Vertical axial deposition

Vertica.1 Axia.1 Deposition. The vertical axial deposition (VAD) process (18) was developed by a consortium of Japanese cable manufacturers and Nippon Telephone and Telegraph (NTT). This process also forms a cylindrical soot form. However, deposition is achieved end-on without use of a mandrel and subsequent formation of a central hole. Both the core and cladding are deposited simultaneously using more than one torch (Fig. 12). 

There are four principal processes that may be used to manufacture the glass body that is drawn into today s optical fiber. "Outside" processes—outside vapor-phase oxidation and vertical axial deposition— produce layered deposits, of doped silica by varying the concentration of SiCl4 and dopants passing through a torch. The resulting "soot" of doped silica is deposited and partially sintered to form a porous silica boule. Next, the boule is sintered to a pore-free glass rod of exquisite purity and transparency. 

The vapor axial deposition (VAD) process is basically similar to the OVD process, except that the soot is deposited by fixed burners located near the end of a vertically mounted seed rod. This type of process allows for several fixed burners to be used in laydown and the cylindrical soot rod created in this way is drawn upward as the burner assembly deposits additional layers (see Figure 9.18). Again, as in the OVD process, the seed rod is removed before the next step. 

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