Abstract:
An optical-pickup apparatus comprising: a laser-light source including first-and-second-light-emitting points for laser lights with first-and-second wavelengths which points are disposed at positions deviating in a direction optically corresponding to an optical-disc-tracking direction; a diffraction grating including a plurality of periodic structures which are joined to be different in phase from each other in the direction and each of which includes a recess and projection repeated in a direction optically corresponding to an optical-disc-tangential direction, the diffraction grating being configured to generate main-and-sub-luminous fluxes from the laser light; a holder to hold the diffraction grating to be movable in the direction corresponding to the tracking direction; a collimating lens; an objective lens to focus the main-and-sub-luminous fluxes from the collimating lens on the same track of an optical disc; and a photodetector to be applied with reflected lights of the main-and-sub-luminous fluxes from the optical disc, to generate main-and-sub-push-pull signals.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of priority to Japanese Patent Application No. 2008-235065, filed Sep. 12, 2008, of which full contents are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an optical pickup apparatus. 
     2. Description of the Related Art 
     As an optical pickup apparatus compatible with a plurality of types of optical discs different in track pitch, there is known an optical pickup apparatus employing tracking control by an inline differential push-pull method. By the inline push-pull method, laser light emitted from a laser light source is diffracted by a diffraction grating in which regions having periodic structures different in phase from each other are joined, to generate 0th order light and ±1st order diffracted lights. The 0th order light and ±1st order diffracted lights are applied to a recording layer of the optical disc, and reflected lights thereof is received, to generate a main push-pull signal and a sub push-pull signal. Then, by generating a differential push-pull signal to obtain a tracking error signal, from the main push-pull signal and the sub push-pull signal, an offset component accompanying displacement of an objective lens or inclination of the optical disc can be effectively reduced. It is known that, in the diffraction grating, by providing a central region having a periodic structure further different in phase between the regions including the periodic structures different in phase from each other, a visual-field characteristic can be improved which indicates a deterioration rate of the differential push-pull signal when the objective lens is displaced in a tracking direction (radial direction of the optical disc) (Japanese Patent Laid-Open Publication No. 2004-145915, for example.) Also, there is known an optical pickup apparatus including a laser light source for emitting laser light having a wavelength corresponding to each of the optical discs in order to be compatible with both CD (Compact Disc) and DVD (Digital Versatile Disc) (Japanese Patent Laid-Open Publication No. 2007-220175, for example.) 
     For example, in order to be compatible with both a CD and DVD, in a case where a diffraction grating having a plurality of periodic structures different in phase from each other is used with a laser light source for emitting laser lights having two wavelengths, the location of the diffraction grating is, in many cases, fixed such that light emitting points are in a predetermined positional relationship. For example, as shown in  FIG. 14 , in an optical system including a diffraction grating  210  having a plurality of periodic structures, a collimating lens  212 , and an objective lens  214 , the diffraction grating is fixed at a position at which the light emitting point of laser light for DVD coincides with an optical axis of the objective lens  214 . In DVDs, particularly a DVD-RAM (Digital Versatile Disc Random Access Memory) has a tendency that the visual-field characteristic deteriorates. Thus, the light emitting point of the laser light for DVD is made coincide with the optical axis of the objective lens  214 , and therefore, the visual-field characteristic is restrained from deteriorating. 
     On the other hand, since the light emitting point of the laser light for DVD is at such a position as to coincide with the optical axis of the objective lens  214 , the light emitting point of the laser light for CD is at such a position deviating from the optical axis of the objective lens  214 . Thus, the visual-field characteristic deteriorates in a case of CD, and the visual-field characteristic might not satisfy a required level depending on a shift amount of the objective lens  214 . 
     SUMMARY OF THE INVENTION 
     An optical pickup apparatus according to an aspect of the present invention, comprises: a laser light source including a first light emitting point for laser light with a first wavelength and a second light emitting point for laser light with a second wavelength, the first light emitting point and the second light emitting point being disposed at positions deviating in a direction optically corresponding to a tracking direction of an optical disc, the laser light source being configured to selectively emit the laser light with the first wavelength or the laser light with the second wavelength; a diffraction grating including a plurality of periodic structures joined so as to be different in phase from each other in the direction optically corresponding to the tracking direction of the optical disc, each of the plurality of periodic structures including a recess and a projection repeated in a direction optically corresponding to a tangential direction of the optical disc, the diffraction grating being configured to generate a main luminous flux and a sub luminous flux from the laser light; a holder configured to hold the diffraction grating so as to be movable in the direction corresponding to the tracking direction of the optical disc; a collimating lens configured to convert the diffused main luminous flux and sub luminous flux into parallel light; an objective lens configured to focus the main luminous flux and the sub luminous flux output from the collimating lens on the same track of the optical disc; and a photodetector configured to be applied with reflected lights of the main luminous flux and the sub luminous flux focused on the optical disc, to generate a main push-pull signal and a sub push-pull signal. 
     Other features of the present invention will become apparent from descriptions of this specification and of the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For more thorough understanding of the present invention and advantages thereof, the following description should be read in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a diagram illustrating an optical pickup apparatus according to an embodiment of the present invention; 
         FIG. 2  is a diagram illustrating an example of a grating face of a diffraction grating; 
         FIG. 3  is a diagram illustrating a configuration example of a part of a photodetector and a drive signal generation unit; 
         FIG. 4  is a diagram illustrating a configuration example of a holder (first holder); 
         FIG. 5  is a diagram illustrating a configuration example of a holder (second holder); 
         FIG. 6  is a diagram illustrating an example of a fit between holders (first holder and second holder); 
         FIG. 7  is a diagram illustrating a state of a holder housed in a housing; 
         FIG. 8  is a diagram illustrating a state of a holder housed in a housing; 
         FIG. 9  is a diagram illustrating an example of rotation adjustment; 
         FIG. 10  is a diagram illustrating an example of an irradiation spot formed on a track of an optical disc; 
         FIG. 11  is a diagram illustrating an example of an irradiation spot formed on a track of an optical disc; 
         FIG. 12  is a graph illustrating an example of visual-field characteristic of a differential push-pull signal in a case where an optical disc is a DVD-RAM; 
         FIG. 13  is a graph illustrating an example of visual-field characteristic of a differential push-pull signal in a case where an optical disc is a CD-ROM; and 
         FIG. 14  is a diagram illustrating an example of a position between a light emitting point of a laser light source and a diffraction grating. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     At least the following details will become apparent from descriptions of this specification and of the accompanying drawings. 
       FIG. 1  is a diagram illustrating an optical pickup apparatus according to an embodiment of the present invention. An optical pickup apparatus  10  includes a laser light source  20 , a diffraction grating  22 , a holder  23 , a beam splitter  24 , a collimating lens  26 , an objective lens  28 , a sensor lens  32 , a photodetector  34 , a drive signal generation unit  36 , and an objective lens driving unit  38 . 
     The laser light source  20  is a laser diode including a light emitting point  40  (first light emitting point) for laser light having a wavelength (first wavelength) for DVD and a light emitting point  42  (second light emitting point) of laser light having a wavelength (second wavelength) for CD, which points are positioned in a direction optically corresponding to a tracking direction of an optical disc  60  with a predetermined space therebetween. Here, the wavelength for DVD is approximately 630 to 685 nm, while the wavelength for CD is approximately 765 to 839 nm. 
     The diffraction grating  22  includes a grating face  44  for generating 0th order light (main luminous flux) and ±1st order diffracted lights (sub luminous flux) used in the inline differential push-pull method from the laser light emitted from the laser light source  20 .  FIG. 2  is a diagram illustrating an example of the grating face  44 . On the grating face  44 , there are provided a region  51  and a region  52  for generating a phase shift of 180 degrees in a part of the laser light emitted from the laser light source  20 . Specifically, the substantially rectangular regions  51  and  52  have periodic structures including projections and depressions, and the regions are different in phase from each other by 180 degrees in a direction optically corresponding to a tangential direction of the optical disc  60 . In a central region between the regions  51  and  52  on the grating face  44 , regions  53  and  54  that have periodic structures different in phase from the regions  51  and  52  are provided, and joined in a direction optically corresponding to the tracking direction of the optical disc  60  so as to be different in phase from each other. Specifically, the periodic structure of the region  53  is different in phase by 60 degrees from the periodic structure of the region  51  in the direction optically corresponding to the tangential direction of the optical disc  60 , and the periodic structure of the region  54  is different in phase by 120 degrees and 60 degrees from the periodic structures of the regions  51  and  53 , respectively, in the direction optically corresponding to the tangential direction of the optical disc  60 . 
     The holder  23  holds the diffraction grating  22  and adjusts a position of the diffraction grating  22 . Specifically, the holder  23  includes a holder  46  (first holder) allowing the entire holder  23  to move in a direction corresponding to the tracking direction of the optical disc  60  and a holder  48  (second holder) allowing the diffraction grating  22  to rotate. 
     The beam splitter  24  allows the 0th order light and ±1st order diffracted lights having passed through the diffraction grating  22  to pass therethrough toward the collimating lens  26 , and reflects toward the sensor lens  32  reflected lights which are reflected by the optical disc  60 . The collimating lens  26  converts the 0th order light and ±1st order diffracted lights, which are diffused light having passed through the beam splitter  24 , into parallel light. 
     The objective lens  28  condenses the parallel light from the collimating lens  26 , to form irradiation spots respectively corresponding to the 0th order light and ±1st order diffracted lights on a recording layer of the optical disc  60 . Moreover, the objective lens  28  outputs reflected light from the recording layer of the optical disc  60  to the collimating lens  26 . 
     The sensor lens  32  corrects aberration of the reflected light reflected by the beam splitter  24 , to be output to the photodetector  34 . The photodetector  34  outputs a detection signal of a level corresponding to a light amount of the received reflected light. The drive signal generation unit  36  generates a tracking error signal and a focusing error signal based on a detection signal from the photodetector  34 . The objective lens driving unit  38  drives the objective lens  28  in the tracking direction or focusing direction based the tracking error signal and the focusing error signal from the drive signal generation unit  36 . 
       FIG. 3  is a diagram illustrating a configuration example of parts of the photodetector  34  and the drive signal generation unit  36 . The photodetector  34  includes light receiving portions  70  to  72  for DVD. The drive signal generation unit  36  includes subtractors  76  to  79 , an adder  80 , and an amplifier  82 . An irradiation spot  92  formed by irradiation with the 0th order light and irradiation spots  93  and  94  formed by irradiation with the ±1st order diffracted lights are on the same track  90  of the optical disc  60 . The reflected light from the irradiation spots  92  to  94  becomes the 0th order light and ±1st order diffracted lights, respectively, by a diffraction function of the optical disc  60 , to be applied to the light receiving portions  70  to  72 . The light receiving portions  70  to  72  have light receiving surfaces each divided into two parts, for example, and difference occurs among light receiving amounts of the light receiving surfaces according to positional relation between the track  90  and the irradiation spots  92  to  94 . 
     The subtractor  76  performs an arithmetic operation to obtain a difference in light receiving amount between the two light receiving surfaces in the light receiving portion  70 , thereby generating a main push-pull signal (MPP). The subtractors  77  and  78  perform arithmetic operations to obtain a difference in light receiving amount between the two light receiving surfaces in the light receiving portions  71  and  72 , respectively. Then, signals with the same phase from the subtactors  77  and  78  are added at the adder  80 , and the sub push-pull signal (SPP) is generated. The main push-pull signal (MPP) and the sub push-pull signal (SPP) are opposite in phase, and the sub push-pull signal (SPP) amplified at the amplifier  82  is subtracted from the main push-pull signal (MPP) at the subtractor  79 , so as to obtain a differential push-pull signal (DPP) to become a tracking error signal. 
     Here, the offset components, which are generated in the main push-pull signal (MPP) and the sub push-pull signal (SPP) due to displacement of the objective lens  28 , inclination of the optical disc  60  and the like, are the same in phase regardless of the positions of the irradiation spots  92  to  94 . Therefore, the offset component contained in the tracking error signal can be effectively reduced by means of the subtraction at the subtractor  79 . 
     Though  FIG. 3  illustrates only the light receiving portions  70  to  72  for DVD, the photodetector  34  also includes light receiving portions for CD (not shown), and the differential push-pull signal (DPP) in which the offset component is reduced is similarly generated in a case of CD as well. 
     The holder  23  will hereinafter be described in detail. As mentioned above, the holder  23  includes the holders  46  and  48 . The holder  46  is provided with a circular opening portion  110  (first opening portion) for passing therethrough the 0th order light and ±1st order diffracted lights to be output via the diffraction grating  22 , as shown in  FIG. 4 , for example. The holder  48  holds the diffraction grating  22  as shown in  FIG. 5 , for example, and is provided with a circular protruding opening portion  112  (second opening portion) for passing therethrough the 0th order light and ±1st order diffracted lights to be output via the diffraction grating  22 . Here, an outer circumferential diameter of a protruding portion  112   a  formed at an edge of the opening portion  112  is slightly smaller than the diameter of the opening portion  110 . As shown in  FIG. 6 , the protruding portion  112   a  formed at the edge of the opening portion  112  of the holder  48  is fitted in the opening portion  110  of the holder  46 . Moreover, the holders  46  and  48  are housed in a housing  120  as shown in  FIGS. 7 and 8 , and a pushing force is applied by a member for contact  126  having elasticity to the holder  48  from a wall face  124  side so that the holder  46  is brought into contact with the wall face  122 . On the wall face  122  of the housing  120 , there is provided an opening portion (third opening portion) for passing therethrough the 0th order light and ±1st order diffracted lights output from the opening portion  110  of the holder  46 , and on the wall face  124  of the housing  120 , there is provided an opening portion (fourth opening portion) for allowing laser light to enter the diffraction grating  22  held by the holder  48 . 
     In a state shown in  FIGS. 7 and 8 , the holder  46  can move in a direction corresponding to the tracking direction of the optical disc  60 . In an embodiment according to the present invention, in the initial state, the diffraction grating  22  is adjusted in position so that the DVD light emitting point coincides with an optical axis of the objective lens  28 , for example. It is assumed that this initial position is a position of zero displacement, and in a direction corresponding to the tracking direction of the optical disc  60 , displacement in a direction in which the CD light emitting point goes away from the optical axis of the objective lens  28  is positive, while displacement in a direction in which the CD light emitting point gets close to the optical axis of the objective lens  28  is negative. 
     As shown in  FIG. 9 , for example, the holder  48  can rotate using the holder  46  as a supporting board. The rotation adjustment by the holder  48  is carried out in order to form the irradiation spots  92  to  94  on the same track  90  of the optical disc  60 . For example, as shown in  FIG. 10 , when the irradiation spots  92  to  94  are not properly formed on the same track  90 , adjustment is made such that the irradiation spots  92  to  94  are properly formed on the same track  90  as shown in  FIG. 11  by rotating the holder  48 . 
       FIG. 12  is a graph illustrating an example of the visual-field characteristic of the differential push-pull signal (DPP) in a case where the optical disc  60  is a DVD-RAM. In an embodiment according to the present invention, as the initial state, the position of the diffraction grating  22  is adjusted such that the DVD light emitting point coincides with the optical axis of the objective lens  28 . Therefore, as shown in  FIG. 12 , when the displacement of the diffraction grating  22  in the direction corresponding to the tracking direction of the optical disc  60  is zero, the visual-field characteristic becomes the maximum (100%) in a state where a shift amount of the objective lens  28  is zero, and the shift amount is almost symmetrical with respect to a point at which the shift amount is zero. When the diffraction grating  22  is displaced by a predetermined amount in the positive direction, a point at which the visual-field characteristic is the maximum is moved slightly to a positive side from the point at which the shift amount of the objective lens  28  is zero, and deterioration in the visual-field characteristic on a negative side of the shift amount of the objective lens  28  becomes greater by a corresponding amount. On the other hand, when the diffraction grating  22  is displaced by a predetermined amount in the negative direction, the point at which the visual-field characteristic is the maximum is moved slightly to the negative side from the point at which the shift amount of the objective lens  28  is zero, and deterioration in the visual-field characteristic on the positive side of the shift amount of the objective lens  28  becomes greater by a corresponding amount. 
       FIG. 13  is a graph illustrating an example of the visual-field characteristic of the differential push-pull signal (DPP) in a case where the optical disc  60  is a CD-ROM. In the initial state, that is, in a state where the displacement of the diffraction grating  22  is zero, since the CD light emitting point is displaced from the optical axis of the objective lens  28  to the negative side, the visual-field characteristic is the maximum at a point at which the shift amount of the objective lens  28  is slightly on the positive side. Therefore, in the case where the displacement is zero, the deterioration is greater in the characteristic on the negative side of the shift amount of the objective lens  28 , and in an example in  FIG. 13 , the visual-field characteristic falls under 50% when the shift amount is the maximum. When the diffraction grating  22  is displaced in the positive direction, deviation between the CD light emitting point and the optical axis of the objective lens  28  becomes greater, and therefore, the deterioration in the visual-field characteristic on the negative side of the shift amount of the objective lens  28  becomes further greater. On the other hand, when the diffraction grating  22  is displaced to the negative side, the CD light emitting point gets closer to the optical axis of the objective lens  28 , and the deterioration in the visual-field characteristic on the negative side of the shift amount of the objective lens  28  becomes smaller, and in an example in  FIG. 13 , the visual-field characteristic exceeds 50% even when the shift amount is the maximum. 
     As mentioned above, in the optical pickup apparatus  10 , when the laser light source which selectively emits a plurality of laser lights having different wavelengths, the position of the diffraction grating  22  can be adjusted so as to satisfy the required level of the visual-field characteristic in each wavelength. That is, in the optical pickup apparatus  10 , the diffraction grating  22  can be moved in the direction corresponding to the tracking direction of the optical disc  60 , and thus, the position of the diffraction grating  22  can be adjusted so as to satisfy the required level of the visual-field characteristic in each of the wavelengths for CD and for DVD. For example, in a case where the visual-field characteristic changes as shown in  FIGS. 12 and 13 , the required level at which the visual-field characteristic is 50% or more, for example, can be satisfied in both the cases of a DVD-RAM and CD-ROM, by displacing the diffraction grating  22  to the negative side only by a predetermined amount. 
     The holder  23  for holding the diffraction grating  22  can include the two holders  46  and  48 . As a result, in addition to the displacement of the diffraction grating in the direction corresponding to the tracking direction of the optical disc  60  by the holder  46 , the rotation adjustment of the diffraction grating by the holder  48  is made possible. Therefore, it becomes possible to improve accuracy of the differential push-pull signal by adjusting the position of the diffraction grating  22  such that the irradiation spots  92  to  94  are formed on the same track  90  of the optical disc  60 . 
     The holder  23  is supported by the member for contact  126  in the housing  120 . Therefore, the holder  23  can be displaced in the direction corresponding to the tracking direction of the optical disc  60  without providing a special mechanism such as a guide rail, for example. 
     The above embodiments of the present invention are simply for facilitating the understanding of the present invention and are not in any way to be construed as limiting the present invention. The present invention may variously be changed or altered without departing from its spirit and encompass equivalents thereof. 
     For example, in an embodiment according to the present invention, the opening portion  112  of the holder  48  for rotation adjustment is in a circular shape, however, it is not limited to the circular shape but may be a polygonal shape, for example, as long as the shape allows rotation in a state where the opening portion  112  is fitted with the opening portion  110  of the holder  46 . Moreover, contrary to an embodiment according to the present invention, a configuration may be made such that a protruding portion is formed in the opening portion  110  of the holder  46  so as to be fitted with the opening portion  112  of the holder  48 . In this case, if the opening portion  112  of the holder  48  is formed in the circular shape, the opening portion  110  of the holder  46  is not limited to the circular shape. 
     Furthermore, for example, in an embodiment according to the present invention, the central region of the diffraction grating  22  is made up of the two regions  53  and  54  different in phase from each other, however, it is not limited to this as long as it is provided in order to improve the visual-field characteristic, and the central region of the diffraction grating  22  may be made up of a single region having the periodic structure different in phase from the regions  51  and  52 .