  A systematic numerical study of ultrafast nonlinear directional coupler
performance based on soliton self-trapping in a novel type of dual-core optical
fibre is presented. The considered highly nonlinear fibre structure is composed
of a real, intentionally developed soft glass-pair with high refractive index
contrast at the level of 0.4 in the near infrared. Nonlinear propagation of
picojoule level femtosecond pulses was studied numerically with the aim to
identify the best switching performance in input parameter space of 1400 - 1800
nm in terms of excitation wavelengths, and of 75 - 150 fs in terms of pulse
width, respectively. For every combination of excitation wavelength and pulse
width, the switching energies together with the optimal fibre length were
determined and their relation to the input and switching parameters is
discussed. The highest switching contrast of 46 dB in the time window of the
ultrashort soliton was predicted at combination of 1500 nm excitation
wavelength and 75 fs pulse width considering 43 mm fibre length. These results
represent significant improvement both from point of view of switching contrast
and switching energies, which are only at level of 20 pJ, in comparison to the
previously published case of air-glass dual-core photonic crystal fibre.
Moreover, the simpler fibre design without cladding microstructure together
with the all-solid approach holds promise of improved dual-core symmetry and
therefore offers high probability of the successful realization of a low power,
compact and simple switching device.
