Dynamic information storage or retrieval – Specific detail of information handling portion of system – Radiation beam modification of or by storage medium
Reexamination Certificate
1998-01-09
2001-01-09
Edun, Muhammad (Department: 2753)
Dynamic information storage or retrieval
Specific detail of information handling portion of system
Radiation beam modification of or by storage medium
C369S044370, C369S044170
Reexamination Certificate
active
06172958
ABSTRACT:
TECHNICAL FIELD
This invention belongs to the technical field relating to an optical pick-up adapted for carrying out write and read operations of information signals with respect to disc-shaped optical recording media such as optical disc or magneto-optical disc, and a disc player caused to be of structure comprising such optical pick-up and adapted for carrying out recording and reproduction of information signals with respect to disc-shaped recording media as described above.
BACKGROUND ART
Hitherto, as a recording medium for information signals, disc-shaped optical recording media such as optical disc or magneto-optical disc have been proposed. Further, optical pick-up devices for carrying out write and read operations of information signals with respect to such disc-shaped optical recording media have been proposed. In addition, disc players caused to be of structure comprising such optical pick-up and adapted for carrying out recording and reproduction of information signals with respect to disc-shaped optical recording media as described above have been proposed.
Such a disc-shaped optical recording medium is caused to be of structure including a transparent base (substrate) consisting of transparent material such as polycarbonate, and a signal recording layer deposited and formed on one principal surface portion of the transparent base. The optical pick-up is caused to be of structure including a semiconductor laser serving as a light source, an object lens (objective) to which light beams emitted from the semiconductor laser are incident, and a photo-detector.
The light beams incident to the object lens are irradiated, in converged state, onto the signal recording surface of the disc-shaped optical recording medium by the object lens. In this case, these light beams are irradiated from the transparent base (substrate) side of the disc-shaped optical recording medium onto the disc-shaped optical recording medium, and are converged onto the signal recording surface which is the surface portion of the signal recording layer after transmitted through the transparent base. This object lens is caused to undergo movement operation in the state supported by the bi-axial actuator to thereby converge the light beams onto the portion where information signal is recorded on the signal recording surface, i.e., a predetermined recording track. Such recording tracks are formed so as to take spiral shape on the principal surface portion of the disc-shaped optical recording medium.
In the disc-shaped optical recording medium, light beams are irradiated in the state converged after passed through the object lens, whereby write or read operation of information signal is carried out at the portion to which such light beams has been converged.
The light beam converged on the signal recording surface is caused to undergo modulation in light quantity or light polarization direction in dependency upon the information signal recorded on the signal recording surface and is reflected by the signal recording surface. The reflected light thus obtained returns to the object lens.
The reflected light beam reflected by the signal recording surface is received by the photo-detector after passed through the object lens. This photo-detector is a light receiving element such as photo-diode, and serves to receive the reflected light beam which has been passed through the object lens to convert it into an electric signal. On the basis of the electric signal outputted from the photo-detector, reproduction of information signal recorded on the disc-shaped optical recording medium is carried out.
Moreover, on the basis of the electric signal outputted from the photo-detector, a focus error signal indicating distance in the optical axis direction of the object lens between the focal point of the light beam by the object lens and the signal recording surface, and a tracking error signal indicating distance in the radial direction of the disc-shaped optical recording medium between the focal point and the recording track on the signal recording surface are generated. The biaxial actuator is controlled on the basis of the focus error signal and the tracking error signal to allow the object lens to undergo movement operation so that these respective error signals are converged into zero (0).
Meanwhile, since such a disc-shaped optical recording medium is used as an auxiliary memory (unit) for computer and/or recording medium for speech and picture signals, recording density of information signals is being caused to be high.
In order to carry out write and read operations of information signals with respect to the disc-shaped optical recording medium of which recording density has been caused to be high, it is necessary to allow the object lens to have greater numerical aperture (NA), and to allow the wavelength of light emitted of the light source to be shorter to reduce (diameter of) beam spot formed as the result of the fact that the light beams are converged onto the disc-shaped optical recording medium.
However, when the numerical aperture of the object lens becomes greater, respective degrees of allowance of inclination of the disc-shaped optical recording medium, variations (unevenness) in thickness of the transparent base of the disc-shaped optical recording medium, and defocus (deviation in focal point) of the light beams on the disc-shaped optical recording medium are reduced. As a result, write and read operations of information signals with respect to the disc-shaped optical recording medium become difficult.
For example, when inclination (skew) with respect to the optical axis of the object lens of the disc-shaped optical recording medium takes place, wave front aberration in the light beam converged on the signal recording surface takes place, so electric signal (RF output) outputted from the photo-detector is affected.
In this wave front aberration, coma-aberration of the third order (cubic coma-aberration) produced in proportion to the third power of the numerical aperture of the object lens and approximately the first power of inclination angle (skew angle) of the disc-shaped optical medium is dominant. Accordingly, the allowed value with respect to the inclination of the disc-shaped optical recording medium is inversely proportional to third power of the numerical aperture of the object lens. Namely, when the numerical aperture becomes great, the allowed value becomes small.
In optical discs (disc like the so-called “compact discs”) caused to be of structure including a disc-shaped transparent base (substrate) having thickness of 1.2 mm and diameter of 80 mm or 120 mm and formed by polycarbonate, and widely used in general at present, there are instances where inclination of ±0.5° to ±1° takes place.
When the numerical aperture (NA) of the object lens is increased in such a optical disc, wave front aberration as described above takes place in light beams irradiated onto the optical disc. As a result, beam spot on the optical disc takes asymmetrical form, and interference between codes remarkably takes place. Thus, precise signal reproduction becomes difficult.
The quantity of such cubic coma-aberration is proportional to the thickness of the transparent base (substrate) of the optical disc. For this reason, the thickness of the transparent base is thinned (e.g., is caused to be 0.6 mm), thereby making it possible to reduce the cubic coma-aberration to one half. In the case where it is assumed that coma-aberration is caused to be decreased in this way, optical discs including the transparent base having thickness of 1.2 mm and optical discs including the transparent base having thickness of 0.6 mm would be used in mixed state as the above-mentioned optical disc.
Meanwhile, when plane parallel plate having thickness t is inserted into an optical path of a converged light beam converged by the object lens, there takes place spherical aberration proportional to t×(NA)
4
in relation to the thickness t and numerical aperture NA of the object lens.
The object lens is designed so th
Mochizuki Tsutomu
Sugawara Yutaka
Edun Muhammad
Kananen Ronald P.
Rader Fishman & Grauer
Sony Corporation
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