Polarisation Properties of Few-mode Highly Birefringent Fibres

Highly birefringent (HiBi) singlemode step-index fibres (SIF), where birefringence arises from stresses induced by core ellipticity or the presence of stress rods, are widely used in applications where preserving the state of polarization of a signal is crucial, such as fibre gyroscopes, coherent communications, frequency conversion etc. It is known that in such fibres, the assumption of linearly-polarised (LP) modes is not valid [1]. Due to the non-uniform stress field, the electric field of the fundamental mode (FM) has a more complex vectorial nature and comprises two orthogonal linearly-polarised components: a major, (one-spot component) and a minor (four-spot component), with a polarisation extinction ratio (PER) not exceeding 45 dB [1].