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| Semiconductor lasers have been used in the last few decades to experimentally explore a variety of dynamical phenomena, among them regular and extreme pulses, multistability, intermittency, quasiperiodicity and chaos. When a semiconductor laser is operated near the threshold current with a moderate level of optical feedback, the output intensity shows sudden, apparently random, dropouts known as low frequency fluctuations (LFF). In the LFF regime semiconductor lasers exhibit excitable behavior and, through the modulation of the injection current, the system can be easily periodically forced. In this work we study the dynamics of such a system using ordinal time-series analysis. This method takes into account the order of consecutive inter-dropout time intervals and allows us to uncover serial correlations in the sequence of intensity dropouts exhibited by the laser. We transform the sequence of inter-dropout intervals into a sequence of ordinal patterns and we analyze the statistics of the patterns and of the transitions between them. We unveil correlations among several consecutive dropouts and we identify clear changes in the dynamics as the modulation amplitude and frequency vary. |
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