We experimentally investigate a nonlinear switching mechanism in a dual-core highly nonlinear optical fiber. We focus the input stream of femtosecond pulses on one core only, to identify transitions between inter-core oscillations, self-trapping in the cross core, and self-trapping of the pulse in the straight core. A model based on the system of coupled nonlinear Schrödinger equations provides surprisingly good agreement with the experimental findings.

All-optical switching of 75 fs pulses centered at 1560 nm, driven by 270 fs, 1030 nm pulses in a dual-core optical fiber exhibiting high index contrast is presented. The fiber is made of a thermally matched pair of lead silicate and borosilicate glasses used as core and cladding material, respectively. The novel switching approach is based on nonlinear balancing of dual-core asymmetry, by control pulse intensity induced group velocity reduction of the fast fiber channel. Due to the fiber core made of soft glass with high nonlinearity high switching contrast exceeding 20 dB is reached by using control pulses of only few nanojoule energy. The optimum fiber length is 14 mm, which closely match the calculated coupling length at the signal wavelength. The results express significant progress in comparison to similar studies based on self-switching of solitonic pulses in dual-core fibers and represent high application potential.

The improvement potential of ultrafast all-optical switching by soliton self-trapping, using all-solid dual-core fibres with high index contrast, was analyzed numerically. The study of the femtosecond nonlinear propagation was performed based on coupled generalised nonlinear Schrödinger equations considering three fibre architectures: homogeneous cladding all-solid, photonic crystal air-glass, and photonic crystal all-solid. The structural geometries of all three architectures were optimised in order to support high-contrast switching performance in the C-band, considering pulse widths at the 100 fs level. Comparing the three 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. Further optimization of the excitation wavelength and pulse width resulted in lower switching energies and simultaneous improvement of the switching contrasts at the combination of 1500 nm, 75 fs pulses and a fibre length of 43 mm. The spectral aspect in this optimised case expresses a broadband and uniform switching character with a span of over 200 nm and a contrast exceeding 30 dB at more frequency channels.

We present a complex study of pulse-energy-controlled solitonic self-switching of femtosecond pulses at wavelengths of 1700 and 1560 nm in two nonlinear high-index contrast dual-core fibers having different levels of slight asymmetry. In the case of the fiber with higher dual-core asymmetry excited by 1700 nm pulses, the highest switching contrast of 20.8 dB at 40 mm fiber length was demonstrated. It was accompanied by multiple exchanges of the dominant core at the fiber output, which is a strong signature of the soliton-based switching process. In the case of the fiber with lower dual-core asymmetry, excited by 1560 nm pulses, the highest switching contrast of 21.4 dB at 35 mm fiber length was achieved with a broadband character of the switching in the spectral range of 1450–1650 nm. Both demonstrations represent progress in all-optical switching studies at these particular wavelengths thanks to a comparison between their results, which reveals the requirement of a higher level of dual-core symmetry for applicable C-band operation.