JP2010033660A - Optical disk drive and optical head - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical head technique suppressing the increase of the number of additional RF signals in PD signals. <P>SOLUTION: This optical head includes a holographic diffraction element (HOE) which is divided into a number of areas and diffracts the reflected light from the optical recording medium to a number of optical detectors (PD) which detect the diffracted light. The focusing area in the HOE crosses the opening in the radial direction of the disk. It uses an area within the circle of the same radius as an opening centering the position separated by 1.5 to 1.6 times the radius of the opening in the radial direction of the disk from the center of the opening of the HOE as the first push-pull (PP) area, and the other focusing area as the second push-pull area, then inputs the light diffracted in the first push-pull area among the divided four areas and the light diffracted in the second push-pull area opposite to those areas and divided into four into the same optical detector out of the optical detectors. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an optical disc apparatus that records information on an optical disc that is an optical information recording medium or reproduces information from the optical disc, and an optical head device that is incorporated in the optical disc apparatus.

  Optical discs are already widely used as recording media suitable for recording, reproducing and erasing (repeating recording) information. On the other hand, optical discs with various standards have been proposed and put into practical use. Note that optical discs of various standards are classified into CD standards and DVD standards when distinguished by recording capacity. Further, when viewed from the application (data recording format), a read-only type in which information has already been recorded (referred to as ROM), and a write-once (referred to as -R) that can record information only once. It is classified into a type (write-once type) or a rewritable type (recording / reproducing type or rewritable type) that can be repeatedly recorded and erased (referred to as RAM or RW).

  With the diversification of optical disc standards and applications, optical disc recording / reproducing apparatuses record information on optical discs of two or more standards, reproduce recorded information, or erase information that has already been recorded. It is desirable to be possible. Note that an optical disc recording / reproducing apparatus is required as an indispensable requirement to be able to identify the standard of a set optical disc even if it is difficult to record and erase information.

  For this reason, in an optical pickup incorporated in an optical disc information recording / reproducing apparatus, regardless of the standard (type) of the optical disc, at least reflected light from a track or recording mark row unique to the optical disc is obtained, and at least an objective lens (optical pickup) ) Tracking and focus must be controllable.

For example, the HOE described in Patent Document 1 requires a photodetector for detecting a corrected track error signal that corrects an offset when the objective lens is shifted. There was a problem that the number of added PDs in the RF signal increased and SN decreased.
Japanese Patent Laying-Open No. 2007-42150 (FIG. 4)

  An object of the present invention is to provide an optical head technology that suppresses an increase in the number of added PDs in an RF signal.

  In order to solve the above problems, an optical head device of the present invention diffracts reflected light from an optical recording medium into a plurality of photodetectors (PD), and is divided into a plurality of regions, a holographic diffraction element (HOE). And the photodetector for detecting the diffracted light, the focus area in the HOE traverses the opening in the radial direction of the disk, and the opening radius from 1.5 to the radial direction of the disk from the center of the opening of the HOE. 1.6 The area within the circle with the same radius as the opening centered at a position twice as the center is the first push-pull (PP) area, and the other focus area is the second push-pull area. Among the photodetectors, the light diffracted in the divided first push-pull region and the light diffracted in the second push-pull region divided into four located diagonally to each of these regions Enter the same photodetector. Features to be.

  According to the present invention, an optical head technology that suppresses an increase in the number of added PDs in an RF signal can be obtained.

  Embodiments of the present invention will be described below.

Embodiment 1 of the present invention will be described below with reference to FIGS.
FIG. 1 shows an example of a configuration of an optical pickup in which an embodiment of the present invention is applied to an optical disc apparatus.
(General description of a head using a diffraction element)
The information recording / reproducing apparatus shown in FIG. 1, that is, the optical disc apparatus, collects information on the optical disc 1 by condensing the laser light emitted from the optical pickup 100 (PUH actuator) on the information recording layer of the recording medium, that is, the optical disc 1. Information can be recorded and reproduced from the optical disc 1.

  The optical disk 1 is supported by a turntable (not shown) of a disk motor (not shown), and is rotated at a predetermined speed by rotating the disk motor at a predetermined rotation speed.

  A PUH (optical pickup) 100 is moved at a predetermined speed in the radial direction of the optical disc 1 at each time of recording, reproducing or erasing information by a pickup feeding motor (not shown).

  The PUH includes an objective lens 16 for condensing a light beam (laser light) of a predetermined wavelength from the first and second laser diodes described later on the recording surface of the optical disc 1, and an optical disc captured by the objective lens 16. A photodetector (PD = Photo Detector) 19 that receives reflected laser light from one recording surface and outputs a current (or voltage) having a magnitude corresponding to the intensity of the reflected laser light is provided. The photodetector 19 detects light in an on-focus state in which the spot diameter on the recording surface of the optical disk of the predetermined wavelength from the first and second laser diodes (laser light) is minimized. The light spot of a predetermined size (cross-sectional area) is obtained on the device 19 at a position where an on-focus state can be specified.

  The objective lens 16 is driven by a focusing / tracking coil 17 in a direction perpendicular to the surface including the recording surface of the optical disc 1, that is, a focus direction and a direction parallel to the surface including the recording surface of the optical disc 1, and It is arbitrarily moved with respect to each of the track directions. The objective lens 16 is made of plastic, for example, and its numerical aperture NA is, for example, 0.65.

  Between the objective lens 16 and the PD (light detector) 19, the direction of the polarization plane of the laser light guided to the recording surface of the optical disc 1 from a laser diode described later and the reflected laser reflected by the recording surface of the optical disc 1 A λ / 4 plate 15 for changing the direction of the plane of polarization of light by 90 ° is positioned. The λ / 4 plate 15 may be provided integrally with the objective lens 16 and the focusing / tracking coil 17 as shown in FIG. Further, on the side opposite to the objective lens 16 of the λ / 4 plate 15 (the side on which the light beam from the laser diode is incident), a diffractive element that gives predetermined characteristics to the wavefront of the reflected laser light reflected by the optical disc 1 ( A wavefront splitting element, a diffraction element, that is, a hologram optical element) 14 is integrally formed. Here, the characteristic that the diffraction element 14 gives to the reflected laser light includes diffraction in a plurality of directions and a plurality of divisions of the wavefront.

  Between the λ / 4 plate (and diffractive element) 15 and the PD (light detector) 19, light from the laser diode is transmitted toward the optical disk 1 (objective lens 16) and reflected by the recording surface of the optical disk 1. A polarized beam splitter 13 is provided for reflecting the reflected laser light thus reflected toward the light receiving surface of the PD 19.

  The signal detected by the photodetector (PD) 19 is processed in a signal processing unit (arithmetic circuit) 30 provided in the subsequent stage so that it can be used as a data signal used for reproducing information recorded on the optical disc 1. . A part of the signal output from the arithmetic circuit 30 is supplied to a servo circuit (lens position control device) 31, and the position of the objective lens 16 (PUH) is set to a predetermined positional relationship with respect to the recording surface of the optical disc 1. It is used as a control signal for positioning at the position. That is, the objective lens 16 is moved from the servo circuit 31 to the focusing / tracking coil 17, and the position of the light spot focused on the recording surface of the optical disc 1 by the objective lens 16 is minimized on the recording layer of the recording surface of the optical disc 1. A focusing control signal is supplied. Further, the objective lens 16 is moved from the servo circuit 31 to the focusing / tracking coil 17, and the center of the light spot coincides with the center of the record mark row recorded on the optical disc 1 or the track (guide groove) formed in advance. A tracking control signal is supplied.

  In the direction in which laser light can be incident on the objective lens 16 via the polarization beam splitter 13, the first semiconductor laser element 10 that emits laser light having the first wavelength and the laser light having the second wavelength are emitted. A second semiconductor laser element 20 and a collimating lens 11 for collimating the light beams having the first wavelength and the second wavelength are provided.

The first and second semiconductor laser elements 10 and 20 are arranged at an angle of approximately 90 °, and the axis line (mainly) of the laser light (light beam) emitted from the dichroic prism 12 toward the objective lens 16 is mainly used. The light paths are overlapped so that the directions of the rays are substantially the same.

  The first laser element 10 is positioned in a direction in which the emitted light (laser light) passes through the wavelength selection film (selective reflection surface) of the dichroic prism 12, for example. Accordingly, the second laser element 20 is arranged so that the emitted light is reflected by the wavelength selection film of the dichroic prism 12 and superimposed on the axis of the light from the first laser element toward the objective lens 16, for example. . The wavelength of the laser beam emitted from the first laser element 10 is approximately 405 nm (400 to 410 nm), and the wavelength of the laser beam emitted from the second laser element 20 is approximately 650 nm (640 to 670 nm). It is. The wavelength of the laser light emitted from the first laser element 10 is approximately 405 nm (400 to 410 nm), and the wavelength of the laser light emitted from the second laser element 20 is approximately 780 nm (770 to 790 nm). Alternatively, the wavelength of the laser beam emitted from the first laser element 10 is approximately 650 nm (640 to 670 nm), and the wavelength of the laser beam emitted from the second laser element 20 is approximately 780 nm (770 to 790 nm). Needless to say.

  In the PUH (optical disc apparatus) 100 shown in FIG. 1, linearly polarized laser light having a wavelength of 405 nm from the first laser element 10 passes through the dichroic prism 12, passes through the polarization beam splitter 13, and then collimates the lens 11. Becomes parallel light and enters the diffraction element 15 integrated with λ / 4. The diffractive element 15 is formed of, for example, an anisotropic optical crystal, and generates diffracted light for linearly polarized light in a certain direction, but diffracts for linearly polarized light whose polarization direction is rotated by 90 °. Does not produce light. The polarization direction of the light emitted from the first semiconductor laser 10 and incident on the diffractive element 15 is a direction that does not generate diffracted light in the diffractive optical element 15, passes through the diffractive element, and passes through the λ / 4 plate (1 / 4 wavelength plate).

  The laser light (of light having a wavelength of 405 nm) incident on the λ / 4 plate 15 has its polarization plane converted into circularly polarized light, and is focused on the recording layer of the optical disk 1 by the objective lens 16. As the optical disc 1, for example, a next-generation DVD-standard optical disc capable of recording at a higher density than the current DVD-standard optical disc can be used. Also, DVD-RAM discs and DVD-RW discs capable of recording and erasing information according to the current DVD standard, DVD-R discs capable of writing only new information, or DVD-ROMs on which information has already been recorded Needless to say, various types of discs (standards), such as discs, can also be used.

  The reflected laser light (of light having a wavelength of 405 nm) reflected by the recording layer of the optical disc 1 is collimated by the objective lens 16, passes through the λ / 4 plate 15 again, and travels from the laser element 10 toward the optical disc 1. The direction becomes linearly polarized light whose direction of polarization is rotated by 90 ° with respect to the direction of polarization of light, diffracted by the diffraction element 15, converged light by the collimator lens 11, and returned to the polarization beam splitter 13.

  The reflected laser light (of light having a wavelength of 405 nm) returned to the polarization beam splitter 13 is reflected by the polarization beam splitter 13 and condensed on the light receiving surface of the photodetector 19 with a predetermined number of divisions and a collection pattern. Is done.

  The linearly polarized laser beam having a wavelength of 650 nm from the second laser element 20 is collimated by the collimator lens 11, reflected by the mirror surface of the dichroic prism 12, guided to the polarization beam splitter 13, and the polarization beam splitter 13 Then, the collimated lens 11 makes parallel light and enters the diffraction element 15. The laser beam having the second wavelength incident on the diffraction element 15 has the direction of polarization defined in the same manner as the laser beam having the first wavelength, and is transmitted through the diffraction element without being diffracted. 15 is incident.

The laser light (of light having a wavelength of 650 nm) incident on the λ / 4 plate 15 has its polarization plane converted into circularly polarized light, and is focused on the recording layer of the optical disc 1 by the objective lens 16.
The reflected laser beam (of light having a wavelength of 650 nm) reflected by the recording layer of the optical disc 1 is collimated by the objective lens 16, passes through the λ / 4 plate 15 again, and travels from the laser element 20 toward the optical disc 1. The direction becomes linearly polarized light whose direction of polarization is rotated by 90 ° with respect to the direction of polarization of light, diffracted by the diffraction element 15, converged light by the collimator lens 11, and returned to the polarization beam splitter 13.

  The reflected laser light (of light having a wavelength of 650 nm) returned to the polarizing beam splitter 13 is reflected by the polarizing beam splitter 13 and condensed on the light receiving surface of the photodetector 19 with a predetermined number of divisions and a condensing pattern. Is done.

  Incidentally, in the optical pickup (optical disk apparatus) shown in FIG. 1, the reflected laser light is reflected by the polarization beam splitter 13 and a plurality of detection (light reception) regions of the photodetector (light detector) 19 as described above. Depending on the number and the position of each detection region, a predetermined diffraction characteristic is given by the diffraction element 15 so that each detection region can be reached.

  The reflected laser beam divided into a plurality of parts by the diffraction element 15 and given a predetermined diffraction characteristic is received by the collimator lens 11 with a predetermined arrangement and size of a PD (photodetector) 19 in advance. Focused on the area.

(Description of features of this embodiment)
The diffraction element in FIG. 1 is divided into several regions, and each region has a different grating pitch and grating angle. Various patterns have been proposed as the division pattern of the region of the diffractive element, and the amount of light distributed to the servo signal such as focus and tracking and the characteristics of the servo signal are determined by the division pattern of the region.

Due to the eccentricity during reproduction of the optical disk, the center position of the recording mark row or the previously formed track changes in the radial direction. Therefore, when tracking is applied, the objective lens shifts in the radial direction corresponding to the eccentricity of the disk. When playing a grooved disc such as a DVD-R or HD DVD-R disc, if the objective lens shifts in the radial direction, the center of the light intensity is shifted from the center of the objective lens. In this case, an offset is added to the push-pull signal. Normally, after correcting the signal offset due to the electrical offset or optical adjustment shift, the servo is applied so that the tracking control signal becomes zero. At this time, if tracking is performed with a signal having an offset, servo is applied at a position deviated from the center of the mark row or groove. That is, since the light spot is condensed at a position deviated from the center of the mark row or groove, the quality of the servo signal and the reproduction signal is deteriorated. In addition, when the lens shift is large, it is difficult to stably apply the servo, for example, the tracking may be lost.

When the objective lens is shifted, an offset component due to the lens shift component is added to the tracking error signal in addition to the track cross component, and this is referred to as a lens shift component. Here, after focusing, the signal from the area not including the track cross component is only the lens shift component. By subtracting the compensation area signal from the tracking area signal, that is, subtracting the lens shift component from the tracking area signal, the tracking area signal becomes only the track cross component, and the lens does not contain the track cross component. A tracking control signal independent of shift can be obtained. This method is called a compensation push-pull method and is widely used.

The compensation push-pull method requires a compensation push-pull signal detection PD in addition to the PD that receives light from the focus region and the tracking region. Since a signal obtained by adding all signals from all regions becomes a reproduction signal, the compensation push-pull method has a problem in that the number of PDs added increases and the SN of the reproduction signal deteriorates. There is also a problem in that it is necessary to adjust the gain between the tracking signal and the compensation signal for each optical head during assembly adjustment.

In order to solve the above problem, in this embodiment, the diffracted light from the compensation area of the HOE and the diffracted light from the tracking area are incident on the same PD. As a result, the PD for the compensation area is not necessary, and the number of PD additions can be reduced.

A specific method is shown below.
Figure 2 shows the HOE area pattern. The focus area in the HOE is arranged to cross the opening in the radial direction of the disk. As shown in Fig. 2 (a), push-pull (Pull-Pull) area in the circle with the same radius as the opening centered at 1.5 to 1.6 times the opening radius in the radial direction of the disk from the center of the HOE opening. PP) Region 1 (upward-slashed region) and other regions are PP region 2 (downward-slashed region). PP areas 1 and 2 are each divided into four areas as shown in FIG.

FIG. 3 shows the correspondence between HOD and PD, and the area divided into eight parts in FIG. 2 is expressed as four sections A to D.
Here, Fig. 4 shows the tracking error signal in PP region 1 and the tracking error signal in PP region 2 when the objective lens is shifted by 4% of the aperture diameter of the objective lens in the radial direction during DVD-R disc playback. . The horizontal axis is normalized so that one cycle = 20. When the horizontal axis = 0, the beam spot is located at the center of the groove.

Each signal component is expressed as follows using each of the four regions in FIG.
MPP = (region 1-1 + region 1-4)-(region 1-2 + region 1-3)
SPP = (region 2-2 + region 2-3)-((region 2-1 + region 2-4)
CPP = (region 1-1 + region 1-4 + region 2-2 + region 2-3)-(region 1-2 + region 1-3 + region 2-1 + region 2-4)

From FIG. 4, it can be seen that when there is an objective lens shift, the signal component does not become zero even when the beam spot is located at the center of the groove, that is, a lens shift component is generated. Here, as described above, push-pull (PP) is the area within the circle having the same radius as the opening centered at a position 1.5 to 1.6 times the opening radius in the radial direction of the disk from the center of the opening of the HOE. By setting the region 1 and the other region as the PP region 2, MPP and SPP can have the same amount of lens shift component (in reverse signs). Therefore, the lens shift component can be reduced by adding MPP and SPP together. (Fig. 4 CPP = MPP + SPP). Here, in the present invention, the diffracted light from the region 1-1 and the region 2-3, the region 1-2 and the region 2-1, the region 1-3 and the region 2-1, and the region 1-4 and the region 2-2 are reflected. By making each incident on the same PD, MPP and SPP can be added together without increasing the number of PDs. As a result, a tracking error signal with a small lens shift component can be obtained.

Similarly, Fig. 5 shows the tracking error signal in PP area 1 and the tracking error signal in PP area 2 when the objective lens is shifted by 4% of the aperture diameter of the objective lens in the radial direction during DVD-RAM disc playback. . The horizontal axis is normalized so that one cycle = 20. When the horizontal axis = 0, the beam spot is located at the center of the groove. As in FIG. 4, by using the above HOE pattern, MPP and SPP can have the same amount of lens shift component (in reverse signs), and a tracking error signal with little lens shift can be obtained.

The DPD signal in the ROM disk is described by the following formula.
DPD = Ph (region 1-1 + region 2-1 + region 1-3 + region 2-3)-Ph (region 1-2 + region 2-2 + region 1-4 + region 2-4)

* Ph (x)
Represents the phase of x.

A second embodiment according to the present invention will be described. Description of the parts common to the first embodiment is omitted.
When only R and ROM disks are supported but not RAM disks, that is, only disks with a fixed track pitch are supported, the reflected light from the optical recording medium is diffracted by a plurality of photodetectors (PD). For a holographic diffractive element (HOE) divided into regions, the focus area in the HOE traverses the aperture in the radial direction of the disc, and the aperture radius is from the center of the HOE aperture in the radial direction of the disc.

(Laser wavelength / (track pitch * numerical aperture)) -0.05
From
(Laser wavelength / (track pitch * numerical aperture)) +0.05
The region in the circle with the same radius as the opening centered at a position twice as far away is the push-pull (PP) region 1, the other region is the PP region 2, and diffraction is performed in the PP region 1 divided into four. By making the diffracted light and the light diffracted in the four divided PP regions 2 diagonally to each region incident on the same photodetector, an optimum region pattern for the R disk can be obtained. it can.

  As described above, the outline of the embodiment is a holographic diffraction grating divided into a plurality of regions, in which the focus region crosses the opening in the radial direction of the disk, and the opening radius is 1.5 The push-pull region 1 is defined as a push-pull region 1 in a circle having the same radius as the opening centered at a position approximately 1.6 times apart, and the push-pull region 2 is defined as the other region. The diffracted light and the light diffracted by the push-pull region 2 divided into four located diagonally to each region are incident on the same photodetector.

  With R and RAM discs, stable tracking can be applied without the need for offset change compensation when the objective lens is shifted, and the number of PD additions in the playback signal is reduced by two or more compared to the conventional example, so the signal SN is reduced. improves.

  In other words, in the embodiment, the correction track error signal is not necessary, and the number of added PDs in the RF signal is two smaller than that in the case where the correction track error signal is necessary, which is advantageous in terms of SN.

As an effect, stable tracking can be applied to R and RAM disks without the need to compensate for offset changes during objective lens shift.
In addition, this invention is not limited to the said embodiment, In the range which does not deviate from the summary, it can implement in various modifications.
Various inventions can be formed by appropriately combining a plurality of constituent elements disclosed in the above-described embodiments. For example, some components may be deleted from all the components shown in the embodiment. Furthermore, constituent elements according to different embodiments may be appropriately combined.

1 is a block diagram illustrating a configuration example of an optical head device according to an embodiment of the present invention. The HOE pattern schematic of the embodiment. The figure shown in order to demonstrate the response | compatibility of HOE and PD of the embodiment. The figure which shows the tracking error signal in the DVD-R disc of the embodiment. The figure which shows the tracking error signal in the DVD-RAM disc of the embodiment.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Optical disk, 10 ... 1st semiconductor laser element, 11 ... Collimator lens, 12 ... Dichroic prism, 13 ... Polarizing beam splitter, 14 ... Diffraction element, 15 ... Lambda / 4 plate, 16 ... Objective lens, 17 ... Focusing / Tracking coil, 19 ... photodetector, 20 ... second semiconductor laser element, 30 ... signal processing unit, 31 ... servo circuit, 100 ... optical pickup.

Claims (4)

A holographic diffraction element (HOE) divided into a plurality of regions for diffracting reflected light from an optical recording medium into a plurality of photodetectors (PD), and
The photodetector for detecting diffracted light, and
The focus area in the HOE traverses the opening in the radial direction of the disk, and in a circle having the same radius as the opening centered at a position 1.5 to 1.6 times the opening radius in the radial direction of the disk from the center of the HOE opening. The first push-pull (PP) region and the other focus region as the second push-pull region, and the light diffracted in the first push-pull region divided into four parts, An optical head device characterized in that light diffracted in a second push-pull region divided into four that is located diagonally to these regions is incident on the same photodetector among the photodetectors.   Using the HOE, the PP signal component output from the PD is tracked without offset compensation when the objective lens is shifted on the R and RAM disks, based on the characteristics that the R and RAM disks are in opposite phases. The optical head device according to claim 1, wherein:   2. The optical head device according to claim 1, wherein the second push-pull region in the HOE is disposed diagonally with respect to the PP region and the outer periphery of the opening so as to increase a substantial numerical aperture. .   An optical disk device comprising the optical head device according to claim 1.

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