JPH0628693A - Separation type optical head - Google Patents

Abstract

(57) [Summary] [Object] To provide a separation type optical head capable of sufficiently suppressing the offset of a tracking error signal even when the tracking control means is installed and operated in a fixed optical system. [Structure] Wedge prism 40 and prism driving means 41
Is provided in the fixed optical system 10 and the wedge prism 40 is rotated to deflect the light beam emitted from the second surface 40b and to displace the emission position of the light beam to perform tracking control operation. To Then, the displacement of the incident position of the light beam on the objective lens 3 due to the above-mentioned deflection and the displacement of the emission position of the light beam on the second surface 40b are substantially canceled each other.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical head for recording / reproducing information on / from an optical information storage medium such as an optical disk or an optical card. In particular, the optical system is divided into a movable optical system and a fixed optical system. The present invention relates to a separation type optical head.

[0002]

2. Description of the Related Art In recent years, optical discs using laser light,
An optical information device that records and reproduces information on an optical information storage medium such as an optical card has been put into practical use. For this optical information device, there is a demand for speeding up the access operation for sending and positioning the optical head at high speed in order to record information at a predetermined position quickly and reproduce information at a predetermined position quickly. It is rising. As a method of realizing this high-speed access, a separation type optical head in which the optical head is separated into a movable optical system and a fixed optical system has been proposed.

As an example of this separation type optical head, an objective lens for converging and irradiating a light beam on an optical information storage medium, and a two-dimensional driving of this objective lens for performing focusing control operation and tracking control operation. A two-dimensional objective lens driving device and a mirror that converts the traveling direction of the light beam are movable as a movable optical system in a direction traversing the information track, and the remaining light source, lens, prism, photodetector, etc. are fixed optics. There is a system fixed to the base of an optical information device. This separate type optical head needs only to send the movable optical system as an access operation, and the mass of the movable portion can be reduced as compared with the integrated type optical head that sends the entire optical system, so that high speed access can be achieved.

In addition to such a separate type optical head, the objective lens driving device is changed from a two-dimensional type to a one-dimensional type, and the objective lens driving device is caused to perform only focusing control operation and tracking control operation. Has proposed a separation type optical head configured to deflect a light beam by a galvano mirror (rotating mirror) provided in a fixed optical system.

According to the latter split type optical head, the objective lens driving device can be made lighter in weight and the mass of the movable portion can be reduced as compared with the former, so that further high speed access can be achieved.

An example of the latter separation type optical head is disclosed in Japanese Patent Application Laid-Open No. 3-214435.

[0007]

However, in the separation type optical head in which the galvano mirror is provided in the fixed optical system, the galvano mirror is rotated from the initial state to deflect the light beam in order to perform the tracking operation. When irradiating the optical information storage medium through the objective lens, the light beam reflected by the optical information storage medium is also similarly deflected and returned to the fixed optical system and incident on the photodetector at a position deviated from the initial state. become. When the light beam entering the photodetector is displaced in this way, there is a problem in that a general tracking error signal detection method such as the push-pull method causes an offset in the detection signal.

Hereinafter, this problem will be described in detail. FIG. 5 is a diagram showing the principle configuration of the above-mentioned separation type optical head in which a galvano mirror is provided in a fixed optical system. In the figure, 2 is a carriage, which is driven in the x direction by a driving means (not shown) such as a linear motor. On this carriage 2, an objective lens 3 for converging and irradiating a light beam on the optical information storage medium 1 as a minute light spot, an objective lens driving device 4 for driving the objective lens 3, and a traveling of the light beam. A rising mirror 5 for changing the direction is mounted. Hereinafter, the carriage 2, the objective lens 3, the objective lens driving device 4, and the raising mirror 5 are collectively referred to as a movable optical system 6.

Reference numeral 10 denotes a fixed optical system including optical components other than the objective lens 3 and the raising mirror 5 described above, which is fixed to the base portion of the optical information device. In this fixed optical system 10, 11 is a semiconductor laser that emits a light beam, 12 is a collimator lens that converts the light beam from the semiconductor laser 11 into parallel light, and 13 is a light beam that is transmitted from the collimator lens 12. A beam splitter for reflecting the reflected light from the optical information storage medium 1 and an optical path conversion mirror 14 for changing the traveling direction of the light beam transmitted through the beam splitter 13.
A galvano mirror 15 changes (deflects) the traveling direction of the light beam by rotating the flat mirror, and performs tracking control operation by deflecting the light beam from the optical path conversion mirror 14. The light beam deflected by the galvanometer mirror 15 passes through the rising mirror 5 and the objective lens 3 and is focused and irradiated as a minute light spot on the optical information storage medium 1 to record information.

Then, the light beam reflected by the optical information storage medium 1 returns to the beam splitter 13 through the objective lens 3, the rising mirror 5, the galvano mirror 15, and the optical path changing mirror 14. Reference numeral 16 denotes a half prism, which splits the light beam reflected by the beam splitter 13 into transmitted light and reflected light. The light beam that has passed through the half prism 16 is converged by the first converging lens 17 on the first photodetector 18, and is converted into an electric signal. The light beam reflected by the half prism 16 enters the second converging lens 19 and is given astigmatism by the cylindrical lens 20,
It is focused on the second photodetector 21 and converted into an electric signal.

Here, the first photodetector 18 has a light receiving surface of 2
It is divided and the tracking error signal is obtained by the well-known push-pull method. The light receiving surface of the second photodetector 21 is divided into four in a square shape, and a focus error signal can be obtained by the well-known astigmatism method.

In accordance with this focus error signal, the objective lens 3 is driven in the z direction in the figure by the objective lens driving device 4, so that the focus of the objective lens 3 is always located on the recording surface of the optical information storage medium 1. Focusing control operation is performed as described above. Also, depending on the tracking error signal,
The tracking control operation is performed by driving the galvanometer mirror 15 in the direction of the arrow R in the figure to cause the laser spot to follow the information track of the optical information storage medium 1. In the access operation, the carriage 2 is moved in the x direction in the figure,
It is performed by positioning the optical information storage medium 1 on a desired information track.

FIG. 6 is a state explanatory view for explaining a state of a main part when the galvano mirror 15 is rotated from the initial state to deflect the light beam for performing the tracking operation in the separation type optical head of FIG. . In the figure, the galvano mirror 15 is shown in a simplified manner, and the initial state is 15a.
And the state rotated from the initial state is indicated by 15b.

When the galvano mirror 15 is in the initial state 15a, the light beam (outgoing light) 25a directed to the movable optical system 6 travels along the optical path indicated by the solid line, and passes through the rising mirror 5 and the objective lens 3 to the optical information storage medium. 1 is focused and irradiated as a minute laser spot. The light beam (return light) 25b reflected by the optical information storage medium 1 is the forward light 25 described above.
The same optical path as a is incident on the half prism 16 through the objective lens 3, the rising mirror 5, the galvano mirror 15, the optical path conversion mirror 14, and the beam splitter 13. The light beam that has passed through the half prism 16 is converged on the center of the first photodetector 18 via the first converging lens 17.

On the other hand, when the galvano mirror 15 is moved from the initial state 15a to the state of reference numeral 15b for performing the tracking control operation, the light beam (forward light) 26a directed to the movable optical system 6 is deflected by the galvano mirror 15. Therefore, the light travels along an optical path indicated by a broken line (an optical path different from the above-mentioned forward light 25a), and is focused and irradiated as a minute light spot on the optical information storage medium 1 through the raising mirror 5 and the objective lens 3. . The light beam (return light) 26b reflected by the optical information storage medium 1 returns on an optical path different from that of the outgoing light 26a, and the objective lens 3, the rising mirror 5, the galvano mirror 15, the optical path changing mirror 14, and the beam splitter 1 are included.
It is incident on the half prism 16 after passing through 3. The light beam that has passed through the half prism 16 receives the first converging lens 17
Then, the light is focused on the first photodetector 18.

Here, the incident position of the return light 26b with respect to the first photodetector 18 is a position deviated from the incident position of the return light 25b as is clear from the drawing. this is,
This means that an offset occurs in the tracking error signal detected by the first photodetector 18.

As described above, in the separation type optical head in which the galvano mirror is provided in the fixed optical system, when the galvano mirror is rotated from the initial state to perform the tracking operation, the light beam incident on the photodetector is displaced. Cause
Since an offset occurs in the tracking error signal, the positional accuracy of the laser spot with respect to the information track of the optical information storage medium 1 deteriorates. Therefore, there is a problem that it is difficult to record and reproduce information in the optical information storage medium 1.

On the other hand, in the separation type optical head in which the two-dimensional type objective lens driving device is mounted on the carriage, in view of the offset of the tracking error signal, the separation type optical head in which the galvano mirror is provided in the fixed optical system is used. Although it is reduced, since the objective lens is movable in the tracking direction, it is likely to vibrate during the access operation, and the pull-in operation of tracking control needs to wait for the oscillation to settle, resulting in a longer access time. There were challenges. Further, as described above, the mass of the movable portion also increases, and there is a problem in that high speed access cannot be achieved in this respect as well.

The present invention has been made in view of the above circumstances, and an object thereof is to install a tracking control means in a fixed optical system without installing it in a movable optical system and to obtain a tracking error. An object of the present invention is to provide a separation type optical head capable of sufficiently suppressing signal offset.

[0020]

The above object includes an objective lens for converging and irradiating a light beam on an optical storage information medium, and a raising mirror for converting the traveling direction of the light beam in the optical axis direction of the objective lens. A movable optical system movably installed in a direction crossing an information track of the optical storage information medium, a light source for emitting a light beam, and a light beam reflected by the optical storage information medium to receive an electric signal. In a separation type optical head including a fixed optical system including a photodetector for conversion, a first surface on which a light beam from the light source is incident and a second surface for emitting the light beam in the direction of the mirror are non-parallel. And a prism driving means for rotating the wedge prism about a rotation axis substantially parallel to the first surface and the second surface in the fixed optical system. The above By rotating the rust type prism, the emission direction of the light beam emitted from the second surface is deflected, and tracking control is performed so that the light spot condensed on the optical storage information medium follows a predetermined information track. This is accomplished by configuring it to perform an action.

[0021]

The wedge prism is a wedge type prism in which the first surface on which the light beam from the light source is incident and the second surface on which the light beam is emitted in the direction of the objective lens through the raising mirror are not parallel to each other. Has the shape of. The wedge prism is rotated by a prism driving means about a rotation axis that is substantially parallel to the first surface and the second surface. Then, the wedge prism and the prism driving means are installed in the fixed optical system.

By rotating the wedge prism, the light beam emitted from the second surface is deflected, enters the objective lens through the rising mirror, and is condensed into the light spot on the optical storage information medium. Move the position. As a result, a tracking control operation is performed in which the light spot follows a predetermined information track. At this time, the light beam entering the objective lens due to the deflection is displaced from the optical axis (center) of the objective lens and enters.

However, when the wedge prism is rotated, the emission position of the light beam emitted from the second surface can be displaced simultaneously with the above deflection.

Therefore, the displacement of the emission position of the light beam emitted from the second surface due to the rotation of the wedge prism and the displacement of the incidence position of the light beam on the objective lens due to the deflection of the emission direction of the light beam are mutually different. The shape of the wedge prism, the installation angle, and the refractive index were set so that they would almost cancel each other out.

As a result, the amount of displacement of the light beam incident on the objective lens with respect to the optical axis of the objective lens can be greatly reduced, even though the tracking control operation is performed by deflecting the light beam by the fixed optical system, and the optical memory information is stored. Since the positional deviation amount of the light beam reflected by the medium and returning to the fixed optical system can also be reduced, the occurrence of offset in the tracking error signal can be suppressed.

[0026]

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

FIG. 1 is a constitutional view showing the constitution of an embodiment of the separation type optical head according to the present invention. In the figure, 2 is a carriage, which is driven in the x direction by a driving means (not shown) such as a linear motor. On this carriage 2, an objective lens 3 for converging and irradiating a light beam on the optical information storage medium 1 as a minute light spot, and an objective lens 3
An objective lens driving device 4 for driving the optical axis and a rising mirror 5 for converting the traveling direction of the light beam are mounted. Hereinafter, the carriage 2, the objective lens 3, the objective lens driving device 4, and the raising mirror 5 are collectively referred to as a movable optical system 6.

A fixed optical system 10 is composed of optical components other than the objective lens 3 and the raising mirror 5 described above, and is fixed to the base portion of the optical information device. In this fixed optical system 10, 11 is a semiconductor laser that emits a light beam, 12 is a collimator lens that converts the light beam from the semiconductor laser 11 into parallel light, and 13 is a light beam that is transmitted from the collimator lens 12. The beam splitter reflects the return light from the optical information storage medium 1.

Reference numeral 40 denotes a wedge prism, which is a first surface 40a on which the light beam from the beam splitter 13 is incident.
And has a wedge shape in which the second surface 40b from which the transmitted light beam is emitted is formed non-parallel. The wedge prism 40 is rotated by the prism driving means 41 in the arrow R direction about the direction (y direction) perpendicular to the paper surface, which is a direction substantially parallel to the first surface and the second surface. . The light beam that has passed through the wedge prism 40 is reflected by the rising mirror 5.
Then, through the objective lens 3, the optical information storage medium 1 is focused and irradiated as a minute laser spot to record information. As will be described in detail later, by rotating the wedge prism 40, the outgoing direction of the transmitted light beam can be deflected, whereby the light spot is moved on the optical information storage medium 1 to perform the tracking operation.

Then, the light beam reflected by the optical information storage medium 1 returns to the beam splitter 13 via the objective lens 3, the rising mirror 5 and the wedge prism 40. Reference numeral 16 denotes a half prism, which splits the light beam reflected by the beam splitter 13 into transmitted light and reflected light. The light beam that has passed through the half prism 16 is converged by the first converging lens 17 on the first photodetector 18, and is converted into an electric signal. The light beam reflected by the half prism 16 is incident on the second converging lens 19 and is given astigmatism by the cylindrical lens 20, and the second photodetector 21.
Is converted into an electric signal.

Here, the first photodetector 18 has a light receiving surface of 2
It is divided and the tracking error signal is obtained by the well-known push-pull method. The light receiving surface of the second photodetector 21 is divided into four in a square shape, and a focus error signal can be obtained by the well-known astigmatism method. Further, the information signal recorded in the optical information storage medium 1 is detected from the sum of the electric signals from the respective light receiving surfaces of the second photodetector 21.

In accordance with this focus error signal, the objective lens 3 is driven in the z direction in the drawing by the objective lens driving device 4, so that the focus of the objective lens 3 is always located on the recording surface of the optical information storage medium 1. Focusing control is performed as described above. Further, in accordance with the tracking error signal, the wedge prism 40 is driven in the direction of arrow R in the figure by the prism driving means 41 to move the laser spot to the optical information storage medium 1.
The tracking control operation is performed by following the information track of. The access operation is performed by moving the carriage 2 in the x direction in the figure and positioning it on a desired information track of the optical information storage medium 1.

Here, the half prism 16 and the first
The converging lens 17, the first photodetector 18, the second converging lens 19, the cylindrical lens 20, and the second photodetector 21 are collectively referred to as a photodetecting system 23. The photodetecting system shown in FIG. 23
The configuration of is an example, and if a focus error signal, a tracking error signal, and an information signal can be obtained,
Other configurations may be used.

Next, the wedge prism 40 will be described in detail. FIG. 2 is an enlarged configuration diagram of the wedge prism 40. When the light beam is transmitted through the wedge prism 40, the traveling direction of the incident light beam incident on the first surface 40a of the wedge prism 40 and the traveling direction of the outgoing light beam emitted from the second surface 40b are set. The angle (hereinafter referred to as the deflection angle) δ is expressed by the following equation.

[0035]

[Equation 1]

Here, β is the first of the wedge prism 40.
The angle formed by the surface 40a and the second surface 40b, n is the refractive index, and φ is the angle formed by the normal line of the first surface 40a and the incident light beam (hereinafter referred to as the incident angle).

As can be seen from Equation 1, the deflection angle δ of the outgoing light beam changes depending on the incident angle φ of the incident light beam incident on the first surface 40a of the wedge prism 40. That is, by rotating the wedge prism 40 about the direction perpendicular to the paper surface as a rotation axis and changing the incident angle φ, the emission direction of the emitted light beam can be deflected. The light beam deflected and emitted by the wedge prism 40 is
As shown in FIG. 1, the objective lens 3 is passed through the raising mirror 5.
And the light spot is moved on the optical information storage medium 1 to perform the tracking control operation.

The shift amount Δη of the incident position of the light beam generated when the light beam is incident on the objective lens 3 by the above deflection is
Deflection angle δ of the light beam emitted from the wedge prism 40
Then, using the distance L from the wedge prism 40 to the objective lens 3, it can be expressed as the following equation.

[0039]

[Equation 2]

On the other hand, the distance ζ between the emission point P of the optical axis when the light beam is emitted from the wedge-shaped prism 40 and the optical axis of the incident light beam is also expressed by the following equation 3 with respect to the incident angle φ of the incident light beam. Change according to.

[0041]

[Equation 3]

Here, t represents the distance between the first surface 40a and the second surface 40b of the wedge prism 40 as shown in the figure.

Consider a case where the wedge prism 40 is rotated from the initial state and the incident angle φ of the incident light beam incident on the first surface 40a is changed from φ ₀ to φ ₁ . In this case, the position shift amount Δζ of the light beam emitted from the wedge-shaped prism 40 is expressed by Equation 4.

[0044]

[Equation 4]

FIG. 3 shows an incident angle φ of the light beam on the wedge prism 40 when β = 1 °, n = 1.51, and t = 3.3 mm is used as an example of the wedge prism 40. The relationship between the deflection angle δ of the light beam emitted from the wedge-shaped prism 40 and the relationship between the incident angle φ of the incident light beam and the position shift amount Δζ of the emitted light beam are shown. The light beam incident angle φ on the horizontal axis and the deflection angle δ of the outgoing light beam on the vertical axis in FIG. 3 are positive in the counterclockwise direction. Further, the position shift amount Δζ of the outgoing light beam, which is the other vertical axis, is positive when it is shifted upward from the optical axis of the incident light beam in FIG.

As is apparent from the figure, when the light beam is transmitted through the wedge prism 40, the wedge prism 4
The deflection direction of the light beam emitted from 0 is the same regardless of the incident direction of the incident light beam (positive or negative of the incident angle φ) (in the example shown in FIG. 3, the deflection angle δ of the emitted light beam is always negative). Is on the side). On the other hand, whether the position shift amount Δζ of the emitted light beam is positive or negative is the incident angle φ of the incident light beam.
Matches the sign of.

Therefore, for example, when the incident angle φ is changed to the plus side in the example of FIG. 3, the deflection angle δ and the position shift amount Δζ of the light beam emitted from the wedge prism 40 are opposite to each other. . This indicates that the shift amount of the light beam emitted from the wedge prism 40 and entering the objective lens via the raising mirror can be reduced with respect to the optical axis (center) of the objective lens. The shift amount Δε of the light beam with respect to the optical axis of the objective lens is, as shown in formula 5, the shift amount Δη depending on the deflection of the outgoing light beam emitted from the wedge prism 40 (given by formula 2) and It is represented by the difference from the positional shift amount of the emitted light beam (given by Equation 4) Δζ.

[0048]

[Equation 5]

From the above, the angle β between the first surface 40a and the second surface 40b of the wedge prism 40, the surface interval t,
By designing the refractive index n and the initial incident angle φ ₀ of the prism to be optimum values, the shift amount Δε of the light beam with respect to the optical axis of the objective lens can be significantly reduced. As a result, the amount of positional deviation of the light beam reflected by the optical information storage medium 1 and returning to the fixed optical system can be greatly reduced, and the amount of positional deviation of the incident position with respect to the photodetector can also be reduced, resulting in the occurrence of offset in the tracking error signal. Can be suppressed.

In FIG. 4, in the separation type optical head shown in FIG. 1, the wedge type prism 40 is rotated by the prism driving means 41 to perform the tracking control operation to move the light spot on the optical information storage medium 1. It is an example of the result of calculating the relationship between the position correction amount (movement amount) of the light spot at this time and the position shift amount Δε of the incident light beam generated in the objective lens 3 in accordance therewith by the formula (5). In the figure, the solid line indicates the wedge prism 40 (β = 1 °, n = 1.5
1, t = 3.3 mm) is a calculation result when the incident angle is set to φ = 45 ° in the initial state (light spot position correction amount = 0). For comparison, the broken line shows the calculation result when the rotary mirror (galvano mirror) 15 shown in FIG. 5 is used in place of the wedge prism 40 and the prism driving means 41 as the tracking control means. In the calculation, the focal length f of the objective lens 3 is f =
3.3 mm, the objective lens 3 to the wedge prism 4
0 or the distance to the turning mirror 15 was L = 85 mm.

From the figure, the wedge prism 40 designed to have a predetermined shape, size, and installation angle based on the above-mentioned mathematical formulas.
The position correction amount of the light spot is -10
It can be seen that the position shift amount Δε of the incident light beam on the objective lens 3 can be set to 20 μm or less in the range of μm to +10 μm. Further, by using the wedge type prism 40, a rotating mirror (galvano mirror) 15 is used as in the conventional case.
The position shift amount Δε is 1/10 compared to the case where
Can be suppressed below.

As described above, according to the present embodiment, the position shift amount Δε of the incident light beam on the objective lens 3 is caused despite the tracking control operation by deflecting the light beam by the fixed optical system 10. Can be significantly reduced, the amount of positional deviation of the light beam reflected by the optical information storage medium 1 and returning to the fixed optical system can be reduced, and as a result, the amount of positional deviation of the incident position with respect to the photodetector can be reduced. The occurrence of offset can be suppressed. As a result, the positional accuracy of the light spot with respect to the information track of the optical information storage medium 1 can be improved, and information can be recorded / reproduced favorably.

[0053]

As described above, according to the present invention, the position shift amount of the incident light beam in the objective lens is greatly reduced despite the tracking operation by deflecting the light beam by the fixed optical system. It is possible to reduce the amount of positional deviation of the light beam reflected by the optical information storage medium and returning to the fixed optical system. As a result, the amount of positional deviation of the incident position with respect to the photodetector can also be reduced, thus suppressing the occurrence of offset in the tracking error signal. it can.

Since the wedge prism and the prism driving means are installed in the fixed optical system instead of the movable optical system as the tracking control means, the wedge prism does not vibrate during the access operation, and the tracking control is pulled in. The operation is facilitated, and as a result, high speed access can be achieved.

Further, since the wedge prism and the prism driving means are installed not in the movable optical system but in the fixed optical system, the weight of the movable optical system can be reduced, and the high speed access can be achieved.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing an embodiment of a separation type optical head according to the present invention.

FIG. 2 is an enlarged configuration diagram of a wedge prism according to the present invention.

FIG. 3 is a characteristic diagram when the wedge prism according to the present invention is rotated.

FIG. 4 is an explanatory diagram comparing characteristics of a wedge prism according to the present invention and a conventional rotating mirror (galvano mirror).

FIG. 5 is a configuration diagram showing an example of a conventional separation type optical head.

FIG. 6 is a state explanatory view of a conventional separation type optical head.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Optical information storage medium, 2 ... Carriage, 3 ... Objective lens, 5 ... Standup mirror, 6 ... Movable optical system, 10 ... Fixed optical system, 11 ... Semiconductor laser, 18, 21 ... Photodetector,
40 ... Wedge type prism, 41 ... Prism driving means.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Masayuki Inoue 292 Yoshida-cho, Totsuka-ku, Yokohama-shi Hitachi, Ltd. Visual Media Research Laboratory

Claims (2)

[Claims] 1. An optical storage information medium comprising an objective lens for converging and irradiating a light beam and a mirror for converting a traveling direction of the light beam in an optical axis direction of the objective lens, the optical storage information medium traversing an information track. And a fixed optical system including a light source that emits a light beam and a photodetector that receives the light beam reflected by the optical storage information medium and converts it into an electrical signal. And a wedge-shaped prism in which a first surface on which a light beam from the light source is incident and a second surface on which the light beam is emitted toward the mirror are non-parallel, and the wedge-shaped prism The fixed optical system is provided with prism driving means for rotating the prism around a rotation axis substantially parallel to the first surface and the second surface, and the wedge type prism is rotated by the prism driving means. Previous The tracking control operation is performed so that the emission direction of the light beam emitted from the second surface is deflected and the light spot focused on the optical storage information medium follows a predetermined information track. A distinctive optical head. 2. The displacement of the emission position of the light beam emitted from the second surface according to the rotation of the wedge prism and the incidence position of the light beam on the objective lens due to the deflection of the emission direction of the light beam. A split type optical head characterized in that the shape, the installation angle and the refractive index of the wedge type prism are set so that the displacements cancel each other out.