Abstract:
An optical disc apparatus capable of mounting an optical disc includes a light source for emitting light; an objective lens for collecting the light emitted by the light source on the optical disc; a first light distribution section integrally movable with the objective lens, the first light distribution section including a first area and a second area, the first light distribution section outputting the light reflected by the optical disc and transmitted through the first area or the second area as transmission light, outputting the light reflected by the optical disc and diffracted by the first area as first diffraction light, and outputting the light reflected by the optical disc and diffracted by the second area as second diffraction light; a transmission light detection section for detecting the transmission light and outputting a TE1 signal indicating an offset of the detected transmission light; a first diffraction light detection section for detecting the first diffraction light and the second diffraction light, and outputting a TE2 signal indicating a difference between a light amount of the detected first diffraction light and a light amount of the detected second diffraction light; and a control device for generating a tracking error signal for the optical disc based on the TE1 signal and the TE2 signal.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an optical disc apparatus, and in particular to an optical disc apparatus for finding an accurate tracking error signal for an optical disc. 
     2. Description of the Related Art 
     An optical disc is known as an information recording medium for storing a large amount of data. An optical disc can store information on tracks thereof, and also allow information recorded thereon to be reproduced. An optical disc apparatus is capable of mounting an optical is other on and is used for recording information on the optical disc and/or reproducing information stored on the optical disc. In order to allow the optical disc apparatus to record information to or reproduce information from an appropriate track accurately, a laser beam needs to accurately follow the tracks on the optical disc. The operation of the laser beam to follow the tracks on the optical disc is referred to as “tracking”. A tracking error signal shows whether the laser beam is accurately following the tracks on the optical disc. 
     Hereinafter, a conventional optical disc apparatus and a tracking error signal provided by the conventional optical disc apparatus will be described. 
     FIG. 10A shows a conventional optical disc apparatus  1000 . Laser light emitted by a laser light source  1010  is converged on an optical disc  1070  through an optical system  1015 . The light reflected by the optical disc  1070  is detected by a photodetector  1050 . Based on a result detected by the photodetector  1050 , a control device  1085  controls an element or elements among the light source  1010 , the optical system  1015 , and the optical disc  1070  as necessary. The optical system  1015  includes, for example, a polarizing beam splitter  1020  having a splitting face  1025 , a collimator lens  1030 , a quarter-wave plate  1042 , a reflecting mirror  1040 , and an objective lens  1060 . 
     A more specific operation of the optical disc apparatus  1000  will be described. 
     Laser light emitted by the light source  1010  is incident on the polarizing beam splitter  1020 , transmitted through the splitting face  1025  of the polarizing beam splitter  1020 , and then converted into parallel light by the collimator lens  1030 . The parallel light, which is linearly polarized (P wave) is converted into circular polarization, by the quarter-wave plate  1042 , and then reflected by the reflecting mirror  1040 . The reflected light is converged by the objective lens  1060  on a signal face  1074  of the optical disc  1070 . 
     The optical disc  1070  has the signal face  1074  between a substrate  1072  and a protection film  1076 . The signal face  1074  has pits (or grooves) formed in a diameter direction of the optical disc  1070  (indicated by arrow X). The pits each have a depth d and a width w, and are arranged at a pitch p. The diameter direction of the optical disc  1070  is perpendicular to the direction of the light incident on the optical disc  1070  and parallel to the sheet of paper of FIG.  10 A. 
     The light reflected by the signal face  1074 , which is circular polarization, is transmitted through the objective lens  1060 , reflected by the reflecting mirror  1040 , and then converted into linear polarization (S wave) by the quarter-wave plate  1042 . The light is made convergent by the collimator lens  1030 , reflected by the splitting face  1025  of the polarizing beam splitter  1020 , and then collected on the photodetector  1050  as light  1080 . Based on a signal detected by the photodetector  1050 , the control device  1085  controls an element or elements among the light source  1010 , the optical system  1015 , and the optical disc  1070  as necessary. 
     In FIG. 10A, reference numeral  1210  represents an optical axis of the optical disc apparatus  1000 . 
     FIG. 10B shows a structure of the photodetector  1050 . The photodetector  1050  includes sub-photodetectors  1050 A and  1050 B. A separation line  1051  shows the border between the sub-photodetectors  1050 A and  1050 B. The sub-photodetector  1050 A and  1050 B each provide a respective light amount. A tracking error signal  1091   s  (TE1 signal) is obtained by subjecting the light amounts provided by the sub-photodetectors  1050 A and  1050 B to subtraction performed by a subtracter  1091 . A reproduction signal  1092  is obtained by subjecting the light amounts provided by the sub-photodetectors  1050 A and  1050 B to addition performed by an adder  1092 . The separation line  1051  substantially equally divides a convergence spot  1081  on the photodetector  1050 . The control device  1085  controls an element or elements among the light source  1010 , the optical system  1015 , and the optical disc  1070  as necessary, so as to make the level of the TE1 signal zero in order to eliminate a tracking error. 
     FIG. 11A shows another conventional optical disc apparatus  1100 . Laser light emitted by a laser light source  1110  is converged on an optical disc  1170  through an optical system  1115 . The light reflected by the optical disc  1170  is detected by a photodetector  1190 , Based on a result detected by the photodetector  1190 , a control device  1185  controls an element or elements among the light source  1110 , the optical system  1115 , and the optical disc  1170  as necessary. The optical system  1115  includes, for example, a collimator lens  1130 , a quarter-wave plate  1142 , a polarizing holographic element  1145 , and an objective lens  1160 . 
     A more specific operation of the optical disc apparatus  1100  will be described. 
     Laser light emitted by the light source  1110  is converted into parallel light by the collimator lens  1130  and incident on the polarizing holographic element  1145 . 
     The polarizing holographic element  1145  is integrated into a lens holder  1165  together with the objective lens  1160 . The polarizing holographic element  1145  has the quarter-wave plate  1142 . A surface of the polarizing holographic element  1145  is a polarizing holographic face  1150 . 
     The light, which is linear polarization (P wave) incident on the polarizing holographic element  1145  is transmitted through the polarizing holographic face  1150  and converted into circular polarization by the quarter-wave plate  1142 , collected by the objective lens  1160 , and then converged on a signal face  1174  of the optical disc  1170 . 
     The optical disc  1170  has the signal lace  1174  between a substrate  1172  and a protection film  1176 . The signal face  1174  has pits (or grooves) formed in a rotation direction of the optical disc  1170 . The pits each have a depth d and a width w, and arranged at a pitch p. 
     The light reflected by the signal face  1174 , which is circular polarization, is transmitted through the objective lens  1160 , converted into linear polarizatlon (S wave) by the quarter-wave plate  1142 , and then diffracted by the polarizing holographic face  1150 . The diffraction light is transmitted through the collimator lens  1130  and incident on the photodetector  1190 . Based on a signal detected by the photodetector  1190 ; the control device  1185  controls an element or elements among the light source  1110 , the optical system  1115 , and the optical disc  1170  as necessary. 
     FIG. 11B shows a structure of the polarizing holographic face  1150 . The polarizing holographic face  1150  includes two areas  1150   a  and  1150   b  which are separated from each other by a separation line  1152 . The light reflected by the optical disc  1170  is substantially equally divided into two by the separation line  1152 . 
     FIG. 11C shows a structure of the photodetector  1190 . The photodetector  1190  includes two sub-photodetectors  1190 A and  1190 B separated from each other by a separation line  1191 . The light diffracted by the area  1150   a  (FIG. 11B) of the polarizing holographic face  1150  is collected on the sub-photodetector  1190 A as a spot  1181   a.  The light diffracted by the area  1150   b  (FIG. 11B) of the polarizing holographic face  1150  is collected on the sub-photodetector  1190 B as a spot  1181   b.  The sub-photodetectors  1190 A and  1190 B each provide a respective light amount. A tracking error signal  1101   s  (TE2 signal) is obtained by subjecting the light amounts provided by the sub-photodetectors  1190 A and  1190 B to subtraction performed by a subtracter  1101 . A reproduction signal  1102 B is obtained by subjecting the light amounts provided by the sub-photodetectors  1190 A and  1190 B to addition performed by an adder  1102 . The control device  1185  controls an element or elements among the light source  1110 , the optical system  1115 , and the optical disc  1170  as necessary, so as to make the level of the TE2 signal zero in order to eliminate a tracking error. 
     The tracking error signals (TE1 signal and TE2 signal) obtained by the conventional optical disc apparatuses  1000  and  1100  have the following problems. First, the tracking error signal obtained by the conventional optical disc apparatus  1000  (TE1 signal) will be described. 
     Generally in the optical disc  1000 , in which the control device  1085  performs tracking control, when the optical disc  1070  vibrates with respect to the center thereof, the objective lens  1060  follows the vibration and is shifted in the diameter direction K (FIG.  10 A). 
     FIG. 12 (parts (a) through (d)) shows light intensity distributions of a cross-section of the optical disc  1070  when a central axis  1220  (part (e)) of the objective lens  1060  is shifted rightward by distance X with respect the optical axis  1210  of the optical disc apparatus  1000  (FIG.  1 ). The cross-section is taken along the diameter direction of the optical disc apparatus  1070 . Part (e) schematically shows the positional relationship between the optical axis  1210  and the central axis  1220  of the objective lens  1060 . 
     In FIG. 12, part (a) shows a light intensity distribution  1231  before the light emitted by the light source  1030  is transmitted through the objective lens  1060 . The light intensity distribution  1231  exhibits a Gaussian distribution with the optical axis  1210  as the center. At this point, as shown in part (e), the central axis  1220  of the objective lens  1060  is shifted by distance X with respect to the optical axis  1210  of the optical disc apparatus  1000 . 
     Part (b) shows a light intensity distribution  1232  after the light is transmitted through the objective lens  1060 . When the objective lens  1060  has a radius (aperture radius) of length r, the light intensity distribution  1232  is zero at a position farther than distance r from the central axis  1220  of the objective lens  1060 . In other words, the light outer aperture rims  1240  and  1250  of the objective lens  1060  are shielded. 
     Part (c) shows a light intensity distribution  1233  after the light is reflected by the optical disc  1070  and before being incident on the objective lens  1060 . A central axis  1215  of the light reflected by the optical disc  1070  is shifted rightward by distance X with respect to the central axis  1220  of the objective lens  1060 . In other words, the central axis  1215  of the light reflected by the optical disc  1070  is shifted rightward by distance 2X with respect to the optical axis  1210  of the optical disc apparatus  1000 . The light intensity distribution  1233  is spread in the diameter direction of the optical disc  1070  due to the diffraction at the pits on the signal face  1074  of the optical disc apparatus  1070 . 
     Part (d) shows a light intensity distribution  1234  after the light is transmitted through the objective lens  1060 . As in part (b), the light outside the aperture rime  1240  and  1250  of the objective lens  1060  is shielded. 
     When distance X is zero, the tracking of the optical disc  1070  is accurately controlled by controlling the level of the tracking error signal (TE1 signal) obtained by the photodetector  1050  (FIG. 10B) to be zero. However, when distance X is not zero, a tracking offset is generated. 
     As described above, the tracking error signal (TE1 signal) obtained by the photodetector  1050  (FIG. 10B) shows a difference in the light amounts detected by the sub-photodetectors  1050 A and  1050 B. When a distance X exists between the optical axis  1210  and the central axis  1220  of the objective lens  1060 , the light amount detected by the sub-photodetector  1050 A corresponds to an area of a pattern ABCD formed by connecting points A, B, C and D (part (d)), and the light amount detected by the sub-photodetector  1050 B correspond to an area of a pattern CDEF formed by connecting points C, D, E and P. 
     The tracking error signal (TE2 signal) obtained by the photodetector  1190  of the optical disc apparatus  1100  (FIG. 11A) is also shifted in a similar manner when there is a distance between an optical axis of the optical disc apparatus  1100  and a central axis of the objective lens  1160  for the following reason. 
     The tracking error signal (TE2 signal) obtained by the photodetector  1190  (FIG. 11C) shows a difference in the light amounts detected by the sub-photodetectors  1190 A and  1190 B. When a distance X exists between the optical axis of the optical disc apparatus  1100  and the central axis of the objective lens  1160 , the light amount detected by the sub-photodetector  1190 A correspond to an area of a pattern formed by connecting points A, B, C′ and D′ (part (d)), and the light amount detected by the sub-photodetector  1090 B correspond to an area of a pattern formed by connecting points C′, D′, E and F. The tracking error signal provided by the photodetector  1190  (TE2 signal) is not offset as much as the tracking error signal provided by the photodetector  1050  (TE1 signal) but is still offset significantly. 
     FIG. 13A is a graph illustrating the degree of asymmetry of the waveform of the tracking error signal when the laser light crosses the pits (when tracking is off). In FIG. 13A, distance X between the optical adds  1210  of the optical disc apparatus  1000  and the central axis  1220  of the objective lens  1060  is assumed to be 100 μm. The degree of asymmetry is represented as contours. The degree of asymmetry is obtained by expression (H−L)/(H+L), where H is a level of the signal output (indicated by reference numeral  1300 ) shown in FIG. 13B above the ground level GND, and L is a level of the signal output shown in FIG. 13B below the ground level GND. 
     In FIG. 13A, the horizontal axis represents the width of the pits w of the optical disc  1070 , and the vertical axis represents the depth of the pits (d×refractive index of the substrate  1072  of the optical disc  1070 , see FIG.  10 A). The parameters for the calculation obtained for the results shown in FIG. 13A are as follows: the numerical aperture (NA) of the objective lens  1060 =0.60; the wavelength λ of the light source  1010 =0.66 μm; the pitch (P) of the pits of the optical disc  1070 =0.74 μm. At point R (where the width w of the pits is 0.30 μm and the depth of the pits is λ/10), the degree of asymmetry of the tracking error signal is 0.52. This corresponds to the difference between the areas of the pattern ABCD and the pattern CDEF shown in part (d) of FIG.  12 . As can be appreciated, in the optical disc apparatus  1000  including the photodetector  1050 , the central axis  1220  of the objective lens  1060  is shifted with respect to the optical axis  1210  of the optical disc apparatus  1000  in the direction of arrow X (FIG.  1 A). As a result, a significant degree of asymmetry of the tracking error signal occurs, and therefore control of tracking becomes unstable. While tracking control is performed, very large off-track may be undesirably generated. This causes a tracking error signal from an adjacent track to be leaked (i.e., crosstalk is increased) and deteriorates the reproduction performance, or causes a part of a signal mark of an adjacent track to be overwritten or erased. 
     FIG. 14 is a graph illustrating the degree of asymmetry of the waveform of the tracking error signal generated when the photodetector  1190  in the optical disc apparatus  1000  issused. The conditions are the same as above. At point R (where the width w of the pits is 0.30 μm and the depth of the pits is λ/10), the degree of asymmetry of the tracking error signal is 0.18. This corresponds to the difference between the areas of the pattern ABC′D′ (and the pattern C′D′EF shown in part (d) of FIG.  12 . The degree of asymmetry is lower than that provided by the photodetector  1050  but is still sufficiently large to cause the unstable control of tracking, a significant control error (off-track), and other problems. 
     SUMMARY OF THE INVENTION 
     An optical disc apparatus capable of mounting an optical disc according to the present invention includes a light source for emitting light; an objective lens for collecting the light emitted by the light source on the optical disc; a first light distribution section integrally movable with the objective lens, the first light distribution section including a first area and a second area, the first light distribution section outputting the light reflected by the optical disc and transmitted through the first area or the second area as transmission light, outputting the light reflected by the optical disc and diffracted by the first area as first diffraction light, and outputting the light reflected by the optical disc and a diffracted by the second area as second diffraction light; a transmission light detection section for detecting the transmission light and outputting a TE1 signal indicating an offset of the detected transmission light; a first diffraction light detection section for detecting the first diffraction light and the second diffraction light, and outputting a TE2 signal indicating a difference between a light amount of the detected first diffraction light and a light amount of the detected second diffraction light; and a control device for generating a tracking error signal for the optical disc based on the TE1 signal and the TE2 signal. 
     In one embodiment of the invention, the optical disc apparatus further includes a second light distribution section for directing the transmission light toward the transmission light detection section, and directing the first diffraction light and the second diffraction light toward the first diffraction light detection section. 
     In one embodiment of the invention, the transmission light detection section includes a first sub-transmission light detection section and a second sub-transmission light detection section. First transmission light is defined as part of the transmission light, which is detected by the first sub-transmission light detection section, and second transmission light is defined as a part of the transmission light, which is detected by the second sub-transmission light detection section. The offset of the transmission light is defined as a difference between a light amount of the first transmission light and a light amount of the second transmission light. 
     In one embodiment of the invention, the first diffraction light detection section includes a first sub-diffraction light detection section for detecting the first diffraction light and a second sub-diffraction light detection section for detecting the second diffraction light. 
     In one embodiment of the invention, the control device obtains the tracking error signal by TE2−k×TE1. 
     In one embodiment of the invention, the transmission light detection section includes a third area and a fourth area. The first sub-transmission light detection section is provided in the third area, and the second sub-transmission light detection section is provided in the fourth area. A border between the third area and the fourth area is parallel to a rotation direction of the optical disc. 
     In one embodiment of the invention, the first diffraction light detection section includes a fifth area and a sixth area. The first sub-diffraction light detection section is provided in the fifth area, and the second sub-diffraction light detection section is provided in the sixth area. A border between the fifth area and the sixth area is parallel to a rotation direction of the optical disc. 
     In one embodiment of the invention, the control device updates a value of k in accordance with a logical product of a numerical aperture (NA) of the objective lens and a pitch (P) of the optical disc in a diameter direction of the optical disc (NA×P). 
     In one embodiment of the invention, a value of k is 0.5×S 2 /S 1  or less, wherein S 1  is a light amount of the transmission light detected by the transmission light detection section, and S 2  is a light amount of the diffraction light detected by the first diffraction light detection section. 
     In one embodiment of the invention, the control device sets the value of k at zero when the logical product of the numerical aperture (NA) of the objective lens and the pit pitch (P) of the optical disc in the diameter direction of the optical disc (NA×P) is 0.9 times or more of the wavelength of the light incident on the optical disk. 
     In one embodiment of the invention, the control device sets a value of k so that an average output level of TE2−k×TE1 is substantially zero when the control device shifts the objective lens in a diameter direction of the optical disc without performing tracking control. 
     In one embodiment of the invention, the optical disc apparatus further includes an aberration section for providing the transmission light with an aberration. The tranismission light detection section includes a third area, a fourth area, a seventh area and an eighth area. The first sub-transmission light detection section is provided in the third area. The second sub-transmission light detection section is provided in the fourth area. The third sub-transmission light detection section is provided in the seventh area. The fourth sub-transmission light detection section is provided in the light area. A border between the third area and the fourth area is parallel to a rotation direction of the optical disc. A border between the third area and the eighth area is parallel to a diameter direction of the optical disc. A border between the fourth area and the seventh area is parallel to a diameter direction of the optical disc. A border between the seventh area and the eighth area is parallel to a rotation direction of the optical disc. The third area is orthogonal with respect to the seventh area. The fourth area is orthogonal with respect to the eighth area. The control device obtains a focusing error signal for the optical disc based on a difference between a sum of a light amount of the transmission light provided with the aberration and detected by the first sub-transmission light detection section and a light amount of the transmission light provided with the aberration and detected by the third sub-transmission light detection section, and a sum of a light amount of the transmission light provided with the aberration and detected by the second sub-transmission light detection section and a light amount of the transmission light provided with the aberration and detected by the fourth sub-transmission light detection section. 
     In one embodiment of the invention, the first light distribution section includes a ninth area and a tenth area. The first light distribution section outputs the light reflected by the optical disc and diffracted by the ninth area of the first light distribution section as third diffraction light, and outputs the light reflected by the optical disc and diffracted by the tenth area of the first light distribution section as fourth diffraction light. The first diffraction light detection section includes a first sub-diffraction light detection section, a second sub-diffraction light detection section, a third sub-diffraction light detection section: a fourth sub-diffraction light detection section, a fifth sub-diffraction light detection section, and a sixth sub-diffraction light detection section. The first diffraction light is detected by the first sub-diffraction detection section and the second sub-diffraction detection section. The second diffraction light is detected by the fifth sub-diffraction detection section and the sixth sub-diffraction detection section. The third diffraction light is detected by the fourth sub-diffraction detection section and the fifth sub-diffraction detection section. The fourth diffraction light is detected by the second sub-diffraction detection section and the third sub-diffraction detection section. The control device obtains a focusing error signal for the optical disc based on a difference between a total light amount of the diffraction light detected by the first sub-diffraction light detection section, the third sub-diffraction light detection section and the fifth sub-diffraction light detection section, and a total light amount of the diffraction light detected by the second sub-diffraction light detection section, the fourth sub-diffraction light detection section and the sixth sub-diffraction light detection section. 
     In one embodiment of the invention, the optical disc apparatus further includes a second diffraction light detection section. The first light distribution section outputs the light, reflected by the optical, disc and diffracted by the first area of the first light distribution section separately from the first diffraction light, as fifth diffraction light, and outputs the light, reflected by the optical disc and diffracted by the second area of the first light distribution section separately from the second diffraction light, as sixth diffraction light. The second diffraction light detection section includes a seventh sub-diffraction light detection section and an eighth sub-diffraction light detection section. The control device obtains a focusing error signal for the optical disc based on a difference between a light amount of the fifth diffraction light detected by the seventh sub-diffraction light detection section and alight amount of the sixth sub-diffraction light detected by the eighth sub-diffraction light detection section. 
     In one embodiment of the invention, the first light distribution section includes a holographic element having a pattern having sawtooth-lie or step-like shape including three or more steps, the pattern being continuous over sequential cycles. The first light distribution section outputs the light, reflected by the optical disc and diffracted by the first area of the first light distribution section separately from the first diffraction light, as fifth diffraction light, and outputs the light, reflected by the optical disc and diffracted by the second area of the first light distribution section separately from the second diffraction light, as sixth diffraction light. A light amount of the first diffraction light and a light amount of the fifth diffraction light both output by the first light distribution section are different from each other, and a light amount of the second diffraction light and a light amount of the sixth diffraction light both output by the first light distribution section are different from each other. 
     In one embodiment of the invention, the first diffraction light and the second diffraction light output by the first light distribution section are positive first order diffraction light, and the fifth diffraction light and the sixth diffraction light output by the first light distribution section are negative first order diffraction light. 
     In one embodiment of the invention, a light amount of the negative first order diffraction light is substantially zero. 
     In one embodiment of the invention, a light amount output by the first light distribution section is largest for the positive first order diffraction light, second largest for the transmission light, and smallest for the negative first order diffraction light. 
     In one embodiment of the invention, a light amount output by the first light distribution section is largest for the transmission light, second largest for the positive first order diffraction light, and smallest for the negative first order diffraction light. 
     In one embodiment of the invention, a light amount output by the first light distribution section is largest for the transmission light, second largest for the negative first order diffraction light, and smallest for the positive first order diffraction light. 
     In one embodiment of the invention, the optical disc apparatus further includes a second diffraction light detection section. The first light distribution section includes a ninth area and a tenth area. The first light distribution section outputs the light reflected by the optical disc and diffracted by the ninth area of the first light distribution section as third diffraction light, outputs the light reflected by the optical disc and diffracted by the tenth area of the first light distribution section as fourth diffraction light, outputs the light, reflected by the optical disc and diffracted by the first area of the first light distribution section separately from the first diffraction light, as fifth diffraction light, and outputs the light, reflected by the optical disc and diffracted by the second area of the first light distribution section separately from the second diffraction light, as sixth diffraction light. The second diffraction light detection section includes an eleventh area, a twelfth area, a thirteenth area, a fourteenth area, a fifteenth area, and a sixteenth area. A seventh sub-diffraction light detection section is provided in the eleventh area. An eighth sub-diffraction light detection section i s provided in the twelfth area. A ninth sub-diffraction light detection section is provided in the thirteenth area. A tenth sub-diffraction light detection section is provided in the fourteenth area. An eleventh subsidization light detection section is provided in the fifteenth Area. A twelfth sub-diffraction light detect Ion sect Ion is provided in the sixteenth area. The third diffraction light lo detected by the seventh sub-diffraction light detection section and the eighth sub-diffraction light detection section. The fourth diffraction light is detected by the is eleventh sub-diffraction light detection section and the twelfth sub-diffraction light detection section. The fifth diffraction light is detected by the tenth sub-diffraction light detection section and the eleventh sub-diffraction light detection section. The sixth diffraction light is detected by the eighth sub-diffraction light detection section and the ninth sub-diffraction light detection section. The control device obtains a focusing error signal for the optical disc based on a difference between a total light amount of the diffraction light detected by the seventh sub-diffraction light detection section, the ninth sub-diffraction light detection section and the eleventh sub-diffraction light detection section, and a total light amount of the sub-diffraction light detected by the eighth sub-diffraction light detection section, the tenth sub-diffraction light detection section and the twelfth sub-diffraction light detection section. 
     In one embodiment of the invention, the optical disc apparatus further includes a second diffraction light detection section. The first light distribution section includes a ninth area and a tenth area. The first light distribution section outputs the light reflected by the optical disc and diffracted by the ninth area of the first light distribution section as third diffraction light, outputs the light reflected by the optical disc and diffracted by the tenth area of the first light distribution section as fourth diffraction light, outputs the light, reflected by the optical disc and diffracted by the first area of the first light distribution section separately from the first diffraction light, as fifth diffraction light, and outputs the light, reflected by the optical disc and diffracted by the second area of the first light distribution section separately from the second diffraction light, as sixth diffraction light. The second diffraction light detection section includes an eleventh area, a twelfth area, a thirteenth area, a fourteenth area, a fifteenth area, and a sixteenth area. A seventh sub-diffraction light detection section is provided in the eleventh area. An eighth sub-diffraction light detection section is provided in the twelfth area. A ninth sub-diffraction light detection section is provided in the thirteenth area. A tenth sub-diffraction light detection section is provided in the fourteenth area. An eleventh tenth sub-diffraction light detection section is provided in the fifteenth area. A twelfth sub-diffraction light detection section is provided in the sixteenth area. The third diffraction light is detected by the seventh sub-diffraction light detection section and the eighth sub-diffraction light detection section. The fourth diffraction light is detected by the eighth sub-diffraction light detection section and the ninth sub-diffraction light detection section. The fifth diffraction light is detected by the tenth sub-diffraction light detection section and the eleventh sub-diffraction light detection section. The sixth diffraction light is detected by the eleventh sub-diffraction light detection section and the twelfth sub-diffraction light detection section. The control device obtains a focusing error signal for the optical disc based on a difference between a total light amount of the diffraction light detected by the seventh sub-diffraction light detection section, the ninth sub-diffraction light detection section and the eleventh sub-diffraction light detection section, and a total light amount of the sub-diffraction light detected by the eighth sub-diffraction light detection section, the tenth sub-diffraction light detection section, and the twelfth sub-diffraction light detection section. 
     Thus, the invention described herein makes possible the advantages of providing an optical disc apparatus for sufficiently decreasing the degree of asymmetry of a tracking error signal caused by the shift of the central axis of an objective lens with respect to the optical axis of the optical disc apparatus and suppressing off-track, so as to realize satisfactory and stable recording and reproduction. 
     These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a schematic view of an optical disc apparatus according to a first example of the present invention; 
     FIG. 1B shows a structure of a polarizing holographic face in the optical disc apparatus of the first example: 
     FIG. 1C shows a structure of a photodetector in the optical disc apparatus of the first example; 
     FIG. 2 is a contour diagram illustrating the degree of asymmetry of a TE2 signal in the optical disc apparatus of the first example when laser light crosses pits of an optical disc (pit pitch p=1.23 μm); 
     FIG. 3 is a graph illustrating the diffraction light amount ratios of a polarizing holographic element in the optical disc apparatus of the first example; 
     FIG. 4A is a schematic view of an optical disc apparatus according to a second example of the present invention; 
     FIG. 4B shows a structure of a photodetector in the optical disc apparatus of the second example; 
     FIG. 5A shows a structure of a polarizing holographic face in an optical disc apparatus according to a third example; 
     FIG. 5B shows a structure of a photodetector in the optical disc apparatus of the third example; 
     FIG. 6A shows a structure of a polarizing holographic face in an optical disc apparatus according to a fourth example of the present invention; 
     FIG. 6B shows a structure of a photodetector in the optical disc apparatus of the fourth example; 
     FIG. 7A shows a structure of a polarizing holographic face in an optical disc apparatus according to a fifth example of the present invention: 
     FIG. 7B shows a structure of a photodetector in the optical disc apparatus of the fifth example; 
     FIG. 8A shows a structure of a polarizing holographic face in an optical disc apparatus according to a sixth example of the present intention; 
     FIG. 8B shows a structure of a photodetector in the optical disc apparatus of the sixth example; 
     FIG. 9A shows a structure of a polarizing holographic face in an optical disc apparatus according to a seventh example of the present invention; 
     FIG. 9B shows a structure of a photodetector in the optical disc apparatus of the seventh example; 
     FIG. 10A is a schematic view of a first conventional optical disc apparatus; 
     FIG. 10B shows a structure of a photodetector in the first conventional optical disc apparatus: 
     FIG. 11A is a schematic view of a second conventional optical disc apparatus; 
     FIG. 11B shows a structure of a polarizing holographic face in the second conventional optical disc apparatus: 
     FIG. 11C shows a structure of a photodetector in the second conventional optical disc apparatus: 
     FIG. 12 show light intensity distributions in a cross-section along a diameter direction of an optical disc when a central axle of an objective lens is shifted with respect to an optical axis of the optical disc apparatus: 
     FIG. 13A it a contour diagram illustrating the degree of asymmetry of a TE1 signal in the first conventional optical disc apparatus (pit pitch p=0.74 μm): 
     FIG. 13B is a signal waveform diagram illustrating asymmetry of a signal; and 
     FIG. 14 is a contour diagram illustrating the degree of asymmetry of a TE2 signal in the second conventional optical disc apparatus (pit pitch p=0.74 μm). 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, the present invention will be described by way of illustrative examples with reference to the accompanying drawings. 
     EXAMPLE 1 
     An optical disc apparatus  100  according to a first example of the present invention will be described with reference to FIGS. 1A through 1C,  2 ,  3 ,  13 A,  13 B and  14 . 
     FIG. 1A shows the optical disc apparatus  100 . Laser light emitted by a laser light source  110  is converged on an optical disc  170  through an optical system  115 . The light reflected by the optical disc  170  is detected by a photodetector  200 . Based on a result detected by the photodetector  200 , a control device  185  controls an element or elements among the light source  110 , the optical system  115 , and the optical disc  170  as necessary. The optical system  115  includes, for example, a polarizing beam slitted  120  having a splitting face  125 , a collimator lens  130 , a quarter-wave plate  142 , a reflecting mirror  140 , a polarizing holographic element  145 , and an objective lens  160 . 
     A more specific operation of the optical disc apparatus  100  will be described. 
     Laser light emitted by the light source  110  is incident on the polarizing beam splitter  120  and transmitted through the splitting face  125  of the polarizing beam splatter  120 , and then converted into parallel light by the collimator lens  130 . The light source  110  is, for example, a semiconductor laser. The parallel light is reflected by the reflecting mirror  140  and incident on the polarizing holographic element  145 . 
     The polarizing holographic element  145  is integrated into a lens holder  165  together with the objective lens  160 . The polarizing holographic element  145  has the quarter-wave plate  142 . A surface of the polarizing holographic element  145  is a polarizing holographic face  150 . 
     The light (P wave), which in incident on the polarizing holographic element  145 , is transmitted through the polarizing holographic face  150  and converted into circular polarization by the quarterwave plate  142 , collected by the objective lens  160 , and then converged on a signal face  174  of the optical disc  170 . 
     The optical disc  170  has the signal face  174  between a substrate  172  and a protection film  176 . The signal face  174  has pits (or grooves) formed in a rotation direction of the optical disc  170 . The pits each have a depth d and a width w, and arranged at a pitch p. 
     The light reflected by the signal face  174 , which is circularly polarized, to transmitted through the objective lens  160 , converted into linear polarization (S wave) by the quarter-wave plate  142 , and then diffracted by or transmitted through the polarizing holographic face  150 . In this specification, 0th order diffraction is defined to be transmission. Then, the light is reflected by the reflecting mirror  140 , made convergent by the collimator lens  130 , reflected by the splitting face  125  of the polarizing beam splitter  120 , and then collected on the photodetector  200  as light  180 . Based on a signal detected by the photodetector  200 , the control device  185  controls an element or elements among the light source  110 , the optical system  115 , and the optical disc  170  as necessary. The photodetector  200  detects, for example, a focusing error signal or a tracking error signal for the optical disc  170 . 
     In this specification, a holographic element acts as a first light distribution section, and a polarizing beam splitter acts as a second light distribution section. 
     FIG. 1B shows a structure of the polarizing holographic face  150 . The polarizing holographic face  150  includes two areas  150   a  and  150   b  which are separated from each other by a separation line  152 . The areas  150   a  and  150   b  have different holographic patterns. The separation line  152  is parallel to a rotation direction of the optical disc  170 . The light reflected by the optical disc  170  (i.e., a light beam  151 ) is substantially equally divided into two by the separation line  152 . The transmission light (0th order light) or diffraction light (for example, 1st order light) passing through the polarizing holographic face  150  is reflected by the reflecting mirror  140  and made convergent by the collimator lens  130 . Then, the light is reflected by the splitting face  125  of the polarizing beam splitter  120  and collected on the photodetector  200  as the light  180 . 
     FIG. 1C shows a structure of the photodetector  200 . The photodetector  200  includes a transmission light detector  210  for detecting transmission light, and a first diffraction light detector  220  and a second diffraction light detector  230  both for detecting diffraction light. The transmission light detector  210  is provided in a central area of the photodetector  200 . The first diffraction light detector  220  and the second diffraction light detector  230  are provided in a first outer area and a second outer area, respectively, of the photodetector  200  so as to interpose the transmission light detector  210  therebetween. 
     The transmission light detector  210  includes four sub-transmission light detectors  210 A 1 ,  210 A 2 ,  210 B 1  and  210 B 2 . The transmission light detector  210  includes four areas  210 C 1 ,  210 C 2 ,  210 C 3  and  210 C 4 . The sub-transmission light detector  210 A 1  is provided in the area  210 C 1 . The sub-transmission light detector  210 A 2  is provided in the area  210 C 2 . The sub-transmission light detector  210 B 1  lo provided in the area  210 C 3 . The sub-transmission light detector  210 B 2  is provided in the area  210 C 4 . The areas  210 C 1 ,  210 C 2 ,  210 C 3  and  210 C 4  are separated from each other by separation lines  211  and  212  which are perpendicular to each other. The separation line  211  extends parallel to the rotation direction of the optical disc  170 . 
     The first diffraction light detector  220  provided in the first outer area includes two sub-diffraction light detectors  220 A and  220 B. The first diffraction light detector  220  includes areas  220 C 1  and  220 C 2 . The sub-diffraction light detector  220 A is provided in the area  220 C 1  The sub-diffraction light detector  220 B is provided in the area  220 C 2 . 
     The second diffraction light detector  230  provided in the second outer area includes two sub-diffraction light detectors  230 A and  230 B. The second diffraction light detector  230  includes areas  230 C 1  and  230 C 2 . The sub-diffraction light detector  230 A is provided in the area  230 C 1 . The sub-diffraction light detector  230 B is provided in the area  230 C 2 . 
     Positive first order diffraction light diffracted by the area  150   a  of the polarizing holographic face  150  is collected on the sub-diffraction light detector  220 A as a spot  182   a.  Negative first order diffraction light diffracted by the area  150   a  of the polarizing holographic face  150  (FIG. 1B) is focused after the sub-diffraction light detector  230 A and collected on the sub-diffraction light detector  230 A as a spot  183   a.    
     Positive first order diffraction light diffracted by the area  150   b  of the polarizing holographic face  150  (FIG. 1B) is collected on the sub-diffraction light detector  220 B as a spot  182   b.  Negative first order diffraction light diffracted by the area  150   b  of the polarizing holographic face  150  is focused before the sub-diffraction light detector  230 B and collected on the sub-diffraction light detector  230 B as a spot  183   b.  The light transmitted through the polarizing holographic face  150  (0th order light or transmission light) is collected substantially at an intersection of the separation lines  211  and  212  of the transmission light detector  210  (in a central area of the transmission light detector  210 ) as a spot  181 . This light is focused after the detection face of the transmission light detector  210 . 
     The sub-diffraction light detectors  220 A and  220 B of the first diffraction light detector  220  each detect a light amount. A second tracking error signal  2435  (TE2 signal) is obtained by subjecting the detected light amounts to a subtraction performed by a subtracter  243 . A reproduction signal  244   s  is obtained by subjecting the detected light amounts to addition performed by an adder  244 . The TE2 signal corresponds to the TE2 signal detected by the photodetector  1190  shown in FIG.  1 C. 
     Based on detection results of the sub-transmission light detectors  210 A 1 ,  210 A 2 ,  210 B 1  and  210 B 2 , a calculator  241  of the photodetector  200  outputs  210 A 1 + 210 A 2 − 210 B 1 − 210 B 2 . The output from the calculator  241  is a first tracking error signal  241   s  (TE1 signal). The TE1 signal corresponds to the TE1 signal detected by the photodetector  1050  shown in FIG.  10 B. Also based on detection results of the sub-transmission light detectors  210 A 1 ,  210 A 2 ,  210 B 1  and  210 B 2 , a calculator  242  of the photodetector  200  outputs  210 A 1 + 210 B 2 − 210 A 2 − 210 B 1 . The output from the calculator  242  is a third tracking error signal  242   s  (TE3 signal). The TE3 signal is generally referred to as a phase differential TE (tracking error) signal. 
     In this example, the transmission light detector  210 , which is substantially rectangular, is divided into sub-transmission light detectors  210 A 1 ,  210 A 2 ,  210 B 1  and  210 B 2 , which are also substantially rectangular, in this cases the difference between the light amount detected by two sub-transmission light detectors adjacent in a direction parallel to the rotation direction of the optical disc  170  ( 210 A 1  and  210 A 2 ) and the light amount detected by the other two sub-transmission light detectors ( 210 B 1  and  210 B 2 ) is the TE1 signal. The difference between the light amount detected by two sub-transmission light detectors orthogonally provided ( 210 A 1  and  210 B 2 ) and the light amount detected by the other two sub-transmission light detectors ( 210 A 2  and  210 B 1 ) is the TE3 signal. 
     The sub-diffraction light detectors  230 A and  230 B of the second diffraction light detector  230  each detect a light amount. A focusing error signal  245   s  (FE signal) is obtained by subjecting the detected light amounts to subtraction performed by a subtracter  245 . 
     The control device  185  generates a tracking error signal for the optical disc  170  based on the TE1 and TE2 signals. 
     In this example, three types of tracking error signals (TE1, TE2 and TE3 signals) are obtained. These tracking error signals can be used in accordance with the type of the optical disc. For example, in the case of an optical disc having a pit depth corresponding to about ¼ of the wavelength (e.g., DVD-ROM disc), the control device  185  can use a TE3 signal as a tracking error signal with respect to a pit signal (emboss signal). 
     In the case of an optical disc having a guide groove such as for example, a DVD-RAM disc or DVD-R disc, the control device  185  can use a calculation result value of TE2−k×TE1, obtained by using an appropriate constant k, as a tracking error signal, in this case, the control device  185  can update the value of k in accordance with the type of the optical disc. 
     For example, in the case where the optical disc  170  has a pit pitch of 0.74 μm, the TE1 signal shows asymmetry as shown in FIG. 13A for the reason described regarding the photodetector  1050  (FIG. 10B) when the objective lens  160  is shifted in the direction of arrow K (FIG.  1 A). The TE2 signal also shows the asymmetry as shown in FIG. 14 for the reason described regarding the photodetector  1190  (FIG.  11 C). Accordingly, where the shifting amount of the objective lens  160  is X, the level of a true tracking error signal (tracking error signal with no influence of the shifting of the objective lens  160 ) is TE, the total light amount received by the transmission light detector  210  is S 1 , and the total light amount received by the first diffraction light detector  220  is S 2 , the following expressions can be provided. 
     
       
           TE 1/ S   1 = TE+X   expression 1 
       
     
     
       
           TE 2/ S   2 = TE+m×X   expression 2 
       
     
     At point R (where the width w of the pits is 0.30 μm and the depth of the pits is λ/10), coefficient m=0.18/0.52=1/2.89. At point R′ (where the width w of the pits is 0.34 μm and the depth of the pits is λ/12), coefficient m=0.22/0.62=1/2.82. At points other than point R, m is in the vicinity of 1/2.89 (see FIGS.  13 A and  14 ). 
     From expressions 1 and 2, expression 3 is obtained. 
     
       
           TE= ( TE 2− k×TE 1)/ S   2 (1− m )  expression 3 
       
     
     where k is given by expression 4. 
     
       
           k=m×S   2 / S   1   expression 4 
       
     
     When the pit pitch P of the optical disc  170  is 0.74 μm, a tracking error signal with no influence of the shifting of the objective lens  160  is obtained by using, as the tracking error signal, the calculation result of TE2−k×TE1 with k fulfilling expression 4. In this manner, the degree of asymmetry of the tracking error signal caused by the shifting of the objective lens  160  can be suppressed. 
     FIG. 2 is a graph illustrating the degree of asymmetry of the waveform of the TE2 signal when the laser light crosses the pit a (when tracking is off). The optical disc has a pit pitch of 1.23 μm. The degree of asymmetry is represented as contours. The other conditions are the same as those of FIG.  13 A. At point S (the width w of the pits is 0.615 μm and the depth of the pits is λ/12), the degree of asymmetry of the TE2 signal is 0.00. Even at points shifted from point S in the pit depth and pit width, the degree of asymmetry of the TE2 signal is almost zero. This is because when the pit pitch p=1.23 μm, the light intensity distributions  1233  (part (c) of FIG. 12) and  1234  (part (d) of FIG. 12) are almost uniform, and thus the patterns ABC′D′ and C′D′EF have almost equal areas to each other. 
     Accordingly, in the case where the pit pitch of the optical disc is 1.23 μm, when the control device  185  sets k=0 the calculated level of the TE signal (TE2−k×TE1) is equal to that of the TE2 signal. The TE signal is not influenced by the shifting of the objective lens and the degree of asymmetry of the TE signal is sufficiently suppressed. 
     Therefore, in the case where the optical disc  170  has a relatively large pit pitch, such as a DVD-RAM disc or the like, the control device  185  sets k=0. In the case where the optical disc  170  has a relatively small pit pitch as a DVD-R disc, a DVD-RW disc or the like, the control device  185  sets k=m×S 2 /S 1 . The value of m is a constant value in the range of, for example, ½ to ⅕. The optimum value of m can be determined in accordance with the pit pitch of the optical disc  170 , the numerical aperture (NA) of the objective lens  160 , the ratio of the rim intensity of the light incident on the objective lens  160  (i.e., the ratio of the light intensity at the rim of the objective lens  160  with respect to the peak light intensity) or the like. The update of the constant k performed by the control device  185  can be determined in accordance with whether or not the logical product of the numerical aperture (NA) of the objective lens  160  and the pit pitch (P) of the optical disc  170  in the diameter direction thereof (NA×P) is larger than a prescribed value (for example, 0.9 times the wavelength). 
     By switching the value of k as described above, the degree of asymmetry of the TE signal caused by the shifting of the objective lens  160  is sufficiently suppressed even when a different type of optical disc is mounted. Off-track while the tracking control is performed can be solved. The update of the value of k can be performed a plurality of times in accordance with the pitch of the optical disc, instead of once as in the above-described example. The optimum value of k can be determined by learning. In this case, the control device  185  can set the value of constant k so that the average output level of the calculated signal TE2˜k×TE1 (average value of the maximum value and the minimum value of the calculated signal) obtained when the objective lens  160  is shifted in the diameter direction of the optical disc  170  without tracking control is almost zero (ground level). 
     FIG. 3 is a graph illustrating the diffraction light amount ratio of the polarizing holographic element  145 . The polarizing holographic face  150  of the polarizing holographic element  145  does not substantially diffract the light propagating toward the optical disc  170  (P wave) but diffracts the light propagating from the optical disc  170  (S wave). FIG. 3 also shows a phase distribution  19  of the wave surface of the light immediately after being transmitted through the polarizing holographic face  150 . The phase distribution  19 , or the holographic pattern, has a sawtooth-like or step-like shape, the pattern being continuous over sequential cycles. A first step  19   a,  a second step  19   b  and a third step  19   c,  each of which corresponds to one cycle of phase, have width ratios of 37%, 25% and 38%, respectively. A phase difference between the first step  19   a  and the second step  19   b  and the phase difference between the second step  19   b  and the third step  19   a  are each 75 degrees. 
     Due to such a cyclic step-like phase distribution  19 , diffraction light is generated. Where the total of the transmission light and the diffraction light is 100% the ratio of the 0th order light amount (transmission light amount) is 20%, the ratio for the positive first order diffraction light amount is 47.6%, and the ratio for the negative first order diffraction light amount is 12.4%. The rest is allocated to higher order diffraction light The optical disc apparatus  100  in the first example generates a reproduction signal using positive first order diffraction light  182   a  and  182   b  (FIG. 1C) detected by the sub-diffraction light detectors  220 A and  220 B. Accordingly, when the ratio of the positive first diffraction light amount is higher as shown in FIG. 3, a signal having a relatively high S/N ratio can be generated. Generally, the S/N ratio is in proportion to the detection index (detected light amount/{square root over ( )} (number of sub detectors for detecting the light)). In this example, the detection index=47.6/{square root over (2)}=34. The phase differential TE signal (TE3 signal) with respect to the pit signal (emboss signal) generally requires high frequency signal processing, but does not involve any problem in terms of the S/N ratio since the ratio of the 0th order light is about 20%. 
     In the optical disc apparatus  100  in the first example, the light source  110  and the photodetector  200  are separately provided, unlike in the conventional optical disc apparatus  1100 . Therefore, the transmission light can be used in order to obtain a tracking error signal. The optical apparatus  100  in the first example, includes the polarizing beam aplitter  120 , but those stilled in the art would readily conceive various structures without the polarizing beam splitter  120 . 
     In the optical disc apparatus  100  in the first example, the light emitted by the light source  110  is diffracted after being reflected by the optical disc  170 . Therefore, the light can be efficiently incident on the optical disc apparatus  200 . 
     In the above description, ±1st order diffraction light is used as the diffraction light. Higher order diffraction light (e.g., ±2nd or 3rd order diffraction light) can be used. The spot  181  can be focused before the detection face of the transmission light detector  210 . In this case, the light distribution is inverted with respect to the optical axis, and thus the polarity of the TE1 signal is changed. This can be handled by changing “TE1” in the above description into “−TE1”. The same effect as described is provided. 
     EXAMPLE 2 
     FIG. 4A schematically shows an optical disc apparatus  300  according to a second example of the present invention. The optical disc apparatus  300  has the same structure as that of the optical disc apparatus  100  in the first example except that a parallel flat plate  370  is provided between the polarizing beam splitter  120  and a photodetector  400  and that the photodetector  400  had a different structure from that of the photodetector  200 . Identical elements, to those of the first example will bear identical reference numeral and will not be described in detail. The parallel flat plate  370  is provided inclined with respected to an optical axis of converged light  380  incident on the parallel flat plate  370 . By this inclination, the light passing through the parallel flat plate  370  is provided with aberration (astigmatism) by which focal lines extending in ±45 degree directions with respect to a separation line  411  (FIG. 4B) appears on a detection face of the photodetector  400 . The parallel flat plate  370  acts as an aberration section. 
     FIG. 4B shows the photodetector  400 . The photodetector  400  includes a transmission light detector  410  and a diffraction light detector  420 . 
     The transmission light detector  410  includes four sub-transmission light detectors  410 A 1 ,  410 A 2 ,  410 B 1  and  410 B 2 . The transmission light detector  410  includes four areas  410 C 1 ,  410 C 2 ,  410 C 3  and  410 C 4 . The sub-transmission light detector  410 A 1  is provided in the area  410 C 1 . The sub-transmission light detector  410 A 2  is provided in the area  410 C 2 . The sub-transmission light detector  410 B 1  is provided in the area  410 C 3 . The sub-transmission light detector  410 B 2  is provided in the area  410 C 4 . The areas  410 C 1 ,  410 C 2 ,  410 C 3  and  410 C 4  are separated from each other by separation lines  411  and  412  which are perpendicular to each other. The separation line  411  extends parallel to the rotation direction of the optical disc  170 . 
     The diffraction light detector  420  includes two sub-diffraction light detectors  420 A and  420 B. The diffraction light detector  420  includes areas  420 C 1  and  420 C 2 . The sub-diffraction light detector  420 A is provided in the area  420 C 1 . The sub-diffraction light detector  420 B is provided in the area  420 C 2 . 
     Positive first order diffraction light diffracted by the area  150   a  of the polarizing holographic face  150  (FIG. 1B) is focused before the sub-diffraction light detector  420 A and collected on the sub-diffraction light detector  420 A as a spot  382   a.  Positive first order diffraction light diffracted by the area  150   b  of the polarizing holographic face  150  is focused after the sub-diffraction light detector  420 B and collected on the sub-diffraction light detector  420 B as a spot  382   b.  In this example, whether the focal point is before or after the detection face does not matter. The focal point can be before or after the detection face. 
     The light transmitted through the polarizing holographic face  150  (0th order light or transmission light) is collected substantially at an intersection of the separation lines  411  and  412  of the transmission light detector  410  (in a central area of the transmission light detector  410 ) as a spot  381 . In this case, the detection face of the transmission light detector  410  is substantially at a mid point between two focal lines (vertical focal line and horizontal focal line). Accordingly, when the spot  381  passes a focal line inclined clockwise at 45 degrees with respect to the separation line  412  before reaching the detection face of the transmission light detector  410 , the light distribution is syxmnetiic with respect to the focal line. The light distribution of the spot  381  is equivalent to the light distribution which is rotated clockwise at 90 degrees from that of the spot  181  in the first example. 
     The sub-diffraction light detectors  420 A and  420 B of the diffraction light detector  420  each detect a light amount. A second tracing error signal  443   s  (TE2 signal) is obtained by subjecting the detected light amounts to subtraction performed by a subtracter  443 . A reproduction signal  444   s  is obtained by subjecting the detected light amounts to addition performed by an adder  444 . The TE2 signal corresponds to the TE2 signal detected by the photodetector  1190  shown in FIG.  11 . 
     Based on detection results of the sub-transmission light detectors  410 A 1 ,  410 A 2 ,  410 B 1  and  410 B 2 , a calculator  441  of the photodetector  400  outputs  410 A 1 − 410 A 2 + 410 B 1 − 410 B 2 . The output from the calculator  441  is a first tracking error signal  441 . (TE1 signal). The TE1 signal corresponds to the TE1 signal detected by the photodetector  1050  shown in FIG.  10 B. Also based on detection results of the sub-transmission light detectors  410 A 1 ,  410 A 2 ,  410 B 1  and  410 B 2 , a calculator  442  of the photodetector  400  outputs  410 A 1 + 410 B 2 − 410 A 2 − 410 B 1 . The output from the calculator  442  is a third tracking error signal  442   s  (TE3 signal). 
     Like in the first example, the transmission light detector  410 , which is substantially rectangular, is divided into sub-transmission light detectors  410 A 1 ,  410 A 2 ,  410 B 1  and  410 B 2 , which are also substantially rectangular. In this case, the difference between the light amount detected by two sub-transmission light detectors adjacent in a direction parallel to the rotation direction of the optical disc  170  ( 410 A 1  and  410 B 1 ) (as described above, the light distribution is rotated clockwise at 90 degrees with respect to the light distribution in the first example, and therefore the separation line ( 412 ) parallel to the rotation direction of the optical disc  170  in the second example is also rotated at 90 degrees with respect to such a separation line ( 211 ) in the first example), and the light amount detected by the other two sub-transmission light detectors ( 410 A 2  and  410 B 2 ) is the TE1 signals The difference between the light amount detected by two sub-transmission light detectors orthogonally provided ( 410 A 1  and  410 B 2 ) and the light amount detected by the other two sub-transmission light detectors ( 410 A 2  and  410 B 1 ) is the TE3 signal. 
     A focusing error of the objective lens  360  is reflected as an astigmatism of the converged light  381  (difference between ±45 degree directions). Therefore, the third tracking error signal  442   s  calculated by the calculator  442  which outputs  410 A 1 + 410 B 2 − 410 A 2 − 410 B 1  corresponds to a focusing error signal (FE signal). 
     In this example also, three types of tracking error signals (TE1, TE2 and TE3 signals) are obtained. Like in the first example, these tracking error signals can be used in accordance with the type of the optical disc. For example, in the case of an optical disc having a pit depth corresponding to about ¼ of the wavelength (e.g., DVD-ROM disc), the control device.  185  can use a TE3 signal as a tracking error signal with respect to a pit signal (emboss signal). 
     In the case of an optical disc having a guide groove such as for example, a DVD-RAM disc or DVD-R disc, the control device  185  can use a calculation result value of TE2−k×TE1, obtained by using an appropriate constant k, as a tracking error signal. In this case, the control device  185  can update the value of k in accordance with the type of the optical disc. 
     Like in the first example, the degree of asymmetry of the tracking error signal caused by the shifting of the central axis of the objective lens  160  with respect to the optical axis of the optical disc apparatus  300  can be sufficiently suppressed. Off-track while the tracking control is performed can be solved. In this example, negative first order diffraction light is not used. Therefore, the cross-sectional shape of the polarizing holographic element  145  can be changed so as to eliminate the ratio of the negative first order diffraction light and thus increase the ratios of the 0th order and positive first order diffraction light. In this manner, the S/N ratio of the reproduction signal and the phase differential TE signal (TE3 signal) can be further improved compared to that of the first example. 
     As a modification of the second example, a sum of the light amounts detected by the sub-transmission light detectors  410 A 1 ,  410 A 2 ,  410 B 1  and  410 B 2  can be detected as a reproduction signal. In the case where the diffraction light ratios are 70% for the 0th order light and 10% for the positive first diffraction light, the detection index of the reproduction signal is about 35. In this manner, the light amounts can be adjusted so as to be largest for the transmission light, second largest for the positive first order diffraction light, and smallest for the negative first order diffraction light. 
     In the above description, the parallel flat plate  370  is used as the aberration section. The present invention is not limited to such a structure. For example, a wedge-like prism can be used as the aberration section. 
     EXAMPLE 3 
     FIG. 5A shows a structure of a polarizing holographic face  550  of an optical disc apparatus according to a third example of the present invention. FIG. 5B shows a structure of a photodetector  500  of the optical disc apparatus according to the third example of the present invention. The optical disc apparatus according to the third example has the same structure as that of the optical disc apparatus  100  in the first example except for the polarizing holographic face  550  and the photodetector  500 . The other elements will be described using the corresponding reference numerals in FIG.  1 A. 
     In FIG. 5A, the polarizing holographic face  550  is divided into a first area  550   a,  a second area  550   b,  a third area  550   a  and a fourth area  550   d  having different holographic patterns, along separation lines  552  and  553 . The separation line  552  is parallel to the rotation direction of the optical disc  170 , and the separation line  553  is perpendicular to the separation line  552 . A light beam  551  reflected by the optical disc  170  is substantially equally divided into four along the separation lines  552  and  553 . The first area  550   a  is further divided into strip-shaped areas  550 F 11 ,  550 B 11 ,  550 F 12 ,  550 B 12  and  550 F 13  along separation lines parallel to the separation line  553 . The second area  550   b  is further divided into strip-shaped areas  550 B 21 ,  550 F 21 ,  550 B 22 ,  550 F 22  and  550 B 23  along separation lines parallel to the separation line  553 . The third area  550   c  is further divided into strip-shaped areas  55031 ,  550 B 31 ,  550 F 32 ,  550 B 32  and  550 F 33  along separation lines parallel to the separation line  553 . The fourth area  550   d  is further divided into strip-shaped areas  550 B 41 ,  550 F 41 ,  550 B 42 ,  550 F 42  and  550 B 43  along separation lines parallel to the separation line  553 . 
     Negative first order diffraction light passing through the strip-shaped areas having the letter “F” in their reference numerals (e.g.,  550 F 11  or  550 F 22 ) is collected before the photodetector  500 . Negative first order diffraction light passing through the strip-shaped areas having the letter “B” in their reference numerals (e.g.,  550 B 11  or  550 B 22 ) is collected after the photodetector  500 . 
     Referring to FIG. 5B, the photodetector  500  includes a transmission light detector  510 , a first diffraction light detector  520  and a second diffraction light detector  530 . The transmission light detector  510  is provided in a central area of the photodetector  500 . The first diffraction light detector  520  and the second diffraction light detector  530  are provided in a first outer area and a second outer area, respectively, of the photodetector  500  so as to interpose the transmission light detector  510  therebetween. 
     The transmission light detector  510  includes four sub-transmission light detectors  510 A 1 ,  510 A 2 ,  510 B 1  and  510 B 2 . The transmission light detector  510  includes four areas  510 C 1 ,  510 C 2 ,  510 C 3  and  510 C 4 . The sub-transmission light detector  510 A 1  is provided in the area  510 C 1 . The sub-transmission light detector  510 A 2  is provided in the area  510 C 2 . The sub-transmission light detector  510 B 1  is provided in the area  510 C 3 . The sub-transmission light detector  510 B 2  is provided in the area  510 C 4 . The areas  510 C 1 ,  510 C 2 ,  510 C 3  and  510 C 4  are separated from each other by separation lines  511  and  512  which are perpendicular to each other. The separation line  511  extends parallel to the rotation direction of the optical disc  170 . 
     The first diffraction light detector  520  provided in the first outer area includes two sub-diffraction light detectors  520 A and  520 B. The first diffraction light detector  520  includes areas  520 C 1  and  520 C 2 . The sub-diffraction light detector  520 A is provided in the area  520 C 1 . The sub-diffraction light detector  520 B is provided in the area  520 C 2 . 
     The second diffraction light detector  530  provided in the second outer area includes six sub-diffraction light detectors  530 A 1 ,  530 A 2 ,  530 A 3 ,  530 B 1 ,  530 B 2  and  530 B 3 . The sub-diffraction light detectors  530 A 1 ,  530 B 2  and  530 A 3  are electrically conductive to each other. The sub-diffraction light detectors  530 B 1 ,  530 A 2  and  530 B 3  are also electrxically conductive to each other. The second diffraction light detector  530  includes areas  530 C 1 ,  530 C 2 ,  530 C 3 ,  530 C 4 ,  530 C 5  and  530 C 6 . The sub-diffraction light detector  530 A 1  is provided in the area  530 C 1 . The sub-diffraction light detector  530 A 2  is provided in the area  530 C 2 . The sub-diffraction light detector  530 A 3  is provided in the area  530 C 3 . The sub-diffraction light detector  530 B 1  is provided in the area  530 C 4 . The sub-diffraction light detector  530 B 2  is provided in the area  530 C 5 . The sub-diffraction light detector  530 B 3  is provided in the area  530 C 6 . 
     Positive first order diffraction light diffracted by the strip-shaped areas  550 B 11  and  550 B 12  of the first area  550   a  of the polarizing holographic face  550  (FIG. 5A; not adjacent to each other but interposing the area  550 F 12  therebetween) is collected on the sub-diffraction light detector  520 B as a spot  582 B 1 . Negative first order diffraction light diffracted by the strip-shaped areas  50 B 11  and  50 B 12  is collected on the sub-diffraction light detector  530 B 3  while being also on the sub-diffraction light detector  530 B 2  as a spot  583 B 1 . 
     Positive first order diffraction light diffracted by the other strip-shaped areas  550 F 11 ,  550 F 12  and  550 F 13  is collected on the sub-diffraction light detector  520 B as a spot  582 F 1 . Negative first order diffraction light diffracted by the strip-shaped areas  550 F 11 ,  550 F 12  and  550 F 13  is collected on the sub-diffraction light detector  530 B 2  while being also on the sub-diffraction light detector  530 B 3  as a spot  583 F 1 . 
     Positive first order diffraction light diffracted by the strip-shaped areas  550 B 21 ,  550 B 22  and  550 B 23  of the second area  550   b  (FIG. 5A; not adjacent to each other but interposing the areas  550 F 21  and  550 F 22  therebetween) is collected on the sub-diffraction light detector  520 A as a spot  582 B 2 . Negative first order diffraction light diffracted by the strip-shaped areas  550 B 21 ,  550 B 22  and  550 B 23  is collected on the sub-diffraction light detector  530 A 2  while being also on the sub-diffraction light detector  530 A 1  as a spot  583 B 2 . 
     Positive first order diffraction light diffracted by the other strip-shaped areas  550 F 21  and  550 F 22  is collected on the sub-diffraction light detector  520 A as a spot  582 F 2 . Negative first order diffraction light diffracted by the strip-shaped areas  550 F 21  and  550 F 22  is collected on the sub-diffraction light detector  530 A 1  while being also on the sub-diffraction light detector  530 A 2  as a spot  583 F 2 . 
     Positive first order diffraction light diffracted by the strip-shaped areas  550 B 31  and  550 B 32  of the third area  550   c  (FIG. 5A; not adjacent to each other but interposing the area  550 F 32  therebetween) is collected on the sub-diffraction light detector  520 A as a spot  582 B 3 . Negative first order diffraction light diffracted by the strip-shaped areas  550 B 31  and  550 B 32  is collected on the sub-diffraction light detector  530 A 2  while being also on the sub-diffraction light detector  530 A 3  as a spot  583 B 3 . 
     Positive first order diffraction light diffracted by the other strip-shaped areas  550 F 31 ,  550 F 32  and  550 F 33  is collected on the sub-diffraction light detector  520 A as a spot  582 F 3 . Negative first order diffraction light diffracted by the strip-shaped areas  550 F 31 ,  550 F 32  and  550 F 33  is collected on the sub-diffraction light detector  530 A 3  while being also on the sub-diffraction light detector  530 A 2  as a spot  583 F 3 . 
     Positive first order diffraction light diffracted by the strip-shaped areas  550 B 41 ,  55042  and  550 B 43  of the fourth area  550   d  (FIG.  5 A: not adjacent to each other but interposing the areas  550 F 41  and  550 F 42  therebetween) is collected on the sub-diffraction light detector  520 B as a spot  582 B 4 . Negative first order diffraction light diffracted by the strip-shaped areas  550 B 41 ,  550 B 42  and  550 B 43  is collected on the sub-diffraction light detector  530 B 1  while being also on the sub-diffraction light detector  530 B 2  as a spot  583 B 4 . 
     Positive first order diffraction light diffracted by the other strip-shaped areas  550 F 41  and  550 F 42  is collected on the sub-diffraction light detector  520 B as a spot  582 F 4 . Negative first order diffraction light diffracted by the strip-shaped areas  550 F 41  and  550 F 42  is collected on the sub-diffraction light detector  530 B 2  while being also on the sub-diffraction light detector  530 B 1  as a spot  583 F 4 . 
     The light transmitted through the polarizing holographic face  550  (0th order light) is collected substantially at an intersection of the separation lines  511  and  512  of the transmission light detector  510  (in a central area of the transmission light detector  510 ) as a spot  581 . The focal point of the spot  581  is after the detection face of the transmission light detector  510 . 
     The sub-diffraction light detectors  520 A and  520 B of the first diffraction light detector  520  each detect a light amount. A second tracking error signal  5438  (TE2 signal) is obtained by subjecting the detected light amounts to a subtraction performed by a subtracter  543 . A reproduction signal  544   s  is obtained by subjecting the detected light amounts to addition performed by an adder  544 . The TE2 signal corresponds to the TE2 signal detected by the photodetector  1190  shown in FIG.  11 C. 
     The TE2 signal corresponds to a difference between the light amount of the positive first order diffraction light diffracted by the first area  550   a  and the fourth area  550   d  of the polarizing holographic face  550  and the light amount of the positive first order diffraction light diffracted by the second area  550   b  and the third area  550   a  of the polarizing holographic face  550 . The reproduction signal corresponds to a sum of the light amount of the positive first order diffraction light diffracted by the first area  550   a,  the second area  550   b,  the third area  550   a  and the fourth area  550   d.    
     Based on detection results of the sub-transmission light detectors  510 A 1 ,  510 A 2 ,  5101 B and  510 B 2 , a calculator  541  of the photodetector  500  outputs  510 A 1 + 510 A 2 − 510 B 1 − 510 B 2 . The output from the calculator  541  is a first tracking error signal  541 . (TE1 signal). The TE1 signal corresponds to the TE1 signal detected by the photodetector  1050  shown in FIG.  10 B. Also based on detection results of the sub-transmission light detectors  510 A 1 ,  510 A 2 ,  51031  and  510 B 2 , a calculator  542  of the photodetector  500  outputs  510 A 1 + 510 B 2 − 510 A 2 − 510 B 1 . The output from the calculator  542  is a third tracking error signal  542   s  (TE3 signal). 
     In this example also, the transmission light detector  510 , which is substantially rectangular, is divided into sub-transmlssion light detectors  510 A 1 ,  510 A 2 ,  5101 B and  510 B 2 , which are also substantially rectangular. In this case, the difference between the light amount detected by two sub-trenomission light detectors adjacent in a direction parallel to the rotation direction of the optical disc  170  ( 510 A 1  and  510 A 2 ) and the light amount detected by the other two sub-transmission light detectors ( 510 B 1  and  5 S 0 B 2 ) in the TE1 signal. The difference between the light amount detected by two sub-transmission light detectors orthogonally provided ( 510 A 1  and  510 B 2 ) and the light amount detected by the other two sub-transmlssion light detectors ( 510 A 2  and  510 B 1 ) is the TE3 signal. 
     A calculator  545  outputs  530 B 1 + 530 B 3 + 530 A 2 − 530 A 1 − 530 A 3 − 530 B 2 . The output of the calculator  545  is a focusing error signal  545   s  (FE signal). 
     In this example also, three types of tracking error signals (TE1, TE2 and TE3 signals) are obtained. Like in the first example, these tracking error signals can be used in accordance with the type of the optical disc. For example, in the case of an optical disc having a pit depth corresponding to about ¼ of the wavelength (e.g., DVD-ROM disc), the control device  185  can use a TE3 signal as a tracking error signal with respect to a pit signal (emboss signal). 
     In the case of an optical disc having a guide groove such as, for example, a DVD-RAM disc or DVD-R disc, the control device  185  can use a calculation result value of TE2−k×TE1 obtained, by using an appropriate constant k, as a tracking error signal. In this case, the control device  185  can update the value of k in accordance with the type of the optical disc. 
     Like in the first example, the degree of asymmetry of the tracking error signal caused by the shifting of the central axis of the objective lens  160  with respect to the optical axis of the optical disc apparatus  300  can be sufficiently suppressed. Off-track while the tracking control is performed can be solved. In this example, the polarizing holographic face  550  is divided into small strip-shaped areas. Using these small strip-shaped areas, a light component to be collected before the photodetector  500  and a light component to be collected after the photodetector  500  are generated. The resultant diffraction light is detected as an FE signal. Therefore, the adverse influence of dust and stains present on the substrate  172  of the optical disc  170  is negated. Thus, the focusing error control to highly stable. 
     In the above description, the sub-diffraction light detector  530 B 1  is electrically conductive to the sub-diffraction light detectors  530 B 3  and  530 A 2 , and the sub-diffraction light detector  530 B 2  is electrically conductive to the sub-diffraction light detector  530 A 1  and  530 A 3 . The difference between the outputs from the two groups of the sub-diffraction light detector is generated as an FE signal. Alternatively, the sub-diffraction light detectors  530 B 1  and  530 B 3 ,  530 A 1  and  530 A 3  can be electrically conductive to each other, and the sub-diffraction light detector  530 B 2  can be electrically conductive to the sub-diffraction light detector  530 A 2 . In this case, an FE signal can be generated by a difference signal thereof (i.e.,  530 B 1 + 530 B 3 + 530 A 1 + 530 A 3 − 530 B 2 − 530 A 2 ). In this case on the second diffraction light detector  530 , the spots  583 B 1  and  583 F 1  are exchanged with the spots  583 B 4  and  583 F 4 . Or on the second diffraction light detector  530 , the spots  583 B 3  and  583 F 3  are exchanged with the spots  58382  and  583 F 2 . The spots on the first diffraction light detector  520  are exchanged in correspondence therewith. 
     The polarizing holographic face  550  is not necessarily divided into the small strip-shaped areas. When the polarizing holographic face  550  is not divided as shown in FIG. 5A, the first area  550   a  and the third area  550   a  are entirely areas shown with “B”, and the second area  550   b  and the fourth area  550   d  are entirely areas shown with “F”. The spots  583 F 1 ,  583 B 2 ,  583 F 3  and  583 B 4  on the second diffraction light detector  530 , and the spots  582 F 1 ,  582 B 2 ,  582 F 3  and  582 B 4  on the first diffraction light detector  520  are eliminated. Only the spots  583 B 1 ,  583 F 2 ,  583 B 3  and  583 F 4  on the second diffraction light detector  530 , and the spots  582 B 1 ,  582 F 2 ,  582 B 3  and  582 F 4  on the first diffraction light detector  520  are left. 
     EXAMPLE 4 
     FIG. 6A shows a structure of a polarizing holographic face  650  of an optical disc apparatus according to a fourth example of the present invention. FIG. 6B shows a structure of a photodetector  600  of the optical disc apparatus according to the fourth example of the present invention. The optical disc apparatus according to the fourth example has the same structure as that of the optical disc apparatus  100  in the first example except for the polarizing holographic face  650  and the photodetector  600 . The other elements will be described using the corresponding reference numerals in FIG.  1 A. 
     In FIG. 6A, the polarizing holographic face  650  is divided into a first area  650   a,  a second area  650   b,  a third area  650   c  and a fourth area  650   d  having different holographic patterns, along separation lines  652  and  653 . The separation line  652  is parallel to the rotation direction of the optical disc  170 , and the separation line  653  is perpendicular to the separation line  652 . A light beam  651  reflected by the optical disc  170  is substantially equally divided into four along the separation line  652  and  653 . The first area  650   a  is further divided into strip-shaped areas  650 F 11 ,  650 B 11 ,  650 F 12 ,  650 B 12  and  650 F 13  along separation lines parallel to the separation line  653 . The second area  650   b  is further divided into strip-shaped areas  650321 ,  650 F 21 ,  650 B 22 ,  650 F 22  and  650 B 23  along separation lines parallel to the separation line  653 . The third area  650   c  is further divided into strip-shaped areas  650 F 31 ,  650 B 31 ,  650 F 32 ,  650 B 32  and  650 F 33  along separation lines parallel to the separation line  653 . The fourth area  650   d  is further divided into strip-shaped areas  650 B 41 ,  650 P 41 ,  650 B 42 ,  650 F 42  and  650 B 43  along separation lines parallel to the separation line  653 . 
     Negative first order diffraction light passing through the strip-shaped areas having the letter “F” in their reference numerals (e.g.,  650 F 11  or  650 F 22 ) is collected before the photodetector  600 . Negative first order diffraction light passing through the strip-shaped areas having the letter “B” in their reference numerals (e.g.,  650 B 11  or  650 B 22 ) is collected after the photodetector  600 . 
     Referring to FIG. 6B, the photodetector  600  includes a transmission light detector  610 , a first diffraction light detector  620  and a second diffraction light detector  630  the transmission light detector  610  is provided in a central area of the photodetector  600 . The first diffraction light detector  620  and the second diffraction light detector  630  are provided in a first outer area and a second outer area, respectively, of the photodetector  600  so as to interpose the transmission light detector  610  therebetween. 
     The transmission light detector  610  includes four sub-transmission light detectors  610 A 1 ,  610 A 2 ,  6103 B and  610 B 2 . The transmission light detector  610  includes four areas  610 C 1 ,  610 C 2 ,  610 C 3  and  610 C 4 . The sub-transmission light detector  610 A 1  is provided in the area S 10 C 1 . The sub-transmission light detector  610 A 2  is provided in the area  610 C 2 . The sub-transmission light detector  610 B 1  is provided in the area  610 C 3 . The sub-transmission light detector  610 B 2  is provided in the area  610 C 4 . The areas  610 C 1 ,  610 C 2 ,  610 C 3  and  610 C 4  are separated from each other by separation lines  611  and  612  which are perpendicular to each other. The separation line  611  extends parallel to the rotation direction of the optical disc  170 . 
     The first diffraction light detector  620  provided in the first outer area includes two sub-diffraction light detectors  620 A and  620 B. The first diffraction light detector  620  includes areas  620 C 1  and  620 C 2 . The sub-diffraction light detector  620 A is provided in the area  620 C 1  The sub-diffraction light detector  620 B is provided in the area  620 C 2 . 
     The second diffraction light detector  630  provided in the second outer area includes six sub-diffraction light detectors  630 A 1 ,  630 A 2 ,  630 A 3 ,  630 B 1 ,  630 B 2  and  630 B 3 . The sub-diffraction light detectors  630 A 1 ,  630 B 2  and  630 A 3  are electrically conductive to each other. The sub-diffraction light detectors  630 B 1 ,  630 A 2  and  630 B 3  are also electrically conductive to each other. The second diffraction light detector  630  includes areas  630 C 1 ,  630 C 2 ,  630 C 3 ,  630 C 4 ,  630 C 5  and  630 C 6 . The sub-diffraction light detector  630 A 1  is provided in the area  630 C 1 . The sub-diffraction light detector  630 A 2  is provided in the area  630 C 2 . The sub-diffraction light detector  630 A 3  is provided in the area  630 C 3 . The sub-diffraction light detector  630 B 1  is provided in the area  630 C 4 . The sub-diffraction light detector  630 B 2  is provided in the area  630 C 5 . The sub-diffraction light detector  630 B 3  is provided in the area  630 C 6 . 
     Positive first order diffraction light diffracted by the strip-shaped areas  650 B 11  and  650 B 12  of the first area  650   a  of the polarizing holographic face  650  (FIG. 5A; not adjacent to each other but interposing the area  650 F 12  therebetween) is collected on the sub-diffraction light detector  620 B as a spot  682 B 1 . Negative first order diffraction light diffracted by the strip-shaped areas  650 B 11  and  650 B 12  is collected on the sub-diffraction light detector  630 A 2  while being also on the sub-diffraction light detector  630 A 1  as a spot  683 B 1 . 
     Positive first order diffraction light diffracted by the other strip-shaped areas  650 F 11 ,  650 F 12  and  650 F 13  is collected on the sub-diffraction light detector  6208  as a spot  682 F 1 . Negative first order diffraction light diffracted by the strip-shaped areas  650 F 11 ,  650 F 12  and  650 F 13  is collected on the sub-diffraction light detector  630 A 1  while being also on the sub-diffraction light detector  630 A 2  as a spot  683 F 1 . 
     Positive first order diffraction light diffracted by the strip-shaped areas  650 B 21 ,  650 B 22  and  650 B 23  of the second area  650   b  (FIG.  6 A: not adjacent to each other but interposing the areas  650 P 21  and  650 P 22  therebetween) is collected on the sub-diffraction light detector  620 A as a spot  682 B 2 . Negative first order diffraction light diffracted by the strip-shaped areas  650 B 21 ,  650 B 22  and  650 B 23  is collected on the sub-diffraction light detector  630 A 3  while being also on the sub-diffraction light detector  630 A 2  as a spot  683 B 2 . 
     Positive first order diffraction light diffracted by the other strip-shaped areas  650 F 21  and  650 F 22  is collected on the sub-diffraction light detector.  620 A as a spot  682 F 2 . Negative first order diffraction light diffracted by the strip-shaped areas  660 F 21  and  650 P 22  is collected on the sub-diffraction light detector  630 A 2  while being also on the sub-diffraction light detector  630 A 3  as a spot  683 F 2 . 
     Positive first order diffraction light diffracted by the strip-shaped areas  650 B 31  and  650 B 32  of the third area  650   c  (FIG. 6A not adjacent to each other but interposing the area  650 F 32  therebetween) is collected on the sub-diffraction light detector  620 A as a spot  682 B 3 . Negative first order diffraction-light diffracted by the strip-shaped areas  650 B 31  and  650 B 32  is collected on the sub-diffraction light detector  630 B 2  while being also on the sub-diffraction light detector  630 B 3  as a spot  683 B 3 . 
     Positive first order diffraction light diffracted by the other strip-shaped areas  650 F 31 ,  650 F 32  and  650 F 33  is collected on the sub-diffraction light detector  620 A as a spot  682 F 3 . Negative first order diffraction light diffracted by the strip-shaped areas  650 F 31 ,  650 F 32  and  650 F 33  is collected on the sub-diffraction light detector  630 B 3  while being also on the sub-diffraction light detector  630 B 2  as a spot  683 F 3 . 
     Positive first order diffraction light diffracted by the strip-shaped areas  650 B 41 ,  650 B 42  and  650 B 43  of the fourth area  650   d  (FIG. 6A; not adjacent to each other but interposing the areas  650 F 41  and  650 F 42  therebetween) is collected on the sub-diffraction light detector  620 B as a spot  682 B 4 . Negative first order diffraction light diffracted by the strip-shaped areas  650341 ,  650 B 42  and  650 B 43  is collected on the sub-diffraction light detector  630 B 1  while being also on the sub-diffraction light detector  630 B 2  as a spot  683 B 4 . 
     Positive first order diffraction light diffracted by the other strip-shaped areas  650 F 41  and  650 F 42  is collected on the sub-diffraction light detector  620 B as a spot  682 F 4 . Negative first order diffraction light diffracted by the strip-shaped areas  650 F 41  and  650 F 42  is collected on the sub-diffraction light detector  630 B 2  while being also on the sub-diffraction light detector  630 B 1  as a spot  683 F 4 . 
     The light transmitted through the polarizing holographic face  650  (0th order light) is collected substantially at an intersection of the separation lines  611  and  612  of the transmission light detector  610  (in a central area of the transmission light detector  610 ) as a spot  681 . The spot  681  is focused after the detection face of the transmission light detector  610 . 
     The sub-diffraction light detectors  620 A and  620 B of the first diffraction light detector  620  each detect a light amount. A second tracking error signal  643   s  (TE2 signal) is obtained by subjecting the detected light amounts to a subtraction performed by a subtracter  643  A reproduction signal  644   s  is obtained by subjecting the detected light amounts to addition performed by an adder  644 . The TE2 signal corresponds to the TE2 signal detected by the photodetector  1190  shown in FIG.  1 C. 
     The TE2 signal corresponds to a difference between the light amount of the positive first order diffraction light diffracted by the first area  650   a  and the fourth area  650   d  of the polarizing holographic face  650  and the light amount of the positive first order diffraction light diffracted by the second area  650   b  and the third area  650   c  of the polarizing holographic face  650 . The reproduction signal corresponds to a sum of the light amount of the positive first order diffraction light diffracted by the first area  650   a,  the second area  650   b,  the third area  650   c  and the fourth area  650   d.    
     Based on detection results of the sub-transmission light detectors  610 A 1 ,  610 A 2 ,  610 B 1  and  610 B 2 , a calculator  641  of the photodetector  600  outputs  610 A 1 + 610 A 2 − 610 B 1 − 610 B 2 . The output from the calculator  641  is a first tracking error signal  641   s  (TE1 signal). The TE1 signal corresponds to the TE1 signal detected by the photodetector  1050  shown in FIG.  10 B. Also based on detection results of the sub-transmission light detectors  610 A 1 ,  610 A 2 ,  610 B 1  and  610 B 2  a calculator  642  of the photodetector  600  outputs  610 A 1 + 610 B 2 − 610 A 2 − 610 B 1 . The output from the calculator  642  is a third tracking error signal  642   c  (TE3 signal). 
     A calculator  645  outputs  630 B 1 + 630 B 3 + 630 A 2 − 630 A 1 − 630 A 3 − 630 B 2 . The output of the calculator  645  it a focusing error signal (FE signal). 
     In this example also, three types of tracking error signals (TE1, TE2 and TE3 signals) are obtained. Like in the first example, these tracking error signals can be used in accordance with the type of the optical disc. For example, in the case of an optical disc having a pit depth corresponding to about ¼ of the wavelength (e.g., DVD-ROM disc), the control device  185  can use a TE3 signal as a tracking error signal with respect to a pit signal (emboss signal). 
     In the case of an optical disc having a guide groove such as, for example, a DVD-RAM disc or DVD-R disc, the control device  185  can be a calculation result value of TE2−k×TE1, obtained by using an appropriate constant k, as a tracking error signal. In this case, the control device  185  can update the value of k in accordance with the type of the optical disc. 
     Like in the first example, the degree of asymmetry of the tracking error signal caused by the shifting of the central axis of the objective lens  160  with respect to the optical axis of the optical disc apparatus can be sufficiently suppressed. Off-track while the tracking control is performed can be solved. In this example, the polarizing holographic face  650  is divided into small strip-shaped areas. Using these small strip-shaped areas, a light component to be collected before the photodetector  600  and a light component to be collected after the photodetector  600  are generated. The resultant diffraction light is detected as an FE signal. Therefore the adverse influence of dust and stains present on the substrate  172  of the optical disc  170  is negated. Thus, the focusing error control is highly stable. In the fourth example, unlike in the third example, the separation lines for separating the sub-diffraction light detectors  630 A 1 ,  630 A 2  and  630 A 3  and the separation lines for separating the sub-diffraction light detectors  630 B 1 ,  630 B 2  and  630 B 3  are along the diffraction direction of the light. Therefore, when there is a wavelength error or wavelength shift, the spots on the second diffraction light detector  630  move along these separation lines. Thus, a detection error of focusing on the optical disc can be sufficiently avoided. 
     The first and third examples have advantages that there is ample room for rotation adjustment of the photodetector, despite the possibility of an FE detection error due to a wavelength error or wavelength shift. The separation lines between the sub-diffraction light detectors used for detecting an FE signal may or may not be along the diffraction direction of the light in accordance with the design idea. In the first, second, third and the following examples, the separation lines are perpendicular to the diffraction direction. The structures in these examples can be modified so that the separation lines are parallel to the diffraction direction. 
     EXAMPLE 5 
     FIG. 7A shows a structure of a polarizing holographic face  750  of an optical disc apparatus according to a fifth example of the present invention. FIG. 7B shows a structure of a photodetector  700  of the optical disc apparatus according to the fifth example of the present invention. The optical disc apparatus according to the fifth example has the same structure as that of the optical disc apparatus  100  in the first example except for the polarizing holographic face  750  and the photodetector  700 . The other elements will be described using the corresponding reference numerals in FIG.  1 A. 
     In FIG. 7A, the polarizing holographic face  750  is divided into a first area  750   a,  a second area  750   b,  a third area  750   a  and a fourth area  750   d  having different holographic patterns, along separation lines  752  and  753 . The separation line  752  is parallel to the rotation direction of the optical disc  170 , and the separation line  753  is perpendicular to the separation line  752 . A light beam  751  reflected by the optical disc  170  is substantially equally divided into four along the separation lines  752  and  753 . The first area  750   a  is further divided into strip-shaped areas  750 F 11 ,  750 B 11 ,  750 F 12 ,  750 B 12  and  750 F 13  along separation lines parallel to the separation line  753 . The second area  750   b  is further divided into strip-shaped areas  750 B 21 ,  750 F 21 ,  750 B 22 ,  750 F 22  and  750823  along separation lines parallel to the separation line  753 . The third area  750   c  is further divided into strip-shaped areas  750 F 31 ,  750 B 31 ,  750 F 32 ,  750 B 32  and  750 F 33  along separation lines parallel to the separation line  753 . The fourth area  750   d  is further divided into strip-shaped areas  750 B 41 ,  750 F 41 ,  750 B 42 .  750 F 42  and  750 B 43  along separation lines parallel to the separation line  753 . 
     Negative first order diffraction light passing through the strip-shaped areas having the letter “F” in their reference numerals (e.g.,  750 F 11  or  750 F 22 ) is collected before the photodetector  700 . Negative first order diffraction light passing through the strip-shaped areas having the letter “B” in their reference numerals (e.g.,  750 B 11  or  750 B 22 ) is collected after the photodetector  700 . 
     Referring to FIG. 7B, the photodetector  700  includes a transmission light detector  710 , a first diffraction light detector  720  and a second diffraction light detector  730 . The transmission light detector  710  is provided in a central area of the photodetector  700 . The first diffraction light detector  720  and the second diffraction light detector  730  are provided in a first outer area and a second outer area, respectively of the photodetector  700  so as to interpose the transmission light detector  710  therebetween. 
     The transmission light detector  710  includes two sub-transmission light detectors  710 A and  710 B. The transmission light detector  710  includes two areas  710 C 1  and  710 C 2 . The sub-transmission light detector  710 A is provided in the area  710 C 1 . The sub-transmission light detector  710 B is provided in the area  710 C 2 . The areas  710 C 1  and  710 C 2  are separated from each other by a separation line  711 . The separation line  711  extends parallel to the rotation direction of the optical disc  170 . 
     The first diffraction light detector  720  provided in the first outer area includes four sub-diffraction light detectors  720 A 1 ,  720 A 2 ,  720 B 1  and  720 B 2 . The first diffraction light detector  720  includes areas  720 C 1 ,  720 C 2 ,  720 C 3  and  720 C 4 . The sub-diffraction light detector  720 A 1  is provided in the area  720 C 1 . The sub-diffraction light detector  720 A 2  is provided in the area  720 C 2 . The sub-diffraction light detector  720 B 1  in provided in the area  720 C 3 . The sub-diffraction light detector  720 B 2  is provided in the area  720 C 4 . 
     The second diffraction light detector  730  provided in the second outer area includes six sub-diffraction light detectors  730 A 1 ,  730 A 2 ,  730 A 3 ,  730 B 3 ,  730 B 2  and  73033  like in the third example. The sub-diffraction light detectors  730 A 1 ,  730 B 2  and  730 A 3  are electrically conductive to each other The sub-diffraction light detectors  730 B 1 ,  730 A 2  and  73033  are also electrically conductive to each other. The second diffraction light detector  730  includes areas  730 C 1 ,  730 C 2 ,  73 OC 3 ,  730 C 4 ,  730 C 5  and  730 C 6 . The sub-diffraction light detector  730 A 1  is provided in the area  730 C 1 . The sub-diffraction light detector  730 A 2  is provided in the area  730 C 2 . The sub-diffraction light detector  730 A 3  is provided in the area  730 C 3 . The sub-diffraction light detector  730 B 1  is provided in the area  730 C 4 . The sub-diffraction light detector  730 B 2  is provided in the area  730 C 5 . The sub-diffraction light detector  730 B 3  is provided in the area  730 C 6 . 
     Positive first order diffraction light diffracted by the strip-shaped areas  750811  and  750 B 12  of the first area  750   a  of the polarizing holographic face  750  (FIG. 7A; not adjacent to each other but interposing the area  750 F 2  therebetween) is collected on the sub-diffraction light detector  720 B 1  as a spot  782 B 1 . Negative first order diffraction light diffracted by the strip-shaped areas  750 B 11  and  750 B 12  is collected on the sub-diffraction light detector  730 B 3  while being also on the sub-diffraction light detector  73032  as a spot  783 B 1 . 
     Positive first order diffraction light diffracted by the other strip-shaped areas  750 F 11 ,  750 F 12  and  750 F 13  is collected on the sub-diffraction light detector  720 B 1  as a spot  782 F 1 . Negative first order diffraction light diffracted by the strip-shaped areas  750 F 11 ,  750 F 12  and  750 F 13  is collected on the sub-diffraction light detector  730 B 2  while being also on the sub-diffraction light detector  730 B 3  as a spot  783 F 1 . 
     Positive first order diffraction light diffracted by the strip-shaped areas  750 B 21 ,  750 B 22  and  7503 B 23  of the second area  750   b  (FIG. 7A; not adjacent to each other but interposing the areas  750 F 21  and  750 F 22  therebetween) is collected on the sub-diffraction light detector  720 A 2  as a spot  782 B 2 . Negative first order diffraction light diffracted by the strip-shaped areas  750 B 21 ,  750 B 22  and  750 B 23  is collected on the sub-diffraction light detector  730 A 2  while being also on the sub-diffraction light detector  730 A 1  as a spot  783 B 2 . 
     Positive first order diffraction light diffracted by the other strip-shaped areas  750 F 21  and  750 F 22  is collected on the sub-diffraction light detector  720 A 2  as a spot  782 F 2 . Negative first order diffraction light diffracted by the strip-shaped areas  750 F 21  and  750 F 22  is collected on the sub-diffraction light detector  730 A 1  while being also on the sub-diffraction light detector  730 A 2  as a spot  783 F 2 . 
     Positive first order diffraction light diffracted by the strip-shaped areas  750 B 31  and  750 B 32  of the third area  750   c  (FIG.  7 A: not adjacent to each other but interposing the area  750 F 32  therebetween) is collected on the sub-diffraction light detector  720 A 1  as a spot  782 B 3 . Negative first order diffraction light diffracted by the strip-shaped areas  750 B 31  and  750 B 32  is collected on the sub-diffraction light detector  730 A 2  while being also on the sub-diffraction light detector  730 A 3  as a spot  783 B 3 . 
     Positive first order diffraction light diffracted by the other strip-shaped areas  750 F 31 ,  750 F 32  and  750 F 33  is collected on the sub-diffraction light detector  720 A 1  as a spot  782 F 3 . Negative first order diffraction light diffracted by the strip-shaped areas  750 F 31 ,  750 F 32  and  750 F 33  is collected on the sub-diffraction light detector  730 A 3  while being also on the sub-diffraction light detector  730 A 2  as a spot  783 F 3 . 
     Positive first order diffraction light diffracted by the strip-shaped areas  750 B 41 ,  750 B 42  and  750 B 43  of the fourth area  750   d  (FIG.  7 A: not adjacent to each other but interposing the areas  750 F 41  and  750 F 42  therebetween) is collected on the sub-diffraction light detector  720 B 2  as a spot  782 B 4 . Negative first order diffraction light diffracted by the strip-shaped areas  750 B 41 .  750 B 42  and  750 B 43  is collected on the sub-diffraction light detector  730 B 1  while being also on the sub-diffraction light detector  73092  as a spot  783 B 4 . 
     Positive first order diffraction light diffracted by the other strip-shaped areas  750 P 41  and  750 F 42  is collected on the sub-diffraction light detector  720 B 2  as a spot  782 P 4 . Negative first order diffraction light diffracted by the strip-shaped areas  750 F 41  and  750 F 42  is collected on the sub-diffraction light detector  730 B 2  while being also on the sub-diffraction light detector  730 B 1  as a spot  783 F 4 . 
     The light transmitted through the polarizing holographic face  750  (0th order light) is collected at a substantial center of the separation line  711  as a spot  781 . The spot  781  is focused before the detection face of the transmission light detector  710 . The sub-transmission light detectors  710 A and  710 B of the transmission light detector  710  each detect a light amount. A tracking error signal  741   s  (TE1 signal) is obtained by subjecting the detected light amounts to a subtraction performed by a subtracter  741 . A reproduction signal  742   s  is obtained by subjecting the detected light amounts to addition performed by an adder  742 . The TE1 signal corresponds to the TE1 signal detected by the photodetector  1050  shown in FIG.  10 B. 
     In this example also, the transmission light detector  710 , which is substantially rectangular, is divided into sub-transmission light detectors  710 A and  710 B, which are also substantially rectangular. In this case, the difference between the light amounts detected by the sub-transmission light detectors  710 A and  710 B separated from each other by the separation line  711  which extends parallel to the rotation direction of the optical disc  170  is the TE1 signal. The sum of the light amounts detected by the sub-transmission light detectors  710 A and  710 B is the reproduction signal. 
     Based on detection results of the sub-diffraction light detectors  720 A 1 ,  720 A 2 ,  720 B 1  and  720 B 2 , a calculator  743  of the photodetector  700  outputs  720 A 1 + 720 A 2 − 720 B 1 − 720 B 2 . The output from the calculator  743  is a second tracking error signal  743   s  (TE2 signal). The TE2 signal corresponds to the TE2 signal detected by the photodetector  1190  shown in FIG.  1 C. Also based on detection results of the sub-diffraction light detectors  720 A 1 ,  720 A 2 ,  720 B 1  and  720 B 2 , a calculator  745  of the photodetector  700  outputs  720 A 1 + 720 B 2 − 720 A 2 − 720 B 1 . The output from the calculator  744  is a third tracking error signal  743   s  (TE3 signal). 
     Based on detection results of the sub-diffraction light detectors  730 A 1 ,  730 A 2 ,  730 A 3 ,  730 B 1 ,  730 B 2  and  730 B 3 , a calculator  745  outputs  730 B 1 + 730 B 3 + 730 A 2 − 730 A 1 − 730 A 3 − 730 B 2 . The output of the calculator  745  is a focusing error signal  745   s  (FE signal). 
     Like in the first example, the phase distribution of the wave surface of the light immediately after being transmitted through the polarizing holographic face  750  has a sawtooth-like or step-like shape. The phase distribution  19 , or the holographic pattern, has a sawtooth-like or step-like shape, the pattern being continuous over sequential cycles. In this example, the phase difference between the first step and the second step, and the phase difference between the second step and the third step are significantly small. In this manner, the diffraction light amount ratio can be 70% for the 0th order light, 15% for the positive first order diffraction light and 5% for the negative first order diffraction light. Since the diffraction efficiency of the ±1st order diffraction light is small, the diffraction loss is also small. As a result, the total diffraction light amount (i.e., 70+15+5=90%) is larger than that of the first example. Thus, the light amounts can be adjusted so as to be largest for the transmission light, second largest for the positive first order diffraction light, and smallest for the negative first order diffraction light. 
     In this example also three types of tracking error signals (TE1, TE2 and TE3 signals) are obtained. Like in the first example, these tracking error signals can be used in accordance with the type of the optical disc. For example, in the case of an optical disc having a pit depth corresponding to about ¼ of the wavelength (e. g., DVD-ROM disc), the control device  185  can use a TE3 signal as a tracking error signal with respect to a pit signal (emboss signal). 
     In the case of an optical disc having a guide groove such as, for example, a DVD-RAM disc or DVD-R disc, the control device  185  can use a calculation result value of TE2−k×TE1, obtained by using an appropriate constant k, as a tracking error signal. In this case, the control device  185  can update the value of k in accordance with the type of the optical disc. 
     Like in the first example, the degree of asymmetry of the tracking error signal caused by the shifting of the central axis of the objective lens  160  with respect to the optical axis of the optical disc apparatus  300  can be sufficiently suppressed. Off-track while the tracking control is performed can be solved. In this example, the polarizing holographic face  750  is divided into small strip-shaped areas. Using these small strip-shaped areas, a light component to be collected before the photodetector  700  and a light component to be collected after the photodetector  700  are generated. The resultant diffraction light is detected as an FE signal. Therefore, the adverse influence of dust and stains present on the substrate  172  of the optical disc  170  is negated. Thus, the focusing error control is highly stable. 
     In the fifth example, the detected light amount of the 0th order light (transmission light) is used to detect a reproduction signal. The detection index=70/{square root over (2)}=about 50. A higher S/N ratio than that of the first example is guaranteed. 
     EXAMPLE 6 
     FIG. 8A shows a structure of a polarizing holographic face  850  of an optical disc apparatus according to a sixth example of the present invention. FIG. 8B shows a structure of a photodetector  800  of the optical disc apparatus according to the sixth example of the present invention. The optical disc apparatus according to the sixth example has the same structure as that of the optical disc apparatus  100  in the first example except for the polarizing holographic face  850  and the photodetector  800 . The other elements will be described using the corresponding reference numerals in FIG.  1 A. 
     In FIG. 8A, the polarizing holographic face  850  is divided into a first area  850   a,  a second area  850   b,  a third area  850   c  and a fourth area  850   d  having different holographic patterns, along separation lines  852  and  853 . The separation line  852  is parallel to the rotation direction of the optical disc  170 , and the separation line  853  is perpendicular to the separation line  852 . A light beam  851 . reflected by the optical disc  170  is substantially equally divided into four along the separation lines  852  and  853 . The first area  850   a  is further divided into strip-shaped areas  850 F 11 ,  850 B 11 ,  850 F 12 ,  850 B 12  and  850 F 13  along separation lines parallel to the separation line  853 . The second area  850   b  is further divided into strip-shaped areas  850 B 21 ,  850 F 21 ,  850 B 22 ,  850 F 22  and  850 B 23  along separation lines parallel to the separation line  853 . The third area  850   c  is further divided into strip-shaped areas  85031 ,  850 B 31 ,  850 F 32 ,  850 B 32  and  850 F 33  along separation lines parallel to the separation line  853 . The fourth area  850   d  is further divided into strip-shaped areas  850 B 41 ,  850 F 41 ,  850 B 42 ,  850 F 42  and  850 B 43  along separation lines parallel to the separation line  853 . 
     Negative first order diffraction light passing through the strip-shaped areas having the letter “F” in their reference numerals (e.g.,  850 F 11  or  850 F 22 ) is collected before the photodetector  800 . Negative first order diffraction light passing through the strip-shaped areas having the letter “B” in their reference numerals (e.g.,  850 B 11  or  850 B 22 ) is collected after the photodetector  800 . 
     Referring to FIG. 8B, the photodetector  800  includes a transmission light detector  810 , a first diffraction light detector  820  and a second diffraction light detector  830 . The transmission light detector  810  is provided in a central area of the photodetector  800 . The first diffraction light detector  820  and the second diffraction light detector  830  are provided in a first outer area and a second outer area, respectively, of the photodetector  800  so as to interpose the transmission light detector  810  therebetween. 
     The transmission light detector  810  includes two sub-transmission light detectors  810 A and  810 B. The transmission light detector  810  includes two areas  810 C 1  and  810 C 2 . The sub-transmission light detector  810 A is provided in the area  810 C 1 . The sub-transmission light detector  810 B provided in the area  810 C 2 . The areas  810 C 1  and  810 C 2  are separated from each other by a separation line  811 . The separation line  811  extends parallel to the rotation direction of the optical disc  170 . 
     The first diffraction light detector  820  provided in the first outer area includes two sub-diffraction light detectors  820 A and  820 B. The first diffraction light detector  820  includes areas  820 C 1  and  820 C 2 . The sub-diffraction light detector  820 A is provided in the area  820 C 1 . The sub-diffraction light detector  820 B is provided in the area  820 C 2 . 
     The second diffraction light detector  830  provided in the second outer area includes six sub-diffraction light detectors  830 A 1 ,  830 A 2 ,  830 A 3 ,  830 B 1 ,  830 B 2  and  830 B 3  like in the third example. The sub-diffraction light detectors  830 A 1 ,  830 B 2  and  830 A 3  are electrically conductive to each other. The sub-diffraction light detectors  830 B 1 ,  830 A 2  and  830 B 3  are also electrically conductive to each other. The second diffraction light detector  830  includes areas  830 C 1 ,  830 C 2 ,  830 C 3 ,  830 C 4 ,  830 C 5  and  830 C 6 . The sub-diffraction light detector  830 A 1  is provided in the area  830 C 1 . The sub-diffraction light detector  830 A 2  is provided in the area  830 C 2 . The sub-diffraction light detector  830 A 3  is provided in the area  830 C 3 . The sub-diffraction light detector  830 B 1  is provided in the area  830 C 4 . The sub-diffraction light detector  830 B 2  is provided in the area  830 C 5 . The sub-diffraction light detector  830 B 3  is provided in the area  830 C 6 . 
     Positive first order diffraction light diffracted by the strip-shaped areas  850 B 11  and  850 B 12  of the first area  850   a  of the polarizing holographic face  850  (FIG. 8A; not adjacent to each other but interposing the area  850 F 12  therebetween) is collected on the sub-diffraction light detector  820 B as a spot  882 B 1 . Negative first order diffraction light diffracted by the strip-shaped areas  850 B 11  and  850 B 12  is collected on the sub-diffraction light detector  830 B 3  while being also on the sub-diffraction light detector  830 B 2  as a spot  883 B 1 . 
     Positive first order diffraction light diffracted by the other strip-shaped areas  850 F 11 ,  850 F 12  and  850 F 13  is collected on the sub-diffraction light detector  820 B as a spot  382 F 1 . Negative first order diffraction light diffracted by the strip-shaped areas  850 F 11 ,  850 F 12  and  850 F 13  is collected on the sub-diffraction light detector  830 B 2  while being also on the sub-diffraction light detector  830 B 3  as a spot  883 F 1 . 
     Positive first order diffraction light diffracted by the strip-shaped areas  850 B 21 ,  850 B 22  and  850 B 23  of the second area  850   b  (FIG. 8A; not adjacent to each other but interposing the areas  850 F 21  and  850 F 22  therebetween) is collected on the sub-diffraction light detector  820 A as a spot  882 B 2 . Negative first order diffraction light diffracted by the strip-shaped areas  850 B 21 ,  850 B 22  and  850 B 23  is collected on the sub-diffraction light detector  830 A 2  while being also on the sub-diffract ion light detector  830 A 1  as a spot  883 B 2 . 
     Positive first order diffraction light diffracted by the other strip-shaped areas  850 F 21  and  850 F 22  is collected on the sub-diffraction light detector  820 A as a spot  882 F 2 . Negative first order diffraction light diffracted by the strip-shaped areas  850 F 21  and  850 F 22  is collected on the sub-diffraction light detector  830 A 1  while being also on the sub-diffraction light detector  830 A 2  as a spot  883 P 2 . 
     Positive first order diffraction light diffracted by the strip-shaped areas  850 B 31  and  850 B 32  of the third area  850   c  (FIG.  8 A: not adjacent to each other but interposing the area  850 F 32  therebetween) is collected on the sub-diffraction light detector  820 A as a spot  882 B 3 . Negative first order diffraction light diffracted by the strip-shaped areas  850 B 31  and  850 B 32  is collected on the sub-diffraction light detector  830 A 2  while being also on the sub-diffraction light detector  830 A 3  as a spot  88333 . 
     Positive first order diffraction light diffracted by the other strip-shaped areas  850 F 31 ,  830 P 32  and  850 F 33  is collected on the sub-diffraction light detector  820 A as a spot  882 F 3 . Negative first order diffraction light diffracted by the strip-shaped areas  850 F 31 ,  850 P 32  and  850 F 33  is collected on the sub-diffraction light detector  830 A 3  while being also on the sub-diffraction light detector  830 A 2  as a spot  883 F 3 . 
     Positive first order diffraction light diffracted by the strip-shaped areas  850 B 41 ,  850 B 42  and  850 B 43  of the fourth area  850   d  (FIG. 8A; not adjacent to each other but interposing the areas  850 F 41  and  850 F 42  therebetween) is collected on the sub-diffraction light detector  820 B as a spot  882 B 4 . Negative first order diffraction light diffracted by the strip-shaped areas  850 B 41 ,  850 B 42  and  8 S 0 B 43  is collected on the sub-diffraction light detector  830 B 1  while being also on the sub-diffraction light detector  830 B 2  as a spot  883 B 4 . 
     Positive first order diffraction light diffracted by the other strip-shaped areas  850 F 41  and  850 F 42  is collected on the sub-diffraction light detector  820 B as a spot  882 F 4 . Negative first order diffraction light diffracted by the strip-shaped areas  850 F 41  and  850 F 42  is collected on the sub-diffraction light detector  830 B 2  while being also on the sub-diffraction light detector  830 B 1  as a spot  883 F 4 . 
     The light transmitted through the polarizing holographic face  850  (0th order light) is collected at a substantial center of the separation line  811  as a spot  881  The sub-transtission light detectors  810 A and  810 B of the transmission light detector  810  each detect a light amount. A first tracking error signal  841   s  (TE1 signal) is obtained by subjecting the detected light amounts to a subtraction performed by a subtracter  841 . A reproduction signal  842   s  is obtained by subjecting the detected light amounts to addition performed by an adder  842 . The TE1 signal corresponds to the TE1 signal detected by the photodetector  1050  shown in FIG.  10 B. 
     The sub-diffraction light detectors  820 A and  820 B of the first diffraction light detector  820  each detect a light amount. A second tracking error signal  843   s  (TE2 signal) is obtained by subjecting the detected light amounts to a subtraction performed by a subtractor  843 . The TE2 signal corresponds to the TE2 signal detected by the photodetector  1190  shown in FIG.  11 C. 
     Based on detection results of the sub-diffraction light detectors  830 A 1 ,  830 A 2 ,  830 A 3 ,  830 B 1 ,  830 B 2  and  830 B 3 , a calculator  845  outputs  830 B 1 + 830 B 3 + 830 A 2 − 830 A 1 − 830 A 3 − 830 B 2 . The output of the calculator  845  is a focusing error signal  845   s  (FE signal). 
     Unlike in the first example, the phase distribution of the wave surface of the light immediately after being transmitted through the polarizing holographic face  850  has a cyclic rectangular shape (so-called two-level grating shape). The phase difference between a lower step and an upper step is significantly small. Therefore, the diffraction light amount ratio can be 70% for the 0th order light, 10% for the positive first order diffraction light and 10% for the negative first order diffraction light. Since the diffraction efficiency of the ±1st order diffraction light is small, the diffraction loss is also small. As a result, the total diffraction light amount (i.e., 70+10+10=90%) is larger than that of the first example. Thus, the light amounts can be adjusted so as to be larger for the transmission light and smaller for the positive first order diffraction light or the negative first order diffraction light. The light amounts can be adjusted so as to be largest for the transmission light, second largest for the negative first order diffraction light, and smallest for the positive first order diffraction light. 
     In this example, two types of tracking error signals (TE1 and TE2 signals) are obtained. Accordingly, like in the first example, the control device  185  can use a calculation result value of TE2−k×TE1 obtained by using an appropriate constant k, as a tracking error signal in this case, the control device  185  can update the value of k in accordance with the type of the optical disc. 
     Like in the first example, the degree of asymmetry of the tracking error signal caused by the shifting of the central axis of the objective lens  160  with respect to the optical axis of the optical disc apparatus  300  can be sufficiently suppressed. Off-track while the tracking control is performed can be solved. In this example, the polarizing holographic face  850  is divided into small strip-shaped areas. Using these small strip-shaped areas, a light component to be collected before the photodetector  800  and a light component to be collected after the photo detector  800  are generated. The resultant diffraction light is detected as an FE signal. Therefore, the adverse influence of dust and stains present on the substrate  172  of the optical disc  170  is negated. Thus, the focusing error control is highly stable. 
     In the sixth example, the detected light amount of the 0th order light is used to detect a reproduction signal. The detection index=70/{square root over (2)}=about 50. A higher S/N ratio than that of the first example is guaranteed. Since the third tracking error signal (TE3 signal) is not obtained, the control device  185  cannot perform tracking of the pit signal (emboss signal) of the optical disc  170  having a pit depth corresponding to about ¼ of the wavelength, such as, for example, a DVD-ROM disc. 
     In the sixth example, the 0th order light is used to detect a reproduction signal. Alternatively, the detected light amount of the positive first order diffraction light can be used. The light amounts detected by the sub-diffraction light detectors  820 A and  820 B can be added by the adder  844  to obtain the reproduction signal  844   s.  In this case, the phase differential distribution of the wave surface of light immediately after being transmitted through the polarizing holographic face  850  is 20% for the 0th order light, 47.6% for the positive first order diffraction light, and 12.4% for the negative first order diffraction light. The detection index of the reproduction signal is 47.6/{square root over (2)}=34. 
     EXAMPLE 7 
     FIG. 9A shows a structure of a polarizing holographic face  950  of an optical disc apparatus according to a seventh example of the present invention. FIG. 9B shows a structure of a photodetector  900  of the optical disc apparatus according to the seventh example of the present invention. The optical disc apparatus according to the seventh example has the same structure as that of the optical disc apparatus  100  in the first example except for the polarizing holographic face  950  and the photodetector  900 . The other elements will be described using the corresponding reference numerals in FIG.  1 A. 
     In FIG. 9A, the polarizing holographic face  950  is divided into a first area  950   a,  a second area  950   b,  a third area  950   c  and a fourth area  950   d  having different holographic patterns, along separation lines  952  and  953 . The separation line  952  is parallel to the rotation direction of the optical disc  170 , and the separation line  953  is perpendicular to the separation line  952 . A light beam  951  reflected by the optical disc  170  is substantially equally divided into four along the separation lines  952  and  953 . 
     The first area  950   a  is further divided into strip-shaped areas  950 F 11 ,  950 B 11 ,  950 F 12 ,  950 B 12  and  950 F 13  along separation lines parallel to the separation line  953 . The second area  950   b  is further divided into strip-shaped areas  950 B 21 ,  950 F 21 ,  950 B 22 ,  950 F 22  and  950 B 23  along separation lines parallel to the separation line  953 . The third area  950   c  is further divided into strip-shaped areas  950 F 31 ,  950 B 31 ,  950 F 32 ,  950 B 32  and  950 F 33  along separation lines parallel to the separation line  953 . The fourth area  950   d  is further divided into strip-shaped areas  950 B 41 ,  950 F 41 ,  950 B 42 ,  950742  and  950 B 43  along separation lines parallel to the separation line  953 . 
     Negative first order diffraction light passing through the strip-shaped areas having the letter “F” in their reference numerals (e.g.,  950 F 11  or  950 F 22 ) is collected before the photodetector  900 . Negative first order diffraction light passing through the strip-shaped areas having the letter “B” in their reference numerals (e.g.,  950 B 11  or  950 B 22 ) is collected after the photodetector  900 . 
     Referring to FIG. 9B, the photodetector  900  includes a transmission light detector  910 , a first diffraction light detector  920  and a second diffraction light detector  930 . The transmission light detector  910  is provided in a central area of the photodetector  900 . The first diffraction light detector  920  and the second diffraction light detector  930  are provided in a first outer area and a second outer area, respectively, of the photodetector  900  so as to interpose the transmission light detector  910  therebetween. 
     The transmission light detector  910  includes four sub-transmission light detectors  910 A 1 ,  910 A 2 ,  910 B 1  and  910 B 2 . The transmission light detector  910  includes four areas  910 C 1 ,  910 C 2 ,  910 C 3  and  910 C 4 . The sub-transmission light detector  910 A 1  is provided in the area  910 C 1 . The sub-transtission light detector  910 A 2  is provided in the area  910 C 2 . The sub-transmission light detector  910 B 1  is provided in the area  910 C 3 . The sub-transmission light detector  910 B 2  is provided in the area  910 C 4 . The areas  910 C 1 ,  910 C 2 ,  910 C 3  and  910 C 4  are separated from each other by separation lines  911  and  912  which are perpendicular to each other. The separation line  911  extends parallel to the rotation direction of the optical disc  170 . 
     The first diffraction light detector  920  has an area  920 C. The first diffraction light detector  920  is provided in the area  920 C. 
     The second diffraction light detector  930  provided in the second outer area includes six sub-diffraction light detectors  930 A 1 ,  930 A 2 ,  930 A 3 ,  930 B 1 ,  930 B 2  and  930 B 3  like in the third example. The sub-diffraction light detectors  930 A 1  and  930 A 3  are electrically conductive to each other. The sub-diffraction light detectors  930 B 1  and  930 B 3  are also electrically conductive to each other. The second diffraction light detector  930  includes areas  930 C 1 ,  930 C 2 ,  930 C 3 ,  930 C 4 ,  930 C 5  and  930 C 6 . The sub-diffraction light detector  930 A 1  is provided in the area  930 C 1 . The sub-diffraction light detector  930 A 2  is provided in the area  930 C 2 . The sub-diffraction light detector  930 A 3  is provided in the area  930 C 3 . The sub-diffraction light detector  930 B 1  is provided in the area  930 C 4 . The sub-diffraction light detector  930 B 2  is provided in the area  930 C 5 . The sub-diffraction light detector  930 B 3  is provided in the area  930 C 6 . 
     Positive first order diffraction light diffracted by the strip-shaped areas  950 B 11  and  950 B 12  of the first area  950   a  of the polarizing holographic face  950  (FIG. 9A; not adjacent to each other but interposing the area  950 F 12  therebetween) is collected on the first diffraction light detector  920  as a spot  982 B 1 . Negative first order diffraction light diffracted by the strip-shaped areas  950 B 13  and  950 B 12  is collected on the sub-diffraction light detector  930 B 3  while being also on the sub-diffraction light detector  930 B 2  as a spot  983 B 1 . 
     Positive first order diffraction light diffracted by the other strip-shaped areas  950 F 11 ,  950 P 12  and  950 P 13  is collected on the first diffraction light detector  920  as a spot  982 F 1 . Negative first order diffraction light diffracted by the strip-shaped areas  950 F 11 ,  950 F 12  and  950 F 13  lo collected on the sub-diffraction light detector  933 B 2  while being also on the sub-diffraction light detector  930 B 3  as a spot  983 F 1 . 
     Positive first order diffraction light diffracted by the strip-shaped areas  950 B 21 ,  950 B 22  and  950 B 23  of the second area  950   b  (FIG. 9A; not adjacent to each other but interposing the areas  950 F 21  and  950 F 22  therebetween) is collected on the first diffraction light detector  920  as a spot  982 B 2 . Negative first order diffraction light diffracted by the strip-shaped areas  950 B 21 ,  950 B 22  and  950 B 23  is collected on the sub-diffraction light detector  930 A 2  while being also on the sub-diffraction light detector  930 A 1  as a spot  983 B 2 . 
     Positive first order diffraction light diffracted by the other strip-shaped areas  950 F 21  and  950 F 22  is collected on the first diffraction light detector  920  as a spot  982 F 2 . Negative first order diffraction light diffracted by the strip-shaped areas  950 F 21  and  950 F 22  is collected on the sub-diffraction light detector  930 A 1  while being also on the sub-diffraction light detector  930 A 2  as a spot  983 F 2 . 
     Positive first order diffraction light diffracted by the strip-shaped areas  950 B 31  and  950832  of the third area  950   c  (FIG. 9A; not adjacent to each other but interposing the area  950 F 32  therebetween) is collected on the first diffraction light detector  920  as a spot  982 B 3 . Negative first order diffraction light diffracted by the strip-shaped areas  950 B 31  and  950 B 32  is collected on the sub-diffraction light detector  930 A 2  while being also on the sub-diffraction light detector  930 A 3  as a spot  983 B 3   
     Positive first order diffraction light diffracted by the other strip-shaped areas  950 F 31 ,  950 F 32  and  950 F 33  is collected on the first diffraction light detector  920 A as a spot  982 F 3 . Negative first order diffraction light diffracted by the strip-shaped areas  950 F 31 ,  950 F 32  and  950 F 33  is collected on the sub-diffraction light detector  930 A 3  while being also on the sub-diffraction light detector  930 A 2  as a spot  983 F 3 . 
     Positive first order diffraction light diffracted by the strip-shaped areas  950 B 41 ,  950 B 42  and  950 B 43  of the fourth area  950   d  (FIG. 9A; not adjacent to each other but interposing the areas  950 F 41  and  950 F 42  therebetween) is collected on the first diffraction light detector  920  as a spot  982 B 4 . Negative first order diffraction light diffracted by the strip-shaped areas  950 B 41 ,  940 B 42  and  950 B 43  is collected on the sub-diffraction light detector  930 B 1  while being also on the sub-diffraction light detector  930 B 2  as a spot  983 B 4 . 
     Positive first order diffraction light diffracted by the other strip-shaped areas  950 F 41  and  950 P 42  is collected on the first diffraction light detector  920  as a spot  982 F 49  Negative first order diffraction light diffracted by the strip-shaped areas  950 F 41  and  950 P 42  is collected on the sub-diffraction light detector  930 B 2  while being also on the sub-diffraction light detector  930 B 1  as a spot  983 F 4 . 
     The light transmitted through the polarizing holographic face  950  (0th order light) is collected substantially at an intersection of the separation lines  911  and  912  (in a central area of the transmission light detector  910 ) a spot  981 . 
     Based on the detection result of the first diffraction light detector  920 , a reproduction signal lid is obtained. 
     Based on detection results of the sub-transmission light detectors  910 A 1 ,  910 A 2 ,  910 B 1  and  910 B 2 , a calculator  941  of the photodetector  900  outputs  910 A 1 + 910 A 2 − 910 B 1 − 910 B 2 . The output from the calculator  941  is a first tracking error signal  941   s  (TE1 signal). The TE1 signal corresponds to the TE1 signal detected by the photodetector  1050  shown in FIG.  10 B. Also based on detection results of the sub-transmission light detectors  910 A 1 ,  930 A 2 ,  910 B 1  and  910 B 2 , a calculator  942  of the photodetector  900  outputs  910 A 1 + 910 B 2 − 910 A 2 − 910 B 1 . The output from the calculator  942  is a third tracking error signal  9428  (TE3 signal). 
     Based on detection results of the sub-diffraction light detectors  930 A 1 ,  930 A 2 ,  930 A 3 ,  930 B 1 ,  930 B 2 , and  930 B 3 , a detection signal  11   e  corresponding to  930 B 1 + 930 B 3 , a detection signal  11   f  corresponding to  93082 , a detection signal  11   g  corresponding to  930 A 1 + 930 A 3 , and a detection signal  11   h  corresponding to  930 A 2  are obtained. A second tracking error signal (TE2 signal) is obtained by calculation of  11   g + 11   h − 11   e − 11   f.  A focusing error signal (FE signal) is obtained by calculation of  11   e − 11   f − 11   g + 11   h.  The TE2 signal corresponds to the TE2 signal detected by the photodetector  1190  shown in FIG.  11 C. 
     In this example, the phase distribution of the wave surface of the light Immediately after being transmitted through the polarizing holographic face  950  is similar to that of the first example. The ratio of the diffracted light amount allocated for the 0th order light amount (transmission light amount) is 20%, the ratio for the positive first order diffraction light amount is 47.6%, and the ratio for the negative first order diffraction light amount is 12.4%. 
     In this example also, three types of tracking error signals (TE1, TE2 and TE3 signals) are obtained. Like in the first example, these tracking error signals can be used in accordance with the type of the optical disc. For example, in the case of an optical disc having a pit depth corresponding to about ¼ of the wavelength (e.g., DVD-ROM disc), the control device  185  can use a TE3 signal as a tracking error signal with respect to a pit signal (emboss signal). 
     In the case of an optical disc having a guide groove such as, for example, a DVD-RAM disc or DVD-R disc, the control device  185  can use a calculation result value of TE2−k×TE1, obtained by using an appropriate constant k, as a tracking error signal. In the case, the control device  185  can update the value of k in accordance with the type of the optical disc. 
     Like in the first example, the degree of asymmetry of the tracking error signal caused by the shifting of the central axis of the objective lens  160  with respect to the optical axis of the optical disc apparatus  300  can be sufficiently suppressed. Off-track while the tracking control is performed can be solved. In this example, the polarizing holographic face  950  is divided into small strip-shaped areas. Using these small strip-shaped areas, a light component to be collected before the photodetector  900  and a light component to be collected after the photodetector  900  are generated. The resultant diffraction light is detected as an FE signal. Therefore, the adverse influence of dust and stains present on the substrate  172  of the optical disc  170  is negated Thus., the focusing error control is highly stable. In the seventh example, one detector (the first diffraction light detector  920 ) is used to detect a reproduction signal. The detection index is about 47.6. A higher S/N ratio than that of the first example is guaranteed. 
     According to the present invention, two types of tracking error signals (TE1 and TE2 signals), which are conventionally detected, can be simultaneously detected. Thus, the control device  185  generates a sufficiently accurate tracking error signal from the two types of tracking error signals. The control device  185  can use a calculation result value of TE2−k×TE1, obtained by using an appropriate constant k, as a tracking error signal. The polarizing holographic element and the photodetector can be divided in other manners. The diffraction efficiency can be distributed in different manners. The holographic element can be a non-polarizing holographic element or other light distribution element. 
     According to the present invention, using a calculation result value of TE2−k×TE1 as a tracking error signal, the degree of asymmetry of the tracking error signal caused by the shifting of the objective lens when the laser light crosses the pits is sufficiently suppressed. Off track while the tracking control is performed can be solved. Therefore, satisfactory and stable recording and reproduction can be realized. In the case where a light distribution section, such as a polarizing holographic element or the like, has a pattern having sawtooth-like or step-like shape including three or more steps (the pattern being continuous over sequential cycles), the reproduction signal can have a sufficiently high S/N ratio and thus a high signal reproduction performance is obtained. 
     Various other modifications will be apparent to and can be readily made by those skilled in the art without departing from the scope and spirit of this invention. Accordingly, it is not intended that the scope of the claims appended hereto be limited to the description as set forth herein, but rather that the claims be broadly construed.