Fabrication and evaluation of single mode fibres

Abstract

The homogeneous chemical vapour deposition technique, normally used for the manufacture of multimode fibres, has been applied to the fabrication of low loss monomode fibres for operation in the 0.6 to 1.6 µm region. The process enables fibres having cores based on phosphosilicate, germanosilicate, and silica glasses to be drawn in lengths up to 5 km. Because an appreciable amount of energy lies outside the core, a low loss cladding of borosilicate glass must also be deposited. A detailed description of the preform deposition and characterisation is presented, and the fibre drawing process is outlined. Core diameters range between 4 and 10 µm for single mode operation in the 0.6 to 1.5 µm region, whilst the index difference between core und cladding is typically 0.3 %. Losses of 1.5 dB/km have been achieved at 1.06 µm. Fibre characterisation by observation of the far field radiation pattern yields V value, core diameter, and index difference directly. An alternative technique involving loss measurements over a range of wavelengths about the cut off point is prone to serious inaccuracy.

Finally, single mode fibres with extremely low birefringence (less than 3°/m linear retardance) have been developed for instrumentation applications.

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