JPH09223327A - Two focal points optical pickup device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a relatively inexpensive two focal points optical pickup device capable of obtaining a stable reproducing signal. SOLUTION: When an SD system disk is reproduced, a voltage is applied to a TN(twist nematic) liquid crystal plate 8 from a circuit 20, and the twisted angle of a deflection surface is made 0 deg.. Beam from a laser diode 11 is deflected in the vertical direction, and the beam transmitting through the TN liquid crystal plate 8 isn't varied in the deflection surface, and transmits through a polarizing beam splitter 9, and is deflected by a 45 deg. reflection plate 10 by a 90 deg. to be made incident on an objective lens 2 through a 1/4 wavelength plate 12. The light reflected by the disk is made incident on the reflection beam splitter 9 through the wavelength plate 12 and the reflection plate 10. At the point of time, since the deflection surface of the beam is varied by a further 90 deg., the beam becomes the lateral deflection, and is reflected by the reflection beam splitter 9 to be made incident on a focal point and a tracking position detective part 13. Since the voltage isn't applied to the TN liquid crystal plate 8 when a CD is reproduced, the beam emitted from the laser diode 11 transmits through the TN liquid crystal plate 8, and is reflected by the reflection beam splitter 9 to be incident on the objective lens 1.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a DVD, for example.
The present invention relates to a bifocal optical pickup device used in a player capable of reproducing or recording two types of discs having different substrate thicknesses, such as a player that is a dual type of (Digtal Versatile Disc) and a CD.

[0002]

2. Description of the Related Art Due to the recent increase in the density of optical recording, DV has been replaced with conventional CD-ROM using a music CD as a medium.
D is about to replace the protagonist of information recording.
However, it is said that recording media such as CD-ROMs, which have been popular so far, will be distributed in the market for the next several years, and it is expected that demand for a device that can reproduce these discs by one player will increase in the future. . By the way, the SD type disc used for DVD has a glass substrate with a thickness of 0.
6mm, the thickness of the base of the CD disc, 1.2m
Since it is different from m, it is impossible to read SD and CD with an objective lens having a single focal length. Therefore, the following method has been proposed so far when reproducing a DVD and a CD with a single pickup.

The first method is to mount two objective lenses on a shaft-sliding type pickup. This structure is shown in FIG. The holder 1 that rotates freely around the fixed shaft 28 has an S
Objective lens 2 for D (DVD) and objective lens 3 for CD
Is fixed, and tracking and focusing are controlled by any one of the selected objective lenses. To select the objective lens to use, an excessive current is applied to the tracking coil instantaneously (giving a kick pulse),
Angle between these lenses (45 to 90 degrees)
Only one lens so that one lens is in the operating position.

The second method is to use a hologram lens, the structure of which is shown in FIG. A bifocal hologram lens 4 capable of focusing on both SD and CD is fixed to a lens holder (not shown), and a signal can be transmitted without selecting a lens regardless of the type of the above-mentioned substrate of the disc. It is a reading method. Figure 7 (a) is SD
The laser light is focused on the signal surface of the SD board 16 by the transmitted light 18 of the hologram lens 4. FIG. 7B shows a state in which the CD is being reproduced, and the laser light is focused on the signal surface of the CD substrate 17 by the diffracted light 19 of the hologram lens 4.

The third method is to use a liquid crystal shutter.
The structure is shown in FIG. The objective lens 15 is an SD board 16
It is made so that it is in focus. Figure 8 (a) is SD
Is playing, open the liquid crystal shutter 5,
SD substrate 1 by irradiating the entire surface of the objective lens 15 with laser light
Focus on the 6 signal plane. FIG. 8B shows a state in which the CD is being reproduced, and the laser light has its outer peripheral portion cut off by the liquid crystal shutter 5 and passes only the portion near the center of the objective lens 15. As a result, the depth of focus becomes large, and the focus shift can be made small.
The signal of can also be read.

[0006]

As described above, the conventional device has the following problems. In the case of the axial sliding type twin lens, in order to withstand the high-speed sliding of about 8 times that of the CD-R0M, use a material that is harder and lighter (having a larger elastic modulus and a smaller specific gravity) than conventional materials. And special technology is required to fit the shaft and bearing,
The manufacturing cost is the highest. Also, although the power supply line of the coil is pulled out from the lens holder,
When the position of the lens is changed by rotating by 90 ° to 90 °, the movement of the lens is restricted by the power supply line, which makes it difficult to process the power supply line. Further, in the hologram lens, since it is difficult to form unevenness on the lens surface, the cost of the lens itself becomes considerably high. In the liquid crystal shutter, when reproducing a CD, the amount of light is considerably reduced, and there is a possibility that stable reading cannot be performed. The present invention has been made in view of the above, and the movable part has a lighter weight as compared with a shaft slide type pickup, so that high-speed operation is possible, and a single objective lens is shared by two types of discs. It is an object of the present invention to provide a relatively inexpensive bifocal optical pickup device that can obtain a stable reproduction signal because the amount of light does not decrease as compared with the system.

[0007]

In order to solve the above-mentioned problems and achieve the above-mentioned object, a bifocal optical pickup device of the present invention comprises two objective lenses having different focal lengths, and these objectives are provided. In a bifocal optical pickup device in which a lens is selected and used, a laser beam emitted from a laser diode passes therethrough, and a TN liquid crystal plate that switches a deflecting surface of the passed laser beam and an optical axis of the passing laser beam are arranged. The deflected beam splitter and the 45 ° reflector, the quarter-wave plate disposed between the deflected beam splitter and the 45 ° reflector and the objective lens, and the objective lens of the deflected beam splitter. It is characterized in that it is provided with a focus and a tracking position detector arranged on the opposite side. And the T
Instead of the N liquid crystal plate, a mechanism for rotating the laser diode by 90 ° around its optical axis may be provided, and the deflection beam splitter and the 45 ° reflector may be arranged on the optical axis. The distance between the center lines of the objective lenses is preferably 10 mm or less.

[0008]

In the present invention constructed as described above, the deflection surface of the laser beam emitted from the laser diode is set to the T
Depending on whether or not a voltage is applied to the N liquid crystal plate, it can be arbitrarily changed to 0 ° and 90 °, and when it is 0 °, it passes through the deflecting beam splitter placed behind it and is reflected by the 45 ° reflector behind it. Then, the laser beam is guided to the first objective lens through the quarter-wave plate located above it, and when it is set to 90 °, it is reflected by the deflecting beam splitter arranged behind it, and the laser beam is located above it. Laser light is guided to the second objective lens through the quarter-wave plate. In either case, the reflected light from the disc is detected by the focus and tracking position detector arranged on the opposite side of the deflection beam splitter from the second objective lens.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIGS. FIG. 1 shows the entire optical pickup. A drive system part (upper part in the drawing) for controlling tracking and focusing is mounted on an optical system part (lower part in the drawing) accommodating the optical path switching unit. However, in the figure, the upper part and the lower part are drawn separately for easy understanding. In FIG. 1, reference numeral 1 is a lens holder, and an objective lens 2 for SD and an objective lens 3 for CD are fixed to the holder 1. The lens holder 1 is supported by the fixing portion 7 by the leaf spring 6. 14 is a fixed part 7
The magnet fixed to the coil and the current supplied to the coil fixed to the lens holder 1 controls the tracking and focusing.

Further, reference numeral 8 is a TN liquid crystal plate, 9 is a deflecting beam splitter, 10 is a 45 ° reflecting plate, 11 is a laser diode, 12 is a quarter wavelength plate, 13 is a focus and tracking position detecting section, The operation of will be described later.

FIG. 2 shows only the portion of the present invention that constitutes the present invention so that the operation can be easily understood. The laser diode 11
The light emitted from is incident on the objective lens 2 for SD only when reproducing SD and is incident on the objective lens 3 for CD only when reproducing CD by changing the optical path. The switching of the optical path is performed by controlling the deflecting surface of the laser light depending on whether or not the voltage from the liquid crystal drive circuit 20 is applied to the TN liquid crystal plate 8.

FIG. 3 is for explaining the operation principle of the TN liquid crystal plate 8. The TN liquid crystal plate 8 includes a transparent conductive film 22 such as ITO and an alignment film 23 for aligning liquid crystals.
The TN liquid crystal (Twisted Nemati
(c liquid crystal) 24. As shown in FIG. 3A, when no voltage is applied to the TN liquid crystal plate 8, the linearly polarized laser light 25 becomes a laser light 26 which is twisted by 90 ° and exits from the TN liquid crystal plate 8. On the other hand, when a voltage is applied to the TN liquid crystal plate 8, the voltage shown in FIG.
As shown in FIG. 5, the laser beam 27 is transmitted while keeping the deflecting surface before the incidence.

Next, the principle of operation when used in an actual product will be described with reference to FIGS. 4 and 5. First, S
A case of reproducing a D system disc will be described with reference to FIG. In this case, a voltage from the liquid crystal drive circuit 20 is applied to the TN liquid crystal plate 8 so that the twist angle of the deflecting surface becomes 0 °. The light emitted from the laser diode 11 is vertically deflected and enters the TN liquid crystal plate 8. Since the deflecting surface of the laser light after passing through the TN liquid crystal plate 8 does not change, it is vertically deflected and enters the deflecting beam splitter 9. here,
The laser light is transmitted, and it is bent 90 ° by the 45 ° reflector 10 arranged further in the back and passed through the quarter-wave plate 12 and S.
It is incident on the D objective lens 2. The light reflected by the disk passes through the quarter-wave plate 12 and the 45 ° reflection plate 10 again,
It is incident on the deflecting beam splitter 9. At this point,
Since the light deflection surface is changed by 90 °, the light is deflected in the lateral direction. Therefore, the light is reflected by the deflecting beam splitter 9, and the focus and tracking position detecting unit 13
Incident on. In this way, the signal of the SD disc can be read.

Next, FIG. 5 shows the case of reproducing a CD.
It will be described based on. In this case, no voltage is applied to the TN liquid crystal plate 8 and the twist angle of the deflecting surface is set to 90 °. The light emitted from the laser diode 11 is vertically deflected and enters the TN liquid crystal plate 8. The deflecting surface of the laser beam after passing through the TN liquid crystal plate 8 is changed by 90 ° and enters the deflecting beam splitter 9 as a lateral deflection. Here, the laser light is reflected and enters the objective lens 1 for CD through the quarter-wave plate 12. The light reflected by the disk passes through the quarter-wave plate 12 again and enters the deflecting beam splitter 9. At this point, the deflecting surface of the light is further changed by 90 °, so that the light is deflected in the vertical direction. Therefore, the light passes through the deflecting beam splitter 9 and enters the focus and tracking position detector 13. In this way, the CD
Can read the signal of the disc.

As described above, the optical path of the laser beam can be selected in two ways depending on whether the voltage is applied to the TN liquid crystal plate 8 or not. Therefore, with respect to two types of base disks that require objective lenses having different focal lengths, the optimum objective lenses 1 and 2 can be switched to read a signal without mechanical switching. In the bifocal optical pickup device configured as described above, since the loss in each optical path is only the loss due to the insertion of the TN liquid crystal plate 8, it can be used as a pickup for a writable optical disc.

Instead of using the TN liquid crystal plate 8, the laser diode 11 may be rotated by 90 ° to change the deflecting surface of the light and thereby switch the optical path. Further, if the two objective lenses for SD and CD are too far apart, the movement direction of the pickup is deviated from the movement direction of the track, and the movement of the pickup is not symmetrical, which causes an unreasonable situation. . As a result of a simulation as to how much the distance between the lenses can be widened, it was found that when the distance exceeds 10 mm, the movement direction of the pickup is tilted by 12 ° with respect to the meridian of the disk at the innermost circumference of the disk. . Therefore, in order to maintain good servo characteristics, the distance between the center lines of the two objective lenses is set to 10 mm or less.

[0017]

Since the present invention is configured as described above, the movable part becomes lighter in weight as compared with the shaft sliding type pickup, and thus high speed operation becomes possible. Further, as compared with the system in which a single objective lens is shared by two types of discs, the amount of light does not decrease, so that a stable reproduction signal can be obtained and a higher laser output is required for recording. It can also be used as a rewritable optical disk pickup.

[Brief description of drawings]

FIG. 1 is an exploded perspective view showing an embodiment of the present invention.

FIG. 2 is a schematic configuration diagram showing a main part of an embodiment of the present invention.

FIG. 3 is a schematic configuration diagram for explaining an operation principle of a TN liquid crystal plate.

FIG. 4 is a schematic configuration diagram for explaining the operating principle of the embodiment of the present invention.

FIG. 5 is a schematic configuration diagram for explaining the operation principle of the embodiment of the present invention.

FIG. 6 is a perspective view showing a first conventional example.

FIG. 7 is a schematic configuration diagram for explaining an operation of a second conventional example.

FIG. 8 is a schematic configuration diagram for explaining an operation of a third conventional example.

[Explanation of symbols]

 1 Lens Holder 2, 3 Objective Lens 4 Hologram Lens 5 Liquid Crystal Shutter 6 Leaf Spring 7 Fixed Part 8 TN Liquid Crystal Plate 9 Deflection Beam Splitter 10 45 ° Reflector 11 Laser Diode 12 1/4 Wave Plate 13 Focus and Tracking Position Detector 14 Magnet 15 Objective lens 16 SD board 17 CD board 18 Transmitted light 19 Diffracted light 20 Liquid crystal drive circuit 21 Glass substrate 22 Transparent conductive film 23 Alignment film 24 TN liquid crystal 25, 26, 27 Laser light 28 Fixed axis

Claims (3)

[Claims] 1. A bifocal optical pickup device comprising two objective lenses having different focal lengths, wherein these objective lenses are selected and used, wherein a laser beam emitted from a laser diode passes through, and A TN liquid crystal plate for switching the deflecting surface, a deflecting beam splitter and a 45 ° reflecting plate arranged on the optical axis of the passing laser beam, and between the deflecting beam splitter and the 45 ° reflecting plate and the objective lens. A bifocal optical pickup device comprising: a 1/4 wavelength plate arranged; and a focus and tracking position detection unit arranged on the opposite side of the deflection beam splitter from the objective lens. 2. A bifocal optical pickup device comprising two objective lenses having different focal lengths, wherein the objective lenses are selected and used, and a mechanism for rotating a laser diode by 90 ° around its optical axis, A deflecting beam splitter and a 45 ° reflecting plate arranged on the optical axis, a ¼ wavelength plate disposed between the deflecting beam splitter and the 45 ° reflecting plate and the objective lens, and the deflecting beam splitter. 2. A bifocal optical pickup device, comprising: a focus and tracking position detector disposed on the side opposite to the objective lens. 3. The distance between the center lines of the objective lens is 10
3. The bifocal optical pickup device according to claim 1, wherein the bifocal optical pickup device has a thickness of not more than mm.