Waves, Solitons and Turbulence in Optical Systems - Abstract
Zajnulina, Marina
The propagation of optical pulses in optical fibres is well-known experimentally, numerically and in some cases even analytically. If an optical system fulfils certain conditions, optical solitons can be formed in it. However, if the propagation of light pulses is described specifically by a Generalised Nonlinear Schrödinger Equation (GLNS) containing the terms of higher-order dispersion, the nonlinear Kerr effect, the Raman effect, and the effect of the pulse self-steepening, the soliton content of pulses formed when an arbitrary initial condition is chosen in no longer well-known, especially when a periodic initial condition is used. Here we use the Soliton Radiation Beat Analysis (SRBA), a numerical technique that allows to obtain information about the energy, the velocity, the phase, and the position of optical solitons that are solutions of a GNLS, to analyse the soliton content of a system with a periodic initial condition. We study a fibre-based system that is capable of optical frequency combs generation. Within this system, the first stage consists of a conventional single-mode fibre, whereas the second stage is a suitably pumped Erbium-doped fibre. The initial optical field is generated by two equally intense continuous-wave (CW) lasers with a frequency separation of 78.125 GHz. The according initial condition equation is given by a deeply-modulated cosine-wave. To be able to control the frequency comb generation, it is necessary to understand how optical pulses evolve out of the initial field in the first stage of the system. For that, we use the SRBA to study the evolution of different initial conditions (a single cosine hump, an Akhmediev breather, and the deeply modulated cosine-wave) and compare the results to draw conclusion about the soliton content of the state generated in the first fibre stage. In case of a deeply-modulated cosine-wave, we observed the build-up of a collective soliton crystal state for low input laser powers and the appearance of separated (free) solitons for high input powers. The transition between the states occurred continuously with increasing input power.