Dynamic information storage or retrieval – Specific detail of information handling portion of system – Electrical modification or sensing of storage medium
Reexamination Certificate
2000-06-16
2003-06-17
Tran, Thang V. (Department: 2653)
Dynamic information storage or retrieval
Specific detail of information handling portion of system
Electrical modification or sensing of storage medium
C369S044260, C369S275400
Reexamination Certificate
active
06580677
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an information recording medium which is reproducible by utilizing near-field light and an information reproducing apparatus for reproducing information recorded on the information recording medium at high density, particularly to an information recording medium and an information reproducing apparatus enabling tracking control.
BACKGROUND OF THE INVENTION
Currently, many of information reproducing apparatus carry out reproduction of information with an information medium of a magnetic disk or an optical disk as an object, particularly, CD (Compact Disk) which is one of optical disks is widely used as a medium enabling information recording at high density and mass production at low cost and recording a large capacity of information. A surface of CD is formed with pits each having a size of about a wavelength of laser beam used in reproduction and a depth of about a quarter of the wavelength and reading operation utilizing interference phenomenon of light is carried out.
In reading recorded information from an optical disk represented by CD, generally, there is utilized a lens optical system used in an optical microscope. In this case, when an information recording density is increased by reducing a size of the pit or a track pitch thereof, a deadlock is reached by the problem of diffraction limit of light in which a spot size of laser beam cannot be made equal to or smaller than a half wavelength and an information recording unit cannot be reduced to a size smaller than the wavelength of laser beam.
Further, not only in CD but also in optical recording disks recorded with information by a magneto-optical recording system and a phase change recording system, record and reproduction of information at high density is realized by a very small spot of laser beam and accordingly, the information recording density is restricted by a diameter of the spot provided by converging laser beam.
Hence, in order to break the restriction imposed by the diffraction limit, there is proposed an information reproducing apparatus using an optical head provided with a very small aperture having a diameter equal to or smaller than a wavelength of laser beam utilized for reproduction, for example, a diameter of about {fraction (1/10)} of the wavelength and utilizing near-field light (including both of near-field light and far-field light) formed at the very small aperture portion.
Originally, as an apparatus utilizing near-field light, there is provided a near-field microscope using a probe having the above-described very small aperture and the near-field microscope is utilized in observing a very small surface structure of a sample. As one of systems of utilizing near-field light in a near-field microscope, there is provided a system in which a very small aperture of a probe and a surface of a sample are made proximate to each other such that a distance therebetween is equal to about a diameter of the very small aperture of the probe and by introducing propagating light via the probe toward the very small aperture of the probe, near-field light is formed at the very small aperture (illumination mode). In this case, scattered light produced by interaction of the formed near-field light and the surface of the sample, is detected by a scattered light detecting system with an intensity and a phase reflecting a fine structure of the surface of the sample and there is enabled observation having high resolution which cannot be realized by a conventional optical microscope.
Further, as other system of a near-field microscope utilizing near-field light, there is provided a system in which a sample is irradiated with propagating light to thereby localize near-field light on the surface of the sample and a very small aperture of a probe is made proximate to the surface of the sample to a degree of a diameter of the very small aperture of the probe (collection mode). In this case, scattered light produced by interaction of the localized near-field light and the very small aperture of the probe, is guided to a scattered light detecting system via the very small aperture of the probe with an intensity and a phase reflecting a fine structure of the surface of the sample to thereby achieve observation with high resolution.
According to the above-described information reproducing apparatus utilizing near-field light, there are utilized these observation systems in the near-field microscope and by unitizing the near-field light, there is enabled reproduction of information of an information recording medium recorded with information with higher density.
In order to realize reproduction of information recorded on the information recording medium with high density by utilizing the above-described near-field light, there is needed a positioning control technology for moving the very small aperture portion of the probe constituting an optical head to an arbitrary position on the information recording medium with high precision.
In the case of a magnetic disk apparatus, generally, as positioning control, there are adopted a servo face servo system and a sector servo system. The servo face servo system is a method in which one face in a plurality of disk faces is used exclusively for servo, a magnetic head for servo is positioned relative to the servo face and a remaining disk face and a magnetic head are used for data. Further, the sector servo system is a method in which servo information is embedded sporadically on data face and by using the servo information detected discretely, a magnetic head is positioned on data tracks.
However, it is difficult to apply positioning control adopted in these magnetic disk apparatus to positioning control in respect of reproduction of a high density information recording medium by near-field light. For example, according to the above-described servo face servo system, positional accuracies of a head for servo and a head for data are determined by mechanical accuracies and accordingly, there is a case in which a positional shift is produced between the both heads by a difference in temperature distribution and the system is not suitable for being adopted as positioning control in respect of an information recording medium with high density formation.
Further, according to the above-described sector servo system, although there causes no positional shift of heads caused by the difference in the temperature distribution which is problematic in the servo face servo system, at a design stage of a control system, the control system needs to deal with as a discrete value system different from a conventional continuous system and in the case of an information reproducing apparatus utilizing near-field light, there is needed high accuracy positioning with regard to an information recording medium with particularly high density formation and accordingly, it is not preferable to use such a complicated control system.
Meanwhile, according to an optical disk apparatus, as positioning,control methods, particularly as tracking error detecting methods, there are adopted a three beam method, a push-pull method and a prewobbling tracking error detecting method. The three beam method is a method in which beam from a laser diode is divided into a total of three beams of a 0-th order beam (main beam) for record and reproduction and two 1-th order beam (sub beam) for tracking by a diffracting grating, two of the sub beams are slightly shifted from a center of a guide groove provided on an optical disk, fluxes of reflected beam from both of them are received by two light receiving faces of an optical detector and an objective lens is controlled by a differential signal thereof.
Further, the push-pull method is a method in which fluxes of reflected beam of beam irradiated to guide grooves provided on an optical disk are detected by a two split detector and a differential signal provided thereby constitutes a track error signal to thereby control an objective lens. The prewobbling tracking error detecting method is a method in which a set of two long pits (pre
Chiba Norio
Kasama Nobuyuki
Mitsuoka Yasuyuki
Niwa Takashi
Oumi Manabu
Adams & Wilks
Seiko Instruments Inc.
Tran Thang V.
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