Patent Publication Number: US-6339564-B2

Title: Optical information recording/reproducing apparatus

Description:
This application is a division of prior application Ser. No. 08/993,703, filed Dec. 18, 1997, now U.S. Pat. No. 6,185,166, which is a Continuation-In-Part application of prior application Ser. No. 08/742,764, filed Nov. 1, 1996, now U.S. Pat. No. 5,793, 725, which is a division of prior application Ser. No. 08/513,578, filed Aug. 10, 1995, now U.S. Pat. No. 5,623,462, which is a File-Wrapper-Continuation of prior application Ser. No. 08/084,362, filed Jun. 30, 1993, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the invention 
     The present invention generally relates to optical information recording/reproducing apparatuses, and more particularly to an optical information recording/reproducing apparatus which optically records information on a recording medium and/or optically reproduces the information from the recording medium. 
     2. Description of the Related Art 
     An optical disk unit is an example of a unit which uses an optical information recording/reproducing apparatus. The optical disk unit can be used as a storage unit of a file system or the like, and is suited for storing programs and large amounts of data. In such an optical disk unit, it is desirable that an optical system thereof can accurately record and/or reproduce the information, and that the number of parts thereof is minimized so as to reduce the cost of the optical disk unit as a whole. 
     Various techniques have been proposed to detect a focal error in the optical disk unit. Generally, the astigmatism technique and the Foucault technique are well known. The Foucault technique is sometimes also referred to as the double knife edge technique. 
     Compared to the astigmatism technique, the Foucault technique is less affected by the external disturbance that occurs when a track on an optical disk is traversed, the birefringence of the optical disk. Accordingly, the mixture of the external disturbance into a focal error signal when the Foucault technique is employed is extremely small compared to the case where the astigmatism technique is employed. In addition, the Foucault technique detects a reflected light beam from the optical disk by a photodetector which is arranged in a vicinity of an image formation point of the optical beam, and for this reason, an abnormal offset is unlikely generated in the focal error signal even if the reflected light beam shifts from an optical axis. Because of these advantageous features obtainable by the Foucault technique, it is desirable to employ the Foucault technique as the focal error detection technique. 
     First, an example of an optical information recording/reproducing apparatus within a conventional magneto-optic disk unit which employs the Foucault technique will be described with reference to FIG.  1 . 
     In an optical system of the optical information recording/reproducing apparatus shown in FIG. 1, a laser beam which is emitted from a laser diode  201  is formed into a parallel beam having an oval cross section in a collimator lens  202 , and is thereafter formed into a light beam having a circular cross section in a true circle correction prism  203 . The light beam from the true circle correction prism.  203  is transmitted through a beam splitter  204 , reflected by a mirror  205 , and is converged on a disk  207  via an objective lens  206 . A reflected light beam from the disk  207  enters the beam splitter  204  via the objective lens  206  and the mirror  205 , but this time the reflected light beam is reflected by the beam splitter  204  and is directed towards a beam splitter  208 . The beam splitter  208  splits the reflected light beam into two light beams, and supplies one light beam to a magneto-optic signal detection system and the other light beam to a servo signal detection system. 
     The magneto-optic signal detection system includes a Wollaston prism  209 , a lens  210  and a 2-part photodetector  211 . One of the two light beams output from the beam splitter  208  is input to the 2-part photodetector  211  via the Wollaston prism  209  and the lens  210 , and the 2-part photodetector  211  detects the magneto-optic signal, that is, the information signal, based on the input light beam. 
     The servo signal detection system includes a condenser lens  212 , a beam splitter  213 , a 2-part photodetector  214 , a composite prism  215  and a 4-part photodetector  216 . The other of the two light beams output from the beam splitter  208  is input to the 2-part photodetector  214  via the condenser lens  212  and the beam splitter  213  on one hand, and is input to the 4-part photodetector  216  via the composite prism  215  on the other. The 2-part photodetector  214  forms a tracking error detection system in the servo signal detection system, and generates a tracking error signal by obtaining a difference between the outputs of the 2-part photodetector  214  according to the push-pull technique. The composite prism  215  and the  4 part photodetector  216  form a focal error detection system in the servo signal detection system, and generates a focal error signal based on outputs of the 4-part photodetector  216  according to the Foucault technique. A focus servo operation controls the relative positional relationship of the objective lens  206  and the disk  207  based on the focal error signal, so that an in-focus position is located on the disk  207 . 
     Next, a description will be given of the push-pull technique, by referring to FIGS. 2 and 3. FIGS.  2 ( a ),  2 ( b ) and  2 ( c ) show the relative positional relationship of the light beam which is irradiated via the objective lens  206  and the track on the disk  207 , and FIGS.  3 ( a ),  3 ( b ) and  3 ( c ) show a spot of the reflected light beam which is formed on the 2-part photodetector  214  in correspondence with FIGS.  2 ( a ),  2 ( b ) and  2 ( c ). 
     FIG.  2 ( b ) shows a case where the spot of the light beam is positioned at the center of a guide groove  207   a  of the disk  207 . In this case, the spot of the reflected light beam on the 2-part photodetector  214  is formed as shown in FIG.  3 ( b ), and a light intensity distribution b is symmetrical to the right and left. If the outputs of the 2-part photodetector  214  are denoted by A and B, a tracking error signal TES is generated based on the following formula (1). 
     
       
         TES=A−B  (1) 
       
     
     In this case, the tracking error signal TES is 0. 
     If the spot of the light beam in FIG.  2 ( b ) shifts to the right as shown in FIG.  2 ( a ), a light intensity distribution a of the reflected light beam becomes unbalanced and the light intensity at the left detector part of the 2-part photodetector  214  becomes larger as shown in FIG.  3 ( a ). For this reason, the tracking error signal TES in this case takes a positive value. 
     On the other hand, if the spot of the light beam in FIG.  2 ( b ) shifts to the left as shown in FIG.  2 ( c ), a light intensity distribution c of the reflected light beam becomes unbalanced and the light intensity at the right detector part of the 2-part photodetector  214  becomes larger as shown in FIG.  3 ( c ). For this reason, the tracking error signal TES in this case takes a negative value. 
     Accordingly, if the spot of the light beam on the disk  207  shifts to the right or left with respect to the central position of the guide groove  207   a , the tracking error signal TES which is obtained in the above described manner changes to a more positive or negative value. Thus, it is possible to carry out an appropriate tracking control operation based on the tracking error signal TES. 
     FIG. 4 shows an example of the shapes of the composite prism  215  and the 4-part photodetector  216 . The 4-part photodetector  216  includes detector parts  216   a ,  216   b ,  216   c  and  216   d . A focal error signal FES is generated from outputs A, B, C and D respectively output from the detector parts  216   a ,  216   b ,  216   c  and  216   d  of the 4-part photodetector  216 , based on the following formula (2). 
     
       
         FES=(A−B)+(C−D)  (2) 
       
     
     Ideally, the focal error signal FES is  0  in a state where the spot of the light beam is in focus on the disk  207 . In this case, the focal error signal FES having an S-curve as shown in FIG. 5 is obtained depending on the distance between the objective lens  206  and the disk  207 . In FIG. 5, the ordinate indicates the focal error signal FES, and the abscissa indicates the distance between the objective lens  206  and the disk  207 . The origin ( 0 ) on the abscissa corresponds to the in-focus position, and the above distance becomes smaller towards the left and larger towards the right in FIG.  5 . 
     FIGS.  6 ( a ),  6 ( b ) and  6 ( c ) show the relative positional relationship of the objective lens  206  and the disk  207 . FIG.  6 ( a ) shows a case where the objective lens  206  is close to the disk  207  and the in-focus position is located above the disk  207  in the figure, FIG.  6 ( b ) shows a case where the in-focus position is located on the disk  207 , and FIG.  6 ( c ) shows a case where the objective lens  296  is far from the disk  207  and the in-focus position is located between the disk  207  and the objective lens  206  in the figure. 
     FIGS.  7 ( a ),  7 ( b ) and  7 ( c ) show beam spots on the 4-part photodetector  216  for each relative positional relationship of the objective lens  206  and the disk  207  shown in FIGS.  6 ( a ),  6 ( b ) and  6 ( c ). FIG.  7 ( a ) shows the beam spots for the positional relationship shown in FIG.  6 ( a ), FIG.  7 ( b ) shows the beam spots for the in-focus positional relationship shown in FIG.  6 ( b ), and FIG.  7 ( c ) shows the beam spots for the positional relationship shown in FIG.  6 ( c ). As shown in FIG.  7 ( b ), the beam spots on the 4-part photodetector  216  have oval shapes in the in-focus position, and a division line E of the 4-part photodetector  216  is positioned at the center of each oval beam spot. 
     However, in the actual disk unit, the distribution of the quantity of the light beam irradiated on the disk  207  may be unbalanced, and errors may exist in the mounting positions of the composite prism  215  and the 4-part photodetector  216 . 
     The light intensity distribution of the light beam which is emitted from the laser diode  201  can generally be approximated by a Gaussian distribution. Hence, if the optical axis of the light beam emitted from the laser diode  201  matches the optical axes of other optical parts, it is possible to obtain a Gaussian distribution in which the center of the light intensity of the light beam input to the objective lens  206  matches the optical axis (point 0) shown in FIG.  8 . However, if the light beam emitted from the laser diode  201  is inclined by an angle θ in FIG. 1, the center of the light intensity of the light beam input to the objective lens  206  is shifted from the optical axis (point 0) in the Gaussian distribution as indicated by a dotted line in FIG.  8 . The “unbalanced distribution” of the light quantity of the light beam irradiated on the disk  207  or “decentering”, refers to such a difference between the optical axis and the center of light beam intensity distribution. 
     On the other hand, the “mounting error” of the composite prism  215 , for example, refers to a positional error of the composite prism  215  in a y-direction in FIG.  4 . If such a mounting error exists, the composite prism  215  cannot accurately split the incident light beam into two equal light beams. Generally, if the division line E of the composite prism  215  shifts a distance Δy in the y-direction from the center of the incident light beam, where the division line E extends in the x-direction in FIG. 4, the value of the mounting error can be obtained from [Δy/(diameter of light beam)]·100 (%). 
     For this reason, if the quantity of the light beam which is split into two in the composite prism  215  changes and a positional error of the division line E of the 4-part photodetector  216  occurs, a focal offset is generated. The generation of the “focal offset” means that the focal error signal FES described by the formula (2) becomes 0 at a position other than the in-focus position. Thus, according to the conventional Foucault technique, the tolerable margin of the focal error detection system is extremely small with respect to the unbalanced distribution of the quantity of light beam irradiated on the disk  207 , the mounting error of the composite prism  215  and the 4-part photodetector  216  and the like. Therefore, there is a problem in that it is extremely difficult to obtain an accurate focal error signal due to the above error factors. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is a general object of the present invention to provide a novel and useful optical information recording/reproducing apparatus in which the problem described above is eliminated. 
     Another and more specific object of the present invention is to provide an optical information recording/reproducing apparatus which records information on and/or reproduces information from an optical recording medium and detects a focal error based on a reflected light beam from the optical recording medium, comprising a composite prism deflecting a part of the reflected light beam to at least two positions excluding a central part of the reflected light beam,.and photodetector means including a plurality of photodetectors for respectively detecting the deflected parts of the reflected light beam and outputting detection outputs, where the focal error is detected based on the detection outputs of the photodetector means. According to the optical information recording/reproducing apparatus of the present invention, it is possible to obtain an accurate focal error signal because the tolerable margin of the focal error detection system can be set large with respect to the unbalanced distribution of the quantity of the light beam irradiated on the optical recording medium, the mounting error of the composite prism, the photodetector and the like. 
     Still another object of the present invention is to provide an optical information recording/reproducing apparatus which records information on and/or reproduces information from an optical recording medium and detects a tracking error and a focal error based on a reflected light beam from the optical recording medium, comprising beam splitter means for splitting the reflected light beam into at least one first beam which is used for detecting the tracking error and at least two second beams which are used for detecting the focal error, and photodetector means including a first photodetector which detects the first beam at a position other than an image formation point of the first beam, and second photodetectors for detecting the second beams approximately at image formation points of the second beams. According to the optical information recording/reproducing apparatus of the present invention, it is unnecessary to provide two independent optical paths even if the focal error is to be detected according to the Foucault technique and the tracking error is to be detected according to the push-pull technique. As a result, it is possible to reduce the space occupied by the optical system within the optical information recording/reproducing apparatus, and to reduce the number of required parts. For this reason, it is possible to reduce both the size and cost of the optical information recording/reproducing apparatus and an optical disk unit to which the optical information recording/reproducing apparatus may be applied. 
     A further object of the present invention is to provide an optical information recording/reproducing apparatus which records information on and/or reproduces information from an optical recording medium and detects a focal error and a tracking error based on a reflected light beam from the optical recording medium, comprising beam splitter means for splitting the reflected light beam into first through fourth light beams which propagate generally in a predetermined direction, and photodetector means for detecting the focal error in response to the first and second light beams, and for detecting the tracking error in response to the third and fourth light beams. According to the optical information recording/reproducing apparatus of the present invention, it is possible to improve the reliability of the focal error detection and tracking error detection. In addition, it is possible to reduce both the size and cost of the optical information recording/reproducing apparatus and an optical disk unit to which the optical information recording/reproducing apparatus may be applied. 
     Another object of the present invention is to provide an optical information recording/reproducing apparatus which records an information signal on and/or reproduces the information signal from an optical recording medium and detects a tracking error, a focal error, the information signal and an address signal based on a reflected light beam from the optical recording medium, comprising beam splitter means for splitting the reflected light beam into first through sixth light beams which propagate generally in a predetermined direction, and photodetector means for detecting the focal error in response to the first and second light beams, and for detecting the tracking error, the information signal and the address signal in response to the third through sixth light beams. According to the optical information recording/reproducing apparatus of the present invention, it is possible to detect the focal error signal, the tracking error signal, the information signal and the address signal by the single photodetector means. For this reason, it is possible to detect all of the necessary signals using a single optical path to the beam splitter means and the single photodetector means. Hence, it is possible to reduce both the size and cost of the optical information recording/reproducing apparatus and an optical disk unit to which the optical information recording/reproducing apparatus may be applied. 
     Still another object of the present invention is to provide an optical information recording/reproducing apparatus which records information on and/or reproduces information from an optical recording medium based on a reflected light beam from the optical recording medium, comprising a light splitting part having a plurality of light splitting stages which split the reflected light beam from the optical recording medium into a plurality of light beams, and a photodetector unit including a plurality of photodetectors which receive the plurality of light beams from the light splitting part, where the photodetector unit includes a plurality of photodetectors which receive light beams used to detect a focal error, at least one photodetector which receives a light beam used to detect a tracking error, and a plurality of photodetectors which receive light beams used to detect magneto-optic information recorded on the optical recording medium. According to the optical information recording/reproducing apparatus of the present invention, the number of optical parts forming the apparatus can be effectively reduced, thereby simplifying the production process and reducing the cost of the apparatus. Further, it is possible to suppress noise and obtain a focal error signal, a tracking error signal and a magneto-optic signal having a high signal quality. 
     Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram showing an example of a conventional optical information recording/reproducing apparatus; 
     FIGS.  2 ( a ),  2 ( b ) and  2 ( c ) are diagrams showing the relative positional relationship between a light beam which is irradiated via an objective lens and a track on an optical disk for explaining the push-pull technique; 
     FIGS.  3 ( a ),  3 ( b ) and  3 ( c ) are diagrams showing a spot of a reflected light beam which is formed on a 2-part photodetector; 
     FIG. 4 is a perspective view showing an  10  example of the shapes of a composite prism and a  4 part photodetector; 
     FIG. 5 is a diagram showing the relationship of a distance between the objective lens and the disk and a focal error signal FES; 
     FIGS.  6 ( a ),  6 ( b ) and  6 ( c ) are diagrams showing the relative positional relationship of the objective lens and the disk; 
     FIGS.  7 ( a ),  7 ( b ) and  7 ( c ) are diagrams showing a spot of a reflected light beam which is formed on the 4-part photodetector; 
     FIG. 8 is a diagram showing a Gaussian distribution; 
     FIG. 9 is a perspective view showing an important part of a first embodiment of an optical information recording/reproducing apparatus according to the present invention; 
     FIG. 10 is a perspective view showing an important part of a second embodiment of the optical information recording/reproducing apparatus according to the present invention; 
     FIG. 11 is a perspective view showing an important part of a third embodiment of the optical information recording/reproducing apparatus according to the present invention; 
     FIG. 12 is a perspective view showing an important part of a fourth embodiment of the optical information recording/reproducing apparatus according to the present invention; 
     FIGS.  13 ( a ),  13 ( b ),  13 ( c ) and  13 ( d ) are diagrams showing simulation results describing the relationship of a focus position and a focal error signal FES in the prior art; 
     FIGS.  14 ( a ),  14 ( b ),  14 ( c ) and  14 ( d ) are diagrams showing simulation results describing the relationship of the focus position and the focal error signal FES in the first or third embodiment; 
     FIG. 15 is a diagram showing the relationship of a detector shift and a focal offset in the prior art; 
     FIG. 16 is a diagram showing the relationship of the detector shift and the focal offset in the first or third embodiment; 
     FIG. 17 is a diagram showing a fifth embodiment of the optical information recording/reproducing apparatus-according to the present invention; 
     FIGS.  18 ( a ) and  18 ( b ) are diagrams showing a composite prism of the fifth embodiment on an enlarged scale; 
     FIG. 19 is a perspective view showing an important part of the fifth embodiment; 
     FIGS.  20 ( a ) and  20 ( b ) are diagrams showing a composite prism of a sixth embodiment of the optical information recording/reproducing apparatus according to the present invention on an enlarged scale; 
     FIG. 21 is a perspective view showing an important part of the sixth embodiment; 
     FIGS.  22 ( a ) and  22 ( b ) are diagrams showing a composite prism of a seventh embodiment of the optical information recording/reproducing apparatus according to the present invention on an enlarged scale; 
     FIG. 23 is a perspective view showing an important part of the seventh embodiment; 
     FIG. 24 is a diagram showing an eighth embodiment of the optical information recording/reproducing apparatus according to the present invention; 
     FIG. 25 is a perspective view showing an important part of the eighth embodiment; 
     FIG. 26 is a plan view showing a photodetector unit of the eighth embodiment; 
     FIG. 27 is a perspective view showing an important part of a first modification of the eighth embodiment of the optical information recording/reproducing apparatus according to the present invention; 
     FIG. 28 is a plan view showing a photodetector unit of the first modification of the eighth embodiment; 
     FIG. 29 is a perspective view showing an important part of a second modification of the eighth embodiment of the optical information recording/reproducing apparatus according to the present invention; 
     FIG. 30 is a perspective view showing a composite prism of the second modification of the eighth embodiment; 
     FIG. 31 is a plan view showing a photodetector unit of the second modification of the eighth embodiment; 
     FIG. 32 is a perspective view showing an important part of a ninth embodiment of the optical information recording/reproducing apparatus according to the present invention; 
     FIG. 33 is a cross sectional view showing an important part of a holographic optical element of the ninth embodiment; 
     FIG. 34 is a perspective view for explaining the functions of the holographic optical element by itself; 
     FIG. 35 is a plan view for explaining the construction of the holographic optical element; 
     FIG. 36 is a cross sectional view showing an important part of a holographic optical element of a tenth embodiment of the optical information recording/reproducing apparatus according to the present invention; 
     FIG. 37 is a perspective view for explaining desirable functions of the holographic optical element by itself; 
     FIG. 38 is a diagram showing an eleventh embodiment of the optical information recording/reproducing apparatus according to the present invention; 
     FIGS.  39 ( a ) and  39 ( b ) are diagrams showing a composite prism of the eleventh embodiment; 
     FIG. 40 is a perspective view showing an important part of the eleventh embodiment; 
     FIG. 41 is a diagram showing a twelfth embodiment of the optical information recording/reproducing apparatus according to the present invention; 
     FIG. 42 is a perspective view showing a Wollaston prism of the twelfth embodiment; 
     FIG. 43 is a diagram showing an integral part made up of a composite prism and the Wollaston prism of the twelfth embodiment; 
     FIG. 44 is a perspective view showing an important part of the twelfth embodiment; 
     FIG. 45 is a diagram showing a thirteenth embodiment of the optical information recording/reproducing apparatus according to the present invention; 
     FIG. 46 is a perspective view showing an important part of the thirteenth embodiment; 
     FIG. 47 is a plan view showing a photodetector unit in a state where an objective lens and a magneto-optic disk are close to each other; 
     FIG. 48 is a plan view showing the photodetector unit in a state where a laser beam is in focus on the magneto-optic disk; 
     FIG. 49 is a plan view showing the photodetector unit in a state where the objective lens and the magneto-optic disk are far away from each other; 
     FIGS. 50A,  50 B and  50 C respectively are diagrams for explaining spots formed on photodetectors of the photodetector unit by light beams output from a composite prism; 
     FIG. 51 is a plan view showing the photodetector unit in a high temperature state; 
     FIG. 52 is a plan view showing a photodetector unit used in a first modification of the thirteenth embodiment of the optical information recording/reproducing apparatus according to the present invention; 
     FIG. 53 is a plan view showing a photodetector unit used in a second modification of the thirteenth embodiment of the optical information recording/reproducing apparatus according to the present invention; 
     FIG. 54 is a perspective view showing an important part of a third modification of the thirteenth embodiment of the optical information recording/reproducing apparatus according to the present invention; 
     FIG. 55 is a perspective view showing a composite prism of the third modification of the thirteenth embodiment; 
     FIG. 56 is a plan view showing a photodetector unit of the third modification of the thirteenth embodiment; 
     FIG. 57 is a perspective view showing an important part of a fourteenth embodiment of the optical information recording/reproducing apparatus according to the present invention; 
     FIG. 58 is a perspective view showing an important part of a fifteenth embodiment of the optical information recording/reproducing apparatus according to the present invention; 
     FIG. 59 is a perspective view showing an important part of an sixteenth embodiment of the optical information recording/reproducing apparatus according to the present invention; 
     FIGS. 60A through 60D respectively are circuit diagrams showing circuits for obtaining a focal error signal, a tracking error signal, a magneto-optic signal and an identification signal of the first modification of the eighth embodiment; and 
     FIG. 61 is a perspective view showing a composite prism which may be used in place of the composite prisms shown in FIGS.  30  and  55 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 9 is a perspective view showing an important part of a first embodiment of an optical information recording/reproducing apparatus according to the present invention. A composite prism  15  includes tapered parts  15   a  and  15   b , and a central part  15   c  having no taper. On the other hand, a 4-part photodetector unit  16  includes 2-part photodetectors  16   a  and  16   b , and a central part  16   c  which includes no photodetector part. The composite prism  15  and the 4-part photodetector unit  16  are provided in place of the composite prism  215  and the 4-part photodetector  216  in the optical system of the optical information recording/reproducing apparatus shown in FIG. 1, for example, and detect the focal error. 
     The reflected light beam which is obtained via the beam splitters  204  and  208 , the condenser lens  212  and the beam splitter  213  is input to the composite prism  15 . Out of the reflected light beam which is input to the composite prism  15 , the light beams transmitted through the tapered parts  15   a  and  15   b  of the composite prism  15  form spots on the corresponding 2-part photodetectors  16   a  and  16   b  of the 4-part photodetector unit  16 . Accordingly, by carrying out the operation of the formula (2) described above using the outputs of the 2-part photodetectors  16   a  and  16   b , it is possible to obtain a focal error signal FES similarly to the conventional case. 
     On the other hand, out of the reflected light beam, the light beam which is transmitted through the central part  15   c  of the composite prism  15  is input to the central part  16   c  of the 4-part photodetector unit  16 . As a result, out of the reflected light beam input to the composite prism  15 , the light beam which is transmitted through the central part  15   c  of the composite prism  15  is not input to the 2-part photodetectors  16   a  and  16   b  of the 4-part photodetector unit  16 , that is, not input to a light sensitive part of the 4-part photodetector unit  16 . 
     In this embodiment, the spots which are formed on the 2-part photodetectors  16   a  and  16   b  of the 4-part photodetector unit  16  have oval shapes with a major axis greater than that of the conventional case. In other words, the oval spots are longer in a direction perpendicular to the division line E of each of the 2-part photodetectors  16   a  and  16   b . For this reason, the focal offset which is generated by the positional error of the division lines E is extremely small. 
     Next, a description will be given of a second embodiment of the optical information recording/reproducing apparatus according to the present invention, by referring to FIG.  10 . FIG. 10 is a perspective view showing an important part of the second embodiment. 
     In FIG. 10, a composite prism  25  has a trapezoidal column shape and includes tapered parts  25   a  and  25   b , and a central part  25   c  which has no taper. On the other hand, a 4-part photodetector unit  26  includes 2-part photodetectors  26   a  and  26   b , and a central part  26   c  which includes no photodetector part. The composite prism  25  and the 4-part photodetector unit  26  are provided in place of the composite prism  215  and the 4-part photodetector  216  in the optical system of the optical information recording/reproducing apparatus shown in FIG. 1, for example, and detect the focal error. 
     The reflected light beam which is obtained via the beam splitters  204  and  208 , the condenser lens  212  and the beam splitter  213  is input to the composite prism  25 . Out of the reflected light beam which is input to the-composite prism  25 , the light beams transmitted through the tapered parts  25   a  and  25   b  of the composite prism  25  form spots on the corresponding 2-part photodetectors  26   a  and  26   b  of the 4-part photodetector unit  26 . Accordingly, by carrying out the operation of the formula (2) described above using the outputs of the 2-part photodetectors  26   a  and  26   b , it is possible to obtain a focal error signal FES similarly to the conventional case. 
     On the other hand, out of the reflected light beam, the light beam which is transmitted through the central part  25   c  of the composite prism  25  is input to the central part  26   c  of the 4-part photodetector unit  26 . As a result, out of the reflected light beam input to the composite prism  25 , the light beam which is transmitted through the central part  25   c  of the composite prism  25  is not input to the 2-part photodetectors  26   a  and  26   b  of the 4-part photodetector unit  26 , that is, not input to a light sensitive part of the 4-part photodetector unit  26 . 
     In this embodiment, the spots which are formed on the 2-part photodetectors  26   a  and  26   b  of the 4-part photodetector unit  26  have oval shapes with a major axis greater than that of the conventional case. In other words, the oval spots are longer in a direction perpendicular to the division line E of each of the 2-part photodetectors  26   a  and  26   b . For this reason, the focal offset which is generated by the positional error of the division lines E is extremely small. 
     Next, a description will be given of a third embodiment of the optical information recording/reproducing apparatus according to the present invention, by referring to FIG.  11 . FIG. 11 is a perspective showing an important part of the third embodiment. In FIG. 11, those parts which are the same as those corresponding parts in FIG. 9 are designated by the same reference numerals, and a description thereof will be omitted. 
     In this embodiment, a light absorbing or blocking layer  15   c A is formed on the central part  15   c  of a composite prism  15 A so as to absorb or block the light beam which has the wavelength of the light emitted from the laser diode  201  shown in FIG.  1 . This light absorbing or blocking layer  15   c A may be formed on the front surface or the rear surface of the composite prism  15 A at the central part  15   c . In addition, this embodiment uses the same 4-part photodetector  216  used in the conventional case shown in FIG.  1 . 
     In this case, the reflected light beam which is obtained via the beam splitters  204  and  208 , the condenser lens  212  and the beam splitter  213  is input to the composite prism  15 A. Out of the reflected light beam which is input to the composite prism  15 A, the light beams transmitted through the tapered parts  15   a  and  15   b  of the composite prism  15 A form spots on the corresponding detector parts  216   a ,  216   b ,  216   c  and  216   d  of the 4-part photodetector  216 . Accordingly, by carrying out the operation of the formula (2) described above using the outputs of the detector parts  216   a ,  216   b ,  216   c  and  216   d , it is possible to obtain a focal error signal FES similarly to the conventional case. 
     On the other hand, out of the reflected light beam, the light beam which is input to the central part  15   c  of the composite prism  15 A is absorbed or blocked light absorbing or blocking layer  15   c A and will not be input to the 4-part photodetector  216 . As a result, out of the reflected light beam input to the composite prism  15 A, the light beam which is input to the central part  15   c  of the composite prism  15 A is not input to the detector parts  216   a ,  216   b ,  216   c  and  216   d  of the 4-part photodetector  216 , that is, not input to a light sensitive part of the 4-part photodetector  216 . 
     In this embodiment, the spots which are formed on the detector parts  216   a ,  216   b ,  216   c  and  216   d  of the 4-part photodetector  216  have oval shapes with a major axis greater than that of the conventional case. In other words, the oval spots are longer in a direction perpendicular to the division line E of the 4-part photodetector  216 . For this reason, the focal offset which is generated by the positional error of the division line E is extremely small. 
     Next, a description will be given of a fourth embodiment of the optical information recording/reproducing apparatus according to the present invention, by referring to FIG.  12 . FIG. 12 is a perspective showing an important part of the fourth embodiment. In FIG. 12, those parts which are the same as those corresponding parts in FIG. 10 are designated by the same reference numerals, and a description thereof will be omitted. 
     In this embodiment, a light absorbing or blocking layer  25   c A is formed on the central part  25   c  of a composite prism  25 A which has a triangular prism shape so as to absorb or block the light beam which has the wavelength of the light emitted from the laser diode  201  shown in FIG.  1 . This light absorbing or blocking layer  25   c A may be formed on the front surface or the rear surface of the composite prism  25 A at the central part  25   c . In addition, this embodiment uses a 4-part photodetector  216 A shown in FIG.  12 . 
     In this case, the reflected light beam which is obtained via the beam splitters  204  and  208 , the condenser lens  212  and the beam splitter  213  is input to the composite prism  25 A. Out of the reflected light beam which is input to the composite prism  25 A, the light beams transmitted through the tapered parts  25   a  and  25   b  of the composite prism  25 A form spots on the corresponding detector parts  216   a ,  216   b ,  216   c  and  216   d  of the 4-part photodetector  216 A. Accordingly, by carrying out the operation of the formula (2) described above using the outputs of the detector parts  216   a ,  216   b ,  216   c  and  216   d , it is possible to obtain a focal error signal FES similarly to the conventional case 
     On the other hand, out of the reflected light beam, the light beam which is transmitted through the central part  25   c  of the composite prism  25 A is absorbed or blocked light absorbing or blocking layer  25   c A and will not be input to the 4-part photodetector  216 A. As a result, out of the reflected light beam input to the composite prism  25 A, the light beam which is input to the central part  25   c  of the composite prism  25 A is not input to the detector parts  216   a ,  216   b ,  216   c  and  216   d  of the 4-part photodetector  216 A, that is, not input to a light sensitive part of the 4-part photodetector  216 A. 
     In this embodiment, the spots which are formed on the detector parts  216   a ,  216   b ,  216   c  and  216   d  of the 4-part photodetector  216 A have oval shapes with a major axis greater than that of the conventional case. In other words, the oval spots are longer in a direction perpendicular to the division lines E of the 4-part photodetector  216 A. For this reason, the focal offset which is generated by the positional error of the division lines E is extremely small. 
     FIGS.  13 ( a ) through  13 ( d ) show simulation results describing the relationship of the focal position and the focal error signal FES in the prior art shown in FIG.  4 . In FIGS.  13 ( a ) through  13 ( d ), a bold solid line indicates a case where a detector shift is 0, a solid line indicates a case-where the detector shift is +10 μm, a dotted line indicates a case where the detector shift is +20 μm, a bold dotted line indicates a case where the detector shift is −10 μm, and a bold and fine dotted line indicates a case where the detector shift is −20 μm. The “detector shift” refers to the shift of the division line E of the 4-part photodetector  216  in the y-direction in FIG. 4, and an upward shift in FIG. 4 is taken as a positive (+) shift and a downward shift in FIG. 4 is taken as a negative (−) shift. 
     FIG.  13 ( a ) shows a case where the mounting error of the composite prism  215  is 5%, FIG.  13 ( b ) shows a case where the mounting error is 10%, FIG.  13 ( c ) shows a case where the inclination angle θ of the light beam emitted from the laser diode  201  is 0.5°, and FIG.  13 ( d ) shows a case where the inclination angle θ of the light beam emitted from the laser diode  201  is 1.0°. The case where the inclination angle θ is 0.5° corresponds to the case where the shift of the light beam from the optical axis at the objective lens  206  is 0.25 mm, and the case where the inclination angle θ is 1.0° corresponds to the case where the shift of the light beam from the optical axis at the objective lens  206  is 0.50 mm. Accordingly, if the detector shift indicated by the dotted line in FIG.  13 ( a ) is +20 μm, for example, it may be seen that a focal offset of approximately 2.0 μm is generated. 
     On the other hand, FIGS.  14 ( a ) through  14 ( d ) show simulation results describing the relationship of the focal position and the focal error signal FES in  10  the first embodiment shown in FIG. 9 or the third embodiment shown in FIG.  11 . In FIGS.  14 ( a ) through  14 ( d ), a bold solid line indicates a case where the detector shift is 0, a solid line indicates a case where the detector shift is +10 μm, a dotted line indicates a case where the detector shift is +20 μm, a bold dotted line indicates a case where the detector shift is −10 μm, and a bold and fine dotted line indicates a case where the detector shift is −20 μm. 
     FIG.  14 ( a ) shows a case where the mounting error of the composite prism  15  or  15 A is 5%, FIG.  14 ( b ) shows a case where the mounting error is 10%, FIG.  14 ( c ) shows a case where the inclination angle θ of the light beam emitted from the laser diode  201  is 0.5°, and FIG.  14 ( d ) shows a case where the inclination angle θ of the light beam emitted from the laser diode  201  is 1.0°. The case where the inclination angle θ is 0.5° corresponds to the case where the shift of the light beam from the optical axis at the objective lens  206  is 0.25 mm, and the case where the inclination angle θ is 1.0° corresponds to the case where the shift of the light beam from the optical axis at the objective lens  206  is 0.50 mm. Accordingly, even if the detector shift indicated by the dotted line in FIG.  14 ( a ) is +20 μm, for example, it may be seen that only an extremely small focal offset of approximately 0.8 μm is generated. In other words, the focal offset is less than one-half the focal offset of the conventional case. 
     FIG. 15 is a diagram showing the relationship of the detector shift and the focal offset in the prior art based on the simulation results of FIGS.  13 ( a ) through  13 ( d ). In FIG. 15, black circular marks indicate experimental data. In FIG. 15, a coarse dotted line shows a case where the mounting error of the composite prism  215  is 5%, a fine dotted line indicates a case where the mounting error of the composite prism  215  is 10%, a two-dot chain line indicates a case where the shift of the light beam from the optical axis at the objective lens  206  is 0.25 mm, and a one-dot chain line indicates a case where the shift of the light beam from the optical axis at the objective lens  206  is 0.50 mm. As may be seen from FIG. 15, the focal offset is generated in each case where the detector shift occurs. 
     On the other hand, FIG. 16 is a diagram showing the relationship of the detector shift and the focal offset in the first or third embodiment based on the simulation results of FIGS.  14 ( a ) through  14 ( d ). In FIG. 16, black circular marks indicate experimental data. In FIG. 16, a coarse dotted line shows a case where the mounting error of the composite prism  15  or  15 A is 5%, a fine dotted line indicates a case where the mounting error of the composite prism  15  or  15 A is 10%, a two-dot chain line indicates a case where the shift of the light beam from the optical axis at the objective lens  206  is 0.25 mm, and a one-dot chain line indicates a case where the shift of the light beam from the optical axis at the objective lens  206  is 0.50 mm. As may be seen from FIG. 16, the focal offset which is generated is extremely small or approximately 0 in each case where the detector shift occurs. Accordingly, it can be seen that the focal offset in the first or third embodiment is extremely small compared to that of the prior art. 
     In FIG. 1, the arrangement of the 4-part photodetector  216  along the optical axis must be set approximately to the image formation point position of the condenser lens  212 , due to the operating principle of the Foucault technique. On the other hand, the arrangement of the 2-part photodetector  214  along the optical axis must be set at a position shifted from the image formation point position of the condenser lens  212 , due to the operating principle of the push-pull technique. In other words, the 2-part photodetector  214  must be set at the so-called far field. 
     For the above reasons, it is necessary to split the reflected light beam into two by use of the beam splitter  213 , and independently provide an optical path which is used to carry out the Foucault technique and an optical path which is used to carry out the push-pull technique. As a result, if the focal error is to be detected using the Foucault technique and the tracking error is to be detected using the push-pull technique, the optical system occupies a relatively large space because of the need to provide two independent optical paths, and furthermore, the number of parts required becomes large. 
     Accordingly, a description will hereinafter be given of embodiments of the optical information recording/reproducing apparatus according to the present invention which reduce the space of the optical system occupying within the optical information recording/reproducing apparatus and reduce the number of required parts, so that the size and cost of the optical information recording/reproducing apparatus and the optical disk unit using the same can both be reduced. 
     First, a description will be given of a fifth embodiment of the optical information recording/reproducing apparatus according to the present invention, by referring to FIGS. 17 through 19. In FIG. 17, those parts which are the same as those corresponding parts in FIG. 1 are designated by the same reference numerals, and a description thereof will be omitted. 
     In this embodiment, it is unnecessary to provide the beam splitter  213  and the 2-part photodetector  214  shown in FIG. 1, as may be seen from FIG.  17 . In addition, a composite prism  35  and a photodetector unit  36  are provided in place of the composite prism  215  and the 4-part photodetector  216 . In other words, this embodiment uses the central part of the reflected light beam which is not used in the first through fourth embodiments, for detecting the tracking error by the push-pull technique. 
     FIGS.  18 ( a ) and  18 ( b ) show the composite prism  35  on an enlarged scale. FIG.  18 ( a ) shows a perspective view of the composite prism  35 , and FIG.  18 ( b ) shows a plan view of the composite prism  35 . As shown in FIGS.  18 ( a ) and  18 ( b ), the composite prism  35  includes tapered first and second parts  35   a  and  35   b , and a third part  35   c  which has a convex surface with a slight curvature. Hence, a reflected light beam  30  which is obtained via the beam splitter  208  is split into three light beams  30   a ,  30   b  and  30   c.    
     FIG. 19 is a perspective view, on an enlarged scale, showing an important part of FIG.  17 . The photodetector unit  36  includes a first photodetector  36   a , a second photodetector  36   b , and a third photodetector  36   c . The first photodetector  36   a  includes photodetectors  37   a  and  37   b . The second photodetector  36   b  includes photodetectors  37   c  and  37   d . The third photodetector  36   c  includes photodetectors  37   e  and  37   f.    
     Out of the reflected light beam  30  which is refracted and condensed via the condenser lens  212 , the light beam  30   a  which is transmitted through the first part  35   a  is deflected depending on the taper angle of the first part  35   a  and is irradiated on the first photodetector  36   a  of the photodetector unit  36 , while the light beam  30   b  which is transmitted through the second part  35   b  is deflected depending on the taper angle of the second part  35   b  and is irradiated on the second photodetector  36   b  of the photodetector unit  36 . In addition, the light beam  30   c  which is transmitted through the third part  35   c  is refracted depending on the curvature of the third part  35   c  and is irradiated on the third photodetector  36   c  of the photodetector unit  36 . In other words, the light beams  30   a  and  30   b  are only subjected to the refraction function of the condenser lens  212 , but the light beam  30   c  is subjected to the refraction function of the condenser lens  212  and the third part  35   c  itself. Therefore, image formation points  300   a  and  300   b  of the respective light beams  30   a  and  30   b  are different from an image formation point  300   c  of the light beam  30   c . That is, distances L1 and L2 from the condenser lens  212  to the image formation points  300   a  and  300   b  of the respective light beams  30   a  and  30   b  are different from a distance L3 from the condenser lens  212  to the image formation point  300   c  of the light beam  30   c.    
     In FIG. 19, the photodetector unit  36  is arranged on a plane which is perpendicular to the optical axis of the reflected light beam  30  and includes the image formation points  300   a  and  300   b . Because of this arrangement, the first and second photodetectors  36   a  and  36   b  which are used to generate the focal error signal FES based on the Foucault technique are respectively provided at the positions of the image formation points  300   a  and  300   b  of the light beams  30   a  and  30   b . On the other hand, the third photodetector  36   c  which is used to generate the tracking error signal TES based on the push-pull technique is provided at a position deviated from the position of the image formation point  300   c  of the light beam  30   c . Hence, it is possible to generate the focal error signal FES using the Foucault technique and to generate the tracking error signal TES using the push-pull technique by use of a simple optical system. The generation itself of the focal error signal FES and the tracking error signal TES may be made similarly to the prior art, and a description thereof will be omitted. 
     The requirement is that the distances L1 and L2 between the condenser lens  212  and the respective image formation points  300   a  and  300   b  of the light beams  30   a  and  30   b  are different from the distance L3 between the condenser lens  212  and the image formation point  300   c  of the light beam  30   c , and the construction and arrangement of the composite prism  35  and the photodetector unit  36  are not limited to those of the above embodiment. 
     Next, a description will be given of a sixth embodiment of the optical information recording/reproducing apparatus according to the present invention, by referring to FIGS. 20 and 21. In FIGS. 20 and 21, those parts which are the same as those corresponding parts in FIGS. 18 and 19 are designated by the same reference numerals, and a description thereof will be omitted. 
     In this embodiment, a composite prism  45  shown in FIGS.  20 ( a ) and  20 ( b ) is used in place of the composite prism  35  shown in FIG.  18 . 
     FIGS.  20 ( a ) and  20 ( b ) show the composite prism  45  on an enlarged scale. FIG.  20 ( a ) shows a perspective view of the composite prism  45 , and FIG.  20 ( b ) shows a plan view of the composite prism  45 . As shown in FIGS.  20 ( a ) and  20 ( b ), the composite prism  45  includes tapered first and second parts  45   a  and  45   b , and a third part  45   c  which has a concave surface with a slight curvature. Hence, a reflected light beam  30  which is obtained via the beam splitter  208  is split into three light beams  30   a ,  30   b  and  30   c.    
     FIG. 21 is a perspective view, on an enlarged scale, showing an important part of this embodiment. The photodetector unit  36  is the same as the photodetector unit  36  used in the fifth embodiment. 
     Out of the reflected light beam  30  which is refracted and condensed via the condenser lens  212 , the light beam  30   a  which is transmitted through the first part  45   a  is deflected depending on the taper angle of the first part  45   a  and is irradiated on the first photodetector  36   a  of the photodetector unit  36 , while the light beam  30   b  which is transmitted through the second part  45   b  is deflected depending on the taper angle of the second part  45   b  and is irradiated on the second photodetector  36   b  of the photodetector unit  36 . In addition, the light beam  30   c  which is transmitted through the third part  45   c  is refracted depending on the curvature of the third part  45   c  and is irradiated on the third photodetector  36   c  of the photodetector unit  36 . In other words, the light beams  30   a  and  30   b  are only subjected to the refraction function of the condenser lens  212 , but the light beam  30   c  is subjected to the refraction function of the condenser lens  212  and the third part  45   c  itself. Therefore, image formation points  300   a  and  300   b  of the respective light beams  30   a  and  30   b  are different from an image formation point  300   c  of the light beam  30   c . That is, distances L1 and L2 from the condenser lens  212  to the image formation points  300   a  and  300   b  of the respective light beams  30   a  and  30   b  are different from a distance L3 from the condenser lens  212  to the image formation point  300   c  of the light beam  30   c.    
     In other words, the image formation point  300   c  of the light beam  30   c  is located between the composite prism  35  and the photodetector unit  36  in the fifth embodiment, but the image formation point  300   c  of the light beam  30   c  in this embodiment is located beyond the photodetector unit  36  in FIG. 21 along the traveling direction of the light beam. 
     In FIG. 21, the photodetector unit  36  is arranged on a plane which is perpendicular to the optical axis of the reflected light beam  30  and includes the image formation points  300   a  and  300   b , similarly to the fifth embodiment shown in FIG.  19 . Because of this arrangement, the first and second photodetectors  36   a  and  36   b  which are used to generate the focal error signal FES based on the Foucault technique are respectively provided at the positions of the image formation points  300   a  and  300   b  of the light beams  30   a  and  30   b . On the other hand, the third photodetector  36   c  which is used to generate the tracking error signal TES based on the push-pull technique is provided at a position deviated from the position of the image formation point  300   c  of the light beam  30   c . Hence, it is possible to generate the focal error signal FES using the Foucault technique and to generate the tracking error signal TES using the push-pull technique by use of a simple optical system. 
     Next, a description will be given of a seventh embodiment of the optical information recording/reproducing apparatus according to the present invention, by referring to FIGS. 22 and 23. In FIGS. 22 and 23, those parts which are the same as those corresponding parts in FIGS. 18 and 19 are designated by the same reference numerals, and a description thereof will be omitted. 
     In this embodiment, a composite prism  55  and a photodetector unit  56  shown in FIG. 23 are used in place of the composite prism  35  and the photodetector unit  36  shown in FIGS.  18 ( a ) and  18 ( b ). 
     FIGS.  22 ( a ) and  22 ( b ) show the composite prism  55  on an enlarged scale. FIG.  22 ( a ) shows a perspective view of the composite prism  55 , and FIG.  22 ( b ) shows a plan view of the composite prism  55 . As shown in FIGS.  22 ( a ) and  22 ( b ), the composite prism  55  includes tapered first and second parts  55   a  and  55   b , and a flat third part  55   c  which has not taper. Hence, a reflected light beam  30  which is obtained via the beam splitter  208  is split into three light beams  30   a ,  30   b  and  30   c.    
     FIG. 23 is a perspective view, on an enlarged scale, showing an important part of this embodiment. The photodetector unit  56  includes a first photodetector  56   a , a second photodetector  56   b , and a third photodetector  56   c . The first photodetector  56   a  includes photodetectors  37   a  and  37   b . The second photodetector  56   b  includes photodetectors  37   c  and  37   d . The third photodetector  56   c  includes photodetectors  37   e  and  37   f . The third photodetector  56   c  is arranged on a plane different from a plane on which the first and second photodetectors  56   a  and  56   b  are arranged. 
     Out of the reflected light beam  30  which is refracted and condensed via the condenser lens  212 , the light beam  30   a  which is transmitted through the first part  55   a  is deflected depending on the taper angle of the first part  55   a  and is irradiated on the first photodetector  56   a  of the photodetector unit  56 , while the light beam  30   b  which is transmitted through the second part  55   b  is deflected depending on the taper angle of the second part  55   b  and is irradiated on the second photodetector  56   b  of the photodetector unit  56 . In addition, the light beam  30   c  which is transmitted through the third part  55   c  is transmitted as it is and is irradiated on the third photodetector  56   c  of the photodetector unit  56 . In other words, all of the light beams  30   a ,  30   b  and  30   c  are only subjected to the refraction function of the condenser lens  212 . Therefore, image formation points  300   a ,  300   b  and  300   c  of the respective light beams  30   a ,  30   b  and  30   c  are all located on the same plane. That is, distances L1, L2 and L3 from the condenser lens  212  to the image formation points  300   a ,  300   b  and  300   c  of the respective light beams  30   a ,  30   b  and  30   c  are the same. However, since the third photodetector  56   c  in this embodiment is arranged on the plane which is different from the plane on which the first and second photodetectors  56   a  and  56   b  are arranged, the image formation point  300   c  of the light beam  30   c  and the position of the third photodetector  56   c  do not match. 
     In other words, the image formation point  300   c  of the light beam  30   c  is located between the composite prism  35  and the photodetector unit  36  in the fifth embodiment, but the image formation point  300   c  of the light beam  30   c  in this embodiment is located beyond the third photodetector  56   c  in FIG. 23 along the traveling direction of the light beam. 
     In FIG. 23, the first and second photodetectors  56   a  and  56   b  of the photodetector unit  56  are arranged on a plane which is perpendicular to the optical axis of the reflected light beam  30  and includes the image formation points  300   a  and  300   b , similarly to the fifth embodiment shown in FIG.  19 . Because of this arrangement, the first and second photodetectors  56   a  and  56   b  which are used to generate the focal error signal FES based on the Foucault technique are respectively provided at the positions of the image formation points  300   a  and  300   b  of the light beams  30   a  and  30   b . On the other hand, the third photodetector  56   c  which is used to generate the tracking error signal TES based on the push-pull technique is provided at a position deviated from the position of the image formation point  300   c  of the light beam  30   c . Hence, it is possible to generate the focal error signal FES using the Foucault technique and to generate the tracking error signal TES using the push-pull technique by use of a simple optical system. 
     The generation of the focal error signal FES based on the Foucault technique is not limited to that of the embodiment using two light beams, and it is of course possible to use more than two light beams for the generation of the focal error signal FES. Similarly, the generation of the tracking error signal TES based on the push-pull technique is not limited to that of the embodiment using one light beam, and it is of course possible to use more than one light beam for the generation of the tracking error signal TES. 
     Next, a description will be given of an eighth embodiment of the optical information recording/reproducing apparatus according to the present invention, by referring to FIGS. 24,  25  and  26 . In FIGS. 24 and 25, those parts which are the same as those corresponding parts in FIG. 17 are designated by the same reference numerals, and a description thereof will be omitted. 
     In this embodiment, an analyzer  208 A shown in FIGS. 24 and 25 is used together with the composite prism  35  and the photodetector unit  36  shown in FIG.  17 . 
     For example, an analyzer  21  disclosed in a Japanese Laid-Open Patent Application No. 63-127436 may be used as the analyzer  208 A. In this eighth embodiment, the light beam is split into three light beams by the analyzer  208 A, and each of the three light beams are further split into three light beams by the composite prism  35 , thereby resulting in nine (3×3=9) light beams being output from the composite prism  35 . The nine light beams from the composite prism  35  are irradiated on corresponding ones of nine photodetectors  66   a  through  66   i  which form the photodetector unit  66 . 
     FIG. 26 shows a plan view of the photodetector unit  66 . The focal error signal FES can be generated according to the Foucault technique based on outputs of the photodetectors  66   a ,  66   b ,  66   d ,  66   e ,  66   g  and  66   h  of the photodetector unit  66 . The photodetectors  66   a ,  66   d  and  66   g  receive the three light beams from the first part of the composite prism  35 , while the photodetectors  66   b ,  66   e  and  66   h  receive the three light beams from the second part of the composite prism  35 . The image formation points of these six light beams match the positions of the photodetectors  66   a ,  66   b ,  66   d ,  66   e ,  66   g  and  66   h . On the other hand, the tracking error signal TES can be generated according to the push-pull technique based on outputs of the photodetectors  66   c ,  66   f  and  66   i . The photodetectors  66   c ,  66   f  and  66   i  receive the three light beams from the third part of the composite prism  35 . The image formation points of these three light beams are deviated from the positions of the photodetectors  66   c ,  66   f  and  66   i.    
     As shown in FIG. 26, the photodetector  66   a  includes photodetector parts  37   a  and  37   b , the photodetector  66   b  includes photodetector parts  37   c  and  37   d , . . . , and the photodetector  66   i  includes photodetector parts  37   q  and  37   r . Accordingly, if the outputs of these photodetector parts  37   a  through  37   i  are denoted by the same reference numerals as these parts, the focal error signal FES using the Foucault technique can be generated based on any one of the following formulas (3a) through (3d) by calculation. 
     
       
         FES=[( 37   a )+( 37   g )+( 37   m )+( 37   d )+( 37   j )+( 37   p )]−[( 37   b )+( 37   h )+( 37   n )+( 37   c )+( 37   i )+( 37   o )]  (3a) 
       
     
     
       
         FES=[( 37   a )+( 37   p )]−[( 37   b )+( 37   o )]  (3b) 
       
     
      FES=[( 37   m )+( 37   d )]−[( 37   n )+( 37   c )]  (3c) 
     
       
         FES=[( 37   a )+( 37   d )+( 37   m )+( 37 p)]−[( 37   b )+( 37   c ) +( 37   n )+( 37   o )]  (3d) 
       
     
     The focal error signal FES obtained by the formula (3a) has a high signal-to-noise (S/N) ratio and is uneasily affected by external disturbances because all of the beams irradiated on the photodetector unit  66  is used and consequently a large amount of light is used to detect the focal error signal FES. 
     On the other hand, the focal error signal FES obtained by the formula (3b) or (3c) enables easy and simple adjustments since only two beams are used to detect the focal error signal FES and thus only two beams need to be irradiated on corresponding two photodetectors  66   a  and  66   h  or,  66   b  and  66   g , of the photodetector unit  66  so that each beam irradiates a division line separating the two photodetector parts forming each of the photodetectors  66   a  and  66   h  or,  66   b  and  66   g.    
     Furthermore, the focal error signal FES obtained by the formula (3d) can obtain, to a certain extent, the above described effects obtainable with respect to the focal error signals FES obtained by the formulas (3a) and (3b) or (3c). In addition, in the case of the focal error signal FES obtained by the formula (3d), the required adjustments are simpler compared to the case of the focal error signal FES obtained by the formula (3a). Moreover, the focal error signal FES obtained by the formula (3d) has a higher S/N ratio and is less affected by external disturbances as compared to the focal error signal FES obtained by the formulas (3b) or (3c). 
     The focal error signal FES according to the above described formulas (3a) through (3d) can be generated by use of known adders and subtracter. 
     The tracking error signal TES using the push-pull technique can be generated based on one of the following formulas (4a) and (4b) by calculation. 
     
       
         TES=[( 37   e )+( 37   k )+( 37   q )]−[( 37   f )+( 371 )+( 37   r )]  (4a) 
       
     
     
       
         TES=[( 37   e )+( 37   q )]−[( 37   f )+( 37   r )]  (4b) 
       
     
     The tracking error signal TES obtained by the formula (4a) has a high S/N ratio and is uneasily affected by external disturbances because all of the beams irradiated on the photodetector unit  66  is used and consequently a large amount of light is used to detect the tracking error signal TES. 
     On the other hand, the tracking error signal TES obtained by the formula (4b) enables easy and simple adjustments since only two beams are used to detect the tracking error signal TES and thus only two beams need to be irradiated on corresponding two photodetectors  66   c  and  66   i  of the photodetector unit  66  so that each beam irradiates a division line separating the two photodetector parts forming each of the photodetectors  66   c  and  66   i.    
     Furthermore, by the function of the analyzer  208 A, a magneto-optic signal (information signal) MO which is recorded on the disk  207  can be reproduced based on one of the following formulas (5a) and (5b) by calculation. 
     The tracking error signal TES according to the above described formulas (4a) and (4b) can be generated by use of known adders and subtracter. 
      MO=[( 37   a )+( 37   b )+( 37   e )+( 37   f )+( 37   c )+( 37   d )]−[( 37   m )+( 37   n )+( 37   q )+( 37   r )+( 37   o )+( 37   p )]  (5a) 
     
       
         MO=[( 37   a )+( 37   b )+( 37   c )+( 37   d )]−[( 37   m )+( 37   n ) +( 37   o )+( 37   p )]  (5b) 
       
     
     According to the magneto-optic signal MO obtained by the formula (5a), it is possible to obtain an average signal amplitude which is relatively large. On the other hand, according to the magneto-optic signal MO obtained by the formula (5b), it is possible to obtain a relatively high resolution. 
     The magneto-optic signal MO according to the above described formulas (5a) and (5b) can be generated by use of known adders and subtracter. 
     The disk  207  is also recorded with positional information which indicates a track, sector and the like on the disk  207 . This positional information is often referred to as an identification signal ID, and this identification signal ID can be recorded in the form of pits on the disk  207  as is well known. The identification signal ID can be reproduced based on the following formula (6) by calculation. In other words, the magneto-optic signal MO is reproduced by obtaining a difference between the outputs of the adders, while the identification signal ID is reproduced by obtaining a sum of the outputs of the adders. 
     
       
         ID=[( 37   a )+( 37   b )+( 37   e )+( 37   f )+( 37   c )+( 37   d ]+[( 37   m )+( 37   n )+( 37   q )+( 37   r )+( 37   o )+( 37   p )]  (6) 
       
     
     The identification signal ID according to the above described formula (6) can be generated by use of known adders. 
     According to the eighth embodiment, the magneto-optic signal detection system and the servo signal detection signal can be provided approximately on a single optical path, and it is therefore possible to further reduce both the size and cost of the optical information recording/reproducing apparatus compared to the fifth through seventh embodiments. As is evident from a comparison of FIGS. 17 and 24, the Wollaston prism  209 , the lens  210  and the 2-part photodetector  211  required in FIG. 17 are omitted in FIG.  24 . 
     Next, a description will be given of a first modification of the eighth embodiment of the optical information recording/reproducing apparatus according to the present invention, by referring to FIGS. 27 and 28. In FIGS. 27 and 28, those parts which are the same as those corresponding parts in FIGS. 25 and 26 are designated by the same reference numerals, and a description thereof will be omitted. 
     In this first modification of the eighth embodiment, a photodetector unit  66 A shown in FIG. 27 is used in place of the photodetector unit  66  shown in FIG.  25 . 
     FIG. 28 shows a plan view of the photodetector unit  66 A. As shown in FIG. 28, the photodetector unit  66 A includes 2-part photodetectors  66 A- 1 ,  66 A- 2  and  66 A- 3 , and photodetectors  66 A- 4  and  66 A- 5 . The 2-part photodetector  66 A- 1  includes photodetector parts A and B. The 2-part photodetector  66 A- 2  includes photodetector parts C and D. The 2-part photodetector  66 A- 3  includes photodetector parts E and F. The photodetector  66 A- 4  includes a photodetector part G, and the photodetector  66 A- 5  includes a photodetector part H. 
     Accordingly, if the outputs of these photodetector parts A through H are denoted by the same reference characters as these parts, the focal error signal FES using the Foucault technique can be generated based on the following formula (7) by calculation. 
     
       
         FES=(A+C)−(B+D)  (7) 
       
     
     The focal error signal FES according to the above described formula (7) can be generated by use of known adders and subtracter. 
     In addition, the tracking error signal TES using the push-pull technique can be generated based on the following formula (8) by calculation. 
     
       
         TES=E−F  (8) 
       
     
     The tracking error signal TES according to the above described formula (8) can be generated by use of a known subtracter. 
     Furthermore, by the function of the analyzer  208 A, the magneto-optic signal MO which is recorded on the disk  207  can be reproduced based on the following formula (9) by calculation. 
     
       
         MO=G−H  (9) 
       
     
     The magneto-optic signal MO according to the above described formula (9) can be generated by use of a known subtracter. 
     Moreover, the identification signal ID which is recorded on the disk  207  can be reproduced based on the following formula (10) by calculation. 
     
       
         ID=G+H  (10) 
       
     
     The identification signal ID according to the above described formula (10) can be generated by use of a known adder. 
     As may be seen from FIG.  28  and the formulas (7) through (10) described above, this first modification of the eighth embodiment can obtain the focal error signal FES, the tracking error signal TES, the magneto-optic signal MO and the identification signal ID by use of a smaller number of photodetectors (or photodetector parts) and a smaller number of adders as compared to the eighth embodiment. Hence, due to the smaller number of photodetectors (or photodetector parts) used, a stray capacitance of the photodetectors can be reduced compared to the eighth embodiment. In addition, due to the smaller number of adders used, a noise generated by the adders within a large scale integrated circuit (LSI) can be reduced compared to the eighth embodiment. 
     Compared to servo signals such as the focal error signal FES and the tracking error signal TES, the magneto-optic signal MO and the identification signal ID require higher frequency bands, and consequently, higher signal-to-noise (S/N) ratios. The stray capacitance and noise described above affect the S/N ratios of the magneto-optic signal MO and the identification signal ID, however, this first modification of the eighth embodiment can further improve the S/N ratios of the magneto-optic signal MO and the identification signal ID compared to the eighth embodiment due to the smaller number of photodetectors (or photodetector parts) and the smaller number of adders used to reproduce these signals MO and ID. 
     Next, a description will be given of a second modification of the eighth embodiment of the optical information recording/reproducing apparatus according to the present invention, by referring to FIGS. 29 through 31. In FIG. 29, those parts which are the same as those corresponding parts in FIG. 25 are designated by the same reference numerals, and a description thereof will be omitted. 
     In this second modification of the eighth embodiment, the analyzer  208 A is used together with a composite prism  35 B shown in FIG. 30 and a photodetector unit  66 B shown in FIG.  29 . 
     The reflected light beam from the disk  207  is split into three light beams by the analyzer  208 A, and each of the three light beams are further split into five light beams by the composite prism  35 B, thereby resulting in fifteen (3×5=15) light beams being output from the composite prism  35 B. The fifteen light beams from the composite prism  35 B are irradiated on corresponding ones of nine photodetectors  66   aa  through  66   ii  which form the photodetector unit  66 B. 
     FIG. 30 shows a perspective view of the composite prism  35 B. As shown in FIG. 30, the composite prism  35 B includes tapered first and second parts  35 B- 1  and  35 B- 2 , a central third part  35 B- 3  which has a convex surface with a slight curvature, and peripheral fourth and fifth parts  35 B- 4  and  35 B- 5  which have convex surfaces with a slight curvature matching that of the third part  35 B- 3 . In other words, the third, fourth and fifth parts  35 B- 3 ,  35 B- 4  and  35 B- 5  are all parts of a single convex surface having a slight curvature. The first and second parts  35 B- 1  and  35 B- 2  function similarly to the first and second parts  35   a  and  35   b  of the composite prism  35 . 
     FIG. 31 shows a plan view of the photodetector unit  66 B. As shown in FIG. 31, the photodetector unit  66 B includes photodetectors  66   aa  through  66   ii . The photodetector  66   aa  includes photodetector parts  37   aa  and  37   bb , the photodetector  66   bb  includes photodetector parts  37   cc  and  37   dd , . . . , and the photodetector  66   ii  includes photodetector parts  37   qq  and  37   rr.    
     The three light beams output from the third part  35 B- 3  of the composite prism  35 B are respectively irradiated on the photodetectors  66   cc ,  66   ff  and  66   ii . The three light beams output from the fourth part  35 B- 4  of the composite prism  35 B are respectively irradiated on the photodetectors  66   cc ,  66   ff  and  66   ii . Further, the three light beams output from the fifth part  35 B- 5  of the composite prism  35 B are respectively irradiated on the photodetectors  66   cc ,  66   ff  and  66   ii . On the other hand, the three light beams output from the first part  35 B- 1  of the composite prism  35 B are respectively irradiated on the photodetectors  66   aa ,  66   dd  and  66   gg . The three light beams output from the second part  35 B- 2  of the composite prism  35 B are respectively irradiated on the photodetectors  66   bb ,  66   ee  and  66   hh.    
     The image formation points of the six light beams output from the-first and second parts  35 B- 1  and  35 B- 2  match the positions of the photodetectors  66   aa ,  66   bb ,  66   dd ,  66   ee ,  66   gg  and  66   hh . On the other hand, the image formation points of the three light beams output from each of the third, fourth and fifth parts  35 B- 3 ,  35 B- 4  and  35 B- 5  are deviated from the positions of the photodetectors  66   cc ,  66   ff  and  66   ii.    
     If the outputs of the photodetector parts  37   aa  through  37   ii  of the photodetector unit  66 B are denoted by the same reference numerals as these parts, the focal error signal FES using the Foucault technique, the tracking error signal TES using the push-pull technique and the identification signal ID can be obtained by calculations similarly to the eighth embodiment described above based on the formulas (3a) through (3d), (4a), (4b) and (6). 
     Furthermore, the magneto-optic signal MO recorded on the disk  207  can be reproduced based on one of the following formulas (11a) and (11b) by calculation. 
      MO=[( 37   aa )+( 37   bb )+( 37   ee )+( 37   ff )+( 37   cc )+( 37   dd )]−[( 37   mm )+( 37   nn )+( 37   qq )+( 37   rr )+( 37   oo )+( 37   pp )]  (11a) 
     
       
         MO=[( 37   ee )+( 37   ff )]−[( 37   qq +( 37   rr )]  (11b) 
       
     
     According to the magneto-optic signal MO obtained by the formula (11a), it is possible to obtain an average signal amplitude which is relatively large. On the other hand, according to the magneto-optic signal MO obtained by the formula (11b), it is possible to obtain a relatively high resolution. Moreover, the resolution obtainable according to the magneto-optic signal MO obtained by the formula (11b) is further improved compared to that obtainable according to the formula (5b). The reason for this further improved resolution of the magneto-optic signal MO using the composite prism  35 B having the shape shown in FIG. 30 may be understood from the teachings of the Proceedings of Magneto-Optical Recording International Symposium &#39;96, J. Magn. Soc. Jpn., Vol. 20, Supplement No. S 1  (1996), pp. 233-238. 
     The magneto-optic signal MO according to the above described formulas (11a) and (11b) can be generated by use of known adders and subtracter. 
     In this second modification of the eighth embodiment, it is of course possible to use the photodetector unit  66 A shown in FIG. 28 in place of the photodetector unit  66 B. In this case, the focal error signal FES, the tracking error signal TES, the magneto-optic signal MO and the identification signal ID can be obtained by calculations similarly to the first modification of the eighth embodiment described above based on the formulas (7) through (10). 
     In the eighth embodiment and the first and second modifications thereof, it is of course possible to arrange the composite prism  35  or  35 B between the analyzer  208 A and the condenser lens  212 . 
     In the fifth through eighth embodiments and the modifications of the eighth embodiment, the image formation point of the light beam used to generate the focal error signal FES according to the Foucault technique and the image formation point of the light beam used to generate the tracking error signal TES according to the push-pull technique are made mutually different by use of the composite prism. However, the method of making the image formation points of the light beams mutually different is not limited to that using the composite prism, and it is also possible to use a holographic optical element, for example. 
     Next, a description will be given of a ninth embodiment of the optical information recording/reproducing apparatus according to the present invention, by referring to FIGS. 32 and 33. In FIGS. 32 and 33, those parts which are the same as those corresponding parts in FIG. 19 are designated by the same reference numerals, and a description thereof will be omitted. 
     In this embodiment, a holographic optical element  75  shown in FIG. 32 is used in place of the composite prism  35  shown in FIG.  19 . 
     FIG. 32 is a perspective view showing an important part of this embodiment on an enlarged scale. The holographic optical element  75  includes first and second parts  75   a  and  75   b . The cross sectional shape of the first part  75   a  along a line A-A′ in FIG. 32 is a sawtooth grating as shown in FIG.  33 . The second part  75   b  has a cross sectional shape similar to that of the first part  75   a , but the cross sectional shape of the second part  75   b  is in point symmetry to that of the first part  75   a  with respect to the center of the holographic optical element  75 . The sawtooth gratings of the first and second parts  75   a  and  75   b  are sometimes also referred to as blazed gratings. 
     The holographic optical element  75  separates the reflected light beam  30  into 0th order diffracted light, ±1st order diffracted lights, and high-order diffracted lights of ±2nd order or higher. In this embodiment, the cross sectional shape of the holographic optical element  75  is designed so that the effect of high-order diffracted lights of ±2nd order or higher are small when detecting the light. With regard to the ±1st order diffracted lights, the above described sawtooth cross sectional shapes of the first and second parts  75   a  and  75   b  are designed so that, for example, the quantity of the emitted +1st order diffracted light is larger than that of the emitted −1st order diffracted light, that is, so that the effects of the −1st order diffracted light which is a divergent ray is minimized. 
     Accordingly, this embodiment uses a +1st order diffracted light  30 - 1  which is diffracted by the grating of the first part  75   a , a +1st order diffracted light  30 - 2  which is diffracted by the grating of the second part  75   b , and a 0th order diffracted light  30 - 3  which passes through the first and second parts  75   a  and  75   b  without being affected by the gratings thereof. In addition, the grating patterns of the first and second parts  75   a  and  75   b  are designed such that the +1st order diffracted light  30 - 1  which is emitted from the first part  75   a  is refracted twice via the condenser lens  212  and the first part  75   a  before being imaged at an image formation point  300   a , and the +1st order diffracted light  30 - 2  which is emitted from the second part  75   b  is refracted twice via the condenser lens  212  and the second part  75   b  before being imaged at an image formation point  300   b . On the other hand, since the  0 th order diffracted light  30 - 3  passes through the holographic optical element  75  as it is without being affected by the grating patterns, the 0th order diffracted light  30 - 3  is refracted only by the condenser lens  212  and is imaged at an image formation point  300   c.    
     In FIG. 32, the photodetector unit  36  is arranged on a plane which is perpendicular to the optical axis of the reflected light beam and includes the image formation points  300   a  and  300   b . Because of this arrangement, the first and second photodetectors  36   a  and  36   b  which are used to generate the focal error signal FES based on the Foucault technique are respectively provided at the positions of the image formation points  300   a  and  300   b  of the +1st-order diffracted lights  30 - 1  and  30 - 2 . On the other hand, the third photodetector  36   c  which is used to generate the tracking error signal TES based on the push-pull technique is provided at a position deviated from the position of the image formation point  300   c  of the 0th order diffracted light  30 - 3 . Hence, it is possible to generate the focal error signal FES using the Foucault technique and to generate the tracking error signal TES using the push-pull technique by use of a simple optical system. The generation itself of the focal error signal FES and the tracking error signal TES may be made similarly to the prior art, and a description thereof will be omitted. 
     The requirement is that the distances L1 and L2 between the condenser lens  212  and the respective image formation points  300   a  and  300   b  of the +1st order diffracted lights  30 - 1  and  30 - 2  are different from the distance L3 between the condenser lens  212  and the image formation point  300   c  of the 0th order diffracted light  30 - 3 , and the construction and arrangement of the holographic optical element  75  and the photodetector unit  36  are not limited to those of this embodiment. 
     Next, a description will be given of the functions of the holographic optical element  75  by itself, that is, for the case where no condenser lens  212  exists, by referring to FIGS. 34 through 36. 
     As described above, the holographic optical element  75  includes the first and second parts  75   a  and  75   b  which are provided with independent patterns for deflecting, converging and diverging the light. More particularly, the patterns of the first and second parts  75   a  and  75   b  are respectively set so that the +1st order diffracted light  30 - 1  from the first part  75   a  converges to a point P′ (−x, 0) and the +1st order diffracted light  30 - 2  from the second part  75   b  converges to a point P(x, 0). Points P and P′ are located on a plane η which is a distance f away from the holographic optical element  75  along the optical axis. In other words, the function of the first part  75   a  is to image the parallel incident light at a focal point O at a focal distance f, and to converge the light to the point P′ by deflecting the light by a distance x in the negative x-direction. 
     FIG. 35 shows a plan view of the holographic optical element  75 . Since the patterns of the first and second parts  75   a  and  75   b  are in point symmetry with respect to the origin 0 in FIG. 35, the pattern of the first part  75   a , for example, is made up of concentric grooves or projections having a center at the point P′ (−x, 0). A radius r i  of an ith concentric groove or projection can be obtained from the following formula (12), where λ denotes the wavelength of the light output from the light source. 
     
       
         r i ={square root over ((2+L ·f·λ·i))}  (12) 
       
     
     In addition, the cross sectional shape of the first part  75   a  is determined so that the ratios of the 0th order diffracted light and the +1st order diffracted light with respect to the total quantity of light become predetermined values. 
     In the above ninth embodiment, the cross sectional shape of the holographic optical element  75  is designed so that the effects of the high-order diffracted lights of ±2nd order diffracted lights or higher are small when detecting the light. In addition, with respect to the ±1st order diffracted lights, the cross sectional shapes of the first and second parts  75   a  and  75   b  of the holographic optical element  75  are set to the sawtooth shape shown in FIG. 33 so that the quantity of the emitted +1st order diffracted light is larger than that of the emitted −1st order diffracted light, that is, so that the effects of the −1st order diffracted light which is a divergent ray are minimized. However, it is of course possible to design the cross sectional shape of the holographic optical element  75  so that the quantity of the emitted −1st order diffracted light is larger than that of the emitted +1st order diffracted light, that is, so that the −1st order diffracted light which is a divergent ray is positively used and the effects of the +1st order diffracted light are minimized. 
     In a tenth embodiment of the optical information recording/reproducing apparatus according to the present invention, the holographic optical element  75  used has a cross sectional shape shown in FIG. 36 along the line A-A′ in FIG.  32 . An important part of this embodiment is essentially the same as FIG. 32, and an illustration thereof will be omitted. Contrary to the ninth embodiment, this embodiment positively uses the −1st order diffracted lights. For this reason, the first and second photodetectors  36   a  and  36   b  for generating the focal error signal FES based on the Foucault technique are provided at the image formation points of the −1st order diffracted lights. On the other hand, the third photodetector  36   c  for generating the tracking error signal TES based on the push-pull technique is provided at a position deviated from the image formation point  300   c  of the 0th order diffracted light, that is, between the holographic optical element  75  and the photodetector unit  36 . As a result, it is possible to generate the focal error signal FES using the Foucault technique and to generate the tracking error signal TES using the push-pull technique by use of a simple optical system. 
     According to the structure shown in FIG. 34, the +1st order diffracted light obtained from the first part  75   a  of the holographic optical element  75  may overlap the −1st order diffracted light obtained from the second part  75   b , and the −1st order diffracted light obtained from the first part  75   a  may overlap the +1st order diffracted light obtained from the second part  75   b . For this reason, the holographic optical element  75  may be constructed so that by itself the holographic optical element  75  acts on the light as shown in FIG.  37 . In FIG. 37, those parts which are the same as those corresponding parts in FIG. 34 are designated by the same reference numerals, and a description thereof will be omitted. 
     In FIG. 37, the patterns of the first and second parts  75   a  and  75   b  are set so that the +1st order diffracted light  30 - 1  from the first part  75   a  converges to a point Q′ (−x, y), the −1st order diffracted light from the first part  75   a  is projected in a semicircular shape about a point R′ (x, −y), the +1st order diffracted light  30 - 2  from the second part  75   b  converges to a point Q(x, y), and the −1st order diffracted light is projected in a semicircular shape about a point R(−x, −y). The points Q, Q′, R and R′ are located on the plane η which is the distance f from the holographic optical element  75  along the optical axis. 
     Next, a description will be given of an eleventh embodiment of the optical information recording/reproducing apparatus according to the present invention, by referring to FIGS. 38 through 40. FIG. 38 shows the eleventh embodiment, FIGS.  39 ( a ) and  39 ( b ) show a composite prism of the eleventh embodiment, and FIG. 40 shows a perspective view of an important part of the eleventh embodiment. In FIG. 38, those parts which are the same as those corresponding parts in FIG. 17 are designated by the same reference numerals, and a description thereof will be omitted. 
     In this embodiment, the spot of the light beam irradiated on the disk  207  via the objective lens  206  has a diameter of approximately 1 μm, for example. In addition, the outputs of the 2-part photodetector  211  are used to generate an address signal ADR via an adder  311 A, and the outputs of the 2-part photodetector  211  are also used to reproduce the magneto-optic signal (information signal) MO via a differential amplifier  311 B. 
     In this embodiment, a composite prism  85  splits the reflected light beam which is obtained via the condenser lens  212  into first through fourth light beams  87   a  through  87   d . These first through fourth light beams  87   a  through  87   d  are irradiated on a photodetector unit  86 . The photodetector unit  86  includes a first photodetector  86   a  which has 4 light receiving parts A through D for receiving the first and second light beams  87   a  and  87   b , a second photodetector  86   b  which has a light receiving part E for receiving the third light beam  87   c , and a third photodetector  86   c  which has a light receiving part F for receiving the fourth light beam  87   d . As shown in FIG. 40, the first, second and third photodetectors  86   a ,  86   b  and  86   c  are arranged on the same plane. The first through third photodetectors  86   a  through  86   c  may or may not be separated from each other within the photodetector unit  86 . 
     FIG.  39 ( a ) shows a perspective view of the composite prism  85  on an enlarged scale, and FIG.  39 ( b ) shows a plan view of the composite prism  85 . As shown, the composite prism  85  includes a first emission surface  85   a  for emitting the first light beam  87   a , a second emission surface  85   b  for emitting the second light beam  87   b , and third and fourth emission surfaces  85   c  and  85   d  for respectively emitting the third and fourth light beams  87   c  and  87   d . In FIG.  39 ( a ), the first emission surface  85   a  has a downward inclination to the right, the second emission surface  85   b  has a downward inclination to the left, and the third and fourth emission surfaces  85   c  and  85   d  form a mountain shape. In other words, the third emission surface  85   c  has a downward inclination to the right, the fourth emission surface  85   d  has a downward inclination to the left, and the third and fourth emission surfaces  85   c  and  85   d  connect to form the mountain shape. 
     The first emission surface  85   a  and the third emission surface  85   c  are inclined towards the same direction, and an inclination angle α 1  of the first emission surface  85   a  relative to a reference plane is smaller than an inclination angle α 3  of the third emission surface  85   c . For example, the reference plane is the back surface of the composite prism  85 , which is approximately perpendicular to the traveling direction of the incoming reflected light beam. On the other hand, the second emission surface  85   b  and the fourth emission surface  85   d  are inclined towards the same direction, and an inclination angle α 2  of the second emission surface  85   b  is smaller than an inclination angle α 4  of the fourth emission surface  85   d.    
     In FIG. 40, the first light beam  87   a  which is emitted from the first emission surface  85   a  of the composite prism  85  is received by the light receiving parts A and D of the first photodetector  86   a . In addition, the second light beam  87   b  which is emitted from the second emission surface  85   b  of the composite prism  85  is received by the light receiving parts B and C of the first photodetector  86   a . Hence, a focal error signal FES is generated according to the Foucault technique based on the formula (2) described above. More particularly, the outputs of the light receiving parts A and C are added in an adder  321 , the outputs of the light receiving parts B and D are added in an adder  322 , and the outputs of these adders  321  and  322  are supplied to a differential amplifier  323  which outputs the focal error signal FES. 
     On the other hand, the third light beam  87   c  which is emitted from-the third emission surface  85   c  of the composite prism  85  is received by the light receiving part E of the second photodetector  86   b , and the fourth light beam  87   d  which is emitted from the fourth emission surface  85   d  of the composite prism  85  is received by the light receiving part F of the third photodetector  86   c . Hence, a tracking error signal TES is generated according to the push-pull technique based on the formula (1) described above. More particularly, the outputs of the light receiving parts E and F are supplied to a differential amplifier  331 , and the tracking error signal TES is output from this differential amplifier  331 . 
     According to this embodiment, it is unnecessary to split the optical path into two by the beam splitter  213  shown in FIG. 1, even though the Foucault technique is used to generate the focal error signal FES and the push-pull technique is used to generate the tracking error signal TES. For this reason, it is possible to reduce the space occupied by the optical system within the optical information recording/reproducing apparatus. In addition, it is possible to reduce both the number of parts and the cost of the optical information recording/reproducing apparatus because this embodiment does not require the beam splitter  213  and the photodetector  214  shown in FIG.  1 . Furthermore. compared to the case where the astigmatism technique is used to generate the focal error signal FES, it is possible to reduce the diameter of the beam spot formed on the photodetector and prevent effects of the external disturbance, thereby making it possible to improve the reliability of the optical information recording/reproducing apparatus. 
     Moreover, if the photodetector unit  86  is adjusted to detect a predetermined focal error signal FES, it is possible to employ a structure that would automatically receive the third light beam  87   c  by the light receiving part E of the photodetector  86   b  and receive the fourth light beam  87   d  by the light receiving part F of the photodetector  86   c . Hence, there is an additional advantage in that no adjustment is required in this case for the detection of the tracking error signal TES. 
     Next, a description will be given of a twelfth embodiment of the optical information recording/reproducing apparatus according to the present invention, by referring to FIGS. 41 through 44. FIG. 41 shows the twelfth embodiment, and in FIG. 41, those parts which are the same as those corresponding parts in FIG. 38 are designated by the same reference numerals, and a description thereof will be omitted. 
     In this embodiment, an integral part  90  is provided in place of the composite prism  85  shown in FIG.  38 . In addition, the beam splitter  208 , the Wollaston prism  209 , the condenser lens  210  and the photodetector  211  shown in FIG. 38 are not provided in FIG.  41 . 
     The reflected light beam which is obtained via the beam splitter  204  is converted by the condenser lens  212  and is input to the integral part  90  which functions as a beam splitter means. Hence, the reflected light beam is split into first through sixth light beams  91   a  through  91   f , and these first through sixth light beams  91   a  through  91   f  are irradiated on a photodetector unit  86 A. 
     The integral part  90  integrally comprises a Wollaston prism  92  shown in FIG.  42  and the composite prism  85  shown in FIG.  39 . In other words, the Wollaston prism  92  is positioned immediately before the composite prism  85  along the traveling direction of a reflected light beam  89 , and is adhered on the back of the composite prism  85  as shown in FIGS. 43 and 44. 
     The Wollaston prism  92  is made up of two triangular prisms  93  and  94  which are cut from a crystal and adhered together. The size of the Wollaston prism  92  corresponds to the central mountain shaped part of the composite prism  85 . The Wollaston prism  92  is adhered on the back of the composite prism  85  immediately behind a mountain part  85   e  of the composite prism  85 . In addition, the Wollaston prism  92  extends for the full width of the mounting part  85   e . Hence, the Wollaston prism  92  splits the incoming reflected light beam in a direction in which a vertex  85   f  of the mountain part  85   e  extends. 
     On the other hand, the photodetector unit  86 A includes a first photodetector  86 Aa, a second photodetector  86 Ab- 1 . a third photodetector  86 Ab- 2 , a fourth photodetector  86 Ac- 1  and a fifth photodetector  86 Ac- 2  which are provided on a single plane as shown in FIG.  44 . The first photodetector  86 Aa includes four light receiving parts A through D for receiving the first and second light beams  91   a  and  91   b . The second photodetector  86 Ab- 1  includes a light receiving part E 1  for receiving the third light beam  91   c , and the third photodetector  86 Ab- 2  includes a light receiving part E 2  for receiving the fourth light beam  91   d . The fourth photodetector  86 Ac- 1  includes a light receiving part F 1  for receiving the fifth light beam  91   e , and the fifth photodetector  86 Ac- 2  includes a light receiving part F 2  for receiving the sixth light beam  91   f.    
     Out of the reflected light beam  89  which is input to the integral part  90  via the condenser lens  212 , a light component  89 - 1  which passes above the upper part of the Wollaston prism  92  in FIG.  43  and reaches the composite prism  85  directly is refracted. by the first emission surface  85   a  and is emitted from the first emission surface  85   a  as the first light beam  91   a . As shown in FIG. 44, this first light beam  91   a  is received by the light-receiving parts A and D of the first photodetector  86 Aa. 
     On the other hand, out of the reflected light beam  89  which is input to the integral part  90  via the condenser lens  212 , a light component  89 - 2  which passes below the lower part of the Wollaston prism  92  in FIG.  43  and reaches the composite prism  85  directly is refracted by the second emission surface  85   b  and is emitted from the second emission surface  85   b  as the second light beam  91   b . As shown in FIG. 44, this second light beam  91   b  is received by the light receiving parts B and C of the first photodetector  86 Aa. 
     A focal error signal FES is generated according to the Foucault technique based on the formula (2) described above, similarly to the eleventh embodiment shown in FIG.  38 . 
     Out of the reflected light beam  89  which is input to the integral part  90  via the condenser lens  212 , a light component  89 - 3  which reaches the Wollaston prism  92  is split into a p-polarized light (wave)  95  and an s-polarized light (wave)  96 . The p-polarized light  95  is, deflected by an angle β with respect to an extension line  97  of the light component  89 - 3  towards the first emission surface  85   a . On the other hand, the s-polarized light  96  is deflected by an angle β with respect to the extension line  97  towards the second emission surface  85   b.    
     The p-polarized light  95  and the s-polarized light  96  output from the Wollaston prism  92  is input to the composite prism  85 . The angle β is small, and the p-polarized light  95  and the s-polarized light  96  propagate within the mountain part  85   e  of the composite prism  85 . The p-polarized light  95  and the s-polarized light  96  reach the third and fourth emission surfaces  85   c  and  85   d  and are refracted thereby, and are thereafter emitted from the third and fourth emission surfaces  85   c  and  85   d.    
     In other words, in FIG. 44 the p-polarized light  95  is emitted from the third emission surface  85   c  as the third light beam  91   c . This third light beam  91   c  irradiates the light receiving part E 1  of the second photodetector  86 Ab- 1 . On the other hand, the s-polarized light  96  is emitted from the third emission surface  85   c  as the fourth light beam  91   d . This fourth light beam  91   d  irradiates the light receiving part E 2  of the third photodetector  86 Ab- 2 . 
     Similarly in FIG. 44, p-polarized light  95  is. emitted from the fourth emission surface  85   d  as the fifth light beam  91   e . This fifth light beam  91   e  irradiates the light receiving part F 1  of the fourth photodetector  86 Ac- 1 . On the other hand, the s-polarized light  96  is emitted from the fourth emission surface  85   e  as the sixth light beam  91   f . This sixth light beam  91   f  irradiates the light receiving part F 2  of the fifth photodetector  86 Ac- 2 . 
     A tracking error signal TES is obtained based on the outputs of the light receiving parts E 1 , E 2 , F 1  and F 2  of the second through fifth photodetectors  86 Ab- 1  through  86 Ac- 2 . More particularly, the tracking error signal TES is obtained through adders  332  and  333  and the differential amplifier  331  shown in FIG. 44, by calculating TES=(E 1 +E 2 )−(F 1 +F 2 ). 
     In addition a magneto-optic signal (information signal) MO is obtained based on the outputs of the light receiving parts E 1 , E 2 , F 1  and F 2  of the second through fifth photodetectors  86 Ab- 1  through  86 Ac- 2 . More particularly, the magneto-optic signal MO is obtained through adders  312  and  313 , and the differential amplifier  311 B shown in FIG. 44, by calculating MO (E 1 +F 1 )−(E 2 +F 2 ). 
     Furthermore, an address signal ADR is. obtained based on the outputs of the light receiving parts E 1 , E 2 , F 1  and F 2  of the second through fifth. photodetectors  86 Ab- 1  through  86 Ac- 2 . More particularly, the address- signal ADR is obtained through the adder  332 , the adder  333 , and the adder  311 A shown in FIG. 44, by calculating ADR=(E 1 +E 2 )+(F 1 +F 2 ). 
     According to this embodiment, it is possible to detect all of the focal error signal FES, the tracking error signal TES, the magneto-optic signal MO and the address signal ADR by use of a single optical path of the reflected light beam  89  and a single photodetector unit  86 A. 
     By comparing FIG. 41 to FIG. 38, it may be seen that this twelfth embodiment shown in FIG. 41 does not have the optical path which extends horizontally from the beam splitter  208  in FIG.  38 . For this reason, the space occupied by the optical system within the optical information recording/reproducing apparatus and the number of required parts are further reduced in this embodiment when compared to the eleventh embodiment. In other words, both the size and cost of the optical information recording/reproducing apparatus in this embodiment can further be reduced when compared to those of the eleventh embodiment. 
     In addition, if the photodetector unit  86 A is adjusted to detect a predetermined focal error signal FES, it is possible to employ a structure that would automatically receive the third through sixth light beams  91   c  through  91   f  by the corresponding light receiving parts E 1 , E 2 , F 1  and F 2  of the second through fifth photodetectors  86 Ab- 1  through  86 Ac- 2 . Hence, there is an additional advantage in that no adjustment is required in this case for the detection of the tracking error signal TES and the magneto-optic signal MO. 
     Of course, independent Wollaston prism and composite prism may be used in place of the integral part  90  which integrally comprises the Wollaston prism  92  and the composite prism  85 . In other words, the independent Wollaston prism may be provided at a position to confront the back of the composite prism with a gap formed therebetween. 
     According to the fifth embodiment of the optical information recording/reproducing apparatus described above in conjunction with FIGS. 24,  25  and  26 , the reflected light beam from the disk  207  is split into nine light beams which are irradiated on. the photodetector unit  66 . But the optical information recording/reproducing apparatus can be made to have a more simple construction which is more compact, if the focal error signal FES, the tracking error signal TES, the magneto-optic signal MO and the identification signal ID can be obtained based on detections of a smaller number of light beams at the photodetector unit. Hence, a description will now be given of further embodiments of the optical information recording/reproducing apparatus which can obtain the signals FES, TES, MO and ID based on a smaller number of light beams-without greatly deteriorating the signal qualities. 
     FIG. 45 shows a thirteenth embodiment of the optical information recording/reproducing apparatus according to the present invention. As shown in FIG. 45, a magneto-optic head unit of an optical information recording/reproducing apparatus is arranged under a magneto-optic disk  505 . A laser beam emitted from a laser diode  501  which is used as a coherent light source is converted into a parallel-ray by a collimator lens  502 , and is input to a beam splitter  503 . The parallel ray transmitted through the beam splitter  503  is converged on a magnetic layer of the magneto-optic disk  505  by an objective lens  504 . The converged laser beam is reflected by the magneto-optic disk  505 , and is input again to the beam splitter  503 . This time, the reflected light beam is reflected by the beam splitter  503  towards. a photodetector unit  509 . More particularly, the reflected light beam is reflected by the beam splitter  503  and is input to a Wollaston prism  506  which is an analyzer. 
     FIG. 46 shows a perspective view of an important part of the thirteenth embodiment, that is, a signal detection system, on an enlarged scale. FIG. 46 shows a state where the light beam output from the Wollaston prism  506  is projected on the photodetector unit  509 , viewed from the back surface side of the photodetector unit  509 . The reflected light beam input to the Wollaston prism  506  is separated into two polarized light beams and input to a composite prism  507 . As shown in FIG. 46, the composite prism  507  is an optical element having three deflection surfaces  571 ,  572  and  573  arranged side by side from the left to right. The deflection surfaces  571  and  573  on both sides of the composite prism  507  have tapered surfaces for deflecting the light beam in upward and downward directions by mutually different angles. On the other hand, the deflection surface  572  at the central part of the composite prism  507  has a convex shape having a curvature with respect to the optical axis. Each of the two polarized light beams input to the composite prism  507  having the structure described above is spatially split into three by the three deflection surfaces  571 ,  572  and  573 , thereby outputting a total of six light beams from the composite prism  507 . The six light beams output from the composite prism  507  are projected on the photodetector unit  509  via a condenser lens  508 . For the sake of convenience, the illustration of the condenser lens  508  shown in FIG. 45 is omitted in FIG.  46 . 
     FIGS. 47 through 49 respectively show plan views of the photodetector unit  509 . FIG. 47 shows a state where the objective lens  504  and the magneto-optic disk  505  are close to each other. FIG. 48 shows a state where the laser beam is in focus on the magneto-optic disk  505  In addition, FIG. 49 shows a state where the objective lens  504  and the magneto-optic disk  505  are far away from each other. For the sake of convenience, FIGS. 47 through 49 show the photodetector unit  509  viewed from a back surface side of the photodetector unit  509 . 
     As may be seen from FIGS. 47 through 49, the photodetector unit  509  is a light receiving element, and photodetectors of the photodetector unit  509  are arranged in three stages from the top to bottom in the vertical direction. An upper stage portion of the photodetector unit, 509  has a photodetector including a photodetector part G for detecting a magneto-optic signal MO and an identification signal ID, and a 2-part photodetector including photodetector parts A and B for detecting a focal error signal FES. A middle stage portion of the photodetector unit  509  has a 2-part photodetector including photodetector parts E and F for detecting a tracking error signal TES. Further, a lower stage portion of the photodetector unit  509  has a 2-part photodetector including photodetector parts C and D for detecting the focal error signal FES, and a photodetector including a photodetector part H for detecting the magneto-optic signal MO and the identification signal ID. 
     The 2-part photodetector in the lower stage portion and including the photodetector parts C and D is arranged below the photodetector in the upper stage portion and including the photodetector part G for detecting the magneto-optic signal MO and the identification signal ID. On the other hand, the 2-part photodetector in the upper stage portion and including the photodetector parts A and B is arranged above the photodetector in the lower stage portion and including the photodetector part H for detecting the magneto-optic signal MO and the identification signal ID. The photodetector parts A and B are separated by a vertical division line, and the photodetector parts C and D are separated by a vertical division line. The photodetector parts E and F are separated by a horizontal division line. In the following description, outputs of the above described photodetector parts A through H are denoted by the same reference characters as these parts. 
     As shown in FIG. 47, the spots of the six light beams are formed on the photodetector unit  509 , and the outputs A through H are obtained from the corresponding photodetector parts A through H. The focal error signal FES using the Foucault technique can be generated based on the following formula (13) by calculation, using known adders and subtracter. 
      FES=(A+C)−(B+D)  (13) 
     The tracking error signal TES using the push-pull technique can be generated based on the following formula (14) by calculation using a known subtracter. 
     
       
         TES=E−F  (14) 
       
     
     The magneto-optic signal MO can be reproduced based on the following formula (15) by calculation, using known adders and subtracter. 
     
       
         MO=(A+B+H)−(C+D+G)  (15) 
       
     
     In addition, the identification signal ID can be reproduced based on the following formula (16a) or (16b) by calculation, using known adders. 
     
       
         ID=(A+B+H)+(C+D+G)  (16a) 
       
     
     
       
         ID=A+B+C+D+E+F+H+G  (16b) 
       
     
     When the identification signal ID is reproduced based on the formula (16a), the number of adders provided in the LSI is relatively small, thereby making it possible to improve the S/N ratio of the identification signal ID. On the other hand, the identification signal ID may become distorted depending on the shape and depth of the pits formed in the magneto-optic disk  505 . Hence, when the identification signal ID is reproduced based on the formula (16b), it is possible to positively prevent the identification signal ID from becoming distorted because all of the diffracted light beams from the pits of the magneto-optic disk  505  are used to detect the identification signal ID. 
     According to this embodiment, a beam splitter means comprises a plurality of beam splitting stages for splitting the reflected light beam from the magneto-optic disk  505 -into a plurality of light beams which are projected onto a single light receiving element. More particularly, the beam splitter means comprises the Wollaston prism  506  and the composite prism  507  in this embodiment, and the reflected light beam from the magneto-optic disk  505  is split into six light beams. The six split light beams are projected onto the single photodetector unit  509 . As a result, the optical information recording/reproducing apparatus can be formed by a relatively small number of parts, and the arrangement of these relatively small number of parts can be simply adjusted with ease. Furthermore, the cost of the optical information recording/reproducing apparatus can be reduced due to the relatively small number of parts used. In addition, since the photodetector unit  509  is formed by only five photodetectors (eight photodetector parts A through H), it is possible to reduce the stray capacitance of the photodetectors and to reduce the noise generated by the adders and subtracter within the LSI which obtains the focal error signal FES, the-tracking error signal TES, the magneto-optic signal MO and the identification signal ID, as compared to the embodiment shown in FIGS. 25 and 26, for example. Consequently, the signal qualities of these signals FES, TES, MO and ID can be improved. 
     In addition, the magneto-optic signal MO and the identification signal ID are reproduced using also the outputs of the 2-part photodetectors (photodetector parts A, B, C and D) which detect the light beams for obtaining the focal error signal FES, in addition to using the outputs of the photodetectors (photodetector parts G and H) which detect the light beams for obtaining the magneto-optic signal and the identification signal ID. Thus, because of the small number of photodetector parts used to reproduce the magneto-optic signal MO and the identification signal ID, the SIN ratio of these signal MO and ID is improved. On the other hand, the required frequency bands of the magneto-optic signal MO and the identification signal ID are higher than those of the tracking error signal TES and the focal error signal FES which requires a frequency band lower than that of the tracking error signal TES. Further, when two or more kinds of signals are obtained based on the detection of the same light beam, the circuit construction of the circuit used to obtain these signals becomes simpler as the frequency bands of these signal become wider apart from each other. Therefore, by obtaining-the magneto-optic signal MO and the identification signal ID based on-the detection of the same light beam that is used to obtain the focal error signal FES, it is possible to simplify the circuit construction also from this point of view. 
     Furthermore, because the magneto-optic signal MO and the identification signal ID are reproduced using also the outputs of the 2-part photodetectors (photodetector parts A, B, C and D) which detect the light beams for obtaining the focal error signal FES in addition to using the outputs of the photodetectors (photodetector parts G and H) which detect the light beams for obtaining the magneto-optic signal and the identification signal ID, the resolution of the magneto-optic signal MO and the identification signal ID is improved. It is taught in a Japanese Laid-Open Patent Application No. 7-6379 that the resolution of the magneto-optic signal is improved when a central portion of the reflected light beam from a recording medium is masked and the magneto-optic signal is detected from a peripheral portion of the reflected light beam. In this embodiment, the light beam used to detect the focal error signal FES is the light beam transmitted through the tapered deflection surfaces  571  and  573  of the composite prism  507  as shown in FIG.  46 . In other words, out of the reflected light beam from the magneto-optic disk  505 , the light beam transmitted through the central deflection surface  572  of the composite prism  507  is not used, and only the light beam transmitted through the tapered deflection-surfaces  571  and  573  on both sides of the central deflection surface  572  of the composite prism  507  is used to detect the focal error signal FES. As a result, the resolution of the magneto-optic signal MO and the identification signal ID is improved by obtaining the magneto-optic signal MO and the identification signal ID using the light beam which is transmitted through the tapered. deflection surfaces  571  and  573  of the composite prism  507  and is used to detect the focal error signal FES. 
     Next, a description will be given of the light beam that is used to detect the focal error signal FES in this embodiment. FIGS. 50A through 50C are diagrams for explaining the shape of spots formed by the split light beams. FIG. 50A shows a plan view of the deflection surfaces  571 ,  572  and  573  of the composite prism  507 , together with light beams “a” and “b” when the two polarized light beams are transmitted through the deflection surfaces  571 ,  572  and  573 . FIGS. 50B and 50C respectively show plan views of the photodetector part G of the photodetector unit  509  on which the light beams “a” and “b” are projected, together with spots “aa” and “bb” of the light beams “a” and “b” irradiated on the photodetector part G and the photodetector parts A and B via the deflection surface  571 . FIGS. 50B and 50C show the photodetector viewed from the back surface side of the photodetector unit  509 . 
     The area of the light beam “b” transmitted through the deflection surface  571  is smaller than that of the light beam “a”, which means that the light beam “b” is deflected more. In this state, the length of the spot “bb” formed on the photodetector parts A and B along the major axis is longer than that of the spot “aa” formed on the photodetector part G. According to the Foucault technique, the longer the length of the spot along the major axis, the easier it is to perform the required adjustments of the arrangement of the optical elements and the photodetectors, and the-focal error signal FES is less affected by changes with time, temperature changes and the like. Hence, in this embodiment, the locations of the 2-part photodetector including the photodetector parts A and B and the 2-part photodetector including the photodetector parts C and D are determined so that out of the light beams transmitted through the deflection surfaces  571  and  573 , the light beams which are deflected more are projected onto the 2-part photodetector including the photodetector parts A and B and the 2-part photodetector including the photodetector part C and D. 
     Next, a description will be given of the effects of the temperature on this embodiment. When the temperature of the optical information recording/reproducing apparatus rises, a housing which fixes the photodetector unit  509  undergoes a thermal expansion, thereby making the photodetectors of the photodetector unit  509  farther away from the composite prism  507 . For similar reasons, the objective lens  504  become farther away from the magneto-optic disk  505 . As a result, the reflected light beam from the magneto-optic disk  505  is once converged and thereafter spreads before being projected onto the photodetector unit  509 . The spot formed on the 2-part photodetector including the photodetector parts A and B and the spot formed on the 2-part photodetector including the photodetector parts C and D are projected on the corresponding division lines of the 2-part photodetectors when the 2-part photodetectors are located at the focal positions of the light beams. However, if the 2-part photodetectors are located farther away from the focal positions of the light beams, the spots of the light beams are formed on the outer sides of the corresponding division lines of the 2-part photodetectors as shown in FIG. 49, and in this case, the spots of the light beams are only formed on the photodetector parts B. and D of the 2-part photodetectors. 
     When the temperature of the magneto-optic head rises; the separation angle of the light beams output from the Wollaston prism  506  also changes. FIG. 51 shows a plan view of the photodetector unit  509  at a high temperature, viewed from the back surface side of the photodetector unit  509 . The Wollaston prism  506  is made of a birefringence material such as crystal and lithium-niobate, and the separation angle of the light beams changes depending on the temperature. For example, a description will be given of the Wollaston prism  506  having a separation angle which becomes narrow at high temperatures. As shown in FIG. 51, the spots of the light beams formed on the 2-part photodetectors located on the right and left move towards the inner side of the photodetector unit  509  due to the temperature characteristic of the Wollaston prism  506 . In other words, the separation angle of the light beams output from the Wollaston prism  506  becomes narrow due to the temperature rise, and the spots of the light beams move toward the inner side of the photodetector unit  509  to become projected on the corresponding division lines of the 2-part photodetectors respectively including the photodetector parts A and B and the photodetector parts C and D. 
     In the case where the separation angle of the light beams output from the Wollaston prism  506  used becomes narrow at high temperatures and the 2-part photodetectors located on the right and left of the photodetector unit  509  are positioned farther away from the focal positions of the light beams, the light beams used are such that the spots of the light beams are formed on the outer side of the corresponding division lines of the 2-part photodetectors. Hence, when the temperature rises and the 2-part photodetectors become farther away from the Wollaston prism  506 , the separation angle of the light beams output from the Wollaston prism  506  becomes narrow such that the spots of the light beams are projected on the corresponding division lines of the 2-part photodetectors of the photodetector unit  509 . As a result, it is possible to carry out a stable automatic focusing operation based on an accurate focal error signal FES even when the temperature becomes high. 
     On the other hand, in the case where the separation angle of the light beams output from the Wollaston prism  506  used or, a Rochon prism (not shown) which is use in place of the Wollaston prism  506 , becomes wide at high temperatures and the 2-part photodetectors located on the right and left of the photodetector unit  509  are positioned farther away from the focal positions of the light beams, the light. beams used are such that the spots of the light beams are formed on the inner side of the corresponding division lines of the 2-part photodetectors. Hence, when the temperature rises and the 2-part photodetectors become farther away from the Wollaston prism  506 , the separation angle of the light beams output from the Wollaston prism  506  becomes wide such that the spots of the light beams are projected on the corresponding division lines of the 2-part photodetectors of the photodetector unit  509 . As a result, it is possible to carry out a stable automatic focusing operation based on an accurate focal error signal FES even when the temperature becomes high. 
     Next, a description will be given of a first modification of the thirteenth embodiment of the optical information recording/reproducing apparatus according to the present invention, by referring to FIG.  52 . In this first modification of the thirteenth embodiment, the basic-construction of the optical information recording/reproducing apparatus is basically the same as that of the thirteenth embodiment shown in. FIGS. 45 and 46, but a photodetector unit  591  is used in place of the photodetector unit  509 . FIG. 52 shows a plan view of the photodetector unit  591  used in this first modification of the thirteenth embodiment. In FIG. 52, those parts which are the same as those corresponding parts in FIGS. 46 and 47 are designated by the same reference characters, and a description thereof will be omitted. 
     FIG. 52 shows the photodetector unit  591  in a state where the laser beam is in focus on the magneto-optic disk  505 , viewed from the back surface side of the photodetector unit  591 . The light beams reaching the photodetector unit  591  via the Wollaston prism  506  and the composite prism  507  is obtained via the same optical path as that of the thirteenth embodiment. Photodetectors-of the photodetector unit  591  are arranged in three stages from the top to bottom in the vertical direction. An upper stage portion of the photodetector unit  591  has a photodetector including a photodetector part G for detecting a magneto-optic signal MO and an identification signal ID, and a 2-part photodetector including photodetector parts AA and BB for detecting a focal error signal FES. A middle stage portion of the photodetector unit  591  has a 2-part photodetector including photodetector parts E and F for detecting a tracking error signal TES. Further, a lower stage portion of the photodetector unit  591  has a 2-part photodetector including photodetector parts C and D for detecting the focal error signal FES, and a photodetector including a photodetector part H for detecting the magneto-optic signal MO and the identification signal ID. 
     This first modification of the thirteenth embodiment, the light beam which forms a spot having an oval shape which is longer along the major axis and the light beam which forms a spot having an oval shape which is shorter along the major axis on the photodetector unit  591  are used to obtain the focal error signal FES. In other words, one of the two light beams obtained by the separation made at the Wollaston prism  506  or Rochon prism is used to detect the focal error signal FES. As a result, the obtained focal error signal FES is less affected by the temperature characteristic of the Wollaston prism  506  or Rochon prism when compared with the thirteenth embodiment described above. 
     As described above, the separation angle of the two light beams output from the Wollaston prism  506  or Rochon prism changes with temperature. For example, the separation-angle of the two light beams output from the Wollaston prism  506  becomes narrow at high temperatures. Hence, if both the two light beams separated and output from the Wollaston prism  506  are used to detect the focal error signal FES, the focal error signal FES is greatly affected by the temperature. But if only one of the two light beams separated and output from the Wollaston prism  506  is used to detect the focal error signal FES as in the case of the first modification of the thirteenth embodiment, it is possible to detect the focal error signal FES without being affected by the temperature characteristic of the Wollaston prism  506  or Rochon prism. 
     In this first modification of the thirteenth embodiment, if the outputs of the photodetector parts AA, BB and C through H of the photodetector unit  591  are denoted by the same reference characters as these parts, the focal error signal FES according to the Foucault technique, the tracking error signal TES according to the push-pull technique, the magneto-optic signal MO and the identification signal ID can be obtained based on the following formulas (17) through (20a) or (20b) by calculation, using known adders and subtracter. 
     
       
         FES=(AA+C)−(BB+D)  (17) 
       
     
     
       
         TES=E−F  (18) 
       
     
     
       
         MO=(AA+BB+C+D)−(G+H)  (19) 
       
     
     
       
         ID=(AA+BB+C+D)+(G+H)  (20a). 
       
     
     
       
         ID=AA+BB+C+D+E+F+G+H  (20b) 
       
     
     When the identification signal ID is reproduced based on the formula (20a), the number of adders provided in the LSI is relatively small, thereby making it possible to improve the S/N ratio of the identification signal ID. On the other hand, the identification signal ID may become distorted depending on the shape and depth of the pits formed in the magneto-optic disk  505 . Hence, when the identification signal ID is reproduced based on the formula (20b), it is possible to positively prevent the identification signal ID from becoming distorted because all of the diffracted light beams from the pits of the magneto-optic disk  505  are used to detect the identification signal ID. 
     Next, a description will be given of a second modification of the thirteenth embodiment of the optical information recording/reproducing apparatus according to the present invention, by referring to FIG.  53 . In this second modification of the thirteenth embodiment, the basic construction of the optical information recording/reproducing apparatus is basically the same as that of the thirteenth embodiment shown in FIGS. 45 and 46, but a photodetector unit  592  is used in place of the photodetector unit  509 . FIG. 53 shows a plan view of the photodetector unit  592  used in this second modification of the thirteenth embodiment. In FIG. 53, those parts which are the same as those corresponding parts in FIGS. 46 and 47 are designated by the same reference characters, and a description thereof will be omitted. 
     FIG. 53 shows the photodetector unit  592  in a state where the laser beam is in focus on the magneto-optic disk  505 , viewed from the back surface side of the photodetector unit  592 . The light beams reaching the photodetector unit  592  via the Wollaston prism  506  and the composite prism  507  is obtained via the same optical path as that of the thirteenth embodiment. Photodetectors of the photodetector unit  592  are arranged in three stages from the top to bottom in the vertical direction. An upper stage portion of the photodetector unit  592  has a photodetector including a photodetector part G for detecting a magneto-optic signal MO and an identification signal ID, and a 2-part photodetector including photodetector parts A and B for detecting a focal error signal FES. A middle stage portion of the photodetector unit  592  has a 2-part photodetector including photodetector parts E 1  and F 1  and a 2-part photodetector including photodetector parts E 2  and F 2  for detecting a tracking error signal TES. Further, a lower stage portion of the photodetector unit  592  has a 2-part photodetector including photodetector parts C and D for detecting the focal error signal FES, and a photodetector including a photodetector part H for detecting the magneto-optic signal MO and the identification signal ID. 
     The 2-part photodetectors respectively including the photodetector parts E 1  and F 1  and the photodetector parts E 2  and F 2  receive the light beams output from the deflection surface  572  of the composite prism  507 . If the outputs of the photodetector parts A through D, E 1 , E 2 , F 1 , F 2 , G and H of the photodetector-unit  592  are denoted by the same reference characters as these parts, the focal error signal FES according to the Foucault technique, the tracking error signal TES according to-the pushpull technique, the magneto-optic signal MO and the identification signal-ID can be obtained based on the following formulas (21a) through (24a) or (24a′) by calculation using known adders and subtracter. 
     
       
         FES=(A+C)−(B+D)  (21a) 
       
     
     
       
         TES=(E 1 +E 2 )−(F 1 +F 2 )  (22a) 
       
     
     
       
         MO=(E 1 +F 1 +G)−(E 2 +F 2 +H)  (23a) 
       
     
     
       
         ID=(E 1 +F 1 +G)+(E 2 +F 2 +H)  (24a) 
       
     
     
       
         ID=A+B+C+D+E 1 +E 2 +F 1 +F 2 +G+H  (24a′) 
       
     
     When the identification signal ID is reproduced based on the formula (24a), the number of adders provided in the LSI is relatively small, thereby making it possible to improve the S/N ratio of the identification signal ID. On the other hand, the dentification signal ID may become distorted depending on the shape-and depth of the pits formed in the magneto-optic disk  505 . Hence, when the identification signal ID is reproduced based on the formula (24a′), it is possible to positively prevent the identification signal ID from becoming distorted because all of the diffracted light beams from the pits of the magneto-optic disk  505  are used to detect the identification signal ID. 
     According to this second modification of the thirteenth embodiment, the magneto-optic signal MO and the identification signal ID are reproduced using the light beams which are used to detect the tracking error signal TES. 
     Alternatively, it is possible to reproduce the magneto-optic signal MO and the identification signal ID using the light beams which are used to detect both the tracking error signal TES and-the focal error signal FEB. In this case, the focal error signal FES according to the Foucault technique, the tracking error signal TES according to the push-pull technique, the magneto-optic signal MO and the identification signal ID can be obtained based on the following formulas (21b) through (24b) by calculation using known adders and subtracter. 
     
       
         FES=(A+C)−(B+D)  (21b) 
       
     
     
       
         TES=(E 1 +E 2 )−(F 1 +F 2 )  (22b) 
       
     
     
       
         MO=(A+B+E 2 +F 2 +H)−(C+D+E 1 +F 1 +G)  (23b) 
       
     
     
       
         ID=(A+B+E 2 +F 2 +H)+(C+D+E 1 +F 1 +G)  (24b) 
       
     
     When the identification signal ID is reproduced based on the formula (24b), the number of adders provided in the LSI is relatively small, thereby making it possible to improve the S/N ratio of the identification signal ID. 
     Next, a description will be given of a third modification of the thirteenth embodiment of the optical information recording/reproducing apparatus according to the present invention, by referring to F 1 GS.  54  through  56 . In F 1 G.  54 , those parts which are the same as those corresponding parts in F 1 G.  46  and  53  are designated by the same reference numerals, and a description thereof will be omitted. 
     In this third modification of the thirteenth embodiment, the analyzer  506  is used together with a composite prism  547  shown in F 1 G.  55  and the photodetector unit  592  shown in F 1 G.  54 . 
     The reflected light beam from the magneto-optic disk  505  is split into two light beams by the analyzer  506 , and each of the two light beams are further-split into five light beams by the composite prism  547 , thereby resulting in ten (2×5=10) light beams being output from the composite prism  547 . The ten light beams from the composite prism  547  are irradiated on corresponding ones of six photodetectors which form the photodetector unit  592 . 
     F 1 G.  55  shows a perspective view of the composite prism  547 . As shown in F 1 G.  55 , the composite prism.  547  includes tapered first and second parts  547 - 1  and  547 - 2 , a central third part  547 - 3  which has a convex surface with a slight curvature, and peripheral fourth and fifth parts  547 - 4  and  547 - 5  which have convex surfaces with a slight curvature matching that of the third part  547 - 3 . In other words, the third, fourth and fifth parts  547 - 3 ,  547 - 4  and  547 - 5  are all parts of a single convex surface having a slight curvature. The first and second parts  547 - 1  and  547 - 2  function similarly to the first and second parts  571  and  573  of the composite prism  507 . 
     F 1 G.  56  shows a plan view of the photodetector unit  592 . As shown in F 1 G.  56 , the photodetector unit  592  includes the photodetectors respectively including the photodetector parts G and H, and the 2-part photodetectors respectively including the photodetector parts A and B, C and D, E 1  and F 2 , and E 2  and F 2 . 
     The two light beams output from the third part  547 - 3  of the composite prism  547  are respectively irradiated on the 2-part photodetector including the photodetector parts. E 1  and F 1  and the 2-part photodetector including the photodetector parts E 2  and F 2 . The two light beams output from the fourth part  547 - 4  of the composite prism  547  are respectively irradiated on the 2-part photodetector including the photodetector parts E 1  and F 1  and the 2-part photodetector including the photodetector parts E 2  and F 2 . Further, the two light beams output from the fifth part  547 - 5  of the composite prism- 547  are respectively irradiated on the 2-part photodetector including the photodetector parts E 1  and F 1  and the 2-part photodetector including the photodetector parts E 2  and F 2 . On the other hand, the two light beams output from the first part  547 - 1  of the composite prism  547  are respectively irradiated on the photodetector including-the photodetector part G and the 2-part photodetector including the photodetector parts A and B. The two light beams output from the second part  547 - 2  of the composite prism  547  are respectively irradiated on the 2-part photodetector including the photodetectors C and D and the photodetector including the photodetector part H. 
     The image formation points of the four light beams output from the first and second parts  547 - 1  and  547 - 2  match the positions of the corresponding photodetectors in the upper and lower stages of the photodetector unit  592 . On the other hand, the image formation points of the two light beams output from each of the third, fourth and fifth parts  547 - 3 ,  547 - 4  and  547 - 5  are deviated from the positions of the corresponding 2-part photodetectors in the middle stage of the photodetector unit  592 . 
     If the outputs of the photodetector parts A through D, E 1 , E 2 , F 1 , F 2 , G and H of the photodetector unit  592  are denoted by the same reference characters as these parts, the focal error signal FES using the Foucault technique, the tracking error signal TES using the push-pull technique, the magneto-optic signal MO, and the identification signal ID can be obtained by calculations based on the formulas (25) through (28a) or (28b) using known adders and subtracter. 
     
       
         FES=(A+C)−(B+D)  (25) 
       
     
     
       
         TES=(E 1 +E 2 )−(F 1 +F 2 )  (26) 
       
     
     
       
         MO=(E 1 +F 1 )−(E 2 +F 2 )  (27) 
       
     
     
       
         ID=(E 1 +F 1 )+(E 2 +F 2 )  (28a) 
       
     
     
       
         ID=A+B+C+D+E 1 +E 2 +F 1 +F 2 +G+H  (28b) 
       
     
     When the identification signal ID is reproduced based on the formula (28a), the number of adders provided in the LSI is relatively small, thereby making it possible to improve the S/N ratio of the identification signal ID. On the other hand, the identification signal ID may become distorted depending on the shape and depth of the pits formed in the magneto-optic disk  505 . Hence, when the identification signal ID is reproduced based on the formula (28b), it is possible to positively prevent the identification signal ID from becoming distorted because all of the diffracted light beams from the pits of the magneto-optic disk  505  are used to detect the identification signal ID. 
     According to the magneto-optic signal MO obtained by the formula (27), it is possible to obtain a relatively high resolution. The reason for this further improved resolution of the magneto-optic signal MO using the composite prism  547  having the shape shown in F 1 G.  55  may be understood from the teachings of the Proceedings of Magneto-Optical Recording International Symposium &#39;96, J. Magn. Soc. Jpn., Vol. 20, Supplement No. S1 (1996), pp. 233-238. 
     Next, a description will be given of a fourteenth embodiment of the optical information recording/reproducing apparatus according to the present invention, by referring to F 1 G.  57 . F 1 G.  57  shows a perspective view of an important part of the fourteenth embodiment of the optical information recording/reproducing apparatus, that is, a signal detection system, on an enlarged scale. In F 1 G.  57 , those parts which are-the same as those corresponding parts in F 1 G.  46  are designated by the same reference numerals, and a description thereof will be omitted. 
     F 1 G.  57  shows a state where the light beam output from the Wollaston prism  506  is projected on the photodetector unit  509 . The reflected light beam input to the Wollaston prism  506  is separated into two polarized light beams and input to a composite prism  517 . As shown in F 1 G.  57 , the composite prism  517  is an optical element having three deflection surfaces  571 ,  572   a  and  573  arranged side by side from the left to right. The deflection surfaces  571  and  573  on both sides of the composite prism  517  have tapered surfaces for deflecting the light beam in-upward and downward directions by mutually different angles. On the other hand, the deflection surface  572   a  at the central part of the composite prism  517  has a concave shape having a curvature with respect to the optical axis, and directs the two split light beams towards the photodetector parts E and F of the 2-part photodetector forming the photodetector unit  509 . Each of the two polarized light beams input to the composite prism  517  having the structure described above is spatially split into three by the three deflection surfaces  571 ,  572   a  and  573 , thereby. outputting a total of six light beams from the composite prism  517 . The six light beams output from. the composite prism  517  are projected on the photodetector unit  509  via a condenser lens  508 . For the sake of convenience, the illustration of the condenser lens  508  shown in F 1 G.  45  is omitted in F 1 G.  57 . 
     Otherwise, the functions and effects of this embodiment are similar to those of the thirteenth embodiment described above, and the focal-error signal FES, the tracking error signal TES, the magneto-optic signal MO and the identification signal ID can be obtained based on the formulas described with reference to the thirteenth embodiment. Furthermore, this embodiment may employ the photodetector unit  591  or  592  of the first or second modification of the thirteenth embodiment described above, so as to obtain similar functions and effects as those of the first or second modification of the thirteenth embodiment. 
     Next, a description will be given of a fifteenth embodiment of the optical information recording/reproducing apparatus according to the present invention, by referring to F 1 G.  58 . F 1 G.  58  shows a perspective view of an important part of the fifteenth embodiment of the optical information recording/reproducing apparatus, that is, a signal detection system, on an enlarged scale. In F 1 G.  58 , those parts which are the same as those corresponding parts in F 1 G.  46  are designated by the same reference numerals, and a description thereof will be omitted. 
     F 1 G.  58  shows a state where the light beam output from the Wollaston prism  506  is projected on a photodetector unit  593 . The reflected light beam input to the Wollaston prism  506  is separated into two polarized light beams and input to a composite prism  527 . As shown in F 1 G.  58 , the composite prism  527  is. an optical element having three deflection surfaces  571   b ,  572   b  and  573   b  arranged side by side from the left to right. The deflection surfaces  571   b  and  573   b  on both sides of the composite prism  527  have tapered surfaces for deflecting the light beam in upward and downward directions by mutually different angles. On the other hand, the deflection surface  572   b  at the central part of the composite prism  527  has a flat surface which is perpendicular to the optical axis, and directs the two split light beams towards the photodetector parts E and F of a 2-part photodetector forming the photodetector unit  593 . Each of the two polarized light beams-input to the composite prism  527  having the structure described above is spatially split into three by the three deflection surfaces  571   b ,  572   b  and  573   b , thereby outputting a total of six light beams from the composite prism  527 . The six light beams output from the composite prism  527  are projected on the photodetector unit  593  via a condenser lens  508 . For the sake of convenience, the illustration of the condenser lens  508  shown in F 1 G.  45  is omitted in F 1 G.  58 . 
     The photodetector unit  593  has a stepped shape such that the 2-part photodetector in the middle stage and including the photodetector parts E and F is arranged closer to the composite prism  527  than the photodetectors in the upper and lower stages of the photodetector unit  593 . 
     Otherwise, the functions and effects of this embodiment are similar to those of the thirteenth embodiment described above, and the focal error signal FES, the tracking error signal TES, the magneto-optic signal MO and the identification signal ID can be obtained based on the formulas described with reference to the thirteenth embodiment. Furthermore, this embodiment may employ the photodetector unit  591  or  592  of the first or second modification of the thirteenth embodiment described above by modifying the photodetector  591  or  592  to have stepped shape similar to that of the photodetector  593 , so as to obtain similar functions and effects as those of the first or second modification of the thirteenth embodiment. 
     Of course, the photodetector unit  593  may have a stepped shape such that the 2-part photodetector in the middle stage and including the photodetector parts E and F is arranged farther away from the composite prism  527  than the photodetectors in the upper and lower stages of the photodetector unit  593 . 
     Next, a description will be given of a sixteenth embodiment of the optical information recording/reproducing apparatus according to the present invention, by referring to F 1 G.  59 . F 1 G.  59  shows a perspective view of an important part of the sixteenth embodiment of the optical information recording/reproducing apparatus, that is, a signal detection system, on an enlarged scale. In F 1 G.  59 , those parts which are the same as those corresponding parts in F 1 G.  46  are designated by the same reference numerals, and a description thereof will be omitted. 
     F 1 G.  59  shows a state where the light beam output from the Wollaston prism  506  is projected on a photodetector unit  593 . The reflected light beam input to the Wollaston prism  506  is separated into two polarized light beams and input to a holographic optical element  537 . As shown in F 1 G.  59 , the holographic optical element  537  is an optical element having two diffraction surfaces  537   a  and  537   b  arranged side by side from to each other. The diffraction surfaces  537   a  and  537   b  of the holographic optical element  537  have gratings with sawtooth-shaped cross sections which are provided symmetrically about a center point of the holographic optical element  537 . 
     The two polarized light beams input to the holographic optical element  537  are respectively separated mainly into ±1st and ±0th order diffracted light beams by the diffraction surfaces  527   a  and  537   b , and a total of six light beams are projected onto the corresponding photodetectors of the photodetector unit  509  via a condenser lens  508 . Actually, high order diffracted light beams of ±2nd order or greater are generated from the holographic optical element  537 , but the light quantity of the high order diffracted light beams is small and negligible. For the sake of convenience, the illustration of the condenser lens  508  shown in F 1 G.  45  is omitted in F 1 G.  59 . 
     Otherwise, the functions and effects of this embodiment are similar to those of the thirteenth embodiment described above, and the focal error signal FES, the tracking error signal TES, the magneto-optic signal MO and the identification signal ID can be obtained based on the formulas described with reference to the thirteenth embodiment. Furthermore, this embodiment may employ the photodetector unit  591  or  592  of the first or second modification of the thirteenth embodiment described above, so as to obtain similar functions and effects as those of the first or second modification of the thirteenth embodiment. 
     In the thirteenth through sixteenth embodiments and the modifications thereof, it is of course possible to arrange the composite prism  507 ,  547 ,  517  or  527  or the holographic optical element  537 , the condenser lens  508 , and the analyzer  506  in an arbitrary order. In other words, the analyzer and the composite prism or holographic optical element may be arranged in an arbitrary order in the optical path for directing the reflected light beam from the optical recording medium to the photodetector unit. 
     F 1 GS.  60 A through  60 D respectively are circuit diagrams showing circuits for obtaining the focal error signal FES, the tracking error signal TES, the magneto-optic signal MO and the identification signal ID of the first modification of the eighth embodiment based on the formulas (7) through (10) described above. 
     F 1 G.  60 A shows a circuit including adders  801  and  802  and a subtracter  803  which are connected as shown to generate the focal error-signal FES based on the outputs of the photodetector parts A through D. 
     F 1 G.  60 B shows a circuit including a subtracter  804  to generate the tracking error signal TES based on the outputs of the photodetector parts E and F. 
     F 1 G.  60 C shows a circuit including a subtracter  805  to reproduce the magneto-optic signal MO based on the outputs of the photodetector parts G and G. 
     F 1 G.  60 D shows a circuit including an adder  806  to reproduce the identification signal ID based on the outputs of the photodetector parts G and H. 
     The circuits for obtaining the focal error signal FES, the tracking error signal TES, the magneto-optic signal MO and the identification signal ID based on other formulas described above can similarly be constructed using known adders and subtracters, and the illustration of such circuits will be omitted in this application since the connections of the adders and subtracters are evident from the formulas. 
     F 1 G.  61  is a perspective view showing a composite prism which may be used in place of the composite prisms  35 B and  547  shown in F 1 GS.  30  and  55 . 
     As shown in F 1 G.  61 , a composite prism  135 B includes tapered first and second parts  135 B- 1  and  135 B- 2 , a central third part  135 B- 3  which has a concave surface with a slight curvature, and peripheral fourth and fifth parts  135 B- 4  and  135 B- 5  which have concave surfaces with a slight curvature matching that of the third part  135 B- 3 . In other words, the third, fourth and fifth parts  135 B- 3 ,  135 B- 4  and  135 B- 5  are all parts of a single concave surface having a slight curvature. The first and second parts  135 B- 1  and  135 B- 2  function similarly to the first and second parts  35 B- 1  and. 35 B- 2  of the composite prism  35 B shown in F 1 G.  30 . This composite prism  135 B may be used in the optical system shown in F 1 GS.  29  and  54 , for example, and substantially the same effects are obtainable as in the embodiments shown in F 1 GS.  29  and  54 . 
     Further, the present invention is not limited to these embodiments, but various variations and modifications may be made without departing from the scope of the present invention.