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Laser Over Power Check and Write Calibration Verification

IP.com Disclosure Number: IPCOM000107341D
Original Publication Date: 1992-Feb-01
Included in the Prior Art Database: 2005-Mar-21
Document File: 3 page(s) / 116K

Publishing Venue

IBM

Related People

Call, D: AUTHOR [+4]

Abstract

A single auxiliary photo-detector is used for read laser power control in most optical drives. Over-power is detected when the incident laser power on this detector is greater than a pre-set threshold value. However, this over-power detection depends on the same detector used for laser power control. If a fault exists, for example, in the auxiliary detector with the efficiency slowly decreasing, the fault will not be detected with the over-power detection. With the auxiliary detector efficiency slowly decreasing, the read power level could increase to a level resulting in degradation of customer data. A method is described in this article that will detect faults in the laser power control that can result in degrading of customer data. Also, write power calibration is verified.

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Laser Over Power Check and Write Calibration Verification

       A single auxiliary photo-detector is used for read laser
power control in most optical drives.  Over-power is detected when
the incident laser power on this detector is greater than a pre-set
threshold value.  However, this over-power detection depends on the
same detector used for laser power control.  If a fault exists, for
example, in the auxiliary detector with the efficiency slowly
decreasing, the fault will not be detected with the over-power
detection.  With the auxiliary detector efficiency slowly decreasing,
the read power level could increase to a level resulting in
degradation of customer data.  A method is described in this article
that will detect faults in the laser power control that can result in
degrading of customer data.  Also, write power calibration is
verified.

      Illustrated in the figure is a method for checking faults in
the laser power control. The check is performed after the completion
of write power calibration while the head is still positioned over
the manufacture tracks that are used for write power calibration.
Step 1 is the beginning of the check which is done at the completion
of write power calibration while the head is still over the
manufacture tracks.  In step 2, the laser DAC is set to the erase DAC
value determined from the write power calibration. In step 3, the
track on which write calibration was performed is erased.  In step 4,
the laser DAC is set to the write DAC value found during the write
power calibration. In step 5, several sectors on the track erased in
step 3 are written with the data read from a control track.  The
control track was read before write calibration.  The sectors could
also be written with any other data that could provide a stress.  In
step 6, the write bias magnet (BM) is left on for one or more
revolutions with the laser power at the full read level and passing
over the written data.  If the read power is too high, the stress of
leaving the bias magnet on, while at this power level, will result in
degradation of the data written.  In step 7, the first sector written
is read in a write verify mode with the ECC off.  The write verify is
a stressed read condition with the PLL window shifted in order to
insure some safety margin in the written data.  Any other stress
condition could also be used.  Thus, if the data read is marginal,
the shift in PLL window will result...