We report the dynamics of dissipative solitons in a ring cavity passively mode-locked fiber laser with a strict control of the polarization state. We study the relation between the polarization state of the pulses propagating in the cavity and the regimes of generation. We have found that at pulse ellipticities between 5° and 15°, the laser generates one bunch of pulses in the cavity, while at higher ellipticities the laser generates multiple bunches. At constant ellipticity we rotated the polarization azimuth and observed a regime transition from the generation of noise-like pulses (NLP) to that of soliton crystal. The NLP regime was found when the azimuth was rotated towards smaller low-power transmission through the polarizer. The number of solitons in the soliton crystal also depended on the azimuth in a straightforward way: the higher the initial transmission, the bigger the number of solitons.

We report on the dynamics of noise-like pulses at the ns scale in a passively mode-locked fibre laser, which grow in complexity as wave retarder adjustments are performed. We can observe that the laser operating in the fundamental mode can be tuned to get different shapes of the noise-like pulse. Following a regime of a very stable waveform, regimes characterized by a much more variable (but still compact) waveform are observed. Then we can get the fragmentation of the main bunch and expulsion of sub-packets and, finally, a variety of puzzling dynamics with increasing complexity are evidenced. Although the collective behaviour of the multiple waveforms is at first sight random we can observe some well-defined patterns in the kinematics of light bunches at the global cavity scale. These results may be useful to unravel the subtle mechanisms at play in complex dissipative nonlinear systems such as passively mode-locked fibre lasers.

Conventional mode locking is characterized by the generation of a stable train of optical pulses. Even in the noise-like pulsing regime of fiber lasers, sometimes described as partial mode locking, a periodic train of waveforms is still generated. In this work we study the dynamics of a figure-eight fiber laser away from the stable noise-like pulsing regime. By analyzing sequences of time-domain measurements performed with ns resolution, we reveal a wide range of puzzling dynamics, in which sub-structures emerge and drift away from the main bunch, decay or grow in a step-like manner, before vanishing abruptly. In some cases, sub-packets also concentrate in the central part of the period, forming one or multiple wide clouds that merge or split over time scales of seconds or minutes. Spontaneous transitions between these multiple states occur in a non-periodic manner, so that no quasi-stationary behavior is found over long time scales. These results provide a dramatic illustration of the extremely rich dynamics taking place in fiber lasers at the frontier of mode locking.

This work presents a novel numerical stability analysis of a collection of pseudo-spectral methods, also known as split-step methods, for solving pulse propagation modeled by the nonlinear Schrödinger equation in the nonlinear fiber optics formalism. In order to guarantee the convergence of different pulse propagation dynamics, the numerical solutions of the pseudospectral methods (split-step Fourier method, symmetric split-step Fourier method, fourth-order Runge-Kutta in the interaction picture method, and an optimization of split-step Fourier schemes for pulse propagation over long distances) are tested by the validation of the conservation laws that govern this system. The presented numerical results are an illustrative guide to consider in the selection of an appropriate numerical method in future investigations of a wide variety of propagation problems that involve the interplay of the linear and nonlinear contribution in the nonlinear Schrödinger equation, in order to accurately reproduce a specific phenomenology using this formalism.

We present a systematic experimental study of supercontinuum (SC) spectra produced by propagating noise-like pulses (NLPs) from an erbium-doped figure-eight laser through sections of different lengths of standard single-mode fibre (SMF) and of high-nonlinearity fibre (HNLF), as well as their combinations. With an average input power that does not exceed 35 mW, very broad and smooth SC spectra extending over several hundreds of nm are typically observed, even when only SMF is used. However, maximal broadening and flatness are obtained through a section of about 300 m of SMF followed by 50 m of HNLF; the spectrum then spans from ~1260 nm up to ~2070 nm, with a bandwidth of 431 nm at 5.7 dB from the maximum (over 800 nm at 20 dB). Our analysis indicates that nonlinear processes are still operating even after propagation through several hundreds of meters of fibre, extending and flattening the spectrum, its maximal bandwidth being eventually limited by the strong fibre attenuation in the 2 µm region.