Dynamic information storage or retrieval – With servo positioning of transducer assembly over track... – Optical servo system
Reexamination Certificate
1998-06-30
2001-02-06
Tran, Thang V. (Department: 2753)
Dynamic information storage or retrieval
With servo positioning of transducer assembly over track...
Optical servo system
C369S044410, C369S112040
Reexamination Certificate
active
06185167
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical head that optically records information on an information-recording medium such as an optical disc or reproduces or deletes information therefrom, and to an information recording and reproduction apparatus that uses this optical head.
2. Related Art of the Invention
An optical head that uses a push pull signal to generate a tracking error signal is commonly used due to an advantage that it can be implemented in an optical system of a simple configuration. It, however, has a disadvantage that an offset occurs in a tracking error a signal as an objective moves in a tracking direction. A technique that compensates for this disadvantage is disclosed in Japanese Patent Application Laid-Open No. 8-306057 and Japanese Patent Application No. 7-280372.
The conventional techniques are described below with reference to the drawings.
FIG. 18
shows a configuration of a first conventional technique that is an optical head in Japanese Patent Application Laid-Open No. 8-306057. In this figure,
74
is the optical axis of the optical head;
75
is a light source;
76
and
77
are half mirrors;
78
is an objective;
79
is an actuator that moves the objective in the X and Y directions;
80
is an optical disc that is an information-recording medium;
81
is a six-piece light-receiving element (In the figure, a sub-figure showing the locational relationship between the element
81
and the optical axis
74
and a sub-figure showing a detection state are shown in parallel);
81
a
to
81
c
are parting lines of the six-piece light-receiving element
81
;
82
a
to
82
f
are divided light-receiving areas;
83
is a differential amplifier that is an operation circuit;
84
is a luminous flux on the six-piece light-receiving element
81
;
88
is a focus control section; and
89
is a tracking control section.
Next, the operation of the optical head of the above configuration is described. Light from the light source
75
is reflected by the half mirror
76
and converged by the objective
78
on an information-recording surface of the optical disc
80
to form a light spot. Information tracks are formed on the optical disc
80
, and their direction is perpendicular to the sheet of the drawing in FIG.
18
. Reflected light from the optical disc
80
is transmitted through the objective
78
and half mirror
76
and separated into two luminous fluxes by the half mirror
77
. Luminous fluxes reflected from the half mirror
77
are incident on the focus control section
88
, and transmitted luminous fluxes enter the six-piece light-receiving element
81
.
An addition provided by the connections in the figure and a differential operation performed by the differential amplifier
83
generate a tracking error signal, which is then guided to the tracking control section
89
. The focus control section
88
detects a focus error signal and controls the actuator
79
so that light is converged on the information-recording surface of the optical disc. Based on the detected tracking error signal, the tracking control section
89
controls the actuator
79
in such a way that the light spot is guided to the center of the information tracks, and moves the objective
78
in the X-positive and -negative direction using the optical axis
74
of the optical head as a reference.
FIGS.
19
(
a
) and
19
(
b
) show the position of the luminous flux
84
on the six-piece light-receiving element
81
; FIG.
19
(
a
) shows that the objective
78
is located at a reference position, and FIG.
19
(
b
) shows that the objective moves in the X-positive direction. Two shaded parts in the luminous flux
84
show area in which a zero-order diffracted luminous flux and a positive and a negative first order diffracted luminous fluxes, which are diffracted by the optical disc
80
, interfere with each other.
In FIG.
19
(
a
), the luminous flux
84
is symmetrical relative to the parting line
81
a,
so an output signal form the differential amplifier
83
is a tracking error signal without offset. On the other hand, in FIG.
19
(
b
), the luminous flux
84
is moved to the right to lose its symmetry relative to the parting line
81
a,
and the area of the luminous flux contained in each of areas
82
b,
82
d
and
82
f
increases while the area of the luminous flux contained in each of areas
82
a,
82
c,
and
82
e
decreases. When a signal detected in each light-receiving area is represented by its area name and the tracking error signal is referred to as TE, the operation performed by the differential amplifier
83
is as follows:
TE=
82
a+
82
e+
82
d−k
*(82
b+
82
f+
82
c
) [Equation 3]
wherein (k) is a correction coefficient. A push pull signal component appears in the areas
82
c
and
82
d
mainly containing the shaded interfering areas, whereas an offset component appears in the other areas mainly due to the movement of the luminous flux. Thus, by setting the correction coefficient (k) at an appropriate value, the operation in Equation 3 provides a tracking error signal in which an offset is corrected that is caused by the movement of the objective.
Next, a second conventional technique that is the optical head in Japanese Patent Application No. 7-280372 is described. The configuration of this optical head is similar to that of the first conventional technique except for the division of the light-receiving element. Thus, its configuration diagram is omitted and only the configuration of the multi-piece light-receiving element is described.
FIG.
20
(
a
) is a top view of an eight-piece light-receiving element. Reference numerals
85
a
to
85
c
designate parting lines of the eight-piece light-receiving element
85
;
86
a
to
86
h
are divided light-receiving areas;
87
is a luminous flux on the eight-piece light-receiving element
85
; and the shaded part in the figure is an area that is not exposed to light.
The operation of the optical head according to the second conventional technique is almost the same as that of the optical head according to the first conventional technique. Thus, the description of the operation is omitted and only the features obtained by configuring the eight-piece light-receiving element as described above are explained. FIG.
20
(
b
) schematically shows the distribution of the amount of light in the luminous flux
87
which is generated when the optical disc is tilted in the radial direction, wherein the magnitude of light intensity is represented by the density of diagonal lines. In this figure, the light intensity increases with increasing density of diagonal lines. This figure indicates that asymmetrical light intensity occurs at the center of the luminous flux due to the inclination of the optical disc in the radial direction, and this asymmetry of the intensity distribution results in an offset in the push pull signal. If a shaded portion is present in the center as shown in FIG.
20
(
a
), the effect of the asymmetrical light intensity shown in FIG.
20
(
b
) can be reduced. When a signal detected in each area is represented by the corresponding area name, the tracking error signal TE can be obtained by the following equation:
TE=
86
c+
86
e
−(86
d+
86
f
)−
k*{
86
a+
86
g−
(86
b+
86
h
)} [Equation 4]
wherein (k) is a correction coefficient.
As described above, the first conventional technique can correct an offset in the tracking signal caused by the movement of the objective in the tracking direction, and the second conventional technique can reduce an offset in the tracking signal caused by the inclination of the optical disc in the radial direction.
The conventional techniques, however, divide a luminous flux on the light-receiving element to detect a possible tracking error, so separate optical systems are required to detect a focus and a tracking error signals. Consequently, despite the simple configuration of the detecting optical system of a trackin
Arai Akihiro
Hayashi Takao
Nagata Takayuki
Nakamura Tohru
Matsushita Electric - Industrial Co., Ltd.
Ratner & Prestia
Tran Thang V.
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