Abstract:
An optical pickup unit comprising: an objective lens that focuses laser light on an optical disc; a lens holder that holds the objective lens; a coil that is fitted on the lens holder and capable of driving the lens holder; and a reinforcement member that is fitted on the lens holder and reinforces strength of the lens holder.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a continuation of application Ser. No. 11/669,754 filed on Jan. 31, 2007, which claims the benefit of priority to Japanese Patent Application Nos. 2006-24281, filed Feb. 1, 2006, and 2006-281176, filed Oct. 16, 2006, of which full contents are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an optical pickup unit and an optical disc apparatus, capable of reproducing data recorded on an optical disc and of recording data onto the optical disc. 
     2. Description of the Related Art 
     Of late years, proposed is an optical pickup unit not shown, a so-called optical head whose lens holder is mounted with two objective lenses (see, e.g., Japanese Patent Application Laid-open Publication No. 9-120573 (p. 2, FIGS. 1-3). 
     However, with respect to a conventional optical pickup unit having a lens holder with an objective lens, if the distances are unequal between the coils around the lens holder and the objective lens, it has been feared that heat transfers to the objective lens may become non-uniform when heat is generated in the coils. It is a problem that non-uniform heat transfers to the objective lens may induce uneven temperature gradients in the objective lens. It is feared, in the case of non-uniform heat transfers to the objective lens, that thermal expansions of the objective lens may not become uniform, resulting in occurrence of aberrations on a spot of laser light focused by the objective lens to be applied to the optical disc. 
     Since the conventional optical pickup unit is of a structure where the objective lens is warmed up gradually from its portions closer to the coils, the objective lens is expected to have large temperature gradients when checking the temperature characteristics of the objective lens. It is difficult for the objective lens with large temperature gradients to have substantially uniform thermal expansions or thermal contractions, as a result of which aberrations may occur on a spot of laser light focused by the objective lens to be applied to the optical disc. 
     Furthermore, with respect to the conventional optical pickup unit having a lens holder with two objective lenses, occurrence of higher-order resonance has been feared. The higher-order resonance of the optical pickup unit means that the lens holder with lenses is subjected to significant vibrations in frequency regions of about 10 KHz or more for example. In the case of the occurrence of a higher-order resonance in the lens holder with lenses, when the optical disc apparatus having the optical pickup unit makes random access to an optical disc, it is feared that the optical pickup unit may not be able to perform a normal seeking operation to the optical disc due to its runaway. The seek means moving the optical pickup unit to a designated position on the disc. Required is an optical pickup unit capable of preventing the occurrence of runaway. 
     It is also feared, in the case of the occurrence of a significant higher-order resonance in the lens holder with lenses of the optical pickup unit, that moving members such as the lens holder may be deformed. To suppress the higher-order resonance occurring in the lens holder with lenses of the optical pickup unit, an increase in rigidity of the lens holder, etc., is required. 
     The optical pickup unit is also facing the needs for a reduction in weight, an improvement in responsivity and a lowering of the price. 
     SUMMARY OF THE INVENTION 
     An optical pickup unit according to an aspect of the present invention may comprise: an objective lens that focuses laser light on an optical disc; a lens holder that holds the objective lens; a coil that is fitted on the lens holder and capable of driving the lens holder; and a reinforcement member that is fitted on the lens holder and reinforces strength of the lens holder. 
     In an optical pickup unit according to an aspect of the present invention, the reinforcement member may be formed from a non-magnetic material. 
     In an optical pickup unit according to an aspect of the present invention, the lens holder may be formed substantially in a shape of a box having an opening, and the reinforcement member may be formed substantially in a plate shape corresponding to the opening, and a lens holder assembly may be configured such that the reinforcement member is fitted in the opening. 
     In an optical pickup unit according to an aspect of the present invention, the lens holder may further comprise: a lens fitting portion on which the objective lens is fitted; and a member fitting portion on which the reinforcement member is fitted, and the lens fitting portion and the member fitting portion may be formed at substantially opposite sides of the lens holder, respectively. 
     In an optical pickup unit according to an aspect of the present invention, the objective lens may further comprise: a first objective lens for a first-wavelength laser light; and a second objective lens for a second-wavelength laser light having a wavelength different from that of the first-wavelength laser light, the lens holder may further comprise: a lens fitting portion on which the first objective lens and the second objective lens are fitted; and a member fitting portion on which the reinforcement member is fitted, and the lens fitting portion and the member fitting portion may be formed at substantially opposite sides of the lens holder, respectively, and a lens assembly may be configured such that the first objective lens and the second objective lens are fitted on the lens fitting portion and such that the reinforcement member is fitted on the member fitting portion. 
     In an optical pickup unit according to an aspect of the present invention, the reinforcement member may be provided with a lightening portion configured to reduce weight of the reinforcement member. 
     In an optical pickup unit according to an aspect of the present invention, the reinforcement member is formed with a laser light passing hole that allows the laser light to pass therethrough. 
     In an optical pickup unit according to an aspect of the present invention, the reinforcement member may be provided with a positioning protrusive end edge and the lens holder may be provided with a positioning bulged inner wall corresponding to the positioning protrusive end edge. 
     In an optical pickup unit according to an aspect of the present invention, the reinforcement member may abut against a wall which is a constituent of the lens holder. 
     In an optical pickup unit according to an aspect of the present invention, the lens holder may further comprise: a first piece on which the objective lens is fitted; and a second piece on which the reinforcement member is fitted. 
     In an optical pickup unit according to an aspect of the present invention, the lens holder may further comprise: a first piece on which the objective lens is fitted; and a second piece on which the reinforcement member is fitted, and a thermal conductivity of a material forming the first piece may be different from that of a material forming the second piece. 
     In an optical pickup unit according to an aspect of the present invention, the lens holder may further comprise: a first piece on which the objective lens is fitted; and a second piece on which the reinforcement member is fitted, and the coil may further comprise a first-direction driving coil fitted on the first piece; and a second-direction driving coil fitted on the second piece, and the first piece may be provided with a first coil fitting portion around which the first-direction driving coil is wound, and the second piece may be provided with a second coil fitting portion around which the second-direction driving coil is wound. 
     In an optical pickup unit according to an aspect of the present invention, the reinforcement member may be secured to the lens holder with an adhesive. 
     In an optical pickup unit according to an aspect of the present invention, the adhesive may include a one-part adhesive. 
     In an optical pickup unit according to an aspect of the present invention, the adhesive may include an electron beam curable adhesive. 
     In an optical pickup unit according to an aspect of the present invention, the adhesive may include an adhesive containing a thermoset resin. 
     In an optical pickup unit according to an aspect of the present invention, the adhesive may include an epoxy resin. 
     In an optical pickup unit according to an aspect of the present invention, the adhesive may include an acrylic resin. 
     An optical disc apparatus according to an aspect of the present invention may comprise an optical pickup unit according to an aspect of the present invention. 
     According to the present invention, the occurrence is prevented of such deficiency that the lens holder provided with the objective lens may significantly vibrate resulting in a deformation of the lens holder. By the reinforcement member that reinforces the strength of the lens holder, the vibrations occurring in the lens holder are suppressed. Thus, the optical pickup unit allowing for vibrations is configured. 
     According to the present invention, the occurrence is prevented of such deficiency that malfunctions of the optical disc apparatus may occur. 
     The other features of the present invention will become apparent from the following description of this specification and the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For more thorough understanding of the present invention, the following description should be read in conjunction with the accompanying drawings, in which: 
         FIG. 1  is an exploded perspective view of an embodiment of a lens assembly of an optical pickup unit according to the present invention; 
         FIG. 2  is a perspective view of the top side of the lens assembly making up the optical pickup unit; 
         FIG. 3  is a perspective view of the bottom side of the lens assembly making up the optical pickup unit; 
         FIG. 4  is a perspective view of the top side of the optical pickup unit; and 
         FIG. 5  is a perspective view of the bottom side of the optical pickup unit. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is an exploded perspective view of an embodiment of a lens assembly of an optical pickup unit according to the present invention;  FIG. 2  is a perspective view of the top side of the lens assembly making up the optical pickup unit;  FIG. 3  is a perspective view of the bottom side of the lens assembly making up the optical pickup unit;  FIG. 4  is a perspective view of the top side of the optical pickup unit; and  FIG. 5  is a perspective view of the bottom side of the optical pickup unit. 
     Directions will be described. The side of a lens holder  5  ( FIGS. 1 ,  2 , and  4 ) on which objective lenses  31  and  32  are fitted refers to the top side of the lens holder  5 , or the top side of a lens holder assembly  6 , or the top side of a lens assembly  7 , or the top side of an optical pickup unit  1 . The side of the lens holder ( FIGS. 1 ,  3 , and  5 ) on which a heat transfer improving member  50  for reinforcement is fitted refers to the bottom side of the lens holder  5 , or the bottom side of the lens holder assembly  6 , or the bottom side of the lens assembly  7 , or the bottom side of the optical pickup unit  1 . 
     The direction from a coil  43  ( FIGS. 1 to 4 ) toward a coil  44  or the direction from the coil  44  toward the coil  43 , or the direction from a coil  45  toward a coil  46  or the direction from the coil  46  toward the coil  45  refers to the front-to-rear direction D 1  of the lens holder  5 , or the front-to-rear direction D 1  of the lens holder assembly  6 , or the front-to-rear direction D 1  of the lens assembly  7 , or the front-to-rear direction D 1  of the optical pickup unit  1 . As used herein, the front-to-rear direction D 1  is defined as a first direction D 1 . 
     The direction from the objective lenses  31  and  32  ( FIGS. 1 to 3 ) toward the heat transfer improving member  50  for reinforcement or the direction from the heat transfer improving member  50  for reinforcement toward the objective lenses  31  and  32  refers to the top-to-bottom direction D 2  of the lens holder  5 , or the top-to-bottom direction D 2  of the lens holder assembly  6 , or the top-to-bottom direction D 2  of the lens assembly  7 , or the top-to-bottom direction D 2  of the optical pickup unit  1 . As used herein, the top-to-bottom direction D 2  is defined as a second direction D 2 . 
     The direction from a coil  41  ( FIGS. 1 to 45 ) toward a coil  42  or the direction from the coil  42  toward the coil  41 , refers to the left-to-right direction D 3  of the lens holder  5 , or the left-to-right direction D 3  of the lens holder assembly  6 , or the left-to-right direction D 3  of the lens assembly  7 , or the left-to-right direction D 3  of the optical pickup unit  1 . As used herein, the left-to-right direction D 3  is defined as a third direction D 3 . 
     The definitions of the “front”, “rear”, “top”, “bottom”, “left”, and “right” used herein are merely given for convenience&#39; sake to describe the optical pickup unit  1  and an optical disc apparatus not shown. 
     Optical pickup is generally abbreviated to “OPU”. The optical pickup unit may also be abbreviated to “OPU”. As used herein, the OPU is the abbreviation of the optical pickup unit for the sake of convenience. 
     The objective lens is abbreviated to “OBL”. The OBLs  31  and  32  serve to focus laser light emitted from corresponding light emitting elements not shown onto a signal portion of an optical disc. The OBLs  31  and  32  are formed of a substantially colorless, transparent glass material. For example, a substantially colorless, transparent synthetic resin material is used, and the OBLs  31  and  32  may be formed based on injection molding superior in mass production capabilities. 
     The OPU  1  ( FIGS. 4 and 5 ) housed in the optical disc apparatus is used to reproduce or record data such as information from or on the optical disc not shown. The optical disc can be e.g., a CD-type optical disc and a DVD-type optical disc (both not shown). “CD” is the abbreviation of “Compact Disc” (trademark). “DVD” is the abbreviation of “Digital Versatile Disc” (registered trademark). 
     The optical disc will be described in detail. The optical disc can be, e.g., data read-only optical discs such as “CD-ROM” and “DVD-ROM”, data recordable optical discs such as “CD-R”, “DVD-R”, and “DVD+R”, and optical discs capable of data writing/erasing and data rewriting such as “CD-RW”, “DVD-RW”, “DVD+RW” (registered trademark), “DVD-RAM”, “HD DVD” (registered trademark), and “Blu-ray Disc” (registered trademark). 
     “ROM” of “CD-ROM” or “DVD-ROM” is the abbreviation of “Read Only Memory”. “CD-ROM” or “DVD-ROM” is a disc dedicated to reading of data/information therefrom. “R” of “CD-R”, “DVD-R”, or “DVD+R” is the abbreviation of “Recordable”. “CD-R”, “DVD-R”, or “DVD+R” is a disc enabling writing of data/information thereon. “RW” of “CD-RW”, “DVD-RW”, or “DVD+RW” is the abbreviation of “Re-Writable”. “CD-RW”, “DVD-RW”, or “DVD+RW” is a disc enabling rewriting of data/information thereon. “DVD-RAM” is the abbreviation of “Digital Versatile Disc Random Access Memory”. “DVD-RAM” is a disc enabling writing/reading and erasing of data/information. 
     “HD DVD” is the abbreviation of “High Definition DVD”. “HD DVD” is a disc compatible with a conventional DVD-type disc having a larger storage capacity than the conventional DVD-type disc. Infrared laser is used for the conventional CD. Red laser is used for the conventional DVD. However, blue-violet laser is used when reading data/information recorded on the optical disc in the form of “HD DVD”. “Blu-ray” means blue-violet laser employed to achieve high density recording, as against red laser used for conventional reading/writing of signals. 
     The optical disc may be e.g., an optical disc not shown whose both disc sides have respective signal surfaces to enable data writing/erasing and data rewriting. The optical disc may be e.g., an optical disc not shown having two-layered signal surfaces to enable data writing/erasing and data rewriting. The optical disc may be e.g., an optical disc not shown for “HD-DVD” having three-layered signal surfaces to enable data writing/erasing and data rewriting. The optical disc may be e.g., an optical disc not shown for “Blu-ray Disc” having four-layered signal surfaces to enable data writing/erasing and data rewriting. The optical disc may be e.g., an optical disc not shown having a labeled side to which laser light is applied to enable various writing on the label, etc. 
     The OPU  1  is used when reproducing data recorded on the various optical discs or recording data on the various writable or rewritable optical discs. The OPU  1  supports the CD-type optical disc, the DVD-type optical disc, etc. This OPU  1  is configured to be able to support plural different types of optical discs. 
     The OPU  1  ( FIGS. 4 and 5 ) includes a couple of OBLs  31  and  32  ( FIGS. 1 to 3 ) each focusing laser light on a signal surface not shown of an optical disc of Blu-ray Disc type, DVD type, CD type, etc.; a lens holder  5  holding the OBLs  31  and  32 ; a plurality of coils  41 ,  42 ,  43 ,  44 ,  45 , and  46  fitted on the lens holder  5 , capable of driving the lens holder  5 ; and a heat transfer improving member  50  fitted on the lens holder  5 , allowing heat generated in the coils  41 ,  42 ,  43 ,  44 ,  45 , and  46  when the coils  41 ,  42 ,  43 ,  44 ,  45 , and  46  are energized as a result of current flowing through the coils  41 ,  42 ,  43 ,  44 ,  45 , and  46  to be radiated while allowing the heat to be transferred substantially uniformly to the OBLs  31  and  32  via the lens holder  5 . 
     By fitting the OPU  1  with the heat transfer improving member  50 , thermal effects on the OBLs  31  and  32  are suppressed. By fitting the lens holder  5  with the heat transfer improving member  50 , the heat is effectively radiated that is generated in the coils  41 ,  42 ,  43 ,  44 ,  45 , and  46  when the coils  41 ,  42 ,  43 ,  44 ,  45 , and  46  are energized as a result of current flowing through the coils  41 ,  42 ,  43 ,  44 ,  45 , and  46 . By fitting the lens holder  5  with the heat transfer improving member  50 , the heat is substantially uniformly transferred with ease via the lens holder  5  to the OBLs  31  and  32  that is generated in the coils  41 ,  42 ,  43 ,  44 ,  45 , and  46  when the coils  41 ,  42 ,  43 ,  44 ,  45 , and  46  are energized as a result of current flowing through the coils  41 ,  42 ,  43 ,  44 ,  45 , and  46 . The occurrence of the deficiency is thus prevented that the OBLs  31  and  32  may not expand substantially uniformly when the coils  41 ,  42 ,  43 ,  44 ,  45 , and  46  are energized, whereupon aberrations may occur on a spot of laser light focused by the OBLs  31  and  32  to be applied to the optical disc. 
     When current flow through the coils  41 ,  42 ,  43 ,  44 ,  45 , and  46 , the whole lens holder  5  is warmed up substantially evenly to raise the temperature of the lens holder  5 . Accordingly, the temperature of lens peripheral portions  31   e  and  32   e , respectively, become substantially equal to the temperature of substantially central portions  31   c  and  32   c  of the OBLs  31  and  32 , respectively. It is thus prevented that the thermal expansions of the OBLs  31  and  32  may cause the aberrations on the spot of laser light focused by the OBLs  31  and  32  to be applied to the optical disc. 
     A driving part  2  ( FIGS. 4 and 5 ) of the OPU  1  is configured to include, e.g., the plurality of coils  41 ,  42 ,  43 ,  44 ,  45 , and  46  that generate electromagnetic forces when energized as a result of current flowing therethrough; a plurality of magnets  61 ,  62 ,  63 ,  64 ,  65 , and  66  corresponding to the plurality of coils  41 ,  42 ,  43 ,  44 ,  45 , and  46 , respectively, to generate magnetic forces at all times; a first yoke  81  ( FIG. 5 ) fitted with the magnets  61  and  62 ; a second yoke  82  fitted with the magnets  63 ,  64 ,  65 , and  66 ; the first OBL  31  which is positioned in the vicinity of the coil  41  ( FIG. 4 ) and through which laser light passes; the second OBL  32  which is positioned in the vicinity of the coil  42  and through which laser light passes; the heat transfer improving member  50  ( FIG. 5 ) to suppress the thermal effects on the couple of OBLs  31  and  32 ; the lens holder  5  ( FIGS. 1 to 5 ) fitted with the plurality of coils  41 ,  42 ,  43 ,  44 ,  45 , and  46  ( FIG. 4 ), the couple of OBLs  31  and  32 , and the heat transfer improving member  50 ; and a plurality of suspensions  70  ( FIGS. 4 and 5 ) resiliently supporting the lens holder  5 . 
     “Yoke” means a member structurally supporting the magnetic connection for example. The yoke serves to reduce the leak of magnetic forces arising from the magnets. The driving part  2  of the OPU  1  is configured as a so-called actuator  2 . “Actuator” means e.g., a driving device converting energy into a translational motion or a rotational motion. The actuator is abbreviated to “ACT”. When a focal point of laser light focused by the OBLs  31  and  32  is adjusted to be on a signal surface layer, the lens assembly  7  fitted with the OBLs  31  and  32  is moved back and forth and up and down by the actuator  2 . 
     Corresponding to the first-direction driving coil  41  ( FIG. 5 ), the first-direction driving magnet  61  is fixedly fitted on the first yoke  81 . Corresponding to the first-direction driving coil  42 , the first-direction driving magnet  62  is fixedly fitted on the first yoke  81 . Corresponding to the second-direction driving coil  43 , the second-direction driving magnet  63  is fixedly fitted on the second yoke  82 . Corresponding to the second-direction driving coil  44  ( FIG. 4 ), the second-direction driving magnet  64  is fixedly fitted on the second yoke  82 . Corresponding to the second-direction driving coil  45  ( FIGS. 4 and 5 ), the second-direction driving magnet  65  is fixedly fitted on the second yoke  82 . Corresponding to the second-direction driving coil  46  ( FIG. 4 ), the second-direction driving magnet  66  is fixedly fitted on the second yoke  82 . 
     The coils  41 ,  42 ,  43 ,  44 ,  45 , and  46  are formed by winding a thin linear conductor around coil fitting portions  11 ,  12 ,  23 ,  24 ,  25 , and  26  by means of a jig not shown for example. A thin enamel-coated electric wire for example was used as the conductor. The coils  41 ,  42 ,  43 ,  44 ,  45 , and  46  of two-layer winding type for example are formed by performing the winding work of the thin linear conductor coated with enamel. Depending on the design/specifications of the OPU  1 , coils not shown of other forms may be used in place of the coils  41 ,  42 ,  43 ,  44 ,  45 , and  46  shown in  FIGS. 1 to 5 . Single-layer wound coils ( 41 ,  42 ,  43 ,  44 ,  45 , and  46 ) may be used as the coils  41 ,  42 ,  43 ,  44 ,  45 , and  46 . The coils ( 41 ,  42 ,  43 ,  44 ,  45 , and  46 ) may be e.g., coils not shown formed by plating a circuit conductor on a substrate having a glass layer portion and a resin layer portion such as an epoxy resin layer. For example, a printed coil may be used as the coil. 
     As used herein, parentheses ( ) imparted to the reference numerals are used for convenience&#39;s sake to describe constituent elements of slightly different shapes from those shown. 
     The lens holder  5  fitted with the OBLs  31  and  32 , the coils  41 ,  42 ,  43 ,  44 ,  45 , and  46 , and the heat transfer improving member  50  is resiliently supported by the plurality of suspensions wires  70  ( FIGS. 4 and 5 ) in a movable manner. The metal suspension wires  70  are electrically connectably fitted on a control board  90 , i.e., a so-called circuit board  90  provided with a circuit conductor. 
     Used as the suspension wires  70  is an electric wire in the form of a thin conductor superior in a resiliently supporting performance. The metal suspensions wires  70  are inserted into wire attachment holes  5   w  of wire fitting portions  5   v  projecting from the first piece  10  of the lens holder  5  ( FIGS. 1 to 3 ) and are soldered to one ends of the coils  41 ,  42 ,  43 ,  44 ,  45 , and  46  to be fitted on the lens holder  5 . At that time, while the suspension wires  70  are inserted in the wire attachment holes  5   w  of a pair of the wire fitting portions  5   v ,  5   v , an adhesive such as an electron beam curable adhesive is applied to the wire attachment holes  5   w  so that the adhesive such as the electron beam curable adhesive cures to fix the suspension wires  70  to the lens holder  5 . The adhesive used is e.g., an adhesive of the same type as the adhesive for fixing the first OBL  31  and the second OBL  32  to the first piece  10  making up the lens holder  5  ( FIGS. 1 and 2 ). The lens holder  5  fitted with the OBLs  31  and  32  are resiliently supported by the plurality of metal suspension wires  70  ( FIGS. 4 and 5 ) in a movable manner. 
     A board body  91  of the circuit board  90  is formed from a synthetic resin material having excellent insulating properties. For example, a metal circuit conductor is formed on the board body  91  of the synthetic resin, on top of which an insulating film is disposed to make up the circuit board  90  (all not shown). The circuit board is called e.g., a PWB (printed wired board/printed wiring board). 
     A moving part  3  ( FIGS. 4 and 5 ) of the OPU  1  is configured to include e.g., the six coils  41 ,  42 ,  43 ,  44 ,  45 , and  46  generating electromagnetic forces when energized as a result of current flowing therethrough; the first OBL  31  which is positioned in the vicinity of the coil  41  ( FIG. 4 ) and through which laser light passes; the second OBL  32  which is positioned in the vicinity of the coil  42  and through which laser light passes; the heat transfer improving member  50  ( FIG. 5 ) suppressing the thermal effects on the couple of OBLs  31  and  32 ; the lens holder  5  ( FIGS. 1 to 5 ) fitted with the six coils  41 ,  42 ,  43 ,  44 ,  45 , and  46  ( FIG. 4 ), and the couple of OBLs  31  and  32 , and the heat transfer improving member  50 ; and the six suspension wires  70  ( FIGS. 4 and 5 ) resiliently supporting the lens holder  5 . 
     The circuit board  90  and the suspension wires  70  are electrically connected, and the suspension wires  70  and the coils  41 ,  42 ,  43 ,  44 ,  45 , and  46  are electrically connected, so that current flows from the circuit board  90  via the suspension wires  70  to the coils  41 ,  42 ,  43 ,  44 ,  45 , and  46  to move the lens holder  5  fitted with the coils  41 ,  42 ,  43 ,  44 ,  45 , and  46 , the objective lenses  31  and  32 , and the heat transfer improving member  50 . 
     The heat transfer improving reinforcement member  50  is formed with a pair of laser light passing holes  51   a  and  51   b  ( FIGS. 1 ,  3 , and  5 ) allowing laser light to pass therethrough. The laser light passing holes  51   a  and  51   b  formed on the heat transfer improving reinforcement member  50  serves also as a lightening part for reducing the weight of the heat transfer improving reinforcement member  50 . 
     The heat transfer improving member  50  ( FIGS. 1 to 3 ) serves also as the reinforcement member  50  for reinforcing the rigidity and strength of the lens holder  5 . 
     This prevents the occurrence of the deficiency that the lens holder  5  fitted with the couple of OBLs  31  and  32  vibrates significantly to cause the plastic deformation of the lens holder  5  upon seeking of the OPU  1  for example. By allowing the heat transfer improving member  50  to serve also as the reinforcement member  50  reinforcing the rigidity and the strength of the lens holder  5 , the vibrations occurring in the lens holder  5  are suppressed. Thus, the OPU  1  allowing for the vibrations is configured. 
     As shown in  FIGS. 3 and 5 , the lens holder  5  is in the shape of a substantially rectangular box having a bottom opening  5   a . The heat transfer improving reinforcement member  50  ( FIGS. 1 and 3 ) is in the shape of a substantially flat plate ( FIG. 1 ) corresponding to the bottom opening  5   a  ( FIG. 3 ) of the lens holder  5 . The heat transfer improving reinforcement member  50  is fitted in the bottom opening  5   a  ( FIG. 3 ) of the lens holder  5  to make up the lens holder assembly  6 . 
     This improves the rigidity of the lens holder assembly  6  provided with the substantially rectangular box-shaped lens holder  5 . By fitting the substantially flat plate-shaped heat transfer improving reinforcement member  50  in the bottom opening  5   a  of the substantially rectangular box-shaped lens holder  5 , the heat transfer improving reinforcement member  50  functions as e.g., a substantially flat plate-shaped lid for the substantially rectangular box-shaped lens holder  5 . For example, when comparing a strength between a box-shaped object with its lid opened and a box-shaped object with its lid closed, the box-shaped object with its lid closed is stronger than the box-shaped object with its lid opened. Based on this, by fitting the substantially flat plate-shaped heat transfer improving reinforcement member  50  in the bottom opening  5   a  of the substantially rectangular box-shaped lens holder  5 , improvements are achieved in the rigidity and strength of the lens holder assembly  6  including the lens holder  5  and the heat transfer improving reinforcement member  50 . The improved rigidity of the lens holder assembly  6  prevents the occurrence of higher-order resonance in which the lens holder  5  provided with the plurality of objective lenses  31  and  32  vibrates significantly when the OPU  1  seeks for example. Thus, the OPU  1  is configured that allows for the higher-order resonance and that is easy to control. 
     As used herein, the higher-order resonance means the state where the moving part  3  making up the OPU  1  resonates to such an extent that it deforms. 
     The lens holder  5  ( FIGS. 1 and 2 ) making up the lens holder assembly  6  includes lens fitting portions  13   a  and  13   b  fitted with the OBLs  31  and  32  and member fitting portions  17  and  27  ( FIG. 3 ) fitted with the heat transfer improving reinforcement member  50 . The member fitting portions  17  and  27  ( FIG. 3 ) are formed at one side of the lens holder  5 , which is substantially opposite to the lens fitting portions  13   a  and  13   b  ( FIG. 1 ) formed at the other side of the lens holder  5 . The lens fitting portions  13   a  and  13   b  are formed on the top side of the lens holder  5  ( FIGS. 1 and 2 ), while the member fitting portions  17  and  27  are formed on the bottom side of the lens holder  5  ( FIG. 3 ). The OBLs  31  and  32  making up the lens assembly  7  ( FIG. 1 ) are fitted on an upper portion  5   c  of the lens holder  5 , whereas the heat transfer improving reinforcement member  50  ( FIG. 3 ) is fitted on a lower portion  5   d  of the lens holder  5  substantially opposite to the upper portion  5   c  of the lens holder  5 . 
     By configuring the lens assembly  7  in this manner, the thermal effects on the OBLs  31  and  32  are suppressed. Since in the lens holder  5 , the heat transfer improving reinforcement member  50  is fitted on the member fitting portions  17  and  27  substantially opposite to the lens fitting portions  13   a  and  13   b  fitted with the OBLs  31  and  32 , heat transferred to the OBLs  31  and  32  becomes substantially uniform and the temperature gradients in the OBLs  31  and  32  become decreased. Most of heat generated from the coils  41 ,  42 ,  43 ,  44 ,  45 , and  46  mounted on the lens holder  5  is transferred to the heat transfer improving reinforcement member  50  fitted on the member fitting portions  17  and  27  of the lens holder  5 , to be radiated therefrom. In the lens holder  5 , since the OBLs  31  and  32  are fitted on the lens fitting portions  13   a  and  13   b  substantially opposite to the member fitting portions  17  and  27  fitted with the heat transfer improving reinforcement member  50 , part of heat is transferred to the OBLs  31  and  32  fitted on the lens fitting portions  13   a  and  13   b  of the lens holder  5 . 
     The thus configured OPU  1  suppresses the “temperature unevenness” which may occur in the OBLs  31  and  32 . Due to having suppressed “temperature unevenness” in the OBLs  31  and  32 , the OBLs  31  and  32  thermally expand substantially uniformly. The substantially uniform thermal expansions of the OBLs  31  and  32  prevent aberrations from occurring on a spot of laser light focused by the OBLs  31  and  32  to be applied to an optical disc. Thus, the OPU  1  is configured in which the aberrations hardly occur and that is easy to control. 
     There are used, as the OBLs  31  and  32  ( FIG. 1 ), two different types of OBLs  31  and  32 , i.e., the first OBL  31  for a first-wavelength laser light, and the second OBL  32  for a second-wavelength laser light having a wavelength different from that of the first-wavelength laser light. 
     The lens holder  5  ( FIGS. 1 and 2 ) includes the first lens fitting portion  13   a  fitted with the first OBL  31 , the second lens fitting portion  13   b  fitted with the second OBL  32 , and the member fitting portions  17  and  27  ( FIG. 3 ) fitted with the heat transfer improving reinforcement member  50 . The member fitting portions  17  and  27  are formed at one side of the lens holder  50 , which is substantially opposite to the first lens fitting portion  13   a  and the second lens fitting portion  13   b  formed at the other side of the lens holder  5 . 
     The first OBL  31  and the second OBL  32  are fitted on the lens fitting portions  13   a  and  13   b  ( FIG. 1 ) of the lens holder  5 , and the heat transfer improving reinforcement member  50  is fitted on the member fitting portions  17  and  27  of the lens holder  5 , to make up the lens assembly  7  ( FIGS. 2 and 3 ). 
     This allows a well-balanced lens assembly  7  taking heat measures to be made up. The lens assembly  7  of the OPU  1  is configured to include the plurality of coils  41 ,  42 ,  43 ,  44 ,  45 , and  46  that generate electromagnetic forces when energized as a result of current flowing therethrough; the first OBL  31  which is positioned in the vicinity of the coil  41  and through which the first-wavelength laser light passes; the second OBL  32  which is positioned in the vicinity of the coil  42  and through which the second-wavelength laser light passes; the heat transfer improving reinforcement member  50  suppressing the thermal effects on the couple of OBLs  31  and  32 ; and the lens holder  5  fitted with the plurality of coils  41 ,  42 ,  43 ,  44 ,  45 , and  46 , the couple of OBLs  31  and  32 , and the heat transfer improving reinforcement member  50 . 
     Due to the first OBL  31  and the second OBL  32  fitted on the lens fitting portions  13   a  and  13   b  disposed on the top side of the lens holder  5 , the weight of the side of the lens fitting portions  13   a  and  13   b  toward the top of the lens holder  5  increases. However, the heat transfer improving reinforcement member  50  is fitted on the member fitting portions  17  and  27 , that are at the bottom side of the lens holder  5 , and that are at one side of the lens holder  5 , which is opposite to the lens fitting portions  13   a  and  13   b  at the other side of the lens holder  5 . Therefore, the weight of the side of the member fitting portions  17  and  27  at the bottom of the lens holder  5  is increased, and thereby the balance of the lens assembly  7  as a whole is kept. The heat transfer improving reinforcement member  50  acts as a balancer. Since the balance of the whole lens assembly  7  fitted with the plurality of OBLs  31  and  32  is kept by mounting the lens assembly  7  with the heat transfer improving reinforcement member  50 , the occurrence is easily prevented of the deficiency that the OPU  1  fitted with the lens assembly  7  may suffer from the runaway upon seeking of the OPU  1  for example. 
     In the case of the lens holder  5  fitted with the couple of OBLs  31  and  32 , i.e., the first OBL  31  and the second OBL  32 , yawing occurs easily when the lens holder  5  is driven. By fitting the lens holder  5  with the heat transfer improving member  50  acting also as the reinforcement member  50 , yawing occurring in the lens holder  5  is suppressed. 
     For example, the electron beam curable adhesive not shown that cures by irradiating with electron beams was used as an adhesive fixing the first OBL  31  and the second OBL  32  to the first piece  10  making up the lens holder  5  ( FIGS. 1 and 2 ). Otherwise, for example, an ultraviolet curable adhesive that cures by irradiating with ultraviolet rays was used as the adhesive. 
     The first OBL  31  is fitted on the first lens fitting portion  13   a  of the first piece  10  making up the lens holder  5 , and the electron beam curable adhesive is applied to a peripheral portion  13   c  of the first lens fitting portion  13   a  through a peripheral portion  31   e  of the first OBL  31 , and the electron beams are applied to the electron beam curable adhesive, whereby the first OBL  31  is fixed to the first lens fitting portion  13   a  in a short period of time. Otherwise, the first OBL  31  is fitted on the first lens fitting portion  13   a  of the first piece  10  making up the lens holder  5 , and the ultraviolet curable adhesive is applied to a peripheral portion  13   c  of the first lens fitting portion  13   a  through a peripheral portion  31   e  of the first OBL  31 , and the ultraviolet rays are applied to the ultraviolet curable adhesive, whereby the first OBL  31  is fixed to the first lens fitting portion  13   a  in a short period of time. 
     The second OBL  32  is fitted on the second lens fitting portion  13   b  of the first piece  10  making up the lens holder  5 , and the electron beam curable adhesive is applied to a peripheral portion  13   d  of the second lens fitting portion  13   b  through a peripheral portion  32   e  of the second OBL  32 , and the electron beams are applied to the electron beam curable adhesive, whereby the second OBL  32  is fixed to the second lens fitting portion  13   b  in a short period of time. Otherwise, the second OBL  32  is fitted on the second lens fitting portion  13   b  of the first piece  10  making up the lens holder  5 , and the ultraviolet curable adhesive is applied to a peripheral portion  13   d  of the second lens fitting portion  13   b  through a peripheral portion  32   e  of the second OBL  32 , and the ultraviolet rays are applied to the ultraviolet curable adhesive, whereby the second OBL  32  is fixed to the second lens fitting portion  13   b  in a short period of time. 
     This allows the first OBL  31  and the second OBL  32  to accurately and promptly be fixed to the first piece  10  making up the lens holder  5 . Since the first OBL  31  is fixed with high accuracy to the first lens fitting portion  13   a  of the first piece  10  making up the lens holder  5 , laser light is applied with high accuracy to the signal surface portion of the optical disc. Since the second OBL  32  is fixed with high accuracy to the second fitting portion  13   b  of the first piece  10  making up the lens holder  5 , laser light is applied with high accuracy to the signal surface portion of the optical disc. Since the first OBL  31  and the second OBL  32  are fixed promptly to the first piece  10  making up the lens holder  5 , prompt work is ensured of adhesion of the first OBL  31  and the second OBL  32  to the first piece  10  making up the lens holder  5 . Thus, the adhesion step in the assembly process of the OPU  1  is speeded up. This leads to a reduction in the price of the OPU  1 . 
     The ultraviolet curable adhesive as one type of the electron beam curable adhesive can be, e.g., an adhesive of OPTOCAST (trade name) series manufactured by EMI Ltd, USA. A specific ultraviolet curable adhesive can be OPTOCAST3400, OPTOCAST 3415, etc., manufactured by EMI Ltd., USA. The ultraviolet curable adhesive such as OPTOCAST3400, OPTOCAST3415, etc., is an epoxy adhesive and is a one-part ultraviolet curable adhesive. The epoxy ultraviolet curable adhesive is of low contraction properties and high heatproof properties and is superior in chemical resistance and humidity resistance. Use of the one-part ultraviolet curable adhesive eliminates the necessity of a work of mixing two different types of liquids performed when a two-part ultraviolet curable adhesive is used. Thus, the adhesive applying step becomes prompt and effective. 
     The ultraviolet curable adhesive as one type of the electron beam curable adhesive can be, e.g., optical UV adhesives NOA60, NOA83H, etc., manufactured by Norland Products Inc, USA. The ultraviolet curable adhesive such as the optical UV adhesives NOA60, NOA83H, etc., is an acryl adhesive and is a one-part ultraviolet curable adhesive. The acryl ultraviolet curable adhesive is of a short curing time and is curable within several seconds. “UV” means “ultraviolet”. “Ultraviolet radiation” means “ultraviolet rays”. The ultraviolet curable adhesive is called an UV curable adhesive, etc. Depending on the design specifications of the optical pickup unit, the adhesion step may be performed using e.g., a two-part ultraviolet curable adhesive. The two-part ultraviolet curable adhesive can be e.g., a two-part epoxy ultraviolet curable adhesive. 
     This OPU  1  is an OPU  1  handling three different wavelengths of the first-wavelength laser light, the second-wavelength laser light whose wavelength is different from that of the first-wavelength laser light, and third-wavelength laser light whose wavelength is different from those of the first-wavelength laser light and the second-wavelength laser light. The first-wavelength laser light is e.g., blue-violet laser light for “HD-DVD” and “Blu-ray Disc” having a wavelength of about 390 nm (nanometers) to 420 nm with its reference wavelength of substantially 405 nm. The second-wavelength laser light is e.g., red laser light for “DVD” having a wavelength of about 630 nm to 685 nm with its reference wavelength of substantially 635 nm or 650 nm. The third-wavelength laser light is e.g., infrared laser light for “CD” having a wavelength of about 770 nm to 830 nm with its reference wavelength of substantially 780 nm. 
     For example, the first OBL  31  for the first-wavelength laser light to serve “Blu-ray Disc” only. The first OBL  31  has a numerical aperture of substantially 0.85. The numerical aperture refers to the product of the refractive index of a medium in front of the OBL and the sign of the angle at which the effective radius (the radius of entrance pupil) of the OBL is viewed from an object point by an optical instrument. The numerical aperture is used to represent the performances of the OBL. The numerical aperture is abbreviated to “NA”. 
     For example, the second OBL  32  for three different types of laser lights, i.e., the first-wavelength laser light for “HD DVD”, the second-wavelength laser light for “DVD”, and the third-wavelength laser light for “CD”. The second OBL  32  has the numerical aperture of substantially 0.6. A broadband quarter-wave plate with aperture limit not shown is disposed on an optical path of laser light passing through the second OBL  32 . By virtue of the disposition of the broadband quarter-wave plate with aperture limit, the second OBL  32  functions as e.g., one having the numerical aperture of about 0.37 to 0.95, substantially 0.45 to 0.65. 
     As shown in  FIG. 1 , the first lens fitting portion  13   a  fitted with the first OBL  31  is concavely formed on the upper portion  5   c  of a body  5   b  of the lens holder  5 . The second lens fitting portion  13   b  fitted with the second OBL  32  is concavely formed on the upper portion  5   c  of a body  5   b  of the lens holder  5 . 
     The coil fitting portion  11  fitted with the coil  41  is convexly formed from a side surface portion  5   e  of the body  5   b  of the lens holder  5  toward the outside of the body  5   b . The coil fitting portion  12  fitted with the coil  42  is convexly formed from the side surface portion  5   e  of the body  5   b  of the lens holder  5  toward the outside of the body  5   b.    
     The coil fitting portion  23  fitted with the coil  43  is convexly formed from a frame portion  5   f  making up the holder  5  toward the outside of the frame portion  5   f . The coil fitting portion  24  fitted with the coil  44  is convexly formed from a frame portion  5   f  making up the holder  5  toward the outside of the frame portion  5   f . The coil fitting portion  25  fitted with the coil  45  is convexly formed from the frame portion  5   f  making up the holder  5  toward the outside of the frame portion  5   f . The coil fitting portion  26  fitted with the coil  46  is convexly formed from the frame portion  5   f  making up the holder  5  toward the outside of the frame portion  5   f.    
     Between the first lens fitting portion  13   a  and the coil fitting portion  11  is disposed a first heat transfer cutoff aperture  14   a  that prevents heat generated in the coil  41  from being transferred to the first OBL  31 . The heat dissipating gap  14   a  lies between the body  5   b  of the lens holder  5  and the coil fitting portion  11 . The heat transfer cutoff aperture  14   a  is disposed in the lens holder  5  to cut off a path of heat. 
     This facilitates the prevention of the transfer of heat generated in the coil  41  ( FIGS. 1 and 2 ) to the first OBL  31 . By virtue of the first heat transfer cutoff aperture  14   a  disposed between the first lens fitting portion  13   a  of the lens holder  5  and the coil fitting portion  11  of the lens holder  5 , heat generated in the coil  41  hardly transfers to the first OBL  31 . Heat generated in the coil  41  when energized as a result of current flowing therethrough transfers from the coil  41  to the coil fitting portion  11  of the lens holder  5 . Since the first heat transfer cutoff aperture  14   a  is disposed between the coil fitting portion  11  convexly formed on the side surface portion  5   e  of the body  5   b  of the lens holder  5  and the first lens fitting portion  13   a  concavely formed on the upper portion  5   c  of the body  5   b  of the lens holder  5 , it is prevented that most of heat may be transferred from the coil fitting portion  11  of the lens holder  5  to the first lens fitting portion  13   a  of the lens holder  5 . Thus, thermal effects on the first OBL  31  is suppressed. 
     Between the second lens fitting portion  13   b  and the coil fitting portion  12  is disposed a second heat transfer cutoff aperture  14   b  that prevents heat generated in the coil  42  from being transferred to the second OBL  32 . The heat dissipating gap  14   b  lies between the body  5   b  of the lens holder  5  and the coil fitting portion  12 . The heat transfer cutoff aperture  14   b  is disposed in the lens holder  5  to cut off a path of heat. 
     This facilitates the prevention of the transfer of heat generated in the coil  42  to the second OBL  32 . By virtue of the second heat transfer cutoff aperture  14   b  disposed between the second lens fitting portion  13   b  of the lens holder  5  and the coil fitting portion  12  of the lens holder  5 , heat generated in the coil  42  hardly transfers to the second OBL  32 . Heat generated in the coil  42  when energized as a result of current flowing therethrough transfers from the coil  42  to the coil fitting portion  12  of the lens holder  5 . Since the second heat transfer cutoff aperture  14   b  is disposed between the coil fitting portion  12  convexly formed on the side surface portion  5   e  of the body  5   b  of the lens holder  5  and the second lens fitting portion  13   b  concavely formed on the upper portion  5   c  of the body  5   b  of the lens holder  5 , it is prevented that most of heat may be transferred from the coil fitting portion  12  of the lens holder  5  to the second lens fitting portion  13   b  of the lens holder  5 . Thus, thermal effects on the second OBL  32  is suppressed. 
     The heat transfer improving reinforcement member  50  ( FIG. 1 ) is formed by punching a metal plate of a nonferrous metal material superior in heat conduction properties and heat radiation properties. The first piece  10  and the second piece  20  making up the lens holder  5  are formed from a synthetic resin material reducing the weight of the lens holder  5  and having an excellent moldability, using injection molding superior in mass production capabilities. 
     This prevents uneven temperature gradients from occurring in each of the OBLs  31  and  32 . By mating and fitting the nonferrous metal heat transfer improving reinforcement member  50  with and on the lens holder  5  configured with the synthetic resin first piece  10  and second piece  20  in assembled relation, flow of heat originating from the coils  41 ,  42 ,  43 ,  44 ,  45 , and  46  becomes uniform. Thus, heat generated in the coils  41 ,  42 ,  43 ,  44 ,  45 , and  46  is transferred via the lens holder  5  substantially uniformly to the OBLs  31  and  32 . 
     Since in general the nonferrous metals are resistant to corrosion, the nonferrous metal heat transfer improving reinforcement member  50  is mounted on the OPU  1  having a rising temperature without developing its corrosion over a long period of time. The nonferrous metals can be, e.g., aluminum or magnesium. Nonferrous metal alloys can be, e.g., aluminum alloys or magnesium alloys. To be concrete, the nonferrous metal materials such as aluminum alloys can be, e.g., nonferrous metal materials containing about 0.01% to 0.6% of performance improving components such as magnesium (Mg), manganese (Mn), etc. 
     Since in general the metal material molded part has a rigidity higher than that of the resin material molded part, an improved rigidity is imparted to the lens holder assembly  6  configured by mating and fitting the nonferrous heat transfer improving reinforcement member  50  with and on the synthetic resin holder  5 . The improved rigidity of the lens holder assembly  6  prevents the occurrence of the higher-order resonance that the lens holder assembly  6  provided with the OBLs  31  and  32  may vibrate remarkably upon seeking of the OPU  1  for example. Thus, the OPU  1  is configured that allows for the higher-order resonance and that is easy to control. 
     The heat transfer improving reinforcement member  50  ( FIG. 1 ) is formed as a non-magnetic member not affected by magnetic forces arising at all times from magnetic materials such as the magnets  61 ,  62 ,  63 ,  64 ,  65 , and  66  ( FIGS. 4 and 5 ) and electromagnetic forces generated in the coils  41 ,  42 ,  43 ,  44 ,  45 , and  46  ( FIGS. 1 to 4 ) when energized as a result of current flowing therethrough. 
     In order to prevent a short circuit from occurring as a result of a contact of the metal coils  41 ,  42 ,  43 ,  44 ,  45 , and  46  with the metal heat transfer improving reinforcement member  50 , the metal coils  41 ,  42 ,  43 ,  44 ,  45 , and  46  fitted on the lens holder  5  are spaced apart from and in non-contact with the metal heat transfer improving reinforcement member  50  ( FIG. 3 ). In order to insulate current flowing through the coils  41 ,  42 ,  43 ,  44 ,  45 , and  46  from the metal heat transfer improving reinforcement member  50 , the lens holder  5  is formed of a synthetic resin material having excellent insulating properties. 
     The lens holder  5  fitted with the coils  41 ,  42 ,  43 ,  44 ,  44 ,  45 , and  46  is formed of a synthetic resin material having excellent insulating properties. The lens holder  5  is formed of a synthetic resin material having a less specific gravity than the metal materials and suited for the weight reduction. Detailedly, the lens holder  5  is formed from a thermoplastic resin such as a liquid crystal polymer having an excellent moldability, based on the injection molding superior in mass production capabilities. The liquid crystal polymer can be, e.g., VECTRA (registered trademark), etc., manufactured by Polyplastic Co. Products of VECTRA can be, e.g., grade A410, S471, etc. 
     The heat transfer improving reinforcement member  50  ( FIG. 1 ) is formed by punching a metal plate of an aluminum material such as an aluminum alloy into a substantially flat plate with lightening portions. The heat transfer improving reinforcement member  50  is formed as an aluminum plate. 
     By forming the heat transfer improving reinforcement member  50  from the aluminum material, uneven temperature gradients are prevented from occurring in the OBLs  31  and  32  ( FIGS. 1 to 3 ). The aluminum material molded part has excellent heat conduction properties, that is, the aluminum material molded part excels in heat conduction. The thermal conductivity of the aluminum material molded part is about three times that of the iron material molded part. For example, the thermal conductivity of the iron material molded part is about 47 kcal/m·hr·° C., whereas the thermal conductivity of the aluminum material molded part is about 180 kcal/m·hr·° C. In this manner, the aluminum material molded part conducts heat easily. Thus, heat generated in the coils  41 ,  42 ,  43 ,  44 ,  45 , and  46  when the coils  41 ,  42 ,  43 ,  44 ,  45 , and  46  are energized as a result of current flowing through the coils  41 ,  42 ,  43 ,  44 ,  45 , and  46  is effectively transferred to the aluminum heat transfer improving reinforcement member  50  to be radiated therefrom. 
     When the coils  41 ,  42 ,  43 ,  44 ,  45 , and  46  generate heat at the same time with current supplied to the coils  41 ,  42 ,  43 ,  44 ,  45 , and  46  at the same time, the heat transfer improving reinforcement member  50  of the nonferrous metal such as aluminum is first warmed up. Afterward, heat rises up from the lower portion  5   d  of the lens holder  5  toward the upper portion  5   c  thereof to be transferred substantially uniformly to the OBLs  31  and  32 . 
     The aluminum material molded part has a high specific rigidity. Since in general the molded part of an aluminum material such as an aluminum alloy has a higher rigidity than the synthetic resin material molded part, the rigidity of the lens holder assembly  6  is improved that is configured by mating and fitting the aluminum heat transfer improving reinforcement member  50  with and on the synthetic resin lens holder  5 . The improved rigidity of the lens holder assembly  6  prevents the occurrence of the higher-order resonance that the lens holder assembly  6  provided with the OBLs  31  and  32  vibrates remarkably when the OPU  1  seeks for example. Thus, the OPU  1  is configured that allows for the higher-order resonance and that is easy to control. 
     The aluminum material molded part is suited for weight reduction. For example, the specific gravity of aluminum is about one third that of iron. For example, the specific gravity of iron is about 7.87, whereas that of aluminum is about 2.71. The weight reduction of the OPU  1  is achieved by forming the substantially flat plate-like heat transfer improving reinforcement member  50  with lightening portions by punching a metal plate of an aluminum material. In spite of the heat transfer improving reinforcement member  50  being mounted on the lens holder  5 , the operating performance responsivity of the lens holder  5  is prevented from being lowered. 
     The aluminum material molded part is a non-magnetic part free from magnetization and not affected by the magnetic field. This prevents the aluminum heat transfer improving reinforcement member  50  from magnetically affecting the coils  41 ,  42 ,  43 ,  44 ,  45 , and  46  and the magnets  61 ,  62 ,  63 ,  64 ,  65 , and  66 . Thus, the OPU  1  is configured that no adverse effects are exerted magnetically on the plurality of coils  41 ,  42 ,  43 ,  44 ,  45 , and  46  capable of driving the lens holder  5 , and on the plurality of magnets  61 ,  62 ,  63 ,  64 ,  65 , and  66 . 
     It is preferred to use, as the nonferrous metal material whose main component is aluminum, an aluminum alloy containing about 0.05% to 0.6% of at least one performance improving component selected from a group consisting of magnesium (Mg) and manganese (Mn) for example. 
     The nonferrous metal material having aluminum as its main component can be, e.g., an aluminum-magnesium (Al—Mg) type alloy that is superior in workability and corrosion resistance. More specifically, the nonferrous metal material having aluminum as its main component can be, e.g., a 5000 series material defined based on “JIS H4000”. To be concrete, the aluminum alloy can be e.g., 5005 (A5005), 5052 (A5052), 5056 (A5056), 5083 (A5083), or 5086 (A5086) defined based on “JIS H4000”. 
     The nonferrous metal material having aluminum as its main component can be, e.g., an aluminum-manganese (Al—Mn) type alloy that is superior in workability and corrosion resistance. More specifically, the nonferrous metal material having aluminum as its main component can be, e.g., a 3000 series material defined based on “JIS H4000”. To be concrete, the aluminum alloy can be e.g., 3003 (A3003), 3004 (A3004), or 3005 (A3005) defined based on “JIS H4000”. 
     Depending on the design/specifications of the optical pickup unit  1 , the heat transfer improving reinforcement member  50  whose main component is aluminum may be replaced by the heat transfer improving reinforcement member  50  whose component is a nonferrous metal other than aluminum. For example, in place of the heat transfer improving reinforcement member  50  whose main component is aluminum, the heat transfer improving reinforcement member  50  having copper as its main component was used. The heat transfer improving reinforcement member  50  made mainly of copper serves as a balancer. Since copper is a heavy material with a specific gravity of about 8.92, the balance is kept of the lens holder  5  fitted with the two OBLs  31  and  32  on the upper portion  5   c  of the body  5   b  of the lens holder  51 , by fitting the heat transfer improving member  50  whose main component is copper onto the lower portion  5   d  of the lens holder  5  fitted with the two OBLs  31  and  32 . 
     The heat transfer improving reinforcement member  50  used in lieu of the aluminum heat transfer improving reinforcement member  50  is formed by punching a metal plate of a copper material such as a copper alloy into a substantially flat plate with lightening portions. The heat transfer improving reinforcement member  50  is formed as a copper plate. 
     By forming the heat transfer improving reinforcement member  50  from the copper material, it is prevented that uneven temperature gradients may occur in the OBLs  31  and  32 . The copper material molded part has excellent heat conduction properties. For example, the thermal conductivity of a pure copper material molded part is at least six times that of the iron material molded part. For example, the thermal conductivity of the iron material molded part is about 0.150 cal/cm·/sec/° C., whereas that of the pure copper material molded part is about 0.938 cal/cm·/sec/° C. For example, the thermal conductivity of the copper material molded part is at least about 1.5 times that of the aluminum material molded part. The thermal conductivity of the aluminum material molded part is about 0.534 cal/cm·/sec/° C., whereas that of the pure copper material molded part is about 0.938 cal/cm·/sec/° C. In this manner, the copper material molded part conducts heat easily. Thus, heat generated in the coils  41 ,  42 ,  43 ,  44 ,  45 , and  46  when the coils  41 ,  42 ,  43 ,  44 ,  45 , and  46  are energized as a result of current flowing through the coils  41 ,  42 ,  43 ,  44 ,  45 , and  46  is effectively transferred to the copper heat transfer improving reinforcement member  50  to be radiated therefrom. 
     When the coils  41 ,  42 ,  43 ,  44 ,  45 , and  46  generate heat at the same time with current supplied to the coils  41 ,  42 ,  43 ,  44 ,  45 , and  46  at the same time, the heat transfer improving reinforcement member  50  of the nonferrous metal such as copper is first warmed up. Afterward, heat rises up from the lower portion  5   d  of the lens holder  5  toward the upper portion  5   c  thereof to be transferred substantially uniformly to the OBLs  31  and  32 . 
     The molded part of a copper material selected from various types of copper materials for use in the optical pickup unit  1  is a non-magnetic part free from magnetization and not affected by the magnetic field. This prevents the copper heat transfer improving reinforcement member  50  from magnetically affecting the coils  41 ,  42 ,  43 ,  44 ,  45 , and  46  and the magnets  61 ,  62 ,  63 ,  64 ,  65 , and  66 . Thus, the OPU  1  is configured that no adverse effects are exerted magnetically on the plurality of coils  41 ,  42 ,  43 ,  44 ,  45 , and  46  capable of driving the lens holder  5 , and on the plurality of magnets  61 ,  62 ,  63 ,  64 ,  65 , and  66 . 
     The nonferrous metal material whose main component is copper can be, e.g., a tough pitch copper that is superior in workability and anticorrosion. The tough pitch copper can be, e.g., C1100, etc., defined based on “JIS H3100”. The nonferrous metal material having copper as its main component can be, e.g., an oxygen free copper that is superior in workability and corrosion resistance. The oxygen free copper can be, e.g., C1020, etc., defined based on “JIS H3100”. The nonferrous metal material made mainly of copper can be, e.g., a phosphorous-deoxidized copper that is superior in workability and anticorrosion. The phosphorous-deoxidized copper can be, e.g., C1201, etc., defined based on “JIS H3100”. 
     The heat transfer improving reinforcement member  50  is provided with a pair of gate trace preventing portions  52 ,  52  in the shape of through-holes that prevent a pair of injection gate trace portions not shown of the lens holder  5  from interfering with the heat transfer improving reinforcement member  50 . The pair of through-hole-shaped gate trace preventing portions  52 ,  52  disposed in the heat transfer improving reinforcement member  50  serves also as the lightening portions for reducing the weight of the heat transfer improving reinforcement member  50 . 
     As shown in  FIG. 3 , the aluminum alloy heat transfer improving reinforcement member  50  is fitted on the lens holder  5  such that the member  50  abuts against walls  5   g  and  5   h  making up the synthetic resin lens holder  5  (i.e., the walls  5   g  and  5   h  are constituents of the synthetic resin lens holder  5 ). 
     This allows heat originating from the coils  41 ,  42 ,  43 ,  44 ,  45 , and  46  fitted on the lens holder  5  to be transferred via the walls  5   g  and  5   h  of the lens holder  5  to the heat transfer improving reinforcement member  50 . More specifically, heat arising from the coils  41  and  42  fitted on the lens holder  5  is securely transferred via the wall  5   g  of the lens holder  5  to the heat transfer improving reinforcement member  50 . Heat arising from the coils  43 ,  44 ,  45 , and  46  fitted on the lens holder  5  is securely transferred via the wall  5   h  of the lens holder  5  to the heat transfer improving reinforcement member  50 . 
     Since the heat transfer improving reinforcement member  50  abuts against the walls  5   g  and  5   h  making up the lens holder  5 , heat is prevented from accumulating in the lens holder  5 . Heat originating from the coils  41  and  42  are transferred to the heat transfer improving reinforcement member  50  abutting against the wall  5   g  of the lens holder  5  to be effectively radiated from the heat transfer improving reinforcement member  50 . Heat originating from the coils  43 ,  44 ,  45 , and  46  are transferred to the heat transfer improving reinforcement member  50  abutting against the wall  5   h  of the lens holder  5  to be effectively radiated from the heat transfer improving reinforcement member  50 . 
     Using an adhesive not shown, the heat transfer improving reinforcement member  50  is secured to the lower portion  5   d  of the lens holder  5 . The adhesive used was an adhesive containing a thermoset resin. More specifically, use was made of an epoxy resin that is the thermoset resin as the adhesive for fixing the heat transfer improving reinforcement member  50  to the lens holder  5 . To be concrete, used as the adhesive was e.g., an ultraviolet curable adhesive of the epoxy type that becomes cured by irradiating with ultraviolet rays. Depending on the design/specifications of the optical pickup unit  1 , an ordinary epoxy adhesive for example may be used instead of the ultraviolet curable adhesive. 
     The heat transfer improving reinforcement member  50  is provided with a pair of positioning apertures  58 ,  58  ( FIG. 1 ) for facilitating the positioning relative to the lens holder  5  ( FIG. 3 ). The pair of positioning apertures  58 ,  58  disposed in the heat transfer improving reinforcement member  50  serves also as lightening portions for reducing the weight of the heat transfer improving reinforcement member  50 . The lens holder  5  (FIG.  3 ) is provided with a pair of positioning protuberances  28 ,  28  corresponding to the pair of positioning apertures  58 ,  58  disposed in the heat transfer improving reinforcement member  50 . 
     By engaging the pair of positioning apertures  58 ,  58  of the heat transfer improving reinforcement member  50  with the pair of positioning protuberances  28 ,  28  of the lens holder  5 , the heat transfer improving reinforcement member  50  is mounted on the lens holder  5  with high accuracy. 
     The heat transfer improving reinforcement member  50  is provided with a pair of positioning protrusive end edges  59 ,  59  ( FIG. 1 ) for facilitating the positioning relative to the lens holder  5  ( FIG. 3 ) and defining the direction of fitting of the heat transfer improving reinforcement member  50  relative to the lens holder  5 . The lens holder  5  ( FIG. 3 ) has a pair of positioning bulged inner walls  29 ,  29  corresponding to the pair of positioning protrusive end edges  59 ,  59  disposed on the heat transfer improving reinforcement member  50 . 
     By engaging the pair of positioning protrusive end edges  59 ,  59  of the heat transfer improving reinforcement member  50  with the pair of positioning bulged inner walls  29 ,  29  of the lens holder  5 , the heat transfer improving reinforcement member  50  is mounted on the lens holder  5  with high accuracy and in a correct direction. 
     Heat of the lens holder  5  is radiated via the heat transfer improving reinforcement member  50  mounted accurately on the lens holder  5  to the exterior of the lens holder assembly  6 . Thus, the thermal conductivity of the whole lens holder assembly  6  is improved. 
     As shown in  FIG. 1 , the lens holder  5  is configured to include the first piece  10  mounted with the plurality of OBLs  31  and  32 , and the second piece  20  mounted with the heat transfer improving reinforcement member  50 . 
     This suppresses the amount of heat transferred to the OBLs  31  and  32 . By dividing the lens holder  5  into two pieces, i.e., the first piece  10  mounted with the plurality of OBLs  31  and  32  and the second piece  20  mounted with the heat transfer improving reinforcement member  50 , heat transfer from the second piece  20  toward the first piece  10  becomes indirect. Due to the indirect heat transfer from the second piece  20  toward the first piece  10 , a suppressed amount of heat is transferred to the first piece  10  mounted with the OBLs  31  and  32 . The suppressed amount of heat transferred to the first piece  10  leads to a suppressed amount of heat to the OBLs  31  and  32  mounted on the first piece  10 . Irrespective of the complex shape of the lens holder  5 , the lens holder  5  is easily fabricated based on the injection molding by dividing the lens holder  5  into two pieces, i.e., the first piece  10  and the second piece  20  mated with the first piece  10 . 
     Although the lens holder  5  is configured to be of a two-piece structure having the first piece  10  and the second piece  20  in  FIGS. 1 to 3 , a one-piece lens holder ( 5 ) may be employed instead of the two-piece lens holder  5 , depending on the design/specifications of the OPU  1 . 
     As shown in  FIG. 1 , the lens holder  5  is configured to include the first piece  10  fitted with the first OBL  31  and the second OBL  32  and the second piece  20  fitting with the single heat transfer improving reinforcement member  50 . The thermal conductivity of a material forming the first piece  10  is different from that of a material forming the second piece  20 . 
     This suppresses the amount of heat transferred to the first OBL  31  and the second OBL  32 . Since the thermal conductivity of the material forming the first piece  10  is different from that of the material forming the second piece  20 , a suppressed amount of heat is transferred to the first piece  10  mounted with the first OBL  31  and the second OBL  32 . For example, the material of the first piece  10  and the material of the second piece  20  may properly be selected so as to suppress the amount of heat transfer delivered to the first piece  10  fitted with the OBLs  31  and  32 . 
     The lens holder  5  ( FIG. 1 ) is configured to include the first piece  10  fitted with the couple of OBLs  31  and  32  and the second piece  20  fitted with the single heat transfer improving reinforcement member  50 . 
     The coils  41 ,  42 ,  43 ,  44 ,  45 , and  46  used were the pair of first-direction driving coils  41  and  42  mounted on the first piece  10  and the two pairs of second-direction driving coils  43 ,  44 ,  45 , and  46  mounted on the second piece  20 . 
     The first-direction driving coils  41  and  42  driving the lens assembly  7  along the first direction D 1  are tracking coils driving the lens assembly  7  along the tracking direction. “Tracking” means locating a spirally described track by tracking and observing minute pits (holes or recesses), grooves, wobbles, etc., disposed on the optical disc using light (all not shown). When a tracking servo of the lens assembly  7  fitted with the OBLs  31  and  32  is provided for the optical disc, the lens assembly  7  fitted with the OBLs  31  and  32  is moved in the front-to-rear direction D 1 . 
     The second-direction driving coils  43 ,  44 ,  45 , and  46  driving the lens assembly  7  along the second direction D 2  are focus/tilt coils driving the lens assembly  7  relative to the optical disc along the focus direction or performing tilt adjustment of the lens assembly  7 . “Focus” means a focal point or focusing. “Tilt” means deviation between the signal surface of the optical disc and the optical axis of laser light issued from the light emitting element and passing through the OBL. When a focus servo of the lens assembly  7  fitted with the OBLs  31  and  32  is provided for the optical disc, the lens assembly  7  fitted with the OBLs  31  and  32  is moved in the top-to-bottom direction D 2 . 
     The coil fitting portion  11  wound with the coil  41  protrudes from the side surface portion  5   e  of the first piece  10 . The coil fitting portion  12  wound with the coil  42  protrudes from the side surface portion  5   e  of the first piece  10 . The side surface portion  5   e  of the first piece  10  is provided with the first coil fitting portions  11  and  12  in a pair wound with the first-direction driving coils  41  and  42 , respectively. 
     The coil fitting portion  23  wound with the coil  43  protrudes from the frame portion  5   f  of the second piece  20 . The coil fitting portion  24  wound with the coil  44  protrudes from the frame portion  5   f  of the second piece  20 . The coil fitting portion  25  wound with the coil  45  protrudes from the frame portion  5   f  of the second piece  20 . The coil fitting portion  26  wound with the coil  46  protrudes from the frame portion  5   f  of the second piece  20 . The frame portion  5   f  of the second piece  20  is provided with the second coil fitting portions  23 ,  24 ,  25 , and  26  in two pairs wound with the second-direction driving coils  43 ,  44 ,  45 , and  46 , respectively. 
     Thus, the OPU  1  is configured that takes for heat measures and that has a suppressed price. For example, using the lens holder of a structure in which coils ( 41 ,  42 ,  43 ,  44 ,  45 , and  45 ) are wound around and across the first piece and second piece, both of the pieces being constituents of the lens holder ( 5 ), it was a difficult work to effectively and correctly wind the coils ( 41 ,  42 ,  43 ,  44 ,  45 , and  46 ) around and across the first piece and the second piece. A difficult coil winding work may result in a lot of time taken for the coil winding work and hence in a rise in the price of the OPU ( 1 ). 
     However, effective and correct coil winding work is ensured by winding the first-direction driving coils  41  and  42  around the first coil fitting portions  11  and  12  of the first piece  10  and by winding the second-direction driving coils  43 ,  44 ,  45 , and  46  around the second coil fitting portions  23 ,  24 ,  25 , and  26  of the second piece  20 . Since the coil winding work is composed of two separate winding works, i.e., the work of winding the first-direction driving coils  41  and  42  around the first coil fitting portions  11  and  12 , respectively, of the first piece  10  of the lens holder  5  and the work of winding the second-direction driving coils  43 ,  44 ,  45 , and  46  around the second coil fitting portions  23 ,  24 ,  25 , and  26 , respectively, of the second piece  20  of the lens holder  5 , the coil winding work is performed without any trouble and effectively. Thus, the OPU  1  has a reduced manufacturing cost. 
     The coils  41 ,  42 ,  43 ,  44 ,  45 , and  46  are formed by directly winding electric wires such as conductor wires around the coil fitting portions  11 ,  12 ,  23 ,  24 ,  25 , and  26  of the lens holder  5 . The coils  41 ,  42 ,  43 ,  44 ,  45 , and  46  formed by winding the electric wires such as conductor wires are configured as two-layer winding type coils  41 ,  42 ,  43 ,  44 ,  45 , and  46 . 
     When the OPU  1  is assembled, the OPU  1  has on its top side a covering plate  100  for protecting various components for example. The covering plate  100  is formed by press molding e.g., a thin metal plate superior in heat radiation properties. The OPU  1  may have on its top side a black covering plate  100  of a synthetic resin for example, in place of the covering plate  100  made of the thin metal plate. 
     The OPU  1  is housed in an optical disc apparatus. The optical disc apparatus includes the OPU  1 . 
     This prevents the occurrence of deficiency that aberrations may arise on a spot of laser light focused by the OBL  31  or the OBL  32  to be applied to an optical disc, and that, as a consequence, malfunctions of the optical disc apparatus may occur. The OPU of the present invention is not intended to be limited to the one shown. 
     For example, a substantially flat plate-shaped aluminum heat transfer improving reinforcement member ( 50 ) may be disposed immediately below the OBLs ( 31  and  32 ). The substantially flat plate-shaped aluminum heat transfer improving reinforcement member ( 50 ) may be disposed between the first piece ( 10 ) and the second piece ( 20 ) making up the lens holder ( 5 ). The substantially flat plate-shaped aluminum heat transfer improving reinforcement member ( 50 ) may be formed so as to surround the OBLs ( 31  and  32 ). 
     By optimizing the position to fit the aluminum heat transfer improving reinforcement member  50  on the lens holder  5  or by optimizing the shape of the aluminum heat transfer improving reinforcement member  50 , is carried out the position adjustment of the center of gravity of the moving part  3  of the OPU  1  or the improvement in the higher-order resonance characteristics. 
     Although the OPU  1  of  FIG. 4  has the lens holder  5  fitted with the two OBLs  31  and  32 , i.e., the first OBL  31  and the second OBL  32 , the OPU is also available (not show) that has a lens holder fitted with only one OBL without being fitting with the two OBLs ( 31  and  32 ), depending on the design/specifications of the OPU  1 , for example. 
     The light emitting element emitting laser light may be e.g., a dual-wavelength light emitting element not shown capable of emitting laser lights of two different wavelengths or a triple-wavelength light emitting element not shown capable of emitting laser lights of three different wavelengths. 
     Although the embodiments of the present invention have hereinabove been described, the above embodiments are merely for facilitating the understanding of the present invention and are not in any way to be construed as limiting the present invention. The present invention may variously be changed or altered without departing from its spirit and encompasses equivalents thereof.