# Fabrication of Alumina-Doped Optical Fiber Preforms by an MCVD-Metal Chelate Doping Method
## 3 Results and Discussion  
Figure 3 shows the oxidation process of Al(acac)\({}_{3}\) at T\({}_{\text{dep}}\) 1850 \({}^{\circ}\)C and an oxygen flow rate of 2400 sccm. As the Al(acac)\({}_{3}\) vapor passes through the hot zone, alumina particles (soot) are formed and then deposited on the substrate tube wall downstream of the oxyhydrogen burner due to thermophoretic forces. It was observed that the alumina soot was deposited further from the oxyhydrogen burner (i.e., longer taper region). This can be attributed to the high total gas flow rate (\(Q_{T}\)) used during the oxidation process (3840 sccm). The length over which the deposition takes place is proportional to \(Q_{T}\)/\(\alpha\)[19], where \(\alpha\) is the thermal diffusivity. Since the hot zone created by our oxyhydrogen burner is small (~2 cm in length), and \(Q_{T}\) is high, the residence time of Al(acac)\({}_{3}\) inside the hot zone is short. This results in the formation of predominantly fine alumina particles that can be uniformly distributed along the substrate tube. Continuous movement of the hot zone results in fusion of the deposited alumina particles and incorporation into the silica soot. The deposited alumina/silica layers are then vitrified (in oxygen atmosphere) into transparent glassy material. Figure 4a,b shows the refractive index profiles for P1 (T\({}_{\text{Al}}\) 175 \({}^{\circ}\)C) and P2 (T\({}_{\text{Al}}\) 185 \({}^{\circ}\)C), respectively. Figure 4c,d displays the longitudinal uniformity values of P1 (average \(\Delta\)n = 0.0036) and P2 (average \(\Delta\)n = 0.0124), respectively. As can be seen from the figure, P1 and P2 showed good longitudinal uniformity with a slight variation in the refractive index difference (percent relative standard deviation of index difference (%RSD) 1% and 2%, respectively). This good longitudinal uniformity can be ascribed to the high total gas flow rate (\(Q_{T}\)) used and the short hot zone and  
Figure 3: The Al(acac)\({}_{3}\) oxidation process via MCVD, where fine white particles (or alumina soot) are deposited along the glass substrate tube while the Al(acac)\({}_{3}\) vapor passes through the hot zone. The figure also shows the ceramic heater used to prevent the condensation of Al(acac)\({}_{3}\) vapor prior to reaching the hot zone. residence time, which result in the production of fine and uniform alumina particles, as discussed above. As is illustrated in Figure 5a-d, the core (d\({}_{\text{core}}\)) and preform (d\({}_{\text{preform}}\)) diameters for P1 were 1.2 mm (%RSD 2.4%) and 15.0 mm (%RSD 1.2%), respectively, whereas those for P2 were 1.44 mm (%RSD 1.4%) and 15.4 mm (%RSD 1.0%), respectively. This shows that it is possible to fabricate alumina-doped silica preforms with high uniformity of \(\Delta\)n and core size using Al(acac)\({}_{3}\) and a chelate vapor delivery system. Figure 4: (**a**,**b**) show the refractive index profiles for P1 and P2 along the preform length; (**c**,**d**) illustrate the longitudinal variation in the refractive index difference (\(\Delta\)n) for P1 and P2, respectively. For preform P3, the deposition of alumina was performed in both the downstream and upstream directions. The upstream deposition mode is normally used to utilize a higher temperature without sintering the produced soot [20,21]. In P3, it was observed that the downstream deposition showed the same behavior as P2. In upstream deposition, however, more alumina with large particle size distribution is produced. In this case, the alumina particles are deposited behind the moving burner and are partially sintered since the burner is moving away towards the reactant inlet. In addition, the upstream deposition mode provides higher temperature and a longer hot zone, which, in turn, result in longer residence time and enhanced conversion or oxidation of Al(acac)\({}_{3}\). This causes more particle nucleation and agglomeration and, hence, larger particle size distribution in the produced alumina soot [20].