Patent Publication Number: US-2007097835-A1

Title: Optical pick-up having a rotary arm actuator

Description:
TECHNICAL FIELD OF THE INVENTION  
      The present invention relates to an optical pick-up having a first stationary part and a second pivoting part, wherein the second part is provided with an optical system for defining a beam path for a laser beam from a laser source towards an optically readable information carrier. The invention also relates to an optical drive comprising such a pick-up device.  
     BACKGROUND OF THE INVENTION  
      Optical pick-ups can be designed in the form of a pivoting optical device, in which a laser diode is mounted in a stationary structure separate from the actual swing arm of the pivoting device. The provision of the laser diode in the swing arm itself would present some major disadvantages, such as increased arm mass, complicated handling of the heat generated by the laser diode, complex wiring, etc. Therefore, it is often preferred to have the laser diode mounted in a stationary structure separate from the swing arm.  
      In an optical pick-up as described above, different parts of the asymmetric emission from the laser diode are focused on the information carrier for different positions of the swing arm. This can be accomplished by having the fast axis of the laser diode (i.e. the most divergent dimension) parallel to the plane in which the swing arm pivots during scanning of the information carrier, i.e. the plane of the information carrier. A small motion in the orthogonal direction is also allowed for, by virtue of the divergence of the emission from the laser diode in this direction. Light emitted by the laser diode travels along the optical system in the pivoting part of the pick-up, and is directed towards the information carrier by means of a folding mirror.  
      For this kind of optical pick-up, focus error detection can be accomplished by the what is called Foucault method using a double wedge or roof prism. This method is well known to those skilled in the art.  
      However, no push-pull tracking-error signal can be produced in such case even if a double-roof prism is incorporated and the detecting photo diodes are divided both laterally and vertically. This kind of division can only be used for generating a signal for the rotation angle of the swing arm, which can be employed for controlling the driving of the arm.  
      Thus, there is a problem in the prior art concerning how to generate a push-pull tracking-error signal for rotary arm actuators of the above-mentioned kind.  
     SUMMARY OF THE INVENTION  
      An object of the present invention is therefore to provide a solution by which one-beam push-pull tracking can be performed in an optical pick-up using a rotary arm actuator.  
      This object is met by an arrangement and an optical drive as set forth in the claims that follow.  
      Hence, according to the present invention, an optical pick-up is provided in which there is an optical system for defining a beam path from a laser source to an optical focusing unit for optical read-out of an information carrier. The optical system is mounted in a part of the device that is pivoting about a first pivot axis. A first folding mirror is provided for folding the beam path by 90° in a plane which is parallel to said pivot axis, and a second folding mirror is provided for folding the beam path by 90° in a plane that is orthogonal to said pivot axis. In this way, the light beam reflected from the information carrier is rotated such that is it imaged onto a detection unit rotated by 90°. Thereby, any push-pull asymmetry in the reflected beam can be detected in order to generate a push-pull tracking-error signal. Splitting the reflected beam by means of a roof-prism will give different power in the two portions of the beam, thus allowing the generation of this tracking-error signal.  
      In one embodiment of the invention, a double roof-prism is used for dividing the reflected beam into four sub-beams, together with a detection unit divided both laterally and vertically. In addition to the push-pull tracking-error signal, the device is then capable of generating a rotation angle position signal which can be used for controlling the pivot position of the swing arm.  
      Hence, the basic idea of the present invention is the use of an extra folding mirror in the optical beam path in order to rotate the reflected beam by 90°, such that any push-pull asymmetry in the beam can be detected. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The detailed description that follows will be better understood when read in conjunction with the accompanying drawings, in which:  
       FIG. 1  is a schematic perspective view of an arrangement for an optical pick-up according to the present invention;  
       FIG. 2  is a schematic side view in cross-section, illustrating a detail of  FIG. 1 ;  
       FIG. 3  is a diagram showing the dimensional relation between a collimating lens of the optical pick-up and a far field radiation pattern from a laser diode;  
       FIG. 4  schematically shows a perspective view of a polarizing beam splitter with a roof-prism and the arrangement of an extra folding mirror according to the present invention; and  
       FIG. 5  schematically shows a perspective view of a polarizing beam splitter with double roof-prism, according to the present invention. 
    
    
      On the drawings, like parts are designated by like reference numerals throughout.  
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
      By way of introduction, a pivoting optical device of a general kind will be described with reference to  FIG. 1 .  
      In  FIG. 1 , a pivoting optical device is shown in the form of an optical disc drive of general design. The optical disc drive of  FIG. 1  comprises a base plate  1  supporting a spindle motor  3  for rotating an optical disc  5  about a spindle axis  7 . The optical disc  5  has an information-carrying surface at its lower side. A peripheral outer surface  11  of the spindle motor  3  has a pivoting optical device attached to it, spaced from the base plate  1 . It comprises a first part  15  and a second part  17 . The second part  17  comprises an optical system which is pivotally movable relative to the first part  15  about a first pivot axis  19 , said optical system defining an optical laser beam path  21  the general direction of which is indicated by a dash-dot line and generally extends in the longitudinal direction of the second part  17 . Bearing means  23  are provided in order to provide the pivotability to the second part  17 . A laser source  25  is attached to the first part  15  for emitting a laser beam  27  in the general longitudinal direction of the second part  17  along the beam path  21  (see  FIG. 2 ).  
      The laser source  25  is located substantially on the optical laser beam path of the second part  17 . To this end, the bearing means  23  presents an open center region  29  in order to allow the laser beam  27  to pass from the laser source  25  into the second pivoting part  17 .  
      The second part  17  is also pivoting relative to the first part  15  about a second pivot axis  31 , substantially orthogonally intersecting the first pivot axis  19  at a point of intersection P. The laser source  25  is located substantially at this point of intersection P.  
      Suitably, the bearing means is of the gimbal type, comprising an intermediate bearing element  33  which is pivotally supported by the first part  15  and which in turn pivotally supports the second part  17 . The point of intersection P is located at the center point of the intermediate bearing element  33 .  
      The laser source  25  is a semiconductor laser diode unit of a type known per se. The laser source  25  emits radiation having a far field radiation pattern  35  which is generally elliptical, as indicated in  FIG. 3  which shows a transverse cross-section of the beam  27 . The elliptical far field pattern has a major axis  35 L and a minor axis  35 S. The laser source  25  is arranged in such a manner that the major axis  35 L is generally parallel to the second pivot axis  31 , and the minor axis  35 S is generally parallel to the first pivot axis  19 .  
      The optical system of the second part  17  comprises an optical collimator in the form of a collimating lens  37  at the point of entry of the laser beam  27  into the second part  17 . The collimating lens  37  is positioned entirely within the elliptical far field pattern  35  of the radiation emitted by the laser source  25  at all pivotal positions of the second part  17 . This arrangement of the collimating lens is schematically shown in  FIG. 3 .  
      The pivoting optical device described so far is a swing arm for supporting an optical focusing unit  39  close to its free end  41  for reading/writing information from/to the information surface  9  of the optical disc  5 . The second part  17  is a rigid swing arm for pivoting, scanning motion about a swing axis constituted by the first pivot axis  19 , and for pivoting, focusing motion about a focusing axis constituted by the second pivot axis  31 . As mentioned above, the focusing axis intersects the swing axis substantially orthogonally at the point of intersection P for moving the optical pick-up unit  39  in substantially orthogonal focusing and scanning directions F and S, respectively, relative to the information surface  9  on the optical disc  5 . Hence, the major axis  35 L of the far field radiation pattern  35  is generally parallel to the focusing axis  31 , and the minor axis  35 S thereof is generally parallel to the swing axis  19 .  
      The embodiment of a swing arm device shown in  FIG. 1  is of a type in which the second part is a rigid swing arm structure  17  that pivotally moves as a whole about the swing axis  19  and the focusing axis  31 . To enable these pivotal movements, magnetic scanning and focusing means are provided, comprising the first part  15  which is of magnetically permeable material and acts as a stator structure and a number of movable magnetic coils  45 ,  47 A,  47 B, which are provided at the free end  41  of the swing arm structure  17  for scanning and focusing, respectively. The movable magnetic scanning coil comprises a cylindrical scanning coil  45  having a generally rectangular shape in cross-section and having a central opening  49 . The movable focusing coils are two substantially identical cylindrical focusing coils  47 A,  47 B, having a generally rectangular shape in cross-section. The scanning coil  45  has been bonded at an outer side surface against the free end  41  of the swing arm structure  17  in a position in which the central axis thereof is generally parallel to the scanning movements S of the swing arm structure. Each focusing coil  47 A,  47 B has been bonded at a portion of their outwardly facing axial end surface against an outer side surface of the scanning coil  45 , which is remote from the swing arm structure  17 , and the two focusing coils  47 A,  47 B are disposed in the manner generally shown in  FIG. 1 . The first part  15  supports stationary magnetic means comprising an elongate permanent magnet  51  facing the focusing coils  47 A,  47 B and spaced from them by an air gap. The magnetically permeable stator or first part  15  has a stator part  53  passing through the central opening  49  of the scanning coil  45  with clearance. The permanent magnet is magnetically polarized in a radial direction relative to the swing axis  19 , and the arrangement is such that a substantially radially directed permanent magnetic field is established across the air gaps which are present between the scanning coil  49  and the stator part  53  and between the focusing coils  47 A,  47 B and the stator  15 , respectively. The stator  15  is rigidly associated with the spindle motor  3 .  
      The stator core  15  and supporting portions of the gimbal-type bearing means  23  are suitably integrated into a combined unit. Such a combined unit is made of a suitable magnetically permeable material such as soft iron, and comprises a temporarily removable part, namely the part  53 , to enable insertion of the scanning coil  45  into the central opening  49 . This combined unit is provided with an interconnecting supporting beam part  55  carrying the bearing means  23  near its free end and may be comprised of a stack of stator laminations which may be integrated with the motor stator of the spindle motor  3 .  
      As can be seen from  FIG. 1 , the first part  15  is generally U-shaped in plan view at its free end  57 , comprising two legs  59 ,  61  and a connecting part  63 . Pivoting pins  65 ,  67  pivotally support the intermediate part  33  in the legs  59 ,  61 . In turn, the second part  17  is pivotally carried by the intermediate bearing part  31  by two pivoting pins  69 ,  71 . The laser diode is inserted in a matching opening in the connecting part  63  of the U-shaped end of the first part  15  in such a way that the active diode surface is situated at the point of intersection P of the swing axis  19  and the focusing axis  31  of the second part  17 .  
       FIG. 3  shows a projection of the collimating lens  37  in the form of a circular shaded area, projected onto the local far field radiation pattern shown as a differently shaded area. The projection of  FIG. 3  shows an orthogonal plane through the plane of the collimating lens  37 . The collimating lens remains within the boundaries of the far field radiation pattern  35  in all operational positions of the swing arm  17 . The focusing amplitudes of the swing arm for optical disc drives are much smaller than the swing amplitudes in the orthogonal plane. The elliptical far field pattern of the radiation from a laser diode is therefore well suited for a pivoting optical device in optical disc drives.  
      Typically, a polarizing beam splitter  110  or some other light-deflecting means will be provided between the laser diode  25  and the collimating lens  37 , in order to direct light reflected from the information carrier towards an array of photodiodes or photodetector or the like referenced by  112  in  FIG. 4  for read-out. To accommodate for the displacement of the image on the photodiodes when the swing arm pivots about its pivot axis, the photodiodes  112 A are divided into sections parallel to the swing plane of the swing arm. It is to be noted that rotation of the swing arm leads to a displacement of the image on the photodiodes in the same plane as the swing arm rotation. The image on the photodiodes does not move in the orthogonal direction.  
      In order to have the possibility of generating a push-pull tracking-error signal that is independent of the rotation of the swing arm, an extra folding mirror  122  is provided. Hence, one folding mirror  121  has the purpose of directing the laser beam  27  onto the information carrier, and the extra folding mirror  122  has the purpose of rotating the beam such that it is imaged onto the photo-detectors rotated by 90° compared to a situation where there is no extra folding mirror. In other words, the first folding mirror  121  is provided for folding the optical beam path by 90° in a plane which is parallel to the swing axis  19  of the device, and the second folding mirror  122  is provided for folding the beam path by 90° in a plane which is orthogonal to this swing axis  19 . In this way, the photo-detectors can be used for generating the push-pull signal required for proper tracking of the information carrier. A difference in light intensity between two opposite edges of the laser beam after reflection from the information carrier (due to a tracking error) can now be resolved through a difference in signal strength from two adjacent photo-detectors. The arrangement of two folding mirrors  121 ,  122  is indicated in  FIG. 1 , but is better seen from  FIGS. 4 and 5 .  
      It should be pointed out that, when using a polarizing beam splitter (PBS), a quarter-wave plate (λ/4-plate) is typically positioned between said PBS and the information carrier. In this case, the λ/4-plate should be positioned between the second folding mirror and the objective lens.  
      Preferably, the collimating lens employed in the optical system has an aspherical surface in order to provide adequate collimation of the emission from the laser diode.  
      When a polarizing beam splitter (PBS)  110  is arranged between the laser diode  25  and the collimating lens  37 , the collimating lens will only be perpendicular to the beam splitter at one position. For any other orientation or rotation of the second, pivoting part, the collimating lens  37  will have an angle with respect to the PBS  110 . This will give rise to aberrations, mainly astigmatism. However, it is quite straightforward to compensate for this, for example by selecting appropriate numerical apertures for the collimating lens and/or by providing an aspherical surface in front of the PBS.  
      In a further embodiment of the present invention, shown in  FIG. 5 , the set-up is provided with a double roof-prism between the PBS and the photodiodes referred to by  112 B, together with a division of the photodiodes  112 B both laterally and vertically. In addition to the push-pull tracking-error signal, the inventive device is then capable of also providing a rotation angle position signal, which can be used for controlling the swing arm position.  
      Although a bearing means  23  having a generally circular shape has been described, it is to be understood that other shapes and types may be employed for the bearing means. For example, the gimbal-type bearing may be of rectangular shape. Other examples of bearing means include spherical bearings. It will be understood that the present invention can be employed regardless of the type of bearing, provided that the pivoting functions described above are implemented.  
      In conclusion, an optical read-out device has been disclosed, comprising a first (stationary) part and a second (pivoting) part. The second part carries an optical system and is pivotally movable relative to the first part about a first pivot axis, wherein the optical system defines a beam path for a laser beam in a generally longitudinal direction of the second part. Further, a laser source is located substantially at the point where the first pivot axis and the beam path intersect. In order to direct the laser beam from the diode laser onto an optically readable information carrier and at the same time allow the generation of a push-pull tracking-error signal, a first folding mirror is provided for folding the beam path by 90° in a first plane which is parallel to the first pivot axis, and a second folding mirror is provided for folding the beam path by 90° in a second plane which is orthogonal to the first pivot axis.  
      Moreover, a double roof-prism can be employed together with a photodetector array in order to simultaneously generate both the tracking-error signal and a rotation angle position signal for the swing arm.  
      Hence, a pivoting optical readout device has been described in which two folding mirrors  121 ,  122  are used in an optical path  21  between an optical information carrier  9  and a photodetector unit  112 A (or  112 B), for rotating a beam of light reflected from the information carrier by 90° such that a push-pull error-tracking signal can be generated.