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The material compositions of optical fibers are almost the same in all high-silicia fiber-fabrication processes; the principal oxide in high-silica fibers is SiO2 and the dopants are usually GeO2, P2OS, B2O3, and F. In the VAD process, the deposition properties of the oxide products play an important role in making graded-index profiles since a characteristic behavior of GeO2, which will be described in detail in Chapters 3.3 and 3.6, is utilized in preform synthesis. Germanium oxide is used as the main dopant ofr fabricating low-loss fibers and phosphorous oxide is generally used to reduce the sintering temperature. Boron oxide can be used for making special fibers such as the single-polarization, single-mode fiber. In practice B2O3 is not desirable because it gives rise to high transmission loss in the long wavelength band. Typically, low-loss fibers fabricated by the VAD process consist of SiO2-GeO2 for the core and SiO2 for the cladding.