JPH0237609B2 - - Google Patents

Description

[Detailed Description of the Invention] [Technical Field of the Invention] This invention relates to a digital audio disk,
The present invention relates to a focus shift detection device for an automatic focus adjustment device for an optical disk head that reads or writes information from an information recording medium such as a video disk.

[Prior Art] Conventionally, as a device of this kind, a method called the so-called areal density method is known, which detects a focus shift by detecting changes in the diameter of a reflected light beam from an optical disk, which is an information recording medium. (See Publication No. 18247). FIG. 1 is a schematic diagram showing this conventional automatic focus adjustment device for an optical disk head, in which 1 is a light source such as a semiconductor laser, 2 is an emitted light beam emitted from the light source 1, 3 is a condenser of the emitted light beam 2, An optical disk 4 which is an information recording medium placed near the focal point position of the optical disk 4
6 is a reflected beam that passes through the objective lens 3 from the condensing spot 5 on the optical disk 4 surface, and 7 is a beam splitter that separates the emitted beam 2 and the reflected beam 6. Ivy 8 is a two-split photodetector placed near the optical axis position P of the reflected light beam 6 separated by the beam splitter 7, which detects an inner sensitive area and an outer sensitive area divided by a circular dividing line 9. Consists of sensitive areas. 10 is a photodetector 8
11 is a focus shift detection circuit consisting of a differential amplifier that amplifies the difference in received light output between the inner region and the outer region of the two-split photodetector 8; 12 is an objective lens drive circuit; 13 is a focus actuator that moves the objective lens 3 in the axial direction.

Next, its operation will be explained. A laser beam 2 emitted from a light source 1 is directed to an optical disk 4 by an objective lens 3.
The light is focused on an information track (not shown) on the surface to form a minute light focusing spot 5. The reflected light flux from the condensing spot 5 re-enters the objective lens 3 and returns along the optical path to the beam splitter 7.
The light beam is separated from the output light beam by , and the direction is changed by 90 degrees to reach the two-split photodetector 8 . Since this photodetector 8 is placed closer to the focal point P of the reflected light beam 6, the reflected light beam spot 10 focused on the photodetector 8 has a certain size. The center of the circular dividing line 9 of the photodetector 8 is made to coincide with the center of the reflected light beam 6, and the light to the photodetector 8 when the optical disk 4 is at the focused position of the objective lens 3 (hereinafter referred to as "in-focus") is determined. The position of the photodetector 8 and the size of the dividing line 9 are set so that the reflected light spot 10 receives equal amounts of light in its inner and outer regions. Therefore, the output Ef of the differential amplifier, that is, the focus shift detection circuit 11 during focusing becomes zero, and when the optical disk 4 approaches the objective lens 3 from the in-focus position, the reflected light beam condensing point P becomes the photodetector. 8, the reflected light spot 10 to the photodetector 8 becomes larger,
The amount of light received by the outer region is larger than the amount of light received by the inner region, and the focus shift detection circuit 11 outputs
Ef becomes negative. On the other hand, when the optical disk 4 moves away from the focal point position, the reflected light beam condensing point P approaches the photodetector 8, the reflected light spot 10 to the photodetector 8 becomes smaller, and the amount of light received by the inner region depends on the outer region. This becomes larger than the amount of received light, and the output Ef of the focus shift detection circuit 11 becomes positive. In this way, the displacement of the optical disk 4 in the optical axis direction causes a displacement of the reflected light beam condensing point P on the optical axis, which causes a change in the diameter of the reflected light spot 10 to the photodetector 8, and detects a focus shift. This results in a change in the output Ef of the circuit 11. The output Ef of the focus shift detection circuit 11 energizes the objective lens drive circuit 12, drives the focus actuator 13, moves the objective lens 3 in a direction that compensates for the focus shift ΔF, and automatically Focusing can be performed. FIG. 2 is a diagram showing this focus deviation detection characteristic, where the vertical axis is the output Ef of the focus deviation detection circuit 11, and the horizontal axis is the deviation △F of the optical disc 4 from the in-focus position (the direction of moving away is positive). There is. In the flat part A of this characteristic diagram, the reflected light flux convergence point P approaches the photodetector 8,
This is the dead zone corresponding to the situation in which the reflected light spot 10 is completely contained within the inner region of the photodetector 8. As the optical disk 4 moves further away, the reflected light beam convergence point P comes to the position of the photodetector 8. As △F further increases from that position, the reflected light flux convergence point P comes to the opposite side of the photodetector 8, and as △F increases, the diameter of the reflected light spot 10 increases, and Ef
decreases. That is, the characteristics are approximately the same on the left and right sides with dead zone A as the center.

The focus shift detection device having the characteristics shown in Figure 2 above uses a divided photodetector divided into an inner area and an outer area, so it is difficult to use a device that uses multiple non-divided photodetectors. Although higher sensitivity can be obtained compared to the conventional method, it has the following two problems. The first problem is that, as shown in Fig. 2, the characteristic intersects the zero line at Ef = 0, which is the detection operation level, and the point B at the in-focus position △F = 0. Point B also intersects at the symmetrical point C. Therefore, the photodetector 8, the circuit 11, 1
When the focus servo loop consisting of the focus actuator 2 and the focus actuator 13 changes from the open state to the closed state by a switch (not shown) at the time of device startup,
If the optical disk 4 deviates far from the in-focus position C
If it is to the right of the point, positive feedback will occur with the loop closed, and the objective lens 3 will be driven further away from the optical disk 4 and will no longer be pulled into the focal point B. The second problem is that the focus shift detection characteristics shown in FIG.
It is not symmetrical to . That is, the detection sensitivity differs before and after the in-focus position, which causes the operation of the focus servo system to become unstable, and it is difficult to design the focus servo system to correct this.

[Summary of the Invention] This invention was made in order to eliminate the drawbacks of the conventional ones as described above. 2. Separate into optical paths, provide an optical path difference for the separated reflected light beams, and receive each reflected light beam on the same plane at a position farther from one light convergence point and closer to the other light convergence point. By providing separate photodetectors, it is possible to provide a defocus detection device for an optical head that is highly sensitive and has symmetrical and good defocus detection characteristics at the focused position.

[Embodiment of the Invention] Figures 3 and 4 are schematic diagrams showing an embodiment of the present invention. The explanation will be omitted. Reference numeral 14 denotes a stepped mirror having two reflecting surfaces 16 and 17 parallel to the surface having a stepped boundary line 15, and is arranged so that the stepped boundary line 15 crosses the center of the reflected light beam 6. 6
8 constitutes a beam splitting element that separates the light beam into approximately half a beam of a first reflected beam 6' and a second reflected beam 6''. A two-split first photodetector 8' for receiving the first reflected light beam 6' and a first photodetector 8' for receiving the first reflected light beam 6' are arranged on the same plane at a position near and far from the condensing point P'' of the second reflected light beam 6''. The two photodetectors consist of a second photodetector 8'' divided into two for receiving reflected light beams 6'', 9' and 9'' are photodetectors 8',
8″ circular dividing line, 10′, 10″ is photodetector 8′,
8'' is a reflected light spot of the first reflected light beam 6' and the second reflected light beam 6''; 11 is a differential amplifier 18;
19 and an adder 20.

Next, its operation will be explained. Beam splitter 7
The reflected light beam 6 separated by the stepped mirror 1
The reflected light beam 4 is divided into a first reflected light beam 6' and a second reflected light beam 6'', each having a semicircular cross section, at a stepped boundary line 15 placed at a position crossing the center of the reflected light beam 6''. The light beam is reflected by the two parallel reflecting surfaces 16 and 17, which are located at different positions on the optical path due to the step 15, and becomes two parallel reflected light beams that are spatially divided into two. 6',
The positions of the condensing points P' and P'' of 6'' differ in the optical axis direction due to the optical path difference caused by the step 15. Near the middle of the focal points P' and P'' at the time of focusing, there is a two-split light detector for receiving each reflected beam at a position where the cross-sectional beam diameters of the first and second reflected beams 6' and 6'' are equal. 8′, 8″ are placed, and the respective photodetectors 8′, 8″ are placed.
The center positions of the circular dividing lines 9', 9'' of 8'' and the center positions of the reflected light spots 10', 10'' are approximately aligned, and each of the two-split photodetectors 8', 8'' when in focus is The amount of light received by the inner region and the amount of light received by the outer region are set to be equal. In this way, when the disk 4 approaches the focal point position, the reflected light beam 6',
6'' tracks P', P'' move to the left, and as the disk 4 moves away from the focal point position, the focusing points P', P'' move to the right.As the focusing points P', P'' move to the right. FIG. 5 shows changes in the reflected light spots 10', 10''.
FIG. 5 shows the reflected light spot 10' to the photodetector 8,
10'' is an operation explanatory diagram showing the change in the shape of the optical sensor 10'', in which c is when focusing, b is when the disk 4 approaches the focus point and the condensing point P'' is at the position of the photodetector 8, a is is closer than that, d is the reflected light spot 10' when it is further away from the focused point and the focal point P' is at the position of the photodetector 8, and e is the reflected light spot 10' when it is further away from it.
10''.The changes in the outputs Ef' and Ef'' of the differential amplifiers 18 and 19 when they change in this way are shown in the sixth figure.
FIG. 7 shows changes in the output of the adder 20, that is, the output Ef of the focus shift detection circuit 11. In Figures 6 and 7, the vertical axis is the signal output Ef',
Ef'' and Ef, the horizontal axis indicates the deviation from the focused position of the disk 4, that is, the focus deviation △F (positive in the direction of moving away), and the solid line in Fig. 6 is the output of the differential amplifier 18.
The broken line indicates the output Ef'' of the differential amplifier 19.
As is clear from the figure, the signal outputs Ef' and Ef'' have symmetrical characteristics rotated by 180 degrees around the origin, and the output signal of the focus shift detection circuit 11 is the sum of them.
Ef is also symmetrical. In addition, in FIGS. 6 and 7, each point indicated by a to e corresponds to the position of a to e in FIG. 5. In this way, the focus shift detection characteristics shown in Figure 7 are completely symmetrical around the in-focus point, so the detection sensitivity near the in-focus point is equal before and after the in-focus point, and the detection characteristics are greatly improved. . In addition, even if the focus shift becomes considerably large, the detection characteristics will not cross the zero line, and △F
If is positive, Ef is positive no matter how large it is,
If ΔF is negative, Ef is always negative. Therefore, no matter what position the objective lens 3 is at when the focus servo is started, when the servo circuit is turned on, the focus actuator is always driven in the servo pull-in direction, and the servo pull-in operation is performed.

In the above embodiments, the stepped mirror 14 is constructed integrally with two reflecting surfaces parallel to the surface, but this is made of glass or plastic that is formed into a predetermined stepped shape, and the surface is coated with metal plating, etc. It can be constructed by applying a reflective film, but it is also possible to construct a stepped mirror by bonding two mirrors together, or by forming a stepped structure on the back side of an optical medium such as transparent glass or plastic. It may also be of a back reflective type with a reflective surface.

Furthermore, the beam splitting element does not necessarily have to be a stepped mirror, but may be used, for example, as shown in FIG. You may use the mirror-like optical element 24 which has. That is, by placing the boundary line 21 at a position that crosses the center of the reflected light beam, the reflected light beam 6 is divided into first and second reflected light beams 6' and 6'' each having half a light beam, and the inclined reflection plane 22,
It is sufficient to spatially separate the light by reflecting it at 23 to create an optical path difference, thereby spatially separating the condensing points P' and P'', and placing the photodetector 8 at an appropriate position between them.

Furthermore, in the above embodiment, an example was shown in which a two-split photodetector having a circular dividing line was used, but as shown in FIGS. 9 and 10, parallel three-split photodetectors 8', 8'' The central area of each photodetector may be used as an inner area, the areas on both sides of the common connection may be used as outer areas, and the sum of the differences in the light reception outputs of these two areas may be used as the focus shift detection output Ef.
In addition, the three-split photodetector 8' in FIGS. 9 and 10,
Two-split photodetectors 8', 8'' may be used in which the central area of 8'' is the inner area surrounded by rectangular dividing lines 9', 9'' as shown in FIGS. 11 and 12. By doing so, there is no need to commonly connect the regions on both sides.

Although not shown in the above explanation, in the above-mentioned optical disk head, in order to make the light condensing spot 5 accurately follow the information track on the optical disk 4, a track deviation of the light converging spot 5 is detected. A tracking servo circuit is provided for driving the objective lens 4 in a direction perpendicular to the track by a tracking actuator in accordance with the deviation. Therefore, the reflected light beam 6 is displaced by the displacement of the condensing spot 5 in the direction perpendicular to the track due to the tracking servo operation. In order to eliminate the influence of this displacement on the beam splitting effect, the beam splitting element 1 in the embodiments shown in FIGS.
By aligning the directions of the boundary lines 15, 21 of the boundaries 15, 24 with the direction orthogonal to the track, that is, the direction of displacement of the reflected beam 6 caused by the tracking servo operation, It is possible to prevent characteristic deterioration due to detachment. or,
In order to eliminate the influence on the light receiving output of the photodetectors 8', 8'' due to the displacement of the reflected light spots 10', 10'' due to this tracking servo operation, the three-split photodetectors 8', 8'' shown in FIG. ” dividing line direction, 2 in Fig. 11
The long side direction of the rectangular dividing lines 9', 9'' of the divided photodetectors 8', 8'' is set in the direction perpendicular to the track of the optical disk, that is, the direction of the reflected light spots 10', 10'' generated by the tracking servo operation. Furthermore, as shown in FIG. 13, each photodetector 8', 8'' of each of the embodiments shown in FIGS. 9, 10, 11, and 12 is divided. By providing an inclination to the lines 10', 10'', each photodetector 8',
8'' in the direction of arrow 25 in the figure,
The relationship between the diameter of the reflected light spots 10', 10'' and the distance between the dividing lines 9', 9'' can be finely adjusted.

[Effects of the Invention] (a) The linearity of the detection characteristics near the in-focus point is good.

(b) There is only one zero cross point, making it easy to pull in the servo system.

(c) Since complementary detection is used, there are small changes in characteristics due to positional shifts of the photodetector over time and changes in the light emission pattern of the LD.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram showing a conventional automatic focusing device for an optical disk head, FIG. 2 is a focus shift detection characteristic diagram thereof, and FIGS. 3 and 4 are schematic diagrams showing an embodiment of the present invention. Figure 5 is an explanatory diagram of its operation, Figures 6 and 7 are its focus shift detection characteristic diagrams,
FIG. 8 is a schematic diagram showing another embodiment of this invention, and FIGS. 9, 10, 11, and 12 are only a photodetector and a focus shift detection circuit of other embodiments of this invention, respectively. FIG. 13 is a schematic diagram showing only a photodetector according to another embodiment of the present invention. In the figure, 1 is a light source, 3 is an objective lens, 4 is an optical disk which is an information recording medium, 7 is a beam splitter,
8', 8'' are divided photodetectors, 9, 9', 9'' are dividing lines thereof, 11 is a focus shift detection circuit, 12 is an objective lens drive circuit, 13 is a focus actuator, 14, 24 are luminous fluxes Separation elements 15 and 21 are their boundaries, and 16, 17, 22 and 23 are reflective surfaces. The same reference numerals in the figures indicate the same or corresponding parts.

Claims (1)

[Scope of Claims] 1. A light source, an objective lens for condensing the light beam emitted from the light source onto a track of an information recording medium, and the objective lens for condensing the light beam emitted from the light source and the light beam from the condensing spot on the recording medium. A beam splitter is placed at a position shifted in the optical axis direction from the convergence point position of the separated reflected light beam, and the beam splitter separates the reflected light beam that has passed through the beam splitter. a photodetector configured to obtain different received light outputs depending on the size of the light, and a focus shift for extracting a signal corresponding to a focus shift of a condensing spot of the objective lens on the recording medium from the output of the photodetector. In a defocus detection device for an optical head equipped with a detection circuit, the reflected light beam separated by the beam splitter is divided into two semicircular sections at the center in a cross section perpendicular to the optical axis of this light beam, and spatially A beam splitting element is provided that separates the first and second reflected beams on different optical paths, and the photodetector is placed far from the convergence point of one of the two reflected beams separated by the beam splitting element and at a point where the other beam is focused. two photodetectors, first and second, each of which is divided into one inner region and one or more outer regions, each receiving the separated reflected light beams at a position closer to the light spot; and the focus shift detection circuit is the first focus shift detection circuit.
the electrical output of the inner region of the photodetector minus the electrical output of its entire outer region; and the electrical output of the entire outer region of the second photodetector minus the electrical output of its inner region. 1. A focus shift detection device for an optical head, characterized in that the focus shift detection circuit obtains a signal according to the focus shift by adding the electrical output of the focus shift. 2. The optical head defocus detection device according to claim 1, wherein the beam separation element is a stepped mirror consisting of two parallel reflection planes having a stepped boundary line at a position crossing the center of the cross section of the reflected beam. . 3. The light beam separating element is a mirror-like optical element consisting of two reflective planes that are bent at a straight line that crosses the center of the cross section of the reflected light beam and are tilted to each other. Optical head defocus detection device. 4. The focal point of the optical head according to any one of claims 1 to 3, wherein the first and second photodetectors have a structure divided into two such that each inner region is approximately circular. Misalignment detection device. 5. The light detector according to any one of claims 1 to 3, wherein each of the first and second photodetectors has a substantially parallel three-part structure with an inner region at the center and outer regions at both sides. Head defocus detection device. 6. The focal point of the optical head according to any one of claims 1 to 3, wherein the first and second photodetectors have a structure divided into two so that the inner regions thereof are approximately rectangular. Misalignment detection device. 7. The optical head according to claim 2 or 3, wherein the stepped boundary line or the bent boundary line of the beam splitting element is arranged so as to substantially coincide with a direction orthogonal to the track of the information recording medium. Defocus detection device. 8. Claim 5, wherein the first and second photodetectors are arranged so that the direction of the dividing line or the long side direction of the rectangular dividing line substantially coincides with the direction orthogonal to the track of the information recording medium. 7. A defocus detection device for an optical head according to claim 6. 9. Claims 5 and 6, wherein the two long sides of the two dividing lines of the first and second photodetectors or the two long sides of the rectangular two-spectrum detection element have adjustment inclinations. Optical head defocus detection device.