Patent Application
20060221785
This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2005-103605, filed on Mar. 31, 2005, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to an optical disk apparatus for playing back information using a laser beam, more particularly relates to an optical disk apparatus to reduce DC offset occurring due to light reflected by a layer to be not played back and leaking to a photodetector when an optical disk having a double-layer information recording layer is played back.
2. Description of the Related Art
The next-generation high-density optical disks having record capacity of three or four times of that of the currently distributed DVD (Digital Versatile Disc) have been developed. Optical disks (referred to as HD DVD hereinafter) optimized to a substrate thickness of 0.6 mm and used for a wavelength 400 nm band blue-violet semiconductor laser and an objective lens of a numerical aperture 0.65 have been developed in terms of compatibility with existing CD (Compact Disc) and DVD, facility of manufacturing of a small-size optical head apparatus for a slim line book-size personal computer, lower disk manufacturing cost. In this HD DVD, a double layer disk have been developed in order to increase an amount of recording information similarly to DVD.
However, the double layer disk has a problem that DC offset occurs on a servo signal and a playback signal due to interlayer crosstalk. In other words, when one of information recording layers is played back, an undesired light reflected by the other information recording layer (referred to as a non-playback layer hereinafter) leaks to a photodetector. The playback signal of the undesired light is combined with a servo signal and a playback signal, resulting in DC offset.
As thus described, the DC offset occurring on the playback signal appears notably in the double layer optical disk of rewritable type. In particular, in the case that data is written intensively at a part of the non-playback layer, when the light beam passes through the data region (recording region) of the non-playback layer during playback of the playback layer, the reflectivity of the disk varies greatly, resulting in causing DC offset on the playback signal. This becomes a problem causing characteristic deterioration of the playback signal.
An offset reduction method described in Japanese Patent Laid-Open No. 9-161282 is effective for reducing DC offset of a focusing error signal occurring due to interlayer crosstalk. In an example of an optical record/playback apparatus mentioned in Japanese Patent Laid-Open No. 9-161282, there is provided an auxiliary light receiving region for detecting light run over a main light receiving region when a light beam is largely defocused. A playback signal is generated as a sum signal of output signals from all light receiving regions. The light run over the main light receiving region in defocusing is detected by the auxiliary light receiving region, and the detected signal is subtracted from the sum signal. This allows for making the falling edge of a focusing error signal to be precipitous when the light beam defocuses significantly. Therefore, the focusing error signal on the double-layer disk is difficult to be influenced by the other layer, so that DC offset in the focused position is reduced.
By the way, in the double-layer DVD, the thickness of the two layers is defined to 55 μm±15 μm for the wavefront aberration of the beam spot on the information recording layer to be kept not more than tolerance. When the similar reference is applied to HD DVD, the thickness of the double-layer becomes as small as about 25 μm. This is because the wavefront aberration occurring due to the thickness deviation of the substrate is almost proportional to four power of the objective lens numerical aperture and inversely proportional to the laser wavelength.
As above described, the thickness of the double-layer becomes small in HD DVD, so that influence of interlayer crosstalk becomes remarkable in comparison with DVD. Therefore, the system design interweaving an anti-interlayer crosstalk measure is required for HD DVD than DVD.
As mentioned above, the thickness of the double layer in HD DVD is small in comparison with DVD, and thus this influences a servo signal for interlayer crosstalk and a playback signal remarkably. The DC offset occurring due to the focusing error signal can be reduced by a method described in Japanese Patent Laid-Open No. 9-161282. However, it is necessary to take a reduction countermeasure against DC offset occurring on a playback signal in playing back the double-layer disk.
The object of the present invention is to provide an optical disk apparatus having good playback signal characteristics by reducing DC offset of a playback signal occurring due to interlayer crosstalk in playing back the double-layer disk.
An aspect of the present invention provides an optical disk apparatus adopted to play back an optical disk, comprising a laser source to emit a laser beam to an optical disk; a condenser lens to condense the laser beam reflected by the optical disk, a photodetector irradiated by the condensed laser beam and including a main light receiving part and an auxiliary light receiving part disposed adjacently to the main light receiving part, and a signal processor to output a difference between an output of the main light receiving part and an output of the auxiliary light receiving part as a playback signal representing information recorded on an information recording layer of the optical disk.
Embodiments of the present invention are explained in conjunction with the drawing in detail hereinafter.
An optical system and a playback signal output system of the first embodiment are shown in
A photodetector 106 having light receiving regions 106a to 106f is disposed in association with the diffracting device 202 so that the light diffracted by the region 202a of the diffracting device 202 is led to the light receiving regions 106a and 106b of the photodetector 106 and the light diffracted with the regions 202b and 202c are led to the light receiving regions 106f and 106e of the photodetector 106, between which an array of the regions 106a to 106d are arranged. The signals output from the light receiving regions 106a and 106b by the light beam diffracted by the region 202a are used for obtaining a focusing error signal by a single knife edge method. Based on this focusing error signal, an objective lens actuator (not shown) positions the objective lens 204 in an optical axis direction.
The signals output from the light receiving regions 106e and 106f by the light beams diffracted with the regions 202b and 202c are used for obtaining a tracking signal by a push-pull method or a DPD (Differential Phase Detection) method. Based on this (tracking error signal, a tracking device (not shown) positions the objective lens 204 in the disk radial direction.
The light receiving regions 106a to 106f of the photodetector 106 shown in
HFS=Sa+Sb+Se+Sf−(Sc+Sd) (1)
This playback signal is a signal indicating information recorded on the optical disk 205. The auxiliary light receiving regions 106c and 106d are provided for reducing DC offset of the focusing error signal occurring due to interlayer crosstalk. The playback signal is generated by subtracting a sum signal of signals Sc and Sd from these auxiliary light receiving regions 106c and 106d from a sum signal from the other light receiving regions 106a, 106b, 106e and 106f with the operational amplifier (signal processor) 11. The method of generating a focusing error signal by a single knife edge method and the method of generating a tracking error signal by a push-pull method or DPD method are executed by the block circuit of
FES (single knife edge method)=Sb+G1*Sc−(Sa+G2−Sd) (2)
TES (push-pull method)=Sf−Se (3)
TES (DPD method)=phase (Sf)−phase (Se) (4)
In
The light receiving regions 106a to 106f are light receiving regions necessary for generating the focusing error signal and tracking error signal. A light receiving region for reducing DC offset occurring on the playback signal needs not to be provided newly, so that the configuration is extremely simplified.
Effect of the above calculation method will be explained.
When a monolayer disk is played back, light does not leak to the auxiliary light receiving regions 106c and 106d. Therefore, the output signals from the auxiliary light receiving regions 106c and 106d are zero. In this case, the playback signal may be generated by the equation (1).
As described above, according to the method of the present invention, DC offset occurring on the focusing error signal and playback signal in the double-layer disk can be reduced effectively.
As a playback signal output unit shown in
HFS=Sa+Sb+Se+Sf−G(Sc+Sd) (5)
where G represents a given gain of the amplifier 12.
The first embodiment uses a single knife edge method as a method of detecting a focusing error. However, the present invention is not limited to this method.
The diffracting device 1401 is divided into six regions 1401a to 1401f by a dividing line 1501 in a disk radial direction and division curves 1502 and 1503 reflecting±1st light diffracted from a land/groove disk as shown in
Two light beams diffracted by the regions 1401a and 1401b of the diffracting device are led to the light receiving regions 1402a to 1402h, and used for generating a focusing error signal by a double knife edge method. Four light beams diffracted by the regions 1401c to 1401f of the diffracting device are led to the light receiving regions 1402i to 14021, and used for generating a focusing error signal by a push-pull method or a DPD method. The output signals from all light receiving regions are used for producing a playback signal.
FES (double knife edge method)=Sa+Sd+Sf+Sg−G1*(Sb+Sc+Se+Sh) (6)
where G1 represents a given gain of the amplifier 15.
The tracking error signal (TES) based on the push-pull method or DPD method is generated according to the following equations (7) and (8), respectively.
TES (push-pull method)=Si+Sj−(Sk+Sl) (7)
TES (DPD method)=phase(Si+Sk)−phase(Sj+Sl) (8)
The method of playing back the playback signal (HFS) is executed according to the following equation (9).
HFS=Sa+Sb+Sc+Sd+Si+Sj+Sk+Sl−G2*(Se+Sf+Sg+Sh) (9)
where G2 represents a given gain of the amplifier 16.
Like the first embodiment, a playback signal generating method of the present invention directed to reduction of interlayer crosstalk uses only light receiving regions for generating the focusing error signal and tracking error signal and needs not to provide newly a light receiving surface to make it possible to be executed in simple configuration.
The beam patterns of undesired leakage light incident on the photodetector from a non-playback layer are shown in
An optical system of the third embodiment of the present invention is shown in
The laser beam is converted from a linearly polarized light beam to a circularly-polarized light beam with the quarterwave plate 1805 and focused on the information recording layer of the optical disk 1808 with the objective lens 1807. The laser beam reflected by the information recording layer follows a path opposite to the outward path and is converted into a parallel light beam with the objective lens 1807. The parallel light beam is diffracted by the division type diffracting device 1806. The diffracted light beam is converted from the circularly-polarized light beam into the linearly-polarized light beam perpendicular to that in the outward path with the quarterwave plate 1805, and reflected by the polarization beam splitter 1803. The reflected light beam is conversed with the condenser lens 1810 and incident on the photodetector 1811 for generating a servo signal/playback signal.
The division shape of the division type diffracting device may be similar to that of the first and second embodiments. In the third embodiment, a five-division type diffracting device shown in
Assuming that output signals from the light receiving regions 1811a to 1811h are Sa, Sb, Sc, Sd, Se, Sf, Sg, Sh respectively, a focusing error signal based on the single knife edge method (FES), a tracking error signal based on the compensation push-pull method or DPD method (TES), and a playback signal (HFS) are produced according to the following equations (10), (11) and (12) by a block circuit shown in
FES (a knife edge method)=Sb+G1*Sc−(Sa+G2*Sd) (10)
TES (compensation push-pull method)=Se−Sh−G3*(Sf−Sg) (11)
TES (DPD method)=phase(Se+Sf)−phase(Sg+Sh) (12)
HFS (a playback signal)=Sa+Sb+Se+Sf+Sg+Sh−G4*(Sc+Sd) (13)
Since the undesired light from the non-playback layer expands over the main light receiving regions 1811a to 1811b and auxiliary light receiving regions 1811e, 1811h, the DC offset due to interlayer crosstalk can be reduced by generating a playback signal according to the equation (13), similarly to the first and second embodiments. As a result, when playing back a double-layer disk, an optical disk apparatus having good playback signal quality can be realized.
According to the present invention, an optical disk apparatus of high reliability reducing DC offset occurring on a playback signal due to interlayer crosstalk, and having good playback signal quality can be realized.
Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2005-103605 | Mar 2005 | JP | national |
