Insights into the polarization behaviour of a long high birefringence photonic crystal fibre under low energy, ultrashort pulse excitation

A highly nonlinear polarization-maintaining (PM) photonic crystal fibre (PCF) utilized for supercontinuum (SC) generation is studied. The effect of fibre microgeometry on fibre parameters is determined using electron microscopy and finite element analysis. The positions of the slow and fast axes of the PM-PCF were experimentally determined in situ at the input and the output by using near and far field imaging of modes. Furthermore the polarization resolved transmission characteristics were probed with ultrashort optical pulses from a modelocked Ti:Sapphire laser at sub-100 pJ input energies. The polarization response was fitted with a simple incoherent pulse fragment model, to reveal the fibre´s twist and input axis orientation. Experimental measurement of SC generation in the 2-m long PM-PCF is also presented. The spectrum spans greater than 1.5 octaves despite the low input energies. The effects of polarization state in these experiments are discussed.