Optical: systems and elements – Lens – With light limiting or controlling means
Reexamination Certificate
1999-12-13
2001-08-21
Schwartz, Jordan M. (Department: 2873)
Optical: systems and elements
Lens
With light limiting or controlling means
C369S112240, C369S118000
Reexamination Certificate
active
06278560
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to an optical device for scanning optical record carriers of a first type with a radiation beam having a first numerical aperture and record carriers of a second, different type with a radiation beam having a second numerical aperture, comprising a radiation source for providing a radiation beam having one optical axis, an optical system for converging the radiation beam to a focused beam so as to form a spot on the record carrier which is being scanned.
BACKGROUND OF THE INVENTION
The amount of information that can be stored on an optical record carrier depends, inter alia, on the size of the spot formed by the scanning device on the information layer of the record carrier. The information density and, hence, the amount of stored information can be increased by decreasing the size of the spot. The spot size can be reduced by increasing the numerical aperture of the radiation beam forming the spot.
An optical device of the above type is known from European patent application No. 0762398. An objective lens in the optical head of the device focuses a radiation beam from a radiation source to a spot on the record carrier. An adjustable diaphragm positioned between the radiation source and the objective lens adapts the numerical aperture of the radiation beam to the type of record carrier which is being scanned. The diaphragm is in the form of an electrically operated liquid crystal shutter. The shutter transmits only the central part of the incident radiation beam when scanning a first type of record carrier and transmits the entire radiation beam when scanning a second type of record carrier, thereby changing the numerical aperture of the beam incident on the record carrier. A disadvantage of the shutter is that it affects the wavefront quality of the transmitted radiation beam because of the different optical properties of the shutter in the central part and in the annular part around it. Another disadvantage is that the shutter constitutes an optical element which adds to the manufacturing cost of the scanning device.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a scanning device that does not have the above disadvantages.
In accordance with the invention, this object is achieved by a scanning device as described in the opening paragraph, which is characterized in that the optical system comprises a first fixed stop and, at a different position on the optical axis, a second fixed stop, the first and the second stop determining the first and the second numerical aperture, respectively. The fixed stops do not affect the quality of the wavefront transmitted by the stops and can easily be integrated with other elements in the optical path, e.g. in the form of a non-transparent coating with a central aperture applied to a surface of a lens. When changing between different types of record carrier, the optical properties or position of one or more elements in the optical head will be changed to obtain a proper spot on the record carrier. The changes may include a change in the axial position of the objective lens to allow for a change in the axial position of the spot, a change in the mutual distance of elements in the optical head or a change in wavelength. Such changes vary the shape of the radiation beam through the optical elements between a first and a second shape. If the first and the second stop are arranged at appropriate positions in the optical path, the first stop will limit the diameter of the radiation beam when it has the first shape, whereas the second stop will limit the diameter of the radiation beam when it has the second shape. If the optical head is provided with two or more radiation sources and a beam-combining element so as to bring the radiation beams of the radiation sources on a common optical path, the first and the second stop are located on the common path.
In a preferred embodiment of the scanning device, the second stop is arranged in the path of the focused beam and the second numerical aperture is smaller than the first numerical aperture. Counter-intuitively, the second stop determines the smaller numerical aperture and does not affect the larger numerical aperture.
In a special embodiment, the optical system comprises an objective system having a first and a second optical element, the first element being further remote from the radiation source than the second element. The objective system is suitable for focusing a high numerical aperture beam. The second stop is preferably a diaphragm arranged on the first optical element.
REFERENCES:
patent: 5790506 (1998-08-01), Morita et al.
patent: 5920532 (1999-07-01), Yagi et al.
patent: 0762398A1 (1997-12-01), None
Schwartz Jordan M.
U.S. Philips Corporation
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