Polarisation-maintaining fibre using a novel glass system

A new glass dopant system, AlPO₄ is used to form the stress-producing regions of a polarisation-maintaining optical fibre. The advantages of this glass system are that it can match the refractive index of silica and does not have a fundamental absorption peak near the optical frequencies of interest, allowing it to be placed in close proximity to the fibre core to optimise birefringence. This material was used for the cladding of a preform, which was then flattened to yield an elliptical fibre upon drawing. This fibre demonstrated an extinction ratio at 1.3 mu m of 42.5 dB for a 10 m length and 22 dB for 500 m for h=1.3*10^-5 m^-1. The beat length at lambda =0.488 mu m was about 9 mm, corresponding to a modal birefringence of 5.3*10^-5.