A method for carrying out a plasma deposition process, said method comprising the steps of: i) providing a hollow substrate tube; ii) supplying a supply flow of dopant-containing glass-forming gases to the substrate tube of step i), wherein the supply flow comprises a main gas flow and one or more secondary gas flows, said main gas flow mainly comprising the glass-forming gases and said one or more secondary gas flows mainly comprising precursors for dopant(s); iii) inducing a plasma by means of electromagnetic radiation in at least a part of the substrate tube of step ii) to create a reaction zone in which deposition of one or more glass layers onto the interior surface of the substrate tube takes place; iv) moving the reaction zone back and forth in longitudinal direction over the substrate tube between a reversal point located near the supply side and a reversal point located near the discharge side of said substrate tube; wherein each forth and each back movement is called a stroke; wherein the flow of at least one secondary gas flow is interrupted one or multiple times during step iii); each of said interruption having a start point and an end point as a function of the axial position of the plasma along the length of the substrate tube; wherein said start point and said end point of each of said interruption both lie within the same stroke; wherein the amount of gas of the at least one secondary gas flow is tuned with a mass flow controller (MFC); wherein a controllable valve is present in the flow path of the at least one secondary gas flow, with the controllable valve blocking or passing the respective gas flow, said valve being located between the MFC and the substrate tube, and being used to interrupt the at least one secondary gas flow; wherein there is no decrease in the amount of gas of the at least one secondary gas flow when there is an interruption; and wherein the interruption during a stroke has a duration of maximally 10% of the total time of said stroke. i) providing a hollow substrate tube; ii) supplying a supply flow of dopant-containing glass-forming gases to the substrate tube of step i), wherein the supply flow comprises a main gas flow and one or more secondary gas flows, said main gas flow mainly comprising the glass-forming gases and said one or more secondary gas flows mainly comprising precursors for dopant(s); iii) inducing a plasma by means of electromagnetic radiation in at least a part of the substrate tube of step ii) to create a reaction zone in which deposition of one or more glass layers onto the interior surface of the substrate tube takes place; iv) moving the reaction zone back and forth in longitudinal direction over the substrate tube between a reversal point located near the supply side and a reversal point located near the discharge side of said substrate tube; wherein each forth and each back movement is called a stroke; wherein said start point and said end point of each of said interruption both lie within the same stroke; wherein the amount of gas of the at least one secondary gas flow is tuned with a mass flow controller (MFC); wherein a controllable valve is present in the flow path of the at least one secondary gas flow, with the controllable valve blocking or passing the respective gas flow, said valve being located between the MFC and the substrate tube, and being used to interrupt the at least one secondary gas flow; wherein there is no decrease in the amount of gas of the at least one secondary gas flow when there is an interruption; and wherein the interruption during a stroke has a duration of maximally 10% of the total time of said stroke. <EOS>