Polarisation independent (<0.01 dB) switching in a 2x2 fused fiber null coupler using asymmetric dumbbell cross-section

Polarisation independent (<0.01dB) switching at 1.55µm is achieved in a strongly guiding tapered fibre null coupler acousto-optic switch. A strongly fused asymmetric dumbbell cross-section gives the form birefringence required to render the null coupler polarisation insensitive