Optical pickup and tilt control device including a plurality of photosensitive units for generating signals indicating whether tilt correction is necessary

An optical pickup for reading information recorded on a recording surface of an optical information storage medium has a light beam source for emitting a linear light beam, and an optical system including an objective lens for converging the linear light beam as a linear image on the recording surface and collecting and emitting a light beam reflected from the recording surface. A tilt control device includes a parallel flat glass plate for refracting the reflected light beam from the optical system, a plurality of photodetector units each having a plurality of photosensitive surfaces for photoelectrically converting the light beam applied thereto from the parallel flat glass plate into a plurality of respective detected signals, an error generator for processing the detected signals from the photodetector units into a tilt error signal indicative of whether the linear light beam is applied perpendicularly to the recording surface, and an actuator responsive to the tilt error signal for controlling the objective lens positionally with respect to the optical information storage medium to apply the linear light beam perpendicularly to the recording surface.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an optical pickup for reading recorded 
information from an optical information storage medium such as a compact 
disc, a laser video disc, or the like, and a tilt control device for use 
with such an optical pickup. 
2. Description of the Prior Art 
Some optical pickups for reading recorded information from an optical 
information storage medium such as a compact disc, a laser video disc, or 
the like have a tilt detecting device for detecting whether a light beam 
emitted from a light source is applied perpendicularly to the recording 
surface of the optical information storage medium. 
One tilt detecting device for use with an optical pickup is shown in FIGS. 
1(A) and 1(B) of the accompanying drawings. As shown in FIGS. 1(A) and 
1(B), the tilt detecting device, generally designated by the reference 
numeral 200, has a semiconductor laser 31 for emitting a laser beam toward 
an optical disc DK, two photodetectors 32, 33 for detecting and 
photoelectrically converting a light beam reflected from a recording 
surface S.sub.2 of the optical disc DK into detected electric signals, and 
a differential amplifier 34 for calculating the difference between the 
detected signals from the photodetectors 32, 33 to produce a tilt error 
signal TE indicative of whether the laser beam is applied perpendicularly 
to the recording surface S.sub.2 of the optical disc DK. 
If the semiconductor laser 31 and the photodetectors 32, 33 face properly 
to the optical disc DK, as shown in FIG. 1(A), then the two photodetectors 
32, 33 detect equal intensities of the reflected light beam, and hence the 
differential amplifier 34 produces output signal of zero. If the optical 
disc DK is tilted with respect to the semiconductor laser 31 and the 
photodetectors 32, 33 as shown in FIG. l(B), the photodetector 32 detects 
a higher intensity of the reflected light beam than the photodetector 33, 
so that the differential amplifier 34 produces a positive output signal, 
thus detecting the tilt of the optical disc DK. A suitable control unit 
including an actuator may be connected to the tilt detecting device 100, 
thus making up a tilt control device for correcting the position of the 
optical pickup so that the laser beam will be applied perpendicularly to 
the recording surface S.sub.2 of the optical disc DK. 
The tilt control device is disadvantageous in that its semiconductor laser 
31 and the photodetectors 32, 33 are required in addition to the optical 
pickup which is used to reproduce an information signal recorded on the 
optical disc DK. Therefore, the entire assembly of the optical pickup and 
the tilt control device has a complex structure and a large size. 
SUMMARY OF THE INVENTION 
It is an object, of the present invention to provide an optical pickup 
which is relatively simple in structure. 
Another object of the present invention is to provide a tilt control device 
of a relatively simple structure for an optical pickup. 
According to the present invention, there is provided an optical pickup for 
reading information recorded on a recording surface of an optical 
information storage medium, including a light beam source for emitting a 
linear light beam, an optical system for converging the linear light beam 
as a linear image on the recording surface and collecting and emitting a 
light beam reflected from the recording surface, refracting unit for 
refracting the reflected light beam emitted from the optical system, and a 
plurality of photodetector units each having a plurality of photosensitive 
surfaces for photoelectrically converting the light beam applied thereto 
from the refracting unit into a plurality of respective detected signals 
indicative of a tilt of the optical system with respect to the optical 
information storage medium. 
According to the present invention, there is also provided a tilt control 
device for use in an optical pickup for reading information recorded on a 
recording surface of an optical information storage medium, including a 
light beam source for emitting a linear light beam, an optical system 
including an objective lens for converging the linear light beam as a 
linear image on the recording surface and collecting and emitting a light 
beam reflected from the recording surface, refracting unit for refracting 
the reflected light beam emitted from the optical system, a plurality of 
photodetector units each having a plurality of photosensitive surfaces for 
photoelectrically converting the light beam applied thereto from the 
refracting unit into a plurality of respective detected signals, 
processing unit for processing the detected signals from the photodetector 
units into a tilt error signal indicative of whether the linear light beam 
is applied perpendicularly to the recording surface, and control unit 
responsive to the tilt error signal for controlling the objective lens 
positionally with respect to the optical information storage medium to 
apply the linear light beam perpendicularly to the recording surface. 
The reflected light beam from the optical system is refracted by the 
refracting unit and then applied to the photosensitive surfaces of the 
photodetector units. The detected signals from the photodetector units are 
processed by the processing unit into the tilt error signal indicative of 
whether each of the photodetector units is too close to or far from the 
recording surface of the optical information storage medium. The 
photosensitive surfaces of the photodetector units can produce the 
detected signals using a portion of the reflected light beam applied 
thereto. The photodetector units may also be used to detect a focus error 
signal. Therefore, the optical pickup may be relatively simple in 
structure. 
The above and other objects, features, and advantages of the present 
invention will become apparent from the following description when taken 
in conjunction with the accompanying drawings which illustrate preferred 
embodiments of the present invention by way of example.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
1st Embodiment 
FIGS. 2 through 8(A), 8(B), 8(C) show a tilt control device for use with an 
optical pickup, according to a first embodiment of the present invention. 
As shown in FIG. 2, the tilt control device, generally designated by the 
reference numeral 100, has a semiconductor laser I for emitting a linear 
laser beam having a predetermined length across its optical axis, a beam 
splitter 2 having a reflecting surface S, for reflecting the laser beam 
from the semiconductor laser 1 toward an optical disc DK as an optical 
information storage medium, a collimator lens 3 for converting the laser 
beam from the beam splitter 2 into a parallel laser beam, and an objective 
lens 4 for converging the parallel laser beam from the collimator lens 3 
as a linear image or a linear beam spot on a recording surface S.sub.2 of 
the optical disc DK. 
The tilt control device 100 also has a photodetector 5A for detecting a 
portion of a linear beam which has been reflected by the recording surface 
S.sub.2, traveled back through the objective lens 4 and the collimator 
lens 3, passed through the reflecting surface S, in the beam splitter 
S.sub.2, and reached a surface S.sub.3, and photoelectrically converting 
the detected linear beam portion into a plurality of detected electric 
signals, an error generator 6A for generating a tilt error signal TE 
indicating whether the optical pickup, namely, the objective lens 4, is 
tilted with respect to the optical disc DK, based on the detected electric 
signals from the photodetector 5A, a driver 7 for producing a drive signal 
based on the tilt error signal TE from the error generator 7, and an 
actuator 8 responsive to the drive signal for controlling the tilt of the 
objective lens 4 with respect to the optical disc DK. 
Another portion of the linear beam that has fallen on the surface S.sub.3 
is read as an RF signal indicative of the information recorded on the 
optical disc DK, and the RF signal is sent to an RF amplifier 9. The RF 
signal is amplified at a suitable gain by the RF amplifier 9, which 
applies the amplified RF signal to a signal demodulator (not shown). 
FIG. 3 shows the relationship between signal pits SP and a linear beam spot 
LS on the optical disk DK, the linear beam spot LS being formed on the 
recording surface S.sub.2 by the laser beam applied thereto. The linear 
beam spot LS extends linearly perpendicularly to a track direction, i.e., 
a circumferential direction, and covers only one track or row of signal 
pits SP but does not reach adjacent tracks. 
FIGS. 4(A), 4(B), 4(C), and 4(D) show the photodetector 5A in detail. As 
shown in FIGS. 4(A), 4(B), 4(C), and 4(D), the photodetector 5A includes a 
photodetector element 13 for detecting a linear beam spot LS reflected 
from the optical disc DK and two parallel flat glass plates 11, 12 mounted 
on and covering portions of the photodetector element 13. The 
photodetector element 13 has three photosensitive surfaces a.sub.1, 
a.sub.2, a.sub.3 successively arranged across the photodetector element 13 
at one end thereof, a single photosensitive surface a.sub.4 extending 
across the photodetector element 13 and spaced from the three 
photosensitive surfaces a.sub.1, a.sub.2, a.sub.3, and three 
photosensitive surfaces a.sub.5, a.sub.6, a.sub.7 successively arranged 
across the photodetector element 13 at the opposite end thereof and spaced 
from the photosensitive surface a.sub.4. The parallel flat glass plates 
11, 12 cover the three photosensitive surfaces a.sub.1, a.sub.2, a.sub.3 
and a.sub.5, a.sub.6, a.sub.7 in their entirety. Each of the parallel flat 
glass plates 11, 12 has a thickness d.sub.1 and an absolute refractive 
index n.sub.1 and an optical path n.sub.1 .times.d.sub.1. The 
photosensitive surfaces a.sub.1 -a.sub.7 photoelectrically convert the 
detected reflected linear beam spot LS into respective detected electric 
signals I.sub.1 -I.sub.7. The signals I.sub.1 -I.sub.3, I.sub.5 -I.sub.7 
are used to detect a tilt error, and the signal I.sub.4 represents an 
information signal indicative of the information recorded on the optical 
disc DK. As shown in FIG. 4(B), the reflected linear beam spot LS from the 
optical disc DK is applied across the photosensitive surfaces a.sub.1 
-a.sub.7. 
Operation of the tilt control device according to the first embodiment will 
be described below with reference to FIGS. 5(A), 5(B), 5(C) through B(A), 
8(B), 8(C). 
FIGS. 5(A), 5(B), 5(C) show the reflected linear beam LS that is applied to 
the photodetector 5A, as viewed in the direction indicated by the arrow I 
in FIG. 4(A). In FIG. 5(A), the reflected linear beam spot LS is shown as 
being applied to the photodetector 5A when the distance between a 
photodetector unit 13.sub.1 composed of the photosensitive surfaces 
a.sub.1 -a.sub.3 and the optical disc DK is shorter than the focal length 
of the objective lens 4, i.e. , when the photodetector unit 13.sub.1 is 
too close to the optical disc DK. In FIG. 5(B), the reflected linear beam 
spot LS is shown as being applied to the photodetector 5A when the 
distance between the photodetector unit 13.sub.1 and the optical disc DK 
is equal to the focal length of the objective lens 4. In FIG. 5(C), the 
reflected linear beam spot LS is shown as being applied to the 
photodetector 5A when the distance between the photodetector unit 13.sub.1 
and the optical disc DK is longer than the focal length of the objective 
lens 4, i.e., when the photodetector unit 13.sub.1 is too far from the 
optical disc DK. In FIGS. 5(A), 5(B), and 5(C), the reflected linear beam 
LS applied to the photodetector 5A travels through the parallel flat glass 
plate 11 along a trajectory indicated by the solid-line arrows, and 
travels outside of the parallel flat glass plate 11 along a trajectory 
indicated by the brokenline arrows. The reflected linear beam LS has 
widths b.sub.1, b.sub.2, b.sub.3 when they are applied as shown in FIGS. 
5(A), 5(B), and 5(C), respectively, and these widths b.sub.1, b.sub.2, 
b.sub.3 satisfy the relationship b.sub.1 &lt;b.sub.2 &lt;b.sub.3 because of the 
refraction by the parallel flat glass plate 11. The above process holds 
true for the reflected linear beam LS which is applied through the 
parallel flat glass plate 12 to a photodetector unit 13.sub.2 composed of 
the photosensitive surfaces a.sub.5 -a.sub.7. 
FIG. 6 shows the photodetector 5A and the error generator 6A connected to 
the photodetector 5A. As shown in FIG. 6, the error generator 6A has two 
adders 23, 24 and three subtractors 25, 26, 27. The adder 23 has one input 
terminal connected to the photosensitive surface a.sub.1 and the other 
input terminal connected to the photosensitive surface a.sub.3. The 
subtractor 25 has a negative input terminal connected to the 
photosensitive surface a.sub.2 and a positive input terminal connected to 
the output terminal of the adder 23. The adder 24 has one input terminal 
connected to the photosensitive surface a.sub.5 and the other input 
terminal connected to the photosensitive surface a.sub.7. The subtractor 
26 has a negative input terminal connected to the photosensitive surface 
a.sub.6 and a positive input terminal connected to the output terminal of 
the adder 24. The subtractor 27 has a positive input terminal connected to 
the output terminal of the subtractor 25 and a negative input terminal 
connected to the output terminal of the subtractor 26. Therefore, the 
error generator 6A produces an output signal TE indicated by: 
EQU TE=(I.sub.1 +I.sub.3 -I.sub.2)-(I.sub.5 +I.sub.7 -I.sub.6) (1) 
as an output signal from the output terminal of the subtractor 27. 
If the distance between the photodetector unit 13.sub.1 and the optical 
disc DK and the distance between the photodetector unit 13.sub.2 and the 
optical disc DK are the same as each other, then the reflected laser beam 
spot LS is applied to the photodetector 5A as shown in FIGS. 7(A), 7(B), 
and 7(C). The reflected laser beam spot LS has a width b.sub.4 on the 
photosensitive surfaces of each of the photodetector units 13.sub.1, 
13.sub.2. Therefore, the output signal from the subtractor 25 and the 
output signal from the subtractor 26 are equal to each other, and the 
output signal TE of the error generator 6A is TE =0. 
If the recording surface S.sub.2 of the optical disc DK is inclined with 
respect to the linear laser beam applied thereto, e.g., if the 
photodetector unit 13.sub.1 is farther from the optical disc DK and the 
photodetector unit 13.sub.2 is closer to the optical disc DK, then the 
reflected laser beam spot LS is applied to the photodetector 5A as shown 
in FIGS. B(A), 8(B), and 8(C). The reflected laser beam spot LS has a 
width b.sub.5 on the photosensitive surfaces of the photodetector unit 
13.sub.1, and the reflected laser beam spot LS has a width b.sub.6 on the 
photosensitive surfaces of the photodetector unit 13.sub.2, with the width 
b.sub.5 being larger than the width b.sub.6 (b.sub.5 &gt;b.sub.6). 
The photosensitive surfaces and the optical system are designed such that 
when the linear laser beam spot or linear image on the recording surface 
S.sub.2 of the optical disc DK is focused at the focal point of the 
objective lens 4, the respective detected signals I.sub.1, I.sub.2, 
I.sub.3, I.sub.5, I.sub.6, I.sub.7 satisfy the following equations: 
EQU FE.sub.1 =I.sub.1 +I.sub.3 -I.sub.2 =0 (2) 
and 
EQU FE.sub.2 =I.sub.5 +I.sub.7 -I.sub.6 =0 (3). 
Then, when either one of the photodetector units 13.sub.1, 13.sub.2 is 
closer to the optical disc DK, as shown in FIG. 5(A), because of the 
refraction by the parallel flat glass plate 11 or 12, the detected signal 
I.sub.2 from the photosensitive surface a.sub.2 is larger than the sum of 
the detected signals I.sub.1, I.sub.3 from the photosensitive surfaces 
a.sub.1, a.sub.3. As a result, the following relationship is satisfied: 
EQU FE.sub.1 =I.sub.1 +I.sub.3 -I.sub.2 &lt;0 (4) 
and similarly 
EQU FE.sub.2 =I.sub.5 +I.sub.7 -I.sub.6 &lt;0 (5). 
Conversely, when either one of the photodetector units 13.sub.1, 13.sub.2 
farther from the optical disc DK, as shown in FIG. 5(C), because of the 
refraction by the parallel flat glass plate 11 or 12, the detected signal 
I.sub.2 from the photosensitive surface a.sub.2 is smaller than the sum of 
the detected signals I.sub.1, I.sub.3 from the photosensitive surfaces 
a.sub.1, a.sub.3. As a result, the following relationship is satisfied: 
EQU FE.sub.1 =I.sub.1 +I.sub.3 -I.sub.2 &gt;0 (6) 
and similarly 
EQU FE.sub.2 =I.sub.5 +I.sub.7 -I.sub.6 &gt;0 (7). 
When the reflected linear beam spot LS is applied to the photodetector 5A 
as shown in FIGS. 8(A) through 8(C), therefore, the output signal from the 
error generator 6A is indicated by: 
EQU TE=(I.sub.1 +I.sub.3 -I.sub.2)-(I.sub.5 +I.sub.7 -I.sub.6)&gt;0(8). 
When the photodetector unit 13.sub.1 is closer to the optical disc DK and 
the photodetector unit 13.sub.2 LS farther from to the optical disc DK, 
the output signal from the error generator 6A is indicated by: 
EQU TE&lt;0 (9). 
Accordingly, the output signal from the error generator 6A can be employed 
as a tilt error signal. 
This is because the parallel flat glass plates 11, 12 each having a 
predetermined optical path and a refractive action cover the entire 
photosensitive surfaces a.sub.1, a.sub.2, a.sub.3 and a.sub.5, a.sub.6, 
a.sub.7 causing the reflected linear beam spot LS to have widths b.sub.1, 
b.sub.2, b.sub.3 (b.sub.1 &lt;b.sub.2 &lt;b.sub.3), respectively, on the 
photosensitive surfaces when the optical disc DK is inclined and not 
inclined with respect to the objective lens 4. If the photosensitive 
surfaces were not covered with the parallel flat glass plates 11, 12, then 
the reflected linear beam spot LS would fall on the photosensitive 
surfaces along the trajectory as indicated by the broken lines in FIGS. 
4(A), 4(B), and 4(C). Although the output signals FE.sub.1, FE.sub.2 are 
FE.sub.1 =0, FE.sub.2 =0 when the laser beam spot is focused on the 
optical disc DK, the output signals FE.sub.1, FE.sub.2 would be FE.sub.1 
&gt;0, FE.sub.2 &gt;0 when the laser beam spot is out of focus on the optical 
disc DK irrespective of whether it is overfocused or underfocused. 
Therefore, it would be impossible to determine whether either one of the 
photodetector units 13.sub.1, 13.sub.2 is closer to or farther from the 
optical disc DK, thus failing to determine which direction the optical 
disc DK is tilted in with respect to the objective lens 4 from the tilt 
error signal TE. 
2nd Embodiment 
FIG. 9 shows a tilt control device according to a second embodiment of the 
present invention. The tilt control device according to the second 
embodiment is similar to the tilt control device according to the first 
embodiment except for a photodetector and an error generator. Therefore, 
only a photodetector and an error generator in the tilt control device 
according to the second embodiment are shown in FIG. 9. 
As shown in FIG. 9, the photodetector, generally denoted at 5B, includes a 
photodetector element 14 for detecting a linear beam reflected from the 
optical disc DK and two parallel flat glass plates (not shown) mounted on 
and covering photodetector units 14.sub.1, 14.sub.2 in their entirety, of 
the photodetector element 14. The photodetector unit 14.sub.1 is composed 
of three photosensitive surfaces a.sub.8, a.sub.9, a.sub.10 successively 
arranged across the photodetector element 14 at one end thereof. A single 
photosensitive surface a.sub.11 extends across the photodetector element 
14 and is spaced from the photosensitive surfaces a.sub.8, a.sub.9, 
a.sub.10. The photodetector unit 142 is composed of three photosensitive 
surfaces a.sub.12, a.sub.13, a.sub.14 successively arranged across the 
photodetector element 14 at the other end thereof and spaced from the 
photosensitive surface a.sub.11. The central photosensitive surfaces 
a.sub.9, a.sub.13 of the photodetector units 14.sub.1, 14.sub.2, which 
detect a central portion of the reflected linear beam spot LS applied 
thereto, are connected to positive and negative input terminals of an 
error generator 6B. The error generator 6B generates an output signal 
indicative of the difference between output signals from the central 
photosensitive surfaces a.sub.9, a.sub.13, as follows: 
EQU TE=I.sub.9 -I.sub.13 (10) 
In FIG. 9, the photodetector unit 14.sub.1 is farther from the optical disc 
DK than the photodetector unit 14.sub.2. Since the reflected beam spot LS 
is diffused as a whole, the relationship I.sub.9 &lt;I.sub.13 is satisfied. 
Therefore, 
EQU TE&lt;0 (11). 
Conversely, when the photodetector unit 14.sub.1 is closer to the optical 
disc DK than the photodetector unit 14.sub.2, 
EQU TE&gt;0 (12). 
Accordingly, the output signal TE from the error generator 6B can be 
employed as a tilt error signal. 
3rd Embodiment 
FIGS. 10(A) and 10(B) show a tilt control device according to a third 
embodiment of the present invention. The tilt control device according to 
the third embodiment is similar to the tilt control device according to 
the first embodiment except for a photodetector and an error generator. 
As shown in FIGS. 10(A) and 10(B), the photodetector, generally denoted at 
5C, includes a photodetector element 17 and two parallel flat glass plates 
15, 16 mounted on and covering photodetector units 17.sub.1, 17.sub.2, in 
their entirety, of the photodetector element 17. The photodetector unit 
17.sub.1 is composed of two parallel photosensitive surfaces a.sub.16, 
a.sub.17 extending across the photodetector element 17 at one end thereof. 
A single photosensitive surface a.sub.18 extends across the photodetector 
element 17 and is spaced from the photosensitive surfaces a.sub.17, 
a.sub.19. The photodetector unit 17.sub.2 is composed of two parallel 
photosensitive surfaces a.sub.19, a.sub.20 extending across the 
photodetector element 17 at the other end thereof and spaced from the 
photosensitive surface a.sub.18. Each of the parallel flat glass plates 
15, 16 has a thickness d.sub.2 and an absolute refractive index n.sub.2 
and an optical path n.sub.2 .times.d.sub.2. The photosensitive surfaces 
a.sub.16 -a.sub.20 photoelectrically convert the detected reflected linear 
beam spot LS into respective detected electric signals I.sub.16 - 
I.sub.20. The signals I.sub.16, I.sub.17, I.sub.19, I.sub.20 are used to 
detect a tilt error, and the signal I.sub.18 represents an information 
signal indicative of the information recorded on the optical disc DK. As 
shown in FIG. 10(B), the reflected linear beam spot LS from the optical 
disc DK is applied across the photosensitive surfaces a.sub.17 -a.sub.19, 
and has opposite ends falling on the photosensitive surfaces a.sub.16, 
a.sub.20, respectively. 
Operation of the tilt control device according to the third embodiment will 
be described below with reference to FIG. 10(B). 
When the distance between the optical disc DK and the objective lens is 
proper, the photosensitive surfaces a.sub.16, a.sub.17, a.sub.19, a.sub.20 
produce their output signals which meet the following equations: 
EQU FE.sub.1 =I.sub.16 -I.sub.17 0 (13) 
and 
EQU FE.sub.2 =I.sub.19 -I.sub.20 =0 (14) 
and 
EQU TE=FE.sub.1 -FE.sub.2 =0 (15) 
where TE is the output signal from the error generator. 
When the linear laser beam is not applied perpendicularly to the optical 
disc DK, the output signal TE becomes TE&gt;0 or TE&lt;0 because of the 
refraction by the parallel flat glass plate 15 or 16. The tilt of the 
objective lens with respect to the optical disc DK can be controlled based 
on the output signal or tilt error signal TE. 
4th Embodiment 
FIGS. 11(A) through 11(E) show a tilt control device according to a fourth 
embodiment of the present invention. The tilt control device according to 
the fourth embodiment is similar to the tilt control device according to 
the first embodiment except for a photodetector and an error generator. 
As shown in FIGS. 11(A) and 11(B), the photodetector, generally denoted at 
5D, includes a photodetector element 19 and a parallel flat glass plate 18 
mounted on and covering a portion of the photodetector element 19. The 
photodetector element 19 has a photodetector unit 19, composed of three 
photosensitive surfaces a.sub.21, a.sub.22, a.sub.23 successively arranged 
across the photodetector element 19 at one end thereof, a single 
photosensitive surface a.sub.24 extending across the photodetector element 
19 and spaced from the photodetector unit 19.sub.1, and a photodetector 
unit 19.sub.2 composed of three photosensitive surfaces a.sub.25, 
a.sub.26, a.sub.27 successively arranged across the photodetector element 
19 at the other end thereof and spaced from the photosensitive surface 
a.sub.24. The central photosensitive surface a.sub.24 is covered in its 
entirety by the parallel flat glass plate 18. The parallel flat glass 
plate 18 has a thickness d.sub.3 and an absolute refractive index n.sub.3 
and an optical path n.sub.3 .times.d.sub.3. The photosensitive surfaces 
a.sub.21 -a.sub.27 photoelectrically convert the detected reflected linear 
beam spot LS into respective detected electric signals I.sub.21 -I.sub.27. 
The signals I.sub.21 -I.sub.23, I.sub.25 -I.sub.27 are used to detect a 
tilt error, and the signal I.sub.24 represents an information signal 
indicative of the information recorded on the optical disc DK. As shown in 
FIG. 11(B), the reflected linear beam spot LS from the optical disc DK is 
applied across the photosensitive surfaces a.sub.21 -a.sub.27. The central 
photosensitive surfaces a.sub.22, a.sub.26 of the photodetector units 
19.sub.1, 19.sub.2 are connected to positive and negative input terminals 
of an error generator 6D. The error generator 6D generates an output 
signal indicative of the difference between output signals from the 
central photosensitive surfaces a.sub.22, a.sub.26, as follows: 
EQU TE=I.sub.22 -I.sub.26 (26). 
The reflected beam spot LS is applied to the photodetector 5D along the 
trajectory indicated by the solidline arrows in FIGS. 11(C) through 11(E). 
The output signal TE from the error generator 6D can be employed as a tilt 
error signal. 
5th Embodiment 
In each of the first through fourth embodiments described above, the linear 
beam spot LS is applied to the optical disc DK so that it does not fall on 
adjacent tracks at the same time, as shown in FIG. 3. According to a fifth 
embodiment, however, a longer linear beam spot LS is applied to the 
optical disc across a plurality of tracks of signal pits SP, as shown in 
FIG. 12 (A) . According to the fifth embodiment, as shown in FIG. 12(B), a 
photodetector 5E has a photosensitive surface a.sub.31 for reading an 
information signal recorded on the optical disc DK, a photodetector unit 
20.sub.1 composed of three photosensitive surfaces a.sub.28, a.sub.29, 
a.sub.30 spaced from the photosensitive surface a.sub.31, and a 
photodetector unit 20.sub.2 composed of three photosensitive surfaces 
a.sub.32, a.sub.33, a.sub.34 spaced from the photosensitive surface 
a.sub.31. The three photosensitive surfaces a.sub.28, a.sub.29, a.sub.30 
and the three photosensitive surfaces a32, a.sub.33, a.sub.34 are covered, 
in their entirety, with respective parallel flat glass plates (not shown) 
as with the first embodiment. The photodetector units 20.sub.1, 20.sub.2 
jointly serve to produce a tilt error signal. If laser beam reflections 
from other tracks than those tracks from which a tilt error signal is to 
be derived are also applied to the photodetector unit 20.sub.1 or 
20.sub.2, then the photodetector unit 20.sub.1 or 20.sub.2 may be arranged 
such that it cannot detect signals in a high-frequency range. 
6th Embodiment 
As shown in FIG. 13, a photodetector 5F according to a sixth embodiment of 
the present invention has a plurality of juxtaposed photosensitive 
surfaces 22.sub.1 -22.sub.n for reading information signals from the 
optical disc DK using a very long linear beam spot LS applied to the 
optical disc DK. The photodetector 5F is thus capable of reading a 
plurality of tracks on the optical disc DK at the same time. The 
photodetector 5F also has a photodetector unit 21.sub.1 composed of three 
photosensitive surfaces a.sub.35, a.sub.36, a.sub.37, and a photodetector 
unit 21.sub.2 composed of three photosensitive surfaces a.sub.38, 
a.sub.39, a.sub.40 spaced from the photodetector unit 21.sub.1. The 
photodetector units 21.sub.1, 21.sub.2 are covered in their entirety with 
respective parallel flat glass plates (not shown) as with the first 
embodiment. 
In the above embodiments, certain photosensitive surfaces of the 
photodetector are covered with a parallel flat glass plate. However, they 
may be covered with a lens, a Fresnel lens, a cylindrical lens, or an 
optical element with a varying distribution of refractive indexes. Such a 
lens or an optical element may be spaced from the photosensitive surfaces. 
In the illustrated embodiments, the photosensitive surfaces for producing a 
tilt error signal and an information signal are disposed in the same 
plane. However, the photosensitive surface for producing an information 
signal may be positioned somewhere else, and a reflected light beam may be 
guided thereto by a half-silvered mirror or some other optical element. 
While each of the illustrated photodetectors is shown as including two 
photodetector units, it may be composed of three or more photodetector 
units. 
Although certain preferred embodiments of the present invention have been 
shown and described in detail, it should be understood that various 
changes and modifications may be made therein without departing from the 
scope of the appended claims.