Abstract:
An optical head having a phase-shifting diffraction grating includes a diffraction grating for dividing the light from a light source into a plus first order diffracted light, a zero order light, and a minus first order diffracted light. An objective lens focuses light from the light source passing through the diffraction grating onto an optical recording medium, and a photodetector receives a reflected light from the optical recording medium. A signal processing circuit generates a track error signal according to an output of the photodetector. The phase difference between lights on different regions of the diffraction grating is π/2 radians allowing the optical head to detect a land/groove position on an optical recording medium even if the land and groove have identical widths.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an optical head apparatus and in particular, to an optical head apparatus causing no offset in a track error signal even if an objective lens is shifted and capable of detecting a land/groove position. 
     2. Description of the Related Art 
     In a conventional optical head apparatus, the push-pull method is known as one of the track error signal detecting methods. The push-pull method is realized by a simple configuration of an optical system and an electric circuit but an offset is caused in the error detecting signal if an objective lens is shifted. 
     To cope with this, there is known a method to use a diffraction grating to generate three beams of 0-th order light, plus and minus 1 st -order diffracted lights, so that the offset of the track error signal caused by the objective lens shift is cancelled by a difference between the 0-th order light and the plus and minus 1 st -order diffracted lights. An optical recording medium has a land and a groove. In this method, the 0-th order light is applied to the land (or the groove) and the plus and minus 1 st -order diffracted lights are applied to the adjacent grooves (or lands). However, in an optical recording medium having a track pitch different from a design, it is impossible to apply the three focal spots as mentioned above and accordingly, it is impossible to cancel the track error signal offset upon an objective lens shift. 
     Japanese Patent Publication (Unexamined) No. 9-81942 discloses a method to use a diffraction grating consisting of two regions one of which has a phase delayed by π from the phase of the other, so as to generate three beams of 0-th order light and plus and minus 1 st -order diffracted lights so that differences between the 0-th order light and the plus and minus 1 st -order lights are used to cancel a track error signal offset at an objective lens shift. In this method, the 0-th order light and the plus and minus 1 st -order diffracted lights is applied to a single land (or groove). Consequently, even in an optical recording medium having a track pitch different from a predetermined design, the arrangement of the three focal spots is not changed, enabling to cancel the offset of the track error signal caused by the objective lens shift. 
     FIG. 17 shows a configuration of a conventional optical head apparatus using the aforementioned method. 
     A light emitted from a semiconductor laser  51  is made into parallel lights by a collimator lens  52  and divided by a diffraction grating  53   d  into 0-th order light and plus and minus diffracted lights. Approximately half of these lights are passed through a beam splitter  54  and focused by an objective lens  55  on a disc  56 . The three lights reflected from the disc  56  are introduced via the objective lens  55  into the beam splitter  54 , where about half of the lights is reflected to be received via a composite lens  57  by a photo detector  58   d . The composite lens  57  consists of a convex lens and a cylindrical lens. The photo detector  58   d  is arranged in an intermediate position between two focal lines of the composite lens  57 . 
     FIG. 18 is a plan view of the diffraction grating  53   d . The diffraction grating  53   d  is divided into a region  78   a  and a region  78   b . The line of this division is a straight line in a tangential direction (parallel to the track) passing through the optical axis of the incident light  59 . The phase difference between the region  78   a  and the region  78   b  is π. Accordingly, there is a phase difference of π between the plus and minus 1 st -order diffracted lights from the region  78   a  and the plus and minus 1 st -order diffracted lights from the region  78   b.    
     FIG. 19 shows an arrangement of the focal spots on the disc  56 . The 0-th order light, the plus 1 st -order diffracted light, and the minus 1 st -order diffracted light respectively correspond to focal spots  79   a ,  79   b , and  79   c , which are arranged on a single track  61  (land or groove). The focal spots  79   b  and  79   c  have two peaks having an identical intensity in a radial direction (vertical direction to the track). 
     FIG. 20 shows light receiving blocks of the photo detector  58   d  and a light spot arrangement on the photo detector  58   d . A light spot  80   a  corresponds to the 0-th order light which is received by the light receiving block divided into four light receiving sections  81   a  to  81   d  by two straight lines of tangential direction passing through the optical axis and the radial direction. A light spot  80   b  corresponds to the plus 1st-order diffracted light, which is received by a light receiving block divided into a light receiving sections  81   e  and  81   f  by a tangential line passing through the optical axis. A light spot  80   c  corresponds to the minus 1 st -order diffracted light, which is received by a light receiving block divided into light receiving sections  81   g  and  81   h  by a tangential line passing through the optical axis. The focal spots  79   a ,  79   b , and  79   c  are arranged in the tangential direction on the disc  56 , but the light spots  80   a ,  80   b , and  80   c  on the photo detector  58   d  are arranged in the radial direction by the function of the composite lens  57 . 
     If it is assumed that outputs of the light receiving sections  81   a  to  81   h  are V 81   a  to V 81   h , the focus error signal can be obtained from the calculation (V 81   a +V 81   d )−(V 81   b +V 81   c ) according to the astigmatism. The track error signal can be obtained by the differential push-pull method as follows: {(V 81   a +V 81   b )−(V 81   c +V 81   d )}−K{( 81   e +V 81   g )−(V 81   f +V 81   h )} (wherein K is a constant). Moreover, the reproduction signal can be obtained from the calculation of V 81   a +V 81   b +V 81   c +V 81   d.    
     FIG. 21 shows a phase change of the 0-th order light, the plus and minus 1 st -order diffracted lights from the disc  56  caused by a position difference between the focal spot  79   a  on the disc  56  and the track  61 . The focal spot  79   a  is formed by a beam  66   d.    
     FIG. 21A, case (1), the light beam  66   d  is applied to a groove  67   a . Here, if the 0-th order light is assumed to have phase 0, the plus and minus 1 st -order diffracted lights have a phase of −π/2. In FIG. 21A, case (2), the light beam  66   d  is applied to a boundary of the groove  67   a  and the land  67   b . Here, with respect to case (1), the plus 1 st -order diffracted light has a phase delayed by π/2, and the minus 1 st -order diffracted light has a phase advancing by π/2. Accordingly, if the 0-th order light has a phase 0, the plus 1 st -order diffracted light has a phase of plus and minus π, and a minus 1 st -order diffracted light has a phase 0. In FIG. 21A, case (3), the light beam  66   d  is applied to the land  67   b . Here, with respect to case (2), the plus 1 st -order diffracted light has a phase delayed by π/2, and the minus 1 st -order diffracted light has a phase advancing by d π/2. Accordingly, if the 0-th order light has phase 0, the plus and minus 1 st -order diffracted lights have a phase of π/2. In FIG. 21A, case (4), the light beam  66   d  is applied to a boundary between the land  67   b  and the groove  67   a . Here, with respect to case (3), the plus  1   st -order diffracted light has a phase delayed by π/2, and the minus 1 st -order diffracted light has a phase advancing by π/2. Accordingly, if the 0-th order light has phase 0, the plus 1 st -order diffracted light has phase 0 and the minus 1 st -order diffracted light has phase plus and minus π. 
     FIG. 21B shows a region  82   a  containing both of the 0-th order light and the plus 1 st -order diffracted light and a region  82   b  containing both of the 0-th order light and the minus 1 st -order diffracted light. These regions  82   a  and  82   b  have light intensities as follows. In FIG. 21A, case (1), the phase difference between the 0-th order light and the plus 1 st -order diffracted light and the phase difference between the 0-th order light and the minus 1 st -order diffracted light are both π/2. Accordingly, the light intensity of region  82   a  is equal to the light intensity of region  82   b . In FIG. 21A, case (2), the phase difference between the 0-th order light and the plus 1 st -order diffracted light is π, and their intensities are weakened by interference. On the other hand, the phase difference between the 0-th order light and the minus 1 st -order diffracted light is 0 and their intensities are intensified by interference. Accordingly, the intensity of the region  82   a  is low and the intensity of the region  82   b  is high. In FIG. 21A, case (3), the phase difference between the 0-th order light and the plus 1 st -order diffracted light and the phase difference between the 0-th order light and the minus 1 st -order diffracted light are both π/2. Accordingly, the light intensity of region  82   a  is equal to the light intensity of region  82   b . In FIG. 21A, case (4), the phase difference between the 0-th order light and the plus 1 st -order diffracted light is 0 and their intensities are increased by interference. On the other hand, the phase difference between the 0-th order light and the minus 1 st -order diffracted light is π and their intensities are weakened by interference. Accordingly, the intensity of the region  82   a  is high and the intensity of the region  82   b  is low. 
     FIG. 22 shows a focal spot  79   b  and phase changes of the 0-th order light and the plus and minus 1 st -order diffracted light from the disc  56  caused by a position shift of the track  61 . The focal spot  79   b  is formed by a light beam  66   e . The light beam  66   e  has a phase at the right side shifted by π from a phase at the left side. 
     In FIG. 22A, case (1), the light beam  66   e  is applied to the groove  67   a . Here, if it is assumed that the 0-th order light has a phase −π/2 at the left half and π/2 at the right half, the plus and minus 1 st -order diffracted light has a phase −π at the left side and phase 0 at the right side. In FIG. 22A, case (2), the light beam  66   e  is applied to a boundary between the groove  67   a  and the land  67   b . Here, with respect to case (1), the plus 1 st -order diffracted light has a phase delayed by π/2 and the minus 1 st -order diffracted light has a phase advanced by π/2. Accordingly, if it is assumed that the 0-th order light has a phase −π/2 at the left side and a phase π/2 at the right side, then the plus 1 st -order diffracted light has a phase π/2 at the left side and a phase −π/2 at the right side, and the minus 1 st -order diffracted light has a phase −π/2 at the left side and a phase π/2 at the right side. In FIG. 22A, case (3), the light beam  66   e  is applied to the land  67   b . Here, with respect to case (2), the plus 1 st -order diffracted light has a phase delayed by π/2 and the minus 1 st -order diffracted light has a phase advanced by π/2. Accordingly, if it is assumed that the 0-th order light has a phase −π/2 at the left side and a phase π/2 at the right side, then each of the plus and minus 1 st -order diffracted light has a phase 0 at the left side and a phase π at the right side. In FIG. 22A, case (4), the light beam  66   e  is applied to the boundary between the land  67   b  and the groove  67   a . Here, with respect to case (3), the plus 1 st -order diffracted light has a phase delayed by π/2 and the minus 1 st -order diffracted light has a phase advanced by π/2. Accordingly, if it is assumed that the 0-th order light has a phase −π/2 at the left side and a phase π/2 at the right side, then the plus 1 st -order diffracted light has a phase −π/2 at the left side and a phase π/2 at the right side, and the minus 1 st -order diffracted light has a phase π/2 at the left side and a phase −π/2 at the right side. 
     FIG. 22 b  shows a region  83   a  containing both of the 0-th order light and the plus 1 st -order diffracted light and a region  83   b  containing both of the 0-th order light and the minus 1 st -order diffracted light. These regions have light intensities as follows. In FIG. 22A, case (1), the phase difference between the 0-th order light and the plus 1 st -order diffracted light and the phase difference between the 0-th order light and the minus 1 st -order diffracted light are both π/2. Accordingly, the light intensity of region  83   a  is equal to the light intensity of region  83   b . In FIG. 22A, case (2), the phase difference between the 0-th order light and the plus 1 st -order diffracted light is 0, and their intensities are increased by interference. On the other hand, the phase difference between the 0-th order light and the minus 1 st -order diffracted light is π and their intensities are weakened by interference. Accordingly, the intensity of the region  83   a  is high and the intensity of the region  83   b  is low. In FIG. 22A, case (3), the phase difference between the 0-th order light and the plus 1 st -order diffracted light and the phase difference between the 0-th order light and the minus 1 st -order diffracted light are both π/2. Accordingly, the light intensity of region  83   a  is equal to the light intensity of region  83   b . In FIG. 22A, case (4), the phase difference between the 0-th order light and the plus 1 st -order diffracted light is π and their intensities are weakened by interference. On the other hand, the phase difference between the 0-th order light and the minus 1 st -order diffracted light is 0 and their intensities are increased by interference. Accordingly, the intensity of the region  83   a  is low and the intensity of the region  83   b  is high. 
     FIG. 23 shows various waveforms related to the track error signal. The horizontal axis represents a positional difference between the focal spot on the disc  56  and the track  61 . Arrows a to d respectively correspond to the cases (1) to (4) in FIG.  21 A and FIG.  22 A. 
     The region  82   a  in FIG. 21B corresponds to the light receiving sections  81   c  and  81   d  of the photo detector  58   d . The region  82   b  in FIG. 21B corresponds to the light receiving sections  81   a  and  81   b  of the photo detector  58   d . Here, the waveform of (V 81   a +V 81   b )−(V 81   c +V 81   d ) is shown by a solid line in FIG.  23 A. The region  83   a  in FIG. 22B corresponds to the light receiving section  81   f  of the photo detector  58   d . The region  83   b  in FIG. 22B corresponds to the light receiving sections  81   e  of the photo detector  58   d . Here, the waveform of (V 81   e −V 81   f ) is shown by a solid line in FIG.  23 B. Similarly, the waveform of (V 81   g −V 81   h ) is as shown by a solid line in FIG.  23 C. From the waveforms of FIG.  23 B and FIG. 23C, the waveform of (V 81   e +V 81   g )−(V 81   f +V 81   h ) is as shown by a solid line in FIG.  23 D. Because of the waveforms of FIG.  23 A and FIG. 23D having phases reversed to each other, the waveform of {(V 81   a +V 81   b )−(V 81   c +V 81   d )}−K{(V 81   e +V 81   g )−(V 81   f +V 81   h )} is as shown by a solid line in FIG.  23 E. 
     When the objective lens is shifted in the radial direction, the light spots  80   a  to  80   c  on the photo detector  58   d  are also shifted in the radial direction. If it is assumed that the light spots  80   a  to  80   c  are shifted upward in FIG. 20, the outputs of the light receiving sections  81   a  and  81   b  are increased and the outputs of the light receiving sections  81   c  and  81   d  are decreased. Accordingly, the waveform of (V 81   a +V 81   b )−(V 81   c +V 81   d ) is as shown by a dotted line in FIG.  23 A. Moreover, the output of the light receiving section  81   e  is increased and the output of the light receiving section  81   f  is decreased. Accordingly, the waveform of (V 81   g −V 81   h ) is as shown by a dotted line in FIG.  23 B. Similarly, the waveform of (V 81   g −V 81   h ) is as shown by a dotted line in FIG.  23 C. From the dotted lines in FIG.  23 B and FIG. 23C, the waveform of (V 81   e +V 81   g )−(V 81   f +V 81   h ) becomes as shown by a dotted line in FIG.  23 D. The waveforms of FIG.  23 A and FIG. 23D have phases reversed to each other but DC components at the objective lens shift have identical signs. Accordingly, the track error signal {(V 81   a +V 81   b )−(VB 1   c +V 81   d )}−K{(V 81   e +V 81   g )−(V 81   f +V 81   h )} has a waveform as shown by a solid line in FIG.  23 E. That is, even if the objective lens is shifted, no offset is caused in the track error signal. 
     Here, in the optical head apparatus, when accessing the land (or the groove), in order to prevent run-away of the track servo, it is preferable to pull in the track servo after confirming that the focal spot is on the land (or the groove). For this, it is necessary to provide a land/groove position detecting function for detecting on which of the land and groove the focal spot resides. 
     However, in a conventional optical head apparatus, there is a problem that it is not always possible to detect the land/groove position. In case the groove  67   a  and the land  67   b  have different widths, the level of (V 81   a +V 81   b +V 81   c +V 81   d ) varies depending on whether the focal spot  79   a  is on the groove  67   a  or on the land  67   b . This enables to detect the land/groove position. However, in case when the groove  67   a  and the land  67   b  have identical widths, the level of (V 81   a +V 81   b +V 81   c +V 81   d ) is identical when the focal spot  79   a  is on the groove  67   a  and when on the land  67   b . Accordingly, it is impossible to detect the land/groove position. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide an optical head apparatus causing no offset in a track error signal even if an objective lens is shifted and capable of detecting a land/groove position even if the land and the groove have identical widths. 
     In order to achieve the aforementioned object, the optical head apparatus according to the present invention comprises: a light source; a diffraction grating for dividing the light from this light source into a plus 1 st -order diffracted light, a 0-th order light, and a minus 1 st -order diffracted light; an objective lens for focusing the light passing through the diffraction grating onto an information recording plane of an optical recording medium; a photo detector for receiving a reflected light from said optical recording medium; and a signal processing circuit for generating a track error signal according to an output of said photo detector. The diffraction grating has a light incident plane divided into a plurality of regions and is set so that adjacent regions have diffracted light phases different from each other. The plus 1 st -order diffracted light, the 0-th order light, and the minus 1 st -order diffracted light are controlled to be arranged on a single land or groove of the optical recording medium. In the present invention the diffraction grating is constructed so that the adjacent regions have diffracted light phases shifted by π/2 from each other. Moreover, the photo detector has a plurality of light receiving blocks for receiving the plus 1 st -order diffracted light and the minus 1 st -order diffracted light corresponding to the plurality of regions of the diffraction grating. Furthermore, the signal processing circuit generates, according to outputs of the plurality of light receiving blocks, a signal for deciding whether a focal spot on the optical recording medium is arranged on the land or groove. 
     Here, the diffraction grating may be divided into two regions by a straight line parallel to a track of the optical recording medium. Moreover, the diffraction grating may be divided into three regions by straight lines parallel to the track of the optical recording medium. Furthermore, the diffraction grating may be divided into four regions by a straight line parallel to the track of the optical recording medium and a straight line vertical to the track of the optical recording medium. Moreover, the present invention may comprise a holographic optical element between the optical recording medium and the photo detector, for diffracting a reflected light from the optical recording medium. All these are intended for achieving the aforementioned object. 
     In the optical head apparatus according to the present invention, a light emitted from the light source is divided by the diffraction grating having adjacent regions with phases shifted by π/2 from each other, into three parts: 0-th order light, and plus and minus 1 st -order diffracted lights, so that three focal spots are arranged on a single track of the optical recording medium. A land/groove position is detected according to the light quantity of the plus and minus 1 st -order diffracted lights from the respective regions of the diffraction grating reflected by the optical recording medium. With this configuration, the plus and minus 1 st -order diffracted lights are used to generate a signal having a phase shifted by π/2 with respect to a track error signal, so that the signal can be used for a land/groove position detection. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram showing a configuration of an optical head apparatus according to a first embodiment of the present invention. 
     FIG. 2 is a plan view showing a diffraction grating used in the optical head apparatus according to the first embodiment of the present invention. 
     FIG. 3 shows an arrangement of a focal spot on a disc in the optical head apparatus according to the first embodiment of the present invention. 
     FIG. 4 shows light receiving sections of a photo detector and an arrangement of a focal spot on the photo detector in the optical head apparatus according to the first embodiment of the present invention. 
     FIG. 5A shows phase changes of the 0-th order light and the plus and minus 1 st -order diffracted lights from the disc caused by a track position shift in the first embodiment; and FIG. 5B shows reflected lights from the disc in the first embodiment. 
     FIG. 6A to FIG. 6F show waveforms related to a track error signal and a land/groove position detecting signal in the optical head apparatus according to the first embodiment of the present invention. 
     FIG. 7 is a plan view showing a diffraction grating used in an optical head apparatus according to a second embodiment of the present invention. 
     FIG. 8 shows a focal spot arrangement on the disc in the optical head apparatus according to the second embodiment of the present invention. 
     FIG. 9 shows light receiving sections of a photo detector and a light spot arrangement on the photo detector in the optical head apparatus according to the second embodiment of the present invention. 
     FIG. 10A shows phase changes of the 0-th order light and the plus and minus 1 st -order diffracted lights from the disc caused by a track position shift in the second embodiment; and FIG. 10B shows reflected lights from the disc in the second embodiment. 
     FIG. 11A to FIG. 11H show waveforms related to a track error signal and a land/groove position detecting signal in the optical head apparatus according to the second embodiment of the present invention. 
     FIG. 12 is a plan view showing a diffraction grating used in an optical head apparatus according to a third embodiment of the present invention. 
     FIG. 13 shows a focal spot arrangement on the disc in the optical head apparatus according to the third embodiment of the present invention. 
     FIG. 14 shows light receiving sections of a photo detector and a light spot arrangement on the photo detector in the optical head apparatus according to the third embodiment of the present invention. 
     FIG. 15A shows phase changes of the 0-th order light and the plus and minus 1 st -order diffracted lights from the disc caused by a track position shift in the third embodiment; and FIG. 15B shows reflected lights from the disc in the third embodiment. 
     FIG. 16A to FIG. 16H show waveforms related to a track error signal and a land/groove position detecting signal in the optical head apparatus according to the third embodiment of the present invention. 
     FIG. 17 is a block diagram showing a configuration of a conventional optical head apparatus. 
     FIG. 18 is a plan view showing a diffraction grating used in the conventional optical head apparatus. 
     FIG. 19 shows a focal spot arrangement on a disc in the conventional optical head apparatus. 
     FIG. 20 shows light receiving sections of a photo detector and a light spot arrangement on the photo detector in the conventional optical head apparatus. 
     FIG. 21A shows phase changes of the 0-th order light and the plus and minus 1 st -order diffracted lights from the disc caused by a track position shift in the conventional optical head apparatus; and FIG. 21B shows reflected lights from the disc in the conventional optical head apparatus. 
     FIG. 22A shows phase changes of the 0-th order light and the plus and minus 1 st -order diffracted lights from the disc caused by a track position shift in the conventional optical head apparatus; and FIG. 22B shows reflected lights from the disc in the conventional optical head apparatus. 
     FIG. 23A to FIG. 23E show waveforms related to a track error signal in the conventional optical head apparatus. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Description will now be directed to a first embodiment of the present invention with reference to FIG. 1 to FIG.  6 . 
     FIG. 1 shows a configuration of an optical head apparatus according to the first embodiment. A semiconductor laser  1  emits a light which is made into a parallel light by a collimator lens  2  and divided by a diffraction grating  3   a  into a 0-th order light, plus and minus 1 st -order lights. About half of the lights passes through a beam splitter  4  and focused by an objective lens  5  on a disc  6 . The three lights reflected from the disc  6  pass through the objective lens  5  and about half of the lights is reflected by the beam splitter  4  and received via a composite lens  7  by a photo detector  8   a . The composite lens  7  is a combination of a convex lens and a cylindrical lens. The photo detector  8   a  is arranged at an intermediate position between two focal lines of the composite lens  7 . 
     FIG. 2 is a plan view of the diffraction grating  3   a . The diffraction grating  3   a  is divided into an area  10   a  and an area  10   b  by a straight line in the tangential direction passing through the optical path of the incident light  9 . The area  10   a  has a phase shifted by π/2 from a phase of area  10   b . The remaining parts are constructed in the same way as in the conventional apparatus. Accordingly, if it is assumed in FIG. 2 that the plus 1 st -order diffracted light is a light diffracted upward and the minus 1 st -order diffracted light is a light diffracted downward, the plus 1 st -order diffracted light from the area  10   a  is delayed by π/2 from the plus 1 st -order diffracted light from the area  10   b , and the minus 1 st -order diffracted light from the area  10   a  is advanced by π/2 from the minus 1 st -order diffracted light from the area  10   b.    
     FIG. 3 shows a focal spot arrangement on the disc  6 . Focal spots  12   a ,  12   b , and  12   c  respectively correspond to the 0-th order light, the plus 1 st -order diffracted light and the minus 1 st -order diffracted light from the diffraction grating  3   a , and they are arranged on a single track  11  (land or groove). The focal spot  12   b  has two peaks in the radial direction with a low intensity at the left and a high intensity at the right. The focal spot  12   c  has two peaks in the radial direction with a high intensity at the left and a low intensity at the right. 
     FIG. 4 shows light receiving sections of the photo detector  8   a  and a light spot arrangement on the photo detector  8   a . A light spot  13   a  corresponds to the 0-th order light from the diffraction grating  3   a , which is received by light receiving sections  14   a  to  14   d  divided by two lines: a tangential direction line passing through the optical axis and a radial direction line. A light spot  13   b  corresponds to the plus 1 st -order diffracted light from the diffraction grating  3   a , which is received by light receiving sections  14   e  and  14   f  divided by a tangential direction line passing through the optical axis. A light spot  13   c  corresponds to the minus 1 st -order diffracted light from the diffraction grating  3   a , which is received by light receiving sections  14   g  and  14   h  divided by a tangential direction line passing through the optical axis. A sequence of the focal spots  12   a  to  12   c  is in a tangential direction, but an optical system is provided so that the sequence of the light spots  13   a  to  13   c  on the photo detector  8   a  is made into a radial direction by function of the composite lens  7 . 
     If outputs to the light receiving sections  14   a  to  14   h  are assumed to be V 14   a  to V 14   h , the focus error signal can be obtained according to the astigmatism from the calculation of (V 14   a +V 14   d )−(V 14   b +V 14   c ). The track error signal is obtained according to the differential push-pull method from the calculation of {(V 14   a +V 14   b )−(V 14   c +V 14   d )}−K{(V 14   e +V 14   g )−(V 14   f +V 14   h )} (wherein K is a constant). The land/groove position detecting signal can be obtained from the calculation of (V 14   e +V 14   h )−(V 14   f +V 14   g ). Moreover, the reproduction signal can be obtained from the calculation of V 14   a +V 14   b +V 14   c +V 14   d . These calculations can be carried out by a corresponding signal processing system (not depicted). 
     Phase differences of the 0-th order light, the plus and minus 1 st -order diffracted lights from the disc  6  caused by a position shift between the focal spot  12   a  and the track  11  on the disc  6  are as shown in FIG.  21 . 
     FIG. 5 shows a phase change of the 0-th order light and the plus and minus 1 st -order diffracted lights from the disc  6  caused by a position shift between the focal spot  12   b  and the track  11  on the disc  6 . The focal spot  12   b  is formed by a light beam  16   a . The light beam  16   a  has a phase at the left side delayed by π/2 from the phase at the right side. 
     In FIG. 5A, case (1), the light beam  16   a  is applied to a groove  17   a . Here, if it is assumed that the 0-th order light has a phase −π/4 at the left side and a phase π/4 at the right side, the plus and minus 1 st -order diffracted lights have a phase −3 π/4 at the left side and −π/4 at the right side. In FIG. 5A, case (2), the light beam  16   a  is applied to a boundary between the groove  17   a  and the land  17   b . Here, with respect to case (1), the plus 1 st -order diffracted light has a phase delayed by π/2 and the minus 1 st -order diffracted light has a phase advanced by π/2. Accordingly, if it is assumed that the 0-th order diffracted light has a phase −π/4 at the left side and a phase π/4 at the right side, the plus 1 st -order diffracted light has a phase 3 π/4 at the left side and −3π/4 at the right side, and the minus 1 st -order diffracted light has a phase −π/4 at the left side and π/4 at the right side. In FIG. 5A, case (3), the light beam  16   a  is applied to the land  17   b . Here, with respect to case (2), the plus 1 st -order diffracted light has a phase delayed by π/2 and the minus 1 st -order diffracted light has a phase advanced by π/2. Accordingly, if it is assumed that the 0-th order diffracted light has a phase −π/4 at the left side and a phase π/4 at the right side, the plus and minus 1 st -order diffracted lights have a phase π/4 at the left side and 3 π/4 at the right side. In FIG. 5A, case (4), the light beam  16   a  is applied to a boundary between the land  17   b  and the groove  17   a . Here, with respect to case (3), the plus 1 st -order diffracted light has a phase delayed by π/2 and the minus 1 st -order diffracted light has a phase advanced by π/2. Accordingly, if it is assumed that the 0-th order diffracted light has a phase −π/4 at the left side and a phase π/4 at the right side, then the plus 1 st -order diffracted light has a phase −π/4 at the left side and π/4 at the right side, and the minus 1 st -order diffracted light has a phase 3 π/4 at the left side and −3 π/4 at the right side. 
     FIG. 5B shows an area  15   a  containing the 0-th order light and the plus 1 st -order diffracted light, and an area  15   b  containing the 0-th order light and the minus 1 st -order diffracted light, each having an intensity as follows. In FIG. 5A, case (1), the phase difference between the 0-th order light and the plus 1 st -order diffracted light is 0 and they intensify each other by interference, whereas the phase difference between the 0-th order light and the minus 1 st -order diffracted light is π and they weaken each other by interference. Accordingly, the area  15   a  has a high intensity and the area  15   b  has a low intensity. In FIG. 5A, case (2), the phase difference between the 0-th order light and the plus 1 st -order diffracted light and the phase difference between the 0-th order light and the minus 1 st -order diffracted light are both π/2. Accordingly, the area  15   a  and the area  15   b  have identical intensities. In FIG. 5A, case (3), the phase difference between the 0-th order light and the plus 1 st -order diffracted light is π and they weaken each other by interference, whereas the phase difference between the 0-th order light and the minus 1 st -order diffracted light is 0 and they intensify each other by interference. Accordingly, the area  15   a  has a low intensity and the area  15   b  has a high intensity. In FIG. 5A, case (4), the phase difference between the 0-th order light and the plus 1 st -order diffracted light and the phase difference between the 0-th order light and the minus 1 st -order diffracted light are both π/2. Accordingly, the area  15   a  and the area  15   b  have identical intensities. 
     FIG. 6 shows various waveforms associated with a track error signal and a land/groove position detecting signal. The horizontal axis represents a position shift between the focal spot and the track  11  on the disc  6 , and arrows ‘a’ to ‘d’ correspond to cases (1) to (4), respectively. 
     In the same way as in the conventional optical head apparatus, the waveform of (V 14   a +V 14   b )−(V 14   c +V 14   d ) is as shown by a solid line in FIG.  6 A. The region  15   a  in FIG. 5B corresponds to the light receiving section  14   f  of the photo detector  8   a . The region  15   b  in FIG. 5B corresponds to the light receiving section  14   e  of the photo detector  8   a . Here, the waveform of (V 14   e −V 14   f ) is shown by a solid line in FIG.  6 B. Similarly, the waveform of (V 14   g −V 14   h ) is as shown by a solid line in FIG.  6 C. From the waveforms of FIG.  6 B and FIG. 6C, the waveform of (V 14   e +V 14   g )−(V 14   f +V 14   h ) is as shown by a solid line in FIG.  6 D. From the waveforms of FIG.  6 A and FIG. 6D, the track error signal represented by {(V 14   a +V 14   b )−(V 14   c +V 14   d )}−K{(V 14   e +V 14   g )−(V 14   f +V 14   h )} can be expressed by a waveform as shown by a solid line in FIG.  6 E. 
     On the other hand, because the waveforms B and C have reversed phases to each other, the land/groove detecting signal obtained from the calculation of (V 14   e +V 14   h )−(V 14   f +V 14   g ) has a waveform as shown by a solid line in FIG.  6 F. The signal of FIG. 6F has a phase shifted by π/2 from the track error signal of FIG. 6E so as to be negative and positive when the light beam  16   a  is applied to the groove  17   a  and to the land  17   b , respectively, thus enabling to detect a land/groove position. 
     When the objective lens  5  is shifted in a radial direction, the light spots  13   a  to  13   c  on the photo detector  8   a  are also shifted in the radial direction. If it is assumed that the light spots  13   a  to  13   c  are shifted upward in FIG. 4, the light receiving sections  14   a  and  14   b  increase their outputs and the light receiving sections  14   c  and  14   d  decrease their outputs. Accordingly, the waveform of (V 14   a +V 14   b )−(V 14   c +V 14   d ) is as shown by a dotted line in FIG.  6 A. The light receiving section  14   e  increases its output and the light receiving section  14   f  decreases its output. Accordingly, the waveform of V 14   e −V 14   f  is as shown by a dotted line in FIG.  6 B. Similarly, the waveform of the V 14   g −V 14   h  is as shown by a dotted line of FIG.  6 C. From the waveforms of B and C, the waveform of (V 14   e +V 14   g )−(V 14   f +V 14   h ) is as shown by a dotted line of FIG.  6 D. In A and D, DC components at an objective lens shift have identical signs. Accordingly, the waveform of {(V 14   a +V 14   b )−(V 14   c +V 14   d )}−K{(V 14   e +V 14   g )−(V 14   f +V 14   h )} is as shown by a solid line in FIG.  6 E. That is, even if the objective lens is shifted, no offset is generated in the track error signal. On the other hand, the waveforms of B and C have reversed phases to each other but their DC components have identical signs at an objective lens shift. Accordingly, the land/groove position detecting signal expressed by (V 14   e +V 14   h )−(V 14   f +V 14   g ) has a waveform as shown by a solid line in FIG.  6 F. That is, no affect is caused by the objective lens shift. 
     Next, description will be directed to a second embodiment of the present invention with reference to FIG. 7 to FIG.  11 . 
     The optical head apparatus according to the second embodiment has a configuration identical to that of FIG. 1 except for that the diffraction grating  3   a  is replaced by a diffraction grating  3   b  and the photo detector  8   a  is replaced by a photo detector  8   b.    
     FIG. 7 is a plan view showing the diffraction grating  3   b . The diffraction grating  3   b  is divided into three regions  18   a ,  18   b , and  18   c , by two straight lines in a tangential direction symmetric to the optical axis of the incident light  9 . The regions  18   a  and  18   c  have phases shifted by π/2 from the phase of region  18   b . If it is assumed in FIG. 7 that the plus 1 st -order diffracted light is a light diffracted upward and the minus 1 st -order diffracted light is a light diffracted downward, the plus 1 st -order diffracted light from the region  18   a  and that from the region  18   c  have a phase advanced by π/2 with respect to the plus 1 st -order diffracted light from the region  18   b , and the minus 1 st -order diffracted light from the region  18   a  and that from the region  18   c  have a phase delayed by π/2 with respect to the minus 1 st -order diffracted light from the region  18   b.    
     FIG. 8 shows a focal spot arrangement on the disc  6 . Focal spots  19   a ,  19   b , and  19   c  correspond to the 0-th order light, the plus 1 st -order diffracted light, and the minus 1 st -order diffracted light, respectively, and are arranged on a single track  11  (land or groove). The focal spot  19   b  and  19   c  have a longer diameter in the radial direction compared to the focal spot  19   a.    
     FIG. 9 shows light receiving sections of the photo detector  8   b  and a light spot arrangement on the photo detector  8   b . A light spot  20   a  corresponds to the 0-th order light from the diffraction grating  3   b , which is received by four light receiving sections  21   a  to  21   d  divided by two lines: a tangential direction line passing through the optical axis and a radial direction line. A light spot  20   b  corresponds to the plus 1 st -order diffracted light from the diffraction grating  3   b , which is received by light receiving sections  21   e  to  21   h  divided by a tangential direction line passing through the optical axis and two straight lines parallel to this line and symmetric with respect to the optical axis. A light spot  20   c  corresponds to the minus 1 st -order diffracted light from the diffraction grating  3   b , which is received by light receiving sections  21   i  to  21   l  divided by a tangential direction line passing through the optical axis and two lines parallel to this and symmetric with respect to the optical axis. A sequence of the focal spots  19   a  to  19   c  on the disc  6  is in a tangential direction, but an optical system is provided so that the sequence of the light spots  20   a  to  20   c  on the photo detector  8   b  is in a radial direction by function of the composite lens  7 . 
     If outputs to the light receiving sections  21   a  to  21   l  are assumed to be V 21   a  to V 21   l , the focus error signal can be obtained according to the astigmatism from the calculation of (V 21   a +V 21   d )−(V 21   b +V 21   c ). The track error signal is obtained according to the differential push-pull method from the calculation of {(V 21   a +V 21   b )−(V 21   c +V 21   d )}−K{(V 21   e +V 21   f +V 21   i +V 21   j )−(V 21   g +V 21   h +V 21   k +V 21   l )} (wherein K is a constant). The land/groove position detecting signal can be obtained from the calculation of (V 21   e +V 21   h +V 21   j +V 21   k )−(V 21   f +V 21   g +V 21   i +V 21   l ). Moreover, the reproduction signal can be obtained from the calculation of V 21   a +V 21   b +V 21   c +V 21   d . These calculations can be carried out by a corresponding signal processing system (not depicted). 
     Phase differences of the 0-th order light, the plus and minus 1 st -order diffracted lights from the disc  6  caused by a position shift between the focal spot  19   a  and the track  11  on the disc  6  are as shown in FIG.  21 . 
     FIG. 10 shows a phase change of the 0-th order light and the plus and minus 1 st -order diffracted lights from the disc  6  caused by a position shift between the focal spot  19   b  and the track  11  on the disc  6 . The focal spot  19   b  is formed by a light beam  16   b . The light beam  16   b  has a phase at the outer left and at the outer right advanced by π/2 with respect to the phase at the center portion. 
     In FIG. 10A, case (1), the light beam  16   b  is applied to a groove  17   a . Here, if it is assumed that the 0-th order light has a phase π/4 at the outer left and the outer right and a phase −π/4 at the center portion, then the plus and minus 1 st -order diffracted lights have a phase −π/4 at the outer left and outer right and −3 π/4 at the center portion. In FIG. 10A, case (2), the light beam  16   b  is applied to a boundary between the groove  17   a  and the land  17   b . Here, with respect to case (1), the plus 1 st -order diffracted light has a phase delayed by π/2 and the minus 1 st -order diffracted light has a phase advanced by π/2. 
     Accordingly, if it is assumed that the 0-th order diffracted light has a phase π/4 at the outer left and the outer right and a phase −π/4 at the center portion, the plus 1 st -order diffracted light has a phase −3 π/4 at the outer left and outer right and 3 π/4 at the center portion, and the minus 1 st -order diffracted light has a phase π/4 at the outer left and outer right and −π/4 at the center portion. In FIG. 10A, case (3), the light beam  16   b  is applied to the land  17   b . Here, with respect to case (2), the plus 1 st -order diffracted light has a phase delayed by π/2 and the minus 1 st -order diffracted light has a phase advanced by π/2. Accordingly, if it is assumed that the 0-th order diffracted light has a phase π/4 at the outer left and outer right and a phase −π/4 at the center portion, the plus and minus 1 st -order diffracted lights have a phase 3 π/4 at the outer left and outer right and a phase π/4 at the center potion. In FIG. 10, case (4), the light beam  16   b  is applied to a boundary between the land  17   b  and the groove  17   a . Here, with respect to case (3), the plus 1 st -order diffracted light has a phase delayed by π/2 and the minus 1 st -order diffracted light has a phase advanced by π/2. Accordingly, if it is assumed that the 0-th order diffracted light has a phase π/4 at the outer left and outer right and a phase −π/4 at the center portion, then the plus 1 st -order diffracted light has a phase π/4 at the outer left and right and a phase −π/4 at the center portion, and the minus 1 st -order diffracted lights has a phase −3 π/4 at the outer left and right and a phase  3 π/4 at the center portion. 
     FIG. 10B shows an outer region  22   a  and an inner region  22   b  containing the 0-th order light and the plus 1 st -order diffracted light, and an inner region  22   c  and an outer region  22   d  containing the 0-th order light and the minus 1 st -order diffracted light, each having an intensity as follows. In FIG. 10A, case (1), the phase difference between the 0-th order light and the plus 1 st -order diffracted light is π at the outer portions and they weaken each other by interference, and 0 at the center portion and they intensify each other by interference, whereas the phase difference between the 0-th order light and the minus 1 st -order diffracted light is 0 at the center portion so as to intensify each other by interference, and π at the outer portions so as to weaken each other by interference. Accordingly, the region  22   a  has a low intensity; the region  22   b  has a high intensity; the region  22   c  has a high intensity; and the region  22   d  has a low intensity. In FIG. 10A, case (2), the phase difference between the 0-th order light and the plus 1 st -order diffracted light and the phase difference between the 0-th order light and the minus 1 st -order diffracted light are both π/2 at the outer portions as well as at the center portion. Accordingly, the regions  22   a ,  22   b ,  22   c  and  22   d  have identical intensities. In FIG. 10A, case (3), the phase difference between the 0-th order light and the plus 1 st -order diffracted light is 0 at the outer portions so that they intensify each other by interference, and π at the center portion so that they weaken each other, whereas the phase difference between the 0-th order light and the minus 1 st -order diffracted light is π at the center portion so that they weaken each other and 0 at the outer portions so that they intensify each other by interference. Accordingly, the region  22   a  has a high intensity; the region  22   b  has a low intensity; the region  22   c  has a low intensity; and the region  22   d  has a high intensity. In FIG. 10A, case (4), the phase difference between the 0-th order light and the plus 1 st -order diffracted light and the phase difference between the 0-th order light and the minus 1 st -order diffracted light are both π/2 at the outer portions as well as at the center portion. Accordingly, the regions  22   a ,  22   b ,  22   c , and  22   d  have identical intensities. 
     FIG. 11 shows various waveforms associated with a track error signal and a land/groove position detecting signal. The horizontal axis represents a position shift between the focal spot and the track  11  on the disc  6 , and arrows ‘a’ to ‘d’ correspond to cases (1) to (4) of FIG. 10, respectively. 
     In the same way as in the conventional optical head apparatus, the waveform of (V 21   a +V 21   b )−(V 21   c +V 21   d ) is as shown by a solid line in FIG.  11 A. The region  22   a  in FIG. 10B corresponds to the light receiving section  21   h  of the photo detector  8   b ; the region  22   b  in FIG. 10B corresponds to the light receiving section  21   g  of the photo detector  8   b ; the region  22   c , to the light receiving section  21   f ; and the region  22   d , to the light receiving section  21   e . Here, the waveform of (V 21   e +V 21   f )−(V 21   g +V 2   h ) is shown by a solid line in FIG.  11 B. Similarly, the waveform of (V 21   i +V 21   j )−(V 21   k +V 21   l ) is as shown by a solid line in FIG.  11 C. From the waveforms of FIG.  11 B and FIG. 15C, the waveform of (V 21   e +V 21   f +V 21   i +V 21   j )−(V 21   g +V 21   h +V 21   k +V 21   l ) is as shown by a solid line in FIG.  11 D. From the waveforms of FIG.  11 A and FIG. 11D, the track error signal represented by {(V 21   a +V 21   b )−(V 21   c +V 21   d )}−K{(V 21   e +V 21   f +V 21   i +V 21   j )−(V 21   g +V 21   h +V 21   k +V 21   l )} can be expressed by a waveform as shown by a solid line in FIG.  11 E. 
     On the other hand, the waveforms of (V 21   e +V 21   h )−(V 21   f +V 21   g ) is as shown by a solid line in FIG.  11 F. Similarly, the waveform of (V 21   i +V 21   l )−(V 21   i +V 21   k ) is as shown by a solid line in FIG.  11 G. Because the waveforms of F and G have reversed phases to each other, the land/groove detecting signal expressed by (V 21   e +V 21   h +V 21   j +V 21   k )−(V 21   f +V 21   g +V 21   i +V 21   l ) has a waveform as shown by a solid line in FIG.  11 H. The signal of FIG. 11H has a phase shifted by π/2 from the track error signal of FIG. 11E so as to be negative and positive when the light beam  16   b  is applied to the groove  17   a  and to the land  17   b , respectively, thus enabling to detect a land/groove position. 
     When the objective lens  5  is shifted in a radial direction, the light spots  20   a  to  20   c  on the photo detector  8   b  are also shifted in the radial direction. If it is assumed that the light spots  20   a  to  20   c  are shifted upward in FIG. 9, the light receiving sections  21   a  and  21   b  increase their outputs and the light receiving sections  21   c  and  21   d  decrease their outputs. Accordingly, the waveform of (V 21   a +V 21   b )−(V 21   c +V 21   d ) is as shown by a dotted line in FIG.  11 A. The light receiving section  21   e  increases its output and the light receiving section  21   h  decreases its output. Accordingly, the waveform of (V 21   e +V 21   f )−(V 21   g +V 21   h ) is as shown by a dotted line in FIG.  11 B. Similarly, the waveform of the (V 21   i +V 21   j )−(V 21   k +V 21   l ) is as shown by a dotted line of FIG.  11 C. From the waveforms of B and C, the waveform of (V 21   e +V 21   f +V 21   i  +V 21   j )−(V 21   g +V 21   h +V 21   k +V 21   l ) is as shown by a dotted line of FIG.  11 D. In A and D, DC components at an objective lens shift have identical signs. Accordingly, the track error signal expressed by {(V 21   a +V 21   b )−(V 21   c +V 21   d )}−K{(V 21   e +V 21   f +V 21   i +V 21   j )−(V 21   g +V 21   h +V 21   k +V 21   l )} has a waveform as shown by a solid line in FIG.  11 E. That is, even if the objective lens is shifted, no offset is generated in the track error signal. On the other hand, the light receiving section  21   e  increases its output and the light receiving section  21   h  decreases its output. Accordingly, the waveform of (V 21   e +V 21   h )−(V 21   f +V 21   g ) is as shown by a solid line in FIG.  11 F. Similarly, the waveform of (V 21   i +V 21   l )−(V 21   j +V 21   k ) is as shown by a solid line in FIG.  11 G. Because the waveforms of F and G have reversed phases to each other, the land/groove position detecting signal expressed by (V 21   e +V 21   h +V 21   j +V 21   k )−(V 21   f +V 21   g +V 21   i +V 21   l ) has a waveform as shown by a solid line in FIG.  11 H. 
     Next, description will be directed to a third embodiment of the present invention with reference to FIG. 12 to FIG.  16 . 
     The optical head apparatus according to the third embodiment has a configuration identical to that of FIG. 1 except for that the diffraction grating  3   a  is replaced by a diffraction grating  3   c  and the photo detector  8   a  is replaced by a photo detector  8   c.    
     FIG. 12 is a plan view showing the diffraction grating  3   c . The diffraction grating  3   c  is divided into four regions  23   a  to  23   d  by two straight lines in a tangential direction and in a radial direction. The regions  23   a  and  23   b  have phases shifted by π/2 from each other and the regions  23   c  and  23   d  have phases shifted by π/2 from each other. Moreover, the phases of the regions  23   a  and  23   c  are shifted by π/2 from each other and the phases of the regions  23   b  and  23   d  are shifted by π/2 from each other. If it is assumed in FIG. 12 that the plus 1 st -order diffracted light is a light diffracted upward and the minus 1 st -order diffracted light is a light diffracted downward, then the plus 1 st -order diffracted light from the region  23   a  and that from the region  23   d  have a phase advanced by π/2 with respect to the plus 1 st -order diffracted light from the regions  23   b  and  23   c , whereas the minus 1 st -order diffracted light from the region  23   a  and that from the region  23   d  have a phase delayed by π/2 with respect to the minus 1 st -order diffracted light from the regions  23   b  and  23   c.    
     FIG. 13 shows a focal spot arrangement on the disc  6 . Focal spots  24   a ,  24   b , and  24   c  correspond to the 0-th order light, the plus 1 st -order diffracted light, and the minus 1 st -order diffracted light, respectively, and are arranged on a single track  11  (land or groove). The focal spot  24   b  and  24   c  have four side lobes in the direction of plus and minus 45 degrees with respect to the tangential direction and radial direction. 
     FIG. 14 shows light receiving sections of the photo detector  8   c  and a light spot arrangement on the photo detector  8   c . A light spot  25   a  corresponds to the 0-th order light from the diffraction grating  3   c , which is received by four light receiving sections  26   a  to  26   d  divided by two lines: a tangential direction line passing through the optical axis and a radial direction line. A light spot  25   b  corresponds to the plus 1 st -order diffracted light from the diffraction grating  3   c , which is received by four light receiving sections  26   e  to  26   h  divided by a tangential direction line passing through the optical axis and a radial direction line. A light spot  25   c  corresponds to the minus 1 st -order diffracted light from the diffraction grating  3   c , which is received by four light receiving sections  26   i  to  26   l  divided by a tangential direction line passing through the optical axis and a radial direction line. A sequence of the focal spots  24   a  to  24   c  on the disc  6  is in a tangential direction, but an optical system is set so th at the sequence of the light spots  25   a  to  25   c  on the photo detector  8   c  is in a radial direction by function of the composite lens  7 . 
     If outputs to the light receiving sections  26   a  to  261  are assumed to be V 26   a  to V 26   l , the focus error signal can be obtained according to the astigmatism from the calculation of (V 26   a +V 26   d )−(V 26   b +V 26   c ). The track error signal is obtained according to the differential push-pull method from the calculation of {(V 26   a +V 26   b )−(V 26   c +V 26   d )}−K{(V 26   e +V 26   f +V 26   i +V 26   j )−(V 26   g +V 26   h +V 26   k +V 26   l )} (wherein K is a constant). The land/groove position detecting signal can be obtained from the calculation of (V 26   e +V 26   h +V 26   j +V 26   k )−(V 26   f +V 26   g +V 26   i +V 26   l ). Moreover, the reproduction signal can be obtained from the calculation of V 26   a +V 26   b +V 26   c +V 26   d . These calculations can be carried out by a corresponding signal processing system (not depicted). 
     Phase differences of the 0-th order light, the plus and minus 1 st -order diffracted lights from the disc  6  caused by a position shift between the focal spot  24   a  and the track  11  on the disc  6  are as shown in FIG.  21 . 
     FIG. 15 shows a phase change of the 0-th order light and the plus and minus 1 st -order diffracted lights from the disc  6  caused by a position shift between the focal spot  24   b  and the track  11  on the disc  6 . The focal spot  24   b  is formed by a light beam  16   c . The light beam  16   c  has a phase at the upper left and at the lower right advanced by π/2 with respect to the phase at the upper right and lower left. 
     In FIG. 15A, case (1), the light beam  16   c  is applied to the groove  17   a . Here, if it is assumed that the 0-th order light has a phase π/4 at the upper left and at the lower right and a phase −π/4 at the upper right and lower left, then the plus and minus 1 st -order diffracted lights have a phase −π/4 at the upper left and lower right and −3π/4 at the upper right and lower left. In FIG. 15A, case (2), the light beam  16   c  is applied to a boundary between the groove  17   a  and the land  17   b . Here, with respect to case (1), the plus 1 st -order diffracted light has a phase delayed by π/2 and the minus 1 st -order diffracted light has a phase advanced by π/2. Accordingly, if it is assumed that the 0-th order diffracted light has a phase π/4 at the upper left and the lower right and a phase −π/4 at the upper right and lower left, then the plus 1 st -order diffracted light has a phase −3 π/4 at the upper left and lower right and 3 π/4 at the upper right and lower left, whereas the minus 1 st -order diffracted light has a phase π/4 at the upper left and lower right and −π/4 at the upper right and lower left. In FIG. 15A, case (3), the light beam  16   c  is applied to the land  17   b . Here, with respect to case (2), the plus 1 st -order diffracted light has a phase delayed by π/2 and the minus 1 st -order diffracted light has a phase advanced by π/2. Accordingly, if it is assumed that the 0-th order diffracted light has a phase π/4 at the upper left and lower right and a phase −π/4 at the upper right and lower left, then the plus and minus 1 st -order diffracted lights have a phase 3 π/4 at the upper left and lower right and a phase π/4 at the upper right and lower left. In FIG. 15A, case (4), the light beam  16   c  is applied to a boundary between the land  17   b  and the groove  17   a . Here, with respect to case (3), the plus 1 st -order diffracted light has a phase delayed by π/2 and the minus 1 st -order diffracted light has a phase advanced by π/2. Accordingly, if it is assumed that the 0-th order diffracted light has a phase π/4 at the upper left and lower right and a phase −π/4 at the upper right and lower left, then the plus 1 st -order diffracted light has a phase π/4 at the upper left and lower right and a phase −π/4 at the upper right and lower left, whereas the minus 1 st -order diffracted light has a phase −3 π/4 at the upper left and lower right and a phase 3 π/4 at the upper right and lower left. It should be noted that in the figures, only the phases at the upper left and the upper right are shown. 
     FIG. 15B shows an upper region  27   a  and a lower region  27   c  containing the 0-th order light and the plus 1 st -order diffracted light, and an upper region  27   b  and a lower region  27   d  containing the 0-th order light and the minus 1 st -order diffracted light, each having an intensity as follows. In FIG. 15A, case (1), the phase difference between the 0-th order light and the plus 1 st -order diffracted light is π at the upper regions and they weaken each other by interference, and 0 at the lower regions and they intensify each other by interference, whereas the phase difference between the 0-th order light and the minus 1 st -order diffracted light is 0 at the upper regions so that they intensify each other by interference, and π at the lower regions so that they weaken each other by interference. Accordingly, the region  27   a  has a low intensity; the region  27   b  has a high intensity; the region  27   c  has a high intensity; and the region  27   d  has a low intensity. In FIG. 15A, case (2), the phase difference between the 0-th order light and the plus 1 st  order diffracted light and the phase difference between the 0-th order light and the minus 1 st -order diffracted light are both π/2 at the upper regions as well as at the lower regions. Accordingly, the regions  27   a ,  27   b ,  27   c , and  27   d  have identical intensities. In FIG. 15, case (3), the phase difference between the 0-th order light and the plus 1 st -order diffracted light is 0 at the upper regions so that they intensify each other by interference, and π at the lower regions so that they weaken each other, whereas the phase difference between the 0-th order light and the minus 1 st -order diffracted light is π at the upper regions so that they weaken each other and 0 at the lower regions so that they intensify each other by interference. Accordingly, the region  27   a  has a high intensity; the region  27   b  has a low intensity; the region  27   c  has low intensity; and the region  27   d  has a high intensity. In FIG. 15A, case (4), the phase difference between the 0-th order light and the plus 1 st -order diffracted light and the phase difference between the 0-th order light and the minus 1 st -order diffracted light are both π/2 at the upper regions as well as at the lower regions. Accordingly, the regions  27   a ,  27   b ,  27   c , and  27   d  have identical intensities. 
     FIG. 16 shows various waveforms associated with a track error signal and a land/groove position detecting signal. The horizontal axis represents a position shift between the focal spot and the track  11  on the disc  6 , and arrows ‘a’ to ‘d’ correspond to cases (1) to (4) of FIG. 15, respectively. 
     In the same way as in the conventional optical head apparatus, the waveform of (V 26   a +V 26   b )−(V 26   c +V 26   d ) is as shown by a solid line in FIG.  16 A. The region  27   a  in FIG. 15B corresponds to the light receiving section  26   h  of the photo detector  8   c ; the region  27   b  in FIG. 15B corresponds to the light receiving sections  26   f  of the photo detector  8   c ; the region  27   c , to the light receiving section  26   g ; and the region  27   d , to the light receiving section  26   e . Here, the waveform of (V 26   e +V 26   f )−(V 26   g +V 26   h ) is shown by a solid line in FIG.  16 B. Similarly, the waveform of (V 26   i +V 26   j )−(V 26   k +V 26   l ) is as shown by a solid line in FIG.  16 C. From the waveforms of FIG.  16 B and FIG. 16C, the waveform of (V 26   e +V 26   f +V 26   i +V 26   j )−(V 26   g +V 26   h +V 26   k +V 26   l ) is as shown by a solid line in FIG.  16 D. From the waveforms of FIG.  16 A and FIG. 16D, the track error signal represented by {(V 26   a +V 26   b )−(V 26   c +V 26   d )}−K{(V 26   e +V 26   f +V 26   i +V 26   j )−(V 26   g +V 26   h +V 26   k +V 26   l )} can be expressed by a waveform as shown by a solid line in FIG.  16 E. 
     On the other hand, the waveforms of (V 26   e +V 26   h )−(V 26   f +V 26   g ) is as shown by a solid line in FIG.  16 F. Similarly, the waveform of (V 26   i +V 26   l )−(V 26   j +V 26   k ) is as shown by a solid line in FIG.  16 G. Because the waveforms of F and G have reversed phases to each other, the land/groove detecting signal expressed by (V 26   e +V 26   h +V 26   j +V 26   k )−(V 26   f +V 26   g +V 26   i +V 26   l ) has a waveform as shown by a solid line in FIG.  16 H. The signal of FIG. 16H has a phase shifted by π/2 from the track error signal of FIG. 16E so as to be negative and positive when the light beam  16   c  is applied to the groove  17   a  and to the land  17   b , respectively, thus enabling to detect a land/groove position. 
     When the objective lens  5  is shifted in a radial direction, the light spots  25   a  to  25   c  on the photo detector  8   c  are also shifted in the radial direction. If it is assumed that the light spots  25   a  to  25   c  are shifted upward in FIG. 14, the light receiving sections  26   a  and  26   b  increase their outputs and the light receiving section  26   c  and  26   d  decrease their outputs. Accordingly, the waveform of (V 26   a +V 26   b )−(V 26   c +V 26   d ) is as shown by a dotted line in FIG.  16 A. The light receiving sections  26   e  and  26   f  increase their outputs and the light receiving sections  26   g  and  26   h  decrease their outputs. Accordingly, the waveform of (V 26   e +V 26   f )−(V 26   g +V 26   h ) is as shown by a dotted line in FIG.  16 B. Similarly, the waveform of the (V 26   i +V 26   j )−(V 26   k +V 26   l ) is as shown by a dotted line of FIG.  16 C. From the waveforms of B and C, the waveform of (V 26   e +V 26   f +V 26   i  +V 26   j )−(V 26   g +V 26   h +V 26   k +V 26   l ) is as shown by a dotted line of FIG.  16 D. In A and D, DC components at an objective lens shift have identical signs. Accordingly, the track error signal expressed by {(V 26   a +V 26   b )−(V 26   c +V 26   d )}−K{(V 26   e +V 26   f +V 26   i +V 26   j )−(V 26   g +V 26   h +V 26   k +V 26   l )} has a waveform as shown by a solid line in FIG.  16 E. That is, even if the objective lens is shifted, no offset is generated in the track error signal. On the other hand, the light receiving sections  26   e  and  26   f  increase their outputs and the light receiving sections  26   g  and  26   h  decrease their outputs. Accordingly, the waveform of (V 26   e +V 26   h )−(V 26   f +V 26   g ) is as shown by a solid line in FIG.  16 F. Similarly, the waveform of (V 26   i +V 26   l )−(V 26   j +V 26   k ) is as shown by a solid line in FIG.  16 G. Because the waveforms of F and G have reversed phases to each other, the land/groove position detecting signal expressed by (V 26   e +V 26   h +V 26   j +V 26   k )−(V 26   f +V 26   g +V 26   i +V 26   l ) has a waveform as shown by a solid line in FIG.  16 H. 
     Here, if the disc  6  has an eccentricity, the sequence of the focal spots on the disc  6  is shifted from the tangential direction. In the first embodiment of the present invention, the phases of the waveforms in FIG.  6 B and FIG. 6C are shifted in opposite directions to each other with respect to the phase of the waveform in FIG.  6 A. If the waveform of FIG. 6A is expressed by A (sin X+C) (wherein C represents a DC component at an objective lens shift), the waveforms of FIG.  6 B and FIG. 6C can be expressed by {−cos(X+Δ)+C} and {B cos(X−Δ)+C} (wherein Δ is a phase shift amount caused by an eccentricity), respectively. Then, the waveform of FIG. 6D can be expressed as follows: 
     
       
           B {−cos( X +Δ)+ C}+B {cos( X −Δ)+ C}= 2 B (sin  X  sin  Δ+C ) 
       
     
     The waveform of FIG. 6E can be expressed as follows: 
     
       
           A (sin  X+C )−2 KB (sin  X  sin Δ+ C )=( A −2 KB  sin Δ)sin  X +( A− 2 KB ) C   
       
     
     The condition for causing no offset in the track error signal when the objective lens is shifted is K=A/2B. Consequently, the waveform of FIG. 6E is A(1−sin Δ)sin X. That is, the eccentricity changes the amplitude of the track error signal by (1−sin Δ). On the other hand, in the second and the third embodiments of the present invention, the waveforms of FIG.  11 B and FIG. 11C as well as FIG.  16 B and FIG. 16C consists of only DC components at the objective lens shift. Accordingly, the eccentricity will cause no change in the amplitude of the track error signal. 
     Here, in the first, the second, and the third embodiments, a light reflected from the disc  6  is received by a photo detector divided into a plurality of regions. However, it is also possible to provide between the disc  6  and the photo detector a holographic optical element divided into a plurality of regions so that diffracted lights from the plurality of regions are received by a plurality of light receiving sections of a photo detector. 
     In the optical head apparatus having the aforementioned configuration, a diffraction grating divided into a plurality of regions having phases shifted by π/2 to each other divides a light emitted from a light source into a 0-th order light and plus and minus 1 st  order diffracted lights and three focal spots are arranged on a single track of an optical recording medium, so as to generate, according to the plus and minus 1 st -order diffracted lights reflected from the optical recording medium, a signal having a phase shifted by π/2 with respect to a track error signal for use in detecting a land/groove position. Thus, it is possible to detect a land/groove position while preventing a offset of a track error signal at an objective lens shift. 
     The invention may be embodied in other specific forms without departing from the spirit or essential characteristic thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein. 
     The entire disclosure of Japanese Patent Application No. 9-299079 (Filed on Oct. 30 th , 1997) including specification, claims, drawings and summary are incorporated herein by reference in its entirety.