JPH09204685A - Optical pickup - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress the jitter of a detection signal and control the tracking accurately by a method wherein the axis of a light beam entering a light converging means is so inclined as to cancel the astigmatism given to the light beam. SOLUTION: A light beam emitted from a semiconductor laser device 11 is applied to a polarization beam splitter 13 through a grating 12. The grating 12 is composed of a parallel glass plate 22 which is an inclined parallel plate and a diffraction grating 24 which are housed and fixed in a holder 23. By diffracting the light beam emitted from the semiconductor laser device 11, at least three diffraction beams, i.e., a 0 degree diffraction beam, a +1 degree diffraction beam and a -1 diffraction beam, are generated. In an optical pickup 10, by suppressing the astigmatism of a light beam applied to a magneto-optical disc, the generation of the jitter harmful to a detected magneto-optical signal can be effectively avoided without deteriorating a tracking error signal.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical pickup for recording and / or reproducing on an optical recording medium such as an optical disc, a magneto-optical disc and a phase change type disc.

[0002]

2. Description of the Related Art Such an optical pickup is constructed, for example, as shown in FIG. The light beam emitted from the semiconductor laser element 2 as the light source is split into three light beams by the grating 3 and is incident on the polarization beam splitter 4. The light beam, a part of which is transmitted through the dielectric multilayer film 4 a of the polarization beam splitter 4, enters the collimator lens 5, is converted into a parallel light beam, and enters the objective lens 6. The objective lens 6 focuses this parallel light beam on the signal recording surface of the magneto-optical disk MO.

The return light beam reflected by the signal recording surface again passes through the objective lens 6 and the collimator lens 5 and enters the polarization beam splitter 4. The light beam containing the recording signal, a part of which is reflected by the dielectric multilayer film 4a of the polarization beam splitter 4, is made into at least three light beams by the Wollaston prism 7 in the direction orthogonal to the direction of division by the grating 3. Is divided into The light beam emitted from the Wollaston prism 7 passes through the multi-lens 8 and enters the photodetector 9.

The optical pickup 1 having such a structure
In the above, the light beam is incident on the inside of the photodetector 9, and based on the detection signals from the respective light-receiving surfaces provided separately on the light-receiving element inside the photodetector 9, the magneto-optical disk Signals such as an MO read signal, a focus error signal, and a tracking error signal are generated. The reading of the magneto-optical disk MO is performed by utilizing the fact that the polarization plane of the light beam is rotated by the Kerr effect by the signal recording surface of the magneto-optical disk MO.

[0005]

By the way, the magneto-optical disk MO as a medium for recording and / or reproducing the signal of the optical pickup 1 is a groove formed along the recording track. It has a metal signal recording surface provided with a groove and a translucent disk substrate (base) that protects the signal recording surface. The disk substrate is made of transparent resin such as polycarbonate. Similarly, in the phase change type disc, the above-mentioned pregroove is formed on the signal recording surface thereof.

The disc substrate made of a transparent resin such as polycarbonate has a property of transmitting light and birefringence, and has a different refractive index depending on the polarization direction of an incident ray, so that it is incident. Astigmatism will be added to the light beam. Further, the above-described pre-groove acts as a diffraction grating on incident light to generate a phase distribution, and this phase distribution causes astigmatism.
Therefore, there is a problem that the signal reading performance is deteriorated by these two types of astigmatism.

In view of the above points, the present invention provides an optical pickup capable of correcting astigmatism, suppressing jitter of a detection signal and accurately performing tracking control, and is preferably used for such an optical pickup. The object is to provide a holder for a diffraction element.

[0008]

According to the first aspect of the present invention, a light source for emitting a light beam and a signal recording surface of a disk-shaped recording medium made of a transparent resin substrate for emitting the light beam are provided. A light focusing means for focusing the light beam upward, a light separating means for separating a light beam emitted from the light source and a return light beam from the signal recording surface of the disk-shaped recording medium through the light focusing means, A photodetector for receiving the return light beam from the signal recording surface separated by the separating means, and the optical axis incident on the light converging means is a compound of the transparent resin of the disk-shaped recording medium. To cancel the astigmatism given to the light beam by refraction,
This is achieved by an optical pickup having a tilted configuration.

According to the second aspect of the present invention, the above object is to provide a light source for emitting a light beam, and a light focusing for focusing the light beam on a signal recording surface of a disk-shaped recording medium made of a transparent resin. Means, light separating means for separating the light beam emitted from the light source and the returning light beam from the signal recording surface of the disc-shaped recording medium via the light converging means, and the light separating means separated by the light separating means. A photodetector that receives the return light beam from the signal recording surface, and a non-optical detector that is disposed in the divergent optical path from the light source to the photodetector and is given to the light beam by the birefringence of the transparent resin of the disk-shaped recording medium. This is achieved by an optical pickup having an optical element that cancels point aberration.

Further, according to the third aspect of the present invention, the above object is to provide a light source for emitting a light beam and the light beam,
Light focusing means for focusing on the signal recording surface of the disc-shaped recording medium provided with a pre-groove, a light beam emitted from the light source, and return from the signal recording surface of the disc-shaped recording medium via the light focusing means The optical axis comprises: a light splitting means for splitting the light beam, and a photodetector for receiving the return light beam from the signal recording surface split by the light splitting means, and which is incident on the light focusing means. Is achieved by an optical pickup having an inclined structure so as to cancel the astigmatism given to the light beam by the pre-groove of the disc-shaped recording medium.

Further, according to the fourth aspect of the present invention, the above object is to focus a light source for emitting a light beam and the light beam on a signal recording surface of a disc-shaped recording medium having a pre-groove. And a light separating means for separating the light beam emitted from the light source and the returning light beam from the signal recording surface of the disk-shaped recording medium via the light collecting means, and the light separating means. A photodetector that receives the returned light beam from the signal recording surface, and an astigmatism that is placed in the divergent optical path from the light source to the photodetector and that is given to the light beam by the pregroove of the disk-shaped recording medium. And an optical element that cancels aberrations.

Further, according to the fifth aspect of the present invention, the above object is to provide a light source for emitting a light beam and a signal recording surface of the disc-shaped recording medium, which is made of a transparent resin having a pre-groove for the light beam. A light focusing means for focusing the light beam upward, a light separating means for separating a light beam emitted from the light source and a return light beam from the signal recording surface of the disk-shaped recording medium through the light focusing means, A photodetector for receiving the return light beam from the signal recording surface separated by the separating means, and the optical axis incident on the light converging means is a compound of the transparent resin of the disk-shaped recording medium. This is achieved by an optical pickup having a tilted structure so as to cancel astigmatism imparted to the light beam by refraction and pregroove.

Further, according to the sixth aspect of the present invention, the above object is to provide a light source for emitting a light beam and a signal recording surface of the disc-shaped recording medium, which is made of a transparent resin having a pre-groove for the light beam. A light focusing means for focusing the light beam upward, a light separating means for separating a light beam emitted from the light source and a return light beam from the signal recording surface of the disk-shaped recording medium through the light focusing means, A photodetector that receives the return light beam from the signal recording surface separated by the separating means, and a birefringent property of the transparent resin of the disk-shaped recording medium that is arranged in the divergent optical path from the light source to the photodetector. And an optical element that cancels the astigmatism imparted to the light beam by the pre-groove.

According to the first and second configurations, astigmatism generated by the birefringence of the transparent resin forming the disc-shaped recording medium in the light incident on the disc-shaped recording medium is canceled, Astigmatism harmful to the signal detection of the light beam incident on the photodetector is reduced, and the occurrence of jitter in the detection signal can be kept below the allowable value.

According to the third and fourth structures, the astigmatism generated by the pregrooves formed on the disc-shaped recording medium in the light incident on the disc-shaped recording medium is canceled out, so that the photodetector is detected. Astigmatism harmful to the signal detection of the light beam incident on is reduced, and the occurrence of jitter in the detection signal can be made equal to or less than the allowable value.

According to the fifth and sixth configurations, astigmatism and / or the disc-like shape caused by the birefringence of the transparent resin forming the disc-like recording medium in the light incident on the disc-like recording medium. By canceling the astigmatism generated by the pre-groove formed on the recording medium, the astigmatism harmful to the signal detection of the light beam incident on the photodetector is further reduced, and the occurrence of jitter in the detection signal is allowed. It can be less than or equal to the value.

[0017]

Preferred embodiments of the present invention will be described below in detail with reference to the accompanying drawings. Note that the embodiments described below are preferred specific examples of the present invention, and therefore, various technically preferable limitations are added. However, the scope of the present invention particularly limits the present invention in the following description. It is not limited to these embodiments unless otherwise stated.

1 and 2 show a preferred embodiment of an optical pickup according to the present invention. FIG. 1 is a front view and FIG. 2 is a side view. This optical pickup 10
Is a light source 11 and a grating 1 as a diffraction element.
2. Beam splitter 13 as light splitting means, Wollaston prism 17 as light splitting element, collimator lens 14, rising mirror 21 as optical path bending mirror, objective lens 15 as light focusing means, multi-lens 18, light detection It is equipped with a container 19.

The light source 11 is, for example, a semiconductor laser element, and is adapted to emit a laser beam. The light beam emitted from the semiconductor laser device 11 remains as divergent light,
The polarized beam splitter 13 as the light separating means is irradiated through the grating 12 as the diffraction element.
As will be described later, the grating 12 is one in which a parallel plate glass 22 as an inclined parallel plate and a diffraction grating 24 are housed and fixed in a holder 23.
By diffracting the light beam emitted from
And at least three diffracted beams of plus and minus first order diffracted light are generated. In the drawing, one light beam is shown for simplification. In this case, the direction in which the three divided light beams are arranged is perpendicular to the paper surface of FIG. The tilted support structure of the parallel plate 22 will be described later in detail.

Next, the beam splitter 13 is, for example, a polarization beam splitter in this embodiment, for separating a light beam containing a magneto-optical signal component. The polarization beam splitter 13 is formed with a dielectric multilayer film 13a inclined by 45 degrees with respect to the optical axis on a boundary surface formed by combining inclined surfaces of two prisms, and the polarization characteristic thereof is, for example, the transmittance TS of the S polarization component. Is about 10% or less, the transmittance TP of the P-polarized component is about 63%, the reflectance RS of the S-polarized component is about 90%, and the reflectance RP of the P-polarized component is about 35%.

Further, the polarization beam splitter 13 is integrally provided with a Wollaston prism 17 as a splitting element. The light beam transmitted by the beam splitter 13 is collimated by the collimator lens 14.
This collimated light beam is used as an optical path bending mirror,
The raising mirror 21 having a reflecting surface inclined at an angle bends its optical path by 90 degrees as shown in FIG.

The light beam incident on the objective lens 15 is condensed by the objective lens 15 at a certain point on the signal recording surface of the magneto-optical disc MO as a disc-shaped recording medium. That is, the track on the signal recording surface is irradiated with the Oth-order diffracted light, and the + 1st-order diffracted light and the -1st-order diffracted light are emitted before and after the O-th diffracted light. Magneto-optical disk MO
The reflected light from the signal recording surface, that is, the return light beam contains a magneto-optical signal component whose polarization plane is rotated based on the Kerr effect.

In the Wollaston prism 17, the return light beam reflected by the signal recording surface of the magneto-optical disc MO enters the beam splitter 13 again through the objective lens 15 and the collimator lens 14, and its dielectric multilayer film. The light reflected by 13a enters. This light is detected in the direction of 45 degrees with respect to the plane of polarization, and is detected by the magneto-optical disk MO.
Are divided into an optical component containing an MO signal due to Kerr rotation on the signal recording surface and an optical component not receiving the detection action.

The multi-lens 1 on which at least three light beams emitted from the Wollaston prism 17 are incident
The incident surface of the cylindrical lens 8 is, for example, a cylindrical surface, and imparts astigmatism to the incident light beam. Further, the emission surface of the multi-lens 18 is formed in a concave shape so that the optical path length of the light beam emitted from the multi-lens 18 to the photodetector 19 is adjusted.

The light receiving element 19c of the photodetector 19 has its light receiving surface divided into a plurality of light receiving portions as shown in FIG. That is, the light receiving element 19c has four light receiving portions A, B, C and D formed at the center thereof, and light receiving portions E, F and I respectively arranged at the upper, lower, left and right positions thereof in FIG. , J. As a result, in the optical pickup 10, when the light beam to which the astigmatism is given by the multi-lens 18 in FIG. 1 is incident on the four-divided light receiving portions A, B, C and D in the central portion of FIG. By finely moving the first objective lens 15 in the optical axis direction by an objective lens actuator (biaxial actuator) (not shown), the return light beam can be brought into a focused state.
This focusing control is performed by the four-division light receiving parts A, B,
Based on the output signals of C and D, (A + C)-(B + D)
This is performed by finely moving the objective lens 15 so that the focus error signal based on the above calculation becomes zero.

Further, of the returned light beams, two side beams (plus / minus primary light) based on the three beams emitted from the light source 11 of FIG. 1 and divided by the grating 12 are respectively shown in FIG. It is incident on the light receiving portions E and F of. In the optical pickup 10, this light receiving portion E
Based on the output signals of F and F, the objective lens 1 is adjusted so that the tracking error signal by the calculation of E−F becomes zero.
5 in tracking direction (radial direction of magneto-optical disk MO)
By making a slight movement to, tracking control is performed.

Of the returned zero-order light, the light beams split by the Wollaston prism 18 are incident on the light receiving portions I and J of FIG. 3, respectively. In the optical pickup 10, the magneto-optical signal is detected by taking the difference between the output signals of the light receiving sections I and J. In the optical pickup 10, as described above, by reducing the astigmatism of the light beam incident on the magneto-optical disk MO, harmful tracking jitter is generated in the detected MO signal, and the tracking error signal is deteriorated. It can be effectively prevented without

By the way, in the optical pickup 10, the magneto-optical disk MO comprises a metal signal recording surface having a so-called pre-groove, and a translucent disk substrate (base) for protecting the signal recording surface. have. The disk substrate is made of transparent resin such as polycarbonate. Therefore, the disk substrate
Due to the birefringence of the transparent resin,
Astigmatism will be generated with respect to the incident light beam.

Furthermore, the pregroove on the signal recording surface has, for example, a width of about 1.1 μm and a depth of λ / 8.
Grooves of a degree (λ is the wavelength of the light beam from the light source) are arranged at predetermined intervals. As a result, this pre-groove similarly produces astigmatism with respect to the incident light beam, as will be described later.

First, astigmatism due to the birefringence of the transparent resin of the disk substrate is generated as follows. That is,
For example, as shown in FIG. 4, the coordinate system is set so that the incident direction of the light beam from the light source in the magneto-optical disk MO is the radial direction x, the tangential direction y, and the vertical direction z, and the magneto-optical disk is moved from the objective lens. The incident polarized light (electric field direction) of the incident light beam condensed at the MO incident position is linearly polarized light in the x direction. Further, the birefringence of the disk substrate is nx, ny, n in the x, y, z directions, respectively.
z, nx = ny = 1.5 (= n), nz = 1.5-
Modeling with (5 × 10 ⁻⁴ ), the focus position and the difference (astigmatic difference) of each of the light rays a, b, c, and d at both ends of the incident light beam in FIG. .

Here, regarding the light ray (wavefront normal) passing through the inside of the disk substrate, the light rays c and b have different polarization directions as shown in FIG. In the substrate, both proceed at an angle of θk. Since the wavefront normal direction in the anisotropic medium having birefringence and the light ray direction which is the flow of energy are different, when the light ray angles θsc and θsb are obtained for the light rays c and b,

[Equation 2] And for ray b,

(Equation 3) Becomes Thus, based on FIG. 7, when the thickness of the disk substrate is set to t (= 1.2 mm) and the astigmatic difference ΔZ (converted into air) is calculated,

(Equation 4) Becomes Here, assuming that the NA of the objective lens is 0.45, θk = 17.45 degrees and ΔZ = 0.533 (μm). Further, regarding the RMS (root mean square) value W ₂₂ of astigmatism, the wavelength λ is set to 780 nm,

(Equation 5) Becomes

On the other hand, astigmatism due to the pre-groove is generated as follows. That is, as shown in FIG. 8, the pregroove acts as a diffraction grating on the incident light beam from the objective lens, and is divided into, for example, 0th order light and ± 1st order light. And these 0
When the secondary light and the ± first-order light overlap each other, the intensity distribution as shown in the lower part of FIG. 8 is exhibited, and the phase distribution as shown in FIG. 9 is generated. This phase distribution has a shape very similar to the wavefront aberration (see FIG. 8) when an appropriate defocus is added to the astigmatism. Therefore, astigmatism is generated by this phase distribution, and the jitter value of the reproduced signal is greatly changed by changing the defocus applied.

Assuming that there is no birefringence due to the optical disk, the result of simulating the fluctuation of the jitter value by a computer is as shown in FIG. in this case,
The curve A is a curve B when there is no aberration and no equalizer.
Intentionally adds astigmatism (0.75μ on the disc)
m) with no equalizer, curve C intentionally adds astigmatism (0.75 μm on the disc) with an equalizer, and curve D shows the case without aberration and with an equalizer. . The equalizer for shaping the waveform of the reproduction signal used in the above simulation is EF.
This is configured so that the frequency component of the 3T cycle of the M signal is emphasized. According to this simulation, it can be seen that the curve C becomes bilaterally symmetrical and the width of defocus at which the jitter value becomes 10% becomes wider.

Therefore, in order to cancel the astigmatism due to the pre-groove, the amount of astigmatism intentionally added may be appropriately set as follows. That is, the factor that changes the phase distribution in FIG. 9 is the angle of the diffracted light, that is, λ / N.
It is considered to be the ratio of A to the track pitch Tp of the magneto-optical disk MO, and the phase difference between the 0th order light and the ± 1st order light. When λ / NA proportional to the angle of the diffracted light is kept constant and the track pitch Tp is changed, the amount of astigmatism becomes as shown in FIG.

(Equation 6) And is also proportional to the phase difference ψ between 0th order light and ± 1st order light. Here, the phase difference ψ between 0th order light and ± 1st order light
Is the reciprocal phase depth of the pre-groove, a is the groove width / Tp, and Arg is the angle of deviation,

(Equation 7) Becomes Therefore, the astigmatism generated by the pre-groove has K as a constant,

(Equation 8) Will be proportional to On the other hand, by changing the track pitch Tp and the groove depth and width, a computer simulation is performed as shown in FIG.
As a result, the constant K is calculated by comparing with Equation 8 so that the astigmatism due to the pre-groove is expressed as

[Equation 9] Becomes

Actually, in the optical pickup of the mini disk (MD) as the magneto-optical disk MO, the track pitch Tp = 1.6 μm and the wavelength λ = 7 from the standard.
Since 80 nm and the objective lens NA = 0.45, the range in which the groove width and the depth satisfy the standard is 1.0 to 1.2 μm for the groove width and λ / 7 to λ / for the groove depth. 9 and the astigmatism value W ₂₂ in this range is 0.0147 to 0.0270 (λr
ms).

Thus, for example, in the case of an MD optical pickup, the astigmatism due to the birefringence of the transparent resin of the disc substrate is 0.0141 ± 0.00118 (λrms), and the astigmatism due to the pre-groove is 0. .020
9 ± 0.0062 (λrms), the overall astigmatism is 0.0350 ± 0.0074 (λrm).
s). In the present invention, the amount of astigmatism to be corrected is calculated based on the above-mentioned equations 8 and 9 or in consideration of some manufacturing technical tolerances, and the light focusing means is set to a degree to cancel it. The optical element is formed so as to cancel the incident ascending optical axis or based on the amount of astigmatism similarly obtained.

For example, specifically, the optimum value of astigmatism correction in MD (mini disk) (a type of magneto-optical disk) can be actually obtained as follows. That is, regarding the defocus characteristics of the jitter value J1 of the reproduced signal and the jitter value J2 of the signal after the equalizer, if the astigmatism correction is small, the overall astigmatism becomes negative, so on the optical disc. The shape of the spot formed by the light beam is as shown in the lower part of FIG. 14 with respect to the defocus. Since the factor of the jitter deterioration on the near defocus side is the MTF deterioration, the jitter improvement by the equalizer is effective. . On the other hand, the deterioration factor on the far side of the defocus is due to the crosstalk from the adjacent tracks, so that the equalizer has almost no effect of improving the jitter. Here, the relationship between the jitter value and the defocus in the actual MD is as shown in FIG.

On the other hand, when the astigmatism correction is appropriate, the overall astigmatism becomes almost 0. Therefore, the spot shape is defocused as shown in the lower part of FIG. The defocus tolerance is the widest because the effect of jitter improvement by the equalizer is also symmetrical with respect to the defocus. Here, the relationship between the jitter value and the defocus in the actual MD is as shown in FIG.

When the astigmatism correction is excessive, the overall astigmatism becomes positive, so that the spot shape is opposite to that when the correction is insufficient, as shown in FIG.
As shown in the lower part of No. In this case, since the deterioration factor of the jitter on the far side of defocus is the MTF deterioration,
It is effective to improve the jitter by an equalizer. On the other hand, the deterioration factor on the near defocus side is due to the crosstalk from the adjacent tracks, so that the equalizer has almost no effect of improving the jitter. Where the actual M
The relationship between the jitter value and the defocus at D is shown in FIG.
It becomes as shown in.

Regarding the relationship between the jitter value and the defocus in the actual MD shown in FIGS. 15, 17 and 19, the equalizer having the characteristic shown in FIG. 20 is used. As a result, the astigmatism correction has an optimum value in the cases of FIGS. 16 and 17, and the astigmatism due to the birefringence of the transparent resin of the disk substrate and the pregroove is substantially eliminated.

On the other hand, in order to cancel the astigmatism, as shown in FIG.
The optical axis of the light beam incident on is tilted. For this purpose, a parallel plate, for example a correction plate 2 made of parallel plate glass
2 is the optical axis L with respect to the optical axis L in the holder 23.
The angle θ between the parallel flat plate 22 and the plane of incidence of the parallel plate 22 is fixed as follows.

First, when the astigmatism generated by the optical disk is only due to birefringence by the disk substrate, for example, the focal length fo of the objective lens 15 is fo =
In 3.4, the focal length fc of the collimator lens 14 is = 1
If 0.8 is set, the correction amount δ required for the parallel plate (correction plate) 22 is, for example, that 0.5 μm of astigmatism occurs in the optical disc.

(Equation 10) Becomes Here, for example, the thickness t of the parallel plate 22 is 0.7.
(Mm) and refractive index N = 1.5, substituting into the following equation,

[Equation 11] θ = about 8 degrees (tilt angle of the parallel plate 22 with respect to the optical axis L)
Is required.

When the optical axis of the light beam incident on the objective lens 15 is tilted so as to cancel the astigmatism,
Specifically, for example, as shown in FIG. 21, in order to correct the astigmatic difference of about 5 μm, the raising mirror 21 is tilted by an angle θ = 0.7 degrees with respect to the optical axis L. Good. Alternatively, in order to correct the same astigmatism, as shown in FIG. 22, even if the collimator lens 14 is tilted by an angle θ = 1.5 degrees with respect to the optical axis L, astigmatism is caused by the off-axis aberration. Aberration can be corrected. In addition to such an optical element, when a hologram element is used in the optical system instead of the grating 12, for example,
This hologram element may be tilted by an angle determined by its thickness and refractive index (not shown).

Further, the hologram element may be configured not to incline but to impart astigmatism to the light beam in a direction of canceling harmful astigmatism by designing the hologram pattern.

Furthermore, in the same optical system as the above-mentioned optical pickup, the radius of curvature r = 2000 mm is set separately from the collimator lens and the rising mirror without tilting.
Even if the cylindrical lens is inserted, it is possible to correct astigmatism under the same conditions as above. In this case, the cylindrical lens is not tilted. As an optical element that exhibits the same function as this,
The following can also be used: For example, a hologram element in which a mologram surface is formed so as to exhibit the same effect by the diffracting action may be used instead of this cylindrical lens. Further, as such an optical element, when astigmatism is corrected under the same conditions as described above, for example, FIG.
Each beam splitter shown in (a), (b), and (c) of 3 can be utilized. These beam splitters are configured in a cube shape, and may be polarized beam splitters or non-polarized beam splitters.

The beam splitter 51 shown in FIG. 23 (a).
Is a beam splitter 1 of FIG.
The dielectric multi-layered film 51a or the like, which is arranged at the same position as that of No. 3 and serves as the light separation film, has the same structure. The beam splitter 51 is composed of two triangular prisms and a thin glass material 54 arranged so as to be sandwiched between these prisms. The glass materials 52 and 53 forming the two prisms are made of the same material or a material having the same refractive index, and the glass material 54 is made of a material having a different refractive index. As a result, astigmatism is given to the light beam incident from the incident surface (the lower surface in the figure), and the astigmatism in the opposite direction to the harmful astigmatism generated in the optical disc or the like is applied to the light beam. It is designed to be corrected by giving it to. Specifically, for example, the beam splitter 51 is formed by bonding a first glass material 52 and a second glass material 53 with a third glass material 54 sandwiched therebetween. These first to third glass materials 52, 53, 54 are each made of ordinary optical glass made of an optically isotropic material. Then, the first glass material 52 and the second glass material are formed of the same material, and those having a refractive index n = 1.5 are selected, and the third glass material 54 has a refractive index n = 1. 7 are selected. The inclination angle θ1 of the bonding surface with respect to the optical axis is 45 degrees. Furthermore, the third glass material 5
The thickness t in FIG. 4 is 0.05 mm.

FIG. 23 (b) shows another beam splitter configuration example. In the figure, the beam splitter 61 is formed by bonding the inclined surfaces of glass materials 62 and 63 having a triangular prism shape, and a light separation film 61a is formed on the bonding surface. The glass materials 62 and 63 are made of the same material and have the same refractive index.
Are arranged at an inclination angle of 45 degrees with respect to the light source 11 and the optical disc. Regarding the angles θ2 and θ3 formed between the light separating film 61 and the light separating films 61 by the end portions of the glass materials 62 and 63 which are prisms having a triangular prism shape to be bonded to each other, one is 45 degrees and the other is 45 degrees. It is set to be slightly different from 45 degrees. As a result, astigmatism is given to the light beam incident from the incident surface (the lower surface in the figure), and the astigmatism in the opposite direction to the harmful astigmatism generated in the optical disc or the like is applied to the light beam. It is designed to be corrected by giving it to. In this case, specifically, the glass materials 62 and 63 are made of the same isotropic material and have the same refractive index n = 1.5. However, although the angle θ2 of the glass material 62 with respect to the illustrated light separation film 61a is 45 degrees, the angle θ3 of the glass material 63 with respect to the illustrated light separation film 61a is set to 45 degrees ± 2.6 degrees. The distance l from the light source 11 to the incident surface is 5
mm. The beam splitter 61
Is arranged at the same position as the polarization beam splitter 13 in the optical system of FIG. 1, but the grating 12 may or may not be provided.

Further, FIG. 23 (c) shows another example of the structure of the beam splitter. This beam splitter 71 is used in the optical system of the optical pickup shown in FIG.
It is arranged similarly to the beam splitter 61 of (b). In the figure, a beam splitter 71 is configured by bonding two triangular prism-shaped glass materials 72, 73 to each other with their inclined surfaces bonded together.
The two glass materials 72 and 73 are selected so that their refractive indexes are slightly different. This allows the incident surface (lower surface in the figure)
Astigmatism is imparted to the light beam incident from the optical beam, and astigmatism in the direction opposite to the harmful astigmatism generated in the optical disc or the like is given to the light beam to correct it. Has become. Specifically, two glass materials 7
A light separation film 71a is formed on the bonding surface of the two and 73. This glass material 72 is made of an isotropic material and has a refractive index n =
A material having a refractive index n = 1.5006 made of an isotropic material is selected as the glass material 73. The angle of inclination of the bonding surface with respect to the optical axis is 45 degrees, and the distance 1 from the light source 11 to the incident surface is set to 5 mm.

Each such beam splitter 51, 6
By using 1, 71, astigmatism can be corrected in the optical pickup of FIG. 1 under the same conditions as in the cases of FIGS.

In the optical disc, not only the astigmatism due to the birefringence of the disc substrate but also the astigmatism due to the diffractive action of the pre-groove obtained by the above-mentioned method is added, and the astigmatism is added to the above optical element. You may make it correct by. That is, it has been conventionally difficult to correct a predetermined astigmatism by using an optical element, but it is difficult to obtain the astigmatism to be corrected. In the present invention, this can be obtained by the above-mentioned method. Similarly, by adding the amount of aberration based on the astigmatic difference of the semiconductor laser element, which is the light source, to each of the above optical elements so that all the astigmatism that occurs in the entire optical system of the light source and the optical disc can be corrected. You may comprise. In this respect, the amount of astigmatism of the light source is
Since it is determined according to the semiconductor laser, there is no difficulty in obtaining the astigmatism in the pregroove when obtaining this.

Optical pickup 10 in the present embodiment
The light beam emitted from the light source 11 is split by the grating 12 into three light beams in the direction perpendicular to the paper surface of FIG. 1, and enters the polarization beam splitter 13. About 60% or more of the P-polarized component of this light beam passes through the dielectric multilayer film 13a and is incident on the collimator lens 14, is made into a parallel light beam, and then is incident on the objective lens 15. . In the objective lens 15, this light beam is focused on the signal recording surface of the magneto-optical disk 16, and on this signal recording surface, the main beam and the two beams that are deviated by half a track back and forth with the main beam sandwiched therebetween. Side beams form a spot.

The light beam applied to the signal recording surface of the magneto-optical disk MO has its polarization plane rotated by the Kerr effect, and the reflected light containing such a magneto-optical signal is returned to the objective lens 15 again. Then, the light enters the polarized beam splitter 13 through the collimator lens 14. Here, most of the components including the magneto-optical signal of the return light and about 35% of the other optical components are reflected by the dielectric multilayer film 13a and reflected toward the left direction in FIG. . Further, the return light emitted from the polarization beam splitter 13 is split by the Wollaston prism 17, enters the multilens 18, is given astigmatism, and enters the photodetector 19.

Then, the optical pickup 1 of this embodiment
At 0, the correction plate 22 which is a parallel flat plate is arranged so as to be inclined by θ with respect to the optical axis L. As a result, even if the light incident on the magneto-optical disc MO is given astigmatism by the birefringence of the transparent resin of the disc substrate and the pregroove, this astigmatism is canceled.
Therefore, unlike the related art, the magneto-optical signal or the like does not have jitter exceeding the allowable range, and accurate signal detection and servo control can be performed.

Next, a diffractive element holder suitable for use in the optical pickup 10 will be described with reference to FIGS. 24 to 28. 24 is a plan view of the grating 12 of FIG. 1, FIG. 25 is a bottom view thereof, FIG. 26 is a central longitudinal sectional view of the grating 12, and FIG. 27 is a side sectional view. Here, FIG. 28 shows a slide base 31 of the optical pickup 10. For example, a long frame 32 made of aluminum die-casting has a portion 33 for accommodating an objective lens actuator for finely moving the objective lens, and an optical pickup. It is provided with a portion for housing 10. The slide base 31 is supported by two parallel axes A1 and A2, and slides in the arrow direction to perform a seek operation.

In the portion of the frame 32 that accommodates the optical elements constituting the optical pickup 10, openings are formed so as to insert and fix the optical elements toward the direction behind the paper in the drawing. There is. As shown in the drawing, the insertion hole 34 for inserting and fixing the grating 12 of the frame body 32 has a bent outer shape so as to have a horizontally long portion in the lower portion and a portion in the lateral direction shorter than this portion in the upper portion. Is provided. As a result, the grating 12
It is designed so that it will not be accidentally mounted upside down, and the grating 12 can be positioned simply by inserting it from the front side of the paper surface of FIG.

As shown in FIG. 26, the grating 12
The diffractive element holder 23 (hereinafter referred to as "holder 23") is composed of a brim-shaped first portion 25 that is long in the lateral direction and a second portion 26 that is shorter in the lateral direction than that of the synthetic resin. It is formed by molding. In the center of the holder 23, continuous through holes 25a and 26a through which a light beam can pass are provided along the optical axis L of the optical pickup 10 in the vertical direction in FIG.

As shown in FIG. 24, the second portion 26 has an opening 26c formed in the upper surface, which is one surface, and the opening 26c has a depth capable of accommodating the diffraction grating 24,
It communicates with the through hole 26a. And the opening 26a is
It is formed into a substantially parallelogram shape, and one side thereof is inclined by an angle C with respect to the horizontal direction in the figure as shown in the figure. That is, in other words, the opening 26a is formed as shown in FIG.
The optical pickup 10 is formed by rotating it by an angle C in a plane orthogonal to the direction of the optical axis of the optical pickup 10.

Further, as shown in FIG. 26, the inner diameter of the through hole 26a is made smaller than the inner diameter of the opening 26c to form the inclined surface 26e and the step portion 26d.
The diffraction grating 24 is placed on this step portion 26d, and its side surface and
By filling the adhesive 24a between the inclined surface 26e,
The diffraction grating 24 can be easily positioned and fixed at a predetermined angle C. In this way, the diffraction grating 24 is rotated by the angle C because the two plus / minus first-order diffracted lights divided by the diffraction grating 24 are on the recording surface of the magneto-optical disc MO with respect to the main beam spot. This is for forming side spots at positions accurately shifted by 1/4 track forward and backward. This ensures proper positioning and minimizes difficult adjustments so that the tracking error signal can be obtained at the maximum level.

Further, an opening 25c is formed on the back surface of the first portion 25 of the holder 23. This opening 25
c has a size and a depth capable of accommodating the parallel plate glass 22 as a parallel plate, and the gap 2 is provided on the left and right of FIG.
It has 7,27. Further, the opening 25c is formed in the through hole 2
5a, and the inner diameter of the through hole 25a is set to the opening 2
By making it smaller than the inner diameter of 5c, the inclined surface 25d and the inclined step portion 25b are formed. Here, the inclination angle θ of the inclined step portion 25b matches the angle θ with respect to the optical axis L that can cancel the astigmatism of the light beam generated in the above-described magneto-optical disk.

Thus, by simply inserting the parallel plate glass 22 from below in FIG. 26, the parallel plate glass 22 is positioned at an appropriate angle θ, so that the gaps 27, 27 are formed between the inclined surface 25d and the side surface of the parallel plate glass 22. Positioning and fixing are performed only by filling the adhesives 22a.

In this way, with the diffraction grating 24 and the parallel plate glass 22 fixed, the holder 23 is moved to the position shown in FIG.
The diffraction grating 24 and the parallel plate glass 22 can be positioned and fixed to the optical axis of the optical pickup 10 of FIG. 1 by inserting the same into the opening 12 of the frame 32 and applying an adhesive. Therefore, when such a holder 23 is not provided, the optical pickup 10 can be assembled very easily as compared with the case where the diffraction grating 24 and the parallel plate glass 22 are positioned and fixed to the frame body 32.

Further, as shown in FIG. 25, the holder 23 is provided with a through hole 25f communicating with the internal space S formed by the diffraction grating 24 and the parallel plate glass 22. As a result, the sealed air expands and contracts due to changes in the temperature of the external environment, etc., so that the grating 12 is not destroyed.

The present invention is not limited to the above embodiment. Although FIG. 1 shows an optical pickup for magneto-optical detection, the present invention can be applied to an optical pickup such as a compact disc for detecting a pit signal. In this case, the polarization beam splitter may be a beam splitter having no polarization. Also, the Wollaston prism may be omitted. Although the Wollaston prism is used in FIG. 1, other elements such as a hologram element or a combination of a quarter wavelength plate and another beam splitter may be used as the light splitting element. Further, in some cases, the optical pickup may be configured by a diverging optical system that does not use the correlator lens 14.

Further, in the above embodiment, the parallel plate, the collimator lens, and the rising mirror are tilted to tilt the light beam incident on the objective lens with respect to the optical axis. However, other optical elements may be tilted. , For other optical elements,
Means such as a configuration for correcting astigmatism may be adopted.

Further, in the above-described embodiment, the light beam incident on the objective lens (light focusing means) is inclined with respect to the optical axis, or is generated by the substrate of the optical disc or the pre-groove depending on the configuration of the optical element itself. Astigmatism is corrected. However, in addition to this, when astigmatism occurs in the light source, the astigmatism may be corrected in consideration of the amount. In this case, since the astigmatism generated in the light source is determined by the element as the light source, it can be easily calculated.
If the astigmatism generated by the optical disk is obtained using the method according to the present invention, the astigmatism generated by the entire optical system including the optical disk can be obtained.

[0066]

As described above, according to the present invention, it is possible to provide an optical pickup capable of suppressing the jitter of a detection signal and accurately performing tracking control, and a diffractive element suitably used for such an optical pickup. A holder can be provided.

[Brief description of drawings]

FIG. 1 is a schematic front view showing the configuration of a preferred embodiment of an optical pickup according to the present invention.

FIG. 2 is a schematic side view of the optical pickup of FIG.

FIG. 3 is a diagram showing an example of an arrangement of light receiving parts of a photodetector of the optical pickup of FIG.

4 is a schematic plan view showing a coordinate system of a magneto-optical disk in the optical pickup of FIG.

5 is a partially enlarged perspective view showing incident light with respect to the magneto-optical disk in the optical pickup of FIG.

6 is a partially enlarged sectional view showing light incident on a disk substrate of the magneto-optical disk of FIG.

7 is a partial enlarged cross-sectional view showing an astigmatic difference due to birefringence due to incident light in FIG.

8 is a schematic perspective view showing diffraction by the pre-groove of the magneto-optical disk of FIG. 5 and a schematic plan view showing an intensity distribution by this diffraction.

9 is a schematic perspective view showing a phase distribution due to diffraction of a pre-groove of the magneto-optical disk of FIG.

10 is a schematic perspective view showing wavefront aberration of the magneto-optical disk of FIG.

11 is a graph showing the relationship between the jitter value and the defocus of the magneto-optical disk of FIG.

12 is a schematic plan view showing a mutual relationship between spots of 0th-order light and ± 1st-order light due to pregrooves of the magneto-optical disk of FIG.

13 is a schematic plan view showing an example of a mutual relationship between spots of 0th-order light and ± 1st-order light due to pregrooves of the magneto-optical disc of FIG.

14A and 14B are graphs showing a relationship between a jitter value and defocus when astigmatism correction is insufficient in the optical pickup of FIG. 1 and a diagram showing a spot shape.

FIG. 15 is a graph showing a relationship between a jitter value and defocus when astigmatism correction for MD is insufficient.

16A and 16B are graphs showing a relationship between a jitter value and defocus in the case of optimal astigmatism correction in the optical pickup of FIG.

FIG. 17 is a graph showing a relationship between a jitter value and defocus in the case of optimal astigmatism correction for MD.

18A and 18B are graphs showing a relationship between a jitter value and defocus when astigmatism is overcorrected and spot shapes in the optical pickup of FIG.

FIG. 19 is a graph showing a relationship between a jitter value and defocus when astigmatism is overcorrected with respect to MD.

20 is a graph showing characteristics of the equalizer used in the cases of FIGS. 15, 17 and 19. FIG.

21 is a partially enlarged view showing an example of a configuration in which the optical axis of the optical pickup of FIG. 1 is tilted.

22 is a partially enlarged view showing an example of a configuration in which the optical axis of the optical pickup of FIG. 1 is tilted.

23 is a schematic diagram showing a configuration example of a beam splitter capable of correcting harmful astigmatism in the optical pickup of FIG.

FIG. 24 is a plan view of a grating of the optical pickup shown in FIG.

FIG. 25 is a bottom view of the grating of FIG. 24.

FIG. 26 is a front center vertical cross-sectional view of the grating of FIG. 24.

FIG. 27 is a side view of the grating of FIG. 24.

28 is a bottom view of a slide base on which the grating of FIG. 24 is mounted.

FIG. 29 is a diagram showing an example of a conventional optical pickup.

[Explanation of Codes] 10 ... Optical pickup, 11 ... Laser light source, 12 ... Grating, 13 ... Polarization beam splitter, 14 ... Collimator lens, 15 ...
Objective lens, MO ... Magneto-optical disk, 17 ... Wollaston prism, 18 ... Multi-lens, 19 ...
..Photodetectors, 21 ... Stand-up mirrors, 22 ...
Parallel plate (correction plate), 23 ... Holder for diffraction element,
24 ... Diffraction grating. 51, 61, 71 ... Beam splitter.

Claims (30)

[Claims] 1. A light source for emitting a light beam, a light focusing means for focusing the light beam on a signal recording surface of a disk-shaped recording medium made of a transparent resin substrate, and a light beam emitted from the light source. Light separating means for separating the return light beam from the signal recording surface of the disk-shaped recording medium via the light focusing means, and light for receiving the return light beam from the signal recording surface separated by the light separating means A detector, and the optical axis incident on the light focusing means is tilted so as to cancel the astigmatism imparted to the light beam by the birefringence of the transparent resin of the disk-shaped recording medium. An optical pickup characterized by being configured. 2. A light source for emitting a light beam, a light focusing means for focusing the light beam on a signal recording surface of a disk-shaped recording medium made of transparent resin, a light beam emitted from the light source, and Optical separation means for separating the return light beam from the signal recording surface of the disk-shaped recording medium via the light focusing means, and light detection for receiving the return light beam from the signal recording surface separated by the light separation means And an optical element that is disposed in the divergent optical path from the light source to the photodetector and that cancels the astigmatism imparted to the light beam by the birefringence of the transparent resin of the disk-shaped recording medium. Optical pickup to do. 3. The optical element includes an optical path bending mirror that bends a light beam from a light source in a direction in which the light focusing means is arranged, and the light beam is reflected by a birefringence of a transparent resin of the disk-shaped recording medium. The optical pickup according to claim 2, wherein the optical path bending mirror is arranged so as to be inclined with respect to the optical axis so as to cancel the astigmatism given to the optical axis. 4. The optical element includes a collimator lens for converting a light beam from a light source into a parallel light beam, and the astigmatism given to the light beam by the birefringence of the transparent resin of the disk-shaped recording medium. The optical pickup according to claim 2, wherein the collimator lens is arranged so as to be tilted with respect to the optical axis so as to cancel. 5. The optical element is arranged in the optical path from the light source to the photodetector, and cancels astigmatism imparted to the light beam by the birefringence of the transparent resin of the disk-shaped recording medium, The optical pickup according to claim 2, further comprising a cylindrical lens having an incident surface. 6. The optical element is arranged in an optical path from the light source to the photodetector, and is configured to cancel astigmatism given to a light beam due to birefringence of a transparent resin of the disk-shaped recording medium. The optical pickup according to claim 2, further comprising a beam splitter that is formed. 7. The optical element is arranged in the optical path from the light source to the photodetector, and is configured to cancel the astigmatism given to the light beam by the birefringence of the transparent resin of the disc-shaped recording medium. The optical pickup according to claim 2, further comprising a hologram element that is formed. 8. The optical element is arranged in the optical path from the light source to the photodetector, and cancels astigmatism imparted to the light beam by the birefringence of the transparent resin of the disk-shaped recording medium, 3. The incident plane has a slanted parallel plate.
An optical pickup according to item 1. 9. A first positioning portion for fixing a plate-like diffraction grating on one surface so as to have a slight rotation angle in a plane orthogonal to the optical axis, and on the other surface. 9. The optical element according to claim 8, further comprising: a diffractive element holder having a second positioning portion for fixing the plane of incidence of the parallel plate with respect to the optical axis by tilting. pick up. 10. A light source for emitting a light beam, a light focusing means for focusing the light beam on a signal recording surface of a disc-shaped recording medium having a pre-groove, and a light beam emitted from the light source. Light separating means for separating the return light beam from the signal recording surface of the disk-shaped recording medium via the light focusing means, and light for receiving the return light beam from the signal recording surface separated by the light separating means A detector, and the optical axis incident on the light focusing means is tilted so as to cancel the astigmatism given to the light beam by the pregroove of the disk-shaped recording medium. An optical pickup featuring. 11. A light source for emitting a light beam, a light focusing means for focusing the light beam on a signal recording surface of a disc-shaped recording medium having a pre-groove, and a light beam emitted from the light source. Light separating means for separating the return light beam from the signal recording surface of the disk-shaped recording medium via the light focusing means, and light for receiving the return light beam from the signal recording surface separated by the light separating means An optical pickup comprising a detector and an optical element arranged in a divergent optical path from the light source to the photodetector, for canceling the astigmatism given to the light beam by the pregroove of the disc-shaped recording medium. . 12. The RMS value W _{22 of} astigmatism given to the light beam by the pre-groove is: track pitch of the optical disc = Tp, and the light beam incident on the optical disc is at least three due to the diffracting action of the pre-groove of the optical disc. When diffracted by two light beams, the phase difference between the 0th order light and the ± first order light is ψ, the angle of diffracted light is λ / NA, the reciprocal phase depth of pregroove is φ, and the groove width is Tp = a. , Arg is an angle (degree), then 2. The optical element is configured so as to cancel the astigmatism thus determined.
The optical pickup described in 1. 13. The optical element includes an optical path bending mirror that bends a light beam from a light source in a direction in which the light converging means is arranged, and a non-optical beam given to the light beam by a pre-groove of the disc-shaped recording medium. The optical pickup according to claim 11, wherein the optical path bending mirror is arranged so as to be inclined with respect to the optical axis so as to cancel the point aberration. 14. A collimator lens for converting a light beam from a light source into a parallel light beam is provided as the optical element so as to cancel astigmatism given to the light beam by the pre-groove of the disk-shaped recording medium. The optical pickup according to claim 11, wherein the collimator lens is arranged so as to be inclined with respect to the optical axis. 15. The optical element is arranged in an optical path from the light source to the photodetector, and an incident surface is formed so as to cancel astigmatism given to a light beam by a pregroove of the disc-shaped recording medium. The optical pickup according to claim 11, further comprising a cylindrical lens that is formed. 16. A beam splitter arranged as an optical element in an optical path from the light source to the photodetector, and configured to cancel astigmatism imparted to a light beam by a pregroove of the disc-shaped recording medium. It has, It has characterized by the above-mentioned.
An optical pickup according to item 1. 17. The hologram element as the optical element, which is arranged in an optical path from the light source to the photodetector and is configured to cancel astigmatism imparted to a light beam by a pregroove of the disk-shaped recording medium. 12. The method according to claim 11, wherein
An optical pickup according to item 1. 18. The optical element is arranged in the optical path from the light source to the photodetector, and the incident surface is inclined so as to cancel the astigmatism given to the light beam by the pregroove of the disk-shaped recording medium. 12. It has a parallel plate which was formed.
An optical pickup according to item 1. 19. A first positioning portion for fixing a plate-like diffraction grating on one surface so as to have a slight rotation angle in a plane orthogonal to the optical axis, and on the other surface. The optical element according to claim 18, further comprising: a diffractive element holder having a second positioning portion for fixing the plane of incidence of the parallel plate with respect to the optical axis by inclining. pick up. 20. A light source for emitting a light beam, a light focusing means for focusing the light beam on a signal recording surface of a disk-shaped recording medium made of a transparent resin having a pre-groove, and emitted from the light source. Optical beam separating means for separating the optical beam from the signal recording surface of the disk-shaped recording medium through the optical focusing means, and the optical beam returning from the signal recording surface separated by the optical separating means. And a photodetector for receiving light, and an optical axis incident on the light converging means is an astigmatism imparted to the light beam by the birefringence of the transparent resin of the disk-shaped recording medium and the pregroove. So as to cancel
An optical pickup having a tilted structure. 21. A light source for emitting a light beam, a light focusing means for focusing the light beam on a signal recording surface of a disc-shaped recording medium made of a transparent resin having a pre-groove, and a light focusing means for emitting the light beam. Optical beam separating means for separating the optical beam from the signal recording surface of the disk-shaped recording medium through the optical focusing means, and the optical beam returning from the signal recording surface separated by the optical separating means. A photodetector that receives the light, and an optical that is disposed in the divergent optical path from the light source to the photodetector and that cancels the astigmatism imparted to the light beam by the birefringence of the transparent resin of the disk-shaped recording medium and the pregroove. An optical pickup having an element. 22. The optical element includes an optical path bending mirror that bends a light beam from a light source in a direction in which the light converging means is arranged, and the birefringence and the pregroove of the transparent resin of the disk-shaped recording medium. 22. The optical pickup according to claim 21, wherein the optical path bending mirror is arranged so as to be inclined with respect to the optical axis so as to cancel the astigmatism given to the light beam by the. 23. The optical element comprises a collimator lens for converting a light beam from a light source into a parallel light beam, which is given to the light beam by the birefringence of the transparent resin of the disk-shaped recording medium and the pre-groove. 22. The optical pickup according to claim 21, wherein the collimator lens is arranged so as to be inclined with respect to the optical axis so as to cancel astigmatism. 24. The optical element, which is arranged in an optical path from the light source to the photodetector, cancels the astigmatism imparted to the light beam by the birefringence of the transparent resin of the disk-shaped recording medium and the pregroove. The optical pickup according to claim 21, further comprising a cylindrical lens having an incident surface formed thereon. 25. The optical element is arranged in an optical path from the light source to the photodetector, and cancels astigmatism imparted to a light beam by a birefringence of a transparent resin of the disk-shaped recording medium and a pregroove. 22. The optical pickup according to claim 21, comprising a beam splitter configured as described above. 26. The optical element, which is arranged in an optical path from the light source to the photodetector, cancels astigmatism imparted to a light beam by birefringence and pregroove of a transparent resin of the disk-shaped recording medium. 22. The optical pickup according to claim 21, which has a hologram element configured as described above. 27. The optical element is arranged in an optical path from the light source to the photodetector, and cancels the astigmatism imparted to the light beam by the birefringence of the transparent resin of the disk-shaped recording medium and the pregroove. 22. The optical pickup according to claim 21, wherein the optical pickup has a parallel flat plate whose incident surface is inclined. 28. A first positioning portion for fixing a plate-like diffraction grating on one surface so as to have a slight rotation angle in a plane orthogonal to the optical axis, and on the other surface. 28. The optical element according to claim 27, further comprising: a diffractive element holder having a second positioning portion for fixing the plane of incidence of the parallel plate with respect to the optical axis by tilting. pick up. 29. A first positioning portion provided on one surface for fixing the plate-shaped diffraction grating so as to have a slight rotation angle in a plane orthogonal to the optical axis, A diffractive element holder, characterized in that it has a second positioning portion which is provided on the other surface and is used to incline and fix the plane of incidence of the parallel plate with respect to the optical axis. 30. The holder for a diffraction element according to claim 29, wherein a space formed by the plate-shaped diffraction grating and the parallel flat plate communicates with the outside.