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Interferometric Testing of Substrate Vertical Birefringence

IP.com Disclosure Number: IPCOM000109460D
Original Publication Date: 1992-Aug-01
Included in the Prior Art Database: 2005-Mar-24
Document File: 1 page(s) / 57K

Publishing Venue

IBM

Related People

Bates, K: AUTHOR [+3]

Abstract

The performance of an optical storage device is strongly dependent upon the characteristics of the optical media. Vertical birefringence is an important performance limiting property of the optical substrate. Vertical birefringence is defined as the difference between the index of refraction normal to the substrate and the index of refraction in the plane of the substrate. When linearly polarized light converging from a typically high numerical aperture (NA) lens used in optical storage traverses the optical substrate, an astigmatism is generated. This astigmatism degrades the quality of the focussed optical spot, and thus corrupts drive performance. Vertical birefringence, however, is difficult to measure.

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Interferometric Testing of Substrate Vertical Birefringence

      The performance of an optical storage device is strongly
dependent upon the characteristics of the optical media.  Vertical
birefringence is an important performance limiting property of the
optical substrate.  Vertical birefringence is defined as the
difference between the index of refraction normal to the substrate
and the index of refraction in the plane of the substrate.  When
linearly polarized light converging from a typically high numerical
aperture (NA) lens used in optical storage traverses the optical
substrate, an astigmatism is generated.  This astigmatism degrades
the quality of the focussed optical spot, and thus corrupts drive
performance.  Vertical birefringence, however, is difficult to
measure.

      Disclosed is a method for quantifying the vertical
birefringence present is the substrate of an optical media.  The
method is illustrated in Figure 1.  The output test beam of a
commercially available Twyman-Green or Fizeau interferometer passes
through an objective lens and the substrate under test, and impinges
upon a reference sphere.  This reference sphere causes the beam to
retrace its path back into the interferometer.  The objective lens
used in this procedure should be identical to that used in the
optical drive in  order to match the cone angle of the beam incident
upon the substrate.  The interferometer will measure the magnitude of
the astigmatism.  Care should be...