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Writing Algorithm for the Reduction of Sensitivity Shifts with Time in Optical Recording

IP.com Disclosure Number: IPCOM000113122D
Original Publication Date: 1994-Jul-01
Included in the Prior Art Database: 2005-Mar-27
Document File: 2 page(s) / 169K

Publishing Venue

IBM

Related People

Cheng, D: AUTHOR [+3]

Abstract

At the present time, the material set of choice for reversible magneto-optic media are amorphous rare-earth transition metal alloys mainly because of the ease with which their magnetic properties can be varied by changing composition, the ease of manufacture, the low noise associated with the absence of grain boundries in the amorphous phase, and the high figure of merit in readout via the polar Kerr effect. However, a primary concern in the use of these materials is the observation of changes in the written mark size with erase cycling, usually referred to as "Sensitivity Shifts with Time" or SST.

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Writing Algorithm for the Reduction of Sensitivity Shifts with Time
in Optical Recording

      At the present time, the material set of choice for reversible
magneto-optic media are amorphous rare-earth transition metal alloys
mainly because of the ease with which their magnetic properties can
be varied by changing composition, the ease of manufacture, the low
noise associated with the absence of grain boundries in the amorphous
phase, and the high figure of merit in readout via the polar Kerr
effect.  However, a primary concern in the use of these materials is
the observation of changes in the written mark size with erase
cycling, usually referred to as "Sensitivity Shifts with Time" or
SST.  This becomes especially critical when Pulse-Width-Modulation or
PWM encoding is applied to optical disk drives because information is
stored in the size of the written marks instead of their position, as
in pulse-position-modulation or PPM encoding.  SST has been shown
experimentally to be the direct result of structural relaxation of
the amorphous phase driven by the thermal heating required when
writing and erasing the media.

      Several possible methods to reduce or eliminate the effects of
SST exist but all have some drawbacks.  First, alternative material
sets which are crystalline in nature, including Co/Pt multilayers and
garnet faraday materials, exhibit greatly reduced thermal relaxation
phenomena.  However, these materials are at an early stage of
development and lack in one or more of the areas described above when
applied to optical recording.  Second, bulk thermal annealing of the
amorphous phase will eliminate SST effects, but requires temperatures
in excess of those suitable for the polymer substrate materials
comprising the disk substrate.  Finally, it is possible to compensate
to some degree for the effects of SST in the drive readback channel
by building electronics which will "sense" the increase (decrease) in
the mark size (i.e., correlated motion of the leading and trailing
edges of the mark) and correct for SST.  However, such channels can
only compensate for about half of the SST effect.

      We propose an alternative solution to the SST problem described
above based on the idea that SST can be minimized by ensuring that
the process used to "write" the magnetic mark on the optical disk
maximizes the thermal gradient in the media, thereby making the mark
formation process insensitive to the relaxation phenomena; i.e., the
thermal...