Browse Prior Art Database

Producing Linear Focus Error Signals in Optical Storage

IP.com Disclosure Number: IPCOM000107773D
Original Publication Date: 1992-Mar-01
Included in the Prior Art Database: 2005-Mar-22
Document File: 2 page(s) / 48K

Publishing Venue

IBM

Related People

Rabedeau, M: AUTHOR

Abstract

Disclosed is a means by which very nearly linear focus error signals can be obtained from optical storage systems that employ a focussed spot-size measuring method for generating the focus servo signal.

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Producing Linear Focus Error Signals in Optical Storage

       Disclosed is a means by which very nearly linear focus
error signals can be obtained from optical storage systems that
employ a focussed spot-size measuring method for generating the focus
servo signal.

      In optical storage systems that employ the focussed spot-size
measuring method for producing the focus servo signal, the read/write
laser beam that is reflected from the recording medium is projected,
slightly out of focus, on the center of a light detector having three
individual elements similar to those shown in Fig. 1.  According to
current practice, a focus error signal (FES) used in maintaining the
laser beam focussed on the recording medium is generated by
electronically combining the signals from the individual elements of
the light detector as indicated below:
      FES1 = (B-(A+C))/(A+B+C),
where A, B and C are the signals from the correspondingly lettered
detector elements.  A much more nearly linear focus error signal can
be obtained if the signals from the individual detector elements are
combined as indicated below:
      FES2 = (B(A+C))/B

      Fig. 2 is a plot of typical focus error signals as a function
of defocus of the recording medium for both algorithms.  The solid
line shows the FES obtained by the conventional algorithm, and the
dotted line shows the FES obtained by the algorithm disclosed here.