Dynamic information storage or retrieval – Dynamic mechanism subsystem – Specified detail of transducer assembly support structure
Reexamination Certificate
2000-07-21
2003-12-16
Davis, David (Department: 2652)
Dynamic information storage or retrieval
Dynamic mechanism subsystem
Specified detail of transducer assembly support structure
Reexamination Certificate
active
06665259
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical disk apparatus. In particular, the present invention relates to an optical disk apparatus of the type which includes a lens system for forming a laser spot on a disk, wherein the lens system includes a first lens mounted on an actuator supported by a carriage, and a second lens mounted on a slider held in facing relation to the disk. In this specification, the “optical disk apparatus” refers not only to a device designed exclusively for reading out data from an optical disk but also to a magneto-optical disk apparatus capable of repeatedly writing data to a magneto-optical disk by magnetic field modulation, light pulse modulation, etc.
2. Description of the Related Art
In general, an optical disk apparatus is arranged to store more data than many other data-storing devices. Thus, various kinds of optical disk apparatus have been developed and widely used as an external storage unit for e.g. computers.
The current data storing capacity of an optical disk apparatus, however, is not large enough to support the full-scale “Multimedia Era” to come. Hence, it is necessary to increase the data-recording density of the optical disk apparatus. For this purpose, the following two techniques may be employed. The first one is to use a semiconductor laser of a shorter wavelength (i.e., a blue semiconductor laser) for a light source, and the second one is to increase the numerical aperture (NA) of an objective used for the optical head.
Blue semiconductor lasers, however, are rather expensive, and their output efficiency is not high enough. Further, they may fail to perform the required functions when the temperature varies. Thus, to use a blue semiconductor laser is not a good idea.
The second option (increasing the NA of the objective) is also disadvantageous in the following points. To increase the NA of an objective, it is necessary to reduce the radius of curvature of the lens surfaces (incidence surface and exit surface). However, to provide the lens surfaces with a small radius of curvature will require a difficult procedure. Further, even if the incidence surface and the exit surface are accurately made and have the intended radius of curvature, they may be tilted or offset relative to each other. Still further, even if the objective lens itself is correctly made (i,e., with the correct radius of curvature and without any erroneous positioning of the two surfaces), the installment of the objective to a supporting member may fail to be performed accurately, thereby rendering the objective slant or offset relative to the supporting member. It is also probable that the optical disk being rotated in operation may be tilted with respect to the objective.
The above-described inappropriateness may give rise to coma (proportional to the NA to the third power). Further, when the optical disk does not have a uniform thickness, spherical aberration (proportional to the NA to the fourth power) will result. The occurrence of coma and spherical aberration may hinder the data-recording and data-reading operations. Since coma and spherical aberration are proportional to the NA of the objective, increasing the NA of an objective may better be avoided.
In this connection, attention should be drawn to e.g. Japanese Patent Application No. 10(1998)-185283 which discloses an optical system consisting of two objectives. It should be noted that this application is laid open on Jan. 21, 2000.
According to the teaching of the above Japanese application, the overall NA of the optical system is increased. In this way, the coma and spherical aberration caused by the increased NA of a single objective are advantageously reduced or even eliminated.
Referring now to
FIG. 13
of the accompanying drawings, the principal portions of the optical disk apparatus disclosed in the above Japanese Patent Application (10-185283) will be described below.
Specifically, the optical disk apparatus includes a carriage
10
′ which is caused to move radially of an optical disk D′. The carriage
10
′ carries an actuator
20
′ which in turn holds a first lens
31
′. The actuator
20
′ is a two-dimensional actuator which is movable perpendicularly to the disk D′ for focus control and radially of the disk D′ for tracking control. A second lens
32
′ is arranged above the first lens
31
′, supported by an elongated suspension member
40
′ fixed to the upper surface of the carriage
10
′. The second lens
32
′ is held by a slider
41
′ mounted on the upper end of the suspension member
40
′.
The first lens
31
′ and the second lens
32
′ are arranged vertically so that their optical axes coincide with each other. With such an arrangement, even if the NA of each lens is rather small (meaning that the lens is easy to process), the overall NA of the optical system as a whole can be rendered sufficiently large. Further, in the optical head, the slider
41
′ is pivotably attached to the upper end of the suspension member
40
′. Thus, when the slider
41
′ is brought close to the disk D′, the slider can automatically adjust its posture to be held in close facing relation to the surface of the rotating disk D′. In this manner, the coma due to the tilting of the disk D′ is advantageously prevented from occurring. In addition, the spherical aberration due to an uneven thickness of the disk D′ is avoided by adjusting the distance between the first and the second lenses
31
′,
32
′ by moving the actuator
20
′ perpendicularly to the disk D′.
While the above optical head has various advantages, it may suffer the following problem. Ideally, the optical axis of the first lens
31
′ coincides with the axis of the second lens
32
′, as shown in FIG.
14
. However, since the slider
41
′ is supported by the elongated suspension member
40
′, the optical axis L
1
′ of the first lens
31
′ may be horizontally displaced from the optical axis L
2
′ of the second lens
32
′ (see FIG.
15
). This may be caused in part by an error in fixing the slider
41
′ to the suspension member
40
′ and/or an error in fixing the suspension member
40
′ to the carriage
10
′. Another cause of the deviation of the axes L
1
′, L
2
′ may be thermal expansion of the suspension member
40
′ or slider
41
′. In this case, the deviation of the optical axes L
1
′, L
2
′ may begin to occur after the optical disk apparatus is turned on. Thereafter, the deviation may expand as the suspension member
40
′ and the slider
41
′ are being heated up.
FIG. 16
is a graph showing the relation between the aberration and the discrepancy between the two optical axes L
1
′, L
2
′. This graph clearly shows that the aberration increases as the two optical axes are spaced further away from each other. As stated above, the actuator
20
′ carrying the first lens
31
′ is movable to perform the focus control and the tracking control. Considering this function, the discrepancy between the two axes L
1
′, L
2
′ may need to be smaller than 40 &mgr;m for example, so that sufficient data-reading and data-writing margins are ensured. However, the extent of the thermal expansion of the suspension member
40
′ and/or slider
41
′ is often unpredictable. Thus, it is difficult or even impossible to assembly the optical head in a manner such that the discrepancy between the two optical axes L
1
′, L
2
′ is to be below 40 &mgr;m.
SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention to provide an optical disk apparatus with an optical head which includes an objective lens system consisting of a plurality of lenses, wherein the offset between the optical axes of the respective lenses is adjusted.
Another object of the present invention is to provide
Davis David
Fujitsu Limted
Greer Burns & Crain Ltd.
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