Patent Publication Number: US-2007097833-A1

Title: Optical head unit and optical disc apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2005-313271, filed Oct. 27, 2005, the entire contents of which are incorporated herein by reference.  
     BACKGROUND  
      1. Field  
      One embodiment of the invention relates to an information recording/reproducing apparatus (optical disc apparatus) which records, reproduces and erases information on/from a recordable, playable and erasable optical disc by using a laser beam, and an optical pickup (optical head) used in the optical disc apparatus.  
      2. Description of the Related Art  
      A long time has been passed since the commercialization of an optical disc capable of recording or playing back information in a noncontact manner by using a laser beam, and an optical disc apparatus (an optical disc drive) capable of recording and reproducing information on/from an optical disc (a recording medium). Optical discs with several kinds of recording density called CD and DVD have achieved widespread use.  
      As optical discs of various standards have been developed and used for various purposes, an optical disc apparatus is required to be capable of recording information on an optical disc of two or more standards, reproducing prerecorded information, and erasing recorded information. Besides, it is demanded as an essential condition of an optical disc recording/reproducing apparatus to be capable of detecting a standard of an optical disc set in the apparatus, even if it is difficult to record and erase information.  
      Therefore, an optical pickup incorporated in an optical disc information recording/reproducing apparatus is required at least to be capable of capturing a reflected ray from a track or a string of record marks peculiar to an optical disc, and controlling the track and the focus of an object lens (optical pickup), regardless of the standards (types) of an optical disc.  
      DVD and HD DVD optical discs are different in the pitch in the radial direction of a track, a guide groove, or a string of record marks, depending on the standards. Therefore, in a track error control to align a laser beam condensed by an object lens with the center of a track or a string of record marks, a method of dividing a laser beam reflected on an optical disc into a required number of beams by a diffraction element has been widely used to detect a focus error and a tracking error by using a diffraction rating, for example.  
      For example, Japanese Patent Application Publication (KOKAI) No. 2002-100063 describes a method of reducing an influence of a tracking offset included in the beams of light divided by a diffraction grating, when detecting a focus error by dividing a diffraction grating into several fine areas.  
      However, in the method described in the Publication No. 2002-100063, the amounts of 2-divided beams of light are made substantially equal by precisely combining two diffraction elements with different diffraction angles, there is a one-to-one correspondence between the divided areas of a diffraction element and the light-receiving areas of a photodetector.  
      Thus, it is difficult to obtain a signal from the areas with different focus/tracking, or to obtain a signal across the areas. This likely causes the output signal to be buried in noise. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
      A general architecture that implements the various feature of the invention will now be described with reference to the drawings. The drawings and the associated descriptions are provided to illustrate embodiments of the invention and not to limit the scope of the invention.  
       FIG. 1  is an exemplary diagram showing an example of an optical disc apparatus in accordance with an embodiment of the invention;  
       FIGS. 2A and 2B  are exemplary diagrams showing a pattern of dividing a luminous flux by a diffraction element (hologram), and a pattern of a light-receiving area of a photodiode (photodetector), of an optical head of the optical disc apparatus shown in  FIG. 1 , according to an embodiment of the invention; and  
       FIG. 3  is an exemplary diagram showing an example of a layout of a light-receiving area of a photodetector of an optical head of the optical disc apparatus shown in  FIG. 1 , according to an embodiment of the invention. 
    
    
     DETAILED DESCRIPTION  
      Various embodiments according to the invention will be described hereinafter with reference to the accompanying drawings. In general, according to one embodiment of the invention, an optical head unit including a diffraction pattern of a diffraction element to guide a reflected laser beam divided into a predetermined number to a photodetector, in order to provide an optical head unit and an optical disc apparatus which provide a stable reproducing signal irrespectively of the standards of recording media, when reproducing information from a recording medium of optional standard.  
      According to an embodiment,  FIG. 1  shows an example of an information recording/reproducing apparatus (an optical disc apparatus) according to an embodiment of the invention.  
      An optical disc apparatus  1  shown in  FIG. 1  includes an optical pickup (optical head unit)  10 , which can record information in a not-shown recording layer, for example, organic film, metallic film or phase changing film, of a recording medium  100  (an optical disc), read information from the recording layer, or erase information recorded in the recording layer. In addition to the optical head unit  10 , though not described in detail, the optical disc unit  1  has mechanical elements, such as a not-shown head moving mechanism which moves the optical head unit  10  along the recording surface of an optical disc D, and a disc motor (not shown) which rotates the optical disc D at a predetermined speed. As explained later, the optical disc unit  1  also includes a signal processor to process the output of a photodetector incorporated in the optical head unit  10 , and a controller to control the mechanical elements of the optical head unit  10 .  
      The optical head unit  10  includes an object lens  11 , which is placed close to the optical disc  100 , and captures a laser beam reflected from the recording layer of the optical disc  100 , as well as condensing a laser beam from a light source, for example, a laser diode (LD)  12  or a semiconductor laser element, on the recording layer L 0  or L 1 . The wavelength of the laser beam emitted from the laser diode (LD)  12  is 400 to 410 nm, preferably 405 nm.  
      The laser beam from the laser diode (LD)  12  passes through a polarization beam splitter (PBS)  19  provided at a predetermined position, and is collimated (paralleled) by a collimator lens (CL)  15 , and guided to the object lens (OL)  11  through a diffraction element  17 , in which an optical dividing element or a hologram plate (hologram optical element (HOE)) is combined with a λ/4 plate (¼ wavelength plate, or polarization control element).  
      The laser beam guided to the object lens  11  is given a predetermined convergence by the object lens, and condensed on one of the recording layers L 0  and L 1  of the optical disc  100 . Each of the recording layers L 0  and L 1  has a guide groove, a track, or a string of record marks (recorded data) formed concentrically or spirally with a pitch of 0.34 μm to 1.6 μm, for example. The object lens  11  is made of plastic, and has a numerical aperture (NA) of 0.65, for example.  
      The laser beam given a predetermined convergence by the object lens  11  passes through a cover layer of an optical disk (not described in detail), and is condensed on one of the recording layers (or in the vicinity of that layer). The laser beam from the light source  12  provides a minimum optical spot at the focal position of the object lens  11 .  
      The object lens  11  (optical head unit  10 ) is placed at a predetermined position in the direction of track orthogonal to the tracks of each recording layer of the optical disc  100 , and at a predetermined position in the direction of focus, or the direction of the thickness of the recording layer, by a not-shown object lens driving mechanism including a driving coil and a magnet, for example. The position of the object lens  11  is controlled to align a minimum optical spot of a laser beam with the center of a track (a string of recording marks), by moving the object lens  25  in the direction of a track. This is called a tracking control. The position of the object lens  11  is also controlled to make the distance from the object lens  11  to the recording layer identical to the focal distance of the object lens  11 , by moving the object lens  11  in the direction of focus. This is called a focus control.  
      The laser beam reflected on the recording layer L 0  or L 1  of the optical disc is captured by the object lens  11 , converted to a beam having a substantially parallel section, and sent back to the diffraction element  17 .  
      As the diffraction element  17  serves also as a λ/4 plate, the reflected laser beam sent back to the polarization beam splitter  19  through the diffraction element  17  is reflected on the plane of polarization (not described in detail) of the polarization beam splitter  19 , because the direction of polarization of the laser beam toward the recording layer of the optical disc  100  is rotated 90 by degrees.  
      The laser beam reflected on the polarization beam splitter  19  is given astigmatic aberration by a cylindrical lens  23  having a power tilted 45° to a tangential or radial direction, and forms an image on the light-receiving surface of the photodiode (photodetector (PD))  14  by the convergence given by the collimator lens  15 . In this time, when passing through the diffraction element  17 , the reflected laser beam is divided into a predetermined form and a predetermined number to meet the form and layout of the detection area (light-receiving area) previously given to the light-receiving surface of the photodetector  14 .  
      The current output from each light-receiving area (explained later in detail with reference to  FIG. 2  or  FIG. 3 ) is converted into a voltage by a not-shown I/V amplifier, and processed to be usable as a HF (reproducing) signal, a track error signal TE, and a focus error signal FE. Though not described in detail, the HF (reproducing) signal is converted to a predetermined signal format, or output to a temporary storage device or an external storage device through a given interface.  
      The signal obtained by the signal processing circuit  21  is also used as a servo signal to optionally move the object lens  11  of the optical head unit  10  through a servo circuit  22 , in the direction (the optical axis direction) orthogonal to the plane including the recording surface of the optical disc  100 , so that the distance from the object lens  11  to the recording layer L 0  or L 1  of the optical disc  100  becomes the same as the focal distance of the object lens  11 , and in the direction orthogonal to the direction of a track or a record mark (a string of record marks) previously formed on the recording surface of the optical disc.  
      The servo signal is generated based on a tracking error signal indicating changes in the position of the object lens  11 , according to the well-known focus error detection method, so that an optical spot having a predetermined size at a focal position of the object lens  11  becomes a predetermined size on recording layer L 0  or L 1  of the optical disc  100 ; and based on a track error signal indicating changes in the position of the object lens  11 , according to the well-known track error detection method, so that the optical spot is guided to substantially the center of a string of record marks or a track.  
      Namely, the object lens  11  is controlled to provide an optical spot condensed by the object lens  11  in a minimum size on each of the recording layer L 0  or L 1  of the optical disc  100 , at the focal distance, at substantially the center of the track or the string of record marks formed on the recording layer of the optical disc  100 .  
       FIGS. 2A and 2B  show an example of a pattern of diffracting a luminous flux by a hologram element incorporated in the optical head of the optical disc apparatus shown in  FIG. 1 , and characteristics of layout and form (a pattern of arrangement) of light-receiving areas of a photodetector.  FIG. 3  is a schematic diagram showing an example of a layout of a light-receiving area of a photodetector applied to the optical head shown in  FIGS. 2A and 2B .  
      As shown in  FIGS. 2A and 2B , the diffraction element (HOE combined with the λ/4 plate)  17  has substantially circular concentric patterns forming two optical diffraction areas, as shown in the magnified part Y ( FIG. 2B ). Namely, the optical diffraction areas includes an outside circular area t and an inside circle area s. As shown in  FIGS. 2A and 2B , each light diffraction area can diffract the laser beam reflected on optional recording layer of the optical disc  100 , in a desired direction to meet the patterns of the light-receiving surface of the photodetector  14  shown in  FIG. 3 . In the present invention, a reflected laser beam passing through the circle area s of the diffraction element  17  is diffracted to a 4-divided pattern (A, B, C, D on the right-hand side of  FIG. 3 , namely a pattern obtained by dividing a square at substantially the center of the vertical and horizontal sides), and a reflected laser beam passing through the circular area t is diffracted to a 4-divided pattern of a photodetector (E, F, G, H on the left-hand side of  FIG. 3 ). The light-receiving areas (patterns A to H of the light-receiving surface) are divided by a division line along a radial direction orthogonal to a tangential direction of a track or guide groove or recording mark string of the optical disc  100 , and a division line along a tangential direction orthogonal to the radial direction.  
      The characteristics, such as the form, the ratio of area, the number of divisions and the direction of diffraction, required by the diffraction element  17  can be optionally set by combining with the layout of the light-receiving area of the photodetector  14 , as long as the diffraction element can improve the S/N of a tracking error signal obtained by a phase difference detection method (DPD, a first signal detection method) and a push pull method (PP, a second signal detection method) used to detect a tracking error, when reproducing information recorded on an optical disc having a track with different pitches, and a compensated tracking error signal (CPP, obtained by a third signal detection method); as long as the diffraction element can be used to detect a fourth signal to detect a signal used as a focus error signal, and to detect a fifth signal to detect a signal used as a signal for correction of a disc tilt and a spherical aberration (disc thickness unevenness); and as long as the diffraction element can detect a reflected beam from an optional recording layer of an optical disc having two or more recording layers.  
      The size of the boundary circle defined in the diffraction element  17  shown as the magnified part Y in  FIG. 2B  is determined based on the pitch of the guide groove (track) previously formed on the recording surface of an optical disc (recording medium) reproducible by the optical disc apparatus  1 .  
      When a reproducible optical disc is of a common DVD standard, for example, the track pitch is 0.68 μm, for example.  
      If a reproducible optical disc is of a HD DVD standard with the recording density higher than a current DVD standard optical disc, the track pitch in the track of data area is 0.3-0.7 μm, for example, 0.34-0.44 μm, typically 0.40 μm in many cases. In an optical disc of HD DVD standard, the track pitch in a system lead-in area is set to 0.68 μm.  
      Therefore, although not shown in the drawing, the diameters of the concentric boundary circles of the diffraction element shown in  FIGS. 2A and 2B  are defined in the area which includes the area where diffracted rays of a laser beam reflected from a track with a wide pitch (e.g., 0.68 μm) are overlapped, and include no diffracted rays of a laser beam reflected from a track with a narrow pitch (e.g., 0.40 μm).  
      The characteristics required by the diffraction element  17  shown in  FIG. 2B  is magnified as the part Y in  FIG. 2A  are not particularly restricted, as long as the diffraction element can divide a reflected ray from an optionally recording layer of the optical disc  100 , so that the luminous flux at the center of the reflected ray (the main light beam, or the component passing through substantially the center of the object lens  11 ) coincides with the center of division, at least in the radial and tangential directions. Making the diffraction element as concentric circles is useful for generating a light beam, which is divided at a predetermined distance (a radius) from the center of the divisions in the radial and tangential directions (for the compensated Push Pull (TE), tilt detection, or spherical aberration correction).  
      For example, a first FE (Focus Error) signal can be generated by the well-known astigmatic aberration method by using the light diffracted by the pattern inside the boundary circle (defining the area of the inside circle), and a second FE (Focus Error) signal can be generated by the astigmatic aberration method by using the light diffracted by the pattern outside the boundary circle, and sa (a spherical aberration correcting signal) explained hereinafter can be obtained by using the difference between the obtained focus error signals.  
       FIG. 3  shows the detail of the pattern of a light-receiving area of the photodetector  14 . The diffracting direction of each light beam diffracted by the diffraction element  17  and guided to each light-receiving area of the photodetector can be optionally defined. Here, a reflected laser beam passing through the circle area s of the diffraction element  17  is diffracted to a 4-divided pattern (A, B, C, D on the right-hand side of  FIG. 3 ), and a reflected laser beam passing through the circular area t is diffracted to a 4-divided pattern of a photodetector (E, F, G, H on the left-hand side of  FIG. 3 ). When the laser beam is diffracted by this method, a servo signal is obtained as follows.  
      As a signal obtained by combining the output of each light-receiving area of the photodetector  14 , there are a focus error signal FE (by the astigmatic aberration method), a tracking error signal PP (TE) by the Push-Pull method, a tracking error signal DPD (TE) by the difference phase detection method, a tracking error signal CPP by the compensated track error (Compensated Push-Pull method) considering the influence of the lens shift of the object lens  11 , and a spherical aberration error signal (sa). Assuming the outputs from the light-receiving areas A-H of the photodetector  14  to be SA to SH, these signals are obtained by  
       FE   =       (     SA   +   SC     )     -     (     SB   +   SD     )           
           PP   ⁡     (   TE   )       =       (     SE   +   SF     )     -     (     SH   +   SG     )         ,       or   ⁢     
     (     SA   +   SE   +   SB   +   SF     )     -     (     SD   +   SC   +   SH   +   SG     )           
         DPD   ⁡     (   TE   )       =       p   ⁢           ⁢     h   ⁡     (     SA   +   SE   +   SC   +   SG     )         -     p   ⁢           ⁢     h   ⁡     (     SB   +   SF   +   SD   +   SH     )               
               CPP   ⁡     (   TE   )       =       ⁢       (     SE   +   SF     )     -     (     SH   +   SG     )     -                     ⁢     k   ⁢           ⁢     1   ⁡     [       (     SA   +   SB     )     -     (     SD   +   SC     )       ]                   
             sa   =       ⁢       (     SA   +   SC     )     -     (     SB   +   SD     )     -                     ⁢     k   ⁢           ⁢     2   ⁡     [       (     SE   +   SG     )     -     (     SF   +   SH     )       ]                   
 
      k (k 1 ≠k 2 ) is an optional constant (a correction coefficient determined based on the factors, such as the wavelength and intensity of a laser beam from a light source, and the divisions of an area of a diffraction element, and either positive or negative).  
      As explained hereinbefore, by using the light-receiving optical system defined by the invention, it is possible to improve the S/N of a tracking error signal (PP) obtained by the Push Pull method and a tracking error signal (DPD) obtained by the phase difference detection method, used to detect a tracking error when reproducing information recorded on an optical disc (a recording medium) having a track with two or more different pitches, and a compensated tracking error signal (CPP); and it is possible to easily obtain various signals usable for detection of signals for correction of focus error, and spherical aberration (disc thickness unevenness). The characteristics of the diffraction element, such as the diffraction pattern, the number of divisions and the direction of diffraction, can be easily set.  
      As explained here, according to the invention, a diffraction pattern of a diffraction element to guide an optional number of reflected laser beams divided into a predetermined number to a photodetector is combined preferably as one unit, and it is easy to design an optical head unit to obtain a focus error signal, a track error signal, a track error signal for correction (in a system with a lens shift), and a reproducing signal (RF), from a reflected laser beam from an optical disc.  
      Particularly, when reproducing a signal from various optical discs with different pitches of a track or a string of record marks peculiar to each optical disc, it is possible to obtain an optical head difficult to be influenced by the pitches of a track or a string of record marks.  
      Namely, it is unnecessary to completely divide an area of a reflected ray from a recording medium (an optical disc) for FE (detection of a focus error), TE (detection of a track error), etc., and the flexibility of designing an optical head unit is enlarged.  
      Further, an optical head unit is easily applicable to several types of recording medium, and particularly a 3-wavelength compatible optical head unit can be easily configured.  
      Moreover, detection of spherical aberration (sa) is possible. If a spherical aberration correction mechanism is separately provided, spherical aberration by a multilayer structure can be corrected.  
      According to at least one of the embodiments of the invention, it is possible to define a diffraction pattern of a photodetector which can preferably take out a reflected beam from optical discs of various standard (kinds) to meet the type of signal to be extracted, and a diffraction pattern of a diffraction element or a hologram polarization element to guide a reflected laser beam divided into a predetermined number by the photodetector.  
      Therefore, it is possible to simplify a pattern of an arrangement of a light-detecting area of a photodetector to extract a signal from a reflected laser beam from an optical disc, according to the kinds and standards of an optical disc.  
      Therefore, an optical head unit and an optical disc apparatus with stable characteristics can be obtained at low cost.  
      While certain embodiments of the inventions have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel methods and systems described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the methods and systems described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.