# Fabrication of Alumina-Doped Optical Fiber Preforms by an MCVD-Metal Chelate Doping Method
## 1 Introduction  
Alumina (Al\({}_{2}\)O\({}_{3}\)) is an interesting dopant material for silica optical fiber technology. It does not only function as a refractive index raiser but also helps dissolve other co-dopants such as rare-earth elements in the silica matrix. Thus, alumina is very essential to the fabrication of rare-earth-doped fiber amplifiers and high-power fiber lasers. The use of alumina was first reported by [PERSON] and [PERSON] [1] in 1972 for passive fiber applications, followed by [PERSON] and [PERSON] [2], [PERSON] et al. [3], [PERSON] [4], [PERSON] et al. [5; 6], and [PERSON] et al. [7]. The fabrication techniques used were outside vapor deposition (OVD), vertical axial deposition (VAD), and modified chemical vapor deposition (MCVD). With the rapid progress on rare-earth specialty fibers, more research is being focused on incorporating alumina in silica, particularly by the MCVD method. The most common technique for doping alumina is conventional MCVD with solution doping (non-vapor phase) (e.g., [8]). This technique has been very successful in the fabrication of fiber amplifiers such as erbium-doped fiber (EDF). However, the technique suffers from limitations, including the inability to deliver advanced fiber design requirements such as high dopant concentration, large core, and precise wave guiding structure. Furthermore, due to the nature of the MCVD solution doping process that involves several stages, it tends to degrade the quality of the preform or fiber. Another alternative technique is MCVD with a chelate delivery system, which was first reported by [PERSON] et al. [9]. This technique offers in situ vapor phase deposition of dopants in a controllable process, thus improving the quality of the fabricated preform or fiber. Since the study by [PERSON] et al., several research groups have reported on the use of MCVD with chelate delivery systems, e.g., [10; 11; 12; 13; 14; 15; 16; 17]. In most of these studies, anhydrous aluminum chloride (AlCl\({}_{3}\)) has been used as the precursor for alumina, with few using aluminum acetylacetonate (Al(C\({}_{5}\)H\({}_{7}\)O\({}_{2}\))\({}_{3}\); Al(acac)\({}_{3}\)) as a precursor, e.g., [16]. AlCl\({}_{3}\) is commonly used due to its relatively high vapor pressure. In this paper, we report on the fabrication of alumina-doped preforms using Al(acac)\({}_{3}\). Even though Al(acac)\({}_{3}\) has lower vapor pressure than AlCl\({}_{3}\), it exhibits some advantages over AlCl\({}_{3}\), including its low cost, non-corrosive properties, and chemical stability. However, the conversion of Al(acac)\({}_{3}\) to Al\({}_{2}\)O\({}_{3}\) requires more oxygen (compared to AlCl\({}_{3}\)) in order to eliminate incomplete oxidation of Al(acac)\({}_{3}\) which can lead to carbon contamination. The total gas flow rate (oxygen and carrier gas) \(Q_{T}\) is thus considerably higher compared to that in the standard MCVD process. This entails the study of the chemistry and the deposition mechanism involved, as well as the effect of various process parameters on the deposition and incorporation efficiency of the Al\({}_{2}\)O\({}_{3}\) particles formed during the process. In this work, we studied the fabrication process of alumina-doped silica fiber preforms with different Al(acac)\({}_{3}\) sublimator temperature (T\({}_{\rm{Al}}\)\({}^{\circ}\)C) and deposition direction (i.e., downstream and upstream). Other parameters such as total oxygen flow, deposition temperature, carriage speed (\(V_{b}\)), and spindle rotation were fixed. The aim is to achieve the highest possible doping amount of alumina, which, in turn, allows for greater incorporation of rare-earth elements into silica fibers. The fabricated preforms were checked for radial and longitudinal uniformity. The efficiency of the fabrication process was also determined by comparing the Al\({}_{2}\)O\({}_{3}\) concentration obtained from energy-dispersive X-ray spectroscopy (EDX) analyses with that derived theoretically.