JPH0329121A - Optical head - Google Patents

Abstract

PURPOSE:To detect a stable focus error signal strong to a temp. drift, etc., by obtaining the differences between the respective electric signals generated in photodetecting regions existing at the diagonal positions of quadrisected photodetectors in an arithmetic circuit. CONSTITUTION:Two light spots of approximately the same light quantity and the same direction are formed approximately jointly on the quadrisected photodetectors 7 by an optical element 6 and the positions of the quadrisected photodetectors 7 are so set as to receive approximately the same light quantity in the four photodetecting regions of the quadrisected photodetectors 7. The sums of the electric signals generated by the diagonal positions of the for photodetecting regions of the quadrisected photodetectors 7 are obtd. and the difference between the same is obtd. by a differential amplifier 8. Further, the dividing lines of the quadrisected photodetectors 7 are previously adjusted to receive the equal light quantity in the four photodetecting regions, by which the change in the light quantity received by the four photodetecting regions of the quadrisected photodetectors is obviated if the sum of the electric signals generated from the photodetecting regions existing in the diagonal positions is obtd. The focus error signal having the high accuracy is detected in this way.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to optical devices such as optical disk devices.

Conventional Technology The modern era is said to be the information age, and the technology development of high-density, large-capacity memory, which forms the core of this era, is actively underway. In addition to the aforementioned high density and large capacity, the capabilities required of memory include high reliability. Optical disk memories are attracting the most attention as a device that satisfies these requirements, and the present invention relates to an optical head in such an optical disk memory.

Conventionally, many reports have been made regarding techniques related to focus error signal detection means in optical heads, and one example is Japanese Patent Laid-Open No. 59-60739.

Hereinafter, the above-mentioned conventional optical head will be explained with reference to the drawings.

FIG. 5 is a schematic diagram of a conventional optical head and a diagram illustrating its operating principle. In Fig. 5, l is a light source, 2 is an objective lens, 3 is an information recording medium, 4 is a half mirror, 5 is a convex lens, 8 is a differential amplifier, 9 is a knife edge, 10 is a two-split photodetector, and 1l is a split The line l2 is a light spot.

The operation of the conventional example configured as described above will be explained below.

After the light emitted from the light source 1 is reflected by the half mirror 4,
The objective lens 2 focuses the light onto the information recording medium 3.

FIG. 5(A) shows a case where the objective lens 2 is focused on the information recording medium 3.

The reflected light from the information recording medium 3 follows the opposite path, passes through the half mirror 4, and is converged by the convex lens 4, but half of the converged light beam is blocked by the knife edge 9, and the remaining light beam is divided into two beams. An optical slot H2 is formed on the dividing line 11 of the detector 10.

Here, a focus error signal can be obtained by taking the difference between the electrical signals generated in each light receiving area of the two-split photodetector 10 using the differential amplifier 8. Sometimes the focus error signal is zero.

FIG. 5(B) shows a case where the information recording medium 3 is displaced in a direction away from the objective lens 2.

At this time, after the reflected light from the information recording medium 3 passes through the objective lens 2, it becomes a converged light compared to the case of FIG. A semicircular light spot 12 is formed, and the focus error signal obtained by the differential amplifier 8 is negative.

FIG. 5(C) shows a case where the information recording medium 3 is displaced in a direction approaching the objective lens 2. At this time, information recording medium 3
After the reflected light passes through the objective lens 2, it becomes a divergent light compared to the case shown in FIG.
Above, a semicircular light spot l2 is formed on the light receiving area a, and a positive focus error signal is obtained by the differential amplifier 8.

Problems to be Solved by the Invention However, in the above configuration, 50% of the light amount is lost due to the knife edge 9. Furthermore, it is vulnerable to relative displacement between optical components due to temperature changes, etc., and has the problem of not being able to obtain a stable focus error signal. This will be explained using FIG. FIG. 6(A) shows only the photodetector portion of FIG. 5(A), and the numbers and names are the same as in FIG. 8. Figure 6 (A
) is the case when focusing, and the focus error signal obtained by the differential amplifier 8 is zero. FIG. 6(B) shows a case where the two-split photodetector lO is slightly displaced in the direction of arrow O due to a change in environmental temperature or the like.

13 is the position of the dividing line before displacement, and it is assumed that the position of the light spot 12 remains unchanged. In this case, since light is incident only on the light receiving area b, the focus error signal, which should originally be zero, becomes negative. That is, if the position of the two-split photodetector lO is slightly displaced due to factors such as temperature changes, an accurate focus error signal cannot be obtained and stable information recording and reproduction is impossible.

The present invention has been made in view of the above-mentioned prior art, and provides a focus error detection means and an optical head that can suppress a decrease in detection ability of a focus error signal caused by minute displacement of a photodetector due to changes in environmental temperature, etc. The purpose is to

Means for Solving the Problems In order to achieve the above objects, the optical head of the present invention includes a light source, a light source that directs the light from the light source onto an information recording medium (6) a light source, and a light beam reflected on the information recording medium. A means for separating the light beam from the condensing means, an information signal detection means on the information recording medium, a tracking error signal detection means, and a means for spatially dividing the incident light beam into two parts.
It has a substantially planar boundary surface including the optical axis of the incident light beam to be divided,
One lens consists of a first lens that converges or diverges approximately half of the incident light beam along the optical axis of the incident light beam, and the other lens deflects approximately half of the incident light beam in a direction parallel to the boundary surface and different from the optical axis of the incident light beam. a light incidence surface consisting of a planar light refractive surface;
an optical element that has approximately the same optical axis as the first lens and a light exit surface that is a convex lens-shaped second lens that converges the light flux from the light entrance surface; Between two focal points of the two light beams formed at different positions in the optical axis direction of the optical element, a substantially cross-shaped beam is provided at a position where the light intensity of the two light beams is received substantially uniformly in the four light receiving areas. A 4-split photodetector with a dividing line and the sum of the electrical signals generated in the diagonally located light-receiving areas of the 4-split photodetector are calculated. The focus error signal detection means is composed of a calculation circuit that calculates a difference.

Effect of the present invention With the above-described configuration, when the focusing means is focused on the information recording medium, two semicircular light spots of the same size and in the same direction are formed on the 4-split photodetector. ing. Furthermore, by adjusting the dividing line of the 4-split photodetector so that the four light-receiving areas receive the same amount of light,
Even if the 4-split photodetector is displaced in either direction of the dividing line due to environmental temperature changes, the amount of light received by the 4 light-receiving areas of the 4-split photodetector will be generated from the light-receiving areas located diagonally. If the electric signals are summed, there will be no change, and a highly accurate focus error signal can be detected.

EXAMPLE Hereinafter, an optical head according to an example of the present invention will be described with reference to the drawings. FIG. 1 is a diagram showing a schematic configuration of an optical head in an embodiment of the present invention.

In FIG. 1, 1 is a light source, 2 is an objective lens, 3 is a disk that is an information recording medium, 4 is a half mirror, 6 is an optical element, 6a is a surface A that is the light incidence surface of the optical element 6, and 6b is an optical Surfaces B and 6c, which are the light incident surfaces of the element 6, are surfaces C, which are the light exit surfaces of the optical element 6.

Further, 7 is a four-split photodetector, and 8 is a differential amplifier that is an arithmetic circuit.

The optical element 6 is shown in FIG. 2 in an enlarged perspective view. The optical element 6 has a dividing surface that passes through the optical axis of the incident light beam so as to divide the incident light beam into approximately two parts. 6a is a surface A that is a lens surface, and 6b is a dividing surface that is perpendicular to the optical axis of the incident light beam. This is surface B which is a light refractive surface on a plane that is not perpendicular. Further, 6c is a convex lens having the same optical axis as the lens on surface A6a, which converges the light transmitted through surfaces A6a and B6b. FIG. 3 shows four side views including the view in the direction of arrow A in FIG. 2. In FIGS. 2 and 3, the external shape of the optical element 6 is quadrangular when viewed from the optical axis side of the incident light beam, but this may be circular or any other shape. Also, in Figure 1,
In order to simplify the drawing, outlines unnecessary for explaining the functions of the optical element 6, information signal detection means, and tracking signal detection means are omitted. The operation of an embodiment of the optical head configured as described above will be described below. In Figure 1,
The light from the light source 1 is converted into parallel light by a parallel light conversion means (not shown), passes through a half ξ mirror 4, and forms a light spot on the disk 3 by the objective lens 2. disc 3
The reflected light from the mirror follows the opposite path, passes through the half ξ mirror 4, and enters the optical element 6. In the optical element 6,
The light beam is divided into two parts with substantially the same amount of light by the surface A6a and the surface B6b. Here, since the surface A6a has a convex lens shape, the light passing through it converges, and further converges at the surface C6c to form a focal point P. Further, the light passing through the surface B6b is refracted and then converged at the surface C6c to form a focal point Q. The focal point P and the focal point Q are formed on the same plane as the dividing plane of the optical element 6, as shown in the I arrow view. The light receiving surface of the 4-split photodetector 7 is located between the focus P and the focus Q, and on the light receiving surface,
As shown in the view along arrow G, semicircular light spots T and S are formed that are substantially congruent and have substantially the same amount of light. Here, if the four light-receiving regions of the 4-split photodetector 7 are configured to receive substantially the same amount of light, the electrical signals generated in the four light-receiving regions can be obtained by connecting or adding the diagonals of each other. 2
The focus error signal can be detected by taking the difference between the two electrical signals using the differential amplifier 8. As shown in FIG. 1, when the objective lens 2 is focused on the disk 3, the focus error signal becomes zero.

Here, consider a case where the disk 3 is slightly displaced in the direction of G and approaches the objective lens 2. In this case, the optical element 6
The light flux that enters becomes diverging light, and the focal points P and Q are slightly displaced in the direction of G. As a result, the diameter of the light spot T on the 4-split photodetector 7 becomes smaller, and the light spot S
becomes larger in diameter.

That is, the amount of light decreases in the light-receiving areas a and C, and increases in the light-receiving areas b and d. Therefore, the focus error signal output from the differential amplifier 8 becomes positive.

Further, when the disk 3 is slightly displaced in the direction opposite to G and moves away from the objective lens 2, a negative focus error signal can be obtained based on the principle opposite to that when the disk 3 approaches the objective lens 2.

FIG. 4 shows a case where the four-split photodetector 7 is slightly displaced in the direction of the arrow H with the objective lens 2 focused on the disk 3. Here, 11 is a dividing line in one direction of the 4-split photodetector 7, and 13, which is indicated by a broken line, is a dividing line before the dividing line 11 is displaced. After the displacement, the amount of light decreases in light receiving areas a and b, and increases in light receiving areas C and d. However, since the light spots T and S have substantially congruent shapes, the same direction, and substantially the same amount of light, the diagonal sums of the amounts of light received in the four light receiving areas do not change and are equal. Therefore, the focus error signal obtained by the differential amplifier 8 does not change compared to before the small displacement of the 4-split photodetector 7, and it is possible to detect a stable signal that is less affected by temperature drift and the like.

As described above, according to this embodiment, the optical element 6 provides four
On the split photodetector 7, two light spots are formed that are substantially congruent, have substantially the same amount of light, and the same direction, and four-split light detection is performed so that the four light-receiving areas of the four-split photodetector 7 receive substantially the same amount of light. By setting the position of the detector 7, calculating the sum of the electrical signals generated at the diagonals of the four light-receiving areas of the 4-split photodetector 7, and calculating the difference between them using the differential amplifier 8, a simple configuration is possible. Therefore, an optical head capable of detecting a stable focus error signal without loss of light quantity tn and resistant to temperature drift etc. can be realized with a small number of components.

Note that in this embodiment, the surface A6 of the optical element 6
Although a is a convex lens, the same effect can be obtained even if a concave lens is used. Moreover, even if both surface A6a and surface C6c are convex lenses, their curvatures do not need to be the same. Furthermore, if at least one of the surfaces A6a and C6c is made into an aspherical shape to eliminate aberrations, the optical element 6
It is possible to further improve the characteristics of

Further, the optical element 6 may be manufactured by polishing a glass material, but if it is manufactured using a molding method such as pressing or molding from resin or glass, it will be more effective in mass production and a low-cost optical head can be realized. It is possible.

Effects of the Invention The present invention serves as a focus error signal detection means in an optical head, and has a substantially planar boundary surface including an optical axis of the incident light beam that spatially divides the incident light beam into two, one of which is approximately half of the incident light beam. The first lens converges or diverges along the optical axis of the incident light beam, and the other consists of a planar light refractive surface that deflects approximately half of the incident light beam in a direction parallel to the boundary surface and different from the optical axis of the incident light beam. an optical element consisting of a light entrance surface and a light exit surface which is a second lens in the shape of a convex lens that has substantially the same optical axis as the first lens and converges the light flux from the light entrance surface; Between the two focal points of the two incident and split light beams formed at different positions in the optical axis direction of the optical element, the four light-receiving areas are provided at positions where the light amounts of the two light beams are received almost uniformly. , a four-division photodetector having a substantially cross-shaped dividing line, and the sum of the electric signals generated in the diagonally located light-receiving areas among the four light-receiving areas of the four-division photodetector, and the sum is calculated. By using an arithmetic circuit to calculate the difference between the electrical signals taken, an excellent optical system with a simple configuration consisting of a small number of components can detect a stable focus error signal with no light loss and withstand temperature drift. It is possible to realize a head.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram of an optical head according to an embodiment of the present invention, FIGS. 2 and 3 are schematic diagrams of its main parts, FIG. 4 is a schematic diagram for explaining the operation, and FIGS. 5 and 6 The figure is a schematic diagram of a conventional optical head. 1...Light source, 2...Objective lens, 3.
...Disk, 4...Half ξler, 6
......Optical element, 7...4-split photodetector, 8...Differential amplifier, 11...Dividing line, 13... Parting line before displacement.

Claims (6)

[Claims] (1) It has a substantially planar boundary surface including the optical axis of the incident light beam that spatially divides the incident light beam into two, one of which converges or diverges approximately half of the incident light beam along the optical axis of the incident light beam. a light incident surface comprising a first lens, the other being a planar light refractive surface that deflects approximately half of the incident light beam in a direction parallel to the boundary surface and different from the optical axis of the incident light beam;
1. An optical element comprising: a light exit surface comprising a second lens in the shape of a convex lens that has substantially the same optical axis as the first lens and converges a light beam from the light entrance surface. (2) The optical element according to claim (1), wherein the first and second lenses have different curvatures. (3) The optical element according to claim (1), wherein at least one of the first and second lenses has an aspherical shape. (4) The optical element according to claim (1), wherein the material is resin or glass and the shape is formed by molding. (5) It is composed of an optical element, a 4-split photodetector having a substantially cross-shaped dividing line, and an arithmetic circuit, and the 4-split photodetector is configured to detect two light beams incident on the optical element and split. The optical element is provided between two focal points formed at different positions in the optical axis direction at a position where the light intensity of the two light beams is received substantially uniformly in the four light receiving areas, and the arithmetic circuit is connected to the four-split photodetector. A focus error signal detection device characterized by calculating the sum of electrical signals generated in diagonally located light receiving areas among the four light receiving areas, and calculating the difference between the summed electric signals. (6) a light source, a condensing means for condensing the light from the light source onto an information recording medium, a means for separating the light beam reflected on the information recording medium from the condensing means, and a condensing means for condensing the light from the light source onto an information recording medium; An optical head comprising: an information signal detecting means; a tracking error signal detecting means; and a focus error signal detecting device according to claim 5.