The VAD Vapor Phase Axial Deposition process is a type of external deposition process. The VAD process was invented by Japanese NT&T company in 1977, based on OVD, by Ryuno Izawa and others. The basic process principle of VAD is the same as that of OVD process. Which also includes the process of first depositing the powder rod and then dehydrating and sintering. The difference is that it does not deposit radially on the outer surface of the target rod, but deposits axially at the top of the quartz seed rod. During the deposition process, the seed rod continuously rotates and moves upwards, and deposition and sintering can occur simultaneously.  Hence it is also known as the continuous manufacturing process of prefabricated rods.
Process: The VAD process includes two processes: loose body deposition and loose body dehydration sintering.