Dynamic information storage or retrieval – With servo positioning of transducer assembly over track... – Optical servo system
Reexamination Certificate
1998-12-24
2002-01-22
Psitos, Aristotelis M. (Department: 2753)
Dynamic information storage or retrieval
With servo positioning of transducer assembly over track...
Optical servo system
Reexamination Certificate
active
06341105
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a light beam condensing apparatus to introduce a condensing spot of a laser beam onto an optical recording medium such as optical disks, and to a method of driving the optical recording medium by applying the apparatus.
2. Description of the Prior Art
There has been frequently studied a technique to improve recording density of an optical disk so as to provide a large capacity optical disk. Thus, it has been known that reduction of a condensing spot diameter of a laser light beam for recording and regeneration is very effective in the improvement of the recording density, and mean surface recording density substantially increases while being inversely proportional to the square of the condensing spot diameter d
SPOT
. The condensing spot diameter d
SPOT
is proportional to a wavelength &lgr; of a laser to be used, and is inversely proportional to numerical aperture NA of an objective lens serving as a condenser, as shown in the following expression (1):
d
SPOT
=k·(&lgr;/NA) (1)
where the proportional constant k is defined by a wave front distribution of light wave incident on the lens. According to the expression (1), there are available three ways to reduce the condensing spot diameter d
SPOT
, i.e., the first way of reducing the wavelength of the laser to be used, the second way of increasing the numerical aperture of the objective lens serving as the condenser, and the third way of using super resolution in a condensing optical system.
A description will now be given of a method for providing a small condensing spot diameter by utilizing the super resolution in the condensing optical system. This method has been often disclosed in articles such as 1) Yamanaka et al., “High Density Recording in Optical Disk by Super Resolution” in
Optics
, Vol.18, No.12, (1989), or 2) H. Ando, “Phase-Shifting Apodizer of Three or More Portions” in
Japanese Journal of Applied Physics
, Vol.31, (1992). In these methods disclosed in the articles, it is possible to reduce the condensing spot diameter on the basis of the same principle, as shown in
FIGS. 1 and 2
.
FIG. 1
shows a configuration of an optical system of a conventional super resolution optical head as an example. In
FIG. 1
, reference numeral
101
means a laser oscillator,
102
means a collimate lens,
103
is a beam forming prism,
105
is an objective lens,
106
is a recording medium, and
107
is a shading plate.
A description will now be given of the operation. Laser light from the laser oscillator
101
serving as a light source is collimated through the collimate lens
102
and the beam forming prism
103
, resulting in parallel light. A laser beam
104
serving as the parallel light is focused and condensed by the objective lens
105
on a recording surface of the recording medium
106
. Here, the shading plate
107
is disposed across the laser beam
104
so as to partially shade the laser beam
104
. At the time, the condensing spot diameter d
SPOT
of the laser beam
104
is varied according to a position and a shape of the shading plate
107
, that is, a width and a length thereof.
A description will now be given of the principle in the reduction of the condensing spot diameter by the super resolution with reference to FIG.
2
. As shown in
FIG. 2
, in case the shading plate
107
is longer than a beam diameter D of the collimate beam
104
, a condensing spot diameter d
SPOT
t in a cross direction of the shading plate is defined as a ratio of the beam diameter D to the width &Dgr;W of the shading plate
107
if the width of the shading plate
107
is defined as &Dgr;W. Further, a condensing spot diameter d
SPOT
r in a longitudinal direction of the shading plate
107
is substantially irrelevant to the width &Dgr;W. Here, as the width &Dgr;W becomes larger, sidelobes
108
in a condensing spot becomes higher while the condensing spot diameter d
SPOT
t of a mainlobe
109
becomes smaller.
FIG. 3
shows a relation between &Dgr;W/D and the condensing spot d
SPOT
t. As understood from
FIG. 3
, as &Dgr;W/D is more increased, the condensing spot diameter d
SPOT
t is more reduced, and concurrently intensity of the sidelobe is more increased. Since increase of the sidelobe causes an increase of crosstalk, it is impossible to allow the sidelobe to become so large. Here, &Dgr;W/D=0 if the shading plate
107
is not employed. At the time, if the condensing spot diameter is set to d
SPOT
0, it is possible to reduce the condensing spot diameter d
SPOT
t to 10% degree as compared with d
SPOT
0 when the sidelobe intensity can be in a range of 0.1 times the mainlobe or less. In such a way, it is possible to reduce the condensing spot diameter with the constant laser wavelength &lgr; and the constant numerical aperture NA of the lens by shading a vicinity of an intermediate portion of the collimate beam in a super-resolution optical head. When the shading plate
107
is coplanarly rotated by 90°, the condensing spot diameter d
SPOT
t is left as it is d
SPOT
0, and the condensing spot diameter d
SPOT
r is reduced.
As set forth above, the principle of the super resolution utilizes the character of focusing light wave that it is possible to vary the intensity distribution at the condensing spot by modulating a wave front of the collimate beam
104
on an entrance surface of the objective lens
105
. That is, the shading plate
107
shown in
FIG. 2
corresponds to space modulation which is performed so as to set an amplitude distribution of the collimate beam
104
on the entrance surface of the objective lens
105
to zero in the vicinity of the intermediate portion of the collimate beam
104
. Accordingly, laser power at a shaded portion is lost.
Further, on the basis of the principle of the super resolution, it is also possible to vary the intensity distribution of the condensing spot by modulating a phase distribution of the collimate beam
104
on the entrance surface of the objective lens
105
. That is, it is possible to form a condensing spot shape by providing appropriate phase shift according to a position on the entrance surface of the objective lens
105
. This method is employed in the article 2) as described before. In this case, the collimate beam
104
is not shaded so that there is no partial loss of the laser power due to the shading.
Alternatively, in another known technique, a distribution is caused in indexes of refraction in order to provide phase modulation to transmitted light. Assumed that there is difference &Dgr;n between the indexes of refraction sensed by the transmitted light at two portions of a modulation plate when light having the wavelength &lgr; passes through the modulation plate having a thickness of L. Consequently, in the light beam passing through both the portions, there is generated a phase difference &Dgr;&phgr; expressed by the following expression (2):
&Dgr;&phgr;=2&pgr;(L/&lgr;)·&Dgr;n (2)
A phase of the transmitted light is modulated by the phase difference. It must be noted that a method of the phase modulation of the transmitted light should not be limited to a method to provide a difference in an optical path length by the difference in the indexes of refraction. It is similarly possible to provide the difference in the optical path length by varying the thickness of the modulation plate so as to perform the phase modulation of the transmitted light.
The recording density of the optical disk can be expressed by the product of recording density in a direction parallel to a recording track (i.e., track recording density BPI) and recording density in a direction perpendicular to the recording track (i.e., track density TPI). Therefore, it is possible to improve surface recording density of the optical disk by improving the BPI and the TPI, respectively. The conventional embodiment shown in
FIG. 2
is provided to improve the BPI. For example, if a concentrically circular shading plate to shade the intermediate portion exclusively is employed instead of the
Leydig , Voit & Mayer, Ltd.
Psitos Aristotelis M.
LandOfFree
Light beam condensing apparatus and method of driving... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Light beam condensing apparatus and method of driving..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Light beam condensing apparatus and method of driving... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2858470