JPH10143878A - Optical pickup and optical disk device using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the occurrence of offset due to objective lens displacement even when a tracking error is detected by a push-pull system by integrating a diffraction grating and a 1/4 wavelength plate with the same actuator as an objective lens. SOLUTION: The diffraction grating 4 and the 1/4 wavelength plate 5 are fixed to the same actuator 10 as the objective lens 6 to be integrated. Thus, even when the objective lens 6 is displaced in the disk radial direction at a tracking control time, the relative position of the division line of the diffraction grating 4 for an optical reflection light beam isn't displaced. The light beam transmitting through the diffraction grating 4 and the 1/4 wavelength plate 5 is converged on an optical disk 7 through the objective lens 6. Then, the reflection beam arrives at the diffraction grating 4 through an original path. At this time, the incident light beam advances in the prescribed direction by that ±1st-order diffracted light are separated from respective areas of the grating since the polarization direction of the incident light beam is orthogonally intersected with the outward light. Thereafter, the ± 1st-order diffracted light are reflected by a polarizing beam splitter 3 to be made incident on an 8 division detector 9 through a detection lens 8.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical pickup suitable for recording an information signal on an optical information recording medium (hereinafter referred to as an optical disk for simplicity) or reproducing the recorded information signal. The present invention relates to an optical disk device that has been used.

[0002]

2. Description of the Related Art In recent years, as an effective means for downsizing and simplifying an optical pickup, many configurations having a diffraction grating and a hologram element in a detection system have been disclosed. For example, the optical pickup described in Japanese Patent Application Laid-Open No. 8-77578 is called spot size detection by arranging a hologram element immediately below an objective lens and further providing a multi-segmented light detector near a laser light source. A focus error signal is detected by a method, and a tracking error signal can be detected by a method called a push-pull method. When a hologram element or a diffraction grating is arranged in the optical path as in this example, it is possible to greatly reduce and simplify the optical pickup.

[0003]

The optical disks generally used at present are roughly classified into two types according to the difference in the disk structure. That is, a continuous guide groove is provided in advance on the information recording surface of the disc,
A so-called recordable disc on which information signals can be recorded or erased along the guide grooves, and a concave / convex pit row corresponding to the information signal are previously formed on the disc. There are two types of discs, so-called read-only discs, which can only reproduce an information signal from a disc. In addition, depending on the type of disc, there is a disc in which a recordable area and a read-only area are mixed in one disc. Under such circumstances, it is naturally desirable that the same optical disk device can freely record or reproduce data without discriminating or restricting a plurality of types of disks having different disk structures.

However, in general, the push-pull method, which is the optimum tracking error signal detection method for a recordable disk provided with a guide groove, is not suitable for a so-called read-only disk having no continuous guide groove. The most common three-spot method as a tracking error signal detection method for a read-only disc cannot be applied to a recordable disc. That is, when recording or reproducing a recordable disc and a read-only disc, respectively, a separate tracking error signal detection method suitable for each disc structure is required.

Conventionally, to solve such a problem, a tracking error signal detection method dedicated to a recordable disc using a push-pull method or a tracking error signal detection method dedicated to a read-only disc is used. An optical pickup that can detect only a signal is generally used, and a configuration in which the same optical pickup is mounted on a plurality of types of tracking error signal detection systems has not been devised.

In view of such a situation, in the present invention, by using a diffraction grating, the size and simplification of the optical pickup are equal to or larger than that of the conventional small optical pickup. It is an object of the present invention to provide an optical pickup equipped with a tracking error signal detection method suitable for a type disc and a high-performance optical disc apparatus using the optical pickup and supporting various optical discs having different disc structures.

[0007]

In order to solve the above-mentioned problems, the present invention employs a differential phase detection system (hereinafter abbreviated as DPD system) as a tracking error signal detection system suitable for a read-only disc. did. This DPD method divides a reflected light beam from an optical disk into four crosses, and scans the light intensity modulation signal detected from each of the divided regions when a minute spot on the optical disk scans the information pit row on the disk. This is a method of generating a tracking error signal by a predetermined arithmetic processing from the phase difference. In particular, this method has recently attracted attention as a detection method suitable for detecting a tracking error signal of a read-only disc composed of an uneven signal pit array. The present invention has devised an optical pickup optical system capable of detecting a tracking error signal by both the DPD method and the push-pull method by combining one 4-split diffraction grating and a multi-split photodetector.

That is, in an optical pickup including a light source that emits a light beam, an objective lens, and a photodetector having a plurality of independent detection areas, an optical path is provided between the objective lens and the photodetector. A diffraction grating having a substantially cross-shaped dividing line, wherein the direction and the grating pitch of the grating in each region are so divided that the diffraction directions of the light beams in the respective regions divided by the substantially cross-shaped dividing line are different from each other. Is set, and the light beam diffracted in each area of the diffraction grating is independently detected in a predetermined detection area of the photodetector. Further, in such an optical pickup, a knife edge method or a double knife edge method is used as a focus error signal detecting means, and a push-pull type first tracking error signal detecting means is used as the tracking error signal detecting means; A second tracking error signal detecting means of a type is provided, and the first and second tracking error signal detecting means are appropriately switched according to a difference in the structure of the optical disk. Further, by displacing the diffraction grating integrally with the objective lens, a relative positional relationship with the objective lens is always maintained. With such a configuration, the offset generated in the tracking error signal by the push-pull method due to the tracking displacement of the objective lens can be significantly reduced as described later. Further, the diffraction grating is formed by an optical member having a predetermined polarization anisotropy, and a light beam having the same linear polarization as the light beam incident on the optical disk does not diffract and has a linear polarization orthogonal to the predetermined direction. The light beam is set to be diffracted at a predetermined diffraction efficiency, and a quarter-wave plate is provided between the diffraction grating and the objective lens, so that the forward light illuminated on the optical disk via the objective lens is provided. Is not diffracted, but only the return light reflected from the optical disc is selectively diffracted to obtain high light use efficiency.

Finally, the optical disk apparatus is provided with the above-described optical pickup and detecting means for detecting a difference in the type of the optical disk, and the first and second tracking error signal detecting systems are provided in accordance with the detection result. Are provided as appropriate.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a first embodiment of the present invention will be described with reference to FIG. FIG. 1 is a schematic front view of an optical pickup showing a first embodiment of the present invention.

A light beam emitted from a semiconductor laser light source 1 reaches a diffraction grating 4 via a collimator lens 2 and a polarizing beam splitter 3. FIG. 2 is a plan view showing one embodiment of the grating pattern of the diffraction grating 4. As can be seen from the example in the figure, the diffraction grating 4 has four regions 4 with a cross-shaped boundary line.
a, 4b, 4c, and 4d. Each region is set at a predetermined grating pitch or grating direction, so that the ± 1st-order diffracted lights diffracted in each region travel in different directions. Moreover, in this embodiment, the diffraction grating is formed of an optical member having polarization anisotropy, and a light beam having the same polarization direction as the forward light reaching the objective lens 6 via the diffraction grating 4 hardly diffracts.
Only a light beam having a polarization direction orthogonal to it is diffracted. Further, a 波長 wavelength plate 5 is arranged in the optical path between the diffraction grating 4 and the objective lens 5. With this configuration, the polarization directions of the forward light and the backward light can be made orthogonal to each other. As a result, unnecessary diffracted light is not generated on the forward light, and ± 1st-order diffracted light required for signal detection only from the backward light is used. Can be separated and generated. Note that the present invention is not limited to the combination of the diffraction grating made of the polarization anisotropic member as shown in FIG. 1 and the quarter-wave plate. Alternatively, a diffraction grating formed of an unsupported member may be used. Further, the grating pattern of the diffraction grating 4 is not limited to the pattern shown in FIG. 2 and may be set freely according to the arrangement of the detection surface of the photodetector and the detection optical system as described later. .

Further, in this embodiment, the diffraction gratings 4 and 1/4
The wave plate 5 is fixed to the same actuator 10 as the objective lens 9, and is driven by the actuator 10 integrally with the objective lens 9. With such a configuration, for example, the objective lens 9 can be used during tracking control.
Does not change the relative position of the dividing line of the diffraction grating 4 with respect to the optical disk reflected light beam even if the tracking error signal is detected by the push-pull method as described later. The detected tracking error signal has almost no offset due to the displacement of the objective lens, and a good signal can be always detected. However, the present invention is not limited to such a configuration, and calibration of fixing the grating 4 or the quarter-wave plate 5 to the chassis of the optical pickup without attaching the grating 4 and the quarter-wave plate 5 to the actuator 10 may of course be performed.

The light beam transmitted through the diffraction grating 4 and the quarter-wave plate 5 is focused on an optical disk 7 by an objective lens 6. The light beam reflected from the optical disk 7 reaches the diffraction grating 4 via the objective lens 6 and the quarter-wave plate 5 again. At this time, since the polarization direction of the light beam incident on the diffraction grating is orthogonal to the forward light, the ± 1st-order diffracted light is separated and generated from each of the regions 4a, 4b, 4c, and 4d of the grating, and each of the light beams is separated by a predetermined amount. Proceed in the direction of. Each of these diffracted lights is reflected by the polarization beam splitter 3 and is detected by the detection lens 8.
And enters a predetermined detection surface of the eight-segment photodetector 9.

FIG. 3 is a schematic perspective view showing an example of the arrangement of the detection surfaces of the eight-segment photodetector 9. The eight-segment light detector 9 is, for example, a four-segment detection surface 9a arranged in a strip shape as shown in FIG.
9b, 9c, and 9d, and square or rectangular detection surfaces 9e, 9f, and 9a having a detection area larger than the rectangular detection surfaces.
It consists of 9g and 9h. For example, of the ± 1st-order diffracted lights separated and generated in the area 4a of the diffraction grating 4, +
The first-order diffracted light is converged on the boundary between the detection surfaces 9a and 9b to form a light spot 100a. On the other hand, the -1st-order diffracted light is condensed on a light detection surface 9g which is point-symmetric with respect to the detection surface 9a with respect to the central optical axis, and forms a light spot 101a. Similarly, among the ± 1st-order diffracted lights separated and generated in the region 4b of the diffraction grating 4, the + 1st-order diffracted light is condensed on the boundary between the detection surfaces 9c and 9d to form a light spot 100b, and the −1st-order diffracted light is used as the light detection surface. Light spot 101b condensed on 9f
To form Further, of the ± 1st-order diffracted lights separated and generated in the region 4c of the diffraction grating 4, the −1st-order diffracted light is condensed on the light detection surface 9e to form a light spot 101c, and the separation and generation occurs in the region 4d of the diffraction grating 4. The -1st-order diffracted light of the ± 1st-order diffracted light is condensed on the light detection surface 9h to form a light spot 101d. Thus, the diffraction grating 4
Each of the diffracted lights separated and generated from each of the four divided areas is condensed on a separate light detection surface, and the light intensity is detected independently for each area.

FIG. 4 is a schematic diagram of a photodetector and a signal generation circuit drawn to explain a mechanism for outputting a focus error signal or a tracking error signal from a light intensity signal detected on each detection surface of the photodetector 9. It is a block diagram. Each error signal is detected as follows. First, the light spots 100a and 100b are focused on the boundary between the light detection surfaces 9a and 9b and on the boundary between 9c and 9d, respectively.
The sum signal of the output signals of the detection surfaces 9a and 9d and the detection surfaces 9b and 9
By inputting the sum signal of the output signals of c to the operational amplifier 200 and detecting the difference signal, it is possible to detect a focus error signal by the so-called double knife edge method. On the other hand, the output signals from the detection surfaces 9e, 9f, 9g, and 9h correspond to the light intensity modulation signals of the respective divided areas when the reflected light beam from the optical disk 7 is divided into four crosses. By inputting each of these output signals to a predetermined phase difference detection circuit 202 via a predetermined signal delay circuit 201, a tracking error signal by a so-called DPD method can be detected. (Because the DPD method is a known technique, the detailed description is omitted.) Further, the operational amplifier 203 detects the sum signal of the output signals from the detection surfaces 9e and 9h, and the operational amplifier 204 similarly detects the sum signal. When the sum signals of the output signals from 9f and 9g are detected, these signals correspond to the light intensity modulation signals obtained in each divided area when the reflected light beam from the optical disk 7 is divided into two in the radial direction of the disk. When a difference signal between these signals is detected by the operational amplifier 205, a tracking error signal by a so-called push-pull method can be detected.

As described above, by subjecting each detection signal obtained from one 8-split photodetector to a predetermined arithmetic processing, the detection method differs from that of the focus error signal.
Different types of tracking error signals can be obtained simultaneously. Therefore, discs having different structures from each other (that is, a recordable disc having a continuous guide groove as described above and a read-only disc without a guide groove)
When recording or reproduction is performed for each of the methods, the use of the optical pickup of the present embodiment makes it possible to appropriately select and switch the optimum tracking error signal detection method according to the structure of the disk.

In this embodiment, the information signal recorded on the disk is a detection surface 9 for detecting a tracking error signal.
e, 9f, 9g, and 9h are detected from the sum signal of the output signals. However, the present invention is not limited to the above configuration. Of course, the recording information signal is detected from the output signals of the detection surfaces 9a, 9b, 9c, 9d for detecting the focus error signal, and the output from all the detection surfaces is detected. A configuration for detecting the recording information signal from the sum signal of the signals may be used.
However, if a method of detecting the recording information signal from the detection surface for tracking error signal detection without detecting the recording information signal from the detection surfaces 9a, 9b, 9c, 9d for detecting the focus error signal as in the present embodiment is used, In addition to avoiding the problem of a decrease in the frequency characteristics of the detection signal that may occur when the converging spot is irradiated on the boundary of the detection surface, the detection surfaces 9a, 9b, 9c,
Since 9d can be used as a detection surface dedicated to focus error signal detection, there is no need to request a high band for the band of the detection surface itself, and also compare the band of the current-voltage conversion amplifier connected to its output terminal. There is an advantage that an inexpensive one for a low band can be used.

By the way, the arrangement of the detection surface of the photodetector 9 is shown in FIG.
4 is not limited to the pattern shown in FIG. 4, but the reflected light beam from the optical disc 7 is divided into four crosses, and ± 1st-order diffracted light separated and generated from each area is independently detected. Any pattern may be used as long as the focus error signal can be detected by a so-called knife edge method from the diffracted light of one or two of the regions. Therefore, of the ± 1st-order diffracted lights separated and generated from the respective regions of the diffraction grating 4, the combination of which diffracted light is irradiated on which detection surface of the photodetector 9 or the boundary between the detection surfaces and the detection surface is also naturally described above. The present invention is not limited to the embodiment, and a designer may freely design.

In the embodiment of FIG. 4, the boundary 220 between 9a and 9b and the boundary 221 between 9c and 9d are not parallel among the four divided detection surfaces 9a, 9b, 9c and 9d for detecting the focus error signal. , Are slightly inclined with respect to each other to form a C-shape. This is because the laser beam emitted from the semiconductor laser light source 1 fluctuates in wavelength, and as a result, the diffraction angle of the ± 1st-order diffracted light separated and generated by the diffraction grating 4 changes, and the light is detected on the light detection surfaces 9a, 9b, 9c, 9d. Focusing spot 100a
This is set for the purpose of minimizing the offset of the focus error signal generated when the optical detector 9 is displaced or when the photodetector 9 is displaced relatively to the condensed spots 100a and 100b. Therefore, the inclination angle is not limited to the embodiment shown in FIG. 4, but may be freely designed by a designer.

Further, as can be seen from FIG. 3, the present invention has a structure in which ± 1st-order diffracted light separated and generated by the diffraction grating 4 is received by a photodetector. Therefore, it is advantageous that the diffraction efficiency of ± 1st-order diffracted light is as high as possible. In general, in a concave-convex (phase-type) diffraction grating, when the optical path length difference between a convex portion and a concave portion substantially coincides with an odd multiple of a half wavelength of a light beam, a value of 0 is used.
The efficiency of the next-order light becomes almost zero, and the diffraction efficiency of ± 1st-order diffracted light becomes maximum. Therefore, the diffraction grating 4 in the present invention
By setting the grating depth as described above, relatively high detection efficiency can be obtained, and unnecessary stray light can be prevented from being generated.

FIG. 5 is a block diagram showing a schematic configuration of an optical disk device using the optical pickup of the present invention.
In FIG. 5, since the optical pickup has the same configuration as that of the embodiment shown in FIG. 1, only the main part is shown and other parts are omitted. A light intensity signal obtained from each detection surface of the photodetector 9 is sent to a signal generation circuit 50, and a focus error signal and a tracking error signal are detected by the above-described arithmetic processing. Among the detected error signals, the focus error signal is directly input to the actuator drive circuit 51. On the other hand, as the tracking error signal, signals detected by two types of the DPD system and the push-pull system are obtained, but these are not directly input to the actuator driving circuit 51. The optical disc apparatus of the present invention is provided with a disc discriminating circuit 52 for discriminating the type of disc, and the disc discriminating circuit 52 discriminates whether the disc being reproduced at that time is a recordable type or a read-only type. . Then, the switching circuit 53 is switched based on the discrimination result, and the tracking error signal obtained by the detection method suitable for the disc structure is selected and supplied to the actuator drive circuit 51. The actuator drive circuit 51 outputs a predetermined actuator drive signal from the supplied focus error signal and tracking error signal, and drives the actuator 10 to displace the objective lens 6, the diffraction grating 4 and the quarter-wave plate 5 together. Let it.

Incidentally, in the optical disk generally used at present, the optimal tracking error signal detection method differs depending on the structure of the disk, and the optimal objective lens may differ due to the difference in the thickness of the disk substrate. Further, there is a disc in which the optimum light beam wavelength differs depending on the recording medium. In the former case, for example, a plurality of objective lenses corresponding to the respective substrate thicknesses and a mechanism for appropriately switching the objective lenses are provided. In addition to switching to the detection method, the objective lens may be switched to the optimal one. In the latter case, an optical pickup having two light sources having different wavelengths may be used, and the light source to be turned on may be switched according to the type of the disc.

Incidentally, the configuration of the optical pickup described so far has been such that the diffraction grating 4 is disposed immediately below the objective lens 6 and is driven integrally with the objective lens. However, as described above, the present invention employs the diffraction grating 4 and the objective lens 6.
Is not limited to a configuration in which the diffraction grating 4 is driven together, and the diffraction grating 4 may be arranged anywhere in the return optical path from the objective lens 6 to the photodetector 9. FIG. 6 is a schematic front view of an optical pickup showing such an embodiment. The same components as those in the embodiment of FIG. 1 are denoted by the same reference numerals.

In this embodiment, a diffraction grating 4 is disposed in the optical path between the polarizing beam splitter 3 and the detection lens 8.
This embodiment has exactly the same configuration as the embodiment of FIG. 1 except for the difference in the arrangement position.
Various error signals can be detected based on exactly the same principle as the signal detection principle described in (1). In this embodiment, it is not necessary to drive the diffraction grating 4 and the quarter-wave plate 5 together with the objective lens 6, so that no load is placed on the actuator 10 and the diffraction grating is provided in the optical path where only the reflected light from the disk travels. 1, the diffraction grating 4 need not be formed with a polarization anisotropic optical member in order to obtain a high light use efficiency as described in the embodiment of FIG. Since it can be used, there is an advantage that an optical pickup can be manufactured relatively inexpensively.

In each of the embodiments described above, a combination of linear gratings is used as the grating pattern of the diffraction grating 4, but the present invention is not limited to this. For example, the grid pattern may be curved, so-called holographic grid. When a diffraction grating having such a curved pattern is used, the diffraction grating itself can have a lens function.
For example, the degree of freedom in designing the optical system of the optical pickup can be increased by omitting the step.

[0026]

As described above, according to the present invention, in addition to the push-pull method, a read-only disk called the DPD method can be used while taking advantage of the advantages of the optical pickup using the diffraction grating, such as compactness and simplicity. It is possible to obtain a high-performance optical pickup and an optical disk apparatus compatible with various optical disks, which also include suitable tracking error signal detecting means.

[Brief description of the drawings]

FIG. 1 is a schematic front view showing a first embodiment of an optical pickup according to the present invention.

FIG. 2 is a schematic plan view showing an example of a specific grating pattern of the diffraction grating 4 according to the present invention.

FIG. 3 is a schematic perspective view showing an embodiment of a detection surface arrangement of a multi-segment photodetector according to the present invention.

FIG. 4 is a block diagram showing an embodiment of a signal generator for a focus error signal and a tracking error signal in the present invention.

FIG. 5 is a block diagram showing an embodiment of the optical disk device of the present invention.

FIG. 6 is a schematic front view showing a second embodiment of the optical pickup of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Semiconductor laser light source, 2 ... Collimate lens, 3 ... Polarization beam splitter, 4 ... 4 division | segmentation diffraction grating, 5 ... 1/4
Wave plate, 6 objective lens, 7 optical disk, 8 detection lens, 9 split photodetector, 10 actuator

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Tohru Sasaki 292 Yoshida-cho, Totsuka-ku, Yokohama-shi, Kanagawa Prefecture Inside the Hitachi, Ltd.Video Information Media Division (72) Inventor Yukio Fukui 292 Yoshida-cho, Totsuka-ku, Yokohama-shi, Kanagawa Hitachi, Ltd.Video and Media Division (72) Inventor Takeshi Nakao 1-280, Higashi-Koigabo, Kokubunji-shi, Tokyo Inside the Central Research Laboratory of Hitachi, Ltd. (72) Inventor Ken Shimano 1, 2-280, Higashi-Koigabo, Kokubunji, Tokyo, Japan Hitachi, Ltd., Central Research Laboratory

Claims (6)

[Claims] 1. A light source for emitting a light beam, an objective lens for condensing the light beam emitted from the light source and irradiating a minute spot on an optical information recording medium, and a light having a plurality of independent detection areas. In an optical pickup having a detector, a diffraction grating having a substantially cross-shaped parting line in an optical path between the objective lens and the photodetector, each of which is divided into four by the substantially cross-shaped parting line A diffraction grating in which the direction of the grating and the pitch of the grating in each region are set so that the diffraction directions of the light beams in the region are different from each other, and the light of the light beam diffracted in each region of the diffraction grating is set. An optical pickup, wherein the intensity is independently detected in each detection area of the photodetector. 2. The optical pickup according to claim 1, wherein the focus error signal is detected by a knife edge method or a double knife edge method. 3. A first tracking error signal detection method comprising a push-pull method as a tracking error signal detection means, and a light beam diffracted in each area divided by the diffraction grating into four parts. And a second tracking error signal detection method comprising a differential phase detection method for obtaining a tracking error signal through a predetermined operation from the phase difference between the detected signals. 3. The optical pickup according to claim 1, wherein the first and second tracking error signal detection methods are appropriately switched in response to the request. 4. The apparatus according to claim 1, wherein said diffraction grating is displaced integrally with said objective lens so that a relative positional relationship with said objective lens is always fixed. Optical pickup. 5. The diffraction grating has a polarization anisotropy in which a light beam having a predetermined linear polarization is not diffracted, and a light beam having a linear polarization in a direction orthogonal to the linear polarization is diffracted at a predetermined diffraction efficiency. 5. The optical pickup according to claim 1, wherein a quarter-wave plate is provided between the diffraction grating and the objective lens. 6. An optical pickup according to claim 3, 4 or 5, and means for detecting the type of optical information recording medium,
An optical disc apparatus comprising means for detecting a difference in the type of an optical information recording medium and appropriately switching between the first and second tracking error signal detection methods according to the detection result.