Dynamic information storage or retrieval – With servo positioning of transducer assembly over track... – Optical servo system
Reexamination Certificate
1999-07-28
2002-10-01
Tran, Thang V. (Department: 2653)
Dynamic information storage or retrieval
With servo positioning of transducer assembly over track...
Optical servo system
C369S275300
Reexamination Certificate
active
06459659
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical recording medium and a fabrication method thereof, and to a near-field recording/reproducing device for recording/reproducing data on/from the optical recording medium, and to a method for, and in particular to a recording medium for recording/reproducing information by using an evanescent field transmitting an electromagnetic interaction controlled in the visible light and infrared regions, overcoming a diffraction limit, and having an ultra-high recording density, and a fabrication method thereof, and to a near-field optical recording/reproducing device for recording/reproducing information on the recording medium, and a method therefor.
2. Description of the Background Art
As the multimedia era in which information of various types, such as audio and video motion picture and text files are integrated has begun, there is an increasing demand for information recording and storage media having a large capacity which can rapidly process and store a large amount of information. Especially, if a two-way picture communication, such as a high definition motion picture and a video-on-demand (VOD) which are expected to be widely distributed is to be realized, the information recording and storage media must be increased in capacity. In order to meet the demand, various recording/reproducing methods have been suggested for the commonly-used recording media, and a magnetic recording method and an optical recording method are commonly used among the methods.
A prime example of the magnetic recording method is a method of recording/reproducing information on a hard disk by using a hard disk drive (HDD).
In order to store more information on the hard disk, the recording density of the hard disk should be improved. For this, a flying height between a head and a recording medium must be lowered. Here, in order to reduce the flying height, a lower portion of the head must be located very closely to the recording medium. However, when the lower portion of the head approaches to closely to a surface of the recording medium, the head scratches it, so called “a head crash” takes place, thereby damaging the recording medium and reducing the reliability thereof.
In addition to the method of improving the recording density by lowering the flying height between the head and the recording medium, there is a method of reducing a size of a MR-head (magnetic-resistive head) in the hard disk drive.
In the case of a longitudinal magnetic recording medium which has been popularly used, if the size of the particles composing each magnetic domain is lesser than a critical size, a recording bit causes magnetic inversion due to thermal fluctuation in an environment of a normal temperature. As a result, the recording medium cannot be well operated. This phenomenon is called a “super-paramagnetic limit”. It is thus impossible to record and retain information on the magnetic disk at an ultra-high recording density exceeding the limit. If the study for the high recording density of the magnetic recording medium proceeds according to the current tendency, it is expected that a limit value thereof will reach to 40 Gbit/in
2
around 2005.
On the other hand, the optical recording method is a method of recording/reproducing information on a recording medium, an optical pickup not being contacted with the recording medium. It does not have such a problem as the head crash occurring in the magnetic recording method. Accordingly, an optical recording medium such as a compact disk (CD) using the optical recording method can partially replace the magnetic recording media such as a magnetic tape and a magnetic disk using the magnetic recording method. In addition, the optical recording medium according to the optical recording method can be easily installed in the optical recording device using the optical recording method, and is convenient to carry. For instance, even if a CD drive is operated in a moving vehicle, a head of the optical recording device and the optical recording medium do not physically damage each other, and thus the information recorded on the optical recording medium is retrieved in safe manner. Also, the optical recording medium has a narrow track pitch, as compared with the magnetic recording medium, thereby achieving the higher recording density.
Nevertheless, a storage amount of the CD which has been widely distributed is only 650 megabytes. Accordingly, it is too small to deal with a large amount of information such as motion picture. In addition, in the case of a digital versatile disk (DVD) which has been recently commercialized, a recording capacity thereof is seven times as much as that of the CD (4.7 GB). However, a new recording medium having a larger capacity than DVD is required in order to freely deal with the motion pictures like movies. Thus, many studies have been made in association with the ultra-high recording density of the recording medium.
The ultra-high recording density will now be described exemplifying optical recording media, such as a read only memory (ROM) and a rewritable memory (-RAM/RW). The most important factor influencing the recording density of the optical recording media is a spot size of a laser light beam. That is, the smaller the spot of the laser beam is, the more information we can record in the optical recording medium. For this, the wavelength of a laser light source must be shortened, and a numerical aperture (NA) of an objective lens of the optical pickup must be increased.
However, although the wavelength of the laser light source is shortened and the numerical aperture of the objective lens is increased, the size of the spot of the laser beam can be narrowed merely to the extent of the wavelength of the laser source. For example, in order to shorten the wavelength of the laser light source, when a laser diode having a wavelength of 400 nm is used as a light source of the digital versatile disk DVD, instead of a laser diode having a wavelength of 650 nm which has been used for the DVD, the amount of information recorded per unit area of the DVD can be approximately 2.5 times as much as an amount of the information of the recording medium when the laser diode having the wavelength of 650 nm is used.
However, even if the short wavelength laser diode is employed as a light source for recording information on the optical recording media, such as the CD and the DVD, there is still a recording limit. In addition, it is impossible to improve the recording density of the recording media over a predetermined limit by overcoming the diffraction limit, due to a property of the diffraction resulting from the wave-nature of the light.
Accordingly, in order to process information of a terabyte, the ultra-high recording methods such as a near-field optics, a volume hologram, a photo-chemical hole burning and a three-dimensional optical recording are suggested on the basis of a totally different principle from the conventional art.
However, in the cases of the volume hologram and the photo-chemical hole burning, there is a remarkable limit in an environment of using the recording medium. Practically, the optical recording medium using the near-field cannot be easily used.
The background of inventing the near-field method will now be explained.
In general, in a diffraction theory, the spot size (diameter) of a focused light is determined by the optical wavelength and the numerical aperture of the objective lens. An upper limit of the recording density of the recording medium is determined by how small the light focusing spot is formed. According to the diffraction phenomenon, as a beam size of the light is reduced by using a lens, the beam is more widened. It is represented by the following expression (1).
θ
=
2
λ
π
d
≅
λ
d
(
1
)
Here, “&thgr;”, “d” and “&lgr;” indicate the diffraction angle, the waist of the beam, and the wavelength of the light, respectively. According to the diffraction theory, if the
Jhe Won Ho
Kim Ki Hyun
Kim Myong R.
Chou Chienowei (Chris)
Jhe Won Ho
Oppenheimer Wolff & Donnelly LLP
Tran Thang V.
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