Abstract: Methods and devices for the production of glass fiber preforms are disclosed. The methods and devices are based on a modified chemical vapour deposition process wherein the reacting gases are reacted in a hot zone within a substrate tube. Glass layers are deposited by a first heating instrument moving with respect to the substrate tube. The reaction gases are subjected to a temperature treatment before entering the hot zone of the main burner to encourage an evenly distributed chemical reaction, deposition and doping concentration over the length of the substrate tube. This is based on the position of the substrate tube with respect to the heating instrument and the resulting different heating of the reaction gases.