Birefringence in single-mode optical fibres

Abstract

The birefringence properties of single-mode optical-fibre waveguides are discussed.

A fibre birefringence measurement system has been developed specifically to measure the birefringence of fibres as a function of temperature, wavelength, twist and bending. By incorporating a birefringence modulator into the system, the measurement speed and sensitivity is substantially increased, particularly for low birefringences.

The fibre spinning technique has been invented to provide a means of manufacturing ultra-low birefringence fibres with extremely high yield. It is shown that spinning averages the intrinsic local birefringence to produce very low overall retardation. A coupled-mode analysis to describe the birefringence properties of spun and twisted fibres is developed and confirmed experimentally. It is now possible to manufacture ultra-low birefringence fibres routinely.

The coupled-mode analysis is subsequently extended to treat the effects of bending, twist and magnetic fields on both linearly- and circularly-birefringent fibres. The predictions are subsequently confirmed experimentally and a thorough understanding of extrinsic effects on fibre birefringence has been established. The design criteria of polarisation maintaining fibres are discussed.

A novel technique is also described for the measurement of polarisation mode-dispersion, based on the variation of birefringence with wavelength. The dramatic reduction of polarisation-dispersion achieved by spinning is demonstrated, thereby indicating the high suitability of spun fibres to high-bandwidth communications. The separate identification of the stress and waveguide effects contributing to the intrinsic fibre birefringence has been achieved by observing the changes in birefringence as a function of wavelength and temperature.

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