Dynamic information storage or retrieval – Specific detail of information handling portion of system – Radiation beam modification of or by storage medium
Reexamination Certificate
1999-04-08
2002-08-06
Hindi, Nabil (Department: 2653)
Dynamic information storage or retrieval
Specific detail of information handling portion of system
Radiation beam modification of or by storage medium
C369S044240
Reexamination Certificate
active
06430141
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a device for reading from and/or writing to optical recording media in which both zeroth-order and first-order diffraction beams fall onto a photodetector.
BACKGROUND OF THE INVENTION
Devices of this type have the disadvantage that in some instances complex diffraction patterns are produced on the photodetector on account of the superposition of the zeroth- and plus/minus first-order diffraction beams, which diffraction patterns have undesirable interfering influences on signals derived from the output signals of the photodetector even in the event of slight displacement of the beam incident on the photodetector relative to the optical axis.
SUMMARY OF THE INVENTION
The object of the invention is to obtain a reduction in the interfering influences in a signal derived from photodetector signals, a signal such as, for example, a focus error signal, in particular one obtained by the astigmatism method, or a track error signal. In this case, the interfering influences can be caused, inter alia, by the superposition of diffraction beams of the zeroth and first or higher diffraction order, by the displacement of the light spot on the photodetector as a consequence of desired or undesired displacement of one or more optical elements relative to the optical axis, or by a combination of these or further interfering influences, for example ones governed by the design.
This object is achieved by means of the measures specified in independent claims.
According to the invention, a beam splitting means is provided, which splits the light spot falling onto the photodetector into two separate partial spots. In this case, this beam splitting means is advantageously arranged in the beam path upstream of the photodetector and generates two partial beams which engender mutually separate light spots on the photodetector. This has the advantage that the partial spots in each case illuminate areas of identical size in different detector zones even in the event of relative displacement with respect to the optical axis. Situated between the partial spots is an unilluminated boundary zone, which advantageously coincides with a boundary between two detector zones, with the result that although the boundary zone is displaced in the event of displacement of the light spot essentially perpendicularly with respect to the said boundary, the boundary nonetheless remains in the unilluminated boundary zone. The detector zones separated by the boundary thus receive an unchanged intensity component even in the event of displacement of the light spot; an interference component in the derived signal is not event brought about in the first place. The optical recording medium is generally in the form of a disc and provided with an information-carrying layer. This layer usually has concentrically or spirally arranged information tracks having a predetermined distance from one another and a predetermined depth. Arranged on the information track are information items in the form of elongate elements of greater or lesser length, which are also called spots or pits, and which may be depressions, elevations and be reflective to a greater or lesser extent or have optically different properties in another suitable manner. The scanning beam generation means generally has a laser diode and corresponding optical elements. The focusing means serves to focus the scanning beam on an information-carrying layer of the optical recording medium. It is often designed, moreover, in such a way that a radial movement, that is to say in a perpendicular direction with respect to the information track, is simultaneously possible for the purpose of tracking the scanning beam on the information track.
According to the invention, the detector zones are separated by a boundary line running in a corresponding manner to the track direction of the recording medium. This has the advantage that a tracking signal can be derived in accordance with the so-called push-pull method, which signal is largely free from interference components that may be caused by a displacement of the light spot relative to the optical axis. In addition to the tracking method mentioned, it is also possible advantageously to employ any other tracking method in which the output signals of the detector zones, which are separated in accordance with the track direction, are combined and evaluated as track error signal. In the case of the method mentioned, the difference zero corresponds to optimal tracking, and a value of greater than or less than zero corresponds to a deviation from the track to the left or right.
According to a further advantageous refinement of the present invention, the device has a photodetector comprising at least four detector elements, and an astigmatism generation means. The astigmatism generation means serves to generate astigmatism in the beam falling onto the photodetector, which enables a focus error signal to be generated. With optimal focusing of the scanning beam onto the recording medium, a circular light spot is produced on the photodetector. In the event of defocusing, the light spot assumes an elliptical form. Therefore, the photodetector has four detector elements generally arranged as four quadrants, the output signals from diagonally opposite detector elements being combined and the difference between the diagonal sums being used as the focus error signal. The astigmatism generation means is a cylindrical lens, for example, but in this case it is also possible, however, to use any other element which generates corresponding astigmatism in the beam falling onto the photodetector. One advantage of this refinement is that a focus error signal is obtained in accordance with the astigmatism method, which signal is largely free from interfering influences caused by movement of the light spot relative to the photodetector. Such movements of the light spot can generally be caused by undesired displacement of one or more optical elements from the respectively optimal position. This is governed for example by ageing, temperature expansion or by sub-optimal adjustment or the like.
According to the invention, the beam splitting means has a light-influencing strip. This has the advantage that it becomes possible to separate the beam into two partial beams in a simple manner, the boundary zone of the photodetector not being covered by any of the partial beams. The light-influencing strip is advantageously an opaque strip; the boundary zone of the photodetector is thus shaded. The opaque strip is arranged in the beam path from the recording medium to the photodetector, to be precise parallel or perpendicular to the track direction and, at the same time, parallel to a boundary between two detector zones of the photodetector, a so-called “dark line”.
However, the light-influencing strip that is provided may likewise be a light-deflecting strip. An advantageous variant consists in designing the strip as a prism. That component of the light beam which falls onto the prism is deflected onto a zone situated outside the photodetector, as a result of which it is likewise possible to obtain shading of the boundary zone. Such a light-deflecting strip is inexpensive to produce.
A further advantageous variant of a light-influencing strip consists in arranging polarization-influencing elements adjoining the strip, while the strip itself has no influence on the polarization of the light passing through it. The light whose polarization is uninfluenced can then be filtered out by means of an analyser. Polarization-influencing elements are, for example, quarter- or half-wave plates which convert linearly polarized light into circularly polarized light or rotates the polarization. direction. The analyser used is, for example, a polarization filter, a polarizing beam splitter or another suitable optical element.
It is advantageous for the beam splitting means, in this case the light-influencing strip, in particular, to be situated between a beam splitter and the astigmatism-generating element. This has the advantage t
Dang Lieu Kim
Dietrich Christoph
Richter Hartmut
Schroeder Heinz-Jörg
Hindi Nabil
Laks Joseph J.
Thomson Licensing S.A.
Wein Frederick A.
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