Dynamic information storage or retrieval – Specific detail of information handling portion of system – Radiation beam modification of or by storage medium
Reexamination Certificate
2002-02-19
2004-09-14
Huber, Paul W. (Department: 2653)
Dynamic information storage or retrieval
Specific detail of information handling portion of system
Radiation beam modification of or by storage medium
C369S044230, C369S053120
Reexamination Certificate
active
06791934
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an optical information reproducing apparatus for reproducing a next-generation high density optical disc, as well as to an optical head and an objective lens optical system which are incorporated therein.
Recent years, high-density recording of an optical disc has been steadily developed, and in a digital versatile disc (DVD) the storage capacity of both of a read-only memory disc (ROM) and a rewritable disc (RAM) is as high as 4.7 GB. In addition to this, in recent years at which satellite broadcasting is to be digitized immediately, the optical disc is expected to be large capacity of 20 GB or more where high definition moving picture can be recorded for two hours or more.
A size of a light beam spot that directly restricts a recording density of the optical disc is given as &lgr;/NA when a wavelength of a light beam is represented as &lgr; and a numerical aperture of an objective lens is represented as NA. Accordingly, the wavelength must be set short or the numerical aperture must be set large in order to realize an optical disc with large capacity. With respect to the wavelength, development of a blue-violet laser diode which emit a light beam of 405 nm has been advanced, and it has been forecasted to realize an optical disc with capacity of 12 GB that is about 2.6 times as large as the present DVD for which a light beam of 650 nm is used. In order to further increase the capacity to 20 GB or more, NA must be increased to be 1.3 times as large as the present DVD, that is, 0.77 or more.
As the conventional technology to increase NA as described above, there has been, for example, Japanese Patent Laid-Open No. 11(1999)-195229. In this technology, NA is increased to 0.85 as the maximum value by use of an objective lens which is composed of two lens elements in two groups. Accompanied with the increase of NA at this time, there is a problem that a spherical aberration and a coma, which are caused by an error of a substrate thickness of a disc and an inclination thereof, increase. To cope with such a problem, the coma due to the inclination of the disc is decreased by decreasing the substrate thickness to 0.1 mm, and the spherical aberration due to the error of the substrate thickness is compensated by detecting the substrate thickness based on a difference of a focus error signal between a disc surface and a recording surface and by changing an interval between the two lenses in response to the substrate thickness. Herein, an interval between a final surface of the lens system and the surface of the substrate surface of the disc when a recording film is focused, that is, a working distance, is 0.13 mm, and an effective pupil diameter of the two-element objective lens is 3 mm &phgr;.
In the above described prior arts, when the working distance is very short and a focus servo comes off by any chance during a recording/reproduction operation, the lens collides against the disc and the disc may be damaged. Moreover, there is a problem that a permissible limit of the interval between the two lenses and decentering of the two lenses relative to each other are very strict and adjustment is difficult.
The simplest and essential means for widen the working distance of the high NA lens in the above described prior arts is to use one lens in stead of the two lenses. This means is described by use of FIGS.
1
(
a
) and
1
(
b
). FIG.
1
(
a
) shows a state where the conventional two lenses
101
and
102
collect a light beam, and FIG.
1
(
b
) is a schematic view showing a difference of the working distance when the light beam collection is performed by one high NA lens
103
. In both of FIGS.
1
(
a
) and
1
(
b
), luminous flux is collected in a recording film
105
through a protection layer
104
of a disc by the same NA. In FIG.
1
(
a
), in order to respectively distribute refractive power to the two lenses
101
and
102
, the second lens
102
will be inserted in the luminous flux collected by the first lens
101
. Therefore, compared to the case of FIG.
1
(
b
) where the luminous flux is collected by one lens, an effective pupil diameter D
2
of the lens is smaller than an effective pupil diameter D of FIG.
1
(
b
), and a working distance WD
1
is obliged to be shorter than a working distance WD
2
of FIG.
1
(
b
). On the contrary, the working distance can be further widened by collecting the luminous flux by one lens than by collecting the luminous flux by the conventional two lenses. However, as a matter of course, the reason why the two lenses are necessary has heretofore been existed. This is a problem in manufacturing the lens. In a lens offering a large NA, necessary precision concerning a decentering between both surfaces of the lens and an error of a surface interval between both surfaces of the lens is very strict, and large aberration occurs by a slight error. To avoid the aberration, by dispersing the necessary refractive power conventionally to the two lenses, manufacturing of the respective lenses is made easier. Accordingly, to acquire a high NA with one lens, either a manufacture technology to increase a positional precision between both surfaces of the lens or means for compensating the aberration caused by a manufacture error is necessary.
With respect to means for compensating the aberration, there is, for example, means described in Japanese Patent Laid-Open No. 12(2000)-182254, which compensates a spherical aberration caused by the error of the surface interval. In this Japanese Patent Laid-Open No. 12(2000)-182254, a spherical aberration at a light convergence spot is optically detected, and luminous flux made incident on an objective lens is slightly diverged or converged, whereby a spherical aberration compensation optical system for causing spherical aberration is driven and spherical aberration of an optical system is compensated. The error of the surface interval between the first and second surfaces of one lens with a high NA can be compensated by combining such technologies. At this time, the light beam itself made incident on the lens does not show aberration, and spherical aberration occurs in an objective lens by allowing the light beam, which changes its diversion/conversion state, to be incident on the objective lens. Accordingly, even if the lens moves from an optical axis in a radius direction of a disc by a tracking operation, an axis of the spherical aberration generated does not swerve substantially.
With respect to coma caused by decentering, for example, a detection method of coma is described in Japanese Patent Laid-Open No. 12(2000)-214048. In this Japanese Patent Laid-Open No. 12(2000)-214048, the coma in a radius direction of a disc is detected based on difference of a push-pull signal between an inner side of luminous flux and an outer side thereof, and coma in a tangent direction is detected based on difference of a push-pull signal in a diagonal direction among four segmented regions split in a radius direction of the disc and a tangent direction thereof. As compensation means, for example, in Japanese Patent Laid-Open No. 13(2001)-4972, a technology is described, in which coma is given by giving a phase distribution having a sign reverse to that a W-shaped phase distribution given in the form of a fourth order function with a shift. By combining these technologies, the detection of the coma and the compensation thereof are made possible. However, in this case, the coma does not occur in the objective lens unlike the case of the foregoing spherical aberration compensation, and the coma occurs in a compensation device of the coma. Accordingly, if the objective lens deviates from an optical axis by a tracking operation, an axis of the coma deviates in accordance with the deviation of the objective lens. In this case, since the axis of the coma of the objective lens to be compensated and the axis of the coma compensated swerve from each other, astigmatism occurs substantially owing to a difference of the axes. This can be understood by the following equations simply. Whe
Ariyoshi Tetsuo
Maruyama Koichi
Shigematsu Kazuo
Shimano Takeshi
Takeuchi Shuichi
A. Marquez, Esq. Juan Carlos
Fisher Esq. Stanley P.
Hitachi , Ltd.
Huber Paul W.
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