Abstract:
An optical disk apparatus adopted to play back an optical disk, includes 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.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     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.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     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.  
         [0004]     2. Description of the Related Art  
         [0005]     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.  
         [0006]     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.  
         [0007]     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.  
         [0008]     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.  
         [0009]     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.  
         [0010]     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.  
         [0011]     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.  
         [0012]     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.  
       BRIEF SUMMARY OF THE INVENTION  
       [0013]     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. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
       [0014]      FIG. 1  is a diagram of an optical system of a first embodiment.  
         [0015]      FIGS. 2A and 2B  are diagrams showing beam profiles of light reflected by a playback layer and a non-playback layer and landing on the photodetector.  
         [0016]      FIG. 3  shows a block diagram for explaining playback signal calculation.  
         [0017]      FIG. 4  is a block circuit for playback signal calculation and focusing error signal calculation according to the first embodiment.  
         [0018]      FIG. 5  is a diagram of an optical system according to a second embodiment.  
         [0019]      FIG. 6  is a diagram of a 6-division diffraction element and a 12-division photodetector according to the second embodiment.  
         [0020]      FIGS. 7A, 7B  and  7 C show beam profiles on the photodetector when the disk is far from a focusing position, at the focusing position, and near than the focusing position.  
         [0021]      FIG. 8  is a block circuit for playback signal calculation and focusing error signal calculation according to the second embodiment.  
         [0022]      FIGS. 9A and 9B  show beam profiles of light reflected by a playback layer and a non-playback layer and landing on the photodetector.  
         [0023]      FIG. 10  is a diagram of an optical system according to a third embodiment.  
         [0024]      FIG. 11  is a diagram of a division type diffraction element driven integrally with an objective lens.  
         [0025]      FIG. 12  shows a block circuit for playback signal calculation and focusing error signal calculation according to the third embodiment.  
         [0026]      FIGS. 13A and 13B  show beam profiles on a photodetector surface in defocusing. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0027]     Embodiments of the present invention are explained in conjunction with the drawing in detail hereinafter.  
       First Embodiment  
       [0028]     An optical system and a playback signal output system of the first embodiment are shown in  FIG. 1 . The laser beam emitted from a semiconductor laser  201  is diffracted with a diffracting device  202 . The 0th light of the laser beam from a diffracting device  202  is converted into a parallel light beam with a collimating lens  203  and focused on an information recording layer of an optical disk  205  with an objective lens  204 . The light reflected by the information recording layer progresses in a reverse direction with respect to an outward path and converted into a parallel light beam with the objective lens  204 . The parallel light beam is converted into a convergence light beam with the collimating lens  203  and incident on the diffracting device  202 . The diffracting device  202  is divided into three division regions  202   a  to  202   c . The region  202   a  and the regions  202   b  and  202   c  are divided by a division line in a disk radial direction indicating a radial direction of the optical disk. The regions  202   b  and  202   c  are divided by a division line in a disk tangential direction indicating a tangential direction of the optical disk  205 .  
         [0029]     A photodetector  106  having light receiving regions  106   a  to  106   f  is disposed in association with the diffracting device  202  so that the light diffracted by the region  202   a  of the diffracting device  202  is led to the light receiving regions  106   a  and  106   b  of the photodetector  106  and the light diffracted with the regions  202   b  and  202   c  are led to the light receiving regions  106   f  and  106   e  of the photodetector  106 , between which an array of the regions  106   a  to  106   d  are arranged. The signals output from the light receiving regions  106   a  and  106   b  by the light beam diffracted by the region  202   a  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.  
         [0030]     The signals output from the light receiving regions  106   e  and  106   f  by the light beams diffracted with the regions  202   b  and  202   c  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.  
         [0031]     The light receiving regions  106   a  to  106   f  of the photodetector  106  shown in  FIG. 1  generate output signals Sa, Sb, Sc, Sd, Se and Sf corresponding to incident light beams, respectively. The signals Sa, Sb, Se and Sf are supplied to a noninverting input terminal of an operational amplifier  11  serving as a signal processor, and the signals Sc and Sd are input to an inverting input terminal thereof. As a result, a playback signal (HFS) is played back according to the following equation (1): 
 
 HFS=Sa+Sb+Se+Sf −( Sc+Sd )  (1) 
 
         [0032]     This playback signal is a signal indicating information recorded on the optical disk  205 . The auxiliary light receiving regions  106   c  and  106   d  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  106   c  and  106   d  from a sum signal from the other light receiving regions  106   a ,  106   b ,  106   e  and  106   f  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  FIG. 4  according to the following equations (2), (3) and (4). 
 
 FES  (single knife edge method)= Sb+G 1* Sc −( Sa+G 2− Sd )  (2) 
 
 TES  (push-pull method)= Sf−Se   (3) 
 
 TES  ( DPD  method)=phase ( Sf )−phase ( Se )  (4) 
 
         [0033]     In  FIG. 4 , an amplifier  13  corresponds to G1 of the equation (2) and amplifies the signal Sc with an amplification factor G1. An amplifier  14  corresponds to G2 of the equation (2) and amplifies the signal Sc with an amplification factor G2. The method of generating a tracking error signal uses a push-pull method if the optical disk is DVD-RAM, for example, and a DPD method if it is DVD-ROM.  
         [0034]     The light receiving regions  106   a  to  106   f  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.  
         [0035]     Effect of the above calculation method will be explained.  FIG. 2A  shows a beam profile of the light reflected by the playback layer and landing on a photodetector surface, and a beam profile of undesired light reflected by the 1st information recording layer (non-playback layer) and landing on the photodetector surface, when the light beam is focused on the 0-th information recording layer (playback layer).  FIG. 2B  shows a beam profile when the light beam is focused on the 1st information recording layer. In either case, it is found that an undesired light from the non-playback layer extends over the main light receiving region  106   a  and  106   b  and the auxiliary light receiving region  106   c  and  106   d . If the playback signal is generated by calculating a difference between the sum of the signals of the main light receiving regions  106   a  and  106   b  and the sum of the signals of the auxiliary light receiving regions  106   c  and  106   d  according to the equation (1), it is found that influence of undesired leakage light can be reduced.  
         [0036]     When a monolayer disk is played back, light does not leak to the auxiliary light receiving regions  106   c  and  106   d . Therefore, the output signals from the auxiliary light receiving regions  106   c  and  106   d  are zero. In this case, the playback signal may be generated by the equation (1).  
         [0037]     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.  
         [0038]     As a playback signal output unit shown in  FIG. 3 , it is preferable that an amplifier  12  is provided after the photodetector  106  to improve an effect of reducing the interlayer crosstalk by adjusting a level of the signal. The method of calculating a playback signal (HFS) in this case is executed according to the following equation (5): 
 
 HFS=Sa+Sb+Se+Sf−G ( Sc+Sd )  (5) 
 
         [0039]     where G represents a given gain of the amplifier  12 .  
       Second Embodiment  
       [0040]     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.  FIG. 5  shows an optical system according to the second embodiment, which uses a double knife edge method as the method of detecting a focusing error. The second embodiment differs from the first embodiment in a division configuration of a diffracting device and a light receiving surface configuration of a photodetector. As shown in  FIG. 5 , a diffracting device  1401  is divided into six division regions, and the light receiving surface of the photodetector  1402  is divided into twelve division regions. The diffracting device  1401  and photodetector  1402  are shown in  FIG. 6  in detail.  
         [0041]     The diffracting device  1401  is divided into six regions  1401   a  to  1401   f  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  FIG. 6 . The photodetector  1402  has main light receiving regions  1402   a  to  1402   d  for focusing error detection, auxiliary light receiving regions  1402   e  to  1402   h , and light receiving regions  1402   i  to  14021  for tracking error detection.  
         [0042]     Two light beams diffracted by the regions  1401   a  and  1401   b  of the diffracting device are led to the light receiving regions  1402   a  to  1402   h , and used for generating a focusing error signal by a double knife edge method. Four light beams diffracted by the regions  1401   c  to  1401   f  of the diffracting device are led to the light receiving regions  1402   i  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.  
         [0043]      FIGS. 7A, 7B  and  7 C show patterns of light beams (signal light beams) from the playback layer in defocusing.  FIGS. 7A, 7B  and  7 C show beam profiles on the photodetector when the disk is far from a focusing position, at the focusing position, and near than the focusing position. Assuming that the output signals from the light receiving regions  1402   a  to  14021  are Sa to Sl respectively, the focusing error signal (FES) is generated by a block circuit shown in  FIG. 8 , for example, according to the following equation (6): 
   FES  (double knife edge method)= Sa+Sd+Sf+Sg−G 1*( Sb+Sc+Se+Sh )  (6)  
         [0044]     where G1 represents a given gain of the amplifier  15 .  
         [0045]     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) 
 
         [0046]     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−G 2*( Se+Sf+Sg+Sh )  (9) 
 
         [0047]     where G2 represents a given gain of the amplifier  16 .  
         [0048]     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.  
         [0049]     The beam patterns of undesired leakage light incident on the photodetector from a non-playback layer are shown in  FIGS. 9A and 9B  to explain effect of the playback signal generating method.  FIG. 9A  shows a beam profile of light reflected by the playback layer and landing on the photodetector surface, and a beam profile of the undesired light reflected by the first information recording layer (the non-playback layer) and landing on the photodetector surface, when the light beam focuses on the information recording layer (the playback layer).  
         [0050]      FIG. 9B  shows a beam profile on the photodetector surface when the light beam is focused on the first information recording layer. In either case, it is found that undesired light reflected by the non-playback layer expands over the main light receiving regions  1402   a  to  1402   d  and auxiliary light receiving regions  1402   e  and  1402   h . Accordingly, if the playback signal is generated by calculating differences between the signals of the main light receiving regions  1402   a  to  1402   d  and the signals of the auxiliary light receiving regions  1402   e  and  1402   h  as indicated by the equation (9), it is found that influence of undesired leakage light can be reduced. When a monolayer disk is played back, light is leaked to the auxiliary light receiving regions  1402   e  to  1402   h  to output no signal therefrom. Therefore, there is no problem at all in playing back the monolayer disk.  
       Third Embodiment  
       [0051]     An optical system of the third embodiment of the present invention is shown in  FIG. 10 . This embodiment differs from the first and second embodiments with respect to a configuration that a diffracting device  1805  for generating a servo signal/playback signal and a quarter-wavelength plate  1806  are driven integrally with an objective lens  1807 . The linearly polarized laser beam emitted from the semiconductor laser  1801  is converted into a parallel light beam with a collimator lens  1802 , transmitted through a polarization beam splitter  1803 , and reflected by an up-rise mirror  1804 . Subsequently, the laser beam is incident on the diffracting device  1806  and the quarterwave plate  1805  driving integrally with the objective lens  1807 .  
         [0052]     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.  
         [0053]     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  FIG. 11  is used. The light beam diffracted by a diffracting device region  1806   a  is led to light receiving regions  1811   a  to  1811   d  to be used for producing a focusing error signal by a single knife edge method. The light beams diffracted with the diffracting device region  1806   b  to  1806   e  are led to light receiving regions  1811   e  to  1811   h  respectively, to be used for generating a tracking error signal by a compensation push-pull method or a DPD method.  
         [0054]     Assuming that output signals from the light receiving regions  1811   a  to  1811   h  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  FIG. 12 . The compensation push-pull method is a method for reducing offset of the tracking error signal caused by radial shifting of the objective lens. The detail principle of this method is described by Toshiba review Vol. 57 No. 7 p 32-p 34 (2002), the entire contents of which are incorporated herein by reference. 
 
 FES  (a knife edge method)= Sb+G 1* Sc −( Sa+G 2* Sd )  (10) 
 
 TES  (compensation push-pull method)= Se−Sh−G 3*( Sf−Sg )  (11) 
 
 TES  ( DPD  method)=phase( Se+Sf )−phase( Sg+Sh )  (12) 
 
 HFS  (a playback signal)= Sa+Sb+Se+Sf+Sg+Sh−G 4*( Sc+Sd )  (13) 
 
         [0055]      FIG. 13A  shows a beam profile of light reflected by the playback layer and landing on the photodetector surface and a beam profile of the undesired light reflected by the first information recording layer (the non-playback layer) and landing on the photodetector surface, when the light beam focuses on the information recording layer (the playback layer).  FIG. 13B  shows a beam profile on the photodetector surface when the light beam is focused on the first information recording layer.  
         [0056]     Since the undesired light from the non-playback layer expands over the main light receiving regions  1811   a  to  1811   b  and auxiliary light receiving regions  1811   e ,  1811   h , 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.  
         [0057]     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.  
         [0058]     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.