We demonstrate ultrafast soliton-based nonlinear balancing of dual-core asymmetry in highly nonlinear photonic crystal fiber at sub-nanojoule pulse energy level. The effect of fiber asymmetry was studied experimentally by selective excitation and monitoring of individual fiber cores at different wavelengths between 1500 nm and 1800 nm. Higher energy transfer rate to non-excited core was observed in the case of fast core excitation due to nonlinear asymmetry balancing of temporal solitons, which was confirmed by the dedicated numerical simulations based on the coupled generalized nonlinear Schrödinger equations. Moreover, the simulation results correspond qualitatively with the experimentally acquired dependences of the output dual-core extinction ratio on excitation energy and wavelength. In the case of 1800 nm fast core excitation, narrow band spectral intensity switching between the output channels was registered with contrast of 23 dB. The switching was achieved by the change of the excitation pulse energy in sub-nanojoule region. The performed detailed analysis of the nonlinear balancing of dual-core asymmetry in solitonic propagation regime opens new perspectives for the development of ultrafast nonlinear all-optical switching devices.

A systematic numerical study of ultrafast nonlinear directional coupler performance based on soliton selftrapping 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.

Improvement potential of ultrafast all-optical switching by soliton self-trapping using all-solid dual-core fibers with high index contrast was analyzed numerically. Study of the femtosecond nonlinear propagation was performed based on the coupled generalized nonlinear Schrödinger equations considering three different fiber architectures: homogeneous cladding all-solid, photonic crystal air-glass and photonic crystal all-solid. The structure geometries of the all three alternatives were optimized in order to support high-contrast switching performance in C-band considering 100 fs level pulse widths. Comparing the three different structural alternatives, the lowest switching energies at common excitation parameters (1700 nm and 70 fs pulses) were predicted for the homogeneous cladding dual-core structure. The further optimization of the excitation wavelength and pulse width resulted in lower switching energies at simultaneous improvement of the switching contrasts at combination of 1500 nm,75 fs pulses and at 43 mm fiber length. The spectral aspect in this optimized case expresses broadband and uniform switching character spanning over 200 nm and exceeding 30 dB contrast at more frequency channels.

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