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Browse Prior Art Database

Spherical Aberration Compensation in a Remote Focus Actuator

IP.com Disclosure Number: IPCOM000110895D
Original Publication Date: 1994-Jan-01
Included in the Prior Art Database: 2005-Mar-26
Document File: 2 page(s) / 63K

Publishing Venue

IBM

Related People

Stahl, R: AUTHOR

Abstract

Seek performance in an optical disk drive is limited by the moving mass of the coarse actuator. In a typical drive, the moving mass includes a carriage with voice coil motor, a focus actuator with voice coil motor, a tracking actuator with voice coil motor, a beam bender, and an objective lens. A proposal for reducing the mass of the actuator removes the focus function off of the moving actuator and places it back at the fixed portion of the optical head. Focus at the media is controlled by placing a pair of lenses in the fixed part of the head such that the collimated beam is brought to a real focus using one positive lens, and then converted back to a collimated beam by a second positive lens. To change focus out of the objective lens, the separation of the focus pair is changed (Fig. 1).

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Spherical Aberration Compensation in a Remote Focus Actuator

      Seek performance in an optical disk drive is limited by the
moving mass of the coarse actuator.  In a typical drive, the moving
mass includes a carriage with voice coil motor, a focus actuator with
voice coil motor, a tracking actuator with voice coil motor, a beam
bender, and an objective lens.  A proposal for reducing the mass of
the actuator removes the focus function off of the moving actuator
and places it back at the fixed portion of the optical head.  Focus
at the media is controlled by placing a pair of lenses in the fixed
part of the head such that the collimated beam is brought to a real
focus using one positive lens, and then converted back to a
collimated beam by a second positive lens.  To change focus out of
the objective lens, the separation of the focus pair is changed (Fig.
1).

      Depth of focus is limited by the level of aberrations
introduced by the lenses.  As the focus pair is moved away from the
nominal position, the level of aberrations increases.  The largest
contributor is spherical aberration.

      In a typical lens, spherical aberrations increase in value as
you move either direction away from nominal focus (converging or
diverging wavefront).  This would mean that both the second lens of
the focus lens pair, and the objective contribute positive spherical
aberration whether the focus pair is adjusting for a disk-too-close
or disk-too-far from the objective...