Patent Publication Number: US-7898927-B2

Title: Optical pickup device

Description:
RELATED APPLICATIONS 
     This application is a Divisional of U.S. application Ser. No. 12/303,578, filed on Dec. 5, 2008, now U.S. Pat. No. 7,751,297, which is the U.S. National Phase under 35 U.S.C. §371 of International Application No. PCT/JP2007/067422, filed on Sep. 6, 2007, which in turn claims the benefit of Japanese Application No. 2006-339617, filed on Dec. 18, 2006 and Japanese Application No. 2007-164510, filed on Jun. 22, 2007, the disclosures of which Applications are incorporated by reference herein. 
    
    
     TECHNICAL FIELD 
     The invention relates to an optical pickup device that is used in an optical information processor for performing processing such as recording of information onto an optical information recording medium and playback or erasure of information recorded on the optical information recording medium. 
     BACKGROUND ART 
     Reading recorded information from an optical information recording medium (optical disc) such as a CD (Compact Disc) and a DVD (Digital Versatile Disc) is conducted by converging a light beam emitted from a light source such as a semiconductor laser device on a recording track of the optical disc by using an objective lens and converting reflected light from the optical disc to an electric signal by a photodetector. In order to accurately converge a light beam on a desired recording track of a rapidly spinning optical disc, a focus error signal and a tracking error signal are detected and the position of the objective lens is controlled according to surface displacement, eccentricity, and the like of the optical disc. 
     A differential push-pull (DPP) method is known as a typical method for detecting a tracking error signal. In the DPP method, a light beam is separated into three beams: a main beam; a +1 st  order diffracted beam; and a −1 st  order diffracted beam. These three beams are respectively converged on three adjacent guide grooves formed at a prescribed pitch on the optical disc. Push-pull signals respectively obtained by detecting reflected light of the three beams and performing an arithmetic operation have a phase difference of 180 degrees between the main beam and the +1 st  and −1 st  order diffracted beams. Therefore, by performing arithmetic processing of each push-pull signal, only offset components included in the push-pull signals are selectively cancelled each other, whereby an excellent tracking error signal can be detected. Accordingly, the DDP method has been widely used especially in a DVD recording optical pickup (e.g., see Patent document 1). 
     There are various standards for currently used optical discs, and a guide groove pitch varies depending on the standards of the optical discs. For example, optical discs such as a write once type DVD-R (Recordable) and an erasable type DVD-RW (Disk ReWritable) have a guide groove pitch of 0.74 μm, and optical discs such as an erasable type DVD-RAM (Random Access Memory) has a guide groove pitch of 1.23 μm. An optical pickup device that enables recording and playback of two or more types of optical discs of different standards has been demanded. The following optical pickup device is proposed in view of this demand (e.g., see Patent document 2). 
     In the optical pickup device disclosed in Patent document 2, a special diffraction grating for separating a light beam is divided into three regions, and the phase of grating grooves periodically provided in each region is sequentially shifted by 90 degrees. A tracking error detection method using such a special diffraction grating is called an in-line DPP method, and the in-line DPP method enables stable tracking error detection on a plurality of optical information recording media having different guide groove pitches. 
     Patent document 1: Japanese Patent Publication for Opposition No. 4-34212 
     Patent document 2: Japanese Laid-Open Patent Publication No. 2004-145915 
     DISCLOSURE OF THE INVENTION 
     Problems to be Solved by the Invention 
     However, a conventional optical pickup device using the conventional in-line DPP method has the following problems. 
       FIG. 18  shows convergence spots of light beams that are converged on an optical information recording medium by a conventional optical pickup device. A convergence spot  101  corresponding to a +1 st  order diffracted beam has higher intensity on the right side in a radial direction X of the optical information recording medium and has lower intensity on the left side. On the other hand, a convergence spot  102  corresponding to a −1 st  order diffracted beam has lower intensity on the right side and has higher intensity on the left side. This can be explained as follows: 
     As shown in  FIG. 19 , in a special diffraction grating used in the conventional in-line DPP method, the phase of grating grooves  119   a  in a region  119  is ahead of that of grating grooves  120   a  in a central region  120  by 90 degrees, and the phase of grating grooves  121   a  in a region  121  is behind that of the grating grooves  120   a  in the central region  120  by 90 degrees. Accordingly, the phase of the +1 st  order diffracted beam that has passed through the region  119  is ahead of that of the +1 st  order diffracted beam that has passed through the central region  120  by 90 degrees, and the phase of the +1 st  order diffracted beam that has passed through the region  121  is behind that of the +1 st  order diffracted beam that has passed through the central region  120  by 90 degrees. The phase relation of the grating grooves and diffracted beams is opposite for the −1 st  order diffracted beam. In other words, the phase of the −1 st  order diffracted beam that has passed through the region  119  is behind that of the −1 st  order diffracted beam that has passed through the central region  120  by 90 degrees, and the phase of the −1 st  order diffracted beam that has passed through the region  121  is ahead of that of the −1 st  order diffracted beam that has passed through the central region  120  by 90 degrees. 
     Accordingly, the +1 st  order diffracted beam has larger intensity distribution on the region  121  side where the phase is retarded, and the convergence spot  101  corresponding to the +1 st  order diffracted beam on the optical information recording medium has higher intensity on the right side and lower intensity on the left side. On the other hand, the −1 st  order diffracted beam has larger intensity distribution on the region  119  side where the phase is retarded, and the convergence spot  102  corresponding to the −1 st  order diffracted beam has lower light intensity on the right side and higher intensity on the left side. 
       FIG. 20  shows signal waveforms of a DPP signal obtained by the above convergence spots. In  FIG. 20 , the ordinate indicates signal strength and the abscissa indicates a relative position of the convergence spot on the optical information recording medium. MPP is a push-pull signal of a main beam corresponding to a 0 th  order diffracted beam, SPP 1  is a push-pull signal of a preceding sub-beam corresponding to a +1 st  order diffracted beam, SPP 2  is a push-pull signal of a succeeding sub-beam corresponding to a −1 st  order diffracted beam, and DPP is a tracking error signal (differential push-pull signal) obtained from MPP, SPP 1 , and SPP 2  as shown by the formula (1):
 
 DPP=MPP−k ×( SPP 1 +SPP 2)   (1)
 
where k is an arbitrary amplification factor. Since the respective convergence spots corresponding to the +1 st  order diffracted beam and the −1 st  order diffracted beam have left-right asymmetric intensity distribution, the phase difference of SSP 1  and SSP 2  from MPP is shifted from 180 degrees. If there is a signal strength difference between SPP 1  and SPP 2 , DPP is shifted from a proper value and therefore each convergence spot cannot be formed on the same guide groove, hindering stable tracking error signal detection by the in-line DPP method. Note that, the positions where SPP 1 , SPP 2 , and DPP have proper values are shown by chain line in  FIG. 20 .
 
     The invention is made to solve the above conventional problems and it is an object of the invention to implement an optical pickup device for conducting stable tracking error detection on a plurality of optical information recording media having different guide groove pitches while maintaining the advantages of the in-line DPP method. 
     Means for Solving the Problems 
     In order to achieve the above object, an optical pickup device of the invention includes a diffraction grating that is divided into four regions having different phases from each other. 
     More specifically, an optical pickup device according to the invention is an optical pickup device for recording information onto an optical information recording medium and reading and erasing information recorded on the optical information recording medium. The optical pickup device includes: a light source; a diffraction grating for separating a light beam emitted from the light source into at least three light beams; and a photodetector for receiving the separated light beams reflected from the optical information recording medium. The diffraction grating is divided into a first region, a second region, a third region, and a fourth region having periodic structures of different phases by dividing lines extending in a direction parallel to a tangential direction of tracks of the optical information recording medium. The second region and the third region are located between the first region and the fourth region sequentially from the first region side. The periodic structure of the second region has a phase difference of approximately 180 degrees from the periodic structure of the third region, and the periodic structure of the first region has a phase difference of approximately 180 degrees from the periodic structure of the fourth region. 
     The optical pickup device of the invention includes such a diffraction grating that the periodic structure of the second region has a phase difference of approximately 180 degrees from the periodic structure of the third region and the periodic structure of the first region has a phase difference of approximately 180 degrees from the periodic structure of the fourth region. Therefore, the phase of a +1 st  order diffracted beam that has passed through the first region is advanced with respect to that of the +1 st  order diffracted beam that has passed through the second region. The phase of the +1 st  order diffracted beam that has passed through the fourth region is advanced with respect to that of the +1 st  order diffracted beam that has passed through the third region. The phase of the −1 st  order diffracted beam, on the other hand, is retarded in both cases. Accordingly, unlike the conventional in-line DPP method, spot shapes do not become left-right asymmetric, but the intensity distribution of convergence spots becomes left-right asymmetric with respect to an extending direction of guide grooves. An optical pickup device for conducting stable tracking error detection on a plurality of optical information recording media having different guide groove pitches can thus be implemented. 
     In the optical pickup device of the invention, a distance between the dividing line dividing the first region and the second region from each other and the dividing line dividing the second region and the third region from each other is preferably equal to a distance between the dividing line dividing the second region and the third region from each other and the dividing line dividing the third region and the fourth region from each other. 
     In the optical pickup device of the invention, the light beams preferably include a 0 th  order diffracted beam, a +1 st  order diffracted beam, and a − 1   st  order diffracted beam. 
     In the optical pickup device of the invention, it is preferable that a plurality of guide grooves are periodically formed on a recording surface of the optical information recording medium, and each of the light beams is converged on one of the plurality of guide grooves. 
     Preferably, the optical pickup device of the invention further includes an arithmetic processing circuit for detecting a tracking error signal by a differential push-pull method based on an output signal of the photodetector. 
     In the optical pickup device of the invention, it is preferable that the photodetector includes at least three light receiving elements respectively corresponding to the reflected light beams, and each of the light receiving elements is divided into a plurality of light receiving regions. 
     In the optical pickup device of the invention, a center of the light beam emitted from the light source is preferably positioned in the second region or the third region. 
     In the optical pickup device of the invention, it is preferable that the light source includes a first light source and a second light source, and a straight line connecting a center of a light beam emitted from the first light source and a center of a light beam emitted from the second light source crosses at least one of the dividing line dividing the first region and the second region from each other, the dividing line dividing the second region and the third region from each other, and the dividing line dividing the third region and the fourth region from each other. 
     In the optical pickup device of the invention, it is preferable that the light source includes a first light source and a second light source, and a straight line connecting a center of a light beam emitted from the first light source and a center of a light beam emitted from the second light source crosses the dividing line dividing the second region and the third region from each other. 
     In the optical pickup device of the invention, it is preferable that the light source includes a plurality of light sources, and a center of at least one of light beams respectively emitted from the plurality of light sources is positioned in the second region or the third region. 
     In the optical pickup device of the invention, the periodic structure of the first region of the diffraction grating preferably has a phase difference of 10 degrees to 350 degrees from the periodic structure of the second region. More preferably, the periodic structure of the first region of the diffraction grating has a phase difference of approximately 90 degrees from the periodic structure of the second region. 
     Preferably, the optical pickup device of the invention further includes an objective lens for converging the at least three light beams onto a recording surface of the optical information recording medium as independent convergence spots, and a region of the diffraction grating on which a range of the emitted light beam corresponding to an effective beam diameter determined by an aperture diameter of the objective lens is incident is a region including the first region, the second region, the third region, and the fourth region. 
     In this case, a sum of a width of the second region and a width of the third region is preferably in a range of 10% to 40% of the effective beam diameter. 
     Effects of the Invention 
     The invention can thus implement an optical pickup device for conducting stable tracking error detection on a plurality of optical information recording media having different guide groove pitches while maintaining the advantages of the in-line DPP method. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of an optical pickup device according to an embodiment of the invention; 
         FIG. 2  is a circuit diagram of a photodetector of the optical pickup device according to the embodiment of the invention; 
         FIG. 3  is a plan view of a diffraction grating of the optical pickup device according to the embodiment of the invention; 
         FIG. 4  is a plan view showing the shapes of convergence spots formed on a recording surface of an optical information recording medium by the optical pickup device according to the embodiment of the invention; 
         FIG. 5  is a graph showing signal waveforms obtained by the optical pickup device according to the embodiment of the invention; 
         FIG. 6  is a plan view showing an example of a positional relation between the diffraction grating of the optical pickup device according to the embodiment of the invention and the respective centers of light beams; 
         FIG. 7  is a plan view showing an example of a positional relation between the diffraction grating of the optical pickup device according to the embodiment of the invention and the respective centers of light beams; 
         FIG. 8  is a plan view showing an example of a positional relation between the diffraction grating of the optical pickup device according to the embodiment of the invention and the respective centers of light beams; 
         FIG. 9  is a plan view showing an example of a positional relation between the diffraction grating of the optical pickup device according to the embodiment of the invention and the respective centers of light beams; 
         FIG. 10  is a plan view showing an example of a positional relation between the diffraction grating of the optical pickup device according to the embodiment of the invention and the respective centers of light beams; 
         FIG. 11  is a plan view showing a modification of the diffraction grating of the optical pickup device according to the embodiment of the invention; 
         FIG. 12  is a plan view showing a modification of the diffraction grating of the optical pickup device according to the embodiment of the invention; 
         FIG. 13  is a plan view showing a modification of the diffraction grating of the optical pickup device according to the embodiment of the invention; 
         FIGS. 14A through 14C  are graphs showing relations between the shift amount of an objective lens and variation in differential push-pull signal amplitude in the optical pickup device of the embodiment of the invention, wherein  FIGS. 14A through 14C  respectively show the case where the phase difference between first and second regions is 0 degrees, 90 degrees, and 180 degrees; 
         FIG. 15  is a graph showing a relation between the phase difference between first and second regions of the diffraction grating of the optical pickup device according to the embodiment of the invention and the change rate of differential push-pull signal amplitude; 
         FIG. 16  is a graph showing a relation between the width of second and third regions of the diffraction grating of the optical pickup device according to the embodiment of the invention and the change rate of differential push-pull signal amplitude; 
         FIG. 17  is a graph showing a relation between the width of the second and third regions of the diffraction grating of the optical pickup device according to the embodiment of the invention and the push-pull signal amplitude; 
         FIG. 18  is a plan view showing the shapes of convergence spots formed on a recording surface of an optical information recording medium by a conventional optical pickup device; 
         FIG. 19  is a plan view of a diffraction grating of the conventional optical pickup device; and 
         FIG. 20  is a graph showing signal waveforms obtained by the conventional optical pickup device. 
     
    
    
     DESCRIPTION OF THE REFERENCE NUMERALS 
       11  light source 
       12  diffraction grating 
       12 A first region 
       12 B second region 
       12 C third region 
       12 D fourth region 
       12   a  grating groove 
       12   b  grating groove 
       12   c  grating groove 
       12   d  grating groove 
       15  half mirror 
       16  photodetector 
       17  integrated circuit board 
       18  collimating lens 
       19  objective lens 
       21 A light receiving element 
       21 B light receiving element 
       21 C light receiving element 
       23  arithmetic processing circuit 
       24  subtracter 
       25  subtracter 
       26  subtracter 
       27  adder 
       28  amplifier 
       29  subtracter 
       31  light beam 
       31   a  main beam 
     BEST MODE FOR CARRYING OUT THE INVENTION 
     An embodiment of the invention will be described with reference to the accompanying drawings.  FIG. 1  schematically shows a structure of an optical pickup device according to an embodiment of the invention. 
     As shown in  FIG. 1 , the optical pickup device of the embodiment includes a light source  11 , such as a semiconductor laser element, for emitting a light beam  31  for performing recording of information onto an optical information recording medium  51  and playback of information recorded on the optical information recording medium  51 , a diffraction grating  12  for diffracting and separating the light beam  31  into at least three light beams (not shown): a main beam that is a 0 th  order diffracted beam; a sub-beam that is a +1 st  order diffracted beam; and a sub-beam that is a −1 st  order diffracted beam, a half mirror  15  for guiding the separated light beams  31  to the optical information recording medium  51 , and an integrated circuit board  17  having a photodetector  16  for receiving the light beams  31  reflected from the optical information recording medium  51 . 
     A collimating lens  18  and an objective lens  19  are placed between the half mirror  15  and the optical information recording medium  51 . The light beam  31  emitted from the light source  11  is first diffracted and separated by the diffraction grating  12  into at least three light beams: a 0 th  order diffracted beam; a +1 st  order diffracted beam; and a −1 st  order diffracted beam. These beams are then reflected by the half mirror  15  and reach the objective lens  19  through the collimating lens  18 . The 0 th  order diffracted beam, the +1 st  order diffracted beam, and the −1 st  order diffracted beam thus obtained by the diffraction grating  1  are then independently converged on a recording surface of the optical information recording medium  51  by the objective lens  19  to form three convergence spots. 
       FIG. 2  shows a circuit structure of the integrated circuit board  17  having the photodetector  16  in the optical pickup device of  FIG. 1 . As shown in  FIG. 2 , the integrated circuit board  17  has light receiving elements  21 A,  21 B, and  21 C and an arithmetic processing circuit  23  for performing an arithmetic operation of signals from the light receiving elements. A main beam  31   a  and two sub-beams  31   b  and  31   c  separated from the emitted light beam  31  by the diffraction grating  12  are received by the light receiving elements  21 A,  21 B, and  21 C, respectively. Each of the light receiving elements  21 A,  21 B, and  21 C is divided into a plurality of light receiving regions. 
     Signals detected by the light receiving elements  21 A,  21 B, and  21 C are applied to the arithmetic processing circuit  23 . The arithmetic processing circuit  23  has subtracters  24 ,  25 , and  26  for receiving signals from the light receiving elements  21 A,  21 B, and  21 C, respectively, and an adder  27 , an amplifier  28 , and a subtracter  29  for receiving outputs from the subtracters  24 ,  25 , and  26 . The subtracters  24 ,  25 , and  26  receive signals from the light receiving elements  21 A,  21 B, and  21 C and output push-pull signals MPP, SPP 1 , and SPP 2 , respectively. The adder  27 , the amplifier  28 , and the subtracter  29  of the arithmetic processing circuit  23  will be described later. 
     In the circuit structure of  FIG. 2 , each light receiving element is divided into two light receiving regions. However, each light receiving element may be divided into three or more light receiving regions. In  FIG. 2 , each beam on each light receiving element is schematically shown to have a circular shape. However, the beam shape is not limited to this. When an astigmatic method is used for a focus error signal, each beam on each light receiving element is rotated by approximately 90 degrees. Each light receiving element therefore needs to be rotated by 90 degrees in advance in this case. 
     The optical pickup device of this embodiment is characterized in the diffraction grating  12  for diffracting the light beam  31  emitted from the light source  11 , and is characterized especially in a periodic structure of the diffraction grating  12 .  FIG. 3  shows a periodic structure, that is, a grating pattern, of the diffraction grating  12 . 
     As shown in  FIG. 3 , a grating surface of the diffraction grating  12  is divided into four regions: a first region  12 A; a second region  12 B; a third region  12 C; and a fourth region  12 D by three dividing lines D 1 , D 2 , and D 3  extending in an extending direction of guide grooves of the optical information recording medium  51  (hereinafter, referred to as Y direction), that is, in a direction substantially parallel to a tangential direction of tracks of the optical information recording medium  51 . Note that, in this case, the parallel direction means a parallel direction in view of an optical system provided between the diffraction grating  12  and the optical information recording medium  51 . The first region  12 A and the second region  12 B are adjacent to each other with the dividing line D 1  interposed therebetween. The second region  12 B and the third region  12 C are adjacent to each other with the dividing line D 2  interposed therebetween. The third region  12 C and the fourth region  12 D are adjacent to each other with the dividing line D 3  interposed therebetween. 
     As shown in  FIG. 3 , the first region  12 A, the second region  12 B, the third region  12 C, and the fourth region  12 D have grating grooves  12   a,    12   b,    12   c,  and  12   d  periodically provided along a radial direction of the optical information recording medium  51  (hereinafter, referred to as X direction), respectively. Note that, in this embodiment, the grating grooves  12   a,    12   b,    12   c,  and  12   d  have the same width, and the portions (protruding portions) between the grating grooves  12   a,    12   b,    12   c,  and  12   d  have the same width. 
     The phase of the periodic structure formed by the grating grooves  12   a  in the first region  12 A is substantially 90 degrees ahead of that of the periodic structure formed in the second region  12 B (a phase difference of substantially +90 degrees). In other words, the arrangement cycle of the grating grooves  12   a  in the first region  12 A is shifted by quarter cycle from the arrangement cycle of the grating grooves  12   b  in the second region  12 B in +Y direction. The phase of the periodic structure formed in the fourth region  12 D is substantially 90 degrees behind that of the periodic structure formed in the second region  12 B (a phase difference of substantially −90 degrees). In other words, the arrangement cycle of the grating grooves  12   d  in the fourth region  12 D is shifted by quarter cycle from the arrangement cycle of the grating grooves  12   b  in the second region  12 B in −Y direction. Accordingly, the periodic structure in the first region  12 A has a phase difference of substantially 180 degrees from the periodic structure in the fourth region  12 D. The phase of the periodic structure in the third region  12 C is shifted by substantially 180 degrees from that of the periodic structure in the second region  12 B. In other words, the arrangement cycle of the grating grooves  12   c  in the third region  12 C is shifted by half cycle from the arrangement cycle of the grating grooves  12   b  in the second region  12 B in +Y direction. 
     Note that the phase difference of the periodic structure between the regions does not have to be exactly 90 degrees or 180 degrees. Since the convergence spots on the recording surface of the optical information recording medium  51  need only have a shape described below, the phase difference between regions may include an error of about ±10 degrees. 
     In this embodiment, as shown in  FIG. 3 , the center (the center of a light emitting point) L 1  of the light beam emitted from the light source  11  is positioned on the dividing line D 2  within the assembly accuracy range of the device. 
     The emitted light beam incident on the diffraction grating  12  is separated into a main beam and sub-beams having a prescribed phase difference by the respective periodic structures formed in the first region  12 A, the second region  12 B, the third region  12 C, and the fourth region  12 D. The main beam and the sub-beams are then guided to the optical information recording medium  51 . 
     Hereinafter, the reason why the optical pickup device of this embodiment can stably detect tracking errors on optical information recording media having different guide groove pitches will be described. 
       FIG. 4  shows the shapes of respective convergence spots of the main beam  31   a  and two sub-beams  31   b  and  31   c  of the emitted light beam which are generated by the diffraction grating  12  on the recording surface of the optical information recording medium  51 . In  FIG. 4 , X direction shows the radius direction of the optical information recording medium and Y direction shows the extending direction of the guide grooves. 
     In the diffraction grating  12 , the diffraction grating in the second region  12 B has a phase difference of 180 degrees from the diffraction grating in the third region  12 C. Therefore, diffracted light that has passed through the second region  12 B and diffracted light that has passed through the third region  12 C cancel each other, and the respective convergence spots of the sub-beams  31   b  and  31   c  on the recording surface of the optical information recording medium  51  in  FIG. 4  have lower intensity in the center. Since the respective convergence spots of the sub-beams  31   b  and  31   c  need only have lower intensity in the center, the phase difference between the second region  12 B and the third region  12 C may include an error of about ±10 degrees from 180 degrees. 
     The phase of the diffraction grating in the first region  12 A is 90 degrees ahead of that of the diffraction grating in the second region  12 B. The phase of the diffraction grating in the fourth region  12 D is 90 degrees ahead of that of the diffraction grating in the third region  12 C. Accordingly, the phase of the +1 st  order diffracted beam that has passed through the first region  12 A is advanced by 90 degrees from that of the +1 st  order diffracted beam that has passed through the second region  12 B. The phase of the +1 st  order diffracted beam that has passed through the fourth region  12 D is advanced by 90 degrees from that of the +1 st  order diffracted beam that has passed through the third region  12 C. The phase of the −1 st  order diffracted beam, on the other hand, is retarded by 90 degrees. Accordingly, unlike the conventional in-line DPP method, spot shapes do not become left-right asymmetric, but the intensity distribution of the convergence spots becomes left-right asymmetric with respect to Y direction. In this case as well, the phase difference between the first region  12 A and the second region  12 B and the phase difference between the fourth region  12 D and the third region  12 C may include an error of about ±10 degrees from 90 degrees. 
     As shown in  FIG. 4 , a plurality of guide grooves  51   a  are periodically formed on the recording surface of the optical information recording medium  51 . The respective convergence spots of the main beam  31   a,  the sub-beam  31   b,  and the sub-beam  31   c  of the light beam  31  converged by the objective lens  19  are located on the same guide groove  51   a,  as shown in  FIG. 4 . 
     The main beam  31   a,  the sub beam  31   b,  and the sub beam  31   c  are reflected at the respective convergence spots, and the reflected light beams corresponding to the respective convergence spots are respectively received by the light receiving elements  21 A,  21 B, and  21 C of the photodetector  16 . The light receiving elements  21 A,  21 B, and  21 C output a push-pull signal MPP corresponding to the main beam  31   a,  a push-pull signal SPP 1  corresponding to the sub-beam  31   b,  and a push-pull signal SPP 2  corresponding to the sub-beam  31   c,  respectively. 
     Offset components of the push-pull signals MPP, SPP 1 , and SPP 2  resulting from a radial shift of the objective lens  19  (a shift in the radius direction of the optical information recording medium) and a tilt of the optical information recording medium  51  are generated on the same side (the same phase) for each of the radial shift of the objective lens  19  and the tilt of the optical information recording medium  51 . Accordingly, a differential push-pull (DPP) signal obtained by cancelling the offsets resulting from the radial shift of the objective lens  19  and the tilt of the optical information recording medium  51  can be detected by performing an arithmetic operation shown by the formula (2) by using the adder  27 , the amplifier  28 , and the subtracter  29  shown in  FIG. 2 :
 
 DPP=MPP−k ×( SPP 1 +SPP 2)   (2)
 
where k is an amplification factor of the amplifier  28 .
 
       FIG. 5  shows respective output waveforms of the push-pull signals MPP, SPP 1 , and SPP 2  and the DPP signal obtained by the formula (2). In  FIG. 5 , the ordinate indicates signal strength and the abscissa indicates a relative position of the convergence spot on the optical information recording medium  51 . As shown in  FIG. 5 , SPP 1  and SPP 2  have a phase difference of exactly 180 degrees from MPP. Even if there is a signal strength difference between SPP 1  and SPP 2 , the DPP signal obtained by the formula (2) has a proper value and each convergence spot can therefore be formed on the same guide groove. 
     As shown in  FIG. 2 , the inputs of the adder  27  are respectively connected to the respective outputs of the subtracters  25  and  26 , and the input of the amplifier  28  is connected to the output of the adder  27 . The inputs of the subtracter  29  are respectively connected to the output of the subtracter  24  and the output of the amplifier  28 . The arithmetic operation shown by the formula (2) can be performed with this structure. The coefficient k in the formula (2) is used to correct the difference in light intensity among the main beam  31   a,  the sub-beam  31   b,  and the sub-beam  31   c  that are reflected from the optical information recording medium  51 . When the light intensity ratio of the main beam  31   a,  the sub-beam  31   c,  and the sub-beam  31   c  is a:b:b, the coefficient k is a/2b. In other words, the coefficient k is a constant that is determined according to the type of the optical information recording medium  51 . Note that a signal processing circuit having a conventional structure may be used in this embodiment. 
     The optical information recording medium  51  is not limited to a specific type, and DVDs including a DVD-ROM, a DVD-RAM, a DVD-R, and a DVD-RW and CDs including a CD-ROM, a CD-R, and a CD-RW may be used as the optical information recording medium  51 . The wavelength of the light beam  31  can be determined according to the type of the optical information recording medium  51 , and is in the range of about 650 nm to about 780 nm in the case of a DVD and a CD. As for DVDs, stable tracking error signal detection can be performed on both a DVD having a guide groove pitch of 0.74 μm such as a DVD-R and a DVD having a guide groove pitch of 1.23 μm such as a DVD-RAM. 
     In this embodiment, the diffraction grating  12  is placed between the light source  11  and the half mirror  15  in the optical system shown in  FIG. 1 . However, the diffraction grating  12  may alternatively be placed, for example, between the half mirror  15  and the collimating lens  18 . The optical system of  FIG. 1  may be replaced with an optical system in which a light source and a photodetector are integrated (for example, an optical system that does not use a half mirror), and the diffraction grating may be placed between the light source and the collimating lens. 
     In this embodiment, the grating grooves in each region of the diffraction grating  12  are formed along X direction, that is, the radius direction of the optical information recording medium. However, the grating grooves may alternatively be formed in a direction oblique to X direction. In this embodiment, the second region  12 B and the third region  12 C of the diffraction grating  12  have the same width. However, the second region  12 B and the third region  12 C of the diffraction grating  12  need not necessarily have the same width. 
     In this embodiment, a single light beam is emitted from the light source. However, the same effects can be obtained even when an optical pickup device has a plurality of light sources and a plurality of light beams are emitted from the light sources. 
     For example, as shown in  FIG. 6 , the center L 1  of a light emitting point of a first light source may be positioned on the dividing line D 2  and the center L 2  of a light emitting point of a second light source may be positioned on the dividing line D 3  within the assembly accuracy range of the device. Alternatively, as shown in  FIG. 7 , the center L 1  of a light emitting point of a first light source may be positioned on the dividing line D 2  and the center L 2  of a light emitting point of a second light source may be positioned within the fourth region  12 D near the dividing line D 3  within the assembly accuracy range of the device. Alternatively, as shown in  FIG. 8 , the center L 1  of a light emitting point of a first light source may be positioned within the second region  12 B and the center L 2  of a light emitting point of a second light source may be positioned within the third region  12 C within the assembly accuracy range of the device. Alternatively, as shown in  FIG. 9 , the center L 1  of a light emitting point of a first light source may be positioned within the first region  12 A near the dividing line D 1  and the center L 2  of a light emitting point of a second light source may be positioned within the fourth region  12 D near the dividing line D 3  within the assembly accuracy range of the device. 
     The number of light sources is not limited to two, and three or more light sources may be provided. For example, in the case where three light sources are provided, as shown in  FIG. 10 , the center of a light emitting point of at least one light source is positioned within the second region  12 B or the third region  12 C within the assembly accuracy range of the device and the center of a light emitting point of the remainder of the light sources need not be specifically limited. 
     In this embodiment, the entire diffraction grating  12  is divided into the first region  12 A, the second region  12 B, the third region  12 C, and the fourth region  12 D. However, a region within an effective beam diameter range determined by the aperture diameter of the objective lens in the diffraction grating  12  need only be divided into the first to fourth regions, and the region outside the effective beam diameter range may have a different dividing state. For example, as shown in  FIG. 11 , the second region  12 B and the second region  13 B may not be formed in the region outside the effective beam diameter range. 
     In the example shown in this embodiment, the phase difference between the periodic structures of the first region  12 A and the periodic structure of the second region  12 B is 90 degrees and the phase difference between the periodic structure of the fourth region  12 D and the periodic structure of the third region  12 C is 90 degrees. However, the phase difference between the first region  12 A and the second region  12 B and the phase difference between the third region  12 C and the fourth region  12 D may be any value as long as the phase difference between the first region  12 A and the fourth region  12 D is substantially 180 degrees and the phase difference between the second region  12 B and the third region  12 C is substantially 180 degrees. In view of the manufacturing error of the periodic structure of the diffraction grating  12 , however, the phase difference between the first region  12 A and the second region  12 B and the phase difference between the fourth region  12 D and the third region  12 C are preferably in the range of 10 degrees and 350 degrees, and more preferably in the range of 70 degrees and 290 degrees. 
       FIG. 12  shows an example in which the phase difference between the first region  12 A and the fourth region  12 D is substantially 180 degrees, the phase difference between the second region  12 B and the third region  12 C is substantially 180 degrees, and the phase difference between the first region  12 A and the second region  12 B and the phase difference between the fourth region  12 D and the third region  12 C are substantially 45 degrees.  FIG. 13  shows an example in which the phase difference between the periodic structure of the first region  12 A and the periodic structure of the fourth region  12 D is substantially 180 degrees, the phase difference between the periodic structure of the second region  12 B and the periodic structure of the third region  12 C is substantially 180 degrees, and the phase difference between the first region  12 A and the second region  12 B and the phase difference between the fourth region  12 D and the third region  12 C are substantially 180 degrees. The same effects as those of the diffraction grating of  FIG. 3  can be obtained in these cases as well. 
       FIGS. 14A through 14C  show relations between the shift amount of the objective lens  19  and the DPP signal amplitude in the case where a DVD-RAM is used as the optical information recording medium  51 .  FIGS. 14A through 14C  show examples in which the phase difference between the first region  12 A and the second region  12 B is 0 degrees, 90 degrees, and 180 degrees, respectively. Note that, in  FIGS. 14A through 14C , the ordinate shows the DPP signal amplitude normalized assuming that the DPP signal amplification value is 100% when the shift amount of the objective lens  19  is 0 μm. The phase difference between the periodic structure of the first region  12 A and the periodic structure of the fourth region  12 D is substantially 180 degrees and the phase difference between the periodic structure of the second region  12 B and the periodic structure of the third region  12 C is substantially 180 degrees. 
     As shown in  FIG. 14 , in the case where the phase difference between the periodic structure of the first region  12 A and the periodic structure of the second region  12 B is 0 degrees, the DPP signal amplitude changes more with shifting of the objective lens than in the case where the phase difference between the first region  12 A and the second region  12 B is 180 degrees. 
       FIG. 15  shows a relation between the phase difference between the first region  12 A and the second region  12 B and the change rate of DPP signal amplitude. The change rate of DPP signal amplitude is shown as a ratio of the DPP signal amplitude obtained when the shift amount of the objective lens is 300 μm to the DPP signal amplitude obtained when the shift amount of the objective lens is 0 μm. 
     As shown in  FIG. 15 , the change rate of DPP signal amplitude is closest to 100% when the phase difference between the periodic structure of the first region  12 A and the periodic structure of the second region  12 B is 180 degrees. The change rate of DPP signal amplitude decreases as the phase difference gets closer to 0 degrees or 360 degrees. 
     It is preferable in the optical pickup that the DPP signal amplitude is constant even if the objective lens is shifted. It is therefore more preferable that the change rate of DPP signal amplitude is closer to 100%. The phase difference between the periodic structure of the first region  12 A and the periodic structure of the second region  12 B can therefore be in the range of 10 degrees to 350 degrees. For more uniform DPP signal amplitude, however, it is preferable that the phase difference between the first region  12 A and the second region  12 B is in the range of 70 degrees to 290 degrees. 
     In order to reduce the change rate of DPP signal amplitude, it is preferable that a light beam that is effectively used includes a portion that has passed through the first region  12 A of the diffraction grating  12 , a portion that has passed through the second region  12 B, a portion that has passed through the third region  12 C, and a portion that has passed through the fourth portion  12 D. In other words, it is preferable that a region of the diffraction grating  12  on which a range of the emitted light beam corresponding to the effective beam diameter determined by the aperture diameter of the objective lens  19  is incident includes the first region  12 A through the fourth region  12 D. 
     More specifically, in the case where the light source is a DVD-type light source, it is preferable that the sum of the width W 1  of the second region  12 B and the width W 2  of the third region  12 C of the diffraction grating  12 , that is, the width (W 1 +W 2 ), is in the range of 10% to 40% of the effective beam diameter determined by the aperture diameter of the objective lens  19 . The effects obtained by this structure will now be described. 
     First, description will be given to the case where the width (W 1 +W 2 ) is smaller than 10% of the effective beam diameter determined by the aperture diameter of the objective lens  19 .  FIG. 16  shows a relation between the change rate of DPP signal amplitude and the ratio of the width (W 1 +W 2 ) to the effective beam diameter determined by the aperture diameter of the objective lens  19  in a DVD-RAM. Note that the change rate of DPP signal amplitude is shown as a ratio of the DPP signal amplitude obtained when the shift amount of the objective lens is 300 μm to the DPP signal amplitude obtained when the shift amount of the objective lens is 0 μm. As shown in  FIG. 16 , when the width (W 1 +W 2 ) is smaller than 10% of the effective beam diameter determined by the aperture diameter of the objective lens  19 , the change rate of DPP signal amplitude is smaller than 66%. The DPP signal amplitude thus decreases by 34% or more of the DPP signal amplitude obtained when the shift amount of the objective lens is 0 μm. Such a large change in DPP signal amplitude is not preferable. 
     Next, description will be given to the case where the width (W 1 +W 2 ) is larger than 40% of the effective beam diameter determined by the aperture diameter of the objective lens  19 .  FIG. 17  shows a relation between the sum (SPP amplitude) of signal amplitude of the push-pull signal SPP 1  and signal amplitude of the push-pull signal SPP 2  and the ratio of the width (W 1 +W 2 ) to the effective beam diameter determined by the aperture diameter of the objective lens  19 . As shown in  FIG. 17 , when the width (W 1 +W 2 ) is larger than 40% of the effective beam diameter determined by the aperture diameter of the objective lens  19 , the signal amplitude becomes closer to 0. Since the absolute value of the signal amplitude is reduced, characteristics of the optical pickup device are degraded. 
     It is therefore preferable that the width (W 1 +W 2 ) is in the range of 10% to 40% of the effective beam diameter determined by the aperture diameter of the objective lens. 
     When two light sources of DVD-type and CD-type are used, it is preferable that the width (W 1 +W 2 ), that is, the sum of the width W 1  of the second region  12 B and the width W 2  of the third region  12 C of the diffraction grating  12 , is in the range of 10% to 35% of the DVD effective beam diameter determined by the aperture diameter of the objective lens  19 . 
     As has been described above, the optical pickup device of this embodiment can be used for various optical information recording media having different guide groove pitches and achieves tracking error signal detection that enables more stable recording and playback. In other words, the optical pickup device of this embodiment can implement size reduction, simplification, cost reduction, improved efficiency, and the like in DVD- and CD-type recording devices and playback devices. Moreover, the optical pickup device of this embodiment is very useful as an optical pickup device having a signal detection function such as a playback signal, a recording signal, and various servo signals which are used in an optical head device that serves as a main part of an optical information processor for performing processing, such as recording, playback, and erasure of information, on an optical information recording medium such as an optical disc. 
     INDUSTRIAL APPLICABILITY 
     The invention can implement an optical pickup device for performing stable tracking error detection on a plurality of optical information recording media having different guide groove pitches while maintaining the advantages of the in-line DPP method. The optical pickup device of the invention is useful as, for example, an optical pickup device that is used in an optical information processor for performing processing such as recording of information onto an optical information recording medium and playback or erasure of information recorded on an optical information recording medium.