Abstract:
An optical pickup actuator independently drivable in focusing, tilting and tracking directions. The optical pickup includes a lens holder movably installed on a base, and mounted with an objective lens for transmitting an incident light to an optical information recording medium; and a magnetic circuit to independently driving the lens holder in the focusing, tilting and tracking directions. The magnetic circuit includes opposing magnets; a focusing coil and tilting coils interposed between the magnets, mounted inside the lens holder, and wound around respective axes parallel to the focusing direction; and a plurality of tracking coils mounted outside the lens holder, interposed between the magnets and wound around respective axes transverse to the focusing direction. The magnetic fields of the magnets are guided by outer yokes partially encompassing the magnets and inner yokes positioned within the focusing coil and the tilting coils.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of Korean Application No. 2005-31093, filed Apr. 14, 2005, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     Aspects of the present invention relate to an optical pickup which records and/or reproduces information onto and/or from a disc type optical information storage medium. More particularly, aspects of the present invention relate to an optical pickup actuator and an optical recording and/or reproducing apparatus having the same.  
         [0004]     2. Description of the Related Art  
         [0005]     An optical recording and/or reproducing apparatus is provided with an optical pickup which records optical information on or reproduces the recorded optical information from an optical recording medium by irradiating a laser beam onto the optical recording medium, such as a compact disk (CD), a digital versatile disk (DVD), etc.  
         [0006]     The optical pickup requires an objective lens, which focuses an incident light to the optical recording medium, and an actuator, which drives the objective lens in a focusing direction and a tracking direction.  
         [0007]     The actuator includes a base, a lens holder on which the objective lens is mounted, and a magnetic circuit for driving the lens holder in the focusing direction and the tracking direction, while supporting the lens holder to move with respect to the base.  
         [0008]     The magnetic circuit includes focusing coils driving the objective lens in the focusing direction, tracking coils driving the objective lens in the tracking direction, and magnets installed, facing the coils for interactions. The actuator drives the focusing coils and the tracking coils independently to control movement of the objective lens in the focusing direction and the tracking direction.  
         [0009]     In recent years, attention has been focused on developing and establishing a standard for DVDs holding a large amount of data (greater than 4.7 GB per side) and high definition (HD)-DVDs having higher density than the 4.7 GB and capable of recording high definition moving images (greater than 23 GB per side). In case of the high-density optical recording medium, the thickness of an optical recording medium needs to be reduced as the NA (numerical aperture) of an objective lens is increased. For instance, although the thickness of a conventional CD is about 1.2 mm, the thickness of a conventional DVD is about 0.6 mm. Thus, it is highly possible that a next generation HD-DVD may be as thin as about 0.1 mm.  
         [0010]     However, as the rotation speed increases, the high-density, thin optical recording medium generates eccentricity problems due to manufacturing errors. As a result of the eccentricity, the optical recording medium may not rotate parallel to a plane, but may deformed in a direction transverse to the plane.  
         [0011]     Since focusing control and tracking control of an objective lens in corresponding directions are not sufficient to accurately focus a light to a recording side of an optical disk, there is a need to develop an actuator, which is capable of driving the objective lens not only in the tracking direction, but also in the focusing and tilting directions.  
       SUMMARY OF THE INVENTION  
       [0012]     An aspect of the present invention is to provide an optical pickup actuator with a simple structure, which drives an objective lens in a tilt direction and thus provides high sensitivity, and an optical recording and/or reproducing apparatus having the same.  
         [0013]     To achieve the above and/or other aspects and advantages of the present invention, there is provided an optical pickup actuator, including: a base, a lens holder movably installed on the base, and mounted with an objective lens emitting a light to an optical information recording medium, and a magnetic circuit for independently driving the lens holder in a focusing direction, a tilting direction and a tracking direction.  
         [0014]     According to an aspect of the present invention, the magnetic circuit includes: a focusing coil installed on the lens holder; a pair of tilting coils installed on the lens holder, and driving independently of each other; at least one pair of tracking coils installed on the lens holder, and driving independently of each other; and magnets interacting with the focusing coil, the tilting coils and the tracking coils, thereby generating a driving force in the focusing direction, the tilting direction and the tracking direction.  
         [0015]     The magnets may be arranged on both sides of the lens holder, and may comprise a pair of axially magnetized magnet which are magnetized in unipolar fashion in a perpendicular direction to the tracking direction of the optical information storage medium.  
         [0016]     One of the focusing coils may be installed on the lens holder and wound around an axis of the objective lens.  
         [0017]     One pair of tilting coils may be installed, and the one pair of tilting coils may be spaced apart by a predetermined distance from each other in a tracking direction of the optical information storage medium.  
         [0018]     The focusing coil and the tilting coils may be layered up in the focusing direction.  
         [0019]     The focusing coil may be disposed under the tilting coils with respect to the focusing direction, and the focusing coil may have a greater thickness than the tilting coils in the focusing direction.  
         [0020]     Operating sides of the focusing coil and the tilting coils may be arranged in parallel with the tracking direction of the optical information storage medium.  
         [0021]     The tracking coils may be installed in parallel pairs on outer sides of the lens holder along the tracking direction of the optical information storage medium.  
         [0022]     The magnets may comprise a pair of axially magnetized magnets disposed on opposite sides of the lens holder to generate an electromagnetic force to operating sides of the tracking coils which are parallel to the focusing direction.  
         [0023]     Each axially magnetized magnet of the pair may be arranged to have a magnetic flux of an opposite direction from the other axially magnetized magnet of the pair.  
         [0024]     The magnetic circuit may further include: inner yokes installed inside the focusing coil and the tilting coils; and outer yokes installed outside the focusing coil and the tilting coils.  
         [0025]     The outer yokes may be installed on the base and support a respective one of the axially magnetized magnets.  
         [0026]     The outer yokes may be installed to partially encompass an exposed outside of each axially magnetized magnet in the tracking direction and thereby provide a magnetic flux of an opposite direction of the axially magnetized magnet.  
         [0027]     The outer yokes may be installed to encompass a part of the axially magnetized magnets to generate a driving force in the tracking direction, facing each of the operating sides of the tracking coils, which are spaced apart in the tracking direction, among the operating sides of the adjacent tracking coils.  
         [0028]     The inner yokes may comprise: a first inner yoke pair disposed inside the focusing coil and one of the tilting coils; a second inner yoke pair disposed the inside of the focusing coil and the other tilting coil; and a third inner yoke pair disposed between the first and the second inner yoke pairs.  
         [0029]     The first, second, and third inner yoke pairs may be arranged at regular intervals in the tracking direction.  
         [0030]     The focusing coil and the tilting coils may be attached to an inner wall of the lens holder.  
         [0031]     The tracking coils may be installed on outer surfaces of the lens holder.  
         [0032]     Another aspect of the present invention provides an optical pickup actuator, including: a base, a lens holder to mount an objective lens for recording and/or reproducing information onto and/or from an optical information storage medium; a support member supporting the lens holder to be movable with respect to a base; and a magnetic circuit independently driving the lens holder in a focusing direction and a tilting direction.  
         [0033]     The magnetic circuit may include: a focusing coil wound around the focusing direction; a pair of tilting coils installed on opposite sides of the objective lens on the lens holder and spaced out in a tracking direction of the optical information storage medium; and a pair of axially magnetized magnets installed on opposite sides of the lens holder to be parallel to the tracking direction of the optical information storage medium.  
         [0034]     The focusing coil may be layered under the tilting coils with respect to the focusing direction.  
         [0035]     The pair of axially magnetized magnets may be arranged so that opposed sides thereof have a same magnetic flux.  
         [0036]     The magnetic circuit may further include: two tracking coil pairs, each pair being installed on the opposed sides of the axially magnetized magnets of the lens holder to drive the lens holder in the tracking direction.  
         [0037]     The tracking coils may be arranged in such a manner that adjacent sides among parallel sides in the focusing direction function as effective coils by an interaction with the axially magnetized magnets.  
         [0038]     The magnetic circuit may further include: three inner yoke pairs installed inside the focusing coil and the tilting coils; and a pair of outer yokes installed on the base and supporting each of the axially magnetized magnets.  
         [0039]     A sectional shape of the outer yokes in the focusing direction may be U-shaped to partially encompass the axially magnetized magnets.  
         [0040]     Parts extended from the outer yokes to be adjacent to the axially magnetized magnets in the tracking direction may be arranged to face the operating sides of the tracking coils, which are spaced apart in the tracking direction.  
         [0041]     Another aspect of the present invention provides an optical recording and/or reproducing apparatus, including: an optical pickup having an objective lens, and an actuator driving the objective lens, the optical pickup moving in a radial direction of an optical information storage medium for recording and/or reproducing information onto and/or from the optical information storage medium; and a controller for controlling a focusing servo, a tracking servo, and a tilting servo of the optical pickup. The actuator may include: a lens holder mounting the objective lens; and a magnetic circuit independently driving the lens holder in a focusing direction, a tilting direction and a tracking direction.  
         [0042]     The magnetic circuit may comprise: a pair of tilting coils installed on the lens holder, being spaced away from each other along the tracking direction of the optical information storage medium; a focusing coil installed on the lens holder, being layered under the tilting coils in the focusing direction; a plurality of tracking coil pairs installed on an outside surface of the lens holder and being parallel to the tracking direction; a pair of axially magnetized magnets disposed on both sides of the lens holder to have a magnetic flux of opposite directions; a pair of outer yokes partially encompassing and supporting a part of the axially magnetized magnets; and a plurality of inner yoke pairs disposed inside the focusing coil and the tilting coils and being parallel to the tracking direction.  
         [0043]     Additional aspects and/or advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0044]     These and/or other aspects and advantages of the invention will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which:  
         [0045]      FIG. 1  is a schematic perspective view of an optical pickup actuator according to an embodiment of the present invention;  
         [0046]      FIG. 2  is a plan view of the optical pickup actuator of  FIG. 1 ;  
         [0047]      FIG. 3  is a perspective view of a magnetic circuit of the optical pickup actuator of  FIG. 1 ;  
         [0048]      FIG. 4  is a sectional view of a lens holder of  FIG. 1 ;  
         [0049]      FIG. 5A  and  FIG. 5B  are diagrams for explaining a focusing operation in which the lens holder is driven in a focusing direction;  
         [0050]      FIG. 6A  and  FIG. 6B  are diagrams explaining a focusing and a tilting operation in which the lens holder is driven in a focusing direction and a tilting direction, respectively;  
         [0051]      FIG. 7A  and  FIG. 7B  are diagrams explaining a tracking operation in which the lens holder is driven in a tracking direction; and  
         [0052]      FIG. 8  is a schematic diagram of an optical recording and/or reproducing apparatus having an optical pickup actuator according to an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0053]     Reference will now be made in detail to the present embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below in order to explain the present invention by referring to the figures.  
         [0054]     Referring to  FIGS. 1, 2  and  3 , an optical pickup actuator according to an embodiment of the present invention includes a base  10  having a support holder  11 , a lens holder  20  having a lens mounting hole  21  formed therein, on which an objective lens  30  is mounted, a suspension or support member  13  connecting the lens holder  20  and the support holder  11 , and a magnetic circuit driving the lens holder  20  in a focusing direction F, a tilting direction T, and a radial or tracking direction R independently of each other.  
         [0055]     The objective lens  30  is used for recording/reproducing information onto/from an optical disk with a predetermined recording density. The objective lens  30  may be designed to record/reproduce information onto/from a low-density optical disk, such as a DVD or a CD. Also, the objective lens  30  may be designed to record/reproduce information onto/from a higher density optical disk than a DVD, i.e., an HD-DVD or a Blu-Ray disk.  
         [0056]     The lens mounting hole  21  on which the objective lens  30  is mounted is provided at a central part of the lens holder  20 . As shown in  FIG. 4 , a resting shoulder  21   a  is formed at a relatively deep position of the lens mounting hole  21  from an upper side of the lens holder  20  facing an optical disk (not shown).  
         [0057]     In addition, the upper side of the lens holder  20  is opened by the lens mounting hole  21 , and a lower side of the lens holder  20  has a hex shaped opening through a coil mounting part  25 . A focusing coil  41  and tilting coils  42 ,  43  are formed inside of the coil mounting part  25 . The lens holder  20  may be made of injection molded plastic materials.  
         [0058]     The lens holder  20  is movably installed on the support holder  11  by the support member  13 . The support member  13  may be formed of elastically transformable spring wires having a predetermined rigidity. A connection board  28  to which the support member  13  is connected is provided on an outer surface of the lens holder  20 . For instance, the support member  13  may be soldered to the connection board  28 . Thus, a current may be supplied to the magnetic circuit through the support member  13 .  
         [0059]     The magnetic circuit drives the objective lens  30  independently in the focusing (F), tracking (R) and tilting (T) directions. As shown in  FIGS. 2-4 , the magnetic circuit includes a focusing coil  41 , a pair of tilting coils  42 ,  43 , two pairs of tracking coils  44 ,  45 , and magnets. The magnets may include a pair of unipolar (axially magnetized) magnets  61 ,  63 .  
         [0060]     The focusing coil  41  is wound around the objective lens  30  and about an axis parallel to the focusing direction (F). The focusing coil  41  may be wound on a rectangular coil form, and may be closely attached to an inner wall of the coil mounting part  25 . The focusing coil may be layered to have a predetermined height in the focusing direction. The focusing coil  41  drives the lens holder  20  in the focusing direction by an interaction between sides of the focusing coil  41  parallel to the R direction and the axially magnetized magnets  61 ,  63 . When a current is applied through the focusing coil  41 , the focusing coil  41  moves upwardly or downwardly along the focusing direction according to a direction of current flow through the focusing coil  41 , consequently moving the coil mounting part  25  and the objective lens  21  in the focusing direction.  
         [0061]     The tilting coils  42 ,  43  are spaced apart by a predetermined distance in the R direction. And, the tilting coils  42 ,  43  are arranged on opposite sides of the objective lens  30  to be symmetrical with each other in the R direction, are layered with the focusing coil  41  in the focusing direction, and are wound around respective axes extending in the focusing direction.  
         [0062]     These tilting coils  42 ,  43  are disposed on the focusing coil  41 , and are closely adhered to opposite inner walls of the coil mounting part  25 . The tilting coils  42 ,  43  may have a smaller thickness in the focusing direction than the focusing coil  41 . Thus, the force being applied in the focusing direction of the lens holder  20  is generated by the focusing coil  41 . Moreover, as currents are applied to the tilting coils  42 ,  43  in opposite directions from each other, each tilting coil  42 ,  43  generates a force of an opposite direction with respect to the focusing direction. In this manner, the tilting coils  42 ,  43  are driven independent of the focusing coil  41 , and move the lens holder  20  in the T direction using a relatively smaller force than the force generated by the focusing coil  41 .  
         [0063]     As the focusing coil  41 , and the tilting coils  42 ,  43  are formed in close contact with the inner walls of the coil mounting parts  25  of the lens holder  20 , a sidewall  20   a  of the lens holder  20  can be supported. Therefore, firmness of the sidewall  20   a  of the lens holder  20  is reinforced, enabling movement of a secondary resonance frequency at a high frequency and sufficient gain margin of the lens holder  20  in driving.  
         [0064]     The two pairs of tracking coils  44 ,  45  are provided on outer surfaces of the lens holder  20  in parallel relation with the R direction. The tracking coil pairs  44 ,  45  are effective coils whose parallel sides in the F direction are subject to a force in the R direction. The tracking coil pair  44  includes tracking coils  44   a  and  44   b  and the tracking coil pair  45  includes tracking coils  45   a  and  45   b . The tracking coils  44   a ,  45   a , face each other having the lens holder  20  therebetween, receive co-directional electrical current and generate force in a same direction. The tracking coils  44   b ,  45   b  face each other i having the lens holder  20  therebetween, and generate a force in the same direction when receiving a current in an opposite direction as the tracking coils  44   a ,  45   a.    
         [0065]     As shown in  FIGS. 7A and 7B , operating sides C 1 , C 2  of the tracking coils  44   a ,  44   b ,  45   a ,  45   b , which neighbor with each other in the R direction, are arranged to face the axially magnetized magnets  61 ,  63 . The operating sides C 3 , C 4 , which are spaced from each other in the R direction, are arranged to face outer yokes  73 , which will be described below. According to such structure, a force in the R direction is also generated at the operating sides C 3 , C 4  due to the influence of flux coming out of the axially magnetized magnets  61 ,  63  and moving toward the neighboring outer yokes  73 . That is, suppose that north polarity of each axially magnetized magnet  61 ,  63  faces the tracking coils  44 ,  45 . Then, the outer yokes  73  function as south polarity magnets. Therefore, the sides C 1 , C 2 , C 3  and C 4  of the tracking coils  44 ,  45  ( FIG. 7A ) parallel to the F direction can be used as effective coils, all receiving force in the R direction.  
         [0066]     Referring again to  FIGS. 2 and 3 , the axially magnetized magnets  61 ,  63  are disposed on opposite sides of the lens holder  20 , being parallel with the R direction. The axially magnetized magnets  61 ,  63  are arranged to have magnetic flux of opposite directions. In the embodiment shown with respect to  FIGS. 2 and 3 , north polarities of axially magnetized magnets  61 ,  63  face each other. Both axially magnetized magnets  61 ,  63  are involved with the independent movements of the lens holder  20  in the focusing, tilting and tracking directions, and the axially magnetized magnets  61 ,  63  are arranged for common use.  
         [0067]     The magnetic circuit includes three inner yoke pairs, i.e., first, second and third inner yoke pairs  71 ,  72  and  73 , respectively, and a pair of outer yokes  75 .  
         [0068]     The first inner yoke pair  71  is installed inside the tilting coil  42  and the focusing coil  41  on the base  10 . The first inner yoke pair  71  may be arranged at the effective coil portion where the focusing coil  41  and the tilting coil  42  face the axially magnetized magnets  61 ,  63 , namely, on opposite sides of an area parallel to the R direction. The first inner yoke pair  71  is made of a suitable material and may be made of a same metallic material as the base  10 , thereby being one body together. The first inner yoke pair  71 , while being separated from the focusing coil  41  and the tilting coil  42 , guides the lines of magnetic flux generated from the focusing coil  41  and the tilting coil  42  in the F and R directions to maximize an intensity of an effective magnetic field.  
         [0069]     The second inner yoke pair  72  is spaced apart by a predetermined distance from the first inner yoke pair  71  in the R direction. The second inner yoke pair  72  is disposed inside of the focusing coil  41  and the tilting coil  43 . However, the configuration and function of the second inner yoke pair  72  are similar to the configuration and function of the first inner yoke pair  71  and a detailed description thereof will not be repeated.  
         [0070]     The third inner yoke pair  73  is disposed between the first inner yoke pair  71  and the second inner yoke pair  72 . The third inner yoke pair  73  faces sides of the focusing coil  41  which are parallel to the R direction. Thus, the third inner yoke pair guides the lines of magnetic flux generated from the focusing coil  41  in the F direction to maximize the intensity of an effective magnetic field. The third inner yoke pair  73  may be made of a same material and shape as the first inner yoke pair  71 . The base  10  may be separately formed or may be integrally formed with one, two or all of the first, second and third inner yoke pairs.  
         [0071]     The inner yoke pairs  71 ,  72 , and  73  are installed by insertion through the opened bottom of the lens holder  20 , that is a side opposite the lens hole  21 , and the inner yoke pairs  71 ,  72  and  73  remain separated inside the lens holder  20 .  
         [0072]     The outer yoke pair  75  is fixed at the base  10 , and may be connected to the base  10  as one body. The outer yokes  75  are arranged on a surface of each of the axially magnetized magnets  61 ,  63  that faces away from the lens holder  20 . One of the outer yoke pair  75  supports each axially magnetized magnet  61 ,  63 , respectively. The outer yoke pair  75  guides the lines of magnetic flux generated from each axially magnetized magnet  61 ,  63 , and focuses the magnetic flux to the lens holder  20 , thereby maximizing the intensity of an effective magnetic field.  
         [0073]     A sectional shape of the outer yokes may be “U” shaped. That is, each yoke of the outer yoke pair  75  has a pair of extended parts  75   a  that extend in a direction perpendicular to the F direction to partially encompass opposite sides of a respective one of the axially magnetized magnets  61 ,  63 . As shown in  FIG. 7A , the extended parts  75   a  are installed, facing the sides C 3 , C 4  which are separated from each other in the R direction of the tracking coils  44 ,  45 . Therefore, the sides C 1 , C 2  adjacent to the tracking coils  44 ,  45  receive force generated from the interaction between the tracking coils  44 ,  45  and the axially magnetized magnets  61 ,  63 , whereas the sides C 3 , C 4  receive force in the R direction by a magnetic force applied to each extended part  75   a . In the embodiment of the present invention, since north (N) polarity of each axially magnetized magnet  61 ,  63  is faced with the tracking coils  44 ,  45 , the adjacent extended parts  75   a  in the R direction bear south (S) polarities. Therefore, by partially encompassing the outsides of the axially magnetized magnets  61 ,  63  in the tracking direction by extending the outer yokes, the force that the tracking coils  44 ,  45  receive in the R direction increases.  
         [0074]     Detailed operational effects of the optical pickup actuator according to the embodiment of the present invention shown in  FIG. 3  will now be described. First, the direction of a driving force generated by an interaction between the focusing coil  41  and the axially magnetized magnets  61 ,  63  is described. As can be seen in  FIG. 5A , the focusing coil  41  is disposed between the axially magnetized magnets  61 ,  63 . As shown in the drawing, if seen from the top, a current is applied to the focusing coil  41  in the counterclockwise direction. Since the north polarity of the axially magnetized magnet  61  faces the north polarity of the axially magnetized magnet  63 , the corresponding magnetic fields repel each other Where a current is applied to the focusing coil  41  in a counterclockwise direction, the sides of the focusing coil  41  parallel to the R direction receive force upwardly, according to Fleming&#39;s left hand rule. Consequently, as depicted in  FIG. 5A , the lens holder  20  and the objective lens  30 , and the driving unit including the focusing coil  41 , the tilting coils  42 ,  43  and the tracking coils move upwardly by the force (F) applied to the focusing coil  41 .  
         [0075]     Referring to  FIG. 5B , where a current is applied to the focusing coil  41  in a clockwise direction, the sides of the focusing coil  41  in parallel with the R direction receives force downwardly. Therefore, the position of the object lens  30  on the lens holder  20  in the F direction is adjustable by controlling the polarity and amount of a current applied to the focusing coil  41 .  
         [0076]     Referring to  FIG. 6A , a current is applied to the focusing coil  41  to drive the lens holder  20  upwardly, similar to  FIG. 5A . At the same time, currents of opposite directions are applied to the tilting coils  42 ,  43 . Then, the tilting coils  42 ,  43  receive force of the opposite directions with respect to the axially magnetized magnets  61 ,  63 , respectively. That is, the tilting coil  42  to which a current is applied in the clockwise direction receives a downwardly applied force with respect to the focusing direction. On the other hand, the tilting coil  43  to which a current is applied in the counterclockwise direction receives an upwardly applied force. Therefore, each tilting coil  42 ,  43  is driven independently of the focusing coil  41 , thereby tilting the lens holder  20  in the T 1  direction.  
         [0077]     As shown in  FIG. 6B , the lens holder  20  can be driven downwardly in the focusing direction by applying a current to the focusing coil  41  in the same direction as shown in  FIG. 5B . Where currents are applied to the tilting coils  42 ,  43 , respectively, in the opposite directions from the directions shown in  FIG. 6A , the tilting coil  42  receives an upwardly applied force, whereas the tilting coil  43  receives a downwardly applied force. Therefore, the lens holder  20  is driven downwardly by the focusing coil  41  and at the same time, the lens holder  20  is tilted in the T 2  direction by the tilting coils  42 ,  43 .  
         [0078]     Thus, in order to tilt the objective lens  30  mounted on the lens holder  20  in the tilting direction while moving the objective lens  30  in the focusing direction, it is necessary to apply a current to the focusing coil  41 , and adjust the polarity and amount of a current that is applied to each of the tilting coils  42 ,  43  in an opposite direction from each other. The lens holder  20  can also be tilted by applying currents of opposite directions to the tilting coils  42 ,  43  only, without applying a current to the focusing coil  41 .  
         [0079]     A tracking operation in which the lens holder  20  is driven in the tracking direction by an interaction between the tracking coils  44 ,  45  and the axially magnetized magnets  61 ,  63 , respectively, will now be described.  
         [0080]     As can be seen in  FIG. 7A , the axially magnetized magnets  61 ,  63  facing two pairs of tracking coils  44 ,  45  are arranged in such a manner that the north polarities of the magnets  61  and  63  face with each other. In this state, each tracking coil pair  44   a ,  44   b , and  45   a ,  45   b  has long sides C 1 , C 2 , C 3  and C 4  in the F direction. Among the long sides C 1 -C 4 , the adjacent sides C 1 , C 2  are arranged to face north polarities of the axially magnetized magnets  61 ,  63 , whereas the other sides C 3 , C 4  are arranged to face the extended parts  75   a  of the outer yoke pair  75  corresponding to south polarity.  
         [0081]     Next, currents of opposite directions are applied to the tracking coil pairs  44   a ,  44   b , and  45   a ,  45   b , respectively, as shown in  FIG. 7A . Then, currents are equally applied to the adjacent sides C 1 , C 2  in the downward direction, whereas currents are applied to the sides C 3 , C 4 , which are spaced apart, in the upward direction. Here, suppose that the direction of a magnetic field from the axially magnetized magnets  61 ,  63  is in the F direction. Then, each side C 1 , C 2 , C 3  and C 4  receives force in a direction opposite the R direction.  
         [0082]     As shown in  FIG. 7B , where currents are applied to the tracking coil pairs  44   a ,  44   b , and  45   a ,  45   b , respectively, in opposite directions from the directions shown in  FIG. 7A , force in the direction R is generated to each side C 1 , C 2 , C 3  and C 4 .  
         [0083]     Therefore, the object lens  30  mounted on the lens holder  20  may be driven in the R direction, according to the polarity and amount of a current applied to the tracking coils  44 ,  45 .  
         [0084]     As set forth above, the focusing coil  41  and the tilting coils  42 ,  43  are independently installed and driven to control the lens holder  20  in the focusing and tilting directions. In this way, the adaptability of the objective lens to the system, and the sensitivity of the optical pickup may be improved.  
         [0085]     Moreover, the rigidity of the injection molded lens holder may be reinforced by tightly attaching the focusing coil and the tilting coils to the inside of the lens holder. Hence, the sensitivity of the optical pickup can be increased even at a high speed, and the movement of a secondary resonant frequency at a high frequency is secured sufficiently.  
         [0086]      FIG. 8  is a schematic diagram showing an optical recording and/or reproducing apparatus to which the optical pickup actuator according to embodiments of the present invention may be applied.  
         [0087]     Referring to  FIG. 8 , the optical recording and/or reproducing apparatus includes a spindle motor  110  rotating an optical medium such as an optical disk D, an optical pickup  120  movably installed in the radial direction of the optical medium D recording and/or reproducing information onto and/or from the optical medium D, a driving unit  130  driving the spindle motor  110  and the optical pickup  120 , and a controller  140  controlling a focusing servo, a tracking servo and a tilting servo of the optical pickup  120 . As shown in  FIG. 8 , the optical medium D is chucked to a turntable  112  by a clamp  124 .  
         [0088]     The optical pickup  120  includes an optical pickup system comprising an object lens  30  focusing a light emitted from a light source onto the optical medium, and an optical pickup actuator driving the object lens  30  about three axes. The optical pickup actuator described with respect to  FIGS. 1-7B  may be adopted to the optical recording/reproducing apparatus shown in  FIG. 8 .  
         [0089]     In the optical recording/reproducing apparatus, a reflected light from the optical medium D is detected through a photodetector provided to the optical pickup  120 , and undergoes photoelectric transformation to become an electrical signal Vp. The electrical signal Vp is input to the controller  140  through the driving unit  130 . The driving unit  130  controls rotation speed of the spindle motor  110 , amplifies an input signal, and drives the optical pickup  120 .  
         [0090]     The controller  140  sends a focusing servo command, a tracking servo command, and a tilting servo command, each being adjusted based on an input signal from the driving unit  130 , to the driving unit  130  again to realize the focusing, tracking and tilting operations of the optical pickup  120 .  
         [0091]     Although the embodiments of the present invention described herein disclose the actuator with one objective lens  30  for one lens holder  20 , a number of objective lenses may be increased. For example, two lens mounting holes may be formed in the lens holder  20 , and an objective lens mounted on only one of the holes, or a plurality of objective lenses may be mounted on the two lens mounting holes. The magnetic circuit may be effectively used in this case as well, to adjust the position of the objective lens being used in a particular application.  
         [0092]     In a case where more than two objective lenses are mounted on the lens holder  20 , the actuator thereof can be applied to an optical pickup operating compatibly with a plurality of kinds of optical disks, such as, CD, DVD, and HD-DVD, for recording/reproducing information.  
         [0093]     In conclusion, the optical pickup actuator according to embodiments of the present invention independently controls the objective sensor lenses in the focusing, tilting and tracking directions. Therefore, adaptability at a high speed is improved, and the sensitivity of the optical pickup can be improved. Moreover, the rigidity of the injection molded lens holder is reinforced by tightly attaching the focusing coil and the tilting coils to the inside of the lens holder. Hence, it becomes possible to secure the gain margin and the movement of a secondary resonant frequency (which is caused due to physical properties of the lens holder) at a high frequency. Overall, the optical pickup actuator of the present invention may be advantageously used for a high-speed optical recording/reproducing apparatus.  
         [0094]     The foregoing embodiments and advantages are merely exemplary and are not to be construed as limiting the present invention. The present teaching can be readily applied to other types of apparatuses. Although a few embodiments of the present invention have been shown and described, it would be appreciated by those skilled in the art that changes may be made in this embodiment without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.