Spherical abberation compensator and optical pickup using...

Details Spherical abberation compensator and optical pickup using... Spherical abberation compensator and optical pickup using...

2002-07-17

2004-04-20

Hindi, Nabil (Department: 2655)

Dynamic information storage or retrieval

Specific detail of information handling portion of system

Radiation beam modification of or by storage medium

Reexamination Certificate

active

06724710

ABSTRACT:

CROSS REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of Korean Application No. 2001-43785 filed Jul. 20, 2001, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a spherical aberration compensator which compensates for spherical aberration resulting from a thickness deviation of a recording medium and an optical pickup using the spherical aberration compensator.
2. Description of the Related Art
Recording/reproduction density of a recording medium increases as a size of a light spot focused on the recording medium by an optical pickup becomes smaller. The size of the light spot is proportional to a wavelength (&lgr;) of light used by the optical pickup and is inversely proportional to the numerical aperture (NA) of an objective lens. Therefore, to implement a high-density recording medium, there is a need for an optical pickup with a short wavelength light source, such as a blue semiconductor laser, and an objective lens having a larger NA. Recently, there is an increasing interest in a format for increasing recording capacity up to 22.5 GB with a 0.85-NA objective lens and for reducing a thickness of a recording medium to 0.1 mm so as to prevent degradation of performance caused by tilting of the recording medium. Here, the thickness of the recording medium refers to a distance from a light receiving surface of the recording medium to an information recording surface.
As is apparent from equation (1) below, spherical aberration W
40d
is proportional to the fourth power of the NA of an objective lens, and to a deviation of the thickness of a recording medium &Dgr;d. For this reason, if an objective lens with an NA of about 0.85 is used, the recording medium must have a uniform thickness with a deviation less than ±3 &mgr;m. However, it is very difficult to manufacture the recording medium within the above thickness deviation range.
W
40
⁢
d
=
n
2
-
1
8
⁢
n
3
⁢
(
NA
)
4
⁢
Δ
⁢
 
⁢
d
(
1
)
FIG. 1
is a graph showing a relationship between the thickness deviation of a recording medium and wavefront aberration (optical phase difference (OPD)) caused by the thickness deviation where a 400-nm light source and an objective lens having an NA of 0.85 are used. As shown in
FIG. 1
, the wavefront aberration increases proportionally to the thickness deviation. Thus, if an objective lens having an NA as large as 0.85 is used, there is a need to compensate for the spherical aberration caused by the thickness deviation of the recording medium.
Japanese Patent Laid-open Publication No. 12-57616 discloses an optical pickup for detecting a thickness deviation of a recording medium. Japanese Patent Laid-open Publication No. 12-30281 describes a technique of compensating for wavefront aberration occurring where a recording medium is tilted with respect to the optical axis, using a liquid crystal compensator.
FIG. 2
shows a principle of compensating for spherical aberration resulting from thickness deviation of a recording medium
10
in a conventional optical pickup. Referring to
FIG. 2
, an objective lens assembly
20
(hereinafter, simply “objective lens”) is designed with a group of three lens elements
21
,
22
, and
23
for a larger NA. The three lens elements
21
,
22
, and
23
are accommodated in a single bobbin
24
and are aligned on the same optical axis X—X. A phase difference compensator
30
for compensating for spherical aberration is disposed on the optical axis X—X on a side of the objective lens
20
opposite the recording medium
10
.
In
FIG. 2
, a curve A drawn near the objective lens
20
shows a wavefront due to spherical aberration resulting from manufacturing errors in the recording medium
10
and/or the lens elements
21
,
22
, and
23
. This type of spherical aberration is referred to as “spherical aberration from an objective lens”. A curve B drawn to overlap the phase difference corrector
30
shows the wavefront after spherical aberration compensation by the phase difference compensator
30
. Another curve A
1
drawn near the objective lens
20
shows the wavefront where the optical axis of the objective lens
20
is radially displaced (or shifted) from the optical axis of the phase difference compensator
30
.
The phase difference compensator
30
generates an inverse spherical aberration that offsets spherical aberration from the objective lens due to manufacturing errors in the recording medium
10
and/or lens elements
21
,
22
, and
23
. The phase difference compensator
30
uses a liquid crystal as a medium to adjust the degree of phase delay and differentially delays the phase of light by locally working the liquid crystal medium, to generate the inverse spherical aberration. Localized phase delay by the driving of liquid crystals is disclosed in Japanese Patent Laid-open Publication No. 12-30281 which is incorporated herein by reference.
The phase difference compensator
30
is operated by a separate driving circuit
40
, and the driving circuit
40
operates according to a recording medium thickness variation signal, which is dynamically detected. The thickness deviation of the recording medium may be calculated from a focus error signal detected by an astigmatic lens disposed on an optical path. As an example, a technique disclosed in Japanese Laid-open Patent Publication No. 12-57616 may be applied to calculate the thickness deviation.
FIG. 3
is a graph showing a relationship between spherical aberration from an objective lens and inverse spherical aberration offsetting the spherical aberration from the objective lens.
FIG. 3
shows theoretical data for a case where the objective lens
20
and the phase difference compensator
30
are coaxially positioned. Accordingly, the aberration B produced by the phase difference compensator
30
has the same magnitude as the corresponding aberration A of the objective lens
20
but has an opposite sign. Therefore, the aberration of the objective lens
20
is completely offset by the inverse aberration of the phase difference compensator
30
, thereby eliminating the spherical aberration, as indicated by C in FIG.
3
.
In an optical pickup, the relative position of the objective lens
20
with respect to the phase difference compensator
30
varies while the objective lens
20
is driven by an actuator (not shown) to trace a track of the recording medium
10
. As a result of the relative displacement of the objective lens
20
from the phase difference compensator
30
, wavefront mismatching occurs, as shown in FIG.
4
. In particular, where an optical axis X
2
—X
2
of the objective lens
20
is shifted from the optical axis X—X of the phase difference compensator
30
by a predetermined distance &Dgr;L, the aberration is abnormally compensated due to the wavefront matching and wavefront degradation results rather than aberration compensation. As shown in
FIG. 4
, as the optical axis X
2
—X
2
of the objective lens
20
is separated from the optical axis X—X of the phase difference compensator
30
, the spherical aberration is incompletely compensated as indicated by a sum of the curves A
1
and B
1
which yields the curve C
1
as shown in
FIG. 4
, causing a more serious spherical aberration at the periphery of the objective lens
20
. The curves A
1
and B
1
represent aberration of the objective lens
20
and inverse aberration produced by the phase difference compensator
30
, respectively. The worsening of the spherical aberration due to the relative displacement between the optical axes of the objective lens and the phase difference compensator is a problem of conventional compensators designed to compensate only for thickness deviation of the recording medium, without considering a shifting of the objective lens.
SUMMARY OF THE INVENTION
To solve the above and other problems, it is an object of the present invention to provide a spherical aberration compensator which effectively compensates for spher

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