Composition-Property Correlation in B2O3-SiO2 Preform Rods Produced Using Modified Chemical Vapor Deposition Technique

Abstract

Due to unique optical properties of high birefringent (Hi-Bi) fibers for sensing and coherent optical communications, there is a strong interest in process optimization at preform fabrication and fiber drawing stages. Boron-doped silica cladding acts as stress-applying part resulting in polarization properties of Hi-Bi fibers that are strongly dependent on chemical composition. Using modified chemical vapor deposition (MCVD) technique, B 2 O 3 -doped silica preform rods were synthesized under different precursor gas flow conditions. Qualitative information about B 2 O 3 -SiO 2 system composition was derived from etching behavior in nonbuffered HF solution and subsequent microstructural examination using scanning electron microscope. Significant degree of B 2 O 3 incorporation was seen in case of high BCl 3 :SiCl 4 ratio and mild oxygen-deficient processing conditions. Increasing the B 2 O 3 content to $26 mol% led to a corresponding increase in coefficient of thermal expansion (CTE) to a maximum value of 2.35 ppm/K. The value of refractive index (RI), on the other hand, was found to decrease with increased B 2 O 3 incorporation. A qualitative correlation between B 2 O 3 and SiO 2 system composition and physical properties such as CTE and RI was established.