Radiant energy – Photocells; circuits and apparatus – Photocell controls its own optical systems
Reexamination Certificate
2000-05-24
2004-05-04
Allen, Stephone B. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controls its own optical systems
C250S216000, C359S717000, C369S112010
Reexamination Certificate
active
06730896
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an optical pickup device which records or reproduces information on a recording medium by converging a light beam onto the recording medium.
BACKGROUND OF THE INVENTION
It is desirable that an objective lens of an optical pickup device converges a light beam onto an information recording layer of a recording medium without aberration. Moreover, a decrease of the beam diameter of the light beam converged onto the information recording layer can increase the recording density of the recording medium and hence the recording capacity of the recording medium is increased.
One example of a method of decreasing the light beam increases the numerical aperture (NA). In a prior art, for example, the numerical aperture is 0.45 for a CD (compact disk) and 0.6 for a DVD (digital versatile disk. In recent years, the beam diameter is decreased by further increasing the numerical aperture.
However, for example, if the numerical aperture is increased to 0.85, it is difficult to form an objective lens by a single lens. Namely, the higher the numerical aperture, the more limited fabrication tolerance and assembling tolerance of the lens. Consequently, the objective lens formed by a single lens has difficulty in limiting the aberration within the tolerance and can not decrease the beam diameter.
Hence, if an objective lens and a spherical lens are combined to form an objective lens by two pieces of lenses, the fabrication tolerance and assembling tolerance of the lens are increased, thereby achieving a high numerical aperture.
Meanwhile, for the recording medium, the information recording layer is covered with a cover glass to provide protection against dust and scratches. More specifically, a light beam emitted from the objective lens passes through the cover glass, and converges on the information recording layer located under the cover glass and is hence focused. At this time, the beam diameter is minimized. However, if the thickness of the cover glass is not a predetermined value, the spherical aberration (SA) occurs and the beam diameter is increased. In this case, information can not be correctly read or written with respect to the information recording layer.
Moreover, the spherical aberration is proportional to the thickness error &Dgr;d in the cover glass and the fourth power of the numerical aperture NA.
SA∝&Dgr;d·NA
4
(1)
Therefore, even when the thickness error &Dgr;d is the same, the spherical aberration SA is increased with an increase of the numerical aperture NA. For instance, when the numerical aperture is 0.85, the spherical aberration SA is about four times larger than a spherical aberration when the numerical aperture is 0.6. Thus, as in the case of a numerical aperture of 0.85, when the numerical aperture is high, it is necessary to compensate for the spherical aberration caused by the thickness error in the cover glass.
On the other hand, Japanese laid-open publication (Tokukaihei) No. 8-212579 (published on Aug. 20, 1996) proposes a method of limiting an increase of the beam diameter by cancelling the spherical aberration caused by the thickness error in the cover glass and variations in the thickness of the objective lens.
In a conventional technique disclosed in the above publication, an objective lens unit
51
shown in
FIG. 13
is provided. This objective lens unit
51
includes a first lens
52
and a second lens
53
. The first lens
52
is held by a first holder
54
. The second lens
53
is hemispherical and held by a second holder
55
. Incidentally, the first holder
54
and the second holder
55
form a capacitor. Moreover, a cover glass
56
b
is provided on a side facing the objective lens unit
51
of a recording medium
56
, and an information recording layer
56
a
is provided on the other side of the recording medium
56
. Besides, a light beam
57
passed through the objective lens unit
51
is made to converge onto the information recording layer
56
a.
The electrostatic capacity C of the capacitor is given by
C∝S/d
(2)
wherein S is the area where the first holder
54
and the second holder
55
face each other and d is the distance between the first holder
54
and the second holder
55
.
Thus, it is possible to control the distance d between the first holder
54
and the second holder
55
by detecting the electrostatic capacity C.
Moreover, in order to prevent a crash of the recording medium
56
and the objective lens unit
51
during a rotation of the recording medium
56
, it is necessary to provide a work distance WD between the recording medium
56
and the objective lens unit
51
. As shown in the paper “High-numerical-aperture lens system for optical storage, Optics Letters, Vol. 18, No. 4, pp. 305-307, (1993)”, the spherical aberration SA caused by this work distance WD is given by
SA
=−(
WD
/8)·
n
2
(
n
2
−1)sin
4
&thgr;
0
(3)
wherein n is the refractive index of the second lens
53
and sin&thgr;
0
is the numerical aperture NA of the first lens
52
.
The objective lens unit
51
formed by the first lens
52
and the second lens
53
is optically designed to eliminate the above-mentioned spherical aberration for a certain work distance WD. When a displacement from the above work distance WD is referred to as &Dgr;WD, a spherical aberration SA
1
caused by this work distance &Dgr;WD is given by
SA
1
=−(&Dgr;
WD
/8)·
n
2
(
n
2
−1)sin
4
&thgr;
0
(4)
according to equation (3).
Besides, as shown in the above-mentioned paper, a spherical aberration SA
2
caused by the thickness error &Dgr;d in the cover glass
56
b
is given by
SA
2
=(&Dgr;
d
)
2
/(8
a
)
n
(
n
−1)sin
4
&thgr;
0
(5)
wherein a is the radius of curvature of the spherical surface of the second lens
53
.
Therefore, even if there are a thickness error &Dgr;d in the cover glass
56
b
and variations in the thickness of the objective lens unit
51
, it is possible to cancel the occurrence of spherical aberration by cancelling out the spherical aberrations SA
1
and SA
2
. In other words, the work distance WD needs to be controlled to an optimum value by changing the work distance WD according to the thickness error &Dgr;d in the cover glass
56
b
and the variations in the thickness of the objective lens unit
51
.
In the above-mentioned conventional structure shown in
FIG. 13
, the distance between the first lens
52
and the recording medium
56
is adjusted to be uniform by a focusing operation. Furthermore, the distance between the first lens
52
and the second lens
53
is controlled to an optimum value by detecting the electrostatic capacity between the first holder
54
and the second holder
55
. As a result, the work distance WD between the second holder
55
and the recording medium
56
is controlled to an optimum value. Consequently, even if there are a thickness error &Dgr;d in the cover glass
56
b
and variations in the thickness of the objective lens unit
51
, it is possible to limit the occurrence of spherical aberration.
However, in the above-mentioned conventional structure, in order to detect the electrostatic capacity C of the capacitor formed by the first holder
54
and the second holder
55
, it is necessary to lead conductors out of the first holder
54
and the second holder
55
, respectively. On the other hand, in general, a focusing operation and a tracking operation for displacing the objective lens in the optical axis direction and a direction perpendicular to the optical axis are performed in an optical pickup. Therefore, the conductors viciously affect the performance of the focusing operation and the tracking operation of the objective lens unit
51
. Namely, deterioration of the frequency characteristics and a tilt of the optical lens unit
51
occur. Moreover, since the conductors are long, a phase lag occurs in detecting the electrostatic capacity C due to the inductance of the conductors, etc., causing a problem that the frequency band for the detection can not be increas
Allen Stephone B.
Conlin David G.
Edwards & Angell
Sharp Kabushiki Kaisha
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