JP2004288373A - Optical pickup device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical pickup device in which good tracking error signals are obtained for the optical disks having different track pitches. <P>SOLUTION: The device is provided with a first light source 4A and a second light source 4B having respectively different wavelengths, a diffraction grating 10 which diffracts the light beams emitted from the first and the second light sources and divides the light beams into three light beams and a first objective lens 5 and a second objective lens 6 which are used to focus the light beams emitted from the light sources having respectively different wavelengths onto the respective signal recording surfaces of a first or a second optical disk. Moreover, the optical pickup device is provided with a two-axis driving means 7 which drives the lenses 5 and 6 in two-axis directions with respect to the first and the second optical disks and a first optical detection section and a second optical detection section 8 which receive reflected light beams from the first and the second optical disks and convert the beams into electric signals. The lenses 5 and 6 have focal distances satisfying the following equation, i.e., f<SB>1</SB>×arcsin (λ<SB>1</SB>/d)/TP<SB>1</SB>×K<SB>1</SB>=f<SB>2</SB>×arcsin (λ<SB>2</SB>/d)/TP<SB>2</SB>×K<SB>2</SB>. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an optical pickup device for reproducing information signals from optical disks having different track pitches.

  When reproducing an information signal from an optical disc, the 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. Examples of the tracking control method include a three-spot (three-beam) method using three beams of a main beam for reading an information signal and two auxiliary beams, a differential push pull (DPP) method, a so-called differential push-pull method, and the like.

  The three-spot method is a method of obtaining a tracking error signal from the output balance of an auxiliary beam that connects a beam spot to the left and right of a main beam on a track of an information signal layer. In the three-spot method, when the beam spot is connected to a position shifted by a 1/4 track pitch in the left and right track directions of the main beam, the amplitude of the tracking error signal increases. It is desirable to set the auxiliary beam as described above.

  The differential push-pull method is a method of obtaining a tracking error signal using each push-pull signal of a main beam on a track of an information signal layer and auxiliary beams connecting beam spots to the left and right of the main beam. In the differential push-pull method, the amplitude of the tracking error signal increases when the beam spot is connected to a position shifted by 1/2 track pitch in the left and right track directions of the main beam. It is desirable to set the auxiliary beams so as to connect them.

JP-A-3-242833

  When reproducing an information signal from an optical disc, the optical pickup device increases the jitter and error rate even with a slight diffraction-limited aberration. Therefore, it is most important to manage the aberration.

  In optical disks, it is necessary to reduce the beam spot diameter by increasing the numerical aperture NA of the objective lens or shortening the wavelength λ of the light beam emitted from the light source in order to achieve a higher recording density. Is done.

  However, the coma caused by the tilt of the optical disk, so-called skew, is in proportion 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. This is the main factor that hinders an increase in the numerical aperture NA. 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.

  Also, coma is proportional to the thickness of the substrate of the optical disk. Therefore, coma can be reduced by reducing the thickness of the substrate. However, since the objective lens causes spherical aberration in proportion to the substrate thickness dimension, the objective lens is set so as to cancel the spherical aberration only for an optical disc having a predetermined substrate thickness dimension, and the substrate thickness dimension differs. There has been a problem that a spherical aberration is generated with respect to an optical disc in accordance with a difference in a substrate thickness dimension.

  Therefore, high recording density optical discs having both a substrate thickness dimension and a track pitch of about に 対 し て of a compact disc have been proposed. However, a compatible optical disc capable of reproducing optical discs having different specifications from each other has been proposed. It is considered difficult to realize a disk reproducing device.

  By the way, when an information signal is reproduced from a compact disk, when the tracking control of the objective lens is performed using, for example, a three-spot method, the optical pickup device sets the track pitch to TP, and sets the beam spot of the main beam and the beam spot of the auxiliary beam. , The angle formed by the auxiliary beams 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 calculated by λ = dsinθ.

  In the case of performing tracking control using the three-spot method as described above, as shown in FIG. 3, the beam spot of the auxiliary beam is formed at a position shifted by 1/4 TP with respect to the track direction. A tracking error signal can be obtained.

  However, when an information signal is reproduced from a high recording density optical disk having a track pitch of 1/2, as shown in FIG. 4, the position of the beam spot of the auxiliary beam is a position of 1/2 of the track pitch. Therefore, there is a problem that the tracking error signal using the three-spot method becomes zero and tracking control cannot be performed.

  Therefore, when reproducing two types of optical discs having different track pitches, the optical pickup device sets the tracking error signal from one of the optical discs so as to obtain a good tracking error signal from the other optical disc. There is a problem that it becomes impossible to obtain.

  Accordingly, it is an object of the present invention to provide an optical pickup device that can obtain an optimum tracking error signal according to an optical disc when the optical discs have different track pitches.

In order to achieve the above object, an optical pickup device according to the present invention provides a first optical disk and a second optical disk having different track pitches to irradiate light beams having different wavelengths to be irradiated. A first light source and a second light source that emit light, a diffraction grating that diffracts a light beam emitted from the first light source and the second light source to split the light beam into three beams, and a first light source or a second light source. A first objective lens and a second objective lens for focusing light beams having different wavelengths emitted from the light source on respective signal recording surfaces of the first optical disk or the second optical disk; Driving means for driving the objective lens and the second objective lens in two axial directions with respect to the first optical disk and the second optical disk, and reflection from the first optical disk and the second optical disk First a light detector and the second light detector, the first objective lens and a second objective lens for converting into an electric signal by receiving the
f ₁ · arcsin (λ ₁ / d) / TP ₁ · K ₁
= F ₂ · arcsin (λ ₂ / d) / TP ₂ · K ₂
Is satisfied.

Where f ₁ : focal length of the first objective lens, f ₂ : focal length of the second objective lens, λ ₁ : wavelength of the first light source, λ ₂ : wavelength of the second light source, d : Pitch of the diffraction grating, TP ₁ : track pitch of the optical disc reproduced by the first objective lens, TP ₂ : 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.

  The first optical disk and the second optical disk have different substrate thicknesses.

In the optical pickup device configured as described above, each of the focal lengths of the first and second objective lenses is f ₁ · arcsin (λ ₁ / d) / TP ₁ · K ₁ = f ₂ · arcsin (λ ₂ / d) By setting / TP ₂ · K ₂ to be satisfied, the beam spot of the auxiliary beam is connected to 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 favorably obtained according to at least two types of optical disks having different track pitches.

  Hereinafter, a specific embodiment of the present invention will be described with reference to the drawings.

  First, an optical pickup device 1 to which the present invention is applied reads information signals from a compact disk 2 and a high-density optical disk 3 whose track pitches are 1.6 μm and 0.74 μm and whose information signal layers are in the same reading direction. This is a so-called compatible optical pickup device for reproducing.

  As shown in FIG. 1, an optical pickup device 1 to which the present invention is applied includes a light source unit 4 having a first laser diode 4A and a second laser diode 4B, which are one type of semiconductor laser that emits laser light, and a laser. A first objective lens 5 and a second objective lens 6 for focusing light on respective signal recording surfaces of the compact disc 2 and the high recording density optical disc 3, and the first objective lens 5 and the second objective lens 6; A biaxial actuator 7 for driving the compact disc 2 and the high recording density optical disc 3 in two directions, and a photodiode for receiving reflected laser light from the compact disc 2 and the high recording density optical disc 3 and converting the reflected laser light into an electric signal And a photodetector 8 including a first photodetector 8A and a second photodetector 8B.

  The diffused laser light emitted from the laser diode 4 has its luminous flux width restricted by the collimator lens 9, is converted into parallel light, and is 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. The light is incident on the second objective lens 6. The first objective lens 5 converges the laser light on the signal recording surface of the compact disc 2. The second objective lens 6 converges the laser beam 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 disk 2 is condensed on the photodetector 8 located on the image forming point via the first objective lens 5, the beam splitter 11, the detection lens 12, and the cylindrical lens 13. Is done. The reflected laser light from the signal recording surface of the high-density optical disc 3 passes through a second objective lens 6, a beam splitter 11, a detection lens 12, and a cylindrical lens 13 to a photodetector 8 located on an image forming point. Focused on top.

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

  When reproducing an information signal from the compact disk 2, the optical pickup device 1 obtains a tracking error signal by the first objective lens 5 using the three-spot method and reproduces the information signal from the high-density optical disk 3. In this case, the tracking error signal is obtained by the second objective lens 6 using the differential push-pull method.

The optimum displacement of the auxiliary beam with respect to the track is 1/2 track pitch when three spots are used, and 1/4 track pitch when the differential push-pull method is used. For this reason, the track pitch of the optical disc reproduced by the first objective lens 5 is TP ₁ , the track pitch of the optical disc reproduced by the second objective lens 6 is TP _2, and the focal length of the first objective lens 5 is Assuming that f ₁ and the focal length of the second objective lens 6 are f ₂ , the first objective lens 5 and the second objective lens 6
_{f 2 / TP 2 = f 1} / TP 1 · 2 ··· Formula 1
Are set to the focal lengths f ₁ and f ₂ , respectively, so that the first objective lens 5 and the second objective lens 6 satisfactorily track the compact disc 2 and the high recording density optical disc 3. Controlled.

The track pitch TP ₁ of the compact disc 2 is 0.74, the track pitch of a high recording density optical disk 3 is 1.6. Therefore, by setting f ₁ = 3, f ₂ = 3.24 is calculated from Expression 1.

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

Next, a case where a plurality of light sources have different wavelengths will be described. Tracking control using a three-spot method using the first objective lens 5 and the second objective lens 6 and tracking control using a differential push-pull method using the first and second objective lenses 5 and 6 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 Formula 2
Can be calculated by

However, in Equation 3, the wavelength of the laser light emitted from the first laser diode 4A is λ ₁ , the wavelength of the laser light emitted from the second laser diode 4B is λ _2, and the pitch of the diffraction grating is d. And

When the tracking is controlled by the first objective lens 5 using the three-spot method and the tracking control is performed by the second objective lens 6 by the differential push-pull method, the focus of the first objective lens 5 is adjusted. The distance f ₁ and the focal length f _{2 of} the second objective lens 6 are:
_{f 1 · arcsin (λ 1 /} d) / TP 1 = f 2 · arcsin (λ 2 / d) / TP 2 · 2 ··· Formula 3
Can be calculated by

Therefore, in the first objective lens 5 and the second objective lens 6, the focal lengths f ₁ and f ₂ are 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 Equation 4
Can be calculated by That is, the first objective lens 5 and the second objective lens 6 are set to focal lengths f ₁ and f ₂ , respectively, such that the general formula 4 is satisfied.

However, in the above equation 4, K ₁ : a coefficient of the first objective lens 5, and K ₂ : a coefficient of the second objective lens 6, these K ₁ and K ₂ are 4 when a three-spot method is used. Set to 2 when using the dynamic push-pull method.

As described above, the optical pickup apparatus 1 to which the present invention is applied 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 differential push-pull method. , The first objective lens 5 and the second objective lens 6 are set to the focal lengths f ₁ and f ₂ such that the above equation 4 is satisfied. Tracking error signals can be detected. Therefore, the optical pickup device 1 can reproduce information signals from the compact disc 2 and the high-density optical disc 3 having different track pitches, respectively.

  Note that the optical pickup device to which the present invention is applied is not limited to the optical pickup device 1 of the above-described embodiment. What is necessary is just to prepare three detectors together.

  Further, the optical pickup device to which the present invention is applied is suitably applied to a recording and / or reproducing apparatus for an optical disk that reproduces and / or records an information signal on an optical disk.

  According to the optical pickup device to which the present invention is applied, it is possible to detect an 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 information signals from at least two types of optical disks having different track pitches.

1 is a schematic configuration diagram illustrating an optical pickup device to which the present invention has been applied. It is a top view showing the optical pickup device to which the present invention is applied. FIG. 3 is a diagram for explaining positions of a track and a beam spot of an auxiliary beam. FIG. 3 is a diagram for explaining positions of a track and a beam spot of an auxiliary beam.

Explanation of reference numerals

Reference Signs List 1 optical pickup device, 4 light source unit (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 driving means) 8 Photodetector (optical detecting means) 8A First photodetector 8B Second photodetector 10 Diffraction grating

Claims (2)

A first light source and a second light source that emit light beams having different wavelengths to be irradiated on the first optical disk and the second optical disk having different track pitches;
A diffraction grating that diffracts a light beam emitted from the first light source and the second light source to split the light beam into three beams;
A first light beam, which is emitted from the first light source or the second light source and has a different wavelength, is focused on a signal recording surface of each of the first optical disk and the second optical disk; An objective lens and a second objective lens;
Biaxial driving means for driving the first objective lens and the second objective lens in biaxial directions with respect to the first optical disc and the second optical disc;
A first light detection unit and a second light detection unit that receive reflected light from the first optical disk and the second optical disk and convert the light 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 ₂
An optical pickup device having a focal length such that the following holds.
Where f ₁ : focal length of the first objective lens, f ₂ : focal length of the second objective lens, λ ₁ : wavelength of the first light source, λ ₂ : wavelength of 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, and TP 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.   2. The optical pickup device according to claim 1, wherein the first optical disk and the second optical disk have different substrate thicknesses.

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