CVD Production of Optical Fibers

Two major processes are available for the production of optical fibers (or liquid phase) and CVD. Solgel is being evaluated but has yet to evolve into a viable production process for that application. 

The direct melt process economically produces thick optical fibers (250-400 pm in diam.), which is advantageous, but their relatively high attenuation (3-20 dB/km) due to impurities is not. As a result, they are limited to short distance multimode applications. 

CVD yields fibers with low loss ( 0.5 dB/km) which are suitable for both mono- and multimode long-distance applications. It has emerged as the strongest technology for high-volume, low- cost production. ] 

Two variations of the general process are used. The first is a classical thermal CVD method. The reactants are usually the halides, i.e. 

A similar deposition system uses a plasma which is produced by a traveling microwave cavity. No other source of heat is required. The deposition system is shown schematically in Fig. 16.12. The reactants are the same as in the thermal CVD process. Pressure is maintained at approximately 1 Torr. In this case, the deposition occurs at lower temperature, no soot is formed and a compact glass is produced directly. A main advantage of this method is the more accurate grading of the refractive index of the cladding material. 

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