JP3208297B2 - Optical pickup - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an optical pickup for reproducing or reproducing and recording information recorded on an information recording medium such as an optical disk, an optical card or a magneto-optical disk.

[0002]

2. Description of the Related Art A three-beam method is known as a tracking control method. In the three-beam method, a light beam from a single light source is incident on a three-beam diffraction grating to generate a main beam and two sub-beams positioned so as to sandwich the main beam. This is a method of irradiating and guiding the reflected light to light receiving elements to obtain a tracking control signal.

[0003] Here, as shown in FIG.
The center of focus is centered, and the allowable range of the deviation width in the height direction at the position of the sub beam 52 caused by the inclination of the optical disk 53 is:
It is within the depth of focus of the sub-beam 52 (the column portion near the focal point). Therefore, as shown in FIG.
As shown in FIG. 6A, the optical disc 53 has a smaller distance between the main spot 51a and the sub spot 52a than a case where the distance between the sub spot 52a and the sub spot 52a is wider.
Can have a wider allowable range.

[0004] Further, as shown in FIG.
When a is located at the center of the track 54, the two sub-spots 52a, 52a need to be located on opposite sides of the track 54. When the track of the information recording medium is linear like an optical card, the interval between the sub-spots 52a, 52a is not particularly problematic, but in the case of the disk-shaped recording medium 53, the track 54 has a curvature. Therefore, if the distance between the sub-spots 52a, 52a is increased, it is difficult for the sub-spots 52a, 52a to be located on opposite sides of the track 54.

Under such circumstances, the distance between the main spot 51a and the sub-spots 52a, 52a is usually set to 20 μm or less.

Incidentally, for example, the distance from the surface of the transparent substrate constituting the disk to the recording layer is different between a compact disk and a digital disk, such as 1.2 mm for the former and 0.6 mm for the latter. On the other hand, there is a demand for compatibility for reproducing two types of discs having different standards with a single optical pickup.

A bifocal optical head has been proposed as a conventional compatible optical pickup of this type (1994).
September 2015, Japan Society of Applied Physics: 19p-S4: 19p
-S-5). In this bifocal optical head, a light beam (635 nm to 650 nm) from one light source is
nm) is separated into a 0th-order diffracted light and a 1st-order diffracted light, and two focal points are simultaneously generated. In particular,
As shown in FIG. 11, the grating lens 101 has a region where the light beam reflected by the half mirror 106 emitted from the light source 105 is transmitted as it is, and a region where the light beam is diffracted in a spreading direction. The light beam (0th-order diffracted light) transmitted through the 0.6 mm disk 102 as it is is used, and the light beam diffracted in the spreading direction (the first By using (folding light), the objective lens 104
The focusing positions of the light beams that have passed through are different. The light reflected by the optical disk is transmitted through the half mirror 106 and is incident on the light receiving element 107.

However, in the above-described conventional optical pickup, the light beam diffracted in the expanding direction is not used for the 0.6 mm disk 102, and the 1.2 mm disk 103 is not used as it is. Since the transmitted light beam is not used, the utilization efficiency of the light beam is low. Therefore, there is a disadvantage that a high-output light source (semiconductor laser) is required. Further, 635 nm
Since the semiconductor laser oscillating in the band has a short life, especially when the output is increased, it becomes shorter.
In the conventional optical pickup that uses the semiconductor laser even during reproduction of data, sufficient reliability cannot be ensured.

When the 1.2 mm disk 103 is a compact disk, for example, when the data is reproduced using a semiconductor laser oscillating in the 635 nm band, the compact disk has the smallest aberration with respect to the laser light having a wavelength of 780 nm. Because the wavelength is short, the refractive index of the transparent substrate changes by the short wavelength, spherical aberration is likely to occur, and the S / N of the RF signal (pit signal) and the reliability of the servo deteriorate. Also have.

Therefore, as shown in FIGS. 9A and 9B,
Semiconductor laser 11 oscillating in 635 nm band and 780 nm
By providing two light sources of a semiconductor laser 12 oscillating in a band and providing a grating lens 13 whose diffraction angle is different due to a difference in wavelength, a focusing position of a light beam required for a 0.6 mm optical disc 15 is provided. , And 1.
It is conceivable to configure so as to selectively form the focal point of the light beam required for the 2 mm optical disc 16.

[0011]

However, the difference in the diffraction angle due to the difference in wavelength is caused by the three-beam diffraction grating 1 which is responsible for the tracking control accuracy in the three-beam method described above.
7 also occurs. The three-beam diffraction grating 17 is shown in FIG.
As shown in FIG. 2, a transparent substrate is formed with concavities and convexities. When a light beam having a wavelength λ from a distance L is incident on a diffraction grating having a concavo-convex pitch Λ, zero-order diffracted light (transmitted light) In the relationship between the (virtual) light source and the virtual light source of the ± 1st-order diffracted light, the following equation is established in relation to the distance S (hereinafter, S ₁ for a short wavelength and S ₂ for a long wavelength). However,
For simplicity, the thickness of the substrate is not considered.

[0012]

Λ / Λ = sin θ θ = tan ⁻¹ (S / L)

Accordingly, when light beams (short wavelength λ ₁ , long wavelength λ ₂ ) whose wavelengths are different from each other from the distance L are incident on the three-beam diffraction grating 17, S ₁ <S with respect to the distances S ₁ and S _2. so that the relation of S ₂ is established.

As described above, the distance between the main spot 51a and the sub-spots 52a on the information recording medium is set to 20 μm or less. Also,
The magnification of the objective lens 14 is set to 1/6 to 1/5 in order to obtain a light focusing characteristic close to the diffraction limit. Therefore, the sub-spot interval is ± 20 μm and the magnification of the objective lens is 1/5.
If it is set to 5, the interval S of the virtual light sources needs to be ± 110 μm.

Assuming that the long wavelength λ _{2 is} 780 nm, the distance L from the virtual light source to the three-beam diffraction grating 17 is 5 mm, and the distance S ₂ between the virtual light sources is 110 μm, the three-beam diffraction grating 17 Is 35μ
m. Then, when a light beam having a short wavelength λ ₁ = 635 nm is applied to the three-beam diffraction grating 17 having Λ = 35 μm from the same distance L = 5 mm as described above,
Spacing S ₁ sub-beams of the virtual light source is a ± 91μm.

The distance between the spots on the light receiving elements 20 and 21 is
It is almost equal to the interval of the virtual light source. The spot size is almost the same regardless of the wavelength. Therefore, 780n
m light beam so as to have a spot interval of 20 μm on the optical disk, that is, about 110 μm on the light receiving element.
10B, the light spot of the 780 nm light beam is received on each light receiving portion of the light receiving element 21 to the fullest extent as shown in FIG. 10B. As shown, the light spot of the 635 nm light beam protrudes from the light receiving portions at both ends of the light receiving element 20 and is received. That is, when a light beam of 635 nm is used, there is a problem that the main spot and the sub spot cannot be separated, and tracking control with sufficient reliability cannot be performed.

Further, the optical pickup shown in FIG. 9 cannot meet the requirement that the inclination of the arrangement direction of the three spots with respect to the track be set to be unique for each type of optical disc.

In view of the above circumstances, the present invention uses two light beams having different wavelengths, and makes the three-spot intervals substantially the same for the light beams of each wavelength, or if the inclination of the arrangement direction of the three spots is different. It is an object of the present invention to provide an optical pickup capable of improving the reliability of tracking control by making it possible to perform tracking control.

[0019]

An optical pickup according to the present invention comprises a first light source for emitting a light beam of a first wavelength having a first polarization direction, and a second light source having a second polarization direction. A second light source that emits a light beam having a wavelength of ??? and a first light source that exhibits a three-beam diffraction function only for a light beam in a first polarization direction.
A polarization-dependent three-beam diffraction grating, a second polarization-dependent three-beam diffraction grating exhibiting a three-beam diffraction function only for a light beam in the second polarization direction, a first light source and a second Drive selection means for selectively driving any one of the light sources.

Thus, the first polarization-dependent three-beam diffraction grating is provided with a grating pattern corresponding to the light beam of the first wavelength, and the second polarization-dependent three-beam diffraction grating is provided. , A grating pattern corresponding to the light beam of the second wavelength can be provided, and the distance between the three spots by the light beam of each wavelength can be made the same, or the inclination of the arrangement direction of the three spots can be set independently. It is possible to do.

The two polarization-dependent three-beam diffraction gratings are arranged such that a linear portion having a refractive index different from the refractive index of the transparent substrate is formed on the transparent substrate only at a predetermined pitch for a light beam in a specific polarization direction. It can be obtained by forming with.
That is, since a light beam having a specific polarization direction has a refractive index in the polarization direction, the refractive index in the polarization direction has a linear portion different from the original refractive index of the transparent substrate. The transparent substrate can function as a diffraction grating for the light beam.

The type of the information recording medium to be used is known in advance by the user, and a predetermined operation is performed to selectively drive either the first light source or the second light source by the driving means. Or the drive selection means may be:
At first, when the information recording medium is loaded in the player provided with the optical pickup, both the first light source and the second light source are driven, and after the type of the loaded information recording medium can be determined, the type of the information recording medium is determined. It may be configured to selectively drive either the first light source or the second light source.

Further, if the first light source and the second light source are arranged on the same plane, wiring to each light source can be easily performed.

The above-described diffraction grating for polarization-dependent three beams can be easily manufactured by using the proton exchange method.

[0025]

Embodiments of the present invention will be described below with reference to the drawings.

FIGS. 1A and 1B are schematic sectional views of an optical pickup according to the present invention. FIG. 1A shows an optical pickup 15 in which the distance from the disk surface to a recording layer 15a is 0.6 mm. FIG. 3B shows a case where an optical disk 16 having a distance of 1.2 mm from the disk surface to the recording layer 16a is used.

The optical pickup comprises a first light source 11 and a second light source.
Are provided. First light source 11
Is a semiconductor laser that oscillates in the 635 nm band.
As the second light source 12, a semiconductor laser oscillating at 780 nm is used. In this embodiment, the semiconductor laser oscillating in the 635 nm band, which is the first light source 11, has a TM mode (for example, an A including an active layer having a quantum well structure having a tensile strain in at least the well layer).
1GaInP-based semiconductor laser).
The semiconductor laser that oscillates in the 780 nm band, which is the second light source 12, oscillates in the TE mode (for example, an AlGaAs-based semiconductor laser), so that the polarization directions of both light beams are different from each other. In addition, these two light sources 11, 12
Are selectively driven by drive selection means (not shown). Further, both light sources 11,
Numeral 12 is arranged on the same plane parallel to the optical axis of the light beam in this embodiment, so that wiring to each light source can be easily performed.

Here, the direction of the optical axis is defined as the x direction, and the direction perpendicular to the plane of the drawing is defined as the y direction.
The direction perpendicular to the direction is defined as the z direction. The first light source 11 of the TM mode is disposed so that the polarization direction of the light beam thereof coincides with the y direction, and the second light source 1 of the TE mode is provided.
2 is arranged such that the polarization direction of the light beam coincides with the z direction.

The light beams emitted from the light sources 11 and 12 are first and second polarization-dependent three beams that generate three beams for tracking control only for light beams having specific polarization directions different from each other. Diffraction gratings 171 and 172, hologram element 18 for generating astigmatic light for focus control, grating lens 13 having a grating pattern having a concave lens function, and objective lens 14 for condensing a light beam
And is condensed on the recording layers 15a and 16a of the optical disc 15 or the optical disc 16. And the optical disk 1
The reflected light reflected by the recording layers 5a and 16a of the optical disk 5 or the optical disk 16 traces the objective lens 14 and the lattice lens 13 in opposite directions, and reaches the hologram element 18. Then, the hologram element 18 causes 635n
m is diffracted and guided to the first light receiving element 20,
The 780 nm light beam is further diffracted and guided to the second light receiving element 21.

First polarization-dependent three-beam diffraction grating 171
Receives a light beam emitted from the first light source 11 or the second light source 12 and applies a three-beam light beam to the TM mode first light source 11 whose polarization direction is the y direction. T functions as a diffraction grating while the polarization direction is the z direction.
With respect to the light beam of the second light source 12 in the E mode, it functions simply as a transparent substrate.

On the other hand, the second polarization-dependent three-beam diffraction grating 172 receives the light beam emitted from the first light source 11 or the second light source 12, and the polarization direction is the TM direction.
For the light beam of the first light source 11 in the mode, it functions as a mere transparent substrate, while for the light beam of the second light source 12 in the TE mode in which the polarization direction is the z direction, 3
It functions as a beam diffraction grating.

Here, for example, as shown in FIG.
In the NbO ₃ crystal substrate, only the refractive index ne in the Z-axis direction is increased by the proton exchange with respect to the refractive index distribution before the proton exchange. The light follows the refractive index in the direction of polarization. Generally, when an anisotropic crystal is ion-exchanged, the refractive index changes only in one direction.

Therefore, the first polarization-dependent three-beam diffraction grating 171 that should function as a diffraction grating for a 635 nm TM-mode light beam (y-polarized light) is shown in FIG.
As shown in (b), the direction in which the refractive index increases (Z
) Coincide with the y direction, and proton exchange linear portions 171a long in the y direction are formed at a predetermined pitch.

Further, a second mode to function as a diffraction grating for a 780 nm TE-mode light beam (polarized light in the z-direction).
The polarization-dependent three-beam diffraction grating 172 of FIG.
As shown in (d), the direction in which the refractive index increases (Z
Are aligned with the z-direction, and the proton-exchange linear portions 172a ... long in the y-direction are formed at a predetermined pitch.

The method for forming the proton exchange linear portion will be described later. FIG. 4 shows an embodiment in which an X-cut substrate is used.

With the above configuration, when the first light source 11 is used, the first polarization-dependent three-beam diffraction grating 171 functions as a three-beam diffraction grating. The recording layer 15a of the optical disc 15 is irradiated with three beams based on the grating pattern of the beam diffraction grating 171. On the other hand, when the second light source 12 is used, the second polarization-dependent three-beam diffraction grating 172 functions as a three-beam diffraction grating, and the grating pattern of the second polarization-dependent three-beam diffraction grating 172 is used. Are formed on the recording layer 16a of the optical disk 16.

Accordingly, the grating pattern of the first polarization-dependent three-beam diffraction grating 171 is adjusted to a light beam of 635 nm, and the grating pattern of the second polarization-dependent three-beam diffraction grating 172 is adjusted to 780 nm. By setting the pattern according to the beam, as shown in FIG.
The interval between the three beam spots (shown by hatched circles) by the 635 nm light beam and the interval between the three beam spots (shown by white circles) by the 780 nm light beam can be made the same. it can.

As a result, as shown in FIG. 2A, a three-beam spot of a 635 nm light beam formed on the light receiving element 20 and, as shown in FIG. 780nm light beam 3
The beam spots are formed without protruding on the light receiving portions at both ends.

Further, the inclinations of the two beam spots on the optical disk can be independently set with light beams of respective wavelengths. In FIG. 5, the inclination of the arrangement direction of the three beam spots by the light beam of 780 nm is made larger by the angle φ than the inclination of the arrangement direction of the three beam spots by the light beam of 635 nm. This is because the light beam (635 nm) of the first light source 11 has a shorter wavelength than the light beam (780 nm) of the second light source 12 and is condensed with an effectively larger NA. This is because if the sub-spot of the 635 nm light beam is slightly in contact with the track center line, a further inclination of the angle φ is required. Note that in order to obtain the angle φ, the case where an X-cut substrate is used is shown in FIG.
As shown in (b), the proton exchange linear portions 172a may be formed to be inclined at an angle φ with respect to the Y-axis (X-axis in the case of a Y-cut substrate) direction.

Next, the polarization dependence 3 using the proton exchange method
A method of manufacturing the beam diffraction grating will be briefly described.
On an X-cut (or Y-cut) LiNbO ₃ crystal substrate,
After depositing a Ta (tantalum) film to a thickness of 300 to 1000 °, a photoresist is spin-coated on the Ta film. Next, the photoresist is patterned by a general photolithography technique to form an opening corresponding to the proton exchange linear portion, and the Ta film in that portion is exposed. Then, the exposed portion of the Ta film is removed by dry etching using a fluorine-based gas, and the L portion of the portion is removed.
The surface of the iNbO ₃ crystal substrate is exposed. After that, the photoresist is removed. Next, a pyrophosphoric acid film is applied on the Ta film, and LiN exposed at the opening of the Ta film.
Proton exchange is performed on the bO ₃ crystal substrate. Thereafter, the Ta film is removed with a hydrofluoric acid-based aqueous solution.

In this embodiment, the LiNbO ₃ crystal substrate is used as the polarization-dependent three-beam diffraction grating. However, the present invention is not limited to this. For example, a LiTaO ₃ crystal plate can be used. Further, an ion exchange method can also be used. For proton exchange, benzoic acid or the like can be used in addition to pyrophosphoric acid.

Further, in the optical system shown in FIG. 1, the optical axis is aligned, but, for example, between the light sources 11 and 12 and the second three-beam diffraction grating 172 or the first three-beam diffraction grating. A reflection mirror may be provided between the grating 171 and the hologram element 18 so that the optical system deflects the optical axis of the light source in different directions. In particular, when the deflection angle is approximately 90 °, the semiconductor laser chip as the light source and the light receiving element can be arranged on a parallel surface, and wire bonding for wiring becomes easy.

[0043]

As described above, according to the present invention, by providing two light sources and independently driving them, the life of the optical pickup can be improved and highly reliable servo control corresponding to each recording medium can be performed. There is an effect that the reliability of tracking control can be improved by eliminating a shift in the interval between the three beam spots due to a difference in the wavelength of the light source or changing the inclination of the arrangement direction of the three spots.

[Brief description of the drawings]

FIGS. 1A and 1B are schematic cross-sectional views of an optical pickup according to an embodiment of the present invention. FIG. 1A shows a case where a 0.6 mm optical disk is used, and FIG. 1B shows a case where a 1.2 mm optical disk is used. The cases where they are used are shown.

FIG. 2 is an explanatory diagram showing spots formed on a light receiving element of the optical pickup according to the embodiment of the present invention;
FIG. 7A shows the case of a 635 nm light beam, and FIG. 7B shows the case of a 780 nm light beam.

FIG. 3 is a schematic diagram showing a refractive index distribution of a LiNbO ₃ crystal according to an embodiment of the present invention before and after proton exchange.

FIG. 4 is a diagram showing a pattern of a polarization-dependent three-beam diffraction grating according to the embodiment of the present invention, wherein FIG.
(B) is a plan view and a sectional view in the case of a 635 nm light beam, and (c) and (d) are a plan view and a sectional view in the case of a 780 nm light beam.

FIG. 5 is an explanatory diagram showing a spot arrangement on an optical disk when the polarization-dependent three-beam diffraction grating according to the embodiment of the present invention is used.

6A and 6B are explanatory diagrams showing the relationship between the three-beam spot interval and the allowable tilt of the optical disk, wherein FIG. 6A shows a case where the spot interval is narrow, and FIG. 6B shows a case where the spot interval is wide. Is shown.

FIG. 7 is an explanatory diagram showing a relationship between a track on an optical disc and a three-beam spot.

8A and 8B are explanatory diagrams showing a difference in virtual light source formation position due to a difference in wavelength of a light beam incident on a diffraction grating. FIG. 8A shows a case where the wavelength is short, and FIG. Each shows a long case.

FIG. 9 is a schematic cross-sectional view of an optical pickup for performing reproduction or reproduction and recording of two types of optical disks by utilizing the difference in wavelength between two light sources. FIG. 9A uses a 0.6 mm optical disk. FIG. 3B shows a case where a 1.2 mm optical disk is used.

10A and 10B are diagrams showing spot arrangements on a light receiving element formed by the optical pickup of FIG. 9; FIG. 10A shows a case where the wavelength is short, and FIG. Is shown.

FIG. 11 is a schematic cross-sectional view showing a conventional optical pickup for performing reproduction or reproduction and recording of two types of optical disks using one light source.

[Explanation of symbols]

 Reference Signs List 11 first light source 12 second light source 13 grating lens 14 objective lens 15 optical disk 16 optical disk 20 light receiving element 21 light receiving element 171 first 3-beam diffraction grating 172 second 3-beam diffraction grating

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Akira Ibaraki 2-5-5 Keihanhondori, Moriguchi-shi, Osaka Sanyo Electric Co., Ltd. (72) Keiichi Yoshinori 2-5-1 Keihanhondori, Moriguchi-shi, Osaka No. 5 Sanyo Electric Co., Ltd. (56) References JP-A-9-73654 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G11B 7/135 G02B 5/18 G02B 5 / 30-5/32

Claims (4)

(57) [Claims] 1. A first light source for emitting a light beam of a first wavelength having a first polarization direction, and a second light source for emitting a light beam of a second wavelength having a second polarization direction. Light source and the first
A first polarization-dependent three-beam diffraction grating that exhibits a three-beam diffraction function only for a light beam in the polarization direction, and a third diffraction grating that exhibits a three-beam diffraction function only for a light beam in the second polarization direction. An optical pickup comprising: a second polarization-dependent three-beam diffraction grating; and a drive selection unit for selectively driving either the first light source or the second light source. 2. The diffraction grating for polarization-dependent three-beams, wherein a linear portion having a refractive index different from the refractive index of the transparent substrate is formed at a predetermined pitch on the transparent substrate only for a light beam in a specific polarization direction. The optical pickup according to claim 1, wherein the optical pickup is formed. 3. The light source according to claim 1, wherein the first light source and the second light source are arranged on the same plane.
An optical pickup according to claim 1. 4. The optical pickup according to claim 1, wherein the polarization-dependent three-beam diffraction grating is manufactured by proton exchange.