Incipient mode locking dynamics in an all-normal dispersion ytterbium-doped fiber ring laser

Abstract

In this work we study experimentally the temporal dynamics of an all-normal dispersion all-fiber ring laser far from stable mode locking. Temporal mapping relying on segmented memory capabilities of a fast oscilloscope is used for this purpose. A regime is found where radiation fills the cavity and is characterized by peaks that successively emerge, grow in amplitude while compressing temporally, before decaying abruptly, and localized low-intensity components mediating their interactions across the cycles. The highest-amplitude peaks ephemerally dominate intracavity radiation, before being superseded by other emerging peaks. As radiation covers the whole period and no permanent pulse ever emerges, the regime can be viewed as an intermediate stage between continuous-wave and mode locking operations. The interest of using segmented memory possibilities for proper characterization of this unconventional regime is highlighted.

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

“…In the regimes described here, spikes displaying complex cyclic evolution are dominating background radiation; in spite of their dramatic evolution over the cycles, however, these intense, compact structures are not short-lived, they do not vanish at any stage of the recorded sequences. This is in striking contrast with [30], and means that these regimes find themselves closer to mode locking compared to those previous results. Temporal mapping was performed using an ultrafast digital oscilloscope in segmented memory data acquisition mode; this method allows retrieving the absolute time of the measurements for their proper synchronization, a valuable asset considering the severe distortions and the temporal drifts undergone by the waveforms over the cycles.…”

“…From this figure, an emission period (round-trip time) T r = 59.5 ns is determined. Although figure 2(b) shows an intense peak dominating background radiation and repeating over a few successive periods, one should refrain from concluding that stable mode locking takes place: indeed, such peak could be an ephemeral feature appearing as part of a quasi-CW regime, as reported in [30]. On the other hand, the average spectrum (figure 2(c)), centered at 1062 nm with a spectral width of ∼0.82 nm, is also compatible with quasi-CW operation and could indicate that there is no mode locking at play; however, it is important to remark that the average spectrum (as measured with an OSA) does not provide a complete spectral characterization, which would involve the acquisition and analysis of real-time spectral evolution over the successive cavity periods.…”

“…Over broad regions of angular adjustments of the polarization controller plates (corresponding to saturable absorber action through NPR and polarizer in the ring cavity), quasi-CW operation characterized by the existence of ephemeral spikes was found, as described in [30]. However, in a reduced number of narrow angular regions, the observed spikes were found to be significantly more persistent, although still presenting ample variations.…”

“…In a previous study, we characterized a particular type of quasi-CW (i.e. closer to CW than to mode locking) operation in an ANDi laser, dominated by ephemeral spikes which were continually emerging from background radiation and subsequently vanishing; this study revealed that even such regimes very far from mode locking can actually display complex puzzling dynamics, whose underlying physics is yet to be elucidated [30]. In the present work, we analyze a few examples of dynamics taking place over different time scales in an ANDi fiber laser, which are midway between CW and mode-locked operations, in order to illustrate the broad diversity of nonstationary regimes accessible with these sources.…”

“…zero on the fast time axis), the information on the trajectory of the main pulse (whose temporal position evolution is driven by dispersive, nonlinear and dissipative effects) is lost, and the trajectories of all the other structures that constitute the intensity profile are only observed relative to that main pulse. Secondly, there may exist in the signal no suitable permanent main pulse to be used as a reference, as illustrated in [30].…”