Dynamic information storage or retrieval – With servo positioning of transducer assembly over track... – Optical servo system
Reexamination Certificate
2002-03-08
2004-06-29
Edun, Muhammad (Department: 2655)
Dynamic information storage or retrieval
With servo positioning of transducer assembly over track...
Optical servo system
C369S044270, C369S044410, C369S053230
Reexamination Certificate
active
06757225
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of performing a data-recording and/or data-reading operation with respect to an optical storage disk. In this specification, an “optical storage disk” may refer to, unless otherwise specified, any type of data storage medium with which desired information is written or read optically. For instance, the optical storage medium may be a read-only disk, magneto-optical disk or phase change optical disk.
2. Description of the Related Art
One way to improve the data storage capacity of an optical disk is to increase the data recording density of the disk. A higher storage density may be achieved by making smaller the laser spot formed on the optical disk in recording data or reading data. Typically, the laser beam emitted from a light source is passed through an objective lens unit before it strikes upon the recording layer of the storage disk. As passing through the lens unit, the laser beam converges to make a small light spot on the storage disk. The storage disk may include a transparent substrate upon which a recording layer is formed. The laser beam emitted from a light source may be arranged to pass through the transparent substrate and then strike upon the recording layer. As known in the art, the diameter of the light spot increases in proportion to &mgr;/NA, where &mgr; is the wavelength of the laser beam and NA is the numerical aperture of the objective lens. As seen from this formula, the wavelength of the laser beam should be shorter, or the NA of the lens unit should be higher (or both) in order to make smaller the light spot.
However, the reduction of the wavelength (shorter than 400 nm for example) increases the laser beam energy. This is disadvantageous because the properties of the optical components incorporated in the disk apparatus may be damaged due to the high energy. In addition, a laser beam of a shorter wavelength is readily absorbed as passing through the transparent substrate of the storage disk. As a result, the reflection light of a shorter wavelength may fail to provide appropriately strong information signals. In light of these, a laser beam whose wavelength is shorter than 400 nm is not suitable for size reduction of the light spot.
When the NA increases, on the other hand, the data-processing operation may suffer large spherical aberration. As known in the art, the spherical aberration caused by the thickness error of the disk substrate increases in proportion to the thickness error with a factor of the 4th power of the NA of the lens. Unfavorably, the occurrence of spherical aberration produces an out-of-focus laser spot, as in the case where the objective lens is “defocused.” (In this specification, “defocus” is used for describing the positional deviation of the objective lens from a prescribed point along the optical axis of the lens.) Therefore, when a high NA is desired, it is necessary to prevent spherical aberration from occurring due to the thickness error of the disk substrate.
JP-A-2000-21014 teaches that an objective lens is moved along the optical axis of the lens to cope with both the aberration caused by the defocusing of the lens and the aberration caused by the thickness error of the disk substrate. According to the disclosed method, a compensation signal to move the objective lens is generated based on the detection of interference rays contained in the reflection light from the storage disk. The disclosed light interference occurs by the interaction of 0-order diffracted rays (whose incident angle is equal to its reflection angle) with 1-order diffracted rays (whose incident angle is smaller than its reflection angle).
Referring to
FIG. 8
of the accompanying drawings, when the data-storing lands L of the disk D are irradiated by a laser beam, 0-order diffracted rays Di
0
and 1-order diffracted rays Di
1
will appear. As illustrated in the figure, these diffracted rays partially interfere with each other (see the hatched area F). When the substrate thickness of the storage disk D is normal, the intensity of the interference light (as measured in the hatched area F) exhibits no variation, as shown in FIG.
10
A. When the substrate thickness is not normal (namely, unduly small or large), the light intensity in the hatched areas F is not uniform (see FIGS.
10
B and
10
C), and spherical aberration occurs.
The same problem is encountered when the objective lens is defocused. Specifically, when the objective lens is deviated negatively (that is, the distance between the objective lens and the disk D is too small), a pattern as shown in
FIG. 10B
is observed. When the lens is deviated positively (i.e., the distance between the lens and the disk D is too large), a pattern as shown in
FIG. 10C
is observed. As seen from the graph in
FIG. 8
(the graph shows light intensity variations measured along the single-dot chain line DL), the light intensity varies in a similar manner for both a case where the objective lens is positively defocused and a case where the disk substrate has an unduly large thickness. Though not depicted, the light intensity curve for the negative lens defocus exhibits a similar variation to that of the light intensity curve for the disk substrate having an unduly small thickness.
The circular figure LA (the lowest illustration) in
FIG. 8
shows the layout of light-receiving sections of an optical detector disclosed in the above JP document (2000-21014). Based on the detected intensity of interference light, the detector LA generates compensation signals for the defocus of the lens and for the substrate thickness error. The detector LA is provided with two light-receiving areas La each of which is divided into two sections A and B for detection of interference light in the hatched areas F. The section A is responsible for receiving light from the inner half (i.e., the half closer to the center of the 0-order diffracted ray Di
0
) of the hatched area F, while the section B is responsible for receiving light from the outer half (i.e., the other half farther from the center of the 0-order diffracted ray Di
0
) of the hatched area F. The compensation signal generated by the detector LA corresponds to the difference between the light intensity I
A
detected by the section A and the light intensity I
B
detected by the section B. It should be noted here that the prior art compensation signal is generated without distinguishing the light intensity variation stemming from the lens defocusing and the light intensity variation stemming from the thickness error of the disk substrate. When the disk substrate has an unduly large thickness or the objective lens is positively defocused, the light intensity in the hatched area F becomes greater in an outer part of the area F. Thus, in this case, the compensation signal becomes negative (−). On the other hand, when the disk substrate has an unduly small thickness or the objective lens is negatively defocused, the compensation signal becomes positive (+). In accordance with the polarity and magnitude of the compensation signal, the objective lens may be moved along the optical axis toward or away from the storage disk, so that the compensation for the lens defocus and the substrate thickness error is made.
In the above method, the objective lens is moved along its optical axis for making the compensation. In this manner, while the intensity variation caused by the defocusing of the objective lens can be corrected (see FIG.
9
A), the intensity variation caused by the thickness error of the disk substrate may remain uncorrected (see FIG.
9
B). This difference results from the non-negligible discrepancy between the intensity curve stemming from the lens defocusing (see the graph in
FIG. 8
) and the intensity curve stemming from the substrate thickness error. Specifically, the aberration due to the thickness error increases in proportion to the thickness variation with a factor of the 4th power of the NA, whereas the aberration due to the lens defocusing increases in proport
Edun Muhammad
Greer Burns & Crain Ltd.
LandOfFree
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