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By using a thick-walled SiO(2) tube (25-mm outside diameter, 1-mm inside diameter), a soft-glass core containing Nd or Yb-Er has its alkali (and to a lesser extent its alkaline earth) components volatilized during drawing to give a good-quality laser fiber with Nd- or Yb-Er-doped cores.
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Snitzer et al. (1989) studied this question.
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