Optics: measuring and testing – Focal position of light source
Reexamination Certificate
1998-09-30
2001-05-08
Font, Frank G. (Department: 2877)
Optics: measuring and testing
Focal position of light source
Reexamination Certificate
active
06229600
ABSTRACT:
FIELD OF THE INVENTION
The invention is related to the field of optical system for information storage and more specifically to detection systems for detecting the beam traveling from an optical disk.
BACKGROUND OF THE INVENTION
The invention relates to a spherical-aberration detection system for measuring spherical aberration of an optical beam, to an optical device for scanning record carriers using the detection system and to a device for measuring the thickness of a transparent layer.
The measurement of spherical aberration has recently become relevant in the field of optical recording. The information density on optical record carriers may be increased by increasing the numerical aperture (NA) of the radiation beam used for reading and writing information on the record carrier. Record carriers are often scanned through a transparent layer protecting the information layer of the record carrier. A small variation of the thickness of the transparent layer causes a substantial change in the spherical aberration incurred by a high-numerical aperture radiation beam traversing the transparent layer. This spherical aberration may be reduced by using a dual lens objective system. Such a system has a first lens and a second lens, the second lens being a plano-convex lens arranged between the first and lens and the record carrier, and a small spacing between the plano surface and the record carrier. In some applications the plano-convex lens is referred to as a solid immersion lens.
The article “High density optical disk system using a new two-element lens and a thin substrate disk”by F. Maeda et al, published in the proceedings of ISOM96 p. 342-344 discloses an optical recording system having such a dual-lens objective system. The spherical aberration due to variations in the thickness of the transparent layer are compensated by changing the axial position of the plano-convex lens of the objective system. The system determines the spherical aberration in the beam reflected from the record carrier and uses this value to position the plano-convex lens. The article proposes two methods to determine the amount of spherical aberration in the reflected beam. In the first method, the envelope magnitude of the information signal read from the record carrier is measured, which will have a maximum value when the spherical aberration is at a minimum value. In the second method the shape of the focus error signal as a function of the focus error is analyzed, and the position of the plano-convex lens is optimized to obtain the desired shape.
The above citations are hereby incorporated herein in whole by reference.
SUMMARY OF THE INVENTION
The inventor recognizes that first method of maeda has as a disadvantage that it requires the presence of an information signal, making the method unsuitable for writing on an unwritten record carrier. The second method has as a disadvantage that the shape of the focus error signal must be analyzed, which requires wobbling the objective system through the point of best focus. During wobbling the reading and writing performance of the optical disk system is reduced.
It is an object of the invention to provide a spherical-aberration detection system that does not have the above disadvantages.
The object is achieved when, in accordance with a first aspect of the invention, the spherical-aberration detection system for measuring spherical aberration of an optical beam includes a plurality of focus-detection systems, each for supplying a focus error signal pertaining to radiation from one of a plurality of concentric zones in a cross-section of the optical beam, and a signal processor for deriving a measure of the spherical aberration from the plurality of focus error signals.
The invention is based on the recognition that marginal rays and paraxial rays of a beam having spherical aberration have different focal points. The detection system according to the invention has several focus-detection systems for measuring the positions of the focal points for different parts of the beam cross-section. If the beam has no spherical aberration, the focal points of the marginal and paraxial rays will coincide, and the different focus-detection systems will measure the same position of the focal point. In the presence of spherical aberration, the positions of the focal points will be different, and the differences between the positions provides a measure for the spherical aberration in the beam.
In a simple form the spherical-aberration detection system includes only two focus-detection systems for determining the position of the focal point of the paraxial and marginal rays of the beam, respectively. The difference between the two positions is a measure for the spherical aberration in the beam.
In a special embodiment of the spherical-aberration detection system two focus detection systems each include a quadrant detector being detector, one quadrant arranged around the other quadrant detector. The inner quadrant captures mainly rays from an inner zone and the outer quadrant captures mainly rays from a zone around the inner zone.
A special embodiment of the detection system according to the invention includes a beam splitter arranged in the optical beam for splitting the optical beam in sub-beams pertaining to the concentric zones. Each of the focus-detection systems is arranged in the path of a sub-beam.
The focus-detection systems may be based on any focus detection method, such as e.g. the astigmatic focus detection method, the Foucault focus-detection method or the beam-size focus detection method. When the astigmatic method is used, the spherical-aberration detection system need not include the above beam splitter.
In accordance with a second aspect of the invention, the device for optically scanning an information layer of a record carrier. The devise has an objective system for focusing a radiation beam onto the information layer, a spherical-aberration detection system having an output for a signal representing a spherical aberration in radiation coming from the record carrier, wherein the spherical aberration detection system has the features of the spherical aberration detection system according to the invention.
A third aspect of the invention relates to a device for measuring the thickness of a transparent layer having a front surface and a rear surface. The device has an objective system for focusing a radiation beam through the entrance surface onto the rear surface, a spherical-aberration detection system having an output for a signal representing a spherical aberration in radiation coming from the transparent layer, and a calculating circuit for deriving a thickness of the transparent layer from the spherical aberration.
The objects, advantages and features of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings, in which.
REFERENCES:
patent: 5708638 (1998-01-01), Braat et al.
“High Density Optical Disk System Using a New Two-Element Lens and a Thin Substrate Disk” by F. Maeda et al, Published in the Proceedings of ISOM96 p. 342-344.
Belk Michael E
Font Frank G.
Nguyen Tu T.
U.S. Philips Corporation
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