Ignac Bugar

We conducted a comprehensive experimental investigation of dual-wavelength switching of 1560 nm, 75 fs pulses (referred to as signal) driven by 1030 nm, 270 fs pulses (referred to as control) using two dual-core fibers with high refractive index contrast and different levels of asymmetry. The study explores the influence of fiber length, control pulse energy, and control-signal pulse delay on switching performance. For the fiber with higher dual-core asymmetry, we achieved an exceptional switching contrast of 41.6 dB at a 14 mm fiber length, exhibiting a homogeneous character within the spectral range of 1450–1650 nm. In contrast, the study of the weaker dual-core asymmetry fiber revealed a maximum switching contrast of 10.7 dB at a 22 mm fiber length, albeit under lower control pulse energy. These observations confirm that the switching mechanism is based on the nonlinear balancing of dual-core asymmetry, wherein the control pulse induces an enhancement of the effective refractive index in the fast fiber core, facilitating the switching of the signal pulse. This work demonstrates high switching contrasts with only a 0.4–0.6 nJ control pulse energy requirement, providing experimental confirmation of a previously reported theoretical model. For the first time, the dual-wavelength switching performance of dual-core fibers with varying levels of asymmetry is compared. The results reveal key directions for the further development of dual-core fibers in view of their potential applications.

Effective all-optical switching using dual-core fibers is studied both experimentally and theoretically. C-band femtosecond pulses are switched by synchronized shorter wavelength ones and interpreted in terms of nonlinear balancing of the couplers mismatch.

A novel type dual-core fiber (DCF), composed of two in-house synthesized glasses with high index contrast enabling significant anomalous dispersion [1] and cm range coupling length was optimized and studied in the near infrared. Following promising results of numerical simulations [2], we performed an experimental study in the 1500–1900 nm spectral range by using a tunable ultrafast laser source generating 70-fs pulses at a few μJ energy level. Continuous control of attenuation and polarization direction of the input field was ensured under separate excitation of a single fiber core, made of the low index glass with an effective mode area of 1.73 μm ² at 1500 nm. The dual-core cladding is made of high index glass including the 1.4-μm gap between the two cores. The series of camera pictures, imaging the output facet of the dual-core fiber at different input pulse energies, indicated nonlinear switching at distinct excitation conditions. A change in the dominancy of the output core of 32 mm long DCF was registered when 1700 nm, 70 fs pulses with increasing energies in range of 100–300 pJ (measured before in-coupling optics) were launched into the fiber. Output spectra were registered separately for the two cores, when the nonlinear switching was observed according to the camera registrations. The spectra were processed by plotting the curves representing a ratio R(λ) of the spectral intensities of the two cores: R(λ) = S t (λ)/S b (λ) − 1, where S t (λ) represent the spectra registered from the top core (dominant at low pulse energies) and S b (λ) are the spectra originating from the bottom core (Fig. 1a). In Fig. 1b, the excitation energy dependence of R(λ) is depicted, with positive and negative values representing the cases of the dominancy of the top and the bottom cores, respectively. The spectra reflect the similar energy dependence as the series of camera images, shown in the insets of Fig. 1b. In the case of low excitation energy, the spectral intensity ratio curves reveal a dominancy of the top core, while at higher pulse energies a majority of launched pulse energy is located in the bottom core. According to our knowledge, the presented simultaneously recorded spatial and spectral study is the first experimental demonstration of the nonlinear switching of a whole ultrashort pulse in a DCF in a solitonic propagation regime. Furthermore, in contrast to previously reported cases, the switching identified here is accompanied by rather moderate spectral broadening at pulse energies in the sub-nJ range.

A new approach on the way of high contrast solitonic directional coupler is introduced in the frame of this paper. An all-solid dual-core photonic crystal fibre (PCF) structure is analysed by means of numerical simulations of linear propagation regime in near infrared spectral region. The PCF structure is suggested on the base of all-solid strategy combining two thermally matched in-house synthesized glasses with refraction index contrast at the level of 0.4. The extensive numerical study was performed by proper alteration of the structural parameters of a hexagonal lattice PCF structure of hexagonal rods in order to have sufficient measure of anomalous dispersion and its proper profile in spectral range of 1.5 - 2.0 μm. The all-solid dual-core PCF structure with optimized pitch and rod radius values are than compared to the previously published optimized dual-core air-glass fibre and its potential for ultrafast all-optical switching application is discussed.