Causes of focus-error feedthrough in optical-disk systems: Astigmatic and obscuration methods

Bruce E. Bernacki, M. Mansuripur

Research output: Contribution to journalArticlepeer-review

4 Scopus citations


High-density magneto-optical recording systems require sensitive and robust focus position sensors that are immune to transient changes in the amplitude and the phase of the light diffracted from pregrooved media during the seek operation. The false focus-error signal produced by track crossing during seeking is termed feedthrough. Total immunity to feedthrough is never achieved, although some focus-error detection methods, notably the obscuration method, perform better in this regard. The astigmatic focus-error detection method is usually operated with a large astigmatic foci separation distance to facilitate detector alignment and to permit push-pull tracking, which increases pattern noise and contributes to its poor resistance to feedthrough. Pattern noise is caused by the projection of the intensity pattern at the exit pupil of the objective lens onto the detector plane, at which it produces false focus-error signals. The obscuration method, a diffraction-limited method of focus-error sensing, evens out this pattern noise and is therefore more resistant to feedthrough. We present numerical modeling results that compare the feedthrough performance of the astigmatic and the obscuration methods of focus-error detection.

Original languageEnglish (US)
Pages (from-to)735-743
Number of pages9
JournalApplied optics
Issue number5
StatePublished - Feb 10 1994


  • Feedthrough
  • Pattern noise
  • Servo cross talk

ASJC Scopus subject areas

  • Atomic and Molecular Physics, and Optics
  • Engineering (miscellaneous)
  • Electrical and Electronic Engineering


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