Abstract

Induced broadband supercontinua via long pulses in solid-core optical fibres has been the subject of huge research in the recent years, after the evidence presented by Solli et al. that the output spectra contain statistically rare rogue events with large intensities and enhanced redshift [1]. In this work, we report new missing details on the dynamics that proceed the rogue-soliton generation [2]. The propagation of intense pulses in optical fibres can be described in terms of the nonlinear Schrödinger equation [3]. Pure Kerr-nonlinearity results in a spatio-temporal modulation of the refractive index that will follow the variation of the pulse intensity. In the anomalous dispersion regime, the amplification of the background noise leads to a random modulation of the refractive index, which corresponds to a disordered optical temporal potential with linear modes. Since the pulse envelope is slowly-varying along the direction of propagation, the spatial-dependence of the refractive index modulation is frozen over short intervals. Hence, the system becomes the 1-D temporal analogue of the transverse-disorder Anderson waveguides [4], where linear temporal Anderson localised modes could be formed. Figure 1(a) depicts how the linear fundamental mode associated with a nonlinear superGaussian pulse is adiabatically compressed significantly due to Anderson localisation until z ≈ 6 m in a solid silica-core photonic crystal fibre. The temporal position of localisation is completely random, and it relies on the shape of the input noise. Since the potential is evolving, the localisation process is halted at that position, and the fundamental mode attempts to sustain at other places for few centimetres. These localised modes seed the emission of solitary waves as shown in panels (b,c), a phenomenon known as solitonisation of Anderson localisation [5]. Then due to formation of multiple optical-event horizons (OEHs) [6], the soliton-induced potential barriers impede the flowing of dispersive waves and reflect them back after collision. Interestingly, this kind of collision can result in a self-frequency redshift accompanied by a deceleration in the time domain even in the absence of Raman nonlinearity, as depicted in Fig. 1(d). This deceleration is stronger for solitons at the leading edge, since they are trailed by a large number dispersive waves. Raman nonlinearity allows the solitons to temporally approach each other with close group velocities, so they could strongly nonlinearly interact for a long distance, and a rogue-soliton is generated after exchanging energy between them, as displayed in panel (c). We believe that this work will potentially lead to novel routes for controlling of extreme nonlinear waves via linear-disorder optimisation.

© 2017 IEEE