Dear Bimal,

Optical fibre preform deposition includes many ways of fabricating porous preforms (MCVD, OVD, ...), mostly for silica fibres. The point of this step is to fabricate a layer or a cylinder of porous silica. Precursors (e.g. SiCl4) are oxidized in a oxygen atmosphere at high temperature to give SiO2 soot. These soot agglomerate. This is the fibre deposition step.

In order to turn these soot into a fibre, a sintering step is necessary. This step consist in heating the soot under controlled atmosphere to reduce the surface of the porous material. It results in forming a glass layer or cylinder (depending on the technique you use), also called fibre preform.

I hope this explanation helps...