JP2000293883A - Optical pickup - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the degradation of the wave aberration of a sub-beam caused by the track deviation of an objective lens by making the optical grating of an optical pickup for converging light flux to the recording plane of a corresponding recording medium and correcting the wave aberration caused by a substrate thickness difference by adjusting a distance between physical images into hologram element having the phase grating of curves. SOLUTION: This device has light sources LD1 and LD2 respectively having light emission wavelengths λ1 and λ2 for recording, reproducing or erasing information to plural recording media 9a and 9 at least. Besides, the device has an optical grating 1A for converging the light flux to the recording planes of the recording media 9a and 9, diffracting an objective lens 7A of optical characteristics optimized for the wavelength λ1 and the light flux of the wavelength λ2 to the recording medium 9a and tracking a light spot on the recording plane to a prescribed position together with 0th-order light flux. The optical element 1A for correcting the spherical aberration caused by the substrate thickness difference is made into hologram element having the phase grating of curves.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to an optical pickup device.

[0002]

2. Description of the Related Art In addition to a conventionally known CD, a DV for recording / reproducing with a laser beam having a shorter wavelength (650 nm).
D (digital video disc) has been put to practical use. Since the recording / reproducing density can be increased as the wavelength of the laser beam becomes shorter, the realization of an optical disk capable of recording / reproducing at a shorter wavelength is intended. For this reason, it is expected that various types of optical disks used with laser beams of different wavelengths will coexist in the future, and an optical pickup that can support these types of optical disks is intended. It is desirable that such an optical pickup capable of supporting various optical discs be capable of recording and / or reproducing various kinds of optical discs, or erasing them, and be compact. For example, if an objective lens for forming an optical spot can be shared between the DVD and the CD, the optical pickup can be made compact and low in cost. In this case, as a method of forming a good spot on the recording surface of each recording medium, an objective lens is optimally designed for a parallel incident light beam (object distance infinity) for a DVD, and a light spot is formed on the recording surface of a CD. When forming, a method is known in which a divergent light beam (finite object distance) is incident on an objective lens to remove "spherical aberration caused by a difference in thickness between a CD substrate and a DVD substrate" when using a CD. In an optical pickup device, tracking control is performed so that a light spot is always positioned on a track. As such a tracking control method, a ± 1st-order light beam is generated as a sub-beam in addition to a 0th-order light beam by a diffraction grating. A "three-beam method" and an "operating push-pull method" for obtaining a track error signal using the three beams generated in this way are known. When the tracking method for generating such a sub-beam is applied to the optical pickup device of the above-described “shared objective lens for a plurality of light beams”, when the objective lens is “track shifted”, the zero-order light beam (main (Beam) without any problem, the wavefront aberration of the sub-beam may be greatly deteriorated, making it difficult to obtain an appropriate track error signal.

An example will be described. FIG. 4 shows a "finite optical pickup device" conventionally known for a CD. The divergent luminous flux from the light source LD0, which is a “semiconductor laser”, is transmitted through the diffraction grating 1 and becomes ± 0
It is split into three beams of the next light beam (sub beam). Each light beam is reflected by the semi-transparent mirror 3, deflected by the deflecting prism 5, enters the objective lens 7, is converted into a convergent light beam by the action of the objective lens 7, passes through the substrate of the recording medium 9, and falls on the recording surface 9 A. Three light spots are formed. Each light beam reflected by the recording surface 9A becomes a "return light beam", is reflected by the deflecting prism 5, passes through the semi-transparent mirror 3, and is received by the light receiving means 8
It is led to. The light receiving means 8 receives the three return light beams, generates a focus error signal and a track error signal, and also generates a reproduction signal when performing reproduction. The focus error signal and the track error signal are input to "control means" (not shown), and are provided for focusing control and tracking control. The generation of the track error signal involves the return light beam of the sub beam.

More specifically, the objective lens 7
Has its both surfaces known aspheric ^{expression: x = (y 2 / R} ) / [1 + √ {1- (1 + K) (y / R) 2}] + A 4 · y 4 + A 6
_{^{· Y 6 + A 8 · y}} 8 + A is represented by ^₁₀ · y ₁₀ + ... is a "non-spherical". In the above equation, x is the coordinate in the optical axis direction, y is the coordinate in the optical axis orthogonal direction, R is the radius of curvature on the optical axis, K, A ₄ , A ₆ , A ₈ ,
A ₁₀ ,. . Is a constant (referred to as “surface coefficient” together with R). These R, K, A ₄ , A ₆ , A ₈ , A ₁₀ ,. . Indicates the first surface (side of the light source) and the second surface (side of the recording medium)
It is given as follows. Table 1 Surface coefficient First surface Second surface R 2.22949 -41.05138 K -0.753317 0.0 A ₄ 0.501200E-02 0.172860E-01 A ₆ 0.494956E-03 -0.454549E-02 A ₈ 0.576542E-04 0.422958E-02 A ₁₀ 0.591456E-04 -0.803649E-03 The center thickness is 2.0 mm, and the refractive index is n = 1.7188 (785 nm).
In the above data, for example, “E-03” means 10 to ³ , and this numerical value depends on the numerical value immediately before it. Hereinafter, the same applies. This objective lens has focal length: f = 3mm, object distance:
It is designed as 50mm, NA = 0.5, substrate thickness: 1.2mm. The diffraction grating 1 that generates three beams is a “linear grating”,
Lattice spacing: 24 (μm). In this optical system, on the recording surface 9A, the zero-order light beam (main beam) and the ± first-order light beams (sub-beams) form light spots separated from each other by approximately 20 μm. At this time, the “wavefront aberration of the light spot” on and off the axis of the objective lens (at the time of track deviation) is as follows. Table 2 Wavefront aberration (λ) On the axis of the objective lens Axis deviation: 0.4 mm Axis deviation: −0.4 mm 0th-order light beam 0.000 0.005 0.005 + 1st-order light beam 0.004 0.007 0.007 -1st-order light beam 0.004 0.007 0.007 As is clear from this, the objective lens The main beam and the sub-beam are not largely degraded by the aberration with respect to the track deviation, and are condensed on the recording surface 9A in a state where the aberration is sufficiently small. Therefore, for a CD as a recording medium,
Reproduction can be performed satisfactorily.

Here, consider a case where an "objective lens optimally designed for DVD" is used instead of the above-mentioned objective lens 7 in the optical pickup device as shown in FIG. The objective lens used for DVD with a substrate thickness of 0.6 mm is
The incident light beam is set to parallel light, and is designed with NA = 0.6. Specifically, both the first and second surfaces are aspherical surfaces represented by the above-mentioned “aspherical expression”, and specific lens data are as follows. Table 3 Surface coefficient First surface Second surface R 2.11952 45.58887 K 0.08986 0.0 A ₄ -0.344961E-02 0.121533E-01 A ₆ -0.459445E-03 0.373814E-03 A ₈ -0.590344E-04 -0.107044E-02 A ₁₀ -0.232290E-04 0.612050E-03 The center thickness is 2.0 mm, and the refractive index is n = 1.7269 (635 nm).
When “this objective lens for DVD” is used instead of the objective lens 7 in FIG.
A semiconductor laser of 5 nm is used separately from the light source LD0, and the light beam from the semiconductor laser is "parallelized by a coupling lens not shown in FIG. 4" so as to be incident on the DVD objective lens ( For example, by switching the direction of the deflecting prism 5, the incident light beam to the objective lens may be switched between a short wavelength and a long wavelength. FIG. 5B shows that the parallel light beam incident on the DVD objective lens 7A passes through the substrate of the DVD 9a and is recorded on the recording surface 9a.
The state where light is converged on 0 is shown. When the above-described “DVD objective lens 7A” is used as the objective lens 7 in FIG. 4, “the substrate thickness: 1.2 mm C” as the recording medium 9 is used.
In order to converge the light beam from the light source LD0 on the recording surface 9A of “D” by the objective lens 7A with good NA = 0.5, FIG.
As shown in (a), a good spot having an on-axis wavefront aberration of 0.005λ can be obtained by using a “finite system with an object distance of about 52 mm”. When the light beam from the light source LD0 is separated into a main beam (zero-order light beam) and two sub-beams (± first-order light beams) by a diffraction grating, on-axis and off-axis wavefront aberrations for the main beam and each sub-beam are as follows. It looks like this. Table 4 Wavefront aberration (λ) On axis of objective lens Axis shift: 0.4 mm Axis shift: -0.4 mm 0th-order light flux 0.005 0.053 0.053 + 1st-order light flux 0.041 0.068 0.068 -1st-order light flux 0.041 0.068 0.068 The pitch of the diffraction grating 1 is 24 μm. , The distance between the main beam and the sub beam on the recording surface 9A is about 20μ.
m. As described above, when the track of the objective lens 7A is displaced, the sub-beam undergoes wavefront degradation, and the difference from the axial wavefront aberration is remarkable, making it difficult to obtain a good track error signal.

[0006]

SUMMARY OF THE INVENTION According to the present invention, recording and the like are performed on a plurality of recording media having different substrate thicknesses by light beams from light sources having different emission wavelengths, and an objective lens is used in common for light beams of each wavelength. In an optical pickup device that performs tracking control using a sub-beam, it is an object to effectively reduce the deterioration of the wavefront aberration of the sub-beam due to the track deviation of the objective lens, and to always perform good tracking control. .

[0007]

According to the present invention, there is provided an optical pickup device having a substrate thickness: t1, t2, t3,. . (T1 <t
2 <t3,. . ) Each having a plurality of recording media D)
1, D2, D3. . . An optical pickup device that performs one or more of recording, reproducing, and erasing information, that is, recording or reproducing or erasing, or recording and reproducing, recording and erasing, reproducing and erasing, or recording, reproducing, and erasing; It has a plurality of light sources, a single objective lens, and one or more optical gratings (claim 1). Assuming that a plurality of types of recording media Di for which one or more of recording, reproduction, and erasing are performed are N types (i = 1 to N), these have different substrate thicknesses ti (i = 1 to N). As described above, the thickness of the substrate is ti <ti + 1, and tN is the thickest. The “plurality of light sources” is the recording medium Di.
One is provided corresponding to each of. These N light sources are referred to as LD1,. . LDi,. . If LDN is used, the light emission wavelengths: λi of these light sources are different from each other. A semiconductor laser can be suitably used as the light source LDi. The “single objective lens” is used in common for N types of light beams having the wavelength: λi, and condenses any of these N types of light beams on the recording surface of the recording medium Di corresponding to the wavelength. . This objective lens
At a wavelength: λ1 (a light beam used for the recording medium D1 having the minimum substrate thickness: t1), the optical characteristics of the recording medium D1 are optimized. Of course, if the objective lens can be used, the wavelength: λi can be used for other recording media Di (i ≠ 1).
May be optimized. The “one or more optical gratings” are control signals for diffracting a light beam having a wavelength of λi (i = 2, 3,...) And tracking a light spot on a recording surface together with a zero-order light beam to a predetermined position. ± 1st order luminous flux is generated as a sub beam for use. That is, the optical grating Hi
(I = 2, 3,...) Is used for each of the light sources LD2 to LDN. Of course, if desired, the light source L
An optical grating H1 for dividing the light beam from D1 into a main beam and a sub beam can also be used. That is, wavelength:
Tracking control using a sub-beam is performed for the light beams of λ2 to λN, but for the light beam of wavelength λ1,
Tracking control using a sub-beam may be performed, or single-beam tracking control such as a normal push-pull method may be performed. As described above, since the substrate thickness of the recording medium is different for each recording medium, the light flux having the wavelength: λi (i = 2, 3,...) Is used for the corresponding recording medium Di (i = 2, 3,. When the light is focused on the recording surface of (1), the form of the light beam incident on the objective
If the same light flux is used, spherical aberration occurs due to a difference in substrate thickness (substrate thickness: difference from t1). Therefore, in order to correct the spherical aberration caused by such a difference in substrate thickness, the luminous flux having the wavelength: λi (i = 2 to N) is corrected by “adjustment of the distance between object images”. The invention according to claim 1 is characterized in that an optical grating Hi (i = 2, 3,...) For a light beam having a wavelength: λi (i = 2, 3,...) Has a curved phase grating. Hologram element ".

In the optical pickup device according to the first aspect, a light beam having a wavelength of λi (i = 2, 3,...) Is applied to a recording surface of a corresponding recording medium Di (i = 2, 3,...). When condensing, one or more "coupling lenses for coupling at least one light beam other than the light source LD1" can be provided as means for correcting spherical aberration caused by a difference in substrate thickness by adjusting the distance between object images (claim). Item 2). Of course, a coupling lens for coupling the light beam of the light source LD1 can also be used. As the coupling lens, a lens having "a wavefront aberration proportional to the angle of view as a performance of a single unit" can be used (claim 3). 4. The optical pickup device according to claim 1, wherein the light source receives a return light beam reflected by a recording surface of a recording medium irradiated with a light spot having an emission wavelength of λi and a light spot having a wavelength of λi. The unit PDi and the detection hologram element HOEEi for diffracting the returning light beam Ri toward the corresponding light receiving unit PDi are unitized to form a light source / light receiving unit Ui, and N (U1 to UN) such units are used. (Claim 4).
In this case, the detection hologram element HOEE1 and / or the detection hologram element HOEEi (i =
2, 3,. . ) Is a “polarization hologram” and the optical grating H
i (i = 2, 3,...) is also a polarization hologram, so that each detection hologram element HOEEi and the optical grating Hi have a “diffraction effect on polarized lights having different vibration directions”. 5). The above-mentioned claim 4 or 5
In the optical pickup device described above, the optical grating Hi and the hologram HOEEi (i = 2, 3,...) Can be formed on the same substrate. Claim 1
6. In the optical pickup device as described in any one of Nos. 6, there are N types of recording media Di, and N can be an integer of 3 or more, but N = 2, that is, recording is performed. The medium can be two types of recording media D1 and D2 (claim 7). In this case, the recording medium D1 can have a substrate thickness: t1 = 0.6 mm (for example, CD), and the other recording medium D2 can have a substrate thickness: t2 = 1.2 mm (for example, DVD) (claim). Item 8).

Here, the above-mentioned "hologram element having a curved phase grating" will be described. Hologram element (Ho
lographic Optical Elements) is a type of optical grating formed by the interference of the wavefronts (spherical or aspherical wavefronts) of two coherent light beams, and records the phase difference between the two wavefronts at the time of manufacturing. The phase of light is represented by the following equation.

Φi = φc ± (φo−φr) Φi: phase of incident light Φc: phase of diffracted light Φo: phase of object light at the time of manufacturing Φr: phase of reference light at the time of manufacturing “(φo−φr)” "Corresponds to the" wavefront phase difference "recorded on the hologram element surface. This is set as φ (x, y) = (φo−φr), and multiplied by the reciprocal of the wave number of the production wave (2π / λc: λc is the wavelength of the production wave) to obtain (λc / 2π) φ (x, y) = Φ2pt + φpoly φ2pt: phase difference due to exposure to spherical waves from two point light sources φpoly: phase difference added by exposure to aspherical wavefronts, and “φpoly” is calculated using a constant c _ij using φpoly.
= Σc _ij x ** i * y ** j (x ** i is "i of x
To the power of y ** j can be expressed as "y to the power of j").
By giving the constant coefficient: c _ij , the phase difference: φpoly can be specified. Since the phase difference: φ2pt is uniquely determined according to the wavelength, the “curve phase grating” of the hologram element can be specified by giving a constant coefficient: c _ij .

[0011]

FIG. 1 is a diagram for explaining an embodiment of an optical pickup device according to the present invention. In order to avoid complication, the same reference numerals in FIGS. 4 and 5 are used for those which are considered to be free from confusion. The recording media indicated by reference numerals 9 and 9a have different substrate thicknesses. In this embodiment, the recording medium 9 has a substrate thickness: t2.
= 1.2 mm, and the recording medium 9a has a substrate thickness: t1 =
It is a 0.6mm DVD. The light source LD1 has an emission wavelength: λ1 = 6
35 nm semiconductor laser, light source LD2 has emission wavelength: λ2 = 78
5nm semiconductor laser. The deflecting prism 5 is rotatable around an axis by driving means (not shown).
By the 0-degree rotation, the attitude for the light source LD1 and the attitude for the light source LD2 can be switched. The figure shows a state where recording or the like is performed on the recording medium 9 which is a CD. When the light source LD2 emits light, a divergent light beam having a wavelength of λ2 = 785 nm is emitted.
This light beam is separated into a main beam and a sub-beam by the optical grating 1A, reflected by the semi-transparent mirror 3, deflected by the deflecting prism 5, incident on the objective lens 7A, converted into a convergent light beam by the action of the objective lens 7A, and recorded. Three light spots are formed on the recording surface 9A through the substrate of the medium 9. Each light beam reflected by the recording surface 9 </ b> A becomes a “return light beam”, is reflected by the deflecting prism 5, and
And is guided to the light receiving means 8. The light receiving means 8 receives the three return light beams, generates a focus error signal and a track error signal, and also generates a reproduction signal when performing reproduction.
The focus error signal and the track error signal are input to "control means" (not shown), and are provided for focusing control and tracking control.

When recording or the like is performed on a recording medium 9a which is a DVD, the deflecting prism 5 is rotated by 180 degrees from the position shown in FIG. Then, the deflecting reflection surface faces the side of the semi-transparent mirror 4. When the light source LD1 emits light, a divergent light beam having a wavelength of λ1 = 635 nm is emitted, and is converted into a parallel light beam by the coupling lens 2. This parallel light beam is reflected by the semi-transparent mirror 4 and is deflected by a deflecting prism 5.
And is incident on the objective lens 7A, converted into a convergent light beam by the action of the objective lens 7A, and transmitted through the substrate of the recording medium 9a to form a light spot on the recording surface 9a0. The light beam reflected by the recording surface 9A becomes a “return light beam”, is reflected by the deflecting prism 5, passes through the semi-transparent mirror 4, and is guided to the return light beam detection system 6. The return light beam detection system 6 receives the return light beam, generates a focus error signal and a track error signal, and also generates a reproduction signal when performing reproduction.
The focus error signal and the track error signal are input to "control means" (not shown), and are provided for focusing control and tracking control. As described above, the objective lens 7A is used in common for the light beam of the wavelength λ1 = 635 nm from the light source LD1 and the light beam of the wavelength λ2 = 785 nm from the light source LD2, but the wavelength λ1 incident as a parallel light beam. Is optimized to focus the light beam on the recording surface 9a0 of the recording medium 9a. In order to correct spherical aberration due to a difference in substrate thickness when a light beam having a wavelength of λ2 from the light source LD2 is focused on the recording surface 9A of the recording medium 9 by the objective lens 7A, the light source LD2 is positioned at an object distance to the objective lens 7A. Finite system with finite
It has become. The optical grating 1A is configured as a “hologram element having a curved phase grating”, and the phase grating is set so as to effectively reduce the wavefront aberration in the sub beam.

That is, the optical pickup device shown in FIG. 1 according to the first embodiment has a substrate thickness: t1, t2. (T1 <t2), a plurality of recording media 9a (D1), 9 (D1
Light source LD having emission wavelengths: λ1 and λ2 for performing at least one of recording, reproduction, and erasing of information with respect to 2).
1, LD2 and a light beam having a wavelength: λi (i = 1, 2) are condensed on the recording surfaces of the recording media 9a, 9 corresponding to the wavelengths, and are optically transmitted to the recording medium 9a (D1) at a wavelength: λ1. A single objective lens 7A whose characteristics have been optimized, and ± 1 as a control signal sub-beam for diffracting a light beam having a wavelength of λ2 and tracking a light spot on a recording surface to a predetermined position together with a zero-order light beam. Optical grating 1 for generating next light flux
A (H2), an optical pickup device for correcting a spherical aberration caused by a substrate thickness difference by adjusting an object-image distance when a light beam having a wavelength: λ2 is converged on a recording surface of a corresponding recording medium 9. , The optical grating 1A (H2) is a hologram element having a curved phase grating. Further, two types of recording media 9a (D
1) and 9 (D2) (claim 7), and the recording medium 9
a is a DVD having a substrate thickness of t1 = 0.6 mm, and the recording medium 9 is a CD having a substrate thickness of t2 = 1.2 mm.

FIG. 2 shows another embodiment of the optical pickup device of the present invention. The light source LD1 has an emission wavelength: λ
1 = 650nm semiconductor laser (chip), light source LD2
Is a semiconductor laser (chip) having an emission wavelength: λ2 = 785 nm. The light beam of wavelength: λ1 emitted from the light source LD1 passes through the hologram member 21 and
1, the light is converted into substantially parallel light, passes through the beam splitter 3A, is deflected by the deflecting prism 5, enters the objective lens 7A, and is incident on the recording surface 9a0 of the recording medium 9a (DVD having a substrate thickness of t1 = 0.6 mm). Form spots. Objective lens 7
A is optimized for substrate thickness: t1 = 0.6 mm,
It is also possible to design such that the spherical aberration is corrected under a certain condition even for the substrate thickness: t2 = 1.2 mm. on the other hand,
When the recording medium 9 is used, the light source LD2 is turned on. The emitted light beam passes through the hologram member 22, is converted into a “predetermined divergent light beam” by the coupling lens 32, is reflected by the beam splitter 3A, is deflected by the deflecting prism 5, and is incident on the objective lens 7A. 9
A light spot is formed on the recording surface 9A of (CD having a substrate thickness of t2 = 1.2 mm). The coupling lens 32 is provided with a light source LD so as to realize a “divergent light flux” such that spherical aberration due to a substrate thickness difference in the objective lens 7A is favorably corrected.
2 is converted into the above-mentioned "predetermined divergent light beam". By doing as described above, the spherical aberration due to the difference in substrate thickness between the recording media 9a and 9 is corrected, and an optical spot in which the wavefront deterioration is suppressed with respect to the axial deviation (track deviation) of the objective lens 7A is obtained. The beam splitter 3A may be of a type that separates reflected light and transmitted light by wavelength or a type that separates reflected light by polarized light. The light beam reflected on the recording surface of the recording medium 9a (or 9) becomes a return light beam, follows the original path, and
(Or 22) and is diffracted, and is incident on the light receiving means 91 (or 92). The hologram members 21 and 21
2, and the light receiving surfaces of the light receiving means 91 and 92 are appropriately divided according to the servo signal generation method used. In the embodiment of FIG. 2, the hologram member 21 is used to diffract the return light beam toward the light receiving means 91. The hologram member 22 has a function of diffracting the return light beam toward the light reception means 92 and a light source. It has a function of separating the light beam from the LD 2 into a main beam and two sub beams.
As shown in FIG. 2, the light source LD1, the hologram member 21, and the light receiving unit 91 are unitized as a light source / light receiving unit U1, and the light source LD2, the hologram member 22, and the light receiving unit 92 are formed as a light source / light receiving unit U2. Unitized.

FIG. 3 shows a specific example of the hologram member 22.
The form is shown. In the embodiment shown in FIG. 3A, the hologram member is formed by forming a hologram element 222 having a curved phase grating as an “optical grating” on one surface (light source LD2 side) of a transparent parallel plate 221 made of glass or the like. The hologram element 223 for detection is formed on the other surface. In the example of FIG. 3A, the detection hologram element 2
23 is a "polarization hologram". Polarization holograms
An optical element having a hologram effect that differs depending on the polarization state of light incident thereon. In the example of FIG. 3A, the detection hologram element 223 does not act on the luminous flux (the main beam and the two sub-beams) incident from the light source LD2 side, but transmits these light beams as they are. . In this case, a quarter-wave plate is provided in the optical path between the hologram member and the recording medium, and by passing through this quarter-wave plate, the return light beam has its polarization plane rotated 90 degrees from the initial state. ing. Therefore, the hologram function of the detection hologram element 223 acts on the three return light beams, and the return light beam is diffracted toward the light receiving unit 92. In the example shown in FIG. 3B, the hologram element 222a having a curved phase grating as an optical grating in the hologram member is formed on the light source LD2 side of the transparent parallel flat plate 221a, and the detection hologram element 223a is provided on the recording medium side. Is formed. Both the hologram element 222a and the detection hologram element 223a are "polarization holograms". Also in this case, in order to make the polarization plane of the light beam emitted from the light source LD2 side orthogonal to the polarization plane of the return light beam,
A quarter-wave plate is provided in the optical path between the hologram member and the recording medium. In this example, the hologram element 222a is a polarization hologram element, and applies a hologram effect (a function of separating a main beam and a sub beam) only to the light beam from the light source side, and transmits the return light beam as it is. The detection hologram element 223a does not exert a hologram effect on three light beams incident from the light source side, but performs a hologram effect (diffraction toward the light receiving means 92) on the three return light beams. Let it work. FIG.
In the example of (a), since the hologram element 222 is not a polarization hologram, the detection hologram element 22 is used so that the returning light beam is not affected by the hologram effect of the hologram element 22.
The light beam diffracted by 3 avoids the hologram element 222. For this reason, the parallel plate 221 needs to have a certain thickness. In the example shown in FIG. 3B, the hologram element 222a and the detection hologram element 223a are both polarization holograms, so that the hologram effect is exerted exclusively on one of the light beam from the light source and the return light beam. Hologram element 223a for detection
The return light beam diffracted by the hologram element 222a
There is no need to avoid. For this reason, the thickness of the parallel plate 221a is thinner than the thickness of the parallel plate 221. Although not described above, the hologram member 21 in the embodiment shown in FIG. 2 also has a detection hologram element formed on one side of a transparent parallel plate, and this detection hologram element is also a polarization hologram. Alternatively, the hologram effect may be applied only to the return light beam (diffraction toward the light receiving means). Of course, with respect to the light beam from the light source LD1, a main beam and two sub-beams may be generated by the hologram element, and tracking control may be performed using the sub-beams. In this case, the hologram member 21 is moved to the position shown in FIG. It is needless to say that the configuration shown in FIG. Of course, FIG.
In the examples of (a) and (b), the hologram element 222,
222a and the detection hologram elements 223 and 223a,
A normal hologram element may be used.
In the example of (b), the hologram element 22
Since the hologram effect of 2a acts, there is a possibility that the S / N ratio in the light receiving means 92 may decrease. On the other hand, if a polarization hologram is used as in the above example, the polarization hologram has high efficiency, so that the light use efficiency in the optical pickup device can be improved. Since the light source LD1 and the light source LD2 have different emission wavelengths, two wavelengths: λ1,
Use of a λ / 4 wavelength plate designed to be a λ / 4 wavelength plate has the advantage of requiring only one λ / 4 wavelength plate. The 波長 wavelength plate may be made of a birefringent material such as quartz, or may be made of a deposited film or the like. Further, the coupling lenses 31 and 32 can be shared. That is, as shown in FIG. 6, a (common) coupling lens 30 is provided on the light source side of the semi-transparent mirror 3. The light source side of the coupling lens is configured as shown in the figure.
That is, the beam splitter 40 is provided on the light source side of the coupling lens 30, and the light beam from the light source LD2 is
The light from the light source LD1 is incident on the coupling lens via the beam splitter 40 while the light is incident on the coupling lens 30 via A and the beam splitter 40. The optical pickup device according to the embodiment described above with reference to FIGS. 2 and 3 has a substrate thickness: t
The emission wavelengths λ1 and λ2 for performing one or more of recording, reproducing, and erasing of information on a plurality of recording media 9a (D1) and 9 (D2) having 1,1 and t2 (t1 <t2), respectively. Light source LD1 and light source LD2 having a wavelength of λ
The luminous flux of 1, λ2 is focused on the recording surfaces 9a0, 9A of the recording media 9a, 9 corresponding to the wavelength, and at least the recording media 9a
In contrast, a single objective lens 7A whose optical characteristics are optimized at a wavelength of λ1 and a light beam of a wavelength of λ2 are diffracted,
An optical grating 222 or 222 that generates a ± first-order light beam as a control signal sub-beam for tracking a light spot on a recording surface to a predetermined position together with a zero-order light beam.
a (H2), an optical pickup for correcting a spherical aberration caused by a difference in substrate thickness by adjusting an object-image distance when a light beam having a wavelength: λ2 is converged on a recording surface 9A of a corresponding recording medium 9. In the apparatus, the optical grating 222 or 222
a (H2) is a hologram element having a curved phase grating (claim 1). Further, when a light beam having a wavelength of λ2 is focused on the recording surface 9A of the corresponding recording medium 9 (D2), a means other than the light source LD1 is used as means for correcting spherical aberration caused by a difference in substrate thickness by adjusting the distance between object images. (Coupling lens 32) for coupling the luminous flux. A light source LD1 having an emission wavelength of λ1;
The recording medium 9a (D
The light receiving means 91 (PD1) for receiving the return light beam R1 reflected by the recording surface of 1), and the detection hologram element 2 for diffracting the return light beam R1 toward the corresponding light receiving means 91
1 (HOEE1) are integrated as a light source / light receiving unit U1, a light source LD2 having an emission wavelength: λ2, and a wavelength:
A light receiving means 92 (PD2) for receiving the return light beam R2 reflected by the recording surface of the recording medium 9 (D2) irradiated with the light spot of λ2, and a corresponding light receiving means 9 for receiving the return light beam R2.
Hologram element 223 (H
OEE2) is integrated as a light source / light receiving unit U2 (claim 4), and the detection hologram element 223a
(HOEE2) is a polarization hologram, and the optical grating 22
2a (H2) is also a polarization hologram, and the detection hologram element HOEE2 and the optical grating H2 have a diffractive effect on polarized lights having different vibration directions from each other (claim 5), and the optical grating 222a (H2) and the detection hologram are different. 223a (H
OEE2) are formed on the same substrate 221a (claim 6). The recording medium is composed of two types of recording medium 9a.
(D1) and 9 (D2) (claim 7), and the recording medium 9a is a DVD having a substrate thickness: t1 = 0.6 mm,
Is a CD having a substrate thickness of t2 = 1.2 mm (claim 9). Of course, it goes without saying that the optical pickup of the present invention can be implemented as having three or more light sources.

[0016]

EXAMPLES Specific examples will be described below.

In each of the embodiments, the recording medium has a substrate thickness:
DVD with t1 = 0.6 mm and substrate thickness: C with t2 = 1.2 mm
D. The emission wavelength of the light source LD1: λ1 is 635n.
m, the emission wavelength of the light source LD2: λ2 is 785 nm. Furthermore, an objective lens 7A common to the light beams of each wavelength
Is the data shown in Table 3 above. Example 1 Example 1 is a specific example of the embodiment shown in FIG.

As described above, the wavefront aberration when a linear grating is used as the optical grating is as shown in Table 4. When the objective lens 7A has a track shift, the sub-beam undergoes wavefront degradation, and the The difference from the above wavefront aberration is also remarkable, and it becomes difficult to obtain a good track error signal. Example 1
Uses a "hologram element having a curved phase grating" as the optical grating 1A. The “constant coefficient: c _ij ” that specifies the “curved phase grating” of the hologram element having the curved phase grating is as follows. Table 5 Phase coefficient: c _ij C10 2.7108E-2 C31 1.3129E-4 C01 8.5210E-4 C22 6.7910E-4 C20 -1.5445E-4 C13 9.0418E-5 C11 -2.5446E-5 C04 6.0460E-4 C02 -7.3592E-5 C50 1.3651E-2 C30 -4.9614E-3 C41 2.3764E-4 C21 -6.5507E-4 C32 3.6502E-3 C12 6.8472E-3 C23 1.7530E-2 C03 -2.1330E-3 C14- 7.9661E-2 C40 2.2932E-4 C05 1.3428E-2 At this time, as the recording medium 9, “substrate thickness: 1.2 mm C
In order to converge the light beam from the light source LD2 on the recording surface 9A of the "D" by the objective lens 7A with good NA = 0.5, the "object distance: about 52 mm finite system" Aberration: A good spot of 0.005λ can be obtained.
The light beam from the light source LD2 is converted by the hologram element 1A into a main beam (zero-order light beam) and two sub beams (± first-order light beams).
, The on-axis and off-axis wavefront aberrations for the main beam and each sub beam are as follows.

Table 6 Wavefront aberration (λ) On-axis of objective lens Axis shift: 0.4 mm Axis shift: -0.4 mm 0th-order light 0.005 0.053 0.053 + 1st-order light 0.031 0.058 0.050 -1st-order light 0.031 0.051 0.057 Case shown in Table 4 ( Wavefront aberration is approximately 0.01λ both on-axis and off-axis compared to when a linear grating is used as the optical grating.
It has been improved.

Embodiment 2 Embodiment 2 is directed to an optical system configuration as shown in FIG. The coupling lens 30 has a focal length f = 20 mm, which is designed to exert a collimating action on a light beam having a wavelength of λ1 (635 nm) from the light source LD1. Therefore, the light beam from the light source LD1 becomes a “parallel light beam” after passing through the coupling lens. The coupling lens also converts the luminous flux from the light source LD2 into a divergent luminous flux that becomes a “finite system with an object distance of about 52 mm” with respect to the objective lens 7A. In this case, the wavefront aberration of the main beam and the sub-beam by the light source LD2 when the linear grating is used as the optical grating 1 is as follows. Table 7 Wavefront aberration (λ) On-axis of objective lens Axis shift: 0.4 mm Axis shift: -0.4 mm 0th-order light 0.009 0.054 0.054 + 1st-order light 0.045 0.070 0.070 -1st-order light 0.045 0.070 0.070 About the same. In the second embodiment, an optical grating having the following phase coefficient is used.

Table 8 Phase coefficient C10 -9.1950E-3 C01 1.1205E-7 C20 -8.7213E-12 C11 -1.2991E-6 C02 -1.1743E-6 C30 -1.6981E-4 C21 4.7745E-7 C12 -1.3651 E-4 C03 6.2261E-6 At this time, the light beam from the light source LD2 is
At A, the main beam (zero-order light beam) and two sub beams (±
When the beam is separated into primary beams, the on-axis and off-axis wavefront aberrations for the main beam and each sub beam are as follows.

Table 9 Wavefront Aberration (λ) On-Axis of Objective Lens Axis Displacement: 0.4 mm Axis Displacement: −0.4 mm 0th-Order Light 0.009 0.054 0.054 + 1st-Order Light 0.024 0.058 0.059 -1st-Order Light 0.024 0.059 0.058 For the cases shown in Table 7, In comparison, the wavefront aberration is effectively improved both on-axis and off-axis.

Embodiment 3 Embodiment 3 is, for example, an embodiment as shown in FIG. 2, in which a first surface is an aspherical surface and a second surface is a coupling lens 32 for coupling a light beam from a light source LD2. Is a plane, and is an example using a shape whose shape is specified as follows. Table 10 Lens surface shape of coupling lens Surface coefficient Surface 1 (side of optical recording medium) Surface 2 (side of light source) R 6.40628 ∞ K 0.0 0.0 A4 -0.508842E-3 0.0 A6 -0.187545E-4 0.0 A8 0.121451E -4 0.0 A10 -0.356265E-5 0.0 The center thickness is 2.0 mm, and the refractive index of the material is n = 1.582 for a wavelength of 785 nm. The focal length is 11 mm, the object distance of the coupling lens itself is 25 mm, and the wavefront aberration is designed to be proportional to the angle of view.

By combining such a coupling lens with the objective lens 7A (Table 3), the luminous flux from the light source LD2 is converted into a divergent luminous flux that forms a finite system with an object distance of about 52 mm with respect to the objective lens 7A. I do. At this time, when the main beam and the two sub-beams are generated by the above-described optical grating using the linear grating, on-axis and off-axis wavefront aberrations for these beams are as follows.

Table 11 Wavefront Aberration (λ) On-Axis of Objective Axis Displacement: 0.4 mm Axis Displacement: −0.4 mm 0th-Order Light 0.004 0.022 0.022 + 1st-Order Light 0.078 0.084 0.084 -1st-Order Light 0.078 0.084 0.084 A hologram element having the following phase coefficient is used as the grating. Table 12 Phase coefficient C10 4.0040E-2 C31 -3.9205E-3 C01 4.1594E-4 C22 -8.9545E-4 C20 -4.2904E-4 C13 -2.4321E-3 C11 2.6239E-4 C04 1.1743E-3 C02 5.5791 E-5 C50 5.7950E-1 C30 -4.4007E-2 C41 -2.0485E-2 C21 1.5324E-3 C32 4.9527E-1 C12 1.8123E-2 C23 -5.2728E-2 C03 9.9048E-3 C14 -2.0132E -1 C40 5.7553E-3 C05 -1.5280E-1 The on-axis and off-axis wavefront aberrations for each beam at this time are
It is as follows. Table 13 Wavefront aberration (λ) On the objective lens axis Axis deviation: 0.4 mm Axis deviation: -0.4 mm 0th-order light 0.004 0.022 0.022 + 1st-order light 0.050 0.059 0.057 -1st-order light 0.051 0.056 0.058 Compared with the case of Table 12, Wavefront aberrations for the main beam and the sub beam are effectively improved.

[0026]

As described above, according to the present invention, a novel optical pick-up device can be realized. This optical pickup device uses a hologram element having a curved phase grating as an optical grating used for generating a sub-beam used for tracking control, and effectively reduces the deterioration of the wavefront aberration of the sub-beam caused by the track shift of the objective lens. So
Good recording and reproduction can always be realized for a plurality of types of recording media having different substrate thicknesses.

[Brief description of the drawings]

FIG. 1 is a diagram showing one embodiment of an optical pickup device of the present invention.

FIG. 2 is a diagram for explaining another embodiment of the optical pickup device of the present invention.

FIG. 3 shows a hologram member 2 in the embodiment of FIG.
It is a figure for explaining an example.

FIG. 4 is a diagram illustrating an example of an optical pickup device.

FIG. 5 is a diagram for explaining formation of a light spot by light beams having different wavelengths when an objective lens is shared with two types of recording media.

FIG. 6 is a diagram illustrating an example of an embodiment of an optical pickup device corresponding to a second embodiment.

[Explanation of symbols]

LD1, LD2 Light source (semiconductor laser) 1A Optical grating (hologram element having curved phase grating) 2 Coupling lens 7A Objective lens 9, 9a Recording medium

Claims (8)

[Claims] A substrate thickness: t1, t2, t3,. . (T1 <
t2 <t3,. . ) Each having a plurality of recording media D)
1, D2, D3. . A light source LDi (i = 1, 2, 3,...) Having an emission wavelength: λi for performing at least one of recording, reproduction, and erasing of information; , 3,...) On a recording surface of a recording medium Di corresponding to a wavelength, and a single objective lens whose optical characteristics are optimized at least at a wavelength of λ1 with respect to at least the recording medium D1; Wavelength: λi (i = 2, 3,.
An optical grating Hi (i = 2, i.e., generating a ± primary light beam as a control signal sub-beam for tracking a light spot on the recording surface to a predetermined position together with the next light beam)
3,. . When a light beam having a wavelength: λi (i = 2, 3,...) Is condensed on a recording surface of a corresponding recording medium Di (i = 2, 3,. An optical pickup device for correcting spherical aberration caused by the above by adjusting the distance between object images, wherein the optical grating Hi (i = 2, 3,...) Is a hologram element having a curved phase grating. Optical pickup device. 2. The optical pickup device according to claim 1, wherein a light beam having a wavelength: λi (i = 2, 3,...) Is transmitted to a recording surface of a corresponding recording medium Di (i = 2, 3,...). When the light is condensed at a point, one or more coupling lenses for coupling at least one light beam other than the light source LD1 are provided as means for correcting spherical aberration caused by a difference in substrate thickness by adjusting the distance between object images. Optical pickup device. 3. The optical pickup device according to claim 2, wherein the coupling lens has a wavefront aberration that is proportional to the angle of view as a single unit performance. 4. An optical pickup device according to claim 1, wherein the light source LDi has an emission wavelength: λi (i = 1, 2, 3,...).
The light receiving means PDi (i = 1, 2, 3,...) For receiving the return light beam Ri reflected by the recording surface of the recording medium Di irradiated with the light spot of wavelength: λi, and the return light beam Ri Hologram element HOEEi (i = 1, 2, 2) diffracted toward the light receiving means PDi
3,. . ), And each light source LDi, light receiving means PDi, and detection hologram element HOEEi are a light source / light receiving unit Ui (i = 1, 2).
2, 3,. . An optical pickup device characterized by being integrated as (1). 5. The optical pickup device according to claim 4, wherein the detection hologram element HOEE1 and / or the detection hologram element HOEEi (i = 2, 3,...) Are polarization holograms, and the optical grating Hi (i = 2, 3, ...) are also polarization holograms, and each of the above-mentioned detection hologram elements HOEEi and optical grating Hi
Is an optical pickup device having a diffractive effect on polarized lights having different vibration directions. 6. The optical pickup device according to claim 4, wherein the optical grating Hi and the hologram HOEEi (i = 2,
3,. . ) Are formed on the same substrate. 7. The optical pickup device according to claim 1, wherein the recording media are two types of recording media D1 and D2. 8. The optical pickup device according to claim 7, wherein the substrate thickness: t1 = 0.6 mm and the substrate thickness: t2 = 1.2 m.
m, an optical pickup device.