JPH06243503A - Beam splitter and optical head - Google Patents

Abstract

PURPOSE:To provide a beam splitter capable of simplifying a photodetecting system. CONSTITUTION:This beam splitter is provided with a planar first surface 2 splitting an incident beam to a reflected beam and a transmitted beam, and a second surface 3 having, positive refractive power converging and emitting the transmitted beam. In such a manner, by using the beam splitter having a converging action, a detective optical system in an optical system for an optical head is simplified, and the number of parts constituting the optical head is reduced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical head used for optically writing or reading information on an information recording medium such as a video disc.
In particular, a beam splitter that integrates a separation function of separating a reflected light beam from the information recording medium and a function of a detection optical system used to form a condensed spot of the reflected light beam from the information recording medium on the photodetector. It relates to an optical head.

[0002]

2. Description of the Related Art An optical head is a key device in an optical disk device that records or reproduces information on an information recording medium using light. This optical head is composed of many optical components such as lenses and prisms, and requires expensive and highly accurate assembly and adjustment.

A typical example of an optical head is shown in FIG. That is, the divergent light flux from the semiconductor laser 34 becomes a substantially parallel light flux by the collimator lens 35, passes through the beam splitter 36, and is focused on the recording surface of the information recording medium 38 by the objective lens 37. The light beam reflected from the information recording medium 38 is again made into a substantially parallel light beam by the objective lens 37 and is separated from the light source direction by the beam splitter 36. Next, the convex lens 39, the cylindrical lens 40, and the concave lens 41
And enters the photodetector 42. Here, the convex lens 39 has a function of converting a parallel light beam into a convergent light beam and forming an image on the photodetector 42, and the cylindrical lens 40 has a function of giving astigmatism to the convergent light beam. Further, the concave lens 41 in front of the photodetector 42 has a function of increasing the size of the spot on the photodetector 42, but the concave lens 41 is necessary only for downsizing the optical system. Therefore, it is not necessarily an optical component necessary for the structure of the optical head. The photodetector 42 is arranged at an intermediate position of the astigmatism generated by the cylindrical lens 40, and the shape of the spot formed on the photodetector 42 changes when the objective lens 37 is out of focus. From this, the focus error signal of the objective lens 37 can be obtained.

On the other hand, in order to reduce the size and cost of the optical head, many techniques for reducing the number of optical parts used in the optical system have been proposed. For example, FIG. 6 is a schematic diagram of an optical system of an optical head of a compact disc player. A divergent light beam from a semiconductor laser 43 is first reflected by a beam splitter 44 and collected by an objective lens 45 on a recording surface of an information recording medium 46. Be illuminated. The reflected light from the information recording medium 46 is converged by the objective lens 45, and this time it is transmitted through the beam splitter 44 and is incident on the photodetector 47. Astigmatism occurs when the convergent light flux to the photodetector 47 passes through the beam splitter 44, and it is used as a signal for focus error detection.

[0005]

The optical head for a compact disc can be made to have a very small number of parts, but since the beam splitter 44 is arranged during diverging light, it is used as the beam splitter 44. When a polarizing beam splitter is used, the characteristics of the polarizing film cannot be fully exhibited.

Therefore, in an optical system using a polarization beam splitter as a light beam separating means, a divergent light beam from the semiconductor laser 34 is converted into a parallel light beam by using a collimator lens 35 as shown in FIG. It is necessary to make it incident. As a result, the convex lens 39 and the cylindrical lens 40 shown in FIG. 5 are used as a lens system for condensing the reflected light flux from the disc on the photodetector 47.
Since a so-called detection optical system including the concave lens 41 and the concave lens 41 is required, the optical system requires assembly and adjustment and is expensive.

On the other hand, as a method for making the beam splitter have the function of another optical element, Japanese Patent Laid-Open No. 62-26654 proposes a means for forming a hologram on the surface of the polarization beam splitter. Had a drawback that the allowable tolerance between the surfaces was small. Therefore, in view of the above problems, an object of the present invention is to provide a beam splitter and an optical head that can simplify the detection optical system.

[0008]

A beam splitter according to a first aspect of the present invention has a flat first surface for separating incident light into reflected light and transmitted light and has a positive refracting power for collecting and emitting transmitted light. It has a second surface having. A beam splitter according to a second aspect is the beam splitter according to the first aspect, wherein the second surface is constituted by a rotationally symmetric spherical surface or an aspherical surface having a positive refractive power, and the rotational symmetry axis of the spherical surface or the aspherical surface and the first surface. The normals are non-parallel to each other.

The beam splitter of claim 3 is the same as that of claim 2.
In the above, the axis of rotational symmetry of the second surface coincides with the optical axis of the light beam refracted by the first surface.
A beam splitter according to a fourth aspect is the beam splitter according to the first aspect, wherein the second surface is constituted by a toric surface having a positive refractive power, and a line of intersection of two symmetrical planes of the toric surface orthogonal to each other and a first surface. The normals are non-parallel to each other.

The beam splitter of claim 5 is the same as that of claim 4
At 2 in which the toric faces of the second face are orthogonal to each other
At least one of the cross sections in the two planes of symmetry has an aspherical cross-sectional shape. A beam splitter according to claim 6 is the beam splitter according to claim 4 or claim 5, wherein a line of intersection of two symmetrical planes orthogonal to each other of the toric surface of the second surface coincides with the optical axis of the light beam refracted by the first surface. It is a feature.

According to another aspect of the optical head of the present invention, there is provided a light source, and collimating means for converting a light beam emitted from the light source into substantially parallel light.
Condensing means for condensing substantially parallel light on the information recording medium, light receiving means for receiving emitted light from the information recording medium, and beam splitter for separating the emitted light from the information recording medium and making it incident on the light receiving means. And the beam splitter is constituted by the beam splitter according to claim 1, claim 2, claim 3, claim 4, claim 5, or claim 6.

[0012]

According to the beam splitter of the first aspect, when the incident light is incident on the planar first surface, it is separated into reflected light and transmitted light. Further, the transmitted light is emitted as convergent light on the second surface having a positive refractive power. Thus, the first surface has the function of separating the light flux and the second surface has the function of the detection lens for condensing the transmitted light flux, so that the detection optical system is simplified when used in the optical head. Therefore, the number of parts constituting the optical head can be reduced.

According to a second aspect of the beam splitter of the present invention, in the first aspect, the second surface is constituted by a rotationally symmetric spherical surface or aspherical surface having a positive refractive power, and a spherical or aspherical rotational symmetry axis, Since the normals of the first surface are not parallel to each other, the same effect as in claim 1 is obtained, and in the case of an aspherical surface, the light when the focusing means for focusing on the information recording medium of the optical head is defocused Irregularity of the illuminance of the spot on the light receiving means on the detection side is suppressed.

According to the beam splitter of claim 3, in claim 2, the axis of rotational symmetry of the second surface coincides with the optical axis of the light beam refracted by the first surface, and therefore has the same effect as in claim 2. There is. According to a fourth aspect of the beam splitter of the present invention, in the first aspect, the second surface is constituted by a toric surface having a positive refractive power, and a line of intersection of two symmetrical planes of the toric surface orthogonal to each other Since the normals of the surfaces are not parallel to each other, in addition to the effect of claim 1, the first surface has the effect of separating the light beam, and one of the light beams separated by the second surface has a desired astigmatism. Since it has a function of making a convergent light beam having an aberration, it is not necessary to separately provide an astigmatism generating means such as a cylindrical lens for focus error detection, and the number of parts can be reduced.

According to the beam splitter of claim 5, in claim 4, at least one of the cross sections in two symmetrical planes orthogonal to each other of the toric surface of the second surface is an aspherical surface. In addition to the function of claim 4,
When the optical head is used, it is possible to suppress unevenness in the illuminance of the spot that occurs when the light collecting means for collecting light on the information recording medium is defocused.

According to a sixth aspect of the beam splitter, in the fourth or fifth aspect, the line of intersection of two symmetrical planes orthogonal to each other of the toric plane of the second plane is refracted by the first plane. Since it coincides with the optical axis, it has the same effect as in claim 4 or claim 5. According to the optical head of the seventh aspect, the light beam emitted from the light source is made into substantially parallel light by the collimating means, is condensed by the condensing means and is incident on the information recording medium. The light emitted from the information recording medium is incident on the first surface of the beam splitter to separate the light flux, and is further condensed on the second surface to enter the light receiving means. In this case, when the beam splitter according to claim 1, claim 2 or claim 3 is used, it is possible to simplify the condensing means of the detection optical system between the beam splitter and the light receiving means. With the beam splitter according to claim 5 or 6,
Further, the astigmatism generating means can be omitted.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment of the present invention will be described with reference to FIG. FIG. 1 shows the optical path of this embodiment. In FIG. 1, 1 is a beam splitter, 2 is a first surface, 3 is a second surface, 4 is an optical axis of an incident light beam, 5 is an optical axis of a light beam refracted by the first surface 2, and 6 is a first optical surface. It is the optical axis of the light beam reflected by the surface 2 of 1.

The beam splitter 1 of the embodiment is formed of a transparent body having a refractive index. The first surface 2 is a surface that acts as a beam splitter that splits the incident light beam 4 into a reflected light beam 6 and a refracted light beam 5 that becomes transmitted light, is formed in a plane, and is inclined by 45 degrees with respect to the optical axis 4 of the incident light beam. It has been configured.

The second surface 3 has a positive refracting power for condensing and emitting the refracted light beam 5, and is composed of a rotationally symmetric aspherical surface. The axis of rotational symmetry of this aspherical surface is the modulus of the first surface 2. The optical axis 5 of the light beam that is not parallel to the line and is refracted by the first surface 2
The refracted light beam 5 by the first surface 2 can be condensed. With this configuration,
The beam splitter 1 has a function of separating a light beam and a function of a detection lens that collects a light beam that passes through the beam splitter 1.

FIG. 2 is a schematic diagram of an optical head when the beam splitter 1 of the embodiment shown in FIG. 1 is used. In FIG. 2, the luminous flux emitted from the semiconductor laser 7 which is the light source is
The collimating lens 8 serving as a collimating means forms a substantially parallel light beam, which is reflected by the first surface 2 of the beam splitter 1, and then is condensed on the information recording medium 12 by the objective lens 11 serving as a light collecting means. The reflected light from the information recording medium 12 becomes a substantially parallel light beam by the objective lens 11, and this time it is refracted by the first surface 2 of the beam splitter 1,
It becomes a convergent light beam by the second surface 3, is given astigmatism by the cylindrical lens 14, passes through the concave lens 15, and then forms an image on the photodetector 16 which is a light receiving means.

The photodetector 16 is a cylindrical lens 14.
Is located at the intermediate position of the astigmatism generated by
When the objective lens 11 defocuses, the shape of the spot on the photodetector 16 changes, so that a focus error signal can be obtained. In this case, since the second surface 3 of the beam splitter 1 is formed of an aspherical surface, unevenness in the illuminance of the spot on the photodetector 16 when the objective lens 11 is defocused is suppressed, and as a result, Detector 16
It is possible to obtain a focus error signal that is less likely to be affected by the position deviation of.

As described above, by using the beam splitter 1 of the first embodiment, it is possible to eliminate the convex lens which is required in the conventional detection optical system. That is, the beam splitter 1 realizes the functions of the beam splitter and the convex lens in the optical system of the conventional optical head with a single element. Although a method utilizing astigmatism is used as the focus error detecting means in this embodiment, the beam splitter 1 of this embodiment has a similar effect of reducing the number of elements in other focus detecting methods. Needless to say.

As a modification, the second surface 3 may be a spherical surface. A second embodiment of the present invention is shown in FIG.
Will be described. FIG. 3 is a diagram showing the optical path of the beam splitter of this embodiment. In FIG. 3, 17 is a beam splitter, 18 is a first surface, 19 is a second surface, 20 is an optical axis of a light beam incident on the beam splitter 17, and 21 is a light beam of a light beam reflected by the first surface 18. Axis, 22 is the first surface 1
It is the optical axis of the light beam refracted at 8. Here, a plane including the optical axis 20 of the incident light flux, the optical axis 21 of the reflected light flux, and the optical axis 22 of the refracted light flux is defined as a YZ plane, and the optical axis 22 of the refracted light flux is perpendicular to the YZ plane.
3 is an optical path diagram on the YZ plane, where the plane including X is the XZ plane. Reference numeral 23 is an image point for a light beam on the YZ plane, and 24 is an image point for a light beam on the XZ plane.

The first surface 18 of the beam splitter 17 has a function as a beam splitter for splitting the incident light beam 20 and is inclined by 45 degrees with respect to the incident light beam 20. The second surface 19 is composed of a toric surface having a positive refractive power, and two symmetric surfaces of the toric surface orthogonal to each other are one YZ surface and the other XZ surface. The line of intersection is non-parallel to the normal to the first face 18,
Further, it coincides with the optical axis 22 of the light beam refracted by the first surface 18. Further, the second surface 19 forms the light flux on the YZ plane at the image point 23 and the light flux on the XZ plane at the image point 24. There are different configurations. Furthermore, Y of the second surface 19
The shape in the Z plane is composed of an aspherical surface, and this aspherical surface is optimized so as to reduce the illuminance unevenness of the focused spot at a position intermediate between the two image forming points 23 and 24.

With such a configuration, the beam splitter 17 of this embodiment has a function of separating the light beams and a function of converting one of the separated light beams into a convergent light beam having a desired astigmatism. FIG. 4 shows a schematic view of an optical head using the second embodiment. In FIG. 4, the divergent light beam from the semiconductor laser 7 becomes a parallel light beam by the collimator lens 8, is reflected by the first surface 18 of the beam splitter 17, and is condensed on the information recording medium 12 by the objective lens 11. The reflected light beam from the information recording medium 12 becomes a parallel light beam by the objective lens 11, is refracted by the first surface 18 of the beam splitter 17, and becomes a convergent light beam having astigmatism by the second surface 19 to form a concave lens. It reaches the photodetector 16 via 15. The photodetector 16 is arranged between the two image points formed by the beam splitter 17, and is capable of detecting an error signal with respect to the focus shift of the objective lens 11.

In the case of this embodiment, since the beam splitter 17 has a function of generating astigmatism in addition to the condensing function as described above, the optical system of the first embodiment is used for generating astigmatism. It is possible to eliminate the use of the cylindrical lens 14 used. Further, since one of the cross sections of the two symmetrical planes of the toric surface orthogonal to each other is formed by an aspherical surface, it is possible to suppress unevenness in the illuminance of the spot that occurs when the objective lens 11 is defocused. It is possible.

As described above, when the optical system is constructed by using the beam splitter 17 of this embodiment, the convex lens and the cylindrical lens in the conventional optical system are unnecessary. That is, the beam splitter 17 of this embodiment realizes the functions of the beam splitter, the convex lens, and the cylindrical lens in the conventional optical system with a single element.

In the present invention, the second surfaces 3 and 19 are composed of curved surfaces as means for allowing the beam splitter 17 to have the function of condensing the light flux. However, like the so-called refractive index distribution type lens, The same effect as that of the present invention can be realized by the method of giving a gradient to the refractive index of the material of the optical element of the present invention. Although the preferred embodiments of the present invention have been described above, the present invention is not limited to the embodiments, and various improvements and design changes can be made without departing from the scope of the present invention. Needless to say.

[0029]

According to the beam splitter of claim 1,
Since the first surface has the function of separating the light flux and the second surface has the function of the detection lens that collects the transmitted light flux, the detection optical system can be simplified when used in the optical head. ,
This has the effect of reducing the number of parts that make up the optical head.

According to the beam splitter of claim 2, in claim 1, the second surface is constituted by a rotationally symmetric spherical or aspherical surface having a positive refractive power, and a spherical or aspherical rotational symmetry axis, Since the normals of the first surface are non-parallel to each other, the same effect as in claim 1 is obtained, and in the case of an aspherical surface, the light when the focusing means for focusing on the information recording medium of the optical head is defocused. Irregularity of the illuminance of the spot on the light receiving means on the detection side is suppressed.

According to the beam splitter of claim 3, in claim 2, the rotational symmetry axis of the second surface coincides with the optical axis of the light beam refracted by the first surface. There is. According to a fourth aspect of the beam splitter of the present invention, in the first aspect, the second surface is constituted by a toric surface having a positive refractive power, and a line of intersection of two symmetrical planes of the toric surface orthogonal to each other Since the surface normals are not parallel to each other, in addition to the effect of claim 1, the first surface has the effect of separating the light beam, and one of the light beams separated by the second surface has a desired astigmatism. Since it has a function of making a convergent light beam having an aberration, it is not necessary to separately provide an astigmatism generating means such as a cylindrical lens for focus error detection, and the number of parts can be reduced.

According to the beam splitter of claim 5, in claim 4, at least one of the cross sections of two toric surfaces of the second surface orthogonal to each other is aspherical. In addition to the effect of claim 4,
When the optical head is used, it is possible to suppress unevenness in the illuminance of the spot that occurs when the light collecting means for collecting light on the information recording medium is defocused.

According to a sixth aspect of the beam splitter, in the fourth or fifth aspect, the line of intersection of two symmetrical planes orthogonal to each other of the toric plane of the second plane is the light beam refracted by the first plane. Since it coincides with the optical axis, it has the same effect as claim 4 or claim 5. According to the optical head of claim 7, when the beam splitter of claim 1, claim 2 or claim 3 is used, the condensing means of the detection optical system between the beam splitter and the light receiving means can be simplified. If the beam splitter according to claim 4, claim 5 or claim 6 is used, the astigmatism generating means can be further omitted.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of a first embodiment of the present invention.

FIG. 2 is a schematic view of an optical head when the beam splitter is used.

FIG. 3 is an explanatory diagram of a second embodiment.

FIG. 4 is a schematic view of an optical head when the beam splitter is used.

FIG. 5 is a schematic view of a conventional optical head.

FIG. 6 is a schematic diagram of a conventional optical head for a compact disc.

[Explanation of symbols]

 1 Beam Splitter 2 First Surface 3 Second Surface

Claims (7)

[Claims] 1. A beam splitter having a flat first surface for separating incident light into reflected light and transmitted light, and a second surface having a positive refractive power for condensing and emitting the transmitted light. . 2. The second surface is composed of a rotationally symmetric spherical surface or aspherical surface having a positive refractive power, and a rotational symmetry axis of the spherical surface or aspherical surface and a normal line of the first surface are not parallel to each other. The beam splitter according to claim 1, wherein 3. The axis of rotational symmetry of the second surface coincides with the optical axis of the light beam refracted by the first surface.
Beam splitter described. 4. The second surface is constituted by a toric surface having a positive refractive power, and a line of intersection between two symmetric surfaces of the toric surface orthogonal to each other and a normal line of the first surface are not parallel to each other. The beam splitter according to claim 1, wherein the beam splitter is provided. 5. The beam splitter according to claim 4, wherein at least one of the cross sections of two symmetrical planes orthogonal to each other of the toric surface of the second surface has an aspherical cross-sectional shape. 6. The toric surface of the second surface, the line of intersection of two symmetry planes orthogonal to each other is coincident with the optical axis of the light beam refracted by the first surface. Beam splitter. 7. A light source, a collimating means for collimating a light beam emitted from the light source into substantially parallel light, a condensing means for condensing the substantially parallel light on an information recording medium, and a light emitted from the information recording medium. A light receiving means for receiving light and a beam splitter for separating the light emitted from the information recording medium and making the light incident on the light receiving means are provided, and the beam splitter is used in claim 1, claim 2, claim 3, claim 4, and claim 4. Claim 5 or Claim 6
An optical head comprising the beam splitter described.