JPH0554179B2 - - Google Patents

Description

Detailed Description of the Invention [Technical Field of the Invention] The present invention relates to an automatic focusing device for an optical disk head that reads or writes information from an information recording medium such as a digital audio disk, a video disk, or a computer optical disk. The present invention relates to an automatic focusing device using a point aberration optical system.

[Prior art]

In recent years, information optically recorded on an information recording disk is reproduced by a head using a laser beam, or conversely, information is recorded. Optical video disc devices, optical audio disc devices, etc. are being actively developed. In the head of this type of optical disk device (hereinafter referred to as an optical disk head), in order to accurately record and reproduce information, a focused laser beam is always correctly focused onto the information recording track of the disk, which is the information recording medium. An automatic focusing device is required for illumination. As such an automatic focusing device, a method is known in which information detection and focus position detection are performed using an astigmatism optical system. FIG. 1 is an optical path diagram schematically showing an automatic focusing device for an optical disk head using this conventional astigmatism optical system, and FIG. 2 is a schematic diagram showing a servo mechanism below the photodetector. In the figure 1
is a light source such as a semiconductor laser, 2 is an emitted light beam emitted from the light source 1, 3 is an objective lens, and 4 is a digital audio signal, video signal, etc. placed near the focal point position of the objective lens, and information such as a track is recorded therein. 5 is a reflected light beam that passes through the objective lens 3 from a condensing spot on the disk 4, 6 is a beam splitter that separates the emitted light beam, and 7 is an astigmatism for the reflected light beam 5. A cylindrical lens is an optical element that gives an aberration, and its cylindrical axis direction is
(perpendicular to the plane of the paper), and the direction perpendicular to this within the lens plane is y. 8 is a photodetector divided into four parts 8a, 8b, 8c, and 8d by a dividing line having an angle of 45 degrees with the x and y directions, and 9 is a reflected light spot when focused by the reflected light beam 5 projected thereon. , 9x is the reflected light spot when the disk 4 approaches the in-focus position, 9y is the reflected light spot when the disk 4 is moving away from the focused position, 10 is a focal position detection circuit consisting of adders 11, 12 and a differential amplifier 13, 1
4 is an information detection circuit consisting of an adder; 15 is a focus actuator that moves the objective lens in the optical axis direction; 16 is a lens drive circuit that energizes the focus actuator 15 to drive the objective lens; 17 is not shown; There is no information reproducing circuit.

Next, its operation will be explained. A laser beam 2 emitted from a light source 1 is converged by an objective lens 3 to form a condensing spot on a track of a disk 4. The reflected light beam 5 containing information read from the information track on the disk 4 from this condensing spot is separated from the output light beam 2 by a beam splitter 6, and is converged in one direction by a cylindrical lens 7 to an astigmatic point. It is converted into an aberrational beam. In other words, the cylindrical lens 7 has no lens action in the plane that includes the cylindrical axis x direction and the optical axis, and the objective lens 3 converges the light to a point P in the y direction perpendicular to x and the plane that includes the optical axis (the plane of the paper). ), the light is focused on a point Q by the convergence effect of the cylindrical lens 7. Therefore, the light distribution shape of the reflected light beam subjected to astigmatism is a long line in the x direction at point Q, a long line in the y direction at point P, and the major axis is in either the x or y direction between them. It can be oval or circular. A 4-split photodetector 8 is placed at a position where the light distribution shape of the reflected light beam subjected to astigmatism becomes circular when the disk 4 is at the focused position of the objective lens 3 (hereinafter referred to as "in-focus"). There is. Therefore, when the photodetector 8 is in focus, a circular reflected light spot 9 appears, and when the optical disk 4 is displaced in a direction closer to the objective lens 3 than when it is focused, an elongated oval reflected light spot 9x in the x direction appears. When the light is displaced in a direction away from the focused state, a reflected light spot 9y having an elongated oval shape in the y direction is incident. When the circular reflected light spot 9 is formed, the light receiving areas 8a, 8b, 8c, and 8d have the same light receiving area, and the reflected light spot 9x has the major axis in the x direction.
When the reflected light spot 9y has a long axis in the y direction, the light receiving areas 8a and 8c are the light receiving areas 8b and 8d.
However, the light receiving area is larger than other areas. Therefore, the adder 11 calculates the sum of the light receiving outputs of the light receiving areas 8a and 8c.
, and the output of the adder 12 which calculates the sum of the light receiving outputs of the light receiving areas 8b and 8d, from the differential amplifier 13, the output signal, that is, the output signal _Ef of the focal position detection circuit 10, When the reflected light spot 9 is circular, it is zero, and when the optical disc 4 is closer than when in focus, and the reflected light spot 9x is elongated in the x direction, it is positive, and when the optical disc 4 is equidistant than when it is in focus, it is elongated in the y direction. At the reflected light spot 9y, the output becomes negative, and the output is approximately proportional to the deviation from the focused position. Therefore, the lens drive circuit 16 is controlled by the output E _f of the focus position detection circuit 10, and the focus actuator 15 is energized, so that the objective lens 3
Focusing is performed automatically by moving the lens in the direction of the optical axis. Further, the read information from the optical disk 4 included in the reflected light beam 5 is guided to a reproducing circuit 17 where it is extracted by an information detecting circuit 14 that calculates the sum of the light receiving outputs of each light receiving area.

However, the above-mentioned conventional device has the following problems in practical use. That is, although the output of the differential amplifier 13, E _f , at the time of focusing is zero, the intensity of the reflected light to the photodetector 8 is not zero. Light receiving area 8
The output is simply zero because the sum of the intensities of incident light to a and 8c and the sum of intensities of incident light to 8b and 8d are balanced. At this time, if the distribution of the reflected light flux on the photodetector 8 surface changes for some reason, the above balance will be disrupted, and it will appear as if the focal point position has changed even though the focal point position has not changed. An output is produced in differential amplifier 13. The cause of this is a variation in the light emission distribution of the light source 1.
When the light source 1 is a semiconductor laser, the emission distribution changes due to changes in excitation current and deterioration over time.
The luminous flux distribution is a single-peaked mountain-shaped intensity distribution with the maximum intensity at the center, and the center of the four divisions coincides with the luminous flux center, so even slight variations in the distribution greatly affect the balance. Another factor is the influence of diffracted light components from pre-groups (guide tracks) and recording pits in add-on disks (recordable disks). The pattern distribution of these diffracted lights is not necessarily symmetrical about the optical axis, but changes depending on the degree of track deviation and whether or not a signal is recorded in the pregroove. As a result, the circular distribution at the time of focus collapses, resulting in an offset of the tracking point.

[Summary of the invention]

This invention was made in order to eliminate the drawbacks of the conventional ones as described above, and by configuring the photodetector so that the light beam reflected thereon during focusing is hardly used for detecting the focal position, It is an object of the present invention to provide an automatic focusing device with a small offset of a tracking point.

[Embodiments of the invention]

An embodiment of the present invention will be described below with reference to the drawings. FIG. 3 is a schematic diagram showing the configuration and connection example of a photodetector used in an automatic focus adjustment device that is an embodiment of the present invention; other parts are shown in FIGS. 1 and 2.
It has the same configuration as the figure. In the figure, the same symbols as in Figure 2 indicate the same or corresponding parts, and 18 is 5.
It is a split photodetector, with a central region 18e whose central part is separated from the outer part by a circular dividing line, and an outer quadrant where the outer part is divided into four by dividing lines having an angle of 45 degrees with the x and y directions. areas 18a, 18b, 18c,
18d. That is, the central region 18e is newly provided, and its position and size are set to be approximately equal to, or slightly smaller than, the position and size of the reflected light spot 9 upon focusing. The focus position detection circuit 10 has four
Only the divided outer regions 18a, 18b, 18c, and 18d are connected, and the central region 18e is connected to the information reproducing circuit 17.

Next, we will explain the operation of optical disk 4.
comes too close to the objective lens 3, the photodetector 18
The reflected light spot becomes 9x, the output E _f of the differential amplifier 13 becomes positive, and the objective lens 3 is controlled to move away from the optical disk 4. Conversely, if the optical disk 4 moves too far away from the focal point position,
The reflected light spot becomes 9y, the output E _f of the differential amplifier 13 becomes negative, and the objective lens 3 is controlled to approach the optical disk 4. When the optical disk 4 is at the focused position, most of the reflected light spot 9 is incident on the central region 18e, and the outer region 18
Almost no light is incident on a, 18b, 18c, and 18d. Therefore, in the focused state, the focus position detection circuit 10
Both the input and output of are close to zero. Of course, at the time of focusing, since the reflected light spot 9 is circular, a small predetermined amount of light flux is transmitted to the outer areas 18a, 18 of the photodetector 20.
b, 18c, 18d, the adder 11,
The output of 12 is balanced, and the output of differential amplifier 13
The output E _f is zero. A reproduced high frequency signal is obtained from the central region 18e.

In this way, the photodetector 18 shown in FIG.
Is no reflected light incident on b, 18c, 18d?
The structure is such that only a small amount of light enters even if it enters, so even if the distribution of the reflected laser beam changes,
Zero point drift does not occur. That is, in the conventional example shown in FIG. 1, when focusing, the strong light incident on the light receiving areas 8a and 8c and the strong light incident on the light receiving areas 8b and 8d are balanced to produce the signal E _f in the example. Therefore, changes in the luminous flux distribution are directly linked to zero point drift. On the other hand, in this invention, as mentioned above, during focusing, almost no reflected light flux enters the outer area of the photodetector for detecting the focus position, so there is no way in principle that zero point drift will occur even if the reflected light flux distribution changes. That's why.

Therefore, it is possible to construct an automatic focus adjustment device that is resistant to changes in the distribution of the semiconductor laser beam and is not easily affected by changes in the distribution of the reflected beam due to diffraction from the pre-group of a recordable disk, a recordable and erasable disk, or the like.

In the above embodiment, the reproduced signal is extracted from the central region 18e of the photodetector, but the reproduced signal may be extracted from the sum of the received light outputs of the outer regions 18a, 18b, 18c, and 18d.

〔Effect of the invention〕

As described above, in the present invention, when the convergent light on the optical disk is in focus, almost no reflected light enters the outer quadrant of the photodetector, so the distribution of the light flux on the photodetector is Even if the focus changes, the disturbance in the focus detection signal can be minimized.Also, since the photodetector is configured into five planar parts, there is little gap between each element of the photodetector, so there is less loss of light amount and operation is improved. It has the effect of being accurate.

[Brief explanation of the drawing]

Fig. 1 is a schematic optical path diagram showing an automatic focus adjustment device for an optical disk head using a conventional astigmatic optical system, Fig. 2 is a schematic diagram showing the servo mechanism below the photodetector, and Fig. 3 1 is a schematic diagram showing the configuration of a photodetector and its connections in an embodiment of the present invention. In the figure, 1 is a light source, 3 is an objective lens, 4 is an optical disk that is an information recording medium, 6 is a beam splitter, 7 is a cylindrical lens that is an astigmatism optical element, 8 is a 4-split photodetector, and 10 is a cylindrical lens that is an astigmatism optical element. Focus position detection circuit, 11 and 12 are its adders, 13 is a differential differential amplifier, 14 is an information detection circuit, 15 is a focus actuator, 16 is a lens drive circuit, 18 is a 5-split photodetector, and 19e is its center Area, 19a, 1
9b, 19c, and 19d are outer regions. The same reference numerals in the figures indicate the same or corresponding parts.

Claims (1)

[Claims] 1. A light source, an objective lens that focuses the emitted light beam from the light source onto a track of the information recording medium, and separates the emitted light beam from the light source and the reflected light beam from the condensing spot on the recording medium through the objective lens. A beam splitter that applies astigmatism to this separated reflected light beam, an optical element that receives the reflected light beam that passes through this optical element, and outputs different detection outputs depending on the shape of the received reflected light spot. It is equipped with a photodetector and a focus position detection circuit that extracts a signal corresponding to a deviation from the focal point position of the objective lens of the recording medium from the output of the photodetector, and the output of the detection circuit detects the objective lens. In an automatic focusing device for an optical disk head that is driven in the axial direction, the photodetector has a central region separated from the outer region by a circular dividing line, and a central region separated from the outer region by a circular dividing line. , has a planar five-division configuration with a four-division outer region divided into four by mutually orthogonal dividing lines forming an angle of approximately 45° with respect to the optical element axis, and an information recording medium is attached to the objective lens. If you get too close, the reflected light spot will
It has an elongated shape that occupies a part of the central area and a part of the two opposing outer areas, and if the information recording medium is too far away from the objective lens, the reflected light spot will be reflected when the information recording medium is Reflected light spots when you get too close to the lens
When the information recording medium is in focus, the reflected light spot forms a circle whose diameter is approximately equal to the central area, and most of the spot is incident on the central area and on the outer area. The difference in the sum of the received light outputs of the respective opposing areas of the four-divided outer area is extracted by the focal position detection circuit as a signal corresponding to the deviation from the in-focus position. An automatic focus adjustment device for an optical disk head, characterized in that: