# Fabrication of Alumina-Doped Optical Fiber Preforms by an MCVD-Metal Chelate Doping Method
## 3 Results and Discussion
In both P2 and P3, the effect of \(Q_{\rm T}\)/\(\alpha\) was the same where the length of deposition was observed to be long (i.e., taper region). It is worth mentioning that during the sintering process for P3, a red glow with intensity increasing towards the exhaust tube was observed along the substrate tube (Figure 6a). This is indicative of the high concentration of alumina and was manifest in the collapsed preform where the core had an opaque center stretching from about the middle of the preform to the outlet with increasing opacity towards the outlet (Figure 6b). This may be attributed to the deposition of Al\({}_{2}\)O\({}_{3}\) in the tiny spaces between silica soot particles. The high temperatures encountered during sintering and collapse may then be enough to cause some aluminum diffusion  
Figure 5: (a,b) present the refractive index profiles for P1 and P2, respectively, obtained at the inlet position and showing the measurements for d\({}_{\rm preform}\) and d\({}_{\rm core}\); (c,d) present the longitudinal uniformity of d\({}_{\rm preform}\) and d\({}_{\rm core}\) for P1 and P2, respectively. into the silica, thereby producing regions of alumina-rich silicates. The slow cooling of the produced materials as the burner moves away from these regions promotes solidification to a crystalline (rather than an amorphous) phase, causing opacity of the core. Another explanation is the formation of alumina-rich silicates by phase separation and crystallization. This may take place when the binary oxide mixture (Al2O3/SiO2) encounters a suitable temperature during sintering or collapse, provided that the aluminum concentration is high enough [22]. It should be pointed out that even though the upstream deposition yielded more alumina, the uniformity in the longitudinal refractive index difference significantly deteriorated. However, the uniformity could be improved with a cooling mechanism, as reported by [PERSON] et al. [23]. Figure 7a illustrates the refractive index profile for P3. The variation in the longitudinal refractive index difference is rather small along the first 13 cm from the inlet (%RSD 4%, Figure 7b). It is, however, significantly high along the full length of the preform. The highest core-to-cladding refractive index difference was found to be 0.027 and was for the region of the preform adjacent to the outlet. It was noticed that the opacity in the core was pronounced when the core-to-cladding refractive index difference was greater than 0.015 (\(\sim\)6 mol.% Al2O3). This is consistent with the conclusion that the opaque regions in the core of the preform are attributable to the formation of Al-rich aluminosilicate crystalline phase. One possible aluminosilicate crystalline phase is mullite (3 Al2O3.2 SiO2). X-ray diffraction and Raman spectroscopy analyses carried out by [PERSON] et al. [24] indicated that high-temperature annealing of aluminosilicate fibers and preforms gives rise to the formation of crystalline mulite phase. It should be mentioned that a rapid cooling rate may prevent crystallization in the core of the preform, although such a rate is typically much higher than that achieved during the MCVD process. The energy-dispersive X-ray spectroscopy (EDX) method was used to investigate the Al distribution and content across the core of the preforms and fibers. The EDX point identification analyses were carried out to support and complement the afore-discussed refractive index profile results. The EDX results for preform P3 are illustrated in Figure 8a,b. As can be seen from Figure 8a, the Al distribution across the core region is uniform and matches the RIP where the Al content gradually increases, reaches a maximum, and then gradually decreases. The Al concentration is highest in the center of the core and was found to range from 8.6 to 9.9 wt.%. Figure 6: (**a**) P3 sintering process with an oxygen flow rate of 1000 sccm, temperature of 2000–2100 \({}^{\circ}\)C, and \(V_{b}\) of 125 mm/min. The red glow is an indicator of the high alumina content in the preform.