Abstract:
According to a method for designing an optical pickup including a monolithic laser diode that emits a laser light beam for DVD recording and a laser light beam for CD playback, two photodetective portions provided on the same substrate for receiving the laser light beams, and an objective lens, for each of the emitting portions of the monolithic laser diode for emitting the DVD recording and CD playback laser light beams respectively, the specification of the error of the emission direction thereof is determined according to the following formula: |(Δθ ⊥ 1 ×Δθ 1 ×Dpd 1 /Dob 1 −Δθ ⊥ 2 ×Δθ 2 ×Dpd 2 /Dob 2 )×Kpd 2 |≦7, where, let the variables corresponding to the DVD recording laser light beam be followed by a subscript “1” and variables corresponding to the CD playback laser light beam be followed by a subscript “2”, Δθ ⊥ represents the emission angle; Δθ represents the sensitivity, to the emission angle, of the deviation of the gravity center of the intensity of the light exiting from the object lens; Dob represents the diameter of the light beam immediately after exiting from the object lens; Dpd represents the diameter of the spot formed by the light beam on the photodetective portion; Kpd represents the sensitivity of the deviation of light reception balance to the deviation of the photodetective portion; and a represents the tolerable value for the remnant deviation of light reception balance.

Description:
[0001]     This application is based on Japanese Patent Application No. 2005-257473 filed on Sep. 6, 2005, the contents of which are hereby incorporated by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a method for designing an optical pickup incorporated in an optical disc apparatus.  
         [0004]     2. Description of Related Art  
         [0005]     An optical disc apparatus that plays back from and records to an optical disc such as a CD or DVD incorporates an optical pickup. Conventionally, different optical pickups have been developed to cope with recording to and playback from different types of discs.  
         [0006]     For example, a DVD recorder employs an optical pickup that can handle recording to a DVD and playback from a CD.  FIGS. 1 and 2  show the optical system of an optical pickup for a DVD recorder,  FIG. 1  showing it as viewed from the side and  FIG. 2  showing it as viewed from the top. It should be noted that the coordinate axes (X, Y, and Z) shown in these diagrams are common to other diagrams wherever applicable.  
         [0007]     A laser diode  1  emits laser light. The structure of the laser diode  1  is shown in  FIG. 3 . A submount  5  is fixed on a projection  4  formed to project from a disc-shaped stem  3 . In an upper end part of the submount  5 , a monolithic laser diode (hereinafter “monolithic LD”)  6  is arranged. Moreover, on the submount  5 , behind the monolithic LD  6 , a PIN diode  7  is formed. The monolithic LD  6  emits laser light of different wavelengths, namely at a DVD wavelength (in a 650 nm band) and at a CD wavelength (in a 780 nm). To achieve control, called APC (automatic power control), for keeping the output of the laser light emitted from the monolithic LD  6  constant, the PIN diode  7  receives the laser light emitted rearward from the monolithic LD  6 . The stem  3  is penetrated by a plurality of terminals  2 , which are connected by leads  8  to the monolithic LD  6  and to the PIN diode  7 , which thus receive their drive current via the terminals  2 .  
         [0008]      FIG. 4  shows the monolithic LD  6  as viewed from the direction in which laser light is emitted (from the direction indicated by arrow L in  FIG. 3 ). The monolithic LD  6  has, integrated into a single chip, a laser structure capable of emitting at two wavelengths, namely at a DVD wavelength (in a 650 nm band) and at a CD wavelength (in a 780 nm). The monolithic LD  6  has a common negative electrode  9 , a GaAs substrate  10 , an active layer  11 , a DVD-side p electrode  14 , and a CD-side p electrode  15 . On the GaAs substrate  10 , the individual layers including the active layer  11  are laid (downward in the diagram) on one another, and, to the lowermost layer thereof, the DVD-side p electrode  14  and the CD-side p electrode  15  are connected. In the active layer  11 , a DVD-side laser emitting portion  12  and a CD-side laser emitting portion  13  are formed. To the DVD-side p electrode  14 , a DVD-side positive electrode  16  formed on the submount  5  is connected, and, to the CD-side p electrode  15 , a CD-side positive electrode  17  formed on the submount  5  is connected. Moreover, to the GaAs substrate  10 , the common negative electrode  9  is connected. When electric current is passed through the electrodes, the DVD-side laser emitting portion  12  emits laser light (in a 650 nm wavelength band) for DVD recording, and the CD-side laser emitting portion  13  emits laser light (in a 780 nm wavelength band) for CD playback. The laser light for DVD recording has a higher output than the laser light for CD playback. The positions of the DVD-side laser emitting portion  12  and the CD-side laser emitting portion  13  are determined by a semiconductor wafer process, and hence with high accuracy.  
         [0009]     Emitted from the laser diode  1  structured as described above, the laser light for DVD recording or CD playback is then split, by a grating  18 , into one main beam and two sub beams. The laser light then passes through a PBS (polarizing beam splitter)  19  and then through a quarter-wave plate  20 , is then reflected on a upward-directing mirror  21 , and then enters a collimator lens  22 . By the collimator lens  22 , the laser light is formed into parallel light, and then the laser light enters an aperture  23 . The aperture  23  is wavelength-selective so as to let the laser light for DVD recording (in a 650 nm wavelength band) pass therethrough intact but restrict the aperture for the laser light for CD playback (in a 780 nm wavelength band). The laser light that has passed through the aperture  23  is focused on a recording surface of a disc  25  by an objective lens  24 .  
         [0010]     The laser reflected from the disc  25  passes through the objective lens  24 , then through the aperture  23 , and then through the collimator lens  22 , and is then reflected on the upward-directing mirror  21 . The laser light then passes through the quarter-wave plate  20 , and then enters the PBS  19 . The laser light that enters the PBS  19  here has passed through the quarter-wave plate  20  twice, and is therefore now reflected by the PBS  19  to enter a cylindrical lens  26 .  
         [0011]     As shown in  FIG. 5A , the cylindrical lens  26  has a concave cylindrical surface, and is arranged so that this concave cylindrical surface faces the PBS  19 . Moreover, as shown in  FIG. 5B , which shows the cylindrical lens  26  as viewed from the direction indicated by arrow A in  FIG. 2 , the cylindrical lens  26  is arranged so that its central generatrix S lies on an XY plane and is inclined 45° from the X direction.  
         [0012]     Having passed through the cylindrical lens  26  described above, the laser light is then received by a photodetector  27 .  FIG. 6  shows the photodetector  27  as viewed from the direction indicated by arrow B in  FIG. 2 . The photodetector  27  has, formed on a single silicon substrate  28 , a plurality of photodetective portions  29  to  34  that cope with laser light of two wavelengths. More specifically, one photodetective portion  30  having its photodetective surface divided into four parts receives the main beam of the laser light for DVD recording; two photodetective portions  29  and  31  having their photodetective surfaces each divided into two parts receive the sub beams of the laser light for DVD recording; one photodetective portion  33  having its photodetective surface divided into four parts receives the main beam of the laser light for CD playback; and two photodetective portions  32  and  34  having their photodetective surfaces undivided receive the sub beams of the laser light for CD playback. These photodetective portions individually convert the laser light they have received into electrical signals, based on which the RF signal recorded on the disc is reproduced and a focus error signal and a tracking error signal are produced.  
         [0013]     Inconveniently, however, the optical pickup configured as described above has the following disadvantages.  
         [0014]      FIG. 7  is a diagram showing how the monolithic LD  6  described above emits laser light. The intensity distribution of the laser light emitted from the DVD-side laser emitting portion  12  or the CD-side laser emitting portion  13  reflects how the laser emitting portion itself is shaped in the active layer  11 , and thus has an elliptic pattern elongate in the direction perpendicular to the active layer  11 . The intensity distribution of the laser light exhibits a Gaussian distribution both in the directions parallel and perpendicular to the active layer  11 , and the angle encompassing the part of such a distribution pattern where the light intensity is equal to or higher than a predetermined proportion (for example, one half) of its peak value is called the radiation angle. In the following description, the radiation angle (θ// in  FIG. 7 ) in the direction parallel to the active layer  11  will be referred to as the parallel radiation angle, and the radiation angle (θ⊥ in  FIG. 7 ) in the direction perpendicular to the active layer  11  will be referred to as the perpendicular radiation angle.  FIG. 8  shows the light intensity distribution in the direction perpendicular to the active layer  11 , as observed with the laser light for CD playback and the laser light for DVD recording, the latter having a higher output than the former. With the laser light for DVD recording, which has the higher output, the perpendicular radiation angle is small, resulting in a sharper distribution pattern; by contrast, with the laser light for CD playback, which has the lower output, the perpendicular radiation angle is large, resulting in a gentler distribution pattern.  
         [0015]     Here, as shown in  FIG. 9 , the direction (hereinafter referred to simply as “emission direction) in which the intensity of the laser light emitted from each emitting portion of the monolithic LD  6  has the peak value may deviate, in the direction perpendicular to the active layer  11  of the monolithic LD  6 , from the reference axis along the optical path leading from each emitting portion of the monolithic LD  6  to the center of the photodetective portion  30  (for the DVD main beam) or the photodetective portion  33  (for the CD main beam) of the photodetector  27 . In the following description, the angle of this deviation will be referred to as the emission angle (Δθ⊥ in  FIG. 9 ). This deviation is attributable to an error in the emission direction at each emitting portion resulting from a fabrication error in the monolithic LD  6 , and to an error in the fitting of the monolithic LD  6  itself resulting from an error in the submount  5  or the stem  3 .  
         [0016]     The intensity distribution of the laser light immediately after exiting from the objective lens  24 , as observed when the emission direction deviates downward in the X-axis direction from the reference axis as shown in  FIG. 9 , is shown in  FIG. 10A . Moreover, with respect to both of a case where the emission direction is not deviated from the reference axis and a case where it is deviated as shown in  FIG. 9 , the cross-section, as viewed in the Z-axis direction, of the intensity distribution of the laser light immediately after exiting the objective lens  24  is shown in  FIG. 10B . In  FIG. 10 , the symbol “Dob” represents the beam diameter of the laser light immediately after exiting the objective lens  24 . As these diagrams show, when the emission direction of the laser light deviates in the X-axis direction from the reference axis, the intensity distribution of the laser light immediately after exiting the objective lens  24 , as having been restricted by the aperture  23  and the member that keeps the objective lens  24  in position, becomes lopsided in the Y-axis direction, and thus the gravity center of the light intensity deviates in the Y-axis direction.  
         [0017]     As the light intensity of the laser light immediately after exiting the objective lens  24  deviates in this way, so the light intensity distribution at the photodetective portions of the photodetector  27  deviates. With respect to a case where the emission direction deviates downward in the X-axis direction from the reference axis as shown in  FIG. 9 , the light intensity distribution and the gravity center (indicated with the symbol “×”) of the light intensity as observed at the photodetective portion  30  (for the DVD main beam) and the photodetective portion  33  (for the CD main beam) are shown in  FIG. 11A . As this diagram shows, when the emission direction of the laser light deviates in the X-axis direction from the reference axis, the light intensity distribution at the photodetective portions becomes lopsided in the Y-axis direction, and thus the gravity center of the light intensity deviates in the Y-axis direction from the center of the photodetective portions.  
         [0018]     Here, as shown in  FIG. 11A , let the four photodetective parts of the photodetective portion  30  be represented by “a”, “b”, “c”, and “d”, and let the four photodetective parts of the photodetective portion  33  be represented by “A”, “B”, “C”, and “D”. Then, the photodetector  27  needs to be adjusted so that the light reception balances expressed by formulae (1) and (2) below equal, ideally, zero. 
 
 PDY   1 =(( I   a   +I   b )−( I   c   +I   d ))/( I   a   +I   b   +I   c   +I   d )×100  (1) 
 
 PDY   2 =(( I   A   +I   B )−( I   C   +I   D ))/( I   A   +I   B   +I   C   +I   D )×100  (2) 
 
 where PDY 1  (%) represents the light reception balance at the DVD photodetective portion; PDY 2  (%) represents the light reception balance at the CD photodetective portion; and Ii represents the light intensity at the photodetective part i. 
 
         [0019]     The state that appears when an adjustment is made in the state shown in  FIG. 11A  by moving the photodetector  27  in the Y-axis direction so that the gravity of center of the light intensity in the DVD photodetective portion  30  is located at the center of the photodetective portion  30  is shown in  FIG. 11B . This makes the light reception balance (formula (1) above) in the DVD photodetective portion  30  equal to zero, but leaves the gravity center of the light intensity in the CD photodetective portion  33  deviated from the center of the photodetective portion  33 , and accordingly leaves the light reception balance there (formula (2) above) non-zero. This is called the remnant light reception balance deviation. Due to errors in the emission direction of the emitting portions of the monolithic LD and errors in the fitting of the monolithic LD itself, the laser emission angle varies from one individual optical pickup to another, and, depending on the magnitude of the emission angle, the deviation of the gravity center of the light intensity in the state shown in  FIG. 11A  may be so great that, after the adjustment of the photodetector, the just-mentioned remnant light reception balance deviation exceeds the tolerable range. When the remnant light reception balance deviation exceeds the tolerable range, the reading of the disc and the servo operation are adversely affected, and this necessitates a further adjustment. Since the remnant light reception balance deviation cannot be eliminated through an adjustment involving the rotation of the photodetector, however, the optical pickup is then evaluated as defective, and this leads to a lower yield. Instead, an adjustment may be made in part of the optical system other than the photodetector, but this leads to increased cost.  
         [0020]     Incidentally, JP-A-2003-22543 discloses a method for adjusting light reception balance whereby a deviation of light reception balance attributable to a deviation of the direction of laser emission from an LD from the reference axis running in the direction parallel to the active layer thereof is adjusted by inclining the LD.  
       SUMMARY OF THE INVENTION  
       [0021]     An object of the present invention is to provide a method for designing an optical pickup that permits yield enhancement and cost reduction in optical pickups.  
         [0022]     To achieve the above object, according to the present invention, in a method for designing an optical pickup including: 
        a monolithic laser diode that emits 
            a first light beam for recording to a disc and     a second light beam for playing back from a disc or for recording to a disc;    
            a substrate;     a first photodetective portion provided on the substrate for receiving the first light beam;     a second photodetective portion provided on the substrate for receiving the second light beam; and     an objective lens,     for each of the emitting portions of the monolithic laser diode for emitting the first and second light beams respectively, the specification of the error of the emission direction thereof is determined according to at least one of formulae (A) and (B) below: 
 
|(Δθ⊥ 1 ×Δθ 1   ×Dpd   1   /Dob   1 −Δθ⊥ 2 ×Δθ 2   ×Dpd   2   /Dob   2 )× Kpd   2 |≦α  (A) 
 
|(Δθ⊥ 1 ×Δθ 1   ×Dpd   1   /Dob   1 −Δθ⊥ 2 ×Δθ 2   ×Dpd   2   /Dob   2 )× Kpd   1 |≦α  (B) 
 
 where, let the variables corresponding to the first light beam be followed by a subscript “1” and variables corresponding to the second light beam be followed by a subscript “2”, 
    Δθ ⊥ represents the emission angle;     Δθ represents the sensitivity, to the emission angle, of the deviation of the gravity center of the intensity of the light exiting from the object lens;     Dob represents the diameter of the light beam immediately after exiting from the object lens;     Dpd represents the diameter of the spot formed by the light beam on the photodetective portion;     Kpd represents sensitivity of the deviation of light reception balance to the deviation of the photodetective portion; and     α represents the tolerable value for the remnant deviation of light reception balance.        
 
         [0037]     When the specification of the error of the emission direction for each of the emitting portions of the monolithic laser diode is determined, it is preferable to take into consideration the variation of the sensitivity, to the emission angle, of the deviation of the gravity center of the intensity of the light exiting from the object lens.  
         [0038]     With this method, it is possible to reduce the number of optical pickups that are evaluated as defective due to the remnant light reception balance deviation exceeding the tolerable range after the position adjustment of their photodetective portions. Thus, it is possible to achieve yield enhancement. Moreover, it is no longer necessary to make an adjustment to bring to within the tolerable range the remnant light reception balance deviation exceeding the tolerable range after the position adjustment of their photodetective portions. Thus, it is possible to achieve cost reduction in optical pickups. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0039]      FIG. 1  is a side view of the optical system in a conventional optical pickup and in an optical pickup according to the invention;  
         [0040]      FIG. 2  is a top view of the optical system in a conventional optical pickup and in an optical pickup according to the invention;  
         [0041]      FIG. 3  is a perspective view of a laser diode;  
         [0042]      FIG. 4  is a diagram showing a monolithic LD as viewed from the direction in which laser light is emitted;  
         [0043]      FIG. 5A  is a perspective view of a cylindrical lens;  
         [0044]      FIG. 5B  is a plan view of the cylindrical lens;  
         [0045]      FIG. 6  is a diagram showing a photodetector;  
         [0046]      FIG. 7  is a diagram showing how the monolithic LD emits laser light;  
         [0047]      FIG. 8  is a diagram showing the intensity distribution of the laser light emitted from the monolithic LD, as observed in the direction perpendicular to the active layer;  
         [0048]      FIG. 9  is a diagram showing a deviation of the emission direction of the laser light emitted from the monolithic LD;  
         [0049]      FIG. 10A  is a diagram showing the intensity distribution of the laser light immediately after exiting from the objective lens, as observed when the emission direction deviates downward in the X-axis direction from the reference axis;  
         [0050]      FIG. 10B  is a cross-sectional view, as viewed in the Z-axis direction, showing, with respect to both of a case where the emission direction is not deviated from the reference axis and a case where it is deviated, the intensity distribution of the laser light immediately after exiting the objective lens;  
         [0051]      FIG. 11A  is a diagram showing the light intensity distribution and the gravity center of the light intensity as observed at the DVD main beam photodetective portion and the CD main beam photodetective portion when the emission direction of the laser light deviates downward in the X-axis direction from the reference axis;  
         [0052]      FIG. 11B  is a diagram showing the state that appears when an adjustment is made in the state shown in  FIG. 11A  by moving the photodetector in the Y-axis direction so that the gravity of center of the light intensity in the DVD main beam photodetective portion is located at the center of the photodetective portion;  
         [0053]      FIG. 12  is a diagram showing the optical system for measuring the intensity distribution of the laser light immediately after exiting from the objective lens;  
         [0054]      FIG. 13  is a graph showing the relationship between the emission angle and the deviation of the gravity center of the intensity of the laser light immediately after exiting the objective lens; and  
         [0055]      FIG. 14  is a graph showing the relationship between the perpendicular radiation angle of the DVD recording laser light and the sensitivity, to the emission angle, of the deviation of the gravity center of the intensity of the light exiting from the objective lens.  
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0056]     Hereinafter, an embodiment of the present invention will be described with reference to the drawings. The following description deals with, as an example, the optical pickup for a DVD recorder that has been discussed previously in connection with the related art. The optical system of the optical pickup is just as described previously and shown in  FIGS. 1 and 2 , and therefore no description thereof will be repeated.  
         [0057]     A description will be given below of the derivation of a formula that represents the value of the previously described remnant light reception balance deviation when, as shown in  FIG. 9 , the emission direction of the laser light emitted from each emitting portion of the monolithic LD  6  deviates in the direction perpendicular to the active layer  11  of the monolithic LD  6  from the reference axis along the optical path leading from each emitting portion of the monolithic LD  6  to the center of the photodetective portion  30  (for the DVD main beam) or the photodetective portion  33  (for the CD main beam) of the photodetector  27 .  
         [0058]     As described previously, when the laser emission direction deviates from the reference axis, the light intensity gravity center immediately after exiting from the objective lens  24  deviates as shown in  FIGS. 10A and 10B . Here, let Δθ represent the ratio of the variation of the deviation of the light intensity gravity center immediately after exiting from the objective lens  24  to the variation of the deviation of the laser emission direction from the reference axis, that is, the variation of the emission angle. Then, the deviation of the light intensity gravity center immediately after exiting from the objective lens  24  is given by Δθ⊥×Δθ.  
         [0059]     When the light intensity gravity center immediately after exiting from the objective lens  24  deviates, accordingly, as shown in  FIG. 11A , the light intensity gravity center in the photodetective portion  30  (for the DVD main beam) and in the photodetective portion  33  (for the CD main beam) deviates from the center of those emitting portions. The deviation of the light intensity gravity center is proportional to the diameter of the laser light, and hence, let Dob represent the laser diameter immediately after exiting from the objective lens  24  and let Dpd represent the laser spot diameter on the photodetective portion  30  or  33 , then, the deviation of the light intensity gravity center on the photodetective portion  30  or  33  is given by Δθ⊥×Δθ×Dpd/Dob.  
         [0060]     Next, consider a case where, as shown in  FIG. 11B , an adjustment is made by moving the photodetector  27  in the Y-axis direction so that the light intensity gravity center in the DVD photodetective portion  30  is located at the center of the photodetective portion  30 . In this case, the deviation of the light intensity gravity center in the CD photodetective portion  33  from the center thereof as observed after the adjustment of the photodetector  27  is expressed as Δθ ⊥ 2 ×Δθ 2 ×Dpd 2 /Dob 2 −Δθ ⊥ 1 ×Δθ 1 ×Dpd 1 /Dob 1 . Here, a subscript “1” indicates that the variable preceding it corresponds to the DVD recording laser light and a subscript “2” indicates that the variable preceding it corresponds to the CD playback laser light. This applies throughout the following description.  
         [0061]     Now, the ratio of the variation of the light reception balance (formula (2) noted previously) to the variation of the Y-axis direction position of the CD photodetective portion  33  is defined as the sensitivity Kpd 2  of the deviation of the light reception balance to the deviation of the photodetective portion. Then, the remnant light reception balance deviation in the CD photodetective portion  33  is given by (Δθ ⊥ 2 ×Δθ 2 ×Dpd 2 /Dob 2 −Δθ ⊥ 1 ×Δθ 1 ×Dpd 1 /Dob 1 )×Kpd 2 .  
         [0062]     Usually, an adjustment is made, as described above, by moving the photodetector  27  so that the light reception balance in the DVD photodetective portion  30  equals zero; axis instead, however, an adjustment may be made by moving the photodetector  27  in the Y-direction so that the light intensity gravity center in the CD photodetective portion  33  is located at the center thereof and thus that the light reception balance in the CD photodetective portion  33  equals zero. In this case, through the same reasoning as used above, the remnant light reception balance deviation in the DVD photodetective portion  30  is given by (Δθ ⊥ 1 ×Δθ 1 ×Dpd 1 /Dob 1 −Δθ ⊥ 2 ×Δθ 2 ×Dpd 2 /Dob 2 )×Kpd 1 .  
         [0063]     Here, the tolerable value of the remnant light reception balance deviation that does not adversely affect the reading from a disc or the servo operation is about 25%; however, when consideration is given to the reliability test conducted after the shipment of the optical pickup and the deterioration of accuracy with time, the tolerable value of the remnant light reception balance deviation at the time of the adjustment of the photodetector needs to be 7%.  
         [0064]     Accordingly, in a case where an adjustment is made by moving the photodetector  27  so that the light reception balance in the DVD photodetective portion  30  equals zero, it is necessary that formula (3) below be fulfilled. 
 
|(Δθ⊥ 1 ×Δθ 1   ×Dpd   1   /Dob   1 −Δθ⊥ 2 ×Δθ 2   ×Dpd   2   /Dob   2 )× Kpd   2 |≦7  (3) 
 
         [0065]     On the other hand, in a case where an adjustment is made by moving the photodetector  27  so that the light reception balance in the CD photodetective portion  33  equals zero, it is necessary that formula (3) below be fulfilled. 
 
|(Δθ⊥ 1 ×Δθ 1   ×Dpd   1   /Dob   1 −Δθ⊥ 2 ×Δθ 2   ×Dpd   2   /Dob   2 )× Kpd   1 |≦7  (4) 
 
         [0066]     Now, how an optical pickup is designed according to formulae (3) and (4) will be described by way of practical examples.  
         [0067]     Suppose here that an optical system as shown in  FIGS. 1 and 2  is being designed and test-fabricated. The laser diameter Dob immediately after exiting from the objective lens  24  depends on the numerical aperture and the focal length of the objective lens  24 ; here, with the DVD recording laser light, the laser diameter was Dob 1 =4 [mm], and, with the CD playback laser light, the laser diameter was Dob 2 =3.14 [mm]. The spot diameter Dpd at the photodetective portion of the photodetector  27  varies with the distance of the cylindrical lens  26  from the photodetector  27 ; here, in the DVD photodetective portion  30 , the spot diameter was Dpd 1 =0.1 [mm], and, on the CD photodetective portion  33 , the spot diameter was Dpd 2 =0.08 [mm].  
         [0068]     With the optical system configured as described above, the sensitivity Δθ, to the emission angle, of the deviation of the gravity center of the intensity of the light exiting from the objective lens was measured. The emission angle can be varied on a simulation basis by moving the objective lens  24  in the Y-axis direction as viewed in  FIG. 1 ; thus, while the objective lens  24  is swung in the Y-axis direction as viewed in  FIG. 1 , the laser light intensity distribution and the light intensity gravity center deviation as observed immediately after exiting from the objective lens  24  were measured. The measurement of the laser light intensity distribution and the light intensity gravity center deviation as observed immediately after exiting from the objective lens  24  was conducted by the use of an optical system including a CCD  36  and a microscope lens system  35  as shown in  FIG. 12 ; specifically, the image on the surface of the objective lens  24  was imaged through the optical system on the CCD  36 , and the intensity of the image was measured. From the monolithic LD  6  was emitted, at a time, the DVD recording laser light (with an output of 135 [mW]) and, at another time, the CD playback laser light (with an output of 8 [mW]), and, while the objective lens  24  is swung in the Y-axis direction as viewed in  FIG. 1 , the laser light intensity gravity deviation immediately after exiting the objective lens  24  was measured. An example of the results of the measurement is shown in  FIG. 13 . It should be noted that along the horizontal axis of the graph shown in  FIG. 13  is taken the amount of swing of the objective lens  24  as converted into the equivalent emission angle according to formula (5) below. 
 
Δθ⊥=tan −1 ( Y/f )  (5) 
 
 where Y represents the amount of swing of the objective lens, and f represents the focal length of the collimator lens. 
 
         [0069]     The gradient of each regression line in the graph represents the sensitivity Δθ, to the emission angle, of the deviation of the gravity center of the intensity of the light exiting from the objective lens; specifically, with the DVD recording laser light, Δθ 1 =43.1 [μm/deg], and, with the CD playback laser light, Δθ 2 =11.0 [μm/deg]. Here, as described earlier and shown in  FIG. 8 , the light intensity distribution in the direction perpendicular to the active layer  11  of the monolithic LD  6  shows the following features: with the DVD recording laser light, which has the higher output, the perpendicular radiation angle is small, resulting in a sharper distribution pattern; with the laser light for CD playback, which has the lower output, the perpendicular radiation angle is large, resulting in a gentler distribution pattern. Accordingly, the intensity distribution of the laser light immediately after exiting from the objective lens  24 , as narrowed around the peak value of the light intensity, exhibits a sharp distribution pattern with the DVD recording laser light and a gentle distribution pattern with the CD playback laser light. Hence, so long as the emission angles of the DVD recording laser light and of the CD playback laser light vary equally, the variation of the deviation of the light intensity gravity center in the light intensity distribution immediately after exiting from the objective lens  24  is larger with the DVD recording laser light than with the CD playback laser light, and the sensitivity Δθ, to the emission angle, of the deviation of the gravity center of the intensity of the light exiting from the objective lens is greater with the DVD recording laser light than with the CD playback laser light.  
         [0070]     Here, due to fabrication errors, the perpendicular radiation angle of the monolithic LD  6  varies from one individual LD to another. To study the effect of the perpendicular radiation angle on the sensitivity Δθ, to the emission angle, of the deviation of the gravity center of the intensity of the light exiting from the objective lens, samples of the monolithic LD  6  with different perpendicular radiation angles were prepared, and, while the perpendicular radiation angle was varied by the use of those samples, the sensitivity Δθ, to the emission angle, of the deviation of the gravity center of the intensity of the light exiting from the objective lens was measured in the same manner as described above.  FIG. 14  is a graph showing the results obtained as the radiation angle of the DVD recording laser light (with an output of 135 [mW]) was varied from 17.3 to 19.1 [deg]. The curve shown here is a regression curve of the results. As this graph shows, the sensitivity Δθ, to the emission angle, of the deviation of the gravity center of the intensity of the light exiting from the objective lens does vary as the perpendicular radiation angle θ⊥ varies. This is because, as the perpendicular radiation angle varies, the shape of the light intensity distribution in the direction perpendicular to the active layer  11  of the monolithic LD  6  and the shape of the light intensity distribution immediately after exiting from the objective lens  24  as narrowed around the peak value of the light intensity distribution also vary.  
         [0071]     Next, with the above-mentioned test-fabricated samples of the optical system, the sensitivity Kp of the deviation of the light reception balance to the deviation of the photodetective portion was measured. Specifically, with the photodetective portion irradiated with a spot, while the photodetector  27  is moved in the Y-axis direction, the light reception balance given by formula (1) or (2) above is measured, and the ratio of the variation of the light reception balance to the amount of movement of the photodetector  27  was calculated. Here, from the monolithic LD  6  was emitted, at a time, the DVD recording laser light (with an output of 135 [mW] and a perpendicular radiation angle of 18 to 21 [deg]) and, at another time, the CD playback laser light (with an output of 8 [mW] and a perpendicular radiation angle of 27 to 36 [deg]), and the sensitivity of the deviation of the light reception balance to the deviation of the photodetective portion was measured in the DVD photodetective portion  30  and in the CD photodetective portion  33 . The results were Kpd 1 =1.97 [%/μm] and Kpd 2 =1.75 [%/μm], indicating that almost no variation due to the variation of the perpendicular radiation angle was observed.  
         [0072]     Next, assuming that an adjustment is made by moving the photodetector  27  so that the light reception balance in the DVD photodetective portion  30  equals zero, based on the parameters measured as described above, the specification of the error of the emission direction in each emitting portion of the monolithic LD  6  is determined so as to fulfill formula (3) above. The parameters used in this determination are listed in Table 1. Here, with respect to the DVD recording laser light, assuming that the monolithic LD  6  used was fabricated with fabrication errors such that the DVD-side vertical radiation angle varied between 18 to 21 [deg], and according to the regression curve described previously and shown in  FIG. 14 , the sensitivity Δθ 1 , to the emission angle, of the deviation of the gravity center of the intensity of the light exiting from the objective lens was assumed to vary between 26.4 to 53.9 [μm/deg]. On the other hand, with respect to the CD playback laser light, assuming that the monolithic LD  6  used was fabricated with fabrication errors such that the CD-side vertical radiation angle varied between 27 to 36 [deg], while the perpendicular radiation angle of the CD playback laser light was varied between 27 and 36 [deg], the sensitivity Δθ 2 , to the emission angle, of the deviation of the gravity center of the intensity of the light exiting from the objective lens was measured, and, since Δθ 2  did not vary so much as with the DVD recording laser light, its average value 11.0 [μm/deg] was used. For higher precision, the variation of Δθ 2  may be taken into consideration.  
                                                                                           TABLE 1                                       DVD   CD                        Unit   Value       Unit   Value                            Δθ 1     μm/deg   24.6-53.9   Δθ 2     μm/deg   11           Dpd 1     mm   0.1   Dpd 2     mm   0.08           Dob 1     mm   4   Dob 2     mm   3.14           Kpd 1     %/μm   1.97   Kpd 2     %/μm   1.75                      
 
         [0073]     In this way, with consideration given to the variation of the sensitivity Δθ 1 , to the emission angle, of the deviation of the gravity center of the intensity of the light exiting from the objective lens corresponding to the DVD recording laser light, the specification of the error of the emission direction in each emitting portion of the monolithic LD  6  is determined so as to fulfill formula (3) above. Here, each emission angle is expressed as the sum of the error of the emission direction of the emitting portion of the monolithic LD  6  and the error in the fitting of the monolithic LD  6  itself attributable to errors in the submount  5 , the stem  3 , etc., and is given by formula (6) below. 
 
Δθ⊥ i =Δθ⊥ Li +Δθ⊥ M  (where i=1 or 2)  (6) 
 
 where Δθ⊥ Li  represents the error in the emission direction of the emitting portion of the monolithic LD, and Δθ⊥ M  represents the error in the fitting of the monolithic LD. 
 
         [0074]     Here, it is assumed that the error in the fitting of the monolithic LD is controlled within a variation of ±0.5 [deg] (Δθ⊥ M =−0.5 to 0.5 [deg]), and, with consideration given to the variation of the sensitivity Δθ 1 , to the emission angle, of the deviation of the gravity center of the intensity of the light exiting from the objective lens corresponding to the DVD recording laser light, the specification of the error of the emission direction of each emitting portion of the monolithic LD  6  is determined so as to fulfill formula (3) above, specifically such that DVD-side Δθ⊥ L1 =−2.0 to 1.0 [deg] and CD-side Δθ⊥ L2 =−2.5 to 2.5 [deg]. By the use of a monolithic LD given such a specification in terms of the error in the emission direction of each emitting portion, when Δθ⊥ 1 =Δθ⊥ L1 +Δθ⊥ M =−2.0-0.5=−2.5 [deg], Δθ⊥ 2 =Δθ⊥ L2 +Δθ⊥ M =2.5-0.5=2.0 [deg], and Δθ 1 =53.9 [μm/deg], the left side of formula (3) above equals its maximum value, namely 6.9 [%], and thus formula (3) is always fulfilled.  
         [0075]     Thus, even when the error in the emission direction of each emitting portion of a monolithic LD or the error in the fitting of the monolithic LD itself varies, and thus the perpendicular radiation angle of DVD recording laser light varies, and as a result the sensitivity Δθ 1 , to the emission angle, of the deviation of the gravity center of the intensity of the light exiting from the objective lens varies, so long as the photodetector  27  is adjusted by being moved so that the light reception balance in the DVD photodetective portion  30  equals zero, the remnant light reception balance deviation always falls within the tolerable range. Thus, it is possible to greatly reduce the number of optical pickups that are evaluated as defective, leading to enhanced yields. Moreover, there is no need to perform adjustment other than that of the position of the photodetector to bring the remnant light reception balance deviation exceeding the tolerable range to within the tolerable range, leading to lower cost.  
         [0076]     In a case where the photodetector  27  is adjusted by being moved so that the light reception balance in the CD photodetective portion  33  equals zero, instead of formula (3), formula (4) above is used to determine the specification of the error in the emission direction of each emitting portion of the monolithic LD  6 .  
         [0077]     Although the above description deals with a monolithic LD that emits laser light for recording to a DVD and laser light for playback from a CD, it should be understood that the present invention is applicable to a monolithic LD that emits laser light for recording from both emitting portions. In this case, the variation, resulting from the variation of the perpendicular radiation angle, of the sensitivity, to the emission angle, of the deviation of the gravity center of the intensity of the light exiting from the objective lens is measured and considered with respect to both laser beams (the variation of both Δθ 1  and Δθ 2  is taken into consideration).  
         [0078]     Needless to say, the present invention is applicable to any types of discs other than DVDs and CDs.