JPS618746A - Tracking error detector of optical disk device - Google Patents

Abstract

PURPOSE:To eliminate the influences of a blind sector due to the division gap of photodetector into two to detect the tracking error with high precision by arranging an optial system which spreads the reflected light, which is made incident on the photodetector divided into two, in the direction orthogonal to the division gap. CONSTITUTION:The linearly polarized laser light of a semiconductor laser 16 passes a collimator 18, a polarizing beam splitter (PBS)20, a lambda/4 plate 22, and an object lens 24 and is converged on a recording face 10a. The reflected light is reflected on the BPS20 and passes a cylindrical concave lens 28 to form an image C spread in the direction orthogonal to a division gap A on a photodetector 30 divided into two. Outputs of photodetecting elements 30a and 30b are inputted to a differential amplifier 32, and a tracking error signal TER is sent to a tracking servo 34. Since the optical image C and a bit shadow D on photodetecting elements 30a and 30b are sufficiently spread, the tracking error is detected with high precision.

Description

Detailed Description of the Invention [Field of the Invention] The present invention detects the movement of a pit shadow or a diffraction image contained in reflected light from a disk recording surface using a two-split photodetector. The tracking error is detected from the difference in the output of each photodetector element of the device. The present invention relates to a tracking error detection device for an optical disc device.

[Technical background of the invention and prior art]

A large number of pits are formed on the recording surface of the disc along concentric or spiral trunks.

There is an optical disc device that reads digital information using an optical head that follows the track.

Note that the term "tracks" used herein includes not only those with guides such as grooves, but also those without guides. This type of device uses a tracking servo system that detects tracking errors and moves the optical head to the correct position relative to the track in order to cause the optical head to follow the track.

The push-pull method has been widely known as one of the methods for detecting tracking errors.

In this push-pull method, the light reflected by the disk signal surface is guided as parallel light to the two-split photodetector, and the movement of the shadow of the pit or the diffraction image included in this reflected light is measured for each light beam making up the two-split photodetector. Detection is performed based on changes in the amount of light incident on the detection element.

However, in a two-split photodetector, a splitting gap exists between the two photodetectors, and this splitting gap usually has a width of several tens of μm. On the other hand, when the tracking error is zero (second), the pit shadow is located on this division gap, but the width of the pit shadow is also small.

All or most of this shadow will fall within the splitting gap. Therefore, when the tracking error is near zero, the output of the two-split photodetector becomes small, making it difficult to improve tracking accuracy.

[Purpose of the invention]

The present invention was made in view of these circumstances, and when detecting tracking errors using the push-pull method,
It is an object of the present invention to provide a tracking error detection device for an optical disc device that eliminates the influence of a dead zone caused by a division gap of a two-split photodetector and makes it possible to detect tracking errors with high accuracy.

[Structure of the invention]

According to the present invention, the reflected light from the disk recording surface is guided to the two-split photodetector, and the tracking error is detected from the amount of movement of the shadow of the second pit on the recording surface included in the reflected light. A tracking error in an optical disc device that performs tracking correction, characterized in that the optical disc device includes an optical system that spreads the reflected light incident on the two-split photodetector in a direction perpendicular to a division gap of the two-split photodetector. The detection device achieves this objective.

[Embodiment]

Fruit 71 shown below! A detailed explanation of the present invention will be given based on an i aspect.

Fig. 1 is a conceptual diagram showing one embodiment of the present invention, and Fig. 2 is a conceptual diagram showing one embodiment of the present invention.
FIG. 3 is a diagram showing the shape of reflected light incident on a split photodetector. .

In FIG. 1, reference numeral 10 is an optical disc.

A metal or the like is deposited on one side of a transparent substrate such as glass, and this deposited layer is further covered with a protective layer. A large number of pits are formed in this deposited layer along concentric or spiral trunks. That is, this vapor deposited layer becomes the recording surface 10a on which digital information is recorded. This optical disc 10
is rotationally driven by a motor 12.

14 is an optical head. The optical head 14 includes a semiconductor laser 16 that outputs linearly polarized laser light. Collimator 18. Polarizing beam splitter 20. λ/4 plate 22, objective lens 2
4 to guide the laser beam to the recording surface 10a. Recording surface 10
The reflected light R by a is.

The light becomes linearly polarized light having a phase different by 90° from the light emitted from the laser 16 and is reflected by the beam splitter 20.

28 is a cylindrical concave lens as an optical system according to the present invention; 30
3 is a two-split photodetector using a semiconductor, and 32 is a subtracter, which constitute a part of the optical head 14. The two-split photodetector 30 includes a pair of photodetecting elements 30a, 301) facing each other with a splitting gap A interposed therebetween. Cylindrical concave lens 2
8 is arranged so that there is no lens effect in the direction parallel to the dividing gap A. The difference between the outputs of the photodetecting elements 30a and 30b is determined by a subtracter 32, and this difference becomes a TER signal indicating a tracking error.

34 is a tracking servo device, which moves the optical head 1 in the direction where the tracking error becomes zero based on the TER signal.
Move 4.

Note that in FIG. 1, a focus error detection system that detects the reflected light R from the recording surface 10a that has a focus error of the objective lens 24, and a focus servo system that moves the objective lens in the optical axis direction are omitted.

In this embodiment, the reflected light R from the recording surface 10a is converted into a parallel beam by the objective lens 24, and the cylindrical concave lens 28
guided by. In the absence of this cylindrical concave lens 28, the reflected light R becomes circular as shown by the virtual line B due to the second factor. When a pit falls within the spot of the laser beam focused on the recording surface 10a, for example, in the case of a phase type pit, the phase of the reflected light R inside the pit is different from the phase of the reflected light R outside the pit, so they weaken each other. Fit. Therefore, a shadow of the pit appears on the light receiving surface of the two-split photodetector 30. When the cylindrical concave lens 28 is black, the width of this shadow is /JS small, and when the tracking error is zero, all or most of this shadow falls within the dividing gap A. On the other hand, if the cylindrical concave lens 28 is provided as in this embodiment, one reflected light R will be divided into the dividing gap A (
-Spreads in orthogonal directions. At this time, C in Figure 2 is 2
The shape of the reflected light R incident on the split photodetector 30 is shown. Therefore, the shadow of the pit is also enlarged as shown in D in the same figure, and becomes large enough to cover the two photodetecting elements 30a and 30b. As a result, even when the tracking error is close to zero, the movement of the shadow of the pit can greatly change the amount of light received by each photodetector element 30a, 30b, making it possible to detect the tracking error with high accuracy.

In this embodiment, the cylindrical concave lens 28 is used as the optical system, but the same effect can be obtained by using a spherical concave lens instead.

Figures 3.4.5 are diagrams showing optical systems in different embodiments. The embodiment of FIG.

A double prism 36 is used as an optical system.

This double prism 36 is arranged so that the ridge line E is parallel to the division gap A and located at the center of the reflected light R. The reflected light R is split into two at the center by the double prism 36 and guided to the photodetecting elements 30a and 30b, respectively.

Therefore, in this case, tracking errors can be detected without being affected by the division gap A at all.

In the embodiment shown in FIG. 4, the optical system is divided into two convex lenses 38.
a, 38b. Each divided convex lens 38
a and 38b include optical axes F and G, respectively;
G enters the light receiving portions of the photodetecting elements 30'a and 30b, and the cemented surfaces H of both lenses are arranged parallel to the dividing gap A.

According to this embodiment, the two-split photodetector 3 has a small light-receiving surface.
0 can be used.

The embodiment shown in FIG. 5 has two optical systems with slightly different reflection angles.
It is formed by two mirrors 40a and 40b. In this case two mirrors 4
The ridgelines of the reflecting surfaces 0a and 40b are arranged parallel to the dividing gap A, and the light reflected by each reflecting surface is directed to the photodetecting element 30a.

It enters the light receiving section 30b.

Note that in the embodiments shown in FIGS. 3 and 5, the double prism 36
The two lights after passing through and the two mirrors 40a.

After the two lights reflected by 40b cross each other, 2
Of course, they may be separated at the light receiving portion positions of the split photodetector 30.

〔Effect of the invention〕

The present invention is equipped with an optical system that spreads the reflected light from the recording surface that enters the two-split photodetector in a direction perpendicular to the splitting gap of the two-split photodetector, so that the shadows of pits on the recording surface are eliminated. The inconvenience that tracking error detection accuracy decreases due to entering the division gap is eliminated. Therefore, a highly accurate tracking servo system can be constructed.

[Brief explanation of the drawing]

The first factor is a conceptual diagram of one embodiment of the present invention, and Figure 2 is the second factor.
A diagram showing the shape of reflected light incident on the split photodetector, and the third
．． 4.5 are diagrams showing different embodiments. 10... Optical disc, 10a... Recording surface. 14... Optical head, 28... Cylindrical concave lens. 30...2-split photodetector. 30 a, 30 b = "ft, detection element. 36...Double prism l 38a, 38b...Divided convex lens. 11Qa, 40b...Mirror, A...Divided gap. D...Shadow of pit, R...Reflected light. Patent applicant Fuji Photo Film Co., Ltd. Agent Patent attorney Yama 1) Yu Fumi Figure 1 Figure 2 Figure 3

Claims (6)

[Claims] (1) The reflected light from the disk recording surface is guided to a two-split photodetector, and the tracking error is detected from the amount of movement of the shadow of the pit on the recording surface included in this reflected light, and the tracking correction of the optical head is performed. A tracking error detection device for an optical disc device, comprising: an optical system that spreads the reflected light incident on the two-split photodetector in a direction perpendicular to a dividing gap of the two-split photodetector. (2) A tracking error detection device for an optical disc device according to claim 1, wherein the optical system is formed of a cylindrical concave lens arranged so as not to have a lens effect in a direction parallel to the dividing gap. (3) A tracking error detection device for an optical disk device whose optical system is formed by a spherical concave lens. (4) A tracking error detection device for an optical disc device according to claim 1, wherein the optical system is formed of a double prism arranged so that its ridgeline is parallel to the dividing gap. (5) The optical system is formed of two split convex lenses whose optical axes are joined by a plane parallel to the split gap, and each optical axis of each split convex lens receives light from two photodetecting elements of a two-split photodetector. A tracking error detection device for an optical disc device according to claim 1, wherein the tracking error detection device is arranged so as to pass through the area. (6) A tracking error detection device for an optical disc device according to claim 1, wherein the optical system is formed of two mirrors in which the ridgelines of the two reflective surfaces are arranged parallel to the dividing gap.