Radiant energy – Photocells; circuits and apparatus – Photocell controls its own optical systems
Reexamination Certificate
2001-11-14
2004-11-23
Pyo, Kevin (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controls its own optical systems
C369S044230, C369S112100
Reexamination Certificate
active
06822209
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a focal point dislocation detection method of detecting a focal point dislocation generated in a converging optical system, and to an optical pickup apparatus in which the focal point dislocation detection method is employed.
BACKGROUND OF THE INVENTION
Recently, there is a demand for an optical disc having a high recording density, in response to an increase in an amount of information. The high recording density of the optical disc has been achieved by increasing linear recording density in an information recording layer of the optical disc or by narrowing a track pitch of the optical disc. In response to the trend for the high recording density of the optical disc, it is necessary to have a small beam diameter of a light beam that is converged on the information recording layer of the optical disc.
In order to make the beam diameter of the light beam small, (a) the light beam, which is directed from an objective lens (a converging optical system of a light pickup apparatus for recording/reproducing the optical disc), has a high numerical aperture (NA), or (b) the light beam has a short wave length.
As to the short wave length of the light beam, it is believed that the short wave length of the light beam can be realized by using, as a light source, a bluish purple semiconductor laser, which has been developed paving the way for its commercial use, instead of a red semiconductor laser.
On the other hand, in order to realize the objective lens having the high numerical aperture, suggested is a method in which an objective lens is coupled with a semispherical lens so as to constitute an objective lens with the two lenses (a couple of lenses), thereby increasing the high numerical aperture.
In general, the information recording layer of the optical disc is covered with a cover glass so that the information recording layer can be protected from being attached by a dust or being damaged. Therefore, a light beam that has passed through the objective lens of the optical pickup apparatus passes through the cover glass, then is converged to make a focal point on the information recording layer that is underneath the cover glass.
When the light beam passes the cover glass, spherical aberration (SA) is generated. The spherical aberration is obtained by an equation (1) as follows:
SA∝d·NA
4
(1).
As indicated by the equation (1), the spherical aberration is proportional to a thickness of the cover glass d, and the fourth power of NA. Because the objective lens is usually so designed to compensate the spherical aberration, the light beams that has passed through the objective lens and the cover glass, has a sufficiently small spherical aberration.
However, if the cover glass has a thickness that is different from a predetermined thickness, the spherical aberration is generated in the light beam converged on the information recording layer, thereby enlarging its beam diameter, causing such a problem that correct reading and writing of the information are impossible.
Moreover, the equation (1) indicates that a larger error &Dgr;d of the thickness of the cover glass gives a larger error &Dgr;SA of the spherical aberration, thereby making it impossible to read and write the information correctly.
Furthermore, a multilayer optical disc, in which the information recording layers are laminated, has been developed for a commercial use, so as to record the information in a still higher density in terms of a thickness direction of the optical disc. For example, a DVD (Digital Versatile Disc) having two information recording layers has been developed as the multilayer optical disc. For an optical pickup apparatus for recording/reproducing the multilayer optical disc, it is necessary that the light beam is converged sufficiently small for each information recording layer of the optical disc.
For the multilayer optical disc having the plural information recording layers, thicknesses from a surface of the optical disc (surface of the cover glass) to the respective information recording layers are different from each other. Because of this, each information recording layer has respectively a different spherical aberration, which has been generated when the light beam passes the cover glass of the optical disc. In this case, for example, a difference (error &Dgr;SA) between the spherical aberration generated in the information recording layers adjoining each other is proportional to a layer-to-layer space t between the adjacent information recording layers, according to the equation (1).
For the DVD having the two information recording layers, used is an optical pickup apparatus that has an objective lens NA of which is as small as 0.6. Because of this, in the DVD having the two information recording layers, the error &Dgr;d of the thickness of the cover glass does not largely affect the error of the spherical aberration &Dgr;SA, even though the error &Dgr;d of the thickness of the cover glass becomes large to some extent, according to the equation (1).
Therefore, with a DVD apparatus provided with the conventional optical pickup apparatus having the numerical aperture NA of about 0.6, it is possible to converge the light beam sufficiently small on each information recording layer, because the error &Dgr;SA of the spherical aberration, which is generated due to the error &Dgr;d of the thickness of the cover glass of the DVD, is small.
However, even if the error &Dgr;d of the thickness of the cover glass is constant, the greater the NA is, the larger spherical aberration SA is generated. For example, when NA=0.85, an approximately four-time larger spherical aberration SA is generated, compared with the case where NA=0.6. Therefore, the equation (1) shows that the respective spherical aberration due to the error in the thickness of the cover glass becomes larger as the NA becomes higher, for example, when NA=8.5.
Similarly, in case of the multilayer optical disc, the spherical aberration have a greater differences (error &Dgr;SA) as the objective lens of the optical pickup apparatus has a larger NA, even if the layer-to-layer distance t between the adjacent information recording layers is constant. For example, when NA=0.85, a approximately four-time larger difference is generated between the spherical aberration SA, compared with the case where NA =0.6. Therefore, according to the equation (1), it is indicated that the difference between the respective spherical aberration between the respective information layers gets greater as the NA becomes higher, for example, when NA=8.5.
Thus, it is a problem for an objective lens having a high NA, which is inevitably affected by the error of the spherical aberration, that the information is read in a low accuracy. Thus, it is necessary to compensate for the spherical aberration in order to realize the high recording density by using the objective lens of the high NA.
As a method for detecting and compensating the spherical aberration, for example, the U.S. patent application, Ser. No. 09/456,414 (filed on Dec. 8, 1999) discloses an optical pickup apparatus for detecting and compensating the spherical aberration. The optical pickup apparatus takes advantages of a feature that the light beam in the vicinity of an optical axis is converged in a different position from a position where the light beam outside the vicinity of the optical axis is converged, in accordance with the spherical aberration, when the light beam is converged on the information recording layer of the optical disc.
The optical pickup apparatus disclosed in the publication, in which an optical element such as a hologram is used to separate a light beam, which is to be detected, into the light beam in the vicinity of the optical axis and the light beam outside the vicinity of the optical axis so as to detect the dislocation of the convergence of one of the light beams, when the spherical aberration is generated, the convergence of which is dislocated from the information recording layer
Nakano Ikuo
Tadano Hiroyuki
Conlin David G.
Daley, Jr. William J.
Edwards & Angell LLP
Pyo Kevin
Sharp Kabushiki Kaisha
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