Radiant energy – Photocells; circuits and apparatus – Photocell controls its own optical systems
Reexamination Certificate
2002-05-10
2004-06-29
Porta, David (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controls its own optical systems
C369S044260
Reexamination Certificate
active
06756575
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an aberration detecting method for detecting an aberration occurring in a optical convergence system in an optical pickup device, an optical recording and reproduction method using the aberration detecting method, and an optical recording and reproduction apparatus.
BACKGROUND OF THE INVENTION
There has been a requirement of an optical disk having greater recording density in proportion to the increase of in the volume of information. The recording density of optical disks has been increased by increasing the linear recording density and narrowing the pitch of tracks. To increase the recording density of optical disks as above, it is necessary to reduce the beam diameter of light beam converging on a information recording layer of the optical disk.
Possible methods to reduce the beam diameter of the light beam are such as: increasing a numerical aperture (NA) of light beam projected from an objective lens as an optical convergence system in the optical pickup device which records and reproduces in and from optical disks; and shortening the wavelength of the light beam converged on the information recording layer of the optical disk.
It is considered that shortening the wavelength of the light beam can be realized by switching the light source from a red semiconductor laser to a violet semiconductor laser, the latter brought into full-scale commercial production in a reasonably short time.
Meanwhile, one of the proposed methods to realize an objective lens having large numerical aperture is to incorporate a hemispherical lens into the objective lens so as to constitute the objective lens by two lenses (two groups of lenses).
An optical disk is generally arranged such that an information recording layer is covered by a cover glass for resisting dust and scratches. Thus after passing through the objective lens of the optical pickup device, the light beam passes through the cover glass and then converges on the information recording layer under the cover glass so as to come into a focus.
When the light beam is passing through the cover glass, a spherical aberration (SA) occurs. The spherical aberration SA is indicated as:
SA∝d·NA
4
(1)
so that the spherical aberration SA is proportional to the thickness d of the cover glass and the fourth power of the NA of the objective lens. Generally, the objective lens is designed to cancel out the spherical aberration so that the spherical aberration of the light beam having passed through the objective lens and the cover glass is small enough.
However, when the thickness of the cover glass is different from a predetermined value, the light beam converging on the information recording layer has the spherical aberration so that the beam diameter becomes large, and hence writing and reading of the information cannot be properly done.
Moreover, the equation (1) shows that the larger the error &Dgr;d of the thickness of the cover glass is, the larger the error &Dgr;SA of the spherical aberration is, and thus proper writing and reading of the information is impaired.
Also, for the sake of increasing the density of recorded information in the direction of the thickness of an optical disk, a multilayer optical disk formed by depositing information recording layers such as, for instance, a DVD (Digital Versatile Disk) having two information recording layers is created so as to have already been commercially available. The optical pickup device for recording and reproducing in and from this kind of multilayer optical disk has to be arranged such that the light beam converges at a sufficiently small point in each of the information recording layers of the optical disk.
In the above-identified multilayer optical disk, the distance from the surface of the disk (surface of the cover glass) to the information recording layer is different in each of the layers so that the extent of the spherical aberration, which occurs when the light beam passes through the cover glass of the optical disk, is also different in each of the layers. For instance, the difference (error &Dgr;SA) of the spherical aberration between neighboring information recording layers is proportional to an interlayer distance t (equivalent to d) between the neighboring information recording layers, according to the equation (1).
In the DVD having two information recording layers, the NA of the objective lens of the optical pickup device is only around 0.6, thus according to the equation (1), it is understood that a not-so-great amount of the error &Dgr;d of the thickness of the cover glass causes little influence on the error &Dgr;SA of the spherical aberration.
Thus, in a conventional DVD device using the optical pickup device with the NA around 0.6, the error &Dgr;SA of the spherical aberration caused by the error &Dgr;d of the thickness of the cover glass is small so that the light beam converges at a sufficiently small point in each of the information recording layers.
However, provided that the error &Dgr;d of the thickness of the cover glass is consistent, the more the NA increases, the greater the error of the spherical aberration SA becomes. For instance, the spherical aberration SA when the NA is 0.85 is around 4 times greater than the case that the NA is 0.6. Thus according to the equation (1), when the NA is large such as the NA=0.85, the spherical aberration generated by the error of the thickness of the cover glass is great.
Similarly, in the multilayer optical disk, provided that the interlayer distance t between the adjacent layers is consistent, the larger the NA of the objective lens of the optical pickup device is, the greater the difference (the error &Dgr;SA) of the spherical aberration is. For instance, the spherical aberration SA when the NA is 0.85 is around 4 times greater than the case that the NA is 0.6. Thus according to the equation (1), when the NA is large such as the NA=0.85, the difference of the spherical aberration between the information recording layers is great.
It is thus understood that the performance of the objective lens having a high NA is certainly influenced by the error of the spherical aberration so that the spherical aberration causes the degradation in reading accuracy, and hence it is necessary to compensate the spherical aberration to realize an optical disk having greater recording density by the objective lens having a high NA.
Accordingly, there are methods to detect so as to compensate the spherical aberration. For instance, Japanese Laid-Open Patent Application, No. 2000-171346 (Tokukai 2000-171346; published on Jun. 23, 2000) discloses an optical pickup device which detects so as to compensates the spherical aberration. The objectives of this optical pickup device are achieved through the use of the property such that light beam around the optical axis converges at a point which is different from a point where light beam surrounding the beam around the optical axis converges, when the light beam converges on the information recording layer of the optical disk.
According to the optical pickup device disclosed in the above-identified application, the light beam to be detected is divided into: a light beam around the optical axis; and a light beam surrounding the beam around the light axis, using an optical element such as a hologram. Then the drift of either one of the light beams from a predetermined convergence point on the information recording layer is detected when the spherical aberration occurs, and in accordance with the result of the detection, the spherical aberration is compensated so that it is possible to sufficiently reduce the diameter of the light beam converging on each of the information recording layers of the optical disk.
Incidentally, an optical recording medium has sections for recording information, etc., such as lands, grooves and pits. For being certainly captured by a light spot formed on the optical recording medium due to the projection of light beam, the widths of the lands, grooves and pits are formed so as to be narrowe
Nakano Ikuo
Tadano Hiroyuki
Conlin David G.
Edwards & Angell LLP
Lee Patrick J.
Penny, Jr. John J.
Porta David
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