We report numerical and experimental studies of instabilities in a “noise-like pulse” dissipative soliton laser generating an output spectrum of 1000 nm bandwidth, and with two orders of magnitude variation in intracavity spectral width over one roundtrip. Simulations identify the origin of the laser instability as the sensitivity to noise of nonlinear soliton dynamics. Our experiments use real-time time and frequency domain measurements, and our simulations reproduce quantitatively both the full extent of intracavity supercontinuum broadening, as well as the probability distributions of temporal and spectral fluctuations, including rogue waves.
Ultrafast mode-locked lasers are well-known to display a rich variety of unstable dissipative soliton dynamics resulting from the interplay of nonlinearity, dispersion and dissipation. Although laser instabilities have been known and studied in depth for many years, their properties have recently received greatly renewed attention because of the development of time and frequency domain techniques that allow laser dynamics and instabilities to be measured in real-time. This has allowed the variations in circulating pulse characteristics to be examined on a roundtrip to roundtrip basis, providing a new window into understanding these instabilities and how they develop based on the cavity configuration being used.
A technique of this kind that has proven both straightforward to implement and powerful is the photonic time stretch or dispersive Fourier transform (DFT) which has been used in a number of important applications including the measurement of soliton rogue waves, modulation instability and supercontinuum noise. The DFT allows direct access to shot-to-shot measurement of the mode-locked fibre laser spectrum and, via computation of the associated autocorrelation function, can also provide complementary time-domain information in cases where multiple pulse states are observed.
In this paper, we report results of DFT measurements which have been used to reveal previously unreported behavior in a mode-locked fiber laser designed to operate with soliton-similariton dynamics. In particular, we observe instabilities including soliton explosions, chaotic evolution and oscillation in the relative phase of bound-state multi-pulse molecules, and what we believe to be a previously-unobserved regime of operation associated with the intermittent appearance of short-lived stable single pulses within of otherwise chaotic dynamics. Our results - obtained in a laser believed to be a particularly stable design - suggest that instabilities such as soliton explosions and intermittence are a universal feature of dissipative soliton systems transitioning from noise to stability.
Numerical modelling based on purely scalar nonlinear Schröodinger equation propagation is applied to a dissipative soliton laser operating in the soliton-similariton regime and generating parabolic pulses. The model is shown to reproduce a range of instabilities that have been reported in recent experiments. Here, we study in detail the laser stability characteristics as a function of the parameters of the gain medium and the saturable absorber, allowing us to readily identify clear regimes where stable single solitons and soliton molecules are observed. Outside these regimes, we reproduce a wide range of instabilities linked with soliton molecule internal motion, soliton explosions and intermittence.
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