This paper presents the new technique for preparing the fiber preforms with large core and large N A by the conventional MCVD process. Under the condition of doping no Boron

Using the technique by decreasing the flow of SiCl

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step by step in the core of the preforms

the problem of splitting caused by the different thermal expansion coefficient of the materials during the deposition and the depression as germanium is excessively doped in the core of the preforms for high refractive index defference An has been solved. By changing the moving velocity of the torch

the rate of deposition has been increased and the time for preparing the preforms has been shortened. The NA of the fibers to be drawn can be up to 0.30.