as to an optical fiber application_number US 84663704 A description using the pcvd technique a preform was produced from which a graded index type optical fibre having a refractive index contrast of about 0 02 was drawn glass layers were deposited on the inside of the substrate tube by having reactive glass forming gases flow into the substrate tube on one side and subsequently cause said gases to react using a plasma generated inside the substrate tube a plasma generator or resonator surrounding the substrate tube was used for coupling the required high frequency energy into the plasma said plasma generator was reciprocated at a constant velocity along the substrate tube with the velocity linearly decreasing to 0 from the deceleration point x to the reversal points y this velocity profile which was maintained at a constant value during the deposition process is shown in fig1 after the deposition process the substrate tube was contracted into a solid preform in a usual manner the refractive index profile was measured at various positions along the preform thus formed using a so called index profiler type photon kinetics model 2600 the changes in the core diameter the maximum u03b4 value and the profile parameter u03b1 are schematically shown in.