Routes to Chaos in External Cavity Semiconductor Lasers

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Semiconductor injection lasers are frequently operated in external cavities for purposes such as spectral narrowing and mode stabilization. However, the external cavity laser is capable of exhibiting a wide range of dynamical behavior, including self-pulsing instabilities and chaos. Currently there is limited understanding of these phenomena, particularly with respect to the crucial question of whether the behavior is stochastic (noise-driven), deterministic or a combination of both. Here we describe experimentally and theoretically the evolution of coherence collapse at low levels, and the onset of another form of coherence collapse as well as self-pulsations and chaos at high levels of feedback (> 1%). Our experiments have used GaAs/GaAlAs index-guided lasers coupled to long (10-60 cm) linear external cavities, without frequency-selective elements such as gratings or etalons. The theoretical analyses of these experiments are based on the usual rate equations for the carrier density and complex optical electric field, the latter equation having an additional time-delayed feedback term for weak external cavities and several such terms for strongly coupled cavities. Good agreement between experiment and theory has been obtained when considering low-feedback coherence collapse, as well as self-pulsations, quasiperiodicity and period doubling in strong feedback situations with tilt asymmetries. Double external cavity lasers can exhibit extreme stability but are susceptible to the same complex dynamical behavior as asymmetric single external cavity systems.