Abstract:
An optical pickup for an optical storage device includes a stem, a substrate mounted on the stem, and a laser diode mounted on the substrate. The substrate is integrally formed with an optical signal detector and an error signal detector for focusing error detection and tracking error detection. A cap is mounted on the stem so as to accommodate the substrate and the laser diode. A beam splitter unit including a polarization beam splitter and a beam splitting element is mounted on the cap. A hologram for diffracting a reflected beam toward the error signal detector is interposed between the cap and the beam splitter unit. The substrate is biased at a given potential, and has an insulating film opposed to the stem.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to an optical pickup (optical head) for an optical device such as an optical disk drive, optical card drive, optical scanner, and microscope unit.  
         [0003]     2. Description of the Related Art  
         [0004]     An optical disk inclusive of a magneto-optical disk has received attention as a memory medium that becomes a core in the recent rapid development of multimedia, and it is usually accommodated in a cartridge case to be provided as an optical disk cartridge for practical use. The optical disk cartridge is loaded into an optical disk drive to perform reading/writing of data from/to the optical disk by means of an optical pickup.  
         [0005]     The optical pickup in a recent optical disk drive intended to realize size reduction is composed of a fixed optical assembly and a movable optical assembly, wherein the fixed optical assembly includes a laser diode, a beam splitter for reflecting and transmitting a laser beam, and a photodetector for receiving reflected light from an optical disk, whereas the movable optical assembly includes an actuator having a carriage and an objective lens mounted on the carriage. The carriage is movable in the radial direction of the optical disk along a pair of rails by means of a voice coil motor (VCM).  
         [0006]     A write-power laser beam output from the laser diode of the fixed optical assembly is first collimated by a collimator lens, next transmitted by the beam splitter, next reflected by a beam raising mirror of the actuator, and finally focused on the optical disk by the objective lens, thereby writing data onto the optical disk. On the other hand, data reading is performed by directing a read-power laser beam onto the optical disk. That is, reflected light from the optical disk is first collimated by the objective lens, next reflected by the beam splitter of the fixed optical assembly, and finally detected by the photodetector, thereby converting the detected optical signal into an electrical signal.  
         [0007]     In general, recording media such as an optical disk and a magneto-optical disk are exchanged for use with an optical disk drive. Further, these recording media have warpage or undulation due to strain in forming the media, resulting in the tendency of eccentricity or inclination of the recording media. Accordingly, focusing error detection and tracking error detection must be carried out to read out information recorded on the recording media. A conventional optical pickup for a magneto-optical disk employs many optical components including a plurality of lenses and a plurality of polarization beam splitters, so as to perform the detection of information recorded on the magneto-optical disk and also perform focusing error detection and tracking error detection.  
         [0008]     U.S. Pat. No. 5,708,644 discloses an optical pickup using a beam splitter unit having a polarization beam splitter and a beam splitting element to reduce the size of an optical system. In this U.S. Patent, a hologram for separating off a focusing error signal and a tracking error signal from a reflected beam is mounted on the lower surface of the beam splitter unit. Further, a laser diode, a first photodiode for detecting a magneto-optical signal, a second photodiode for detecting the focusing error signal, and a third photodiode for detecting the tracking error signal are mounted on a stem.  
         [0009]     Thus, the first, second, and third photodiodes are mounted on the stem in the optical pickup described in the above U.S. Patent, so that there is a problem of insufficient integration of the photodiodes. To solve this problem, it is considered to provide a silicon (Si) substrate integrally formed with these photodiodes.  
         [0010]     In an optical pickup for a magneto-optical disk, a PIN-photodiode is generally used as each photodiode to meet the requirement for a high-speed response signal. Accordingly, in the case of forming a PIN-photodiode integrally with an Si substrate, a reverse bias voltage is applied to the Si substrate to increase a response speed as a photodetecting element. The application of a reverse bias voltage means applying a bias voltage to the cathode of the photodiode.  
         [0011]     The stem (optical base) on which the Si substrate is mounted is bonded to a drive base, so that the stem is at the same potential as a ground potential. Therefore, the lower surface of the Si substrate must be insulated from the stem, so as to apply a reverse bias voltage to the Si substrate. Further, the laser diode chip is mounted on the upper surface of the Si substrate, so that an insulating layer must be interposed between the lower surface (electrode surface) of the laser diode chip and the upper surface of the Si substrate. However, in the case that there is a potential difference between the reverse-biased Si substrate and the electrode surface of the laser diode chip, especially in the case that there are high-frequency variations in potential, the potential of the Si substrate is influenced by variations in potential of the electrode of the laser diode.  
         [0012]     Such high-frequency variations in potential occur especially in writing data, and have adverse effects on a photodiode for detection of a focusing error signal, a photodiode for detection of a tracking error signal, and a photodiode for monitoring an output from the laser diode, thus causing instability in detecting signals output from these photodiodes.  
         [0013]     A region on the Si substrate except the photodiodes (photodetecting regions) also has sensitivity to light, and generates electrical charge when receiving light. This electrical charge has an influence on signal currents generated in the photodiode regions, causing a problem that high-quality signal currents cannot be obtained. This is due to the fact that all of the light quantities of the laser beam output from the laser diode cannot be transmitted or reflected by each optical component, but a part of the laser beam remains in the optical unit to become stray light.  
         [0014]     This stray light may enter the photodiode for detection of a magneto-optical signal, the photodiode for detection of a focusing error signal, the photodiode for detection of a tracking error signal, and a photodiode for automatic power control (APC), causing adverse effects on signal currents. As a known technique for shielding such stray light, a metal film is provided on the entire surface of the substrate except the photodiode regions. The metal film is usually formed of aluminum common to the material of wiring on the substrate. However, reflected light from the optical components is further reflected by the metal film to result in an increase in stray light.  
       SUMMARY OF THE INVENTION  
       [0015]     It is therefore an object of the present invention to provide an optical pickup in which an optical signal detecting mechanism is compact and integrated, and the quality of a signal detected by a photodetector is improved.  
         [0016]     It is another object of the present invention to provide an optical storage device including an optical pickup in which the quality of a signal detected by a photodetector is improved and low-cost production is allowed.  
         [0017]     It is a further object of the present invention to provide an optical pickup and an optical storage device which can solve the problem of stray light incident on the photodiode regions to improve the quality of a regenerative signal and the quality of a servo signal.  
         [0018]     In accordance with an aspect of the present invention, there is provided an optical pickup comprising a stem; a substrate mounted on the stem; a laser diode mounted on the substrate; and a photodetector provided on the substrate for detecting return light from an object to be irradiated; the substrate being biased at a given voltage and having an insulating member opposed to the stem.  
         [0019]     Preferably, the photodetector comprises a first photodetector for detecting a regenerative signal, a second photodetector for detecting a servo signal from a laser beam focused on the object, and a third photodetector for monitoring an output from the laser diode. The optical pickup further comprises a beam splitter unit having a polarization beam splitter and a beam splitting element formed of a birefringent crystal. The beam splitter unit further has a hologram lens for focusing monitor light to the third photodetector.  
         [0020]     Preferably, each of the first and second photodetectors comprises a PIN-photodiode. The laser diode has a first electrode opposed to the substrate with a first insulating film, a conductor film, and a second insulating film being interposed between the first electrode and the substrate. Preferably, the substrate comprises an Si substrate, and each of the insulating member, the first insulating film, and the second insulating film comprises an SiO 2  film. Preferably, the conductor film and the stem are connected by a first wire. The laser diode further has a second electrode, and the first and second electrodes of the laser diode are connected to the stem by second and third wires, respectively.  
         [0021]     In accordance with another aspect of the present invention, there is provided an optical storage device capable of at least reading information stored in an optical storage medium, comprising a base; a carriage movable along the optical storage medium; a stem mounted on the base; a substrate mounted on the stem; a laser diode mounted on the substrate; an objective lens mounted on the carriage for focusing a laser beam from the laser diode onto the optical storage medium; and a photodetector provided on the substrate for detecting at least a regenerative signal from a reflected beam from the optical storage medium; the substrate being biased at a given potential and having an insulating member opposed to the stem.  
         [0022]     In accordance with a further aspect of the present invention, there is provided an optical pickup comprising a stem; a substrate mounted on the stem; a laser diode for outputting a laser beam; a photodetector provided on the substrate for detecting return light from an object to be irradiated with the laser beam; a dummy photodetecting region provided on the substrate adjacent to the photodetector; a dummy electrode formed in the dummy photodetecting region so as to surround the photodetector; and wiring for connecting the dummy electrode to a ground potential.  
         [0023]     Preferably, the dummy photodetecting region comprises a first dummy photodetecting region provided on the substrate adjacent to a photodetector for detecting a regenerative signal, and a second dummy photodetecting region provided on the substrate adjacent to a photodetector for detecting a servo signal. The optical pickup further comprises a light shielding film having light absorptivity formed on the substrate so as to cover at least the first and second dummy photodetecting regions. Preferably, the light shielding film comprises a polyimide film.  
         [0024]     In accordance with a still further aspect of the present invention, there is provided an optical storage device capable of at least reading information stored in an optical storage medium, comprising a base; a carriage movable along the optical storage medium; a stem mounted on the base; a substrate mounted on the stem; a laser diode for outputting a laser beam; an objective lens mounted on the carriage for focusing the laser beam from the laser diode onto the optical storage medium; a photodetector for detecting at least a regenerative signal from return light from the optical storage medium; a dummy photodetecting region provided on the substrate adjacent to the photodetector; a dummy electrode formed in the dummy photodetecting region so as to surround the photodetector; and wiring for connecting the dummy electrode to a ground potential.  
         [0025]     In accordance with a still further aspect of the present invention, there is provided an optical pickup comprising a stem; a substrate mounted on the stem; a laser diode for outputting a laser beam; a photodetector provided on the substrate for detecting return light from an object to be irradiated with the laser beam; and a metal layer provided on the substrate so as to cover at least a region adjacent to the photodetector, the metal layer having a surface modified so as to have light absorptivity.  
         [0026]     Preferably, the metal layer comprises an anodized aluminum film.  
         [0027]     In accordance with a still further aspect of the present invention, there is provided an optical storage device capable of at least reading information stored in an optical storage medium, comprising a base; a carriage movable along the optical storage medium; a stem mounted on the base; a substrate mounted on the stem; a laser diode for outputting a laser beam; an objective lens mounted on the carriage for focusing the laser beam from the laser diode onto the optical storage medium; a photodetector provided on said substrate for detecting at least a regenerative signal from a reflected beam from the optical storage medium; and a metal layer provided on the substrate so as to cover at least a region adjacent to the photodetector, the metal layer having a surface modified so as to have light absorptivity.  
         [0028]     The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing some preferred embodiments of the invention.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0029]      FIG. 1  is an exploded perspective view of a magneto-optical disk drive;  
         [0030]      FIG. 2  is a top plan view of the magneto-optical disk drive;  
         [0031]      FIG. 3  is a bottom plan view of a movable optical assembly in the magneto-optical disk drive;  
         [0032]      FIG. 4  is a perspective view of the movable optical assembly;  
         [0033]      FIG. 5  is a schematic illustration of an optical pickup according to a preferred embodiment of the present invention;  
         [0034]      FIG. 6  is an elevational view of an optical unit according to a first preferred embodiment;  
         [0035]      FIG. 7  is a top plan view of the optical unit shown in  FIG. 6 ;  
         [0036]      FIG. 8  is an enlarged sectional view of an LD chip mounting portion in the optical unit;  
         [0037]      FIG. 9  is a top plan view of an optical unit according to a second preferred embodiment;  
         [0038]      FIG. 10  is a top plan view of an optical unit according to a third preferred embodiment;  
         [0039]      FIG. 11  is an elevational view of the optical unit shown in  FIG. 10 ; and  
         [0040]      FIG. 12  is a top plan view of an optical unit according to a fourth preferred embodiment. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0041]     Referring to  FIG. 1 , there is shown an exploded perspective view of a magneto-optical disk drive  2  according to a preferred embodiment of the present invention. The magneto-optical disk drive  2  is composed generally of a load/eject mechanism unit  4  and a read/write mechanism unit  6 . The load/eject mechanism unit  4  includes a chassis  8  having a bottom plate  8   a  and a pair of side plates  8   b , and a cartridge holder  10  mounted on the chassis  8  so as to be vertically movable with respect to the bottom plate  8   a  of the chassis  8 .  
         [0042]     An insert opening  12  for allowing insertion of a magneto-optical disk cartridge in the direction shown by an arrow C is defined by the cartridge holder  10  and the bottom plate  8   a  of the chassis  8 . The cartridge holder  10  is formed with a guide groove  14 . The guide groove  14  is composed of a first portion obliquely extending from one end of the insert opening  12  laterally inward of the cartridge holder  10  and a second portion extending from an inward end of the first portion to the rear end of the cartridge holder  10  in parallel to the longitudinal direction of the cartridge holder  10 . A first slider  16  and a second slider  18  are slidably engaged with the guide groove  14 .  
         [0043]     A continuous slit  23  is formed at one side portion of the cartridge holder  10  to thereby form a first spring arm  20  and a second spring arm  22  integral with the cartridge holder  10 . Similarly, a slit  25  is formed at the other side portion of the cartridge holder  10  to thereby form a third spring arm  24  integral with the cartridge holder  10 . A bias magnetic field generating device  26  is mounted on the cartridge holder  10 .  
         [0044]     A cartridge identification sensor  28  for detecting a write protected condition of the cartridge and the kind of the cartridge is mounted on the bottom plate  8   a  of the chassis  8 . Further, an eject motor  32  for ejecting the magneto-optical disk cartridge inserted in the cartridge holder  10  is mounted on the bottom plate  8   a  at its rear end portion opposite to the insert opening  12 .  
         [0045]     Although not shown, a vertically moving mechanism for the cartridge holder  10  is provided between the chassis  8  and the cartridge holder  10 . When the magneto-optical disk cartridge is fully inserted into the cartridge holder  10 , the cartridge holder  10  is moved toward the bottom plate  8   a  of the chassis  8  by the vertically moving mechanism, so that the magneto-optical disk cartridge is pressed on the bottom plate  8   a . In this condition, a shutter of the magneto-optical disk cartridge is opened, and a magneto-optical disk (to be hereinafter described) exposed is chucked to a spindle motor (to be hereinafter described). Such a vertically moving mechanism for the cartridge holder  10  is known in the art, so any further description thereof will be omitted herein.  
         [0046]     The load/eject mechanism  4  is provided with a flexible printed circuit board (FPC)  30 . The FPC  30  is branched at its front end portion into three parts, i.e., a first FPC  30   a , a second FPC  30   b , and a third FPC  30   c . The first FPC  30   a  is connected to the bias magnetic field generating device  26 . The second FPC  30   b  is connected to the eject motor  32 . The third FPC  30   c  is connected to the cartridge identification sensor  28 . The read/write mechanism unit  6  includes a base  34  formed of metal. The load/eject mechanism unit  4  is mounted on the base  34 . A spindle motor  36  is fixed to the base  34 .  
         [0047]     A movable optical assembly  38 , a fixed optical assembly  40 , and a printed circuit board  42  are mounted on the base  34 . The movable optical assembly  38  includes a carriage  44  on which an objective lens  46  is mounted. A connector  48  to be connected to a printed circuit board (not shown) mounted on the upper side of the load/write mechanism unit  4  is mounted on the printed circuit board  42 . Reference numeral  50  denotes an FPC for transferring a signal to the spindle motor  36  and a signal to the movable optical assembly  38 .  FIG. 2  is a plan view showing an assembled condition of the magneto-optical disk drive  2  shown in  FIG. 1 .  
         [0048]     As shown in  FIG. 3 , the movable optical assembly  38  is a preassembly constructed by inserting first and second guide rails  52  and  54  and two center yokes  60  through the carriage  44 , and fixing a side yoke  58  to each center yoke  60 . A permanent magnet  62  is fixed to each side yoke  58 . Thus, two magnetic circuits  56  are formed by the two side yokes  58 , the two center yokes  60 , and the two permanent magnets  62 . A pair of coils (not shown) are mounted on the carriage  44 , and these coils are connected to an FPC  64 .  
         [0049]     These magnetic circuits  56  and coils constitute a voice coil motor (VCM). The VCM is driven by supplying a current through the FPC  64  to the coils, so that the carriage  44  is moved along the first and second guide rails  52  and  54 . While the carriage  44  is linearly driven by the VCM in this preferred embodiment, an arm to be rotationally driven may be used in place of the carriage  44 , so as to move a light beam across the tracks on an optical storage medium.  
         [0050]     Referring to  FIG. 4 , there is shown a perspective view of the movable optical assembly  38  in relation to a magneto-optical disk  70 . The objective lens  46  is mounted on the carriage  44 . A pair of coils  72  are fixed to the opposite sides of the carriage  44 . Each coil  72  is inserted in a gap defined between the corresponding center yoke  60  and the corresponding permanent magnet  62 .  
         [0051]     Referring to  FIG. 5 , there is shown a schematic illustration of an optical pickup according to a preferred embodiment of the present invention. The optical pickup is configured by the movable optical assembly  38  having the objective lens  46  and the fixed optical assembly  40  mounted on the base  34  shown in  FIG. 1 . The fixed optical assembly  40  includes a stem (optical base)  74  to be mounted on the base  34 . The stem  74  is formed by pressing a metal plate such as an iron plate. The upper surface of the stem  74  is plated with gold.  
         [0052]     A silicon (Si) substrate  76  is mounted on the stem  74 . An SiO 2  film  77  is formed on the lower surface of the Si substrate  76  to insulate the Si substrate  76  from the stem  74 . The resistivity of the Si substrate  76  is 10 15  Ωcm, and it is a semiconductor substrate. Any other semiconductor substrates such as a germanium (Ge) substrate and a GaAs substrate may be used in place of the Si substrate  76 .  
         [0053]     The SiO 2  film  77  is formed by thermal oxidation of silicon, i.e., by heating the lower surface of a Si wafer. Accordingly, no special insulating member is required, and the SiO 2  film  77  can be formed at a low cost. The SiO 2  film  77  may be replaced by any other dielectric films having good heat conductivity, such as aluminum nitride (AlN) and silicon carbide (SiC). The Si substrate  76  is bonded to the stem  74  by using Au—Sn solder. To improve the bonding property of the Si substrate  76  to the stem  74 , the upper surface of the stem  74  is plated with gold as mentioned above. Similarly, the lower surface of the Si substrate  76  is also plated with gold.  
         [0054]     Referring to  FIG. 7 , there is shown a plan view of an optical unit  75  according to a first preferred embodiment. The optical unit  75  includes the stem  74  and the Si substrate  76  mounted on the stem  74 . The Si substrate  76  is integrally formed with a PIN-photodiode  78  for detecting a magneto-optical signal (MO signal), PIN-photodiodes  80   a  and  80   b  for detecting a focusing error signal, PIN-photodiodes  82   a  and  82   b  for detecting a tracking error signal, and a PIN-photodiode  84  for monitoring the power of a laser diode (LD) chip  86 .  
         [0055]     Aluminum wiring for leading out the signals from the PIN-photodiodes  78 ,  80   a ,  80   b ,  82   a ,  82   b , and  84  is patterned on the Si substrate  76 . The LD chip  86  is mounted on the Si substrate  76 . The LD chip  86  has a central oscillation wavelength of 685 nm and a beam spread angle of about  200 . The LD chip  86  is a chip cut from a wafer. Reference numeral  88  denotes an electrode formed on the lower surface of the LD chip  86 . The electrode  88  is connected to a terminal  96  by a wire  92 . An electrode  90  formed on the upper surface of the LD chip  86  is connected to a terminal  98  by a wire  94 .  
         [0056]     The PIN-photodiode  78  for detection of a MO signal is connected to terminals  104  and  106  respectively by wires  100  and  102 . The PIN-photodiode  80   a  for detection of a focusing error signal is connected to terminals  112  and.  114  respectively by wires  108  and  110 . Similarly, the PIN-photodiode  80   b  for detection of a focusing error signal is connected to terminals  120  and  122  respectively by wires  116  and  118 . The PIN-photodiode  82   a  for detection of a tracking error signal is connected to a terminal  126  by a wire  124 . Similarly, the PIN-photodiode  82   b  for detection of a tracking error signal is connected to a terminal  130  by a wire  128 . The PIN-photodiode  84  for detection of monitor light is connected to a terminal  134  by a wire  132 . These terminals  96 ,  98 ,  104 ,  106 ,  112 ,  114 ,  120 ,  122 ,  126 ,  130 , and  134  are hermetically sealed with glass and thereby insulated from the stem  74 .  
         [0057]     Referring again to  FIG. 5 , a laser beam is output from the LD chip  86  in the horizontal direction, and a reflection prism  140  for reflecting the laser beam in the vertical direction is therefore mounted on the Si substrate  76 . The reflection prism  140  is formed of BK7 glass (manufactured by Shott Inc.), and has a reflection surface coated with a phaseless reflecting film. The reflection prism  140  is fabricated by glass molding, polishing, etc.  
         [0058]     A cap  142  formed of Kovar is welded to the stem  74 . The Si substrate  76  and the LD chip  86  are accommodated in the cap  142 . The cap  142  has an opening  143  for forming a reciprocative optical path of a light beam. The opening  143  is closed by a glass plate  144 . Accordingly, the inside of the cap  142  is enclosed. A hologram  160  formed on the lower surface of a glass substrate  162  is bonded by adhesive to the upper surface of the cap  142 . Mass production of the hologram  160  can be made by forming a plurality of hologram patterns on a single glass substrate by etching and separating these hologram patterns by dicing to obtain individual holograms.  
         [0059]     The hologram  160  has a patterned diffraction grating for dividing a focusing error signal and a tracking error signal. A reflected beam from the magneto-optical disk  70  is diffracted by the diffraction grating of the hologram  160 , and then divisionally focused on the PIN-photodiodes  80   a  and  80   b  for detection of a focusing error signal and on the PIN-photodiodes  82   a  and  82   b  for detection of a tracking error signal, formed on the Si substrate  76 .  
         [0060]     A beam splitter unit  146  is fixedly mounted on the glass substrate  162  opposite to the hologram  160  by optical adhesive. The beam splitter unit  146  includes a glass block  148  having a cylindrical surface  150  and an inclined surface  152 , and a glass block  154  bonded by adhesive to the glass block  148 . A polarizing light spitting film  151  is formed on the cylindrical surface  151  of the glass block  148 . The glass block  154  has a concave cylindrical surface  156  just fitted with the cylindrical surface  150 , and a cylindrical reflection surface  158 . The concave cylindrical surface  156  of the glass block  154  is bonded by optical adhesive to the cylindrical surface  150  of the glass block  148 .  
         [0061]     Preferably, the inclined surface  152  of the glass block  148  and the cylindrical reflection surface  158  of the glass block  154  are coated with a reflecting film. A glass plate  166  formed with a focusing hologram lens  164  is bonded by adhesive to the lower surface of the glass block  148 . Further, a Wollaston prism  168  for splitting the reflected beam into a P-polarized light component and an S-polarized light component is bonded by adhesive to the lower surface of the glass block  154 .  
         [0062]     The LD chip  86  is bonded by Pb—Sn solder to the Si substrate  76 . The temperature of soldering of the LD chip  86  to the Si substrate  76  must be lower than the temperature of soldering of the Si substrate  76  to the stem  74 . Accordingly, Au—Sn solder is used to bond the Si substrate  76  to the stem  74  and heated to about 320° C. On the other hand, Pb—Sn solder is used to bond the LD chip  86  to the Si substrate  76  and heated to about 240° C. Reference numeral  170  denotes a collimator lens formed from a normal glass lens. The collimator lens  170  has a focal length of 10 mm, and it is an aspherical lens in which the focal length is corrected by an amount corresponding to the total thickness of the beam splitter unit  146  and the hologram substrate  162 .  
         [0063]     Referring to  FIG. 8 , there is shown an enlarged sectional view of an LD chip mounting portion. An SiO 2  film  176  is formed on the upper surface of the Si substrate  76 , and a conductor film  178  of a gold plating film is formed on the SiO 2  film  176 . An SiO 2  film  180  is formed on the conductor film  178 , and the LD electrode  88  of a gold plating film is formed on the SiO 2  film  180 . The SiO 2  films  176  and  180  are formed by sputtering, CVD, or electron beam process.  
         [0064]     However, the SiO 2  film  180  is formed selectively on a part of the conductor film  178  to be required for bonding to the LD chip  86  rather than the entire surface of the conductor film  178 , in consideration of wire connection to the outside of the LD chip  86 . A gold plating film  172  is formed on the lower surface of the LD chip  86 . The gold plating film  172  and the LD electrode  88  are bonded together by Pb—Sn solder  174 . Referring again to  FIG. 7 , the conductor film  178  is connected to the stem  74  by a wire  182 . Accordingly, the conductor film  178  is at the same electric potential as that of the stem  74 . Since the stem  74  is mounted on the base  34  of the magneto-optical disk drive, the conductor film  178  is at a ground potential.  
         [0065]     In operation, a P-polarized laser beam output from the LD chip  86  is reflected by the reflection prism  140  to change its optical path from the horizontal direction to the vertical direction. The laser beam reflected by the reflection prism  140  is passed through the hologram  160  and then passed through the polarizing light splitting film  151  with a transmittance of about 70%. The laser beam reflected by the polarizing light splitting film  151  is reflected by the inclined surface  152  and then focused onto the PIN-photodiode  84  for detection of monitor light by the focusing hologram lens  164 . Then, the power of the LD chip  86  is controlled to a predetermined level according to an output signal from the PIN-photodiode  84 .  
         [0066]     The laser beam passed through the polarizing light splitting film  151  is converted into a collimated beam by the collimator lens  170 , and the collimated beam is focused onto the magneto-optical disk  70  by the objective lens  46 . A reflected beam from the surface of the magneto-optical disk  70  undergoes Kerr rotation according to information written on the magneto-optical disk  70  to thereby include an S-polarized light component. The reflected beam is reconverted into a collimated beam by the objective lens  46  and then converged by the collimator lens  170  to enter the beam splitter unit  146 .  
         [0067]     The P-polarized light component in the reflected beam is passed through the polarizing light splitting film  151  with a transmittance of about 70%, and about 30% of the P-polarized light component is reflected by the polarizing light splitting film  151 . On the other hand, the S-polarized light component in the reflected beam is reflected by the polarizing light splitting film  151  with a reflectance of about 97%. Although the proportion of the S-polarized light component in the reflected beam is very small, the proportion of the S-polarized light component can be increased by reflecting most of the S-polarized light component on the polarizing light splitting film  151 .  
         [0068]     The beam reflected by the polarizing light splitting film  151  is totally reflected downward by the cylindrical reflection surface  158  of the glass block  154  to enter the Wollaston prism  168 . The beam is then split into a P-polarized light component and an S-polarized light component by the Wollaston prism  168 , and these components are detected by the PIN-photodiode  78 . The PIN-photodiode  78  includes a photodiode for detecting a P-polarized light component and a photodiode for detecting an S-polarized light component. Signals detected by these two photodiodes are subjected to differential detection by a method well known in the art to thereby detect a magneto-optical signal. On the other hand, the reflected beam transmitted by the polarizing light splitting film  151  enters the hologram  160  to undergo diffraction. The diffracted beams from the hologram  160  enter the PIN-photodiodes  80   a  and  80   b  for detection of a focusing error signal and the PIN-photodiodes  82   a  and  82   b  for detection of a tracking error signal.  
         [0069]     The hologram  160  may be fabricated by direct drawing using an electron beam or a laser beam. While it is necessary to tilt a hologram pattern to expect high efficiency in the direct drawing, such a requirement can be met by multiple drawing. Another fabrication method for a hologram includes the steps of preliminarily directly drawing a large hologram pattern, reducing the hologram pattern by using a stepper to prepare a mask, and transferring the hologram pattern by a photolithography. In this case, the hologram pattern is fabricated by ion beam etching using a photoresist or the like as a mask.  
         [0070]     According to the optical pickup of the above preferred embodiment, fluctuations in oscillation wavelength due to temperature changes as the emission characteristic of the LD chip  86  can be reduced to reduce the influence of chromatic aberration of an optical system. Further, since the Si substrate  76  is insulated from the stem  74  by the Sio 2  film  77 , a deterioration in radiation characteristic can be reduced. Further, the conductor film  178  is provided between the LD chip  86  and the Si substrate  76 , and the conductor film  178  is set at the same electric potential as a ground potential. Accordingly, it is possible to avoid the crosstalk between a drive signal to the LD chip  86  in writing data and output signals from the PIN-photodiodes  80   a  and  80   b  for detection of a focusing error signal, the PIN-photodiodes  82   a  and  82   b  for detection of a tracking error signal, and the PIN-photodiode  84  for detection of monitor light.  
         [0071]     Further, since the potential of the conductor film  178  present under the LD chip  86  is set to a ground potential, the emission characteristic of the LD chip  86  can be improved. Further, since the electrodes  88  and  90  of the LD chip  86  are connected directly to the terminals  96  and  98  provided on the stem  74  by the wires  92  and  94 , respectively, possible runaround of a signal to each PIN-photodiode can be avoided.  
         [0072]     Referring to  FIG. 9 , there is shown a plan view of an optical unit  75 A according to a second preferred embodiment of the present invention. In the following description of the second preferred embodiment and subsequent preferred embodiments, substantially the same parts as those of the optical unit  75  according to the first preferred embodiment shown in  FIGS. 6 and 7  will be denoted by the same reference numerals, and the description thereof will be omitted to avoid repetition.  
         [0073]     A dummy photodetecting region  186  is formed adjacent to the PIN-photodiode  78  for detection of a MO signal on the Si substrate  76 . A dummy photodetecting region  190  is formed adjacent to the PIN-photodiodes  80   a  and  82   a  on the Si substrate  76 . A dummy photodetecting region  194  is formed adjacent to the PIN-photodiodes  80   b  and  82   b  on the Si substrate  76 . Further, a dummy photodetecting region  198  is formed adjacent to the PIN-photodiode  84  for detection of monitor light on the Si substrate  76 . These dummy photodetecting regions  186 ,  190 ,  194 , and  198  function as PIN-photodiodes.  
         [0074]     The dummy photodetecting region  186  is formed with a dummy electrode  188  surrounding the PIN-photodiode  78 . The dummy photodetecting region  190  is formed with a dummy electrode  192  surrounding the PIN-photodiodes  80   a  and  82   a . The dummy photodetecting region  194  is formed with a dummy electrode  196  surrounding the PIN-photodiodes  80   b  and  82   b . The dummy photodetecting region  198  is formed with a dummy electrode  200  surrounding the PIN-photodiode  84 .  
         [0075]     The dummy electrode  200  is electrically connected to a terminal  202 , and the terminal  202  is electrically connected to the stem  74 . The dummy photodetecting regions  194  and  196  are electrically connected by aluminum wiring  206 . The dummy photodetecting regions  186  and  190  are electrically connected by aluminum wiring  208 . The dummy photodetecting regions  190  and  198  are electrically connected by aluminum wiring  210 . Accordingly, all of the dummy photodetecting regions  194 ,  186 ,  190 , and  198  are electrically connected to the stem  74  by the wire  204  to establish the same electric potential as a ground potential.  
         [0076]     The dummy electrodes  188 ,  192 ,  196 , and  200  are located on the dummy photodetecting regions  186 ,  190 ,  194 , and  198  so as to surround the PIN-photodiodes  78 ,  80   a  and  82   a ,  80   b  and  82   b , and  84 , respectively. Accordingly, electrical charge generated in each dummy photodetecting region does not leak into the electrode of the adjacent PIN-photodiode, but flows into the corresponding dummy electrode. Since each dummy electrode is electrically connected to the stem  74  having a ground potential, there is no adverse effect on the signal detected by each PIN-photodiode.  
         [0077]     In the case that the intensity of stray light in recording or erasing information, or in the case that an optical modulation rate is high, there is a possibility that the electrical charge in each dummy photodetecting region may be reduced in responsiveness to leak into an Mo signal and a servo signal, for example. To prevent this possibility and further improve the quality of a signal current, there is provided an optical unit  75 B according to a third preferred embodiment of the present invention as shown in  FIGS. 10 and 11 . More specifically, the optical unit  75 B includes a light shielding film  212  having light absorptivity formed on the Si substrate  76  except the mounting portions for the LD chip  86 , the reflection prism  140 , and the PIN-photodiodes  78 ,  80   a ,  80   b ,  82   a ,  82   b , and  84  and also except the pad portions for connection of the aluminum wiring.  
         [0078]     Preferably, the light shielding film  212  is formed from a polyimide film. The thickness of the polyimide film is about 1 to 5 am, preferably about 2 to 3 am. By adopting a polyimide film as the light shielding film  212 , it can be easily patterned by applying a resist on the substrate and next performing exposure and development in the photodiode fabrication step. Alternatively, the light shielding film  212  may be formed of a resist material or the like. Further, the light shielding film  212  hardly contains gas discharging components, so that there is almost no possibility of contamination of the PIN-photodiodes  78 ,  80   a ,  80   b ,  82   a ,  82   b , and  84  in hermetically sealing the cap  142 . Further, a current generating stray light can be suppressed to thereby suppress fluctuations in supply voltage and contribute to a reduction in power consumption.  
         [0079]     While the light shielding film  212  is formed on the Si substrate  76  except the above-mentioned portions in the preferred embodiment shown in  FIG. 10 , the light shielding film in the present invention may be formed so as to cover at least the dummy photodetecting regions  186 ,  190 ,  194 , and  198 . The stray light is caused by the incidence of surface reflection light from the hologram  160  and the beam splitter unit  146  into the optical unit  75 B. However, since the light shielding film  212  having light absorptivity is present in the optical unit  75 B, a photocurrent is not induced by the stray light except from the PIN-photodiodes  78 ,  80   a ,  80   b ,  82   a ,  82   b , and  84 . Further, there is no possibility that the stray light may be reflected on the light shielding film  212  to become a new stray light component. As a result, undue noise components are not mixed into a magneto-optical signal current or a servo signal current, thereby obtaining a high-quality output signal.  
         [0080]     Referring to  FIG. 12 , there is shown a plan view of an optical unit  75 C according to a fourth preferred embodiment of the present invention. In the optical unit  75 C shown in  FIG. 12 , the dummy photodetecting regions  186 ,  190 ,  194 , and  198  shown in  FIGS. 9 and 10  are not provided, but a metal film  214  having a surface modified so as to have light absorptivity is formed on the Si substrate  76 . The thickness of the metal film  214  is set to 2 to 8 am, preferably 4 to 6 am so as not to transmit light. Preferably, the metal film  214  is formed of the same material as that of the wiring patterned on the Si substrate  7 , 6 . In this case, the metal film  214  can be formed in the same process as the wiring forming process, thus improving the workability. Usually, the wiring pattern is formed of aluminum, so that the metal film  214  is preferably formed of aluminum.  
         [0081]     In the case that the metal film  214  is formed of aluminum, the surface of the aluminum film is preferably modified to prevent the reflection of stray light and to have light absorptivity, thereby avoiding reflected scattering light. More specifically, the surface modification is performed by masking the surface of the Si substrate  76  so that only a light shielding region (Al film) is exposed, and next anodizing the surface of the light shielding aluminum film to form a black coating, thereby obtaining a light absorbing surface. The aluminum film thus having an anodized surface is a light shielding film improved in heat resistance and reliability, so that the stray light component entering the Si substrate  76  can be almost cut off and the reflected light from the surface of the metal film  214  can also be suppressed. As a result, it is also possible to suppress the generation of a new stray light component due to re-reflection of reflected scattering light from the surface of the metal film  214  inside the optical unit  75 C. The material of the metal film  214  is not limited to aluminum, but any other materials having light absorptivity may be used.  
         [0082]     While the metal film  214  is formed on the almost entire surface of the Si substrate  76  in the preferred embodiment shown in  FIG. 12 , the metal film  214  may be formed on only regions adjacent to the PIN-photodiodes  78 ,  80   a ,  80   b ,  82   a ,  82   b , and  84 .  
         [0083]     Having thus described the optical pickup of the present invention in relation to a magneto-optical disk drive, the application of the present invention is not limited to the magneto-optical disk drive. For example, the optical pickup of the present invention is applicable also to any other types of optical storage devices using an optical pickup for driving an optical storage medium such as CD, DVD, and optical card. Further, the optical pickup of the present invention is applicable also to a microscope unit and various inspection devices, for example.  
         [0084]     According to the present invention as described above, the substrate integrally formed with a plurality of photodiodes is mounted on the stem with an insulating film being formed on the lower surface (mount surface) of the substrate, so that the radiation characteristic of the LD chip mounted on the upper surface of the substrate can be improved to thereby obtain a stable emission characteristic. Further, a conductor film is provided under the lower surface of the LD chip, and the electric potential of the conductor film is set to a ground potential, thereby reducing the crosstalk between a drive signal to the LD chip and output signals from the photodiodes to obtain a good servo signal and regenerative signal. Accordingly, the radiation from the LD chip can be facilitated to allow a stable writing operation, and an LD chip drive signal component mixing into a servo signal can be suppressed to thereby improve the quality of the servo signal and allow stable control. Thusly, it is possible to provide an optical pickup which can ensure high reliability and low cost.  
         [0085]     According to the preferred embodiment employing a dummy photodetecting region formed adjacent to each photodiode, a photocurrent induced by stray light incident on the dummy photodetecting region can be guarded by a dummy electrode formed in the dummy photodetecting region, thereby avoiding adverse effects on a regenerative signal detector and/or a servo signal detector to obtain a good regenerative signal and servo signal. According to the preferred embodiment employing a light shielding film or metal film having light absorptivity so formed as to cover the upper surface of the substrate, a stray light component reflecting on the surface of the film can be suppressed to prevent the generation of new stray light. Accordingly, the regenerative signal and the servo signal can be improved in quality to thereby provide an optical pickup having high reliability.  
         [0086]     The present invention is not limited to the details of the above described preferred embodiments. The scope of the invention is defined by the appended claims and all changes and modifications as fall within the equivalence of the scope of the claims are therefore to be embraced by the invention.