Dynamic information storage or retrieval – With servo positioning of transducer assembly over track... – Optical servo system
Reexamination Certificate
1998-11-23
2002-06-18
Hindi, Nabil (Department: 2653)
Dynamic information storage or retrieval
With servo positioning of transducer assembly over track...
Optical servo system
C369S044340, C369S053280
Reexamination Certificate
active
06407968
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an optical information recording/reproducing device for recording/reproducing information on/from an optical disk using a laser beam, and more particularly relates to an optical information recording/reproducing device for recording/reproducing information on/from both land sections and groove sections of the optical disk.
BACKGROUND OF THE INVENTION
An optical disk device as an example of an optical information recording/reproducing device performs recording/reproducing of information in the following manner. That is, a light beam emitted from a semiconductor laser (light source) is converged by an objective lens (converging means) to form a light spot, and a tracking of the resulting light spot is performed along tracks formed on the optical disk, whereby information is recorded or reproduced based on the resulting signal.
To realize the described recording/reproducing operations, on the optical disk for recording thereon and reproducing therefrom information, tracks (guide grooves) are formed beforehand so that a tracking of the light spot can be performed along the tracks. Hereinafter, the groove sections where the tracks are formed are simply referred to as the groove sections, and the regions between adjacent grooves are referred to as land sections.
In the conventional optical disk devices, recording/reproducing of information are performed with respect to either the land sections or the groove sections of the optical disk. In recent years, however, the method for recording information both on the groove sections and the land sections on the optical disk has been proposed, which realizes an improved recording density of twice as high as that of the conventional optical disks.
However, in the described optical disk devices designed for recording/reproducing information on/from both the groove sections and the land sections on the optical disk, a problem arises in that an interference occurs as a tracking error signal is transferred to a focus error signal. This is known as a track interference, or a crosstalk between error signals, and hereinafter referred to as a crosstalk between error signals.
Due to the described crosstalk between error signals, a problem arises in that an optimal focus offset amount differs between (1) when a tracking is carried out with respect to the groove sections with the light spot resulting from converging light emitted from the semiconductor laser by the objective lens and (2) when a tracking is carried out with respect to the land sections with the light spot. For this difference in optimal focus offset amount, if the same servo control amount is used for both cases, an optimal focal position cannot be obtained, resulting in the problem that optimal recording/reproducing cannot be performed.
The reason why a difference in focus offset amount exists between when tracking the land sections and when tracking the groove sections will be explained in reference to FIG.
10
.
FIG. 10
shows the focus error signal and the tracking error signal obtained from the optical pickup and these servo error signals are shown in the state where only the focus servo is set ON. The servo error signals (focus error signal and tracking error signal) are shown so as to correspond to error signals at respective positions of the spot as converged on the optical disk
66
by the objective lens (groove sections
66
a
and land sections
66
b
shown in the figure) of the optical disk
66
.
As shown in the
FIG. 10
, generally, the focus error signal is affected by the tracks on an optical disk
66
, and has the same period as the tracking error signal, but has a different phase from that of the tracking error signal. This can be observed when the frequency band f
F
of the focus servo is smaller than the track cross frequency f
TC
generated due to the eccentricity of the track.
The described deviations in the focus error signal is known as a crosstalk between error signals. The crosstalk occurs by the following mechanism. On the photodetector which generates a servo error signal, a reflected light from the optical disk is affected by aberrations of the optical components of the optical pickup, particularly the objective lens. As a result, an asymmetrical property is attributed to the reflected light, and thus tracking error signal leaks into the focus error signal, thereby generating a crosstalk between the error signals.
According to the servo control of the optical disk device, generally, the tracking servo is set ON after setting ON the focus servo. Therefore, as can be seen from the servo error signal shown in
FIG. 10
, by the effect of the crosstalk between error signals, when a tracking of the land sections
66
b
is performed with the light spot, the focal point is L as indicated in FIG.
10
. On the other hand, when a tracking of the groove sections
66
a
is performed with the light spot, the focal point is G as indicated in FIG.
10
.
As described, the focal point differs between when tracking the land sections
66
b
and when tracking the groove sections
66
a
. Therefore, if the focus servo is carried out with the same amount of servo control for both the groove sections
66
a
and the land sections
66
b
, a deviation in the direction of an optical axis (focus offset) occurs as indicated by 1+g, and an optimal focus offset amount differs.
Accordingly, the inventors of the present application disclose in the Japanese Unexamined Patent Publication No. 180429/1996 (Tokukaihei 8-190429) (U.S. patent application Ser. No. 08/539,523) the structure of switching the amount of servo control of the focus servo between when tracking the groove sections and when tracking the land sections.
Specifically, the respective focus offset amounts for the land sections and the groove sections are stored. Then, when tracking the groove sections, the focus servo is carried out by compensating the focus error signal based on the focus offset amount set for the groove sections. On the other hand, when tracking the land sections, the focus servo is carried out by compensating the focus error signal based on the focus offset amounts set for the land sections. As a result, the information can be recorded/reproduced in the tracking area of the optical disk under just-in-focus conditions.
The focus offset amount may be set by the following methods {circle around (1)} and {circle around (2)}.
Method {circle around (1)}: The focus offset amount is set based on the crosstalk between error signals at a time of assembling the optical pickup.
As described earlier, an optimal focal position differs between the land sections and the groove sections for the crosstalk between error signals of the optical pickup. Therefore, using the reference disk, by the crosstalk between the error signals obtained at a time of assembling the optical pickup, the respective optimal focal positions for the land sections and the groove sections are determined.
The method {circle around (2)}: the focus offset amount is set based on a reproducing signal obtained at a time of starting up an optical disk device.
After activating the optical disk device, before carrying out recording/reproducing information, a reproducing of signal (test reading) is carried out with variable focus offset, and the respective focus offset amounts for the land sections and the groove sections which maximize the resulting reproducing signal are calculated respectively.
The described method {circle around (1)} of setting the focus offset amount based on the error signal crosstalk obtained at a time of assembling the optical pickup and the method {circle around (2)} of setting the focus offset amount based on a reproducing signal generated when starting the optical disk device have the following problems.
When adopting the method {circle around (1)}, in an event that an amount of crosstalk between error signals varies due to changes over time of the optical pickup, an appropriate focus offset amount cannot be obtained, thereby presenting the problem that an optimal focus servo c
Fuji Hiroshi
Nakata Yasuo
Ogata Nobuo
Hindi Nabil
Nixon & Vanderhye P.C.
Sharp Kabushiki Kaisha
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