Dynamical behavior of delay-coupled semiconductor lasers

Author: Cristina Martinez Gonzalez
Title: Dynamical behavior of delay-coupled semiconductor lasers
Director: M. C. Torrent and J. García-Ojalvo
Presentation date: January 30, 2009
Link to text: http://www.tdx.cat/handle/10803/108903

Abstract: In this Thesis we have presented the results of a set of experimental investigations of the dynamics of delay-coupled semiconductor lasers. The characterization of these systems includes topics such as the processing of information by coupled lasers, the choice of a leader in their lag-synchronized dynamics, or how chaos arises in two coupled semiconductor lasers. • The transition to chaos of a semiconductor laser through a quasiperiodic state was reported in the past adding an optical feedback or through mutual coupling. We studied the quasiperiodic route to chaos of two semiconductor lasers mutually coupled through two independent unidirectional paths. The transition from unidirectional to bidirectional coupling was implemented by increasing the coupling strength of one of the paths, while keeping the other constant. We have seen how the transition from stable unidirectional injection to chaos arises through a quasiperiodic state. Adjusting the currents and temperatures of the lasers in a unidirectional configuration, we maximize the injection by choosing optical frequencies sufficiently close to each other. In this regime, the output intensity of the receiver laser is forced to oscillate at the relaxation oscillation frequency of the driver, with a delay time introduced by the interaction. When the back injection is slightly increased, the external modes of the other cavity are excited and the laser output becomes quasiperiodic. This quasiperiodic behavior is produced by a competition between the external compound-cavity modes and the relaxation oscillation frequencies of both lasers. When the mutual coupling is sufficiently high the chaotic behavior appears. • We have examined how a system of two coupled chaotic oscillators behaves under conditions of lag synchronization. Our experimental setup consists on two semiconductor lasers coupled via mutual injection of their emitted fields, and allows for the control of the coupling directionality. Our results show that the laser leading the dynamics changes depending on the coupling scenario. When one of the lasers has autonomous chaotic dynamics, in the form of irregularly spaced sudden power dropouts (low-frequency fluctuations), and this dynamics is injected into a solitary laser (which is stable in the absence of injection), the injecting laser obviously leads the dynamics. Such role, however, can be transferred to the other laser by converting the coupling from unidirectional to bidirectional. The transition occurs via a state in which the two lasers alternate randomly the leader and laggard roles. This type of behavior is not only restricted to the low-frequency dynamics that we have studied experimentally, but also to fully developed chaotic dynamics (coherence collapse) that occurs for higher pump currents, as shown by numerical simulations. Our model also shows that the type of dynamics can be changed in a continuous way by acting upon the optical feedback strength affecting the laser with independent dynamics. We have discussed the potential of this system for bidirectional chaotic communications. Our experimental results show that whenever one of the lasers leads the dynamics, the other laser (the laggard) is able to operate as a chaos-pass filter. However, we have not been able to send information bidirectionally in an effective way. Numerical simulations show that, even though the maximum cross-correlation is similar in both the unidirectional and bidirectional cases, sudden synchronization losses in the latter situation prevent the system from being used as a reliable setup for bidirectional chaotic communications.