Dynamic information storage or retrieval – Specific detail of information handling portion of system – Radiation beam modification of or by storage medium
Patent
1994-05-19
1995-10-17
Epps, Georgia Y.
Dynamic information storage or retrieval
Specific detail of information handling portion of system
Radiation beam modification of or by storage medium
369 4423, 369 4441, 369 13, 369112, G11B 700
Patent
active
054597072
DESCRIPTION:
BRIEF SUMMARY
The 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 magneto-optic recording medium upon which items of data are stored above one another both in 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 polarization beam splitter onto a first photodetector and, after being deflected by the polarization beam splitter, onto a second photodetector.
A known optical recording medium is, for example, the CD disc in which a light reflecting aluminium layer follows on the transparent layer. The light reflecting aluminium 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 aluminium 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 recognises 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, 20, 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 electro-magnet, which magnetises the region heated by the laser beam in the one or the other direction of magnetisation, 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 magnetisation determined by the electro-magnet is maintained: it is, so to speak, frozen in. The individual bits are stored in this manner in domains of different directions of magnetisation. Thereby, the one direction of magnetisation of a domain corresponds, for example, to a logical one, while the opposite direction of magnetisation 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 magnetised mirror by a measurable angle. In dependence upon the direction in which the mirror is magnetised, 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 magnetised 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 magnetisation of the currently scanned domain.
The optical scanning device recognises 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 this recording medium by means of pits and also in the magnetic layer of the disc. Because the pits and the magnetic domains lie above one another, items of d
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Deutsche Thomson-Brandt GmbH
Epps Georgia Y.
Herrmann Eric P.
Hindi Nabil
Tripoli Joseph S.
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