Dynamic information storage or retrieval – With servo positioning of transducer assembly over track... – Optical servo system
Reexamination Certificate
2000-06-29
2002-02-12
Edun, Muhammad (Department: 2651)
Dynamic information storage or retrieval
With servo positioning of transducer assembly over track...
Optical servo system
C369S112010
Reexamination Certificate
active
06347066
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
The present invention relates to an optical pickup and an optical disk apparatus, applicable, for example, to an optical disk apparatus adapted to access a high-density recorded optical disk. In the present invention, an optical path length difference generator means is disposed in an optical path to produce an optical path length difference between two luminous fluxes obtained through resolution of return light, hence realizing a simplified structure, which is capable of preventing any characteristic deterioration of a focus error signal that may otherwise be caused by horizontal deviation.
2. Description of Prior Art
In any conventional optical disk apparatus known heretofore, it has been customary that focus control of a laser beam to be irradiated to an optical disk is executed on the basis of a focus error signal of a level changed in accordance with the amount of a focus error. Detection of such a focus error signal is performed by the use of an astigmatism method, Foucault method, SSD (Spot Size Detection) method or the like.
Out of these detection methods mentioned above, when the astigmatism method is applied to an optical disk where land-groove recording is adopted, a positional deviation is detected of a “just focus” position between a land portion and a groove portion. Therefore, in regard to any optical disk adopting such land-groove recording, principally the Foucault method or SSD (Spot Size Detection) method is applied.
FIG.
11
(A) is a schematic diagram showing an exemplary optical pickup in which the Foucault method is applied. In this optical pickup
1
, a semiconductor laser
2
emits a laser beam L
1
therefrom, and a collimator lens
3
converts the laser beam L
1
into substantially parallel light rays. A beam splitter
4
reflects the incoming laser beam L
1
, which is incident thereon from the collimator lens
3
, toward an optical disk
5
, and then an objective lens
6
condenses the laser beam, which is obtained from the beam splitter
4
, onto an information recording plane surface of the optical disk
5
.
As a result, return light L
2
is obtained from the optical disk
5
. Then, this return light L
2
is incident upon the beam splitter
4
by way of the optical path of the laser beam L
1
in the reverse direction. The beam splitter
4
transmits the return light L
2
therethrough to separate the optical path of the laser beam L
1
from that of the return light L
2
. A collimator lens
7
converts the return light, which is emitted from the beam splitter
4
, into a converged luminous flux, and then a half mirror
8
splits the return light, which has been converted into such a converged luminous flux, into two luminous fluxes.
A light sensor
9
receives and senses the return light reflected by the half mirror
8
. In the optical disk apparatus, the sensed result of the light received by the light sensor
9
is processed through current-to-voltage conversion to thereby generate a reproduced signal RF whose level is changed in accordance with pit trains or the like formed on the optical disk
5
. The reproduced signal RF thus obtained is processed to reproduce the data recorded on the optical disk
5
.
In a case in which the Foucault method is employed, a Foucault prism
10
is disposed in the optical path of the return light converted into a converged luminous flux as mentioned. The Foucault prism
10
is so shaped that its center protrudes, hence resolving the return light into two luminous fluxes respectively having outgoing directions that are inclined obliquely to the optical axis. In this optical pickup
1
, the return light is resolved substantially symmetrically with respect to the optical axis of the return light, and the luminous fluxes mutually intersect in the outgoing directions.
In the optical pickup
1
, the return light L
2
emitted from the Foucault prism
10
is received by a predetermined light sensor
11
. When the light receiving plane of the light sensor
11
and the information recording plane surface of the optical disk
5
are held in a conjugate relation, as shown in FIG.
11
(C), the two luminous fluxes form respective focal points on the light receiving plane of the light sensor
11
. Consequently, two beam spots SP
1
and SP
2
are formed by the two luminous fluxes (hereinafter this state will be referred to as a “just focus state”).
When an objective lens
6
is moved toward the information recording plane of the optical disk
5
and the emission point of the return light recedes equivalently from the objective lens
6
, the two beam spots SP
1
and SP
2
formed on the light receiving plane are positionally changed in a manner to approach the optical axis. Also, the shapes thereof are enlarged as shown in FIG.
11
(B), since the outgoing directions of such two beam spots are so inclined as to mutually intersect by the Foucault prism
10
.
To the contrary, when the objective lens
6
is moved away from the information recording plane of the optical disk
5
and the emission point of the return light equivalently approaches the objective lens
6
, the two beam spots SP
1
and SP
2
formed on the light receiving plane are positionally changed in a manner to recede from the optical axis and also the shapes thereof are enlarged as shown in FIG.
11
(D).
Utilizing such relationship, there are formed, as shown in FIGS.
11
(A),
11
(B), and
11
(C), in the light sensor
11
, light receiving planes
11
A and
11
B defined by respectively dividing the light receiving plane into two areas a, b and c, d in directions where the respective focal points of the beam spots SP
1
and SP
2
are changed with reference to the return-light focal point in the just focus state. In the optical disk apparatus, the sensed results of the received light in such areas a to d are processed through current-to-voltage conversion, and the results of the current-to-voltage conversion are represented by codes which correspond respectively to the areas a to d, thereby generating a focus error signal FE expressed by an arithmetic equation of
FE=
(
a+d
)−(
b+c
).
Further, the objective lens
6
is so moved as to reduce the level of the focus error signal FE to zero as indicated by an arrow A, hence executing focus control.
FIG. 12
is a perspective view showing principal portions of a light accumulator applied to an optical pickup that is based on the SSD method. In this optical pickup, a laser beam L
1
emitted from the light accumulator
15
is condensed on an optical disk by means of an objective lens, and return light L
2
obtained from the optical disk is received via the objective lens and then is introduced to the light accumulator
15
.
The light accumulator
15
reflects the laser beam L
1
, which is emitted from a semiconductor laser diode chip
17
, onto an inclined surface
16
A of a prism
16
produced by cutting a glass material, and then sends the laser beam L
1
toward the objective lens.
The light accumulator
15
introduces the return light L
2
, which is reflected by way of the optical path of the laser beam L
1
in the reverse direction, from the inclined surface
16
A into the prism
16
, and then separates the return light L
2
into transmitted light and reflected light by the lower plane of the prism
16
. The light accumulator
15
of
FIG. 12
enables a light sensor
18
to receive the transmitted light obtained through the lower plane of the prism
16
. The light accumulator
15
further reflects the reflected light from the lower plane of the prism
16
by the upper plane thereof and, after transmitting the reflected light through the lower plane, enables the light sensor
19
to receive the reflected light. Thus, the optical pickup using this light accumulator
15
is so structured that the diameters of beam spots formed on the respective light receiving planes of the light sensors
18
and
19
are substantially equalized to each other in a just focus state. If the distance to the optical disk is changed from the just focus position, the d
Edun Muhammad
Kananen Ronald P.
Rader & Fishman & Grauer, PLLC
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