Dynamic information storage or retrieval – Specific detail of information handling portion of system – Radiation beam modification of or by storage medium
Reexamination Certificate
1999-03-22
2001-08-21
Hindi, Nabil (Department: 2651)
Dynamic information storage or retrieval
Specific detail of information handling portion of system
Radiation beam modification of or by storage medium
C369S112160, C369S112170, C369S112190, C369S112260, C369S110040
Reexamination Certificate
active
06278681
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to an optical head for recording data into and reproducing data from a medium such as a phase-change type optical disc and a photo-electro-magnetic tape, a method of fabricating such an optical head, and an apparatus for fabricating such an optical head.
2. Description of the Related Art
FIG. 1
illustrates an optical pickup as an example of a conventional optical head equipped with a micro-prism. The illustrated pickup is one having been suggested in Proceedings of the 6th Sony Research Forum, “The optical features of an integrated “Laser Coupler” optical pickup in the CD system”, Yoshiyuki Matsumoto et al., 1996, pp. 541-546.
A light having been emitted from a laser diode chip
101
is reflected at a first plane
104
a
of a micro-prism
104
by half an amount, and the thus reflected light is focused onto an optical disc
106
through a lens
105
. A light having been reflected at the optical disc
106
advances on the same optical path in an opposite direction, and then, is refracted at the first plane
104
a
to thereby enter the micro-prism
104
.
The micro-prism
104
includes a second surface
104
b
having a half-mirror coating in the right half thereof Hence, a light having entered the second surface
104
b
of the micro-prism
104
transmits through the second surface
104
b
in half an amount, and then, is received in a front light-receiving section
103
a
of a photodiode chip
103
. The remaining half of a light is reflected at the second surface
104
b,
and enters a third surface
104
c
of the micro-prism
104
.
The second surface
104
b
of the micro-prism has a non-reflective coating applied thereto in the left half. Hence, a light having been reflected at the third surface
104
c
of the micro-prism transmits through the second surface
104
b,
and then, is received at a rear light-receiving section
103
b
of the photodiode chip
103
.
Alight emitted from the laser diode chip
101
varies in an amount due to degradation with the lapse of time and variation in temperature, even if a constant current is applied to the laser diode chip
101
. Hence, a light backwardly emitted from the laser diode chip
101
is received a light-receiving section (not illustrated) formed on a sub-mount
102
, and a signal detected by the light-receiving section is fed back to a current to be applied to the laser diode chip
101
, to thereby keep an amount of light emitted from the laser diode chip
101
constant.
As illustrated in
FIG. 2
, the front light-receiving section
103
a
is comprised of four light-receiving sections
103
aa,
103
ab,
103
ac
and
103
ad
defined by three divisional lines extending in a direction from the laser diode chip
101
to the micro-prism
104
(that is, a y-axis direction illustrated in
FIGS. 1 and 2
) in parallel with one another. Similarly, the rear light-receiving section
103
b
is comprised of four light-receiving sections
103
ba,
103
bb,
103
bc
and
103
bd
defined by similar three divisional lines. Herein, signals to be detected in the light-receiving sections
103
aa
to
103
bd
are represented with S
103
aa
to S
103
bd,
respectively. A focus signal FE
100
is detected in accordance with the following equation.
FE
100
=S
103
aa
−S
103
ab
−S
103
ac
+S
103
ad
−S
103
ba
+S
103
bb
+S
103
bc
−S
103
bd
A track error signal TE
100
is detected in accordance with the following equation.
TE
100
=S
103
aa
+S
103
ab
−S
103
ac
−S
103
ad
−S
103
ba
−S
103
bb
+S
103
bc
+S
103
bd
However, the conventional optical head illustrated in
FIG. 1
is accompanied with the following problems.
The first problem is that it is unavoidable for an optical head to be thick.
For instance, Japanese Unexamined Patent Publication No. 6-333290 has suggested such an optical pickup as illustrated in FIG.
3
. The illustrated optical pickup is designed to include a mirror
107
to reflect a light reflected at the first surface
104
a,
in parallel with the photodiode chip
103
, in order to make the optical pickup thinner.
However, as long as a light directing from the laser diode chip
101
to the lens
105
is to be reflected at the first surface
104
a
of the micro-prism
104
, the pickup cannot have a thickness smaller than a sum of thicknesses of the mirror
107
, the micro-prism
104
, and the photodiode
103
.
In addition, the optical pickup illustrated in
FIG. 3
has to include a second mirror
108
at which a light reflected from the mirror
107
is reflected towards the optical disc
106
, as well as the first mirror
107
, resulting in an increase in the number of parts and complexity in a structure.
The second problem is that a light can be utilized in an amount only by 25% at greatest.
This is because, among a light emitted from the laser diode chip
101
to the lens
105
, a light not reflected but refracted at the first surface
104
a
of the micro-prism
104
, and a light having been reflected at the optical disc
106
, and then, not refracted but reflected at the first surface
104
a,
are consumed in loss.
There may be employed a quarter wavelength plate in order to change polarizing directions in incoming and outgoing optical path for increasing a light utilization efficiency. However, a surface having polarization can be formed only at a surface through which mediums having almost the same indexes of refraction make contact with each other. Hence, it might occur to those skilled in the art that a pillar-shaped micro-prism having a cross-section of a right-angled isosceles triangle and having almost the same index of refraction as that of the micro-prism
104
is adhered to the first surface
104
a
of the micro-prism
104
. However, such a structure would be accompanied newly with a problem that a focus error signal cannot be detected, because a light having been reflected at the optical disc
106
straightly advances without being refracted at the first surface
104
a.
The third problem is poor productivity of the micro-prism
104
.
This is because that the first surface
104
a
of the micro-prism has to be polished accurately at an angle of 45 degrees, and further because the second surface
104
b
has be coated with a half-mirror coating in one half, and with non-reflective coating in the other half.
The fourth problem is that if a light-emitting point of the laser diode chip
101
at which a light is emitted is shifted in a z-axis direction, there will be generated focus offset. This is because a height of the light-emitting point of the laser diode chip
101
is dependent on a thickness of the sub-mount
102
on which the laser diode chip
101
is mounted.
A light reflected at the optical disc
106
is designed to be converged on the third surface
104
c
of the micro-prism, when the optical disc
106
is located on a light-converging point of the lens
105
. As illustrated in
FIG. 4
, if the light-emitting point of the laser diode chip
101
is shifted in a z-axis direction by a distance of “q”, there is not generated an optical path difference in an incoming path, but there is generated an optical path difference D in an outgoing path, defined as the following equation, with respect to an optical path indicated with a solid line and an optical path indicated with a broken line. As a result, a light having been reflected at the optical disc
106
is not focused on the third surface
104
c
of the micro-prism
104
.
D
=[(2
n
2
−1)
½
−n
2
]q
/(
n
2
−1)
The fifth problem is that the track error signal is likely to be mixed into the focus error signal.
This is because it is unavoidable for an optical pickup to be assembled containing focus offset therein, due to limited assembling accuracy. The focus offset is removed by means of a particular circuit, when an optical pickup is incorporated into a drive. However, after all, the optical pickup is assembled in such a manner that a beam spot formed on the front light-receiving section
Hindi Nabil
McGinn & Gibb PLLC
NEC Corporation
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