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Buildup of Noise-Like Pulses in Ultrafast Fiber Lasers

Noise-like pulses (NLP) - the emission of sub-nanosecond incoherent pulses in ultrafast lasers — are a topic of widespread attention, owing to their fundamental and applied interest [1]. Their existence seems paradoxical, combining the high temporal localization of circulating pulses akin to mode-locking, with a fundamental instability that entails large fluctuations [2]. NLPs can be considered as incoherent dissipative solitons [3]. It is therefore of considerable interest to investigate the self-emergence of these pulses in the laser. We undertake this study experimentally using the time-stretch dispersive Fourier transform (DFT), recording the pulse spectral and temporal information in real time over successive cavity roundtrips [4]. The buildup dynamics of noise-like pulses in both anomalous and normal-dispersion fiber lasers are compared, revealing markedly different stages and timescales in the evolution process.