Optical pickup with a two-detector arrangement

An optical pickup for detecting a distance to an optical disk on which information is optically stored includes a tracking error detecting device for detecting a tracking error by receiving a part of the light reflected from the disk and a focusing error detecting device for detecting the remaining part of the light reflected from the disk, structured such that the tracking error detecting device receives 60% or more of the reflected light.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention generally relates to an optical information processing 
device and particularly to an optical pickup which receives a light beam 
reflected from a recording medium to control its position with respect to 
the recording medium. More specifically, the present invention relates to 
an optical pickup for carrying out tracking and focusing at high accuracy. 
2. Description of the Prior Art 
An optical pickup for optically reading information stored in an optical 
disk is well known. Such an optical pickup is normally structured to carry 
out tracking and focusing controls so as to have a light beam properly 
follow and stay focused on a track along which information is recorded. 
For this purpose, the optical pickup receives a light beam reflected from 
the optical disk to produce a position control signal, which is used to 
control the position of the optical pickup with respect to the optical 
disk. One of the methods for detecting the position of the optical pickup 
with respect to the disk is the knife-edge method. 
FIGS. 1a and 1b show the overall structure of the typical prior art optical 
pickup system for detecting tracking and focusing conditions on the basis 
of the knife-edge method. As shown, a light beam emitted from a light 
source 1, such as a semiconductor laser, is collimated (parallel) by a 
coupling lens 2 and the thus collimated beam is reflected by a polarizing 
beam splitter 3 to pass through a 1/4 wave length plate 4. The light beam 
is then focused by an objective lens 5 onto the surface of a disk 6 
thereby forming a spot having the diameter of approximately 1.6 microns. 
A reflected light beam from the surface of the disk 6 passes through the 
objective lens 5 to be also converted into a collimated light beam which 
is then converted into a linearly polarized light beam whose plane of 
polarization is perpendicular to that of the incident light beam by the 
1/4 wave length plate 4. Thereafter, the light beam passes through the 
beam splitter 3 and is made convergent by a lens 7. As shown in FIG. 1b, a 
half of this convergent light beam 10 is incident on a tracking error 
detecting device 8 which is comprised of a pair of light receiving 
elements C and D arranged on both sides with respect to the direction of 
track T, and the remaining half of the beam 10 is incident on a focus 
error detecting device 9 comprised of a pair of light receiving elements A 
and B arranged on both sides of a knife edge defined by the tracking error 
detecting device 8. 
The principle of focusing error detecting operation of the optical pickup 
shown in FIG. 1a will be described with reference to FIGS. 2a through 2c. 
The top end of the tracking error detecting device 8 serves as a knife 
edge, and, under the in-focus condition as shown in FIG. 2a, outputs from 
the respective light receiving elements A and B are equal. However, when 
the disk 6 moves away from the objective lens 5 as shown in FIG. 2b, the 
output from the element A becomes smaller than the output from the element 
B; on the other hand, when the disk 6 moves closer to the objective lens 5 
as shown in FIG. 2c, the output from the element A becomes larger than the 
output from the element B. In this manner, the focus error condition may 
be detected by comparing the outputs from both of the elements A and B. 
Regarding the tracking error detecting operation, when the spot is formed 
in registry with a track as shown in FIG. 3a, outputs from the respective 
elements C and D are equal. However, as shown in FIG. 3b when the spot is 
shifted from a track, outputs from the respective elements C and D become 
unequal. That is, one of the outputs become larger than the other 
depending upon the direction of shift with respect to the track. 
In such a prior art optical pickup, accuracies required for focusing error 
detection and for tracking error detection are 1 and 0.1 micron, 
respectively. In the prior art knife edge method, the tracking error 
detecting device receives approximately 50% of the light beam and the 
error in this signal is approximately 0.05 microns, which is approximately 
half of the required accuracy of 0.1 micron. This indicates that higher 
accuracies are required for other parts of the optical system, which tends 
to push up the cost. 
SUMMARY OF THE INVENTION 
It is therefore a primary object of the present invention to provide an 
improved optical pickup. 
Another object of the present invention is to provide an optical pickup 
capable of detecting a tracking error at high accuracy. 
A further object of the present invention is to provide an optical pickup 
capable of detecting a tracking error and a focusing error optimally. 
Other objects, advantages and novel features of the present invention will 
become apparent from the following detailed description of the invention 
when considered in conjunction with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to FIG. 4, there is graphically shown a measured result 
indicating how the level of error in tracking signal varies depending on 
how much the tracking error detecting device 8 receives of the light from 
the light beam 10. In the graph of FIG. 4, the abscissa is taken for the 
ratio of the area of the light beam 10 intersected by the tracking error 
detecting device 8 to the cross-sectional area of the light beam 10 (or % 
amount of light received by the tracking error detecting device 8) and the 
ordinate is taken for the tracking accuracy or the level of error in 
tracking signal. As shown in FIG. 4, with the % amount of received light 
at 50, the error level in tracking signal is approximately 0.05 microns. 
The tracking error level decreases rapidly as the % amount of received 
light increases and the error level becomes as small as can be neglected 
when the % amount of received light reaches 60% or more. 
FIGS. 5a and 5b schematically show the structure of one embodiment of the 
present invention. As shown, the illustrated optical pickup is 
structurally similar in many respects to that described above, and thus 
FIGS. 5a and 5b only show the main portion of the present invention. In 
the present invention, the tracking error detecting device 8 is arranged 
to extend beyond the optical axis of the lens 7. In the preferred 
embodiment, the tracking error detecting device 8 is arranged to intersect 
the light beam 10 such that the % amount of intersected area is 60% or 
more. That is, as shown in FIG. 5b, the tracking error detecting device 8 
is comprised of the pair of light receiving elements C and D which are 
arranged side by side horizontally with the vertical boundary line aligned 
perpendicular to the optical axis of the lens 7 and thus in parallel with 
a track on the disk 6 so as to receive 60% or more of the light of the 
light beam 10. Thus, the occurrence of tracking error may be detected when 
the light beam 10 moves sideways with respect to the vertical boundary 
line or partition line between the pair of light receiving elements C and 
D. 
FIG. 6 shows another embodiment of the present invention in which the 
tracking error detecting device 8 and the focusing error detecting device 
9 are disposed in different optical paths. That is, the tracking error 
detecting device 8 is disposed in a first optical path which extends 
straight through the beam splitter 3 from the optical path of lens 5 and 
the focusing error detecting device 9 is disposed in a second optical path 
which extends as bent over 90.degree. by the beam splitter 3. In the first 
optical path is disposed a 1/4 wave length plate 10 having its surface 
opposite to the tracking error detecting device 8 is structured in the 
form of a half mirror 11. Thus, the tracking error detecting device 8 
receives all of the light passing through the plate 10 in the first 
optical path. In the second optical path is disposed the convergent lens 7 
and a knife edge element 13 which cuts half of the light passing through 
the lens 7 to be applied to the focusing error detecting device 9 because 
the knife edge of element 13 is arranged at the optical axis of lens 7 and 
thus at the partition line between the paired light receiving elements A 
and B. 
It is to be noted that various modifications may be made from the structure 
of FIG. 6. For example, the convergent lens 7 may be disposed in the first 
optical path as interposed between the beam splitter 3 and the 1/4 wave 
length plate 10. Moreover, the elements in the first optical path and the 
elements in the second optical path may be interchanged. 
In order to obtain an information signal from the light beam reflected from 
the disk 6, use may be made of a sum of output signals from the focusing 
error detecting light receiving elements A and B, which will be simply 
indicated as A+B, a sum of output signals from the tracking error 
detecting light receiving elements C and D, which will be simply indicated 
as C+D, or a sum of all of the light receiving elements A, B, C and D, 
which will be simply indicated as A+B+C+D. 
FIG. 4 also shows the relation between the % amount of light received by 
the tracking error detecting device 8 and the phase shift of information 
signal. As shown, it has been found that if the % amount of light received 
by the tracking error detecting device 8 is 50% or less, an appreciable 
phase shift of information signal is produced thereby adversely affecting 
the accuracy of reading out the stored information. As described 
previously, in the prior art optical pickup employing the knife edge 
method, the amount of light received by A+B is 50% and equal to the amount 
of light received by C+D. Thus, this indicates the presence of an 
appreciable phase shift in the resultant information signal. If it is 
desired to obtain an information signal from A+B+C+D, since different 
elements are required to be used in the tracking and focusing devices 8 
and 9, there will be produced an error in the resultant information signal 
due to nonuniformity in sensitivity. 
Under the circumstances, in accordance with this aspect of the present 
invention, since the tracking error detecting device 8 is disposed to 
intersect 60% or more of the light beam 10, it is structured to obtain an 
information signal from the paired light receiving elements C and D of the 
tracking error detecting device 8. With this structure, an information 
signal free of phase shift can be obtained. Thus, this embodiment 
structurally corresponds to that illustrated in FIGS. 5a and 5b. In the 
present embodiment, however, output signals from the tracking error 
detecting light receiving elements C and D are used not only for obtaining 
a tracking error signal but also for obtaining an information signal. In 
other words, an information signal is obtained by C+D and tracking error 
and focusing error signals are obtained by C-D and A-B, respectively. 
FIG. 7 shows another embodiment of the present invention in which the 
tracking error detecting device 8 is disposed out of the optical path of 
lens 7. In this embodiment, the tracking error detecting device 8 does not 
serve as a knife edge, and, instead, a reflecting mirror 14 is provided in 
the optical path of lens 7, interposed between the lens 7 and a convex 
lens 15. Thus, the light beam from the lens 7 is partly reflected by the 
mirror 14 toward the tracking error detecting device 8 disposed above and 
the remaining light beam is allowed to impinge upon the focusing error 
detecting device 9. In this case, the top edge of mirror 14, in effect, 
serves as a knife edge. 
A further aspect of the present invention will be described with particular 
reference to FIG. 8. In accordance with this aspect of the present 
invention, it is so structured that the tracking error detecting device 8 
receives 60-90% of the light beam 10 from the disk 6 and the focusing 
error detecting device 9 receives the remaining or 40-10% of the light 
beam 10. 
As described previously, in the optical pickup of the above-described type, 
the accuracy required for tracking error detection is approximately 
.+-.0.1 microns and the accuracy required for focusing error detection is 
approximately .+-.1 micron. Thus, the accuracy required for tracking error 
detection is higher by an order of magnitude. As graphically shown in FIG. 
8, according to the study of the present inventors, it has been found that 
as the amount of light received by the tracking error detecting device 8 
is increased, the level of tracking signal increases and thus the 
detection accuracy increases; on the other hand, if the amount of received 
light is decreased, the signal level decreases and thus the detection 
accuracy becomes deteriorated. 
In the graph of FIG. 8, the abscissa is taken for the percentage of the 
amount of light received by the tracking error detecting device 8 out of 
the total amount of light of the light beam 10 from the disk 6 and the 
ordinate is taken for the level of signal obtained from the tracking error 
detecting device 8 or focusing error detecting device 9. As shown by the 
solid line 13 in the graph of FIG. 8, as the amount of light received by 
the tracking error detecting device 8, which requires the detection 
accuracy of .+-.0.1 microns, increases, the signal level increases 
approximately linearly, indicating that the detection accuracy may be 
maintained at a high level. On the other hand, if the amount of light 
received by the tracking error detecting device 8 is increased, the amount 
of light received by the focusing error detecting device 9 decreases, and, 
thus, as indicated by the dotted line 12 in the graph of FIG. 8, the 
signal level for focusing error detection decreases thereby becoming 
increasingly difficult to maintain the required detection accuracy. 
In the optical pickup of the above-described type, there is typically a 
noise having the level in the order of 10 mV, and, thus, there must be 
provided a noise margin of at least 50 mV, which is indicated in the graph 
of FIG. 8 by the one-dotted line. As is obvious from these considerations, 
in order to carry out a position control operation, including tracking and 
focusing, stably as well as reliably while maintaining the tracking error 
detection accuracy at .+-.0.1 microns and the focusing error detection 
accuracy at .+-.1 micron, it is important that both of the tracking error 
signal 13a and the focusing error signal 12 be larger than the noise 
margin of 50 mV. In order to satisfy such a requirement, as is apparent 
from the graph of FIG. 8, the % amount of light of the light beam 10 
received by the tracking error detecting device 8 must be set in a range 
from 60 to 90%, and thus the % amount of light received by the focus error 
detecting device 9 is set in a range from 40 to 10% correspondingly. 
While the above provides a full and complete disclosure of the preferred 
embodiments of the present invention, various modifications, alternate 
constructions and equivalents may be employed without departing from the 
true spirit and scope of the invention. Therefore, the above description 
and illustration should not be construed as limiting the scope of the 
invention, which is defined by the appended claims.