Graded-index optical fibres with germanium-boron and with germanium-phosphorus doped silica core have been fabricated. The dependence of the infrared absorption edge on the boron concentration was investigated. For the germanium-phosphorus doped fibre a broad transmission window with losses below 1.5 dB/km between 1500 and 1720 nm was found.
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Zwick et al. (1979) studied this question.