Patent Publication Number: US-2006002276-A1

Title: Optical disc apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2004-193198, filed Jun. 30, 2004, the entire contents of which are incorporated herein by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to an optical disc apparatus, which can simplify a signal playback system with little crosstalk, when playing back information from an optical disc having information recorded in two or more recording layers.  
      2. Description of the Related Art  
      An optical disc used as an information recording medium is available in a play-only type represented by CD and DVD-ROM, a write-once type represented by CD-R and DVD-R, and a rewritable type used for an external memory of computer and a record-playback video (video recording).  
      Nowadays, to increase the recording capacity of the current standard DVD, research is being conducted to increase the number of recording layers to two or more. To do this, it is necessary to make the space layer between the recording layers thinner than in a current standard DVD. However, it is known that crosstalk between layers increases when the space layer is thin.  
      It is also known that there is an extreme change in the difference (magnification) between crosstalk produced in a second recording layer while playing back a first recording layer, and crosstalk produced in a first layer while playing back a second layer. This lowers the efficiency of zero-order light and the detection performance in a signal playback system using a ±first order diffraction light by combining an astigmatic aberration method and spot size method as a focus error detection system.  
      In a next-generation DVD (hereinafter referred to as an HD DVD), if, in order to increase the recording capacity, the space layer is made thinner than in a current dual layer DVD-ROM among standard DVDs having two or more recording layers, there is a problem that the crosstalk from the space layer is increased and a preferable playback signal cannot be obtained in a certain focus error detection system.  
      When the crosstalk difference is great, it is necessary to increase the dynamic range of the signal playback system. This increases the cost of the signal playback system, and raises the noise level.  
     BRIEF SUMMARY OF THE INVENTION  
      According to an aspect of the present invention, there is provided an optical disc apparatus comprising:  
      a light source;  
      an object lens which condenses light from the light source on at least one recording layer of a recording medium having at least two recording layers;  
      a condenser lens which imposes a predetermined convergence on light reflected from one recording layer of a recording medium having at least two recording layers; and  
      a photodetector which has a light-receiving a surface at a position displaced a certain distance from the focal point of the condenser lens, receives light reflected from one recording layer of a recording medium having at least two recording layers, and outputs a signal corresponding to the intensity of the light. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
      The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention, and together with the general description given above and the detailed description of the embodiments given below, serve to explain the principles of the invention.  
       FIG. 1  is a schematic illustration for explaining an example of a recording medium (an optical disc) applicable to an optical head unit according to an embodiment of the present invention;  
       FIG. 2  is a schematic illustration for explaining an essential part of an optical head unit according to an embodiment of the present invention;  
       FIGS. 3A and 3B  are schematic illustrations for explaining the cause of a crosstalk from a space layer occurring when reflected laser beams are obtained from two recording layers by using the optical head unit shown in  FIG. 2 ;  
       FIG. 4  is a graph for explaining changes in the crosstalk from the space layer explained with reference to  FIG. 3  depending on the thickness of the space layer distance between recording layers of an optical disc, and the position of a photodetector;  
       FIG. 5  is a graph showing an example of a preferable position of a light-receiving surface of a photodetector defined according to the a value of the crosstalk from the space layer explained with reference to  FIG. 3 ;  
       FIG. 6  is a graph for explaining suppression of the crosstalk from the space layer as a result of providing a photodetector at the position explained with reference to  FIG. 5 ;  
       FIG. 7  is a schematic illustration showing an example of an optical head unit which satisfies conditions for a preferable position of a photodetector explained with reference to  FIG. 4  to  FIG. 6 ; and  
       FIG. 8  is a schematic sketch showing an example of an optical head unit which satisfies conditions for a preferable position of a photodetector explained with reference to  FIG. 4  to  FIG. 6 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      Hereinafter, an embodiment of the present invention will be explained in detail with reference to the accompanying drawings.  
       FIG. 1  is a schematic illustration of an example of an optical disc suitable for recording or playing back information with an optical head unit of the present invention to be explained with reference to  FIG. 2 .  
      As shown in  FIG. 1 , an optical disc (a recording medium)  1  has a first information recording layer  3  including a phase-change recording film, on a first substrate  2  made of polycarbonate.  
      On the first information recording layer  3 , a space layer  4  having a predetermine transmittivity relative to a wavelength of laser beam emitted from a semiconductor laser unit  20  of an optical head unit  11 , is stacked. On the space layer  4 , a second information recording layer  5  is stacked. The space layer  4  is usually made of adhesive or ultraviolet hardening resin. The second information recording layer  5  is covered by a second substrate  6  made of polycarbonate.  
      In the optical disc  1 , the information recording layers  3  and  5  may be a play-only (read-only) layer composed of a reflection film made of metal, or a record-playback (re-writable) layer composed of phase change film. Further, in the optical disc  1 , only one of two recording layers  3  or  5  may be a play-only layer and the other may be a record-playback layer.  
      As described above, the optical disc  1  can be formed either by stacking the first substrate  2  and second substrate  6  in order, or by sticking two substrates having the information recording layer  3  (or  5 ) with a predetermined thickness to the space layer  4  to be faced to each other by means of adhesive material. Each of the substrates with the information recording layer formed on one side is about 0.6 mm thick, and the whole optical disc is about 1.2 mm thick (a thickness of the substrate for reference).  
      The optical disc  1  is a so-called one-side dual layer disc. The first information recording layer  3  is a semitransparent having a predetermined transmittivity relative to a wavelength of laser beam emitted from the semiconductor laser unit  20 . Therefore, the first information recording layer  3  can transmit the remaining amount of light as well as reflecting a certain amount of light. Thus, when light is radiated to the optical disc  2  from the direction of the substrate  2 , information can be recorded on one of the information recording layers  3  and  5 , or information can be played back from one of the recording layers, by adjusting a focus to one of the first and second recording layers  3  and  5  (by controlling the position of an object lens  24 , so that the distance between the object lens  24  and optical disc  1  relative to the position, where a laser beam  100  is condensed by convergence of the laser beam  100  through the object lens  24 , becomes the same as the distance to one of the recording layers  3  and  5 ).  
      The space layer  4  has a function to optically interrupt a crosstalk from the other information recording layer  5  (or  3 ), while the information of one information recording layer  3  (or  5 ) is being played back. The crosstalk is a leakage light (beam) leaked from a difference information layer. In this sense, an interval between two information recording layers  3  and  5  is preferably separated as far as possible, and the space layer  4  is preferably thick. However, in that case, the load of the optical system for recording and playing back becomes heavy.  
      Namely, when the thickness from the surface of the substrate  2  to the center of the space layer  4  is defined as a load of an object lens described later with reference to  FIG. 2 , an aberration is caused by a thickness error in the half thickness of the space layer  4 , in any case of recording or playing back information in/from the information recording layer  3  (or  5 ).  
      Therefore, from the viewpoint of aberration in the recording/playing optical system, the space layer  4  is preferably thin. Namely, the thickness of the i space layer  4  is decided at a trade-off point between an aberration of the recording/playing back optics and a crosstalk between the information recording layers  3  and  5 .  
       FIG. 2  is a schematic illustration for explaining an example of an optical head unit, which records information in the optical disc shown in  FIG. 1 , or plays back information from the optical disc. In  FIG. 2 , only the essential part of the optical head unit is explained.  
      As shown in  FIG. 2 , an optical head unit  11  has a semiconductor laser (a light source)  20  to output a laser beam or a purple light beam of 400-410 nm. A wavelength of the laser beam may be 440 nm or lower, for example, 405 nm.  
      A laser beam  100  emitted from the semiconductor laser  20  is collimated by a collimator lens  21 . The collimated parallel beam passes through a polarizing beam splitter  22  and quarter-wavelength plate  23 , and is guided by the object lens  24  to the recording surface of the optical disc  1  explained later with reference to  FIG. 2 .  
      The laser beam  100  guided by the optical disc  1  is condensed on one of the first and second recording layers, by the convergence given by the object lens  24  and the distance between the object lens and optical disc  1 .  
      The laser beam  100  condensed on the recording layer of the optical disc  1  is reflected on the recording layer, returned to the object lens  24  as a reflected laser beam  101 , and returned to a polarizing beam splitter  22  through the quarter-wavelength plate  23 .  
      The reflected laser beam  101  returned to the polarizing beam splitter  22  is reflected toward the condenser lens  25  on the polarizing surface of the polarizing beam splitter  22 , and forms an image on the light-receiving surface of a photodetector  26  as a convergent beam having a beam spot size corresponding to the focal distance defined by the convergence given by the condenser lens  25 .  
      The light-receiving part of the photodetector is usually divided into several sections, each of which outputs a current corresponding to the light intensity. The current output from each light-receiving section is converted into a voltage by a not-shown I/V amplifier, and processed by a processor  27  to be usable as a HF (playback) signal, focus error signal and track error signal. The HF (playback) signal is converted into a predetermined signal format, or outputted to a temporary storage or external storage through a given interface.  
      The focus error signal and track error signal among those obtained by the processor  27  are converted by a servo driver  28  into signals usable as a focus control signal and track control signal to operate an actuator  29 , which displaces the position of the object lens  24 , and supplied to the actuator  29 . Therefore, the object lens  24  held by the actuator  29  is optionally moved in the vertical direction to approach to and separate from the information recording layer  3  (or  5 ) of the optical disc  1 , and/or in the disc radial direction. Namely, the object lens  24  is controlled by the servo driver  28  to follow the information track on the optical disc  1 .  
      Description will be given on optimization of the elements of the optical head unit  11  for recording or playing back information in/from a new-standard optical disc called a next-generation DVD (hereinafter referred to as an HD DVD).  
      Assuming that the wavelength of a light source (the output of a semiconductor laser unit) is 405 nm and the numerical aperture NA of the object lens  24  is 0.65, the suitable thickness of the space layer  4  of the optical disc  1  is 15-25 μm as a specification of HD DVD considering the trade-off mentioned above. Conditions on the thickness of the space layer  4  described below are applicable to an optical disc having more than three information recording layers or a disc having two or more space layers.  
      Description will now be given on a crosstalk upon playback from the space layer of a dual layer optical disc with reference to  FIGS. 3A and 3B . In  FIGS. 3A and 3B , a disc having two record-playback layers is taken as an example, but the result is the same as for a disc having more than three recording layers.  
      As shown in  FIG. 3A , when a laser beam given convergence by the object lens  24  is focused on a record-playback layer  5  (L 1 ) provided farther from the object lens  24 , the reflected laser beam  101 a (a solid line) from the information recording layer  5  (L 1 ) becomes a parallel light after passing through the object lens  24 , and is condensed on the condenser lens  25  and focused at a predetermined position near the photodetector  26 . The reflected laser beam  101   b  (a dotted line) from the information recording layer  3  (L 0 ) provided near the object lens  24  is focused at a position farther than the focal point of the reflected beam from the information recording layer  5  (L 1 ).  
      On the other hand, as shown in  FIG. 3B , when a laser beam given convergence by the object lens  24  is focused on a record-playback layer  3  (L 0 ) provided near the object lens  24 , the reflected laser beam  101   c  (a solid line) from the information recording layer  3  (L 0 ) becomes a parallel light after passing through the object lens  24 , and is condensed on the condenser lens  25  and focused at a predetermined position near the photodetector  26 , or at substantially the same position as the focal point of the reflected laser beam  101   a  from the information recording layer  5  (L 1 ) shown in  FIG. 3A . In this case, the reflected laser beam  101   d  (a dotted line) from the information recording layer  5  (L 1 ) provided farther from the object lens  24  is focused at a position in front of the focal point of the reflected beam from the information recording layer  3  (L 0 ), or at the point near the condenser lens  25 .  
      For an optical disc (dual layer) emitting a reflected laser beam as described above, consider placing the light-receiving surface of the photodetector  26  so that the photodetector is located at a position different from the focal point of the condenser lens (the position indicated by the dotted line as  126  in  FIGS. 3A and 3B ), as in an optical head unit which uses a photodetector both for focusing and for RF (a playback signal) by using astigmatic aberration for detecting a focus error. In this case, the sectional beam size of a reflected laser beam from L 0  (the second recording layer  3 ) causing crosstalk when the beam is condensed on L 1  (the first recording layer  5 ) is largely different from the sectional beam size of a reflected laser beam from L 1  (the first recording layer  5 ) when the beam is condensed on L 0  (the second recording layer), as indicated by S 1  and S 0  in  FIGS. 3A and 3B .  
      In other words, the crosstalk from a non-focused-record-playback layer (the different layer either  3  or  5 ) differs depending on a recording layer on which a laser beam is condensed (focused, so-called on-focus) by the object lens  24 .  
      This is seen from the crosstalk CT 1  caused by a reflected laser beam from L 1  when the beam is condensed on L 0  indicated by the curve A (◯ and dotted line) in  FIG. 4 , and the crosstalk CT 0  caused by a reflected laser beam from L 0  when the beam is condensed on L 1  indicated by the curve B (Δ and dotted line). In  FIG. 4 , the horizontal axis is plotted as a thickness of the space layer  4  between dual layers.  
      It is seen from the curves A and B that the signal amplitudes of two crosstalks (CT 1  and CT 0 ) are largely different. Thus, it is necessary to consider the larger crosstalk when designing dynamic range of an optical disk drive. This causes an increase in noise.  
      Conversely, consider placing the photodetector  26  at the position  26  indicated by the solid line in  FIGS. 3A and 3B , that is, the position closer to the focal point of the condenser lens  25  than the position  126  indicated by the dotted line. In this case, as seen from  FIGS. 3A and 3B , the beam spot of the reflected laser beam from L 0  when the beam is condensed on L 1  is substantially the same as the beam spot of the reflected laser beam from L 1  when the beam is condensed at L 0 . As for the crosstalk to each reflected laser beam, as indicated by the curves a (◯ and solid line, CT 1 ) and curve b (Δ and solid line, CT 0 ), the difference between the crosstalk CT 1  caused by a reflected laser beam from L 1  when the beam is condensed at L 0 , and the crosstalk CT 0  caused by a reflected laser beam from L 0  when the beam is condensed at L 1 , becomes small compared with the case indicated by the curves A and B.  
      Therefore, the crosstalk caused by leak from two recording layers can be set within almost certain range regardless of the layer being recorded or played back. In this case, it is also possible to decrease a maximum value of crosstalk in an optical disc drive. It is suitable to set the crosstalk difference to twice or less from the viewpoint of dynamic range and a noise eliminating circuit.  
      It is known however that a signal for detecting a track error becomes small in a signal playback system (the object lens  24 , condenser lens  25  and photodetector  26  in the optical head unit  11 ) capable of realizing the crosstalk indicated by the curve a (◯ and sold line, CT 1 ) and curve b (Δ and solid line, CT 0 ).  
      Thus, it is preferable to set the size of the light-receiving part of the photodetector to a value obtainable by A/M 2  (the area A of a light-receiving part of a photodetector) and the detection lateral magnification M (a square of the ratio of the focal distance of the condenser lens  25  to the focal distance fo of an object lens) defined in Japanese Industrial Standards (JIS) X 6241 (1997). For example, when the focal distance fc of the condenser lens  25  is  80  mm, the area A of the light-receiving part of the photodetector is 16900 μm 2  and the lateral magnification M is 26.67, A/M 2  is obtained by 
 
 A/M   2 =16900/(26.07) 2 =23.76≈23.8 [μm 2 ]
 
      As shown by Δ in  FIG. 5 , it is preferable to place the light-receiving surface of the photodetector  26  at a position of −1 mm relative to the focal point of the condenser lens  25  (“−1” indicates a position toward this side, that is, a position toward the condenser lens  25 ). It is confirmed that the difference (magnification) in the crosstalk when the light-receiving surface of the photodetector  26  is placed at the position indicated by A in  FIG. 5 , that is, the difference between the crosstalk CT 1  caused by a reflected beam from L 1  when the beam is focused at L 0 , and the crosstalk CT 0  caused by a reflected beam from L 0  when the beam is focused on L 1 , is almost within two times regardless of the thickness of the space layer  4  between the recording layers  3  and  5 .  
       FIG. 7  is a schematic illustration showing an example of an optical head unit which satisfies conditions for a preferable position of a photodetector explained in  FIG. 4  to  FIG. 6 . The same components (elements) and similar configuration as/to those of the schematic illustration of  FIG. 2, 700  is added to the same reference numerals, and detailed explanation will be omitted.  
      As shown in  FIG. 7 , a laser beam with a wavelength of 405 nm emitted from a light source (a semiconductor laser unit)  720  is collimated by a collimator lens  21 . The collimated parallel beam passes through a polarizing beam splitter  722  and quarter-wavelength plate  723 , and is guided to the object lens  724 . The laser beam guided to the object lens  724  is given a predetermined convergence in the object lens  724 , and condensed on the first recording layer  3  (close to the object lens, L 0 ) or second recording layer  5  (at the back of the space layer  4 , L 1 ).  
      The reflected laser beam from the first or second recording layer  3  (L 0 ) or  5  (L 1 ) of the optical disc  1  is captured by an object lens  724 , transmitted through the quarter-wavelength plate  723 , and reflected toward a beam splitter  730  by a polarizing beam splitter  722 .  
      The reflected laser beam from the polarizing beam splitter  722  is given a predetermined convergence in a condenser lens  725 , divided into two parts by the beam splitter  730 , and condensed on a first photodetector  731  used for generating a focus error signal, and a second photodetector  732  used for generating a tracking error signal, respectively.  
      The second photodetector  732  for detection of tracking error is preferably placed at the position explained in  FIG. 5 . Namely, it is preferable to place a photodetector having a light-receiving surface area of 23.8 μm 2  at a position of −1 mm relative to the focal distance fc of the condenser lens, as defined by A/M 2 , that is, the area A of a photodetector and a square of the ratio of a focal distance of a condenser lens to the focal distance fo of an object lens. This is caused by that the photodetector  732  for obtaining a tracking error signal can decrease dynamic range based on the detection principle and accuracy. (Which channel is “+” or “−” can be specified in a two-channel detector.)  
      The output from the second photodetector  732  for detecting a tracking error is added by a not-shown adder, and processed as an RF (playback) signal according to a predetermined regulation, or through a predetermined interface, and sent to a not-shown temporary storage or external storage.  
      A wavefront conversion element such as a hologram is effective as a method of placing a photodetector near a focal point of a detection system (a condenser lens). Namely, in an ordinary astigmatic optical system, it is difficult to place a photodetector at a focal point of a condenser lens.  
      When a focus error is detected by a knife edge method without using a wavefront conversion means, a tracking error signal is also obtained from an output of a photodetector to detect an RF signal. Thus, the beam spot becomes very small on the light-receiving surface of a photodetector, and easy to be affected by an offset of an object lens.  
       FIG. 8  explains an example of using a wavefront conversion means in the optical head unit shown in  FIG. 7 . The same components (elements) and similar configuration as/to those of the schematic illustration of  FIGS. 2 and 7 ,  800  is added to the same reference numerals, and detailed explanation will be omitted.  
      As shown in  FIG. 8 , a laser beam with a wavelength of 405 nm emitted from a light source (a semiconductor laser unit)  820  is collimated by a collimator lens  21 . The collimated parallel beam passes through a polarizing beam splitter  822  and quarter-wavelength plate  823 , and is guided to the object lens  824 . The laser beam guided to the object lens  824  is given a predetermined convergence in the object lens  824 , and condensed on the first recording layer  3  (close to the object lens side, L 0 ) or second recording layer  5  (at the back of the space layer  4 , L 1 ).  
      The reflected laser beam from the first or second recording layer  3  (L 0 ) or  5  (L 1 ) of the optical disc  1  is captured by an object lens  824 , transmitted through the quarter-wavelength plate  823 , and reflected toward a beam splitter  830  by a polarizing beam splitter  822 .  
      The reflected laser beam from the polarizing beam splitter  822  is given a predetermined convergence in a condenser lens  825 . The advancing direction of the laser beam is changed to a predetermined position of a photodetector  840  by a wavefront conversion element or a hologram element  850  provided at a predetermined position between a photodetector  840  and a condenser lens  825 . The laser beam is condensed on a light-detection area of the not-shown photodetector  840  for detecting a focus error used for generation of a focus error signal, and a light-detecting area for detecting a tracking error used for generation of a tracking error signal. The output from the light-detecting area for detecting a tracking error is added by a not-shown adder, and processed as an RF (playback) signal according to a predetermined regulation, or through a predetermined interface, and sent to a not-shown temporary storage or external storage.  
      The hologram element  850  is given a pattern which can give a specific sectional beam shape (a beam spot size) to a reflected laser beam give a predetermined convergence by the condenser lens  825 , assuming that a knife edge method is used for detecting a focus error and a push-pull method is used for detecting a tracking error. Various well known methods are usable as a focus error detection method and a tracking error detection method.  
      As described hereinbefore, according to the present invention, a playback signal can be stably obtained from an optical disc having two or more recording layers without using a focus error detection method. When a playback signal is obtained from two or more recording layers, crosstalk can be controlled to twice, decreasing the load on a signal playback system.  
      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.  
      In the detailed description of the invention, an optical disc drive is taken as an example for explaining the embodiment of the invention. But, it will be apparent that the invention is also applicable to a movie camera and portable audio equipment storing musical data.