Dynamic information storage or retrieval – With servo positioning of transducer assembly over track... – Optical servo system
Reexamination Certificate
2000-03-08
2003-03-04
Tran, Thang V. (Department: 2653)
Dynamic information storage or retrieval
With servo positioning of transducer assembly over track...
Optical servo system
C369S044230, C369S112280
Reexamination Certificate
active
06529454
ABSTRACT:
TECHNICAL FIELD
This invention relates to an integrated optical component, used in an optical pickup for recording and/or reproducing signals for an optical disc, such as a magneto-optical disc, an optical pickup device employing this integrated optical component and an optical disc device provided with the optical pickup device.
BACKGROUND ART
Up to now, an optical pickup for a magneto-optical disc, constructed as shown in
FIG. 1
, has been put to practical use.
An optical pickup
1
, shown in
FIG. 1
, is constructed as an optical pickup for e.g., a mini-disc (MD), and includes an astigmatism correcting plate
3
a
, a grating
3
b
, a beam splitter
4
, a collimator lens
5
, a optical path raising mirror
6
and an objective lens
7
, arranged in this order in an optical path of a light beam radiated from a semiconductor laser element
2
as a light source and proceeding towards an optical disc D, and a Wollaston prism
8
a
, a multi-lens
8
b
and a photodetector
9
, arranged in this order in the optical path of the return light from the optical disc D separated by a separator film
4
a
of the beam splitter
4
, these optical components being mounted separately from one another.
In the optical pickup
1
, constructed as described above, the light beam radiated from the semiconductor laser element
2
is corrected for astigmatism by the astigmatism correcting plate
3
a
and subsequently split by the grating
3
b
into three light beams, namely a main beam and two side beams, which are respectively incident on the beam splitter
4
.
A portion of the light beam incident on the beam splitter
4
is transmitted through the separator film
4
a
of the beam splitter and is turned into a collimated beam by the collimator lens
5
. The collimated light beam then has its optical path warped by the optical path raising mirror
6
and is converged by the objective lens
7
so as to be illuminated on a signal recording surface of the optical disc D. At this time, three spots are formed on the signal recording surface of the optical disc D by the respective light beams split by the grating
3
b.
When the light beam illuminated on the signal recording surface of the optical disc D is reflected by the signal recording surface of the optical disc D, it has its polarization plane rotated, under the magnetic Kerr effect, depending on the state of magnetization (recording state) of a portion of the signal recording surface irradiated with the light beam.
The return light beam, reflected by the signal recording surface of the optical disc D, again falls on the beam splitter
4
via the objective lens
7
, optical path raising mirror
6
and collimator lens
5
.
Another portion of the light beam incident on the beam splitter
4
is reflected by the separator film
4
a
of the beam splitter
4
to fall on the Wollaston prism
8
a.
The Wollaston prism
8
a
is made up of two uniaxial crystals, bonded together, and separates the incident light into three light beams, namely a p-polarized light beam, an s-polarized light beam and a p+s polarized light beam (direction of polarization relative to the separator film
4
a
of the beam splitter
4
), having respective different reflection angles, based on the difference in the orientation on the junction surface of the optical axes of the two uniaxial crystals. The return light, falling on the Wollaston prism
8
a
, is split into the three light beams by the Wollaston prism
8
a
, afforded with astigmatism and extended in optical path length by the multi-lens
8
b
and is received by the light receiving surface of the photodetector
9
for signal detection.
Of the return light, received by the light receiving surface of the photodetector
9
, the p-polarized light and the s-polarized light, obtained on splitting by the Wollaston prism
8
a
, is used as basis to detect the magneto-optical signals. That is, the return light, obtained on reflection on the signal recording surface of the optical disc D with rotation of the plane of polarization and on separation by the Wollaston prism
8
a
into the p-polarized light and the s-polarized light, is received by the light receiving surface of the photodetector
9
, whereby the state of magnetization on the signal recording surface of the optical disc D (recording state) is detected as changes in light intensity.
On the other hand, of the return light, received by the light receiving surface of the photodetector
9
, the p+s polarized light, separated by the Wollaston prism
8
a
and afforded with the astigmatism by the multi-lens
8
b
, is used as basis to detect focussing error signals by the so-called astigmatic method. Also, of the return light, received by the light receiving surface of the photodetector
9
, the two side beams, as split by the above-mentioned grating
3
b
, are used as basis to detect tracking error signals by the so-called three-spot method.
In the present optical pickup
1
, the objective lens
7
is adapted to perform fine movements, based on pre-set servo signals, so that the light beam from the semiconductor laser element
2
will form a spot at a correct position on the signal recording surface of the optical disc D to reproduce correct recording signals, in order to detect correct magneto-optical signals.
That is, the so-called tracking servo of causing fine movements of the objective lens
7
along the radius of the optical disc D is effected based on the above-mentioned tracking error signals, in order for the spot of the light beam to follow the recording track of the optical disc D. On the other hand, the so-called focussing servo of causing fine movements of the objective lens
7
along the optical axis towards and away from the signal recording surface of the optical disc D is effected based on the above-mentioned focussing error signals so that the light beam will form a correct spot on the signal recording surface of the optical disc D.
Meanwhile, in the optical pickup
1
, constructed as described above, the recording information written on the optical disc D is read out by plural separately mounted optical components, such that the optical pickup cannot be reduced in size or in the number of the components, thus complicating the assembling steps or the optical adjustment steps of the optical pickup to raise the production cost.
On the other hand, in the replay-only optical pickup, adapted for reading out the recording information from e.g., a compact disc (CD), an integrated optical component, obtained on integration of the semiconductor laser element as a light source and a photodetector etc, is used to reduce the size of the optical pickup and that of the optical disc device having the optical pickup built-in therein.
Meanwhile, the conventional integrated optical component, employing a non-polarization optical system as an optical system, is highly effective for use on a replay-only optical pickup. However, if the integrated optical component is to be applied to an optical pickup adapted for recording and/or reproducing a magneto-optical disc, the following problem arises.
That is, if such integrated optical component is used in an optical pickup for recording and/or reproduction of a magneto-optical disc, only the focussing error signals, not dependent on the direction of polarization, need to be detected by the light receiving element of the integrated optical component, whereas the magneto-optical signals and tracking error signals, dependent on the direction of polarization, need to be detected by a photodetector provided independently of the integrated optical component. The result is that, in this optical pickup, not only can the number of component parts not be reduced sufficiently, but detection signals are respectively detected from the two optical components, that is the integrated optical component and the photodetector, thus increasing the number of lead lines for signal lead-out and complicating the assembling operation to raise the mounting cost.
Moreover, since the polarization splitting means, such as Wollaston prisms, or cylindrical len
Asoma Yoshito
Nishi Noriaki
Toyota Kiyoshi
Kananen, Esq. Ronald P.
Rader & Fishman & Grauer, PLLC
Sony Corporation
Tran Thang V.
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