Dynamic information storage or retrieval – With servo positioning of transducer assembly over track... – Optical servo system
Reexamination Certificate
1997-10-08
2001-06-19
Tran, Thang V. (Department: 2651)
Dynamic information storage or retrieval
With servo positioning of transducer assembly over track...
Optical servo system
C369S112280
Reexamination Certificate
active
06249493
ABSTRACT:
BACKGROUND OF THE INVENTION
a. Field of the Invention
The invention relates to a method and a device for recognition of a focussing status of a light beam incident on an information carrier.
b. Description of the Prior Art
The method according to the invention for recognition of a focussing status can be employed in a corresponding recording or replay device for contactless scanning of optical information carriers. In a recording or replay device of this type, an information carrier, for example a CD, is illuminated by a scanning light beam. A light beam reflected by the information carrier carries the information stored on the information carrier. In the case of CDs, by far the most common optical information carrier, usually elongate indentations are made on a plane reflecting face. These indentations, also referred to as pits, form a track in the form of a spiral or concentric circles. The reflected light beam has a different intensity, depending on whether the scanning light beam is incident on the CD at an indentation or at a plane location.
In order to obtain an unambiguous signal, the scanning light beam needs to be focussed very well. On the one hand, the light spot on the information carrier face carrying the information should not be too large, so that neighbouring pits or tracks are not picked up at the same time, and on the other hand the light spot should not fall short of a particular minimum size. This minimum area is necessary, for example in the case of a CD, so that, for incidence on a pit, a sufficient area around the pit is also illuminated. The pit gap conventionally used in a CD then causes a drop in intensity, due to destructive interference, in the reflected light beam.
Conventional optical scanning systems are therefore provided with an autofocus system for automatically setting or correcting the focussing of the scanning light beam. A known design for recognizing the focussing status of a light beam incident on an information carrier uses astigmatism.
This design is explained with the aid of a conventional scanning system with reference to FIG.
6
. In
FIG. 6
, a scanning light beam
14
is emitted by a light source
12
, in particular a semiconductor laser. The scanning light beam
14
passes through a beam splitter
29
and is focussed using a converging lens
13
in such a way that the focus falls on the information track
30
of an information carrier
11
. A light beam
15
reflected by the information carrier
11
carries the information read from the information track
30
, passes again through the converging lens
13
and is reflected down onto a cylindrical lens
31
by the beam splitter
29
. In the reflected light beam
15
, the cylindrical lens
31
produces artificial astigmatism, so that the reflected light beam
15
is converted into an irregular light beam. This light beam does not have a cross-section with symmetry of rotation about the propagation direction. It does not have a single focal point, but has two focal lines
33
,
34
which are spaced apart and one of which extends in the x direction of the cylinder axis of the cylindrical lens
31
while the other extends in the y direction perpendicular thereto.
If the information carrier
11
moves in a direction in which the distance to the converging lens
13
becomes smaller, the cross-section of the light beam at point
32
becomes broader in the x direction. If, however, the information carrier
11
is moved away from the converging lens
13
, the cross-section of the reflected light beam
15
at point
32
becomes broader in the y direction. This means that the so-called astigmatism and the concomitant change in the cross-section of the reflected light beam
15
can be used as a focussing error signal, in order to control or regulate the distance from the converging lens
13
to the information carrier
11
so that the pits in the information-carrying face of the information carrier
11
always lie within the focus of the scanning light beam. The change in the cross-section of the reflected light beam at point
32
can be scanned using a light-sensitive detector
18
, in particular a photodiode.
DE 40 02 015 C2 discloses a further scanning system, in which the focussing status of the light beam incident on the information carrier can be recognized. In the scanning system presented there, a prism with parallelogram-shaped configuration is used instead of the cylindrical lens.
The use of a prism is proposed in the arrangement described there in order to achieve an optical system with reduced dimensions. On the one hand, in the optical system proposed there, the beam splitter can be omitted, since the prism itself constitutes one. On the other hand, the overall arrangement is made shorter by the reflected light beam being reflected to and fro repeatedly in the prism itself.
However, in order to ensure the beam-splitting function as well as the multiple reflection of the reflected light beam, elaborate alignment of the prism is necessary.
A likewise elaborate alignment is necessary when using a cylindrical lens. Furthermore, a cylindrical lens has the disadvantage that it requires a relatively high degree of outlay on production. In order to achieve a sufficiently large focal spot, the cylindrical lens is furthermore combined with a negative lens, i.e. a diverging lens.
The object of the invention is to provide a method for recognizing a focussing status by utilizing astigmatism, in which significant astigmatism can be achieved in the reflected light beam simply and cost-efficiently without elaborate alignment. A further intention is to provide a corresponding recording or replay device.
SUMMARY OF THE INVENTION
This object is achieved in terms of the method and device described below.
One key idea of the present invention consists in that the reflected light beam is split by means of a birefringent element into an ordinary and an extraordinary fractional beam, and in that the focussing status of the scanning light beam is inferred from the cross-sectional shape of the extraordinary fractional beam. For use in the birefringence of a crystal, according to the invention significant astigmatism, or a status similar to astigmatism, can be achieved so that an accurate measure of the focussing status of the scanning light beam on the information carrier can be derived therefrom.
The birefringence effect will be explained in brief below. For further details, reference may be made, for example, to Bergmann/Schäfer, “Lehrbuch der Experimentalphysik” [Textbook of Experimental Physics], Vol. 3 “Optik” [Optics], 8th edition, pages 524 to 566. A converging light beam will be considered, which is passed through a birefringent, plane-parallel plate. To simplify the explanation, it will be assumed that the optical axis of the plane-parallel plate is parallel to its incidence and emergence faces. It will furthermore be assumed that the ordinary refractive index no is greater than the extraordinary refractive index n
e
, that is to say that there is optically negative behaviour. The reverse case which is likewise possible will not be considered further here. For the ordinary part of the light beam, the plate is isotropic, and the usual laws of refraction are valid. For the extraordinary part, only two incidence planes for the incident light beam will be considered below, namely an incidence plane parallel to the optical axis and an incidence plane perpendicular to the optical axis. The marginal rays in the perpendicular incidence plane experience in the n
e
mode, that is to say in the extraordinary part, a smaller refractive index, given by the extraordinary refractive index and the angle with respect to the optical axis of the plate, than that in the parallel incidence plane. Consequently, the marginal rays in the perpendicular incidence plane, when considering the n
e
mode, are refracted less than the marginal rays in the parallel incidence plane. When these marginal rays of the n
e
mode emerge from the plate, they form a focal point which is further away than the focal poi
Deutsche Thomson-Brandt GmbH
Kiel Paul P.
Tran Thang V.
Tripoli Joseph S.
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