Dynamic information storage or retrieval – With servo positioning of transducer assembly over track... – Optical servo system
Reexamination Certificate
2001-01-05
2003-12-30
Tran, Thang V. (Department: 2653)
Dynamic information storage or retrieval
With servo positioning of transducer assembly over track...
Optical servo system
C369S044120
Reexamination Certificate
active
06671236
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a light receiving and emitting compound element which is suitable for modifying focus errors and tracking errors of an optical pick-up device to be used for recording and reproducing an optical disk such as a CD-ROM, a CD-R, a DVD-ROM or an MD and correcting an image forming position to automatically form an image of a microdisplay provided in front of eyes on a human retina, and to an optical pick-up device using the light receiving and emitting compound element. More specifically, the present invention relates to a photodetector capable of accurately detecting reflected light corresponding to the situation of reflection on an object without precisely adjusting the position of an optical system to align a light emitting portion with a light receiving portion and of detecting a shift in a direction of a focal point and a shift in an xy plane (in a tracking direction), and to an optical pick-up device using the photodetector.
BACKGROUND OF THE INVENTION
A pit for recording information of an optical disk acting as an information recording medium has a size of approximately 0.9 &mgr;m, and a train (track) of the pit is arranged with a small pitch of approximately 1.6 &mgr;m. In an actual optical pick-up device, therefore, a pit should be detected while correcting a shift in a focus or the like which is caused by a displacement in a direction perpendicular to a direction of the track, a rotation slippage of the optical disk or the like. As a method for correcting the shift in the focus, there have conventionally been used an astigmatism method comprising the steps of separating a received signal light reflected by an optical disk from a transmitted signal light through a half lens or a beam splitter, forming an astigmatism through a cylindrical lens or the like and receiving the light by means of a 4-split sensor to detect a shift in a focus, a Foucault method for detecting a shift in a focus depending on whether or not a light reflected by an optical disk forms a focal point on an apex angle of a prism by using the prism, for example, and the like.
As an example of an optical pick-up device using the astigmatism method, for example, as shown in
FIG. 15
, a light transmitted from a light source such as a laser diode (which will be hereinafter referred to as an LD)
1
is diffracted through a diffraction grating
6
to generate a beam for a tracking servo (a 3-beam method), the beam is reflected by a half mirror
7
and is collected on a pit of an optical disk
10
through a lens system
3
including a collimator
3
a,
an objective lens
3
b
and the like, the reflected light is received by a light receiving element
2
such as a photodiode through the objective lens
3
b,
the collimator
3
a,
the half mirror
7
and a concave lens
8
, and a size of the pit of the optical disk
10
is detected while carrying out a focus servo and a tracking servo so that information is read out.
In this example, a half mirror plane is formed as the half mirror
7
on a surface of a thick transparent substrate without using a cylindrical lens. Consequently, 50% of the light transmitted from the LD
1
is reflected by the surface and is transmitted to the optical disk
10
without a distortion, and half of the light reflected by the optical disk
10
and returned is refracted through the thick half mirror
7
and is then transmitted to the light receiving element
2
. In this case, the half mirror
7
has an inclination in a constant direction in order to reflect the light transmitted from the LD
1
toward the optical disk
10
side. All convergent beams reflected and returned which enter an x-axis of
FIG. 15
have equal angles of incidence with respect to the inclined surface of the half mirror
7
, while convergent beams entering a y-axis have different angles of incidence with respect to the inclined surface in a y-axis direction. For this reason, while a position of a convergent point is changed, all the beams in an x direction which enter the x-axis are refracted in the same manner and are simply moved in parallel so that a distortion is not generated. Consequently, an astigmatism is generated. By utilizing the astigmatism, the focus servo is carried out.
The example using a hologram applying the Foucault method, moreover, as shown in
FIG. 16
, a laser beam transmitted from the LD
1
is diffracted through the diffraction grating
6
to generate a beam for a tracking servo (the 3-beam method) and is then collected on a pit of the optical disk
10
through a hologram (diffracting element)
9
provided on the top surface of the diffraction grating
6
and the lens system
3
including the collimator
3
a
and the objective lens
3
b,
the reflected light is transmitted through the objective lens
3
b,
the collimator
3
a
and the hologram
9
, and a primary diffraction signal diffracted through the hologram
9
is received by the light receiving element
2
so that information is read out in the same manner described above. The hologram
9
has the functions of a plane beam splitter and a Foucault prism in the conventional optical pick-up device and uses the principle of the Foucault method.
These methods have a problem in that the light emitting element
1
and the light receiving element
2
are provided in different positions and are therefore aligned with much difficulty. More specifically, an area of the light receiving element
2
should be very reduced in order to recognize a shift in a focal point or the like. In order to receive the reflected light through the very small light receiving element
2
, the light receiving element
2
should be provided accurately in the convergent position of the reflected light. As a method for solving such a problem, there has been known a structure shown in FIG.
17
.
A structure shown in FIG.
17
(
a
) is referred to as a so-called L-SCOOP (Self Coupled Optical Pickup) method utilizing a change in an amount of detection through a light receiving element
61
for a monitor provided behind the LD
1
because a light irradiated from the LD
1
and reflected by the optical disk
10
and then returned is exactly returned to the LD
1
and an oscillation state of the LD
1
is changed by the reflected light thus returned. A structure shown in FIG.
17
(
b
) is referred to as a so-called E-SCOOP method in which a change in the oscillation state of the LD
1
through the reflected light is directly detected depending on a change in a driving current or terminal voltage of the LD in the same manner.
In the case in which a light beam transmitted from a light source is irradiated on an object such as an optical disk and the reflected light is received to detect information or the like as in the above-mentioned optical pick-up device, light paths for reciprocation should be separated by using a beam splitter, a hologram or the like and a light emitting element and a light receiving element should be assembled by separate parts. For this reason, it is necessary to carefully adjust mutual optical axes of the light emitting element and the light receiving element and a distance therebetween, thereby carrying out assembly. However, the work is very complicated and only the skilled can carry out the assembly.
In the optical pick-up device using the astigmatism method, an expensive part such as a hologram is not required and it is sufficient that a half mirror plane is formed on the surface of the transparent substrate. Although parts are inexpensive, this structure also requires a half mirror and uses the 3-beam method to require a diffraction grating. All these parts are discrete and the number of the parts is increased, resulting in an increase in a cost. In addition, the relationship between the LD and the light receiving element which are aligned carefully should be prevented from being changed due to a temperature or the like. Therefore, a case or the like should be manufactured by an expensive material such as engineering plastics which is deformed by heat with difficulty. Consequently, the cost of the parts
Tanaka Haruo
Tsumori Masahiko
Arent Fox Kintner & Plotkin & Kahn, PLLC
Rohm & Co., Ltd.
Tran Thang V.
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