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 fig.

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.