Abstract: A semiproduct for use in the manufacture of light conducting fibers comprising a core of synthetic quartz glass having a thickness of 6 to 400 mm, the quartz glass containing less than 10 ppm hydroxyl ions and having, in the near infrared spectral range, an optical loss totaling less than 4 dB/km, measured in the mass, the core being fused with a jacket having a wall thickness of 2 to 20 mm and consisting essentially of synthetic quartz glass containing more than 4,000 ppm of fluorine, the jacket having a length of at least 200 mm; a method of producing the same and light conducting fibers derived therefrom.