Optical head devices for use in recording and/or reproduction of information

An optical head device for use in recording and/or reproduction of information comprises a light beam generating portion, a lens element for causing a light beam obtained from the light beam generating portion to impinge upon a record medium and receiving the light beam from the recording medium, and a photodetecting portion to which the light beam from the record medium received by the lens element is guided and which has three photosensitive elements separated from each other by a couple of parallel dividing portions and disposed to form a common light receiving plane on which a beam spot is formed by the light beam guided to the photodetecting portion for detecting the light beam guided to the photodetecting portion to produce respective detection output signals from which a tracking error signal is produced, wherein each of the parallel dividing portions extends along a direction selected to be at an angle within an angle rqnge of 0 to 90 degrees to a direction of movement of a diffraction pattern appearing in the beam spot formed on the common light receiving plane in accordance with a record track in the record medium.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to optical head devices for use in 
recording and/or reproduction of information, and more particularly, to an 
optical head device which causes a light beam to impinge upon a record 
medium and guides the light beam from the record medium to a photodetector 
for reproducing information recorded on the record medium and obtaining a 
tracking error signal used for maintaining the light beam incident upon 
the record medium in correct tracking relation to a record track formed in 
the record medium. 
2. Description of the Prior Art 
In an optical disc player for reproducing information recorded on an 
optical record disc, an optical head device is provided within an optical 
arrangement to read an information signal from a record track formed in he 
optical record disc by the use of a laser light beam impinging upon the 
record track. The optical head device is required to produce the laser 
light beam, to cause the laser light beam to be focused correctly on the 
record track, which is usually very narrow in width, so as to trace 
faithfully the same, and to guide properly a reflected laser light beam 
obtained from the optical record disc to a photodetector. For fulfilling 
these requirements, the optical head device comprises a precise 
arrangement of various optical components including a semiconductor laser 
for generating a laser light beam, an objective lens facing the optical 
record disc to focus the laser light beam incident upon the optical record 
disc and to receive the reflected laser light beam from the optical record 
disc, other lenses, mirrors, prisms, photosensitive elements constituting 
the photodetector and so on. These components are supported to be movable 
in both directions along an optical axis of the objective lens and 
perpendicular to the optical axis of the objective lens in order to 
maintain focus control so that the laser light beam incident upon the 
optical record disc remains in correct focus relation to the record track 
and to maintain tracking control, so that the laser light beam incident 
upon the optical record disc remains in correct tracking relation to the 
record track. 
In the focus control performed in the optical head device, a focus error 
signal which varies in response to variations in the focus condition of 
the laser light beam incident upon the record track in the optical record 
disc is produced based on detection output signals obtained from the 
photodetector to which the reflected laser light beam from the optical 
disc is guided and the objective lens is moved along its optical axis in 
accordance with the focus error signal. Further, in the tracking control 
performed in the optical head device, a tracking error signal which varies 
in response to variations in the tracking condition of the laser light 
beam incident upon the record track in the optical record disc is produced 
based on the detection output signals obtained from the photodetector to 
which the reflected laser light beam from the optical disc is guided. The 
optical axis of the objective lens is centered onto the record track in 
accordance with the tracking error signal. 
For producing the focusing and tracking error signals, the photodetector 
provided in the optical head device comprises, for example, four 
photosensitive elements d.sub.1, d.sub.2, d.sub.3 and d.sub.4 disposed 
close to one another, as illustrated in FIG. 1. The photosensitive 
elements d.sub.1 to d.sub.4 are so arranged that the photosensitive 
elements d.sub.1 and d.sub.3 are aligned along a direction X corresponding 
to the tangential direction of the record track in the optical record disc 
and the photosensitive elements d.sub.2 and d.sub.4 are also aligned along 
the direction X. The reflected laser light beam coming through the 
objective lens from the record track in the optical record disc forms a 
beam spot Q on the photosensitive elements d.sub.1 to d.sub.4 as indicated 
by a dot-dash line in FIG. 1. The photosensitive elements d.sub.1 to 
d.sub.4 produce respective detection output signals S.sub.1, S.sub.2, 
S.sub.3 and S.sub.4 each dependent on a portion of the beam spot Q formed 
on each of the photosensitive elements d.sub.1 to d.sub.4. 
A reproduced information signal is produced by summing up the detection 
output signals S.sub.1 to S.sub.4, and the focus error signal is obtained 
based on a difference between an added signal obtained by adding the 
detection output signal S.sub.1 to the detection output signal S.sub.4 and 
an added signal obtained by adding the detection output signal S.sub.2 to 
the detection output signal S.sub.3, in the same manner as disclosed in, 
for example, U.S. Pat. No. 4,023,033 and U.S. Pat. No. 4,079,247. 
The tracking error signal will now be considered. The record track in the 
optical record disc is composed of a series of pits having a depth which 
is a quarter of the wavelength .lambda. of the laser light beam incident 
upon the record track. The laser light beam irradiating the record track 
is diffracted and reflected by the pits. Then, the laser light beam which 
is modulated by the record track is reflected back through the objective 
lens as the reflected laser light beam and reaches the photodetector to 
form the beam spot Q on the photosensitive elements d.sub.1 to d.sub.4. 
Accordingly, the reflected light beam produces a diffraction pattern 
corresponding to the positional relationship between one of the pits 
forming the record track and a beam spot formed by the laser light beam 
irradiating the record track. The diffraction pattern thus produced is 
moved in a direction corresponding to the direction X in relation to the 
photosensitive elements d.sub.1 to d.sub.4 at speed corresponding to the 
movement of the record track relative to the laser light beam irradiating 
the record track. 
FIGS. 2A, 2B and 2C illustrate the manner in which the foregoing process 
takes place. In each of FIGS. 2A to 2C, a designates positional 
relationships between a pit P and a beam spot L formed by the laser light 
beam irradiating the pit P, and b designates diffraction patterns (hatched 
portions) formed on an exit pupil surface of the objective lens by the 
reflected laser light beam in dependence on each of the positional 
relationships designated by a. Four quarters d.sub.1, d.sub.2, d.sub.3 and 
d.sub.4 shown at b indicate portions in which quantities of light are 
detected by the four photosensitive elements d.sub.1, d.sub.2, d.sub.3 and 
d.sub.4, respectively. The pit P moves from a position t.sub.1 to a 
position t.sub.2 in relation to the beam spot L. FIG. 2A shows a condition 
in which the beam spot L is displaced to the right in relation to the pit 
P. FIG. 2B illustrates a condition in which the beam spot L is positioned 
centrally on the pit P, namely, a correct tracking condition in which the 
laser light beam arrives properly at the record track in the optical 
record disc. FIG. 2C shows a condition in which the beam spot L is 
displaced to the left in relation to the pit P. 
It is understood from FIGS. 2A to 2C that when the beam spot L is centrally 
positioned on the pit P, that is, when the laser light beam incident upon 
the record track is in the proper tracking condition, a diffraction 
pattern in which quantities of light are symmetrically distributed among 
another quarters d.sub.1 and d.sub.2 and among another quarters d.sub.3 
and d.sub.4, is produced, and when the beam spot L is displaced rightward 
or leftward in relation to the pit P, a diffraction pattern is produced in 
which the distribution of the quantities of light among the quarters 
d.sub.1 and d.sub.2 and among the quarters d.sub.3 and d.sub.4, is 
asymmetric, with the light distribution out of symmetry in reversed 
patterns in the case where the beam pattern is disposed to the right and 
in the case where the beam pattern is disposed to the left, respectively. 
Based on the foregoing, the detection output signals S.sub.1 to S.sub.4 
obtained from the photosensitive elements d.sub.1 to d.sub.4, 
respectively, may be supplied to and processed by a signal generating 
circuit for producing a signal which varies in dependence on the 
positional relationships between the beam spot L and the pit P, namely, 
the tracking error signal. 
FIG. 3 shows one example of the signal generating circuit by which the 
tracking error signal is produced based on the detection output signals 
S.sub.1 to S.sub.4 obtained from the photosensitive elements d.sub.1 to 
d.sub.4, together with the reproduced output of an information signal 
recorded on the record medium, that is, a reproduced information signal. 
In the circuit shown in FIG. 3, the detection output signals S.sub.1 and 
S.sub.4 obtained from the photosensitive elements d.sub.1 and d.sub.4, 
respectively are added to each other by an adder 11, and the detection 
output signals S.sub.2 and S.sub.3 obtained from the photosensitive 
elements d.sub.2 and d.sub.3, respectively, are added to each other by an 
adder 12. Output signals from the adders 11 and 12 are supplied to an 
adder 13 to produce an output signal S.sub.5 and supplied also to a 
subtracter 14 to produce an output signal S.sub.6. The output signal 
S.sub.5 obtained from the adder 13 serves as the reproduced information 
signal, and the output signal S.sub.6 obtained from the subtracter 14 
comprises a signal residing in a frequency band of the information signal 
recorded on the record track in the optical record disc and varying 
whenever the beam spot L on the optical record disc passes through the pit 
P and serves as the tracking error signal which indicates deviations of 
the beam spot L from the center of the record track formed by the series 
of pits. 
Further, in the circuit shown in FIG. 3, the output signal S.sub.5 obtained 
from the adder 13 is supplied to pulse generators 15 and 16. The pulse 
generator 15 produces a pulse signal S.sub.7 in response to each rising 
edge of the output signal S.sub.5 and the pulse generator 16 produces a 
pulse signal S.sub.8 in response to each falling edge of the output signal 
S.sub.5. The output signal S.sub.6 obtained from the subtracter 14 is 
supplied to sampling-hold circuits 17 and 18. In the sampling-hold circuit 
17, the level of the output signal S.sub.6 is sampled by the pulse signal 
S.sub.8 and each level sampled by pulse signal S.sub.8 is held to produce 
an output signal S.sub.9. Similarly, in the sampling-hold circuit 18, the 
level of the output signal S.sub.6 is sampled by the pulse signal S.sub.7 
and each level sampled by pulse signal S.sub.7 is held to produce an 
output signal S.sub.10. Each of the output signals S.sub.9 and S.sub.10 
obtained from the sampling-hold circuits 17 and 18, respectively, has a 
polarity varying from negative to positive or vice versa when the beam 
spot L is moved to traverse the record track and a level representative of 
the deviations of the beam spot L from the center of the record track. 
Consequently, the output signals S.sub.9 and S.sub.10 can be employed as 
tracking control signals. These output signals S.sub.9 and S.sub.10 are 
supplied to a differential circuit 19 to produce a tracking control signal 
S.sub.11 which is more reliable and is then delivered to an output 
terminal 20. 
In the previously proposed optical head device as described above, wherein 
the tracking error signal is produced in such a manner as aforementioned, 
four photosensitive elements constituting the photodetector are required 
to be precisely arranged with respect to each other and with respect to 
the record track in the optical record disc. This, results in complicated 
configurations of the photodetector itself and of devices for positioning 
the photodetector. Further, a bidirectional optical arrangement is 
required to control movement of the reflected laser light beam in each of 
two directions perpendicular to each other and to thereby accurately 
position the reflected beam onto the photodetector. 
Especially, in the case where the previously proposed optical head device 
is equipped with an integrated light beam generating and detecting unit 
which comprises a semiconductor substrate on which a photodetector, a 
semiconductor laser, and a prism for directing a laser light beam 
generated by the semiconductor laser to impinge upon an optical record 
disc and for guiding a reflected laser light beam obtained from the 
optical record disc to the photodetector are provided, as disclosed in the 
Japanese patent application published before examination under publication 
number 62-197931, the disadvantage resulting from the photodetector 
composed of four photosensitive elements which are provided for producing 
the tracking error signal as described above is more serious. 
OBJECTS AND SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide an optical 
head device for use in recording and/or reproduction of information, by 
which a light beam is caused to impinge upon a record medium and the light 
beam from the record medium is guided to a photodetecting portion so that 
detection output signals used for producing at least a reproduced signal 
of information recorded on the record medium and a tracking error signal 
are obtained from the photodetecting portion, and which avoids the 
aforementioned disadvantages and problems encountered with the prior art. 
Another object of the present invention is to provide an optical head 
device for use in recording and/or reproduction of information, by which a 
light beam is caused to impinge upon a record medium and the light beam 
reflected from the record medium is guided to a photodetecting portion so 
that detection output signals used for producing at least a reproduced 
signal of information recorded on the record medium and a tracking error 
signal are obtained from the photodetecting portion, and which can further 
supply the detection output signals of the photodetecting portion from 
which a reliable tracking error signal is obtained under a condition 
wherein the photodetecting portion is simplified in configuration and the 
position on the photodetecting portion upon which the light beam from the 
record medium is incident need not be controlled in two directions 
perpendicular to each other. 
A further object of the present invention is to provide an optical head 
device for use in recording and/or reproduction of information, by which a 
light beam reflected is caused to impinge upon a record medium and the 
light beam from the record medium is guided to a photodetecting portion so 
that detection output signals used for producing at least a reproduced 
signal of information recorded on the record medium and a tracking error 
signal are obtained from the photodetecting portion, and which can further 
supply the detection output signals of the photodetecting portion from 
which a reliable tracking error signal is obtained under a condition 
wherein the photodetecting portion is simplified in configuration and an 
exact bidirectional control for a position on the photodetecting portion 
upon which the light beam from the record medium is incident is not 
required even in the case where an integrated light beam generating and 
detecting unit is employed in the device. 
According to the present invention, there is provided an optical head 
device for use in recording and/or reproduction of information, which 
comprises a light beam generating portion, a lens element for causing a 
light beam obtained from the light beam generating portion to impinge upon 
a record medium and for receiving the light beam from the recording 
medium, and a photodetecting portion to which the light beam from the 
record medium received by the lens element is guided and which has a group 
of photosensitive elements including a central and two side photosensitive 
elements separated from each other by a couple of parallel dividing 
portions each extending along a predetermined direction and disposed to 
form a common light receiving plane on which a beam spot is formed by the 
light beam guided to the photodetecting portion for detecting the light 
beam guided to the photodetecting portion to produce respective detection 
output signals which are of use in such a manner that the tracking error 
signal is produced based on the detection output signal from the central 
photosensitive element and an added signal obtained by adding the 
detection output signals from the side photosensitive elements to each 
other, wherein, in the photodetecting portion, the predetermined direction 
along which each of the parallel dividing portions extends is selected to 
be at an angle within an angle range of 0 to 90 degrees with respect to a 
direction of movement of a diffraction pattern appearing in the beam spot 
formed on the common light receiving plane in accordance with a record 
track in the record medium. 
In one embodiment, the photodetecting portion has first and second groups 
of photosensitive elements each arranged in the same manner as the group 
described above, and the light beam guided to the photodetecting portion 
is caused to be incident upon each of the first and second groups 
successively with a focusing point thereof on an optical path formed 
between the first and second groups. 
In the optical head device of there is provided present invention, since 
the group of photosensitive elements including three photosensitive 
elements separated from each other by two parallel dividing portions each 
extending the predetermined direction in the photodetecting portion, where 
the predetermined direction along which each of the parallel dividing 
portions extends is arranged to be at the angle within the angle range of 
0 to 90 degrees with respect to the direction of movement of the 
diffraction pattern appearing in the beam spot formed on the group of 
photosensitive elements by the light beam from the record medium in 
accordance with the record track in the record medium, the diffraction 
pattern in the beam spot formed on the group of photosensitive elements 
varies in accordance with the positional relationship between the record 
track in the record medium and a beam spot formed on the record disc by 
the light beam impinging thereon in substantially the same manner as that 
shown in FIGS. 2A to 2C and described above. 
Then, each of the central photosensitive elements and two side 
photosensitive elements produces the detection output signal in response 
to the portion of the light beam from the record medium received thereby 
and the tracking error signal is produced based on the detection output 
signal from the central photosensitive element and the added signal 
obtained by adding the detection output signals from the side 
photosensitive elements to each other. 
In the optical head device according to the present invention thus 
arranged, the predetermined direction along which each of the parallel 
dividing portions extends in the photodetecting portion is not required to 
be arranged so exactly and a position on the group of photosensitive 
elements upon which the light beam guided to the photodetecting portion is 
incident is also not required to be controlled so exactly in the 
predetermined direction. Consequently, with the optical head device 
according to the present invention, a reliable tracking error signal is 
obtained under a condition wherein the photodetecting portion is 
simplified in configuration and an exact bidirectional control for a 
position on the photodetecting portion upon which the light beam from the 
record medium is incident is not required even in the case where an 
integrated light beam generating and detecting unit is employed in the 
optical head device. 
Further, in the embodiment which is provided with the photodetecting 
portion having the first and second groups of photosensitive elements, the 
detection output signals from which a focus error signal is also produced, 
in addition to the reproduced signal of information recorded on the record 
medium and the tracking error signal, are obtained from the photodetecting 
portion. 
The above, and other objects, features and advantages of the present 
invention will become apparent from the following detailed description 
which is to be read in conjunction with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIGS. 4 to 7 show one embodiment of optical head device for use in 
recording and/or reproduction of information according to the present 
invention. This embodiment shown in FIGS. 4 to 7 is, in actual use, 
incorporated in a disc player. 
Referring to FIG. 4, the embodiment has a case 30 provided with an opening 
30a to which an objective lens 31 contained in the case 30 faces. On the 
occasion of actual use in the disc player, the embodiment is mounted on 
the upper surface of a movable base member 34 which is engaged to be 
movable with guide members 32 and 33 provided in the disc player. The 
movable base member 34 is driven by a driving mechanism, which is provided 
in the disc player with a pinion engaging with a rack portion 34a formed 
on the movable base member 34 to be moved in a direction along the radius 
of an optical record disc mounted on the disc player under the guidance by 
the guide members 32 and 33. 
The case 30 of the embodiment, contains a movable optical assembly 
including the objective lens 31 and a supporting and driving assembly 
operative to support the movable optical assembly and to move the same in 
predetermined directions. 
As shown in FIG. 5, the movable optical assembly has a supporting member 36 
on which the objective lens 31 and an integrated element mounting board 35 
are mounted. The integrated element mounting board 35 is provided with a 
flexible connecting film 35a extending therefrom. 
In the supporting member 36, a package 37 which contains a light beam 
generating and detecting unit 38 is mounted on the inner surface of the 
integrated element mounting board 35. Further, a mirror 39 is held by the 
supporting member 36 to be disposed under the light beam generating and 
detecting unit 38, and another mirror 40 is also held by the supporting 
member 36 under the objective lens 31 to be disposed on the axis of the 
objective lens 31. 
The movable optical assembly is disposed so as to cause the objective lens 
31 mounted on the supporting member 36 to face the under surface of an 
optical record disc R. In the movable optical assembly thus disposed, the 
light beam generating and detecting unit 38 contained in the package 37 is 
operative to generate a laser light beam directed toward the mirror 39 
which is positioned under the light beam generating and detecting unit 38. 
The laser light beam from the light beam generating and detecting unit 38 
is reflected by the mirror 39 and then reflected further by the mirror 40 
to be directed upward to enter the objective lens 31. The objective lens 
31 is operative to focus the laser light beam from the mirror 40 on a 
record track Rt formed in the optical record disc R. Then, a reflected 
laser light beam obtained from the optical record disc R returns through 
the objective lens 31 to the mirror 40 to be reflected thereby. The 
reflected laser light beam reflected by the mirror 40 is further reflected 
by the mirror 39 to be directed upward to the light beam generating and 
detecting unit 38 contained in the package 37. 
As shown in FIG. 6 in which the light beam generating and detecting unit 38 
contained in the package 37 is enlarged, the package 37 mounted on the 
inner surface of the integrated element mounting board 35 comprises a body 
41 in which the light beam generating and detecting unit 38 is disposed 
and a glass plate member 42 attached to the body 41. The laser light beam 
generated by the light beam generating and detecting unit 38 passes 
through the glass plate member 42 to the mirror 39 and the reflected laser 
light beam reflected by the mirror 39 passes through the glass plate 
member 42 to the light beam generating and detecting unit 38. 
In the light beam generating and detecting unit 38, as shown in FIG. 6, 
first and second photodetectors 51 and 52 are formed to align in a 
semiconductor substrate 50 so as to constitute a photodetecting portion. A 
semiconductor laser 53 is also provided on the surface of the 
semiconductor substrate 50. Further, a protective coating layer 54 is 
formed to cover the surface of the semiconductor substrate 50 except a 
portion thereof on which the semiconductor laser 53 is placed and a prism 
55 is fixed on a portion of the protective coating layer 54 covering the 
photodetecting portion on the semiconductor substrate 50 whereon the first 
and second photodetectors 51 and 52 are formed. With such optical 
elements, the light beam generating and detecting unit 38 is integrated. 
The prism 55 has a semi-transparent surface 55a which faces the 
semiconductor laser 53 and is inclined in relation to the surface of the 
semiconductor substrate 50 on which the semiconductor laser 53 is placed. 
In the light beam generating and detecting unit 38 thus constituted, the 
laser light beam generated by the semiconductor laser 53 is reflected by 
the semi-transparent surface 55a of the prism 55 to pass through the glass 
plate member 42 toward the mirror 39. Then, the laser light beam from the 
semi-transparent surface 55a of the prism 55 is reflected twice by the 
mirrors 39 and 40 and caused to impinge upon the record track Rt in the 
optical record disc R through the objective lens 31. The reflected laser 
light beam obtained from the record track Rt in the optical record disc R 
returns through the objective lens 31 and is reflected twice by the 
mirrors 40 and 39. Then, the reflected laser light beam from the mirror 39 
passes through the semi-transparent surface 55a of the prism 55 to enter 
the prism 55. In the prism 55, a part of the reflected laser light beam 
reaches to the first photodetector 51 and the rest of the reflected laser 
light beam is reflected to reach to the second photodetector 52. The 
reflected laser light beam is arranged to have a focusing point on an 
optical path formed between the first and second photodetectors 51 and 52 
in the prism 55. 
As shown in FIG. 7, each of the first and second photodetectors 51 and 52 
formed in the semiconductor substrate 50 comprises a group of 
photosensitive elements including a central photosensitive element and two 
side photosensitive elements facing each other with the central 
photosensitive element positioned therebetween. In more detail, the first 
photodetector 51 comprises three rectangular photosensitive elements 
D.sub.1, D.sub.2 and D.sub.3 which are separated from each other by a 
couple of parallel dividing portions E.sub.1 and E.sub.2 so that the 
photosensitive element D.sub.2 is positioned at the center and the 
photosensitive elements D.sub.1 and D.sub.3 are positioned at both sides 
of the photosensitive element D.sub.2 and disposed to form a common light 
receiving plane, and similarly, the second photodetector 52 comprises 
three rectangular photosensitive elements D.sub.1 ', D.sub.2 ' and D.sub.3 
' which are separated from each other by a couple of parallel dividing 
portions E.sub.1 ' and E.sub.2 ' so that the photosensitive element 
D.sub.2 ' is positioned at the center and the photosensitive elements 
D.sub.1 ' and D.sub.3 ' are positioned at both sides of the photosensitive 
element D.sub.2 ' and disposed to form a common light receiving plane. 
The semiconductor substrate 50, in which the first and second 
photodetectors 51 and 52 are formed in the above mentioned manner and on 
which the semiconductor laser 53 and the prism 55 are provided, is fixed 
to the inner surface of the body 41 of the package 37 in such a manner 
that a direction along which each of the parallel dividing portions 
E.sub.1 and E.sub.2 in the first photodetector 51 and the parallel 
dividing portions E.sub.1 ' and E.sub.2 ' in the second photodetector 52 
extends is set to be at an angle .theta. within an angle range of 0 to 90 
degrees, for example, 45 degrees to a direction corresponding to the 
tangential direction of the record track Rt in the optical record disc R, 
as indicated with arrows X in FIG. 7. Accordingly, each of the 
photosensitive elements D.sub.1 to D.sub.3 constituting the first 
photodetector 51 and the photosensitive elements D.sub.1 ' to D.sub.3 ' 
constituting the second photodetector 52 also extends to be at the angle 
.theta. to the direction corresponding to the tangential direction of the 
record track Rt in the optical record disc R indicated with the arrows X. 
The part of the reflected laser light beam which comes from the record 
track Rt in the optical record disc R through the objective lens 31 to 
enter the prism 55 reaches the common light receiving plane formed by the 
photosensitive elements D.sub.1 to D.sub.3 constituting the first 
photodetector 51 and forms a beam spot U thereon as shown in FIG. 7 and 
another part of the reflected laser light beam reaches the common light 
receiving plane formed by the to photosensitive elements D.sub.1 ' to 
D.sub.3 ' constituting the second photodetector 52 and forms a beam spot 
U' thereon as shown in FIG. 7. The photosensitive elements D.sub.1 to 
D.sub.3 produce respective detection output signals V.sub.1, V.sub.2 and 
V.sub.3 in response to portions of the beam spot U irradiating the 
photosensitive elements D.sub.1 to D.sub.3, respectively, and the 
photosensitive elements D.sub.1 ' to D.sub.3 ' produce respective 
detection output signals V.sub.1 ', V.sub.2 ' and V.sub.3 ' in response to 
portions of the beam spot U' irradiating the photosensitive elements 
D.sub.1 ' to D.sub.3 ', respectively. These detection output signals 
V.sub.1 to V.sub.3 and V.sub.1 ' to V.sub.3 ' are derived through the 
integrated element mounting board 35 and the flexible connecting film 35a 
from the light beam generating and detecting unit 38. 
The record track Rt in the optical record disc R is also composed of a 
series of pits having a depth which is a quarter of the wavelength of the 
laser light beam incident upon the record track Rt, and therefore the 
laser light beam impinging upon the record track Rt through the objective 
lens 31 is diffracted and reflected by the pits. Accordingly, the 
reflected light beam which returns through the objective lens 31 from the 
record track Rt in the optical record disc R and is guided to the first 
and second photodetectors 51 and 52 so as to form the beam spots U and U' 
on the photosensitive elements D.sub.1 to D.sub.3 and the photosensitive 
elements D.sub.1 ' to D.sub.3 ', respectively, produces a diffraction 
pattern in each of the beam spots U and U', which corresponds to the 
positional relationship between one of the pits forming the record track 
Rt and a beam spot formed by the laser light beam irradiating the record 
track Rt. The diffraction pattern thus produced in each of the beam spots 
U and U' is moved in the direction corresponding to the tangential 
direction of the record track Rt in the optical record disc R, as 
indicated with arrows X in FIG. 7, in relation to each of the first and 
second photodetectors 51 and 52 at speed corresponding to the movement of 
the record track Rt relative to the laser light beam irradiating the 
record track Rt. 
FIGS. 8A, 8B and 8C illustrate the manner in which the movement of the 
diffraction pattern mentioned above takes place. In each of FIGS. 8A to 
8C, a designates positional relationships between a pit P and a beam spot 
L formed by the laser light beam irradiating the pit P, and b designates 
diffraction patterns (hatched portions) formed on an exit pupil surface of 
the objective lens 31 by the reflected laser light beam in dependence on 
each of the positional relationships designated by a. Three divided areas 
D.sub.1 (D.sub.1 '), D.sub.2 (D.sub.2 ') and D.sub.3 (D.sub.3 ') shown at 
b indicate portions in which quantities of light are detected by the 
photosensitive elements D.sub.1 to D.sub.3 constituting the first 
photodetector 51 and portions in which quantities of light are detected by 
the photosensitive elements D.sub.1 ' to D.sub.3 ' constituting the second 
photodetector 52. The pit P moves from a position T.sub.1 to a position 
T.sub.2 in relation to the beam spot L. FIG. 8A shows a condition in which 
the beam spot L is displaced to the right in relation to the pit P. FIG. 
8B illustrates a condition in which the beam spot L is positioned 
centrally on the pit P, namely, a correct tracking condition in which the 
laser light beam arrives properly at the record track Rt in the optical 
record disc R. FIG. 8C shows a condition in which the beam spot L is 
displaced to the left in relation to the pit P. 
It is understood from FIGS. 8A to 8C that when the beam spot L is centrally 
positioned on the pit P, that is, when the laser light beam incident upon 
the record track Rt is in the proper tracking condition, a diffraction 
pattern in which quantities of light are symmetrically distributed among 
the divided area D.sub.2 (D.sub.2 ') and a combination of the divided 
areas D.sub.1 (D.sub.1 ') and D.sub.3 (D.sub.3 ') is produced, and when 
the beam spot L is displaced rightward or leftward in relation to the pit 
P, a diffraction pattern in which no symmetric distribution of the 
quantities of light is obtained among the divided area D.sub.2 (D.sub.2 ') 
and the combination of the divided areas D.sub.1 (D.sub.1 ') and D.sub.3 
(D.sub.3 ') is produced with the light distribution out of symmetry in 
reversed patterns in the case where the beam pattern is disposed to the 
right and in the case where the beam pattern is disposed to the left, 
respectively. Based on the foregoing, it is further understood that, by 
applying the detection output signals V.sub.1, V.sub.2 and V.sub.3 
obtained from the photosensitive elements D.sub.1, D.sub.2 and D.sub.3, 
respectively, and the detection output signals V.sub.1 ', V.sub.2 ' and 
V.sub.3 ' obtained from the photosensitive elements D.sub.1 ', D.sub.2 ' 
and D.sub.3 ', respectively, to a signal generating circuit in which the 
detection output signal V.sub.2 and a combination of the detection output 
signals V.sub.1 and V.sub.3 are processed separately and the detection 
output signal V.sub.2 ' and a combination of the detection output signals 
and V.sub.3 ' are also processed separately, a signal which varies in 
dependence on the positional relationships between the beam spot L and the 
pit P, namely, the tracking error signal can be obtained from the signal 
generating circuit. 
FIG. 9 shows one example of the signal generating circuit by which the 
tracking error signal is produced based on the detection output signals 
V.sub.1 to V.sub.3 obtained from the photosensitive elements D.sub.1 to 
D.sub.3, respectively, and the detection output signals V.sub.1 ' to 
V.sub.3 ' obtained from the photosensitive elements D.sub.1 ' to D.sub.3 
', respectively, together with a reproduced output of an information 
signal recorded on the record track Rt in the optical record disc R, that 
is, a reproduced information signal and a focusing error signal. 
The circuit shown in FIG. 9 is connected with the photosensitive elements 
D.sub.1 to D.sub.3 constituting the first photodetector 51, each of which 
extends tp be at the angle .theta. to the direction of movement of the 
diffraction pattern corresponding to the positional relationship between 
the pit P forming the record track Rt in the optical record disc R and the 
beam spot L formed by the laser light beam irradiating the record track 
Rt, as indicated by an arrow X close to the first photodetector 51, and 
also with the photosensitive elements D.sub.1 ' to D.sub.3 ' constituting 
the second photodetector 52, each of which extends to be at the angle 
.theta. to the direction of movement of the diffraction pattern, as 
indicated by an arrow X close to the second photodetector 52. 
In the circuit shown in FIG. 9, the detection output signals V.sub.1 and 
V.sub.3 obtained from the photosensitive elements D.sub.1 and D.sub.3, 
respectively, are added to each other by an adder 61, the detection output 
signals V.sub.1 ' and V.sub.3 ' obtained from the photosensitive elements 
D.sub.1 ' and D.sub.3 ', respectively, are added to each other by an adder 
62, and the detection output signal V.sub.2 obtained from the 
photosensitive elements D.sub.2 and the detection output signals V.sub.2 ' 
obtained from the photosensitive elements D.sub.2 ' are added to each 
other by an adder 63. Further, an output signal V.sub.4 obtained from the 
adder 61 and an output signal V.sub.4 ' obtained from the adder 62 are 
added to each other by an adder 64. 
Then, an output signal V.sub.5 obtained from the adder 63 and an output 
signal V.sub.6 obtained from the adder 64 are supplied to an adder 65 to 
produce an output signal V.sub.7 and supplied also to a subtracter 66 to 
produce an output signal V.sub.8. The output signal V.sub.7 obtained from 
the adder 65 corresponds to the sum of the detection output signals 
V.sub.1 to V.sub.3 and V.sub.1 ' to V.sub.3 ' and therefore serves as the 
reproduced information signal. The output signal V.sub.8 obtained from the 
subtracter 66 comprises a signal residing in a frequency band of the 
information signal recorded on the record track Rt in the optical record 
disc R and varying whenever the beam spot L on the optical record disc R 
passes through the pit P and serves as the tracking error signal which 
indicates deviations of the beam spot L on the optical record disc R from 
the center of the record track Rt formed by the series of pits P. 
The detection output signal V.sub.2 and the output signal V.sub.4 obtained 
from the adder 61 are supplied to a subtracter 73, and the detection 
output signal V.sub.2 ' and the output signal V.sub.4 ' obtained from the 
adder 62 are supplied to a subtracter 74. Then, output signals obtained 
from the subtracters 73 and 74, respectively, are supplied to a subtracter 
75 to produce an output signal V.sub.14. This output signal V.sub.14 
varies in response to the focusing condition of the laser light beam 
incident upon the record track Rt in the optical record disc R and is 
delivered to an output terminal 76 as the focus error signal. 
Further, in the circuit shown in FIG. 9, the output signal V.sub.7 obtained 
from the adder 65 is supplied to pulse generators 67 and 68. The pulse 
generator 67 produces a pulse signal V.sub.9 in response to each rising 
edge of the output signal V.sub.7 and the pulse generator 68 produces a 
pulse signal V.sub.10 in response to each falling edge of the output 
signal V.sub.7. The output signal V.sub.8 obtained from the subtracter 66 
is supplied to both of sampling-hold circuits 69 and 70. In the 
sampling-hold circuit 69, the level of the output signal V.sub.8 is 
sampled by the pulse signal V.sub.10 and each level sampled by pulse 
signal V.sub.10 is held to produce an output signal V.sub.11. Similarly, 
in the sampling-hold circuit 70, the level of the output signal V.sub.8 is 
sampled by the pulse signal V.sub.9 and each level sampled by pulse signal 
V.sub.9 is held to produce an output signal V.sub.12. Each of the output 
signals V.sub.11 and V.sub.12 obtained from the sampling-hold circuits 69 
and 70, respectively, has a polarity varying from negative to positive or 
vice versa when the beam spot L formed on the optical record disc R is 
moved to traverse the record track Rt and a level representative of the 
deviations of the beam spot L from the center of the record track Rt. 
Consequently, the output signals V.sub.11 and V.sub.12 can be employed as 
tracking control signals. These output signals V.sub.11 and V.sub.12 are 
supplied to a differential circuit 71 to produce a tracking control signal 
V.sub.13 which is more reliable and is then delivered to an output 
terminal 72. 
Although, in the embodiment described above, both the detection output 
signal V.sub.2 from the photosensitive element D.sub.2 and the combined 
signal obtained by adding the detection output signals V.sub.1 and V.sub.3 
from the photosensitive elements D.sub.1 and D.sub.3, which are derived 
from the first photodetector 51, and the detection output signal V.sub.2 ' 
from the photosensitive element D.sub.2 ' and the combined signal obtained 
by adding the detection output signals V.sub.1 ' and V.sub.3 ' from the 
photosensitive elements D.sub.1 ' and D.sub.3 ', which are derived from 
the second photodetector 52, are used for producing the output signal 
V.sub.8 which serves as the tracking error signal, it is possible to use 
either the detection output signal V.sub.2 and the combined signal 
obtained by adding the detection output signals V.sub.1 and V.sub.3, which 
are derived from the first photodetector 51, or the detection output 
signal V.sub.2 ' and the combined signal obtained by adding the detection 
output signals V.sub.1 ' and V.sub.3 ', which are derived from the second 
photodetector 52, for producing the tracking error signal.