Patent Publication Number: US-2011069597-A1

Title: Optical head apparatus, holographic optical device, optical integrated device, optical information processing apparatus, and signal detection method

Description:
TECHNICAL FIELD  
     The present invention relates to an optical head apparatus, a holographic optical device, an optical integrated device, an optical information processing apparatus, and a signal detection method for recording, reproducing or deleting information stored on an optical medium such as an optical disc or an optical card. 
     BACKGROUND ART  
     In recording optical information on optical discs and so on, the servo technology is essential for collecting light to an optical spot at a desired recording and reproducing position. To detect a tracking error signal with the servo technology, it is common to switch among multiple methods according to the type of media on and from which recording and reproducing are to be performed. Therefore, optical head apparatuses are required to detect multiple tracking error signals. For example, optical head apparatuses for DVDs are required to detect a differential phase detection tracking signal (DPD signal) in the case of a reproduction-only DVD-ROM, and a push-pull signal (PP signal) in the case of a recording optical disc represented by DVD-RAMs. 
     Likewise, the Blu-ray system also requires optical head apparatuses capable of detecting the DPD signal and the PP signal. 
     This has led to the proposal of various conventional techniques for detecting the DPD signal and the PP signal (for example, see Patent Literature 1). 
     CITATION LIST  
     Patent Literature 
     [PTL 1] 
     Japanese Unexamined Patent Application Publication No. 2001-229573 
     [PTL 2] 
     Japanese Patent Application No. 2008-168479 
     SUMMARY OF INVENTION 
     Technical Problem 
     Holographic optical devices (also known as holographic optical elements (HOEs)) are widely used for such detection systems since they allow simplification of signal detection optical systems and thus enable implementation of optical head apparatuses that are small in size, low in cost, and high in stability. 
     As an optical head apparatus which includes an HOE capable of detecting the DPD signal and the PP signal, there is an optical head apparatus disclosed in Patent Reference 2. Hereinafter, the optical head apparatus according to Patent Literature 2 is described. Note that although Patent Reference 2 describes a differential push-pull method (DPP method) which is an extension of a push-pull method, the following describes only the push-pull method for simplicity. 
       FIG. 10  is a diagram showing a structure of the optical head apparatus according to Patent Literature 2. 
       FIG. 10  shows a semiconductor laser device  30 , a photoreceptor  40 , a holographic optical device  20 , and others. The semiconductor laser device  30  and the photoreceptor  40  are provided close to each other and fixed to a holding unit  741 . The holding unit  741  is fixed with the holographic optical device  20  with a desired positional relationship via another holding unit (not shown). Note that although the other holding unit may be an optical bench of the optical head apparatus, a unit may be provided into which the holographic optical device  20 , the semiconductor laser device  30 , and the photoreceptor  40  are integrated using a holding member different from the optical bench. Such a unit structure allows the optical system to be stably structured. 
     The optical head apparatus further includes a collimating lens  11  and an object lens  12  that make up a light-collection optical system for collecting laser light on an optical disc  10  that is an information recording medium. In addition, the optical head apparatus includes a lens driving mechanism (not shown) that displaces the object lens  12  in the optical-axis direction of the object lens  12  (z direction) and in the radial direction of the optical disc  10  (x direction). 
     Hereinafter, unless otherwise noted, the optical-axis direction of the light-collection optical system is referred to as Z-axis direction, the radius direction of the optical disc  10  (radial direction) is referred to as X direction, and the track direction of the optical disc  10  (tangential direction) is referred to as Y direction as indicated in  FIG. 10 . Note that even in the case where the optical axis is bent by a mirror, a prism, or the like, the directions of the optical system of the optical head apparatus are defined with reference to the optical axis and a map of the optical disc  10 . 
     A light beam R 0  emitted from the semiconductor laser device  30  passes through the holographic optical device  20  and is collected on the information recording surface of the optical disc  10  by the collimating lens  11  and the object lens  12 . The light reflected from the optical disc  10  is converted by the object lens  12  and the collimating lens  11  into light converging at the light emission point of the semiconductor laser device  30 . This light enters the holographic optical device  20  and is diffracted. The diffracted light enters the photoreceptor  40 , and the photoreceptor  40  detects signals from the diffracted light. 
       FIG. 11  is a diagram showing diffraction regions of the holographic optical device  20  included in the optical head apparatus according to Patent Literature 2. 
     The grating pattern of the holographic optical device  20  is that it is divided into a first diffraction region  261  and a second diffraction region  262  by a straight line L 11  parallel to the X axis and passing through the approximate center of the light beam. R 0  in  FIG. 11  is the light reflected from the optical disc  10  and entering the holographic optical device  20 . R 1  and R 2  are light diffracted by the optical disc  10 , and produce a contrast corresponding to a tracking position in regions interfering with R 0 , that is, overlapping regions. 
       FIG. 12  is a diagram showing photoreception regions of the photoreceptor  40  included in the optical head apparatus according to Patent Literature 2. The photoreceptor  40  has a first photoreception region group  451  and a second photoreception region group  452 . The first photoreception region group  451  includes a first photoreception region  451   a  and a second photoreception region  451   b  facing each other across a first photoreception dividing line L 71  which is approximately parallel to the X axis, whereas the second photoreception region group  452  includes a third photoreception region  452   a  and a fourth photoreception region  452   b  facing each other across a second photoreception dividing line L 72  which is approximately parallel to the X axis. 
     The first diffraction region  261  has a grating pattern that converts light returning from the optical disc  10  into light entering the first photoreception region  451   a  and the second photoreception region  451   b  with a first coma aberration in the X direction across the first photoreception dividing line L 71  of the first photoreception region group  451 . 
     The second diffraction region  262  has a grating pattern that converts light returning from the optical disc  10  into light entering the third photoreception region  452   a  and the fourth photoreception region  452   b  with a second coma aberration which is formed across the second photoreception dividing line L 72  of the second photoreception region group  452  and is opposite in polarity to the first coma aberration caused by the grating pattern of the first diffraction region  261 . 
     Here, the following assumptions are applied: a signal detected in the first photoreception region  451   a  is a first signal S 1 ; a signal detected in the second photoreception region  451   b  is a second signal S 2 ; a signal detected in the third photoreception region  452   a  is a third signal S 3 ; a signal detected in the fourth photoreception region  452   b  is a fourth signal S 4 ; a sum of the first signal S 1  and the fourth signal S 4  is (S 1 +S 4 ); a sum of the second signal S 2  and the third signal S 3  is (S 2 +S 3 ); a sum of the first signal S 1  and the third signal S 3  is (S 1 +S 3 ); and a sum of the second signal S 2  and the fourth signal S 4  is (S 2 +S 4 ). With such assumptions, a focus error (FE) signal in this structure is detected according to the equation below. Note that what is calculated according to the equation is the level (intensity) of a signal. (The same holds true for the other equations.) 
       FE=( S 1 +S 4)−( S 2 +S 3)   (Equation 1)
 
     A tracking error signal TE DPD  according to the DPD method and a tracking error signal TE PP  according to the push-pull method are generated by calculation according to the equations below. 
       TE PP =( S 1 +S 3)−( S 2 +S 4)   (Equation 2)
 
       TE DPD =Phase ( S 1 +S 4,  S 2+ S 3)   (Equation 3)
 
     Here, phase is a function for phase comparison (calculation of phase difference) between two signals. 
     However, the TE signal of the optical head apparatus according to Patent Literature 2 has a problem of being susceptible to assembly errors such as an error in adjusting the photoreceptor  40 . The details are described hereinafter using parts (a) and (b) of  FIG. 13 . 
     The part (a) in  FIG. 13  shows an ideal state, whereas the part (b) in  FIG. 13  shows photoreception regions of the photoreceptor  40  in a state where the photoreceptor  40  is shifted in the tangential direction (Y direction). 
     In the ideal state ((a) in  FIG. 13 ), the light beams R 1  and R 2  generating the push-pull signal enter in such a manner as shown. In this case, the boundary line between the light beams R 1  and R 2  coincides with the first photoreception dividing line L 71  and the second photoreception dividing line L 72 , and thus the signals in each of these two regions are detected without leaking into the other region, thereby enabling detection of the push-pull signal without problems. 
     However, in the case where the photoreceptor  40  is shifted in the tangential direction (Y direction) due to an adjustment error ((b) in  FIG. 13 ), the light beams R 2  leak into the regions of the light beams R 1  (the second photoreception region  451   b  and the fourth photoreception region  452   b ). This causes a decrease in the amplitude of the TE signal TE PP  detected according to the push-pull method shown in Equation 2 above. In addition, the TE signal TE DPD  detected according to the DPD method shown in Equation  3  above also degrades due to crosstalk. 
     As described, the tracking error (TE) signals are susceptible to a shift of the photoreceptor  40  in the tangential direction (Y direction). For this reason, a problem arises that the photoreceptor  40  needs to be adjusted with high precision. 
     Thus, the present invention has been conceived in view of the above problems, and it is an object of the present invention to provide an optical head apparatus, a holographic optical device, an optical integrated device, an optical information processing apparatus, and a signal detection method that enable reduction of the adverse effect of a positional shift of the photoreceptor on tracking signals and enable detection of tracking error signals for more accurate and stable recording and/or reproduction. 
     Solution to Problem 
     In order to achieve the above object, the optical head apparatus according to an aspect of the present invention is an optical head apparatus including: a light source which emits a light beam; a light-collection optical system which receives the light beam and converges the light beam to a minute spot on an information recording medium having tracks; a holographic optical device which diffracts the light beam reflected from the information recording medium; and a photoreceptor which receives the light beam diffracted by the holographic optical device, wherein the photoreceptor includes at least: a first photoreception region in which a first signal S 1  is detected; a second photoreception region in which a second signal S 2  is detected; a third photoreception region in which a third signal S 3  is detected; and a fourth photoreception region in which a fourth signal S 4  is detected, the first photoreception region and the second photoreception region face each other across a first photoreception dividing line, the third photoreception region and the fourth photoreception region face each other across a second photoreception dividing line, the holographic optical device includes a first diffraction region and a second diffraction region, the first diffraction region and the second diffraction region face each other across a region dividing line passing through an optical axis of the light-collection optical system and extending in a radial direction of the information recording medium, the first diffraction region has a grating pattern for generating diffracted light having a first wavefront and entering the first and second photoreception regions, the second diffraction region has a grating pattern for generating diffracted light having a second wavefront and entering the third and fourth photoreception regions, the first wavefront has a first coma aberration in the radial direction of the information recording medium, the first coma aberration having an axis located off the optical axis of the light-collection optical system, and the second wavefront has a second coma aberration in the radial direction of the information recording medium, the second coma aberration having an axis located off the optical axis of the light-collection optical system. 
     With this, the first and second wavefronts have the first and second coma aberrations in the radial direction of the information recording medium, respectively, and the axes of the first and second coma aberrations are located off the optical axis of the light-collection optical system, thereby making it possible to reliably extract tracking signal components even in the case where the position of the photoreceptor is shifted in that direction. 
     Note that the present invention can be realized not only as the optical head apparatus above, but also as an holographic optical device which functions as a diffraction device that diffracts light, the holographic optical device including a first diffraction region and a second diffraction region facing each other across a region dividing line, wherein the first diffraction region generates diffracted light having a first coma aberration in a direction of the region dividing line, the first coma aberration having an axis located off the region dividing line and the second diffraction region generates diffracted light having a second coma aberration in the direction of the region dividing line, the second coma aberration having an axis located off the region dividing line. 
     The present invention can be realized also as an optical integrated device including: a light source which emits a light beam; a holographic optical device which diffracts the light beam reflected from an information recording medium; and a photoreceptor which receives the light beam diffracted by the holographic optical device, wherein the photoreceptor includes at least: a first photoreception region in which a first signal S 1  is detected; a second photoreception region in which a second signal S 2  is detected; a third photoreception region in which a third signal S 3  is detected; and a fourth photoreception region in which a fourth signal S 4  is detected, the first photoreception region and the second photoreception region face each other across a first photoreception dividing line, the third photoreception region and the fourth photoreception region face each other across a second photoreception dividing line, the holographic optical device includes a first diffraction region and a second diffraction region, the first diffraction region and the second diffraction region face each other across a region dividing line passing through an optical axis of a light-collection optical system and extending in a radial direction of the information recording medium, the first diffraction region has a grating pattern for generating diffracted light having a first wavefront and entering the first and second photoreception regions, the second diffraction region has a grating pattern for generating diffracted light having a second wavefront and entering the third and fourth photoreception regions, the first wavefront has a first coma aberration in the radial direction of the information recording medium, the first coma aberration having an axis located off the optical axis of the light-collection optical system, and the second wavefront has a second coma aberration in the radial direction of the information recording medium, the second coma aberration having an axis located off the optical axis of the light-collection optical system. 
     The present invention can be realized also as a signal detection method performed by an optical head apparatus, wherein the optical head apparatus includes: a light source which emits a light beam; a light-collection optical system which receives the light beam and converges the light beam to a minute spot on an information recording medium having tracks; a holographic optical device which diffracts the light beam reflected from the information recording medium; and a photoreceptor which receives the light beam diffracted by the holographic optical device, the photoreceptor includes at least: a first photoreception region in which a first signal S 1  is detected; a second photoreception region in which a second signal S 2  is detected; a third photoreception region in which a third signal S 3  is detected; and a fourth photoreception region in which a fourth signal S 4  is detected, the first photoreception region and the second photoreception region face each other across a first photoreception dividing line, the third photoreception region and the fourth photoreception region face each other across a second photoreception dividing line, the holographic optical device includes a first diffraction region and a second diffraction region, the first diffraction region and the second diffraction region face each other across a region dividing line passing through an optical axis of the light-collection optical system and extending in a radial direction of the information recording medium, the signal detection method includes: generating, in the first diffraction region, diffracted light having a first wavefront and entering the first and second photoreception regions; and generating, in the second diffraction region, diffracted light having a second wavefront and entering the third and fourth photoreception regions, the first wavefront has a first coma aberration in the radial direction of the information recording medium, the first coma aberration having an axis located off the optical axis of the light-collection optical system, and the second wavefront has a second coma aberration in the radial direction of the information recording medium, the second coma aberration having an axis located off the optical axis of the light-collection optical system. 
     In addition, the present invention can be realized also as an optical information processing apparatus including: the optical head apparatus above; and a circuit which performs focus servo using a focus error signal generated by calculating (S 1 −S 2 ) or (S 3 −S 4 ), or both (S 1 −S 2 ) and (S 3 −S 4 ), where (S 1 −S 2 ) is a difference between the first signal S 1  and the second signal S 2  and (S 3 −S 4 ) is a difference between the third signal S 3  and the fourth signal S 4 . 
     Advantageous Effects of Invention 
     With the optical head apparatus and so on according to an implementation of the present invention, it is possible to reduce the adverse effect of a positional shift of the photoreceptor on tracking signals and to detect tracking error signals for more accurate and stable recording and/or reproduction. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram showing a structure of an optical head apparatus according to Embodiment 1 of the present invention. 
         FIG. 2  is a plan view showing a holographic optical device of Embodiment 1 of the present invention. 
         FIG. 3  is a plan view showing a photoreceptor of Embodiment 1 of the present invention. 
         FIG. 4  is a diagram showing calculations of Embodiment 1 of the present invention. 
       [ FIG. 5 ] The parts (a) to (e) in  FIG. 5  are spot diagrams of Embodiment 1 of the present invention. 
         FIG. 6  is a graph showing focus error signals of Embodiment 1 of the present invention. 
         FIG. 7  is a diagram showing a structure of an optical head apparatus according to Embodiment 2 of the present invention. 
         FIG. 8  is a plan view showing a photoreceptor of Embodiment 2 of the present invention. 
         FIG. 9  is a structural diagram of an optical information processing apparatus of Embodiment 3 of the present invention. 
         FIG. 10  is a diagram showing a structure of an optical head apparatus according to Patent Literature 2. 
         FIG. 11  is a plan view showing a holographic optical device of an optical head apparatus according to Patent Literature 2. 
         FIG. 12  is a plan view showing a photoreceptor of an optical head apparatus according to Patent Literature 2. 
       [ FIG. 13 ] The parts (a) and (b) in  FIG. 13  show states of light spots on a photoreceptor of an optical head apparatus according to Patent Literature 2. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, an optical head apparatus, a holographic optical device, an optical integrated device, an optical information processing apparatus, and a signal detection method according to an implementation of the present invention are described in detail with reference to the drawings. 
     Embodiment 1 
     First, the optical head apparatus according to Embodiment 1 of the present invention is described. 
       FIG. 1  is a schematic diagram showing a structure of the optical head apparatus according to Embodiment 1 of the present invention. 
     The optical head apparatus includes: a semiconductor laser device  30  which emits a light beam; a light-collection optical system (a collimating lens  11  and an object lens  12 ) which receives the light beam and converges the light beam to a minute spot on an optical disc  10  having tracks (information recording medium); a holographic optical device  20  which diffracts the light beam reflected from the optical disc  10 ; and a photoreceptor  40  which receives the light diffracted by the holographic optical device. 
     The semiconductor laser device  30  and the photoreceptor  40  are provided close to each other and fixed to a holding unit  741 . The holding unit  741  is fixed with the holographic optical device  20  with a desired positional relationship via another holding unit (not shown). Note that although the holding unit  741  may be an optical bench of the optical head apparatus, a more stable optical system can be provided by integrating the semiconductor laser device  30  and the photoreceptor  40  into an optical integrated device by using a holding member different from the optical bench. In addition, further stabilization can be achieved by integrating the semiconductor laser device  30 , the photoreceptor  40 , and the holographic optical device  20  into an optical integrated device. 
     The collimating lens  11  and the object lens  12  make up a light-collection optical system for collecting laser light on the optical disc  10  that is an information recording medium. The optical head apparatus further includes a lens driving mechanism (not shown) that displaces the object lens  12  in the optical-axis direction of the object lens  12  (z direction) and in the radial direction of the optical disc  10  (x direction). 
     Hereinafter, unless otherwise noted, the optical-axis direction of the light-collection optical system is referred to as Z-axis direction, the radius direction of the optical disc  10  (radial direction) is referred to as X direction, and the track direction of the optical disc  10  (tangential direction) is referred to as Y direction as indicated in  FIG. 1 . Note that even in the case where the optical axis is bent by a mirror, a prism, or the like, the directions of the optical system of the optical head apparatus are defined with reference to the optical axis and a map of the optical disc  10 . 
     First, a light beam emitted from the semiconductor laser device  30  of the optical head apparatus of Embodiment 1 is described. A light beam R 0  emitted from the semiconductor laser device  30  passes through the holographic optical device  20  and is collected on the information recording surface of the optical disc  10  by the collimating lens  11  and the object lens  12 . The light reflected from the optical disc  10  is converted by the object lens  12  and the collimating lens  11  into light converging at the light emission point of the semiconductor laser device  30 . This light enters the holographic optical device  20  and is diffracted. The diffracted light enters the photoreceptor  40 , and the photoreceptor  40  detects signals from the diffracted light. 
     The following is a description of the details on diffraction regions formed on the holographic optical device  20  and photoreception regions formed on the photoreceptor  40 . 
       FIG. 2  is a diagram showing the diffraction regions of the holographic optical device  20  according to the present embodiment. 
     The grating pattern of the holographic optical device  20  is that it is divided into a first diffraction region  261  and a second diffraction region  262  by a straight line (region dividing line) L 11  parallel to the X axis and passing through the approximate center of the light beam. R 0  in  FIG. 2  is the light reflected from the optical disc  10  and entering the holographic optical device  20 . R 1  and R 2  are light diffracted by the optical disc  10 , and produce a contrast corresponding to a tracking position in regions interfering with R 0 , that is, overlapping regions.  FIG. 2  separately shows a region that the light beams R 0  and R 1  both enter (hereinafter referred to as region R 1 ), a region that the light beams R 0  and R 2  both enter (hereinafter referred to as region R 2 ), and a region that only the light beam R 0  enters (hereinafter referred to as region R 0 ). 
       FIG. 3  is a diagram showing photoreception regions of the photoreceptor  40  according to the present embodiment. The photoreceptor  40  has a first photoreception region group  451  and a second photoreception region group  452 . The first photoreception region group  451  includes a first photoreception region  451   a  and a second photoreception region  451   b  facing each other across a first photoreception dividing line L 71  which is approximately parallel to the X axis. The second photoreception region group  452  includes a third photoreception region  452   a  and a fourth photoreception region  452   b  facing each other across a second photoreception dividing line L 72  which is approximately parallel to the X axis. 
     The first diffraction region  261  has a grating pattern that converts light returning from the optical disc  10  into light having a first wavefront and entering the first photoreception region  451   a  and the second photoreception region  451   b  with a first coma aberration in the X direction across the first photoreception dividing line L 71  of the first photoreception region group  451 . Note that the center of the first coma aberration is at a position shifted in the tangential direction (Y direction) from the optical axis. 
     The second diffraction region  262  has a grating pattern that converts light returning from the optical disc  10  into light having a second wavefront and entering the third photoreception region  452   a  and the fourth photoreception region  452   b  with a second coma aberration which is formed across the second photoreception dividing line L 72  of the second photoreception region group  452  and is opposite in polarity to the first coma aberration caused by the grating pattern of the first diffraction region  261 . Note that as in the first diffraction region  261 , the center of the second coma aberration is at a position shifted in the tangential direction (Y direction) from the optical axis. 
     Here, the light has the second coma aberration which is opposite in polarity to the first coma aberration caused by the grating pattern of the first diffraction region  261 , for the purpose of separating a tracking signal and a focus signal which are to be described later. 
     In addition, there is also an advantageous effect of canceling out the offset of a focus error signal generated due to a Y-directional shift of the photoreceptor, by reversing the sign of a later-described calculation performed for generating a focus error signal using the first photoreception region  451   a  and the third photoreception region  452   a  as well as the second photoreception region  451   b  and the fourth photoreception region  452   b.    
     In  FIG. 3 , a spot  601  indicates light diffracted by the first diffraction region  261  and a spot  602  indicates light diffracted by the second diffraction region  262 . The spots  601  and  602  distinguishably show the light diffracted by each of the regions R 0 , R 1 , and R 2 , using the same shading patterns as in  FIG. 2 . 
     The signals from the photoreception regions undergo processing by a calculation circuit shown in  FIG. 4  so that a focus error signal (FE signal) and a tracking error signal (TE signal) are detected. 
     First, the principle of the focus error signal detection is described. 
     The parts (a) to (e) of  FIG. 5  are spot diagrams showing light spots on the photoreceptor  40 , and  FIG. 6  is a graph showing focus error signals obtained. 
     The parts (a) to (e) of  FIG. 5  are spot diagrams each corresponding to one of positions (a) to (e) of the optical disc  10  that are indicated in  FIG. 6 . Note that in  FIG. 6 , the origin point is set to a disc position at which a focused state is reached, that is, the smallest spot is formed on the information recording surface of the optical disc  10  ((c) in  FIGS. 5 and 6 ). 
     As previously described, the FE signal is detected by the circuit shown in  FIG. 4 . Here, the following assumptions are applied: a signal detected in the first photoreception region  451   a  is a first signal S 1 ; a signal detected in the second photoreception region  451   b  is a second signal S 2 ; a signal detected in the third photoreception region  452   a  is a third signal S 3 ; a signal detected in the fourth photoreception region  452   b  is a fourth signal S 4 ; a sum of the first signal S 1  and the fourth signal S 4  is (S 1 +S 4 ); and a sum of the second signal S 2  and the third signal S 3  is (S 2 +S 3 ). With such assumptions, one of the calculations performed by this circuit can be expressed as Equation 4 below. 
       FE=( S 1 +S 4)−( S 2+ S 3)   (Equation 4)
 
     First, in the focused state ((c) in  FIGS. 5 and 6 ), the second signal S 2  (signal detected in the second photoreception region  451   b ), and the first signal S 1  (signal detected in the first photoreception region  451   a ) are in balance and the third signal S 3  (signal detected in the third photoreception region  452   a ) and the fourth signal S 4  (signal detected in the fourth photoreception region  452   b ) are also in balance, making the focus error signal FE in Equation 4 zero. 
     When the optical disc  10  gets closer to the object lens  12  than it is in the focused state ((b) in  FIGS. 5 and 6 ), the spot  601  moves from the first photoreception region  451   a  toward the second photoreception region  451   b  accordingly. Likewise, the spot  602  moves from the fourth photoreception region  452   b  toward the third photoreception region  452   a . As a result, the focus error signal FE in Equation 4 above has a negative value. 
     When the optical disc  10  gets further closer to the object lens  12 , the spot  601  moves almost entirely to the second photoreception region  451   b , and the spot  602  moves almost entirely to the third photoreception region  452   a  as shown in (a) in  FIGS. 5 and 6 . In this state, the focus error signal FE has a minimum value. 
     On the other hand, when the optical disc  10  gets farther from the object lens  12  than it is in the focused state ((d) in  FIGS. 5 and 6 ), the spot  601  moves from the second photoreception region  451   b  toward the first photoreception region  451   a  accordingly. Likewise, the spot  602  moves from the third photoreception region  452   a  toward the fourth photoreception region  452   b . As a result, the focus error signal FE in Equation 4 above has a positive value. When the optical disc  10  gets even farther from the object lens  12 , the spot  601  moves almost entirely to the first photoreception region  451   a , and the spot  602  moves almost entirely to the fourth photoreception region  452   b  as shown in (e) in  FIGS. 5 and 6 . In this state, the focus error signal FE has a maximum value. 
     In such a manner, the focus error signal FE that varies according to the position of the optical disc  10  can be obtained. 
     Note that the distance between the position at which the focus error signal FE has the maximum value and the position at which the focus error signal FE has the minimum value, that is, the range in which the focus error signal is detected, can be designed as desired through adjustment of the amounts of the first and second coma aberrations of the light generated by the holographic optical device  20 . 
     Next, the principle of the tracking error signal detection is described with reference to  FIG. 3 . 
     Here, assuming that a sum of the first signal S 1  and the third signal S 3  is (S 1 +S 3 ) and a sum of the second signal S 2  and the fourth signal S 4  is (S 2 +S 4 ), a tracking error signal TE PP  according to the push-pull method and a tracking error signal TE DPD  according to the DPD method are generated by calculation according to the equations below. 
       TE PP =( S 1+ S 3)−( S 2 +S 4)   (Equation 5)
 
       TE DPD =Phase ( S 1+ S 4,  S 2+ S 3)   (Equation 6)
 
     Here, phase is a function for phase comparison (calculation of phase difference) between two signals. 
     Equation 5 above represents a differential between the interference region of the light beams R 0  and R 1  and the interference region of the light beams R 0  and R 2 , and thus allows detection of the push-pull signal equivalent to that disclosed in the technique according to Patent Literature 2. 
     Equation 6 above compares the phases of the sums of diagonally opposite signals, and thus allows detection of a signal equivalent to the differential phase detection tracking error signal disclosed in the technique according to Patent Literature 2. 
     A feature of the optical head apparatus of the present embodiment is that the light containing the tracking signal components and passing through the regions R 1  and R 2  enters a position located off the first photoreception dividing line L 71  and the second photoreception dividing line L 72 . This is achieved by having the centers of the above-described first and second coma aberrations shifted in the tangential direction (Y direction). This makes it possible, even when the photoreceptor  40  is shifted in the tangential direction (Y direction), to provide an optical head apparatus capable of reliably extracting the tracking signal components and less susceptible to an error in adjusting the photoreceptor  40 . 
     As described thus far, according to Embodiment 1, it is possible to detect a tracking error signal in a manner less susceptible to a shift of the photoreceptor  40  in the tangential direction (Y direction) caused by an adjustment error, for example. 
     Note that although what is described above is the structure in which the centers of the first and second coma aberrations are shifted only in the tangential direction (Y direction), the present invention is not limited to this. Any structure is acceptable as long as the centers of the first and second coma aberrations are located off a straight line passing through the optical axis and extending in the radial direction, that is, as long as the positional vectors of the first and second coma aberrations have a Y-directional component. 
     Embodiment 2 
     Next, the optical head apparatus according to Embodiment 2 of the present invention is described. 
       FIG. 7  is a schematic diagram showing a structure of the optical head apparatus according to Embodiment 2 of the present invention. 
       FIG. 7  shows a semiconductor laser device  30 , a photoreceptor  41 , a holographic optical device  20 , and others. A diffraction grating  24  is formed on a surface of the holographic optical device  20  closer to the semiconductor laser device  30 . The semiconductor laser device  30  and the photoreceptor  41  are provided close to each other and fixed to a holding unit  741 . The holding unit  741  is fixed with the holographic optical device  20  with a desired positional relationship via another holding unit (not shown). Note that although the other holding unit may be an optical bench of the optical head apparatus, a unit (an optical integrated device, for example) may be provided into which the holographic optical device  20 , the semiconductor laser device  30 , and the photoreceptor  41  are integrated using a holding member different from the optical bench. Such a unit structure allows the optical system to be stably structured. 
     The optical head apparatus further includes a collimating lens  11  and an object lens  12  that make up a light-collection optical system for collecting laser light on an optical disc  10  that is an information recording medium. In addition, the optical head apparatus includes a lens driving mechanism (not shown) that displaces the object lens  12  in the optical-axis direction of the object lens  12  (z direction) and in the radial direction of the optical disc  10  (x direction). 
     First, a light beam emitted from the semiconductor laser device  30  of the optical head apparatus of Embodiment 2 is described. A light beam R 0  emitted from the semiconductor laser device  30  is separated into a main beam (R 0   a ) that is zero-order light and two sub beams R 0   b  and R 0   c  that are ± first-order light (not shown) through diffraction at a desired ratio by the diffraction grating  24 . These beams pass through the holographic optical device  20  and are collected on the information recording surface of the optical disc  10  by the collimating lens  11  and the object lens  12 . The light reflected from the optical disk  10  is converted by the object lens  12  and the collimating lens  11  into light converging at the light emission point of the semiconductor laser device  30 . This light enters the holographic optical device  20  and is diffracted. The diffracted light enters the photoreceptor  41 , and the photoreceptor  41  detects signals from the diffracted light. Here, the region of the diffraction grating  24  is set to an appropriate size such that the light diffracted by the holographic optical device  20  is not diffracted. 
     As with the holographic optical device described in Embodiment 1, the holographic optical device  20  has the same first diffraction region  261  and the same second diffraction region  262  as those in  FIG. 2 , and these diffraction regions have the same grating patterns as those in  FIG. 2 . The structure of the photoreceptor  41  is like that shown in  FIG. 8 . 
     The photoreceptor  41  has a first photoreception region group  451  and a second photoreception region group  452 . The first photoreception region group  451  includes a first photoreception region  451   a  and a second photoreception region  451   b  facing each other across a first photoreception dividing line L 71  which is approximately parallel to the X axis. The second photoreception region group  452  includes a third photoreception region  452   a  and a fourth photoreception region  452   b  facing each other across a second photoreception dividing line L 72  which is approximately parallel to the X axis. 
     The photoreceptor  41  also has a third photoreception region group  453  and a fourth photoreception region group  454  on the Y-directional sides of the first photoreception region group  451  and the second photoreception region group  452 . 
     The third photoreception region group  453  includes a fifth photoreception region  453   a  and a sixth photoreception region  453   b  facing each other across a third photoreception dividing line L 73  which is approximately parallel to the X axis. 
     The fourth photoreception region group  454  includes a seventh photoreception region  454   a  and an eighth photoreception region  454   b  facing each other across a fourth photoreception dividing line L 74  which is approximately parallel to the X axis. 
     The previously-described first diffraction region  261  has a grating pattern that forms a spot  601   a  through which the main beam (R 0   a ), which is part of the light returning from the optical disc  10 , enters the first photoreception region  451   a  and the second photoreception region  451   b  with a first coma aberration in the x direction across the first photoreception dividing line L 71  of the first photoreception region group  451 . 
     At this time, the light on the positive side of the X axis in  FIG. 2  is detected in the second photoreception region  451   b , whereas the light on the negative side is detected in the first photoreception region  451   a . With this, a tracking error signal can be detected according to the push-pull method. 
     Furthermore, the second diffraction region  262  has a grating pattern that forms a spot  602   a  through which the main beam (R 0   a ), which is part of the light returning from the optical disc  10 , enters the third photoreception region  452   a  and the fourth photoreception region  452   b  with a second coma aberration which is formed across the second photoreception dividing line L 72  of the second photoreception region group  452  and is opposite in polarity to the first coma aberration caused by the grating pattern of the first diffraction region  261 . 
     At this time, the light on the positive side of the X axis in FIG.  2  is detected in the third photoreception region  452   a , whereas the light on the negative side is detected in the fourth photoreception region  452   b . With this, a tracking error signal can be detected according to the push-pull method. 
     The sub beam R 0   b  enters positions spanning the third photoreception dividing line L 73 . More specifically, the light diffracted in the first diffraction region  261  enters a spot  601   b , and the light diffracted in the second diffraction region  262  enters a spot  602   b . Furthermore, the sub beam R 0   c  enters positions spanning the fourth photoreception dividing line L 74 . More specifically, the light diffracted in the first diffraction region  261  enters a spot  601   c,  and the light diffracted in the second diffraction region  262  enters a spot  602   c.    
     Like from the main beam, the tracking error signal can be detected according to the push-pull method also from the sub beams using signals detected in the two detection regions that the light from the corresponding spots enters. 
     With the optical head apparatus of the present embodiment, a focus error signal is detected using a later-described detection method according to an implementation of the present invention, and a tracking error signal TE DPD  according to the DPD method and a tracking error signal TE DPP  according to the DPP method are generated by calculation according to the equations below. 
       FE=( S 1 +S 4)−( S 2+ S 3)   (Equation 7)
 
       TE DPP =TE MPP   −K ·TE SPP    (Equation 8)
 
       TE DPD =phase ( S 2,  S 1)−phase ( S 3,  S 4)   (Equation 9)
 
     Here, the following assumptions are applied: a signal detected in the fifth photoreception region  453   a  is a fifth signal S 5 ; a signal detected in the sixth photoreception region  453   b  is a sixth signal S 6 ; a signal detected in the seventh photoreception region  454   a  is a seventh signal S 7 ; a signal detected in the eighth photoreception region  454   b  is an eighth signal S 8 ; a sum of the fifth signal S 5  and the seventh signal S 7  is (S 5 +S 7 ); and a sum of the sixth signal S 6  and the eighth signal S 8  is (S 6 +S 8 ). With such assumptions, TE MPP  which is a push-pull signal of the main beam and TE SPP  which is a push-pull signal of the sub beams can be given according to the equations below. 
       TE MPP =( S 1 +S 3)−( S 2 +S 4)   (Equation 10)
 
       TE SPP =( S 5 +S 7)−( S 6 +S 8)   (Equation 11)
 
     K is a constant optimized so that fluctuations of TE MPP  caused by a shift of the object lens  12  are minimized. 
     Furthermore, a signal RF for reading recorded information is also generated. 
     A feature of the optical head apparatus of the present embodiment, too, is that the light containing the tracking signal components and passing through the regions R 1  and R 2  enters a position located off the first photoreception dividing line L 71  and the second photoreception dividing line L 72 . 
     This is achieved by having the centers of the above-described first and second coma aberrations shifted in the tangential direction (Y direction). This makes it possible, even when the photoreceptor  41  is shifted in the tangential direction (Y direction), to provide an optical head apparatus capable of reliably extracting the tracking signal components and less susceptible to an error in adjusting the photoreceptor  41 . 
     In addition, with the optical head apparatus of the present embodiment, the light of the sub beams passing through the regions R 1  and R 2  enters positions located off the third photoreception dividing line L 73  and the fourth photoreception dividing line L 74 . This makes it possible to accept not only the shift of the photoreceptor  41  in the Y direction but also a change in the distance between the sub beams caused by such factors as a change in the wavelength of the semiconductor laser device  30  and a shift of the diffraction grating  24  in the optical-axis direction. 
     As described thus far, according to Embodiment 2, it is possible to detect a tracking error signal in a manner that is less susceptible to a change in the distance between the sub beams and a shift of the photoreceptor  41  in the tangential direction (Y direction) caused by an adjustment error, for example. 
     Note that although what is described above is the structure in which the centers of the first and second coma aberrations are shifted only in the tangential direction (Y direction), the present invention is not limited to this. Any structure is acceptable as long as the centers of the first and second coma aberrations are located off a straight line passing through the optical axis and extending in the radial direction, that is, as long as the positional vectors of the first and second coma aberrations have a Y-directional component. 
     Embodiment 3 
     Next, the optical information processing apparatus (optical disc apparatus) of Embodiment 3 of the present invention is described. 
       FIG. 9  is a diagram showing a structure of the optical information processing apparatus of Embodiment  3  of the present invention. The optical information processing apparatus includes an optical disc  10 , an electric circuit  59 , an optical head apparatus  76 , a driving apparatus  79 , and a rotation mechanism  78 . 
     The rotation mechanism  78  is a mechanism that holds and rotates the optical disc  10 . The optical head apparatus  76  is the optical head apparatus of either Embodiment 1 or Embodiment 2 and includes a unit for finely adjusting the object lens  12 . The optical head apparatus  76  is coarsely adjusted by the driving apparatus  79  to a track of the optical disc  10  where desired information is recorded. The optical head apparatus  76  then sends a signal to the driving apparatus  79 . The electric circuit  59  has all or some of the calculation functions shown in  FIG. 4  and generates a TE signal and an FE signal. Based on these signals, the electric circuit  59  sends a signal for finely adjusting the optical head apparatus  76  and the object lens  12 , and performs focus servo and tracking servo. 
     A reproduction signal is generated as a sum of signals detected by the photoreceptor  40  in either the optical head apparatus  76  or the electric circuit  59 , and is output as a data raw signal after undergoing signal processing such as processing by an equalizer. 
     With the optical information processing apparatus of the present embodiment, the tracking error signal can be stably detected even when the photoreceptor  40  of the optical head apparatus  76  is shifted, and thus tracking servo can be stably performed, enabling favorable recording and reproduction. 
     Although the optical head apparatus, the holographic optical device, the optical integrated device, the optical information processing apparatus, and the signal detection method according to an implementation of the present invention have been described above based on Embodiments 1 to 3, the present invention is not limited to such embodiments. The scope of the present invention also includes what a person skilled in the art can conceive without departing from the scope of the present invention; for example, implementations realized by making various modifications to the above embodiments and implementations realized by arbitrarily combining the constituent elements of the embodiments. 
     INDUSTRIAL APPLICABILITY  
     The optical head apparatus, the holographic optical device, the optical integrated device, the optical information processing apparatus, and the signal detection method according to the present invention can be used for recording information on an information storage medium and reproducing the recorded information, and are useful as video/audio recording and reproduction apparatuses and so on. In addition, they can also be applied for storing data and programs of a computer, storing map data of a car navigation system, and so on. 
     REFERENCE SIGNS LIST  
       10  Optical disc 
       11  Collimating lens 
       12  Object lens 
       20  Holographic optical device 
       24  Diffraction grating 
       30  Semiconductor laser device 
       40  Photoreceptor 
       41  Photoreceptor 
       59  Electric circuit 
       76  Optical head apparatus 
       78  Rotation mechanism 
       79  Driving apparatus