Semiconductor lasers subject to optical feedback are of both scientific and technological importance. The laser and external mirror form a compound cavity which can be modeled using a coupled cavity approach. Using this approach we have calculated the longitudinal mode frequencies and the associated mode lifetimes of all the modes in the system. We have found that for a narrow range of external cavity reflectivities, the mode lifetimes become almost degenerate. In this regime the mode lifetimes become highly sensitive to variations in the refractive index of the laser and external cavity reflectivities. Close inspection of this regime has revealed ever finer detail, suggesting a fractal structure to the mode solutions. The calculations have been undertaken in the 'cold cavity' limit and indicate a catastrophic loss of mode discrimination, even in a system with no inherent material gain. In the short cavity limit the external reflectivity required to induce the loss of mode discrimination varies as a function of external cavity round-trip time.
Synchronized external-cavity diode lasers are used for chaotic optical communication. A 1 GHz message is encoded by current modulation of the chaotic transmitter laser and decoded by a synchronzied chaotic receiver laser diode. The effect of introducing the message by optical injection into the receiver laser instead of direct current modulation is then investigated. Finally, we show how the signal masking can be enhacned by the appropriate choice of external cavity length for a given message frequency.
A nearly single mode vertical cavity surface emitting laser (VCSEL) subject to optical injection have been investigated experimentally. It is found that regions of polarization-resolved chaotic behavior exist for both positive and negative detuning from the stable injection locking regime. Outside the chaos regimes, several nonlinear dynamical phenomena including frequency pushing, nearly degenerate four-wave mixing, injection locking, limit cycle and period doubling, were also observed.
Chaotic dynamics in a self-pulsating laser diode has been shown theoretically to occur by modulation of the laser current. It has been also shown that synchronization of two chaotic self-pulsating lasers can be achieved by small amounts of optical coupling. This result has been obtained with a deterministic model for the laser intensity. We study coherent synchronization of single mode self-pulsating laser diodes by means of a field-equation model that takes into account phase-effects and spontaneous emission noise. It is shown that the size of the coupling required to achieved synchronization is influenced by spontaneous emission noise and by the linewidth enhancement factor. Numerical simulations are then used to identify the optimum regime for efficient synchronization. It is found that good synchronization can be obtained for large values of the bias current, such that the spontaneous emission plays a minor role. The degree of synchronization is studied as a function of the differences between the master and slave laser parameters. Finally, a sinusoidal signal is used to analyze a chaotic communication system based on self-pulsating laser diodes.
Two-transverse-mode vertical cavity surface emitting lasers (VCSELs) subject to external optical injection is experimentally investigated. The spectra of the VCSEL with two parallel-polarized modes, under optical injection, demonstrate significantly different behaviors, compared to the VCSEL with two orthogonally-polarized transverse modes. In a VCSEL with two orthogonally-polarized transverse modes, the experimental results are good qualitative agreement with the theoretical prediction that the external optical injection can be used to achieve single-mode operation. However, in the case of a two parallel-polarized transverse modes VCSEL, single-mode operation cannot be obtained via optical injection. When the injection frequency is close to one of the modes, the coexistence of the injected beam and the mode gives rise to very rich dynamical behavior, however, the other mode is unperturbed by the optical injection.
Chaotic synchronization is investigated using two diode lasers as transmitter and receiver. The transmitter laser is rendered chaotic by application of an optical feedback in an external-cavity configuration. Changes in the spectrum of the synchronized system are shown to be associated with the transition between anticipating and lag synchronization.
This paper presents an overview of work undertaken and directed at the utilization of chaotic laser diodes in secure optical communications systems. Particular emphasis will be given to experimental work using external cavity laser diodes.