Browse Prior Art Database

Method and Apparatus for Reading Optical Disks

IP.com Disclosure Number: IPCOM000108355D
Original Publication Date: 1992-May-01
Included in the Prior Art Database: 2005-Mar-22
Document File: 3 page(s) / 109K

Publishing Venue

IBM

Related People

Owa, S: AUTHOR

Abstract

Disclosed is a method for reading optical ROMs (such as compact discs (CDs), CD-ROMs, and the header parts of magneto-optical disks) by using a shift in the polarization angle caused by the dichroism of pits on the disks. The polarization angle of incident light is set at about 45 or 135 degrees from the direction of the track (the long side of pits). The polarization angle of the reflected light is shifted from that of the incident light. This method makes it possible to read both optical ROMs and magneto-optical disks by the same optical and electronic means.

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Method and Apparatus for Reading Optical Disks

       Disclosed is a method for reading optical ROMs (such as
compact discs (CDs), CD-ROMs, and the header parts of magneto-optical
disks) by using a shift in the polarization angle caused by the
dichroism of pits on the disks.  The polarization angle of incident
light is set at about 45 or 135 degrees from the direction of the
track (the long side of pits).  The polarization angle of the
reflected light is shifted from that of the incident light.  This
method makes it possible to read both optical ROMs and
magneto-optical disks by the same optical and electronic means.

      Optical ROM contains information recorded in the form of a
stream of embossed pits, which are depressions on the reflecting
layer of optical disks.  Usually, the pits are detected by changes in
reflectivity, that is, changes in the intensity of reflected light.

      On the other hand, magneto-optical disks record information by
the magnetization (upwards or downwards) of the reflecting layer,
whose direction is read by the shift of the polarization angle.

      A problem occurs because magneto-optical disks also have a ROM
part (a pit part) as a header of each sector.

      The conventional apparatus (pick-up) for reading
magneto-optical disks has two operating modes: (1) detecting
reflectivity to read the ROM part, and (2) detecting polarization
shifts to read the magneto- optical part.

      This invention utilizes a property of pits other than change in
reflectivity.  Pits have dichroism, which means different
reflectivities for two polarizations of incident light; that is, the
reflectivity Rx for a polarized light beam parallel to the track (the
x-direction) is different from the reflectivity Ry for a light beam
whose polarization is perpendicular to the track (the y-direction).

      Fig. 1 shows the pits and a laser spot for reading, whose
incident polarization angle t1 is defined as the angle to the
direction of the track.  The polarization angle t2 of reflected light
is given by the following relation:
  tan(t2) = (ry/rx)tan(t1) ...