Dynamic information storage or retrieval – With servo positioning of transducer assembly over track... – Optical servo system
Reexamination Certificate
1998-06-17
2001-06-12
Psitos, Aristotelis M. (Department: 2651)
Dynamic information storage or retrieval
With servo positioning of transducer assembly over track...
Optical servo system
C369S112230
Reexamination Certificate
active
06246644
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates, in particular, to an optical pickup for generating focus error information by using an astigmatism system and a method for manufacturing the optical pickup.
In an optical pickup used in an optical disk reproducing apparatus or the like, not only information recorded on a disk is reproduced from reflected light information, but also error information such as focus, tracking or the like for precise recording/reproducing scan of a laser beam can be obtained.
In particular, error information (focus error signal) for executing a focus servo which brings the focus position of a laser light into an in-focus state with respect to a recording face of a disk or the like is obtained by means of an astigmatism system using the output of a quadruple photodetector. Such a configuration is known.
FIG. 1
shows an example of a configuration of an optical pickup disk
20
.
This optical pickup
20
includes a laser diode
22
, a collimator lens
23
, a beam splitter
24
, an objective lens
25
, a condensing lens
26
, a cylindrical lens
27
, a photodetector
28
, and a biaxial mechanism
29
.
A laser beam output from the laser diode
22
is converted to a parallel beam by the collimator lens
23
, then reflected toward a disk
90
by 90 degrees by the beam splitter
24
, and irradiated on the disk
90
via the objective lens
25
.
The objective lens
25
is held by the biaxial mechanism
29
so as to be able to move in the focus direction and the tracking direction. The operations for moving the objective lens
25
in the focus direction and the tracking direction are executed by currents applied to a focus coil and a tracking coil in the biaxial mechanism
29
.
On the, disk
90
, grooves GB are formed as recording tracks. However, both lands LD and grooves GB can be used as the data recording tracks.
A reflected light resulting from reflection on the disk
90
enters the beam splitter
24
via the objective lens
25
. The reflected light is then transmitted through the beam splitter
24
as it is and arrives at the condensing lens
26
. The reflected light is condensed by the condensing lens
26
, and then is incident on the photodetector
28
via the cylindrical lens
27
. As the photodetector
28
, a quadruple detector having light receiving faces A, B, C and D as shown in
FIGS. 2A
to
2
C is provided.
The cylindrical lens
27
is disposed so as to have its mother line inclined by 45 degrees with respect to the track direction of the disk
90
. By utilizing the astigmatism generated by the cylindrical lens
27
, the focus error signal is detected from the output of the quadruple detector.
When the beam spot is in the in-focus state with respect to the recording face of the disk
90
, a spot SP on the quadruple detector becomes a circle as shown in FIG.
2
A.
If the objective lens
25
is located too near the disk
90
as compared with the position of the in-focus state, however, the spot SP on the quadruple detector becomes an ellipse having its longer radius in a direction parallel to the mother line direction of the cylindrical lens
27
(i.e., direction directed from the light receiving face B toward the light receiving face D) as shown in FIG.
2
B. On the contrary, if the objective lens
25
is located too far from the disk
90
as compared with the position of the in-focus state, the spot SP on the quadruple detector becomes an ellipse having its longer radius in a direction parallel to a direction perpendicular to the direction of the mother line of the cylindrical lens
27
(i.e. direction directed from the light receiving face A toward the light receiving face C) as shown in FIG.
2
C.
Denoting outputs corresponding to quantities of light received by the light receiving faces A, B, C and D respectively by SA, SB, SC and SD, therefore, a focus error signal FE can be obtained as
FE=(SA+SC)−(SB+SD).
In other words, if (SA+SC)−(SB+SD) is zero, it can be detected that the objective lens
25
is in the just focus state. If (SA+SC)−(SB+SD) is a positive value, it can be detected that the objective lens
25
is located further apart from the disk
90
than the in-focus position. If (SA+SC)−(SB+SD) is a negative value, it can be detected that the objective lens
25
is located nearer the disk
90
than the in-focus position.
By constructing the focus servo system so as to converge the focus error signal FE toward zero, therefore, the focus position of the objective lens
25
can be controlled properly.
On the recording face of the recording medium, however, a series of pits formed of concave-convex pits or lands/grooves as in the above described disk
90
are formed. The irradiated laser light is modulated by them. As a result, the intensity pattern of the spot on the quadruple detector is changed. For example, the light intensity of the light spot on the receiving portions A and C becomes intense. Or on the contrary, the light intensity of the light spot on the receiving portions B and D becomes intense.
Therefore, there occurs such a phenomenon that the focus error signal FE becomes plus in, for example, the grooves GB, and becomes minus in the lands LD (or vice versa).
FIG. 3A
shows an example of the focus error signal FE obtained when the laser spot traverses the grooves GB/lands LD while being kept in the in-focus state.
In the in-focus state, the focus error signal should originally become constant (zero) irrespective of the grooves GB/lands LD (or irrespective of concave-convex pit train). Due to modulation conducted by the grooves GB/lands LD, the focus error signal FE becomes a signal having an offset GFOF caused by the grooves GB and an offset LFOF caused by the lands LD as illustrated.
In other words, the relation that the focus error signal FE=0 does not necessarily indicate the in-focus state. Unless some countermeasure is taken, the focus servo does not function satisfactorily.
For example, in the case of such a system that information recording and reproducing are conducted only for either the grooves GB or the lands LD, the focus error signal FE is provided with a bias so as to cancel either the offset GFOF caused by the grooves GB or the offset LFOF caused by the lands LD. By doing so, the focus servo system converging to the in-focus state with the focus error signal FE=0 functions normally.
For example, in an example shown in
FIG. 3B
, the function of the focus servo loop for the grooves GB is made effective by giving a focus bias FB to the focus error signal FE so as to cancel the offset GFOF caused by the grooves GB.
In the case where the grooves GB/lands LD are traversed, however, the modulated signal becomes as illustrated. The larger the degree of modulation caused by the grooves GB/lands LD, therefore, the more the stability of the focus servo loop is hampered, resulting in a problem.
Furthermore, in recent years, there has been proposed a system using both the grooves GB and the lands LD as the recording and reproducing tracks for the purpose of increasing the recording capacity.
In the case of such a system, the servo function cannot be made effective by some fixed focus bias. Therefore, such a sophisticated and difficult control as to change over the focus bias according to whether the laser spot is currently in the grooves GB or in the lands LD is needed.
In other words, the following offset changeover processing is conducted. In the case where the focusing is conducted with respect to the lands LD, a focus bias for land for canceling the offset LFOF is given to the focus error signal FE. In the case where the focusing is conducted with respect to the grooves GB, a focus bias for grooves for canceling the offset GFOF is given to the focus error signal FE.
From these problems, it is demanded to make the degree of modulation caused on the focus error signal by the grooves GB/lands LD (or the degree of modulation caused on the focus error signal by a pit train) as small as possible. For example, in the system using ei
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Psitos Aristotelis M.
Sony Corporation
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