Abstract:
A semiconductor laser device has a semiconductor laser element, a starting mirror and a signal photodetector mounted on a surface of laminate ceramic package which is formed by layering a plurality of ceramic sheets having mutually different conductive patterns. The semiconductor laser device and an optical pickup apparatus having the device allow to eliminate the restrictions on arrangement of wire-bonded electrodes and wiring layout and to reduce the adverse effect of heat generated in the photodetector on the semiconductor laser element.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This nonprovisional application claims priority under 35 U.S.C. §119(a) on Patent Application No. 2005-145468 filed in Japan on 18 May 2005, the entire contents of which are incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates to a semiconductor laser device for use in reading information of optical recording media and writing information into optical recording media such as CD (Compact Disc), CD-R (Compact Disc Recordable), DVD (Digital Versatile Disc) and DVD-R (Digital Versatile Disc Recordable). The present invention also relates to an optical pickup apparatus provided with the semiconductor laser device.  
         [0003]     In accordance with the trend of reducing the size and thickness of semiconductor laser devices, development of less expensive semiconductor laser devices has been demanded. Conventionally, there has been the semiconductor laser device disclosed in JP 06-203403 A.  
         [0004]      FIG. 5A  shows a schematic top view of the conventional semiconductor laser device.  FIG. 5B  shows a schematic sectional view of the conventional semiconductor laser device.  
         [0005]     As shown in  FIGS. 5A and 5B , the semiconductor laser device has a lead frame  52  constructed of a die pad portion  59  and a lead terminal portion  60 , and a resin package  53  resin-molded to the lead frame  52 .  
         [0006]     A semiconductor laser element  57  is mounted on a silicon substrate  58  which is die-bonded to a die pad portion  59  of the lead frame  52 . A photodetection portion  56  is formed on the silicon substrate  58 . Specifically, on a surface of the silicon substrate  58  which is located on the side of the semiconductor laser element  57 , the photodetection portion  56  is formed for receiving light reflected on an optical disk. A pad is also formed there for electrically connecting the photodetection portion  56  and the semiconductor laser element  57  to the lead terminal portion  60 .  
         [0007]     The lead terminal portion  60  of the lead frame  52  is electrically connected to the semiconductor laser element  57  and the signal photodetector  56  via thin metal wires  51  and pads.  
         [0008]     After carrying out burn-in and characteristic inspection, a hologram element  54  is fixed to the resin package  53  with use of a UV (ultraviolet) resin  55 . Minute corrugations are formed on the surface of the hologram element  54 .  
         [0009]     According to the semiconductor laser device thus constructed, laser light emitted from the semiconductor laser element  57  is reflected on a mirror and directed toward the optical disk. The laser light is reflected on the optical disk to become optical signals which contain various pieces of information written in the optical disk, and diffracted by the hologram element  54  to be directed toward the signal photodetector  56 . The optical signal is converted into an electrical signal by the signal photodetector  56 , and the electrical signal is outputted to the outside via the thin metal wires  51 .  
         [0010]     In the conventional semiconductor laser device, however, the thin metal wires  51  are allowed to be led only two-dimensionally. Accordingly, there are restrictions on the arrangement of the pads or electrodes for wire-bonding the thin metal wires  51  and on the wiring layout of the thin metal wires  51 .  
         [0011]     Moreover, heat generated in the photodetection portion  56  adversely affects the semiconductor laser element  57  because the semiconductor laser element  57  is placed on the silicon substrate  58  together with the photodetection portion  56 .  
       SUMMARY OF THE INVENTION  
       [0012]     An object of the present invention is to provide a semiconductor laser device which eliminates the restrictions on arrangement of wire-bonded electrodes and wiring layout and reduces the adverse effect of heat generated in the photodetector on the semiconductor laser element, and to provide an optical pickup apparatus provided with the device.  
         [0013]     In order to achieve the object, the present invention provides a semiconductor laser device comprising:  
         [0014]     a semiconductor laser element;  
         [0015]     a starting mirror for reflecting laser light emitted from the semiconductor laser element toward a light-irradiated object; and  
         [0016]     a package in which the semiconductor laser element and the starting mirror are mounted, wherein  
         [0017]     the package is constituted by layering a plurality of ceramic sheets having mutually different conductive patterns.  
         [0018]     According to the thus-constructed semiconductor laser device, the plurality of ceramic sheets constituting the package have mutually different conductive patterns, which allows three-dimensional wiring patterns formed of conductive patterns to be provided in the package. Therefore, it is possible to eliminate restrictions on arrangement of the electrodes formed in the package and restrictions on wiring layout of the thin metal wires which electrically connect the semiconductor laser element with the electrodes.  
         [0019]     When a photodetector for example is mounted in the package, no placement of the semiconductor laser element above the photodetector makes it possible to reduce the adverse effect of the heat generated in the photodetector on the semiconductor laser element. This improves the high-temperature operation characteristic of the semiconductor laser element.  
         [0020]     A hologram element, which diffracts light reflected on the light-irradiated object, may be mounted on the package. In this case, a photodetector may be further mounted in the package, the photodetector receiving the reflected light diffracted by the hologram element.  
         [0021]     Moreover, even if the hologram element is not mounted on the package, a photodetector for receiving the light reflected on the light-irradiated object may be mounted in the package.  
         [0022]     In one embodiment of the present invention, through-holes are respectively provided in the ceramic sheets, and the semiconductor laser element and the starting mirror are placed in the through-holes.  
         [0023]     According to the semiconductor laser device of the embodiment, height of device is decreased since the semiconductor laser element and the starting mirror are placed in the through-holes.  
         [0024]     In one embodiment of the present invention, through-holes are respectively provided in the ceramic sheets, and a stairs-like slope face of the through-holes is formed by accumulating the ceramic sheets having different through-holes in size respectively in such a way that a laser light reflecting surface of the starting mirror mounted on the stairs-like slope face has an angle of approximately 45 degrees with respect to a resonator length direction of the semiconductor laser element.  
         [0025]     According to the semiconductor laser device of the embodiment, the optical axis of the laser light emitted from the semiconductor laser element can be changed by approximately 90 degrees because the laser light-reflecting surface of the starting mirror has an angle of approximately 45 degrees with respect to the resonator length direction of the semiconductor laser element.  
         [0026]     In one embodiment of the present invention, a concave portion is provided in a side surface of the package.  
         [0027]     According to the semiconductor laser device of the embodiment, since the concave portion is provided on the side surface of the package, when a cap for example is mounted on the package, the concave portion allows the cap to be easily mounted on the package by fitting a part of the cap to the concave portion, and a bonding force to be secured between the cap and the package.  
         [0028]     In one embodiment of the present invention, a resonator length direction of the semiconductor laser element forms an angle of approximately 45 degrees with respect to an outer edge of the package.  
         [0029]     According to the semiconductor laser device of the embodiment, it is possible to increase the resonator length of the semiconductor laser element without any increase in the outer edge length of the package because the resonator length direction of the semiconductor laser element forms an angle of approximately 45 degrees with respect to an outer edge of the package.  
         [0030]     In one embodiment of the present invention, a material for the ceramic sheets is made of aluminum nitride.  
         [0031]     According to the semiconductor laser device of the embodiment, it is possible to increase heat radiation of the package because aluminum nitride is used as a material of the ceramic sheet.  
         [0032]     The present invention also provides an optical pickup apparatus comprising the above-stated semiconductor laser device.  
         [0033]     According to the optical pickup apparatus of the invention, by virtue of the semiconductor laser device, the degree of freedom of design can be increased and the high-temperature operation characteristic can also be improved.  
         [0034]     According to the semiconductor laser device of the present invention, three-dimensional wiring patterns formed of conductive patterns are provided in the package because ceramic sheets constituting the package have mutually different conductive patterns. Therefore, it is possible to eliminate the restrictions on the arrangement of the electrodes provided in the package and the restrictions on the wiring layout of the thin metal wires that electrically connect the semiconductor laser element to the electrodes.  
         [0035]     Moreover, when a photodetector for example is mounted in the package, no placement of the semiconductor laser element above the photodetector makes it possible to reduce the adverse effect of the heat generated in the photodetector on the semiconductor laser element. Thus, the high-temperature operation characteristic of the semiconductor laser element can be improved. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0036]     The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein:  
         [0037]      FIG. 1  is a schematic perspective view of a hologram unit that is a semiconductor laser device according to one embodiment of the present invention;  
         [0038]      FIG. 2A  is an in-process view of parts of the hologram laser unit;  
         [0039]      FIG. 2B  is an in-process view of different parts of the hologram laser unit;  
         [0040]      FIG. 2C  is an in-process view of still different parts of the hologram laser unit;  
         [0041]      FIG. 3  is a schematic top view of a modification example of the hologram laser unit;  
         [0042]      FIG. 4  is a schematic top view of another modification example of the hologram laser unit;  
         [0043]      FIG. 5A  is a schematic top view of a conventional semiconductor laser device;  
         [0044]      FIG. 5B  is a schematic sectional view of the conventional semiconductor laser device; and  
         [0045]      FIG. 6  is a schematic structural view of an optical pickup apparatus provided with the semiconductor laser device according to another embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0046]     A semiconductor laser device of the present invention and an optical pickup apparatus provided with the device is described in detail below with reference to drawings.  
         [0047]      FIG. 1  shows a schematic perspective view of a hologram unit that is a semiconductor laser device according to one embodiment of the present invention. A cap  11  in  FIG. 1  is shown in a transparent form so as to comprehensibly show the structure inside the hologram laser unit.  
         [0048]     The hologram laser unit has a semiconductor laser element  7 , a starting mirror  13  that reflects laser light emitted from the semiconductor laser element  7  toward an optical disk, a hologram element  12  that diffracts the light reflected on the optical disk, a signal photodetector  9  that receives the reflected light diffracted by the hologram element  12 , and a laminate ceramic package  5  on the upper surface  17  of which the semiconductor laser element  7 , the starting mirror  13  and the signal photodetector  9  are mounted. The optical disk is one example of a light-irradiated object. The laminate ceramic package  5  is one example of a package. The signal photodetector  9  is one example of a photodetector.  
         [0049]     A concave portion  14  is provided in a center portion of the upper surface  17  of the laminate ceramic package  5 . Moreover, a concave portion  18  and external terminals  10  are provided on side surfaces of the laminate ceramic package  5 .  
         [0050]     An opening of the concave portion  14  has a rectangular shape. The lengthwise direction of the opening of the concave portion  14  is roughly perpendicular to an edge of the laminate ceramic package  5  on the side of the concave portion  18 , and roughly parallel to an edge of the laminate ceramic package  5  on the side of the external terminals  10 . Then, the semiconductor laser element  7  and the starting mirror  13  are placed in the concave portion  14 .  
         [0051]     More in detail, a monitor submount  6 , on which the semiconductor laser element  7  is mounted, is die-bonded to the bottom surface of the concave portion  14 . The resonator length direction of the semiconductor laser element  7  is roughly perpendicular to the edge of the laminate ceramic package  5  on the side of the concave portion  18  and roughly parallel to the edge of the laminate ceramic package  5  on the side of the external terminals  10 . Moreover, a side surface of the concave portion  14 , which faces the laser light-emitting end surface of the semiconductor laser element  7 , has a stairs-like configuration on which the starting mirror  13  is mounted.  
         [0052]     Each of the monitor submount  6 , the semiconductor laser element  7  and the signal photodetector  9  is electrically connected to at least one of electrodes  15  provided on the upper surface  17  of the laminate ceramic package  5  via a thin metal wire  8 . Moreover, the monitor submount  6 , the semiconductor laser element  7  and the signal photodetector  9  are covered with the cap  11  for protection. The electrode  15  is one example of a conductive pattern.  
         [0053]     A convex portion  19  is provided in a lower portion of the cap  11   b  and fit to the concave portion  18  provided on a side surface of the laminate ceramic package  5 . With this arrangement, the cap  11  is positioned and fixed. Moreover, an upper part of the cap  11  is provided with an opening  21  on which a hologram element  12  is placed. After optically adjusting the position of the hologram element  12  on the upper surface of the cap  11 , the hologram element  12  is fixed to the upper surface of the cap  11  with a UV resin or the like.  
         [0054]     The reflecting surface  20  of the starting mirror  13  reflects the laser light emitted from the laser light-emitting end surface of the semiconductor laser element  7 . The reflecting surface  20  is oriented at an angle of approximately 45 degrees with respect to the resonator length direction of the semiconductor laser element  7 . With this arrangement, the laser light reflected on the reflecting surface  20  travels toward a direction roughly perpendicular to the upper surface  17  of the laminate ceramic package  5 . That is, the starting mirror  13  changes the optical axis of the laser light emitted from the laser light-emitting end surface of the semiconductor laser element  7  at an angle of approximately 90 degrees.  
         [0055]     The electrodes  15  are electrically connected to the external terminals  10  (see  FIG. 2B ) via a conductive pattern  4  that is three-dimensionally formed in the laminate ceramic package  5 .  
         [0056]     A diffraction grating  22  is provided on the upper surface of the hologram element  12 , the opposite surface of which is located on the side of the semiconductor laser element  7 . A diffraction grating  23  having a different configuration from that of the diffraction grating  22  is provided on the lower surface of the hologram element  12 , that is, the surface thereof located on the side of the semiconductor laser element  7 .  
         [0057]     The manufacturing method of the hologram laser unit is described below with reference to  FIGS. 2A through 2C .  
         [0058]     First, as shown in  FIG. 2A , viaholes  0 A,  1 A and a punching hole  2 A as an example of a through-hole are provided in a thin ceramic sheet  3 A. Each of the viaholes is also a through-hole and has a conductive pattern on an inner surface thereof so as to electrically connect viaholes of ceramic sheets located above and blow the ceramic sheet.  
         [0059]     As shown in  FIG. 2B , viaholes  0 B,  1 B and a punching hole  2 B, which are through-holes, are provided in a thin ceramic sheet  3 B. Then, a conductive pattern  4  is pattern-printed on the upper surface of the ceramic sheet  3 B with a conductive paste (e.g., Ag paste). The punching hole  2 B is larger than the punching hole  2 A. The conductive pattern  4  is electrically connected to the conductive pattern of the inner surface of the viahole  0 B and the conductive pattern of the inner surface of the viahole  1 B.  
         [0060]     As shown in  FIG. 2C , viaholes  0 C,  1 C, a punching hole  2 C larger than the punching hole  2 A and electrodes  15  are provided in a thin ceramic sheet  3 C. The punching hole  2 C is a through-hole.  
         [0061]     The ceramic sheets  3 A to  3 C are baked together with a ceramic sheet having no punching holes. Thereby, a plate member is obtained which includes a plurality of laminate ceramic packages  5  each provided with a three-dimensional circuit pattern.  
         [0062]     Next, the monitor submount  6 , the semiconductor laser element  7  and the signal photodetector  9  are mounted on prescribed positions of each of the laminate ceramic package  5 .  
         [0063]     Next, the monitor submount  6 , the semiconductor laser element  7  and the signal photodetector  9  are electrically connected to the electrodes  15  via the thin metal wires  8 . Thereafter, the plate member is cut along the dashed lines (dashed lines intersecting the center of the viaholes  0 C) shown in  FIG. 2C , so as to form the external terminals  10  (obtained by dividing the viaholes  0 A,  0 B and  0 C into halves) and the concave portion  18  on the side surfaces of the laminate ceramic package  5 . Thereby, the separated laminate ceramic packages  5  are obtained, on each of which the monitor submount  6 , the semiconductor laser element  7  and the signal photodetector  9  are mounted. Moreover, the electrodes  15  are electrically connected to the external terminals  10  via the conductive pattern  4  (see  FIG. 2B ) or the like.  
         [0064]     Finally, the cap  11  is mounted on the laminate ceramic package  5 , and thereafter the hologram element  12  is secure to the upper surface of the cap  11  with a UV resin or the like. Thereby, the complete hologram laser unit shown in  FIG. 1  is obtained.  
         [0065]     In the above-stated embodiment, the lengthwise direction of the opening of the concave portion  14  is perpendicular to the edge of the laminate ceramic package  5  located on the side of the concave portion  18 . However, it is acceptable to angle the lengthwise direction of the opening of the concave portion  14  at an angle of approximately 45 degrees to the edge of the laminate ceramic package  5  located on the side of the concave portion  18 , as shown in  FIG. 3 . With this arrangement, a semiconductor laser element  7  having a longer resonator length can be placed in the concave portion  14  without any increase in length of the edge of the laminate ceramic package  5  located on the side of the external terminals  10 , which is achieved by only increasing the length in the lengthwise direction of the concave portion  14 .  
         [0066]     It is also acceptable to mount a semiconductor laser element driving IC (integrated circuit)  16  on the upper surface  17  of the laminate ceramic package  5  as shown in  FIG. 4 . Thereby, the hologram laser unit is further integrated, which makes it possible to reduce size and thickness of the optical pickup apparatus.  
         [0067]     Although not shown in the drawings, it is acceptable to mount a high-frequency overlay IC on the upper surface  17  of the laminate ceramic package  5  in the case where a semiconductor laser element of a single oscillation mode necessary for high-frequency overlay is mounted on the upper surface  17  of the laminate ceramic package  5 .  
         [0068]     Moreover, AlN (aluminum nitride) may be used as a material of the laminate ceramic package  5  since thermal conductivity of AlN is greater than that of silicon. Specifically, the laminate ceramic package  5  may be constructed of ceramic sheets of AlN. This construction allows heat of the hologram laser unit to be more effectively released in comparison with a silicon package where which the semiconductor laser element  7 , the signal photodetector  9  and so on are mounted.  
         [0069]     As described above, the hologram laser unit may be mounted on an optical pickup apparatus.  
         [0070]      FIG. 6  shows a schematic structural view of an optical pickup apparatus  230  provided with a semiconductor laser device  200  according to another embodiment of the present invention.  
         [0071]     The optical pickup apparatus  230  has an optical pickup apparatus casing  231 , a collimating lens  234 , a starting mirror  235  and an object lens  236  besides a semiconductor laser device  200 .  
         [0072]     In the semiconductor laser device  200 , the external terminals  10 , which are formed by dividing the viaholes  0 C into halves, are exposed on both sides of the laminate ceramic package  205  to serve as electrodes  218 . Same components in  FIG. 6  as the components of the semiconductor laser device shown in  FIG. 1  are denoted by the same reference numerals as those of the components shown in  FIG. 1 . Description therefor is omitted.  
         [0073]     The collimating lens  234  transforms incident light into parallel light. Specifically, laser light emitted from the semiconductor laser element  7  (see  FIG. 1 ) of the semiconductor laser device  200  is transformed into parallel light  220   a  by the collimating lens  234 .  
         [0074]     The starting mirror  235  bends the optical path of the laser light  220   a , which has passed through the collimating lens  234 , at an angle of 90 degrees. As a result, the laser light  220   a  is conducted to the object lens  236 .  
         [0075]     The object lens  236  condenses the laser light  220   a , which is bent by the starting mirror  235 , onto the surface of an optical recording medium  237  located on the side of the starting mirror  235 .  
         [0076]     The optical pickup apparatus casing (hereinafter referred to as a “casing”)  231  is formed by metal casting or die casting. The collimating lens  234  and the starting mirror  235  are adjusted so that the center of the mounting hole (not shown) of the housing  231  and the optical axis of the semiconductor laser device  200  can accurately coincide with each other, and thereafter fixed to the housing  231 .  
         [0077]     The optical pickup apparatus  230  is assembled by inserting the semiconductor laser device  200  into the mounting portion (not shown) of the housing  231 . At this time, the optical axis of the semiconductor laser device  200  parallel to the direction of emission of the laser light  220   a  is adjusted by bringing a surface of the laminate ceramic package  5 , which surface is located on the side of the hologram element  12 , in contact with the surface formed at the mounting portion of the housing  231 .  
         [0078]     As shown in  FIG. 6 , the laser light  220   a  emitted from the semiconductor laser device  200  is transformed into parallel light by the collimating lens  234 , bent at an angle of 90 degrees by the starting mirror  235 , and condensed on the surface of the optical recording medium  237  located on the side of the starting mirror  235  by the object lens  236 . The optical pickup apparatus  230  employs a starting mirror  235  having a sufficiently large area, on which the laser light  220   a  is incident, so as to reflect the whole laser light  220   a  transmitted through the collimating lens  234 . Specifically, sides of a starting mirror  235  need to be 7 mm or more in length because the effective diameter of the collimating lens  234  is about 5 mm.  
         [0079]     The laser light reflected on the optical recording medium  237  becomes signal light  220   b  containing the information recorded in the optical recording medium  237 . The signal light  220   b  passes through a path opposite to that from the semiconductor laser device  200  to the optical recording medium  237 , specifically, in order of the object lens  236 , the starting mirror  235  and the collimating lens  234 , and returns to the semiconductor laser device  200 . The signal light  220   b  that returns to the semiconductor laser device  200  is diffracted by the hologram pattern (not shown) formed at the hologram element  12 , and received by the photodetector  9  (see  FIG. 1 ). The signal from the photodetector  9  allows obtaining the information recorded in the optical recording medium  237 . Control signals such as a focus error signal and a tracking error signal are also obtained by the photodetector  9 .  
         [0080]     The hologram pattern is divided into a plurality of regions in order to generate the information to be recorded in the optical recording medium  237  and the control signals such as the focus error signal and the tracking error signal.  
         [0081]     It is acceptable to provide a plurality of the hologram patterns. Also, the hologram patterns may diffract different wavelengths from each other. In this case, it is only necessary to separate the light at every wavelength in advance.  
         [0082]     As described above, the optical pickup apparatus  230  shown in  FIG. 6  has the construction in which the hologram element  12  is integrated with the laminate ceramic package  5 . However, the hologram element  12  does not necessarily need integration with the laminate ceramic package  5 . Also, the cap is not necessarily required.  
         [0083]     The invention being thus described, it will be obvious that the invention may be varied in many ways. Such variations are not be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.