Optical recording and reproducing method optical recording...

Details Optical recording and reproducing method optical recording... Optical recording and reproducing method optical recording...

1999-03-08

2001-10-02

Edun, Muhammad (Department: 2651)

Dynamic information storage or retrieval

With servo positioning of transducer assembly over track...

Optical servo system

C369S112230

Reexamination Certificate

active

06298018

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to an optical recording and reproducing method for converging light from a light source onto a signal recording layer of an optical recording medium such as an optical disk, and relates to an optical recording and reproducing device and an optical recording medium.
BACKGROUND OF THE INVENTION
Optical disks, magneto-optical disks, etc. are used in memory devices for computers, and as packaged media for music and video information. In optical recording and reproducing devices for this type of disk, a light beam projected by a light source is converged by an objective lens, and projected onto a recording layer of the disk, where recording and reproducing are performed (this type of optical recording and reproducing device will hereinafter be referred to as “Conventional Example 1”).
In recent years, there is a demand for increased recording density in such optical recording and reproducing devices. One method of achieving increased recording density is to reduce the diameter of the light spot projected onto the recording layer of the optical recording medium.
The reason behind this is that, when reproducing an information signal from an optical recording medium recording small recording marks at high density, a smaller light spot diameter gives rise to less contamination of the signal by signals from marks adjacent to a mark to be reproduced (so-called “crosstalk”), and the small recording marks can thus be reproduced accurately. Further, in recording an information signal in the recording medium, a smaller light spot diameter enables small marks to be recorded accurately, without influencing adjacent marks.
However, the spot diameter of a light beam is proportional to &lgr;/NA, where &lgr; is the wavelength of the light and NA is the numerical aperture. Therefore, in order to reduce the diameter of a light beam spot, it suffices to increase the numerical aperture of the objective lens which converges the light beam onto the surface of the recording medium. However, due to the difficulty of manufacturing objective lenses, there is a limit to how much the numerical aperture can be increased (practically, to around 0.6).
One proposed solution to this difficulty is to decrease the spot diameter by using a composite of lenses for the objective lens (hereinafter referred to as an “objective lens composite”). The following will explain such an objective lens composite in concrete terms with reference to FIG.
5
.
As shown in
FIG. 5
, this type of objective lens composite includes an objective lens
200
having a numerical aperture of NA, and a hemispherical lens
201
having a refractive index of N. A parallel light beam P
1
is projected onto the objective lens
200
, which reduces the beam diameter thereof and projects a converged light beam P
2
. The hemispherical lens
201
has a hemispherical light-incident surface facing the objective lens
200
, and is positioned such that rays of the light beam P
2
strike the foregoing light-incident surface perpendicularly. Further, the opposite surface of the hemispherical lens
201
from the light-incident surface is flat.
In an objective lens composite with the foregoing structure, since the rays of the light beam P
2
exiting the objective lens
200
strike the light-incident surface perpendicularly, there is little reflection or diffraction of these rays as they enter the hemispherical lens
201
. Accordingly, as the light beam P
2
enters the hemispherical lens
201
, it maintains the angle with which it was converged by the objective lens
200
of numerical aperture NA. Here, since the hemispherical lens
201
has a refractive index of N, the wavelength of the light after entering the hemispherical lens
201
is 1/N.
Then, light exiting the flat surface of the hemispherical lens
201
is further converged due to a difference in refractive indices of the hemispherical lens
201
and air, exiting as a light beam P
3
corresponding to a numerical aperture of N×NA (wavelength returns to &lgr;).
In this way, with an objective lens composite like that shown in
FIG. 5
, a light beam effectively having a large numerical aperture can be easily produced. Various optical disk devices using this type of objective lens composite have been proposed.
An optical disk device using such an objective lens composite, shown in
FIG. 6
, is disclosed in Japanese Unexamined Patent Publication Nos. 8-221772/1996 (Tokukaihei 8-221772) and 8-221790 (Tokukaihei 8-221790) (the optical disk device disclosed in these publications will be referred to hereinafter as “Conventional Example 2”).
In the foregoing conventional optical disk device, light projected from an objective lens composite
210
, made up of an objective lens
200
and a hemispherical lens
201
, reaches an optical disk
211
across a gap
212
of at least several &mgr;m, and is projected onto a recording layer
213
, in which information is recorded. Here, light projected from the objective lens composite
210
crosses the gap
212
and reaches the optical disk
211
as a light beam corresponding to a numerical aperture of N×NA, as discussed above.
With the foregoing conventional optical disk device, the light beam projected onto the optical disk
211
has a numerical aperture N times greater (N=refractive index of the hemispherical lens
201
), i.e., a beam spot with a diameter of 1/N times that when the objective lens
200
is used alone.
Another optical disk device using an objective lens composite, shown in
FIG. 7
, is disclosed in
Nikkei Electronics,
Jun. 16, 1997, pp. 99-108 (hereinafter referred to as “Conventional Example 3”).
In the foregoing conventional optical disk device, an objective lens composite
210
, made up of an objective lens
200
(numerical aperture=NA) and a hemispherical lens
201
(refractive index=N), is positioned in close proximity (around &lgr;/4) to a recording layer
213
of an optical disk
211
.
If the objective lens composite
210
and the recording layer
213
are positioned in close proximity with one another, near field effect causes light attempting to exit the flat surface of the hemispherical lens
201
to seep through the flat surface and reach the recording layer retaining the same properties it had inside the hemispherical lens
201
.
As mentioned above, while inside the hemispherical lens
201
, the light beam has a numerical aperture of NA and a wavelength of 1/N of its initial wavelength. Accordingly, the light beam which reaches the recording layer
213
has a wavelength of 1/N times that of normal projected light. Therefore, the light beam projected onto the recording layer
213
has a beam spot of 1/N the diameter of normal projected light.
In this way, Conventional Example 3 makes use of near field effect to guide the light beam, whose wavelength has been reduced by the hemispherical lens
201
, to the recording layer
213
with unchanged properties, thus reducing the size of the beam spot. An example of use of such an objective lens composite in a lithography system is disclosed in U.S. Pat. No. 5,121,256.
As discussed above, with Conventional Examples
2
and 3, the size of the beam spot projected onto the recording layer can theoretically be reduced, thus realizing high density of information recording in the optical disk.
However, in the case of Conventional Example 2 shown in
FIG. 6
, if the numerical aperture of the light beam projected from the objective lens composite
210
is too large, rays near the perimeter of the light beam exiting the hemispherical lens
201
have a large angle of incidence at the flat surface of the hemispherical lens
201
, and are totally reflected therefrom. Thus there is a limit to how much the numerical aperture can be increased.
For example, reflectance at the interface between the hemispherical lens
201
(refractive index=1.5) and air (refractive index=1.0) begins to increase at an angle of incidence of around 33°, and is totally reflected at an angle of incidence of 41.8°.
An increase in reflectance mea

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