Dynamic information storage or retrieval – Specific detail of information handling portion of system – Radiation beam modification of or by storage medium
Reexamination Certificate
1998-11-02
2001-07-03
Huber, Paul W. (Department: 2651)
Dynamic information storage or retrieval
Specific detail of information handling portion of system
Radiation beam modification of or by storage medium
C369S112120, C369S120000
Reexamination Certificate
active
06256285
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a device for optically scanning an information plane, which device comprises a radiation source for supplying a scanning beam, an objective system for focusing the scanning beam to a scanning spot on the information plane, a dividing element comprising at least one sector having an included arc equal to or smaller than 120° arranged in the optical path of the radiation from the information plane for splitting at least a part of said radiation into a sub-beam, and a radiation-sensitive detection system comprising an oblong detector for receiving the sub-beam.
A sector is an area enclosed between two radii and a line connecting endpoints of the radii. The line may have any form, e.g. an arc cut off by the radii or a straight line, or a line having a 90° angle. A bisector of a sector is a straight line which bisects the included angle between the two radii of the sector in equal parts. A figure is said to be oblong when its dimensions in perpendicular directions are not equal.
A device of this type, which in principle is suitable for reading and writing information in an optical record carrier, is known from a publication in the journal Neues aus der Technik, number 4, Dec. 20, 1990, page 7, item 493. The sub-beams formed by a dividing element of the known device are incident on a detection system. The detection signals of the detection system are used for deriving a radial tracking error signals and a focus error signal. It is a disadvantage of the known device that it is less suitable for scanning multi-layer record carriers, as known from inter alia U.S. Pat. No. 5,511,057. The information planes in such a multi-layer record carrier are scanned from one side of the record carrier, i.e. one information plane is scanned through another information plane. Stray light from the other information plane affects the signals derived from the detection system. Moreover, neither is the known device suitable for scanning different types of record carriers having transparent substrate layers of different thicknesses, as is known from inter alia European patent application no. 763 236. Aberrated light rays in this applications affect the signals derived from the detection system.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a device according to the preamble which is less sensitive to stray-light and aberrated light rays.
The device according to the invention is therefore characterized in that the direction of the longest dimension of the detector is substantially perpendicular to the bisector of the sector of the dividing element. The sub-beam formed by a sector forms a spot on the detector. In the geometric approximation the spot has the form of the sector. Stray-light and aberrated rays deflected by the sector form a fan of light radially extending from the centre of the sector and within the angle subtended by the sector. The detector size in the direction of the bisector determines the spatial filtering characteristics of the detector. When, according to the invention, a detector of the detection system has its largest dimension perpendicular to the direction of the bisector, the quantity of stray light or aberrated rays intercepted by the detector will be reduced. A detector is called oblong if its length is at least twice its width.
In a preferred embodiment of the device the angle between the bisector and the direction of the longest dimension is within the range from 75° to 105°, in order to reduce substantially the effect of stray light and aberrated rays.
When the wavelength of the radiation changes, e.g. because of temperature changes of the radiation source or changes in the electrical control of the radiation source, the position of the spot preferably changes in the direction of the longest dimension of the detector. The spot will then be properly detected even when the wavelength changes.
In special embodiments of the scanning device one or more detectors comprise two sub-detectors located symmetrically at both sides of a dividing line. The dividing line is preferably arranged substantially parallel to the direction of the longest dimension of the detector. A change of wavelength will cause a drift of the spot along the dividing line, not affecting the distance between the centre of gravity of the spot's intensity distribution and the dividing line.
The sectors of the dividing element are preferably provided with grating lines and the angle between the direction of the grating lines in each sector and the bisector of the sector is less than 15°. The orientation of the grating lines causes the spots formed on the detectors to have a wavelength drift in a direction substantially perpendicular to the bisector of each sector, i.e. in the direction of the longest dimension of each detector. The direction of grating lines of a grating having curved lines is understood to be the direction of the grating lines averaged over the grating.
The width of a detector in the direction of the bisector of the appertaining sector is preferably in a range from 4 to 8 times &lgr;/NA, where &lgr; is the wavelength of the radiation and NA is the numerical aperture of the beam incident on the detector. A smaller detector affects the quality of the signals derived from the detector output signals, whereas a wider detector captures too much stray light and aberrated rays.
The number of sectors in the dividing element is preferably equal to or larger than two. More preferably, the number of sectors is equal to four, which allows the generation of a focus error signal using the Foucault method, as known from inter alia U.S. Pat. No. 4,665,310. The dividing element may comprise e.g. one 180° sector and two 90° sectors, four 90° sectors, or three 120° sectors.
REFERENCES:
patent: 4665310 (1987-05-01), Heemskerk
patent: 4945529 (1990-07-01), Ono et al.
patent: 5511057 (1996-04-01), Holtslag et al.
patent: WO9628816 (1996-09-01), None
Neues Aus Der Technik, No. 4, Dec. 20, 1990, p. 7, Item 493.
Biren Steven R.
Huber Paul W.
U.S. Philips Corporation
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