Dynamic information storage or retrieval – Specific detail of information handling portion of system – Radiation beam modification of or by storage medium
Patent
1994-09-12
1996-03-26
Ben, Loha
Dynamic information storage or retrieval
Specific detail of information handling portion of system
Radiation beam modification of or by storage medium
369120, 369 13, 369 4441, 360114, G11B 700
Patent
active
055027083
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to an optical scanning device which is suitable for reading both an optical and a magneto-optic recording medium as well as a combined magneto-optic recording medium upon which items of data are stored on top of one another in both a magnetic layer and by means of so-called pits, wherein the light beam from a light source is focused on the recording medium and is reflected from the recording medium through a first .lambda./2 plate to a first polarization beam splitter which guides the light beam onto a first or second photodetector, wherein the output of the first and of the second photodetector are connected to the inputs of a first differential amplifier.
BACKGROUND OF THE INVENTION
A known optical recording medium is, for example, the CD disc in which a light reflecting aluminum layer follows on the transparent layer. The light reflecting aluminum layer has depressions, so-called pits, which represent the items of data stored on the CD disc. The items of data are readable from the CD disc by means of an optical scanning device because the reflective behaviour of the light reflecting aluminum layer depends on the pattern which the depressions form on the disc. Less light is reflected from a depression, frequently also called a groove, than from a raised area which is often also referred to as a land.
From the intensity of the light reflected from the CD disc, the optical scanning device therefore recognizes whether the scanned bit relates for example, to a logical one or a logical zero.
A further optical recording medium of this type, known under the designation of a magneto-optic disc, is described in the article "Magnetooptische Versuche dauern an" in Funkschau 13, 20th Jun. 1986 at pages 37-41.
In contrast to a conventional CD disc, a magneto-optic disc does not have any pits. A magnetic layer, in which items of data are recordable and from which items of data are readable, is located behind the transparent layer. It will now be explained how items of data are written onto a magneto-optic disc.
The magnetic layer is heated above the Curie temperature by means of a laser beam focused onto the disc. Usually however, it is only necessary to heat up the magnetic layer to the compensation temperature which lies somewhat under the Curie temperature. An electromagnet, which magnetizes the region heated by the laser beam in the one or the other direction of magnetization, is arranged behind the focal point on the disc. Because, after switching off the laser beam, the heated spot cools once more below the Curie temperature, the direction of magnetization determined by the electromagnet is maintained: it is, so to speak, frozen in. The individual bits are stored in this manner in domains of different directions of magnetization. Thereby, the one direction of magnetization of a domain corresponds, for example, to a logical one, while the opposite direction of magnetization represents a logical zero.
One makes use of the Kerr effect for reading the items of data. The plane of polarization of a linearly polarized light beam is rotated by the reflection at a magnetized mirror by a measurable angle. In dependence upon the direction in which the mirror is magnetized, the plane of polarization of the reflected light beam is rotated to the right or to the left. However, because the individual domains on the disc act like magnetized mirrors, the plane of polarization of a scanning light beam is rotated by a measurable angle to the left or to the right in dependence upon the direction of magnetization of the currently scanned domain.
The optical scanning device recognizes which bit is present, a logical one or a logical zero, from the rotation of the plane of polarization of the light beam reflected from the disc. In contrast to a CD disc having pits, a magneto-optic disc is erasable and re-writable virtually as often as desired.
A disc shaped recording medium which represents a combination of an optical and a magneto-optic disc is known from DE-OS 37 32 875. Items of data are stored on t
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Ben Loha
Deutsche Thomson-Brandt GmbH
Hayes James B.
Herrmann Eric P.
Tripoli Joseph S.
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