Solution doped preform with improved uniformity and concentration using dual-layer soot deposition

•Solution doped optical fibre preform couple with MCVD is fabricated.•A conventional single-layer is compared with dual-layer soot deposition.•The dual layer soot improves the dopant uptake.•The dual layer soot improves the dopant uniformity.

A new method of soot deposition to improve the characteristics of a solution-doped optical fibre preform is reported. The soot was generated using a modified chemical vapour deposition (MCVD) technique. A better longitudinal uniformity of the core refractive index profile and a higher degree of dopant incorporation were obtained when two layers of soot were deposited in the core. These improvements were further extended by depositing each layer at different temperatures. A variation of 0.15 × 10−2 (%RSD 3.5%) in the refractive index difference along 23 cm of the preform and a core-to-cladding refractive index difference of approximately 0.012 were achieved using a 1.2 M AlCl3 solution.