Optical information recording/reproducing apparatus which detects focal error

An optical information recording/reproducing apparatus 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. The apparatus includes an optical element which deflects a part of the reflected light beam to at least two positions excluding a central part of the reflected light beam, and a photodetector device having a plurality of photodetectors which respectively detect the deflected parts of the reflected light beam and output detection outputs. The focal error is detected based on the detection outputs of the photodetector device.

BACKGROUND 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. 
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. FIG. 2 shows the relative positional relationship of the 
light beam which is irradiated via the objective lens 206 and the track on 
the disk 207, and FIG. 3 shows a spot of the reflected light beam which is 
formed on the 2-part photodetector 214 in correspondence with FIG. 2. 
In FIG. 2, (b) shows a case where the spot of the light beam is positioned 
at the center of a guide groove 207a 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). 
EQU 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 
207a, 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 216a, 216b, 216c and 216d. A focal error signal FES is generated 
from outputs A, B, C and D respectively output from the detector parts 
216a, 216b, 216c and 216d of the 4-part photodetector 216, based on the 
following formula (2). 
EQU 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. 
FIG. 6 shows the relative positional relationship of the objective lens 206 
and the disk 207. In 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, (b) shows a case where the in-focus position is 
located on the disk 207, and (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. 
FIG. 7 shows beam spots on the 4-part photodetector 216 for each relative 
positional relationship of the objective lens 206 and the disk 207 shown 
in FIG. 6. In FIG. 7, (a) shows the beam spots for the positional 
relationship shown in FIG. 6 (a), (b) shows the beam spots for the 
in-focus positional relationship shown in FIG. 6 (b), and (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 218 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 .theta. 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 center line of the composite prism 215 
shifts a distance .DELTA.y in the y-direction from the center of the 
incident light beam, where the center line extends in the x-direction in 
FIG. 4, the value of the mounting error can be obtained from 
.DELTA.y/(diameter of light beam)!.multidot.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 focus error signal because the tolerable margin of 
the focus 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 focus 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 and detects 
a focal error based on a reflected light beam from the optical recording 
medium, comprising an optical element 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 
focus error signal because the tolerable margin of the focus 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. For example, the optical element comprises a 
hologram optical element including a plurality of deflecting parts formed 
with hologram patterns, and the part of the reflected light beam excluding 
the central part of the reflected light beam is deflected to at least two 
positions by the deflecting parts.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 9 is a perspective view showing an essential part of a first 
embodiment of an optical information recording/reproducing apparatus 
according to the present invention. A composite prism 15 includes tapered 
parts 15a and 15b, and a central part 15c having no taper. On the other 
hand, a 4-part photodetector 16 includes 2-part photodetectors 16a and 
16b, and a central part 16c which includes no photodetector part. The 
composite prism 15 and the 4-part photodetector 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 
15a and 15b of the composite prism 15 form spots on the corresponding 
2-part photodetectors 16a and 16b of the 4-part photodetector 16. 
Accordingly, by carrying out the operation of the formula (2) described 
above using the outputs of the 2-part photodetectors 16a and 16b, 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 15c of the composite prism 15 is 
input to the central part 16c of the 4-part photodetector 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 15c of the composite 
prism 15 is not input to the 2-part photodetectors 16a and 16b of the 
4-part photodetector 16, that is, not input to a light sensitive part of 
the 4-part photodetector 16. 
In this embodiment, the spots which are formed on the 2-part photodetectors 
16a and 16b of the 4-part photodetector 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 16a and 16b. 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 essential part of the second embodiment. 
In FIG. 10, a composite prism 25 has a trapezoidal column shape and 
includes tapered parts 25a and 25b, and a central part 25c which has no 
taper. On the other hand, a 4-part photodetector 26 includes 2-part 
photodetectors 26a and 26b, and a central part 26c which includes no 
photodetector part. The composite prism 25 and the 4-part photodetector 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 
25a and 25b of the composite prism 25 form spots on the corresponding 
2-part photodetectors 26a and 26b of the 4-part photodetector 26. 
Accordingly, by carrying out the operation of the formula (2) described 
above using the outputs of the 2-part photodetectors 26a and 26b, 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 25c of the composite prism 25 is 
input to the central part 26c of the 4-part photodetector 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 25c of the composite 
prism 25 is not input to the 2-part photodetectors 26a and 26b of the 
4-part photodetector 26, that is, not input to a light sensitive part of 
the 4-part photodetector 26. 
In this embodiment, the spots which are formed on the 2-part photodetectors 
26a and 26b of the 4-part photodetector 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 26a and 26b. 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 view showing 
an essential 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 15cA is formed on 
the central part 15c of a composite prism 15A so light 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 15cA may 
be formed on the front surface or the rear surface of the composite prism 
15A at the central part 15c. 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 15A. Out of the reflected light beam which is 
input to the composite prism 15A, the light beams transmitted through the 
tapered parts 15a and 15b of the composite prism 15A form spots on the 
corresponding detector parts 216a, 216b, 216c and 216d 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 216a, 
16b, 216c and 216d, 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 15c of the composite prism 15A is absorbed or 
blocked light absorbing or blocking layer 15cA 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 15A, the light beam which is input to the 
central part 15c of the composite prism 15A is not input to the detector 
parts 216a, 216b, 216c and 216d 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 216a, 
216b, 216c and 216d 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 view showing 
an essential 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 25cA is formed on 
the central part 25c of a composite prism 25A 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 25cA may be formed on the front surface or the 
rear surface of the composite prism 25A at the central part 25c. In 
addition, this embodiment uses a 4-part photodetector 216A 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 25A. Out of the reflected light beam which is 
input to the composite prism 25A, the light beams transmitted through the 
tapered parts 25a and 25b of the composite prism 25A form spots on the 
corresponding detector parts 216a, 216b, 216c and 216d of the 4-part 
photodetector 216A. Accordingly, by carrying out the operation of the 
formula (2) described above using the outputs of the detector parts 216a, 
216b, 216c and 216d, 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 25c of the composite prism 25A is 
absorbed or blocked light absorbing or blocking layer 25cA and will not be 
input to the 4-part photodetector 216A. As a result, out of the reflected 
light beam input to the composite prism 25A, the light beam which is input 
to the central part 25c of the composite prism 25A is not input to the 
detector parts 216a, 216b, 216c and 216d of the 4-part photodetector 216A, 
that is, not input to a light sensitive part of the 4-part photodetector 
216A. 
In this embodiment, the spots which are formed on the detector parts 216a, 
216b, 216c and 216d of the 4-part photodetector 216A 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 216A. 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 fifth embodiment of the optical 
information recording/reproducing apparatus according to the present 
invention, by referring to FIG. 13. FIG. 13 is a perspective view showing 
an essential part of this fifth embodiment. In FIG. 13, 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 FIG. 13, a hologram optical element 85 includes lattice forming parts 
85a and 85b having the lattice shape or structure, and a central part 85c 
having no lattice shape or structure. The hologram optical element 85 and 
the 4-part photodetector 16 are provided in place of the composite prism 
215 and the 4-part photodetector 216 and detect the focal error in the 
optical system of the optical information recording/reproducing apparatus 
shown in FIG. 1, for example. 
In other words, when this embodiment is applied to the optical system shown 
in FIG. 1, the reflected light beam obtained via the beam splitters 204 
and 208, the condenser lens 212 and the beam splitter 213 becomes incident 
to the hologram optical element 85. Out of this reflected light beam 
incident to the hologram optical element 85, the light beams transmitted 
through the lattice forming parts 85a and 85b of the hologram optical 
element 85 respectively form spots on the 2-part photodetectors 16a and 
16b of the 4-part photodetector 16. Accordingly, it is possible to obtain 
the focal error signal FES similarly to the conventional case by carrying 
out the operation of the formula (2) described above using the outputs of 
the 2-part photodetectors 16a and 16b. 
Out of the reflected light beam incident to the hologram optical element 
85, the light beam transmitted through the central part 85c of the 
hologram optical element 85 becomes incident to the central part 16c of 
the 4-part photodetector 16. As a result, the light beam transmitted 
through the central part 85c of the hologram optical element 85 will not 
become incident to the 2-part photodetectors 16a and 16b of the 4-part 
photodetector 16, that is, will not become incident to the photosensitive 
part of the 4-part photodetector 16. 
According to this embodiment, the spots formed on the 2-part photodetectors 
16a and 16b of the 4-part photodetector 16 have oval shapes with a 
relatively large major axis when compared to the conventional case 
described above. In other words, the oval spots formed on the 2-part 
photodetectors 16a and 16b of the 4-part photodetector 16 are long in the 
direction perpendicular to the division lines E of the corresponding 
2-part photodetectors 16a and 16b. For this reason, the focal offset which 
is generated by the positional error of the division lines E can be made 
extremely small. 
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 FIG. 14. FIG. 14 is a perspective view showing 
an essential part of this sixth embodiment. In FIG. 14, 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 FIG. 14, a hologram optical element 95 includes lattice forming parts 
95a and 95b having the lattice shape or structure, and a central part 95c 
having no lattice shape oF structure. The hologram optical element 95 and 
the 4-part photodetector 26 are provided in place of the composite prism 
215 and the 4-part photodetector 216 and detect the focal error in the 
optical system of the optical information recording/reproducing apparatus 
shown in FIG. 1, for example. 
In other words, when this embodiment is applied to the optical system shown 
in FIG. 1, the reflected light beam obtained via the beam splitters 204 
and 208, the condenser lens 212 and the beam splitter 213 becomes incident 
to the hologram optical element 95. Out of this reflected light beam 
incident to the hologram optical element 95, the light beams transmitted 
through the lattice forming parts 95a and 95b of the hologram optical 
element 95 respectively form spots on the 2-part photodetectors 26a and 
26b of the 4-part photodetector 26. Accordingly, it is possible to obtain 
the focal error signal FES similarly to the conventional case by carrying 
out the operation of the formula (2) described above using the outputs of 
the 2-part photodetectors 26a and 26b. 
Out of the reflected light beam incident to the hologram optical element 
95, the light beam transmitted through the central part 95c of the 
hologram optical element 95 becomes incident to the central part 26c of 
the 4-part photodetector 26. As a result, the light beam transmitted 
through the central part 95c of the hologram optical element 95 will not 
become incident to the 2-part photodetectors 26a and 26b of the 4-part 
photodetector 26, that is, will not become incident to the photosensitive 
part of the 4-part photodetector 26. 
According to this embodiment, the spots formed on the 2-part photodetectors 
26a and 26b of the 4-part photodetector 26 have oval shapes with a 
relatively large major axis when compared to the conventional case 
described above. In other words, the oval spots formed on the 2-part 
photodetectors 26a and 26b of the 4-part photodetector 26 are long in the 
direction perpendicular to the division lines E of the corresponding 
2-part photodetectors 26a and 26b. For this reason, the focal offset which 
is generated by the positional error of the division lines E can be made 
extremely small. 
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 FIG. 15. FIG. 15 is a perspective view showing 
an essential part of this seventh embodiment. In FIG. 15, those parts 
which are the same as those corresponding parts in FIGS.11 and 13 are 
designated by the same reference numerals, and a description thereof will 
be omitted. 
In FIG. 15, a layer 85cA is formed at a central part 85c of a hologram 
optical element 85A. This layer 85cA absorbs or blocks the light beam 
having the wavelength of the light beam emitted from the laser diode 201 
in the optical system of the optical information recording/reproducing 
apparatus shown in FIG. 1. This layer 85cA may be formed on the front 
surface or the rear surface of the hologram optical element 85A at the 
central part 85c. 
In other words, when this embodiment is applied to the optical system shown 
in FIG. 1 , the reflected light beam obtained via the beam splitters 204 
and 208, the condenser lens 212 and the beam splitter 213 becomes incident 
to the hologram optical element 85A. Out of this reflected light beam 
incident to the hologram optical element 85A, the light beams transmitted 
through the lattice forming parts 85a and 85b of the hologram optical 
element 85A respectively form spots on the detector parts 216a and 216b 
and the detector parts 216c and 216d of the 4-part photodetector 216. 
Accordingly, it is possible to obtain the focal error signal FES similarly 
to the conventional case by carrying out the operation of the formula (2) 
described above using the outputs of the detector parts 216a, 216b, 216c 
and 216d. 
Out of the reflected light beam incident to the hologram optical element 
85A, the light beam incident to the central part 85c of the hologram 
optical element 85A is absorbed or blocked by the layer 85cA and will not 
become incident to the 4-part photodetector 216. As a result, the light 
beam incident to the central part 85c of the hologram optical element 85A 
will not become incident to the detector parts 216a, 216b, 216c and 216d 
of the 4-part photodetector 216, that is, will not become incident to the 
photosensitive part of the 4-part photodetector 216. 
According to this embodiment, the spots formed on the detector parts 216a 
and 216b and the detector parts 216c and 216d of the 4-part photodetector 
216 have oval shapes with a relatively large major axis when compared to 
the conventional case described above. In other words, the oval spots 
formed on the detector parts 216a and 216b and the detector parts 216c and 
216d of the 4-part photodetector 216 are long in the direction 
perpendicular to the division lines E of the corresponding pair of 
detector parts 216a and 216b and pair of detector parts 216c and 216d. For 
this reason, the focal offset which is generated by the positional error 
of the division lines E can be made extremely small. 
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 FIG. 16. FIG. 16 is a perspective view showing 
an essential part of this eighth embodiment. In FIG. 16, those parts which 
are the same as those corresponding parts in FIGS. 12 and 14 are 
designated by the same reference numerals, and a description thereof will 
be omitted. 
In FIG. 16, a layer 95cA is formed at a central part 95c of a hologram 
optical element 95A. This layer 95cA absorbs or blocks the light beam 
having the wavelength of the light beam emitted from the laser diode 201 
in the optical system of the optical information recording/reproducing 
apparatus shown in FIG. 1. This layer 95cA may be formed on the front 
surface or the rear surface of the hologram optical element 95A at the 
central part 95c. 
In other words, when this embodiment is applied to the optical system shown 
in FIG. 1, the reflected light beam obtained via the beam splitters 204 
and 208, the condenser lens 212 and the beam splitter 213 becomes incident 
to the hologram optical element 95A. Out of this reflected light beam 
incident to the hologram optical element 95A, the light beams transmitted 
through the lattice forming parts 95a and 95b of the hologram optical 
element 95A respectively form spots on the detector parts 216a and 216b 
and the detector parts 216c and 216d of the 4-part photodetector 216A. 
Accordingly, it is possible to obtain the focal error signal FES similarly 
to the conventional case by carrying out the operation of the formula (2) 
described above using the outputs of the detector parts 216a, 216b, 216c 
and 216d. 
Out of the reflected light beam incident to the hologram optical element 
95A, the light beam incident to the central part 95c of the hologram 
optical element 95A is absorbed or blocked by the layer 95cA and will not 
become incident to the 4-part photodetector 216A. As a result, the light 
beam incident to the central part 95c of the hologram optical element 95A 
will not become incident to the detector parts 216a, 216b, 216c and 216d 
of the 4-part photodetector 216A, that is, will not become incident to the 
photosensitive part of the 4-part photodetector 216A. 
According to this embodiment, the spots formed on the detector parts 216a 
and 216b and the detector parts 216c and 216d of the 4-part photodetector 
216A have oval shapes with a relatively large major axis when compared to 
the conventional case described above. In other words, the oval spots 
formed on the detector parts 216a and 216b and the detector parts 216c and 
216d of the 4-part photodetector 216A are long in the direction 
perpendicular to the division lines E of the corresponding pair of 
detector parts 216a and 216b and pair of detector parts 216c and 216d. For 
this reason, the focal offset which is generated by the positional error 
of the division lines E can be made extremely small. 
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 FIG. 17. FIG. 17 is a perspective view showing 
an essential part of this ninth embodiment. 
In FIG. 17, a hologram optical element 105 includes lattice forming parts 
105a, 105b, 105c and 105d having the lattice shape or structure. On the 
other hand, a 6-part photodetector 217 includes detector parts 217a, 217b, 
217c and 217d which form a 4-part photodetector, and detector parts 217e 
and 217f arranged above and below this 4-part photodetector. 
When the reflected light beam from the disk is irradiated on the hologram 
optical element 105, the light beams transmitted through the lattice 
forming parts 105a and 105b respectively form spots on the detector parts 
217a and 217b and the detector parts 217c and 217d which form the 4-part 
photodetector of the 6-part photodetector 217. In addition, the light 
beams transmitted through the lattice forming parts 105c and 105d of the 
hologram optical element 105 respectively form spots on the detector parts 
217e and 217f of the 6-part photodetector 217. Accordingly, it is possible 
to obtain the focal error signal FES similarly to the conventional case by 
carrying out the operation of the formula (2) described above using the 
outputs of the detector parts 217a, 217b, 217c and 217d. On the other 
hand, it is possible to obtain the tracking error signal TES similarly to 
the conventional case by carrying out the operation of the formula (1) 
described above using the outputs of the detector parts 217e and 217f. 
When this embodiment is applied to the optical system shown in FIG. 1, the 
hologram optical element 105 and the 6-part photodetector 217 are provided 
in place of the composite prism 215 and the 4-part photodetector 216, to 
detect the focal error. At the same time, it is possible to also detect 
the tracking error, and for this reason, it is possible to omit the beam 
splitter 213 and the 2-part photodetector 214. 
Out of the reflected light beam incident to the hologram optical element 
105, the light beams incident to the lattice forming parts 105c and 105d 
at the central part of the hologram optical element 105 respectively 
become incident to the detector parts 217e and 217f of the 6-part 
photodetector 217. As a result, the light beams transmitted through the 
central part of the hologram optical element 105 will not become incident 
to the detector parts 217a, 217b, 217c and 217d which form the 4-part 
photodetector of the 6-part photodetector 217, that is, will not become 
incident to the part of the 6-part photodetector 217 for obtaining the 
focal error. 
According to this embodiment, the spots formed on the detector parts 217a 
and 217b and the detector parts 217c and 217d forming the 4-part 
photodetector of the 6-part photodetector 217 have oval shapes with a 
relatively large major axis when compared to the conventional case 
described above. In other words, the oval spots formed on the detector 
parts 217a and 217b and the detector parts 217c and 217d of the 4-part 
photodetector are long in the direction perpendicular to the division 
lines E of the corresponding pair of detector parts 217a and 217b and pair 
of detector parts 217c and 217d. For this reason, the focal offset which 
is generated by the positional error of the division lines E can be made 
extremely small. 
Next, a description will be given of a tenth embodiment of the optical 
information recording/reproducing apparatus according to the present 
invention, by referring to FIG. 18. FIG. 18 is a perspective view showing 
an essential part of this ninth embodiment. In FIG. 18, 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 FIG. 18, a hologram optical element 105A includes lattice forming parts 
105a, 105b, 105cA and 105dA having the lattice shape or structure. On the 
other hand, a 6-part photodetector 217A includes detector parts 217a, 
217b, 217c and 217d which form a 4-part photodetector, and detector parts 
217eA and 217fA arranged on the right and left of this 4-part 
photodetector. 
When the reflected light beam from the disk is irradiated on the hologram 
optical element 105A, the light beams transmitted through the lattice 
forming parts 105a and 105b respectively form spots on the detector parts 
217a and 217b and the detector parts 217c and 217d which form the 4-part 
photodetector of the 6-part photodetector 217A. In addition, the light 
beams transmitted through the lattice forming parts 105cA and 105dA of the 
hologram optical element 105A respectively form spots on the detector 
parts 217eA and 217fA of the 6-part photodetector 217A. Accordingly, it is 
possible to obtain the focal error signal FES similarly to the 
conventional case by carrying out the operation of the formula (2) 
described above using the outputs of the detector parts 217a, 217b, 217c 
and 217d. On the other hand, it is possible to obtain the tracking error 
signal TES similarly to the conventional case by carrying out the 
operation of the formula (1) described above using the outputs of the 
detector parts 217eA and 217fA. 
When this embodiment is applied to the optical system shown in FIG. 1, the 
hologram optical element 105A and the 6-part photodetector 217A are 
provided in place of the composite prism 215 and the 4-part photodetector 
216, to detect the focal error. At the same time, it is possible to also 
detect the tracking error, and for this reason, it is possible to omit the 
beam splitter 213 and the 2-part photodetector 214. 
Out of the reflected light beam incident to the hologram optical element 
105A, the light beams incident to the lattice forming parts 105cA and 
105dA at the central part of the hologram optical element 105A 
respectively become incident to the detector parts 217eA and 217fA of the 
6-part photodetector 217A. As a result, the light beams transmitted 
through the central part of the hologram optical element 105A will not 
become incident to the detector parts 217a, 217b, 217c and 217d which form 
the 4-part photodetector of the 6-part photodetector 217A, that is, will 
not become incident to the part of the 6-part photodetector 217A for 
obtaining the focal error. 
According to this embodiment, the spots formed on the detector parts 217a 
and 217b and the detector parts 217c and 217d forming the 4-part 
photodetector of the 6-part photodetector 217A have oval shapes with a 
relatively large major axis when compared to the conventional case 
described above. In other words, the oval spots formed on the detector 
parts 217a and 217b and the detector parts 217c and 17d of the 4-part 
photodetector are long in the direction perpendicular to the division 
lines E of the corresponding pair of detector parts 217a and 217b and pair 
of detector parts 217c and 217d. For this reason, the focal offset which 
is generated by the positional error of the division lines E can be made 
extremely small. 
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. 19 and 20. FIG. 19 shows a cross 
sectional view of this eleventh embodiment, and FIG. 20 shows a 
perspective view of an essential part of this eleventh embodiment. In 
FIGS. 19 and 20, 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 FIG. 19, a laser diode chip 201A, the 6-part photodetector 217 and a 
hologram optical element 105B are mounted on a common housing 218. The 
hologram optical element 105B has the same functions as the hologram 
optical element 105 shown in FIG. 17, but the 6-part photodetector 217 is 
arranged at a position shifted in the direction Y in FIG. 19 from a center 
of an optical axis of the optical system. For this reason, the deflecting 
direction of the light beam incident to the 6-part photodetector 217 needs 
to be shifted in the same direction Y to match the arrangement of the 
6-part photodetector 217. Hence, the hologram patterns of the hologram 
optical element 105B is slightly different from the hologram pattern of 
the hologram optical element 105 shown in FIG. 17. 
The light beam which is emitted from the laser diode chip 210A in the 
direction Z in FIG. 19 first becomes incident to the hologram optical 
element 105B. In general, the light beam incident to a hologram optical 
element is separated into a 0th order light, .+-.1st order lights and high 
order lights. The 0th order light is the light beam which passes through 
the hologram optical element 105B as it is. Hence, the 0th order light is 
transmitted through a beam splitter 204A and becomes incident to an 
objective lens 206A, and is converged as a minute point on the disk 207. 
The reflected light beam from the disk 207 is transmitted through the 
objective lens 206A and becomes incident again to the beam splitter 204A. 
A part of the reflected light beam is reflected by the beam splitter 204A 
towards a magneto-optic signal detection system which is made up of a 
Wollaston prism 209A and a 2-part photodetector 211A, and a magneto-optic 
signal is detected. On the other hand, a part of the reflected light beam 
transmitted through the beam splitter 204A becomes incident to the 
hologram optical element 105B, and is again separated into the 0th order 
light, the .+-.1st order lights and the high order lights. 
In this embodiment, the +1st order light or the -1st order light obtained 
by the second separation is used as the light for detecting the servo 
signal. For example, the +1st order light emitted from a part 105aB of the 
hologram optical element 105B is received by the detector parts 217a and 
217b, and the +1st order light emitted from a part 105bB of the hologram 
optical element 105B is received by the detector parts 217c and 217d. In 
addition, the +1st order light emitted from a part 105cB of the hologram 
optical element 105B is received by the detector part 217e, and the +1st 
order light emitted from a part 105dB of the hologram optical element 105B 
is received by the detector part 105f. Accordingly, it is possible to 
obtain the focal error signal FES similarly to the conventional case by 
carrying out the operation of the formula (2) described above using the 
outputs of the detector parts 217a, 217b, 217c and 217d forming the 4-part 
photodetector of the 6-part photodetector 217. On the other hand, it is 
possible to obtain the tracking error signal TES similarly to the 
conventional case by carrying out the operation of the formula (1) 
described above using the outputs of the detector parts 217e and 217f. 
According to this embodiment, it is possible to reduce the volume (that is, 
the size) of the appartus as a whole when compared to the conventional 
case shown in FIG. 1. Furthermore, it is possible to reduce the cots, and 
improve the reliability of the apparatus by reducing the number of parts 
that are required. 
FIG. 21 shows simulation results describing the relationship of the focal 
position and the focal error signal FES in the prior art shown in FIG. 4. 
In FIG. 21, 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 .mu.m, a 
dotted line indicates a case where the detector shift is +20 .mu.m, a bold 
dotted line indicates a case where the detector shift is -10 .mu.m, and a 
bold and fine dotted line indicates a case where the detector shift is -20 
.mu.m. 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. 
In FIG. 21, (a) shows a case where the mounting error of the composite 
prism 215 is 5%, (b) shows a case where the mounting error is 10%, (c) 
shows a case where the inclination angle .theta. of the light beam emitted 
from the laser diode 201 is 0.5.degree., and (d) shows a case where the 
inclination angle .theta. of the light beam emitted from the laser diode 
201 is 1.0.degree.. The case where the inclination angle .theta. is 
0.5.degree. 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 .theta. is 1.0.degree. 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. 21 (a) is +20 .mu.m, for example, it may be seen that a focal 
offset of approximately 2.0 .mu.m is generated. 
On the other hand, FIG. 22 shows simulation results describing the 
relationship of the focal position and the focal error signal FES in the 
first embodiment shown in FIG. 9, the third embodiment shown in FIG. 11, 
the fifth embodiment shown in FIG. 13, the seventh embodiment shown in 
FIG. 15, the ninth embodiment shown in FIG. 17, the tenth embodiment shown 
in FIG. 18 or, the eleventh embodiment shown in FIGS. 19 and 20. In FIG. 
22, 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 .mu.m, a 
dotted line indicates a case where the detector shift is +20 .mu.m, a bold 
dotted line indicates a case where the detector shift is -10 .mu.m, and a 
bold and fine dotted line indicates a case where the detector shift is -20 
.mu.m. 
In FIG. 22, (a) shows a case where the mounting error of the composite 
prism 15 or 15A or the hologram optical element 85, 85A, 105, 105A or 105B 
is 5%, (b) shows a case where the mounting error is 10%, (c) shows a case 
where the inclination angle .theta. of the light beam emitted from the 
laser diode 201 is 0.5.degree., and (d) shows a case where the inclination 
angle .theta. of the light beam emitted from the laser diode 201 is 
1.0.degree.. The case where the inclination angle .theta. is 
0.5.degree.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 .theta. is 1.0.degree. 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. 22 (a) is +20 .mu.m, for example, it may be seen that 
only an extremely small focal offset of approximately 0.8 .mu.m is 
generated. In other words, the focal offset is less than one-half the 
focal offset of the conventional case. 
FIG. 23 is a diagram showing the relationship of the detector shift and the 
focal offset in the prior art based on the simulation results of FIG. 21. 
In FIG. 23, 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. 23, the focal 
offset is generated in each case where the detector shift occurs. 
On the other hand, FIG. 24 is a diagram showing the relationship of the 
detector shift and the focal offset in the first, third, fifth, seventh, 
ninth, tenth or eleventh embodiment based on the simulation results of 
FIG. 22. In FIG. 24, a coarse dotted line shows a case where the mounting 
error of the composite prism 15 or 15A or the hologram optical element 85, 
85A, 105, 105A or 105B is 5%, a fine dotted line indicates a case where 
the mounting error of the composite prism 15 or 15A 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. 24, 
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, third, fifth, seventh, ninth, tenth or 
eleventh 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 twelfth embodiment of the optical 
information recording/reproducing apparatus according to the present 
invention, by referring to FIGS. 25 through 27. In FIG. 25, 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. 25. 
In addition, a composite prism 35 and a photodetector 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. 
FIG. 26 shows the composite prism 35 on an enlarged scale. In FIG. 26, (a) 
shows a perspective view of the composite prism 35, and (b) shows a plan 
view of the composite prism 35. As shown in FIG. 26, the composite prism 
35 includes tapered first and second parts 35a and 35b, and a third part 
35c 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 30a, 30b and 30c. 
FIG. 27 is a perspective view, on an enlarged scale, showing an essential 
part of FIG. 25. The photodetector 36 includes a first photodetector 36a, 
a second photodetector 36b, and a third photodetector 36c. The first 
photodetector 36a includes photodetectors 37a and 37b. The second 
photodetector 36b includes photodetectors 37c and 37d. The third 
photodetector 36c includes photodetectors 37e and 37f. 
Out of the reflected light beam 30 which is refracted and condensed via the 
condenser lens 212, the light beam 30a which is transmitted through the 
first part 35a is deflected depending on the taper angle of the first part 
35a and is irradiated on the first photodetector 36a of the photodetector 
36, while the light beam 30b which is transmitted through the second part 
35b is deflected depending on the taper angle of the second part 35b and 
is irradiated on the second photodetector 36b of the photodetector 36. In 
addition, the light beam 30c which is transmitted through the third part 
35c is refracted depending on the curvature of the third part 35c and is 
irradiated on the third photodetector 38c of the photodetector 36. In 
other words, the light beams 30a and 30b are only subjected to the 
refraction function of the condenser lens 212, but the light beam 30c is 
subjected to the refraction function of the condenser lens 212 and the 
third part 35c itself. Therefore, image formation points 300a and 300b of 
the respective light beams 30a and 30b are different from an image 
formation point 300c of the light beam 30c. That is, distances L1 and L2 
from the condenser lens 212 to the image formation points 300a and 300b of 
the respective light beams 30a and 30b are different from a distance L3 
from the condenser lens 212 to the image formation point 300c of the light 
beam 30c. 
In FIG. 27, the photodetector 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 300a and 300b. Because of this 
arrangement, the first and second photodetectors 36a and 38b 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 300a and 300b of the light beams 30a and 30b. On the 
ether hand, the third photodetector 36c 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 300c of 
the light beam 30c. 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 300a and 300b of the light 
beams 30a and 30b are different from the distance L3 between the condenser 
lens 212 and the image formation point 300c of the light beam 30c, and the 
construction and arrangement of the composite prism 35 and the 
photodetector 36 are not limited to those of the above embodiment. 
Next, a description will be given of a thirteenth embodiment of the optical 
information recording/reproducing apparatus according to the present 
invention, by referring to FIGS. 28 and 29. In FIGS. 28 and 29, those 
parts which are the same as those corresponding parts in FIGS. 26 and 27 
are designated by the same reference numerals, and a description thereof 
will be omitted. 
In this embodiment, a composite prism 45 shown in FIG. 28 is used in place 
of the composite prism 35 shown in FIG. 26. 
FIG. 28 shows the composite prism 45 on an enlarged scale. In FIG. 28, (a) 
shows a perspective view of the composite prism 45, and (b) shows a plan 
view of the composite prism 45. As shown in FIG. 28, the composite prism 
45 includes tapered first and second parts 45a and 45b, and a third part 
45c 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 30a, 30b and 30c. 
FIG. 29 is a perspective view, on an enlarged scale, showing an essential 
part of this embodiment. The photodetector 36 is the same as the 
photodetector 36 used in the twelfth embodiment. 
Out of the reflected light beam 30 which is refracted and condensed via the 
condenser lens 212, the light beam 30a which is transmitted through the 
first part 45a is deflected depending on the taper angle of the first part 
45a and is irradiated on the first photodetector 36a of the photodetector 
36, while the light beam 30b which is transmitted through the second part 
45b is deflected depending on the taper angle of the second part 45b and 
is irradiated on the second photodetector 36b of the photodetector 36. In 
addition, the light beam 30c which is transmitted through the third part 
45c is refracted depending on the curvature of the third part 45c and is 
irradiated on the third photodetector 36c of the photodetector 36. In 
other words, the light beams 30a and 30b are only subjected to the 
refraction function of the condenser lens 212, but the light beam 30c is 
subjected to the refraction function of the condenser lens 212 and the 
third part 45c itself. Therefore, image formation points 300a and 300b of 
the respective light beams 30a and 30b are different from an image 
formation point 300c of the light beam 30c. That is, distances L1 and L2 
from the condenser lens 212 to the image formation points 300a and 300b of 
the respective light beams 30a and 30b are different from a distance L3 
from the condenser lens 212 to the image formation point 300c of the light 
beam 30c. 
In other words, the image formation point 300c of the light beam 30c is 
located between the composite prism 35 and the photodetector 36 in the 
twelfth embodiment, but the image formation point 300c of the light beam 
30c in this embodiment is located beyond the photodetector 36 in FIG. 29 
along the traveling direction of the light beam. 
In FIG. 29, the photodetector 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 300a and 300b, similarly to the 
twelfth embodiment shown in FIG. 27. Because of this arrangement, the 
first and second photodetectors 36a and 36b 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 300a and 300b of 
the light beams 30a and 30b. On the other hand, the third photodetector 
36c 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 300c of the light beam 30c. 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 fourteenth embodiment of the optical 
information recording/reproducing apparatus according to the present 
invention, by referring to FIGS. 30 and 31. In FIGS. 30 and 31, those 
parts which are the same as those corresponding parts in FIGS. 26 and 27 
are designated by the same reference numerals, and a description thereof 
will be omitted. 
In this embodiment, a composite prism 55 and a photodetector 56 shown in 
FIG. 31 are used in place of the composite prism 35 and the photodetector 
36 shown in FIG. 28. 
FIG. 30 shows the composite prism 55 on an enlarged scale. In FIG. 30, (a) 
shows a perspective view of the composite prism 55, and (b) shows a plan 
view of the composite prism 55. As shown in FIG. 30, the composite prism 
55 includes tapered first and second parts 55a and 55b, and a flat third 
part 55c which has not taper. Hence, a reflected light beam 30 which is 
obtained via the beam splitter 208 is split into three light beams 30a, 
30b and 30c. 
FIG. 31 is a perspective view, on an enlarged scale, showing an essential 
part of this embodiment. The photodetector 56 includes a first 
photodetector 56a, a second photodetector 56b, and a third photodetector 
56c. The first photodetector 56a includes photodetectors 37a and 37b. The 
second photodetector 56b includes photodetectors 37c and 37d. The third 
photodetector 56c includes photodetectors 37e and 37f. The third 
photodetector 56c is arranged on a plane different from a plane on which 
the first and second photodetectors 56a and 56b are arranged. 
Out of the reflected light beam 30 which is refracted and condensed via the 
condenser lens 212, the light beam 30a which is transmitted through the 
first part 55a is deflected depending on the taper angle of the first part 
55a and is irradiated on the first photodetector 56a of the photodetector 
56, while the light beam 30b which is transmitted through the second part 
55b is deflected depending on the taper angle of the second part 55b and 
is irradiated on the second photodetector 56b of the photodetector 56. In 
addition, the light beam 30c which is transmitted through the third part 
55c is transmitted as it is and is irradiated on the third photodetector 
56c of the photodetector 56. In other words, all of the light beams 30a, 
30b and 30c are only subjected to the refraction function of the condenser 
lens 212. Therefore, image formation points 300a, 300b and 300c of the 
respective light beams 30a, 30b and 30c are all located on the same plane. 
That is, distances L1, L2 and L3 from the condenser lens 212 to the image 
formation points 300a, 300b and 300c of the respective light beams 30a, 
30b and 30c are the same. However, since the third photodetector 56c in 
this embodiment is arranged on the plane which is different from the plane 
on which the first and second photodetectors 56a and 56b are arranged, the 
image formation point 300c of the light beam 30c and the position of the 
third photodetector 56c do not match. 
In other words, the image formation point 300c of the light beam 30c is 
located between the composite prism 35 and the photodetector 36 in the 
twelfth embodiment, but the image formation point 300c of the light beam 
30c in this embodiment is located beyond the third photodetector 56c in 
FIG. 31 along the traveling direction of the light beam. 
In FIG. 31, the first and second photodetectors 56a and 56b of the 
photodetector 58 are arranged on a plane which is perpendicular to the 
optical axis of the reflected light beam 30 and includes the image 
formation points 300a and 300b, similarly to the twelfth embodiment shown 
in FIG. 27. Because of this arrangement, the first and second 
photodetectors 56a and 56b 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 300a and 300b of the light 
beams 30a and 30b. On the other hand, the third photodetector 56c 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 300c of the light beam 30c. 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 a fifteenth embodiment of the optical 
information recording/reproducing apparatus according to the present 
invention, by referring to FIGS. 32, 33 and 34. In FIGS. 32 and 33, 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 embodiment, an analyzer 208A shown in FIGS. 32 and 33 is used 
together with the composite prism 35 and the photodetector 36 shown in 
FIG. 25. 
For example, an analyzer 21 disclosed in a Japanese Laid-Open Patent 
Application No.63-127436 may be used as the analyzer 208A. In this 
fifteenth embodiment, the light beam is split into three light beams by 
the analyzer 208A, and each of the three light beams are further split 
into three light beams by the composite prism 35, thereby resulting in 
nine (3.times.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 66a through 66i which form the 
photodetector 66. 
FIG. 34 shows a plan view of the photodetector 66. The focal error signal 
FES can be generated according to the Foucault technique based on outputs 
of 5 the photodetectors 66a, 66b, 66d, 66e, 66g and 66h of the 
photodetector 66. The photodetectors 66a, 66d and 66g receive the three 
light beams from the first part of the composite prism 35, while the 
photodetectors 66b, 66e and 66h 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 66a, 66b, 66d, 
66e, 66g and 66h. On the other hand, the tracking error signal TES can be 
generated according to the push-pull technique based on outputs of the 
photodetectors 66c, 66f and 66i. The photodetectors 66c, 66f and 66i 
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 66c, 66f and 66i. 
As shown in FIG. 34, the photodetector 66a includes photodetector parts 37a 
and 37b, the photodetector 66b includes photodetector parts 37c and 37d . 
. . , and the photodetector 66i includes photodetector parts 37q and 37r. 
Accordingly, if the outputs of these photodetector parts 37a through 37i 
are denoted by the same reference numerals as these parts, the focal error 
signal FES using the Foucault technique can be generated based on the 
following formula (3) by calculation. 
EQU FES=(37a)+(37g)+(37m)+(37d)+(37j)+(37p)!-(37b)+(37h)+(37n)+(37c)+(37i)+(3 
7o)! (3) 
In addition, the tracking error signal using the push-pull technique can be 
generated based on the following formula (4) by calculation. 
EQU TES=(37e)+(37k)+(37q)!-(37f)+(371)+(37r)! (4) 
Furthermore, by the function of the analyzer 208A, a magneto-optic signal 
(information signal) RF which is recorded on the disk 207 can be 
reproduced based on the following formula (5) by calculation. 
EQU RF=(37a)+(37b)+(37e)+(37f)+(37c)+(37d)!-(37m)+(37n)+(37q)+(37r)+(37o)+(37 
p)! (5) 
According to the fifteenth 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 twelfth through fourteenth embodiments. As is 
evident from a comparison of FIGS. 25 and 32, the Wollaston prism 209, the 
lens 210 and the 2-part photodetector 211 required in FIG. 25 are omitted 
in FIG. 32. 
In the twelfth through fifteenth embodiments, 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 hologram optical element, for example. 
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 FIGS. 35 and 36. In FIGS. 35 and 36, those 
parts which are the same as those corresponding parts in FIG. 27 are 
designated by the same reference numerals, and a description thereof will 
be omitted. 
In this embodiment, a hologram optical element 75 shown in FIG. 35 is used 
in place of the composite prism 35 shown in FIG. 27. 
FIG. 35 is a perspective view showing an essential part of this embodiment 
on an enlarged scale. The hologram optical element 75 includes first and 
second parts 75a and 75b. The cross sectional shape of the first part 75a 
along a line A-A' in FIG. 35 is a sawtooth grating as shown in FIG. 36. 
The second part 75b has a cross sectional shape similar to that of the 
first part 75a, but the cross sectional shape of the second part 75b is in 
point symmetry to that of the first part 75a with respect to the center of 
the hologram optical element 75. The sawtooth gratings of the first and 
second parts 75a and 75b are sometimes also referred to as blazed 
gratings. 
The hologram 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 hologram 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 75a and 75b 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 75a, a +1st order 
diffracted light 30-2 which is diffracted by the grating of the second 
part 75b, and a 0th order diffracted light 30-3 which passes through the 
first and second parts 75a and 75b without being affected by the gratings 
thereof. In addition, the grating patterns of the first and second parts 
75a and 75b are designed such that the +1st order diffracted light 30-1 
which is emitted from the first part 75a is refracted twice via the 
condenser lens 212 and the first part 75a before being imaged at an image 
formation point 300a, and the +1st order diffracted light 30-2 which is 
emitted from the second part 75b is refracted twice via the condenser lens 
212 and the second part 75b before being imaged at an image formation 
point 300b. On the other hand, since the 0th order diffracted light 30-3 
passes through the hologram 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 300c. 
In FIG. 35, the photodetector 36 is arranged on a plane which is 
perpendicular to the optical axis of the reflected light beam and includes 
the image formation points 300a and 300b. Because of this arrangement, the 
first and second photodetectors 36a and 36b 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 300a and 300b of 
the +1st order diffracted lights 30-1 and 30-2. On the other hand, the 
third photodetector 36c 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 300c 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 300a and 300b 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 300c of the 
0th order diffracted light 30-3, and the construction and arrangement of 
the hologram optical element 75 and the photodetector 36 are not limited 
to those of this embodiment. 
Next, a description will be given of the functions of the hologram optical 
element 75 by itself, that is, for the case where no condenser lens 212 
exists, by referring to FIGS. 37 through 39. 
As described above, the hologram optical element 75 includes the first and 
second parts 75a and 75b which are provided with independent patterns for 
deflecting, converging and diverging the light. More particularly, the 
patterns of the first and second parts 75a and 75b are respectively set so 
that the +1st order diffracted light 30-1 from the first part 75a 
converges to a point P'(-x, 0) and the +1st order diffracted light 30-2 
from the second part 75b converges to a point P(x, 0). Points P and P' are 
located on a plane .pi. which is a distance f away from the hologram 
optical element 75 along the optical axis. In other words, the function of 
the first part 75a 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. 38 shows a plan view of the hologram optical element 75. Since the 
patterns of the first and second parts 75a and 75b are in point symmetry 
with respect to the origin 0 in FIG. 38, the pattern of the first part 
75a, for example, is made up of concentric grooves or projections having a 
center at the point P'(-x, 0). A radius r.sub.i of an ith concentric 
groove or projection can be obtained from the following formula (6), where 
.lambda. denotes the wavelength of the light output from the light source. 
##EQU1## 
In addition, the cross sectional shape of the first part 75a is determined 
so that the ratios of the Oth order diffracted light and the +1st order 
diffracted light with respect to the total quantity of light become 
predetermined values. 
In the above sixteenth embodiment, the cross sectional shape of the 
hologram 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 75a and 75b of the hologram optical element 75 are set to 
the sawtooth shape shown in FIG. 36 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 hologram 
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 seventeenth embodiment of the optical information 
recording/reproducing apparatus according to the present invention, the 
hologram optical element 75 used has a cross sectional shape shown in FIG. 
39 along the line A-A' in FIG. 35. An essential part of this embodiment is 
essentially the same as FIG. 35, and an illustration thereof will be 
omitted. Contrary to the sixteenth embodiment, this embodiment positively 
uses the -1st order diffracted lights. For this reason, the first and 
second photodetectors 36a and 36b 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 36c for generating the tracking error signal TES based on 
the push-pull technique is provided at a position deviated from the image 
formation point 300c of the 0th order diffracted light, that is, between 
the hologram optical element 75 and the photodetector 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. 37, the +1st order diffracted 
light obtained from the first part 75a of the hologram optical element 75 
may overlap the -1st order diffracted light obtained from the second part 
75b, and the -1st order diffracted light obtained from the first part 75a 
may overlap the +1st order diffracted light obtained from the second part 
75b. For this reason, the hologram optical element 75 may be constructed 
so that by itself the hologram optical element 75 acts on the light as 
shown in FIG. 40. In FIG. 40, those parts which are the same as those 
corresponding parts in FIG. 37 are designated by the same reference 
numerals, and a description thereof will be omitted. 
In FIG. 40, the patterns of the first and second parts 75a and 75b are set 
so that the +1st order diffracted light 30-1 from the first part 75a 
converges to a point Q'(-x, y), the -1st order diffracted light from the 
first part 75a is projected in a semicircular shape about a point R'(x, 
-y), the +1st order diffracted light 30-2 from the second part 75b 
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 .pi. which is the distance f from 
the hologram optical element 75 along the optical axis. 
Next, a description will be given of an eighteenth embodiment of the 
optical information recording/reproducing apparatus according to the 
present invention, by referring to FIGS. 41 through 43. FIG. 41 shows the 
eighteenth embodiment, FIG. 42 shows a composite prism of the eighteenth 
embodiment, and FIG. 43 shows a perspective view of an essential part of 
the eighteenth embodiment. In FIG. 41, 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 embodiment, the spot of the light beam irradiated on the disk 207 
via the objective lens 206 has a diameter of approximately 1 .mu.m, for 
example. In addition, the outputs of the 2-part photodetector 211 are used 
to generate an address signal ADR via an adder 311A, and the outputs of 
the 2-part photodetector 211 are also used to reproduce the magneto-optic 
signal (information signal) RF via a differential amplifier 31lB. 
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 87a through 87d. These first through fourth light beams 87a 
through 87d are irradiated on a photodetector 86. The photodetector 86 
includes a first photodetector 86a which has 4 light receiving parts A 
through D for receiving the first and second light beams 87a and 87b, a 
second photodetector 86b which has a light receiving part E for receiving 
the third light beam 87c, and a third photodetector 86c which has a light 
receiving part F for receiving the fourth light beam 87d. As shown in FIG. 
43, the first, second and third photodetectors 86a, 86b and 86c are 
arranged on the same plane. The first through third photodetectors 86a 
through 86c may or may not be separated from each other within the 
photodetector 86. 
In FIG. 42, (a) shows a perspective view of the composite prism 85 on an 
enlarged scale, and (b) shows a plan view of the composite prism 85. As 
shown, the composite prism 85 includes a first emission surface 85a for 
emitting the first light beam 87a, a second emission surface 85b for 
emitting the second light beam 87b, and third and fourth emission surfaces 
85c and 85d for respectively emitting the third and fourth light beams 87c 
and 87d. In FIG. 42 (a), the first emission surface 85a has a downward 
inclination to the right, the second emission surface 85b has a downward 
inclination to the left, and the third and fourth emission surfaces 85c 
and 85d form a mountain shape. In other words, the third emission surface 
85c has a downward inclination to the right, the fourth emission surface 
85d has a downward inclination to the left, and the third and fourth 
emission surfaces 85c and 85d connect to form the mountain shape. 
The first emission surface 85a and the third emission surface 85c are 
inclined towards the same direction, and an inclination angle 
.alpha..sub.1 of the first emission surface 85a relative to a reference 
plane is smaller than an inclination angle .alpha..sub.3 of the third 
emission surface 85c. 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 85b and the fourth emission surface 85d 
are inclined towards the same direction, and an inclination angle 
.alpha..sub.2 of the second emission surface 85b is smaller than an 
inclination angle .alpha..sub.4 of the fourth emission surface 85d. 
In FIG. 43, the first light beam 87a which is emitted from the first 
emission surface 85a of the composite prism 85 is received by the light 
receiving parts A and D of the first photodetector 86a. In addition, the 
second light beam 87b which is emitted from the second emission surface 
85b of the composite prism 85 is received by the light receiving parts B 
and C of the first photodetector 86a. 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 87c which is emitted from the third 
emission surface 85c of the composite prism 85 is received by the light 
receiving part E of the second photodetector 86b, and the fourth light 
beam 87d which is emitted from the fourth emission surface 85d of the 
composite prism 85 is received by the light receiving part F of the third 
photodetector 86c. 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 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 87c by the light receiving part 
E of the photodetector 86b and receive the fourth light beam 87d by the 
light receiving part F of the photodetector 86c. 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 ninteenth embodiment of the optical 
information recording/reproducing apparatus according to the present 
invention, by referring to FIGS. 44 through 47. FIG. 44 shows the 
nineteenth embodiment, and in FIG. 44, those parts which are the same as 
those corresponding parts in FIG. 41 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. 41. In addition, the beam splitter 208, 
the Wollaston prism 209, the condenser lens 210 and the photodetector 211 
shown in FIG. 41 are not provided in FIG. 44. 
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 91a through 91f, and these 
first through sixth light beams 91a through 91f are irradiated on a 
photodetector 86A. 
The integral part 90 integrally comprises a Wollaston prism 92 shown in 
FIG. 45 and the composite prism 85 shown in FIG. 42. 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. 46 and 47. 
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 85e of the composite prism 85. 
In addition, the Wollaston prism 92 extends for the full width of the 
mounting part 85e. Hence, the Wollaston prism 92 splits the incoming 
reflected light beam in a direction in which a vertex 85f of the mountain 
part 85e extends. 
On the other hand, the photodetector 86A includes a first photodetector 
86Aa, a second photodetector 86Ab-1. a third photodetector 86Ab-2, a 
fourth photodetector 86Ac-1 and a fifth photodetector 86Ac-2 which are 
provided on a single plane as shown in FIG. 47. The first photodetector 
86Aa includes four light receiving parts A through D for receiving the 
first and second light beams 91a and 91b. The second photodetector 86Ab-1 
includes a light receiving part E.sub.1 for receiving the third light beam 
91c, and the third photodetector 86Ab-2 includes a light receiving part 
E.sub.2 for receiving the fourth light beam 91d. The fourth photodetector 
86Ac-1 includes a light receiving part F.sub.1 for receiving the fifth 
light beam 91e, and the fifth photodetector 86Ac-2 includes a light 
receiving part F.sub.2 for receiving the sixth light beam 91f. 
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. 46 and reaches the composite 
prism 85 directly is refracted by the first emission surface 85a and is 
emitted from the first emission surface 85a as the first light beam 91a. 
As shown in FIG. 47, this first light beam 91a is received by the light 
receiving parts A and D of the first photodetector 86Aa. 
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. 46 and 
reaches the composite prism 85 directly is refracted by the second 
emission surface 85b and is emitted from the second emission surface 85b 
as the second light beam 91b. As shown in FIG. 47, this second light beam 
91b is received by the light receiving parts B and C of the first 
photodetector 86Aa. 
A focal error signal FES is generated according to the Foucault technique 
based on the formula (2) described above, similarly to the eighteenth 
embodiment shown in FIG. 41. 
Out of the reflected light beam 89 which is input to the integral part 90 
via the condenser lens 12, a light component 89-3 which reaches the 
Wollaston prism 92 is split into a p-wave 95 and an s-wave 96. The p-wave 
95 is deflected by an angle D with respect to an extension line 97 of the 
light component 89-3 towards the first emission surface 85a. On the other 
hand, the s-wave 96 is deflected by an angle .beta. with respect to the 
extension line 97 towards the second emission surface 85b. 
The p-wave 95 and the s-wave 96 output from the Wollaston prism 92 is input 
to the composite prism 85. The angle D is small, and the p-wave.95 and the 
s-wave 96 propagate within the mountain part 85e of the composite prism 
85. The p-wave 95 and the s-wave 96 reach the third and fourth emission 
surfaces 85c and 85d and are refracted thereby, and are thereafter emitted 
from the third and fourth emission surfaces 85c and 85d. 
In other words, in FIG. 47 the p-wave 95 is emitted from the third emission 
surface 85c as the third light beam 91c. This third light beam 91c 
irradiates the light receiving part E.sub.1 of the second photodetector 
86Ab-1. On the other hand, the s-wave 96 is emitted from the third 
emission surface 85c as the fourth light beam 91d. This fourth light beam 
91d irradiates the light receiving part E.sub.2 of the third photodetector 
86Ab-2. 
Similarly in FIG. 47, p-wave 95 is emitted from the fourth emission surface 
85d as the fifth light beam 91e. This fifth light beam 91e irradiates the 
light receiving part F.sub.1 of the fourth photodetector 86Ac-1. On the 
other hand, the s-wave 96 is emitted from the fourth emission surface 85e 
as the sixth light beam 91f. This sixth light beam 91f irradiates the 
light receiving part F.sub.2 of the fifth photodetector 86Ac-2. 
A tracking error signal TES is obtained based on the outputs of the light 
receiving parts E.sub.1, E.sub.2, F.sub.1 and F.sub.2 of the second 
through fifth photodetectors 86Ab-1 through 86Ac-2. More particularly, the 
tracking error signal TES is obtained through adders 332 and 333 and the 
differential amplifier 331 shown in FIG. 47, by calculating TES=(E.sub.1 
+E.sub.2)-(F.sub.1 +F.sub.2). 
In addition a magneto-optic signal (information signal) RF is obtained 
based on the outputs of the light receiving parts E.sub.1, E.sub.2, 
F.sub.1 and F.sub.2 of the second through fifth photodetectors 86Ab-1 
through 86Ac-2. More particularly, the magneto-optic signal RF is obtained 
through adders 312 and 313, and the differential amplifier 311B shown in 
FIG. 47, by calculating RF=(E.sub.1 +F.sub.1)-(E.sub.2 +F.sub.2). 
Furthermore, an address signal ADR is obtained based on the outputs of the 
light receiving parts E.sub.1, E.sub.2, F.sub.1 and F.sub.2 of the second 
through fifth photodetectors 86Ab-1 through 86Ac-2. More particularly, the 
address signal ADR is obtained through the adder 332, the adder 333, and 
the adder 311A shown in FIG. 47, by calculating ADR=(E.sub.1 
+E.sub.2)+(F.sub.1 +F.sub.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 
RF and the address signal ADR by use of a single optical path of the 
reflected light beam 89 and a single photodetector 86A. 
By comparing FIG. 44 to FIG. 41, it may be seen that this nineteenth 
embodiment shown in FIG. 44 does not have the optical path which extends 
horizontally from the beam splitter 208 in FIG. 41. 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 eighteenth 
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 eighteenth embodiment. 
In addition, if the photodetector 88A 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 91c through 91f 
by the corresponding light receiving parts E.sub.1, E.sub.2, F.sub.1 and 
F.sub.2 of the second through fifth photodetectors 88Ab-1 through 88Ac-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 RF. 
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. 
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.