We report a direct experimental observation of chaotic itinerancy in simultaneous measurements of the light intensity and voltage fluctuations of a laser diode exhibiting low-frequency fluctuations. The distribution of trajectories leading up to (following) an intensity dropout is computed from the experiment and reveals the presence of itinerant mechanisms before (after) dropout initiation. A phase space reconstruction of the trajectory for the optimal path of motion illustrates sudden shifts between low-dimensional attractor ruins and is shown to correspond to simulations of the laser intensity and carrier number.
The dynamical behavior of power dropouts in a semiconductor laser with optical feedback, pumped near threshold current, is strongly influenced by quantum noise. This is clearly demonstrated by experiments with modulations on the pumping current or the feedback strength. For the cases without modulation and with only current modulation, the dropouts occur randomly. However the feedback strength modulation locks the dropout events periodically. By numerically modeling these three cases using the Lang-Kobayashi equations with a stochastic term to take into account spontaneous emission noise, it is shown that the observed behavior of the dropouts can be readily reproduced for all three cases. Noise plays a signifcant role in explaining the observed dropout events. A simple explanation of the observed dropout phenomenon is presented, based on the adiabatic motion of the ellipse formed by the steady state solutions of the rate equations due to slow time modulations of the injection current or the feedback strength.
The phase dynamics of a semiconductor laser with optical feedback is studied by construction of the Hilbert phase from its experimentally measured intensity time series. The Hurst exponent is evaluated for the phase fluctuations and grows from 0.5 to approximately 0.7 (indicating fractional Brownian motion) as the feedback strength is increased. A comparison with numerical computations based on a delay-differential equation model shows excellent agreement and reveals the relative roles of spontaneous emission noise and deterministic dynamics for different feedback strengths.
The nonlinear dynamics of a semiconductor laser with an optical feedback is studied using Hilbert phase analysis, which reveals interesting facets of the nonlinear dynamical behavior, including the formation and interaction of external cavity modes with increasing external feedback. Here we report measurements on two illustrative cases with very different dynamics; the laser is first biased just near threshold, and then far above threshold. We observe 2pi phase jumps at intervals that are multiples of the external cavity round-trip time, indicating the interaction and transfer of energy between external cavity modes.
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