Analysis of the Spike Rate and Spike Correlations in Modulated Semiconductor Lasers With Optical Feedback

We investigate experimentally how changes in the external cavity length and dc current affect the mean interspike-interval (ISI) in a modulated semiconductor laser with optical feedback operating in the low-frequency fluctuations regime. The variation of the mean ISI with the modulation frequency is shown to be more pronounced when time delay and dc current allow for low spike rate. We use the method of ordinal symbolic analysis to examine how time correlations (among three, four and five consecutive laser spikes) change with the spike rate. This method is able to capture subtle changes, otherwise hidden in the dynamics. We find that higher spike rates wash out the effects of the modulation in the time correlations. Simulations using the Lang and Kobayashi model are in good qualitative agreement with the experimental observations.