JPS60626A - Optical pickup - Google Patents

Abstract

PURPOSE:To attain light weight and ease of positioning by forming a face where a crystal having double refraction is stuck to a polarized optical beam splitter as a polarized light separating face and setting the polarized light separating face to a critical angle to a reflected parallel light. CONSTITUTION:The polarized optical beam splitter consists of a triangular prism part 4a and a quadrangular prism part 4b, and they are tiled respectively with the polarized light separating face 13. A vertex angle theta of the triangular part 4a is in parallel with an optical axis 16 and set to an angle so that a light being the S polarized light in the direction of polarization is reflected totally at the critical angle. The parallel light transmitted through a collimator lens 3 is transmitted through the polarized light separating face 13 of the polarized beam splitter 4 without being reflected thereupon, made incident to a 1/4-wavelenght plate 5, becomes a circularly polarized light and is collected on the face of a disc 7 by means of an objective lens 6. A focusing error signal is obtained through the change in the reflectance in the vicinity of the critical angle by using a Foster prism in this way.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical pickup for optically reading out information from recording pits recorded on a disk-shaped information recording medium (hereinafter referred to as a disk) in an optical disk player, etc. The damage that occurs is related to the optical system.

In optical disc players, an optical optical pickup is used to read out information signals from the recording pits recorded on the disc, and an information reading beam is orcused over the recording pits and the existing disc surface, and the information is recorded. It is necessary to track the pits, to always accurately irradiate the recording pits on the disk surface with reading light, and to receive the reflected light from the recording pits.

The astigmatism method is known as a method for detecting deviations in focusing so that the light beam is always focused on the disk surface.

FIG. 1 fA) is a diagram showing an example of a conventional optical pickup using the astigmatism method. In the figure, the laser light source (1
), the diverging laser beam (2) enters the collimator lens (3), is converted into a parallel beam, passes through the polarizing beam splitter +41, 1/4 wavelength plate (5), and is sent to the disk by the objective lens (6). (7) The light is focused on the recording surface. The focused laser beam is reflected by the disk (7) and travels backwards, and after passing through the objective lens, it becomes a parallel beam and becomes 1/
The polarized beam splitter (4) passes through the 4-wavelength plate (5).
separated by.

The light beam is transmitted through the convex lens (8) and the cylindrical lens (9) through the light beam shape converting optical fiber axis, and is received by the photodetector I. The light beam shape conversion optical system a1 is provided to convert a parallel light beam into a convergent astigmatic light beam.

That is, if the axis of the cylindrical lens (9) is oriented in the vertical direction of the figure, and the cylindrical lens (9) has the property of converging the light flux in a direction perpendicular to the paper plane of the figure, then the shape of the light flux is The laser beam incident on the optical system a1 is converted into a convergent beam having rotational symmetry by a convex lens, and is further focused by a cylindrical lens (9) in a direction perpendicular to the plane of the drawing to form a first heat ray parallel to the plane of the paper. (XZa) and then a photodetector (I
There is no ll, the second focal line perpendicular to the paper (12b
) forming stones.

The photodetector (lυ) is the position of the circle of least confusion of the astigmatic light beam located between the first focal line (12a) and the second focal line (x2b) when the disk (7) is at the best focus position. Note that the polarization beam splitter and 1/
The effect of a four-wavelength plate on optical pickup is well known. Figure 1 ([3)] shows an example of the shape of the photodetector aυ and the cross-sectional shape of the light beam on the photodetector when it is in the best focus state. FIG. The photodetector αυ is divided into four by orthogonal linear electrically insulating regions! λ,
In the pest focus state, the beam cross section is circular and divided into 9 photodetectors (11a), (xxb), (oc) (11d).
) are equally distributed among each other. FIG. 1 (fl) is a diagram showing the relationship between the photodetector a0 and the cross-sectional shape of the light beam when the disk (7) is tilted toward the objective lens side from the best focus state. This alignment Figure 1 (A) The focal line of mud 1 in K (
12a) approaches the photodetector (II), the beam cross section has a vertically elongated shape.

FIG. 1(D) is a diagram showing the relationship between the photodetector 11υ and the cross-sectional shape of the light beam when the disk (7) moves away from the best focus state. In this case, the first focal line (12a
) moves away from the photodetector I and the second focal line (x2b) approaches, so the beam cross section shows a horizontally elongated shape. The first of the above
From Figure (B) ((1) and (D) and their explanations, the sum of the outputs of the photodetectors (11a) and (ttc) divided into nine parts, and the sum of the outputs of the same < (llb) and (lid). It will be understood that the two orcasing error signals are obtained by differentially amplifying each other.

However, the conventional optical pickup using astigmatism as shown in Figure 1 above consists of an optical system consisting of two sets of parts: a convex lens (8) and a cylindrical lens (9). Because it has been.

There are many parts, and the assembly and adjustment of the photodetector aυ to the position of the minimum scattering circle is not easy and takes time.
Furthermore, there was a drawback in that the optical pickup protruded laterally, making it difficult to make it smaller and lighter.

In order to solve the above-mentioned drawbacks of the conventional optical pickup using the astigmatism method, this invention uses a polarizing beam splitter made of a crystal having birefringence. explain.

FIG. 2(4) is a diagram showing an embodiment of the present invention. In the figure, (4) is a polarizing beam splitter made of a crystal having birefringence.

Showing Foster prism. The polarizing beam splitter has a triangular prism-like part (4a) and a quadrangular prism-like part (4a).
b), each of which has a polarization separation plane (L
It is bound by 3. The apex angle θ of the triangular prism-shaped portion (4a) is set to be parallel to the optical axis tilt and to totally reflect light whose polarization direction is S-polarized (the vibration plane of the electric field is perpendicular to the plane of the paper) at a critical angle. . In addition, the 1-reflection surface 04 is designed to reflect one incident light, and the apex angle is the light parallel to the optical axis (Lf9).
+! It is set to reflect in a direction perpendicular to 9.

Next, the action of the polarizing beam splitter (4) will be explained in detail using the drawings. Figure 3 shows a polarizing beam splitter (4
) is a diagram showing the effect of As shown in the figure, P-polarized light (third
(indicated by the symbol ←→ in the figure) and S-polarized light (indicated by the symbol ■ in Fig. 3) enter the polarization separation surface 03 at an incident angle θO.
If the polarizing beam splitter (4) is made of a crystal having birefringence, the refractive index for each polarized light is different, and therefore the angle of incidence at which total reflection occurs will be different. Here, since the apex angle θ in FIG. 3 is determined so that the S-polarized light is totally reflected at the critical angle, the S-polarized light component is totally reflected at the polarization separation surface al. However, since the P-polarized light component does not satisfy the conditions for total reflection, it passes through the polarization separation surface 01 as is. FIG. 4 schematically depicts the relationship between the incident angle θ0 and reflectance near the critical angle oS of S-polarized light. When the incident angle is larger than O3, the 8 polarized light components are totally reflected, but when the incident angle is smaller than θS, the reflectance becomes smaller than 1, and the polarization separation surface (1
3, and the proportion of the S-polarized light component reflected by the polarization separation surface is reduced. The change in reflectance around this critical angle of 08 is steep.

Returning to FIG. 2, the operation will be explained. Assuming that the laser light emitted from the laser light source (1) in FIG. 2 is P-polarized,
According to the above explanation, the parallel light that has passed through the collimator lens (3) is
) without being reflected, and enters the 1/4 wavelength plate (5). After passing through the 174 wavelength plate (5), the light becomes circularly polarized light and is focused onto the surface of the disk (7) by the objective lens (6). The light reflected by the surface of the disk (7) passes through the objective lens (6) again, becomes parallel light whose traveling direction is opposite to that of the incident parallel light, and enters the quarter-wave plate (5) again. 1/4
After passing through the wavelength plate (5), it becomes S-polarized light and enters the polarization separation surface (I) of the polarizing beam splitter (4), but since the incident angle of all the incident light is a critical angle, it is totally reflected. Further, the light is sufficiently reflected by the reflective surface 0 of the polarizing beam splitter, and the traveling direction is changed, and the light is incident on the photodetector αυ.

FIG. 2(B) is a diagram showing a cross-sectional shape of a light beam on a photodetector in a just-focus state. The photodetector aυ is divided into two by a linear electrically insulating region, and in the best focus state, the cross section of the light beam is equally distributed to each of the nine circular photodetectors (xxa) (xlb). .

FIG. 5(4) shows a case where the disk (7) is further away from the best focus state. The light reflected by the surface of the disk (7) passes through the objective lens (6) and is then converted into a converging light beam αυa8. Now, when such convergent light enters the polarization beam splitter (4), the angle of incidence on the polarization separation surface α is different for the light flux αη and the light flux O1, and the angle of incidence of the light flux aη is greater than the critical angle. Therefore, it is totally reflected at the polarization separation surface, but the luminous flux (
180 Since the incident angle is less than the critical angle, the polarization separation plane 03
Transparent. In FIG. 5 IA), the transmitted light beam is shown by the shaded area. Figure 5 (B) shows the photodetector α when the de, f screen (7) is further away from the best focus state.
It is a diagram showing the relationship between υ and the cross-sectional shape of a light beam, where the cross-section of the light beam is incident only on a photodetector (11a) divided into nine semicircular parts.

FIG. 6 IA) shows a case where the disk (7) is closer to the objective lens (6) than in the best focus state. In this case, the light reflected on the surface of the disk (7) passes through the objective lens (61) and then becomes a divergent beam (111
21. When such diverging light enters the bright light beam splitter (4), the angle of incidence on the polarization splitting surface is different between the luminous flux (II) and the luminous flux (river), and the luminous flux Q
Since the incident angle of light C11 is less than the critical angle, it is transmitted through the polarization separation surface 0, but the light beam C11 is totally reflected by the polarization separation surface 413 because its incidence angle is greater than the critical angle. In FIG. 6(A), the transmitted light flux is indicated by diagonal lines. Figure 6 (B) shows the disk (
7) is a diagram showing the relationship between the photodetector 01) and the cross-sectional shape of the light beam when it approaches the objective lens (6) from the best focus state. The light flux is incident only on the photodetector (11b), which is divided into nine semicircular parts on the left side.

The photodetector (11a) and (1
It can be seen that the focusing error signal can be obtained by differentially amplifying the output of 1b).

FIG. 7 is a diagram showing another embodiment of the present invention, in which a photodetector aυ is glued and integrated with a polarizing beam splitter (4).

In addition, although the case in which a Foster prism is used as a polarizing beam splitter made of a birefringent crystal has been described above, the present invention is not limited to this, and can be applied to other polarizing beam splitters made of a birefringent crystal such as a Glan-Thompson prism. may also be used.

As described above, in the optical pickup according to the present invention, by using a Foster prism, a focusing error signal can be obtained from changes in reflectance near the critical angle.

There is no need for an optical system that converts the shape of the beam as in the conventional astigmatism method, which reduces weight, and the large diameter of the beam makes it easy to align the photodetector. It is also possible to integrate it with a prelifter.
It has the advantage of being able to use a b-structure that prevents the photodetector from being misaligned8.

[Brief explanation of the drawing]

Figure 1 (4) shows an example of conventional optical pickup using the astigmatism method, and Figure 1 (B) shows an example of the shape of a photodetector and the cross-sectional shape of the light beam in the best 7 orcus state. Figure 1 (n) is a diagram showing the relationship between the photodetector and the cross-sectional shape of the light beam when the disk approaches, and Figure 1 (I) is a diagram showing the relationship between the photodetector and the cross-sectional shape of the light beam when the disk is far away. FIG. 2 (4) is a diagram showing an embodiment of the present invention. FIG. The figure shows the action of the Foster prism. Figure 4 is a schematic diagram showing the change in reflectance near the critical angle of the Foster prism. Figure 5 (A) is a schematic diagram when the disk moves away. B) is a diagram showing the relationship between the photodetector and the cross-sectional shape of the light beam when the disk moves away. FIG. 6(A) is a schematic diagram when the disk approaches. FIG. 6(B) is a diagram showing the relationship between the photodetector and the cross-sectional shape of the light beam when the disk approaches, and FIG. 7 is a diagram showing another embodiment according to the present invention. In the figure, (1) is a laser light source, (3) is a collimator lens, (4) is a polarizing beam splitter, (5) is a quarter-wave plate, (6) is an objective lens, (7) is a disk, and the frame is The beam shape conversion optical system, αυ is the photodetector, the opposite side is the polarization separation plane, (
I4 is a reflective surface, αη inclination is a converging light flux, and α1 (to) is a diverging light flux. Note that the same or corresponding parts in FIG. 9 are designated by the same reference numerals. Agent Masuo Oiwa Figure 1 (A) Figure 2 (A) Figure 3 Figure 4 Figure 6 (8) Figure 7

Claims (3)

[Claims] (1) A light source, a collimator lens that converts the diverging light from the light source into a parallel beam, an objective lens that focuses the parallel beam onto an information recording medium, and an optical path between the collimator lens and the objective lens. The light beam reflected by the information recording medium is transmitted through the objective lens again to form a parallel light beam that is opposite to the parallel light beam. In an optical pickup equipped with a photodetector that receives the reflected light separated by a notch, the polarization beam splitter has a surface on which a birefringent crystal is pasted together, and serves as a polarization separation surface. What is claimed is: 1. An optical big boot tube characterized in that the polarization separation surface is set at a critical angle with respect to the generated reflected parallel light. (2) The optical pickup according to claim 11, wherein the polarizing beam splitter is a Foster prism. (3) An optical pickup according to claim j1, characterized in that a polarizing beam splitter and a photodetector are bonded together.