JPS61280033A - Focus detecting device - Google Patents

Abstract

PURPOSE:To detect a stable focus error signal which has no loss of the quantity of light and is not affected by the fine displacement of a four-division photodetector by forming two light spots by a convex lens and a prism and setting the position of the four-division photodetector so that the quantity of light photodetected in a photodetection area becomes equal. CONSTITUTION:The prism is positioned in convergent luminous flux from the convex lens 5a and the luminous flux converged by he convex lens 5a is split spatially into two by the prism 6; and luminous flux which is not transmitted through the prism 6 is focused on P and light transmitted through the prism 6 is refracted and focused on Q. The photodetection surface of the four-division photodetector 7 is positioned at the midpoint between the focuses P and Q almost perpendicularly to an optical axis of incidence. Two light spots T and S of the same size which are nearly semicircular are formed on the four-division photodetector 7. The position of the four-division photodetector 7 is adjusted in the photodetection surface so that uniform light is received in four photodetection areas. The sum of electric signals obtained in diagonal photodetection areas of the four-division photodetector are calculated and their difference is further calculated by a differential amplifier 8 to obtain the focus error signal.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a focus error detection device for an optical head in a device that optically records and reproduces information, such as an optical disk device.

Conventional Technology The modern era is called the information age, and the technological development of high-density, large-capacity memory, which forms the middle wave of this era, is actively being carried out.

In addition to the above-mentioned high density and large capacity, the capabilities required of memory include high reliability and high speed access, and optical disk memory is attracting the most attention as a device that satisfies all of these requirements. An optical disk memory is a device that optically records information on a recording medium, and recently, a lot of research has been conducted on magneto-optical disks that can also erase recorded information.

A major feature of optical disk memory is that it does not damage the information recording medium because it uses a non-contact recording/reproducing method. Servo system is required. The optical head requires a function to detect a focus error between the information recording medium and the objective lens, and an objective lens actuator for driving the objective lens and performing focus servo.
The former relates to the focus error detection function.

In the past, there have been many reports on techniques related to focus error detection; for example, Japanese Patent Laid-Open No. 59-60
There are publications such as Publication No. 739.

Hereinafter, the conventional focus error detection device as described above will be explained with reference to the drawings.

FIG. 9 is a schematic diagram of a conventional focus error detection device and a diagram illustrating its operating principle. In Figure 9, 1
is a light source, 2 is an objective lens, 3 is an information recording medium, 4 is a half mirror, 15 is a convex lens, 8 is a differential amplifier, 9 is a knife, 1o is a two-split photodetector, 11 is a dividing line, and 12 is a light spot. be.

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 light is focused by the objective lens 2 onto the information recording medium 3.

FIG. 9(m) shows a case where the objective lens 2 is focused on the information recording medium 3. At this time, 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 lens 9, and the remaining light beam is divided into 2
A light spot 12 is formed on the dividing line 11 of the divided photodetector 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.
At the time of focus indicated by , the focus error signal is zero.

FIG. 9(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 convergent light compared to the case shown in FIG. A semicircular light spot 12 is formed, and the focus error signal obtained by the differential amplifier 8 is negative.

FIG. 9(C) shows a case where the information recording medium 3 is displaced in a direction approaching the objective lens 2. FIG. 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 12 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, 60% of the light amount is lost to the knife 9.

Furthermore, it is susceptible to relative displacement between the optical head 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. 10. FIG. 10(M) shows only the photodetector portion of FIG. 9(M), and the numbers and names are based on FIG. 8. FIG. 10(m) shows the case when focusing, and the focus error signal obtained by the differential amplifier 8 is zero.

FIG. 10(B) shows a case where the two-split photodetector 10 is slightly displaced in the direction of arrow 0 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, the focus error signal, which should originally be zero, becomes negative. In other words, if the position of the two-split photodetector 1o 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 it is an object of the present invention to provide a focus error detection device capable of suppressing a decrease in detection ability of a focus error signal caused by minute displacement of a photodetector due to changes in environmental temperature, etc. This is the purpose.

Means for Solving the Problems In order to achieve the above object, the focus error detection device of the present invention includes a convex lens that receives reflected light from an information recording medium, and is located in a convergent light beam formed by the convex lens and the convex lens,
one or more transmissive prisms that spatially split the convergent light beam into two and bend only half of it in a minute direction in the direction of the convergent light beam dividing line; and a focal point of the objective lens on the information recording medium. If the convex lens and the prism match each other, the lens is located at an intermediate position in the optical axis direction between the two focal points formed by the convex lens and the prism, and is installed so that the light intensity of the two light spots is almost uniformly received in the four light receiving areas. A 4-split photodetector having a cross-shaped dividing line is electrically connected to light-receiving areas at diagonal positions among the 4 light-receiving areas of the 4-split photodetector, and the sum is calculated. It consists of a differential amplifier that takes the difference between each electrical signal.

Operation According to the present invention, when the objective lens is focused on the information recording medium with the above-described configuration, on the 4-split photodetector,
Two semicircular light spots of the same size and direction are formed. Furthermore, if you adjust the dividing line position of the 4-split photodetector so that the same amount of light is received in the 4 light-receiving areas, the 4-split photodetector will move in either direction of the dividing line due to environmental temperature changes, etc. Even if the light is displaced, the amount of light received by the four light-receiving areas of the 4-split photodetector will not change if you take the sum of the electrical signals generated from the light-receiving areas at diagonal positions, and if you take the difference between the sums, The output of the differential amplifier obtained, that is, the focus error signal, always maintains stable detection ability.

Embodiment Hereinafter, one embodiment of the present invention will be described based on the accompanying drawings.

FIG. 1 shows a schematic diagram of a focus error detection device in one embodiment of the present invention. The present invention relates to a focus error detection device, and in the embodiment shown in FIG. 1, the information signal detection and tracking error signal detection functions are omitted.

In FIG. 1, 1 is a light source, 2 is an objective lens, 3 is an information recording medium, and 4 is a half mirror, which are the same as those in the conventional example. 5a is a convex lens, 6 is a prism, and 7 is a 4-split photodetector.
An arrow view is also shown. Further, 8 is a differential amplifier.

The prism 6 is located in the convergent light beam formed by the convex lens X'5a, spatially uniformly divides the convergent light beam into two parts, and only one of the halves is slightly bent in the direction of the convergent light beam dividing line. . FIG. 1 shows a case where the objective lens 2 is focused on the information recording medium 3.

The light beam converged by the convex lens 6& is spatially divided into two parts by the prism 6, and then the light beam that does not pass through the prism 6 connects to a focal point P, and the light that passes through the prism 6 is bent and connected to a focal point Q. The light receiving surface of the 4-split photodetector 7 is a focal point P,
It is installed at a non-focal intermediate position, almost perpendicular to the incident optical axis. As a result, as shown in the diagram in the direction of arrow G, two optical spora (T and S) that are approximately semicircular and approximately the same size are formed on the four-split photodetector 7. The 4-split photodetector 7 has 4
The position is adjusted within the light receiving surface so that each light receiving area receives uniform light. Here, among the electrical signals generated in the four light receiving areas of the 4-split optical detector 7, the sum of the electrical signals of the light receiving areas at diagonal positions is calculated, and the difference is calculated by the differential amplifier 8.
The configuration is such that a focus error signal is obtained by taking the image.

The operation of an embodiment of the focus error detection device configured as described above will be explained below. FIG. 1 shows a case where the objective lens 2 is focused on the information recording medium 3, and at this time, the four-split photodetector 7 receives uniform light in four light receiving areas. Therefore, the focus error signal output from the differential amplifier 8 is zero.

FIG. 2 shows the case where the information recording medium 3 in FIG. 1 is displaced in a direction away from the objective lens 2 using only the main parts in FIG. Comparatively, converged light is incident. As a result, the two convergence points by the convex lens 5a are both located at the prism θ.
As a result, light spots S and T are formed on the four-split photodetector T as shown in the diagram along arrow G.

At this time, the output of the differential amplifier 8 becomes unbalanced, and a positive focus error signal can be detected.

FIG. 3 shows a case where the information recording medium 3 shown in FIG. 1 is displaced in a direction approaching the objective lens 2, and the light that is diverging compared to the case of FIG. 1 enters the convex lens 6a. . As a result, the two convergence points formed by the convex lens 51L are both shifted in the direction away from the prism 6, and a light spot (S) is formed on the four-split photodetector 7 as shown in the view along the arrow G.
and T are formed. At this time, a negative focus error signal can be detected from the output of the differential amplifier 8.

FIG. 4 shows a state in which the objective lens 2 is focused on the information recording medium 3, and the 4-split photodetector 7 is placed in the same manner as in FIG. 1.
The figure shows a case where there is a slight displacement in the direction of arrow H. 11 is a dividing line in one direction of the 4-split photodetector 7, indicated by a broken line, 1
3 is a dividing line before the dividing line 11 is displaced. Before the displacement, the amount of light received by the light receiving areas a, b, a, and d of the four-split photodetector 7 was equal, but after the displacement, it decreases in the light receiving areas a, b, and increases in the light receiving areas c, d. However, light subo)
Since Sb and T have approximately the same shape, the same direction, and the same amount of light, the amount of light decreased in the light receiving area a, that is, the light receiving area d.
The amount of light increased in the light receiving area is equal to the amount of light that has already decreased in the light receiving area, that is, the amount of light increased in the light receiving area 0. Therefore, the sum of the electrical signals detected in the light-receiving regions & and C and the sum of the electrical signals detected in the light-receiving regions Amplifier 8
The focus error signal obtained is zero and is not affected by the displacement of the four-split photodetector 7.

As described above, according to this embodiment, the convex lens 5a and the prism 6 form two semicircular light spots on the 4-split photodetector T, approximately congruent, in the same direction, and with the same view. The position of the 4-split photodetector 7 is set so that the amount of light received in the 4 light-receiving areas of the 4-split photodetector 7 is equal. By calculating the sum of the electric signals and calculating the difference between them using the differential amplifier 8, it is possible to detect a stable focus error signal without loss of light quantity and unaffected by minute displacements of the 4-split photodetector 7. It is.

Next, other embodiments of the present invention will be described with reference to the drawings. FIG. 5, FIG. 6, FIG. 7, and FIG. 8 show schematic diagrams of focus error detection devices in other embodiments of the present invention. In FIGS. 5 to 8, only the optical axes of the light beams are shown, and the optical axes that are in contact with the prisms 6, 8&, and 6b are shown by broken lines.

In FIG. 5, a convex lens 6a and a prism 6 are optically bonded together, and the operating principle is the same as that in FIG. 1. Convex lens 5 & and prism 6 are not only optically bonded, but also
It may be processed or molded from a single optical material to form a completely single optical element, thereby stabilizing the assembly accuracy when constructing the optical head.

FIG. 6 shows a case in which the prism 6 is used upside down from that shown in FIG. 6, and the light receiving surface of the four-split photodetector 7 and the prism 6 are optically bonded. In this case as well, the operating principle is the same as in the case of FIG. 1, and there is an advantage that the positional relationship between the four-split photodetector 7 and the prism 6 is stabilized by adhesion.

Although an embodiment using a single prism has been described above, an embodiment using a plurality of prisms is shown in FIGS. 7 and 8.

In the embodiment shown in FIG. 7, two prisms, a prism 61L and a prism 6b, are used. prism 6&,
The prism 6b transmits and bends only a spatial half of the light beam emitted from the convex lens S&.

The detection principle is the same as in the case of FIG. Here, if the light output angle of the prism 61L and the light incidence angle of the prism 6b are made equal, the two optical axes entering the 4-split photodetector 7 can be made parallel to each other, and the 4-split photodetector Since the two light spots formed on the device T are completely congruent when the objective lens is focused, it is possible to further improve focus detection accuracy.

FIG. 8 shows a configuration similar to that of FIG. 7, but with a convex lens 6a.
and a prism 6a are optically bonded, and a prism 6b and a four-split photodetector 7 are optically bonded. Convex lens 6a and prism 6! L may be formed not only by optical bonding but also by processing or molding from a single optical material to form a completely single optical element. Further, the space between the prisms 6& and eb is filled with an optical member 14 having a different refractive index from that of the prisms 6& and 6b, and a convex lens 5a and a prism 61L are formed.

It is also possible to completely integrate the prism 6b and the 4-split photodetector 7, and by doing so, the mutual positional relationship is stabilized, resulting in an extremely high-precision focus error detection device that is not affected by changes in ambient temperature, etc. It becomes possible to configure.

Effects of the Invention The present invention is a focus error detection device that uses a convex lens and one or more prisms to form two semicircular light spots congruent, in the same direction, and with the same light intensity on a 4-split photodetector. Then, set the position of the 4-split photodetector so that the amount of light received by the 4 light-receiving areas of the 4-split photodetector is equal, and
By summing the electrical signals generated in the diagonal photo-receiving areas of the four light-receiving areas of the split photodetector and calculating the difference between them using a differential amplifier, it is possible to detect light in four parts without loss of light intensity. It is possible to stably detect a focus error signal that is not affected by minute displacements of the device.

In addition, by optically bonding the prism, convex lens, and 4-split photodetector, and filling the space with an optical member having a refractive index different from that of the prism, the detection part is completely integrated. Accordingly, it is possible to realize an excellent focus error detection device that can obtain the effect of further improving the precision of the focus error detection device.

[Brief explanation of the drawing]

FIG. 1 is a schematic configuration diagram of a focus error detection device according to an embodiment of the present invention, FIGS. 2, 3, and 4 are schematic configuration diagrams for explaining the operation of the main parts thereof, and FIG. FIG. 6, FIG. 7, and FIG. 8 are schematic diagrams of main parts of other embodiments, and FIG. 9 and FIG. 10 are schematic diagrams of a conventional focus error detection device. 1...Light source, 2...Objective lens, 3.
... Information recording tlXL 4 ... Half mirror 16 & ... Convex lens, e, aa, eb ...
... Prism, 7... Four-split photodetector, 8
... Differential amplifier, 14 ... Optical member. Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9

Claims (5)

[Claims] (1) a light source, an objective lens that focuses light from the light source onto an information recording medium to form a minute light spot, and is located between the objective lens and the light source, and is reflected on the information recording medium; means for separating the light transmitted through the objective lens; a convex lens for converging the light separated by the separating means; and a convex lens located in the convergent light beam by the convex lens, spatially dividing the convergent light beam into two, and one or more transmissive prisms whose half is slightly bent in the direction of the convergent beam splitting line; and the convex lens when the objective lens is focused on the information recording medium. 2 formed by the prism
a four-segment photodetector having a substantially cross-shaped dividing line located at an intermediate position in the optical axis direction of the four focal points and installed so as to receive the light intensity of the two light spots almost uniformly in the four light receiving areas; Of the four light-receiving regions of the split photodetector, the light-receiving regions at diagonal positions are electrically connected to calculate the sum, and a differential amplifier is provided to calculate the difference between the summed electric signals. A focus error detection device characterized by: (2) A focus error detection device according to claim 1, in which a convex lens and one prism are optically bonded. (3) The focus error detection device according to claim 1, which is processed or molded from a single optical material in a shape that integrates a convex lens and one prism. (4) A focus error detection device according to claim 1, in which one prism and a light receiving surface of a four-split photodetector are optically bonded. (5) Optically bond the convex lens and the first prism or
the second prism and the light receiving surface of the four-split photodetector are optically bonded, and the first prism and the second prism are connected between the first prism and the second prism. 2. The focus error detection device according to claim 1, wherein the focus error detection device is filled with an optical member having a refractive index different from that of the focus error detection device.