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Real-time characterization of regimes between continuous-wave operation and mode locking in an all-normal dispersion ytterbium-doped fiber ring laser

In the present work, the temporal dynamics of an all-normal-dispersion ytterbium-doped fiber ring laser operating in regimes intermediate between continuous-wave operation and mode locking are studied experimentally. Exploiting the segmented memory data acquisition possibilities of an ultrafast digital oscilloscope and through the use of a specifically developed algorithm, the real-time waveform evolution anchored in absolute time could be retrieved, without relying on a fixed reference carried by the signal itself, which does not exist in these regimes. By controlling the time spacing between the successively acquired frames, several dynamics taking place over different time scales could be evidenced and described. These measurements highlight in particular the evolution cycles of intense localized structures including spikes that emerge, grow, decay, interact and describe peculiar trajectories in phase-space diagrams, whereas their temporal positions evolve, driven in particular by gain dynamics. In spite of their dramatic variability, these structures are found to be enduring features of these regimes. Analysis of these data helps assessing the degree of partial mode locking associated with these dynamics. The time-domain mapping technique optimized using segmented memory data acquisition is shown to be useful to characterize precisely highly dynamical evolutions such as those revealed in this work, which are dominated by structures that present large and complex variations in amplitude, shape and position, and develop over different time scales.