Patent Document

TECHNICAL FIELD  
       [0001]     The present invention relates to an optical information device and an information recording and reproduction device, and more particularly relates to an optical information device for recording, reproducing, or erasing information to or from an optical recording medium, and to an information recording and reproduction device that uses the optical information device for recording, reproducing, or erasing information to or from an optical recording medium.  
       BACKGROUND ART  
       [0002]     Recent years have witnessed the practical application of a type of optical disk of high density and capacity, called a DVD, and this type of disk has gained widespread acceptance as an information medium capable of handling large amounts of information, such as video images.  
         [0003]      FIG. 13  shows the structure of a conventional optical pickup used in recording and reproduction with an optical recording medium such as this. Here, by irradiating the optical recording medium with three light beams, a tracking error signal is detected (see Patent Document  1 , for example).  
         [0004]     A light source  1  comprising of a semiconductor laser or the like emits a divergent beam  70  of linearly polarized light with a wavelength λ 1  of 405 nm. The divergent beam  70  emitted from the light source  1  is converted into parallel light by a collimating lens  53  with a focal distance fl of 15 mm, after which the beam is incident on a polarized beam splitter  52 . The incident beam  70  passes through the polarized beam splitter  52 , then passes through a quarter-wave plate  54 , and is thereby converted into circularly polarized light, after which it is converted into a focused beam by an objective lens  56  with a focal distance f2 of 2 mm, then passes through a transparent substrate  41  of an optical recording medium  40 , and is converged on an information recording plane  40   b . The opening of the objective lens  56  is limited by an aperture  55 , with the numerical aperture NA set to 0.85. The thickness of the transparent substrate  41  is 0.1 mm. The optical recording medium  40  has an information recording plane  40   b . A continuous groove that serves as a track is formed in the optical recording medium  40 , and the track pitch tp is 0.32 μm.  
         [0005]     The beam  70  reflected by the information recording plane  40   b  passes through the objective lens  56  and the quarter-wave plate  54  and is thereby converted into linearly polarized light that its direction is different by 90 degrees with respect to the outward path, after which the beam is reflected by the polarized beam splitter  52 . The beam  70  reflected by the polarized beam splitter  52  passes through a converging lens  59  with a focal distance f3 of 30 mm and is thereby converted into focused light, then is incident on a photodetector  30  via a cylindrical lens  57 . Astigmatism is imparted to the beam  70  as it passes through the cylindrical lens  57 .  
         [0006]     The photodetector  30  has four light receivers  30   a  to  30   d . The light receivers  30   a  to  30   d  output current signals I 30   a  to I 30   d  corresponding to the light intensity received by each.  
         [0007]     A focus error (hereinafter referred to as FE) signal produced by an astigmatism method is obtained by (I 30   a +I 30   c )−(I 30   b +I 30   d ). A tracking error (hereinafter referred to as TE) signal produced by push-pull method is obtained by (I 30   a  +I 30   d )−(I 30   b +I 30   c ). An information signal recorded to the optical recording medium  40  (hereinafter referred to as RF) is obtained by I 30   a +I 30   b +I 30   c +I 30   d . The FE signal and TE signal are amplified to the desired level and subjected to phase compensation, after which they are supplied to actuators  91  and  92  and subjected to focus and tracking control.  
         [0008]     In general, to increase the volume of information that can be stored in a single optical recording medium  40 , as the track pitch is narrowed, the accuracy of track production has to be increased by a corresponding amount. Actually, however, since a certain absolute amount of error is present, as the track pitch is narrowed, there is a relative increased in the amount of production error with respect to track pitch. Therefore, the effect of this error is far greater than with a DVD.  
         [0009]      FIG. 14  is a graph of the TE signal obtained when the beam  70  was scanned at a right angle to the tracks formed in the optical recording medium  40 . Tn−4, . . . , Tn+4 shown on the horizontal axis indicate the tracks formed in the information recording plane  40   b  of the optical recording medium  40 . The solid lines extending vertically in the graph indicate the central locations of the tracks Tn−4, . . . , Tn+4 when the track pitch is formed consistently as tp. Here, tracks Tn−1 and Tn are formed at locations that are offset from the locations where tracks Tn−1 and Tn are supposed to have been formed, by Δn−1 and Δn, respectively. Δn −1 is +25 nm, and Δn is −25 nm. As a result, the amplitude of the TE signal fluctuates greatly. Here, we will let S 1  be the minimum amplitude in the vicinity of track Tn −1, and S 2  the maximum amplitude. The location of the zero cross point of the TE signal is offset from the centers of tracks Tn −1 and Tn. Here, we will let oft 1  by the offset of track Tn −1, and oft 2  that of track Tn. Specifically, the offset oft 1  and the offset oft 2  express the off-track amount.  
         [0010]     The amount of fluctuation in the TE signal amplitude is defined as ΔPP=(amplitude S 2 −amplitude S 1 )/(amplitude S 2 +amplitude S 1 ), and when a TE signal is detected with a conventional configuration such as that described above, the amount of fluctuation ΔPP is 0.69, the offset oft 1  is +33 nm, and the offset oft 2  is −33 nm, which are large values. When the TE signal amplitude thus fluctuates so greatly (at a large fluctuation ΔPP), there is a decrease in the gain of tracking control in tracks Tn −1 and Tn, tracking control becomes unstable, and information can no longer be recorded and reproduced at high reliability.  
         [0011]     Patent Document 1: Japanese published unexamined patent application H3-005927  
       DISCLOSURE OF THE INVENTION  
       [0012]     It is an object of the present invention to provide an optical pickup head device, optical information device, and information reproduction method with which fluctuation in the TE signal amplitude is reduced and information can be recorded or reproduced at high reliability.  
         [0013]     To solve the above problems, the optical information device pertaining to the present invention comprises a light source for emitting a light beam, a focusing unit for converging the light beam emitted from the light source onto a predetermined information recording plane of an optical recording medium having a plurality of information recording planes, a beam splitting unit for splitting the light beam reflected by the optical recording medium, and a light detection unit having a light receiver that receives the light beam split by the beam splitting unit, for outputting a signal corresponding to the light intensity of the light beams received by the light receiver, wherein a guide groove is formed on at least one of the information recording planes, and the light receiver is entirely disposed within a map formed on the light detection unit by the light beam reflected by an information recording plane other than the predetermined information recording plane (hereinafter referred to as non-convergence plane).  
         [0014]     This reduces fluctuation in the tracking error signal amplitude and allows information to be recording or reproduced at high reliability.  
         [0015]     The optical information device pertaining to the present invention further comprises an astigmatism generation unit disposed along the optical path between the focusing unit and the light receiver.  
         [0016]     The astigmatism generation unit is preferably a cylindrical lens.  
         [0017]     Further, the light detection unit has a plurality of the light receivers, and each of the light receivers is disposed so as to receive the light beam reflected by the same non-convergence plane.  
         [0018]     At least a portion of the plurality of light receivers are disposed roughly adjacent to each other, and the light beams split by the beam splitting unit are each received by said portion of the light receivers.  
         [0019]     The beam splitting unit has at least first to fourth regions, and the light beam split in the first to forth regions is incident on the light receiver that outputs a signal for producing a tracking error signal.  
         [0020]     The light beams split in the first and second regions mainly include 1 st  order diffracted light diffracted by the track of the optical recording medium, the light beams split in the third and fourth regions mainly include zero-order diffracted light diffracted by the track of the optical recording medium, the light detection unit has first to fourth light receivers that receive the light beams split in the first to fourth regions, respectively, and the tracking error signal is expressed by (I 1 −I 2 )−K·(I 4 −I 3 ), where I 1  to I 4  are signals outputted from the first to fourth light receivers, and K is a real number.  
         [0021]     The optical information device pertaining to the present invention is such that the beam splitting unit has at least first to fourth regions, the light beams split in the first and second regions mainly include  1   st  order diffracted light diffracted by the track of the optical recording medium, the light beams split in the third and fourth regions mainly include zero-order diffracted light diffracted by the track of the optical recording medium, the light detection unit has first to fourth light receivers that receive the light beams split in the first to fourth regions, respectively, and a fifth light receiver disposed at a location where the light beams split by the beam splitting unit are not received, and the tracking error signal is expressed by ((I 1 −I 2 )−K·(I 4 −I 3 ))−L·15, where I 1  to I 5  are signals outputted from the first to fifth light receivers, and K and L are real numbers.  
         [0022]     This reduces fluctuation in the tracking error signal amplitude and allows information to be recording or reproduced at high reliability.  
         [0023]     Also, the beam splitting unit is a diffraction grating, and a tracking error signal is produced by using the +1 st  order diffracted light or −1 st  order diffracted light diffracted by the diffraction grating.  
         [0024]     The light receivers for receiving the +1 st  order diffracted light and −1 order diffracted light diffracted by the diffraction grating are disposed substantially in axially symmetric locations on either side of the optical axis of the zero-order diffracted light of the diffraction grating, and a tracking error signal is produced using both +1 st  order diffracted light and −1 st  order diffracted light.  
         [0025]     The optical information device also comprises a light source for emitting a light beam, a focusing unit for converging the light beam emitted from the light source onto a predetermined information recording plane of an optical recording medium having a plurality of information recording planes, a beam splitting unit for splitting the light beam reflected by the optical recording medium, an opening limiting unit disposed in the vicinity of the beam splitting unit, and a light detection unit having a light receiver that receives the light beam split by the beam splitting unit, for outputting a signal corresponding to the light intensity of the light beams received by the light receiver, wherein a guide groove is formed on at least one of the information recording planes, the light detection unit has at least a first light receiver for focus control and a second light receiver for tracking control, and the second light receiver is disposed outside a map formed on the light detection unit by the light beam reflected by an information recording plane other than the predetermined information recording plane (hereinafter referred to as non-convergence plane) and limited by the opening limiting unit.  
         [0026]     This reduces fluctuation in the tracking error signal amplitude and allows information to be recording or reproduced at high reliability.  
         [0027]     The optical information device further comprises an astigmatism generation unit disposed along the optical path between the focusing unit and the light receivers.  
         [0028]     The astigmatism generation unit is preferably a cylindrical lens.  
         [0029]     The second light receiver is disposed, with respect to the first light receiver, in a disposition direction other than the direction in which the light beam is diffracted by the track of the optical recording medium.  
         [0030]     The disposition direction is a direction rotated by approximately 40 to 50 degrees from the direction in which the light beam is diffracted by the track of the optical recording medium.  
         [0031]     The second light receiver has a plurality of light receiving regions disposed roughly adjacent to each other, and the light beams split by the beam splitting unit are received in these light receiving regions.  
         [0032]     Also, the beam splitting unit has at least four regions, and the light beams split in the regions are incident on the second light receiver that outputs a signal for producing a tracking error signal.  
         [0033]     Also, the beam splitting unit is preferably a diffraction grating.  
         [0034]     Also, the opening limiting unit is preferably formed integrally with the beam splitting unit.  
         [0035]     The information recording and reproduction device pertaining to the present invention comprises any of the above optical information devices, a transfer controller for moving the optical information device, a controller for controlling the optical information device and the transfer controller, a recording and reproduction unit for recording and/or reproducing information to or from an optical recording medium using the optical information device, and a rotating unit for rotationally moving the optical information device.  
         [0036]     The present invention provides an optical information device with which fluctuation in the tracking error signal amplitude is reduced and information can be recorded or reproduced at high reliability. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0037]      FIG. 1  is a diagram of the structure of an optical information device in Embodiment 1 of the present invention;  
         [0038]      FIG. 2  is a diagram of the simplified structure of an optical pickup in the optical information device of Embodiment 1 of the present invention;  
         [0039]      FIG. 3  is a diagram of the structure of a beam splitting element that forms part of the optical information device of Embodiment 1 of the present invention;  
         [0040]      FIG. 4  is a diagram of the structure of a photodetector that forms part of the optical information device of Embodiment 1 of the present invention;  
         [0041]      FIG. 5  is a diagram of the structure of a photodetector that forms part of the optical information device of Embodiment 1 of the present invention;  
         [0042]      FIG. 6  is a diagram of the structure of a photodetector that forms part of the optical information device of Embodiment 1 of the present invention;  
         [0043]      FIG. 7  is a diagram of the structure of a photodetector that forms part of the optical information device of Embodiment 1 of the present invention;  
         [0044]      FIG. 8  is a diagram of the structure of a photodetector that forms part of the optical information device of Embodiment  2  of the present invention;  
         [0045]      FIG. 9  is a diagram of the structure of a photodetector that forms part of the optical information device of Embodiment  3  of the present invention;  
         [0046]      FIG. 10  is a diagram of the simplified structure of an optical information device of Embodiment  4  of the present invention;  
         [0047]      FIG. 11  is a diagram of the structure of the opening limiting unit that forms part of the optical information device of Embodiment  4  of the present invention;  
         [0048]      FIG. 12  is a diagram of the structure of a photodetector that forms part of the optical information device of Embodiment  4  of the present invention;  
         [0049]      FIG. 13  is a diagram of the structure of an optical pickup head device that forms part of a conventional optical information device; and  
         [0050]      FIG. 14  is a diagram of the TE signal obtained with a conventional optical information device. 
     
    
     NUMERICAL REFERENCES  
       [0051]      32  to  35  photodetectors  
         [0052]      32   a  to  32   h ,  33   a  to  33   i ,  35   a  to  35   h  light receivers  
         [0053]      40  optical recording medium  
         [0054]      52  polarized beam splitter  
         [0055]      53  collimating lens  
         [0056]      54  wave plate  
         [0057]      56  objective lens  
         [0058]      57  cylindrical lens  
         [0059]      59  converging lens  
         [0060]      60  to  62  beam splitting elements (diffraction gratings)  
         [0061]      70  to  73 ,  71   a  to  71   h  beams  
         [0062]      91 ,  92  actuator  
         [0063]      93  spherical aberration correction unit  
         [0064]      201 ,  202  optical pickup head device  
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0065]     The optical information device pertaining to the present invention and embodiments of an optical pickup device and an optical information reproduction method will now be described through reference to the drawings. Components that are numbered the same in the drawings have either the same structure element or the same action and operation.  
       Embodiment 1  
       [0066]     FIG. 1  is a diagram of the structure of an optical information device in Embodiment 1 of the present invention.  
         [0067]     An optical pickup head device  201  (also called an optical pickup) irradiates an optical recording medium  40  with a laser beam having a wavelength μ of 405 nm, and a signal recorded to the optical recording medium  40  is reproduced. A transfer controller  205  moves the optical pickup device  201  in the radial direction of the optical recording medium  40  in order to record or reproduce information at the desired location on the optical recording medium  40 . A motor  206  that drives the optical recording medium  40  rotates the optical recording medium  40 . A controller  207  controls the optical pickup head device  201 , the transfer controller  205 , and the motor  206 .  
         [0068]     An amplifier  208  amplifies a signal read by the optical pickup head device  201 . An output signal from the amplifier  208  is inputted to a controller  209 . On the basis of this signal, the controller  209  produces a servo signal such as an FE signal, TE signal, and so forth that is required by the optical pickup device  201  in the reading of the signal of the optical recording medium  40 , and the resulting signal is outputted to the controller  207 . The signal inputted to the controller  209  is an analog signal, but this analog signal is digitized (binarized) by the controller  209 . A demodulator  210  analyzes the signal that has been read from the optical recording medium  40  and digitized, and reconstructs the original data (such as a video image or music), and the reconstructed signal is outputted from an output device  214 .  
         [0069]     A detector  211  detects an address signal and so forth on the basis of the signal outputted from the controller  209 , and this signal is outputted to a system controller  212 . The system controller  212  identifies the optical recording medium  40 , decodes recording and reproduction conditions and so forth, and controls the overall optical information device on the basis of optical recording medium manufacture information (optical recording medium management information) and physical format information read from the optical recording medium  40 . When information is to be recorded to or reproduced from the optical recording medium  40 , the controller  207  controls the drive of the transfer controller  205  according to a command from the system controller  212 . As a result, as shown in  FIG. 1 , the transfer controller  205  moves the optical pickup head device  201  to the desired location on the information recording plane formed on the optical recording medium  40  (discussed below), and the optical pickup head device  201  records or reproduces information to or from the information recording plane of the optical recording medium  40 .  
         [0070]      FIG. 2  is a diagram of an example of the structure of the optical pickup head device  201  pertaining to the present invention.  
         [0071]     A light source  1  emits a divergent beam  70  of linearly polarized light with a wavelength μ of 405 nm. The divergent beam  70  emitted from the light source  1  is converted into parallel light by a collimating lens  53  with a focal distance fl of 18 mm, after which the beam passes through a polarized beam splitter  52 , then passes through a quarter-wave plate  54 , and is thereby converted into circularly polarized light. After this, it is converted into a focused beam by an objective lens  56  with a focal distance f 2  of 2 mm, then passes through a transparent substrate formed on the optical recording medium  40 , and is converged on an information recording plane  40   a . The opening of the objective lens  56  is limited by an aperture  55 , with the numerical aperture NA set to 0.85. Information recording planes  40   a  and  40   b  are formed in the optical recording medium  40 , the thickness d 1  of the optical recording medium  40  from its surface to the information recording plane  40   a  is 0.1 mm, the thickness d 2  to the information recording plane  40   b  is 75 μm, and the refractive index n is 1.57. A stepper motor or the like is used to allow the collimating lens  53  to move in the direction of the optical axis, as a spherical aberration correction unit  93  for correcting spherical aberration generated by a difference between the substrate thicknesses d 1  and d 2  of the information recording planes  40   a  and  40   b .  
         [0072]     The beam  70  reflected by the information recording plane  40   a  passes through the objective lens  56  and the quarter-wave plate  54  and is thereby converted into linearly polarized light that its direction is different by 90 degrees with respect to the outward path, after which the beam is reflected by the polarized beam splitter  52 . The beam  70  reflected by the polarized beam splitter  52  is split by a diffraction grating  60  (a beam splitting element) into zero-order diffracted light and 1 st -order diffracted light, through a converging lens  59  with a focal distance f 3  of 30 mm and a cylindrical lens  57 , and is incident on a photodetector  32 . The beam  70  that is incident on the photodetector  32  is imparted with astigmatism while passing through the cylindrical lens  57 .  
         [0073]      FIG. 3  schematically shows the structure of the diffraction grating  60 , and  FIG. 4  the relationship between the photodetector  32 , the beam  70  received by the photodetector  32 , and the beams  70   a  to  70   d .  
         [0074]     The diffraction grating  60  may have a lateral cross sectional shape that is either a simple grooved shape, or a stepped or serrated blazed shape, and has a total of four different regions  60   a  to  60   d . The 1 st -order diffracted light diffracted in the region  60   a  is expressed as  70   a , the 1 st -order diffracted light diffracted in the region  60   b  is expressed as  70   b , the 1 st -order diffracted light diffracted in the region  60   b  is expressed as  70   b , the 1 st -order diffracted light diffracted in the region  60   c  is expressed as  70   c , and the 1 st  -order diffracted light diffracted in the region  60   d  is expressed as  70   d . The various regions are split up and constituted such that regions  60   a  and  60   b  include most of the tracking groove component, which is 1 st -order diffracted light diffracted by the track of the information recording plane  40   a , while regions  60   c  and  60   d include substantially none of this tracking groove component. The diffraction grating  60  is designed so that the diameter of the beam  70  that is incident on the diffraction grating  60  after being reflected by the polarized beam splitter  52  is usually about 2 to 4 mm.  
         [0075]     An FE signal is obtained by astigmatism method using the signals I 32   a  to  132   d  outputted from the photodetector  32 , that is, by (I 32   a  +I 32   c ) - (I 32   b  +I 32   d ). A TE signal is obtained by (I 32   e  - I 32   f ) −K·(I 32   h  - I 32   g ), where K is a real number.  
         [0076]     After the FE signal and TE signal have been amplified and phase-corrected to the desired levels, they are supplied to the actuators  91  and  92  for moving the objective lens  56 , and then subjected to focus and tracking control.  
         [0077]     If the beam  70  has been focused on the information recording plane  40   a , it is greatly defocused on the information recording plane  40   b . Accordingly, of the beam  71  reflected by the information recording plane  40   b , the zero-order diffracted light that has passed through the diffraction grating  60  is greatly defocused on the photodetector  32 . Here, light receivers  32   e  to  32   j  are disposed so that the beam  71  will always be incident on the light receivers  32   e  to  32   h . The purpose of this is to prevent disturbance from occurring in the TE signal due to whether or not the beam  71  is incident on the light receivers  32   e  to  32   h when the thickness varies between layers (between the information recording planes  40   a  and  40   b ), which would result in an inability to control tracking stably. Thus, even when the thickness varies between layers (between the information recording planes  40   a  and  40   b ), the beam  71  will always be incident on the light receivers  32   e  to  32   h , and this design yields a TE signal with less disturbance and allows tracking error to be controlled more stably. Therefore, with the optical information device in this embodiment, fluctuation in the TE signal amplitude is reduced and tracking can be carried out more stably, so information can be recorded or reproduced at high reliability.  
         [0078]     Also, an optical recording medium comprising two layers of information recording plane was described in this embodiment, but the same effect will be obtained with an optical recording medium having more information recording planes. FIGS.  5  to  7  show the relationship between the photodetector  32  and the beam reflected from an optical recording medium having four layers of information recording plane. Information recording planes are expressed as  40   a ,  40   b ,  40   c , and  40   d , and the beams reflected from these respective information recording planes are expressed as  70 ,  71 ,  72 , and  73 . As shown in  FIG. 5 , when the focus is on the information recording plane  40   a , for example, the beams on the photodetector  32  are disposed so that the beam  71  will be incident on the light receivers  32   e  to  32   h , and the diffraction angle is set so that the beams  70   a  to  70   d  split by the regions  60   a  to  60   d  of the diffraction grating  60  will be incident on the light receivers  32   e  to  32   h , the result of which is that tracking error can be controlled more stably. Also, as shown in  FIG. 6 , the light receivers  32   e  to  32   h  may be disposed at positions where the beam  71  is not incident, and the beam  72  is always incident. Also, as shown in  FIG. 7 , these may be disposed at positions where the beams  71  and  72  are not incident, and the beam  73  is always incident, and here again the same effect will be obtained.  
       Embodiment 2  
       [0079]      FIG. 8  is a schematic diagram of the relationship between a photodetector  33  used in this embodiment and the beams  70 ,  71 , and  70   a  to  70   d  received by the photodetector  33 .  
         [0080]     The difference between the optical pickup in this embodiment and the optical pickup in Embodiment 1 is that the photodetector  33  is used instead of the photodetector  32 . The difference between the photodetector  32  and the photodetector  33  is that light receivers  33   i  to  331  are disposed at positions in substantially axial symmetry with light receivers  33   e  to  33   h  with respect to the center of the beam  71  reflected by the information recording plane  40   b , where the beam is not focused. The TE signal when this photodetector  33  is used is obtained by (I 33   e −I 33   j )−(I 33   f −I 33   i )−K·((I 33   h −I 33   k )−(I 33   g −I 33   l )), where K is a real number.  
         [0081]     In this case, stray light from the beam  71  reflected by the information recording plane  40   b  where the light receivers  33   e  to  33   h  for producing the TE signal are incident is canceled out by the light receiver  33   i  on which the beam  71  is similarly incident, which yields a TE signal with less disturbance and allows tracking error to be controlled more stably. Therefore, with the optical information device of this embodiment, fluctuation in the TE signal amplitude is reduced and tracking can be carried out more stably, so information can be recorded or reproduced at high reliability.  
       Embodiment 3  
       [0082]      FIG. 9  is a schematic diagram of the relationship between a photodetector  34  used in this embodiment and the beams  70  and  70   a  to  70   n  received by the photodetector  34 .  
         [0083]     The difference between the optical pickup in this embodiment and the optical pickup in Embodiment 1 is that a diffraction grating  61  (not shown) is used instead of the diffraction grating  60 , and the photodetector  34  is used instead of the photodetector  32 . The diffraction grating  60  in Embodiment 1 could have a lateral cross sectional shape that was either a simple grooved shape, or a stepped or serrated blazed shape, but the diffraction grating  61  in this embodiment has a simple groove-shaped cross section that generates ± diffracted light. This diffraction grating  61  also has a total of four different regions  60   a  to  60   d , just as did the diffraction grating  60  in  FIG. 3 .  
         [0084]     Next, the beam split by the diffraction grating  61  will be described.  70   a  is the +1 st -order diffracted light, and  70   e  the −1 st -order diffracted light, reflected by the information recording plane  40   b  and diffracted in the region  61   a  of the diffraction grating  61 ,  70   b  is the +1 st -order diffracted light, and  70   f  the −1 st  -order diffracted light, diffracted in the region  61   b ,  70   c  is the +1 st -order diffracted light, and  70   g  the −1 st -order diffracted light, diffracted in the region  61   c , and  70   d  is the +1 st -order diffracted light, and  70   h  the −1 st -order diffracted light, diffracted in the region  61   d . The beams  70   a  to  70   h  are incident on the photodetector  34  as shown in  FIG. 9 . The TE signal here is obtained by (I 34   e +I 34   j )−(I 34   f +I 34   i )−K·((I 34   h +I 34   k )−(I 34   g +I 34   l )). In this case, the signal for the groove component of the optical recording medium is obtained as a larger signal than the TE error signal of Embodiment  1 , so tracking can be controlled even more stably. Therefore, with the optical information device in this embodiment, fluctuation in the TE signal amplitude is reduced and tracking can be carried out more stably, so information can be recorded or reproduced at high reliability.  
         [0085]     Also, with the photodetector  34  of this embodiment, even if the number of information recording planes is increased, the same disposition of the photodetection components as in Embodiment  1  can be performed, allowing tracking error to be controlled stably. Embodiment 4  
         [0086]      FIG. 10  is a diagram of an example of the structure of another optical pickup device  202  pertaining to this embodiment.  
         [0087]     The difference from Embodiment 1 is that a diffraction grating  62  is used instead of the diffraction grating  60  (the beam splitting unit), and an opening limiting element  80  (opening limiting unit) and a photodetector  35  (instead of the photodetector  32 ) are provided in the vicinity of the diffraction grating  62 . The opening limiting element  80  has the structure shown in  FIG. 11 . Specifically, it has an oval shape that is longer in the direction corresponding to the tracking direction of the objective lens  56 , and the center of this oval shape and the center of the diffraction grating  62  kept in a substantially coinciding positional relationship. The beam that passes through the region on the outside of this oval shape is blocked and prevented from going into the photodetector  35 . The difference between the diffraction grating  62  and the diffraction grating  60  of Embodiment 1 is that the direction of diffraction is obtained by Θ rotation of the zero-order diffracted light around the center (see  FIG. 12 ). Θ here is approximately 40 to 50 degrees, and preferably 45 degrees. Also, in Embodiments 1 to 3, the shape in which the beam reflected by the information recording plane  40   b  (on which the beam was not focused) was incident on the photodetector  35  in the optical recording medium  40  was expressed schematically as being circular. However, in actual practice, since the beam reflected from the information recording plane passes through the cylindrical lens  57 , the reflected beam from an unfocused information recording plane has an elliptical shape on the photodetector  35 . Also, the orientation of the ellipse of the beam reflected by an unfocused information recording plane on the photodetector  35  is determined by the direction of the curved plane of the cylindrical lens  57 .  
         [0088]     Also, as shown in  FIG. 12 , the light receivers  35   e  to  35   h  that receive the beams  70   a  to  70   d  split by the diffraction grating  62  are disposed in a different direction from the diffraction direction on the track on the information recording plane  40   a  of the zero-order diffracted light that has passed through the diffraction grating  62 . In this embodiment, these light receivers are disposed in a direction rotated by approximately 40 to 50 degrees with respect to the track diffraction direction, and are in a positional relationship in which they receive the beams  70   a  to  70   d  split by the diffraction grating  62 . The TE signal in this optical pickup is obtained by (I 35   e −I 35   f )−K·(I 35   h −I 35   g ), just as in Embodiment 1.  
         [0089]     With an optical pickup structured as above, when the focal point of the objective lens  56  is on the information recording plane  40   b , the map of the beam  71  a reflected by the information recording plane  40   a on the photodetector  35  is the elliptical shape indicated by the dotted line in  FIG. 12 , and when the focal point of the objective lens  56  is on the information recording plane  40   a , the map of the beam  71   b  reflected by the information recording plane  40   b  on the photodetector  35  is an elliptical shape rotated by 90 degrees from  71   a , and the blocking effect of the opening limiting element  80  produces the map indicated by the dotted line shown in  FIG. 12 .  
         [0090]     When the opening limiting element  80 , the diffraction grating  62 , and the photodetector  35  are combined as above, there is no stray light on the light receivers  35   e  to  35   h  of the photodetector  35 , which detect the TE signal, so stable tracking control is possible. Therefore, with the optical information device of this embodiment, fluctuation in the TE signal amplitude is reduced and tracking can be carried out more stably, so information can be recorded or reproduced at high reliability.  
         [0091]     Furthermore, with this embodiment, the opening limiting element  80  was formed separately from the diffraction grating  62 , but the same effect can be obtained by forming it integrally with the diffraction grating. Also, the opening may be limited by giving the diffraction grating  62  a holder shape.  
       Other Embodiments  
       [0092]     Embodiments 1 to 4 described above are just examples, and various modifications are possible without exceeding the gist of the present invention. The following are examples thereof  
         [0093]     In the above embodiments, the diffraction gratings  60  to  62  were split into four regions, but the number of regions into which they are split is not limited to this. Specifically, the constitution may be such that the diffraction grating is divided into a region mainly including the tracking groove component of the information recording plane, and a region including substantially no tracking groove component.  
         [0094]     Also, there is no need to use all of the regions within a beam to produce the TE signal, and a situation in which, for example, no TE signal is used near the center of the beam can also be applied to the present invention, and the same effect as above can be obtained.  
         [0095]     The use of a polarizing optical system was described above, but a non-polarizing optical system may be used instead.  
         [0096]     No FE signal detection systems other than an astigmatism process were described because this is not related to the gist of the present invention, but there is no restriction whatsoever on how the FE signal is detected, and a spot size detection method, a Foucault method, or any other ordinary FE signal detection method can be used.  
         [0097]     Even when there is variance in the track position, width, or depth during the production of the optical recording medium, or when an optical recording medium is used whose TE signal amplitude fluctuates when information is recorded to a track, all the optical information devices given in these embodiments will still reduce fluctuation in the TE signal amplitude and allow stable tracking. Therefore, the yield of the optical recording medium is increased, and a less expensive optical recording medium can be provided.  
         [0098]     Also, because an optical recording medium whose TE signal amplitude fluctuates is permissible, a laser beam can be used to cut out the masters of optical recording medium at higher speed, so this is faster than using an electron beam to cut out masters, and masters can be produced less expensively. This allows the optical recording medium to be provided at a cost that is correspondingly lower.  
         [0099]     With the above-mentioned embodiments, the wavelength of the light source  1  was 405 nm and the numerical aperture NA of the objective lens  56  was set at 0.85, but the advantages of the optical. information device pertaining to these embodiments will be particularly pronounced when tp/0.8&lt;λ/NA &lt;0.5 μm.  
       INDUSTRLAL APPLICABILITY  
       [0100]     The optical information device pertaining to the present invention can be used in applications such as optical information device that require a reduction in the fluctuation of the TE signal amplitude and the ability to record or reproduce information at high reliability.

Technology Category: 3