Dynamic information storage or retrieval – Specific detail of information handling portion of system – Radiation beam modification of or by storage medium
Reexamination Certificate
2000-07-07
2003-03-11
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
C369S044230
Reexamination Certificate
active
06532202
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical element used in optical information processing or optical communications or the like to correct aberrations, and also relates to an optical head and an optical recording/reproduction apparatus.
2. Related Art of the Invention
In recent years, the Digital Versatile Disc (DVD) has been attracting attention as a large capacity optical recording medium since it can record digital information at a recording density about seven times higher than the Compact Disc (CD).
In order to play back high density DVDs, the wavelength of the light source is made shorter than that for the CD (650 nm for DVD compared with 780 nm for CD), and the numerical aperture of the objective lens is made larger than that for the CD (0.6 for DVD compared with 0.45 for CD).
However, since the wavelength is made shorter and the lens NA is increased, as described above, wavefront aberration, especially coma, is generated because of the displacement (tilt) from the normal relative to the optical axis caused by warping or other irregularities of the optical disc, and thus the margin for tilt is smaller than in the case of CD.
To overcome this problem, an optical head which corrects wavefront aberration by using a liquid crystal panel is proposed in Japanese Laid-open No. 9-128785. However, the optical head described in the above cited patent publication can only correct one kind of aberration, for example, the coma caused by radial tilt.
Actual optical discs, however, may become warped in both radial and tangential directions, depending on the manufacturing method used, and the optical head described in the above cited patent publication cannot provide a sufficient correction effect. In view of this problem, an optical head which simultaneously corrects for the radial and tangential directions is proposed in Japanese Laid-open No. 10-247330.
One example of the above prior art optical head will be described with reference to drawings.
FIG. 20
is a diagram showing the construction of an optical head (also called an optical pickup) according to the prior art. As shown in the figure, the optical head comprises a light source
170
, a half-silvered mirror
171
, a liquid crystal panel
172
, an objective lens
173
, an optical disc
174
, a converging lens
175
, a photodetector
176
, a first tilt sensor
177
, a second tilt sensor
178
, and a liquid crystal panel control circuit
179
.
The light source
170
is constructed, for example, from a semiconductor laser device, and outputs recording/playback coherent light directed to a recording layer of the optical disc
174
. The half-silvered mirror
171
is a device for separating light; the liquid crystal panel
172
is a device that has the structure shown in FIG.
21
and the pattern shown in
FIG. 22
, and that corrects aberrations by applying a desired voltage to each electrode and thereby changing the refractive index of the liquid crystal and, hence, the phase of each electrode (the details will be described later); the objective lens
173
is a lens for focusing the light onto the recording layer of the optical disc
174
; the converging lens
175
is a lens for focusing the light, reflected from the recording layer of the optical disc
174
, onto the photodetector
176
; the photodetector
176
is a device that receives the light reflected from the recording layer of the optical disc, and converts the light into an electrical signal; the first tilt sensor
177
is a sensor which detects the radial tilt angle of the optical disc
174
, and generates a signal proportional to the detected tilt angle; the second tilt sensor
178
is a sensor which detects the tangential tilt angle of the optical disc
174
, and generates a signal proportional to the detected tilt angle; and the liquid crystal panel control circuit
179
is a circuit that generates two signals, one for controlling each electrode of the liquid crystal panel according to the signal generated by the first tilt sensor
177
and the other for controlling each electrode of the liquid crystal panel according to the signal generated by the second tilt sensor
178
.
The operation of the thus constructed optical head will be described below. Linearly polarized light emitted from the light source
170
is reflected by the half-silvered mirror
171
, and the light thus redirected in the direction of the optical disc
174
enters the liquid crystal panel
172
. Suppose here that the optical disc
174
is rotating with the disc tilted from the normal in the radial direction relative to the optical axis; then, the first tilt sensor
177
outputs a signal proportional to the amount of the tilt (the radial tilt angle), and the signal is input to the liquid crystal panel control circuit
179
. The liquid crystal panel control circuit
179
then outputs the necessary signal to each electrode portion of the pattern shown in
FIG. 22
of the radial tilt correcting electrode shown in
FIG. 21
to generate a wavefront aberration that compensates for the wavefront aberration caused when the optical disc
174
is tilted in the radial direction, and this signal is input to the liquid crystal panel
172
.
If, at the same time, the optical disc
174
is tilted from the normal in the tangential direction relative to the optical axis, the second tilt sensor
178
outputs a signal proportional to the amount of the tilt (the tangential tilt angle), and the signal is input to the liquid crystal panel control circuit
179
; the liquid crystal panel control circuit
179
then outputs the necessary signal to each electrode portion of the pattern shown in
FIG. 22
of the tangential tilt correcting electrode shown in
FIG. 21
to generate a wavefront aberration that compensates for the wavefront aberration caused when the optical disc
174
is tilted in the tangential direction, and this signal is input to the liquid crystal panel
172
.
As a result, the light entering the liquid crystal panel
172
, as it passes through the liquid crystal panel
172
, is given such wavefront aberrations that compensate for the wavefront aberrations caused when the optical disc is tilted in both the radial and tangential directions. The light passed through the liquid crystal panel
172
is then focused onto the optical disc
174
by means of the objective lens
173
.
Since the light having wavefront aberrations that compensate for the wavefront aberrations caused when the optical disc
174
is tilted is focused through the objective lens, a light spot free from aberrations and thus focused to the diffraction limit is formed on the optical disc. Next, the light reflected from the optical disc
174
emerges with wavefront aberrations proportional to the tilt of the optical disc
174
, but these wavefront aberrations are corrected by the liquid crystal panel
172
.
The light passed through the liquid crystal panel
172
is not directed back to the light source
170
, but is passed through the half-silvered mirror
171
and directed to the converging lens
175
which focuses the light onto the photodetector
176
. The photodetector
176
outputs a focus error signal indicating the focusing condition of the light on the optical disc
174
, and also outputs a tracking error signal indicating the position of the light shone thereon.
One of these signals is supplied to focus control means not shown and, based on the focus error signal, the focus control means controls the position of the objective lens
173
along its optical axis so that the light is always kept in focus on the optical disc
174
.
On the other hand, tracking control means not shown controls the position of the objective lens
173
based on the tracking error signal so that the light is kept focused on the desired track on the optical disc
174
. The photodetector
176
also reproduces information recorded on the optical disc
174
.
The operating principles of the liquid crystal panel
172
and tilt corrections will be described in detail below. First,
FIG. 23
shows the wavefront aberrati
Hotta Naoya
Kadowaki Shin-ichi
Mizuno Sadao
Ogata Daisuke
Saimi Tetsuo
Matsushita Electric - Industrial Co., Ltd.
RatnerPrestia
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