Optical head and optical disk apparatus

Details Optical head and optical disk apparatus Optical head and optical disk apparatus

1999-12-07

2002-03-19

Tran, Thang V. (Department: 2651)

Dynamic information storage or retrieval

Specific detail of information handling portion of system

Radiation beam modification of or by storage medium

C369S112010, C369S112230

Reexamination Certificate

active

06359852

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical head and an optical disk apparatus both utilizing a near-field light, particularly an optical head and an optical disk apparatus both permitting the attainment of high-density recording.
2. Description of the Prior Art
Optical disks used in optical disk drives have been becoming larger in both density and capacity from a compact disk (CD) to a digital video disk (DVD), but a still further increase of capacity is now demanded with improvement in performance of computers and in definition of display devices.
The recording density of an optical disk is basically controlled by the diameter of a light spot formed on a recording medium. Recently, as a technique for making the diameter of a light spot smaller, a technique which utilizes a near-field light in a microscope has been applied to optical recording. As conventional optical disk drives using such a near-field light there are known, for example, those disclosed in the literature Jpn. J. Appl. Phys., Vol. 35 (1996), p. 443 and U.S. Pat. No. 5,497,359.
FIGS. 23A and 23B
show an optical disk drive disclosed in the literature Jpn, J. Appl. Phys., Vol. 35 (1996), p. 443. As shown in
FIGS. 23A and 23B
, this optical disk drive, indicated at
190
, comprises a semiconductor laser
191
which emits a laser beam
191
a
, a coupling lens
192
which shapes the laser beam
191
a
emitted from the semiconductor laser
191
into a collimated beam
191
b
, a probe
194
which has an optical fiber
193
having been ground so as to be tapered off from an incident end
193
a toward an exit end
193
b
and which introduces the collimated beam
191
b
from the coupling lens
192
through the incident end
193
a
, and a recording medium
195
on which data are recorded with a near-field light
191
c
which leaks out from the exit end
193
b
of the optical fiber
193
.
The recording medium
195
has a recording layer
195
a
formed by GeSbTe as a phase change medium, which is heated by incidence thereon of the near-field light
191
c
, thereby inducing a phase change between crystal phase and amorphous phase. Recording is effected by utilizing a change in reflectance between both phases.
The optical fiber
193
is machined so as to have a diameter of 10 &mgr;m at the incident end
193
a
and a diameter of 50 nm at the exit end
193
b
and it is coated with a metallic film
194
b
such as aluminum film through a clad layer
194
a
to prevent light from leaking out to any other portion than the exit end
193
b
. Since the diameter of the near-field light
191
c
is about the same as that of the exit end
193
b
, it is possible to attain a high recording density of ten GB/inch
2
.
In reproduction, as shown in
FIG. 23B
, the near-field light
191
c
which is low in power to an extent not inducing a phase change is radiated to the recording layer
195
a
and reflected light
191
d
therefrom is condensed to a photomultiplier tube (hereinafter referred to simply as “photomultiplier”)
197
through a condenser lens
196
and is detected.
FIG. 24
illustrates an optical head of the optical disk drive disclosed in U.S. Pat. No. 5,497,359. This optical head, indicated at
50
, comprises an objective lens
52
which condenses a collimated light
51
and a spherical bottom-cut SIL (Solid Immersion Lens)
54
which is disposed so that a bottom
54
a
thereof is perpendicular to a convergent light
53
emerging from the objective lens
52
. When the collimated light
51
is converged by the objective lens
52
and a convergent light
53
thus obtained is applied to a semispherical incident surface
54
b
, the convergent light
53
is refracted by the incident surface
54
b
and is focused on the bottom
54
a
, whereby a light spot
55
is formed on the bottom
54
a
. In the interior of the SIL
54
, the wavelength of light becomes shorter in inverse proportion to the refractive index of the SIL
54
, so that the light spot
55
also becomes smaller proportionally. Most of the light focused to the light spot
55
is totally reflected toward the incident surface
54
b
, but a portion thereof leaks out as a near-field light
57
from the light spot
55
to the exterior of the SIL
54
. If a recording medium
56
having a refractive index almost equal to that of the SIL
54
is disposed at a distance sufficiently smaller than the wavelength of light from the bottom
54
a
, the near-field light
57
will be coupled with the recording medium
56
into a propagation light which is propagated within the recording medium
56
. With this propagation light, information is recorded in the recording medium
56
.
If the SIL
54
is constructed so that the collimated light
51
is converged at a position spaced r
(r stands for the radius of the SIL) from a center
54
c
of the semispherical surface
54
b
, which structure is designated a Super SIL structure, it is possible to diminish a spherical aberration caused by the SIL
54
and increase the numerical aperture in the interior of the SIL
54
. Further, it becomes possible to make the light spot
55
very small. More particularly, the light spot
55
is microminiaturized like the following expression:
D
½
=k&lgr;/(n·NAi)=k&lgr;/(n
2
·NAo)
where,
k: a proportional constant (usually 0.5 or so) which is dependent on the intensity distribution of light beam
&lgr;: wavelength of light beam
n: refractive index of SIL
54
NAi: numerical aperture in the interior of SIL
54
NAo: numerical aperture of incident light on SIL
54
Since the collimated light
51
is converged as the light spot
55
without being absorbed on the optical path, there is attained a high light utilization efficiency. Consequently, it is possible to use a light source of a relatively low output and the detection of reflected light can be done even without using the photomultiplier.
According to the above conventional optical disk drive
190
it is possible to form a small light spot of several ten nm or so on the recording medium, but since the optical fiber
193
is tapered, a portion of laser light incident on the optical fiber
193
is absorbed in the interior of the optical fiber, thus giving rise to the problem that the light utilization efficiency becomes as low as 1/1000 or less. Therefore, the use of the photomultiplier
197
for the detection of reflected light
191
d
is unavoidable, with consequent increase in size and cost of the optical head portion. In addition, the response speed of the photomultiplier
197
is low and the optical head portion is heavy, so it is impossible to effect a high-speed tracking and hence impossible to rotate the optical disk at high speed. As a result, there occur various problems, including the problem of a low transfer rate, and thus improvements are needed for practical application of such a conventional optical disk drive.
FIG. 25
is a diagram for explaining a problem of the conventional optical head
50
shown in
FIG. 24
, which diagram is of an analysis made by Suzuki at #OC-
1
of Asia-Pacific Data Storage Conference (Taiwan, '97. 7.). A relation between the refractive index, n, of the SIL
54
and NAo is shown therein. There is a reciprocal relation between NA of incident light on the SIL
54
, i.e., maximum value, &thgr;max, of incident angle &thgr;, and the refractive index, n, of the SIL
54
. It is not that both can be made large each independently. As is seen from the same figure, with an increase in refractive index, n, of the SIL
54
, the maximum value NAomax which the NAo of incident light can take becomes smaller. This is because if NAo increases beyond the maximum value Naomax and the incidence angle becomes larger as a result, then the light concerned enters the recording medium
56
directly without passing through the SIL
54
and consequently the light spot
55
rather expands at the position of the recording medium
56
. For example, when the refractive index, n, is equal to 2, the value of NAomax is 0.44 and the product of the two, n·NAomax, falls under the r

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