Dynamic information storage or retrieval – Specific detail of information handling portion of system – Radiation beam modification of or by storage medium
Reexamination Certificate
2003-01-17
2004-08-17
Patel, Gautam R. (Department: 2655)
Dynamic information storage or retrieval
Specific detail of information handling portion of system
Radiation beam modification of or by storage medium
C369S044230
Reexamination Certificate
active
06778485
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical storage device for accessing an optical recording medium such as a phase change type of optical disk and an optical magnetic disk, and a liquid crystal device preferably applicable to such a optical storage device.
2. Description of the Related Art
An optical disk, such as a phase change type (PD) of optical disk and an optical magnetic disk (MO), is a portable recording medium having a high storage capacity, and is being considered as a preferred recording medium of a personal computer. A possibility of obtaining a higher density and larger capacity for such an optical disk is being pursued.
To implement a larger capacity of an optical disk while a recording area of the optical disk is maintained as it is, there is a need to increase a recording density of the recording area or to establish a multi-layer construction of recording. In order to increase a recording density of the recording area, there is basically a need to reduce a condensing spot of a laser beam to be used, while there is considered means such as a magnetic resolution.
Generally, a diameter of a spot of a laser beam is in proportion to
&lgr;/NA (&lgr;:wavelength of light, NA: numerical aperture).
Therefore, to implement a higher density of recording for an optical disk, there is a need to use a laser (for example, a laser emitting light of blue) which is short in wavelength, or increase NA of an objective lens.
However, in the event that NA of an objective lens is increased, it involves a problem of a spherical aberration due to an unevenness in thickness of transparent protective layers on a surface of an optical disk when the optical disk is manufactured.
Particularly, since the optical disk is constructed as a storage medium which is detachably loaded, there is needed a transparent protective layer on a layer which is essentially necessary for a storage and a pick-up of information, such as a reflecting layer and a recording layer. The unevenness in thickness of the protective layer on the manufacture is of ±50 &mgr;m or so as an unevenness on an individual optical disk (an unevenness as an individual difference) and is of ±10 &mgr;m or so as a variation inside the same optical disk (an unevenness inside an individual). The unevenness in thickness of transparent protective layers on a surface of an optical disk causes a spherical aberration on light condensed on the recording layer, and this spherical aberration has a bad effect on recording and reading of a pit mark.
FIG. 1
is a diagram showing a spherical aberration RMS to an unevenness of a protective layer.
FIG. 1
shows results of calculations in case of NA=0.6 corresponding to the present DVD, and in case of NA=0.85. In a case where an unevenness in thickness of the protective layer on the manufacture is of ±50 &mgr;m, if NA=0.6, it is within an aberration allowance. On the other hand, if NA=0.85, an aberration, which cannot be covered, occurs. Therefore, to implement a high NA of objective lens there is needed a mechanism for correcting the spherical aberration in accordance with an unevenness in thickness of the protective layer.
In order to satisfy such a requirement, there is proposed a scheme in which two objective lenses are used so that a spherical aberration is actively corrected by mechanically altering an interval between the two objective lenses (cf. Japanese Patent Laid Open Gazette Hei. 8-212579).
However, according to this proposal, a further mechanical driving for the objective lens is added. Thus, this is associated with a problem that a weight of a head portion of a pick-up is increased and a larger space is needed.
On the other hand, there is proposed a scheme in which a liquid crystal device is disposed in an optical path so as to correct an aberration (cf. for example, Japanese Patent Laid Open Gazette Hei. 8-212611, Japanese Patent Laid Open Gazette Hei. 9-128785).
As such a liquid crystal device, there are two known two types of an electrode structure of two dimensional matrix configuration and an electrode structure patterned after a pattern associated with the aberration.
To implement a matrix configuration of electrode, there is a need to use a TFT. The TFT matrix panel needs a very complicated manufacturing process, and thus this is associated with such a problem that it is obliged to increase greatly the cost.
On the other hand, with respect to the electrode structure patterned, it is associated with a problem that a phase distribution of light, which is formed by a distribution of index of refraction of the liquid crystal device, is fixed. Thus, there is a need to dispose the liquid crystal device as to an optical axis with great accuracy. This involves such a problem that a strict precision of an alignment is required. In order to correct a spherical aberration through a patterned electrode structure, an electrode structure having a concentric circle of pattern is adopted. However, to correct the spherical aberration with greater accuracy, if the concentric circle of pattern is given with greater definition, this causes a polarization of the baser beam to be disturbed. Thus, this is not suitable for correction of the spherical aberration when the objective lens of a high NA is adopted. Further, in the event that the concentric circle of pattern is given with greater definition, this causes the number of lead wires derived from a ring electrode inside the concentric circle to be increased, and thereby increasing a wiring area for the lead wires. This is associated with a problem on manufacture that the concentric circle of pattern cannot be formed per se.
Further, Japanese Patent Laid Open Gazette Hei. 9-128785 discloses the use of a strip shaped electrode. However, the use of a strip shaped electrode cannot almost correct the aberration from a view point of an aberration correction at the time of a high NA.
In the above description, the necessity for the aberration correction is explained in association with an unevenness in thickness of the protective layer of the optical disk. On the other hand, also when it is intended that an optical storage medium having a multi-layer construction of recording, that is, a plurality of information recording layers in a depth direction, is implemented, there is a need to actively correct the aberration due to the variation in depth.
While the above explanation is made in connection with the optical disk, the above-mentioned problems are applied to, for example, a tape-like shaped optical storage medium too, regardless of the disk configuration.
SUMMARY OF THE INVENTION
In view of the foregoing, it is an object of the present invention to provide an optical storage device capable of effectively correcting an aberration caused by a variation in a depth from a surface of an optical storage medium to a condensing point, even if the depth is varied, for example, in cases of an unevenness in thickness of a protective layer and the multi-layer recording, and a liquid crystal device capable of being preferably adopted to the optical storage device for a use of the aberration correction.
To achieve the above-mentioned objects, the present invention provides an optical storage device comprising:
a light source;
an irradiation optical system for leading light emitted from said light source to condense on a predetermined optical storage medium;
a photo detector for picking up a signal light carrying information stored in said optical storage medium to read the information, said signal light being condensed onto said optical storage medium and reflected on said optical storage medium;
a pick-up optical system for leading said signal light to said photo detector;
a liquid crystal device having first and second liquid crystal layers disposed in mid way of an optical path of said irradiation optical system and extending in parallel with a direction intersecting said optical path, a plurality of first electrodes for driving said first liquid crystal layer,
Hasegawa Shin-ya
Uchida Tatsuo
Yoshikawa Hiroyasu
Fujitsu Limited
Greer Burns & Crain Ltd.
Patel Gautam R.
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