Dynamic information storage or retrieval – Specific detail of information handling portion of system – Radiation beam modification of or by storage medium
Reexamination Certificate
1999-12-13
2002-11-12
Tran, Thang V. (Department: 2653)
Dynamic information storage or retrieval
Specific detail of information handling portion of system
Radiation beam modification of or by storage medium
Reexamination Certificate
active
06480454
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to optical element that changes the phase of an incident light beam and to an optical head and an optical recording and reproducing apparatus using such an optical element.
2. Description of the Prior Art
Optical recording media, such as digital versatile disks (DVDs), can record digital information with high densities, so that they are noted as optical recording media with large capacities. To record and reproduce digital information with high densities, it is necessary to use short wavelength light for recording and reproducing, and make the NA (numerical aperture) of the objective lens large. However, making the wavelength short and the NA of the lens large increases the wave-front aberrations, in particular coma aberration, due to deviations (tilt) from the optical axis, for example caused by warps in the optical recording medium, and there is the problem that the system design margins for the tilt have to be reduced.
In order to solve this problem, optical heads correcting the wave-front aberrations with a liquid crystal panel have been proposed (see e.g. Japanese Patent Application (Tokkai) Hei 9-128785).
Referring to
FIG. 23
, the following explains an example of such a conventional head.
FIG. 23
shows the configuration of a conventional optical head
1
(also called “optical pickup”). The optical head
1
comprises a light source
2
, a half-mirror
3
a
, an objective lens
3
b
, a focus lens
3
c
, an optical element
4
, a tilt sensor
5
, an optical element control circuit
6
, and a photo-detector
7
.
The light source
2
, which can be a semiconductor laser element, outputs coherent light for recording and reproducing toward the recording layer of the optical recording medium
8
(which is a medium for recording information, in which the recorded information can be read out optically, such as a CD or a DVD). The optical element
4
includes a liquid crystal panel, which has a plurality of segment electrodes in a pattern such as shown in FIG.
24
. By applying an appropriate voltage to each of the segment electrodes, the optical element
4
changes the refractive index of the liquid crystal for each of the segment electrodes, and changes the phase of the light passing through each of the electrodes. Thus, the optical element
4
corrects the aberration caused by the tilt.
The following explains how this conventional optical head
1
functions. Linearly polarized light emitted by the light source
2
is reflected by the half-mirror
3
a
and enters the optical element
4
. When the optical recording medium
8
is tilted vertically with respect to the optical axis, a signal depending on the tilt amount (tilt angle) is output by the tilt sensor
5
. Based on the signal from the tilt sensor
5
, the optical element control circuit
6
controls the liquid crystal panel of the optical element
4
so as to generate the necessary phase change for correcting the wave-front aberrations caused when the optical recording medium is tilted.
Thus, the light entering the optical element
4
is subjected to a phase change that corrects the wave-front aberrations caused by tilting of the optical recording medium
8
. The light that has passed through the optical element
4
is focused on the optical recording medium
8
by an objective lens
3
b
. Since light that has been subjected to a phase change that corrects the wave-front aberrations caused by tilting of the optical recording medium
8
is focused by the objective lens
3
b
, a light spot without aberration (constricted to the diffraction limit) is formed on the optical recording medium
8
.
Then, the light that is reflected from the optical recording medium
8
may turn into light with wave-front aberrations, depending on the tilt of the optical recording medium
8
, but the wave-front aberrations are corrected by the optical element
4
. The light that has passed through the optical element
4
passes the half-mirror
3
a
and enters the focus lens
3
c
without returning to the light source
1
, and is focused by the focus lens
3
c
on the photo-detector
7
. The photo-detector
7
outputs the information stored on the optical recording medium
8
. In addition, the photo-detector
7
outputs a focus error signal indicating how well the light focuses on the optical recording medium
8
, and a tracking error signal indicating the irradiation position of the light.
The following explains the principle of the tilt correction with the optical element
4
.
FIG. 25
shows an example of the wave-front aberrations in the best image point of the optical recording medium
8
(for a 1° tilt angle of the optical recording medium
8
, a 0.6 NA of the objective lens, a wavelength of 655 nm, and a 0.6 mm substrate thickness of the optical recording medium
8
). As is shown in
FIG. 25
, if the optical recording medium
8
is tilted, the wave-front aberrations have a substantially semicircular distribution, anti-symetrically to the left and right. By subjecting the incident light to a phase change that cancels the wave-front aberration distribution in
FIG. 25
with the optical lens
4
, the spot on the optical recording medium
8
can be constricted to the diffraction limit, even when the optical recording medium
8
is tilted. Moreover, by subjecting the light that is reflected from the optical recording medium
8
to a phase change that cancels the wave-front aberrations, the photo-detection with the photo-detector
7
becomes more precise.
To subject the incident light to a phase change that cancels the wave-front aberration distribution in
FIG. 25
, the light path length of the optical element
4
has to be changed partially. Since the refractive index of the liquid crystal can be changed by applying a voltage from outside, the light path length of the optical element can be changed partially by partially changing the applied voltage. Thus, the wave-front aberrations shown in
FIG. 25
can be corrected by applying different voltages to different segment electrodes partitioned into a fine pattern as shown in FIG.
24
.
However, with such an optical element
4
, it is necessary to apply a corresponding control signal from the outside to each segment electrode of the liquid crystal panel in the optical element
4
. This means, that from the driving circuit for driving the liquid crystal panel, a flexible circuit board has to be connected to the optical element
4
with the same number of wires as there are segment electrodes in the liquid crystal panel. Consequently, in the case of an optical element
4
having many segment electrodes as shown in
FIG. 24
, many signals have to be supplied, and the flexible circuit board becomes accordingly wider. When such a wide flexible circuit board is connected to the optical element
4
, it becomes very difficult to adjust the parts properly, and this stands in the way of making the optical head
1
smaller. Moreover, to install a small optical element as the optical head
4
without shorting the many wires is very difficult, and the more wires there are, the worse becomes the yield of the step of installing the optical element
4
on the flexible circuit board, which leads to higher costs for the optical head
1
.
In order to solve this problem, an optical element has been proposed having segment electrodes that differ from the above conventional segment electrodes (Tokkai Hei 10-20263).
FIG. 26
shows the shape of these segment electrodes. The shape of these segment electrodes agrees with the shape of the wave-front aberrations caused by tilting of the optical recording medium
8
. Consequently, the wave-front aberrations can be corrected better than with the above-mentioned conventional optical element, even though the number of segment electrodes is reduced.
However, even with the optical element having segment electrodes of the shape shown in
FIG. 26
, the pattern shown in
FIG. 26
has to be partitioned even finer to correct the wave-front aberrations more precisely. Consequently, even in an optical ele
Mizuno Sadao
Nishino Seiji
Ogata Daisuke
Saimi Tetsuo
Wada Hidenori
Merchant & Gould P.C.
Tran Thang V.
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