JPH09153228A - Optical pickup device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide the optical pickup device which is simple in its construction, has a high utilization rate of laser beams and is capable of conducting recording and reproducing operations on optical disks having transparent substrates with different thicknesses. SOLUTION: An InGaAlP type laser diode 11a and a GaAlAs type laser diode 11b, which emit laser beams having different planes of polarization, are selected for a light source 11. Moreover, an objective lens 15 and a polarization type hologram element 16, which has interference fringes reacting to only TE waves, are provided to a converging section 14. The TM waves emitted from the laser diode 11a are converged by the NA of the lens 15 on an optical disk 1a having a thin transparent substrate 3a. On the other hand, the TE waves emitted from the laser diode 11b are converged by the NA compensated for by the element 16 on an optical disk 1b having a thick transparent substrate 3b.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical pickup device for reproducing an optical recording medium such as an optical disk, and more particularly to an optical pickup capable of recording or reproducing on an optical recording medium having a different substrate thickness. It relates to the device.

[0002]

2. Description of the Related Art In recent years, optical discs such as compact discs (CDs) are widely used as data recording media. Then, an optical disc such as a digital video disc (DVD) having a recording density higher than that of a conventional CD is about to be put into practical use. A laser beam is used for recording or reproducing on the optical disc, and the reflected beam of the laser beam focused as a minute spot on the recording layer of the optical disc is obtained to read the data recorded on the optical disc.

When the recording density of an optical disk is improved, it is necessary to increase the numerical aperture (NA) of the objective lens to reduce the spot diameter, and in order to reduce the tilt error that accompanies this, the transparent substrate of the optical disk is required. It is desirable to make thin. Therefore, a thin transparent substrate is used for a high recording density optical disc as compared with a conventional CD. In this way, since the thickness of the transparent substrate is different between the high recording density optical disc and the low recording density optical disc similar to the conventional CD, future optical disc devices will record or record on optical discs having different transparent substrate thicknesses. It is desirable to be able to perform processing such as reproduction.

In order to process optical discs having different substrate thicknesses by one disc device, the numerical aperture (NA) of the objective lens housed in the optical pickup for condensing the laser beam is determined by the substrate thickness of the optical disc. It is desirable to control the focus position (depth of focus) of the laser beam emitted from the optical pickup device by switching according to the above. As the NA switching method of the optical pickup, a lens switching method of exchanging the objective lens itself, a correction plate insertion method of inserting and removing a correction lens into and from the objective lens, an aperture insertion method of switching a diaphragm (aperture) to the objective lens, and an objective lens There is a bifocal lens system in which a diffracting means such as a hologram is provided to have two focal points.

FIG. 6 is a conceptual diagram of a pickup device using a correction plate insertion system. The objective lens 15 of the optical pickup is adjusted according to the optical disc 1a for high recording density having the thin transparent substrate 3a with the thickness d1, and the laser beam emitted from the semiconductor laser 60 is shown in FIG.
As shown in (a), the light is focused on the recording layer 2a of the optical disk 1a for high recording density by the objective lens 15.

On the other hand, for an optical disc 1b for low recording density having a thick transparent substrate 3b having a thickness d2, as shown in FIG.
As shown in (b), the correction lens 71 of negative power is slid by the lens moving guide member 70 to a position where the optical axis coincides with the objective lens 15 to reduce the NA of the condensing portion of the laser beam. By inserting the correction lens 71, the NA of the condensing portion is larger than that in the case of the objective lens 15 alone.
, The focal length of the condensing portion becomes long, and the laser beam is condensed on the recording layer 2b of the optical disc 1b for low recording density. As described above, in this optical pickup, the NA of the condensing portion is controlled by mechanically moving the correcting lens 71 so that one disc device can process optical discs having different transparent substrate thicknesses. Has become.

FIG. 7 is a conceptual diagram of an optical pickup using a bifocal lens system. In this method, a grading lens 80 having a hologram 81 formed on one surface of the objective lens, for example, on the incident surface side is used. For example, when the hologram 81 having negative power is formed, the 0th-order diffracted light of the laser beam emitted from the semiconductor laser 60 is condensed on the optical disc 1a for high recording density having a thin transparent substrate. The first-order diffracted light of the laser beam is focused on the low recording density optical disc 1b provided with a thick transparent substrate. In this way, the grating lens 8
Since two focal points are formed by 0, it is possible to provide an optical disk device capable of processing optical disks having different substrate thicknesses.

[0008]

However, the optical pickup adopting the correction plate insertion method shown in FIG. 6 requires a mechanism for sliding the correction lens, which complicates the structure of the optical pickup. The same applies to the lens switching system and the aperture insertion system, and a mechanical mechanism for moving the lens or the aperture is required. Therefore, it is difficult to reduce the size and weight of the optical pickup. Further, since the optical elements are mechanically moved, it is necessary to control each optical element with high precision, and in order to form a stable laser spot corresponding to a high density optical disc, a mechanism with high precision is not provided. In addition, the size of the optical pickup is increased and the manufacturing cost is increased.

On the other hand, if the bifocal lens system is adopted, no mechanical movement is required, so that a highly accurate and small optical pickup can be realized. However, since the laser beam is focused on two different focal points by the grading lens, the power of the laser beam at each focal point is reduced, and the intensity at each focal point is less than half the intensity of the laser beam emitted from the semiconductor laser. turn into. Therefore, the utilization efficiency of the laser beam is low, the recording or reproducing capability is deteriorated, and the utilization efficiency of electric power is deteriorated.

As described above, in the conventional optical pickup corresponding to the optical discs having different thicknesses, the one that mechanically changes the NA of the condensing portion can maintain the utilization efficiency of the laser beam, but the mechanism becomes complicated and the size is reduced. It is difficult to reduce the price. On the other hand, an optical pickup using a bifocal lens can be miniaturized, but the utilization efficiency of the laser beam is low and the performance deteriorates.

Therefore, it is an object of the present invention to provide an optical pickup which can be miniaturized and reduced in cost, has high utilization efficiency of a laser beam, and has high recording or reproducing ability. Therefore, it is an object of the present invention to provide an optical pickup capable of changing the NA of the condensing unit without using mechanical control and switching the focus with a simple configuration. Further, it is an object of the present invention to provide an optical pickup device which is excellent in recording or reproducing ability and has high power utilization efficiency.

[0012]

In the optical pickup device of the present invention, a polarizing hologram element capable of switching the NA of the condensing portion is adopted, and the polarizing surface differs from the light source portion.
By emitting different types of laser beams, the focal length can be changed depending on the thickness of the transparent substrate of the optical recording medium. Therefore, in the optical pickup device of the present invention, the light source section that emits the first and second laser beams having different polarization planes and the condensing section that condenses the first and second laser beams onto the optical recording medium. And a polarizing hologram element that acts on the first laser beam to change the NA of the condensing unit is provided in the condensing unit.

The polarization plane may be changed by using a wave plate or the like, but in terms of simplifying the structure of the optical pickup, it is different from the first semiconductor laser which emits the first laser beam so that the polarization plane is different. It is desirable to dispose a second semiconductor laser that emits a second laser beam. Furthermore, In
If a GaAlP-based laser diode and a GaAlAs-based laser diode are used, the polarization plane of the emitted laser beam is different, and therefore two laser beams having different polarization planes can be used without changing the optical path such as the beam shaping path.
Further, it is desirable that these semiconductor lasers are arranged adjacent to each other so that they can be regarded as one light source.

By forming an interference fringe having a positive or negative power on the polarizing hologram element, the NA of the condensing portion can be changed. Also, the NA of the condensing portion can be changed by forming an aperture which is a transmitting portion on the central axis in the polarizing hologram element.

Therefore, the optical pickup device of the present invention is
Instead of dividing one laser beam into two focal positions, it is possible to control the focusing position of laser beams having different polarization planes by using a polarization hologram element, so that the intensity of the laser beam applied to the optical recording medium can be controlled. It can be maintained and utilization efficiency can be improved. In addition, since it is only necessary to change the polarization plane of the laser beam emitted to switch the NA of the focusing part, no mechanical control is required, the configuration of the optical pickup device can be simplified, and the size and weight can be reduced. It can be provided at low cost. Further, since the optical system is stable and the optical recording medium can be irradiated with a high-intensity laser beam, it is possible to provide an optical pickup device having high recording or reproducing ability and stable performance.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows an outline of an optical pickup according to an embodiment of the present invention and an optical disk device using the optical pickup. Further, FIG. 2 shows how a laser beam is focused in the optical pickup of this example, and FIG. 3 shows a polarizing hologram element having an interference fringe used in the optical pickup.

The optical pickup 10 of the present example has a light emitting section 20.
And a light receiving section 21. The light emitting section 20 includes a light source 11 capable of emitting two types of laser beams having different polarization planes, and a collimator lens 1 installed in the traveling direction of the laser beam.
2 is provided. The laser beam emitted from the light emitting unit 20 enters the beam splitter 13, and further the condensing unit 1
It is incident on the optical disc 4, and is condensed on the optical disc 1. On the other hand, the light receiving unit 21 includes a photodiode 22 for detecting the laser beam reflected from the optical disc 1, and a lens 23 that focuses the light on the photodiode 22.

The light source 11 of this embodiment comprises two laser diodes 11a and 11b for emitting a laser beam, which are arranged in parallel at an interval of about 100 μm. Therefore, the laser diodes 11a and 11
The optical paths can be configured by assuming that the laser beams emitted from b are emitted from the same light source.
That is, the light emitting unit 20 of this example is the laser diode 11a.
The same collimator lens 12 is used for and 11b, and the common optical system is used thereafter. The laser diode 11a is made of InGaAlP and has a wavelength of 650 nm.
The laser diode 11b is a GaAlAs system and has a wavelength of 780 nm.
The reverse electric (TE) polarized wave is emitted. Therefore, by selecting one of the laser diodes from the light source 11, two laser beams having different polarization planes are emitted.

The condensing unit 14 comprises an objective lens 15 and a polarizing hologram element 16, which are the lens holder 1
It is held together at 7. The objective lens 15 of this example is a lens with a high NA, and as shown in FIG. 2A, the thin (thickness d1) recording layer 2a of the transparent substrate 3a for high recording density is used.
It is designed to match the focal length. The polarizing hologram element 16 of this example is made of LiNbO ₃ or the like, and the interference fringes 16a having a negative blazed focal power as shown in FIG. 3 are formed on the surface thereof. In the polarization hologram element 16 of this example, the orientation of the crystal axis is set so that it acts only on the TE wave.
It has no effect on M waves. Therefore, when the laser beam of the TM wave is emitted from the laser diode 11a by the focusing unit 14 of this example, the laser beam is focused as shown in FIG. On the other hand, the laser diode 11
When a TE-wave laser beam is emitted from b, as shown in FIG.
As shown in (b), since the polarization hologram element 16 acts and the NA of the condensing part 14 becomes low, it is condensed on the recording layer 2b of the transparent substrate 3b (thickness d2) for low recording density. It

Furthermore, the polarizing hologram element 16 of this example.
Since interference fringes are formed on the laser diode 11b so as to increase the intensity of the + direction first-order diffracted light of the wavelength of 780 nm of the laser diode 11b, the laser diode 11b is formed.
Most of the laser beam emitted from the laser is focused on the recording layer 2b.

The laser beam reflected from the recording layer 2 of the optical disk is passed through the condensing section 14 again to the optical pickup 10 again.
And is emitted by the beam splitter 13 toward the light receiving portion 21. In the light receiving unit 21, the laser beam reflected from the optical disc 1 is converted into electric data by the photodiode 22 and output to the control unit 31 provided in the optical disc device 30.

The optical disk device 30 of this embodiment includes the optical pickup 10, the controller 31, the driving circuit 32 for driving which supplies a driving current to the laser diodes 11a and 11b of the light source, and the laser diode 11.
Selector 3 for selecting either a or 11b
3 is provided. The control unit 31 has a function of discriminating a disc from the thickness of the transparent substrate, focus and tracking control of the optical pickup 10, and further the optical pickup 10
This is a system controller equipped with a communication function such as supplying the received data to the external personal computer.
The control unit 31 issues an instruction to supply a driving current to the light source 11 to the driving circuit 32 in response to a request from a personal computer or the like. At this time, the control unit 31 determines the thickness of the transparent substrate 3 of the optical disc 1 set in the disc device 30, switches the selector 33, and selects one of the laser diodes 11a and 11b that is suitable for the set optical disc 1. The laser diode is supplied with a drive current to emit a laser beam.

When an optical disc device 30 employing the optical pickup 10 of the present example is mounted with an optical disc 1a having a thin transparent substrate such as a DVD having a high recording density, the control unit 30 operates as shown in FIG. 2 (a). The thickness of the substrate is discriminated and the selector 33 is switched to the laser diode 11a that emits TM waves, and the drive current from the driving circuit 32 is supplied to the laser diode 11a. Therefore, the TM wave is applied to the thin optical disk 1a having the transparent substrate. Therefore, the polarization hologram element 16 does not act in the condensing unit 14, and the laser beam is condensed on the recording layer 2 a according to the NA of the objective lens 15.

On the other hand, when the optical disc device 30 is loaded with an optical disc 1b having a thick transparent substrate such as a CD having a low recording density, as shown in FIG.
Is switched to the laser diode 11b that emits a TE wave. Therefore, the optical disk 1b having a thin transparent substrate is irradiated with the TE wave. For this reason, since the polarization hologram element 16 acts on the condensing unit 14, the NA of the objective lens 15 is corrected by the negative focal power of the interference fringes 16a of the polarization hologram element 16, and the laser beam is emitted to the recording layer 2b. Collected.

Thus, the optical pickup 10 of this example
Does not need to replace the lens itself to switch the NA of the condensing unit or to insert or remove the lens for correction, but it is sufficient to switch the laser diode serving as the light source and switch the polarization plane of the laser beam. Therefore, no mechanical mechanism is required and it is not necessary to adjust the position of the optical system. Therefore, it is possible to realize an optical pickup with a simple structure, which is compatible with optical disks having different thicknesses, and is small in size.
The weight can be reduced. Further, since the optical path is stable, it is possible to maintain high optical precision and provide a high-performance optical pickup corresponding to a high recording density. Furthermore, since no mechanical mechanism is required, a small optical pickup can be provided at low cost.

Further, the optical pickup 10 of this embodiment uses the polarization hologram element 16 so that the NA of the condenser 14 can be adjusted for each laser beam having a different polarization plane. Therefore, in the optical pickup 10 of this example, the TE-wave laser beam is focused with a low NA, and the TM-wave laser beam is focused with a high NA. Thus, unlike a conventional pickup using a bifocal lens, one laser beam is dispersed into two focal points and the intensity of each focal point does not drop to less than half. The intensity of each laser beam of a wave is focused on an optical disc having a transparent substrate having a different thickness. Therefore, in the optical pickup 10 of this example, the utilization efficiency of the laser beam emitted from each laser diode is very high, and a high-intensity laser spot can be formed on the recording layer of each optical disc. Therefore, the optical pickup of this example has a high recording or reproducing ability for any of optical discs having different thicknesses, and has high power utilization efficiency. Further, since the polarization hologram element 16 of this example is formed with the blazed interference fringes 16a that match the wavelength of the laser diode 11b, there is almost no loss of optical power due to diffraction, and a high-intensity laser beam is obtained. Can be irradiated.

The polarization hologram element 16 for controlling the NA of the condenser 14 is not limited to the above.
For example, the NA of the objective lens 15 may be set low and the polarization hologram element 16 may be provided with an interference fringe having a positive focal power. Furthermore, in the above, the orientation direction of the crystal is set so that the polarization hologram element 16 acts on the TE wave, but it goes without saying that it may be arranged so as to act on the TM wave. In addition, although a blazed diffraction grating is used to reduce the loss of optical power due to diffraction, even if a planar diffraction grating is used, a conventional bifocal lens can be used with several times the intensity. Obtainable.

Further, the polarizing hologram element 16 includes
Instead of the interference fringes, an aperture 16b having a transmission part on the central axis as shown in FIG. 4 may be formed. The polarization hologram 16 of this example is oriented so that it does not act on the TM-wave laser beam similarly to the polarization hologram element in which the interference fringes are formed. Only works against. Then, only the central portion of the aperture 16b passes through the polarizing hologram element 16 and is focused on the optical disc. On the other hand, the laser beam outside the positions A and B of the polarization hologram element 16 in the circumferential direction is not incident on the objective lens 15 or is given a large negative focal power that is not condensed by the objective lens 15. Not collected on top. That is, the polarization hologram element 16 is
Since it functions as a diaphragm for the TE-wave laser beam, the NA of the condensing portion 14 is adjusted, and the same function as that of the polarization hologram element in which the interference fringes are formed is achieved. Of course, a polarizing hologram element may be formed so as to perform a diaphragm function for TM waves.

FIG. 5 shows an example in which the polarizing hologram element 16c is integrated on one surface of the objective lens 15. By thus integrating the polarizing hologram element with the front surface or the back surface of the objective lens, the configuration of the light condensing unit 14 can be further simplified, and the aberration can be improved.

In this example, the InGaAlP system emitting the TM wave and the GaAlA emitting the TE wave are used.
Although the description has been given by using the s-series laser diode, the present invention is not limited to these laser diodes and any laser diode capable of emitting laser beams having different polarization planes may be used. Further, it is possible to change the direction of the laser diode or switch the polarization plane of the laser beam by using a wave plate or the like. When two types of laser diodes having different polarization planes are used as in this example, an optical pickup can be configured with a small number of optical elements using a common optical path, which is suitable for a compact and high-performance optical pickup.

In the above embodiments, the material for the polarizing hologram element is LiNbO ₃ , but the material is not limited to this.

[0032]

As described above, according to the present invention,
By using laser beams having different polarization planes for optical recording media having different transparent substrate thicknesses, it is possible to provide an optical pickup device capable of forming spots at positions suitable for the recording layers of the optical recording media having the respective thicknesses. The optical pickup device of the present invention does not require a mechanical mechanism for switching the NA of the condensing part, and further, the laser beam is
There is no need to disperse and focus on one focal point. Therefore, it is possible to provide an optical pickup device that is small and lightweight, has excellent recording or reproducing performance, and has high utilization efficiency of a laser beam.
As described above, the optical pickup device of the present invention is a pickup device suitable for a compact and high-performance optical disc device capable of operating any of optical recording media having different thicknesses depending on recording density and the like.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a schematic configuration of an optical disc device using an optical pickup according to an embodiment of the present invention.

2 is a diagram showing how a laser beam is condensed by the optical pickup shown in FIG. 1, and FIG. 2 (a) shows how a laser beam is condensed on an optical disc having a thin transparent substrate.
(B) shows a state in which light is focused on a thick optical disk having a transparent substrate.

3 is a diagram showing a polarizing hologram element used in the optical pickup shown in FIG.

4A and 4B are views showing a polarization hologram element having an aperture formed, which is different from FIG. 3, in which FIG. 4A shows a side surface of a light converging portion and FIG. 4B shows a plane of the polarization hologram. It is shown.

FIG. 5 is a cross-sectional view showing an objective lens integrated with a polarizing hologram element, which is different from the above examples.

FIG. 6 is a conceptual diagram of a pickup device using a conventional correction plate insertion method.

FIG. 7 is a conceptual diagram of a conventional pickup device using a bifocal lens system.

[Explanation of symbols]

 1 Optical Disc 2 Recording Layer 3 Transparent Substrate 10 Optical Pickup 11 Light Source Section 11a InGaAlP Laser Diode 11b GaAlAs Laser Diode 12 Collimator Lens 13 Beam Splitter 14 Condensing Section 15 Objective Lens 16 Polarizing Hologram Element 20 Light Emitting Section 21 Photoreceptive Section 22 Photo Diode 23 Condensing lens of light receiving unit 30 Optical disk device 31 Control unit 32 Driving circuit 33 Selector

Claims (6)

[Claims] 1. A light source unit that emits first and second laser beams having different polarization planes, and a light collecting unit that focuses the first and second laser beams on an optical recording medium, An optical pickup device, wherein the condensing unit includes an objective lens and a polarizing hologram element capable of changing a numerical aperture NA of the condensing unit with respect to the first laser beam. 2. The light source unit according to claim 1, comprising a first semiconductor laser that emits the first laser beam and a second semiconductor laser that emits the second laser beam, An optical pickup device characterized in that the first and second semiconductor lasers are arranged adjacent to each other. 3. The light source unit according to claim 2, wherein the light source unit is InG.
an aAlP-based laser diode and a GaAlAs-based laser diode, one of which is the first semiconductor laser, the other is the second semiconductor laser, and the first and second semiconductor lasers Optical pickup device characterized by being installed in parallel 4. The optical pickup device according to claim 1, wherein an interference fringe having a positive or negative power with respect to the first laser beam is formed on the polarizing hologram element. 5. The optical pickup device according to claim 1, wherein the polarizing hologram element is formed with an aperture whose center axis is a transmitting portion. 6. The optical pickup device according to claim 1, wherein the polarizing hologram element is in close contact with at least one surface of the objective lens.