Abstract:
An optical pickup device includes a semiconductor laser for emitting laser light, an objective lens for irradiating luminous flux emitted from the semiconductor laser to an optical disc, a branching element having a plurality of regions for branching luminous flux reflected from the optical disc to a plurality of fluxes, and a photodetector having a plurality of light receiving parts which receive luminous flux branched by said branching element. Luminous flux which enters at least two regions arrayed along a direction which is made substantially coincident with a tangential direction of the optical disc in regard to substantially a center of the branching element is arranged along a direction which is made substantially coincident with a radial direction of the optical disc on the photodetector.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a continuation of U.S. application Ser. No. 12/417,053, filed Apr. 2, 2009, the contents of which are incorporated herein by reference. 
     
    
     INCORPORATION BY REFERENCE 
       [0002]    The present application claims priority from Japanese application JP2008-227741 filed on Sep. 5, 2008, the content of which is hereby incorporated by reference into this application. 
       BACKGROUND OF THE INVENTION 
       [0003]    1. Field of the Invention 
         [0004]    The present invention is related to an optical pickup device and an optical disc apparatus on which the optical pickup device has been mounted. 
         [0005]    2. Description of the Related Art 
         [0006]    Various sorts of conventional technical ideas related to optical pickup devices and optical disc apparatuses have been proposed. For instance, JP-A-2006-344344 has described such a technical idea that “the desirable signals are acquired from the optical disc having the plurality of recording layers in high precision” (refer to page 26, FIGS. 3 and 5). Also, JP-A-2006-344380 has disclosed another technical idea that “even when the optical recordable storage medium having two sets of the information recording planes is employed, the tracking error signal having the less offset amount is detected” (refer to page 14, FIG. 1). Furthermore, Japanese Publication “Shingaku Giho” CPM2005-149 issued by The Institute of Electronics, Information and Communication Engineers has described another technical idea that “the tracking-purpose photodetector is arranged in such a region where the stray light derived from other layers is not present” (2005-10, page 33, FIGS. 4 and 5). This photodetector structure has also been described in JP-A-2004-281026. 
       SUMMARY OF THE INVENTION 
       [0007]    The above-described JP-A-2006-344344 has described the below-mentioned optical system: That is, the optical beam reflected on the optical disc is focused by the focusing lens; the focused optical beam passes through two sheets of the ¼ wavelength plates and the polarization optical element; the widened optical beam is focused by the focusing lens; and then, the focused optical beam is irradiated to the detector. As a result, the conventional technical idea of JP-A-2006-344344 has a problem that the structure of the optical detecting system becomes complex, and the dimension thereof becomes large. Also, the conventional technical idea of JP-A-2006-344380 has the below-mentioned problem: That is, while the 3-spot producing-purpose diffraction grating is arranged after the laser light source, one main optical spot and two optical sub-spots are irradiated onto the optical disc, so that the optical utilization efficiency of the main optical spot which is required to record information is lowered. 
         [0008]    The above-described Japanese Publication and JP-A-2004-281026 have employed the following optical structures, namely, the tracking-purpose photodetector is arranged outside the stray light derived from other layers as to the focus-purpose optical beam produced around the focus-purpose photodetector, and furthermore, the light diffracted at the center portion of the hologram element is skipped outside the stray light derived from other layers. As a result, there is such a problem that the dimension of the photodetector becomes large. 
         [0009]    The present invention has been object to provide an optical pickup device capable of acquiring stable servo signals in such a case that an information recording medium having a plurality of information recording layers is recorded/reproduced, and also to provide an optical disc apparatus on which the above-described optical pickup device has been mounted. 
         [0010]    The above-described object can be achieved for example by one of the present invention. 
         [0011]    In accordance with the present invention, both the optical pickup device and the optical disc apparatus on which the optical pickup device has been mounted can be provided by which the stable servo signals can be acquired in such a case that the information recording medium having the plurality of information recording layers is recorded/reproduced. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is an explanatory diagram for explaining an arrangement of an optical pickup device according to an embodiment 1 of the present invention, and an optical disc. 
           [0013]      FIG. 2  is an explanatory diagram for explaining an optical system related to the embodiment 1 of the present invention. 
           [0014]      FIG. 3  is an explanatory diagram for explaining a diffraction grating related to the embodiment 1 of the present invention. 
           [0015]      FIG. 4  is an explanatory diagram for explaining a light receiving part of a photodetector related to the embodiment 1 of the present invention. 
           [0016]      FIG. 5A  and  FIG. 5B  are diagrams for showing shapes (viewed on detectors) of stray light when an optical dual layer disc is recorded/reproduced in the embodiment 1 of the present invention. 
           [0017]      FIG. 6A  to  FIG. 6C  are explanatory diagrams for explaining behavior of stray light traveled from other layers of the optical dual layer disc. 
           [0018]      FIG. 7A  to  FIG. 7C  are explanatory diagrams for explaining behavior of stray light traveled from other layers of the optical dual layer disc. 
           [0019]      FIG. 8A  and  FIG. 8B  are explanatory diagrams for explaining other diffraction gratings related to the embodiment 1 of the present invention. 
           [0020]      FIG. 9  is an explanatory diagram for explaining a light receiving part related to the embodiment 2 of the present invention. 
           [0021]      FIG. 10A  and  FIG. 10B  are diagrams for showing shapes (viewed on diffraction gratings) of stray light when an optical dual layer disc is recorded/reproduced in an embodiment 2 of the present invention. 
           [0022]      FIG. 11A  and  FIG. 11B  are diagrams for showing shapes (viewed on detectors) of stray light when the optical dual layer disc is recorded/reproduced in the embodiment  2  of the present invention. 
           [0023]      FIG. 12A  and  FIG. 12B  are diagrams for representing such conditions that an objective lens is deviated when the optical dual layer disc is recorded/reproduced in the embodiment 2 of the present invention. 
           [0024]      FIG. 13  is an explanatory diagram for explaining a light receiving part of a photodetector related to an embodiment 3 of the present invention. 
           [0025]      FIG. 14  is an explanatory diagram for explaining a diffraction grating related to the embodiment 3 of the present invention. 
           [0026]      FIG. 15A  and  FIG. 15B  are diagrams for showing shapes (viewed on detectors) of stray light when an optical dual layer disc is recorded/reproduced in the embodiment 3 of the present invention. 
           [0027]      FIG. 16  is an explanatory diagram for explaining an arrangement of an optical reproducing apparatus according to an embodiment 4 of the present invention. 
           [0028]      FIG. 17  is an explanatory diagram for explaining an arrangement of an optical recording/reproducing apparatus according to an embodiment 5 of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0029]    An optical pickup device and an optical disc apparatus, according to the present invention, will now be described in detail with employment of various sorts of embodiments. 
       Embodiment 1 
       [0030]      FIG. 1  is an explanatory diagram for explaining one example as to an arrangement of an optical pickup device  1  according to a first embodiment of the present invention, and an optical disc. 
         [0031]    The optical pickup device  1  has been arranged in such a manner that the optical pickup device  1  can be driven by a driving mechanism  7  along a radial direction (will be referred to as “Rad” direction hereinafter) as indicated in  FIG. 1 . Also, while an objective lens  2  is mounted on an actuator  5  on the optical pickup device  1 , light is irradiated onto an optical disc  100  from the objective lens  2 . The light emmited from the objective lens  2  forms a spot on the optical disc  100  and is reflected from the optical disc  100 . Since the reflection light from the optical disc  100  is detected, a focus error signal and a tracking error signal are produced. 
         [0032]    In the above-described optical pickup device  1 , an optical system thereof is represented in  FIG. 2 . It should be understood that although a description will be made of a BD (Blu-ray Disc), DVD and other recording type of discs may be freely employed. 
         [0033]    In the optical system of  FIG. 2 , an optical beam having a wavelength of approximately 405 nm is emitted from a semiconductor laser  50  as divergent light. The optical beam emitted from the semiconductor laser  50  is reflected on a beam splitter  52 . It should also be noted that a portion of the optical beam passes through the beam splitter  52  and then is entered to a front monitor  53 . Generally speaking, in such a case that information is recorded on a recording type optical disc such as a BD-RE and a BD-R, a light amount of a semiconductor laser is required to be controlled in high precision in order that a predetermined light amount of the semiconductor laser is irradiated onto a recording layer of the recording type optical disc. To this end, when a signal is recorded on the recording type optical disc, the front monitor  53  detects a change in light amounts of the semiconductor laser  50  and feeds back the detected light change amount to a driving circuit (not shown) of the semiconductor laser  50 . As a result, the front monitor  53  can monitor light amounts on the optical disc. 
         [0034]    An optical beam reflected from the beam splitter  52  is converted into a substantially parallel optical beam by a collimator lens  51 . An optical beam passed through the collimator lens  51  is entered to a beam expander  54 . The beam expander  54  is utilized in order to compensate spherical aberration which is caused by a thickness error of a cover layer of the optical disc  100 , since the beam expander  54  changes divergent/convergent situations of an optical beam. An optical beam emitted from the beam expander  54  is reflected on a raising mirror  55 , the reflected optical beam passes through a ¼ wavelength plate  56 , and thereafter, the passed optical beam is focused onto the optical disc  100  by the objective lens  2  mounted on the actuator  5 . 
         [0035]    An optical beam reflected on the optical disc  100  passes through the objective lens  2 , the ¼ wavelength plate  56 , the raising mirror  55 , the beam expander  54 , the collimator  51 , and the beam splitter  52 , and then, is entered to a diffraction grating  11 . The optical beam is divided into a plurality of regions by the diffraction grating  11 , the divided optical beams are giffracted along directions which are different from each other with respect to these plural regions, and then, are focused on a photodetector  10 . While a plurality of light receiving parts have been formed on the photodetector  10 , the plurality of divided optical beams divided by the diffraction grating  11  are irradiated onto the respective light receiving parts. Electric signals are outputted from the photodetector  10  in response to light amounts of optical beams irradiated on the light receiving parts, and these outputted electric signals are calculated so as to produce an RF signal, a focus error signal, and a tracking error signal as reproduction signals. 
         [0036]      FIG. 3  illustratively shows a shape of the above-described diffraction grating  11 . A solid line indicates a boundary line of regions; a two-dot and dash line represents an outer shape of an optical beam of laser light; and a hatched portion indicates an interference region (push-pull pattern) between zero-order diffraction light and ±first-order diffraction light, which have been diffracted by tracks of an optical disc. The diffraction grating  11  has been formed by regions “De”, “Df”, “Dg” and “Dh” (region “A”) in which only the zero-order diffraction light of the diffraction light diffracted by the tracks formed on the optical disc  100 ; regions “Da”, “Db”, “Dc”, and “Dd” (region “B”) into which the zero-order diffraction light and the ±first-order diffraction light of the above-described diffraction light are entered; and also, a region “Di” (region “C”). 
         [0037]    It is assumed that a diffraction efficiency of the diffraction grating  11  except for the region “Di” is selected to be, for instance, zero-order light: +first-order light: −first-order light=0:7:3, and a diffraction efficiency of the region “Di” is selected to be, for example, zero-order light: +first-order light: −first-order light=0:1:0. The photodetector  10  has such a pattern of light receiving parts as represented in  FIG. 4 . 
         [0038]    In this case, the +first-order diffraction light diffracted by the regions “Da”, “Db” “Dc”, “Dd”, “De”, “Df”, “Dg”, “Dh”, and “Di” of the diffraction grating  11  is entered to light receiving parts “a 1 ”, “b 1 ”, “c 1 ”, “d 1 ”, “e 1 ”, “f 1 ”, “g 1 ”, “h 1 ”, and “i 1 ” of the photodetector  10  as shown in  FIG. 4 , respectively. Also, the −first-order diffraction light diffracted by the regions “Da”, “Db” “Dc”, and “Dd” is entered to focus error signal detecting-purpose light receiving parts “r”, “s”, “t”, “u”, and “v”, respectively, whereas the −first-order diffraction light diffracted by the regions “De”, “Df”, “Dg”, and “Dh” is entered to light receiving parts “e 2 ”, “f 2 ”, “g 2 ”, and “h 2 ”, respectively. 
         [0039]    Signals “A 1 ”, “B 1 ”, “C 1 ”, “D 1 ”, “E 1 ”, “F 1 ”, “G 1 ”, “H 1 ”, “I 1 ”, “R”, “S”, “T”, “U”, “V”, “E 2 ”, “H 2 ”, “F 2 ”, “G 2 ”, and “H 2 ”, which have been acquired from the light receiving parts “a 1 ”, “b 1 ”, “c 1 ”, “d 1 ”, “e 1 ”, “f 1 ”, “g 1 ”, “h 1 ”, “i 1 ”, “r”, “s”, “t”, “u”, “v”, “e 2 ”, “f 2 ”, “g 2 ”, and “h 2 ”, are processed based upon the below-mentioned calculations in order to produce a focus error signal, a tracking error signal, and an RF signal. 
         [0000]        FES =( R+T+V )−( S+U )
 
         [0000]        TES ={( A 1+ B 1+ E 1+ F 1)−( C 1+ D 1+ G 1+ H 1)}− kt ×{( E 2+ F 2)−( G 2+ H 2)}
 
         [0000]        RF=A 1+ B 1− C 1+ D 1+ E 1+ F 1+ G 1+ H 1+ i 1+ E 2+ F 2+ G 2+ H 2  [Expression 1]
 
         [0040]    It should be noted in the above-described expression 1 that symbol “Kt” indicates a coefficient by which when the objective lens  2  displaces, a DC component is not generated by the tracking error signal. 
         [0041]    In this case, since the focus error detecting method corresponds to the knife edge method which is known in this field, an explanation of this knife edge method will be omitted. 
         [0042]      FIG. 5A  and  FIG. 5B  indicate stray light from other layers during recording/reproducing a dual layer.  FIG. 5A  shows recording/reproducing of L 0 , and  FIG. 5B  shows recording/reproducing of L 1 . As can be understood from  FIG. 5A  and  FIG. 5B , as to optical beams other than an optical beam diffracted by the region “Di” of the diffraction grating  11 , signal light and stray light emitted from other layers are not superimposed with each other on light receiving parts. Since the signal “I 1 ” detected from the light receiving parts “iI” is not used in order to detect a tracking error signal, but is used in order to detect a reproduction signal, even when stray light is present, there is no practical problem. 
         [0043]    Generally speaking, when signals are actually detected, while an objective lens follows tracks formed on an optical disc, the object lens records/reproduces signals, so that the objective lens is deviated along a radial direction (will be referred to as “Rad” direction hereinafter). When the objective lens displaces, only a stray light component displaces on a photodetector. As a result, if the light receiving part pattern of the photodetector is a normal light receiving part, when the objective lens displaces, there are some possibilities that stray light emitted from other layers. In contrast to the above-explained general situation, in accordance with the present invention, the light receiving parts of the photodetector  10  are optimized with respect to the patterns of the diffraction grating  11 , so that a displacement allowable amount of the objective lens  2  can be increased. In this case, as a technical idea which should be considered, how signal light should be separated from stray light with respect to displacement directions of the objective lens  2 . This technical idea will be described in the following descriptions. 
         [0044]      FIG. 6A  to  FIG. 6C  represent an optical beam which has been diffracted by the region “Dh” of the diffraction grating  11  and then entered to the light receiving part “hi.” Also,  FIG. 7A  to  FIG. 7C  represent an optical beam which has been diffracted by the region “Dd” of the diffraction grating  11  and then entered to the light receiving part “d 1 .”  FIG. 6A  to  FIG. 6C  and  FIG. 7A  to  FIG. 7C  have been separated from each other, depending upon situations of optical spots on an optical disc.  FIG. 6B  and  FIG. 7B  show situations under which the optical beams have been focused on the optical discs, whereas  FIG. 6A ,  FIG. 6C ,  FIG. 7A , and  FIG. 7C  indicate situations under which the optical beams have been defocused. It should also be noted that a relationship among  FIG. 6A  through  FIG. 7C  does not substantially depend upon positions of light receiving parts. The reason why the defocused situations are described is given as follows: That is, it can be interpreted that such a stray light emitted from a dual layer optical disc corresponds to defocused light reflected from such a position which is not a focal position. 
         [0045]    When the optical beam situations indicated in  FIG. 6A  to  FIG. 6C  are compared with those indicated in  FIG. 7A  to  FIG. 7C , it can be understood that optical beam moving directions are different from each other due to defocusing. The optical beam diffracted from the region “Dh” of  FIG. 6A  to  FIG. 6C  is moved along a track direction (will be referred to as “Tan” direction hereinafter) of the optical disc by being defocused. In contrast thereto, the optical beam diffracted from the region “Dd” of  FIG. 7A  to  FIG. 7C  is moved along the radial direction (“Rad” direction). This different moving direction is caused by that the optical beams are blurred in a point symmetrical manner with respect to a center  15  (see  FIG. 3 ) of the optical beams on the diffraction grating  11 , the moving directions of the defocused optical beams are different from each other. As a result, the methods for escaping the stray light are classified, depending upon the regions, which constitute an important aspect. In such a case that regions of the diffraction grating  11  have been separated along the “Tan” direction with respect to the optical beam center  15  (namely, regions “Dh” “De”, “Df”, “Dg”, (region “A”), it is desirable that the stray light is escaped along the “Tan” direction. Since the stray light is escaped in the above-described manner, even when the objective lens  2  displaces along the “Rad” direction, this stray light is not entered to the photodetector  10 . As a consequence, the light receiving parts for detecting the optical beams diffracted from the regions “Dh”, “De”, “Df”, “Dg” of the diffraction grating  11  are arranged along the “Rad” direction, so that adverse influences caused by the stray light diffracted from other regions can be suppressed to the minimum effect. 
         [0046]    Also, in such a cse that regions of the diffraction grating  11  have been separated along the “Ran” direction with respect to the optical beam center  15  (namely, regions “Da”, “Db”, “Dc”, “Dd” (region “B”), it is desirable that the stray light is escaped along the “Rad” direction. As a consequence, the light receiving parts for detecting the optical beams diffracted from the regions “Da”, “Db”, “Dc”, “Dd” of the diffraction grating  11  are arrayed along the “Tan” direction, so that adverse influences caused by the stray light diffracted from other regions can be suppressed to the minimum effect, and also, the photodetector  11  can be made compact. If the optical beams diffracted from the regions “Da”, “Db”, “Dc”, “Dd” are arrayed along the “Rad” direction, when the objective lens  2  displace along the “Rad” direction, then there is such a problem that the stray light is entered to the light receiving parts. 
         [0047]    As previously described, since the right receiving parts of the photodetector  10  are made of such patterns shown in  FIG. 4 , the signal light can be effectively separated from the stray light, and furthermore, the photodetector  10  can be made compact. As apparent from the foregoing description, even when the diffraction grating  11  is made by having patterns as indicated in  FIG. 8A  and  FIG. 8B , a similar effect may be achieved. Moreover, in the first embodiment, the diffraction grating  11  has been arranged at the position after the optical beam has passed through the beam splitter  52 . Alternatively, while the diffraction grating  11  may be replaced by a polarizing diffraction grating, even when the polarizing diffraction grating may be arrayed at a position before the optical beam passes through the beam splitter  52 , a similar effect may be achieved. Also, although the optical disc  100  having the two layers has been explained in the first embodiment, even when such optical discs having 2, or more layers may be employed, similar effects may be obtained. In addition, as apparent from the foregoing description, there is no limitation as to spherical aberration corrections. 
       Embodiment 2 
       [0048]      FIG. 9  shows a photodetector of an optical system of an optical pickup device according to a second embodiment of the present invention. A structural difference of the optical pickup device according to the second embodiment from that of the first embodiment is given as follows: That is, a distance between the objective lens  2  and the diffraction grating  11  shown in  FIG. 2  is made longer than that of the first embodiment, and other structural elements of the second embodiment are similar to those of the first embodiment. 
         [0049]    When the distance between the objective lens  2  and the diffraction grating  11  becomes long, adverse influences caused by stray light are different from each other, depending upon recording/reproducing layers. This reason is given as follows: since the stray light constitutes convergent light during L 0  recording/reproducing operation, and the stray light constitutes divergent light during L 1  recording/reproducing operation, such a condition that a beam diameter of the stray light on the diffraction grating  11  is largely changed may give the adverse influence. 
         [0050]    For example, it is so assumed that beam diameters of both signal light and stray light on the diffraction grating  11  are illustrated as those in  FIG. 10A  and  FIG. 10B .  FIG. 10A  shows the beam diameters of the signal light and the stray light during L 0  recording/reproducing operation.  FIG. 10B  indicates the beam diameters of the signal light and the stray light during L 1  recording/reproducing operation. In these drawings, the signal light is indicated by employing a two-dot and dash line, and the stray light is indicated by employing a dot and dash line  46 . The stray light in such a case is represented in  FIG. 11A  and  FIG. 11B .  FIG. 11A  indicates a shape of the stray light during L 0  recording/reproducing operation, and  FIG. 11B  shows a shape of the stray light during L 1  recording/reproducing operation. As can be understood from the drawings, as to optical beams other than the optical beam diffracted from the region “Di” of the diffraction grating  11 , the signal light is not superimposed with the stray light emitted from other layers on the light receiving parts. It should be understood that since the signal “I 1 ” detected from the light receiving part “iI” is not used in order to detect a tracking error signal, but is used in order to detect a reproduction signal, even when stray light is present, there is no practical problem. Also, although the stray light has been entered to the light receiving parts “r” and “s” which detect the focus error signal, there is no problem in order to detect the servo signal. 
         [0051]    Even in the above-described structure of the photodetector  10 , as indicated in  FIG. 9 , the light receiving parts “a 1 ”, “b 1 ”, “c 1 ”, and “d 1 ” of the photodetector  10  are arranged in a straight line along the “Tan” direction, and also, the light receiving parts “e 1 ”, “f 1 ”, “g 1 ”, “h 1 ”, as well as “e 2 ”, “f 2 ”, “g 2 ”, “h 2 ” thereof are arranged in a straight line along the “Rad” direction. As a result, the stray light can be separated from the signal light, and also, the photodetector  10  can be made compact. 
         [0052]    Now, a description is made why the light receiving parts of the photodetector  10  arranged along the Rad direction are not located in a line symmetrical relationship with respect to the light receiving parts thereof arranged along the “Tan” direction. That is, in such a case that the optical beams are entered as represented in  FIG. 10B , the optical beams are entered to other regions (Da, Db, Dc, Dd) of the diffraction grating  11  due to displacement of the objective lens  2 . In an actual case, even under such a condition that the objective lens  2  has displaced, the stray light from other layers should be escaped.  FIG. 12A  and  FIG. 12B  indicate stray light from L 0  when the objective lens  2  displaces. In this case, if the objective lens  2  displaces, then the stray light for other layers, which has been diffracted by the region “Db” of the diffraction grating  11 , is not entered to the light receiving part “e 1 .” Although this problem may be solved by simply arranging the positions of the light receiving parts already arranged along the “Rad” direction and further arranging along the “Tan” direction, it is desirable that these positions of the light receiving parts already arranged along the “Rad” direction are shifted along the “Rad” direction. Since the above-described positional shifts of the light receiving parts are made, even when the objective lens  2  is deviated, the stable servo signals can be detected, and a compact photodetector may be realized. 
         [0053]    Because of the above-described reason, the light receiving parts of the photodetector  10  arranged along the Rad direction have not been located in a line symmetrical relationship with respect to the light receiving parts thereof arranged along the “Tan” direction. 
         [0054]    As previously explained, even if the distance between the objective lens  2  and the diffraction grating  11  is made long, the light receiving parts of the photodetector  10  are arranged as indicated in  FIG. 9 , so that the signal light can be electively separated from the stray light. In this example, as to signal detecting operations, the signals can be obtained by executing a similar calculation to that of the first embodiment. As apparent from the foregoing description, even when the diffraction grating  11  is made by having patterns as indicated in  FIG. 8A  and  FIG. 8B , a similar effect may be achieved. Moreover, in the second embodiment, the diffraction grating  11  has been arranged at the position after the optical beam has passed through the beam splitter  52 . Alternatively, while the diffraction grating  11  may be replaced by a polarizing diffraction grating, even when the polarizing diffraction grating may be arrayed at a position before the optical beam passes through the beam splitter  52 , a similar effect may be achieved. Also, although the optical disc  100  having the two layers has been explained in the second embodiment, even when such optical discs having 2, or more layers may be employed, similar effects may be obtained. In addition, as apparent from the foregoing description, there is no limitation as to spherical aberration corrections. 
       Embodiment 3 
       [0055]      FIG. 13  shows a photodetector of an optical system of an optical pickup device according to a third embodiment of the present invention. A structural difference of the optical pickup device according to the third embodiment from that of the first embodiment is given as follows: That is, a characteristic of a diffraction grating  11  and a detector  10  employed in the third embodiment are different from those of the above-described first embodiment, and other structural elements of the third embodiment are similar to those of the first embodiment. 
         [0056]    The diffraction grating  11  has been made of such a pattern as indicated in  FIG. 14 . A solid line indicates a boundary line of regions; a two-dot and dash line represents an outer shape of an optical beam of laser light; and a hatched portion indicates an interference region (push-pull pattern) between a zero-order diffraction light and ±first-order diffraction light, which have been diffracted by tracks of an optical disc. The diffraction grating  11  has been formed by regions “De”, “Df”, “Dg” and “Dh” (region “A”) in which only the zero-order diffraction light of the diffraction light diffracted on the tracks of the optical disc; regions “Da”, “Db”, “Dc”, and “Dd” (region “B”) into which the zero-order diffraction light and ±first-order “diffraction light of the above-described diffraction light are entered; and also, a region “Di” (region “C”). In this example, it is assumed that a diffraction efficiency of the diffraction grating  11  is selected to be, for instance, zero-order light: +first-order light: −first-order light=7:3:0. 
         [0057]    In this case, the +first-order diffraction light (otherwise, −first-order diffraction light) diffracted by the regions “Da”, “Db” “Dc”, “Dd”, “De”, “Df”, “Dg”, and “Dh” of the diffraction grating  11  is entered to light receiving parts “a 1 ”, “b 1 ”, “c 1 ”, “d 1 ”, “e 1 ”, “f 1 ”, “g 1 ”, “h 1 ”, and “i 1 ” of the photodetector  10  as shown in  FIG. 14 , respectively. Also, the +first-order diffraction light (otherwise, −first-order diffraction light) diffracted by the regions “ Di 1 ”, “Di 2 ” “Di 3 ”, and “Di 4 ” is entered to focus error signal detecting-purpose light receiving parts “r”, “s 1 ”, “s 2 ” and “t”, respectively. Also, spots “i 1 ”, “i 2 ”, “i 3 ”, and “i 4 ” on the focus error signal detecting-purpose light receiving parts correspond to optical spots diffracted by the regions “Di 1 ”, “Di 2 ”, “Di 3 ”, and “Di 4 ” of the diffraction grating  11 , respectively. In addition, zero-order diffraction light from all regions of the diffraction grating  11  is entered to the light receiving part “i.” 
         [0058]    Signals “A 1 ”, “B 1 ”, “C 1 ”, “D 1 ”, “E 1 ”, “F 1 ”, “G 1 ”, “H 1 ”, “R”, “S 1 ”, “S 2 , “T”, and “I”, which have been acquired from the light receiving parts “a 1 ”, “b 1 ”, “c 1 ”, “d 1 ”, “e 1 ”, “f 1 ”, “g 1 ”, “h 1 ”, “r”, “s 1 ”, “s 2 ”, “t”, and “i”, are processed based upon the below-mentioned calculations in order to produce a focus error signal, a tracking error signal, and an RF signal. 
         [0000]        FES =( R+T )−( S 1+ S 2)
 
         [0000]        TES ={( A 1+ B 1+ E 1+ F 1)−( C 1+ D 1+ G 1+ H 1)}− Kt ×{( E 2+ F 2)−( G 2+ H 2)}
 
         [0000]      RF=I  [Expression 2]
 
         [0059]    It should be noted in the above-described expression 2 that symbol “Kt” indicates a coefficient by which when the objective lens  2  displaces, a DC component is not generated by the tracking error signal. 
         [0060]    In this case, since the focus error detecting method corresponds to the knife edge method which is known in this field, an explanation of this knife edge method will be omitted. 
         [0061]      FIG. 15A  and  FIG. 15B  indicate stray light from other layers during recording/reproducing a dual layer.  FIG. 5A  shows recording/reproducing of L 0 , and  FIG. 5B  shows recording/reproducing of L 1 . As can be understood from  FIG. 15A  and  FIG. 15B , as to optical beams other than an optical beam diffracted by the region “Di” of the diffraction grating  11 , signal light and stray light emitted from other layers of the optical disc are not superimposed with each other on light receiving parts. Since the signal “I” detected from the light receiving part “i” is not used in order to detect a tracking error signal, but is used in order to detect a reproduction signal, even when stray light is present, there is no practical problem. 
         [0062]    In this example, light receiving parts of the photodetector  10  are arranged along the “Rad” direction, which detect optical beams diffracted by the regions “Dh”, “De”, “Df”, “Dg” of the diffraction grating  11 , and also, light receiving parts thereof are arranged along the “Tan” direction, which detect optical beams diffracted by the regions “Da”, “Db”, “Dc”, “Dd” thereof. As a result, an adverse influence caused by the stray light can be suppressed to a minimum influence. Although the light receiving parts “a 1 ” and “b 1 ” have been separated from the light receiving parts “c 1 ” and “d 1 ” in the third embodiment, even when these light receiving parts “a 1 ”, “b 1 ”, “c 1 ”, and “d 1 ” are arrayed in one column along the “Tan” direction, a similar effect may be apparently achieved. 
         [0063]    Since such a method for arranging these light receiving parts of the photodetector  10  is employed, stable servo signals can be detected with respect to optical multi-layer discs. Also, with respect to the conventional technique for detecting the same one optical beam (described in JP-A-2004-281026), since the light is focused on the photodetector, the intervals among the light receiving parts can be shortened in this third embodiment. As a result, the photodetector can be made compact. 
         [0064]    In this case, the positions of the light receiving parts of the knife edge are not limited only to the positions of  FIG. 13 , but even if the light receiving parts are arranged at any positions, similar effects may be apparently achieved. 
         [0065]    As apparent from the foregoing descriptions, as to detections of the focus error signals, similar effects may be achieved even when any regions formed on the diffraction grating  11  are utilized. For instance, even when a focus error signal is detected by employing the −first-order diffraction light diffracted from one of the diffraction grating regions “Da” to “Dh” and “Di 1 ” to “Di 2 ”, or the −first-order diffraction light diffracted from a plurality of diffraction grating regions, it is obvious that similar effects may be obtained. 
         [0066]    Also, even in such a case that the diffraction grating  11  has such patterns as indicated in  FIG. 8A  and  FIG. 8B  and the regions “Di” of the diffraction grating  11  are sub-divided, it is obvious that similar effects may be achieved. In addition, in accordance with the third embodiment, the diffraction grating  11  has been arranged at the position after the optical beam has passed through the beam splitter  52 . Alternatively, while the diffraction grating  11  may be replaced by a polarizing diffraction grating, even when the polarizing diffraction grating may be arrayed at a position before the optical beam passes through the beam splitter  52 , a similar effect may be achieved. Also, although the optical disc  100  having the two layers has been explained in the third embodiment, even when such optical discs having 2, or more layers may be employed, similar effects may be obtained. In addition, as apparent from the foregoing description, there is no limitation as to spherical aberration corrections. 
       Embodiment 4 
       [0067]    A description is made of an optical reproducing apparatus according to a fourth embodiment of the present invention, on which the above-described optical pickup device  1  has been mounted. 
         [0068]      FIG. 16  shows a schematic arrangement of the optical reproducing apparatus. While the optical pickup device  1  has been equipped with a mechanism capable of driving the optical pickup device  1  along the “Rad” direction of the optical disc  100 , the optical pickup device  1  is positionally controlled by the above-explained mechanism in response to an access control signal supplied from an access control circuit  172 . 
         [0069]    A predetermined laser drive current is supplied from a laser turning-ON circuit  177  to a semiconductor laser provided in the optical pickup device  1 , and then, laser light having a predetermined light amount is emitted from the semiconductor laser in response to a reproducing operation. It should be noted that the laser turning-ON circuit  177  may be alternatively assembled in the optical pickup device  1 . 
         [0070]    A signal outputted from the photodetector  10  mounted in the optical pickup device  1  is transferred to both a servo signal producing circuit  174  and an information signal reproducing circuit  175 . In response to the signals supplied from the photodetector  10 , the servo signal producing circuit  174  produces such servo signals as a focus error signal, a tracking error signal, and a tilt control signal; and in response to the servo signals, an actuator driving circuit  173  drives an actuator provided in the optical pickup device  1  so as to positionally control the objective lens  2 . 
         [0071]    In the above-described information signal reproducing circuit  175 , information signals recorded on the optical disc  100  are reproduced based upon the signals detected from the photodetector  10 . A portion of the signals acquired by the servo signal producing circuit  174  and the information signal reproducing circuit  175  is transferred to a control circuit  176 . A spindle motor driving circuit  171 , an access control circuit  172 , the servo signal producing circuit  174 , the laser turning-ON circuit  177 , a spherical aberration correcting element driving circuit  179 , and the like are connected to the control circuit  176 . Thus, this control circuit  176  controls rotations of a spindle motor  180 ; controls an access direction and an access position; servo-controls the objective lens  2 ; controls a light emitting amount of the semiconductor laser employed in the optical pickup device  1 ; and also, corrects spherical aberration which is caused by thickness differences of disc boards. 
       Embodiment 5 
       [0072]    A description is made of an optical recording/reproducing apparatus according to a fifth embodiment of the present invention, on which the above-described optical pickup device  1  has been mounted. 
         [0073]      FIG. 17  shows a schematic arrangement of the optical recording/reproducing apparatus. The optical recording/reproducing apparatus of this fifth embodiment has the below-mentioned structural different point from that of the optical reproducing apparatus explained in  FIG. 16 . That is, while an information signal recording circuit  178  is provided between the control circuit  176  and the laser turning-ON circuit  177 , the control circuit  176  controls turning ON of the laser turning-ON circuit  177  based upon a recording control circuit supplied from the information signal recording circuit  178  so as to write desirable information in the optical disc  100 . 
         [0074]    Although the various sorts of embodiments as to the optical pickup device and the optical disc apparatus of the present invention have been described in the above-explanations, the present invention is not limited only to these embodiments, but may be modified and substituted in various manners. That is, for instance, the structural elements exemplified in the above-described embodiments may be alternatively combined with each other within an applicable technical scope of the present invention.