JPH09161307A - Optical head and optical information reproducing device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To deal with two kinds of optical disks with different substrate thickness by providing a reflection optical member laminating a reflection type hologram giving luminous flux a prescribed phase distribution and a polarizing beam splitter on a bottom part of an objective lens. SOLUTION: The luminous flux transmitting through the liquid crystal optical rotator 6 is made incident on the reflection optical member 8. The reflection optical member 8 whose first reflection surface 81 arranged obliquely by nearly 45 deg. for an optical axis is nearly parallel to the polarizing beam splitter is provided with the reflection type hologram 82. The reflection type hologram 82 generates +1st-order diffracted luminous flux or -1st-order diffracted luminous flux when the luminous flux 4 is reflected by the hologram, and gives the wave surface of the diffracted luminous flux the prescribed phase distribution so as to correct excellently spherical aberration caused when a CD is reproduced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical head suitable for reproducing optical disks having different substrate thicknesses and an optical information reproducing apparatus using the same.

[0002]

2. Description of the Related Art At present, an optical disc represented by a compact disc (hereinafter abbreviated as CD) is widely used for music or computer data. This CD has a diameter of 120 mm, a track pitch of about 1.6 μm, and a minimum pit length of about 0.9 μm. The optical information reproducing device for reproducing this CD is 770 to 80 as a light source.
A 0 nm near-infrared light semiconductor laser is used, and an objective lens that focuses the laser light on the optical disc is generally 0.4.
It has a numerical aperture (NA) of about 5.

On the other hand, in recent years, a semiconductor laser for red light having a wavelength of 630 to 650 nm has been put into practical use, and a numerical aperture (NA) of 0.
A high density optical disc having a recording density four times or more that of a current CD has been proposed by combining with an objective lens of about 6. (For example, 1993 Autumn Meeting of the Institute of Electronics, Information and Communication Engineers, Lecture No. C-364 "Study on High Density CD-ROM Using Red Laser Pickup") By the way, optical disc substrates such as CDs are generally manufactured by injection molding of polycarbonate resin. Therefore, warping of the substrate is unavoidable, and in general, there is a warp corresponding to an inclination of about 0.3 to 0.5 ° in a CD or the like. When the optical disc is tilted, coma aberration is generated in a focused spot that is focused on the disc, and in a remarkable case, signal reproduction becomes impossible. Such a coma aberration amount is the numerical aperture (N
Since it is proportional to the cube of A), even if the disc tilt amount is the same, NA = 0.4 for an objective lens with NA = 0.6, for example.
The coma aberration is about 2.4 times that of the objective lens of No. 5. On the other hand, the amount of coma generated is proportional to the substrate thickness of the optical disc. Therefore, recently, for example, an attempt has been made to suppress the amount of coma aberration as much as a CD even if the NA of the objective lens is increased by using a thin optical disk substrate of about 0.6 mm, which is half the thickness of the conventional CD substrate.

However, the allowable range of the substrate thickness error of the optical disc (the substrate thickness of the optical disc to be reproduced with respect to the optimum substrate thickness of the objective lens) is inversely proportional to the fourth power of the numerical aperture of the objective lens, and generally NA = 0. 0.1 mm for 45
Hereinafter, when NA = 0.6, it is 0.03 mm or less.
Therefore, there is a problem that the optical information reproducing device for the substrate of 0.6 mm cannot reproduce the CD of the substrate of 1.2 mm due to the spherical aberration.

When the above-mentioned high-density optical disk is put to practical use in the future, it is naturally desirable that the optical information reproducing apparatus can also reproduce the conventional CD. As a first conventional technique to meet this demand, an optical recording / reproducing device is disclosed in Japanese Patent Laid-Open No. 182690/1995. This is equipped with an objective lens corresponding to a high-density disc with an optimum substrate thickness of 0.6 mm, and when reproducing a CD (substrate thickness 1.2 mm), a concave lens is provided in the optical path between the objective lens and the beam splitter. The spherical aberration is corrected by inserting a conversion lens having the action of. As the second conventional technique, an optimum substrate thickness of 0.6 mm, which is also compatible with high density discs,
A method has also been proposed in which a spherical aberration is reduced by using an objective lens having an NA of 0.6 and limiting the aperture of the objective lens to reduce the NA to 0.35 when reproducing a CD. (56th Academic Society of Applied Physics, Lecture No. 29a-ZA-6 "Examination of compatibility between two types of discs having different thicknesses") As described above, the spherical aberration generation amount is proportional to the fourth power of NA. It focuses on doing. The size of the spot for reproducing the optical disk is inversely proportional to the NA of the objective lens and proportional to the wavelength of the light source. Therefore, when reproducing a CD using a light source having a wavelength of 635 nm, the NA of the objective lens is 0.35 to 0.35. 0.37 is sufficient. Therefore, the above-mentioned proposed method is designed so that by reducing the NA of the objective lens, the effective spherical aberration can be suppressed and the CD can be reproduced sufficiently even with the objective lens corresponding to the optimum substrate thickness of 0.6 mm.

[0006]

However, the first prior art requires a mechanism for moving the conversion lens in and out of the optical path according to the type of the optical disk to be reproduced, which complicates the structure of the optical head. Further, there is also a problem that due to a slight inclination that occurs when the conversion lens is taken in and out, the aberration increases, the quality of the light spot on the disc deteriorates, and the operating point of focus control shifts.
Similarly, even in the second conventional technique, the problem that the structure of the optical head becomes complicated is inevitable because a mechanism for moving the aperture for reducing the effective NA of the objective lens in and out of the optical path is required. Moreover, the NA of the objective lens
Is limited to 0.35, the spherical aberration generated when reproducing a CD is not small. Furthermore, when an aperture is inserted in the optical path and the objective lens moves in the direction orthogonal to the optical axis following the eccentricity of the optical disc, the reflected light from the optical disc is partially vignetted by the aperture and the entire reflected light is reflected. There is also the problem of not reaching the detector.
That is, when an aperture is provided in the optical path, it is desirable that the aperture move integrally with the objective lens.

An object of the present invention is to realize an optical head capable of stably reproducing two types of optical disks having different thicknesses with a simple structure and an optical information reproducing apparatus using the same.

[0008]

In order to solve the above-mentioned problems of the prior art, in the optical reproducing apparatus of the present invention, a polarized light is used as an aperture member for limiting the NA of the objective lens when reproducing a CD. A reflective optical member, in which a reflective hologram that gives a predetermined phase distribution to a light beam and a polarizing beam splitter are laminated, is provided below the objective lens in order to correct a spherical aberration that occurs when reproducing a CD. Further, an optical rotation means for selectively rotating the polarization direction of the laser light generated by the light source by about 90 ° is provided in the optical path.

That is, when reproducing a high density optical disc having a substrate thickness of 0.6 mm, the polarizing plate does not function as an aperture in the polarization direction of the laser light generated by the light source by the optical rotation means, and the laser light is the polarized beam. The splitter reflects the light and changes the direction to reach the objective lens. On the other hand, when reproducing a CD having a substrate thickness of 1.2 mm, the polarizing plate functions as an aperture, and the laser light is converted into a polarization direction that passes through the polarization beam splitter and reaches the reflection hologram. The laser light that reaches the reflection hologram has a predetermined phase distribution (details will be described later) that compensates for spherical aberration that occurs due to the difference in substrate thickness when the reflection hologram is reflected.

Diffracted light given by (1) is generated, and is condensed on the disk through the objective lens. As a result, a good focused spot with sufficiently reduced aberration can be obtained even during CD reproduction.

Further, by using the liquid crystal as the optical rotatory means, a special mechanism becomes unnecessary. Further, by driving the polarizing plate integrally with the objective lens, it is possible to adopt a configuration in which reflected light is not eclipsed even if the optical disc has eccentricity or the like. Therefore, according to the present invention, it is possible to favorably reproduce both the high density optical disc and the CD with one optical information reproducing device.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described in detail below with reference to the drawings. FIG. 1 shows an embodiment of an optical head which is a main part of the optical disk device of the present invention.
The semiconductor laser 1, which is a laser light source, has a wavelength of about 650.
At nm, it produces a luminous flux 2 which is polarized almost linearly and diverges. Light flux 2 is collimated by collimator lens 3
Is converted into the beam splitter 5 and reaches the liquid crystal optical rotator 6 which is the optical rotator. The liquid crystal optical rotator 6 has a function of selectively rotating the polarization direction of the light flux passing through the liquid crystal optical rotator by approximately 90 ° as described later. The light flux transmitted through the liquid crystal optical rotator 6 then enters the reflective optical member 8. In the reflective optical member 8, the first reflective surface 81, which is arranged at an angle of about 45 ° with respect to the optical axis, is a polarization beam splitter (hereinafter, referred to as PBS for simplicity). A reflection hologram 82 is provided on the back surface of the PBS 81 and on a reflection surface substantially parallel to the polarization beam splitter. This reflection hologram 82 generates a + 1st-order diffracted light beam or a -1st-order diffracted light beam when the light beam 4 reflects the hologram, and satisfactorily corrects the spherical aberration generated during CD reproduction on the wavefront of this diffracted light beam. It has a function of giving a predetermined phase distribution.

The light beam 9a reflected from the PBS 81 or the + 1st-order or -1st-order diffracted light beam 9b generated by the reflection hologram 82 and given the phase distribution is the polarization filter 1
Optical disk 13 or 1 through the objective lens 12 through 1
It is focused on 4 and connects the spots. The light beam reflected by the optical disk 13 or 14 follows the original optical path in the opposite direction, is reflected by the reflecting surface of the beam splitter 5, passes through the detection lens 15 and the cylindrical lens 16, and enters the photodetector 17. The photodetector 17 has a plurality of light receiving regions, and detects the focus, tracking error signal, information signal, etc. by calculating the output signals of these regions.

Here, the objective lens 12 has a substrate thickness of 0.6 m.
It is optimally designed so that the high density optical disc 13 of m can be reproduced. When reproducing the high-density optical disc 13, the liquid crystal optical rotator 6 and the liquid crystal drive circuit 7 cause the light beam 4 incident on the reflecting member 8 to reflect the PBS 81 and become the light beam 9a and pass through the polarizing filter 11 as it is, as will be described later. The polarization direction is set so as to reach the lens 12.

On the other hand, when reproducing a CD 14 having a substrate thickness of 1.2 mm, the liquid crystal optical rotator 6 and the liquid crystal drive circuit 7 make the luminous flux 4
Light is rotated (the polarization direction is rotated),
Reflective hologram 82 which is transmitted through 1 and is provided on the back surface thereof
The polarization direction is set so that the light will be reflected by. Then, the + 1st-order or -1st-order diffracted light beam 9b generated in the reflection hologram 82 enters the polarization filter 11. Polarizing filter 11
Since it has an annular filter portion as will be described later, the peripheral portion of the light flux 9b is shielded by the polarization filter 11, and only the central light flux passes through the polarization filter 11 and enters the objective lens 12. The polarization filter 11 is fixed to the movable part of the actuator integrally with the objective lens 12 as described later.

Next, the operation of the liquid crystal optical rotator will be described.
2 and 3 show that the combination of a liquid crystal optical rotator and a polarization filter can transmit or block light. The liquid crystal 6a enclosed in the liquid crystal optical rotator 6 is a twisted nematic (TN) liquid crystal. Transparent electrodes 6b and 6c are formed on both sides of the liquid crystal 6a. Transparent electrodes 6b, 6
The voltage from the voltage source 7a of the liquid crystal drive circuit 7 is supplied to c through the switch 7b. As shown in FIG. 2, the orientation of the liquid crystal 6a is twisted by 90 ° when no electric field is applied, and has an optical rotatory power of 90 °. Therefore, the polarized light 21 of the incident light 20 is rotated by 90 degrees and becomes the polarized light 22. At this time, if the transmission axis 23a of the polarization filter 23 (the axis indicating the polarization direction where the transmittance is maximum) 23a is aligned with the direction of the polarized light 22, the light is transmitted at a predetermined transmittance. Next, when a voltage is applied to the transparent electrodes 6b and 6c, the liquid crystal 6a is oriented along the electric field, so that the optical rotatory power disappears. At this time, the polarization of the light transmitted through the liquid crystal 2
Since 2 is orthogonal to the transmission axis 23a of the polarization filter 23, it does not transmit light. FIG. 4 shows the operating characteristics of the liquid crystal optical rotator 6 and shows the relationship between the applied voltage and the optical rotation angle. The operation threshold value is about 4V, and the optical activity disappears when a voltage of 6V or more is applied.

The means for rotating the polarization direction of the light beam by a predetermined angle is not limited to the means using the liquid crystal optical rotator as described above. For example, FIG. 5 is a diagram for explaining the operation of the half-wave plate which is the second embodiment of the optical rotation means, and is used in place of the liquid crystal optical rotator 6 of FIG. 1 /
When the optical axis of the two-wave plate and the polarization direction of the incident light 32 are matched, the outgoing light 33 is transmitted without being rotated. Here, the half-wave plate is set at 45 ° around the optical axis as shown by the arrow 31.
When rotated, the polarization direction of the emitted light 34 is rotated by 90 °, whereby the same operation as that of the liquid crystal optical rotator 6 can be performed.

Next, the reflective optical member 8 will be described in detail. The reflective optical member 8 is provided below the objective lens 12 and the polarization filter 11 and has a function of a rising mirror in a normal optical head. First, when reproducing a high density disc having a substrate thickness of 0.6 mm, the polarization direction of the light beam 4 is substantially aligned with the direction perpendicular to the plane of the drawing as shown in FIG. 6 by using an optical rotation means such as the liquid crystal optical rotator. Let
With such a polarization direction, the light beam 4 is reflected by the polarization beam splitter 81 with a reflectance of almost 100%. Then, as it is, the optical path is simply bent in the vertical direction to form a light beam 9a from the polarization filter 11 to the objective lens 12.

On the other hand, when reproducing a CD having a substrate thickness of 1.2 mm, the optical rotation means is driven to rotate the polarization direction of the light beam 4 by about 90 °, and as shown in FIG. 7, it is substantially parallel to the plane of the drawing. To suit. Then, this light beam is converted into the polarization beam splitter 8
1 passes through with a transmittance of almost 100% to reach the reflection hologram 82. Then, a + 1st-order or -1st-order diffracted light beam 9b having a predetermined phase distribution on the wavefront thereof is generated by this reflection hologram, passes through the polarization beam splitter 81 again, and reaches the objective lens 12 from the polarization filter 11. The phase distribution given to the wavefront of the diffracted light beam 9b at this time is 1.2 mm for the substrate thickness of 1.2 mm by using the objective lens 12 optimally designed so that the high-density optical disk 13 having the substrate thickness of 0.6 mm can be reproduced well. The phase distribution should reduce the spherical aberration that occurs when reproducing the CD 14. In general, it is known that the spherical aberration that occurs when the substrate thickness of the disc becomes thicker is reduced by making the parallel light flux incident on the objective lens slightly divergent. (See, for example, Japanese Patent Laid-Open No. 7-182690.) Therefore, spherical aberration can be reduced by giving a phase distribution such that the diffracted light beam 9b becomes a predetermined divergent light beam. That is, the reflection hologram 82 needs to have a property of functioning as a negative lens (concave lens) for diffracted light. FIG. 8 shows an example of the hologram pattern (the locus of the hologram lattice groove) of the reflection hologram having such a lens action. The curved line in the figure shows the locus of the hologram grating groove. As is clear from the figure, the reflection hologram having the lens function as described above has a pattern in which lattice grooves having an elliptical locus in which the radius of curvature and the center of curvature are gradually changed are arranged.

In the above example, the reflection hologram 82
Simply functions as a negative lens (concave lens), but the wavefront aberration (spherical aberration) distribution W (p) due to changes in the disk substrate thickness is approximately expressed by the following analytical expression W (p) = A. ( p4−p2) · NA4 (1) where A = Δt · (n2-1) / 8n3 0 ≦ p ≦ 1 Δt: change in disk substrate thickness n: refractive index of disk substrate NA: numerical aperture of objective lens Therefore, if the hologram pattern of the reflection hologram 82 is set so as to give the diffracted light beam 9b a phase distribution opposite to this wavefront aberration distribution, the spherical aberration described above can be corrected more strictly.

In the reflection hologram as described above, unnecessary diffracted light is generated in addition to the diffracted light flux necessary for actual signal reproduction, and these reduce the light utilization efficiency of the optical head or disturb the signal reproduction. However, such a problem can be sufficiently solved by a known technique such as forming a sawtooth-shaped hologram grating groove. In the above embodiment, the luminous flux 9a
Although a reflection hologram is used as a means for giving a predetermined phase distribution to the wavefront to convert it into a divergent light beam, other optical means may be used as a matter of course. For example, as shown in FIG. 9, a convex mirror 8 is used instead of the reflection hologram.
Even with the configuration in which 3 is provided or the configuration in which a normal negative lens (concave lens) and a flat plate mirror are combined, the reflected light beam can be converted into a predetermined divergent light beam as in the case of using the reflection hologram.

Further, all of the above-mentioned embodiments describe the case where the objective lens optimized for the disc having the thinner substrate is used, but it is naturally optimized for the disc having the thick substrate. Even when an objective lens is used, spherical aberration correction is performed according to the same principle, and two types of disks having different substrate thicknesses can be reproduced. In this case, an optical element such as a reflection hologram or a concave mirror, which gives a phase distribution such that the light beam incident on the objective lens becomes a convergent light beam when reproducing a disk having a thin substrate, may be provided in the reflective optical member 8. .

By the way, even if the reflection hologram as described above is used, when the spherical aberration cannot be sufficiently compensated, or when the wavelength of the laser beam is shifted or the objective lens is displaced, a sufficiently good focused spot is obtained. You may not get it. In order to solve such a problem, when reproducing a disc having a low recording density such as a CD, the effective aberration is reduced by selectively limiting the NA (numerical aperture) of the objective lens to reduce the focused spot quality. Means to prevent the deterioration of In the present invention, the polarization filter 11 is used as an optical means for selectively limiting the NA of the objective lens.

Next, the function of the polarization filter 11 used in the present invention will be described. This polarizing filter 11 is provided with a circular annular polarizing film whose transmittance changes depending on the polarization direction of the incident light beam, and has a structure in which the central portion transmits the light beam with a constant transmittance regardless of the polarization direction. ing. FIG. 10 shows a light beam that passes through the polarization filter 11 when reproducing the high density optical disc 13. The polarization direction a of the light beam 9a coincides with the direction of the transmission axis of the polarization film 11a formed on the peripheral annular portion on the upper surface of the polarization filter 11. Therefore, the entire light beam 9a passes through the polarization filter 11 as it is, and the transmitted light beam 25 enters the objective lens 12.

On the other hand, FIG. 11 shows the case of reproducing the CD 14. The light beam 9b is polarized by the liquid crystal optical rotator 6 in the polarization direction a in FIG.
The light is rotated so as to have a polarization direction b orthogonal to. Therefore, the light around the light beam 9b is blocked by the polarization filter 11,
Only the light beam 26 in the central portion where the polarizing film 11a is not formed enters the objective lens 12. As a result, the effective light beam diameter of the light beam incident on the objective lens 12 can be limited to reduce the effective NA. Although the polarization filter 11 is arranged so as to be orthogonal to the light beams 9a and 9b in the present embodiment, it is inclined by about 5 ° to 10 ° with respect to the light beams in order to reduce the incident angle dependency and wavelength dependency of the polarizing film. You may arrange it.

FIG. 12 shows the objective lens 12 and the polarization filter 1.
3 is a cross-sectional view showing a part of an actuator movable portion 60 for driving the optical axis 1 and the objective lens 12 in the optical axis and a direction orthogonal to the optical axis. The objective lens 12 and the polarization filter 11 are integrally attached to the movable portion 60 of the actuator. With such a structure, even if the objective lens 12 is displaced in the tracking direction (radial direction of the disk), unnecessary vignetting due to the polarization filter 11 does not occur.

FIG. 13 is a block diagram of an optical information reproducing apparatus using the optical pickup shown in FIG. The optical disk 13 or 14 is engaged with the motor 100. The motor drive circuit 117 receives the motor control signal 118 according to the control signal 119 issued from the system controller 121.
Is generated to control the rotation of the motor 100. Further, the laser drive circuit 115 receives the laser drive current 116 according to the control signal 120 issued from the system controller 121.
Is output to turn on the semiconductor laser in the optical pickup 101. Signal 1 from photodetector in optical pickup 101
02 is input to the preamplifier 103, and the servo error signal 1
04 and RF corresponding to the information signal recorded on the optical disc
The signal 107 is output. The servo circuit 105 outputs the actuator drive signal 106 based on the servo error signal 104 and the control signal 111 issued from the system controller 121, and controls the actuator to which the objective lens is attached. On the other hand, the RF signal 107 is the demodulation circuit 10
8 and data signals 109 and 110 are output. Then, the system controller 121 determines the type of the optical disk based on the data signal 109, and controls the liquid crystal control circuit 113 by the control signal 112. A liquid crystal control signal 114 is output from the liquid crystal control circuit 113 to control the liquid crystal optical rotator in the optical pickup 101.

[0028]

As described above, the optical head and the optical information reproducing apparatus of the present invention do not require a special mechanism, and can cope with two types of optical disks having different substrate thicknesses by electrical control. Therefore, the size and cost of the device can be reduced.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of an optical head which is a main part of an optical information reproducing apparatus of the present invention.

FIG. 2 is a configuration diagram of a liquid crystal optical rotator that is a first embodiment of the optical rotator.

FIG. 3 is a configuration diagram of a liquid crystal optical rotator that is a first embodiment of the optical rotator.

FIG. 4 is an operational characteristic diagram of a liquid crystal optical rotator.

FIG. 5 is a configuration diagram of a half-wave plate that is a second embodiment of the optical rotator.

FIG. 6 is a schematic sectional view showing a first embodiment of the reflective optical element of the present invention.

FIG. 7 is a schematic cross-sectional view showing a first embodiment of the reflective optical element of the present invention.

FIG. 8 is a plan view showing an example of a hologram pattern of a reflective hologram used in the present invention.

FIG. 9 is a schematic cross-sectional view showing a second embodiment of the reflective optical element of the present invention.

FIG. 10 is a configuration diagram of an embodiment of a polarization filter.

FIG. 11 is a configuration diagram of an embodiment of a polarization filter.

FIG. 12 is a schematic cross-sectional view showing the configuration of the main part of the actuator.

FIG. 13 is a block diagram showing an embodiment of an optical information reproducing apparatus of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Semiconductor laser, 6 ... Liquid crystal optical rotator, 7 ... Liquid crystal drive circuit, 11 ... Polarization filter, 12 ... Objective lens, 13, 14 ... Optical disk, 30 ... 1/2 wavelength plate, 81 ... Polarization beam splitter, 82 ... Reflection Shape hologram, 101 ... Optical pickup.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Ken Shimano, Ken-ji, Kokubunji, Tokyo 1-280 Higashi-Kengokubo, Ltd. Central Research Laboratory, Hitachi, Ltd. (72) Inventor, Akira Arimoto 1-280, Higashi-Kengokubo, Kokubunji-shi, Tokyo Hitachi, Ltd. Central Research Laboratory (72) Inventor Masayuki Inoue 292, Yoshida-cho, Totsuka-ku, Yokohama-shi, Kanagawa Stock company Hitachi Ltd. multimedia system development headquarters

Claims (7)

[Claims] 1. An optical head capable of reproducing optical disks having different substrate thicknesses, comprising a light source, an objective lens for converging laser light emitted from the light source on the optical disk, and detection means for detecting reflected light from the optical disk. It is characterized in that it is provided with a light-shielding means arranged in the optical path of the light source and the objective lens for shielding a part of the laser light, and an optical element for giving a predetermined phase distribution to the wavefront of the laser light. Optical head. 2. The optical head according to claim 1, wherein:
An optical head comprising an optical rotation means for rotating a polarization direction of laser light in an optical path between a light source and an objective lens. 3. The optical head according to claim 2, wherein:
An optical head characterized in that the optical rotation means is a liquid crystal optical rotator. 4. The optical head according to claim 2 or 3, wherein the light shielding means is a polarizing plate. 5. The optical head according to claim 2, 3 or 4, wherein the optical element that gives a predetermined phase distribution to the wavefront of the laser light is a reflection hologram element having a function of a negative lens. An optical head characterized by. 6. The optical head according to claim 2, 3 or 4 or 5, wherein the optical element for giving a predetermined phase distribution to the wavefront of the laser light is a laminated beam splitter and the reflection hologram element. An optical head having an optical element having the above structure. 7. An optical information reproducing apparatus equipped with the optical head according to claim 1, 2 or 3 or 4 or 5 or 6.