Patent Publication Number: US-2013242715-A1

Title: Optical pickup apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of priority to Japanese Patent Application No. 2011-197474, filed Sep. 9, 2011, 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 apparatus. 
     2. Description of the Related Art 
     In General, an optical pickup apparatus is known that is configured to condense a laser beam onto a signal recording layer of an optical disc to read information recorded in the optical disc (see, e.g., Japanese Laid-Open Patent Publication 2011-34643). This optical pickup apparatus includes: a first laser diode configured to generate a first laser beam having a first wavelength; and a second laser diode configured to generate a second laser beam having a second wavelength different from the first wavelength, in order to read the information stored in two types of optical discs of different standards, for example. The first laser diode and the second laser diode are respectively housed in a first holder and a second holder, made of metal, for example. The first holder and the second holder respectively have functions of dissipating the heat generated when the first laser diode and the second laser diode generate the laser beams, respectively. Thus, the first holder and the second holder are arranged in positions apart from each other inside a housing so as not to be affected by the heat dissipation of each other. 
     In the optical pickup apparatus disclosed in Japanese Laid-Open Patent Publication No. 2011-34643, it is preferable that the enough volumes of the first holder and the second holder are secured to dissipate the heat of the first laser diode and the second laser diode, respectively. If the first holder and the second holder are increased in volume to sufficiently secure the capability of dissipating the heat of the first laser diode and the second laser diode, the optical pickup apparatus is increased in size. Whereas, in the case of downsizing the optical pickup apparatus, the first holder and the second holder are required to be reduced in volume and there is a possibility of a shortage of the dissipating capability of the first holder and the second holder. 
     SUMMARY OF THE INVENTION 
     An optical pickup apparatus according to an aspect of the present invention, includes: a first laser diode configured to generate a first laser beam having a first wavelength; a first holder made of metal configured to incorporate the first laser diode; a second laser diode configured to generate a second laser beam, having a second wavelength different from the first wavelength, in a manner complementary to the first laser beam; a second holder made of metal configured to incorporate the second laser diode; an objective lens configured to condense the first laser beam onto a signal recording layer of a first optical disc as well as condense the second laser beam onto the signal recording layer of a second optical disc of a standard different from that of the first optical disc; optical elements configured to guide the first laser beam and the second laser beam to the objective lens; a housing made of synthetic resin configured to house the first holder, the second holder, the objective lens, and the optical elements such that the first holder and the second holder are adjacent to each other; and a first heat-transfer gel filled in a first space between the first holder and the second holder, so as to transfer heat generated in the first laser diode from the first holder to the second holder as well as transfer the heat generated in the second laser diode from the second holder to the first holder. 
     Other features of the present invention will become apparent from descriptions of this specification and of the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For more thorough understanding of the present invention and advantages thereof, the following description should be read in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a diagram illustrating a configuration of an optical system of an optical pickup apparatus according to a first embodiment of the present invention; 
         FIG. 2  is a diagram illustrating a configuration of a part of an optical system in an optical pickup apparatus according to a first embodiment of the present invention; 
         FIG. 3  is a block diagram illustrating an optical disc device in which a optical pickup apparatus according to a first embodiment of the present invention is used; 
         FIG. 4  is an exploded perspective view illustrating an optical pickup apparatus according to a first embodiment of the present invention; 
         FIG. 5  is an enlarged view illustrating a part of a housing in a first embodiment of the present invention; 
         FIG. 6  is a perspective view illustrating an optical pickup apparatus according to a first embodiment of the present invention; 
         FIG. 7  is a cross-sectional view of an optical pickup apparatus according to a first embodiment of the present invention; and 
         FIG. 8  is a cross-sectional view of an optical pickup apparatus according to another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     At least the following details will become apparent from descriptions of this specification and of the accompanying drawings. 
     First Embodiment 
     ===Optical System of Optical Pickup Apparatus=== 
       FIG. 1  depicts a configuration of an optical system of an optical pickup apparatus according to an embodiment of the present invention.  FIG. 2  depicts a configuration of a part of an optical system of an optical pickup apparatus according to an embodiment of the present invention. 
     An optical pickup apparatus  100  is an apparatus configured to irradiate a rotating optical disc with a laser beam and detect return light of the laser beam reflected by the optical disc. The optical pickup apparatus  100  is mounted on an information recording and reproduction device, such as an optical disc device  500  which will be described later. The optical disc for which the information recording or reproduction is performed by the optical pickup apparatus  100  include the optical disc of BD (Blu-ray Disc) standard (hereinafter referred to as “first optical disc  5 A”), the optical disc of DVD (Digital Versatile Disc) standard (hereinafter referred to as “second optical disc  5 B”), the optical disc of CD (Compact Disc) standard (hereinafter referred to as “third optical disc  5 C”), etc. The optical pickup apparatus  100  includes: a first optical system along the optical path of a first laser beam applied onto the second optical disc  5 B and the third optical disc  5 C; and a second optical system along the optical path of a second laser beam applied onto the first optical disc  5 A. The first optical system and the second optical system will be described later in detail. 
     ===First Optical System=== 
     The first optical system of the optical pickup apparatus according to an embodiment of the present invention will hereinafter be described with reference to  FIGS. 1 and 2 . 
     The first optical system is an optical system for DVD standard and CD standard, and includes a first laser light source  110 , a first diffraction grating  12 , a first half-wave plate  13 , a beam splitter  32 , a collimating lens  33 , a quarter-wave plate  34 , a reflection mirror  35 , a first raising reflection mirror  15 , a first objective lens  16  (objective lens), a coupling lens  24 , a semitransparent mirror  36 , a detecting lens  37 , a photodetector  38 , and a front monitor diode  31 . The first diffraction grating  12 , the first half-wave plate  13 , the beam splitter  32 , the collimating lens  33 , the quarter-wave plate  34 , the reflection mirror  35 , and the first raising reflection mirror  15  correspond to optical elements configuring the first optical system (hereinafter referred to as “optical elements of first optical system”). 
     The first laser light source  110  is configured to selectively generate the first laser beam having two different wavelengths, which are a wavelength of 655 nm, for example, in a red wavelength range (645 nm to 675 nm) wherein the second optical disc  5 B is to be irradiated with the laser beam having a wavelength in this range; and a wave length of 785 nm, for example, in an infrared wavelength range (765 nm to 805 nm) wherein the third optical disc  5 C is to be irradiated with the laser beam having a wavelength in this rage. The first laser light source  110  is formed incorporating, in a first holder  17 , a first laser diode  11 A configured to generate the first laser beam having a wavelength of 655 nm, for example, and a first laser diode  11 B configured to generate the first laser beam having a wavelength of 785 nm, for example. The first holder  17  will be described later in detail. 
     The first diffraction grating  12  is configured to generate zero-order light, plus-first-order diffracted light, and minus-first-order diffracted light from the first laser beam generated in the first laser light source  110 . 
     The first half-wave plate  13  is configured to convert the first laser beam, which is linearly-polarized light, into P-linearly-polarized light, for example. 
     The beam splitter  32  is configured to allow the P-polarized laser beam in the red wavelength range and the infrared wavelength range, for example, to pass therethrough and reflect the laser beam other than the P-polarized laser beam in the red wavelength range and the infrared wavelength range. The beam splitter  32  allows the P-polarized first laser beam in the red wavelength range or the infrared wavelength range incident from the first half-wave plate  13  to pass therethrough. At this moment, it is assumed that the beam splitter  32  reflects a part of the first laser beam in the direction of the front monitor diode  31  so as to adjust the intensity of the first laser beam. The front monitor diode  31  is an optical element configured to receive a part of the first laser beam from the beam splitter  32 , to adjust the intensity of the first laser beam. Since the return light of the first laser beam incident from the collimating lens  33  has converted into the S-polarized laser beam by being reflected by the second optical disc  5 B or the third optical disc  5 C, for example, the beam splitter  32  reflects the return light of the first laser beam in the direction of the coupling lens  24 . 
     The collimating lens  33  is configured to convert the first laser beam incident from the beam splitter  32  into parallel light. 
     The quarter-wave plate  34  is configured to convert the first laser beam incident from the collimating lens  33  from the linearly-polarized light into the circularly-polarized light. The quarter-wave plate  34  converts the return light of the first laser beam incident from the reflection mirror  35  from the circularly-polarized light into the linearly-polarized light. 
     The reflection mirror  35  is configured to reflect the first laser beam incident from the quarter-wave plate  34  in the direction of the first raising reflection mirror  15 . The reflection mirror  35  is configured to reflect the return light of the first laser beam incident from the first raising reflection mirror  15  in the direction of the quarter-wave plate  34 . 
     The first raising reflection mirror  15  is configured to reflect the first laser beam incident from the reflection mirror  35  in the direction perpendicular to a recording face of the second optical disc  5 B or the third optical disc  5 C. The first raising reflection mirror  15  is configured to reflect the return light of the first laser beam incident from the first objective lens  16  in the direction of the reflection mirror  35 . 
     The first objective lens  16  is configured to condense the first laser beam incident from the first raising reflection mirror  15  onto a signal recording layer in the recording face of the second optical disc  5 B or the third optical disc  5 C. 
     The return light of the first laser beam reflected by the signal recording layer of the second optical disc  5 B or the third optical disc  5 C is converted into the parallel light by the first objective lens  16 , thereafter enters the quarter-wave plate  34  via the first raising reflection mirror  15  and the reflection mirror  35 , and is converted by the quarter-wave plate  34  from the circularly-polarized light into the linearly-polarized light. The return light of the first laser beam, which has been converted into the linearly-polarized light, enters the coupling lens  24  via the collimating lens  33  and the beam splitter  32 . 
     The coupling lens  24  is configured to convert the convergence angle of the return light of the first laser beam incident from the beam splitter  32  so that the return light of the first laser beam can be received by the photodetector  38 . 
     The semitransparent mirror  36  is configured to reflect the S-polarized laser beam in a blue wavelength range, and allow the laser beam other than the S-polarized laser beam in the blue wavelength range to pass therethrough, for example. The blue wavelength range will be described later in detail. The return light of the first laser beam incident from the coupling lens  24  is the S-polarized laser beam in the red wavelength range or the infrared wavelength range, and the semitransparent mirror  36  is configured to allow the return light of the first laser beam incident from the coupling lens  24  to pass there through. 
     The detecting lens  37  is configured to condense the return light of the first laser beam incident from the semitransparent mirror  36  onto the photodetector  38 , as well as cause astigmatism in the return light of the first laser beam, thereby generating a focus error signal. On the incident surface side or the emitting surface side of the detecting lens  37 , for example, a cylindrical surface, a flat surface, a concave curved surface, or a convex curved surface is formed, and in an embodiment of the present invention, the detecting lens  37  is configured such that a parallel plate is inclined in a predetermined direction considering the direction of occurrence of astigmatism. 
     The photodetector  38  is configured to perform a photoelectric conversion of the return light of the first laser beam incident from the detecting lens  37 . 
     ===Second Optical System=== 
     The second optical system of the optical pickup apparatus according to an embodiment of the present invention will hereinafter be described with reference to  FIGS. 1 and 2 . 
     The second optical system is an optical system for BD standard, and includes a second laser light source  210 , a second diffraction grating  22 , a second half-wave plate  23 , the semitransparent mirror  36 , the coupling lens  24 , the beam splitter  32 , the collimating lens  33 , the quarter-wave plate  34 , the reflection mirror  35 , a second raising reflection mirror  25 , a second objective lens  26  (objective lens), the detecting lens  37 , the photodetector  38 , and the front monitor diode  31 . For example, the semitransparent mirror  36 , the coupling lens  24 , the beam splitter  32 , the collimating lens  33 , the quarter-wave plate  34 , the reflection mirror  35 , the detecting lens  37 , the photodetector  38 , and the front monitor diode  31  are commonly used between the first optical system and the second optical system. The second diffraction grating  22 , the second half-wave plate  23 , the semitransparent mirror  36 , the coupling lens  24 , the beam splitter  32 , the collimating lens  33 , the quarter-wave plate  34 , the reflection mirror  35 , and the second raising reflection mirror  25  correspond to the optical elements configuring the second optical system (hereinafter referred to as “optical elements of second optical system”). 
     The second laser light source  210  is configured to generate, in a manner complementary to the first laser beam, the second laser beam having 405 nm wavelength, for example, in the blue wavelength range (400 nm to 420 nm) different from the wavelength of the first laser beam generated by the first laser light source  110 , wherein the first optical disc  5 A is to be irradiated with the laser beam having a wavelength in this range. The second laser light source  210  is formed incorporating, in a second holder  27 , a second laser diode  21  configured to generate the second laser beam having a wavelength 405 nm, for example. The second holder  27  will be described later in detail. 
     The second diffraction grating  22  is configured to generate zero-order light, plus-first-order diffracted light, and minus-first-order diffracted light from the second laser beam generated in the second laser light source  210 . 
     The second half-wave plate  23  is configured to convert the second laser beam, which is linearly-polarized light, into S-linearly-polarized light, for example. 
     The semitransparent mirror  36  is configured to reflect the S-polarized second laser beam in the blue wavelength range incident from the second half-wave plate  23  in the direction of the coupling lens  24 . Since the return light of the second laser beam incident from the coupling lens  24  has been converted into the laser beam of the P-polarized laser beam by being reflected by the first optical disc  5 A, for example, the semitransparent mirror  36  allows the return light of the second laser beam to pass therethrough. 
     The coupling lens  24  is configured to convert the divergence angle of the second laser beam incident from the semitransparent mirror  36  so that the second laser beam is condensed onto the signal recording layer of the first optical disc  5 A. The coupling lens  24  is also configured to convert the convergence angle of the return light of the second laser beam incident from the beam splitter  32  so that the return light of the second laser beam can be received by the photodetector  38 . 
     Since the second laser beam incident from the coupling lens  24  is the S-polarized laser beam in the blue wavelength range other than the P-polarized laser light in the red wavelength range and the infrared wavelength range, for example, the beam splitter  32  reflects the second laser beam incident from the coupling lens  24  in the direction of the collimating lens  33 . At this moment, the beam splitter  32  allows a part of the second laser beam to pass therethrough so as to adjust the intensity of the second laser beam. The front monitor diode  31  is the optical element configured to receive a part of the second laser beam from the beam splitter  32 , to adjust the intensity of the second laser beam. Since the return light of the second laser beam incident from the collimating lens  33  is the P-polarized laser beam in the blue wavelength range other than the P-polarized laser beam in the red wavelength range and the infrared wavelength range, for example, the beam splitter  32  reflects the return light of the second laser beam incident from the collimating lens  33  in the direction of the coupling lens  24 . 
     The second laser beam reflected by the beam splitter  32  in the direction of the collimating lens  33  is converted into the parallel light by the collimating lens  33 , and thereafter is converted by the quarter-wave plate  34  from the linearly-polarized light to the circularly-polarized light. The second laser beam, which has been converted into the circularly-polarized light, is reflected by the reflection mirror  35  in the direction of the second raising reflection mirror  25 . It is assumed that the first raising reflection mirror  15  arranged between the reflection mirror  35  and the second raising reflection mirror  25  in the optical path of the second laser beam reflects the laser beam in the red wavelength range and the infrared wavelength range and allows the laser beam in the blue wavelength range to pass therethrough. 
     The second raising reflection mirror  25  is configured to reflect the second laser beam incident from the reflection mirror  35  in the direction perpendicular to the recording face of the first optical disc  5 A. The second raising reflection mirror  25  reflects the return light of the second laser beam incident from the second objective lens  26  in the direction of the reflection mirror  35 . 
     The second objective lens  26  is configured to condense the second laser beam incident from the second raising reflection mirror  25  onto the signal recording layer in the recording face of the first optical disc  5 A. 
     The return light of the second laser beam reflected by the signal recording layer of the first optical disc  5 A is converted into the parallel light by the second objective lens  26 , thereafter enters the quarter-wave plate  34  via the second raising reflection mirror  25  and the reflection mirror  35 , and is converted by the quarter-wave plate  34  from the circularly-polarized light into the linearly-polarized light. The return light of the second laser beam, which has been converted into the linearly-polarized light, enters the detecting lens  37  via the collimating lens  33 , the beam splitter  32 , the coupling lens  24 , and the semitransparent mirror  36 . 
     The detecting lens  37  is configured to condense the return light of the second laser beam incident from the semitransparent mirror  36  onto the photodetector  38 , as well as cause astigmatism in the return light of the second laser beam, thereby generating the focus error signal. 
     The photodetector  38  is configured to perform the photoelectric conversion of the return light of the second laser beam incident from the detecting lens  37 . 
     ===Optical Disc Device=== 
     The optical disc device, in which the optical pickup apparatus according to an embodiment of the present invention is used, will hereinafter be described with reference to  FIG. 3 .  FIG. 3  is a block diagram of the optical disc device in which the optical pickup apparatus according to an embodiment of the present invention is used. 
     An optical disc device  500  includes a spindle motor  502 , a motor drive circuit  503 , the optical pickup apparatus  100 , a thread mechanism  504 , an amplifying circuit  505 , a demodulating circuit  506 , a focus control circuit  507 , a tracking control circuit  508 , a tilt control circuit  509 , a laser driver  510 , a modulating circuit  511 , and a system control device  512 . 
     The spindle motor  502  is configured to rotate the optical disc  5  about a rotation axis  501 . Out of the first optical disc  5 A, the second optical disc  5 B, and the third optical disc  5 C, the optical disc rotated by the spindle motor  502  is referred to as the optical disc  5  for the sake of convenience. 
     The motor drive circuit  503  is configured to control the rotation of the spindle motor  502  in response to a control signal sent from the system control device  512 . 
     The thread mechanism  504  includes a pulse-driven stepping motor, for example, and is configured to move the optical pickup apparatus  100  in the radical direction of the optical disc  5  in response to the control signal sent from the system control device  512 . 
     The laser driver  510  is configured to control the output of the first laser beam and the second laser beam generated in the first laser diode  11 A/ 11 B and the second laser diode  21 , respectively, in response to a signal inputted from the modulating circuit  511 . 
     The modulating circuit  511  is configured to convert data, which is to be recorded in the optical disc  5  and is inputted from the system control device  512 , into a pulse signal for recording. It is assumed that the data to be recorded in the optical disc  5  is supplied at any time from an external device (not shown) such as a personal computer via the system control device  512 , for example. 
     The amplifying circuit  505  is configured to amplify an RF (Radio Frequency) signal contained in an electrical signal outputted from the photodetector  38  of the optical pickup apparatus  100 , and output the amplified signal to the demodulating circuit  506 . 
     The demodulating circuit  506  is configured to demodulate the RF signal inputted from the amplifying circuit  505 , and output the demodulated signal to the system control device  512 . The system control device  512  is configured to output, to the external device, a data signal based on the demodulated signal inputted from the demodulating circuit  506 . 
     The focus control circuit  507 , the tracking control circuit  508 , and the tilt control circuit  509  perform drive control of the first objective lens  16  and the second objective lens  26  of the optical pickup apparatus  100 . 
     ===Optical Pickup Apparatus=== 
     The optical pickup apparatus according to an embodiment of the present invention will hereinafter be described with reference to  FIG. 1 ,  FIG. 2 , and  FIGS. 4 to 7 .  FIG. 4  is an exploded perspective view of the optical pickup apparatus according to an embodiment of the present invention. The central axis of the rotation axis  501  of the spindle motor  502  is denoted by a dashed-dotted line for the convenience of description. The first half-wave plate  13 , the second diffraction grating  22 , and the second half-wave plate  23  are in an invisible state.  FIG. 5  is an enlarged view of a part of a housing according to an embodiment of the present invention. The first half-wave plate  13 , the second diffraction grating  22 , and the second half-wave plate  23  are in the invisible state.  FIG. 6  is a perspective view of the optical pickup apparatus according to an embodiment of the present invention. The central axis of the rotation axis  501  of the spindle motor  502  is denoted by the dashed-dotted line for the convenience of description. The first holder  17 , the second holder  27 , a first heat-transfer gel  90 , and a second heat-transfer gel  91  are in the invisible state, but are denoted by a dotted line for the convenience of description.  FIG. 7  is a cross-sectional view of the optical pickup apparatus as seen from the cross-section along the line A-A′ of  FIG. 6  toward the other direction. 
     The optical pickup apparatus  100  includes a housing  50 , a cover  60 , an actuator  70 , a lens holder  80 , the optical elements of the first optical system, the optical elements of the second optical system, the first heat-transfer gel  90 , and the second heat-transfer gel  91 . In an embodiment of the present invention, the Z-axis is an axis along the longitudinal direction (vertical direction) of the rotation axis  501  of the spindle motor  502  to rotate the optical disc  5 , wherein the direction going upward is given as +Z direction and the direction going downward is given as−Z direction. The Y-axis is an axis along the direction in which the optical pickup apparatus  100  is moved in the radial direction of the optical disc  5 , wherein the direction (back side) away from the rotation axis  501  is given as +Y direction and the direction (front side) toward the rotation axis  501  is given as −Y direction. The X-axis is an axis along the tangential direction orthogonal to both the side faces of the housing  50 , wherein the direction toward one side face is given as −X direction and the direction toward the other side face is given as +X direction. 
     The housing  50  is a case made of synthetic resin to house the optical elements of the first optical system, the optical elements of the second optical system, the first holder  17 , the second holder  27 , and the actuator  70 . The first holder  17  and the second holder  27  will be described later in detail. The housing  50  has an opening  201  formed so as to arrange from the upper side, for example, and house, inside the housing  50 , the optical elements of the first optical system, the optical elements of the second optical system, the first holder  17 , the second holder  27 , the first objective lens  16 , and the second objective lens  26 . The front side of the housing  50  is in a shape formed by hollowing out with a predetermined curvature to avoid the spindle motor, for example. Guide members  53 ,  54 A, and  54 B are disposed on both side faces of the housing  50 . The guide members  53 ,  54 A, and  54 B are members to attach the optical pickup apparatus  100  to a pair of guide shafts for moving the optical pickup apparatus  100  along the radial direction of the optical disc  5 . The guide member  53  is provided on one side face of the housing  50 , for example. The guide members  54 A and  54 B are provided on the other side face of the housing  50 , for example. In the opening  201 , the optical elements of the first optical system, the optical elements of the second optical system, the first holder  17 , the second holder  27 , the first objective lens  16 , and the second objective lens  26  are arranged to have such a positional relationship as described referring to  FIGS. 1 and 2 . The configuration in which the first laser diodes  11 A and  11 B and the second laser diode  21  are arranged in the housing  50  will be described later in detail. 
     In the lens holder  80 , the first objective lens  16  and the second objective lens  26  are provided such that the first objective lens  16  and the second objective lens  26  are arranged above the first raising reflection mirror  15  and the second raising reflection mirror  25 , respectively. The lens holder  80  is attached to an actuator frame so as to be capable of being displaced in the focus direction (Z-axis direction), the tracking direction (Y-axis direction), and the tilt direction, using a plurality of suspension wires  71  arranged on both sides of the lens holder  80 . The actuator  70  is a device configured to drive the first objective lens  16  and the second objective lens  26  in the focus direction (Z-axis direction), the tracking direction (Y-axis direction), and the tilt direction so that the laser beam applied onto the optical disc  5  is focused on the signal recording layer of the optical disc  5 , is caused to follow a signal track of the optical disc  5 , and becomes perpendicular to the signal recording layer of the optical disc  5 . The actuator  70  and the lens holder  80  are arranged in a left-side area as seen from the central axis of the rotation axis  501  toward the back side, for example, in the opening  201  of the housing  50 . It is assumed that the lens holder  80  is in such a shape as to be opened downward so that the first objective lens  16  and the second objective lens  26  are arranged above the first raising reflection mirror  15  and the second raising reflection mirror  25 , respectively, when the lens holder  80  is arranged in the opening  201 . 
     The first laser diodes  11 A and  11 B are configured with the first laser light source  110 , which is a multi-laser unit having a plurality of laser diodes housed in the same package, and this first laser light source  110  is a so-called frame type laser light source, with a synthetic resin package. The first laser light source  110  is housed in the housing  50  in a state where it is incorporated in the first holder  17 . On the other hand, the second laser diode  21  is configured with the second laser light source  210 , and is a so-called can-type laser light source housed in a metallic cylindrical package. The second laser light source  210  is housed in the housing  50  in a state where it is incorporated in the second holder  27 . The laser diode selected to generate the first laser beam, out of the first laser diodes  11 A and  11 B, is referred to as a first laser diode  11  for the convenience of description. The first holder  17  is a fixing member made of metal that is composed of lead, aluminum, or an alloy thereof, having a function of dissipating the heat generated in the first laser diode  11  when generating the first laser beam. The second holder  27 , similarly to the first holder  17 , is a fixing member made of metal that is composed of lead, aluminum, or an alloy thereof, having a function of dissipating the heat generated in the second laser diode  21  when generating the second laser beam. The heat dissipating capability of the first holder  17  and the second holder  27  is determined mainly based on the volumes of the first holder  17  and the volume of the second holder  27 . The volume of the first holder  17  and the volume of the second holder  27  will be described later in detail. 
     The first holder  17  and the second holder  27  are arranged in a right-side area as seen from the central axis of the rotation axis  501  toward the back side, for example, in the opening  201  of the housing  50 . The first holder  17  and the second holder  27  are arranged in the opening  201  in a manner adjacent to each other. The first holder  17  is arranged in the opening  201  so that the side face on the rotation axis  501  side of the first holder  17  is along the part formed by hollowing out with the predetermined curvature of the housing  50 , for example. The second holder  27  is arranged in the opening  201  of the housing  50  in such a manner that a part of the side face on the rotation axis  501  side of the second holder  27  is opposed to a part of the side face opposite to the side face on the rotation axis  501  side of the first holder  17 , for example. The first holder  17  and the second holder  27  are adjacent to each other in the radial direction so that there can be a space between the first holder  17  and the second holder  27 , At this moment, the optical path of the first and the second optical systems are sufficiently secured. The space between the first holder  17  and the second holder  27  corresponds to a first space. 
     The first heat-transfer gel  90  is a gel made by mixing metallic particles of aluminum, iron, etc., into silicon which is a main component, for example. The first heat-transfer gel  90  is filled in the first space so as to transfer the heat generated in the first laser diode  11  from the first holder  17  to the second holder  27  as well as transfer the heat generated in the second laser diode  21  from the second holder  27  to the first holder  17 . It is assumed that the first heat-transfer gel  90  has viscosity enough to prevent the gel from spreading to an area other than the filled area at the time of filling the first space. The heat-transfer will be described later in detail. 
     The cover  60  is a flat plate, for example, made of metal such as stainless steel to cover the opening  201  of the housing  50  from the upper side of the housing  50 . The cover  60  is in a shape along the area other than the area in which the actuator  70  and the lens holder  80  are arranged in the upper face of the housing  50  so as to cover the area other than the area in which the actuator  70  and the lens holder  80  are arranged in the opening  201 , for example. The cover  60  is attached to the edge of the housing  50  by an adhesive (not shown), screws (not shown), etc., for example, in the state of covering the opening  201  from the upper side of the housing  50 . It is assumed that the first holder  17  and the second holder  27  are arranged in the opening  201  so that the height H 1  of the first holder  17  and the height H 2  of the second holder  27  becomes lower than the height H 3  of the edge of the housing  50  with the bottom face of the housing  50  used as a reference, for example. Accordingly, when the opening  201  of the housing  50  is covered by the cover  60 , a space  95  is formed between the cover  60  and the first holder  17  and between the cover  60  and the second holder  27 . This space  95  corresponds to a second space. A hole  61  is formed in the cover  60  at the position opposed to the second holder  27 , for example. The hole  61  will be described later in detail. 
     The second heat-transfer gel  91  is the gel made by mixing metallic particles of aluminum, iron, etc., into silicon which is the main component similarly to the first heat-transfer gel  90 , for example. The second heat-transfer gel  91  is filled in the space  95  so as to transfer the heat generated in either of the first laser diode  11  and the second laser diode  21  from at least either of the first laser diode  11  and the second laser diode  21  to the cover  60 . The second heat-transfer gel  91  is filled in the space  95  of the housing  50  with the cover  60  attached thereto through the hole  61  formed in the cover  60 . It is assumed that the second heat-transfer gel  91  has viscosity enough to prevent the gel from spreading to an area other than the filled area at the time of filling the space  95 . The heat-transfer will be described later in detail. 
     ===Volumes and Heat-Dissipation of First and Second Holders=== 
     Description of the volume of the first holder  17 , the volume of the second holder  27 , and the heat-dissipation of the first laser diode  11  and the second laser diode  21  will hereinafter be made with reference to  FIG. 1 ,  FIG. 2 , and  FIGS. 4 to 7 . 
     The first laser diode  11  generates heat when generating the first laser beam, as described above. The heat is generated by the current supplied to the first laser diode  11  when the first laser diode  11  generates the first laser beam, for example. The second laser diode  21 , similarly to the first laser diode  11 , generates heat when generating the second laser beam. The optical pickup apparatus  100  is required to dissipate heat in order to prevent the first laser diode  11  or the second laser diode  21  from being deteriorated by the heat generated by the first laser diode  11  or the second laser diode  21 . As described above, the first laser diode  11  and the second laser diode  21  generate the first laser beam and the second laser beam in a manner complementary to each other, and generate heat. The total volume of the first holder  17  and the second holder  27  is assumed to be a volume capable of dissipating the heat generated in the first laser diode  11  to such an extent that the deterioration of the first laser diode  11  is suppressed as well as dissipating the heat generated in the second laser diode  21  to such an extent that the deterioration of the second laser diode  21  is suppressed (hereinafter referred to as “volume capable of suppressing deterioration of laser diode”). It is assumed that the above total volume is determined by an experiment, etc., for example. 
     Accordingly, for example, when the first laser diode  11  generates the first laser beam, the heat generated in the first laser diode  11  is transferred from the first laser diode  11  to the first holder  17 , to be dissipated. The heat generated by the heat-dissipation of the first holder  17  is transferred from the first holder  17  to the second holder  27  via the first heat-transfer gel  90 , to be dissipated. The heat generated by the heat dissipation of the second holder  27  is transferred to the cover  60  via the second heat-transfer gel  91 , to be dissipated. On the other hand, for example, when the second laser diode  21  generates the second laser beam, the heat generated in the second laser diode  21  is transferred from the second laser diode  21  to the second holder  27 , to be dissipated. The heat generated by the heat-dissipation of the second holder  27  is transferred from the second holder  27  to the first holder  17  via the first heat-transfer gel  90 , to be dissipated, as well as is transferred from the second holder  27  to the cover  60  via the second heat-transfer gel  91 , to be dissipated. 
     As described above, the first laser diode  11  generates the first laser beam having a wavelength of 655 nm, for example. The first laser diode  11  is incorporated in the first holder  17  made of metal. The second laser diode  21  generates the second laser beam having a wavelength of 405 nm different from the wavelength of the first laser beam, for example, in a manner complementary to the first laser beam. The second laser diode  21  is incorporated in the second holder  27  made of metal. The first objective lens  16  condenses the first laser beam onto the signal recording layer of the second optical disc  5 B or the third optical disc  5 C. The second objective lens  26  condenses the second laser beam onto the signal recording layer of the first optical disc  5 A. The optical elements of the first optical system and the optical elements of the second optical system guide the first laser beam and the second laser beam, respectively, to the first objective lens  16  and the second objective lens  26 , respectively. The first holder  17 , the second holder  27 , the first objective lens  16 , the second objective lens  26 , the optical elements of the first optical system, and the optical elements of the second optical system are housed in the housing  50  made of synthetic resin. The first holder  17  and the second holder  27  are adjacent to each other in the housing  50 . The first heat-transfer gel  90  is filled in the first space between the first holder  17  and the second holder  27 . The heat generated from the first laser diode  11  is transferred from the first holder  17  to the second holder  27 , and the heat generated in the second laser diode  21  is transferred from the second holder  27  to the first holder  17 . Accordingly, the heat generated in the first laser diode  11  and the heat generated in the second laser diode  21  can be dissipated to the first holder  17  and the second holder  27 . Since the housing  50  is made of synthetic resin and is easy to mold, a compact optical pickup apparatus  100  can be provided. Since the housing  50  is made of synthetic resin, the optical pickup apparatus  100  can be reduced in weight. 
     The total volume of the first holder  17  and the second holder  27  is determined depending on the amount of heat generation of each of the first laser diode  11  and the second laser diode  21 . Accordingly, for example, setting the total volume of the first holder  17  and the second holder  27  to the volume capable of suppressing the deterioration of the laser diodes enables reliable dissipation of the heat generated in the first laser diode  11  and the heat generated in the second laser diode  21 , thereby being able to prevent the deterioration of the first laser diode  11  and the second laser diode  21 . Both when dissipating the heat generated in the first laser diode  11  and dissipating the heat generated in the second laser diode  21 , the heat can be dissipated to the first holder  17  and the second holder  27 , and thus the volumes of the first holder  17  and the second holder  27  can be reduced, thereby being able to provide a compact optical pickup apparatus  100 . 
     At least a part of the opening  201  of the housing  50 , in which the first holder  17 , the second holder  27 , the first objective lens  16 , the second objective lens  26 , the optical elements of the first optical system, and the optical elements of the second optical system are exposed, is covered by the metal-made cover  60 . At this moment, the space  95  is formed between the cover  60  and the first holder  17  and between the cover  60  and the second holder  27 , for example. The second heat-transfer gel  91  is filled in the space  95  between the cover  60  and the second holder  27 , for example. Accordingly, for example, the heat of the second holder  27  is dissipated to the cover  60 . Therefore, the optical pickup apparatus  100  can be provided that has high capability of dissipating the heat generated in the first laser diode  11  and the heat generated in the second laser diode  21 . 
     The hole  61  is formed in the cover  60  in the position opposed to the second holder  27 , for example. Thus, for example, after the opening  201  of the housing  50  is covered by the cover  60 , the second heat-transfer gel  91  can be filled in the space  95  between the cover  60  and the second holder  27  through the hole  61  of the cover  60 . Therefore, with the hole  61  formed in the cover  60  in such a position that the second heat-transfer gel  91  can be reliably filled in the space  95 , the second heat-transfer gel  91  can be reliably filled in the space  95  through the hole  61 . Consequently, it is made possible to reliably dissipate the heat generated in the first laser diode  11  and the heat generated in the second laser diode  21 . 
     The first holder  17  and the second holder  27  are arranged in the housing  50  along the radial direction. Thus, for example, in the case where the first holder  17  and the second holder  27  are arranged, in the radial direction, along the direction in which the optical pickup apparatus  100  is moved being guided by a pair of guide shafts, the width in the direction orthogonal to both the side faces of the housing  50  can be shortened, for example, thereby being able to provide a compact optical pickup apparatus  100 . 
     The first holder  17  and the second holder  27  are arranged in the housing  50  so that parts of the faces will be opposed to each other in the radial direction. If parts of the faces of the first holder  17  and the second holder  27  are opposed to each other, then the first holder  17  and the second holder  27  can transfer the heat to each other via the first heat-transfer gel  90 . Thus, it is not necessary to cause the entire area of the face opposed to the second holder  27  in the first holder  17  and the entire area of the face opposed to the first holder  17  in the second holder  27  to be opposed to each other, thereby enhancing the degree of freedom in design of the optical pickup apparatus  100  such as the arrangement of the optical elements with respect to the housing  50  and being able to provide the compact optical pickup apparatus  100 . 
     Other Embodiments 
     In a first embodiment of the present invention, a description has been given of the case where the first holder  17  and the second holder  27  are arranged in the housing  50  so that the height H 1  of the first holder  17  and the height H 2  of the second holder  27  becomes lower than the height H 3  of the edge of the housing  50  with the bottom face of the housing  50  used as a reference and the cover  60  is in a flat plate shape, however, it is not limited thereto. For example, the first holder  17  and the second holder  27  may be arranged in a housing  50 A so that the height H 1  of the first holder  17  and the height H 2  of the second holder  27  become substantially equal to or greater than the height H 31  of the edge of the housing  50 A with the bottom face of the housing  50 A used as a reference, and a cover  60 A may be in such a shape as to cover the first holder  17  and the second holder  27  from the upper side. Hereinafter, a description will be given, with reference to  FIG. 8 , of the case where the first holder  17  and the second holder  27  are arranged in the housing  50 A so that the height H 1  of the first holder  17  and the height H 2  of the second holder  27  become equal to or greater than the height H 31  of the edge of the housing  50 A with the bottom face of the housing  50 A used as a reference, and the cover  60 A is in such a shape as to cover the first holder  17  and the second holder  27  from the upper side.  FIG. 8  is a cross-sectional view of the optical pickup apparatus according to an embodiment of the present invention. Constituents equivalent to those shown in  FIG. 7  are designated by the same reference numerals to omit the descriptions thereof. The first holder  17  and the second holder  27  are arranged in the housing  50 A so that the height H 1  of the first holder  17  and the height H 2  of the second holder  27  become equal to or greater than the height H 31  of the edge of the housing  50 A with the bottom face of the housing  50 A used as a reference. The cover  60 A is a metal-made cover in such a shape as to cover the first holder  17  and the second holder  27  from the upper side. It is assumed that the height H 61  of the part of the cover  60 A that covers the first holder  17  and the second holder  27  is greater than the height H 1  of the first holder  17  and the height H 2  of the second holder  27  with the bottom face of the housing  50 A used as a reference. It is also assumed that the height H 62  of the part of the cover  60 A other than the part that covers the first holder  17  and the second holder  27  is equal to or smaller than the height H 1  of the first holder  17  and the height H 2  of the second holder  27 , with the bottom face of the housing  50 A used as a reference. It is further assumed that the height H 38  of a photodetector  38 A is smaller than the height H 62  of the part other than the part that covers the second holder  27 , with the bottom face of the housing  50 A used as a reference. 
     When the opening of the housing  50 A is covered with the cover  60 A, a space  95 A is formed between the cover  60 A and the first holder  17  and between the cover  60 A and the second holder  27 . A second heat-transfer gel  91 A is filled in the space  95 A through a hole  61 A formed in the cover  60 A in the position opposed to the second holder  27 , for example. Thus, the heat generated in the first laser diode  11  or the second laser diode  21  is dissipated from the first holder and the second holder to the cover  60 A via the second heat-transfer gel  91 A. The height H 31  of the edge of the housing  50 A is equal to or smaller than the height Hi of the first holder  17  and the height H 2  of the second holder  27 , with the bottom face of the housing  50 A used as a reference, and the height H 62  of the part other than the part of the cover  60 A that covers the first holder  17  and the second holder  27  is equal to or smaller than the height H 1  of the first holder  17  and the height H 2  of the second holder  27 , with the bottom face of the housing  50 A used as a reference, thereby being able to provide the compact optical pickup apparatus. 
     The above embodiments of the present invention are simply 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 encompass equivalents thereof. 
     In a first embodiment of the present invention, a description has been given of the configuration in which the second heat-transfer gel  91  is filled in the space  95  through the hole  61 , however, it is not limited thereto. For example, the cover  60  may be attached to the housing  50  after the second heat-transfer gel  91  is heaped up on the top face of the first holder  17  and the second holder  27  so that the second heat-transfer gel  91  is filled in the space  95 , without forming the hole  61  in the cover  60 . In this case, a sufficient amount of the second heat-transfer gel  91  can be filled in the space  95  between the first holder  17  and the second holder  27 , and the cover  60 , thereby being able to provide the optical pickup apparatus  100  that has a high capability of dissipating the heat generated in the first laser diode  11  and the heat generated in the second laser diode  21 . Further, it is not necessary to form the hole  61  in the cover  60  when manufacturing the optical pickup apparatus  100 , thereby being able to reduce the manufacturing process and reduce the manufacturing cost of the optical pickup apparatus  100 .