Polarisation maintaining (PM) fibres are used primarily in fibre optic gyroscopes (FOGs), sensors and in the pigtailing of lasers and modulators. To make a PM fibre, you need to start with a PM preform - Fibercore Limited fabricates 'Bow-Tie' polarisation maintaining preforms using the technique of gas-phase etching.

In order to produce an asymmetric structure, like HiBi, the basic preform fabrication process must be modified. First of all, a layer of boron-rich glass is deposited (by the oxidation of boron tribromide), then the rotation of the lathe is stopped, an etchant is passed through the substrate tube and activated by means of two narrow-zone burners. When the etching is complete, rotation of the lathe is resumed and the inner-cladding and core materials are deposited. Finally, the temperature of the main burner is increased so that the structure collapses under surface-tension into the characteristic bow-tie shape.

Fibre Drawing converts the preform into fibre.