Experimental Investigation of Ultrafast Solitonic All-Optical Switching in a High Index Contrast Dual-Core Optical Fiber

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.