• Silica, chalgenide and halide glass fibers can all be made using a direct double-crucible technique.
• In this method, glass rods for the core and Cladding materials are first made separately by melting mixtures of purified powders, to make the appropriate glass composition.
• These rods are then used as feedback for each of two concentric crucibles.
• The inner crucible contains molten core glass and the outer one contains the cladding glass.
• Subsequent development in the drawing of optical fibers (especially graded index) produced by liquid-phase techniques has concentrated on the double-crucible method.
• In this method the core and cladding glass in the form of separate rods is fed into two concentric platinum crucibles.
• The assembly is usually located in a muffle furnace capable of heating the crucible contents to a temperature of between 800 and 1200°C.
• The fibers are drawn from the molten state through orifices in the bottom of the two concentric crucibles in a continuous production process.
• The crucibles have nozzles in their bases from which the cladding fiber is drawn directly from the melt as shown in figure 1.18.
• Although this method has the advantage of being a continuous process (both melting and drawing), careful attention must be paid to avoid contaminants during the melting.
• The main sources of contamination arise from the furnace environment and from the crucible.
• Silica crucibles are normally used in preparing the glass feed rods, whereas the double concentric crucibles used in the drawing furnace are made from platinum.
• This method has disadvantage of obtaining and maintaining extremely pure glass which limits their ability to produce low loss fibers.
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