We investigate correlations of the intensity fluctuations of two-dimensional arrays of non-identical, locally-coupled lasers, numerically and experimentally. We find evidence of a power-law dependence of spatial correlations as a function of laser pair distance (or coupling strength) near the phase-locking threshold.
We show that vertical-cavity surface-emitting lasers (VCSEL) subject to a polarization preserving optical feedback from a long external cavity may exhibit two different types of low-frequency fluctuations (LFF) regime. In both types, the total intensity shows random drops and then recovers gradually to its original value. But the two LFF types are clearly distinguished by the behavior of the x- and y-linearly polarized (LP) modes. Type I LFF is characterized by the fact that the x- and y-LP modes compete with nearly equal power, while type II LFF is characterized by intensity dropouts in the dominant polarization mode and intensity bursts in the depressed mode. We analyze the parameter regions in which these different LFF occur and characterize them further. Our numerical results suggest that a transition from one LFF type to the other may be experimentally observed by modifying the frequency difference and/or the gain-to-loss ratio of the two LP modes. The LFF we present is an example of vectorial chaos, which may be of use for multichannel secure communication systems using chaotic light.
Semiconductor lasers with optical feedback are prone to exhibit unstable behavior. When working near threshold with moderate to low optical feedback, intensity dropouts are observed. These intensity drops, also called low-frequency fluctuations, occur both in single-mode and multimode semiconductor lasers. In this paper, the dynamics of the power distribution between the longitudinal modes of a multimode semiconductor laser is experimentally and numerically analyzed in the low-frequency fluctuation regime. It is observed that power dropouts of the total intensity, corresponding to drops in the dominant modes of the laser, are invariably accompanied by sudden activations of several longitudinal side modes. These activations are seen not to be simultaneous to the dropouts of the main modes, but to occur after them. The phenomenon is statistically analyzed in a systematic way, and the corresponding delay is estimated, leading to the conclusion that the side mode activation is a consequence of the dropouts of the dominant modes. A multimode extension of the Lang-Kobayashi equations is used to model the experimental setup. Numerical simulations also exhibit a time delay between the side-mode activation and the power dropout of the total intensity.
We demonstrate numerically a secure communication scheme based on the synchronization of two chaotic laser diodes that are respectively subjected to incoherent optical feedback and incoherent optical injection. In this scheme, the optical fields emitted by the two lasers and the fields that are fed back and injected into these two lasers have orthogonal polarizations. Consequently, the external fields do not coherently interact with the lasing fields but only act on the population inversions. Synchronization of both lasers does not require fine tuning of their optical frequencies neither accurate control of the external cavity lengths contrary to the cryptographic systems based on conventional optical feedback. The message encoding/decoding is achieved by chaos shift keying.
We demonstrate numerically and experimentally that low- frequency fluctuations (LFF) in a laser diode subject to delayed optical feedback can be suppressed or stabilized by a second optical feedback with a short delay. The second feedback suppresses LFF by shifting antimodes far away from the external cavity modes in phase space, or by making them disappear, with the consequence that the crises that induce the power dropouts are no longer possible. Moreover, as the second feedback strength increases, the laser undergoes a bifurcation cascade with successive regions where it exhibits chaos or LFF and regions where it locks to a newly-born stable maximum gain mode. This all-optical stabilization technique is easier to implement from an experimental point of view than many existing methods since it does not require modification of any laser parameters or of the first optical feedback.
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