Patent Publication Number: US-6906430-B2

Title: Actuator

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a pickup device of a disc drive for optically writing and reading information on an optical disc and, more particularly, to a pickup device suitable for a thin disc drive. 
     2. Description of Related Art 
     A disc drive for recording information on and reproducing information from an optical disc such as a CD (Compact Disc) and a DVD (Digital Video Disc) includes an optical pickup device. In order to accurately record and reproduce information on and from an optical disc, the pickup device performs a focus control for controlling a distance between an objective lens and an information recording surface of the optical disc and performs a tracking control for controlling the objective lens to follow an information track of the optical disc. 
     The pickup device directs a laser beam, which is emitted from a light source such as a laser diode, to the objective lens by using predetermined optical system components, and irradiates the laser beam onto the information recording surface of the optical disc via the objective lens. Normally, the objective lens is positioned to oppose to the optical disc. A laser beam from the light source is guided by the optical system components to a position below the objective lens along an optical path provided in parallel with the optical disc. Then, the laser beam is changed its path by a raise-up mirror placed under the objective lens to a vertical direction, that is, the direction of the objective lens, and the laser beam reaches the objective lens. The objective lens is mounted on a lens holder and coil substrates are mounted on the lens holder. The lens holder is moved vertically and horizontally with respect to the optical disc by applying driving current to the coil substrates in a magnetic field formed by the magnets which are placed close to each other, so that the focus control and the tracking control are performed. An example of such a pickup device is described in Japanese Patent Laid-Open under No. 2001-229557. 
     However, in the above-described pickup device, since the coil substrates are mounted on the opposing sides of the rectangular lens holder, the optical path of a laser beam from the laser light source cannot be arranged at the same height level as the coil substrates in the horizontal direction. Namely, since the paired coil substrates are positioned on both sides of the objective lens, the coil substrates become obstacles and prevent a laser beam from being guided to the position below the objective lens at the same height level as the coil substrates. For this reason, in the pickup device described in Japanese Patent Laid-Open under No. 2001-229557, for example, the optical path of a laser beam is placed below the lens holder. Namely, the optical path is arranged such that the laser beam is directed to the position below the objective lens at a lower level than the coil substrates and then directed to the objective lens by the raise-up mirror. 
     However, when the optical path of a laser beam is placed at a lower height level than the lens holder, the whole height (thickness in the vertical direction) of the pickup increases. For example, in a thin drive device such as a drive device mounted in a notebook-type personal computer, a thin pickup device has to be used and thus the above thick pickup device cannot be used. 
     SUMMARY OF THE INVENTION 
     The present invention is devised in view of the above-described point, and its object is to provide an actuator for a thin pickup device which is suitable for use in a thin drive device. 
     According to one aspect of the present invention, there is provided an actuator including: a lens holder having an upper face for holding an objective lens and a pair of side faces for holding coil substrates; the objective lens fixed to the upper face; and the pair of coil substrates fixed to the pair of the side faces, wherein the objective lens is fixed at a position displaced from a center of gravity of the lens holder to one end of the lens holder, and one of the pair of the coil substrates is fixed on one of the side faces of the lens holder in a range from the one end of the lens holder to a center portion of the lens holder, and the other one of the coil substrates is fixed on the other one of the side faces of the lens holder in a range from the other end of the lens holder to the center portion of the lens holder. 
     The above actuator is used in a pickup device which is used for a drive device of an optical disc. The actuator is positioned opposing to the optical disc to irradiates a light beam from a light source to an information recording surface of the optical disc. The objective lens is fixed to the lens holder forming the actuator at a position closer to one end thereof. One coil substrate is fixed on one side face of the lens holder in a range from the one end of the lens holder to a center portion of the lens holder, and the other coil substrate is fixed on the other side face of the lens holder in a range from the other end of the lens holder to the center portion of the lens holder. Therefore, near the end of the lens holder on the objective lens side, the coil substrate is not attached to one side of the lens holder, providing a space below the objective lens. By arranging an optical system to guide the light beam to the objective lens via the space, the optical system can be arranged at the same height level as the actuator. As a result, In comparison with the case where the optical system is provided below the actuator, whole thickness of the pickup device can be reduced, and a preferable pickup device for a thin-type drive device may be provided. 
     The lens holder may have a mass portion at a side of the other end, and the mass portion may have a mass larger than a mass of the one end of the lens holder. By this, it is possible to correct the shift of the gravity center of the lens holder when the objective lens is fixed near one end thereof. 
     The lens holder may include a first stopper provided at the one end and a second and a third stoppers provided at the other end, wherein the second stopper contacts a portion of a body mounting the actuator to limit movement of the lens holder in a focus direction of an optical disc, the third stopper contacts the portion of the body to limit the movement of the lens holder in a tracking direction of the optical disc, and the first stopper contacts the portion of the body to limit the movement of the lens holder in both of the focus direction and the tracking direction. 
     In this feature, at the end of the lens holder on the objective lens side, one stopper can limit the strokes of the lens holder in both focus direction and tracking direction. Therefore, the lens holder may be formed thinner. 
     The actuator may further include: an actuator base having a pair of yokes; and a pair of magnets fixed to the pair of the yokes, wherein each of the pair of the magnets may be positioned opposing to each of the pair of the coil substrates. In this feature, by applying a driving current to the coils on the coil substrates within the magnetic field created by the magnets, the objective lens can be moved to perform focus control and tracking control. 
     Each of the pair of the coil substrates may include a tracking coil and a focus coil formed in alignment with each other in a horizontal direction of the coil substrate, and the coil substrates may oppose to each other only in areas where the focus coil is formed. Thus, an appropriate magnetic field can be created between the coil substrates, and a space to arrange the optical system may be ensured. 
     Each of the pair of the coil substrates may include a pair of lands in an area where the focus coil is formed, and the lens holder may further include connection wires which electrically connect the lands on the pair of the coil substrates. Therefore, the coil substrates can be electrically connected by short wires within the area in which the coil substrates oppose to each other. 
     Each of the pair of the magnets may include a magnetization pattern having a boundary area, and an north pole area and a south pole area on both sides of the boundary area. A vertically extending portion of the boundary area may perpendicularly cross the tracking coil of the coil substrate, and a horizontally extending portion of the boundary area may perpendicularly cross the focus coil of the coil substrate. In this feature, the tracking servo and the focus servo can be appropriately performed by the magnetic field created by the magnets and the application of the drive current to the coils. The nature, utility, and further features of this invention will be more clearly apparent from the following detailed description with respect to preferred embodiment of the invention when read in conjunction with the accompanying drawings briefly described below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A  to  1 C are diagrams showing the configuration of a pickup device according to the present invention; 
         FIGS. 2A  to  2 C are diagrams showing the configuration of an actuator in the pickup device of the present invention; 
         FIGS. 3A  to  3 C are diagrams showing the relative positions of coil substrates and magnets in the actuator; 
         FIGS. 4A  to  4 C are diagrams showing a magnetization pattern of the magnets used for the actuator of the present invention; 
         FIGS. 5A and 5B  are diagrams showing the configuration of the coil substrates used for the actuator of the present invention; 
         FIGS. 6A and 6B  are diagrams showing the relative positional relation between the magnets and the coil substrates; 
         FIGS. 7A  to  7 C are diagrams showing the configuration of suspension wires used for the actuator of the present invention; 
         FIGS. 8A  to  8 D are diagrams showing the configuration of a lens holder used for the actuator of the present invention; and 
         FIGS. 9A  to  9 C are diagrams showing stroke limitation performed by stoppers formed on the lens holder. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The preferred embodiment of the present invention will now be described below with reference to the attached drawings. 
       FIGS. 1A  to  1 C show the configuration of a pickup device according to an embodiment of the present invention.  FIG. 1A  is a top view of a pickup device  100 ,  FIG. 1B  is a perspective view of the pickup device  100 , and  FIG. 1C  is a bottom plan view of the pickup device  100 . As shown in  FIGS. 1A  to  1 C, the pickup device  100  has various components mounted on a body  1 . An actuator base  8  is fixed on the body  1 , and the actuator base  8  supports a lens holder  5  movably in a focusing direction F and a tracking direction T by using four suspension wires  10 . The lens holder  5  has coil substrates  7   a  and  7   b  mounted on the opposing sides thereof and an objective lens  4  mounted on the top surface. 
     Magnets  6   a  and  6   b  are fixed on the body  1  so as to be respectively opposed to the paired coil substrates  7   a  and  7   b  mounted on the lens holder  5 . The paired magnets  6   a  and  6   b  are fixed so as to be opposed to each other, and a magnetic field is created in a space between the magnets  6   a  and  6   b  in accordance with the magnetization patterns of the magnets  6   a  and  6   b . The lens holder  5  having the coil substrates  7   a  and  7   b  on the sides is placed in the magnetic field created by the paired magnets  6   a  and  6   b . Thus, when a predetermined driving current is applied to a focus coil and a tracking coil that are formed on the coil substrates  7   a  and  7   b , the lens holder  5  is moved in the focusing direction F and the tracking direction T due to force generated by the currents in the coils and the magnetic field. Due to the movement of the lens holder  5 , the objective lens  4  on the lens holder  5  is moved to perform the focus control and the tracking control. 
     A laser light source  2  is mounted on an end of the body  1 . A laser beam emitted from the laser light source  2  passes through an optical path  3 , which is constituted by an optical system arranged in the body  1 , and reaches the position below the objective lens  4 . A raise-up mirror  9  is placed below the objective lens. The raise-up mirror  9  changes the path of the laser beam upward and the laser beam passes through the objective lens  4  upwardly from below. An optical disc is placed above the objective lens  4 , and the objective lens  4  condenses the laser beam on the information recording surface of the optical disc. 
     Next, referring to  FIGS. 2A  to  2 C, an actuator of the pickup device  100  will be described below.  FIG. 2A  is a top view of the actuator,  FIG. 2B  is a perspective view of the actuator, and  FIG. 2C  is a bottom plan view of the actuator. The actuator  50  is a mechanism for moving the objective lens  4  in the focusing direction F and the tracking direction T, and includes the actuator base  8 , the lens holder  5 , the paired magnets  6   a  and  6   b , and the paired coil substrates  7   a  and  7   b  mounted on the lens holder  5 . 
     As shown in  FIGS. 1A  to  1 C and  2 A to  2 C, the actuator base  8  is fixed on the body  1  of the pickup device  100 . Further, a pair of yokes  11  is formed on the body  1 , and the magnets  6   a  and  6   b  are fixed inside the yokes  11 . In the pickup device  100  of the present invention, the paired yokes  11  are not completely opposed to each other but are displaced from each other in the tracking direction T. This arrangement is made to provide a space for arranging an optical path (optical system)  3  indicated by broken lines in FIG.  2 A. The magnets  6   a  and  6   b  are mounted inside of the yokes  11 , and hence a magnetic field is created in a space between the paired magnets  6   a  and  6   b.    
     Meanwhile, on the actuator base  8 , the lens holder  5  is supported by the four suspension wires  10 . The lens holder  5  is supported by elasticity of the suspension wires  10  so as to be moved in the focusing direction F and the tracking direction T. The objective lens  4  is mounted on the lens holder  5 . By applying driving current to the focus coil and the tracking coil on the coil substrates  7   a  and  7   b , which are mounted on the lens holder  5 , in a magnetic field created by the paired magnets  6   a  and  6   b , force is applied to the coil substrates  7   a  and  7   b  as well as the lens holder  5  integrated with the coil substrates  7   a  and  7   b  in the focusing direction F and the tracking direction T, so that the objective lens  4  is moved with respect to the information recording surface and the information track of the optical disc. 
     As clearly shown in  FIGS. 2A and 2B , in the pickup device  100  of the present invention, the paired coil substrates  7   a  and  7   b  are laterally (i.e., in the tracking direction T) shifted in the opposite directions and are not completely opposed to each other in a horizontal direction. This arrangement is made to place the optical path  3  from the laser light source  2  at the same height level as the magnets  6   a  and  6   b  and the coil substrates  7   a  and  7   b.    
     In an ordinary pickup device, paired magnets and coil substrates are almost entirely opposed to each other in the width direction. Thus, the magnets and the coil substrates cover both sides of an objective lens like walls and a laser beam cannot be directed to the objective lens at the same height level. Hence, the optical path of the laser beam has to be placed at a lower height than the magnets and the coil substrates, thereby increasing the entire thickness of the pickup device. 
     On the contrary, in the actuator  50  of the present invention, the magnets  6   a  and  6   b  and the coil substrates  7   a  and  7   b  are displaced from each other oppositely in the tracking direction T. Namely, the paired magnets  6   a  and  6   b  are only partially opposed to each other, and the coil substrates  7   a  and  7   b  are only partially opposed to each other. Thus, as shown in  FIGS. 2A and 2B , the objective lens  4  partially forms a space in the circumferential direction. The optical path  3  is placed in the space and a laser beam from the laser light source  2  is introduced to a position below the objective lens  4 . With this configuration, it is possible to arrange the magnets  6   a  and  6   b , the coil substrates  7   a  and  7   b , and the optical path  3  of a laser beam from the laser light source  2  at the same height level, thereby reducing the entire height (thickness) of the pickup device. 
       FIGS. 3A  to  3 C show the positional relation between the magnets and the coil substrates.  FIG. 3A  shows the arrangement of the magnets  6   a  and  6   b  with respect to the actuator  50 ,  FIG. 3B  shows the arrangement of the coil substrates  7   a  and  7   b  with respect to the actuator  50 , and  FIG. 3C  shows the arrangement of the magnets  6   a  and  6   b  and the coil substrates  7   a  and  7   b  with respect to the actuator  50 . Besides,  FIGS. 3A  to  3 C show that the magnets and the coil substrates are removed upward from the mounting positions. Namely, in  FIGS. 3A  to  3 C, the magnets and the coil substrates are moved from the illustrated positions in parallel to lower positions and are fixed thereon. 
     As shown in  FIG. 3A , the magnets  6   a  and  6   b  are fixed inside the yokes  11  so as to be opposed to each other. Moreover, as shown in  FIG. 3B , the coil substrates  7   a  and  7   b  are respectively mounted on the sides along the longitudinal direction of the lens holder  5 . As already mentioned, the lens holder  5  is movably supported by the four suspension wires  10  in the space between the magnets  6   a  and  6   b . As a result, as shown in  FIG. 3C , the magnet  6   a  and the coil substrate  7   a  are positioned close to each other, and the magnet  6   b  and the coil substrate  7   b  are positioned close to each other. 
       FIGS. 4A  to  4 C are plan views showing the magnets.  FIG. 4A  shows the magnet  6   a , and  FIG. 4B  shows the magnet  6   b . The magnets  6   a  and  6   b  are produced by magnetizing iron pieces of a predetermined size by using a magnetizing device. With the magnetizing device, illustrated magnetization patterns are formed on the magnets  6   a  and  6   b . As shown in  FIG. 4A , the magnet  6   a  has the magnetization pattern constituted by a magnetized area  18   a  of north pole, a magnetized area  19   a  of south pole, and a boundary area  20   a  between the areas  18   a  and  19   a . Further, as shown in  FIG. 4B , the magnet  6   b  has the magnetization pattern constituted by a magnetized area  18   b  of north pole, a magnetized area  19   b  of south pole, and a boundary area  20   b  between the areas  18   b  and  19   b.    
       FIG. 4C  shows the positional relation between the magnets  6   a  and  6   b . As shown in  FIG. 3A , the magnets  6   a  and  6   b  are fixed respectively on the yokes  11  so as to be opposed to each other.  FIG. 4C  shows the positional relation between the magnetization patterns of the magnets  6   a  and  6   b  in a state in which the magnets  6   a  and  6   b  are opposed to each other on the yokes  11 .  FIG. 4C  shows the relative positional relation between the magnetization patterns of the magnets  6   a  and  6   b , taken from the direction of an arrow  80  in FIG.  3 A. The magnet  6   a  is indicated by broken lines and the magnet  6   b  is indicated by solid lines. 
     As shown in  FIG. 4C , the magnets  6   a  and  6   b  are partially opposed to each other and partially has an overlap in the lateral direction when viewed from the direction of the arrow  80 . At this point, in the perspective view taken from the direction of the arrow  80  in  FIG. 3A , the north pole magnetized area  18   a  of the magnet  6   a  and the north pole magnetized area  18   b  of the magnet  6   b  overlap with each other, and the south pole magnetized area  19   a  of the magnet  6   a  and the south pole magnetized area  19   b  of the magnet  6   b  overlap with each other. In addition, the boundary area  20   a  of the magnet  6   a  and the boundary area  20   b  of the magnet  6   b  overlap with each other. As a result, a substantially recessed magnetic field, which corresponds to the magnetization pattern (hereinafter referred to as a “combined magnetization pattern”) schematically shown in  FIG. 4C , is formed in a space between the magnets  6   a  and  6   b , that is, a space in which the lens holder  5  is placed. 
     Next, the coil substrates will be described.  FIG. 5A  is a plan view of the coil substrate  7   a , and  FIG. 5B  is a plan view of the coil substrate  7   b . The coil substrate  7   a  is a printed wiring board and has a tracking coil  13   a  and a focus coil  14   a  formed on the substrate surface. Further, lands  15   a  and  16   a  are formed on the coil substrate  7   a  to establish continuity with the opposed coil substrate  7   b.    
     Meanwhile, the coil substrate  7   b  is also a printed wiring board and has a tracking coil  13   b  and a focus coil  14   b  formed on the substrate surface. Moreover, lands  15   b  and  16   b  are formed on the coil substrate  7   b  to establish continuity with the opposed coil substrate  7   a . In the state being mounted on the sides of the lens holder  5 , the focus coil  14   a  of the coil substrate  7   a  and the focus coil  14   b  of the coil substrate  7   b  are substantially opposed to each other. 
       FIG. 6A  shows the relative positional relation between the coil substrates  7   a  and  7   b  in a state being mounted on the lens holder  5 .  FIG. 6A  perspectively shows the relative positional relation between the coil substrates  7   a  and  7   b  on the lens holder  5 , viewed in the direction of the arrow  81  in FIG.  3 B. The coil substrate  7   a  is indicated by broken lines and the coil substrate  7   b  is indicated by solid lines. As shown in  FIG. 6A , the coil substrates  7   a  and  7   b  are mounted on the lens holder  5  with a such relative positional relation that the focus coils  14   a  and  14   b  are substantially overlap with each other, when viewed from the side of the lens holder  5 . Further, in that state, the land  15   a  of the coil substrate  7   a  and the land  15   b  of the coil substrate  7   b  are opposed to each other, and the land  16   a  of the coil substrate  7   a  and the land  16   b  of the coil substrate  7   b  are opposed to each other. In a state in which the coil substrates  7   a  and  7   b  are mounted on the lens holder in this manner, the opposed land  15   a  and  15   b  and the opposed land  16   a  and  16   b  are electrically connected via connecting wires  22  (see.  FIGS. 3A  to  3 C). This electric connection applies driving current to the tracking coil  13   b  and the focus coil  14   b  of the coil substrate  7   b.    
       FIG. 6B  shows a magnetic field created by the magnets  6   a  and  6   b  and the arrangement of the tracking coil and the focus coil in the magnetic field. As shown, the magnets  6   a  and  6   b  create a magnetic field corresponding to the combined magnetization pattern of a substantially recessed shape (indicated by broken lines) in FIG.  6 B. The boundary area  20   a  of the magnet  6   a  shown in  FIG. 4A  longitudinally penetrates nearly the center of the rectangular tracking coil  13   a  of the coil substrate  7   a  shown in FIG.  5 A. The boundary area  20   b  of the magnet  6   b  shown in  FIG. 4B  penetrates nearly the center of the rectangular tracking coil  13   b  of the coil substrate  7   b  shown in FIG.  5 B. Further, the boundary areas  20   a  and  20   b  of the magnets  6   a  and  6   b  laterally penetrate nearly the center of the rectangular focus coils  14   a  and  14   b  of the coil substrates  7   a  and  7   b . The lens holder  5  on which the coil substrates  7   a  and  7   b  are attached is placed in the space between the magnets  6   a  and  6   b  so as to establish such relative positional relations. 
     Next, the arrangement of the suspension wires  10  will be described. The suspension wire  10  is made from a linear metal having elasticity, and so on. In the actuator  50  of the present invention, the lens holder  5  is supported by the four suspension wires  10 . As shown in  FIGS. 1A and 3A , the ends of the four suspension wires  10  are fixed on the actuator base  8  and the other ends are fixed on the coil substrates  7   a  and  7   b  integrated with the lens holder  5 . Thus, the lens holder  5  is placed in the space between the magnets  6   a  and  6   b  while being supported by the four suspension wires  10  in the horizontal direction. By the elasticity of the suspension wires  10 , the lens holder  5  holding the objective lens  4  can move in the focusing direction F and the tracking direction T that are perpendicular to the longitudinal direction (jittering direction J) of the suspension wires  10 . 
       FIG. 7A  shows the relative positional relationship between the actuator base  8 , the suspension wires  10  and the lens holder  5 . In the present invention, in order to place the optical path of the laser beam at the same vertical level as the coil substrate  7 , the two opposed coil substrates  7   a  and  7   b  are mounted on the lens holder  5  with being shifted in the lateral direction (tracking direction T). Therefore, the suspension wires  10  are not in parallel with the jittering direction J. Namely, the suspension wires  10  are extended in a direction having a predetermined angle from the jittering direction J, and the ends of the suspension wires  10  are fixed on the actuator base  8 . 
     Referring to  FIGS. 7B and 7C , the above configuration will be described in detail.  FIG. 7B  shows the relative positional relation between a typical actuator base, suspension wires and a lens holder. In  FIG. 7B , a lens holder  121  is supported by four suspension wires  124  from an actuator base  123 . It is to be noted that, since  FIG. 7B  is a plan view showing the top of the actuator, only the two upper suspension wires are shown. An objective lens  120  and a pair of coil substrates  122   a  and  122   b  are mounted on the lens holder  121 . As shown in  FIG. 7B , when the paired coil substrates  122   a  and  122   b  are placed on both sides of the lens holder  121  so as to be completely opposed to each other (namely, without the shift in the tracking direction T, unlike the present invention), the four suspension wires  10  fixed on the actuator base  8  and the lens holder  121  are arranged in parallel with each other. 
     On the other hand,  FIG. 7C  schematically shows the relative positional relation between the actuator base  8 , the suspension wires  10  and the lens holder  5  in the actuator  50  of the present invention. In the case of the present invention, the coil substrates  7   a  and  7   b  are mounted on the sides of the lens holder  5  so as to be partially opposed to each other. Specifically, the coil substrate  7   a  is shifted downward in FIG.  7 C and the coil substrate  7   b  is shifted upward in  FIG. 7C  to create the space for arranging the optical path  3 . 
     Therefore, the suspension wires  10  are not in parallel with each other, but are somewhat opened to the direction of the lens holder  5 . Specifically, as shown in  FIG. 7C , each of the suspension wires  10  is extended to the lens holder  5  at an angle increased by an angle a from the direction of the lens holder  5  (that is, the jittering direction J), and the ends of the suspension wires  10  are fixed on the actuator base  8 . Moreover, as is evident from comparison with  FIG. 7B , the fixing positions  31  and  32  of the suspension wires  10  to the actuator base  8  are shifted by a distance L in the jittering direction J. Namely, a line connecting the fixing positions  31  and  32  of the suspension wires to the actuator base  8  is not arranged in parallel with the coil substrates  7   a  and  7   b . The reason is as follows. Since the supporting positions of the suspension wires  10  on the side of the lens holder  5  are arranged at different levels with respect to the jittering direction J, when the supporting positions of the suspension wires  10  on the side of the actuator base  8  are not shifted in the jittering direction J, the four suspension wires  10  cannot have equal length, thereby breaking the relation between the center of gravity of the lens holder  5  and the spring constants of the four suspension wires  10 . In order to prevent this problem, the supporting positions  31  and  32  of the suspension wires  10  on the side of the actuator base  8  are shifted in the jittering direction J and the four suspension wires are made equal in length. 
     Moreover, when suspension wires of equal lengths are used, the number of components can be reduced, thereby reducing the product cost. However, it is not always necessary to have suspension wires of equal lengths and various lengths are applicable. In that case, the configuration can be made by suitably setting spring constants and the like of the suspension wires. 
     Further,  FIG. 7C  only shows the two upper suspension wires  10  of the four suspension wires  10  in the focusing direction. The two lower suspension wires  10  are held below and in parallel with the two upper suspension wires  10 . 
     In this way, in the actuator  50  of the present invention, the directions of the suspension wires  10  are not arranged in parallel but are somewhat opened to the lens holder  5 . One of the reasons is that, since the width of the lens holder is increased in the tracking direction by shifting the position of the coil substrates, a spring interval (i.e., the interval between the supporting wires  10 ) on the side of the actuator is reduced to have a smaller projection area over the pickup. Another reason is that, by providing suspension wires not being in parallel, the spring constant of the suspension wires is increased in a twisting direction viewed from the arrow  81  of  FIG. 3 , so that a mode frequency (rolling frequency) in the twisting direction can be higher than a parallel spring. Further, the fixing positions  31  and  32  of the suspension wires  10  on the side of the actuator base  8  are shifted in the jittering direction J. 
     Next, the lens holder will be described below.  FIGS. 8A  to  8 D show the configuration of the lens holder  5 .  FIG. 8A  is a top view of the lens holder  5 ,  FIG. 8B  is a bottom plan view of the lens holder  5 ,  FIG. 8C  is a perspective view of the lens holder  5  obliquely viewed from above, and  FIG. 8D  is a perspective view of the lens holder  5  obliquely viewed from below. 
     As shown in  FIGS. 8A  to  8 D, the lens holder  5  has a mounting hole  40  for mounting the objective lens  4 . The mounting hole  40  is shifted from a gravity center  41  of the lens holder  5  to the side of the optical path  3 . The objective lens is normally placed around the center of the lens holder (around the center of gravity of the lens holder, see FIG.  7 B). However, in the present invention, the pickup  100  is configured so that the coil substrates  7   a  and  7   b  mounted on the lens holder  5  are shifted in the tracking direction T to obtain the optical path for directing a laser beam to below the objective lens. Thus, the position mounting the objective lens on the lens holder  5  is shifted to the optical path  3  of the laser beam. Since the optical path  3  of a laser beam has to reach below the objective lens  4 , when the objective lens  4  is positioned around the center of the lens holder  5 , the optical path  3  of the laser beam has to reach around center of the lens holder  5 . Accordingly, the coil substrate  7   b  on the side of the optical path  3  should be shifted more largely to obtain a larger space on the side of the lens holder  5 . On the contrary, as shown in  FIG. 8A , when the position of the objective lens  4  (that is, the position of the mounting hole  40 ) is shifted to the optical path  3 , the optical path  3  is placed closer to the end of the lens holder  5 , thereby reducing a shifting amount of the coil substrate  7   b  on the side of the optical path  3 . Hence, it is possible to reduce the length of the lens holder  5 . 
     Moreover, when the mounting position of the objective lens  4  on the lens holder  5  is shifted in the direction of the optical path  3 , the center of gravity of the lens holder  5  itself is shifted to the optical path  3  side after mounting the objective lens  4 . Therefore, as shown in  FIGS. 8C and 8D , the configuration of the lens holder  5  is devised as follows. Namely, the wall parts  42  and  43  on the opposite side of the objective lens  40  of the lens holder  5  are formed with a larger thickness than a part  44  on the side of the objective lens mounting hole  40 , for example, and the center of gravity of the lens holder  5  is positioned substantially at the center when the objective lens  4  is mounted on the lens holder  5 . In this way, the shift of the center of gravity of the lens holder, that may happen by shifting the position of the objective lens in the direction of the optical path, is cancelled. 
     Further, as shown in  FIG. 8A , the fixing positions  33  and  34  of the suspension wires  10  to the lens holder  5  have a midpoint substantially coinciding with the gravity center  41  of the lens holder  5  supported by the suspension wires  10 . Namely, in  FIG. 8A , the suspension wires  10  are each fixed on the lens holder  5  (to be precise, the coil substrates  7   b  and  7   a  mounted on the lens holder  5 ) at the fixing positions  33  and  34 . In this case, the midpoint of the fixing positions  33  and  34  substantially coincide with the gravity center  41  of the lens holder  5 . As a result, both ends of the lens holder  5  can be held by the suspension wires  10  with being equally balanced, thereby preventing rolling and the like of the lens holder  5 . 
     Further, as shown in  FIGS. 8A  to  8 D, the lens holder  5  includes stoppers  59 ,  51  and  52 . The stoppers  59 ,  51  and  52  play a role of limiting the stroke (movable range) of the lens holder  5  in the focusing direction F and the tracking direction T. Thus, when a focus servo or a tracking servo causes a malfunction, it is possible to prevent the lens holder  5  from moving unlimitedly, making contact with the body  1  and the other members, and damaging the lens holder  5 , the objective lens  4  and so on. 
     In the lens holder  5  of the present invention, the stopper  59  has both functions of a stopper in the focusing direction F and a stopper in the tracking direction T. Namely, the movement of the lens holder  5  in the focusing direction F is limited by the stoppers  59  and  51 , and the movement of the lens holder  5  in the tracking direction T is limited by the stoppers  59  and  52 . 
       FIG. 9A  shows a state in which the stroke is limited in the focusing direction F by the stoppers  59  and  51 . The stopper  59  (indicated by broken lines) makes contact with a cover  55  of the body  1  from below and the stopper  51  similarly makes contact with a cover  56  of the body  1  from below, thereby limiting the movement of the lens holder  5  in the focusing direction F (i.e., the upward direction in FIG.  9 A). 
       FIGS. 9B and 9C  show that the stroke is limited in the tracking direction T by the stoppers  59  and  52 . In  FIG. 9B , the stopper  59  makes contact with the side wall of a recessed part  57  provided on the body  1 . In  FIG. 9C , the stopper  52  makes contact with the side wall of a recessed part  58  provided on the body  1 . Thus, the moving range of the lens holder  5  is limited in the tracking direction T. 
     As described above, in the lens holder  5  of the present invention, the position of the objective lens  4  is shifted in the direction of the optical path  3 . In addition, as well shown in  FIG. 8D , the portion of the lens holder  5  on the side of the mounting hole  40  of the objective lens has a relatively small thickness and the portion of the lens holder  5  on the opposite side to the mounting hole  40  has a relatively large thickness to adjust the center of gravity of the lens holder  5 . Moreover, it is desirable to minimize the number of members provided in the direction of the optical path  3  around the mounting hole  40  so as not to interrupt the optical path of a laser beam reaching below the objective lens  4  from the laser light source  2 . For these reasons, the stopper  59  has stroke limiting functions both in the focusing direction F and the tracking direction T. 
     As described above, according to the actuator of the present invention, the pair of coil substrates mounted on the lens holder is shifted in the longitudinal direction (tracking direction T) of the lens holder to obtain the optical path for directing a laser beam to the position below the objective lens. Thus, it is possible to direct a laser beam to the position below the objective lens at the same height level as the coil substrates and the magnets, thereby reducing the thickness of the whole actuator. 
     The invention may be embodied on other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description and all changes which come within the meaning an range of equivalency of the claims are therefore intended to embraced therein. 
     The entire disclosure of Japanese Patent Application No. 2002-152061 filed on May 27, 2002 including the specification, claims, drawings and summary is incorporated herein by reference in its entirety.