Method for manufacturing a hermetically coated optical fiber

A method of manufacturing a hermetically coated optical fiber comprising the steps of:

• passing a bare optical fiber through a single reaction chamber;
• introducing a first raw material gas into an upstream portion of the reaction chamber through a first inlet tube;
• introducing a second raw material gas into a downstream portion of the reaction chamber through a second inlet tube;
• and
• controlling at least one of flow rate, composition, concentration and temperature of the first and second raw material gasses before they are introduced into the reaction chamber, wherein the first raw material gas is different from the second raw material gas in at least one of flow rate, composition, concentration and temperature before the raw material gasses are introduced into the reaction chamber, whereby at least one of the flow rate, the composition, the concentration and the temperature of the raw material gas in the reaction chamber varies continuously from the upstream portion to the downstream portion.