# Fabrication of Alumina-Doped Optical Fiber Preforms by an MCVD-Metal Chelate Doping Method
## 2 Materials and Methods  
Our chelate delivery system is depicted in Figure 1. The Al(acac)\({}_{3}\) is placed in a sublimator (maximum operating temperature of 220 \({}^{\circ}\)C), and the vapor is carried to the reaction zone by a constant flow of high-purity helium gas using heated stainless steel tubes. A ceramic heater is placed at the end of the delivery tube and just before the MCVD main oxyhydrogen burner in order to prevent any condensation of Al(acac)\({}_{3}\) vapor on the walls of the glass substrate tube. The fabrication process is divided into two major consecutive steps; first is the MCVD process, and second is the Al(acac)\({}_{3}\) oxidation process. These steps are then followed by a standard MCVD procedure including sintering and collapsing of the glass substrate tube to a solid preform. At the MCVD step, a Heraeus F300 synthetic silica substrate tube (25 mm (Outer Diameter), 19 mm (Inner Diameter)) is first rinsed with isopropanol/acetone to remove any organic contaminants. The tube is then mounted onto a glass working lathe and is etched with SF\({}_{6}\) for several passes of the oxyhydrogen burner at a very high temperature. Several layers of high-purity SiO\({}_{2}\) are then deposited and sintered to act as a barrier between the glass substrate tube and the preform's core. Finally, for the preform's core, two layers of unsintered SiO\({}_{2}\) are deposited for doping with alumina. For the Al(acac)\({}_{3}\) oxidation step, Al(acac)\({}_{3}\) vapor is delivered to the glass substrate tube where it is converted to Al\({}_{2}\)O\({}_{3}\) at high temperature and in the presence of high-purity oxygen gas. The deposition of the formed Al\({}_{2}\)O\({}_{3}\) particles occurs on the surface of the deposited unsintered silica layers. Table 1 lists the process parameters used in this work. For Preform 1 (P1), Al(acac)\({}_{3}\) was sublimed at T\({}_{\rm{Al}}\)\({}^{\circ}\)C of 175 \({}^{\circ}\)C, with carrier gas (He) and O\({}_{2}\) flow rates of 1440 and 2400 sccm, respectively. The deposition temperature (T\({}_{\rm{dep}}\)\({}^{\circ}\)C) was fixed at 1850 \({}^{\circ}\)C with a carriage speed (V\({}_{\rm{dep}}\)) of 100 mm/min and spindle rotation of 50 rpm (rotation per minute). A total of 10 layers were deposited in the downstream direction. For Preforms 2 (P2) and 3 (P3), T\({}_{\rm{Al}}\)was fixed at 185 \({}^{\circ}\)C, with P2 having a total of 8 layers deposited in the downstream direction and P3 having 7 layers deposited in each direction (i.e., downstream and upstream). Figure 2 illustrates the downstream and upstream deposition directions. The refractive index profiles (RIPs) and longitudinal uniformity of the fabricated preforms were determined using a preform analyzer (_Photon Kinetics_, P104). The longitudinal uniformity of the fabricated preforms was determined by scanning the preform at 2.0 cm intervals. For each preform, \begin{table}
\begin{tabular}{c c c c c c c c} \hline \multirow{2}{*}{**Preform**} & \multirow{2}{*}{\(\mathbf{T_{Al}}\) (\({}^{\circ}\)C)} & \multicolumn{2}{c}{**Gas Flow (sccm)**} & \multirow{2}{*}{\(Q_{v}\)} & \multirow{2}{*}{\(Q_{m}\)} & \multirow{2}{*}{\(\mathbf{Number\ of\ Passes}\)} & \multirow{2}{*}{**Total \(Q_{m}\) (g) 1**} \\ \cline{3-4} \cline{6-9}  & & **He** & & & & **G/min** 1 ** & \multicolumn{2}{c}{**Forward**} & \multicolumn{2}{c}{**Backward**} \\ \hline P1 & 175 & & & 0.062 & 0.97 \(\times\) 10\({}^{-2}\) & 10 & - & 0.34 \\ \hline P2 & 185 & & & & 8 & - & 0.58 \\ \hline P3 & 185 & & & 0.116 & 1.82 \(\times\) 10\({}^{-2}\) & 7 & 7 & 1.02 \\ \hline \end{tabular}
\end{table} 
Table 1: Process parameters and calculated flow rates for the fabrication process. Figure 2: (**a**) The downstream deposition direction with \(\mathrm{V_{dep}}\) 100 mm/min and \(\mathrm{V_{ret}}\) (burner return speed) 1500 mm/min; and (**b**) downstream and upstream deposition directions (\(\mathrm{V_{dep}}\) 100 mm/min and \(\mathrm{V_{ret}}\) = \(\mathrm{V_{dep}}\) = 100 mm/min).