A few ppm of sodium and potassium contained in natural silica tube has been found to be removed during the deposition process in MCVD fiber, while remaining such alkalies in over-jacketed natural silica tube was pointed out to migrate into the germania doped core part during drawing process and to deteriorate resistivity against hydrogen.

Sodium and potassium contained in natural silica tubes have been found to migrate into the central part of germania doped silica fiber during drawing process after secondary ion mass spectrometry (SIMS) measurements. Migrated such impurities caused the structural change of germania doped silica glass, which raised hydrogen induced loss increases in infrared wavelength region.