Abstract:
An optical pickup apparatus includes an astigmatic element, an angle adjusting element for contradicting propagation directions of luminous fluxes within four different luminous flux regions out of a reflected light, a polarization adjusting element. Two of the luminous flux regions are placed in a direction in which aligned are a set of opposite angles made by the two mutually crossing straight lines respectively parallel to a first convergence direction and a second convergence direction vertical to the first convergence direction by the astigmatic element, and the remaining two luminous flux regions are placed in a direction in which an alternate set of opposite angles are aligned. The polarization adjusting element differentiates polarization directions of luminous fluxes which are selected out of the luminous fluxes within the four luminous flux regions and which are adjacent in a peripheral direction in which the optical axis of reflected light serves as an axis.

Description:
This application claims priority under 35 U.S.C. Section 119 of Japanese Patent Application No. 2008-198903 filed Jul. 31, 2008, entitled “OPTICAL PICKUP APPARATUS AND FOCAL-POINT ADJUSTING METHOD”. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an optical pickup apparatus and a focal-point adjusting method, and more particularly, relates to an optical pickup apparatus and focal-point adjusting method suitable in use at the time of recording to and reproducing from a recording medium stacked thereon with a plurality of recording layers. 
     2. Description of the Related Art 
     In the recent years, along with the increased capacity of optical discs, the multilayered recording layers have been advanced. By including a plurality of recording layers in a single disc, the data capacity of the disc can be increased remarkably. When stacking the recording layers, the general practice until now has been to stack two layers on one side, but recently, to further advance the large capacity, disposing three or more recording layers on one side is also examined. Herein, when the number of recording layers to be stacked is increased, the large capacity of a disc can be promoted. However, on the other hand, the space between recording layers is narrowed, and signal degradation caused by crosstalk between layers increases. 
     If the recording layer is multilayered, a reflected light from the recording layer to be recorded or reproduced (target recording layer) becomes very weak. Therefore, when unnecessary reflected light (stray light) enters a photodetector from the recording layers present above and below the target recording layer, the detection signal is degraded, which may exert an adverse effect on a focus servo and a tracking servo. Therefore, when a large number of recording layers are disposed in this way, the stray light needs to be removed properly so as to stabilize the signals from the photodetector. 
     Herein, a method for removing the stray light includes that which uses a pinhole. In this method, a pinhole is disposed at a convergence position of the signal light. According to this method, a part of the stray light is intercepted by the pinhole, and therefore, the unnecessary stray light component entering the photodetector can be reduced. Another method for removing the stray light includes that which combines ½ wavelength plates and polarized light optical elements. According to this method, a polarization direction of the stray light is changed by the ½ wavelength plates, and the stray light is intercepted by the polarized light optical elements. Thus, the unnecessary stray light component entering the photodetector can be removed. 
     However, in the case of the method for removing the stray light by using a pinhole, the pinhole needs to be positioned accurately at the convergence position of a laser light (signal light) reflected from the target recording layer, and therefore, a task for adjusting the position of the pinhole is difficult, thus posing a problem. If the size of the pinhole is increased to facilitate the task for adjusting the position, the proportion of the stray light passing through the pinhole increases, and the signal degradation caused by the stray light cannot be inhibited effectively. 
     Furthermore, in the case of the method in which the ½ wavelength plates and the polarized light optical elements are combined to remove the stray light, apart from the fact that the ½ wavelength plates and the polarized light optical elements two each are needed to remove the stray light, a user needs to have two lenses, which increases the number of components and the cost, and adjusting the placement of each component is a complex process, thus posing a problem. Furthermore, the user needs to have a space for placing and arraying these components, which results in the enlargement of the optical system, thus posing a problem. 
     SUMMARY OF THE INVENTION 
     An optical pickup apparatus according to a first aspect of the present invention is provided with: a laser light source; an objective lens for converging laser light emitted from the laser light source onto a recording medium; and an astigmatic element for introducing an astigmatism into the laser light reflected by the recording medium. The astigmatic element mutually spaces a first focal line position occurring by the convergence of the laser light in a first direction and a second focal line position occurring by the convergence of the laser light in a second direction vertical to the first direction, into a propagation direction of the laser light. Moreover, the optical pickup apparatus is provided with: an angle adjusting element for mutually contradicting propagation directions of luminous fluxes, out of the laser light reflected by the recording medium, within four different luminous flux regions so that the luminous fluxes within the four luminous flux regions are mutually dispersed; a photodetector for outputting a detection signal when receiving each of the dispersed luminous fluxes; and a polarization adjusting element for adjusting a polarization direction of the luminous fluxes within the four luminous flux regions. When an intersection point of two mutually crossing straight lines respectively parallel to the first direction and the second direction is matched to an optical axis of the laser light, the angle adjusting element sets the four luminous flux regions so that two of the luminous flux regions are placed in a direction in which a set of opposite angles made by the two straight lines are aligned and remaining two luminous flux regions are placed in a direction in which an alternate set of opposite angles are aligned. The polarization adjusting element mutually differentiates polarization directions of luminous fluxes which are selected out of the luminous fluxes within the four luminous flux regions and which are adjacent in a peripheral direction in which the optical axis of laser light serves as an axis. 
     A second aspect of the present invention relates to a focal-point adjusting method for positioning a focal point position of an irradiation light on a target surface. The focal-point adjusting method comprises: introducing an astigmatism into the irradiation light reflected by the target surface so that a first focal line position occurring by the convergence of the irradiation light in a first direction and a second focal line position occurring by the convergence of the irradiation light in a second direction vertical to the first direction are mutually spaced in a propagation direction of the irradiation light; mutually contradicting propagation directions of luminous fluxes, out of the irradiation light reflected by the target surface, within four different luminous flux regions so that the luminous fluxes within the four luminous flux regions are mutually dispersed; receiving each of the dispersed luminous fluxes in a photodetector; and producing a focus error signal by performing an arithmetic process based on an astigmatic method, on a detection signal outputted from the photodetector. When an intersection point of two mutually crossing straight lines respectively parallel to the first direction and the second direction is matched to an optical axis of the irradiation light, the four luminous flux regions are so set that two of the luminous flux regions are placed in a direction in which a set of opposite angles made by the two straight lines are aligned and remaining two luminous flux regions are placed in a direction in which an alternate set of opposite angles are aligned, and polarization directions of luminous fluxes which are selected out of the luminous fluxes within the four luminous flux regions and which are adjacent in a peripheral direction in which the optical axis of laser light serves as an axis are mutually differentiated. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects and novel features of the present invention will become more completely apparent from the description of the embodiment below taken with the following accompanying drawings. 
         FIGS. 1A and 1B  are diagrams each describing a technical principle (a manner in which a light ray advances) according to an embodiment. 
         FIGS. 2A and 2B  are diagrams each describing the technical principle (a manner in which a light ray advances) according to the embodiment. 
         FIGS. 3A and 3B  are diagrams each describing the technical principle (a manner in which a light ray advances) according to the embodiment. 
         FIG. 4  is a diagram describing the technical principle (a manner in which a light ray advances) according to the embodiment. 
         FIGS. 5A to 5D  are diagrams each describing the technical principle (a splitting pattern and a distribution of a luminous flux) according to the embodiment. 
         FIGS. 6A to 6D  are diagrams each describing the technical principle (a splitting pattern and a distribution of a luminous flux) according to the embodiment. 
         FIGS. 7A to 7D  are diagrams each describing the technical principle (a splitting pattern and a distribution of a luminous flux) according to the embodiment. 
         FIGS. 8A to 8D  are diagrams each describing the technical principle (a splitting pattern and a distribution of a luminous flux) according to the embodiment. 
         FIGS. 9A and 9B  are diagrams each describing the technical principle (an angular provision and a distribution of a luminous flux) according to the embodiment. 
         FIGS. 10A to 10D  are diagrams each describing a method for placing a sensor pattern according to the embodiment. 
         FIGS. 11A to 11D  are diagrams each describing an interference between the beams of stray light, a problem of the interference (a leakage of a stray light into a signal light region), and a solution to the interference according to the embodiment. 
         FIG. 12  is a diagram showing an optical system used for a simulation on the leakage of the stray light into the signal light region according to the embodiment. 
         FIGS. 13A to 13C  are diagrams showing simulation results of the leakage of stray light into the signal light region according to the embodiment. 
         FIG. 14  is a diagram showing an optical system of an optical pickup apparatus according to the embodiment. 
         FIGS. 15A to 15F  are diagrams each showing an optical system of a polarization adjusting element according to the embodiment. 
         FIGS. 16A to 16C  are diagrams each showing a configuration example of an angle adjusting element according to the embodiment. 
         FIG. 17  is a diagram showing a preferred applicable range of the technical principle of the embodiment and the present invention. 
         FIGS. 18A to 18D  are diagrams each showing a modified example (modified mode of the sensor pattern) of the embodiment. 
         FIGS. 19A to 19D  are diagrams each showing a modified example (modified mode of the sensor pattern) of the embodiment. 
         FIG. 20  is a diagram showing a modified example (modified mode of an optical system) of the embodiment. 
         FIGS. 21A and 21B  are diagrams each showing a modified example (modified example of the optical system) of the embodiment. 
     
    
    
     However, the diagrams are for the purposes of illustration only, and are not intended to limit the scope of the present invention. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An embodiment of the present invention will be described below with reference to the drawings. 
     Technical Principle 
     Firstly, with reference to  FIG. 1A  to  FIG. 10D , a technical principle applied to this embodiment will be described. 
       FIG. 1A  is a diagram showing a convergence state of a signal light and a stray light, when the laser light (signal light) reflected by a target recording layer enters an astigmatic element, such as an anamorphic lens, in a state of a parallel light. A “stray light  1 ” is a laser light reflected by a recording layer present on a farther side by one layer than the target recording layer when seen from the side of a laser-light entering surface, and a “stray light  2 ” is a laser light reflected by a recording layer present on a nearer side by one layer than the target recording layer.  FIG. 1A  also shows a state when the signal light is focused on the target recording layer. 
     As illustrated, because of the effect of an anamorphic lens, a focal line occurs on a surface S 1  due to the convergence of the signal light in a “curved-surface direction” shown in  FIG. 1 , and furthermore, a focal line occurs on a surface S 2  due to the convergence of the signal light in a “plane-surface direction” shown in  FIG. 1 , which is vertical to the curved-surface direction. Thus, a spot of the signal light becomes minimum (circle of least confusion) on a surface S 0  between the surfaces S 1  and S 2 . In the case of a focus adjustment based on an astigmatic method, the surface S 0  is situated as a light-receiving surface of a photodetector. It should be noted that in order to simplify the description of the astigmatic effect in the anamorphic lens, the “curved-surface direction” and the “plane-surface direction” are simply expressed for the sake of convenience, and in reality, it suffices that the effect for connecting the focal lines in positions different to each other occurs by the anamorphic lens. In this case, the anamorphic lens may also have a curvature in the “plane-surface direction” shown in  FIG. 1A . 
     As shown in  FIG. 1A , a focal line position of the stray light  1  (in  FIG. 1A , a range between the two focal line positions by the astigmatic element is shown as a “convergence range”) is closer to the astigmatic element as compared to the focal line position of the signal light, while a focal line position of the stray light  2  is further away from the astigmatic element as compared to the focal line position of the signal light. 
     FIGS.  1 B(a) to  1 B(d) are diagrams each showing a beam shape of the signal light in the parallel light portion and on the surfaces S 1 , S 0 , and S 2 , respectively. The signal light that has been entered on the astigmatic element in a true circle becomes elliptical on the surface S 1 , then after becoming a substantially true circle on the surface S 0 , it again becomes elliptical on the surface S 2 . Herein, the beam shapes formed on the surface S 1  and on the surface S 2 , the respective long axes are vertical to each other. 
     As shown in FIGS.  1 A and  1 B(a), if eight positions (Positions  1  to  8 : written by encircled numeric figures in  FIGS. 1A and 1B ) are set up in the anti-clockwise direction on the outer circumference of the beam in the parallel light portion, a light ray passing through the positions  1  to  8  each experiences convergence effect by the astigmatic element. The position  4  and the position  8  are positioned on a parting line when a beam cross section of the parallel light portion is split into two by a parallel straight line in the curved-surface direction, while the position  2  and the position  6  are positioned on a parting line when the beam cross section of the parallel light portion is split into two by a parallel straight line in the plane-surface direction. The Positions  1 ,  3 ,  5 , and  7  are in the middle of the outer circular arc sectioned by the positions  2 ,  4 ,  6 , and  8 , respectively. 
     The light ray passing through the position  4  and the position  8  in the parallel light portion enters the surface S 0  after being converged to the focal line in the curved-surface direction on the surface S 1 . Thus, the light ray passing through these positions  4  and  8  passes through the positions  4  and  8  shown in FIG.  1 B(c), on the surface S 0 . Similarly, the light ray passing through the positions  1 ,  3 ,  5 , and  7  in the parallel light portion also enters the surface S 0  after being converged to the focal line in the curved-surface direction on the surface S 1 , and as a result, the light ray passes through the positions  1 ,  3 ,  5 , and  7  shown in FIG.  1 B(c), on the surface S 0 . In contrast to this, the light ray passing through the positions  2  and  6  in the parallel light portion enters the surface S 0  without being converged to the focal line in the curved-surface direction, on the surface S 1 . Thus, the light ray passing through these positions  2  and  6  passes through the positions  2  and  6  shown in FIG.  1 B(c), on the surface S 0 . 
     FIGS.  2 B(a) to  2 B(d) are diagrams each showing beam shapes and light ray passage positions of the stray light  1  in the parallel light portion and on the surfaces S 1 , S 0 , and S 2 , respectively. As shown in FIG.  2 B(a), similar to the case of the aforementioned signal light, if eight positions  1  to  8  are set up on the outer circumference of the stray light  1 , the light ray passing through these eight positions  1  to  8  enters on the surface S 0  after being converged either to the focal line in the curved-surface direction or to the focal line in the plane-surface direction. Thus, the light ray passing through the positions  1  to  8  in the parallel light portion respectively passes through the positions  1  to  8  shown in FIG.  2 B(c), on the surface S 0 . 
     FIGS.  3 B(a) to  3 B(d) are diagrams each showing beam shapes and light ray passage positions of the stray light  2  in the parallel light portion and on the surfaces S 1 , S 0 , and S 2 , respectively. As shown in FIG.  3 B(a), similar to the case of the aforementioned signal light, if eight positions  1  to  8  are set up on the outer circumference of the stray light  2 , the light ray passing through these eight positions enters the surface S 0  without being converged either to the focal line in the curved-surface direction or to the focal line in the plane-surface direction. Thus, the light ray passing through the positions  1  to  8  in the parallel light portion respectively passes through the positions  1  to  8  shown in FIG.  3 B(c), on the surface S 0 . 
       FIG. 4  is a diagram in which the beam shapes and the light ray passage positions in the parallel light portion and on the surfaces S 1 , S 0 , and S 2 , described above, are shown by comparing among the signal light, the stray light  1 , and the stray light  2 . As can be understood by comparing rows shown in  FIG. 4(   c ), luminous fluxes of the signal light, the stray light  1 , and the stray light  2  passing through the position  1  in the parallel light portion pass through outer circumference positions different to one another, on the surface S 0 . Similarly, also the luminous fluxes of the signal light, the stray light  1 , and the stray light  2  passing through the positions  3 ,  4 ,  5 ,  7 , and  8  in the parallel light portion pass through outer circumference positions different to one another, on the surface S 0 . The luminous fluxes of the signal light and the stray light  2  passing through the positions  2  and  6  in the parallel light portion pass through the same outer circumference position on the surface S 0 . Also in this case, the luminous fluxes of the signal light and the stray light  1  passing through the positions  2  and  6  in the parallel light portion pass through outer circumference positions different to one another on the surface S 0 , and the luminous fluxes of the stray light  1  and the stray light  2  passing through the positions  2  and  6  in the parallel light portion pass through outer circumference positions different to one another on the surface S 0 . 
     Subsequently, in consideration of the phenomenon, a relationship between region splitting patterns of the signal light and the stray lights  1  and  2  in the parallel light portion, and irradiating regions of the signal light and the stray lights  1  and  2  on the surface S 0  will be examined. 
     Firstly, as shown in  FIG. 5A , the signal light and the stray lights  1  and  2  in the parallel light portion are split by two straight lines inclined at 45 degrees relative to the plane-surface direction and the curved-surface direction, to form four sections, i.e., luminous flux regions A to D. It should be noted that this splitting pattern corresponds to a region split based on the conventional astigmatic method. 
     In this case, based on the aforementioned phenomenon, the signal light of the luminous flux regions A to D is distributed on the surface S 0  as shown in  FIG. 5B . Furthermore, based on the aforementioned phenomenon, the stray light  1  and stray light  2  in the luminous flux regions A to D are distributed as shown in  FIGS. 5C and 5D , respectively. 
     Herein, if the signal light and the stray lights  1  and  2  on the surface S 0  are extracted for each luminous flux region, the distribution of each light will be as shown in  FIGS. 6A to 6D . In such a case, either one of the stray light  1  or the stray light  2  in the same luminous flux region overlaps the signal light in each luminous flux region all the time. Therefore, if the signal light in each luminous flux region is received by a sensor pattern on a photodetector, at least the stray light  1  or the stray light  2  in the same luminous flux region will simultaneously enter the corresponding sensor pattern, thus causing a degradation of the detection signal. 
     In contrast to this, as shown in  FIG. 7A , the signal light and the stray lights  1  and  2  in the parallel light portion are split by two straight lines parallel to the plane-surface direction and the curved-surface direction, to form four sections, i.e., luminous flux regions A to D. In such a case, based on the aforementioned phenomenon, the signal light of the luminous flux regions A to D is distributed on the surface S 0  as shown in  FIG. 7B . Furthermore, based on the aforementioned phenomenon, the stray light  1  and the stray light  2  of the luminous flux regions A to D are distributed as shown in  FIGS. 7C and 7D , respectively. 
     Herein, if the signal light and the stray lights  1  and  2  on the surface S 0  are extracted for each luminous flux region, the distribution of each light will be as shown in  FIGS. 8A to 8D . In such a case, neither the stray light  1  nor the stray light  2  in the same luminous flux region overlaps the signal light in each luminous flux region. Therefore, after scattering the luminous fluxes (the signal light, and the stray lights  1  and  2 ) within each luminous flux region in different directions, if the configuration is such that only the signal light is received by the sensor pattern, only the signal light will enter the corresponding sensor pattern, and the entry of the stray light can be inhibited. Thus, the degradation of the detection signal due to the stray light can be avoided. 
     As described above, the signal light and the stray lights  1  and  2  are split into the four luminous flux regions A to D by two straight lines parallel to the plane-surface direction and the curved-surface direction, and by dispersing the light passing through these luminous flux regions A to D, and then alienating it on the surface S 0 , it becomes possible to extract only the signal light. The embodiment is based on this principle. 
       FIGS. 9A and 9B  are diagrams each showing distribution states of the signal light and the stray lights  1  and  2  on the surface S 0 , when advancing directions of the luminous fluxes (the signal light, and the stray lights  1  and  2 ) passing through the four luminous flux regions A to D shown in  FIG. 7A  are changed by the same angle in the respectively different directions. Herein, as shown in  FIG. 9A , the advancing directions of the luminous fluxes (the signal light, and the stray lights  1  and  2 ) passing through the luminous flux regions A to D are changed by the same angular amount α (not shown) in the directions Da, Db, Dc, and Dd, respectively. Each of the directions Da, Db, Dc, and Dd are inclined at 45 degrees relative to the plane-surface direction and the curved-surface direction. 
     In such a case, by regulating the angular amount α in the directions Da, Db, Dc, and Dd, the signal light and the stray lights  1  and  2  in each luminous flux region can be distributed on the plane surface S 0 , as shown in  FIG. 9B . As a result, as shown in  FIG. 9B , a signal light region containing only the signal light can be set up on the plane surface S 0 . By setting a sensor pattern of the photodetector in this signal light region, only the signal light of each region can be received by the corresponding sensor pattern. 
       FIGS. 10A to 10D  are diagrams each describing a method for placing the sensor pattern.  FIGS. 10A and 10B  are diagrams each showing a splitting method of the luminous flux and the sensor pattern based on the conventional astigmatic method, while  FIGS. 10C and 10D  are diagrams each showing a splitting method of the luminous flux and the sensor pattern based on the aforementioned principle. Herein, a track direction has an inclination of 45 degrees relative to a planar direction and a curvature direction. In  FIGS. 10A and 10B , for illustration purposes, the luminous flux is sectioned into eight luminous flux regions a to h. Furthermore, the diffracted image due to the track groove is shown by the solid line and the shape of the beam when off focus is shown by the dotted line. 
     In the conventional astigmatic method, sensor patterns P 1  to P 4  (quadratic sensor) of a photodetector are set as shown in  FIG. 10B . In such a case, if detection signal components based on the light intensity of the luminous flux regions a to h are represented by A to H, a focus error signal FE is evaluated by an arithmetic operation of: FE=(A+B+E+F)−(C+D+G+H), and a push-pull signal PP is evaluated by an arithmetic operation of PP=(A+B+G+H)−(C+D+E+F). 
     In contrast to this, in the case of the distribution state in  FIG. 9B , as described above, the signal light is distributed within the signal light region according to the state shown in  FIG. 10C . In such a case, if the distribution of the signal light passing through the luminous flux regions a to h shown in  FIG. 10A  is overlapped on the distribution shown in  FIG. 10C , a distribution as shown in  FIG. 10D  results. That is, the signal light passing through the luminous flux regions a to h in  FIG. 10A  is guided into the luminous flux regions a to h shown in  FIG. 10D , on the surface S 0  on which the sensor pattern of the photodetector is installed. 
     Therefore, if the sensor patterns P 11  to P 18  that are shown to be overlapped in  FIG. 10D  are set to the positions of the luminous flux regions a to h shown in  FIG. 10D , the focus error signal and push-pull signal can be generated by the same arithmetic process as that in  FIG. 10B . That is, also in this case, if the detection signals from the sensor patterns receiving the luminous flux of the luminous flux regions a to h are represented by A to H, similar to the case in  FIG. 10B , the focus error signal FE can be acquired by an arithmetic operation of FE=(A+B+E+F)−(C+D+G+H), and the push-pull signal PP can be acquired by an arithmetic operation of PP=(A+B+G+H)−(C+D+E+F). 
     As described above, according to this principle, if the signal light and the stray lights  1  and  2  in the parallel light portion are split into the four luminous flux regions A to D by two straight lines parallel to the plane-surface direction and the curved-surface direction shown in  FIG. 1A , and the light passing through these luminous flux regions A to D is dispersed, and the dispersed signal light in each luminous flux region A to D is received individually by a light-receiving portion split into two, the focus error signal and push-pull signal can be generated by the same arithmetic process as in the case based on the conventional astigmatic method. 
     According to this principle, as described above, it is possible to set a region which is irradiated by only a signal light. However, for example, as understood with reference to  FIG. 9B , two beams of stray light are mutually superimposed in the region proximate to the signal light. Thus, when the polarization direction of the stray light is the same, these two beams of stray light create mutual interference in the superimposed region. As a result, there may lead to a problem that the stray light amplified by the interference leads to noise by leaking into the detection region of the signal light. 
       FIG. 11A  is a diagram schematically showing the interference between the beams of stray light.  FIG. 11C  provides simulation results in which interference states between the beams of stray light are evaluated by simulating. In  FIG. 11A , only a distribution state of stray light and signal light passing through a luminous flux region A and a luminous flux region B is extracted from a distribution state shown in  FIG. 9B . In  FIG. 11C , a stray light  2  is omitted and only an intensity distribution of a stray light  1  and a signal light is simulated. In  FIG. 11C , the intensity of a central portion of the signal light takes the highest value and, in regions other than the signal light, the light intensity increases towards a black-colored portion. 
       FIG. 12  is a diagram showing an optical system used for the simulation in  FIG. 11C . In  FIG. 12 , reference numeral  10  denotes a semiconductor laser for emitting a laser light at a wavelength of 405 nm;  11  is a polarizing beam splitter for reflecting substantially all of the laser light emitted from the semiconductor laser  10 ;  12  is a collimate lens for converting a laser light into a parallel light;  13  is a ¼ wavelength plate for converting a laser light (linearly polarized light) incident from the collimate lens  12  side into a circularly polarized light;  14  is an objective lens for converging a laser light onto a disc;  15  is a detection lens for introducing an astigmatism into a reflected light from the disc transmitting the polarizing beam splitter  11 ;  16  is an angle adjusting element for imparting an operation described with reference to the preceding  FIG. 9A  to a laser light; and  17  is a photodetector. 
     As described with reference to the preceding  FIG. 9A , the angle adjusting element has an operation which mutually isolates the laser light passing through four luminous flux regions A to D so as to distribute on the light detecting surface the laser light passing through the respective luminous flux regions as shown in  FIG. 9B . It should be noted that in this simulation, it is assumed that only a single recording layer (mirror surface) is present at a deeper end of the target recording layer and that there is no recording layer at a near end of the target recording layer. Thus, in the simulation, a distribution of the stray light  2  does not occur out of the distribution shown in  FIG. 9B . Furthermore, an interval between the target recording layer and the deeper recording layer is taken to be 10 μm. 
     Design conditions for the optical system are shown as follows: 
     (1) Approach-route factor: 10 magnifications; 
     (2) Return-route factor: 18 magnifications; 
     (3) Spectral angle imparted by angle adjusting element  16 : 1.9 degrees; 
     (4) Optical path length between detection surface of photodetector  17  and spectral surface of angle adjusting element  16 : 3 mm; 
     (5) Spot diameter on light detecting surface when angle adjusting element  16  is not disposed: 60 μm; and 
     (6) Displacement distance for each signal light (respectively passing through luminous flux regions A to D) on the light detecting surface when angle adjusting element  16  is disposed: 100 μm. 
     The approach-route factor is a ratio of the focal point distance of the collimate lens to the focal point distance of the objective lens. The return-route factor is a ratio of a synthetic focal point distance of the detection lens and the collimate lens, to the focal point distance of the objective lens. In this optical system, the laser light (signal light) which is reflected by the disc forms a least circle of confusion on the detection surface when the angle adjusting element  16  is removed. The spot diameter in (5) above is the diameter of the least circle of confusion. Furthermore, the displacement distance in (6) above is a distance between an optical axial center of the signal light on the detection surface when the angle adjusting element  16  is removed and an apical position (position of the apex when a fan shape shown in  FIG. 8A  to  FIG. 8D  is a right angle) of the respective signal light when the angle adjusting element  16  is disposed. 
       FIG. 11C  shows a distribution state of light having an intensity of at least 1/30 of the peak intensity, out of the laser light (signal light and stray light  1 ) irradiated onto the detection surface. 
     With reference to  FIG. 11A , as described above, two beams of stray light  1  are mutually superimposed in the region proximate to the signal light. These two beams of stray light  1  have the same polarization direction. As a result, in the superimposed region, the two beams of stray light  1  create mutual interference and the stray light  1  produces an interference band in this region. The resulting interference band has a relatively high light intensity, as shown in  FIG. 11C , and a portion thereof results in noise by leaking into the detection region (sensor region) of the signal light. 
     This problem can be suppressed by causing a lack of correspondence between the polarization directions of the mutually superimposed stray light  1 .  FIG. 11B  is a diagram schematically showing a state when the polarization directions of the mutually superimposed stray light  1  are mutually set orthogonal.  FIG. 11D  shows results of a simulation in which the light intensity distribution on the detection surface at that time is simulated. The conditions for the simulation in  FIG. 11D  are the same as those in  FIG. 11C  with the exception that functional means for mutually setting the polarization directions of the two beams of stray light  1  orthogonal, as described above, is disposed on an incident side of the angle adjusting element  16 . Similarly to  FIG. 11C ,  FIG. 11D  shows a distribution state of light having an intensity of at least 1/30 of the peak intensity, out of the laser light (signal light and stray light  1 ) irradiated onto the detection surface. 
     With reference to  FIG. 11D , it can be seen that the generation of an interference band due to the stray light  1  and the leakage of the stray light relative to the detection region (sensor region) of the signal light can be suppressed by setting the polarization direction of the mutually superimposed stray light  1  orthogonal. Thus, a detection signal without noise caused by the stray light can be obtained. 
       FIG. 13A  shows simulation results obtained by simulating an amount of leakage of stray light  1  relative to the signal light region (see  FIG. 9B ) when an interval ΔL between the target recording layer and the deeper recording layer is changed under the above-described simulation conditions. A vertical axis of  FIG. 13A  shows a ratio (light intensity ratio) between: the amount of all light (including stray light and signal light. Hereafter, this light is referred to as “detected light”) incident on the signal light region; and the amount of light of only signal light. This ratio is used as an indicator of the amount of leakage of stray light  1  relative to the signal light region. As the amount of stray light leakage increases, the light intensity ratio becomes higher. The intensity of the detected light and the signal light is a received light intensity when placing a sensor pattern as shown in  FIG. 13C  on the detection surface of the photodetector  17 . 
     In  FIG. 13A , a graph plotted with a symbol “X” shows simulation results when the polarization directions of the two mutually superimposed beams of stray light  1  coincide in the optical system in  FIG. 12 . A graph plotted with a symbol “▴” shows simulation results when the polarization directions for the two mutually superimposed beams of stray light  1  are set mutually orthogonal. A graph plotted with a symbol “⋄” shows simulation results (in a conventional example) when the angle adjusting element  16  is omitted from the optical system in  FIG. 12 . The intensity of the detected light and the signal light at this time is a received light intensity when placing a sensor pattern shown in  FIG. 13B  on the detection surface of the photodetector  17 . 
     It should be noted that along with the interval ΔL between the target recording layer and the deeper recording layer is changed, relative phases of the signal light and the stray light  1  vary, and as a result, the interference band resulting from the two beams of stray light  1  causes a transition which inverts (light-dark) brightness. Thus, when viewed on a micro-scale, the simulation results shown in  FIG. 13A  show that the light intensity ratio varies minutely along with the change in ΔL. In this case, however, in order to clearly show the amount of leakage of stray light itself, that variation is omitted from  FIG. 13A , and instead, a transition of the light intensity ratio on a macro-scale is illustrated. 
     As understood with reference to  FIG. 13A , when the above principle is used, the leakage of stray light relative to the signal light region is more suppressed as compared to the conventional example. Furthermore, when the polarization directions of the two beams of mutually superimposed stray light  1  are set orthogonal (graph plotted with the symbol “▴” in  FIG. 13A ), it can be seen that the leakage of stray light relative to the signal light region is more suppressed as compared to a case where the polarization direction coincides (graph plotted with a symbol “X” in  FIG. 13A ). In particular, when the polarization directions of the stray light  1  are set orthogonal, the leakage of stray light is conspicuously suppressed if the interval ΔL between the target recording layer and the deeper recording layer is small. Thus, when the polarization directions of the two beams of stray light  1  are set orthogonal as described above, it is possible to conspicuously suppress the influence resulting from the stray light. 
     Embodiment 
     An embodiment based on the principle will be described below. 
       FIG. 14  depicts an optical system of the optical pickup apparatus according to the embodiment. It is noted that in  FIG. 14 , for the sake of convenience, a related circuit configuration is also shown. A plurality of recording layers are stacked and placed on a disc shown in  FIG. 14 . 
     As shown in  FIG. 14 , the optical system of the optical pickup apparatus is provided with: a semiconductor laser  101 ; a polarizing beam splitter  102 ; a collimating lens  103 ; a lens actuator  104 ; a startup mirror  105 ; a ¼ wavelength plate  106 ; an objective lens  107 ; a holder  108 ; an objective lens actuator  109 ; a detection lens  110 ; a polarization adjusting element  111 ; an angle adjusting element  112 ; and a photodetector  113 . 
     The semiconductor laser  101  emits a laser light of a predetermined wavelength. The polarizing beam splitter  102  substantially completely reflects the laser light (S polarized light) entering from the semiconductor laser  101 , and at the same time, substantially completely transmits the laser light (P polarized light) entering from the collimating lens  103  side. The collimating lens  103  converts the laser light entering from the polarizing beam splitter  102  side into a parallel light. 
     The lens actuator  104  displaces the collimating lens  103  in an optical-axis direction according to a servo signal inputted from the servo circuit  203 . This corrects the aberration caused in the laser light. The startup mirror  105  reflects the laser light entering from the collimating lens  103  side in a direction towards the objective lens  107 . 
     The ¼ wavelength plate  106  converts the laser light towards the disc into a circularly polarized light, and at the same time, converts a reflected light from the disc into a linearly polarized light perpendicularly intersecting the polarization direction at the time of heading towards the disc. Thereby, the laser light reflected by the disc is transmitted through the polarizing beam splitter  102 . 
     The objective lens  107  is designed such that the laser light is converged properly in a target recording layer within the disc. The holder  108  holds the ¼ wavelength plate  106  and the objective lens  107  as a single piece. The objective lens actuator  109  is configured by a conventionally well-known electromagnetic driving circuit, and out of the circuit, a coil portion, such as a focus coil, is attached to the holder  108 . 
     The detection lens  110  introduces astigmatism into the reflected light from the disc. That is, the detection lens  110  is equivalent to the astigmatic element of  FIG. 1A . 
     The polarization adjusting element  111  adjusts the polarization direction of the laser light incident from the detection lens  110  side for each luminous flux region. That is, the polarization directions of the luminous flux, out of the laser light passing through the four luminous flux regions A to D shown in  FIG. 9A , passing through the luminous flux regions adjacent in the peripheral direction are mutually set orthogonal. In other words, the polarization adjusting element  111  causes the polarization directions of the laser lights passing through the regions A and D to coincide to each other and makes the polarization directions of the laser lights passing through the regions B and C orthogonal to the polarization directions of the laser lights passing through the regions A and D. 
     The angle adjusting element  112  changes the advancing direction of the laser light entering from the polarization adjusting element  111  side according to the manner described with reference to  FIGS. 9A and 9B . That is, the angle adjusting element  112  changes the advancing direction of the luminous flux, out of the laser light that has been entered, passing through the luminous flux regions A to D of  FIG. 9A  by the same angular amount α, in the directions Da to Dd, respectively. It is noted that the angular amount α is set in a manner that the distribution states of the signal light and the stray lights  1  and  2  on the surface S 0  result in the distribution states in  FIG. 9B . 
     The photodetector  113  has the sensor pattern shown in  FIG. 10D . The photodetector  113  is placed in a manner that this sensor pattern is positioned at a location of the surface S 0  of  FIG. 1A . The eight sensors P 11  to P 18  shown in  FIG. 10D  are disposed in the photodetector  113 , and each of these sensors receives the luminous flux passing through the luminous flux regions a to h of  FIG. 10D . 
     A signal arithmetic circuit  201  performs the arithmetic process, as described with reference to  FIG. 10 , on the detection signals outputted from the eight sensors of the photodetector  113 , and generates a focus error signal and a push-pull signal. Furthermore, the signal arithmetic circuit  201  adds up these detection signals outputted from the eight sensors to generate a reproduction RF signal. The generated focus error signal and push-pull signal are sent to a servo circuit  203 , and the reproduction RF signal is sent to a reproduction circuit  202  and the servo circuit  203 . 
     The reproduction circuit  202  demodulates the reproduction RF signal inputted from the signal arithmetic circuit  201  so as to generate reproduction data. The servo circuit  203  generates a tracking servo signal and a focus servo signal from the push-pull signal and the focus error signal inputted from the signal arithmetic circuit  201 , and outputs these signals to the objective lens actuator  109 . Furthermore, the servo circuit  203  outputs the servo signal in the lens actuator  104  such that the quality of the reproduction RF signal inputted from the signal arithmetic circuit  201  becomes optimum. 
       FIG. 15A  to  FIG. 15C  show an example of the configuration of the polarization adjusting element  111  and the operation thereof. In this configuration example, as shown in  FIG. 15B , an incident surface of the polarization adjusting element  111  is divided into four polarization adjusting regions  111   a  to  111   d . The polarization adjusting element  111  is placed on a later stage of the detection lens  110  so that the laser light (signal light, and stray light  1  and  2 ) that has passed through the light flux regions A to D in  FIG. 9A  is incident on the polarization adjusting regions  111   a  to  111   d , respectively. 
     The polarization adjusting regions  111   b  and  111   c  are transparent regions in which a polarization adjusting operation is not imparted to the laser light. The polarization adjusting regions  111   a  and  111   d  are regions having a ½ wavelength plate structure. On the polarization adjusting regions  111   a  to  111   d , the laser light (signal light, and stray light  1  and  2 ) is incident in a polarization direction as shown by an arrow in  FIG. 15A . 
     The polarization adjusting regions  111   a  and  111   d  are arranged so that an optic axis of the ½ wavelength plate structure is inclined by 45 degrees relative to the polarization direction of the incident laser light. Thus, the polarization direction of laser light portions La and Ld transmitting the polarization adjusting regions  111   a  and  111   d  is in a direction which is rotated by 90 degrees, as shown by a dashed arrow in  FIG. 15C , relative to the polarization direction upon incidence. On the other hand, since the polarization adjusting regions  111   b  and  111   c  do not impart a polarization adjusting operation to the laser light, a polarization direction of the laser light portions Lb and Lc transmitting these regions maintains the polarization direction upon incidence as shown by a dashed arrow in  FIG. 15C . 
     As a result, the polarization directions of laser light portions which are adjacent in the peripheral direction are set mutually orthogonal as shown in  FIG. 15C  since the laser light (signal light, and stray light  1  and  2 ) incident on the polarization adjusting element  111  from the detection lens  110  side transmits the polarization adjusting element  111 . Furthermore, the laser light portions La to Ld in  FIG. 15C  correspond to the laser light passing through the four luminous flux regions A to D shown in  FIG. 9A . 
       FIG. 15D  to  FIG. 15F  are diagrams each showing an example of the configuration and the operation of the polarization adjusting element  111  different from those in  FIG. 15A  to  FIG. 15C . In this configuration example, as shown in  FIG. 15E , the polarization adjusting regions  111   b  and  111   c  are also regions having a ½ wavelength plate structure. 
     The polarization adjusting regions  111   a  and  111   d  are arranged so that an optic axis of the ½ wavelength plate structure is inclined by 67.5 degrees relative to the polarization direction of the incident laser light. Thus, in the polarization adjusting regions  111   b  and  111   c , the optic axis of the ½ wavelength plate structure is inclined by 22.5 degrees relative to the polarization direction of incident laser light. Therefore, the polarization directions of the laser light portions La, Ld, and Lb, Lc transmitting the polarization adjusting regions  111   a  and  111   d  and the polarization adjusting regions  111   b  and  111   c  lead to directions obtained by rotating by 45 degrees in a counterclockwise and clockwise direction, respectively, relative to the polarization direction upon incidence as shown by a dashed arrow in  FIG. 15F . 
     Due to this configuration, the laser light (signal light, and stray light  1  and  2 ), which is incident on the polarization adjusting element  111  from the detection lens  110  side, transmits the polarization adjusting element  111 . As a result, the polarization directions of the laser light portions which are adjacent in the peripheral direction are set mutually orthogonal, as shown in  FIG. 15F . Moreover, the laser light portions La to Ld in  FIG. 15F  correspond to the laser light passing through the four luminous flux regions A to D shown in  FIG. 9A . 
       FIGS. 16A to 16C  are diagrams each showing a configuration example of the angle adjusting element  112 .  FIG. 16A  shows a configuration example in a case that the angle adjusting element  112  is configured by a hologram element having a diffraction pattern, while  FIGS. 16B and 16C  show configuration examples in a case that the angle adjusting element  112  is configured by a multi-faced prism. 
     Firstly, in the configuration example of  FIG. 16A , the angle adjusting element  112  is formed by a square-shaped transparent plate, and has a hologram pattern being formed on the light-entering surface. As shown in  FIG. 16A , the light-entering surface is sectioned into four hologram regions  112   a  to  112   d . The angle adjusting element  112  is placed after the polarization adjusting element  111  so that the laser light (the signal light and the stray lights  1  and  2 ) passing through the luminous flux regions A to D of  FIG. 9A  enters in each of the hologram regions  112   a  to  112   d.    
     The hologram regions  112   a  to  112   d  diffract the entered laser light (the signal light and the stray lights  1  and  2 ) in directions Va to Vd, respectively. The directions Va to Vd coincide with the directions Da to Dd of  FIG. 9A . Thus, by means of diffraction, the hologram regions  112   a  to  112   d  change the advancing direction of the laser light (the signal light and the stray lights  1  and  2 ) entering from the polarization adjusting element  111  to the directions Da to Dd of  FIG. 9A , respectively. A diffraction angle in each region is the same. 
     Herein, the diffraction angle is so adjusted that the laser light (the signal light and the stray lights  1  and  2 ) passing through the hologram regions  112   a  to  112   d  is distributed as shown in  FIG. 9B , on the surface S 0  of  FIG. 1A . Thus, as described above, if the light-receiving surface of the photodetector  113  having the sensor pattern shown in  FIG. 10D  is placed on the surface S 0 , the corresponding signal light can be received properly by the aforementioned eight sensors. 
     At this time, the polarization directions of the two mutually superimposed beams of stray light in a distribution state as shown in  FIG. 9B  are set mutually orthogonal as a result of the operation by the polarization adjusting element  111 . Therefore, suppression of the interference of the two beams of stray light is enabled and, as a result, as shown in the simulation above, it is possible to suppress leakage into the sensor pattern in  FIG. 10D , which is caused as a result of the beams of stay light being interfered and amplified. 
     It is noted that the diffraction efficiency of the hologram regions  112   a  to  112   d  is the same as one another. If the hologram formed in the hologram regions  112   a  to  112   d  is of a step-like structure, the diffraction efficiency is adjusted by the number of steps of the hologram pattern and the height for each step, and the diffraction angle is adjusted by a pitch of the hologram pattern. Therefore, in this case, the number of steps of the hologram pattern and the height for each step are set so that the diffraction efficiency of a previously determined diffraction order reaches an expected value, and also, the pitch of the hologram pattern is adjusted so that the diffraction angle in the diffraction order can provide the distribution shown in  FIG. 9B . 
     It is noted that the hologram formed in the hologram regions  112   a  to  112   d  can also be of a blaze type. In this case, a higher diffraction efficiency can be achieved as compared to the step-like structured hologram. 
     In the configuration example of  FIG. 16B , the angle adjusting element  112  is formed by a transparent body whose light-emitting surface is plane, and the light-entering surface is individually inclined in different directions in four regions.  FIG. 16C  is a view of  FIG. 16B  as seen from the light-entering surface side. As shown in  FIG. 16C , on the light-entering surface of the angle adjusting element  112 , four inclined surfaces  112   e  to  112   h  are formed. If a light ray enters these inclined surfaces from the light-entering surface side, in parallel to an X-axis, the advancing direction of the light will change in the direction of Ve to Vh shown in  FIG. 16C , respectively, due to the refractive effect caused when the light enters the inclined surfaces  112   e  to  112   h . Herein, the refraction angle in the inclined surfaces  112   e  to  112   h  is the same. 
     The angle adjusting element  112  of  FIG. 16B  is placed after the polarization adjusting element  111  so that the laser light (the signal light and the stray lights  1  and  2 ) passing through the luminous flux regions A to D of  FIG. 9A  enters the inclined surfaces  112   e  to  112   h , respectively. If the angle adjusting element  112  is placed in this way, the refraction directions Ve to Vh on the inclined surfaces  112   e  to  112   h  coincide with the directions Da to Dd of  FIG. 9A . Therefore, by means of the refraction, the inclined surfaces  112   e  to  112   h  change the advancing direction of the laser light (the signal light and the stray lights  1  and  2 ) entering from the polarization adjusting element  111  by a constant angle into the directions Da to Dd of  FIG. 9A , respectively. 
     Herein, the refraction angle on each inclined surface is adjusted in a manner that the laser light (the signal light and the stray lights  1  and  2 ) passing through the inclined surfaces  112   e  to  112   h  is distributed as shown in  FIG. 9B , on the surface S 0  of  FIG. 1A . Thus, if the photodetector  113  having the sensor pattern shown in  FIG. 10D  is placed on the surface S 0 , the corresponding signal light can be received properly by the aforementioned eight sensors. Because such a refractive effect has a significantly small dependency on the wavelength as compared to the diffractive effect, the adaptability to a change in the wavelength of a light source or to a multi-wavelength light source is high. 
     It is noted that in the configuration example of  FIG. 16A , the hologram regions  112   a  to  112   d  are imparted with only the diffractive effect of providing an angle for changing the advancing direction of the laser light by a constant angle. However, besides providing the angle, a hologram pattern that simultaneously exhibits an astigmatic effect caused by the detection lens  110  can also be set to the hologram regions  112   a  to  112   d . Furthermore, it may be also possible that a hologram pattern for providing the aforementioned angle is formed on the light-entering surface of the angle adjusting element  112  and the light-emitting surface of the angle adjusting element  112  is imparted with the hologram pattern for imparting the astigmatic effect. Similarly, also in the angle adjusting element  112  of  FIG. 16B , a lens surface may be formed on the light-emitting surface for introducing astigmatism. Alternatively, the inclined surfaces  112   e  to  112   h  can be shaped into curved surfaces, and the inclined surfaces  112   e  to  112   h  may be imparted with an astigmatic lens effect. In this way, the detection lens  110  can be omitted, and reductions in the number of parts and in cost can be achieved. 
     Thus, according to the embodiment, from among recording layers disposed in the disc, the overlapping between the signal light reflected from the target recording layer, and the stray lights  1  and  2  reflected from the recording layers present above and below the target recording layer can be prevented from overlapping one another on the light-receiving surface (the surface S 0  where the signal light spot becomes a circle of least confusion at the time of on-focus) of the photodetector  113 . More specifically, the distribution of the signal light and the stray lights  1  and  2  on the light-receiving surface (surface S 0 ) can be made as shown in  FIG. 9B . Therefore, by placing the sensor pattern shown in  FIG. 10D  in the signal light region of  FIG. 9B , only the corresponding signal light can be received by the sensors P 11  to P 18 . Thus, the degradation of the detection signal due to the stray light can be inhibited. 
     In addition, according to the embodiment, since the polarization directions of two beams of mutually superimposed stray light in the distribution state shown in  FIG. 9B  are mutually set orthogonal as a result of the operation by the polarization adjusting element  111 , the interference between these two beams of stray light can be suppressed, and as a result, as shown in the simulation above, it is possible to suppress leakage into the sensor pattern shown in  FIG. 10D , which is caused as a result of the beams of stay light being interfered and amplified. Therefore, further suppression of deterioration of the detection signal as a result of the stray light is enabled. 
     Furthermore, these effects are achieved only by placing the polarization adjusting element  111  and the angle adjusting element  112  in a light path of the laser light reflected by the disc, i.e., between the detection lens  110  and the photodetector  113  in terms of the configuration in  FIG. 14 . Therefore, according to the embodiment, an influence caused due to the stray light can be removed effectively with a simple configuration. 
     It is noted that as shown in  FIG. 17 , the effect by the aforementioned principle can be demonstrated when the focal line position of the stray light  1  in the plane-surface direction is closer to the astigmatic element than the surface S 0  (the surface where the signal light spot becomes a circle of least confusion), and the focal line position of the stray light  2  in the curved-surface direction is further away from the astigmatic element than the surface S 0 . That is, when this relationship is fulfilled, the distribution of the signal light and the stray lights  1  and  2  becomes the same state as those shown in  FIGS. 8A to 8D , and the overlapping between the signal light, and the stray lights  1  and  2  on the surface S 0  can be prevented. In other words, as long as this relationship is fulfilled, for example, even if the focal line position of the stray light  1  in the plane-surface direction is closer to the surface S 0  than the focal line position of the signal light in the curved-surface direction, or else, even if the focal line position of the stray light  2  in the curved-surface direction is closer to the surface S 0  than the focal line position of the signal light in the plane-surface direction, the effects of the present invention and the embodiment based on the aforementioned principle can be demonstrated. 
     The embodiment of the present invention is thus described above. However, the present invention is not limited thereto, and the embodiment of the present invention can also be modified in various ways apart from the aforementioned description. 
     For example, in the aforementioned embodiment, the eight sensors are placed on the light-receiving surface of the photodetector  113 . However, as shown in  FIG. 18B , the sensors for a focus error signal can be combined together, as a single piece, to form sensors P 21  to P 24 , and as shown in  FIG. 18D , the sensors for a push-pull signal can be combined together, as a single piece, to form sensors P 31  to P 34 . Furthermore, the shapes of the sensors P 21  to P 24  of  FIG. 18B  can also be changed as shown in  FIG. 19B , or the shapes of the sensors P 31  to P 34  of  FIG. 18D  can also be changed as shown in  FIG. 19D . 
     It is noted that if the sensor pattern is to be configured as shown in  FIG. 18B ,  18 D,  19 B, or  19 D, the optical system of the optical pickup apparatus needs to be changed as shown in  FIG. 20 . That is, in this optical system, the laser light transmitted through the angle adjusting element  112  is split by a non-polarizing beam splitter (such as a half mirror)  120 , and the split laser light is received respectively by two photodetectors  121  and  122 . For example, the sensor pattern shown in  FIG. 18B  or  FIG. 19B  is disposed in the photodetector  121 , while the sensor pattern shown in  FIG. 18D  or  FIG. 19D  is disposed in the photodetector  122 . Thus, the focus error signal is generated based on the detection signal from the photodetector  121 , and the push-pull signal is generated based on the detection signal from the photodetector  122 . It is noted that similar to the aforementioned embodiment, either one of the photodetectors  121  or  122  may be the sensor pattern shown in  FIG. 10D . 
       FIGS. 21A and 21B  are diagrams each showing another configuration example when two split laser lights are received individually by the sensor pattern for a focus error signal and the sensor pattern for a push-pull signal. It is noted that in  FIGS. 21A and 21B , for the sake of convenience, only a configuration after the polarizing beam splitter  102  is shown. 
     In the configuration example of  FIG. 21A , the placement of the polarization adjusting element and the angle adjusting element is changed as compared to that in  FIG. 20 . That is, in this configuration example, two polarization adjusting elements  131  and  133  and two angle adjusting elements  132  and  134  are disposed respectively after the non-polarizing beam splitter  120 , and provide the aforementioned polarization adjusting effect and angle adjusting effect to the split laser light. Examples of the polarization adjusting element  131  and  133  include that which is configured as shown in  FIGS. 15B and 15E , and examples of the angle adjusting elements  132  and  134  include that which is configured as shown in  FIGS. 16A to 16C . In this case, either one of the polarization adjusting element  131  and the angle adjusting element  132  or the polarization adjusting element  133  and the angle adjusting element  134  can even be omitted. In a light path where the polarization adjusting element and the angle adjusting element is omitted, a photodetector having the sensor pattern in  FIG. 10B  is applied, and the focus error signal and the push-pull signal are generated by a normal arithmetic process. In this case, the stray light superimposes the signal light, thus causing the detection signal to degrade. 
     In the configuration of  FIG. 20 , the laser light is split by using the non-polarizing beam splitter. However, the laser light can also be split by using another optical means. In the configuration example of  FIG. 21B , the laser light is split by using a diffraction element  141 . Herein, for example, the configuration of the diffraction element  141  is such that a +1-order diffraction light is diffracted by a predetermined angle in one direction from a zero-order diffraction light. In this case, for example, in the photodetector  113 , the sensor pattern shown in  FIG. 18B  or  FIG. 19B  is disposed in an irradiation position of the +1-order diffraction light, while the sensor pattern shown in  FIG. 18D  or  FIG. 19D  is disposed in an irradiation position of the zero-order diffraction light. It is noted that either one of the sensor patterns can also be the sensor pattern shown in  FIG. 10D . 
     Herein, if the diffraction pattern is to be formed in a step-like structured hologram, then as described above, the light amount ratio of the zero-order diffraction light and +1-order diffraction light can be adjusted by adjusting the number of steps and the height for each step. It is noted that in this configuration example, the diffraction element  141  and the angle adjusting element  112  maybe combined together, as a single piece, also, the diffraction element  141 , the polarization adjusting element  111 , and the angle adjusting element  112  may be combined together, as a single piece. Also, if the angle adjusting element  112  is configured as shown in  FIG. 16A , the hologram pattern in the diffraction element  141  is placed on the light-emitting surface of the angle adjusting element  112 , or the hologram pattern on the light-entering surface can be adjusted to a pattern that simultaneously exhibits the diffractive effect in the aforementioned embodiment (angular provision in the luminous flux regions A to D) and the diffractive effect by the diffraction element  141  (splitting of the laser light). In this way, the diffraction element  141  can be omitted, the simplification of the configuration can be implemented while inhibiting a decline in the diffraction efficiency. 
     Furthermore, in the above-described embodiment, although the angle adjusting element  112  is placed on a later stage of the polarization adjusting element  111 , the angle adjusting element  112  may also be placed on a forward stage of the polarization adjusting element  111 . However, when the angle adjusting element  112  is configured by a hologram element having a diffraction pattern as shown in  FIG. 16A , it is preferred as in the above embodiment that the angle adjusting element  112  is placed on a later stage of the polarization adjusting element  111 . The diffraction efficiency of the hologram element changes according to a polarization direction of laser light. As in the above embodiment, when the angle adjusting element  112  is placed on a later stage of the polarization adjusting element  111 , a relationship between a polarization direction of the incident laser light and a pitch direction of the diffraction pattern can be made the same relative to all hologram regions  112   a  to  112   d . Therefore, the diffraction efficiency of the hologram regions  112   a  to  112   d  can be made the same. 
     It is noted that in the aforementioned embodiment, if the objective lens  107  deviates in the tracking direction, the center of the objective lens  107  deviates from the laser light axis, and an offset occurs in the push-pull signal. In this case, for example, in the sensor pattern shown in  FIG. 10D , by evaluating the push-pull signal through an arithmetic operation of PP=A+H−(D+E)−k{B+G−(C+F)} (k: adjustment coefficient), an offset component of the push-pull signal occurring at the time of the tracking operation can be reduced. It is noted that if the sensor patterns of  FIG. 18D  and  FIG. 19D  are used, the arithmetic operation of A+H, D+E, B+G, and C+F can be omitted during calculation of the push-pull signal, which simplifies the arithmetic process. 
     Besides, the embodiment of the present invention may be modified in various ways, where appropriate, within the range of the technological idea set forth in the claims.