Abstract:
An optical pickup device has a semiconductor laser emitting light which is in turn branched via a diffraction grating into at least three beams of light including a main beam and two sub beams which are in turn condensed via an objective lens on an optical disk at a guide groove and reflected by the optical disk to provide three reflections of light which are in turn received by detectors, each divided into two regions, respectively, to generate a tracking error signal. The diffraction grating is divided into three regions including a first region, a second region and a third region located intermediate therebetween, each having a periodical structure out of phase, the periodical structure having grating grooves in a direction determined depending on the phase of the second region to incline relative to a direction perpendicular to the guide groove of the optical disk.

Description:
[0001]     This nonprovisional application is based on Japanese Patent Application No. 2005-219490 filed with the Japan Patent Office on Jul. 28, 2005, the entire contents of which are hereby incorporated by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates generally to optical pickup devices and particularly to optical pickup devices optically recording information on and reproducing information from a plurality of different types of recording media having guide grooves, respectively, different in pitch.  
         [0004]     2. Description of the Background Art  
         [0005]     In recent years, optical disks, optical cards and similar recording media can record large amounts of information signals densely and are accordingly utilized in audio equipment, video equipment, computers and other equipment.  
         [0006]     Recently, in particular, for motion video information and the like used for example in computers, the data that are handled are drastically increasing in amount. Accordingly, a recording pit reduced in size, a guide groove reduced in pitch and the like are provided to provide optical disks with large capacities.  
         [0007]     The above described optical disks and similar recording media have an information signal recorded therein in microns. Accordingly, reproducing such signal therefrom entails causing a beam of light to track a recording guide groove precisely. A variety of methods of detecting a tracking error signal is known.  
         [0008]     For example, Japanese Patent Laying-Open No. 61-094246 describes a differential push-pull (DPP) method employing three beams including a main beam and two sub beams. The DPP method is generally employed for a variety of recording optical disks including compact disk-recordable/rewritable (CD-R/RW).  
         [0009]     The DPP method, however, requires that relative to a guide groove at which the main beam is positioned, the sub beams be positioned offset in the radial direction of an optical disk by ½ of the pitch of the guide groove. This provides a tracking error signal degraded for a different type of optical disk having a guide groove different in pitch.  
         [0010]     As a means for resolving such problem, Japanese Patent Laying-Open No. 09-081942 proposes a method, as will be described hereinafter with reference to  FIGS. 11-14 .  
         [0011]      FIG. 11  schematically shows a structure of an optical system of a conventional optical pickup device  100 .  
         [0012]     With reference to the figure, optical pickup device  100  includes a semiconductor laser  1 , a collimator lens  2 , a diffraction grating  300 A, a beam splitter  4 , an objective lens  5 , a condenser lens  7 , and a photoreceptive unit  8  including photoreceptive elements  8 A- 8 C each divided into two regions. Optical pickup device  100  records information on and reproduces information from an optical disk  6  having a guide groove  61 .  
         [0013]     Semiconductor laser  1  emits a beam of light  30 P which is received by collimator lens  2  and collimated thereby. The collimated light is divided by diffraction grating  300 A serving as an optical branching element into a main beam  30  for recording and reproduction and detecting a servo signal and two sub beams  31  and  32  for tracking. The three beams  30 - 32  are transmitted through beam splitter  4  and condensed by objective lens  5  on a recording medium or optical disk  6  at guide groove  61 .  
         [0014]     The three beams of light are reflected  30 - 32  and again transmitted through objective lens  5 , reflected by beam splitter  4  and incident through condenser lens  7  on the three photoreceptive elements  8 A- 8 C, respectively, which detect a push pull signal MPP of main beam  30  and push pull signals SPP 1  and SPP 2  of sub beams  31  and  32 , respectively. Optical pickup device  100 , as well as the conventional DPP method, obtains a tracking error signal TR by the following operation: 
 
 TR=MPP−k ·( SPP 1 +SPP 2), 
 
 wherein k represents a coefficient correcting a difference in quantity of light between main beam  30  and sub beams  31  and  32 . Furthermore, k·(SPP 1 +SPP 2 ) will also be indicated as a composite sub beam push pull signal SPP. 
 
         [0015]     The conventional optical pickup device  100  shown in  FIG. 11  is characterized by a periodical structure that diffraction grating  300 A has. This feature will now be described with reference to  FIGS. 12-14 .  
         [0016]      FIG. 12  is a perspective view of the structure of diffraction grating  300 A in optical pickup device  100  shown in  FIG. 11 .  
         [0017]     As shown in  FIG. 12 , diffraction grating  300 A is divided into a first region  300   a  and a second region  300   b  by a line extending in a direction Y corresponding to that of guide groove  61  of optical disk  6 . These regions each have a structure having a protrusion and a depression extending in a direction X perpendicular to that of guide groove  61  and periodically repeated such that the periodical structures have phases, respectively, different from each other by 180°.  
         [0018]      FIG. 13  shows a position of main beam  30  and sub beams  31  and  32  condensed on optical disk  6  at guide groove  61  for optical pickup device  100  shown in  FIG. 11 .  
         [0019]     As shown in  FIG. 13 , sub beams  31  and  32  diffracted by the periodical structure of diffraction grating  300 A each have on optical disk  6  at guide groove  61  an optical phase difference of 180° in a half thereof. Consequently, sub beam  31  is divided into spots of condensed light  31   m  and  31   n  and sub beam  32  is divided into spots of condensed light  32   m  and  32   n . Sub beams  31  and  32  each thus form spots of condensed light having two peaks, respectively, in intensity.  
         [0020]      FIG. 14  represents in waveform push pull signals MPP, SPP, SPP 1 , SPP 2  corresponding to a structure of optical disk  6  for optical pickup device  100  shown in  FIG. 11 .  
         [0021]     As shown in the figure, sub beams  31  and  32  provide push pull signals SPP 1  and SPP 2  180 20  out of phase with push pull signal MPP of main beam  30 . Similarly, composite sub beam push pull signal SPP has a waveform 180° out of phase, in opposite phase, with push pull signal MPP of main beam  30 .  
         [0022]     As such if sub beams  31  and  32  are positioned on the same guide groove  61  as main beam  30 , as shown in  FIG. 13 , tracking error signal TR as intended is obtained. Optical pickup device  100  shown in  FIG. 11  can thus accommodate a different type of optical disk having a guide groove different in pitch from optical disk  6 .  
         [0023]     The above described DPP system, however, has a significant disadvantage in practical use. Currently increasingly used digital versatile disks (DVDs) include DVD-R/RW (having a storage capacity of 4.7 GB and a guide groove with a pitch of 0.74 μm), DVD-Random Access Memory (DVD-RAM)  1  (having a storage capacity of 2.6 GB and a guide groove with a pitch of 1.48 μm), DVD-RAM 2  (having a storage capacity of 4.7 GB and a guide groove with a pitch of 1.23 μm), and the like.  
         [0024]     If such a variety of DVDs is subjected to the system disclosed in Japanese Patent Laying-Open No. 09-081942, and objective lens  5  shifts in a direction (X) orthogonal to guide groove  61  of optical disk  6 , then, tracking error signal TR would significantly be reduced in amplitude as compared with that provided in the conventional DPP method. Such reduction in amplitude is significant particularly for DVD-RAM 1 , DVD-RAM 2 , and other similar disks having guide grooves large in pitch.  
         [0025]     An arithmetic circuit, a control circuit and the like for tracking error signal TR receives a signal, which in general is limited in how it varies in amplitude, and if tracking error signal TR significantly varies in amplitude as the objective lens shifts, optical pickup device  100  is significantly reduced in range of practical use.  
         [0026]     The above disadvantage is resolved by a method, as disclosed in Japanese Patent Laying-Open No. 2004-145915. The publication describes an optical pickup device basically identical to the optical system of optical pickup device  100  shown in  FIG. 11  except that diffraction grating  300 A serving as an optical branching element is replaced with a diffraction grating  300 B having a feature as will be described hereinafter with reference to  FIG. 15 .  
         [0027]      FIG. 15  is a perspective view of a structure of diffraction grating  300 B serving as another example of diffraction grating  300 A in optical pickup device  100  of  FIG. 11 .  
         [0028]     As shown in the figure, diffraction grating  300 B has constantly periodically repeated grating grooves formed therein and is divided into at least three regions, i.e., first, second and third regions  300   a ,  300   b  and  300   c , by a line extending in a direction (Y) orthogonal to that of the grating grooves.  
         [0029]     While the second region  300   b  has a periodical structure with a phase similarly as described for diffraction grating  300 A of Japanese Patent Laying-Open No. 09-081942, i.e., 180° out of phase with the first region  100   a , the third region  300   c  intermediate between the first and second regions  300   a  and  300   b  is structured to be 90° out of phase with the first region  300   a.    
         [0030]     Diffraction grating  300 B thus structured can also prevent tracking error signal TR significantly reduced in amplitude as the objective lens shifts for example for DVD-RAM 1  and DVD-RAM 2  having guide grooves with large pitches. Optical pickup device  100  can thus be increased in range of practical use.  
         [0031]     Japanese Patent Laying-Open No. 2004-145915 describes that diffraction grating  300 B divided in three has each region having a periodical structure with a phase offset by an amount limited to 90° and 180°.  
         [0032]     If a diffraction grating has a periodical structure positionally offset between adjacent regions to add a phase difference to diffracted light (or a sub beam), the diffraction grating, having a limited number of grating lines providing protrusions and depressions, cannot provide accurate phase variation at a boundary region. In that case, there is also caused unwanted high-order diffracted light, and hence reduced efficiency in utilization of light and unwanted light acting as noise. Accordingly, achieving an object with a minimized phase difference is desired.  
         [0033]     Furthermore, diffraction grating  300 B that has regions having the periodical structure with their respective phases offset by amounts limited to 180° and 90° has the intermediate or third regions  300   c  limited in optimum width and hence a reduced degree of freedom in design. Japanese Patent Laying-Open No. 2004-145915 is silent on a combination of phase offsets other than other than 180° and 90°, and any relationship between the direction of the grating grooves of the diffraction grating and a phase difference given.  
       SUMMARY OF THE INVENTION  
       [0034]     The present invention contemplates an optical pickup device that can also minimize that degradation in amplitude of a tracking error signal which is caused as an objective lens shifts for a plurality of types of optical disks having guide grooves, respectively, different in pitch despite a diffraction grating divided into multiple regions provided with phases, respectively, offset by a reduced amount.  
         [0035]     The present invention is an optical pickup device having a laser as a source of light emitting light in turn branched via an optical branching element into at least three beams of light including a main beam and two sub beams in turn condensed via an objective lens on an optical recording medium at a guide groove and reflected by the optical recording medium to provide three reflections of light in turn received by different detectors, each divided into two regions, respectively, to obtain a differential signal from the detectors to generate a tracking error signal from the differential signal, the optical branching element being a diffraction grating divided into at least three regions including a first region, a second region and a third region located intermediate between the first and second regions, the first, second and third regions having a periodical structure with their respective phases different from each other, the periodical structure having grating grooves in a direction determined depending on the phase of the second region to incline relative to a direction perpendicular to the guide groove of the optical recording medium.  
         [0036]     Preferably, if relative to the first region the second regions has a phase difference α in degrees, on the optical recording medium the main beam and the sub beams have a distance L therebetween, and the optical recording medium has the guide groove with a pitch M, then the optical branching element has the periodical structure with the grating grooves in a direction inclined relative to the direction perpendicular to the guide groove of the optical recording medium by an angle θ=((180−α)/360)tan −1  (M/L).  
         [0037]     Preferably the second region has the phase difference α of at least 30° and at most 180°.  
         [0038]     Preferably the third region is provided with a phase difference of approximately 1/2 of the phase difference α of the second region.  
         [0039]     Preferably the third region is further divided into at least two regions each having a different phase difference smaller than the phase difference α of the second region.  
         [0040]     Preferably the third region has a phase smoothly varying from that of the periodical structure of the first region to that of the periodical structure of the second region.  
         [0041]     Preferably the third region has the periodical structure of the first region and that of the second region alternately.  
         [0042]     In accordance with the present invention, if a diffraction grating divided into multiple regions provided with phases, respectively, offset by a reduced amount, there can be provided a tracking error signal that is not degraded for a plurality of types of optical disks having guide grooves, respectively, different in pitch, and the signal can also be prevented from significantly reducing in amplitude as the objective lens shifts.  
         [0043]     The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0044]      FIG. 1  schematically shows a configuration of an optical system of an optical pickup device  10  in a first embodiment of the present invention.  
         [0045]      FIG. 2  is a perspective view of a structure of a diffraction grating  3 A in optical pickup device  10  of  FIG. 1 .  
         [0046]      FIG. 3  shows a periodical structure of diffraction grating  3 A in optical pickup device  10  of  FIG. 1 .  
         [0047]      FIG. 4  shows a position of a main beam  30  and sub beams  31  and  32  condensed on a optical disk  6  at a guide groove  61  for optical pickup device  10  of  FIG. 1 .  
         [0048]      FIG. 5  represents push pull signals MPP, SPP, SPP 1 , SPP 2  in waveform corresponding to a structure of optical disk  6  before diffraction grating  3 A is rotated by an angle θ.  
         [0049]      FIG. 6  represents push pull signals MPP, SPP, SPP 1 , SPP 2  in waveform corresponding to the structure of optical disk  6  after diffraction grating  3 A is rotated by angle θ.  
         [0050]      FIG. 7  represents for optical pickup device  10  of  FIG. 1 a  characteristic of tracking error signal TR in how it varies in amplitude for optical disk  6  provided in a variety of DVDs.  
         [0051]      FIGS. 8, 9  and  10  show periodical structures of diffraction gratings  3 B,  3 C and  3 D in first, second and third exemplary variations, respectively, of the first embodiment of the present invention.  
         [0052]      FIG. 11  schematically shows a configuration of an optical system of a conventional optical pickup device  100 .  
         [0053]      FIG. 12  is a perspective view of a structure of diffraction grating  300 A in optical pickup device  100  of  FIG. 11 .  
         [0054]      FIG. 13  shows a position of main beam  30  and sub beams  31  and  32  condensed on optical disk  6  at guide groove  61  for optical pickup device  100  of  FIG. 11 .  
         [0055]      FIG. 14  represents push pull signals MPP, SPP, SPP 1 , SPP 2  in waveform corresponding to a structure of optical disk  6  for optical pickup device  100  of  FIG. 11 .  
         [0056]      FIG. 15  is a perspective view of a structure of diffraction grating  300 B serving as another example of diffraction grating  300 A in optical pickup device  100  of  FIG. 11 . 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0057]     Hereinafter an embodiment of the present invention will now be more specifically described with reference to the drawings. Note that in the figures, identical or like components are identically denoted and will not be described repeatedly.  
       First Embodiment  
       [0058]      FIG. 1  schematically shows a configuration of an optical system of an optical pickup device  10  in a first embodiment of the present invention.  
         [0059]     As shown in  FIG. 1 , the first embodiment provides optical pickup device  10  different from optical pickup device  11  of  FIG. 11  in that diffraction grating  300 A serving as an optical branching element is replaced with a diffraction grating  3 A. Accordingly, the components that overlap  FIG. 11  will not be described repeatedly.  
         [0060]      FIG. 2  is a perspective view of a structure of diffraction grating  3 A in optical pickup device  10  of  FIG. 1 .  
         [0061]     As shown in the figure, the first embodiment provides diffraction grating  3 A divided into first, second and third regions  3   a ,  3   b  and  3   c  by a line having a direction perpendicular to that of its grating grooves, with each region having a structure having a protrusion and a depression which extend in a direction inclined relative to a direction X by an angle θ and are periodically repeated. In other words, the first embodiment provides diffraction grating  3 A having grating grooves in a direction inclined by angle θ relative to directions X and Y. This will furthermore specifically be described with reference to  FIG. 3 .  
         [0062]      FIG. 3  shows the periodical structure of diffraction grating  3 A in optical pickup device  10  of  FIG. 1 .  
         [0063]     As shown in the figure, the first embodiment provides diffraction grating  3 A, by way of example, having the second region  3   b  with its periodical structure 120° out of phase with the first region  3   a , and the third region  3   c  intermediate between the first and second regions  3   a  and  3   b  with its periodical structure 60° out of phase with the first region  3   a.    
         [0064]     Furthermore, if a direction corresponding to that of guide groove  61  of optical disk  6  is represented as direction Y and a direction perpendicular thereto is represented as direction X, then diffraction grating  3 A has grating grooves set in a direction inclined relative to directions X and Y by angle θ.  
         [0065]      FIG. 4 . shows a position of a main beam  30  and sub beams  31  and  32  condensed on optical disk  6  at guide groove  61  for optical pickup device  10  of  FIG. 1 .  
         [0066]     As shown in the figure, sub beams  31  and  32  diffracted by the periodical structure of diffraction grating  3 A each have on optical disk  6  at guide groove  61  an optical phase difference at a portion thereof. Consequently, sub beam  31  is divided into spots of condensed light  31   a  and  31   b  and sub beam  32  is divided into spots of condensed light  32   a  and  32   b . Sub beams  31  and  32  each thus form spots of condensed light providing two peaks in intensity.  
         [0067]     With reference to  FIG. 4 , if relative to the first region  3   a  the second region  3   b  has a phase difference represented by α (in degrees), on optical disk  6  main beam  30  and sub beams  31  and  32  have a distance therebetween represented by L (in μm), and optical disk  6  has guide groove  61  with a pitch represented by M (in μm), then angle θ is defined by: 
 
θ=((180− 60  )/360)tan −1 ( M/L )   (1). 
 
         [0068]     Distance L on optical disk  6  between main beam  30  and sub beams  31  and  32  depends on the optical system of interest. For example, if L=15 μm and a DVD-R/RW has a guide groove with a pitch M=0.74 μm, then, for α=120° as shown in  FIG. 3 , diffraction grating  3 A has an inclination or angle θ=0.47° . This allows optical pickup device  10  of the first embodiment to provide tracking error signal TR invariable in amplitude and hence a completely equivalent signal for the DVD-R/RW, similarly as provided in the conventional DPP method.  
         [0069]     Main beam  30  provides push pull signal MPP and sub beams  31  and  32  provide push pull signals SPP 1 , SPP 2 , SPP, as will be described hereinafter.  
         [0070]      FIG. 5  represents push pull signals MPP, SPP, SPP  1 , SPP 2  in waveform corresponding to a structure of optical disk  6  before diffraction grating  3 A is rotated by angle θ.  
         [0071]      FIG. 5  represents push pull signals MPP, SPP, SPP  1 , SPP 2  in waveform provided when relative to the first region  3   a  the second region  3   b  has phase difference α. If the second region  3   b  has grating grooves +α out of phase with those of the first region  3   a , sub beam  31  (or a first-order, positive, diffracted beam of light) has an optical phase difference of +α added thereto and sub beam  32  (or a first-order, negative, diffracted beam of light) has an optical phase difference of −α added thereto.  
         [0072]     Accordingly, if diffraction grating  3 A has grating grooves in a direction matching that perpendicular to guide groove  61  of optical disk  6 , as assumed as shown in  FIG. 5 , then the first-order, positive, diffracted beam of light provides push pull signal SPP 1 +α out of phase with push pull signal MPP of the main beam and, in contrast, the first-order, negative, diffracted beam of light provides push pull signal SPP 2 −α out of phase therewith.  
         [0073]     In the above described case, as shown in  FIG. 5 , push pull signals SPP 1  and SPP 2  added together, i.e., composite sub beam push pull signal SPP, reduces from a maximum amplitude. Accordingly, diffraction grating  3 A is rotated around an optical axis (or an axis Z) to have its grating grooves in a direction inclined relative to direction X by angle θ to maximize composite sub beam push pull signal SPP in amplitude for optical disk  6  (e.g., DVD-R/RW).  
         [0074]      FIG. 6  represents push pull signals MPP, SPP, SPP 1 , SPP 2  in waveform corresponding to the structure of optical disk  6  after diffraction grating  3 A is rotated by angle θ.  
         [0075]     As shown in the figure, rotating diffraction grating  3 A by angle θ allows push pull signal SPP 1  to have a waveform (180−α) out of phase and push pull signal SPP 2  to have a waveform—(180−α) out of phase. This can provide composite sub beam push pull signal SPP similar to that provided in the conventional DPP method.  
         [0076]      FIG. 7  represents for optical pickup device  10  of  FIG. 1 a  characteristic of tracking error signal TR in how it varies in amplitude for optical disk  6  provided in a variety of DVDs. In the figure, the horizontal axis represents an amount, as represented in micrometers, of shifting an objective lens, and the vertical axis represents tracking error amplitude (in relative value, as represented in %).  
         [0077]     The relative value as represented along the vertical axis represents a relative value of a variation in amplitude with reference to an amplitude of tracking error signal TR provided in the system of Japanese Patent Laying-Open No. 09-081942 when an objective lens has a shift of zero. Such value was obtained for DVD-RAM 1  (having a guide groove with a pitch of 1.48 μm), DVD-RAM 2  (having a guide groove with a pitch of 1.23 μm) and a DVD-R/RW (having a guide groove with a pitch of 0.74 μm).  
         [0078]     With reference to  FIG. 7 , curves A 1  and A 2  represent variations in amplitude in the system of Japanese Patent Laying-Open No. 09-081942 for DVD-RAM 1  and DVD-RAM 2 , respectively. Curves A 3  and A 4  represent variations in amplitude in the system of the first embodiment of the present invention for DVD-RAM 1  and DVD-RAM 2 , respectively. A curve A 5  represents a variation in amplitude for the DVD-R/RW, which is substantially the same characteristic between the system of Japanese Patent Laying-Open No. 09-081942 and that of the first embodiment.  
         [0079]     As shown in  FIG. 7 , the system of the first embodiment (curves A 3  and A 4 ), as compared with that of Japanese Patent Laying-Open No. 09-081942, can also significantly prevent tracking error signal TR from having an amplitude varying increasingly/decreasingly for DVD-RAM 1  and DVD-RAM 2  having guide grooves with large pitches as the objective lens shifts. Although curves A 3  and A 4  show amplitude smaller in absolute value than curves A 1  and A 2 , increasing in gain for example by a signal generation circuit or the like allows them to be used without problem.  
         [0080]     In the above description diffraction grating  3 A has relative to the first region  3   a  the second region  3   b  with a phase difference of 120° and the third region  3   c  with that of 60°. However, it is not limited to such a combination of phase differences; it is similarly effective with combinations for example of: 60° and 30°; 90° and 45°; and the like.  
         [0081]     It should be noted, however, that if relative to the first region  3   a  the second region  3   b  has phase difference α reduced in absolute value, and a DVD-RAM having a guide groove with a large pitch is subjected to reproduction, the tracking error signal is significantly degraded in amplitude as the objective lens shifts. Accordingly, desirably, phase difference α is set in a range of 30°≦α≦180°.  
         [0082]     The first embodiment can thus provide diffraction grating  3 A having regions with their respective phases having a difference appropriately set, and rotated by angle θ to significantly prevent tracking error signal TR from having an amplitude varying increasingly/decreasingly as the objective lens shifts.  
       First Exemplary Variation of First Embodiment  
       [0083]     Optical pickup device  10  of the first embodiment shown in  FIG. 1  with diffraction grating  3 A serving as an optical branching element replaced with a diffraction grating  3 B will now be described as a first exemplary variation with reference to  FIG. 8 .  
         [0084]      FIG. 8  shows a periodical structure of diffraction grating  3 B in the first exemplary variation of the first embodiment of the present invention.  
         [0085]     As shown in the figure, the first exemplary variation provides diffraction grating  3 B divided into first, second and third regions  3   a ,  3   b  and  3   d  by a line having a direction perpendicular to that of its grating grooves, with each region having a structure with a protrusion and a depression that extend in a direction inclined relative to direction X by angle θ and are periodically repeated.  
         [0086]     The first exemplary variation provides diffraction grating  3 B having the third region  3   d  located intermediate between the first and second regions  3   a  and  3   b  and further divided in two to provide regions  3   d   1  and  3   d   2 . If a direction corresponding to that of guide groove  61  of optical disk  6  is represented as direction Y and a direction perpendicular thereto is represented as direction X, then the first exemplary variation provides diffraction grating  3 B having grating grooves set in a direction inclined relative to directions X and Y by angle θ, similarly as has been described for diffraction grating  3 A with reference to  FIG. 3 .  
         [0087]     The first exemplary variation provides diffraction grating  3 B, by way of example, having the second region  3   b  having a periodical structure 120° out of phase with the first region  3   a , and the third region  3   d  with regions  3   d   1  40° out of phase with the first region  3   a  and region  3   d   2  80° out of phase with the first region  3   a . If diffraction grating  3 B has the second region  3   b  out of phase by an amount α, then diffraction grating  3 B has region  3   d   1  set to be out of phase by an amount of α/3 an region  3   d   2  set to be out of phase by an amount of 2α/3.  
         [0088]     While  FIG. 8  shows the third region  3   d  divided in two by way of example, the region is not limited to division in two and may be divided in n and have a phase offset added stepwise, wherein n is an integer of at least 2. In that case, providing regions successively adjacent to the first region  3   a  such that they are successively out of phase by an amount of α/(n+1) will suffice. Thus dividing the third region  3   d  into multiple regions each having a phase offset by an amount added stepwise can also be as effective as or more effective than the first embodiment.  
       Second Exemplary Variation of First Embodiment  
       [0089]     Optical pickup device  10  of the first embodiment shown in  FIG. 1  with diffraction grating  3 A serving as an optical branching element replaced with a diffraction grating  3 C will now be described as a first exemplary variation with reference to  FIG. 9 .  
         [0090]      FIG. 9  shows a periodical structure of diffraction grating  3 C in the second exemplary variation of the first embodiment of the present invention.  
         [0091]     As shown in the figure, the second exemplary variation provides diffraction grating  3 C divided into first, second and third regions  3   a ,  3   b  and  3   e  by a line having a direction perpendicular to that of its grating grooves, with each region having a structure with a protrusion and a depression that extend in a direction inclined relative to direction X by angle θ or an angle (θ−Δ) and are periodically repeated.  
         [0092]     The second exemplary variation provides diffraction grating  3 C having the third region  3   e  located intermediate between the first and second regions  3   a  and  3   b . The third region  3   e  has a structure with a protrusion and a depression periodically repeated and adjacent to both those of the structure of the first region  3   a  and those of the structure of the second region  3   b . Consequently, diffraction grating  3 C has the third region  3   e  different in inclination from the first and second regions  3   a  and  3   b . If a direction corresponding to that of guide groove  61  of optical disk  6  is represented as direction Y and a direction perpendicular thereto is represented as direction X, then the second exemplary variation provides diffraction grating  3 C having grating grooves set in a direction inclined relative to directions X and Y by angle θ, similarly as has been described for diffraction grating  3 A with reference to  FIG. 3 .  
         [0093]     As has been described above, the second exemplary variation can provide diffraction grating  3 C including the third regions  3   e  having a structure with a protrusion and a depression periodically repeated and adjacent to both those of the structure of the first region  3   a  and those of the structure of the second region  3   b . Diffraction grating  3 C thus structured corresponds to diffraction grating  3 B described in the first exemplary variation that has the third region  3   d  divided in n increased indefinitely. This example is also as effective as or more effective than the first embodiment.  
       Third Exemplary Variation of First Embodiment  
       [0094]     Optical pickup device  10  of the first embodiment shown in  FIG. 1  with diffraction grating  3 A serving as an optical branching element replaced with a diffraction grating  3 D will now be described as a third exemplary variation with reference to  FIG. 10 .  
         [0095]      FIG. 10  shows a periodical structure of diffraction grating  3 D in the third exemplary variation of the first embodiment of the present invention.  
         [0096]     As shown in the figure, the third exemplary variation provides diffraction grating  3 D divided into first, second and third regions  3   a ,  3   b  and  3   f  by a line having a direction perpendicular to that of its grating grooves, with each region having a structure with a protrusion and a depression that extend in a direction inclined relative to direction X by angle θ and are periodically repeated.  
         [0097]     The third exemplary variation provides diffraction grating  3 D having the third region  3   f  located intermediate between the first and second regions  3   a  and  3   b  and alternately having the first region  3   a  structure formed of a protrusion and a depression periodically repeated and the second region  3   b  structure formed of a protrusion and a depression periodically repeated. If a direction corresponding to that of guide groove  61  of optical disk  6  is represented as direction Y and a direction perpendicular thereto is represented as direction X, then the third exemplary variation provides diffraction grating  3 D having grating grooves set in a direction inclined relative to directions X and Y by angle θ, similarly as has been described for diffraction grating  3 A with reference to  FIG. 3 .  
         [0098]     The third exemplary variation provides diffraction grating  3 D having the first and second regions  3   a  and  3   b  with a boundary therebetween in the form of a comb, and that boundary can be regarded as the third region  3   f . This example is also as effective as or more effective than the first embodiment.  
         [0099]     Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.