JPH09212904A - Optical pickup device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an optimum tracking error signal according to an optical disk by a method wherein a light detection means by which beam of reflected light from two kinds of optical disks are received so as to be converted into electric signals is installed and two objective lenses are provided with focal lengths satisfying a specific condition. SOLUTION: A laser beam which is radiated from a laser diode is driven in two axial directions, i.e., an F-direction and a T-direction, by a biaxial actuator 7, and it is converged on the recording face of a compact disk 2 and an that of a high-recording-density optical disk 3. reflected laser beams from the disk 2 and the disk 3 are condensed respectively on a photodetector 8 which is situated on an image formation point. In this configuration, a first objective lens 5 and a second objective lens 6 are provided with focal lengths which establish an expression. In the expression, f1 and f2 represent the focal distances of the first and second objective lenses, λ1 and λ2 represent the wavelength of a first light source and that of a second light source, (d) represents the pitch of a diffraction grating, and TP1 and TP2 represent track pitches of both optical disks, as well as k1 and k2 represent the factor of the first objective lens and that of the second objective lens as 4 and 2 respectively.

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 information signals from optical discs having different track pitches.

[0002]

2. Description of the Related Art When an information signal is reproduced from an optical disc, an optical pickup device performs tracking control of an objective lens on a track of an information signal layer in order to reproduce the information signal well. The tracking control method includes a three-spot (three-beam) method using three beams of a main beam for reading an information signal and two auxiliary beams, and a D-beam method.
PP (Differential Push Pul)
1) method There is a so-called operation push-pull method and the like.

The three-spot method is a method for obtaining a tracking error signal from the output balance of an auxiliary beam that connects the beam spots to the left and right of the main beam on the track of the information signal layer. In the three-spot method, since the amplitude of the tracking error signal is large when the beam spot is connected at a position shifted by ¼ track pitch in the left and right track directions of the main beam, the beam spot is connected at this position. It is desirable to set the auxiliary beam so that

The working push-pull method is a method of obtaining a tracking error signal by using each push-pull signal of a main beam on a track of an information signal layer and an auxiliary beam connecting a beam spot to the left and right of this main beam. In this operation push-pull method, the amplitude of the tracking error signal becomes large when the beam spot is connected at a position displaced by 1/2 track pitch in the left and right track directions of the main beam, so the beam spot is connected at this position. It is desirable to set the auxiliary beam so that

[0005]

When an information signal is reproduced from an optical disc, the optical pickup device can manage the aberration because the jitter and the error rate are increased even if the aberration is a slight diffraction limit. Most important.

In order to realize a high recording density, the optical disc has a large numerical aperture NA of the objective lens or a short wavelength λ of the light beam emitted from the light source.
It is necessary to reduce the beam spot diameter.

However, the coma aberration caused by the inclination of the optical disc, so-called skew, is proportional to the cube of the numerical aperture NA of the objective lens and increases in inverse proportion to the wavelength λ of the light source. It is said to be the main factor that prevents the numerical aperture NA of the objective lens from increasing. Therefore, it is difficult to set the numerical aperture NA of the objective lens included in the optical pickup device to 0.6 or more.

Further, since the coma aberration is proportional to the substrate thickness dimension of the optical disc, the coma aberration can be reduced by reducing the substrate thickness dimension. However, since the objective lens causes spherical aberration in proportion to the thickness of the substrate, it is set to cancel the spherical aberration only for the optical disc having the predetermined thickness of the substrate.
There is a problem that spherical aberration occurs in optical discs having different substrate thickness dimensions depending on the difference in the substrate thickness dimensions.

Therefore, a high recording density optical disc has been proposed in which both the substrate thickness and the track pitch are about 1/2 of the compact disc, but it is possible to reproduce optical discs having different specifications. It is difficult to realize a compatible optical disc reproducing device.

By the way, when the optical pickup device reproduces the information signal from the compact disc, the optical pickup device, for example,
When the objective lens is tracking-controlled using the spot method, the track pitch is TP, the distance between the beam spot of the main beam and the beam spot of the auxiliary beam is fθ,
If the angle formed by each auxiliary beam diffracted by the diffraction grating is θ, the pitch of the diffraction grating is d, and the wavelength of the light beam emitted from the light source is λ, the wavelength λ of this light beam is
It is calculated by λ = dsin θ.

When tracking control is performed using the three-spot method as described above, as shown in FIG. 3, the beam spot of the auxiliary beam is 1/4 of the track direction.
A good tracking error signal can be obtained by connecting to a position shifted by TP.

However, when reproducing an information signal from a high recording density optical disc having a track pitch of 1/2, the position of the beam spot of the auxiliary beam is 1/2 of the track pitch, as shown in FIG. To be in position,
There is a problem that the tracking error signal using the 3-spot method becomes zero and tracking control cannot be performed.

Therefore, when the optical pickup device reproduces two types of optical discs having different track pitches, the optical disc is set so that a tracking error signal can be favorably obtained for one of the optical discs, so that the tracking error from the other optical disc can be obtained. There was a problem that it was not possible to obtain a good signal.

Therefore, it is an object of the present invention to provide an optical pickup device capable of obtaining an optimum tracking error signal according to the optical disc when the optical discs have different track pitches.

[0015]

In order to achieve the above object, an optical pickup device according to the present invention comprises a light source means having at least a first light source and a second light source for emitting a light beam, and the light source means. A diffraction grating that diffracts a light beam emitted from the laser beam into three beams, and at least a first objective lens and a second objective lens that focus the light beam on respective signal recording surfaces of at least two types of optical discs.
Lens means having an objective lens, biaxial driving means for driving at least two objective lenses of the lens means in the biaxial directions with respect to the optical disc, and receiving reflected light from at least two types of optical discs. And a light detecting means for converting the electric signal into an electric signal.

The first objective lens and the second objective lens are: f ₁ · arcsin (λ ₁ / d) / TP ₁ · K ₁ = f ₂ · arcsin (λ ₂ / d) / TP ₂ · K _The focal length is such that ₂ holds.

In the above equation, f _{1 is} the focal length of the first objective lens, f _{2 is} the focal length of the second objective lens, λ _{1 is} the wavelength of the first light source, and λ _{2 is} the second light source. , D: the pitch of the diffraction grating, TP ₁ : the track pitch of the optical disc reproduced by the first objective lens, T
P ₂ : The track pitch of the optical disc reproduced by the second objective lens. Further, K _{1 is} a coefficient of the first objective lens, and K _{2 is} a coefficient of the second objective lens.
It is set to 2 when the arithmetic processing of (operation push-pull method) is used.

In the optical pickup device configured as described above, the focal lengths of the first and second objective lenses are f ₁ · arcsin (λ ₁ / d) / TP ₁ · K ₁ = f ₂ · arcsin ( By setting such that λ ₂ / d) / TP ₂ · K ₂ is established, the beam spot of the auxiliary beam is formed at the optimum position with respect to the track on the signal recording surface. Therefore, in this optical pickup device, a tracking error signal can be satisfactorily obtained according to at least two types of optical disks having different track pitches.

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION A specific embodiment of the present invention will be described below with reference to the drawings of an optical pickup device.

First, in the optical pickup device 1 of the embodiment, for example, the track pitches are 1.6 μm and 0.74 μm.
This is a so-called compatible optical pickup device for reproducing an information signal from the compact disc 2 and the high recording density optical disc 3 in which the information signal layer is in the same reading direction at m.

The optical pickup device 1 of the embodiment is
As shown in FIG. 1, a light source unit 4 having a first laser diode 4A and a second laser diode 4B, which is a kind of semiconductor laser that emits a laser beam, a compact disc 2, a high recording density optical disc 3, and a laser beam. Of the first objective lens 5 and the second objective lens 5 for focusing on the respective signal recording surfaces of
Objective lens 6 and the first objective lens 5 and the second objective lens
The biaxial actuator 7 that drives the objective lens 6 in the biaxial direction with respect to the compact disc 2 and the high recording density optical disc 3, and the reflected laser light from the compact disc 2 and the high recording density optical disc 3 is received and converted into an electric signal. It is provided with a photodetector 8 including a first photodetector 8A and a second photodetector 8B such as a photodiode for conversion.

The diffused laser light emitted from the laser diode 4 is collimated by the collimator lens 9 to have its luminous flux width limited to be parallel light, which is then incident on the diffraction grating 10. The laser light is diffracted by the diffraction grating 10 into, for example, three spots, reflected by the beam splitter, and driven by the biaxial actuator 7 in the biaxial directions of the F direction and the T direction. It is incident on the second objective lens 6. The first objective lens 5 is the compact disc 2
The laser light is focused on the signal recording surface of. The second objective lens 6 focuses the laser light on the signal recording surface of the high recording density optical disc 3.

The reflected laser light from the signal recording surface of the compact disc 2 is passed through the first objective lens 5, the beam splitter 11, the detection lens 12 and the cylindrical lens 13 and then on the photodetector 8 located at the image forming point. Is focused on. Further, the reflected laser light from the signal recording surface of the high recording density optical disc 3 is reflected by the second objective lens 6 and the beam splitter 1.
1, through the detection lens 12 and the cylindrical lens 13, the light is focused on the photodetector 8 located on the image formation point.

The first mounted on the biaxial actuator 7
The objective lens 5 and the second objective lens 6 are arranged in a direction orthogonal to the tracking direction T, as shown in FIG. The biaxial actuator 7 connects the objective lens holder 7A holding the first objective lens 5 and the second objective lens 6 to the drive unit 7B via the elastic support member 7C. Then, the biaxial actuator 7 drives the drive unit 7B in accordance with each servo signal supplied from a servo circuit (not shown), so that the first objective lens 5 and the second objective lens 6 are orthogonal to the optical axis. It is driven in the tracking direction T and the focusing direction F parallel to the optical axis.

Then, the optical pickup device 1 obtains a tracking error signal by using the three-spot method by the first objective lens 5 when reproducing the information signal from the compact disc 2, and the information signal from the high recording density optical disc 3 is obtained. When reproducing, the tracking error signal is obtained by using the operation push-pull method by the second objective lens 6.

The shift of the auxiliary beam with respect to the track is optimally 1/2 track pitch when three spots are used and 1/4 track pitch when the operation push-pull method is used. Therefore, the track pitch of the optical disc reproduced by the first objective lens 5 is set to TP.
₁ , the track pitch of the optical disk reproduced by the second objective lens 6 is TP ₂ , the focal length of the first objective lens 5 is f ₁ , and the focal length of the second objective lens 6 is f _2.
If so, the first objective lens 5 and the second objective lens 6 have focal lengths f ₁ and f ₂ that satisfy the following equation: f ₂ / TP ₂ = f1 / TP ₁ · 2. By setting them respectively, the first objective lens 5 and the second objective lens 6 are favorably tracking-controlled with respect to the compact disc 2 and the high recording density optical disc 3.

Track pitch T of the compact disc 2
P ₁ is 0.74, and the track pitch of the high recording density optical disc 3 is 1.6. Therefore, f ₁ = 3
Thus, f ₂ = 3.24 is calculated from Expression 1.

As described above, when the wavelengths of the light sources are the same, the first objective lens 5 and the second objective lens 6
The focal lengths f ₁ and f ₂ can be calculated by the equation 1.

Next, a case where a plurality of light sources have different wavelengths will be described. When performing tracking control using the three-spot method with the first objective lens 5 and the second objective lens 6, and when performing tracking control using the operation push-pull method with the first and second objective lenses 5 and 6. is the focal length f ₂ of the focal length f ₁ and a second objective lens 6 of the first objective lens _{5, f 1 · arcsin (λ 1} / d) / TP 1 = f 2 · arcsin (λ 2 / d ) / TP ₂ ... It can be calculated by Equation 2.

However, in the above expression 3, the wavelength of the laser light emitted from the first laser diode 4A is λ ₁ and the wavelength of the laser light emitted from the second laser diode 4B is λ ₂ , and the diffraction grating Let the pitch be d.

When tracking control is performed by the first objective lens 5 using the three-spot method and tracking control is performed by the second objective lens 6 using the operation push-pull method, the first objective lens 5 is used. focal length f ₁ and the focal length f ₂ of the second objective lens 6 _{is, f 1 · arcsin (λ 1} / d) / TP 1 = f 2 · arcsin (λ 2 / d) / TP 2 · 2 ·· of -It can be calculated by Equation 3.

Therefore, the first objective lens 5 and the second objective lens 5
The objective lens 6 has the focal lengths f ₁ and f ₂ set to values proportional to the track pitch. Then, the focal length f ₂ of the focal length f ₁ and a second objective lens 6 of the first objective lens _{5, f 1 · arcsin (λ 1} / d) / TP 1 · K 1 = f 2 · arcsin (λ ₂ / d) / TP ₂ · K ₂ ... It can be calculated by Equation 4. That is, the first objective lens 5 and the second objective lens 6 are expressed by the general formula 4
The focal lengths f ₁ and f ₂ are set so that

However, in the above equation 4, K _{1 is} a coefficient of the first objective lens 5, K _{2 is} a coefficient of the second objective lens 6, and these K ₁ and K ₂ are in the case of using the three-spot method. 4 and 2 when the operation push-pull method is used.

As described above, the optical pickup device 1 of the embodiment performs tracking control on the compact disc 2 and the high recording density optical disc 3 having different track pitches by the three-spot method and / or the operation push-pull method. At this time, the first objective lens 5 and the second objective lens 6 have focal lengths f ₁ and f ₂ such that the above expression 4 is satisfied.
The optimum tracking error signals for the compact disc 2 and the high recording density optical disc 3 can be detected by setting to. Therefore, the optical pickup device 1 can satisfactorily reproduce information signals from the compact disc 2 and the high recording density optical disc 3 having different track pitches.

The optical pickup device according to the present invention is not limited to the optical pickup device 1 of the above-described embodiment, and for example, when three types of optical discs having different specifications are targeted, the objective lens is used. , Three photodetectors may be prepared together.

Further, the optical pickup device according to the present invention is suitable for being applied to a recording and / or reproducing device of an optical disc for reproducing and / or recording an information signal on an optical disc.

[0037]

As described above, according to the optical pickup device of the present invention, it is possible to detect the optimum tracking error signal according to at least two types of optical disks having different track pitches. Therefore, this optical pickup device can satisfactorily reproduce the information signal from at least two types of optical disks having different track pitches.

[Brief description of drawings]

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

FIG. 2 is a plan view showing the optical pickup device.

FIG. 3 is a diagram shown for explaining positions of a track and a beam spot of an auxiliary beam.

FIG. 4 is a diagram for explaining positions of a track and a beam spot of an auxiliary beam.

[Explanation of symbols]

 1 Optical Pickup Device 4 Light Source Section (Light Source Means) 4A First Laser Diode (First Light Source) 4B Second Laser Diode (Second Light Source) 5 First Objective Lens 6 Second Objective Lens 7 Biaxial Actuator (biaxial drive means) 8 Photodetection section (optical detection means) 8A First photodetector 8B Second photodetector 10 Diffraction grating

Claims (1)

[Claims] 1. When reproducing an information signal from at least two kinds of optical discs having different track pitches, 1 is provided to the left and right of the main beam on the track of the information signal layer of the optical disc.
/ 4 first arithmetic processing for obtaining a tracking error signal from the output balance of the auxiliary beam that connects the beam spots at a position shifted by / 4 tracks and / or to the main beam on the track of the information signal layer of the optical disc An optical pickup device that performs tracking control by a second calculation process that obtains a tracking error signal by using each push-pull signal of an auxiliary beam that connects beam spots at a position shifted by 1/2 track pitch, emits at least a light beam. Light source means having a first light source and a second light source; a diffraction grating for diffracting the light beam emitted from the light source means to divide it into three beams; and a light beam for each of at least two types of optical discs. A lens having at least a first objective lens and a second objective lens for focusing on the signal recording surface. Means, biaxial driving means for driving at least two objective lenses of the lens means in biaxial directions with respect to the optical disc, and reflected light from the at least two types of optical discs to receive an electric signal. And a light detecting means for converting the light into a first objective lens and a second objective lens, wherein f ₁ · arcsin (λ ₁ / d) / TP ₁ · K ₁ = f ₂ · arcsin (λ ₂ / d ) / TP ₂ · K ₂ is an optical pickup device characterized by having a focal length. However, in the above formula, f _{1 is} the focal length of the first objective lens, f _{2 is} the focal length of the second objective lens, λ _{1 is} the wavelength of the first light source, λ _{2 is} the wavelength of the second light source, d is the pitch of the diffraction grating, TP ₁ is the track pitch of the optical disc reproduced by the first objective lens, and TP _{2 is} the track pitch of the optical disc reproduced by the second objective lens. Further, K _{1 is} a coefficient of the first objective lens, and K _{2 is} a coefficient of the second objective lens, which is 4 when the first arithmetic processing is used and 2 when the second arithmetic processing is used. .