Temperature dependences of phase and group birefringence in spun fibres

We consider the application of the spectral method for measuring the beat length of birefringence in spun fibre. We have found that the beat length measured in spun fibres is the geometric mean of phase and group birefringence beat lengths. Temperature measurements of beat lengths and shifts of the entire spectrum as a whole make it possible to separate types of birefringence and to determine their dispersion. We have performed an experiment on a spun fibre and a linear polarisation maintaining fibre, drawn from the same preform. The experimental results confirm the theory.