Effect of Processing Parameters on Boron Doped Silica Preform Rods Fabricated Using Modified Chemical Vapor Deposition (MCVD) Process Through Study of its Coefficient of Thermal Expansion (CTE) and Refractive Index (RI) Changes

Abstract

Various processing techniques are used to synthesize optical fibers, modified chemical vapor deposition (MCVD) being one of them. MCVD technique provides great flexibility in design, composition and dimensional control. In this process, a mixture of halide vapors and oxygen are passed through a rotating fused silica tube, which is heated by axially traversing oxy-hydrogen flame. At elevated temperature, oxidation of halides results in formation of oxide particles, which either deposit inside tube or exit as soot. After collapse process, silica deposited tube transforms into a solid transparent rod, referred as preform. Two types of preforms produced separately are: (i) Ge-doped core, surrounded by silica, and (ii) B-doped preform rod as stress applying part. In order to produce high birefringence fibers of required optical properties in sensor applications, two holes of desired dimensions are drilled across the core on either side of Ge-doped preform diametrically. The boron-doped preform rods are then inserted into these holes to eventually draw fibers containing post-drawn residual stresses. In this work, composition of boron-doped preform rods fabricated under different synthesis conditions were determined by EDS analysis. By considering Si in EDS analysis to be more accurate, B O –SiO system compositions were theoretically 2 3 2 calculated in mole percent for fully oxidized, stiochiometric B O –SiO system. It was 2 3 2 found from oxygen flow rate (F ) and theoretical calculations that oxygen quantity was O2 much less, just adequate, or in excess to that needed for complete conversion of BCl and 3 SiCl to their respective oxides. For low F values, the incorporated amount of B O was 4 O2 2 3 high, whereas high F resulted in low B O co-deposition into the system. For the case O2 2 3 of adequate F , deposited B O (mol%) was almost equal to that calculated theoretically. O2 2 3 The values of coefficient of thermal expansion (CTE) for each boron-doped preform rod was determined and compared with their respective B2O3 content (mol%). There was an overall increase in CTE with increasing amount of B O in the system, and vice versa. The law of additivity of constituents predicted CTE values for a range of B2O3– SiO compositions that matched with the linear experimental trend. Similar behavior was 2 seen in case of refractive index (RI) data obtained for these preform rods. For a couple of samples, the estimated system compositions were found to be slightly off, indicating inaccuracies in the EDS analysis. An attempt to correlate CTE and RI data for various B O –SiO system compositions was not successful due to insufficient available data. 2 3 2 This work suggests that for ~ 6–26 mol percent B O in the preform rod, any change in 2 3 the physical properties such as CTE and RI can be approximated as following a linear behavior. Also, careful use of EDS analysis together with theoretical calculations can transform the overall system composition from elemental to compound form.