shifter.
This phase shifter has a polarization dependence, and when light of a certain polarization enters it, it imparts a phase difference to the transmitted light, and when light of polarization orthogonal to it enters, it does not impart a phase difference to the transmitted light. is there. The material of the substrate 50 is lithium niobate, and the portion corresponding to the dielectric film 41 in FIG. 10 has undergone proton exchange. The dielectric film 52 is for correcting the phase difference.
The thickness Td of the dielectric film 52 and the depth Tp of the proton exchange region 51 are obtained as follows.

[0024]

(Equation 6)

Here, when the liquid crystal optical rotator 6 is controlled so that the polarization direction of the light beam 9 becomes the ordinary light for the phase shifter 10 when reproducing the high density optical disk 13 and becomes the abnormal light when reproducing the CD 14, the phase change of the ordinary light is changed. Since the amount is 0 and the phase change amount of extraordinary light is 0.103λ

[0026]

(Equation 7)

## EQU1 ##

FIG. 12 is a partial sectional view showing the structure of an actuator for driving the objective lens in the optical axis and in two directions orthogonal to the optical axis. The objective lens 12, the polarizing plate 11, and the phase shifter 10 having polarization dependency are integrally attached to the movable portion 60 of the actuator. The size of the hole 61 at the bottom of the movable part 60 is such that when a light beam having the size of the hole 61 enters the objective lens 12, the numerical aperture of the objective lens (N
It suffices that A) be 0.6.

FIG. 13 is a block diagram of an optical disk device using the optical pickup shown in FIG. Optical disk 1
3 or 14 is engaged with the motor 60, and the control signal 7
9, the motor drive circuit 77 generates a motor control signal 78 to control the rotation of the motor 60. The laser drive circuit 75 outputs a laser drive current 76 according to the control signal 80 to turn on the semiconductor laser in the optical pickup 61. A signal 62 from a photodetector in the optical pickup 61 is input to a preamplifier 63, and a servo error signal 64 and an RF signal 67 corresponding to the signal recorded on the optical disc are output. The servo circuit 65 outputs an actuator drive signal 66 based on the servo error signal 64 and the control signal 71 to control the actuator to which the objective lens is attached. RF signal 6
7 is input to the demodulation circuit 68 and the data signal 70 is output. On the other hand, the system controller 81 discriminates the type of the optical disc based on the data signal 69, and controls the liquid crystal drive circuit 7 by the control signal 72. A liquid crystal drive signal 74 is output from the liquid crystal drive circuit 7 to control the liquid crystal optical rotator in the optical pickup 61.

[0030]

As described above, the optical disc device of the present invention does not require a special mechanism and can be adapted to two types of optical discs having different substrate thicknesses by electrical control, thus making the device compact. Therefore, the cost can be reduced.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of an optical pickup which is a main part of an optical disc device of the present invention.

FIG. 2 is a configuration diagram of a liquid crystal optical rotator that is a first embodiment of the optical rotator.

FIG. 3 is a configuration diagram of a liquid crystal optical rotator that is a first embodiment of the optical rotator.

FIG. 4 is an operational characteristic diagram of a liquid crystal optical rotator.

FIG. 5 is a configuration diagram of an example of a polarizing plate.

FIG. 6 is a configuration diagram of an example of a polarizing plate.

FIG. 7 is a configuration diagram of a half-wave plate that is a second embodiment of the optical rotator.

FIG. 8 is an explanatory diagram of reduction of spherical aberration by a phase shifter.

FIG. 9 is a first embodiment of the phase shifter.

FIG. 10 shows a second embodiment of the phase shifter.

FIG. 11 is an example of a phase shifter having polarization dependency.

FIG. 12 is a configuration diagram of a main part of an actuator.

FIG. 13 is a block diagram of an optical disk device of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Semiconductor laser, 6 ... Liquid crystal optical rotator, 7 ... Liquid crystal drive circuit, 10 ... Phase shifter, 11 ... Polarization filter, 12 ... Objective lens, 13, 14 ... Optical disk, 30 ... 1/2 wavelength plate, 61 ... Optical pickup .

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Ken Shimano, Ken-ji, Kokubunji, Tokyo 1-280 Higashi-Kengokubo, Ltd. Central Research Laboratory, Hitachi, Ltd. (72) Inventor, Akira Arimoto 1-280, Higashi-Kengokubo, Kokubunji-shi, Tokyo Central Research Laboratory, Hitachi, Ltd. (72) Inventor, Kuniichi Onishi, 292 Yoshida-cho, Totsuka-ku, Yokohama-shi, Kanagawa Stock Company, Hitachi, Ltd. Multimedia System Development Headquarters

Claims (5)

[Claims] 1. An optical disc device comprising a light source, an objective lens for condensing laser light emitted from the light source onto the optical disc, and a detection means for detecting reflected light from the optical disc, and capable of reproducing optical discs having different substrate thicknesses. An optical disk comprising: a light-shielding unit arranged in the optical path between the light source and the objective lens for shielding a part of the laser light; and a phase shifter for imparting a phase difference to a part of the laser light. apparatus. 2. The optical disk device according to claim 1, further comprising optical rotation means for rotating the polarization direction of the laser light in the optical path between the light source and the objective lens. 3. The optical disk device according to claim 2, wherein the optical rotation means is a liquid crystal optical rotator. 4. The optical disk device according to claim 2 or 3, wherein the light shielding means is a polarizing plate. 5. The optical disk device according to claim 2, 3 or 4, wherein the phase shifter has polarization dependence.
An optical disk device, wherein a phase difference is not generated for laser light of a first polarization direction and a phase difference is generated for laser light of a second polarization direction orthogonal to the laser light of the first polarization direction.